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21 New compounds and structures William T. A. Harrison Department of Chemistry, University of Aberdeen, Aberdeen, UK AB24 3UE. E-mail: w.harrison@abdn.ac.uk 1 Introduction Synthetic and structural solid state chemistry is one of the most thriving and varied areas of the subject. Any survey that attempted to be fully comprehensive would be impossibly unwieldy. Therefore, this review attempts to provide an overview of some of the most significant developments reported in 1998.As in previous years, it focuses largely on new structure types containing extended inorganic lattices. 2 Oxides Many new vanadium oxide phases have been prepared by a variety of synthesis methods. A new polymorph of VO 2 , designated VO 2 (C), containing sheets of vertexsharing VO 5 square pyramids was prepared from VO 2 ·0.5H 2 O by chimie douce methods.1 Two vanadium oxide phases, BaV 7 O 16 ·nH 2 O,2 and Zn 2 (OH)VO 4 ,3 have been prepared by combined hydrothermal/electrochemical methods as single crystals.The former phase contains both VO 4 tetrahedra and distorted VO 6 octahedra and an average vanadium oxidation state of 4.29. The latter contains tetrahedral VV in a new superstructure modification of the adamite [Zn 2 (OH)AsO 4 ] structure type.Cr 0.11 V 2 O 5.16 4 has been prepared by a sol–gel process and has a structure (with ordered Cr and V) related to that of orthorhombic V 2 O 5 . Hydrothermally prepared SrV 12 O 27 ·3H 2 O (average V oxidation state\4.33) contains VO 6 octahedra andVO 5 square pyramids enclosing wide tunnels occupied by Sr2` cations and H 2 O molecules. 5 Fe 2 V 4 O 13 ,6 prepared by cooling from the melt, contains edge-sharing Fe 2 O 10 octahedral dimers and U-shaped [V 4 O 13 ]6~ clusters.A surprising double substitution mechanism (MoVI for VV accompanied by a stoichiometric quantity of additionalO2~ ions) results in the formation of Fe 2 V 3.16 Mo 0.84 O 13.42 . The isostructural FeVMoO 7 and CrVMoO 7 contain7 very unusual[VMoO 7 ]3~ ions.Single crystal studies showed perfect ordering of the VV and MoVI cations and the presence of an isolated Mo––O bond. a-CoV 3 O 8 contains8 VIV and VV and is antiferromagnetic below 8K. Ceramic methods were used to synthesise BiCa 2 VO 6 9 and BiCu 2 VO 6 10 (Fig. 1) which both Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 409–429 409Fig. 1 Structure of BiCu 2 VO 6 as VO 4 tetrahedra and BiO 2 chains. Open circles are Cu atoms. (Reproduced by permission from J. Solid State Chem., 1998, 141, 149.) contain [BiO 2 ]~ chains and [VO 4 ]3~ tetrahedra. However, the di§erent coordination preferences for calcium (irregular seven-fold) and copper (square pyramidal]distorted octahedral) lead to quite distinct structures.BiCa 2 VO 6 (space group Cmc2 1 ) may be ferroelectric. A soft chemical route e§ected the conversion of (NC 7 H 14 ) 4 ·H 2 V 10 O 28 to the lamellar (NC 7 H 14 )·V 4 O 10 ,11 which contains disordered VIV and VV on one square-pyramidal site. Hydrothermally prepared (N 2 C 4 H 12 )·V 4 O 9 12 is also layered and contains a new topology of edge-sharingVIVO 5 square pyramids. La 3 Co 3 O 8 has a structure ‘intermediate’ between perovskite and brownmillerite.13 It contains vertex-linked CoO 6 octahedra and CoO 4 tetrahedra, and orders antiferromagnetically at temperatures below 35 K.La 4 Co 3 O 9 is related to three-layer Ruddlesden –Popper phases, but contains an octahedral/tetrahedral/octahedral layer motif. It may be structurally derived from La 4 Co 3 O 10 by ordering of oxygen vacancies and is antiferromagnetic below 305 K.14 La 2 Ti 2 O 7 ,15 prepared by topotactic dehydration of HLaTiO 4 , possesses a defective two-layer Ruddlesden–Popper structure. Careful synthetic work has resulted in several layered titanates: Na 2~x`yCax@2 La 2 Ti 3 O 10 is a triple-layered phase with unusual electrical properties.16 BaLa 4 Ti 4 O 15 contains17 perovskite-like blocks four TiO 6 octahedra thick.It is the n\5 member of the series (Ba,La)nTin~1 O 3n and shows Ba/La ordering over the 12-fold coordinated sites. Ba 5 Fe 4 Ti 10 O 31 exhibits18 a complex 18-layer hexagonal structure. The octahedral cation sites contain a mixture of FeIII and TiIV, the tetrahedral sites only FeIII. A much simpler phase is NaTiO 2 , which shows19 a symmetry change from trigonal to monoclinic and associated magnetic anomalies on cooling which may be correlated with an increase in the Ti–Ti bonding. 15R SrMn 1~xFexO 3~x (x[0.1) (Fig. 2) crystallises20 in a new 15-layer hexagonal perovskite stacking sequence denoted (cchch) 3 . Face-, edgeand corner-sharing occurs for the octahedral sites. Neutron di§raction measurements showed that the MnIV cations order ferromagnetically (T N \220 K), whilst the FeIII cations remain magnetically disordered at 3K.A new series of cuprates, [M 2 Cu 2 O 3 ]m[CuO 2 ]n (m\n\1), whereMis an alkaline earth or trivalent metal have been reported,21 some of which show a T#[85K Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 409–429 410Fig. 2 15R crystal structure of SrMn 1~xFexO 3~x: circles represent Sr2` cations, octahedra represent (Mn,Fe)O 6 units.The three transition metal sites are labelled. (Reproduced by permission from Inorg. Chem., 1998, 37, 6071.) superconducting transition. Their structures were interpreted in terms of ‘the commensurate version of two fragments due to the reconstruction of the Cu–O ribbons.’ Nd 4 Cu 2 O 7 is an unusual coppper(I) oxide containing two- and four-coordinate copper sites which may be viewed as a cooperatively distorted T@-type [Nd 2 CuO 4 ] structure. 22 Tl(Sr 1.4 La 2.6 )Ni 2 O 9 is a direct23 nickel-containing analogue of the superconducting cuprate Tl(Ba 2~xLa 2`x)Cu 2 O 9 .The structure of Mn 3 Ta 2 O 8 was determined24 ab initio from synchrotron and electron di§raction data and is a complex tetragonal I5]I5]2 fluorite superstructure.SrMnTeO 6 is characterised25 by staggered (Mn,Te) 6 O 18 Kagome� layers along [001] and SrO 6 hexagonal prisms. The ‘condensed’ silicates Fe 4 Si 2 Sn 7 O 16 26 and Cs 4 CuSi 2 O 7 27 were prepared by solid state methods. Structural, Mo� ssbauer and susceptibility measurements indicated that the former phase contains mixed-valent tin (SnII, SnIV).The latter phase contains square planar CuIIO 4 and Si 2 O 7 groups incorporated into CuSi 2 O 7 ribbons. The hexagonal A 4 A@Ir 2 O 9 (A\Sr, Ba; A@\Cu, Zn) family of phases28 are very sensitive to preparation conditions, and may crystallise in commensurate or incommensurate modifications depending on thermal treatment, but their magnetic properties appear to be unchanged. The related Sr 6 Rh 5 O 15 contains29 infinite face-sharing Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 409–429 411Fig. 3 Arrangement of the four di§erent Mo clusters within the hexagonal unit cell in Pr 4 Mo 9 O 18 . (Reproduced by permission from Inorg. Chem., 1998, 37, 6229.) columns built up from four RhIV octahedra interspersed by RhII trigonal prisms.Sr 3 MRhO 6 (M\Y, Sc, In)30 are isostructural with K 4 CdCl 6 and contain infinite chains of alternating RhO 6 octahedra and MO 6 trigonal prisms. The reaction of ammonia and ammonium heptamolybdate has led31 to two polymorphs of the simple molybdate (NH 4 ) 2 MoO 4 , which di§er in their hydrogen bond topologies. The layered molybdate (C 2 H 10 N 2 )[Mo 4 O 12 ] was prepared32 hydrothermally and contains a two-dimensional network of MoV- and MoIV-centred octahedra sandwiching ethylenediammonium cations.Several reduced-molybdenum containing phases containing complex clusters have been prepared. The semiconducting (E! \0.12 eV) phasesM 4 M@3 Mo 26 O 48 (M\Sr, Eu;M@\Al, Ga)33 are built up from Mo 7 groups and infinite Mo 7 –Mo 10 –Mo 7 chains. Pr 4 Mo 9 O 18 is an extremely complex phase (Fig. 3) containing Mo 3 , Mo 7 , Mo 13 and Mo 19 clusters.34 The Mo 7 cluster is planar and the Mo 13 and Mo 19 groupings are hemispherical and spherical, respect- Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 409–429 412ively. A detailed structural study35 aMo 2 O 5 by neutron di§raction and EXAFS measurements revealed the presence of disordered Mo 3 and Mo 6 clusters.Cu 2 Th 4 (MoVIO 4 ) 9 is cubic,36 with a three-dimensional network of ThO 9 and MoO 4 polyhedra delimiting cages occupied by the CuI species. Bi 3 NbO 7 is an oxygendeficient fluorite37 with promising ionic conduction properties up to 875 °C. Tl 8 Nb 27.2 O 72 is related to hexagonal bronzes.38 Pr 2 Nb 11 O 30 was prepared39 by chemical vapour transport methods and contains both six- and seven-coordinated niobium atoms.Se 4 Nb 2 O 13 [or (NbO) 2 (SeO 3 ) 2 (Se 2 O 5 )] is a new non-centrosymmetric phase40 containing SeO 3 and Se 2 O 5 moieties in combination with NbO 6 octahedra. Despite the presence of the asymmetric NbV and SeIV species, the non-linear optical response of this phase is very low. The metallic SbRe 2 O 6 contains41 slabs built up from pairs of ReO 6 octahedra sharing edges [d(Re · · · Re)\2.52Å] interspersed by the antimony species. Pb 6 Re 6 O 19 contains similar Re 2 O 10 groups and is also metallic. 42 A new metastable modification of bismuth oxide, Bi 2 O 3 , has been synthesised at high pressure and temperature.43 It is isostructural with a-La 2 O 3 and slowly transforms to other modifications of Bi 2 O 3 under ambient conditions.K 4 Bi 2 O 5 contains44 isolated [Bi 4 O 10 ]8~ groupings with BiIII showing both pyramidal and see-saw (SF 4 like) coordinations.Ba 2 BiGa 11 O 20 is structurally related to Ba 3 TiAl 10 O 20 but is complicated by disorder of the barium and bismuth cations which may be rationalised in terms of the stereochemically active lone pair of the latter species.45 Pb 2 TeO 5 contains46 isolated chains of distorted TeO 6 octahedra crosslinked by PbO 7 groups.a-Tl 2 Te 2 O 5 shows47 the structural e§ect of the stereochemical activity of both the thallium(I) and tellerium(IV) lone pairs and has strong two dimensional character. Ab initio powder methods were used to solve the hexagonal structure of the technologically important cement phase erroneously described as CaAl 2 O 4 ·10H 2 O (‘CAH 10 ’) which has been implicated in building collapses as a hydration product of CaAl 2 O 4 , the main phase of high alumina cement.48 The actual stoichiometry is better described by the formula CaAl 2 (OH) 8 (H 2 O) 2 ·1.84H 2 O.SrCe 2 O 4 is the first new inorganic structure to result from a combinatorial approach to solid state synthesis.49 Weinberg and co-workers49 reported the preparation and structural characterisation of this new material by ab initio powder methods.Initially, the putative SrCe 2 O 4 was selected from a ‘discovery library’ of some 25 000 chemical compositions on the basis of its photoluminescence properties. The structure contains linear chains of edgesharing CeIVO 6 octahedra. 3 Zeolites and molecular sieves It has been an exceptional year for the synthesis and characterisation of new zeolite structures. The chabazite modification50 of SiO 2 has the lowest framework density of any pure silica polymorph. Microcrystalline single crystal di§raction methods revealed that \[(C 13 H 24 N) 4.1 F 3.3 (OH) 0.8 (H 2 O) 1.6 ][SiO 2 ] 64 , denoted SSZ-23, contains very unusual seven- and nine-membered-ring pore openings.51 Similar methods showed that the borosilicate phase SSZ-42 contains undulating, one-dimensional 12-ring channels.52 It is thermally stable to at least 800 °C.The aluminosilicate framework of the remarkable blue volcanic mineral zeolite, tscho� rtnerite, Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 409–429 413Fig. 4 A unique 3-connected two-dimensional sheet in (CH 3 NH 3 ) 2 Ga 2 Ge 3 O 10 .The five T-atom unit surrounded by three 3-ring units are denoted as 4\1 units. The bridging atoms are omitted for clarity. (Reproduced by permission from J. Am. Chem. Soc., 1998, 120, 11 204.) \Ca 4 (K,Ca,Sr,Ba) 3 Cu 3 (OH) 8 [Si 12 Al 12 O 48 ]·xH 2 O (xq20), contains53 an enormous, new, 96-membered (tscho� rtnerite) cage, as well as double-six-rings, doubleeight- rings, sodalite cages and truncated cuboctahedra.Complex Cu–Ca–O clusters are a feature of the extra-framework content of this phase. The structure of ERS-7 (framework Si:Al ratio\25: 1), containing a new 17-sided ‘picnic-basket’ (46546582) cage was elucidated by simulated annealing (based on six independent tetrahedral atom sites in the asymmetric unit) and refined against synchrotron powder di§raction data.54 Stucky and co-workers have continued their trail blazing investigations into nonaluminium containing molecular sieves.(H 3 NCH 2 CH 2 NH 3 ) 0.5 ZnAsO 4 55 is the first amine-templated arsenate molecular sieve and crystallises in a new topology based on 4.82 sheets. [(H 3 C) 2 NH(CH 2 ) 2 NH(CH 3 ) 2 ]Co 2 Al 4 (PO 4 ) 6 56 contains two-dimensional 8-ring channels.There is a non-random distribution of cobalt and aluminium over the metal sites. No fewer than five distinct analogues of zeolite edingtonite were prepared in the aluminium–cobalt–phosphate and gallium–cobalt–phosphate systems with appropriate organic templates.57 The family of molecular sieves58 of various framework compositions (BeAsO, ZnAsO, GaGeO) denoted UCSB-7 possess two independent sets of three-dimensional crosslinked helical pores separated by a ‘gyroid periodic minimal surface.’ Perhaps the most interesting new framework composition is the gallogermanate system.The gallium: germanium ratio appears to be highly flexible, and both odd- and even-membered ring systems can be formed from the GaO 4 and GeO 4 building blocks.(CH 3 NH 3 ) 2 Ga 2 Ge 3 O 10 59 (Fig. 4) contains very unusual threemembered rings and fibrous-zeolite-like 4\1 subunits which combine to form infinite Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 409–429 414Fig. 5 Perspective view of the framework of gallophosphate Mu2 showing the two types of cage-like voids (one without OH group; the other with 8 T-OH groups); the Ga and P atoms are located at the vertices.(Reproduced by permission from Chem. Commun., 1998, 1769. 9-ring channels. (N 2 C 4 H 11 )GaGe 5 O 12 60 contains 5–1 secondary building units which assemble into a structure containing one-dimensional 10-ring channels. Other gallogermanates60 crystallise as analogues of sodalite (both cubic and triclinic variants), analcime, and known zincoarsenate frameworks.DIPYR-GaPO (Ga 7 P 6 O 28 F 3 N 2 C 10 H 16 ) contains61 14-ring channels with two amines (pyridine and 4,4@-dipyridyl) appearing to co-template this new framework. Mu2, or Ga 32 P 32 O 120 (OH) 16 F 6 ·(C 9 H 21 N 2 ) 6 ·12H 2 O,62 is a new open-framework gallofluorophosphate built up from double-four-ring subunits hosting fluoride ions in a similar manner to the situation in cloverite (Fig. 5). (NH 4 ) 2 Ge 7 O 15 contains both GeO 4 and GeO 6 polyhedra, which fuse together into a new framework containing 9-ring pores.63 4 Oxy-anions The group of Cheetham have continued their exploration of organically templated tin(II) phosphates. The polyhedral building units of the novel structures of [Sn 3 (PO 4 ) 2 (OH)]·0.5[H 3 N(CH 2 ) 6 NH 3 ] and [Sn 3 (PO 4 ) 2 (OH)]·0.5[H 3 N(CH 2 ) 8 NH 3 ] are truncated SnO 4 square pyramids, SnO 3 trigonal pyramids and PO 4 tetrahedra; Sn–O–Sn bonds result from this connectivity.64 The framework of [CN 3 H 6 ]- Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 409–429 415Fig. 6 Polyhedral views of (C 4 H 12 N 2 ) 2 [Fe 6 (HPO 4 ) 2 (PO 4 ) 6 (H 2 O) 2 ] ·H 2 O (top) and [HN(CH 2 CH 2 ) 3 NH] 3 [Fe 8 HPO 4 ) 12 (PO 4 ) 2 (H 2 O) 6 ] (bottom) emphasising the threedimensional Fe/P/O frameworks surrounding channel systems occupied by the organic species. (Reproduced by permission from Chem.Mater., 1998, 10, 2599.) [Sn 4 P 3 O 12 ] is chiral (space group P4 3 or P4 1 ) and enantiomeric crystals can be hand sorted.65 The three-dimensional [Sn 4 P 3 O 12 ]·0.5[NH 3 CH 2 CH 2 CH(NH 3 )CH 2 CH 3 ]· 2H 2 O loses water (but not template) reversibly and appears to show ion-exchange propert66 [C 2 N 2 H 10 ][Sn 2 (PO 4 ) 2 ]·H 2 O contains infinite layers of strictly alternating pyramidal SnO 3 and tetrahedral PO 4 moieties.67 The related Sn 2 (PO 4 )[C 2 O 4 ] 0.5 adopts a one-dimensional structure with respect to Sn/P/O connectivity.68 Lii et al.69 have characterised a family of organically templated iron phosphates (Fig. 6). For example, using piperazine as a template leads to at least three distinct structures: (C 4 H 12 N 2 )[Fe 4 (OH) 2 (HPO 4 ) 5 ], (C 4 H 11 N 2 ) 0.5 [Fe 3 (HPO 4 ) 2 (PO 4 )(H 2 O)] and Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 409–429 416(C 4 H 12 N 2 ) 2 [Fe 6 (HPO 4 ) 2 (PO 4 ) 6 (H 2 O) 2 ]·H 2 O.70,71 Interesting features of these phases include the presence of mixed valent iron (FeII and FeIII) and FeO 6 octahedra and FeO 5 trigonal bipyramids as polyhedral building blocks.Numerous other organically templated phosphates continue to be reported. A novel room-temperature synthesis in silica gel was used to prepare the layered phases R·M(HPO 4 ) 2 ·2H 2 O (R\C 5 NH 6 `, C 3 N 2 H 5 `; M\Al, Ga) which contain vertexsharing trans-MO 4 (H 2 O) 2 octahedra and HPO 4 tetrahedra.72 However, C 3 N 2 H 5 ·Al(HPO 4 ) 2 ·2H 2 O could also be prepared73 solvothermally at 160 °C.CN 3 H 6 ·Al(HPO 4 ) 2 ·2H 2 O 74 (prepared at 120 °C) adopts74 essentially the same structure, and may show ‘template cooperation’ in terms of infinite stacks of guanidinium cations.The layered *,*-Co(en) 3 [Al 3 P 4 O 16 ]·xH 2 O contains more typical AlO 4 tetrahedra and may be converted to the chain-like phase Co(en) 3 [AlP 2 O 8 ]·xH 2 O by hydrothermal treatment.75 The layered (CN 3 H 6 ) 2 ·(VO 2 ) 3 (PO 4 )(HPO 4 ) contains76 the hexagonal tungsten oxide motif of vertex-sharing VVO 6 octahedra capped by phosphate groups with guanidinium cations serving to pillar the sheets.Organically templated zinc phosphates may crystallise as one-dimensional [H 3 N(CH 2 ) 3 NH 3 ·Zn(HPO 4 ) 2 ],77 two-dimensional [H 3 N(CH 2 ) 3 NH 3 ·Zn 2 - (HPO 4 ) 2 (H 2 PO 4 ) 2 ],77 or three-dimensional [H 3 NCH 2 CH 2 NH 3 ·Zn 6 (PO 4 ) 4 - (HPO 4 )(H 2 O)]78 structures in terms of Zn/P/O connectivity.However, the lack of any e§ective control or predictability of such syntheses remains a serious shortcoming of such methods. The first layered, organically templated uranium(VI) phosphates,79 NH(CH 2 CH 3 ) 3 ·(UO 2 ) 2 (PO 4 )(HPO 4 ) and N(CH 2 CH 2 CH 3 ) 4 ·(UO 2 ) 3 (PO 4 )(HPO 4 ) 2 , contain sheets of edge-sharing UO 7 pentagonal pyramids crosslinked by PO 4 tetrahedra.The uranyl [UO 2 ]2` groupings are close to linear. (C 2 H 9 N 2 ) 6 ·Co 3 Mo 4 P 4 O 28 80 is a three-dimensional open-framework phase built up from CoO 6 , MoO 6 and PO 4 groups which surround unusual seven-membered intersecting channels. The ethylenediamine template may be removed by thermal treatment without structure collapse. Several new simple inorganic phosphates have been described.InPO 4 ·H 2 O, containing In 2 O 8 (H 2 O) 2 dimers, represents81 the ‘missing link’ between InPO 4 ·2H 2 Oand InPO 4 . NH 4 CuPO 4 ·H 2 O, which orders antiferromagnetically at very low temperatures, 82 is layered, with CuO 5 square pyramids fusing together into Cu 2 O 8 dimers, which in turn are crosslinked by phosphate groups into infinite sheets. NH 4 VOPO 4 ·H 2 O shows83 strong one-dimensional character based on double chains built up from vertex-linked VO 4 (H 2 O) square pyramids and PO 4 tetrahedra.No magnetic ordering was observed at 4K. High temperature–high pressure methods were used to prepare NH 4 Fe 2 (PO 2 ) 2 which contains84 fully ordered FeIIO 6 and FeIIIO 6 octahedra. b-LiZnPO 4 ·H 2 O85 and NaZnPO 4 -ABW86 crystallise as new modifications of the zeolite-ABW [LiAlSiO 4 ·H 2 O] tetrahedral topology with previously unobserved crystal symmetries for this structure type.Ba 3 V 2 O 3 (PO 4 ) 3 87 and Ba 2 V 5 O 8 (PO 4 ) 4 ,88 prepared by high-temperature methods, are the first mixed-valence (VIV and VV) barium vanadium phosphates. The latter phase contains three di§erent vanadium coordinations–pyramidal, octahedral and tetrahedral.The hydrothermally prepared Ba 2 (VO 2 )(PO 4 )(HPO 4 )·H 2 O is noncentrosymmetric, transparent, and contains unusual VVO 5 trigonal bipyramids, linked by phosphate groups into infinite chains.89 The new bismuth oxyphosphate, Bi 6.67 (PO 4 ) 4 O 4 , contains90 BiO 5 , BiO 6 and BiO 8 polyhedra. Substitution of bismuth Annu. Rep. Prog. Chem., Sect.A, 1999, 95, 409–429 417Fig. 7 (opposite) The structures of (a) [H 3 N(CH 2 ) 3 NH 3 ]U 2 F 10 ·2H 2 O (UFO-1), (b) [H 3 N(CH 2 ) 4 NH 3 ]U 2 F 10 ·3H 2 O (UFO-2), and (c) [H 3 N(CH 2 ) 6 NH 3 ]U 2 F 10 ·2H 2 O (UFO-3) viewed parallel to the uranium fluoride layers (along the [010] direction) showing the location of the organic template in each case. The carbon atoms are represented by dark spheres, the nitrogen atoms by lighter spheres.The occluded water molecules are excluded for clarity. In UFO-1, the [H 3 N(CH 2 ) 3 NH 3 ]2` cation lies on an inversion center and the end carbon atom is disordered around this position. For clarity, this disorder is not represented in the figure and only one orientation of the template is shown. (Reproduced by permission from Chem.Mater., 1998, 10, 3131.) by both alkali metal and alkaline earth cations is possible, leading to the Bi 6.5 A 0.5 (PO 4 ) 4 O 2 (A\Li`, Na`, K`) and Bi 6 M(PO 4 ) 4 O 4 (M\Sr2`, Cd2`, Pb2`, Ca2`) series, respectively. PbIISnIV(PO 4 ) 2 shows91 the expected stereochemical activity of the lead(II) lone pair in a new structure type related to the mineral yavapaiite [KFe(SO 4 ) 2 ].The Li 9 M 3 (P 2 O 7 ) 3 (PO 4 ) 2 (M\Al, Ga, Cr, Fe) series92 are isostructural and display lithuim ion conductivity and ion-exchange properties. Na 2 MnIIP 2 O 7 93 was prepared from a high temperature reaction and is composed of slabs of fused Mn 4 P 4 O 26 cages. Synthetic taranakite,94 K 3 Al 5 (HPO 4 ) 6 (PO 4 ) 2 ·18H 2 O, contains the longest crystallographic axis (94.98Å) of any mineral described hitherto.Slabs of composition K 3 Al 5 (HPO 4 ) 6 (PO 4 ) 2 ·12H 2 O are separated from each other by water layers. The group of Raveau have continued their exhaustive investigations of phosphates of the heavier transition metals. The MoV-containing Li 3 (MoO) 2 (PO 4 ) 3 is related to Na 3 (MoO) 2 (PO 4 ) 3 , but the modified polyhedral connectivity in the lithium phase results in di§erently shaped channels accommodating the smaller Li` cation.95 Similarly, the A 2 Mo 3 O 5 (PO 4 ) 3 (A\Cs, Rb, K, Tl) series96 adopt a three-dimensional structure in which the [Mo 3 P 3 O 17 ] 2 ~ framework is flexible in accommodating di§erently sized univalent cations.The AMoOPO 4 Cl (A\K, Rb) structure97 is strongly layered in nature, and contains sheets of vertex-sharing MoO 5 Cl octahedra and PO 4 tetrahedra, akin to the situation described below for ANbOAsO 4 Cl.Structurally, K 3 Nb 3 WO 9 (PO 4 ) 4 98 is closely related to hexagonal tungsten bronzes. However, electrons are localised in this phase despite the existence of infinite octahedral [MO 3 ] chains. ANb 4 WO 9 (PO 4 ) 3 (A\K, Rb, Cs) and related phases99 are closely related to a defective ReO 3 structure and show electronic conduction properties describable by a ‘variable, range-hopping’ mechanism.The full structures of b-Ti(PO 4 )(H 2 PO 4 )100 and s-Zr(HPO 4 ) 2 101 were determined from neutron powder data. Hydrogen bonds appear to play key structural roles in stabilising the layered structure of the titanium phase, and the three-dimensional zirconium material.Several organo zirconium fluorophosphates102 of formula R·Zr 2 (PO 4 ) 2 (HPO 4 )F·H 2 O (R\1,3-diaminopropane, etc.) crystallise with the threedimensional ‘ZrPO-1’ framework (ZrO 6 and ZrO 5 F octahedra, PO 4 tetrahedra) and show very promising zeolite-like sorption properties.103 The fluoride-rich Na 7 Mn 5 F 13 (PO 4 ) 3 (H 2 O) 3 104 contains MnF 3 O 2 (H 2 O) octahedra showing a stong, static Jahn–Teller distortion along a F–Mn–OH 2 axis and MnF 3 O 3 octahedra only showing a weak, dynamical Jahn–Teller e§ect.Na 2 BP 2 O 7 (OH)105 contains layers of BO 4 /PO 4 tetrahedra enclosing 6- and 8-rings occupied by the sodium cations. Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 409–429 418Annu. Rep. Prog.Chem., Sect. A, 1999, 95, 409–429 419Arsenates are receiving more attention. Careful synthetic work was necessary to prepare the new Zn 9 (AsO 4 ) 6 ·4H 2 O (ZnO 6 , ZnO 5 , ZnO 4 and AsO 4 polyhedra) in reasonable yield.106 Zr 3 (AsO 4 ) 4 (H 2 O) 2 ·2H 2 O contains107 one-dimensional channels occupied by water molecules and may have useful ion-exchange properties. Ba 4 Mn 2 Ag 2 O(AsO 4 ) 4 is a new barium manganese oxide–arsenate incorporating nonlinear O–Ag–O dumbbells in its structure.108 The cubic Rb 3 Sc 2 (AsO 4 ) 3 is distinct from garnets and shows similarity to Na 4 BaCu 3 F 12 in terms of cation positions.109 Tl 2 Cu(AsO 4 )110,111 is constructed from thallium(I) cations and isolated [Cu 2 As 2 - O 8 ]4~ ions (components: AsO 4 tetrahedra and O–Cu–O dumbbells).K 2 Cu 2 (SeO 3 ),112 K 2 Cu 3 (SeO 3 ) 4 ,112 CoSeO 3 ·H 2 O-II,113 CoSeO 3 ·H 2 O-III,113 and Cr(HSeO 3 )(Se 2 O 5 )-II114 all adopt new crystal structures which have the common feature of pyramidal SeO 3 groups. The layered A 2 (MoO 3 ) 3 TeO 3 (A\NH 4 , Cs) tellurites contain115 pyramidal [TeO 3 ]2~ groups and are isostructural with their A 2 (MoO 3 ) 3 SeO 3 selenite counterparts.Mn(ReO 4 ) 2 ·2H 2 O contains116 interconnected ReO 4 tetrahedra andMnO 4 (H 2 O) 2 octahedra. Dehydration leads to the layered phase Mn(ReO 4 ) 2 (ReO 4 tetrahedra and MnO 6 octahedra). Ag 2 Cs(B 15 O 24 ) contains117 the novel, cyclic [B 15 O 24 ]3~ ion built up from twelve BO 3 and three BO 4 units. 5 Halides and oxyhalides O’Hare and co-workers reported the hydrothermal syntheses and single crystal structures of the first fully fluorinated organically templated materials–[H 3 N(CH 2 ) 3 NH 3 ]- U 2 F 10 ·2H 2 O, [H 3 N(CH 2 ) 4 NH 3 ]U 2 F 10 ·3H 2 O, [H 3 N(CH 2 ) 6 NH 3 ]U 2 F 10 ·2H 2 O, and [HN(CH 2 CH 2 NH 3 ) 3 ]U 5 F 24 .118 These layered UFO phases contain UF 8 and UF 9 polyhedra (Fig. 7), sharing edges and/or corners. The inter-lamella organic cations may be ion-exchanged, thus providing a mild-condition route to new metal–uranium –fluorides.More conventional fluorides reported in 1998 include NaLnCu 2 F 8 (Ln\Sm, Eu, Y, Er, Yb)119 which contain sheets of CuF 4 square planes interleaved alternately by Na` and Ln3` cations, both of which occupy square-antiprismatic sites. The light green Ni(NH 3 ) 2 V 2 F 8 120 was prepared adventitiously from the reaction of NH 4 F, vanadium metal and nickel extracted from the monel ampoule.It contains alternating layers of VF 6 and NiF 4 (NH 3 ) 2 octahedra. Ba 0.43(1) Sr 0.57(1) AlF 5 contains121 linear chains of AlF 6 units, whereas SrAlF 5 contains [Al 2 F 10 ]4~ dimers. Ba 2 Cu 2 AlF 11 adopts122 a new crystal structure containing a three-dimensional network of CuF 6 and AlF 6 octahedra which share both edges and vertices. The new phases La 3 Zr 4 F 25 and a-LaZr 3 F 15 emerged from a study of the LaF 3 –ZrF 4 pseudo binary system.123 The former is cubic and contains ZrF 6 octahedra and LaF 8 dodecahedra sharing vertices.The latter has an orthorhombic crystal structure containing LaF 6 trigonal prisms, ZrF 6 octahedra and ZrF 7 monocapped trigonal prisms.Eu3` luminescence properties were reported for both materials. KTbIIITbIV 2 F 12 contains124 perfectly ordered terbium(III) (as [TbF 8 ]5~ cubes) and terbium(IV) (as [TbF 8 ]4~ dodecahedra) cations. The phase transition behaviour of metastable b-CuAlCl 4 and the thermodynamically favoured a-CuAlCl 4 has been studied.125 Both phases contain large van der Waals channels and show reversible adsorption of CO and ethylene.The noncen- Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 409–429 420trosymmetric Ba 2 ScCl 7 contains126 isolated ScCl 6 octahedra and a ‘lonesome’ chloride ion bound only to Ba2`. Ab initio powder methods revealed that Rb 6 Pb 5 Cl 16 127 adopts a structure containing several di§erent metal coordination environments: RbCl 6`2 and PbCl 6`2 double capped boats, RbCl 9 tricapped trigonal prisms, and RbCl 8 cubes.One cation site contains a statistical (1: 3) distribution of Rb and Pb. The sealed-tube reaction128 of MoCl 3 and BiCl 3 led to an unusual light red material containing [Mo 6 Cl 14 ]2~ clusters and [BiCl]2` dumbbells. The Mo atoms are octahedral, and the bismuth atoms are distorted monocapped trigonal prismatic (one short]six very long Bi–Cl bonds).K 2 W 6 Br 14 , Rb 2 W 6 Br 14 and Cs 2 W 6 Br 14 adopt distinct crystal structures129 containing the [W 6 Br 14 ]2~ cluster. The cluster packing is pseudo-ccp for the potassium phase and hcp for the rubidium and caesium phases, with the alkali metal cations occupying octahedral holes.The complex formula of the subbromide Bi 34 Ir 3 Br 37 130 can be broken down into cuboctahedral [IrBi 6 Br 12 ]~ clusters, [IrBi 6 Br 13 ]2~ clusters, Bi 5 ` square pyramids and Bi 6 2` octahedra. KInBr 3 131 is a much simpler phase containing formal In2` cations as a component part of [In 2 Br 6 ]2~ groupings. BaScO 2 F, prepared by solid state reaction, appears to be the first example of a simple perovskite of stoichiometry AIIBIII[XII~] 2 [Y~].132 The oxide and fluoride ions are completely disordered.Ab initio methods showed the structure of SrHg 2 O 2 Cl 2 133 to contain unusual T-shaped HgIIO 3 groupings, as well as more familiar HgIIO 2 dumbbells. Together, these units form infinite sheets. The structure of SbTeO 3 Cl was also determined134 ab initio to reveal SbO 3 and TeO 3 pyramids linking together into strongly covalent [SbTeO 3 ]` sheets which are held together by chloride anions. Supercritical water was the reaction medium for the synthesis of Ba 2 Cu 4 Te 4 O 11 Cl 4 and BaCu 2 Te 2 O 6 Cl 2 .135 The tellurium(IV) atoms adopt pyramidal coordination in both phases.Structural and magnetic data indicated that the former phase contains mixed valent copper(I) and copper(II) whilst the latter material contains only divalent copper.The MNbAsO 5 Cl (M\Rb, Cs) types,136 grown as single crystals from a eutectic NaCl–KCl flux, are strongly layered with alternating NbAsO 5 and MCl slabs making up the structure. The structure can be visualised as ‘coordination intercalation’ of rock salt layers into the a-NbAsO 5 topology, with distorted NbO 5`1 octahedra replaced by NbO 5 Cl groupings.Bi 9 (V 1~xPx) 2 ClO 18 shows137 some resemblance to Aurivillius phases, with a major new feature of large one-dimensional tunnels occupied by chloride ions which show significant ionic mobility. NaW 2 O 2 Br 6 138 and Ag 0.37 WOBr 3 139 both contain trans-WO 2 Br 4 octahedra linked into infinite double chains.LnSCl (Ln\La–Nd)140 and Ln 7 S 6 Cl 9 (Ln\Pr, Nd)141 exemplify the rich and under-explored solid-state chemistry of the lanthanide–sulfide–chloride phase space. The basic lanthanide coordinations of square antiprism, trigonal prism and capped trigonal prism, are complicated by considerations of the S/Cl distribution over the anion sites. Ta 3 SBr 7 142 is the first example of a tantalum sulfide halide not isostructural with its niobium counterpart and may be regarded as a variety of the Cd(OH) 2 packing motif. 6 Sulfides Ozin and coworkers have done a great deal of work on the systematics of formation Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 409–429 421Fig. 8 A labelled portion of two pores from a single layer of [(C 3 H 7 ) 2 NH 2 ] 3 In 6 S 11 H. (Reproduced by permission from Chem.Commun., 1998, 1715.) and structures of microporous tin sulfides.143–146 Some clear correlations between the size and shape of template molecules and the topology of the resulting tin–sulfur framework were observed. A typical phase to emerge from this work was (C 6 N 2 H 14 ) 2 Sn 3 S 7 147 which contains Sn 3 S 4 ‘broken cube’ clusters as a component park of 24-atom rings.A facile, room-temperature synthesis was used. This area of chemistry is rapidly expanding, with respect to the metallic component of the framework. The first open-framework germanium sulfide, d-GeS 2 ,148 was prepared by the polycondensation of molecular [N(CH 3 ) 4 ] 4 Ge 4 S 10 under acidic conditions. Open framework indium sulfides reported last year include [(CH 3 ) 2 NH 2 ] 6 In 10 S 18 which is built up from vertex-linked [In 10 S 20 ]10~ ‘supertetrahedron’ clusters.149 All the In atoms are tetrahedral, with the sulfur atoms existing in terminal, bridging and trigonal geometries.This network encloses a channel system occupied by the highly disordered dimethylamine species. The layered [(C 3 H 7 ) 2 NH 2 ] 3 In 6 S 11 H150 contains pores of atom-to-atom dimensions of 6.9]12.8Å (Fig. 8). ‘Condensed’ metal sulfides are being studied by several groups. Mo 7 S 8 151 is the first example of a Chevrel type phase containg ‘extra’ molybdenum in the channels. LiAuS and NaAuS are not isostructural152 but both contain (AuS)n n~ threads which interweave into a ‘chicken-wire like’ motif involving Au· · ·Au interactions which appear to stabilise the structures.Bi 2 M 4 S 8 (M\Al, Ga)153 contains the hitherto unknown cluster cation Bi 2 4` [d(Bi–Bi)\3.14Å]. Tl 2 Au 4 S 3 is the x\4/3 member of the series A 2~xAuxQ.154 Gold–sulfur and gold–gold interactions are considered to be important in stabilising the structure. NaNbS 6 contains155 infinite anionic chains rather than the isolated anions of related compounds.PdGeS 3 was prepared156 from the elements at 600 °C and contains double chains of GeS 4 tetrahedra sharing corners encapsulating palladium cations in distorted square planar coordination to sulfur. Semiconducting Cu 4 Sn 7 S 16 is a defect spinel,157 which may be reformulated as (Cu 0.75 .0.25 )(Sn 1.75 Cu 0.25 )S 4 . The ternary sulfides A 3 MS 4 (A\Na, Rb; M\Nb, Ta) are Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 409–429 422Fig. 9 Crystal structure of SrSiAl 2 O 3 N 2 . (Reproduced by permission from Z. Anorg. Allg. Chem., 1998, 624, 1154.) based158 on a simple structure of isolated [MS 4 ]3~ ions and alkali metal cations. The A 2 Sn 4 S 9 (A\K, Rb, Cs) family159 contain a previously unseen form of the [Sn 4 S 9 ]2~ network.KIn 5 S 6 and MIn 5 S 7 (M\Na, K)160 contain In`, In3` and covalent pairs of [In 2 ]4` if an ionic model can be assumed to apply. A complicated substitution mechanism of alkali metal cations for In` occurs. Quaternary sulfides are being prepared in increasing numbers and show great structural variety. A 2 Hg 3 F 2 M 2 S 8 (A\Rb, Cs; M\Sn, Ge) contain161 both twocoordinate linear and three-coordinate pseudotrigonal Hg2` cations.Rb 2 Au 6 Sb 4 S 10 consists162 of two interpenetrating [Au 3 Sb 4 S 8 ]~ and [Au 3 S 2 ]~ frameworks. KCuGd 2 S 4 contains163 GdS 6 octahedra and CuS 4 tetrahedra delimiting tunnels occupied by the 8-fold coordinated potassium cations. K 2 Au 2 Ge 2 S 6 has one-dimensional character164 with [Ge 2 S 6 ]4~ units linked by linear, two-coordinate Au` ions.The A 2 Au 2 Cd 2 S 4 series (A\Rb, Cs) contains165 [Cd 2 S 4 ]4~ ions and linear gold atoms. K 3 CeP 2 S 8 ,166 La 8 Ti 10 S 24 O 4 ,167 and La 5 Ti 6 S 3 Cl 3 O 15 168 serve as a small memorial to the work of Jean Rouxel, and contain interesting structural features such as PS 4 tetrahedra166 and distorted TiO 5 S and TiO 5 Cl octahedra.168 7 Nitrides, phosphides, borides Reaction of the elements have led to Ba 3 N, a new binary nitride crystallising in the anti-TiI 3 structure.169 The exotic reaction medium of liquid sodium was used to prepare the complex phase Sr 39 Co 12 N 31 .170 It contains novel, linear CoN 2 units, with Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 409–429 423Fig. 10 ORTEP representation of the unit cell of Rb 4 Sn 2 Ag 4 P 6 Se 18 viewed down the b-axis.Rb` cations have been omitted for clarity. Tin and silver atoms are shown as octant-shaped ellipses, selenium as open cllipses, and phosphorus as crossed ellipses with no shading (90% thermal ellipsoids). (Reproduced by permission from Inorg. Chem., 1998, 37, 2848.) a formal cobalt valence of 1.5. SrTiN 2 , which represents only the third example of a ternary nitride containing titanium,171 was prepared by solid-state methods from the binary nitrides and is isostructural with KCoO 2 . TiN 5 square based pyramids are a feature of this phase.Members of the solid solution BaHf 1~xZrxN 2 172 adopt the same structure and show a paramagnetic to superconducting transition at ca. 8 K. The MnMo 3 N series (M\Fe, Co, n\3; M\Ni, n\2)173 can be obtained by ammonolysis of the metal molybdate precursors MMoO 4 ·nH 2 O at relatively low temperatures (\1200 K) for this type of synthesis. The cubic Ni 2 Mo 3 N structure was Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 409–429 424determined ab initio from powder data. La 3 Br 6 N contains174 isolated bitetrahedral La 6 N 2 units (i.e, a pair of edge-sharing NLa 4 units) bridged by bromide ions into a three-dimensional network.SrSiAl 2 O 3 N 2 175 (Fig. 9) is a new SIALON-type phase containing corner-sharing SiON 3 , AlO 3 N, and AlO 2 N 2 tetrahedra, with the N and O atoms showing perfect long-range order. The new phosphide ScNiP cannot be described176 by a simple ionic model. It contains Sc–Sc bonds and is weakly metallic, in agreement with extended Hu� ckel calculations.Sr 3 Sn 2 P 4 is metallic177 and contains the new cyclic [Sn 12 P 24 ]36~ ion, with six phosphide ions coordinating each Sr2` cation. Cd 4 P 2 Cl 3 is based on fcc packing of cadmium atoms,178 with the tetrahedral holes occupied by Cl, P or P–P pairs. The compound previously known as ‘NaB 6 ’ is actually Na 3 B 20 and contains a novel B 7 pentagonal bipyramidal cluster unit.179 Sr 13 Mg 2 Si 20 contains sheets of silicon atoms in which the novel naphthalene-like Si 10 10~ unit can be identified.180 Rb 12 Nb 6 Se 35 is the first niobium polyselenide181 built up from infinite chains, which are constructed from NbSe 7 , Se 2 2~, Se 3 2~ groups, and the unprecedented Se 3 4~ unit.Cs 2 PdSe 8 was prepared182 solvothermally using ethylenediamine as solvent.The crystal structure contains a novel double helical arrangement of chains built up from [Pd(Se 4 ) 2 ]2~ ions. The layered ternary manganese tellurides AMnTe 2 (A\Li, Na) can be described183 as wurtzite superstructures. The sodium phase was prepared from the lithium material by ion exchange in molten NaCl. The group of Corbett have continued their extensive investigations of the structures and properties of intermetallic and metal rich phases.A key feature of their studies is the comprehensive rationalisation of crystal structures in terms of atomic sizes and bonding e§ects, as applied to, for example, Sc 8 Te 3 ,184 K 2 La 6 I 12 Os,185 and Ca 6.2 Mg 3.8 Sn 7 .186 Ba 3 AlO 4 H may be rationalised187 in terms of Ba2` cations and AlO 4 5~ and H~ anions. HBa 6 octahedra link together by corners into a threedimensional framework containing voids occupied by the aluminate ions.This network can be visualised as an inverse perovskite. Ba 3 TO (T\Ge, Si) are also inverse perovskite types,188 crystallising in the same orthorhombic space group as GdFeO 3 type phases. La 2 INi 2 contains189 graphite-like hexagonal layers of nickel.Chondroudis and Kanatzidis have reported a number of novel selenophosphosphate phases containing various [PySez]n~ groupings. 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ISSN:0260-1818    

DOI:10.1039/a805978k

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

22 Fullerene chemistry P. R. Birkett Fullerene Science Centre, School of Chemistry, Physics and Environmental Science, University of Sussex, Brighton, UK BN1 9QJ 1 Introduction The following are highlights of the large number of publications covering fullerene and carbon nanotube chemistry.A major emphasis has been placed on the preparation and chemistry of new fullerene derivatives and related nanotube materials. 2 Synthesis, separation and biological properties of fullerenes Reviews of the combustion of hydrocarbons producing fullerenes, polyaromatic hydrocarbons and soot1 and of the potential uses of fullerenes for the synthesis of diamonds, diamond films, thin fullerene films for microelectronics and as lubricants are available.2 In situ electron microscopy has led to the direct observation of fullerene formation from graphene sheets3 and the same authors have observed the formation of fullerenes from a methane precursor using a radio frequency plasma enhanced chemical vapour deposition technique.4 Approaches towards the synthesis of fullerenes and related bowl-like structures using traditional organic chemistry techniques have also been reviewed.5 A thirty-two carbon atom cage has been suggested, using theoretical considerations, 6 to be the most stable fullerene-like cluster below [60]fullerene.A new, small fullerene-like cage with D 6) symmetry consisting of 36 carbon atoms, C 36 1, has been prepared using a modified arc-discharge method.7 The electronic and structural properties of the foregoing C 36 have been investigated theoretically using the pseudopotential density functional approach.8 An independent search of 598 closed thirty-six carbon atom cage structures revealed that a cage with D 2$ symmetry would have the lowest energy.9 A second type of fullerene, represented, for example, by C 74 , which are either free radicals or have small HOMO–LUMO gaps (in contrast to the well known, stable closed cage fullerenes which have large HOMO–LUMOgaps such as C 60 ), have been isolated after the electrochemical reduction of their insoluble polymerised solids obtained from fullerene containing soot and subsequent reduction to soluble anions.10 The high yield synthesis of C 74 using the arc-discharge method at high He pressures has also been reported.11 Similarly, an increased yield of higher fullerenes has been achieved by vaporising a variety of doped graphite anodes.12 The Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 431–451 431Fig. 1 Computer generated structures of the D 6) symmetrical isomer of C 36 1, the [2]2] cycloadduct C 120 2 and the dimer (C 60 ) 2 C 2 3. two major isomers of C 84 , i.e. D 2 and D 2$, have been separated for the first time using a recycling HPLC technique.13 In addition, high resolution synchrotron X-ray powder di§raction shows that a mixture of the foregoing two isomers adopts a face-centredcubic (fcc) structure at all temperatures.14 Two approaches to the synthesis of the [2]2] type fullerene dimer, C 120 2 (Fig. 1), have been reported. The first uses a mechanomechanical synthesis procedure by reacting C 60 with KCN in a ‘high-speed’ vibrating mill.15 A modified mechanomechanical method was subsequently used to produce the dimer in similar yield using metallic Li as the reaction catalyst and in addition the Raman spectra of the dimer was reported.16 The second technique involves squeezing the molecular crystal of (ET) 2 C 60 at 200 °C and 5 GPa.17 Theoretical calculations predict that the formation of fullerene dimers of this type may play an important role in the mechanism during the incorporation of helium into C 60 .18 Reaction of C 60 with diazotetrazole results in the formation of a di§erent type of fullerene dimer, (C 60 ) 2 C 2 3, in which the fullerene moieties are linked by a C––C function.19 The selective separation of the fullerenes C 60 and C 70 with unmatched selectivity using a stationary phase consisting of hydroxyphenyltriphenylporphyrin on silica has been reported.20 In contrast, C 60 based stationary phases have proved to be useful in independent studies for the separation of high energy nitroaromatics21 or polychlorinated biphenyls.22 A fullerene/lipid device capable of stable and reversible electron transfer reactions has been prepared.23 Similarly, photoinduced electron transfer in porous polymer membranes modified by the incorporation of fullerene and phospholipids has been found to depend on the nature of the polymer used.24 An EPR study of aqueous Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 431–451 432solutions of C 60 and C 70 reveals thatOH· andO 2 ·~ may be the primary factors for the biological damage caused by fullerenes under photoirradiation conditions.25 Singlet oxygen has also been implicated in the oxidative damage which is induced by fullerenes on photosentisization.26 Fullerene-specific antibodies have been produced after immunization of mice with a C 60 derivative covalently attached to bovine thyroglobulin.27 Moreover, C 60 has been shown to inhibit glutathione S-transferase activity towards ethacrynic acid.28 There is also evidence that C 60 does not remain unchanged in the liver, as previously believed, following the identification of both retinol and retinyl palmitate C 60 adducts which are formed by in vitro cycloaddition reactions.29 3 Endohedral fullerenes A simple formula has been developed that permits an estimation of the oxidation state of a fullerene encapsulated atom based on the positive shift of the atomic orbital energies.30 The production and isolation of the ‘missing’ endohedral fullerenes such as Ca@C 72 ,31 Ca@C 74 ,31 La 2 @C 72 ,32 Eu@C 74 ,33 Ca@C 80 ,34 and Ba@C 80 34 have all been reported. A two step extraction process using first xylene to remove empty fullerenes and second N,N-dimethylformamide has been developed for the isolation of endometallofullerenes.35 A high temperature and pressure protocol also results in the e¶cient extraction of endometallofullerenes.36 Nucleus-independent chemical shifts have been calculated for a range of fullerenes.37 The 3He NMR spectra of He 2 @C 70 38 and Ne 2 @C 70 39 have been reported.The most shielded and deshielded signals in 3He NMR spectroscopy of 3He inside a fullerene result from the reduction of C 60 to C 60 6~ and C 70 to C 70 6~ respectively.40 The former providing compelling evidence for the ability of electrons to move freely about the surface of a spheroidal p-system. 4 Organic chemistry Reviews of electroactive fullerenes, from intercalated systems to covalently linked fullerene–donor dyads,41 and of the family of fulleropyrrolidines are available.42 Two boronic acid groups have been regioselectively introduced into the fullerene cage using saccharides as template molecules.43 The previously reported addition of diethylaminopropyne to C 60 results in a [2]2] cycloadduct which can be ring-opened under acidic conditions, and when followed by oxidative cyclisation in the presence of activated carbon, yields novel fullerene lactones.44 Further work on the reaction of C 60 with propiolates in the presence of triphenylphosphine has resulted in the synthesis of bismethanofullerenes and a [2]2] cycloadduct consisting of a cyclobutene ring fused to the carbon atoms of a 6,6-ring junction.45 A novel ring opening of a methano[60]fullerene under reducing conditions, NaBH 3 CN, resulted in the formation of a 1,2-dihydro[60]fullerylglycine derivative.46 The photoreaction of C 60 with tetramethyl ethylenediaminetetraacetate results in the formation of a number of mono-adducts with mutliple methyl carboxylate groups which, it is suggested by the authors, may have useful biological activity.47 A fullerene modified protein has been Annu. Rep.Prog. Chem., Sect. A, 1999, 95, 431–451 433Fig. 2 Computer generated structures of diisopropylcyclohexyl [60]fullerene alcohol 4 and the diphenyl [60]fullerene alcohol 5 targeted as fullerene inhibitors of HIV-1 protease synthesised by combination of a surface cysteine-containing redox protein with a [60]fullerene derivative labelled with a thiol selective reagent; significant electrochemical interaction between the fullerene and protein redox centre was also reported.48 A dimethylpyrrolidinium [60]fullerene derivative has been synthesised and complexed with DNA resulting in the formation of hybrid DNA/fullerene-based architectures which were imaged using transmission electron microscopy (TEM).49 The successful enzymatic modification of fullerene derivatives by a restricted number of lipases, which were selected from the range of enzymes tested, has been reported for the first time.50 The highly diastereoselective formation of stable cluster opened fulleroids has been reported with the bulkier groups preferentially located above a five-membered ring of the fullerene cage.51 High a¶nity [60]fullerene ligands 4 and 5 (Fig. 2) of the HIV-1 protease, which have an increase of 50-fold binding over previously tested fullerene compounds, have been designed and synthesised using Bingel addition to [60]- fullerene.52 Highly water soluble dendro[60]fullerenes which have 18 peripheral carboxylic acid groups have also been synthesised by the nucleophilic cyclopropanation method.53 Cycloaddition of per-O-acetyl glycosyl azides to [60]fullerene result in the formation of two unseparable stereoisomers, which the authors suggest may have important biological activity.54 The addition of benzyne to C 70 has been repeated and the four main isomeric mono-adducts isolated by HPLC; the unusual addition of a benzyne group across a closed 5,6-ring fusion results in the formation of the third most abundant mono-adduct isomer.55 Several groups have synthesised and examined the properties of fullerene–porphyrin diads with a variety of structural architectures.For example, a zinc porphyrin and C 60 diad was constructed via the cyclopropanation route; almost no electronic Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 431–451 434Fig. 3 A tetraphenylporphyrin with four [60]fullerene substituents 6.(Reproduced by permission from Angew. Chem., Int. Ed., 1998, 37, 1934.) interaction between the fullerene and porphyrin moieties was observed.56 In contrast, two-fold regioselective cyclopropanation leads to a [60]fullerene-porphyrin diad which has significant interaction between the two p–p stacked chromophores owing to the restricted conformational freedom of the covalent linkages.57 A porphyrin with two fullerene substituents has been prepared and a variable temperature NMR study used to show that there is a mixture of two conformers present which are in slow equilibrium.57b,58 Similarly, two [60]fullerene units have been covalently linked to a porphyrin unit using the pyrrolidine methodology; in this example, considerable interaction of the chromophores in the cis-configuration owing to the close proximity of the two [60]fullerene moieties has been reported.59 Four [60]fullerene groups have also been appended to a central porphyrin by the reaction of a bis-adduct of C 60 bearing an aldehyde group with pyrrole under acidic conditions (6, Fig. 3); electrochemical studies of the product indicate that there is electronic interaction between the molecular components in both the ground and excited states.60 An alternative approach to the synthesis of diads of this type was achieved by the reaction of bis(octaethylporphyrinato)ruthenium(II) with C 60 and has resulted in the first example of a porphyrin and [60]fullerene linked directly by a metal bond; the fullerene is g2-coordinated to the the Ru metal.61 Further spectroscopic evidence for photoinduced electron transfer in triads, which mimic similar processes observed in photosynthetic reaction centres, and which consist of a porphyrin covalently linked to a [60]fullerene group by a carotenoid polyene has been obtained at 20 K.62 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 431–451 435Other approaches to the construction of fullerene diads have been developed.A series of methano[60]fullerene based ligands in which a tridentate 2,2@: 6@,2A-terpyridine group is attached to the fullerene have been synthesised and interaction between the fullerene and metal centre observed when they are spatially close.63 The synthesis of other methanofullerenes which incorporate terdentate diamino aryl ligands which can be converted to the nickel(II) compound via an oxidative addition reaction has been achieved.64 The self-assembly of a dipyridyl substituted methano[ 60]fullerene is directed by Pt(II) centers.65 The 5,6-open 2-aminomethylpyridineappended [60]fullerene interacts more strongly with Ag` than the comparable 6,6 closed isomer.66 Other [60]fullerene diads containing cyclohexene-fused tetrathiafulvalene67 and anthraquinone-based dienes have also been synthesised.68 The electron accepting properties of a family of N-methylpyrrolidinium fullerene iodide salts are enhanced when compared to those of the parent fulleropyrrolidines and [60]- fullerene.69 A number of fullerene–crown ether/calixarene hybrids have also now been synthesised.The regioselective bis-addition of a crown ether using the Bingel reaction has resulted in a [60]fullerene conjugate which is capable of binding metal cations.70 A di§erent approach based on azido addition to fullerenes produced a calix[4]arene linked to the fullerene core via ionophoric chains which enables e¶cient complexing of metal cations.71 Other hybrids include [60]fulleropyrrolidines which have calix[4]- arenes72 and crown ether groups.73 Even a C 60 functionalised with a calix[4]-spiro-bis crown has been produced.74 The Bingel reaction of [60]fullerene has been used to synthesise a thermotropic liquid crystalline material which contains ferrocene moieties.75 Regioselective bisaddition to C 60 has been achieved by reaction with bis(b-keto esters) in the presence of DBUand iodine.76 As an extenson of this work, amphiphilic [60]fullerene bis-adducts with a polar head group and long pendant chains have been prepared and found to form stable Langmuir films at the air–water interface.77 It has been found that a bisazafulleroid 8 is converted to the aza-aziridinofullerene 7 when exposed to ambient light whilst the retro-reaction is achieved by thermolysis in refluxing toluene (Fig. 4).78 Isomerization of bis-adducts of C 60 can also be achieved under electrochemical conditions by migration of di(alkoxycarbonyl)methano bridges.79 Similarly, the retro- Bingel reaction under electrochemical conditions has been used to obtain enantiomerically pure samples of [76]fullerene.80 The synthesis and properties of a range of enantiomerically pure bis- and tris-adducts of [60]fullerene, which may find use as chiral auxiliaries in enantioselective synthesis, with inherent chiral addition patterns have been reported.81 The absolute configuration of some chiral fullerene derivatives have been assigned by comparison of experimental circular dichroism (CD) spectra with theoretically predicted spectra.82 The CD spectra of some other enantiomeric [60]fullerene derivatives have also been recorded83 whilst the enantiomers of a cageopened keto lactam [60]fullerene derivative have been separated by chiral HPLC and their CD spectra recorded.84 Reviews of the elecrochemistry of fullerenes and their derivatives85 and the addition of free radicals to [60]fullerene are available.86 The electrochemical properties of a range of covalent derivatives of higher fullerenes, C 70 , C 76 and two isomers of C 78 have been investigated by cyclic voltammetry; in most of the derivatives reduction requires more energy than for the parent fullerene whereas the oxidation becomes progressively Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 431–451 436Fig. 4 A possible mechanism for the conversion of the aza-aziridinofulleroid 7 to the bisazafulleroid 8.The substituent on nitrogen has been omitted for clarity. (Reproduced by permission from Chem. Eur. J., 1998, 4, 2037.) easier with increasing carbon number.87 The selective generation of [70]fullerene anions has been achieved using a variety of reduction conditions and an aqueous colloidal solution of C 70 found to have excellent optical limiting properties.88 The photochemical reduction of a water-soluble N-methylpyrrolidinium [60]fullerene iodide in a titanium dioxide suspension is observed to proceed analagously to that of pure [60]fullerene.89 The formation of organofullerenes from fullerene dianions is reported to result from a combination of (i) electron transfer which initially forms C 60 R~ (R\alkyl group such as Bu5) and is then followed by (ii) formation of C 60 R 2 by an S N 2 reaction; a range of fullerene derivatives with two di§erent alkyl groups have been prepared using this methodology.90 The reaction of [60]fullerene with methoxide anion results in the formation of the C 60 ·~ which then reacts to produce the dimethoxy[60]fullerene; the reaction is found to be highly dependent on the concentration of methoxide anion present.91 The same [60]fullerene radical anion, C 60 ·~, is also produced during the reaction of C 60 with 1-benzyl-1,4-dihydronicotinamide via a photoinduced electron transfer reaction.92 The nucleophilic addition of a variety of lithium acetylides to [60]fullerene followed by quenching under acidic conditions has resulted in the formation of a range of derivatives which have polar functional groups.93 Similarly, addition of Grignard reagents containing protected mercaptoalkyl groups have been successfully employed to produce [60]fullerene derivatives with thiol moieties in the addended group.94 During the addition of propiolates in the presence of trialkylphosphines described earlier,45 the authors also observed the reaction of trialkylphosphine oxides with [60]fullerene, which results in the formation of 1,2-addended [60]fullerenes.95 C 60 F 36 consists of two isomers which have T and C 3 symmetry.96 An epoxide, C 60 F 18 O, other oxides and hydroxides have been isolated as either by-products formed during fluorination of [60]fullerene or as decomposition products of the fluorinated fullerene.97 The reaction of C 60 F 18 with benzene under Friedel–Crafts Annu. Rep.Prog. Chem., Sect. A, 1999, 95, 431–451 437Fig. 5 Computer generated structure of the trisphenylated adduct of [70]fullerene 9 conditions produces a single isomer of C 60 F 15 Ph 3 , where the most accessible fluorines have been replaced.98 Fluorinated [60]fullerene has also been reacted with methyland phenyllithium resulting in mixtures of poly-methylated and -phenylated [60]- fullerene.99 The Friedel–Crafts methodology has been extended to include the reaction of C 60 Cl 6 with allyltrimethylsilane and results in the substitution of all six chlorine atoms by allyl groups.100 Moreover, the first stable [60]fullerene carbocation, C 60 Ph 5 `, has been characterised by in situ NMR experiments and confirms the reaction mechanism to be S N 1 via frontside attack.101 Electrochemical studies of a dihydroxyanthraquinone [60]fullerene derivative suggests that this species may also form a stablised fullerene cation.102 Reaction of the monoarylated azafullerene, ArC 59 N, with iodine monochloride mirrors the reactivity observed for C 60 and produces Cl 4 ArC 59 N which has a pyrrole moiety embedded in the surface of the fullerene cage.103 The foregoing arylated fullerene is produced by the thermal treatment of the dimer, (C 59 N) 2 , with an aromatic species under acidic conditions.104 [70]fullerene reacts with organocopper reagents producing the tris-adduct, C 70 Ar 3 H (9, Fig. 5) which has an indene-like substructure flanked by three sp3 carbons embedded within the fullerene cage; the foregoing derivatives were used to synthesise metal complexes.105 Hydrogenation of C 70 using a Zn(Cu) couple has allowed the separation of C 70 H 2 , C 70 H 4 and C 70 H 8 ; interestingly the authors have suggested that three di§erent reduction mechanisms are in action at any one time as, for example, upon further reduction of C 70 H 2 the same isomer of C 70 H 8 is not produced.106 Hexanitro[60]fullerene, C 60 (NO 2 ) 6 , has been produced by the reaction of [60] fullerene with NO 2 (no information was provided regarding addition sites of the nitro groups) and subsequently reacted with aniline.107 Oxidative sulfation was used to produce a mixture of isomers of hexacyclosulfated fullerene which upon hydrolysis is Annu.Rep.Prog. Chem., Sect. A, 1999, 95, 431–451 438converted to the dodecahydroxyfullerene (again no information was provided regarding the addition sites).108 Electron reduction of [60]fullerene followed by quenching with 1,4-butane sultone produced highly water soluble hexa(sulfobutyl)fullerenes as mixtures of isomers which behave as potent free-radical scavengers in physiologic media.109 Oleum induced oxidative sulfation of [70]fullerene followed by hydrolysis has resulted in the production of a mixture of isomers of octadecahydroxylated [70]fullerene, C 70 (OH) 18 .110 The anti-proliferative e§ect of polyhydroxylated [60] fullerene on smooth muscle cells has been ascribed, in part, to be mediated through the inhibition of tyrosine kinase.111 Significant membrane damage was observed upon the photosensitisation of rat liver microsomes in the presence of polyhydroxylated fullerenes.112 Singlet oxygen does not react with ground state [60]fullerene but does react with the lowest triplet exited state to produce the fullerene epoxide, C 60 O.113 C 60 has been found to oxidise to C 120 O, which has a furanoid bridge between the two fullerene moieties, at ambient temperature and pressure.114 A series of dimeric fullerene oxides have been extracted from fullerene soot using a novel hydrothermally initiated dynamic extraction method.115 Additional oxides of [60]fullerene have also been isolated as by-products of the reaction between C 60 and C 60 O which produces the aforementioned C 120 O.116 Matrix assisted laser desorption-ionization Fourier transform mass spectrometry (MALDI-FTMS) has been shown to be a fast and e¶cient method for the analysis of fullerene oxides.117 5 Polymers A range of fullerene polymers have been synthesised using a variety of techniques.Fullerene functionalised polycarbonates have been produced by the reaction of [60]- fullerene with polycarbonates in the presence of AlCl 3 ; the resultant polymer limits strong 532nm optical pulses more e§ectively than the parent fullerene.118 An alternative approach allows the synthesis of fullerene-containing polymers by the reaction of functional groups, such as carboxylic acid moieties, within a fullerene derivative with, for example, a diamine resulting in the formation of polyamides.119 Fullerene functionalised polydienes have been prepared by grafting C 60 onto a lithiated polymer chain and the fullerene moieties attached to the polymer chain have subsequently been converted to polyhydroxylated derivatives.120 Nucleophilic addition of poly(vinyl alcohol) to [60]fullerene results in polymers with reduced viscosity as the concentration of C 60 within the polymer increases.121 [60]Fullerene end-capped triarm poly(ethylene oxide) stars produce highly expanded fluid Langmuir monolayers.122 Novel copolymers of [60]fullerene/styrene with di§ering contents of [60]fullerene have been synthesised by anionic polymerization.123 Methacrylate containing ‘charmbracelet’ fullerene polymers have been produced by the reaction of [60]fullerene with azide-containing methacrylate monomers; the polymers obtained were subsequently copolymerised under anionic conditions with a Grignard reagent.124 C 60 has also been covalently incorporated into amphiphilic polysulfonates where the fullerene moieties show extreme blue shifted fluorescence due to the highly constrained hydrophobic microenvironment in which they are trapped.125 Star-like polymers of polyoxyethylenes which have C 60 molecules at their core have herbicidal activity.126 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 431–451 439Palm-tree like architectures where five polystyrene branches of equal length are connected to a trunk of polystyrene through a single [60]fullerene molecule have been prepared.127 Two routes have been used to graft [60]fullerene to polythiophene producing novel intramolecular donor–acceptor polymers.128 First, the fullerene was covalently attached to a bithiophene derivative which was electropolymerised and second, polythiophene was prepared and then functionalized with [60]fullerene.Poly(N-vinylcarbazole) has been modified by anionic addition to [70]fullerene and the resulting polymer found to have enhanced stability when compared with the parent polymer.129 Polyaniline emeraldine base, a conducting polymer, has been doped with polysulfonated [60]fullerene which acts as a protonic acid dopant; the resultant polymers have conductivities of about six orders of magnitude higher than that of typical fullerene-doped conducting polymers.130 Conjugated polymer/fullerene blends produce photo-induced changes in the complex index of refraction through charge transfer. 131 Multilayers of sexithienyl polymer and [60]fullerene do not have a stable charge-transfer state at the interfaces of the two layers but strong photoluminesence reduction and a much faster radiative lifetime do provide indirect evidence for the formation of a metastable charge transfer state.132 The dominant long-lived photoexcitations in [60]fullerene–poly(p-phenylene vinylene) polymers consist of interchain spin-correlated polaron pairs which were characterised using transient spectroscopy. 133 Conjugated polymer:fullerene blends have been embedded in a conventional polymer, polystyrene, and the photoexcitations of the composite materials investigated. 134 6 Organometallic chemistry A comprehensive review of fullerene organometallic chemistry is available covering all aspects of the field.135 A computational study explains the ready formation of g2- fullerene complexes due to the orientation of the p-orbitals and the large release of strain energy upon bonding a metal atom.Similarly the p-orbitals have been shown to be poorly oriented for overlap with an exohedral metal atom centred over five- or six-membered rings whereas an endohedral metal atom is well positioned for orbital overlap under a five- or six-membered ring.136 Photolysis of M(CO) 4 (dppe) (M\W or Mo) with C 60 or C 70 results in the formation of g2-complexes of the fullerenes, M(CO) 3 (dppe)(g2-C 60 ) and two isomers of Mo(CO) 3 (dppe)(g2-C 70 ) where the metal centres have distorted octahedral geometries in all cases.137A series of transition metal derivatives, including M(NO)(PPh 3 ) 2 (g2-C 60 ) (M\Co or Rh), RuX(NO)(PPh 3 ) 2 (g2- C 60 ) (X\Cl or H), and TaH(g-C 5 H 5 ) 2 (g2-C 60 ) have been prepared and an additional example of complexation involving adjacent fullerene double bonds, Re 2 H 8 (PMe 3 ) 4 (g2,g2-C 60 ), reported.138 Reaction of [60]fullerene with iridium cycloocta- 1,5-diene dimers, Ir 2 (OR) 2 (cod) 2 (R\Me, Et, Pr/, Ph) containing bridged alkoxide groups results in complexes, Ir 2 (OR) 2 (cod) 2 (g2,g2-C 60 ), which again involve coordination at adjacent double bonds.139 Examples of coordination in which the [60]- fullerene acts as a cyclohexatriene-like ligand have also been produced.Reaction of [60]fullerene with Ru 5 C(CO) 15 with C 60 followed by PPh 3 , dppe or dppf forms Ru 5 C(CO) 11 (PPh 3 )(g2,g2,g2-C 60 ), Ru 5 C(CO) 10 (dppe)(g2,g2,g2-C 60 ), and Ru 5 CAnnu.Rep. Prog. Chem., Sect. A, 1999, 95, 431–451 440Fig. 6 Side view of the crystal structure of [60]fullerene and a tribromohexahomooxacalix[ 3]arene. (Reproduced by permission from Chem. Commun., 1998, 896.) (CO) 10 (g1,g1-dppf)(g2,g2,g2-C 60 ).140 Trisosmium complexes of [60]fullerene, Os 3 (CO) 9 (g2,g2,g2-C 60 ), Os 3 (CO) 8 (PMe 3 )(g2,g2,g2-C 60 ), Os 3 (CO) 8 (PPh 3 ) (g2,g2,g2-C 60 ) and Os 3 (CO) 7 (PMe 3 ) 2 (g2,g2,g2-C 60 ) in addition to some osmium g2-C 60 complexes have also been synthesised and characterised.141 A short review of the use of fullerene-based (including nanotubes) materials for use as new carbon-support media in heterogeneous catalysis by supported metals is available.142 The electrochemical reduction of [60]fullerene in the presence of (PhCN) 2 PdCl 2 , Ir(CO) 2 Cl(C 7 H 9 N) (C 7 H 9 N\p-toluidine) or (CF 3 CO 2 ) 4 Rh 2 produces three di§erent redox active black films which coat the electrode; analysis suggests that the metal atoms within the films are g2-bonded to the fullerene itself.143 A Pt-based catalyst supported on silica was grafted with [60]fullerene and the liquid phase hydrogenation of cinnamaldehyde evaluated; a fairly good selectivity to cinnamyl alcohol was observed.144 New charge-transfer complexes of [60]fullerene derivatives, 1,2-dicyanodihydro[60]fullerene [C 60 (CN) 2 ]145 and 3-aminophenylmethano[ 60]fullerene [C 60 (CHPhNH 2 )],146 with cobaltacene have been prepared; the fullerene derivative in the former complex is reduced to the anion. 7 Inclusion complexes In addition to the many covalent crown ether–calixarene fullerene derivatives described earlier there have been a considerable number of non-covalent supramolecular complexes synthesised during the last year.Single crystal analysis of C 60 and a hexahomooxacalix[3]arene, substituted with bromine atoms on the upper rim, indicates that van der Waals attractive interaction is the main driving force which locates the six-membered rings of the [60]fullerene close to the aromatic rings and the dibenzyl ether oxygen (Fig. 6).147 Crystal structure analysis of the isostructural com- Annu.Rep. Prog.Chem., Sect. A, 1999, 95, 431–451 441Fig. 7 Plot of the cationic and anionic units in [Ni(NH 3 ) 6 ]C 60 ·6NH 3 . (Reproduced by permission from Chem. Commun., 1998, 1205.) plexes, [(calix[6]arene)(C 60 ) 2 ] and [(calix[6]arene)(C 70 ) 2 ] shows that the calixarenes are in the double-cone conformation and that each cavity is occupied by a fullerene molecule.148 Cyclotriveratrylene (CTV) derivatives with pendant aromatic groups have been synthesised and complexed with [60]- and [70]-fullerenes; the CTV derivatives with N-methylpyrrole substituents were found to have the highest association constant with C 60 .149 Similar bis-CTV molecules with rigid acetylenic spacers have also been synthesised and found to complex [60]fullerene e¶ciently.150 Bis-calix[5]- arenes singly bridged at the upper rim having a but-2-enyl linker and carrying allyl groups at all of the other available p-positions are particularly e§ective at complexing [60]fullerene.151A series of bridged calix[5]arenes with rigid alkyne containing spacer groups preferentially bind [70]fullerene.152 During a study to examine which calix[n]- arenes e¶ciently bind fullerenes it was found that calix[n]aryl esters do not interact with [60]fullerene however they become excellent fullerene acceptors upon the addition of specific metal cations and this fact is attributed to a change of conformation in the calix[n]arene ester induced by the metal cation.153 Amphiphilic poly(phenylquinoline) –block-polystyrene rod–coil diblock copolymers self-organise into micrometre scale spherical aggregates from solution, which have large hollow cavities in which it is possible to encapsulate 1010 fullerene molecules per aggregate.154 8 Metal fullerides The optical properties155 and recent developments in the chemistry and physics of metal fullerides have been reviewed.156 A new and versatile method has been developed for the synthesis of fulleride materials based on ion exchange in liquid ammonia.For example, the ammoniate [Ni(NH 3 ) 6 ]C 60 ·6NH 3 was prepared from K 2 C 60 via an ionic exchange resin loaded with Ni2` in liquid ammonia (Fig. 7).157 The same authors have thus far used the same technique to produce [Ba(NH 3 ) 6 ]C 60 ·NH 3 ,158 [Mn(NH 3 ) 6 ]C 60 ·NH 3 ,159 [Cd(NH 3 ) 6 ]C 60 ·NH 3 159 and even organic equivalents, (PhCH 2 NMe 3 ) 2 C 60 ·3NH 3 .160 Using a low-temperature synthesis protocol Cs 4 C 60 has been obtained as a pure single phase for the first time and is found to have an Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 431–451 442orthorhombic distortion of the body-centred tetragonal structure observed for K 4 C 60 .161 A mechanochemical methodology has been used to synthesise alkali and alkaline earth fullerides in organic solvents.162 Coevaporation of [60]fullerene and titanium under ultra high vacuum conditions results in the formation of titanium fulleride films, (TixC 60 ).163 Europium fullerides, EuxC 60 (x ca. 3 and 6) have been prepared and the structures found to have structures analagous to those of other metal fullerides with similar compositions.164 Stable fulleride films with two di§erent structures are believed to have been formed during the examination of the redox behaviour of [60]fullerene films in the presence of 18-crown-6 complexes of alkali metals.165 The crystalline compound [Ph 4 P`] 2 [C 60 ]~[I~]x contains a high symmetry two-dimensional network of cations, which function as an e§ective host completely encapsualting the fulleride ion with both face-to-face and face-to-edge fullerene–phenyl interactions being observed.166 The insertion of an excess of potassium into pure D 2 and D 2$ isomers of [84]fullerene produces the cubic phases with composition, K 8`xC 84 ; there is multiple occupancy of the octahedral sites by potassium.167 9 Nanotubes Owing to the very large number of publications concerning the synthesis, properties, applications and theoretical considerations in the carbon nanotube related area only a representative collection of papers are reviewed here.Where possible reference to key reviews of areas less well covered are included for the readers convenience.168 The synthesis of nanotubes has been a major area of research with a number of major breakthroughs in the control of their preparation.Well-aligned clusters of multi-walled nanotubes (MWNTs) have been grown on nickel coated glass at temperatures below 666 °C using acetylene gas as the carbon source and ammonia gas as both carrier and catalyst gas.169 In contrast, individual high quality single-walled nanotubes (SWNTs) are synthesised, and can subsequently be manipulated individually, by chemical vapour deposition of methane on silicon wafers patterned with micrometre scale islands of catalytic material.170 Aligned bundles of nanotubes result from the pyrolysis of ferrocene or ferrocene–acetylene mixtures using a simple two furnace procedure with Ar as carrier gas (Fig. 8).171 Pyrolysis of organic materials over laser etched cobalt thin films produces aligned nanotubes of uniform length and diameter; the tubes grow perpendicularly to the catalytic substrate and only in the etched regions.172 Very long carbon nanotubes, 2mm in length, are produced by the pyrolysis of acetylene over iron/silica substrates.173 The pyrolysis of tripropylamine in the 0.73nm pores of microporous aluminophosphate crystallites results in the formation of parallel-aligned SWNTs.174 Graphitic multi-walled carbon cube-like nanocages result under arc conditions when graphite is evaporated in the presence of calcium or strontium; the cages are deposited on the cathode surface and range in size from 20 to 100 nm.175 Some SWNTs which were produced using pulsed laser vaporization have been found to contain encapsulated [60]fullerene molecules.176 Carbon nanotubes and polyhedra can be formed spontaneously at temperatures as low as 50 °C by the interaction of nano-porous carbon [produced by pyrolysis of a mixture of poly(furfuryl alcohol) and poly(ethylene glycol] with elemental caesium.177 Several approaches to the synthesis of nanotubes based on metal catalysis in Annu.Rep. Prog. Chem., Sect.A, 1999, 95, 431–451 443Fig. 8 SEM images of aligned nanotubes generated by pyrolysis of ferrocene. (Reproduced by permission from Chem. Commun., 1998, 1525.) addition to those described above have been published.178–182 SWNTs are produced at a rate of 1mg h~1, and without termination of tube growth occurring, provided that the reactant gas can di§use through the product nanotube mat that covers the catalyst, by the decomposition of CO or ethylene over a supported metal catalyst.178 Unusual highly graphitic needle-like tubes, which contain encapsulated Ni, are produced when a sandwich of alternating films of [60]fullerene and Ni deposited on a silica plate is heated.179 The production of magnetic-material free SWNTs, which should make them more suitable for electrical and magnetic studies, is remarkably increased when a Rh–Pt catalyst is used during the arc-discharge process.180 Long and wide ropes, larger than those obtained by arc-discharge or laser vaporisation, of approximately aligned SWNT bundles result from the catalytic decomposition of hydrocarbons.181 Purification of carbon nanotubes remains a major challenge and a number of strategies have now been developed to achieve this.Size exclusion chromatography using a stationary phase with a defined pore size of a dispersion of MWNTs results in the isolation of chemically unmodified, almost impurity free and size separated MWNTs.183 The process also works well for SWNTs.184 A similar process uses Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 431–451 444ultrasonically assisted filtration to remove amorphous and crystalline carbon impurities from SWNTs; sonication prevents filter contamination and provides a fine suspension throughout the process, however there is some sonication induced cutting of the tubes.185 Boiling of SWNTs with concentrated nitric acid removes the majority of impurities, however some tube damage is induced during this technique.186 An unusual method for the purification of nanotubes employs ac electrophoresis in isopropyl alcohol, the nanotubes are separated from other material by their movement towards the electrodes for all applied fields.187 The chemistry of the nanotubes is now beginning to develop.SWNTs are selectively opened at their tips by concentrated HCl and may be filled with, for example, crystals of Ru metal using wet chemistry techniques.188 Similarly SWNTs can be cut into smaller sections using sonication in a mixture of concentrated sulfuric and nitric acids.189 Once opened the tubes can be filled with a variety of materials for example enzymes, proteins190 and DNA.191 The biomolecules appear to be encapsulated within an environment which o§ers some protection and subsequent analysis of the catalytic activity of the immobilised enzymes showed that a significant amount of the enzyme retains its activity.190 Alternatively materials such as UCl 4 can be deposited inside MWNTs via the capillary action of eutectic and non-eutectic mixtures of UCl 4 and KCl.192 By expressing the wetting conditions as a function of polarizabilities it is possible to predict the threshold diameter for capillary filling of di§erent tubes.193 Alternatively, material can be incorporated within the tubes during preparation of the tubes.Using the arc discharge methodology, tubes have been filled with, for example, transition metals such as Cr, rare earth metals like Yb or covalent solids such as S; when graphite rods which contain no sulfur were used in the process there was no generation of tubes which contained encapsulated material leading the authors to propose a growth mechanism which incorporates carbon, metal and sulfur.194 Microcrystals of refactory carbides such as TaC may also be encapsulated and moreover novel metallic b-Sn nanowires can be produced by electrolyis of graphite electrodes immersed in molten LiCl, which contains SnCl 2 .195 A simple technique of high temperature treatment of a microporous carbon, which has been impregnated with a compound or material to be encapsulated such as molybdenum, uranium or cobalt, results in the formation of metal-filled carbon manoparticles.196 The opening of the tube tips has enabled the resultantOHand CO 2 Hgroups (which are assumed to be present) to be functionalised using standard chemical reaction conditions. For example SWNTs have been functionalised at the opened tips by reaction with thionyl chloride and octadecylamine; the derivatised tubes are more soluble in organic solvents than their parent compounds and can subsequently be reacted further along the waists of the molecules.197 Dichlorocarbene, fluorination, Birch reduction, and photolabelling experiments are also reported to successfully functionalise SWNTs.198 The Young’s modulus for a number of tubes has been estimated and found to be larger than that of the in-plane modulus of graphite by observing their free-standing room temperature vibrations in a transmission electron microscope.199 The buckling and collapse of SWNTs has been observed as a result of their incorporation into polymeric films; the tubes are calculated to have compressive strengths approximately two orders of magnitude higher than any known fibre.200 It is suggested that nanotube tips may be used in nanolithography and a cleaving method has been developed to Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 431–451 445reproducibly obtain dome-closed MWNT tips of suitable length and size.201 Covalently functionalised nanotube tips, which are produced by reaction of the carboxylic acid groups present to form amide moieties, have been used as probes in scanning tunneling microscopy (STM) and are sensitive to specific molecular interactions, a fact which should prove especially useful in the study of biological systems.202 Non-functionalisedMWNTshave also been used as high resolution probes for atomic force microscopy (AFM) by the same authors.203 Dry etching pattern transfer in GaAs using carbon nanotubes as tips in AFMhas been investigated independently.204AFM has also been used to manipulate their shapes, for example tubes can be bent or straightened, and the position of individual MWNTs dispersed on a surface.205 The semimetal-like temperature dependence of the electrical conductivity and a finite gap in the IR absorption spectrum is predicted by first principle calculations for a (10,10) tube with broken symmetry.206 Atomically resolved STM images have allowed the examination of the electronic properties of SWNTs as a function of tube diameter and wrapping angle; both metallic and semiconducting tubes were observed.207 Similar atomic resolution images of individual SWNTs reveal that a triangular arrangement of carbon atoms is seen rather than the expected hexagonal arrays.208 The conductance, which is found to be quantized, of MWNTs was measured by dipping a scanning probe microscope mounted tube into a liquid metal.209 One semiconducting SWNT connected to two metal electrodes has been used to construct a field-e§ect transistor which operates at room temperature.210 Calculations show that nanotubes should have ballistic transport properties.211 There has been considerable interest in the production of field-emission devices from nanotubes.Luminescence during electron field emission on SWNTs and MWNTs has been observed and is believed to be due to electron transitions between di§erent electronic levels participating in the field emission.212 Lighting elements with the structure of a triode-type vacuum tube having the conventional thermionic cathodes replaced with MWNT field emitters have been fabricated and stable electron emission, adequate luminance and long life of the tubes demonstrated.213 Similarly nanotubes are reported to compare favourably with other field emission sources owing to their low operating voltages and high current densities.214 Aligned nanotubes have also been filled with catalyst materials and used to electrocatalyse O 2 reduction and methanol oxidation.215 Addition of nanotubes, which act as heat sinks, to conducting polymers increases their electrical conduction by up to eight orders of magnitude.216 References 1 K.-H.Homann, Angew. 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ISSN:0260-1818    

DOI:10.1039/a804878i

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

23 Inorganic and organometallic polymers Michael L. Turner Department of Chemistry, University of She¶eld, She¶eld, UK S3 7HF 1 Introduction Macromolecules containing main group or d-block elements in their backbone continue to attract considerable attention because of the synthetic challenges involved in their preparation and the unusual properties displayed by these materials.1 This review covers developments in inorganic and organometallic polymers published during 1998.It follows the format of previous articles in this series authored by Manners.2 The first two sections cover new developments in main group polymers including the well established polysiloxanes, polyphosphazenes and polysilanes. Following these sections, recent developments in organometallic polymers of the d-block elements are discussed.This review emphasises the synthesis and properties of polymers with inorganic elements within the main chain rather than contained in the side-group structure. Several important reviews of inorganic polymer chemistry have appeared this year, including one from Manners who has reviewed the development of ring-opening polymerisation of strained metallocenophanes for the synthesis of polymetallocenes. 3 A comprehensive review of research on poly(sulfur nitride) carried out from the 1970s to the end of 1997 has appeared. The review covers the synthesis, properties, applications and calculations performed on this remarkable material.4 A great deal of work on inorganic polymers as solid electrolytes has been reported this year and two reviews discuss the relative merits of a number of inorganic polymerbased electrolytes, such as siloxane and phosphazene based systems.5,6 Recent developments in the area of heteroatom-based dendrimers, i.e.dendrimers whose connectivity is based on atoms other than carbon, have also been reviewed.7 2 Polysiloxanes (silicones), polysilanes and other Group 14 containing polymers A comprehensive review on the application of siloxanes in soft lithography has appeared this year.8 A detailed investigation of the chain length distribution, chemical heterogeneity and sequence statistics of poly(dimethylsiloxane)–poly(hydromethylsiloxane) copolymers has been reported.Conventional analytical tools such as SEC, 1H and 29Si NMR spectroscopy and fractionation by precipitation show that the hydrodynamic behaviour of these copolymers is almost identical to that of Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 453–465 453poly(dimethylsiloxane). These results were compared with those obtained from MALDI-TOF-MS. A unique interpretation of the mass spectrum without supporting analytical information was not possible and may give misleading results, especially when using high power lasers.Quantitative interpretation may be achieved by computing the expected spectrum from SEC and NMR results and mapping it on to the measured mass spectrum.9 Time-resolved fluorescence anisotropy measurements on N,N@-bis(2,5-di-tert-butylphenyl)- 3,4,9,10-perylenedicarboximide have been used to study the segmental chain dynamics within bulk polydimethylsiloxane.This work showed that there is a simple correlation between the probe dynamics and the polysiloxane bulk density and that polymer dynamics in supercritical CO 2 may be tuned by controlling the CO 2 pressure at temperatures above the critical point.10 Siloxane polymers with NLO active chromophores covalently bound in the polymer backbone exhibit good second harmonic generation characteristics on poling.Incorporation of a rigid spacer and a large NLO chromophore led to enhanced temporal stabilities at room temperature.11 Si O Me (CH2) n O O OC CO OCmH2m+1 H2m+1CmO O O n 1 Siloxane polymers 1 with mesogenic groups incorporated at each Si atom and linked to the rigid portion of the mesogen by flexible spacers show nematic and smectic C mesophases.The stability of the nematic phase is promoted for short spacers and short aliphatic tails, whereas larger spacers and longer alkyl tails stabilise the formation of the smectic phase. Dilution of the mesogens by incorporation of dimethylsiloxane segments in the polymer backbone destabilises the nematic phase owing to a reduction in the number of mesogen–mesogen interactions.However, it stabilises the smectic phase on weak dilution as the polymer backbone flexibility is increased.12 The optically active phthalocyaninatopolysiloxane 2 displays what the authors call Shish kebab-like chirality, a new type of main chain chirality in polymers. The phthalocyanine rings are stacked on top of each other to give a helical structure resulting from a staggering of neighbouring phthalocyanine rings at a constant angle, always in the same direction.13 Hyperbranched poly(siloxysilanes) have been synthesised by the platinum-on-carbon catalysed hydrosilylation of the AB 2 , AB 4 and AB 6 monomers 3, 4 and 5.Molecular weight averages are around 10 000 with polydispersities of around 2. Intramolecular cyclisation reactions may be responsible for the limited growth of these hyperbranched polymers.End-capping reactions of the terminal Si–H groups with a variety of reagents may lead to materials with a variety of interesting applications.14 High molecular weight vinyl-substituted silphenylene siloxane polymers were prepared by a polycondensation. Elastomers of these materials Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 453–465 454exhibited remarkable thermal and oxidative stability with the vinyl substituted polymer showing loss of only 0.7 wt% after 5 h at 400 °C in nitrogen.15 N N N N N N N N R R R R R R R R Si O O n 2 R = Si(OSiMe2H)2 Me Si(OSi(Me)(OSiMe2H)2)2 Me Si(OSi(Me)(OSiMe2H)2)3 3 4 5 The polymodal molecular weight distribution of poly(methylphenylsilane), obtained from the Wurtz reaction of MePhSiCl 2 with sodium in refluxing toluene, has been explained by a variation in the rates of propagation and termination during the growth of the polymer molecule.Given that the extent of conjugation and most probably the stabilisation energy of a polysilane reaches a maximum at around 35 repeat units, the authors suggest that the probability for chain growth termination is at a maximum at this chain length and decreases as you go to higher degrees of polymerisation.This variation in the probability of termination results in two distinct molecular weight fractions, one at around 35 repeat units and the other at very high molecular weight, the latter fraction should have a very low probability of termination and hence a low polydispersity, which is experimentally observed.The authors suggest that if the Wurtz reaction is carried out at lower temperatures when the rate of termination is lowered, then it should be possible to obtain only the high MW fraction, which is supported by monomodal GPC traces obtained in refluxing THF.16 High molecular weight methylsilane polymers can also be prepared by extended sonication (25–40 h) of MeSi(H)Cl 2 and sodium in hexane or toluene and THF mixtures.These polymers serve as precursors for near stoichiometric SiC with ceramic yields of up to 90%.17 A series of isomeric polymers, [RMeSi]n and [R 2 Si]n [R\(CH 2 ) 6 OMe, (CH 2 ) 5 OEt or (CH 2 ) 4 OPr/], with side groups consisting of seven carbon atoms and one oxygen atom have been prepared in which the position of the ether oxygen in the side chain was varied.The unsymmetrical ether-substituted polymethylsilanes were all amorphous but some underwent an amorphous to amorphous phase transition which is accompanied by a discontinuous thermochromism. Polymer mobility was enhanced Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 453–465 455as the oxygen atom of the ether substituent moved closer to the silicon backbone. In the symmetrical ether-substituted polysilanes a phase transition from a mesophase to a polycrystalline polymer leads to an abrupt shift in the visible absorption spectra.18 The thermal ring-opening polymerisation of Si 4 Cl 8 gives perchloropolysilane, [SiCl 2 ]n.This polymer is virtually insoluble in all solvents and an X-ray di§raction study suggests that the polymer adopts an all trans conformation.Substitution of the chlorine atoms by nucleophiles is possible and in the presence of excess propan-2-ol, [Si(OPr*) 2 ]n is produced with on average 35 repeating units.19 Ring-opening polymerisation of tetraphenylsilole spirooctamethylcyclopentasilane with butyllithium gives a silole-incorporated polysilane of well defined microstructure 6.This polymer shows two absorptions in the UV spectrum at 320 and 360 nm, these features were assigned to the polysilane skeleton and the silole ring, respectively.20 Dehydrobromination of 2,5-dibromosiloles and diethynyldi- and mono-silanes in the presence of a CuI/Pd(PPh 3 ) 4 catalyst gives diethynylsilole-silane copolymers 7. These polymers show intense UV absorptions at around 410 nm, red-shifted from monomeric models by ca. 20 nm. They are insulators in the pure state but can be doped to conduction (10~3Scm~1) with FeCl 3 .21 Palladium catalysed coupling of diiodobenzene derivatives with 1,4-diethynylbenzene and 1,2-diethynyltetra-n-butyldisilane gives a variety of silane-phenyleneethynylene copolymers 8. The supramolecular structures of these polymers become more disordered as the proportion of silicon-containing component increased, resulting in a blue shift of the j.!9 observed in the UV spectra.22 Multiblock copolymers of poly(ethylene oxide) and poly(methylphenylsilane) form vesicles on addition of a THF solution to water, followed by concentration.TEM analysis showed the diameter of the vesicles to range from 100 to 180 nm.23 Si Si Bu Bu Bu Bu OR RO OR RO n m n 8 (m = 0–2, n = 0–2) Si Si Me2 Me2 Si Si Me2 Me2 Si Ph4 Si Si Ph Ph Ph Ph R R m n 6 7 A dendritic polysilane with a total of thirty-one silicon atoms and thirteen silicon atoms in the longest chain has been synthesised by a convergent approach.The UV spectrum of this dendrimer is very similar to that of the analogous linear thirteen silicon chain with two absorption maxima at 260 and 283 nm.24 Dendrimers with alternating Si and Ge atoms in the framework were prepared from Me(PhMe 2 Ge) 3 Si as a core.Divergent growth was achieved by cleavage of the Ge–Ph bond with Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 453–465 456CF 3 SO 3 H, followed by further reaction with Me(PhMe 2 Ge) 2 SiLi. A permethyl substituted dendrimer was synthesised by conversion of the six peripheral Ph groups to methyl groups via a hexachloro-substituted dendrimer. The struture of the permethyl dendrimer was confirmed by a single crystal X-ray di§raction study.25 Network copolymers, poly(cyclohexylsilyne-co-phenylsilyne) [(C 6 H 11 Si)x(PhSi)y]n and poly(cyclohexylsilyne-co-phenylgermyne) [(C 6 H 11 Si)x(PhGe)]n, have been prepared by an electrochemical reduction utilising copper electrodes and a constant applied potential.The molecular weight distributions are narrow and monomodal, whereas those obtained under typical Wurtz coupling conditions are broader and polymodal. 26 Electrochemical synthesis of poly(dialkylstannanes) gives polymers whose properties are consistent with those prepared by Wurtz or dehydropolymerisation.The authors report that these polystannanes are stable to oxygen but are sensitive to moisture in solvents such as THF.27 Demethanative coupling of ArMe 2 GeH with a ruthenium catalyst gives poly(arylmethylgermanes), [ArMeGe]n (Ar\Ph, p-tolyl, p-fluorophenyl, m-xylyl, p-anisyl and p-trifluorotolyl). The spectroscopic and electronic properties of the Ph substituted polymers are identical to those prepared by conventional Wurtz coupling, suggesting that there is little branching in the polymers prepared by demethanative coupling.28 A number of polycarbosilanes have been prepared by hydrosilylation polymerisation.The polymer properties were examined and compared to those of a range of silicon-based IPNs prepared by simultaneous curing of silsesquioxane oligomers using a titanium catalyst.29 Chloromethylsilyl terminated carbosilane dendrimers serve as useful precursors to amphiphilic dendrimers on reaction with thiols and subsequent elaboration.These dendrimers have hydrophobic carbosilane cores and hydrophilic alcohol, dimethylamino or sulfonate groups at the periphery.They act as micelles in solubilising small aromatic molecules (C 6 H 5 R, R\H, Et, Pr) in water.30 Hydrosilylation polymerisation of bis(vinyldimethylsilyl)dichloromethane with dihydrotetramethyldisiloxane or dihydrohexamethyltrisiloxane is reported to give [SiMe 2 CCl 2 SiMe 2 (CH 2 CH 2 )SiMe 2 (OSiMe 2 )x(CH 2 CH 2 )]n (x\1 or 2) with two halogeno groups directly attached to each backbone unit.31 A short communication has reported that optically active allylhydromethylphenylsilane CH 2 \CHCH 2 Si(H)MePh polymerises by hydrosilylation to give predominantly isotactic poly(methylphenylsilylenetrimethylene).32 Coupling of the diynes, MeC- –– – CSiMe 2 ArSiMe 2 C–– – CMe(Ar\1,4-C 6 H 4 , 1,3-C 6 H 4 or 4,4@-C 6 H 4 C 6 H 4 ), with a slight excess of ‘zirconocene’, ‘[Cp 2 Zr]’, gave zirconacyclopentadienyl polymers 9.These polymers hydrolyse rapidly to the parent butadienediyl polymers, [Me 2 SiArSiMe 2 CH–– CMeCMe––CH]n. Heating the zirconium polymers in THF results in a remarkably selective chain scission reaction to form cyclic oligomers, the size of which are determined by the aryl spacer geometry.33 Si Ar Si Cp2 Zr Me Me Me Me Me Me n 9 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 453–465 4573 Polyphosphazenes and polymer systems based on main group elements Alkoxy ether-substituted polyphosphazenes continue to attract considerable interest, owing to their high solid-state conductivities. This year, Allcock et al.34–37 have reported the synthesis of crown ether substituted polyphosphazenes, by reaction of poly(dichlorophosphazene) with the lithium salts of hydroxymethyl crown ethers.High levels of substitution have been achieved with the replacement of up to 96% of the chlorine atoms. Mixed substituent polymers 10 with 75% O O O O O P N O O O x y z y : z = 3:1 x = 1,2,3 n 10 2-(2-methoxyethoxy)ethoxy groups and 25% crown ether groups were also reported and these proved easier to handle than the parent homopolymers.The cosubstituent polymers have higher glass transition temperatures than the 2-(2- methoxyethoxy)ethoxy homopolymer and lower ambient temperature ionic conductivity. The ionic conductivity decreased when a particular cation (Li, Na, K, Rb or Cs) was known to form a stable 1: 1 or 2: 1 complex with the pendant crown ether. These results suggest that in polymer electrolytes such as these a large fraction of the current is carried by the cations.34 The influence of polymer architecture on dimensional stability and solid-state conductivity has also been studied.The conductivities of three di§erent lithium salts in tri-armed star, low molecular weight linear and highly branched alkoxyether-substituted polymers has been examined. Lower molecular weight and highly branched polymers showed higher conductivities than conventional 2-(2-methoxyethoxy)ethoxy substituted linear polymers.35 New side groups for polyphosphazenes reported this year include pendant trialkylamine, 36 sulfone and sulfoxide groups.The polar sulfone and sulfoxides, prepared by oxidation of thioether-containing side groups, raised the glass transition of alkoxyether co-substituent polymers to ca. 19 °C, which precluded their use as solid electrolytes. However these polymers have shown promise as gel electrolytes on addition of propylene carbonate.37 Deprotonation–substitution reactions of [NP(Ph)Me]n have continued to attract interest and reaction of ethylesters with the anion generated on addition of Bu/Li to [NP(Ph)Me]n gives phosphazene polymers with ketone side groups.38 In an initial report, various cycloalkylamines were reacted with [NPCl 2 ]n to give poly(cycloalkylaminophosphazenes). Full substitution proved impossible and these polymers retained some residual P–Cl bonds and hence became insoluble on standing.39 Phosphazene-ethyleneoxy di- and tri-block copolymers have been prepared by the cationic polymerisation of phosphoranimines using the mono- or di-functional macroinitiators [R(CH 2 CH 2 O)nCH 2 CH 2 N(H)(X 2 P––NPCl 3 `)]PCl 6 ~ [R\NH(X 2 P–– Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 453–465 458NPCl 3 `)PCl 6 ~ or OMe].40 Several new spirocyclic phosphazene polymers have been prepared by the reaction of aromatic diols with [NPCl 2 ]n using an alkali metal carbonate as a proton abstractor.No evidence of cross-linking reactions was reported and optically active polymers were prepared starting from enantiomerically pure binaphthol. The specific rotation of these polymers was opposite and much higher than that of the starting diol but declined on heating for extended periods at high temperature.41 Phosphazene polymers with loadings of NLO chromophores of [1 per repeat unit have been prepared by polymer modification reactions. These polymers have high glass transitions ([100 °C) and high refractive indices ([1.7); however, they show low electrooptical coe¶cients on weak poling at 3 eV.42 The first well characterised alkoxy-substituted poly(thionylphosphazenes) have been reported.Replacement of the chlorine atoms of [NS(O)Cl(NPCl 2 ) 2 ]n with tri- fluoroethoxy groups can be accomplished by slow addition of the nucleophile at low temperature ([50 °C).Substitution of up to 95% of the chlorine atoms has been achieved and stable polymers have been isolated by replacement of the remaining, S-bound, chlorine with Bu/NH 2 . The resulting copolymers are all amorphous and the glass transition temperature decreases from [14 to [30 °C on increasing the tri- fluoroethoxy substitution from 50 to 95%.43 Treatment of the cyclic thionylphosphazene, [NS(O)Cl(NPCl 2 ) 2 ], with substoichiometric quantities of GaCl 3 in CH 2 Cl 2 results in an ambient temperature ring-opening polymerisation to give [NS(O)Cl(NPCl 2 ) 2 ]n.Treatment of this polymer with Bu/NH 2 gave [NS(O)(NHBu/)MNP(NHBu/) 2N2 ]n of comparable molecular weight to that obtained by thermally induced ring-opening at 165 °C.44 Thionylphosphazenes 11 (R\H or SiMe 3 ) can be thermally polymerised to give an alternating copolymer of dimethylphosphazene and methyloxothiazene units.This polymer 12 is the first example of a polythionylphosphazene in which the side groups are bound by P–C and S–C bonds and the first example of an inorganic polymer with a repeating unit of one sulfur, one phosphorus and two nitrogen atoms.45 CF3CH2O P N Me Me S N O Me R PN Me Me S N O Me n 11 12 Heat Interfacial polycondensation of bifunctional diazonium salts with bifunctional phosphoric diesters gives a number of poly(arylazophosphonates) 13.All of these polymers are red and in laser ablation experiments give structures with sharp edges, clear contours and flat bottoms.46 2,5-Dialkynylthiophenes, (RC–– – C) 2 C 4 H 2 S (R\Ph, Me 3 Si or Bu5) undergo hydroboration polymerisation on sequential treatment with HBCl 2 and Et 3 SiH.The 11B NMR resonances for the resulting intensely coloured polymers 14 are ca. 50 ppm upfield of those for the analogous monomers, perhaps indicating an electronic interaction between the boron and the thiophene units in the polymer main chain.47 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 453–465 459N N R1 N N P O R3 O P O OR2 O OR2 S B R R Cl n R1 = O,CO; R2 = Me,Et; R3 = (CH2) n,(CH2)2O(CH2)2, (CH2)2( c–C6H10)(CH2)2– p, (CH2)2C6H4(CH2)2– p 13 n 14 4 Polymers containing skeletal d-block elements Metallodendrimers and metallocenyl polymers have attracted a huge amount of interest over the past year and several useful reviews have appeared.A review of metallodendrimers has appeared, this article focusses on the various structural types reported and illustrates the functions that these intriguing molecules may perform.48 Puddephatt reviews his work in the field of precious metal polymers, describing the preparation and properties of linear, hyperbranched and dendritic polymers with gold or platinum atoms in the polymer backbone.49 Convergent and divergent syntheses of Ru(II) and Os(II)-containing dendrimers based on polypyridyl ligands have been reviewed.The electrochemical and spectroscopic properties of the constituent units are only slightly perturbed in the assembled dendrimers.Rapid exoergic energy transfer between adjacent units results in quenching of the luminescence from units with higher energy 3MLCT states.50 Dendrimers with metalloporphyrins as the building block for both the central core and the periphery show interesting co-operative behaviour between the di§erent arms of the dendrimer in response to the bifunctional ligand dabco.51 Two isomeric ruthenium- containing dendrimers have been reported with the location of the metal centres within the dendrimer precisely defined.52 Four new neutral carbosilane dendrimers have been synthesised and functionalised with [Cp*Ru`] centres to yield polycations with charges of 12], 24], 36]and 72]. The structure of the smallest member of this series 15 has been determined by single crystal X-ray di§raction and shows a distinctly non-spherical structure with packing which can be approximated as a body-centred cubic lattice of 23 Å diameter spheres.Electrospray ionisation Fourier transform mass spectrometry gave unequivocal evidence for the nuclearities of these dendrimers by complete resolution of the isotropic distribution.53 A range of platinum-containing polyynes with heteroaromatic rings in the polymer backbone have been prepared using an adaptation of the synthetic work first described by Hagihara.In these polymers the character of the optical excitations is independent of the aromatic segments of the chain. The T 1 state remains strongly localised on the Annu. Rep. Prog. Chem., Sect.A, 1999, 95, 453–465 460Si Si Si Si Cp*Ru Ru Cp* Cp*Ru 15 4 4 12+ (OTf–)12 [Cp*Ru(NCMe)3]+OTfaromatic region, whereas the Tn and S 1 states are delocalised. The extent of delocalisation is larger for the more electron-rich aromatic segments.54 Hypothetical organometallic polymers, [LnMCm]n (m\2–4), of early to mid d-block elements with p-donor ligands (L) have been studied by extended Huckel calculations.The electronic properties of the polymers depend on the electron count at the metal centre and the length of the bridging chain. Suitable synthetic targets for stable, conducting polymers are proposed.55 Three platinum alkynyl polymers, [Pt(PBu/ 3 ) 2 –C–– – C–R–C–– – C]n, with fluorene derivatives (R\fluorenediyl, fluorenonediyl and ferrocenylfluorenediyl) incorporated in the main chain have been reported with the ferrocenylfluorene derivative showing the smallest band gap (2.1 eV).56 Platinum-containing dendrimers have been built up from a triyne core using a divergent approach based on copper-catalysed dehydrohalogenation reactions.The largest dendrimer reported has 21 Pt atoms, 22 benzene rings and 42 alkyne units.57 Zinc porphyrin polymers 16 in which the porphyrin units are connected by diethynylaryl groups, have been synthesised.Absorption and emission spectra of these polymers are significantly red-shifted from those of the monomeric porphyrins, suggesting significant electronic delocalisation in the polymers.58 Polymeric films of the Co(Salen) derivative 17 can be readily oxidatively deposited on platinum electrodes from a saturated monomer solution.These polymer–d-block metal hybrid materials display high conductivity (up to 200 S cm~1) and a high sensitivity to coordinating ligands such as pyridine.59 Polycondensation of a dihalogenated cobaltacyclopentadiene complex using a [Ni(cod) 2 ] catalyst gave regioregular p-conjugated cobaltcyclopentadiene polymers 18.Cyclic voltammetry showed the interaction between adjacent cobalt centres to be weak, presumably due to the large dihedral angle between the benzene ring and the cobaltcyclopentadiene ring in the polymer backbone.60 Reaction of cobalt cyclopentadienyl polymers with elemental sulfur results in the replacement of the CpCo moiety with sulfur to produce thiophene containing copolymers.61 The phase behaviour of a range of metal-containing liquid crystalline polymers 19 have been studied by DSC, dynamic viscosity and X-ray di§raction.Nematic phase stability follows the trend Pd[Ni[Cu[V(––O) for any value of chain length, m. However, the stability interval of the mesophase decreases along the sequence V(––O)[Pd[Ni[Cu, with monotropic phases mostly observed for the copper derivative. It is possible that the di§erences in the observed LC properties are related to the co-ordination geometry at the metal, but this relationship has not yet been clearly demonstrated.62 Oligomeric Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 453–465 461N N N N R R Zn Mes Mes S O O O Co N O N S O O n 16 [R = OC15H31, CON(C8H17)2] 17 Co PPh3 Bun Bun n 18 (CH2)12O CO O M N N O OC O (CH2)m-1CH3 CH3(CH2)m-1 O O n 19 (M=Pd, m=8–11; M=Ni, m=8–12; M=V(O), m=6,8,10) siloxyferrocenyldiethynyl polymers with m-carborane units randomly dispersed in the backbone gave a weight retention of 78% on heating to 1000 °C under nitrogen. The resulting char is ferromagnetic and shows essentially no weight loss on heating to 1000 °C in air.63 Metal catalysed polymerisation of silicon-bridged [1]ferrocenophanes allows access to a wide range of ferrocenyl polymers of controlled architectures.For example polymerisation of [Fe(g-C 5 H 4 )(g-C 5 Me 4 )SiMe 2 ] 20 with PtCl 2 at room temperature gives a regioregular polymer 21, in which it is only the Cp–Si bond which is broken on polymerisation. Copolymerisation of 20 with benzosilacyclobutane gives a random ferrocenylsilane–carbosilane copolymer with two glass transitions at 66 and 147 °C Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 453–465 462SiMe2 Fe Me2 Si Fe Fe Si Me2 n PtCl2 20 21 detected by DSC. The presence of two glass transitions suggests that the polymer microstructure contains blocks of each polymer which are distributed randomly throughout the polymer backbone.Excellent molecular weight control in the platinum- catalysed homopolymerisation of 20 was achieved by adding varying amounts of Et 3 SiH as a chain-capping agent. Furthermore, end-capping the growing chains with a poly(hydromethylsiloxane) gave graft copolymers of poly(siloxane) and poly(ferrocenylsilane). End-capping with cyclo-(HMeSiO) 4 gave star polymers with a central cyclosiloxane core, whereas end-capping with a hydro-terminated poly(dimethylsiloxane), H[SiMe 2 O]nH, gave triblock copolymers of poly(ferrocenylsilane)– poly(siloxane)–poly(ferrocenylsilane).64 Stereoregular ferrocenylsilane polymers are obtained on 60Co c-irradiation of large single crystals of the unsymmetrically substituted [1]ferrocenophane, [Fe(g- C 5 H 4 ) 2 SiMePh].Preliminary assignment of a syndiotactic stereochemistry has been made by comparison with analogous atactic polymer prepared by the thermal polymerisation of [Fe(g-C 5 H 4 ) 2 SiMePh].65 Treatment of the dichlorosilyl-substituted [1]ferrocenophane, [Fe(g-C 5 H 4 ) 2 SiCl 2 ], with a variety of alcohols or phenols (ROH) in the presence of an HCl acceptor results in substitution of the chlorine atoms to give [Fe(g-C 5 H 4 ) 2 Si(OR) 2 ] (R\Me, Et, CH 2 CF 3 , Bu/, C 6 H 13 , C 10 H 21 , Ph, PhBu5, PhNO 2 , PhPh).These new ferrocenophanes undergo thermal or d-block metalcatalysed ring-opening polymerisation to give new poly(ferrocenylsilanes). By varying the side group at Si, polymers with glass transitions as low as [51 °C or as high as ]97 °C have been obtained.66 Tin bridged [1]ferrocenophanes have been prepared with two bulky substituents (Bu5 or C 6 H 2 Me 3 -1,3,5) at tin.Single crystal X-ray di§raction studies show that in these compounds the cyclopentadienyl rings are tilted at angles of 14° (R\Bu5) and 15° (R\C 6 H 2 Me 3 -1,3,5), angles which are significantly smaller than either the silicon or germanium bridged analogues. These new ferrocenophanes undergo thermally induced ring-opening polymerisation to give high molecular weight poly(ferrocenylstannanes) which are stable in air for several days in the solid state.Ring-opening of the Bu5-substituted ferrocenophane is remarkably facile and polymerisation can be carried out at room temperature in toluene solution. Polyferrocenylstannanes show strong redox coupling between adjacent iron centres, mediated by the tin atoms.Unfortunately these ferrocenophanes did not undergo metal-catalysed ring-opening polymerisation with Pt catalysts.67 Vapour phase impregnation of MCM-41 with [1]dimethylsilylferrocenophane gave composite materials which at low loadings consist of ring-opened monomeric and oligomeric ferrocenyl silanes.Higher loadings result in absorption of excess monomer into the pores of the mesoporous silica. This encapsulated monomer can then be thermally polymerised within the channels. Pyrolysis of these composites at 900 °C under nitrogen gives a black magnetic powder. Powder XRD confirms that the Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 453–465 463CH2 S Fe H2 C S Fe BF3•OEt2 n 22 23 hexagonal structure of the MCM-41 is maintained and an increase in contrast suggests that the walls are coated by Fe.68 The melting and crystallisation behaviour of a series of well defined poly(ferrocenylsilanes) is described.A value for the equilibrium melting temperature of 143 °C was obtained and this is significantly higher than the previously reported value of 122 °C for high molar mass poly(ferrocenyldimethylsilane).69 [2]Carbothiaferrocenophanes 22 undergo cationic ring-opening polymerisation using conventional Lewis acid catalysts such as BF 3 ·OEt 2 to give poly(carbothiaferrocene) 23, the high molecular weight fraction of which is insoluble in common organic solvents.70 The synthesis and self assembly of ferrocene block copolymers has been reviewed.Ferrocenyl-siloxane block copolymers assemble in the bulk state to form polyferrocene cylinders in a hexagonal array within a siloxane matrix. Cylinders of these polymers can be solubilised in hexane and consist of a polyferrocene core surrounded by a polysiloxane sheath.71 References 1 J.E. Mark, H. R. Allcock and R. West, Inorganic Polymers, Prentice Hall, New York, 1992. 2 I. Manners, Annu. Rep. Prog. Chem., Sect. A, Inorg. Chem., 1997, 94, 603 and preceding issues. 3 I. Manners, Can. J. Chem., 1998, 76, 371. 4 A. J. Banister and I. B. Gorrell, Adv. Mater., 1998, 10, 1415. 5 W.H. Meyer, Adv. Mater., 1998, 10, 439. 6 V. Chandrasekhar, Adv. Polym. Sci., 1998, 135, 140. 7 H. Frey, C. Lach and K. Lorenz, Adv. Mater., 1998, 10, 279. 8 Y. Xia and G. M.Whitesides, Angew. Chem., Int. Ed. Engl., 1998, 37, 550. 9 S. Servaty, W. Kohler, W.H. Meyer, C. Rosenauer, J. Spickermann, H. J. Rader, G. Wegner and A. Weier, Macromolecules, 1998, 31, 2468. 10 E. D. Niemeyer and F. V. Bright, Macromolecules, 1998, 31, 77. 11 H. Jiang and A. K. Kakkar, Macromolecules, 1998, 31, 2501; 4170. 12 S. Lecommandoux, M. F. Achard and F. Hardouin, Liq. 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ISSN:0260-1818    

DOI:10.1039/a805981k

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

24 Magnetism S. T. Bramwell University College London, Department of Chemistry, Christopher Ingold Laboratories, 20 Gordon Street, London, UK WC1H 0AJ 1 Introduction This year’s report will follow a similar format to the last two years’.1,2 A number of trends can be spotted in the 1998 magnetism literature, and most of these are motivated by interest in physical properties. Thus, magnetoresistance of transition metal oxides remains a theme, but authors are increasingly discussing these materials in terms of a ‘half metallic’ band model, in addition to the traditional ‘double exchange’ mechanism.Another popular subject among physicists, that will surely have some spin-o§s in solid state chemistry, is quantum phase transitions. These are transitions at zero temperature that are driven by quantum, rather than thermal fluctuations.They typically occur in so-called ‘heavy fermion’ materials, which are usually rare earth alloys. Magnetic frustration remains a popular subject, and there has been a noticeable upturn in the number of papers discussing high pressure properties of magnets. Thus, half-metallic behaviour, quantum phase transitions, high pressure properties and frustrated magnets are the special topics chosen for review in Section 2.This will be followed by the usual round-up of results on ionic, covalent and metallic materials (Section 3) and co-ordination complexes and molecular compounds (Section 4). This introductory section is completed by a review of developments in magnetism at large that have made it into the news.Unfortunately, not much magnetochemistry has been successful in this regard, but condensed matter physics has fared better. Ceperly3 has described in Nature how a system of metallic electrons, in contrast to a system of molecules, prefers a lattice structure (the Wigner crystal) at low density and a disordered liquid like structure (the Fermi fluid) at higher density.At intermediate densities, as originally realised by Bloch, the ground state is ferromagnetically polarised. Young et al.4 have reported that lanthanum-doped CeB 6 exhibits the polarised state at room temperature. This material has a weak ferromagnetic moment that disappears at 600 K, a temperature of the order of the Fermi energy. The change from Wigner crystal to Fermi liquid with increasing pressure has also been described by Voss in Science as a quantum phase transition.5 Fisk and Pines6 have reported in Nature on new evidence of a magnetic pairing mechanism in ‘heavy fermion’ materials (see Section 2) that is an alternative to the usual BCS superconductivity where the formation of Cooper pairs is Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 467–480 467mediated by phonons.Another novel aspect of superconductivity has been discovered in Sr 2 RuO 4 (T# \1.2 K), which has been found to have triplet Cooper pairs (S\1), rather than the usual singlets (S\0).7 In general terms, the response of a superconductor to an applied magnetic field is a highly characteristic probe of the superconducting state, and can lead to useful technology such as magnetic levitation.The basic response is diamagnetism (repulsion from sources of magnetic field), which in its purest form is total exclusion of magnetic flux from the sample, the Meissner e§ect. In recent years, however, paramagnetic behaviour (attraction to sources of magnetic field) has been observed in small or granular samples of both conventional and high-T# superconductors.Work by Geim and co-workers published in 19988 has helped finally establish the origin of this ‘Paramagnetic Meissner E§ect’. Careful magnetization measurements on lm sized Al discs showed the paramagnetic state is a field-induced metastable state that involves several quanta of magnetic flux being ‘frozen’ into the sample, and behaving, in essence like a simple paramagnetic moment.The metastability is a surface e§ect that should be common to all superconductors, and depends on quantum confinement induced by the small sample size. Magnetism in space has received some attention, with Nature9 discussing recent astronomical results that demonstrate the existence of neutron stars with superstrong magnetic fields ([1015 G). Nature have also reported on turbulence.This is a phenomenon that is as familiar to astronomers as it is to chemists, and so its universality over many length scales perhaps comes as no surprise. L’vov10 has described new results11 that demonstrate a universal connection between the statistics of turbulent flow and those of a magnetic critical point. At a more practical level, the magnetocaloric e§ect is a method of obtaining refrigeration by repeated magnetization and demagnetization of a magnetic material.Metallic gadolinium has a large magnetocaloric e§ect, but Glantz12 has written in Science how improved magnetocaloric materials (described in last year’s report2) have been discovered by Pecharsky and Gschneider. The materials are formed by alloying gadolinium with germanium and silicon.Finally, New Scientist13 has reported a fast way of synthesising commercially important magnetic oxides. The method, due to Parkin and Pankhurst, involves a thermite-type reaction between Fe and metal oxides, conducted in a strong magnetic field. This gives samples that already have a remnant magnetization, and do not need further processing in this regard. 2 Selected topics Half metallic magnets, double exchange and magnetoresistance The concept of a half metallic magnet was introduced by de Groot et al.in 1983.14 The idea is that the electrons form a spin up and spin down band; however, one is metallic and the other insulating. Double exchange ferromagnets such as LaxSr 1~xMnO 3 can be thought of as half metallic, in that the transfer of an electron between the e' orbitals of Mn3` (d4) and Mn4` (d3) is favoured when the ions have parallel spins, but disfavoured by Hund’s rules when spins are antiparallel. TheMne' bands overlap with the O 2p bands, but for one spin state there is a band gap caused by the Hund rule repulsion.Park et al.15 have used spin resolved photoemission measurements on the Annu. Rep. Prog.Chem., Sect. A, 1999, 95, 467–480 468Fig. 1 Schematic band diagram of the doped manganese perovskites, where J is the Hund rule exchange energy (after ref. [15]). ferromagnetic double exchange perovskite La 0.7 Sr 0.3 MnO 3 to establish the validity of this picture. The spectra distinguish a metallic ‘Fermi cut o§’ at the Fermi level for one spin state from an insulating band gap for the other.In this way Park et al. show that La 0.7 Sr 0.3 MnO 3 is half metallic at 40K (@T C ) with [100% carrier spin polarisation, but insulating at 380K (AT C ). The schematic band diagram is shown in Fig. 1. Perhaps an even clearer example of a half metallic magnet is provided by Sr 2 FeMoO 6 , reported by Kobayashi et al.16 This materials is an ordered double perovskite with Fe3` (d5) and Mo5` (d1) occupying alternate sites in the perovskite lattice.The half metallic character arises from the fact that the Fe3` electrons are localised and the Mo5` electrons are partly delocalised into the conduction band, but the two are connected by antiferromagnetic superexchange to give ferrimagnetic behaviour below T C \425 K. Thus there is 100% spin polarisation of the carriers.Kobayashi et al. find that Sr 2 FeMoO 6 shows giant magnetoresistance of 10% at room temperature, rising to 42% at 4.2 K. Ferromagnetic CrO 2 was predicted to be half-metallic by Schwartz,17 but this was questioned by Klemper et al.18 It is an enigmatic substance that has attracted some attention this year. It has metallic conductivity despite large coulomb interactions that would normally result in a Mott–Hubbard insulator.This year Korotin et al.19 have presented band structure calculations that show why this is. Cr is formally in the d2 state; however, one d electron is localised, and the other is p-bonded with oxygen which leads to a partially filled narrow band. In this regard CrO 2 is like a self-doped double exchange ferromagnet, where ferromagnetic exchange arises from the motion of the second d electron of Cr in the O2p band.Another investigation of CrO 2 20 has shown that pressed powders of CrO 2 exhibit giant negative magnetoresistance of up to 50% at 5K(but falling to 0.1% at room temperature). The authors argue that the e§ect is extrinsic, arising from spin dependent tunneling between the particles.Tunneling magnetoresistance in CrO 2 has also been reported by Hwang and Cheong.21 The samples were made by thermal decomposition of CrO 2 . Continued annealing led to partial decomposition to Cr 2 O 3 , the presence of which was found to enhance the Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 467–480 469magnetoresistance. Returning to the doped manganites, it was originally thought that a double exchange delocalisation of holes in the Mn3` lattice was su¶cient to explain the large magnetoresistance of these materials.However, in 1995 Millis et al.22 showed that the magnetoresistance was less than expected for the double exchange model, and invoked variable lattice distortions (Mn3` is a Jahn–Teller ion, but Mn4` is not) to explain the discrepancy.This year Booth et al.23 have presented XAFS measurements on La 1~xCaxMnO 3 that are used as a structural estimate for the hole concentration. They find that the hole concentration is proportional to the magnetization, which establishes a direct link between magnetism and structural distortions of the MnO 6 octahedra in this material. A dc-magnetization study has been used to measure the critical behaviour of La 0.7 Sr 0.3 MnO 3 .24 The following parameters were determined: T C \354.0(2) K, b\0.37(4), c\1.22(3).Only the b value is consistent with three dimensional Heisenberg behaviour (b\0.365, c\1.336). Other magnetoresistive oxides have also continued to attract interest. Neutron scattering investigations of the Ruddlesdon–Popper phase La 1.2 Sr 1.8 Mn 2 O 7 were reported by Osborn et al.25 They found a strong spin canting below the transition temperature (112 K), indicative of a competition between ferromagnetic double exchange and antiferromagnetic superexchange, and a critical exponent b\0.13 close to the value expected for the 2D Ising model (b\0.125).Lynn et al.26 have used neutron di§raction to characterise the magnetic critical properties of the ferromagnetic pyrochlore Tl 2 Mn 2 O 7 .They report a transition temperature T C \123.2 K, with exponent b\0.312. Majumder and Littlewood27 have considered a general model of magnetoresistance that involves the scattering of charge carriers by magnetic fluctuations. They show that the magnetoresistance scales as (charge density)~2@3 and (magnetization) 2.Thus substances with a small carrier density such as Tl 2 Mn 2 O 7 are expected to show the largest magnetoresistance. Kondo e§ect, heavy fermions and quantum phase transitions The Kondo e§ect is the screening of a localised spin, such as an impurity in a normal metal, by itinerant ones (the conduction electrons). At high temperature the impurity is paramagnetic, while at low temperature it becomes non-magnetic due to the screening.Although the physics of the Kondo e§ect is very well established, there has been little direct experimental evidence for the predicted energy levels of the screened moment. Madhaven et al.28 have determined the local structure of cobalt atoms on a gold crystal, using STM, and have thereby obtained direct evidence of the Kondo resonance.Heavy fermion materials contain both conduction electrons and localised moments, that interact strongly together. The itinerant spins strongly screen the localised ones by the Kondo e§ect, which means that the e§ective mass of the conduction electrons is hugely increased, hence the name. Heavy fermion materials have been studied by physicists for a number of years, but they have recently been thrust into the limelight due to increasing interest in ‘quantum phase transitions’.Just as temperature drives thermal phase transitions by regulating the amplitude of thermal fluctuations, so certain parameters such as pressure, field or composition can drive quantum phase transitions at zero temperature by regulating the amplitude of quantum fluctuations.Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 467–480 470CeCu 6 is a heavy fermion metal in which the antiferromagnetic interactions between the Ce moments competes with the single site Kondo e§ect. As Cu is substituted for Au, the antiferromagnetism wins out and there is a quantum phase transition from a heavy fermion metal, to an ordered antiferromagnet. Schro� der et al.29 have used inelastic neutron scattering to show that the energy scale that governs the charge and spin fluctuations in this material is the temperature, or in other words sAT (the neutron scattering amplitude) is a universal function of E/T where E is the energy transfer.Stockert et al.30 have shown the transition to have two dimensional features, with rods of scattering in reciprocal space.Other heavy fermion materials of interest include CePd 2 Si 2 and CeIn 3 . Mathur et al.31 have suggested that the superconductivity that arises in these materials arises from the formation of Cooper pairs, but the pairing mechanism is magnetic, rather than phonon-mediated, as in the Bardeen–Cooper–Schrei§er mechanism. The e§ect of pressure on magnetic properties There are numerous examples of rare earth chalcogenides in which localised 4f states and extended 5d–6s states compete for electron occupation.The Tm chalcogenides are notable for the tunability of this e§ect. Link et al.32 have studied TmTe by neutron di§raction in pressures of up to 7 GPa, and have found a crossover from an ambient pressure semiconducting antiferromagnetic phase (T N \3.54 K) to a high pressure metallic ferromagnetic phase (T C \14 K) caused by a gradual closing of the band gap.The ferromagnetism is ascribed to the RKKY mechanism of the superexchange mediated by conduction electrons. A related, but more sudden e§ect is observed in the sublattice magnetization of the mixed metal phosphide EuCo 2 P 2 .33 At ambient pressure this material is antiferromagnetic with T N \66.5 K, and the moment caused by localised Eu2`. However, application of pressure causes a transition from Eu2` to non-magnetic Eu3`, with the electron entering the Co d-states.At a critical pressure of 3.1 GPa, the localised moment antiferromagnetism of Eu2` gives way to itinerant electron antiferromagnetism of Co, with T N estimated to be 260 K.The europium monochalcogenides, with the NaCl structure, are localised f-electron semiconductors that show a variety of magnetic behaviour at ambient pressure (EuTe, EuSe are antiferromagnetic, EuSe is ferrimagnetic, and EuS, EuO are ferromagnetic). They are ideal systems for the study of ligand-mediated superexchange interactions, as they have neither itinerant electrons to mediate an RKKY interaction, nor magnetic ions su¶ciently close for direct exchange.Goncharenko and Mirebeau34 have studied the magnetic behaviour of EuS and EuSe by neutron di§raction in applied pressures of up to 20.5GPa (EuS) and 14.8GPa (EuSe). An increase in pressure would be expected to reduce cell constants, increase superexchange and increase ordering temperatures. However, the experimental results are spectacular, with the ordering temperatures increasing much more rapidly than expected from a comparison of the family at ambient pressure.Some of these interesting results are shown in Fig. 2. The Earth’s solid inner core is believed to consist largely of oriented iron crystals in the higher pressure hexagonal close packed (hcp) structure. Diamond anvil cell experiments on hcp-Fe at a pressure of 17GPa have found it to be either ferromagnetic or paramagnetic, like the familiar (body centred cubic) a-Fe.35 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 467–480 471Fig. 2 Pressure dependence of the Curie temperature of EuSe (triangles) and EuS (squares) (from ref. [34]). Fstrated magnetism Interest in highly frustrated pyrochlore magnets continues.Moessner and Chalker36 have analysed the Heisenberg pyrochlore magnet theoretically and shown it to be disordered at all temperatures, in agreement with earlier evidence presented by Reimers.37 In particular there is no ‘Order by Disorder’ – the restoration of long range order by thermal fluctuations – that is prevalent in frustrated spin models. The authors conclude that the model should be classified as a ‘co-operative paramagnet’, following Villain.38 The quantum spin one half Heisenberg magnet on the pyrochlore lattice has been investigated theoretically by Canals and Lacroix39 who conclude that the ground state shows ‘spin liquid’ behaviour.Gaulin et al.40 have reviewed the extensive work of the McMaster group on the pyrochlore antiferromagnetsY 2 Mo 2 O 7 , Tb 2 Ti 2 O 7 and Tb 2 Mo 2 O 7 .Other pyrochlore lattice antiferromagnets include the mixed metal fluorides such as CsMnFeF 6 . The cation ordered substance NH 4 CoAlF 6 41 contains only one magnetic ion (Co) and shows a complex series of magnetic transitions at 0.8, 2.5 and 5.8 K. However the magnetic specific heat data above 6K can be described by the one-dimensional S\1/2 XY model which is consistent with the ordered chain structure of the Co2` sublattice.An interesting approach to a frustrated pyrochlore lattice antiferromagnet is via the defect spinel compound ‘k-MnO 2 ’, that is prepared by removal of Li from LiMn 2 O 4 by acid leaching.42 This material retains the spinel structure down to Li 0.1 MnO 2 , with the Mn3` ions distributed on the B-site spinel lattice that is identical to the pyrochlore lattice.Neutron di§raction shows a very complex type of antiferromagnetic order below T N \32 K, described by a propagation vector k\(1/2 1/2 1/2). The complexity of the order may be attributed to frustration. The general e§ect of local axial anisotropy on the pyrochlore lattice has been discussed theoretically.43 It is shown how, in the presence of anisotropy, the roles of Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 467–480 472ferromagnetic and antiferromagnetic coupling are reversed, such that the ferromagnet is frustrated and the antiferromagnet is not. The temperature-field phase diagram of frustrated pyrochlore ferromagnet (known as ‘spin ice’) has been investigated by Monte Carlo simulation.44 The Ising-like model was found to have a line of first order phase transitions terminating in a critical point.These transitions are unusual, in that they separate phases distinguished only by the intensity of the magnetization. They are thus analogous to the liquid–gas phase transition where the two phases are distinguished only by a di§erence in densities. The spin ice model is relevant to Ho 2 Ti 2 O 7 , Dy 2 Ti 2 O 7 and Yb 2 Ti 2 O 7 .Experiments on these and other frustrated titanate pyrochlores have been briefly reviewed.45 Preparations of large single crystal samples of the rate earth titanate pyrochlores have also been reported.46,47 Gadolinium gallium garnet (‘GGG’), Gd 2 Ga 5 O 12 , is another important three dimensional frustrated magnet, that was described in detail in the 1994 report.48 In zero applied field, single crystal bulk measurements show the material to have no long range order above 25 mK, and a spin glass transition at 125–135 mK.Petrenko et al.49 have used neutron di§raction to investigate the magnetic behaviour of polycrystalline samples. They observe sharp magnetic scattering replacing liquid like scattering below about 140 mK, and suggest that the low temperature phase is not a spin glass, but rather a mixture of a ‘spin liquid’ and a rigid magnetic structure nucleated around impurity centres.Two-dimensional kagome lattice antiferromagnets have received less attention than in previous years. However, Harrison et al.50 have described how long range magnetic order is induced by diamagnetic dilution in the series of model kagome lattice antiferromagnets, AFe 3 (OH) 6 (SO 4 ) 2 (A\alkali metal or hydronium ion), that are related to the mineral jarosite.With A\alkali metal ion the materials show antiferromagnetic transitions at T N[55 K, whereas for the hydronium salt, a spin glass-like transition at 15K is observed.The authors attribute the di§erence to the fact that only the hydronium salt is nearly stoichiometric in iron. This is clear experimental evidence that defects can cause long range order in this system. The physical properties of deuteronium jarosite with 97^1% Fe coverage have been studied in detail by Wills et al.51 This material has a huge Curie–Weiss temperature of ca. 700 K, but a spin glass transition at only 13.8 K.Powder neutron di§raction revealed short-range magnetic correlations of about 19Å at 1.9 K. However, the magnetic contribution to the specific heat below the transition temperature was found to vary as the square of temperature, indicative of two-dimensional propagating modes. This is significant observation, as a T2 dependence is also a characteristic of the well known kagome antiferromagnet SCGO (SrCrxGa 12~xO 19 ).NaTiO 2 has long been viewed as a possible example of a triangular lattice spin one half frustrated magnet. However, Clarke et al.52 have presented new di§raction results to show that it is not a localised moment d1 system, but has a structural phase transition between 200 and 250K that a§ects the magnetic behaviour.Investigations of the related material LiNiO 2 have been reported by Kitaoka et al.53 Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 467–480 4733 Ionic, covalent and metallic materials Oxides Nanoparticulate c-Fe 2 O 3 has been investigated by Marti� nez et al.54 The study was related to the wider interest of a general e§ect in nanoparticulate magnets, in which the saturation magnetization is reduced from the expected value.55 The authors conclude from their magnetization study that the reduction is consistent with a spin-glass like surface phase that freezes at T& \42Kin zero field and shifts in finite field according to the de Almeida–Thouless prediction for a spin glass,56 dT&[H2@3.One dimensional cuprates consisting of CuO 2 chains show some unusual properties.In SrxCuO 2 , every hole introduced by a Sr ion should make a Cu ion nonmagnetic, and one would expect that a few such holes would easily prevent magnetic order. However Shengelayu et al.57 have shown by susceptibility and EPR measurements that Sr 0.73 CuO 2 has long range order below 12 K, with a ferromagnetic moment. The origin of this order is not clearly understood.LiCuO 2 also shows ferromagnetic intrachain coupling, but orders as a three dimensional antiferromagnet below T N \9K, with a moment of 0.9 kB per unit cell. Local spin density calculations58 have shown that a surprisingly high proportion (40%) of this moment is carried by oxygen rather than copper. Another one dimensional copper chain system, CuGeO 3 has been of interest for a number of years as showing the only example of a spin Peierls transition known in an inorganic compound (at 15 K).59 This year, Masuda et al.60 have characterised Cu 1~xMgxCuO 3 .They find that doping with Mg causes a change from spin Peierls to dimerised antiferromagnetic phase at temperatures of a few Kelvin, but above x[0.02, an undistorted antiferromagnetic phase is most stable.The data storage property of ferromagnets, relies on the ability to reverse magnetization by applying a magnetic field. Magnetization reversal with a change in temperature can also occur, but is much rarer. Ren et al.61 have described how in the weakly ferromagnetic material YVO 3 , there is an abrupt magnetization reversal at 77 K, followed by a more gradual one centred around 95 K.The latter is caused by tilting of the antiferromagnetic moments associated with an orthorhombic distortion of the crystal structure, but the cause of the former is more mysterious. The Goodenough–Kanamori rules show how the sign of the superexchange teraction in an insulating material depends on the nature of the metals and the intervening ligand.Although the rules give a chemical recipe for ferromagnetism, it is often hard to achieve the desired arrangement of metal atoms and ligands for purely thermodynamic reasons. Molecular beam epitaxy (MBE) can o§er advantages in this regard. Thus, Ueda et al.62 have discussed how Fe3` and Cr3`, when connected by a 180° oxide bridge, should be coupled by ferromagnetic superexchange.The obvious choice of material is La(Cr,Fe)O 3 ; however, bulk preparations result in phase separation, and a solid solution of Cr and Fe would result in frustrated interactions. To avoid these problems the authors stacked alternate LaCrO 3 and LaFeO 3 layers on single SrTiO 3 crystals by MBE, and thereby synthesised a ferromagnetic superlattice. Oxides that contain Cr2` are quite rare.CrTa 2 O 6 , a new Cr2` oxide with the trirutile structure, has been reported by Saes et al.63 The trirutile structure is quasi-two Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 467–480 474dimensional and highly frustrated. CrTa 2 O 6 was found to have complex antiferromagnetic order below T N \10.3 K. Oxyanions: borates, arsenates and molybdates The borate Fe 2 OBO 3 formally contains Fe2` and Fe3` in equal ratios. However, Bell et al.64 have shown, using neutron and X-ray di§raction, that the two Fe sites are occupied by an exact Fe2.5` valence at all temperatures.Ferrimagnetic behaviour was observed below T C \155 K. Aranda et al.65 have discovered novel magnetic behaviour in the simple manganese arsenate MnAsO 4 . The substance was found to order antiferromagnetically at 14.5(5) K, but only half the Mn3` spins were found to be ordered.The remaining spins (described as ‘idle’ spins) showed spin glass-like behaviour. Orendacova et al.66 have reported quasi one dimensional ferromagnetic Ising behaviour in the mixed metal molybdate KEr(MoO 4 ) 2 . The one dimensional ferromagnetic Ising model is the simplest model of cooperative magnetism, and has interesting properties.Here the Ising nature arises from the orbital magnetism of Er. Chalcogenides Experimental estimation of magnetic exchange constants by bulk measurements is no simple matter. The Curie–Weiss temperature gives an aggregate exchange field that may consist of interactions over several neighbours. Bindilatti et al.67 have used a novel method to extract the exchange constants for a series of Mn chalcogenidesMnX (X\S, Se, Te).The ‘magnetization step method’ involves measuring magnetization versus field isotherms for a dilute salt, in this case Zn 1~xMnxX. The magnetization curve consists of the superposition of curves for single ions and clusters of two and three ions, etc.; this leads to a series of characteristic steps in the magnetization curve that can be analysed to give exchange constants.In this way the authors estimate the first four nearest neighbour exchange constants for the series. For X\S they find J 1 \[16.9 K, J 2 \0.27 K, J 3 \[0.04K and J 4 \[0.41K respectively (Jn represents the nth nearest neighbour exchange constant). Halides The hexagonal perovskite halides are a famous test-bed for magnetic interactions.The distorted hexagonal perovskites ND 4 FeCl 3 and NH 4 FeCl 3 have been characterised by neutron di§raction,68 which shows phase transitions associated with motion of the ammonium ions at 181 and 19 K. Antiferromagnetically coupled ferromagnetic chains order at 3.5 K. Boron carbides Ferromagnetism and superconductivity are two phenomena which are normally mutually exclusive.In TmNi 2 B 2 C they compete for order at low temperature. Eskildsen et al.69 have used small angle neutron scattering to show that superconductivity and antiferromagnetically modulated ferromagnetism coexist in TmNi 2 B 2 C below a Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 467–480 475temperature of 1.5 K. The antiferromagnetic modulation is crucial to the stability of this state, as the magnetic periodicity is short enough to ensure a small net moment on the length scale of the superconducting coherence length.Thus the ferromagnetism is coupled to the superconductivity, and does not destroy it at low temperature. Antimonides Raju et al.70 have described a new itinerant electron antimonide, LaCrSb 3 . The crystal structure, determined by neutron di§raction consists of corrugated CrSb 2 separated by La3` ions and a nearly square planar Sb layer.The material was found to be metallic within the layer, and ferromagnetic below 125 K. This was attributed to itinerant ferromagnetism in the Cr d-bands. 4 Co-ordination complexes and molecular compounds Framework structures: three dimensional systems The magnetic complex CoCuL(H 2 O) 3 ·2H 2 O [L\1,3-propylenebis(oxamato)], has been described as a ‘molecular sponge’ by Chavan et al.71 Blue CoCuL(H 2 O) 3 ·2H 2 O has antiferromagnetically coupled Co(II)–Cu(II) ferrimagnetic chains below ca. 7K. Heating this compound at 130 °C in vacuum yields the dehydrated purple compound CoCuL·H 2 O, which is ferromagnetic below 33 K. The process is reversible on rehydration. The compounds M[N(CN) 2 ] 2 (M\Co, Ni, Cu) have been described by Batten et al.72 They have framework structures related to that of rutile (TiO 2 ) and the Co and Ni materials behave as ferromagnets with ordering temperatures of 9 and 20 K, respectively.The Cu compound on the other hand is nearly paramagnetic. Layer structures: two-dimensional systems Nutall and Day73 have reported magnetization measurements on the layer compounds AFeIIFeIII(C 2 O 4 ) 3 (A\organic cation), in which the Fe2` and Fe3` ions occupy alternate sites in a honeycomb layer formed by oxalate.The compounds are ferrimagnetic and show ‘negative magnetization’, as predicted by Ne� el. Kaneyoshi et al.74 have presented an e§ective field theory of such systems, that shows that two or three compensation points are possible, depending on the anisotropy and relative concentrations of Fe2` and Fe3`.Ikorskii et al.75 have described new molecular magnets based on nickel(II) complexes with 3-imidazoline nitroxides and alcohols. The complexes were formed by crystallization of heterospin bis-chelate NiL 2 with alcohols to give compounds such as NiL 2 (CH 3 OH) 2 .Here, L is the deprotonated nitroxide 4-(3@,3@,3@-trifluoromethyl-2@- oxopropylidene)-2,2,5,5-tetramethyl-3-imidazoline-1-oxyl.These substances consist of polymeric hydrogen bonded layers, and are weakly ferromagnetic below 7K. Surville-Barland et al.76 have described a family of layered compounds of general formula [PPh 4 ] 2 Mn 2 [Cu(L)] 3 ·nH 2 O, where L is ortho-phenylenebis(oxamate) and related ligands. The crystal structure consists of two dimensional honeycomb net- Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 467–480 476works separated by [PPh 4 ]` cations, and ferrimagnetic transitions in the range 8– 12K have been found. Chain structures: one dimensional systems Kou et al.77 have reported a new one dimensional thiocyanato-bridged bimetallic compound, [Cu(en) 2 Mn(NCS) 4 (H 2 O) 2 ]n.The crystal structure consists of alternating Cu2` and Mn2` bridged by NCS ligands. A decrease in the e§ective moment squared (sT) was attributed mainly to zero-field splitting of the single Mn(II) ions superimposed on weak antiferromagnetic intrachainar interactions. Isolated complexes: zero dimensional systems The complex Mn 12 O 12 (CH 3 CO 2 ) 16 (H 2 O) 4 has received a lot of attention in recent years as the prototypical example of a complex that exhibits quantum tunneling of the magnetization (see the 1996 report1).The Mn3` and Mn4` ions in the complex combine to give an overall spin S\10, and tunneling between D^MS[ states leads to characteristic steps in the magnetization curve. This year Fort et al.78 have presented a theory of the process that is essentially a mixed thermal and quantum tunneling mechanism.Briefly, if the complex is ‘frozen’ into the D[S[ state at 2K, then higher D[S@[ states are accessed ermally, before tunneling into D]S@[ state occurs (S@ represents a state with magnetic quantum number DMSD\S). This model is found to reproduce experimental data.The tunneling is actually caused by transverse fluctuations. Prokof’ev and Stamp79 have demonstrated the crucial role that rapidly fluctuating hyperfine and nuclear dipolar fields play in this process. At an experimental level, C� aciu§o et al.80 have used neutron spectroscopy to observe transitions between energy levels for the di§erent orientations of the magnetic angular momentum in the (non-deuterated) complex MFe 8 O 2 (OH) 12 L 6N8` (known as ‘Fe 8 ’, L\1,4,7-triazacyclononane).In this complex the S\10 ground state is split by the ligand field into states made up of the MS components (MS \10, 9, . . .[10). The lowest energy states are almost pure MS \^10, ^9, etc.; but the higher states are composed of mixtures of di§erentMS.Transitions with DMS \0, ^1 are excited by neutron scattering in the dipole approximation. The height of the energy barrier is about 33 K, so by measuring neutron spectra at di§erent temperatures, di§erent populations of the individual states can be selected. The results, illustrated in Fig. 3, are a beautiful example of the power of high resolution neutron scattering. A new tetranuclear nickel compound, [Ni 4 (l-CO 3 ) 2 L 8 ][ClO 4 ] 4 [L\(2- aminoethyl)pyridine], has been reported to have a butterfly-like topology, with frustrated antiferromagnetic interactions. In this regard, it may be regarded as a fragment of the kagome lattice structure (see Section 2).Consistent with this, the ground state of the complex is reported to be an almost degenerate set of levels with overall spin S\0, 1 and 2.81 Spin crossover complexes are not only interesting, but are potentially useful for the memory e§ects they show.Khan and Martinez82 have described a series of iron-based complexes that show thermal hysteresis of the low spin to high spin transition of Fe2` (d6). In Fe(Rtrz) 3 Az·nH 2 O, iron atoms are triply bridged by 4R substituted triazole Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 467–480 477Fig. 3 (a) Neutron scattering spectrum of the complex ‘Fe 8 ’, and (b) the corresponding energy level diagram (from ref. [80]). molecules. The low spin (t 2' 6) complex is violet, while the high spin (t 2' 4e' 2) complex is white. By suitable molecular alloying, the centre of the thermal hysteresis loop can be brought, for example, to room temperature.This suggests a possible use as temperature fluctuation sensors. References 1 S.T. Bramwell, Annu. Rep. Prog. Chem., Sect. A, 1996, 93, 457. 2 S.T. Bramwell, Annu. Rep. Prog. Chem., Sect. A, 1997, 94, 459. 3 D. Ceperly, Nature (London), 1998, 397, 386. 4 D.P. Young, D. Hall, M. E. Torelli, Z. Fisk, J. L. Sarrao, J. D. Thompson, H.-R. Ott, S.B. Osero§, R. G. Goodrich and R. Zysler, Nature (London), 1998, 397, 412. 5 D. Voss, Science, 1998, 282, 221. 6 Z. Fisk and D. Pines, Nature (London), 1998, 394, 22. 7 K. Ishida, H. Mukada, Y. Kitaoka, K. Asayama, Z. Q. Mao, Y. Mori and Y. Maeno, Nature (London), 1998, 396, 658. 8 A.K. Geim, S. V. Dubnis, J. G. S. Lok, M. Henini and J. C. Maan, Nature (London), 1998, 396, 144 9 S.Kulkarni and C. 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ISSN:0260-1818    

DOI:10.1039/a804885a

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

25 Conducting solids, covering ionic and electronic conductors R. A. Jennings School of Chemistry, University of St. Andrews, St. Andrews, Fife, UK KY16 9ST 1 Introduction This year has seen yet another increase in research in the area of ionic conductors, again predominantly due to the commercial interest in Li batteries and solid oxide fuel cells. Whilst the search for new materials continues to attract considerable attention there has been a noticeable increase in research into the mechanisms and factors e§ecting the conducting and intercalation processes in these materials.This exciting aspect of work has been greatly aided by the emergence of the many new ‘in situ’ techniques now available to the solid state scientist. 2 Ionic conductors and intercalation compounds Lithium ion conductors As the market for portable electronic devices continues to flourish, the interest in Li rechargeable batteries as compact, high density energy sources increases each year.Economic and environmental issues have again directed cathode research towards manganese systems with the spinel phase LiMn 2 O 4 being by far the most widely studied. Ein-Eli et al.have performed a thorough study on the e§ects of Cu substitution in the spinel phase.1 The Cu doped materials exhibit a new electrochemical process at 4.9V which in situ X-ray absorption near edge spectroscopy (XANES) has shown to be due to the Cu3`/Cu4` redox couple.2 A shifting to slightly higher voltage of the Mn3`/Mn4` redox couple was also observed. As with previous studies of Ni and Cr substitution3,4 the Cu doping improves the capacity retention but at the cost of a lower initial capacity.Interestingly, what little capacity loss is observed in the highly doped material, is at the 4.9V potential which as the authors point out could be due to electrolyte decomposition rather than the spinel. A detailed structural investigation has indicated that the materials are slightly Cu deficient compared to the starting reagents, this has been explained by the relatively low reactivity of CuO compared to Li 2 O.Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 481–506 481An even higher redox potential has been reported by Kawai et al. with the spinel phase Li 2 CoMn 3 O 8 showing an operating voltage above 5V.5 An initial capacity of [130mAh g~1 ([60mAh g~1 at the plateau centred on 5.1V and[70mAh g~1 at a plateau centred at 3.8 V) is obtained decreasing slightly to [100mAh g~1 after 50 cycles.The high voltage and easy preparation of this material makes it a promising candidate for commercial applications although the authors note that further improvement in capacity at the 5V plateau is still required. Further work has been carried out on a range of other cation substitutions in LiMn 2 O 4 .Co,6 Fe7,8 and Cr9 continue to attract attention, with minor improvements reported, but these are far from being the only substituents studied.10–12 All the previous examples have shown that whilst cation doping leads to improvements in cyclability, a loss of initial capacity is usually observed.As the capacity depends strongly on the Mn valence state (the lower the better), Hosoya et al. have studied the e§ect of reducing the oxygen content in doped spinels as a way of lowering the Mn valency and hence improving capacity.13 All the samples of composition LiCo 1@6 - Mn 11@6 O 4~d showed the expected improvement in cycling performance, but with the reduced oxygen content material (d\0.040) showing an 11% improvement in initial capacity (127Ah kg~1) as compared to the fully oxygenated samples.Many new synthetic routes, especially low temperature sol–gel methods, have also been investigated as a means of improving electrochemical performance.14–16 Prabaharan et al. have used a precursor route involving the complexation of Li and Mn by succinic acid to obtain high quality sub-micron powders.17 The method is simple yet highly e¶cient, producing bulk quantities of spinel with practical capacities of 110mAh g~1.Using related methods several authors have investigated the relationship between physicochemical/electrochemical properties and processing conditions. Lee et al., using adipic acid as the chelating agent, have shown that crystallinity, particle size and specific surface area can be controlled by simple variations of temperature and chelating agent quantity.18 Similarly Sun et al., using glycolic acid, have studied the e§ect of temperature on lattice constant and electrochemical performance. 19 The e§ect on transport properties of various synthetic routes has been investigated by Guan and Liu.20 Both sol–gel and traditional solid state techniques were studied, as was the e§ect of quenching versus slow cooling.Although not directly related to spinel materials, another cause of capacity loss in Li ion batteries is the parasitic reactions taking place at the carbon anode. The additional amount of cathode required to supplement this irreversible capacity leads to a reduction in the specific energy of the battery.To overcome this problem Peramunage and Abraham have reacted LiMn 2 O 4 with n-butyllithium to obtain the over-lithiated material Li 1`xMn 2 O 4 .21 Whilst this is not the first synthesis of an over-lithiated spinel the use of n-butyllithium, as opposed to LiI, has allowed the tailoring of the lithium excess to match the irreversible capacity. The mechanisms and structural changes involved in the intercalation process have also been a topic of much work.Liu et al. have used ex situ X-ray and neutron di§raction studies to follow the Li insertion in LixMn 2 O 4 for x\0–1.0 (in the range 3.7 to 4.4 V).22 They report that initially (up to x\0.1) the spinel accommodates the Li ions in a single phase before converting to a second order transition for x\0.1–0.5.Above this the Li intercalation reverts to a single phase transition with the Li inserting in a random fashion. Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 481–506 482Mukerjee et al. have also studied this process but using in situ synchrotron X-ray di§raction.23 The authors claim that the use of high intensity synchrotron radiation has many advantages over more conventional in situ techniques.Not only does it remove the need for a beryllium window in the electrochemical cell but also by enabling the use of transmission rather than reflection geometries the bulk sample rather than just the few microns of the surface can be examined. Their results are comparable with those of Liu et al. although they report that the structural transitions observed vary noticeably depending on the synthetic conditions used.An in situ X-ray di§raction study has also been performed by Le Cras et al. but on the lower voltage range (2.5–3.5 V).24 Of primary interest was the structural transition from cubic to tetragonal as the Mnundergoes a Jahn–Teller distortion on reduction to Mn3`. The di§raction data collected clearly show that the relative fractions of the cubic and tetragonal phases vary continuously, with the tetragonal peak intensities extrapolating to zero at the beginning of the discharge. Interestingly samples of composition Li 1.23 Mn 1.77 O 4 also show complete two phase behaviour over the full range of Li contents despite having an initial Mn oxidation state well above the expected Jahn–Teller distortion limit of 3.5.For commercial usage, Li batteries will clearly be expected to perform over a range of temperatures, especially at the elevated temperatures commonly obtained inside electronic equipment. Inoue and Sano have studied the capacity fade in spinel materials stored at these elevated temperature at various states of discharge.25 Samples stored at 80 °C in 1M LiPF 6 ethylene carbonate–dimethyl carbonate electrolyte showed 1.2%Mndissolution for all samples studied. However whilst this compared to a 3% loss in capacity of the fully charged cathode, the fully discharged material showed a capacity loss of over 50%.Correlations were found between X-ray di§raction peak widths and capacity losses leading the authors to propose a mechanism of structural disordering to explain the capacity fading.Blyr et al. have performed a similar investigation using three electrode measurements to follow the capacity loss.26 The authors conclude thatMndissolution is the primary cause of the capacity loss and various mechanisms to explain the observations were proposed. Antonini et al. have studied the capacity fade on cycling at elevated temperatures.27 They report that Ga or Cr doping in small quantities reduces deterioration whilst the double substitution of both metals shows an even more marked e§ect (\0.5% fade per cycle). It is also noted that storage at elevated temperatures (in LiBF 4 –ethylene carbonate–propylene carbonate) leads to the formation of a surface layer, which also reduces capacity loss on cycling.Xia et al. have similarly investigated the e§ect of various substitutions, including fluorination, on high temperature performance. Several successful methods were reported and discussion on choice of solvent and e§ect of particle size was also given.28 Another practical consideration of cathode materials is their use as thin films for applications in lithium micro-batteries.Thin films also have the advantage, from a scientific point of view, of allowing the electrochemical properties of the material to be studied without the need for carbon (or any other additive) usually required for electrical conductivity. Thin films of LiMn 2 O 4 produced by Rougier et al. showed excellent cyclability although, due to the unknown morphology and hence density, the exact values of the capacity are not known.29 The authors also reported that the shape of the cyclic voltammogram was extremely sensitive to processing conditions and Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 481–506 483whilst performance was not a§ected by overcharging, over discharge led to a capacity loss from the 4V peak. Pulsed laser deposited films of LiMn 2 O 4 have also been prepared by Morcette et al.30 Whilst good cycling behaviour was also observed the authors proposed and demonstrated the alternative use of the thin films as lithium ion sensors.Two groups have studied the e§ects of acid reaction with LiMn 2 O 4 but for very di§erent reasons. The loss of capacity due to acid attack and Mn dissolution has been investigated by Larcher et al.31 In aqueous media they report the transition from k- to a- and then to c-MnO 2 proceeds by a dissolution/crystallisation mechanism which can be prevented by the addition of soluble Bi salts.In anhydrous solutions the addition of Bi slows down the dissolution process with BiF 3 being precipitated. In contrast Shao-Horn et al. have studied the e§ect of acid digestion of both LiMn 2 O 4 and Li 2 MnO 3 in order to examine the delithiation mechanism as a means of preparing new morphology-modified MnO 2 materials.32 Their results confirm the delithiation mechanism proposed by Hunter33 whilst also demonstrating the pronounced e§ect crystal morphology can have on cathodic properties.LiMn 2 O 4 is far from being the only Mn cathode material to be studied.Levi et al. have proposed a structure for the previously reported Li 0.33 MnO 2 phase, obtained from the reaction of c-MnO 2 and LiNO 3 .34 The proposed structure is similar to that of c-MnO 2 , consisting of [1]2] and [2]2] channels, the major di§erence being in the ordering of the MnO 6 octahedra. A new tunnel-structured phase, Li 0.44 MnO 2 , has been prepared by Armstrong et al.35 Electrochemical measurements show a capacity of 85–90mAh g~1 but no perceptible loss in capacity when cycled over the range 2.8–3.6 V.Detailed analysis of the material has been performed with the structures of chemically lithiated/delithiated samples and the starting material all being derived from neutron di§raction experiments. Ko� tschau and Dahn have reported an in situ study of orthorhombicLiMnO 2 and its modification on cycling.36 It was previously known that the material converted on initial cycling to a cubic structure, which has now been identified as a spinel but with many stacking faults and/or other dislocations. The material then undergoes the familiar cubic to tetragonal transition at 3V although this also has further structural complications.Remarkable improvements in the capacity in layered LiMnO 2 by Co doping have been reported by Armstrong et al.37 The material LiMn 0.9 Co 0.1 O 2 yields a consistent capacity of [200mAh g~1 at 100 lAcm~1, an increase of ca. 50% compared to the undoped material. The substitution of Co also appears to have the additional benefit of suppressing the Jahn–Teller distortion.The first direct synthesis of undoped LiMnO 2 has been reported by Tabuchi et al.38 (previous preparations have all been via an ion exchange route39). The hydrothermal route is simple and gives high purity samples although preliminary electrochemical data show poor capacity retention. Hydrothermal synthesis has also been used by Duncan et al. to prepare todorokite MnO 2 and its related layered structures.40 The performance of 2D vs. 3D structures are compared as are the e§ects of di§erent synthetic routes. The highest discharge capacity observed was 155mAh g~1 corresponding to 0.6Li/Mn for a todorokite sample. Dahn et al.41 have studied Li 2 CrxMn 2~xO 4 previously reported by Davidson.42 At Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 481–506 484high synthesis temperatures the authors reported the phase to adopt a distorted hexagonal layered structure and not the spinel structure claimed by Davidson.At lower temperatures the material adopts the same structure as LiCoO 2 and shows good capacity (150mAh g~1) when cycled at slow rates. A low temperature synthesis of the spinel material Li 4 Mn 5 O 12 has been reported by Kim and Manthiram.43 The material, prepared by the oxidation of Mn2` salts by Li peroxide in an excess of LiOH and subsequent firing at 400–500 °C, show capacities just below the theoretical maximum (163mAh g~1) and excellent cyclability.LiCoO 2 was the first and is still the most widely used material in commercial rechargeable Li batteries. Despite the increased interest in Mn materials LiCoO 2 still attracts a great deal of attention.The first single crystal X-ray di§raction study of LiCoO 2 has been reported by Akimoto et al.44 The structure is in good agreement with previous results from powder neutron di§raction studies with no evidence of superstructures being observed. Shao-Horn et al. have investigated the structure of low temperature synthesised LiCoO 2 .45 It had previously been proposed that low temperature LiCoO 2 was a mixture of layered LiCoO 2 and a lithiated spinel Li 2 Co 2 O 4 structure, however, due to the almost indistinguishable powder patterns this was not clear.The authors used convergent beam electron di§raction and high resolution lattice imaging to confirm that this is the correct interpretation of the structure. The low temperature structure was shown to be dependent only on temperature and not on the synthesis technique, acid delithiated samples were also studied. Peeters et al.have studied LiCoO 2 samples using 6Li, 7Li and 59Co MAS NMR.46 Low temperature synthesised and Li deficient samples showed similar spectra indicating both Li and Co were in octahedral environments, whilst high temperature LiCoO 2 showed evidence of paramagnetic Co.The e§ect of Li extraction and reinsertion were also investigated with the almost identical resulting spectra demonstrating the excellent reversibility of the process. In situ conductivity measurements on LiCoO 2 by an interdigitated microarray have been performed by Nishizawa et al.47 Metallic behaviour was induced on initial Li extraction, in accordance with previous measurements of composite electrodes, but surprisingly the material did not revert back to its original insulating state on Li reinsertion.Reasons for this irreversible hundred-fold increase in conductivity were described and its relevance to electrochemical performance discussed. Choi and Pyun have used the galvanostatic intermittent titration tecique (GITT) to study the electrochemically active surface area of LixCoO 2 samples.48 Their results show a clear reduction of the active area with increasing Li content. Cells have been prepared by Abraham et al.in order to maximise the power output.49 Power densities of up to 5500Wkg~1 were demonstrated, with discharge currents of 100mAcm~3 ([200C rate), which compares to rates of typically c/5 to c/2 for laptop and cellular phone usage.This work clearly demonstrates the potential use of LiCoO 2 for high power applications. New improved synthetic routes to LiCoO 2 continue to be sought with novel ion exchange50 and solution51–53 routes being reported. Continuing their previous work Yan et al. have studied the electrochemical properties of LiCoO 2 prepared by microwave synthesis54. The fine powders had large surface areas leading to good cyclability and capacity to withstand large current densities.Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 481–506 485Fig. 1 Schematic diagram of K`–b-ferrite showing insertion/extraction of Li. The analogous material LiNiO 2 also continues to attract attention. Several doping studies have been reported with Mn substitution giving little improvement55–57 whilst Co substitution has been shown to be more promising owing to the suppression of the phase transition normally associated with the electrochemical cycling of LiNiO 2 .58,59 Peres et al.have studied the structure of the intermediate phase, Li 0.63 Ni 1.02 O 2 , observed during the cycling of Li 0.98 Ni 1.02 O 2 .60 The e§ect that this structure has on the electrochemical performance of LiNiO 2 was further discussed by the authors.Nakai et al. used in situ X-ray absorption fine structure (XAFS) experiments to follow the change in Ni co-ordination during cycling, with particular interest being paid to the Jahn–Teller distortion.61 Local distortion of the NiO 6 octahedra was also investigated and compared in the Co doped material. Fe-based systems are of great interest owing to their huge potential as non-toxic and very cheap cathode materials.Sakurai et al. have used aH`/Li` ion exchange route to form a new low temperature a-LiFeO 2 phase.62 In contrast to high temperature a-LiFeO 2 the new material displays electrochemical behaviour, cycling [0.2Li/Fe in the range 1.5–4.5 V.Preliminary investigations show minimal changes in lattice parameter during cycling. Ito et al. have reported the electrochemical properties of the novel cathode material K`–b-ferrite.63 The structure of this material consists of thin layers of c-Fe 2 O 3 separated by potassium oxide (Fig. 1). The structure allows the Li ions to pass smoothly through the loose-packed K layer and insert readily into the c-Fe 2 O 3 .Owing to the thinness of the c-Fe 2 O 3 layer (only four oxygen layers) Li is not required to intercalate deep into the structure leading to improved kinetics over bulk c-Fe 2 O 3 . Initial electrochemical results show capacities up to 200mAh g~1 with little degradation after 10 cycles. Studies have also been performed on the di§erent polymorphs of Li 3 Fe 2 (PO 4 ) 3 and Li 3 Fe 2 (AsO 4 ) 3 .64 Whilst the structure had little e§ect on the position of the Fe2`/Fe3` redox couple, the di§erent polymorphs showed very di§erent discharge profiles and rate capacities with the NASICON structure showing the most promising behaviour. Following from their previous work on Li 3 Fe 2 (PO 4 ) 3 64 and LixFe 2 (SO 4 ) 3 ,65 Padhi Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 481–506 486and co-workers have prepared the mixed oxyanion phase Li 3 Fe 2 (PO 4 ) 2 (SO 4 ).66 The material exhibits the better features of the two parent compounds, a high voltage (ca. 3.4 V) due to the sulfate and a single-phase lithium insertion reaction associated with the phosphate material.Pe� rez Vicente et al. have performed a detailed study of the structural modification of In 16 Fe 8 S 32 on Li insertion.67 Mo� ssbauer spectroscopy measurements show no reduction of the Fe2` whilst X-ray absorption experiments indicate that there is also no reduction of the In3`. It was proposed that the incoming electrons are incorporated into a delocalised band structure which is consistent with the observation of increased covalency.The discharge curve display two domains, the first showed a small interaction between inserted ions and the second showed a repulsive interaction and a decrease in the di§usion coe¶cient. Of other metals studied, Yu et al. have prepared a range of Na molybdenum compounds by acid treatment of Na 2 MoO 4 .68 The electrochemical properties of these new phases were found to vary strongly with composition and structure.Suzuki et al. have studied the chromium Chevrel phases CrxMo 6 S 8~y.69 The material Cr 1.3 Mo 6 O 7.7 maintains a capacity of ca. 50mAhg~1 between 1.5 and 2.7V for up to 1000 cycles and at high currents an energy density of [140Wh kg~1 was obtained. X-Ray di§raction studies have been performed before and after cycling to investigate possible Li intercalation sites and a reaction model was proposed.Obrovac et al. have prepared a series of compounds LiMO 2 (M\Ti, Mn, Fe, Co, Ni) by simple ball-milling of the respective metals and/or metal oxides in the appropriate ratios.70 Although the electrochemistry of these materials was poor, thought to be due to cation disorder, the authors suggest the potential use of this method for future synthesis.Vanadium oxides are another well studied class of materials. Whilst these materials generally lack the high voltage of LiMn 2 O 4 , LiCoO 2 and other ‘4 volt’ cathodes, they been shown to possess high capacities. Kawakita et al. have investigated a new route to LiV 3 O 8 replacing water by methanol in a sol–gel synthesis.71 It was hoped to reduce the processing temperature by removing the need for a post-synthesis heat treatment to eliminate incorporated water.Whilst the materials showed good electrochemical performance, a post-synthesis heating was still required to remove CH 3 OH and oxygen defects. The authors also studied the change in structure on Li insertion into Li 1`xV 3 O 8 prepared at 680 °C.72 For x\2.0 a single-phase transition was reported with a constant increase in lattice parameters and similarly for x[3.2.For 2.0\x\3.0 a two phase region was observed with no change in lattice parameters. Dai et al. have also reported a low temperature synthesis of LiV 3 O 8 .73 The samples prepared from a solution of LiCO 3 andNH 4 VO 3 and heated at 300–350 °C showed an initial capacity of 300mAh g~1.Structural studies showed a strained V 2 O 3 layer, di§erent crystallite orientations and higher surface areas compared to typical materials prepared at high temperature. A low temperature route to V 2 O 5 , another promising V compound, has been reported by Coustier et al.74 The modified aero–gel method gives highly amorphous V 2 O 5 with capacities in excess of 400mAh g~1 and \0.5% fading per cycle. A composite electrode of polyaniline and V 2 O 5 has been fabricated and studied by Kuwabata et al.75 The polyaniline acts not only as a conducting matrix for the V 2 O 5 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 481–506 487but, due to its redox potential being similar to that of V 2 O 5 , also contributes as an active material improving the overall capacity of the composite.E§ects of composite ratio, film thickness and current densities were investigated as was the intercalation mechanism. McGraw et al. have studied the properties of V 2 O 5 thin films produced by di§erent methods.76 Pulsed laser deposition (PLD) crystalline films show good cyclability between 4.1 and 3.0V but when cycled down to 2.0V show a dramatic capacity fade.By comparison amorphous PLD films showed\2% fade when cycled over the same extended range. Amorphous films prepared by plasma-enhanced chemical vapour deposition (PECVD) showed even better performance with consistent capacities of ca. 225mAh g~1 over 5500 cycles.77 Similar performance was also achieved in chemically lithiated films of composition 2 O 5 prepared by use of a (CF 3 ) 2 CHOLi precursor.78 Zhang and Frech have followed the Li insertion into c-LixV 2 O 5 by in situ Raman spectroscopy.79 The authors highlight many local structural changes observed as Li is intercalated which have been previously undetectable by di§raction techniques. Bergstro � m et al.have used deformation electron density refinements of V 6 O 13 single crystals to observe the electron rearrangement on Li insertion.80 Owing to insu¶cient data quality only limited information was extracted concerning the reduction of one of the three independent V sites and changes in e§ective charge on some of the oxygens.NaV 3 O 8 and its chemically lithiated product have been studied by Spahr et al.81 Whilst the unlithiated material showed a higher initial capacity (310mAh g~1), the lithiated material Li 3 NaV 3 O 8 showed far superior capacity retention over repeated cycling.Discharge currents of up to 50mAh g~1 were achieved with minimal changes in performance. Dai et al. have prepared a new vanadium oxide, (Na 2 O) 0.23 V 2 O 5 by a reflux method.82 A capacity of 220mAh g~1 was achieved between 3.8 and 1.8V with reasonable cyclability.Another sodium vanadium bronze, Na 0.33 V 2 O 5 · 1.3H 2 O, has been synthesised by Millet et al.83 The layered material can intercalate 1.65 Li per unit with the hydrated material showing better electrochemical performance than the dehydrated one. The importance of this ‘structural water’ was discussed with reference to other layered materials.Similar findings were also reported by Yu et al. who observe improved discharge capacities in partially hydrated LiV 3 O 8 , with the small amount of water contained in the structure causing an increase in the interlayer spacing.84 Vanadates such as LiNiVO 4 have also been studied as Li battery electrodes but, owing to their lower voltage profiles, as anodes rather than cathodes.Orsini et al. have reported a new chimie douce synthesis of this material consisting of a precipitation reaction at a well defined pH.85 Amorphous and crystalline samples have been obtained by varying the annealing temperature with all materials showing large capacities,[1000mAh g~1 (7 Li per unit), but poor capacity retention. In situ X-ray di§raction was performed in an attempt to understand the mechanisms involved but further work is still required.Carbonaceous materials continue to be the most widely studied anode materials with new carbon composites, cycling studies and in situ measurements all reported this year. Bindra et al. have performed electrochemical cycling on the super dense phase LiC 2 .86 The material shows a very flat voltage profile with an initial capacity of 910mAh g~1 which immediately drops to values slightly below 373mAh g~1, the theoretical value for LiC 6 , on subsequent cycling.The high pressure synthesis and Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 481–506 488need for the battery to be fabricated in its charged state make the material not ideally suited for commercialisation but the authors point out its potential in applications requiring a large ‘one shot’ primary capacity and partial rechargeability.Wu et al. have prepared carbons by the heat treatment of melamine resins.87 The performance of the carbons vary significantly with firing temperature with the amount of graphene-nitrogen incorporated, particle size and number of micropores all being contributing factors.Maximum performance was achieved at 600 °C, with the addition of phosphoric acid improving capacity at higher firing temperatures. The authors also reported a similar study on carbons derived from polyacrylonitrile.88 Similar to their work on V 2 O 5 /polymer composites (discussed previously in this review, ref. 75) Kuwabata et al. have reported a synthetic carbon/poly(3-n-hexylthiophene) composite for use as an anode.89 The polymer acts as a binder as well as an active material and owing to its electrical conductivity removes the need for mechanical processing of the anode hence reducing any potential fabrication costs.The composite shows an initial capacity of 312mAh g~1 with greater coulombic e¶ciency and reduced irreversible capacity compared to other organic binders.The potential of the huge theoretical capacities of Si continues to be of interest. Wang et al. have attempted to overcome the problems associated with Si cycling by creating nano-sized Si particles encapsulated in amorphous carbon.90 The ball-milled samples show reversible specific capacities up to 1039mAh g~1 with nearly 800mAh g~1 being maintained after 20 cycles.The e§ect of rhombohedral (3R) phase content (compared to the usual hexagonal (2H) phase) on electrochemical performance in natural carbons has been reported by Huang et al.91 The reversible capacity is seen to increase with increasing 3R content with defects along the boundary allowing further intercalation above the theoretical maximum (LiC 6 ).Winter et al. have studied the correlation between irreversible capacity and Brunauer–Emmett–Teller (BET) surface areas.92 The authors report that although capacity loss increases with surface area it is also strongly dependent on the graphite type. Conclusions about the e§ect of the ratio between prismatic and basal surfaces were drawn and simple models to take into account the particle size distribution and morphology were discussed.Buiel et al. have investigated the loss of capacity with increasing firing temperature of hard carbons.93 BET and CO 2 absorption measurements showed a dramatic decrease in surface area and open micropore volume on heating above 1100 °C. It was proposed that the micropores close up to form ‘embedded fullerenes’ that are impenetrable to Li, resulting in the loss of capacity. The e§ects of surface area have also been discussed by Xing et al.who studied the change in structure and properties on ball-milled hard carbons.94 Initially the graphene layers become better stacked as pores were flattened but further milling caused disordering in the carbons. Particle size was observed to decrease and surface areas increase whilst the fracturing of the graphene sheets caused the incorporation of oxygen (presumed to be as surface functional groups).Electrochemical data showed an increase in capacity on milling but also large hysteresis. A similar study has been performed by Wang et al. on graphite powders with capacities as high as 700mAh g~1 being achieved.95 Both groups discussed mechanisms to explain these observations.Takami et al. have analysed the hysteresis in disordered carbons by use of polarisation and impedance measurements.96 They conclude that the large charge resistance Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 481–506 489and slow Li di§usion in extraction is responsible for the hysteresis, with the rectification of the disordered carbon sites responsible for the large resistances.Funabiki et al. have also used impedance measurements to study the charge transfer resistance and other factors on the intercalation mechanism in natural graphites.97 The authors report that the charge transfer takes place on the whole surface, whereas the Li` ion is intercalated from the edge plane and di§uses into the interior. Di§erences are also observed in the kinetics, with the rate of charge transfer being independent of the host structure in contrast to the Li di§usion which is shown to be strongly dependent.Several groups have reported 7Li NMR studies of carbon anodes. Imanishi et al. have studied a highly crystalline carbon fibre and natural graphite98 whilst Dai et al. have investigated hard carbons.99 In both papers, observed peaks are assigned to intercalation processes and their relevance to reversible and irreversible capacities discussed.Menachem et al. have used NMR to investigate the reasons for improved performance in partially oxidised graphite.100 Chemically bonded Li within the solid electrolyte interface (SEI) was also examined. Other studies have been performed on the formation of the SEI between carbons and electrolytes with Barsoukov eal.investigating the growth of passive layers at low temperatures.101 At [20 °C a very thin insulating layer is formed but a large porous precipitate of electrolyte decomposition products was also observed causing an increase in irreversible capacity loss. Further losses were observed on warming to room temperature owing to the formation of a secondary layer comprised of low- and room-temperature deposits.Scott et al. have tried to reduce irreversible capacity losses by producing an ex situ SEI by reaction with n-butyllithium.102 Whilst the treatment is successful in almost completely eliminating the irreversible capacity, the SEI formed is thick and brittle causing damage to the electrode on cycling. Another ex situ method for reducing irreversible capacity has been investigated by Buiel and Dahn.103 Chemical vapour deposition (CVD) of a carbonaceous substance from ethylene gas above 70 °C on hard carbons was observed to give a marked reduction in irreversible capacity but with no change in reversible capacity or voltage profile.The authors believe that the treatment modifies the surface and reduces the samples ability to absorb/react with species in the air that are generally thought to be responsible for irreversible capacities.Wu et al. have reported the improvement of electrochemical properties by the addition of V 2 O 5 to carbon.104 Three factors are proposed for the superior performance of the VO–graphene complex; an increase in separation of the (002) planes, the VO acting as an electron absorbing agent and an increase in the ordering of the carbon.105 Sn based materials have seen a surge in interest this year. Liu et al.have followed the intercalation process of Li into SnO 2 by X-ray di§raction and Raman spectroscopy. 106 Their results confirm the proposed mechanism of initial reaction of Li and SnO 2 to form Li 2 O and Sn metal.The Sn then alloys with further Li giving an irreversible and reversible capacity for the first and second processes respectively. Similar studies have also been conducted on SnO 2 thin films.107 The authors highlight the importance of the Li 2 Omatrix, making comparisons to the poor cycling of pure Sn metal, and propose an explanation for the improved performance.Wang et al. have studied the electrochemical lithiation of SnO by 7Li NMR, confirming that the proposed mechanism is also true for SnO.108 At low Li contents Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 481–506 490the alloys showed good agreement with thermally prepared LixSn reference alloys but at higher concentrations significant di§erences were observed.Similar findings have been reported by Dahn et al. from investigations of the crystal structure of electrochemically produced Li 4.4 Sn.109 The powder di§raction patterns obtained show a mixture of sharp di§raction patterns and broad oscillations. This was compared to thermally obtained Li 4.4 Sn and the di§erences discussed and explained. Brousse et al. have studied SnS 2 and compared its performance to that of SnO 2 .110 Although the SnS 2 material shows the expected lower irreversible capacity the reversible capacity and cycling performance were inferior to those of SnO 2 .This could be due to the relative performances of the Li 2 S and Li 2 O matrices but the authors point out that di§erences in synthesis conditions could also be responsible. The addition of glass forming elements in order to prepare a superior matrix has also attracted further attention.Nagayama et al. have studied SnSO 4 observing the formation of Li 2 SO 4 and LixSn alloys111 whilst Sn 2 P 2 O 7 has been examined by Wan et al.112 The electrochemical properties of the phosphate were found to be highly dependent on the heat treatment with the addition of Mn leading to greatly improved properties.Machill et al. have synthesised materials in the SnO–B 2 O 3 –P 2 O 5 system which although show an improvement over pure SnO are inferior to previously reported materials.113 An in situ Mo� ssbauer spectroscopy study of Sn 2 Fe anodes has been performed by Mao et al.114 The initially reversible reaction occurs by the formation of Li 4.4 Sn and small regions of Fe.On repeated cycling the Fe regions grow leading to the eventual failure of the cell. However by constraining the voltage limit to 0.6V improved cycling is obtained, this is thought to be due to the back reaction to Sn 2 Fe being avoided and hence minimising the disruption to the Fe matrix. Peramunage and Abraham have reported a new single step synthesis to micron sized Li 4 Ti 5 O 12 .115 The material is known to possess excellent cyclability and with a very flat voltage profile at ca. 1.5V can be used as a cathode in a Li cell or alternatively as an anode with a high voltage cathode. Samples prepared by the new method show improved capacity over previously prepared samples especially at high rates; with up to 25–30% improvement at 3–8C rates.The excellent performance of these materials is also demonstrated in a Li 4 Ti 5 O 12 /PAN electrolyte/LiMn 2 O 4 cell.116 Li titanates are also known as fast Li ion conductors. Gover and Irvine have reported the discovery of a new solid solution linking the ramsdellite phases LiTi 2 O 4 and Li 2 Ti 3 O 7 (Li 1`xTi 2~2xO 4 ; 0pxp0.143).117 Although no conductivity measurements were reported, Li temperature factors, obtained from combined X-ray and neutron di§raction studies, are shown to increase with x in accordance with increasing channel vacancies.Several papers have been concerned with the perovskite Li 3xLa 2@3~xTiO 3 and related phases. Ruiz et al. have performed structural characterisation and ionic conductivity studies on various members of the series Li 3xLa 4@3~xTiO 6 (x\0.29, 0.21, 0.09).118 Their results confirm the doubled perovskite structure with 2D ordering on the A-site.All three materials show similar activation energies with the intermediate sample (x\0.21) showing the highest conductivity. Harada et al. have obtained disordered cubic materials by quenching samples from 1350 °C to liquid N 2 temperature.119 Contrary to expectations these disordered samples give higher conductivities than their ordered analogues with a bulk conductiv- Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 481–506 491ity of 1.53]10~3S cm~1 at 25 °C reported for Li 0.36 La 0.55 TiO 3 . The lower conductivities in the ordered samples are thought to be due to higher activation energies caused by a decrease in lattice parameters. 7Li NMR studies on Li 3xLa 2@3~xTiO 3 by Bohnke et al. show strong asymmetry for the spin relaxation times, which would seem to be in contradiction to the 2D ionic motion thought to occur in this system.120 The authors propose that the asymmetry may be explained by a distribution of activation energies for the thermally activated Li` hops, probably due to the disorder in the materials.Moreno et al. have studied the Mn doped system La 0.5`x`yLi 0.5~3xTi 1~3yMn 3yO 3 121 whilst Morales et al. have studied both Mn and Cr doped materials122 and the analogous Pr 0.5`x`yLi 0.5~3xTi 1~3yCr 3yO 3 phase.123 All these materials are mixed conductors with electrical conduction increasing with Mn/Cr content and Li ion conductivity dependant on the Li content.Chung et al. have studied a range of isovalent substitutions for Ti to investigate the e§ect of B-site ionic size on conductivity.124 It was found that the conductivity increased with decreasing ionic size with this reciprocal relationship being due to bond strengths rather than spatial factors (free-space, bottlenecks or TiO 6 octahedra tilting). The smaller B-site ion strengthens the metal to oxygen bond, weakening the competing Li–O bond and hence reducing activation energies.A similar study was performed by Kawakami et al. by substituting larger Sr and smaller Ca for La/Li on the A-site.125 Lattice expansions due to Sr substitution led to an increase in conductivity although for higher Sr substitutions low ionic conductivity was observed owing to the lack of Li` ions and vacancies.The most widely used and studied electrolytes in Li batteries are the polymer elecolytes. Many of these however are liquid- or gel-like polymers, and hence not appropriate for this review, and several of the ‘solid polymer electrolytes’ are simply liquid polymers suspended in a solid matrix. Therefore rather than perform a somewhat limited and selective survey of the topic, lithium polymer electrolytes have not been included in this review.A new series of Li ion conductors has been synthesised by Poisson et al.126 The phases Li 9 M 3 (P 2 O 7 ) 3 (PO 4 ) 2 (M\Fe, Al) consist of layers of [(MP 2 O 7 ) 3 (PO 4 ) 2 ]9~ separated by Li` ions (Fig. 2). Li ion conductivity occurs through tunnels parallel to [001] but with rather low values (1.3]10~4S cm~1 forM\Fe).The high temperature phase transitions of the Li ion conductor Li 2 UBr 6 have been studied by Ma�etka et al.127 The phase transition from trigonal to monoclinic is observed at 505K and both low and high temperature structures calculated from neutron di§raction data. Long and short range order are discussed with reference to the structures and mechanisms for ionic conductivity described.Other alkali metal conductors A new fluorophosphate, Na 3 Al 2 (PO 4 ) 2 F 3 , has been synthesised by Le Meins et al.128 The Na` conductivity is bi-dimensional and is mainly due to long range movement along cavities formed by the arrangement of [Al 2 O 8 F 3 ] bioctahedra and [PO 4 ] tetrahedra (Fig. 3). An increase in conductivity was observed above 500 °C and further studies are currently being undertaken to elucidate the new mechanism.Daidouh et al. have reported two new polymorphs in the series AVP 2 O 8 (A\Na, Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 481–506 492Fig. 2 Structure of Li 9 M 3 (P 2 O 7 ) 3 (PO 4 ) 2 showing tunnels of Li conduction. Rb).129 Both materials consist of layers of P 2 O 7 and VO 5 polyhedra intercalated by the alkali metal ion, with the Na material containing pentagonal tunnels through the layers.Both materials show ionic conductivity with activation energies of 0.49 and 0.85 eV for the Na and Rb phase, respectively. Ionic conductivity in another double phosphate has been investigated by Dridi et al.130 The phases Na 8~2xM 4`x(P 2 O 7 ) 4 (M\Mn, Co, Ni) all show Na` conductivity through tunnels along the c-axis.The electrical properties of the three materials were discussed and correlated to structural features. New layered phases of MGa(CrO 4 ) 2 (M\Na, K, Rb) have been prepared by Saavedra et al.131 All of the phases are ionic conductors with the highest conductivity observed for the Na material with an activation energy of 0.86 eV.Na 3 H(SO 4 ) 2 is reported to become a fast ion conductor at 260 °C.132 The sample exhibits both Na` and H` conductivity and the di§erent mechanisms were discussed with respect to the structure. The new phase KDyHP 3 O 10 has also been reported to be a dual ionic conductor.133 The similarity of the K` and H` activation energies, obtained by both impedance and modulus spectroscopy, suggests a hopping mechanism for the conductivity.Kuhn et al. have studied the e§ect of Na extraction on the conductivity of the tunnel structured Na 0.875~dFe 0.875 Ti 1.125 O 4 .134 The extraction leads to a decrease in the activation energy, interpreted as in increase in ion mobility due to increased vacancies. In addition an electronic contribution due to partial oxidation of Fe3` to Fe4` also appears on Na removal.Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 481–506 493Fig. 3 Structure of Na 3 Al 2 (PO 4 ) 2 F 3 showing possible displacement of Na` from one cavity to another (reproduced by permission from Solid State Ionics, 1998, 111, 67). Oxide ion conductors The commercial interest in solid oxide fuel cells (SOFCs) continues to be the driving force in the expansion of oxide ionic conductor research.Although yttria stabilised zirconia (YSZ) remains the most widely used electrolyte in practical SOFCs the progression towards lower operating temperatures has reduced the scientific interest in this material. Several doping studies have been reported but with no marked improvement in performance.135–137 Practical considerations such as aging,138 mechanical strength139 and electrochemical pre-treating140 have also been investigated. The high conductivity of doped ceria at moderate temperatures makes it a promising replacement for YSZ.Torrens et al. have studied di§erent synthetic methods for preparing the dense ceramics required for maximising conductivity.141 Traditional solid state techniques produced poor quality samples even when sintered at 1650 °C compared to the hydrothermal route which gave dense products at 1400 °C.Ionic conductivities varied by almost a factor of 10 depending on the synthesis route, highlighting the importance of powder quality. Scanlon et al.142 and Hong et al.143 have both reported di§erences in conductivity between grain edges and bulk samples.This has been attributed to variations in dopant concentrations at the surface compared to the bulk although there is disagreement as to the nature of this variation. Dikmen et al. have synthesised a Bi doped ceria for the first time.144 The material prepared by a hydrothermal route exhibits a maximum conductivity of ca. 10~2 Scm~1 at 600 °C with an activation energy of 1.01 eV.Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 481–506 494The major drawback of doped ceria electrolytes is the emergence of electronic conductivity in reducing atmospheres owing to the partial reduction of Ce4`. Mishima et al. have attempted to overcome this problem by fabrication of a doped ceria/YSZ composite electrolyte, the ceria providing high conductivity whilst the YSZ blocks any electronic current.145 Although promising results were obtained, reaction between the two components caused a deterioration in performance.La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 2.85 (LSGM) is perhaps the most promising electrolyte for lower temperature operation but a great deal of further characterisation is still required. Slater et al. have studied the structure of LSGM by neutron di§raction at room146 and higher tempertures.147 The room temperature structure previously thought to be orthorhombic is shown to be monoclinic (pseudo-orthorhombic) undergoing two phase transitions to monoclinic (pseudo-rhombohedral) and then to rhombohedral on heating to 1000 °C.Possible correlation between the tilting of the GaO 6 octahedra and the activation energies of ion conductivity are presented and discussed.Huang and Goodenough have studied wet chemistry routes to LSGM in order to obtain the sub-micron particles required for commercial applications.148 Both the Pechini and sol–gel methods a§ord the desired product but use of hydrothermal methods proved unsuccessful. Ishihara et al. have shown that Co doping of La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 3 further improves the ionic conductivity.149 Excess Co however induces electrical conductivity and so the composition La 0.9 Sr 0.1 Ga 0.8 Mg 0.115 O 3 with almost purely ionic conductivity was proposed as the most suitable for electrolyte applications.Isotope exchange studies have indicated the oxygen exchange behaviour of LSGM is surprisingly more similar to that of YSZ rather than isostructural (but mainly mixed conducting) perovskites.150 Amongst other results it is also concluded that the high ion conductivity is due to the high concentration and mobility of the oxide vacancies.The compatibility of the LSGM electrolyte with di§erent electrodes has also been investigated. Yamaji et al. have studied Pt electrodes151 whilst Ishihara et al.have investigated doped SmCoO 3 as the cathode.152 Performance and interfacial reactions are discussed for both samples with reference to their use in SOFCs. Other systems studied have included Bi 2 O 3 –Nb 2 O 5 , with Castro et al. reporting the new phase Bi 3 NbO 7 .153 The phase is cubic with a defect fluorite structure and shows good conductivity up to 875 °C.Continuing their earlier work, Lee et al. have investigated the conductivity of Y 0.8 Nb 0.2 O 1.7 as a function of temperature and oxygen partial pressure.154 The terial shows good conductivity and a wide PO 2 range of ionic domain suggesting its possible suitability for oxide conductor applications. Bhuvanesh et al. have produced a new oxide ion conductor by a novel route.155 The Li ion conductor La 2@3~xLi 3xTiO 3 has been ion exchanged to give the phase La 2@3~xTiO 3~3x(OH) 3x and can be dehydrated to La 2@3~xTiO 3~3x@2 .The new oxygen deficient perovskite is an oxide ion conductor transforming to a mixed conductor above 500 °C. Several papers have reported work on the BIMEVOX series of materials. The structure of BIMEVOX is an intergrowth of alternating (Bi 2 O 2 )2` sheets and (VO 3.5 )2~ oxygen deficient layers (Fig. 4) with metals substituted on to the V site to stabilise the conducting c-phase. Muller et al. have reported neutron di§raction studies of Bi 4 (V 0.85 Co 0.15 ) 2 O 11~d at various temperatures.156,157 The electrical properties are correlated to the refined structures obtained and the order–disorder transition Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 481–506 495Fig. 4 Idealised structure of BIMEVOX. observed. Abrahams et al. have performed a combined X-ray and neutron di§raction study of c-Bi 2 V 0.9 Ni 0.1 O 5.35 quenched from high temperatures.158 As with the Co material all the oxygen vacancies are located in the V layer. Further work by the authors has combined ac impedance and X-ray di§raction to investigate the relationship between structure and conductivity.159 Two di§erent regions of conductivity are located from Arrhenius plots depending on the composition.Cyclic voltammetry on Bi 2 V 0.9 Cu 0.1 O 5.35 microsamples has been performed by Fafilek.160 The sample was reduced on the first cathodic scan, in agreement with previous results, but this study has revealed that only part of the sample is re-oxidised on the positive sweep.Subsequent cycles show completely reversible reduction and oxidation of the residual oxide, possible explanations for the initial irreversible reduction were given. La doping in several BIMEVOX materials has been studied by Yaremchenko et al.161 Various results for di§erent samples were reported as well as a new experimental arrangement to study Faradaic e¶ciency of oxide ion conductors. Jacobsoone et al.have reported the first fluorinated BIMEVOX material Bi 2 V 1~x~yZnxBiyO 5.5~2.5x~yF 2x.162 Three di§erent forms (a, b, c) were identified depending on composition and temperature, and the e§ect of the fluorination on Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 481–506 496conductivity was discussed.Begue et al. have studied the electrical properties of the BIMEVOX related phases Bi(Bi 12~xTexO 14 )Mo 4~xV 1`xO 20 .163 Conductivities as high as 7.96]10~3Scm~3 at 750 °C were reported. On a practical note, Boivin et al. have demonstrated the use of BIMEVOXmaterials as oxygen separators.164 Nearly 100% e¶ciency was reported in the temperature range 430–600 °C for current densities up to 1000mAcm~2.The mixed conducting perovskites, Sr-doped LaMnO 3 , LaCoO 3 and LaFeO 3 , continue to be the most widely studied electrode materials. Mitterdorfer and Gauckler have investigated the interface reaction between La 0.85 Sr 0.15 MnO 3~d (LSM) and YSZ electrolytes.165 They report that the incorporation of excess Mn or the reduction of sintering temperature can suppress the formation of the La 2 Zr 2 O 7 impurity, which is known to reduce conductivity.The authors conclude that further improvement in performance can be achieved by a deeper understanding of the interfacial processes. The behaviour of LSM in high temperature oxygen atmospheres is clearly important and so Krogh Anderson et al. have performed in situ synchrotron X-ray di§raction studies on LSM under these conditions.166 Their results show that the oxidation of LSM follows first order kinetics and is essentially a single phase process with an average activation energy of oxidation of 160 kJ mol~1.Several studies have focused on the kinetics of oxygen dissociation at LSM electrodes. Tikhonovich et al. have reported that the addition of silver or praseodymia (in the correct oxidation state, di§erent forms of Pr give di§erent results) reduces the overpotential by activating the material.167 Murray et al.have achieved similar results by increasing the triple phase boundary (TPB) by use of a LSM/YSZ composite electrode.168 YSZ grain boundary resistances were found to limit the performance of the composite cathode.The importance of the TPB on performance has been demonstrated by Hurita et al. who have used secondary ion mass spectroscopy (SIMS) to visualise the oxygen reduction sites.169 The active sites were determined to be ‘spots’ located around the TPB. Similar conclusions have been drawn by Endo et al.170 By electrochemical methods they have shown that in LSM the boundary structure is rate determining whilst in La 1~xSrxCoO 3~d (LSC) the surface area is the important factor for reaction rate.Ezin et al. have used isotope exchange reactions to study the reaction mechanisms on the surface of LSC.171 At elevated temperatures (786–875 °C) and oxygen pressures of 1.28]103 Pa it is shown that surface exchange proceeds via an absorption–desorption mechanism.Electrochemical permeability measurements by Figueiredo et al. have confirmed the high ionic conductivity of LSC, enhanced by the Sr substitution. 172 Their results however could not be fitted to a simple model behaviour derived from the expected defect chemistry suggesting that significant over potentials could be expected. Ni doping in LSC and La 1~xSrxFeO 3~d has been investigated by Huang et al.173 The materials showed improved conductivities and thermal expansions well matched to current electrolytes.In addition their readily reversible oxygen loss at T[600 °C make them good catalysts for oxygen reduction and hence ideal for cathodes in SOFCs. Sahibzada et al. have studied the e§ect of Pd incorporation in La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3~d (LSCF).174 Small amounts of impregnated Pd improve Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 481–506 497performance with a 3–4 times lower cathodic impedance and up to a 40% reduction in single cell resistance. Isotope exchange depth profiling (IEDP) and SIMS indicated however that the Pd has a detrimental e§ect on the surface exchange coe¶cient but the authors point out that this may be misleading due to the oxidising conditions used in IEDP e§ecting the results.Weston and Metcalfe have used a novel in situ method to obtain gravimetric and catalytic information on LSCF anodes.175 Weight losses suggested a degree of thermal instability with the material readily being reduced by the methane fuel. Redox studies though have revealed that re-oxidation is relatively fast, implying that careful control of the oxygen flow should allow practical use of LSCF without excessive reduction.The combined use of LSCF electrodes with gadolinium doped ceria electrolytes has been investigate by Bae and Steele.176,177 The introduction of a secondary thin dense layer of LSCF between electrode and electrolyte caused an improvement in area specific resistivities by a factor of 2–3 with values as low as 0.5)cm2 recorded at 520 °C.Kharton et al. have investigated Pb doping in LaFeO 3 as a way of reducing catalyst poisoning by gases such as SO.178 Whilst Pb doping increased the conductivity the materials showed very poor oxygen permeabilities compared to the analogous Sr phases. Kindermann et al. have shown that the addition of Ir greatly reduces the interfacial reaction between LaFeO 3 based perovskites and YSZ although as yet no conductivity data has yet been reported for the Ir doped samples.179 The other class of anode materials widely studied are Ni–YSZ cermats.Anselmi- Tamburinin et al.180 and Aruna et al.181 have both reported materials with marked improvement in performance by use of new combustion synthesis routes.Tsai and Barnett have shown that reduced interfacial resistances can be achieved by the introduction of a thin Ti doped YSZ or yttria doped ceria layer between electrode and electrolyte.182 The interfacial layer also has the added advantage of reducing problems with incompatible thermal expansions. Other ionic conductors Schober has prepared the new proton conductor BaIn 0.5 Sn 0.5 O 2.75 .183 Like the previously reported yttrium analogue the phase shows excellent proton conducting properties but is highly unstable in reducing atmospheres, limiting its practical application.Kabayashi et al. have sulfonated poly(oxy-1,4-phenylene-oxy-1,4-phenylene-carbonyl- 1,4-phenylene) (PEEK) and poly(4-phenoxybenzoyl-1,4-phenylene, Poly-X 2000) (PPBP) to obtain new proton conductors.184 Sulfonated PPBP shows much higher water uptake and conductivity than S-PEEK and this was discussed with reference to the structures and design of future proton conducting polymers.Improvement in the Sc3` conductivity of Sc 2 (WO 4 ) 3 has been achieved by Gd3` doping.185 The larger Gd3` acts as a lattice-forming ion allowing increased mobility of the Sc3` with conductivities up to 6.2]10~5S cm~1 at 600 °C being obtained.Several investigations of Ag` and Cu` conducting glasses have been reported. Fanggao et al. have measured the e§ects of temperature and pressure on the conductivity of (Ag 2 S)x(AgPO 3 ) 1~x glasses in order to investigate the ionic motion.186 Their results are consistent with the random walk model but are an order of magnitude Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 481–506 498Fig. 5 Structure of Bi 9 (V 1~xPx) 2 ClO 18 showing Cl~ ions located within tunnel. smaller than expected from the free volume theory. Doped Cu telleride glasses have been studied by Chowdari et al.187 The Cu` conductivity was found to be strongly dependent on the non-bridging oxygen content as estimated from X-ray photoelectron spectroscopy.Both Chowdari et al.188 and Bolotov et al.189 have studied mixed Ag- and Cuglasses. The work of Bolotov et al. on the CuI–AgI–Ag 2 Se 3 system is particularly notable for the observation of a degenerated mixed cation e§ect. Isotope di§usion tracer techniques have been used to investigate this unusual behaviour more thoroughly. Bazan et al.have reported increased Cu` conductivity in c-CuBr by the addition of CdBr 2 .190 A lowering of the c to b phase transition temperature was also observed. Two new anionic conductors have been reported. El Omari et al. have synthesised Ln 1~yCdyF 3~y (Ln\Ce, Nd) with a tysonite-type structure.191 19F NMR has identi- fied three fluoride sublattices and the ion di§usion and exchange between the lattices is detailed.Mentre and Abraham have reported a new series of oxychlorides Bi 9 (V 1~xPx) 2 ClO 18 .192 The Cl~ ions are located in elliptical tunnels within the Bi–V/P–O network (Fig. 5) and are relatively unbonded. Conductivity measurements confirm the Cl~ mobility which is observed to decrease with increasing P content. 3 Superconductors 12 years after the discovery of high T# superconductivity by Bednorz and Mu� ller superconductors are still making the headlines.193–195 Most of the work however is Annu. Rep.Prog. Chem., Sect. A, 1999, 95, 481–506 499concerned with application196 or theoretical/physical considerations.197 New phases are still being reported although many of these are simply substituted analogues of known families of superconductors.198–201 Chu and co-workers have continued to investigate the Ba–Ca–Cu–O system.202,203 A new family of materials has been identified and the structures more thoroughly examined.The decay products of these air sensitive samples have also been studied with T# values up to 107K observed. Xiong et al. have reported the first bulk superconductivity in Pr 1~xCaxBa 2 Cu 3 O 7~d (Pr-123) (T#,.!9 \98 K) by high pressure synthesis.204 The authors described a mixed valency for the Pr, concluding that hole depletion is the main cause for superconductivity suppression in the system and not pair breaking as previously proposed.The successful fluorination of Y 2 Ba 4 Cu 7 O 14 (YBCO-247) has been achieved by Abakumov et al.205 For high levels of fluorination a significant structural rearrangement was observed with an accompanying increase in T# up to 62 K.Extensive structural characterisation has revealed fluorine entering the anion deficient Cu1 layers resulting in an octahedral co-ordination for some of the Cu1 atoms with a consequential elongation of the apical Cu–O bond. An overview of fluorination techniques and applications in superconducting systems, with special reference to the Sr 2~xAxCuO 3 (A\Ca,Ba) system, has been given by Francesconi et al.206 Oxy-anion substitution have also been studied, with Palmer et al.reporting SO 4 2~ incorporation in to La 1.85 Sr 0.15 CuO 4 .207 No evidence for S2~ was observed in contradiction to previous reports, with the presence of the SO 4 2~ confirmed by X-ray emission spectroscopy. Magnetic measurements showed a reduction of T# with increasing SO 4 2~ content.New non-cuprate systems have also been reported this year including new organic superconductors.208 Istomin et al. have synthesised the new 3K superconductor EuNb 2 S 2 .209 The phase crystallises in a hexagonal structure with electron di§raction also revealing the presence of a superstructure.Khasanova et al. have reported a new superconductor in the K–Bi–O system.210 Perovskites of composition K 1~xBi 1~xO 3 show superconductivity in the range x\0–0.2 with T#,.!9 \10.2K for x\0–0.1. Comparing their results with those of other Bi–O based superconductors the authors conclude that the doping level is the predominant feature in determining the T# of these materials.Intercalation has proved an important technique for the synthesis of new materials. Yamanaka et al. have induced superconductivity in b-HfNCl by electron doping achieved by Li intercalation.211 The maximum T# of 25.5K is the highest reported for an intermetallic and suggests the possibility of even higher T# values in layered nitride species. Choy et al.have demonstrated that intercalation can not only produce new materials but can also be a useful tool for understanding the structural and electronic properties e§ecting T#. From a study of iodine intercalation into Bi 2 Sr 2 CaCu 2 Oy a direct correlation between hole concentration and T# was observed regardless of c-lattice expansion.212 The authors also reported related studies on a range of intercalation guests in the Bi 2 Sr 2 Can~1 CunOy (n\1, 2) systems.213 Similar investigations have been performed by Felser et al.who have used Li intercalation in ZnSe 2 to probe the electronic structure and its relevance to superconductivity.214 Electronic and chemical factors e§ecting superconductivity have generated a lot of Annu. Rep. Prog. Chem., Sect.A, 1999, 95, 481–506 500interest this year with many authors presenting work that has questioned our understanding of these materials. Attfield et al. have shown that T# in the system (Ln 1~xMx) 2 CuO 4 (Ln\lanthanide; M\Ca, Sr, Ba) decreases linearly with A-site disorder.215 Their results further show that in the absence of disorder T# should increase quadratically with A-site ionic radii in single CuO 2 layer systems.The sensitivity of T# to lattice strain has been compared to the metal–insulator transitions observed in the magneto-resistive perovskites Ln 1~xMxMnO 3 . Jennings and Greaves have performed a thorough examination of the (Y 1~yCay)BaSrCu 3~zCrzO 7 system.216 Neutron di§raction combined with bond valence studies have shown that whilst T# remains constant, hole density within the superconducting CuO 2 sheets increases with Ca doping.Their observations were discussed with respect to a proposed increase in optimal hole concentration with Sr content in the YBa 2~xSrxCu 3 O 7 system. Agreements for aspects of this proposal have been provided by Zhao et al.217 Measurements of oxygen isotope shifts in YBa 2~xSrxCu 3 O 7~y have suggested that whilst T# decreases with increasing Sr content the material remains ideally doped.This has been confirmed by doping studiesith both O, by high PO 2 synthesis (i.e. using high O 2 pressure), and with Ca doping leading to a decrease in T#. Anomalous behaviour of T# with hole concentration has also been reported by Shah et al.218 Whilst T# in La 1.5~xCaxBa 1.5 Cu 3 Oz is observed to increase on Ca doping, corresponding to an increase in hole concentration, similar increases in hole concentration in LaCa 0.5`yBa 1.5~yCu 3 Oz resulted in no change of T#.Correlations between T# and hole concentration were further discussed. Liu et al. have also studied hole concentration in the CuO 2 layers by use of X-ray absorption near edge spectroscopy (XANES).219,220 The e§ect of Ca and Sr doping in YBa 2 Cu 3 O 7 with respect to hole concentrations and T# were discussed.Licci et al. have continued their investigation into the di§erences between mechanical and chemically induced pressure.221 Structural determinations indicate that Sr substitution in YBa 2 Cu 3 O 7 simulates the e§ect of external pressure except with regards to the thickness of the CuO 2 –Y–CuO 2 superconducting blocks.Further work however has indicated that the thickness of these blocks is not a key parameter in determining T#. Detailed analysis has suggested that the decrease in T# on Sr doping may be due to the relaxation of the Sr/Ba layer hindering the charge transfer from Cu1 to Cu2. Han et al. have used quasi-hydrostatic pressure to achieve a T# of 255.4K for the phase Tl 1.8 Ba 2.0 Ca 2.6 Cu 3 O 10`d, an almost doubling of the ambient conditions value (T# \129 K).222 The authors draw attention to the good agreement in the location of T#,.!9 and value of dT#/dP with previously reported studies. Through analysis of uniaxial strain and pressure derivatives of T#, Locquet et al.have proposed that compressive epitaxial strain in thin films could generate much larger T# values.223 The authors have demonstrated this theory with the fabrication of strained single crystal thin films of La 1.9 Sr 0.1 CuO 4 .The strained films show a T# value of 49K compared to 25K for bulk samples, nearly a two-fold increase. Whether similar results can be achieved for higher T# materials remains to be seen.Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 481–506 501References 1 Y. Ein-Eli, W.F. Howard, S. H. Lu, S. Mukerjee, J. McBreen, J. T. Vaughey and M. M. Thackeray, J. Electrochem. 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ISSN:0260-1818    

DOI:10.1039/a804887h

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

26 Mechanisms and kinetics in the solid state Nigel A. Young Department of Chemistry, The University of Hull, Hull, UK HU6 7RX 1 Introduction This article deals with work published in 1998 covering the area of inorganic solid state mechanisms and kinetics. Although the net has been drawn fairly widely, it excludes work describing the kinetics and mechanisms of catalytic cycles using solid state supports, UHV surface science, metals, alloys and the majority of semi-conductor work. 2 General mechanistic studies and reviews A number of review articles and editorials have appeared covering the areas of: modelling and simulation of solids,1,2 including multiscale modelling of the mechanics of materials,3 first principles calculations of phase stability and thermodynamic properties, 4 the modelling of ceramic materials,5 a survey of theoretical predictions for electronic materials and their properties,6 integral modelling of metallic materials,7 the simulation of solidification;8 the application of combinatorial techniques to materials science,9 homogeneous and heterogeneous catalysis;10 the utilisation of mollusc shell structures in the development of novel design strategies for nanoscale biomaterials;11 an overview of amorphous materials, especially glasses,12 including amorphous silicon, 13 the structure of and dynamics in glassy and molten silicates,14 nanoscale heterogeneity of glass forming liquids,15 computer simulations of structure and transport in glasses;16 the synthesis and reactivity of solids17 with articles on the negative thermal expansion of materials,18 fluorine insertion in inorganic materials,19 hydrothermal synthesis of zeolites,20 laser ablation for the growth of materials,21 and atomic layer epitaxy;22 the optical and magnetic properties of mixed valence manganites, 23 transition metal oxides;24 solid catalysts and porous solids25 including microporous manganese oxides,26 and derivatised mesoporous solids.27 The importance of kinetic and thermodynamic data for the accurate ab initio prediction of molecular crystal structures has been highlighted.28 The negative thermal expansion observed for some oxides (e.g.ZrW 2 O 8 , the Sc 2 W 3 O 12 family, NbPO 4 , AlPO4-17 and faujasite SiO 2 ) has been explained in terms of a transverse thermal Annu. Rep. Prog. Chem., Sect.A, 1999, 95, 507–533 507motion of oxygen in M–O–M linkages, and this mechanism can give strong negative thermal expansion over broad temperature ranges.18,29 A general overview of the field of solid state ionics has been published,30 as well as a review dealing with the structure, properties and phase transitions of relaxor ferroelectric complex perovskites.31 Atomistic simulations of several basic dislocation mechanisms have been developed,32 a new model has been suggested for the description of interdi§usion in powder mixtures,33 the e§ect of fast di§usional growth of an interphase layer has been investigated and the fast growth is attributable to grain boundary di§usivity and/or accelerated volume di§usion due to vacancy supersaturation,34 the initiation/growth mechanism, product morphology, driving force, reaction kinetics and e§ect of external parameters on grain boundary migration on solid state discontinuous reactions have been discussed,35 a mesoscopic model has been proposed to describe the reaction fronts in solid state reactions,36 the stability of a planar solid state interface at which a chemical reaction occurs has been analysed,37 and the e§ects of contamination from the milling medium have been shown to have a great influence on the sintering behaviour.38 The thermal decomposition pathways of solids (such as the Group 2 carbonates to give oxides and CO 2 ) have been determined using crystal modelling procedures, involving symmetry controlled routes for transforming the reactant into the solid product, lattice energies of the reactants, the conjectured transient intermediate structures and the final products and profiles of energy changes during the proposed decomposition route.39 A review of the preparation of crystalline solid state materials using decomplexation of soluble complexes has been published.40 A new mechanism has been proposed to describe the long range e§ect of ion bombardment in terms of energy transport,41 and materials driven far from equilibrium under irradiation conditions have been modelled using a kinetic Monte–Carlo approach.42 In the area of experimental advances there have been reviews on the use of molten nitrates for the synthesis of solid materials,43 the synthesis, structure and properties of inorganic–organic perovskites,44 microwave processing of ceramics,45 colloidal powder processing for ceramics,46 the application of ceramic hollow spheres,47 the application of synchrotron radiation techniques such as XAFS, and in situ XRD and SAXS/WAXS,48 and modern imaging techniques.49 There have been reports on: the use of Pt markers in the study of solid state reactions in the presence of an electric field;50 the utilisation of scanning acoustic microscopes for the investigation of ferroelectric properties of materials;51 muon spin relaxation has been used to study interstitial and molecular motion;52 the application of a non-linear heating regime for TGA determination of kinetic parameters in solid state reactions;53 the use of fractional heating DSC to the determination of the parameters that control the kinetics of solid state reactions;54 and high temperature adiabatic calorimetry has been used to obtain accurate determinations of the heat capacities of key materials and is claimed to be indispensable for the development of high-temperature enthalpy standards for DSC.55 The application of scanning probe microscopy has been reviewed, with the conclusion that notable advances have been made in the imaging of the topography of oxides with high vertical resolution, the identification of the catalytic sites at the atomic scale, and an understanding of the catalytic behaviour of oxides at the micrometric scale,56 and the use of analytical electron microscopy to the study of discontinuous solid state reactions has also been reviewed.57 Recent advances in the application of multinuclear solid-state NMR for mechanistic studies on heterogeneous catalysis Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 507–533 508and inorganic materials,58 and mercury compounds59 have been reviewed. The use of intense synchrotron radiation has been exploited for the micro single crystal di§raction structural study of the disordered template within a chabazite related cobalt aluminium phosphate (DAF-5),60 the use of ‘white’ beam energy dispersive X-ray di§raction to record in situ di§raction data on a number of important solid state reactions such as intercalation and hydrothermal syntheses,61 the application of spatially resolved X-ray di§raction using a 180 lm diameter X-ray beam to map the a to b to a phase transition in the heat a§ected zone of commercially pure titanium arc welds,62 the use of lNEXAFS and lPES has been reported for a study of lateral doping inhomogeneities in UHV cleaved MoTe 2 , thin films of conductive polymer, and sublimed WO 3 films,63 and an initial design of a high count rate curved linear position sensitive detector for high resolution powder di§raction has been described.64 The modelling of electrical conductivity in high temperature proton conducting oxides has been reported,65 a first principles molecular dynamics simulation of proton di§usion in perovskite oxides has shown that the proton forms an O–H bond with the neighbouring O ion,66 the role of p–d hybridisation in perovskites exhibiting giant magnetoresistance has been investigated by a semi-classical calculation of single particle eigenenergies of electrons in Mn–O–Mn triatomic systems,67 and a model has been proposed to account for the anomalous transport of oxygen in non-stoichiometric perovskites.68 3 General kinetic studies and reviews Time resolved X-ray di§raction has been used to perform in situ kinetic studies on the intercalation of a range of guest molecules in layered lattices,69 and solid state reaction kinetics and product compositions have been determined by XRD.70 Scanning electrochemical microscopy using the substrate-generation-tip-collection mode has been used as a new approach to investigate the kinetics of chemical reactions following electron transfer.71 A symposium has been held on kinetically determined particle shapes and the dynamics of solid–solid interfaces.72 The development of geometrical probabilities to the study of heterogeneous kinetics over the last 60 years has been reviewed.73 A comparative study of kinetic analyses of solid state reactions by means of stepwise isothermal analysis and constant rate thermal analysis has been published,74 together with preliminary results of the dissociative- evaporation scheme and of the method for the calculation of absolute reaction rates,75 a study on non-isothermal transformation kinetics using non-isothermal DSC techniques,76 non-isothermal kinetics of solid-state decomposition using quadrupole mass spectrometry,77 a review of isothermal and non-isothermal kinetics of thermally stimulated reactions of solids,78 isothermal solidification kinetics of di§usion brazing, 79 and some comments about a correction estimation of the kinetic exponent for non-isothermal solid-state processes.80 The determination of the kinetics of solid state reactions such as Na` or Li` ion exchange using a positive feedback between the reaction and fracture have been presented,81,82 as well as a theoretical study of the e§ect of the fractal properties of the solid reagent on the kinetics of nucleation-limited solid state reactions,83 pressure and temperature e§ects on the kinetics of tunnel solid-state reactions,84 the chemical kinetics of phase boundaries in solids,85 the e§ect Annu. Rep.Prog. Chem., Sect. A, 1999, 95, 507–533 509of geometrical assumptions in modelling solid-state transformation kinetics,86 the kinetics of reactions on non-uniform surfaces,87 a kinetic description of di§usionlimited reactions in random catalytic media,88 kinetic analyses of solid-state reactions with a particle size distribution,89 a correlation of macroscopic and microscopic rate constants in solid state chemistry,90 the kinetics of interface di§usion controlled reactions in lamellar eutectoids and reactive multilayers,91 the kinetics of sintering of metal supported catalysts,92 surface induced sinter crystallisation,93 a discussion on the use of the extended Eyring kinetic equation and the compensation e§ect in catalysis,94 an analytical description of devitrification kinetics induced by time-dependent nucleation and growth of crystallites during a quench heating cycle has been proposed,95 and the phenomenological kinetics of irreversible electrochemical dissolution of metal oxide microparticles.96 4 General crystallisation and nucleation studies A number of theoretical or general papers dealing with crystallisation and nucleation have appeared and include: a theoretical description of the kinetics of overall crystallisation when the appearance of the stable crystalline phase is preceded by the formation of a metastable phase;97 a study of the free convection and surface kinetics in crystal growth from solution;98 an investigation of the e§ect of kinetic parameters on the dynamics of continuous crystallisers;99 the deduction of a set of functions to interpret the kinetic behaviour of growth phenomena occurring at the solid/solution interface by physicochemical interactions between a liquid/gaseous bulk phase and a solid surface;100 an AFM investigation of the mechanism of secondary nucleation induced by contact;101 the e§ect of volume changes during precipitation on the reaction kinetics accompanying internal reduction of mixed oxides;102 a method to study crystallisation by electrical resistivity;103 step kinetics by one-dimensional nucleation, 104 subsurface ordering and kinetic coe¶cients for melt growth;105 and the kinetics of monolayer particle deposition.106 5 Non-oxygen containing materials Borides, carbides, silicides, porous silicon, nitrides and phosphides A review of the solid state rare earth metal borocarbides has been published.107 The oxidation behaviour of several TiB 2 cermets have been studied between 700 and 1000 °C, and the products studied by XRD and SEM,108,109 as have the nanocrystallisation kinetics of Fe 85.5 Zr 4 Nb 4 B 5.5 Al,110 and also the high temperature solid state reactivity between iron and chromium monoborides.111 The defect formation,112 etching kinetics,113 and synthesis of nanometer sized SiC powders114 have been reported, as well as a new approach to isothermal precipitation kinetics of carbides.115 The kinetics of formation of titanium carbide in shock compression of titanium and carbon powders,116 during carbothermal reduction of an ultrafine titania/carbon mixture,117 and the mechanosynthesis by a combustion reac- Annu.Rep.Prog. Chem., Sect. A, 1999, 95, 507–533 510tion during ball milling118 have been reported, as well as the factors determining the diameter of SiC whiskers119 and the morphology and growth mechanism of TiC whiskers,120 both prepared via CVD and the growth kinetics of tantalum carbide layers on graphite produced by pyrolysis of TaCl 5 .121 Co 2 Si has been prepared from cobalt oragonosilicon precursors via thermal degradation under argon.122 29Si NMR has been used to characterise porous silicon structures,123 a mechanism for the control and formation of a porous silicon layer on p-type silicon based on a simple extension of Beale’s and Lehmann’s models has been proposed,124 a computer simulation study of the mechanisms of formation and topological synthesis of porous silicon under anodisation in HF solution has been presented,125 the concentration of the aqueous HF used for the electrochemical etching of a silicon wafer to form porous silicon has been shown to have an e§ect on the wavelength of photoluminescence,126 and the mechanism of photoluminescence has been investigated.127,128 The mechanisms of silicon crystal growth have been discussed.129 A kinetic study of the direct nitridisation of pelletised silicon grains has shown that the yield of a-Si 3 N 4 and the overall conversion of silicon are temperature and nitrogen gas concentration dependent, but independent of hydrogen gas,130 an investigation of shock enhanced a to b phase transformation in Si 3 N 4 powders has revealed that the transformation occurs at 200 °C below that for unshocked material and with an apparent activation energy of 154–276 kJ mol~1,131 a theoretical and experimental investigation of the oxidation mechanism of Si 3 N 4 bonded SiC ceramics by CO, CO 2 and steam has been published.132 The initiation of the reaction between lithium nitride and anhydrous metal halides induces a spontaneous exothermic propagating reaction (with temperatures in excess of 1000 °C) to yield TiN, ZrN and HfN.133 The formation mechanism of TiN and Al 2 O 3 from the reaction of in situ injection into molten Al alloys has been determined over the temperature range 1000–1600 °C,134 the experimental Ti–V–N phase diagram data have been thermodynamically assessed, and the thermodynamic calculation is linked with limiting cases for solid state di§usion kinetics.135 A reaction mechanism with energetically branched chains has been proposed for the low threshold initiation of lead azide by a neodymium laser.136 The decomposition of a-Ca 3 N 2 and a-Mg 3 N 2 (which crystallise in the anti-bixbyite structure) to M(OH) 2 on exposure to the atmosphere have been investigated and the appearance of NH 4 ` vibrational modes in the decomposition products has been explained on the basis of the existence of Brønsted acid centres on the surface of the solid, and that IR bands characteristic of NH 3 indicate that Lewis acid centres also exist on the surface.137 The kinetics of the decomposition of a-Mg 3 N 2 has been studied with FTIR microspectroscopy and this has shown that the formation of Mg(OH) 2 is a three dimensional di§usion process, while the proton donation by the hydroxide to the absorbed NH 3 is a quasi-first-order reaction.137 The formation of aluminium nitride from hydrated basic aluminium dicarboxylate complexes has been studied by 27Al magic angle spinning NMR,138 and the oxidation kinetics of AlN powder has been reported.139 The structural evolution of Ni 70 Mo 10 P 20 has been studied by XRD and electron microscopy.140 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 507–533 511Chalcogenides Fullerene like Group 6 sulfides and selenides,141,142 as well as zinc sulfides,143,144 and selenides145 have been prepared. There have also been studies of the kinetics of the intercalation of cobaltacene into layered metal dichalcogenides.146 An investigation of a microporous layered tin sulfide A 2 Sn 3 S 7 (A\tetramethylammonium, quinuclidinium or tert-butylammonium) has shown that aqueous ion-exchange and gas phase exchange allow for intercalation like processes, but that it also behaves as a molecular sieve with a kinetic diameter [3.4Å, showing potential for catalysis, separation and chemical sensing applications.147 A study of the mechanism of formation of TiS 2 and NbS 2 from titanium tetraalkoxides with H 2 S in benzene has shown that it follows a thiolysis condensation route via an alkoxysulfide which is converted to TiS 2 by heat treatment in flowingH 2 S.148 High resolutionTEMhas been used to observe the host-layer restacking in HgxTiS 2 (1.24[x[0.0),149 and the characteristics of the electronic subsystem in intercalated TiS 2 have shown that thermodynamic functions must be taken into account in any explanation.150XRDhas been used to study the decomposition of the spinel-type NiCr 2 S 4 –NiIn 2 S 4 –Cr 2 S 3 –In 2 S 3 system.151 A study of the kinetics of the evaporation of SnS from SnS–Cu 2 S melts has shown that the process is controlled by the mass transport of SnS in the gas phase, and the apparent activation energy for the process is 204.67 kJ mol~1.152 Combustion synthesis studies of transition metal carbosulfides have been carried out to investigate their suitability as high temperature solid lubricants. 153 The crystallisation kinetics of the glassy alloy Sb 0.12 As 0.36 Se 0.52 has been studied using DSC.154 Cyanides and related materials The thermal decomposition and kinetic parameters of cobalt hexacyanoferrate(II), cobalt hexacyanoferrate(III) and cobalt nitrosopentacyanoferrate(II) have been obtained from TGA methods and show that it occurs mainly in two steps, and that the intermediates formed during decomposition undergo further decomposition over a considerable temperature range.155 Sorption kinetics and isotherms for caesium on Cu 2 FeII[FeII(CN) 6 ]·xH 2 Oand Cu 3 [FeIII(CN) 6 ]·xH 2 Ohave shown that the maximum uptake for the former is only 0.073Cs/Fe (at./at.) whereas for the latter it is 1.5 Cs/Fe.156 A reinvestigation of the a to b phase transition of cobalt phthalocyanine (PcCo) has been carried out using energy dispersive XRD and DSC which shows that the phase transition follows a three step model: (i) disordering of the adjacent layers of PcCo molecules; (ii) crystallisation of the b-form from the disordered phase and (iii) crystal growth of the b-phase from the nuclei to an average particle size of 2500Å.157 Halides The ionic conductivity of Ln 1~yCdyF 3~y (yD0) (Ln\Ce, Nd) with tysonite-type structures have been investigated using 19F NMR,158 and the pressure–temperature dependence of the conductivity of TlX (X\Cl, Br, I) have been reported.159 A high Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 507–533 512resolution synchrotron radiation XRD study of the (KxNa 1~x)MgF 3 perovskites has been carried out at high temperature and high pressure. The crystal structure transforms reversibly from orthorhombic to tetragonal to cubic with change of pressure and temperature, as well as with K` concentration.160 Precipitate periodic and aperiodic patterning of non-equilibrium PbI 2 /PbF 2 systems,161 and the electric and dielectric properties of monoclinic BaR 2 F 8 (R\rare earth) single crystals have been reported. 162 Rapid quenching of a CuCl–AlCl 3 melt results in the formation of metastable b-CuAlCl 4 , which on heating to above 100 °C changes to the thermodynamically stable a-CuAlCl 4 form.The structures of a- and b-CuAlCl 4 and a-CuAlBr 4 have been determined by single crystal XRD, and a mechanism for the phase transition based on that for cristobalite-type structures is presented.163Asingle crystal quasielastic neutron scattering investigation of the inverse spinel-type Li 2 MnCl 4 has been used to determine the mechanism of its high ionic conductivity.164 It has been found that the temperature dependent yield stresses of NaCl(Pb) are anomalous, whereas those for NaCl(Ca) are normal, and that the deformation of NaCl(Ca) crystals at 77Koccurs mainly via a dislocation mechanism at slow strain rates.165 The nucleation and growth of silver bromide tabular crystals has been observed in situ for the first time by optical microscopy,166 and the crystallisation kinetics of PbI 2 –PbB 2 –AgI ternary glasses have been measured.167 Solid state chlorometallates have been prepared by reductive dechlorination of solvated niobium pentachloride with erbium and samarium atom vapours, with chlorine edge shared aggregation preferred for Er vapours and face shared condensation patterns from Sm vaporisation.168 Nanoclusters and particles The chemistry and physics of cosmic nano- and micro-particles has been reviewed.169 Pt and Au gold nanostructures have been prepared utilising a reductive deposition method where porous silicon acts as both a reducing agent and as the substrate for the resulting metal nanostructures,170 the growth and shape formation of Pt nanoparticles have been achieved under kinetic control,171 a mechanism has been proposed for the stabilisation of magnetic fluids, such as amorphous cobalt nanoparticles, by long chain surfactant molecules,172 and a study of the kinetic instability of nanocrystalline Al prepared by two di§erent routes has been published.172 Layered Pt based materials have been synthesised through the thermal decomposition of self-assembled metal carbonyl-surfactant phases.173 6 Oxygen containing materials Alkali metal and alkaline earth species A multinuclear (7Li, 23Na, 39K, 87Rb, 133Cs) magic angle NMR study of alkali metal oxides, peroxides and superoxides has been used to study the e§ects of alkali metal–oxygen phases as catalyst components and promoters.174 The thermal decomposition of alkaline earth peroxides have been modelled using a symmetry based sequence of steps by which the reactant structure is converted to the product structure. 175 An oriented CaO[111] film has been grown through the dehydration of a Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 507–533 513Ca(OH) 2 film,176 and atomistic models of CaO/SrO interfaces have been constructed via the sequential deposition of CaO species on to SrO surfaces in conjunction with energy minimisation and dynamics.177 The sodium environments in albite glasses with water concentrations ranging from 0 to 60mol% have been investigated using 23Na o§ resonance quadrupole nutation and MAS NMR spectroscopy.178 A study of the thermal and static contributions to the EXAFS Debye–Waller factors has been carried out on KxCs 1~xSi 2 O 5 glasses.179 Nanocrystalline MgO (1.8–35 nm) has been prepared by a sol–gel method and characterised using multinuclear (13C, 17O, 25Mg) NMR.180 Borates and indium oxides The recombination kinetics in LiB 3 O 5 crystals have been studied by time-resolved luminescence and absorption spectroscopy,181 and the relaxation processes in Li 2 O–B 2 O 3 and Cs 2 O–B 2 O 3 glasses have been investigated by dynamic mechanical spectroscopy.182 A study of the sintering of tin doped indium oxide powders and a mixture of In 2 O 3 and SnO 2 powders with a 1.0wt% V 2 O 5 additive has shown that densification was observed without sacrificing the electroconductivity, and the mechanism of densification was discussed in terms of the liquid phase sintering and formation of a V–In–O compound.183 Alumina and aluminates A novel method for the preparation of phase pure K-b-Al 2 O 3 has been reported using the gel-to-crystallite conversion method, where coarse gels of hydrated Al 2 O 3 ·yH 2 O (80\y\120) were reacted with ethanolic KOH, leading to crystalline K-inserted gibbsite or K-pseudoboehmite.Thermal decomposition of these precursors above 227 °C yielded intermediate K-inserted boehmite and above 1200 °C, K-b-Al 2 O 3 was obtained via a modified K-b-Al 2 O 3 intermediate.184 Proton NMR imaging spectroscopy has been used to investigate the drying of alumina and titania porous catalysts, 185 Al 2 O 3 –TiO 2 composite oxide nanocrystals of varying Al 2 O 3 : TiO 2 ratio prepared using a sol–gel method have been studied using TGA–DSC,186 the phase development of Y 3 Al 5 O 12 , YAlO 3 and Y 4 Al 2 O 9 prepared via a sol–gel route have been investigated using XRD,187 the sol–gel synthesis and gas adsorption properties of a CuCl modified mesoporous alumina have been reported,188 FTIR spectroscopy has been used to study the surface acid–base properties of sol–gel alumina catalysts,189 the formation of porous Al 2 O 3 /Al composites under hydrothermal conditions has been studied using SEM, TEM, XRD and IR spectroscopy of lattice modes and thermal analysis,190 as well as a comparative study of the formation of nickel, cobalt, copper and iron aluminates from a- and b-supported oxides.191 A mechanism of cation equilibration in the NiAl 2 O 4 spinel has been proposed.192 The microstructural properties of boehmite formed in an alkaline medium under hydrothermal conditions have been investigated by XRD, FTIR and TEM, which have shown that the initial product, bayerite, formed at the beginning of the precipitation process is converted to boehmite by the dissolution reprecipitation mechanism.193 A study of the mechanism of Annu.Rep.Prog. Chem., Sect. A, 1999, 95, 507–533 514pseudo-boehmite dehydration has shown that the reagent structure and reaction kinetics have an influence on the transformation sequence.194 Carbonates The surface chemistry and reactivity of vaterite and its crystalline transformation in the presence of MnII have been studied in ultrapure water at room temperature using XRD, SEM, BET, IR, EPR and XPS.The transformation from vaterite to calcite is strongly dependent on the metal abundance in the reaction medium and cannot be considered a simple or direct solid phase transition.195 Silica and silicates A review of cristobalite related oxide structures has been published.196 The use of polarised EXAFS to study fine grained layer silicates has been demonstrated with a study on Garfield nontronite.197 The characterisation of mixed amorphous/crystalline cerium oxide supported on SiO 2 using XRD, TPR and TEM-EDX has been used to obtain information about the CeO 2 dispersion and crystallinity on SiO 2 .198 Amolecular mechanism explaining the chemical phenomena that occur during the preparation of silica supported nickel catalysts has been proposed.199 Other reports include: a molecular dynamics simulation study of the structural development in sol–gel processes for silica systems derived from Si(OH) 4 and Si(OMe)(OH) 3 ;200 a computer modelling study of ionic conductivity in lithium silicates and aluminosilicates;201 a viscoelastic characterisation of three di§erent sol–gel derived silica gels;202 the investigation of silica aerogel evolution using IR and luminescence spectroscopy;203 the sol–gel preparation and crystallisation of 2.5CaO–2SiO 2 glassy powders;204 the reaction between alumina and aluminoborosilicate melt at 1400 °C yielded mullite, the reaction of zirconium silicate with silicon boride yielding zirconium boride;205 the intercalation of methanol and ethanol into Na–fluorotetrasilicic mica was followed dynamically by XRD, and the intercalation of methanol was much faster than that of ethanol, with methanol forming successively single and double layers between the silicate sheets of the mica, whereas ethanol forms similar layers concurrently.206 It has been demonstrated that single crystal langsite (La 3 Ga 5 SiO 14 ) can be used as a high temperature piezoelectric transducer up to 1000 °C.207 The kinetics of wollastonite nucleation in calcium silicate glasses have been determined,208 as have the crystallisation kinetics of apatite in apatite–wollastonite glass.209 An experimental study on the spontaneous crystallisation of quartz in alkaline solutions under hydrothermal conditions has been presented,210 the products of mechanochemical activation of mixtures in the MgO–SiO 2 system ground in a ball mill have been found to be dependent on the water content of the siliceous starting materials,211 the e§ect of addition of lithium ions to ZrO 2 –SiO 2 gels was found to be very e§ective for the preparation of zircon powders at 800–900 °C,212 the electrical conductivity of alkali silicate glasses containing Ca or Al have been studied,213 and the influence of solid state transformation time on the nucleation and growth of silicalite-1 prepared from layered silicate has been studied by XRD, TGA, IR, 29Si and 13CNMR and electron microscopy.214 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 507–533 515Aluminium and silica based microporous, mesoporous, zeolitic and clay materials A number of reviews have appeared in this area dealing with the design of selective catalysts from hybrid silica-based materials,215 the kinetics and mechanism of the crystallisation of microporous materials,216 the use of topological structure control at the meso phase,217 the description of the use of a retrosynthetic tool to reduce any crystalline structure to a set of two or three simple chemical ‘tectons’ from which the whole framework may be reconstituted,218 and the use of electron beam lithography to fabricate ideal, representative models of supported porous catalysts.219 In addition the tethering of transition metal ligands onto the micelle templated silica surfaces,220 coordination complexes impregnated in molecular sieves,221 the organometallic chemistry occurring in the void spaces of zeolites222 and on the surfaces of inorganic oxides, metals and zeolites223 have been reviewed.A number of reports have been published detailing the design and synthesis of sites with specific activity within microporous and mesoporous structures.224–229 The formation of MCM-41 has been studied using in situ FTIR,230 EPR,231 high resolution TEM and XRD,232 and a guest-templated synthesis of a kinetically stable nanosized cage has been proposed as a thermally switchable molecular lock.233 The use of a surfactant based synthetic strategy has been used to control thickness and defects in oriented mesoporous silica films,234 a study of iron containing mesoporous silicas using EXAFS, EPR, Mo� ssbauer and UV/VIS spectroscopies indicates that the iron species are tetrahedrally coordinated,235 the formation of Ti containing MCM-41 using a gas–solid reaction has shown that the Ti species were highly dispersed in the structure with tetrahedral co-ordination, but with a small amount of anatase,236 and the molecular mechanism of capillary condensation of acetonitrile on MCM-41 has been determined using adsorption isotherms and IR spectra.237 During low temperature hydrothermal alkaline conversion of kaolinite in the presence of sodium carbonate a disordered phase has been identified showing structural features between sodalite and cancrinite minerals, and the kinetics of its crystallisation have been determined using XRD and 27Al MAS NMR,238 the kinetics of the hydrothermal synthesis of an orthorhombic zeolite Li 4 [Al 4 Si 4 O 16 ]·4H 2 O from thermally activated kaolinites have been reported,239 mullite type I precursors have been prepared via a sol–gel process,240 in situ isothermal XRD has confirmed that mullite is the final high tperature product from the high temperature phase transition of muscovite-2M(1),241 the synthesis of Na 4 Mg 6 Al 4 Si 4 O 2 F 4 and its SrII uptake kinetics have been carried out,242 a study of sol–gel prepared aluminosilicates has shown that when prepared from tetraethoxysilane (TEO) and hydrated aluminium nitrate in n-propanol, only 0–70% of the aluminium is chemically incorporated in the network, but that this can be increased by decreasing the water content and prehydrolysis of the aluminium nitrate,243 the use of calorimetric data from the sorption of benzene in siliceous faujasite, has been used in the development of a new forcefield for the modelling of aromatics in siliceous zeolites,244 the kinetics of the reversible reduction of faujasite-hosted mononuclear titanium oxides prepared from the reaction of dehydrated zeolites with TiCl 4 at di§erent temperatures has been evaluated,245 the dealumination of H-mordenite in vacua by CCl 4 followed by gallium impregnation using GaCl 3 has been studied by DRIFTS, 71Ga, 29Si and 27Al solid state MAS NMR,246 wollastonite has been synthesised by a solid state reaction from two Spanish Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 507–533 516diatomites, and the results compared with those of a quartz and silica gel.247 An in situ FTIR study on activated solution exchanged Cu-ZSM-5 using CO as a probe molecule has identified a new CuI site, of the form AlOd~Cu`,248 and thin (230–3500 nm) continuous films of ZSM-5 have been synthesised in an organic template free, gel on quartz substrates using surface modification, covering with a monolayer of colloidal silicalite-1 seed crystals, followed by hydrothermal treatment.249 XRD and IR have been used to show that in ZnCl 2 /NaY samples prepared using microwaves and solid state interaction between ZnCl 2 and NaY, the ZnCl 2 is completely dispersed on to the surface of NaY at loadings up to 37.5 wt%, and that the ZnCl 2 interacts in two di§erent ways with the NaY,250 solid stateMASNMRhas been used to investigate Y-type zeolites reacted with chlorofluorocarbons,251 the solid state crystallisation process and mechanism of formation of B-Al-ZSM-5 from a dry gel of B 2 O 3 –Al 2 O 3 –SiO 2 in the presence of ethylamine and water vapour has been studied using 11B, 27Al, 29Si, 13C MAS NMR, XRD and IR,252 and the e§ect of BaO on the crystallisation kinetics of the cordierite glass (MgAl 1.83 Si 2.25 O:) have been determined using XRD,253 Oxygen-17NMRhas been used to study the oxygen exchange kinetics between framework oxygens and oxygens in the channel waters in the zeolite mineral stilbite.254 The extremely high and selective uptake of transition metal cations such as CuII, NiII, CoII and ZnII by diadochy clays of nominal composition [Mg 6 Al 2 (OH) 16 ]- [CO 3 ·4H 2 O] and [Mg 6 Al(OH) 16 ][NO 3 ·2H 2 O] has been reported for the first time.255 XRD, solid state MAS NMR and isothermal TGA have been used to study the oxidation mechanism and kinetics of pure X-phase and O@-sialon (Si 12 Al 18 O 3 N 8 ) powders.256 The microporosity of aluminium intercalated and pillared synthetic saponites with variable layer charges has been studied by high resolution quasiequilibrium adsorption volumetery and low temperature adsorption microcalorimetry. 257A thermal decomposition study of the intercalation of K 3 [Fe(C 2 O 4 ) 3 ] into the layered double hydroxide, Mg 6 Al 3.4 (OH) 18.82 (CO 3 ) 1.51 (NO 3 ) 0.36 ·4.5H 2 O, has suggested that the [Fe(C 2 O 4 ) 3 ]3~ complex intercalates intact.258 Tin oxides The e§ect of changing the doping metal (Cu, Ni, Pd, Pt) on the kinetics of oxygen exchange reactions of polycrystalline SnO 2 thin films has been studied by conductance measurements,259 both pure and CuII and FeIII doped tin oxide nanocrystals have been prepared by a sol–gel process, and their response to CO in dry air has been investigated as a function of annealing temperature,260 the influence of Pd and Pt additives on the microstructural and electrical properties of SnO 2 based sensors has also been studied,261 and a refined interpretation of the electron–hole kinetics in metal oxides such as the ultrafast charge carrier dynamics of SnO 2 nanoclusters has been published. 262 The influence of heating rate (either via a furnace or cw CO 2 laser) on the microstructures and macroscopic properties of sol–gel SnO 2 : Sb coatings has been explored,263 as well as the deactivation mechanisms of gas sensors based on semiconducting oxides,264 and the structure of the topaxial Mg 2 SnO 4 /MgO solid reaction front has been investigated, where there is a positive lattice mismatch of 2.5% due to the di§erence in lattice parameters.265 Tin–antimony oxide catalysts have been prepared by precipitation techniques.266 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 507–533 517Phosphates and antimonates There have been many studies of phosphates reported and these include: the e§ects of organic additives on the growth kinetics and morphology of KH 2 PO 4 crystals;267 the synthesis and characterisation of a new polymorph of lithium zinc phosphate hydrate, b-LiZnPO 4 ·H 2 O;268 a kinetic study using in situ synchrotron XRD of the hydrothermal transformation of microporous lithium zinc phosphates;269 the crystallisation kinetics of a low melting PbO–ZnO–P 2 O 5 glass;270 the e§ect of hydrostatic pressure and temperature on the frequency dependencies of the ac conductivity of ionic AgPO 3 and (Ag 2 S) 0.3 (AgPO 3 ) 0.7 glasses;271 glycine intercalated vanadyl and niobyl phosphates; 272 nucleation and growth stages of vanadyl pyrophosphate;273 the structure and e§ect on catalytic activity of di§ering vanadyl phosphates;274,275 the e§ect of electron irradiation on the physico-chemical and ion exchange characteristics of ammonium molybdophosphate;276 a study of the kinetics and mechanisms of reversible topochemical transformations in Mg(H 2 PO 4 )·2H 2 O;277 the sol–gel synthesis of calcium phosphates from alkyl phosphates;278 the relation between aggregation and heterogeneity of sol–gel derived CaO–P 2 O 5 –SiO 2 ,279 and the study of the incorporation of growth inhibiting diphosphates into the steps of calcite cleavage planes.280 The mechanism of the topotactic reaction of c-ZrPO 4 [O 2 P(OH) 2 ]·2H 2 O with benzene diphosphonic acid to form pillared compounds was only observed in water–acetone mixtures but not in water alone,281 anion exchange between sulfate and hydrogenphosphate has resulted in the formation of a mesoporous zirconium phosphorus complex oxide,282 conductivity relaxation parameters of H` and K` in polycrystallineKDyHP 3 O 10 prepared fromK 2 CO 3 , Dy 2 O 3 andH 3 PO 4 at 280 °C for 12 h have been reported,283 as well as the sol–gel synthesis of phosphate ceramic compounds.284 The mechanism of low thermal expansion in the cation ordered Nasicon related material Sr 0.5 Ti 2 (PO 4 ) 3 ,285 and a high resolution solid state magic angle spinning NMRstudy on the thermal decomposition of layered HSb(PO 4 ) 2 have been reported.286 There have also been reports detailing work on the oriented growth of hydroxyapatite (HA) [Ca 10 (PO 4 ) 6 (OH) 2 ] on (0001) textured titanium with a functionalised self-assembled silane monolayer as a template,287 and the determination of the growth kinetics of theHA(0001) face and bulk supersaturation of a simulated body fluid using real-time Moire phase shift interferometry.288 Broadband and pulsed 1H NMR measurements over the temperature range [173 to 177 °C have been used to assess the structure and mobility of proton groups in hydrous antimony pentoxide (Sb 2 O 5 ·nH 2 O, 2pnp3.2) and have shown that the spin–spin relaxation time of the protons and the electrical conductivity depend on the number of water molecules in position 16d of the pyrochlore structure,289 the conductivity mechanism of AgSbO 3 with the defect pyrochlore structure has been investigated using impedance spectroscopy.290 Sulfates, selenites, selenates and tellurates The Tl 2 SO 4 –Li 2 SO 4 phase diagram has been re-examined using DTA, XRD and classical methods, with several changes reported from the previous study.The mixed TlLiSO 4 sulfate exists under two forms, a low temperature b-form which is orthorhombic, and a high temperature hexagonal a-form, both of which have the stu§ed- Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 507–533 518tridymite structural type, with an a to b transition temperature of 531 °C. The a-form can be classed as a superionic conductor.291 The ionic conductivity of (1[x)Li 2 SO 4 –xM 2 SO 4 (M\Na, K, Rb, Ag; 0.0025\x\0.09) has been studied using complex impedance spectroscopy,292 the complex electrical impedance of Na 3 H(SO 4 ) 2 has been measured between 25 and 316 °C, and the electrical conductivity shows remarkable changes in the temperature range 160–260 °C, with the sample becoming a fast ionic conductor above 260 °C.293 Atomistic simulations of the e§ects of calcium and strontium defects on the surface structure and stability of BaSO 4 have been carried out.294 A kinetic study of the thermal decomposition of polycrystalline powdered ammoniojarosite (obtained from the hydrolysis of bio-oxidised sulfuric pickling waste water) has been carried out using TGA methods.295 The mechanochemical synthesis of sodium selenite (Na 2 SeO 3 ) from SeO 2 and Na 2 CO 3 has been shown to be a zero-order reaction,296 and thermal stability and vapour pressure studies of UTe 3 O 9 and UTeO 5 have been carried out.297 Strontium, yttria and the rare earths A review of the binary rare earth oxides has been published.298 Titania and titanates The grain growth and kinetics of nanocrystalline titania prepared from TiCl 4 have shown that the grain growth activation energy is 54.62^3.73 kJ mol~1 at temperatures [500 °C, and 6.43^3.73 kJ mol~1 at temperatures lower than this.299 The kinetics of the anatase–rutile transformation in TiO 2 in the presence of Fe 2 O 3 have shown that the anatase–rutile transition temperature is lower, and the transformation rate higher in air in the presence of Fe3` than in the corresponding pure TiO 2 .300 The size of sol titania nanoparticles has been shown to be independent of the micellar composition, whereas the kinetics of the sol–gel transition are not.301 Anatase has been prepared by sol–gel synthesis by mixing TiOSO 4 and KOH at pH 1.97, whereas titania was formed when the precipitation method was used.302 A significant number of reports dealing with titanates have been published and include: a combined XRD and NPD study showing that the solid solution linking LiTi 2 O 4 and Li 2 Ti 3 O 7 ramsdellites is best thought of as Li 1`xTi 2~2xO 4 (0\x\0.143);303 a kinetic study of the reaction between TiO 2 and Na 2 CO 3 of relevance to the direct causticization of kraft black liquour;304 the e§ect of annealing on the dielectric permittivity of strontium titanate films in the SrTiO 3 /Al 2 O 3 structure; 305 and an analysis of the mechanisms of the formation and redistribution of space charge in SrTiO 3 .306 In the case of barium titanates there are papers dealing with the preparation and characterisation of nanocrystalline BaTiO 3 307,308 a kinetic study by XRD of the conventional solid-state synthesis of BaTiO 3 ,309 a study of the thermal behaviour of sol–gel derived barium titanate gels,310 gold particle enhanced crystallite growth of thin films of BaTiO 3 using a sol–gel process,311 the formation and reactions of Bi 4 Ti 3 O 12 ultrafine powders from a sol–gel process,312 the abnormal grain growth of BaTiO 3 containing an excess of 0.1, 0.2 and 0.4mol% of TiO 2 .313 The growth mechanism of PbTiO 3 films has been investigated,314 as well as the nucleation and Annu.Rep.Prog. Chem., Sect. A, 1999, 95, 507–533 519growth of highly oriented PbTiO 3 films prepared by a sol–gel process,315 a study of the crystallisation of the di§erent phases of sol–gel processedSm modified lead titanate thin films,316 the controlled crystallisation of lead zirconate titanate glass-ceramics via a sol–gel route,317 the kinetics of formation of the pyrochlore and perovskite phases in sol–gel derived lead zirconate titanate powder,318 the crystallisation behaviour of the series of solid solutions ZrxTi 1~xO 2 and PbyZrxTi 1~xO 2`y prepared by the sol–gel process,319 an Fe K-edge EXAFS study of FeTiO 3 ,320 the mechanochemical reaction between ilmenite (FeTiO 3 ) and aluminium to give TiAl 3 , Fe 4 Al 3 and Al 2 O 3 ,321 the formation of pseudobrookite (Fe 2 TiO 5 ) from Fe 2 O 3 –TiO 2 at ca. 900 °C in air, Ar and Cl 2 atmospheres, including in the last case vapour transport via FeCl 3 and TiCl 4 vapours,322 and the structural characterisation and ionic conductivity properties of three members of the La 1.33~xLi 3xTi 2 O 6 (x\0.29, 0.21 and 0.09) perovskites.323 Static lattice energy minimisation calculations have been used to investigate the defect structure of pyrochlore structured titanates and zirconates of the general form A 2 B 2 O 7 (A\Gd, Y; B\Zr, Ti).324 The behaviour of titanium and zirconium alkoxides towards complexation and water addition has been analysed by water titration and calorimetric experiments.325 Zirconia and zirconates A study of the e§ect of an incorporated dispersion of V 2 O 5 on tetragonal zirconia (t-ZrO 2 ) has shown that the dispersion possibly proceeds by the incorporation of VV ions into the vacant surface sites on the support,326 t-ZrO 2 powders have been prepared by a solid state reaction between sodium metazirconate and sodium metaphosphate, and the yield of t-ZrO 2 increases monotonically with time at all reaction temperatures according to phase boundary controlled kinetics,327 TGA and DSC analyses of mesoporous zirconias prepared from anionic surfactants via a sca§olding reaction reveal interesting di§erences between samples prepared from long-chain sulfates or sulfonates for which dehydration occurs in two steps, compared to phosphates where the dehydration occurs in one step,328 a study of the formation of mesoporous zirconium(IV) oxides having controlled surface areas has shown that the control of the surface areas of the calcined zirconium(IV) oxides arises from the extent of equilibration permitted between the hydrolysed zirconyl oxychloride and the surfactant prior to calcination.329 A study of the kinetics in phosphoric acid media of the lamellar to hexagonal to cubic transformations of mesoporous zirconia prepared from a neutral organic amine has shown that the lamellar to hexagonal transformation is preceded by a loss of the template molecules and the hexagonal to cubic transformation proceeds only when the lamellar form has entirely transformed to the hexagonal phase,330 the synthesis of mesoporous zirconia has been performed by slowly hydrolysing zirconium propoxide in the presence of dodecyl phosphate or sulfate, and hexadecyl sulfonate,331 the characterisation of zirconia films deposited by rf magnetron sputtering,332 and the grain growth kinetics of sol–gel derived alumina–zirconia composites have been reported,333 as has the e§ect of water content in sulfated zirconia catalysts on their activities for n-butane isomerisation.334 The cubic to tetragonal phase transitions in ZrO 2 –Ln 2 O 3 and HfO 2 –Ln 2 O 3 solid solutions have been studied using Raman spectrsocopy,335 and the formation of La 2 Zr 2 O 7 and Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 507–533 520oxygen reduction kinetics of the La 0.85 Sr 0.15 MnyO 3 /O 2 /Y 2 O 3 -stabilised ZrO 2 have been investigated.336 Vanadium oxides and vanadates The structure of VO 2 has been redetermined using single crystal XRDfor both the low (25 °C) and high temperature (200 °C) phases.337 Li 1`xV 3 O 8 prepared via a low temperature sol–gel method had smaller grain size and better electrochemical performance than samples prepared by conventional high temperature synthesis,338 and the electrochemical insertion in sol–gel crystalline V 2 O 5 thin films has shown that 1.8 Li atoms can enter the oxide lattice at a 2V cut-o§ voltage,339 and the thermal expansion of calcium polyvanadate (Ca 2 V 2 O 7 ) has been shown to be dominated by the calcium–oxygen soft polyhedra.340 Niobates and tantalates The relationship between dielectric relaxation behaviour and B-site ordering in lead magnesium niobate ferroelectric ceramics has been considered,341 and the low temperature electrical conductivity of congruent lithium niobate crystals discussed.342 The local structure and nature of the phase transitions in potassium niobate were shown to be governed by both displacive and order–disorder mechanisms using polarised Nb K-edge XAFS,343 the ionic conductivity of ferroelectric BaTiO 3 , LiNbO 3 and KNbO 3 as well as Al 2 O 3 dispersed Li 2 CO 3 ,344 and the reversible oxidation/reduction in CeTaO 4`d studies by TEM and XRD have been published.345 The synthesis, structure and magnetic properties of the new non-stoichiometric perovskite phase, Ca 2 MnNbOy, has been reported which shows semiconducting behaviour, 346 and a detailed study of the calcination and sintering of the perovskite phase Pb(Fe 0.5 Nb 0.5 )O 3 has been reported.347 The structural evolution of the perovskite phase Pb[(MgxZn 1~x) 0.33 Nb 0.67 ]O 3 has been found to be highly dependent on its precursors’ strong tendency to form pyrochlore phases such as Pb 2 Nb 2 O 7 , Pb 3 Nb 2 O 8 and Pb 3 Nb 4 O 13 in the mixture of the raw materials,348 and the e§ect of buoyancy driven convection and crystal growth of KNbO 3 has been observed.349 Chromates, molybdates, tungstates and polymetallates A study of the electrochemical dissolution of CrIII and CrIV oxides has shown that the rate determining step is preceded by two electron oxidation in the solid phase,350 and a TGA study has been used to determine the kinetics of chlorination of Cr 2 O 3 with Cl 2 /N 2 and Cl 2 /O 2 mixtures.351 The parabolic rate constant for the solid-state reaction of La 2 O 3 and CrO 3 to give LaCrO 3 has been determined,352 a study of the thermal decomposition of M@2 CrO 4 (M@\Li, Na, K, Rb, Cs, Ag, Tl), MACrO 4 (MA\Mg, Ca, Sr, Ba) and M@@@2 (CrO 4 ) 3 (M@@@\La, Nd) has been investigated,353 a thermoanalytical study has yielded the activation energies for the solid state reactions of Li 2 CO 3 with Cr 2 O 3 (97.3 kJ mol~1), MoO 3 (120.6 kJ mol~1) and WO 3 (167.1 kJ mol~1).354 Crystalline catalysts of orthorhombic MoO 3 have been prepared via a CVD method, and a chemical etching method has been developed for restructur- Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 507–533 521ing the catalysts, and it is believed that the chemically caved surface structures may be crucial for certain catalytic reactions involving MoO 3 .355 A kinetic analysis of the serial reactions of lead magnesium tungstate ceramics has been reported,356 a relationship between the orientation of the WO 4 2~ tetrahedra in tungstate crystals and their morphology has been demonstrated,357 as well as evidence for pressure dependent ion transport in NH 4 NbWO 6 .358 A structural comparison of Fe 2 V 4 O 13 and Fe 2 V 3.16 Mo 0.84 O 13.42 has revealed that the two structures are closely related and contain isolated Fe 2 O 10 octahedral dimers, and the substitution of MoVI for VV revealed a new mechanism in which a corresponding stoichiometric amount of oxygen was brought into the structure to balance the charges,359 and the use of P 2 W 15 Nb 3 O 62 polyoxoanions to stabilise Rh0 nanoclusters.360 A number of other polymetallates have been studied including: [Nb 3 PW 9 O 40 ]6~; [Nb 6 P 2 W 18 O 77 ]6~ and b-[(NbO 2 ) 3 PW 9 O 37 ]6~;361 [Fe 8 V 10 W 16 O 85 ];362 [CuHPW 12 O 40 ·15H 2 O] and [CaHPW 12 O 40 ·13H 2 O].363 A large number of review articles covering many aspects of polyoxometallate chemistry have been published in the last year,364–367 including a series of articles in Chemical Reviews.368–380 Manganese oxides and manganates The sequence of phase transitions in the La 2 O 3 –MO(MCO 3 )Mn 2 O 3 (M\Ca, Sr, Ba, Cd) and the La 1~xMxMnO 3`d solid solutions have been studied and it has been shown that interaction begins at 650 °C due to the formation of divalent metal manganites, and that the chemical transformation is complete by 1200 °C,381 thermogravimetric studies of oxygen stoichiometry and oxygen transport kinetics in (La 1~xSrx) 0.95 FeyMn 1~yO 3`d (0pxp0.3, 0pyp0.7) have been reported,382 time resolved synchrotron XRD methods have been used to study the kinetics of oxidation of lanthanum strontium manganates(III, IV) in a bow of oxygen,383 the structural and morphological changes associated with charge ordering in La 0.2 Ca 0.8 MnO 3 have been investigated with electron microscopy,384 the liquid-mix disorder in crystalline solids such as ScMnO 3 by SR XRD,385 the synthesis and crystal chemistry of a new manganite member Bi 3.6 Sr 12.4 Mn 8 O 28`d, of the ‘tubular’ family,386 an investigation of charge carriers and magnetic, magnetotransport and optical properties of La 0.67~xYxBaO 0.33 MnO 3 ,387 the epitaxial growth and strain of manganite thin films,388 and the dependence of the lithium ionic conductivity on the B-site ion substitution in (Li 0.5 La 0.5 )Ti 1~xMxO 3 (M\Mn, Sn, Zr, Ge).389 The Li 2 MnO 4 spinel phase has been prepared using an aqueous sol–gel method, and the kinetics of electrochemical insertion–extraction have been investigated using ac impedance spectroscopy, 390 an NPD study has been carried out on two series of Li–Mn–O samples with spinel structure.391 NPD and Rietveld refinements of Li 0.5~3xLa 0.5`x`yTi 1~3yM 3 O 3 (M\Mn, Cr) have shown that the C polymorph is of an ordered perovskite type, containing a three dimensional framework of corner sharing MO 6 octahedra in which the structures are partially collapsed as a result of a cooperative tilting and rotation of the octahedra.392 Impedance spectroscopy has been used to study the oxygen reaction kinetics of La 0.8 Sr 0.2 MnO 3 electrodes on yttria stablised zirconia electrolytes.393 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 507–533 522Iron oxides, ferrates, ferrites and ruthenates A review of the occurrence and constitution of natural and synthetic ferrihydrite has been published.394 The apparent activation energies of 82^6 and 82^7 kJ mol~1 for the formation of BaFeO 3~x (x\0.02–0.08) and BaFe 2 O 4 , respectively, were determined using the Jander equation from XRD data,395 the preparation of LaFeO 3 particles by sol–gel technology from the metal nitrates using urea as the gellificant agent have been presented,396 the morphology and structure of heamite particles have been shown to be dependent on the concentration of CuII, NiII and CrIII,397 the temperature and frequency dependence of the permittivity, dielectric-loss tangent and electrical conductivity of BiFeO 3 have been studied,398 as well as the rheological behaviour of aciculate ultrafine a-FeOOH particles under alkaline conditions,399 and the structure, cation distribution and properties of nanocrystalline spinel type titanomagnetites of composition Fe 2.5 Ti 0.5 O 4 formed via either soft chemistry routes or high energy ball milling.400 A study of the reduction enhancement of Fe 2 O 3 in physical mixtures with Pt/mordenite has shown that the migration of Pt from the zeolite to the iron oxide is crucial,401 the e§ect of urotropin on the chemical and microstructural properties of iron oxide powders has shown that in the absence of urotropin, akaganeite (b-FeOOH) particles showed a range of morphologies, but after 5 h ageing in 0.025M urotropin, acicular b-FeOOH monodispersed particles of decreased size were obtained, but at higher urotropin concentrations (0.25 M) amorphous particles were formed,402 b-FeOOH has also been obtained from precipitation by the addition of urotropin or NaOH to FeCl 3 solutions, but if prolonged ageing took place there was also evidence of a-Fe 2 O 3 ,403 nanosized (1–100 nm) iron oxide particles have been generated by pulsed IR laser pyrolysis of Fe(CO) 5 ,404 a study of the interdi§usion at the interfaces of NiO/a-Fe 2 O 3 has been reported,405 together with the electrical and magnetic properties of the SnIV substituted spinel ferrite, ZnSnx- Fe 2~(4@3)xO 4 (0.00pxp0.3) which have cubic symetry,406 and the magnetic and electrical conduction mechanism of Co 1~xMnxFe 2 O 4 spinel,407 single phase samples of LaBa 2 Fe 3 O 8`x ([0.20pxp0.83) have been synthesised and characterised by oxygen content, XRD, magnetic susceptibility and 57Fe Mo� ssbauer spectroscopy.408 Two new low temperature synthetic routes for the preparation of perovskite related materials have been proposed, based on electrochemical oxidation and the exothermic reaction ofNO 2 withNH 4 ` ions at moderate temperatures.409 The structure of Mg2` doped a-Fe 2 O 3 has been determined using XRD and Rietveld refinement,410 and a 57FeMo� ssbauer spectroscopic study of a single crystal of Fe 3 O 4 has been reported.411 A systematic study of polycrystalline Sr 2 RuO 4`d has been carried out by varying the oxygen stoichiometry,412 Bi 2 Ru 2 O 7 and Bi 3 Ru 3 O 11 have been tested as electrodes on stabilised zirconia at temperatures as low as 227 °C.413 Cobalt oxides and cobaltates The morphology of ultrafine LiCoO 2 powders prepared by a sol–gel process has been shown to be pH dependent from an SEM and AFM study,414 the mechanism of chemical and electrochemical intercalation of O 2 in La 2 CoO 4 has been investigated, 415 a study of the stability, oxygen non-stoichiometry and transformations of Bi 2 Sr 2 CoO 6`d has shown that annealing outside the stability region [[850 °C at Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 507–533 523high oxygen partial pressures and at all temperatures at low oxygen partial pressures (pO 2 1.5]10~3 to 2]10~2 atm)] Bi 2 Sr 3 Co 2 O 9 and the binary compounds of the Bi 2 O 3 –SrO systems are formed and the kinetics at 700–850 °C of this transformation have also been studied,416 the resistivity of La 1~xBaxCoO 3 has been found to decrease with an increase in x, reaching a minimum at x\0.5, and then increases with further increase of x,417 the mechanism and kinetics of oxygen reduction on porous La 1~xSrxCoO 3 using ac impedance have indicated a reaction mechanism in which O 2 is reduced chemically at the porous mixed conductor electrode surface, followed by di§usion of oxygen through the mixed conductor to the electrolyte,418 a study of the kinetics of oxygen isotopic exchange between La 0.7 Sr 0.3 CoO 3~d and oxygen gas has been carried out,419 the monodispersed mixed spinel (CoxMn 1~x) 3 O 4 has been prepared using a room temperature coprecipitation method, with the formation based on an aggregation mechanism,420 the permeation kinetics in a ceramic membrane based on the composition SrFeCo 0.5 O 3.25~d have been measured as a function of both temperature and oxygen pressure gradient across the membrane,421 as well a study of the stability of oxygen permeable membranes of perovskite La 0.3 Sr 0.7 CoO 3~d,422 the crystal structure, thermal and electrical properties of Pr 1~xSrxCoO 3~d perovskite oxides have been reported.423 The isothermal sintering kinetics of nanocrystalline Dy 1~xSrxCoO 3~y (x\0.6) have been studied to give a sintering activation energy of 5.255]104 J mol~1.424 Nickel oxides, hydroxides and nickellates The substitution of a large amount of manganese for nickel in the spinel LiMn 2 O 4 to give LiMn 2~xNixO 4 has been studied using DTA and TGA data, and the variation of Ni content to manganese indicates high thermal stability of the nickel substituted spinel.425 The thermal behaviour of Li 1~yNiO 2 has been studied using TGAand DSC, and the decomposition mechanism elucidated by XRD data from the thermally decomposed products,426 a study of the lithium loss kinetics from polycrystalline LixNi 1~xO in the temperature range 900–1500 °C has shown that at 900 °C the rate of Li 2 O evaporation was controlled by lithium ion di§usion, which was the rate limiting step up to 1300 °C, above this the evaporation process depended on the demixing of Li 2 O and NiO at the particle surface,427 the spinel precipitation in Al-doped Ni 1~xO has been studied by XRD and electron microscopy.428 A Jahn–Teller distortion of the NiIII site in b(III)-HNiO 2 phase has been identified from Ni K-edge EXAFS as in LiNiO 2 and NaNiO 2 .In the c-oxyhydroxide (which contains both NiIII and NiIV), the Jahn–Teller distortion is not observed because of the NiIII/NiIV hopping.429 Copper oxide superconductors and related materials In addition to a review on cuprate superconductors430 there have been many studies reported on cuprate superconducting materials and their analogues including: the crystal growth of the Nd rich Nd 1`xBa 2~xCu 3 Oz solid solution;431 the preparation of Sr 3 Cu 2 O 5 Cl;432 the mechanism of the Bi-2212 to Bi-2223 transformation within the Bi 2 O 3 –SrO–CaO–CuO system;433 the synthesis and characterisation of Gd 1~xPrxBa 2 Cu 3 O 7~y;434 the pressure controlled synthesis and investigation of the Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 507–533 524thermodynamic and kinetic aspects of the formation of Hg 0.8 Re 0.18 Ba 2 Cu 3 O 8`d;435 a study of the transformation from Bi 2 Sr 2 CaCu 2 O 8 to Bi 2 Sr 2 Ca 2 Cu 3 O 10 ;436 a study of heavily Pb doped Bi 2 Sr 2 CaCu 2 O 8`d has shown that the critical current density increases dramatically beyond a Pb content of 0.4 per formula unit where electron microscopy reveals a two-phase microstructure;437 a study of the usefulness of YBa 2 Cu 3 O 7~d as cathode materials in high temperature solid oxide fuel cells;438 the synthetic pathways to, and structural rearrangements of Sr 2~xAxCuO 2 F 2`d (A\Ca, Ba);439 the preparation of YBa 2 Cu 3 O 7~d using a microwave melt texture growth process;440 the processing and characterisation of YBCO grain boundaries obtained by liquid phase removal methods;441 the high temperature deformation of Bridgman melt textured YBCO;442 the thermal stresses and microcracking of YBCO melt processed bulks;443 the simulation of phase transitions in (RE)Ba 2 Cu 3 O 7~d (RE\rare earth);444 the growth and morphology of (M)Ba 2 Cu 3 O 7~d (M\Tm, Lu);445 the microstructures of YBa 2 Cu 3 O 7~d which have been partially melted in air and quenched from ca. 1000 °C;446 the e§ect of Ca doping on the superconductivity in Yb 1~xCaxBa 1.6 Sr 0.4 Cu 3 O 6`d;447 the study of solid oxide electrochemical doping (SOED) of Ca into YBCO;448 the oxygen di§usion and surface exchange kinetics in YBa 2 Cu 3 O 6`d;449 the investigation of the relaxation of T# in YBa 2 Cu 3 O 6.38 with increasing oxygen order in the CuOx planes under 1GPa pressure;450 the Ka/Kb intensity ratios of Cu, Y and Ba in YBa 2 (Cu 1~xZnx) 3 O 7~d (x\0, 0.033) have been used to show that there is little Cu(3d)–O(2p) hybridisation in either of the superconductors; 451 annealing studies on Zn and Fe doped epitaxially grown YBCO thin films have shown that these materials are sensitive to deoxygenation processes, and the oxygen kinetics appear to depend on the valence and site of the dopant;452 and a substantial increase in flux pinning has been reported in Li doped grains following thermal neutron irradiation.453 Zinc oxides The growth kinetics of ZnO nanometer sized particles from colloidal suspensions have been shown to follow the Lifshitz–Slyozov–Wagner theory for Ostwald ripening.454 References 1 E.Kaxiras and S. 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ISSN:0260-1818    

DOI:10.1039/a805982i

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

27 Mechanisms of reactions in solution† N. Winterton The Leverhulme Centre for Innovative Catalysis, Department of Chemistry, University of Liverpool, Liverpool, UK L69 3BX 1 Introduction Full understanding of a chemical change requires the identity and nature of intermediates formed between reactants and products to be identified and knowledge of the processes of bond making and breaking and of electron transfer to be established.Inorganic reaction mechanisms are thus of intrinsic interest. Moreover, they underpin our understanding of key environmental, biological, medical and industrial processes. This review of the 1998 literature does not, except for studies of particular importance or interest, cover homogeneous catalysis or redox processes involving organic substrates, organic reactions of the heavier p-block elements, or fluxional, photochemical, electrochemical, solid-state or heterogeneous processes.A new journal, Inorg. React. Mech., and a special issue of Transition Met. Chem. are devoted to inorganic mechanisms. Activation volumes in solution,1 models for electron transfer,2 the use of density functional theory3 and electron-transfer reactions of polyoxymetalates4 have been reviewed and the application of genetic-algorithm5 and non-linear optimization methods6 to the treatment of kinetic data described.Stopped- flow EXAFS has been used to study7 short-lived intermediates. Studies of processes on the femtosecond timescale8–18 are noted. 2 Redox reactions A series of papers on electron transfer in proteins19–23 and a review by Marcus on theory and experiment in electron-transfer reactions24 have appeared in a special issue of J.Electroanal. Chem. Long range electron transfer Di§erent sites on cytochrome bc 1 are associated with oxidation and reduction,25 with the Fe–S protein component of cytochrome bc 1 acting as an electron shuttle between †Dedicated to the memory of Professor Bob Hay, an esteemed friend and colleague.Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 535ubiquinol and cytochrome c. Proton-coupled electron transfer involving the [Fe 3 S 4 ] cluster in Azobacter vinelandii ferredoxin I has also been studied.26 Electron transfer from the reduced [Fe 2 S 2 ] centre in putidaredoxin to the camphor-oxidising cytochrome P450 occurs27 via Cys-39 and Asp-38 of the former, across the interprotein interface to Arg-112 of the latter, and finally to the haem-group via a propionate.Electron transfer between cytochrome b 5 and hemoglobin (Hb) involves prior complexation28 and is faster for the a-chains than for the b-chains. Binding and redox processes have also been studied29 in complexes between cytochrome c and a horseheart apomyoglobin reconstituted with artificial terminally-functionalised porphyrins.Spacer flexibility a§ects photoinduced electron transfer in semisynthetic myoglobins into which [Ru(bipy) 3 ] moieties have been incorporated, linked via XNHC(O)(CH 2 ) 2 , spacers [X\CH 2 , (CH 2 ) 3 O(CH 2 ) 2 or (CH 2 ) 3 O(CH 2 ) 2 O(CH 2 ) 2 ].30 The rate constant for electron transfer between an iron(II) sub-unit of a mixed-metal Hb hybrid and a zinc(II)- or magnesium(II)-substituted sub-unit varies little with temperature from 5 to ca. 200 K.31 Redox processes have been described involving pseudoazurin (from Achromobacter cycloclastes) and cytochrome c from two sources,19 cytochrome c and cytochrome c peroxidase,20 cytochrome c and dtpa-modified zinc myoglobin21 and galactose oxidase,32 various cytochrome c’s and an iron(II) complex,33 FeIII reduction in Coprinus macrorhizus peroxidase by dithionite,34 and oxidation of a plastocyanin from the green alga Ulva pertusa by [Fe(CN) 6 ]3~ or [Co(phen) 3 ]3`.35 A distance attenuation factor for electron transfer, b, of 1.1^0.4Å~1 is reported36 for mono-, di- and trivaline-bridged [RuII(bipy) 2 (cmbipy)]2`–[CoIII(NH 3 ) 5 ] donor –acceptor pairs.Polymer conformation a§ects37 the quenching kinetics by 1,1@- didodecyl-4,4@-pyridinium in the presence of L-tyrosine esters of RuII* bound to poly(1- vinylimidazole). The distance attenuation factors for electron transfer in Ru-modified b-sheet proteins are smaller than for those with helical structures.22 The increase in the rate on cooling from 220 and 170K of CuI to RuIII electron transfer in the blue copper protein, azurin, isolated from Pseudomonas aeruginosa, with the [Ru(im)(bipy) 2 ]- moiety bound at His-83, has been ascribed38 to reductions in the lengths of hydrogen bonds which mediate the coupling.A reduction in electron-transfer rate from CuI to native and the His-46-Asp mutant forms of azurin, RuIII-modified at His-83, reflects39 changes associated with the binding site for copper rather than driving-force or reorganisation-energy e§ects. Intramolecular (pH-dependent) electron transfer from CuI to [RuIIIL(terpy)]His-59-modified Scenedesmus obliquus plastocyanin (L\bipy, 4,4@-Me 2 bipy, 4,5,4@,5@-Me 4 bipy) occurs40 with a Cu–Ru separation of 15.6Å.Rates of intramolecular electron transfer in [RuIII(NH 3 ) 4 L]-modified manganese(II)-substituted cytochrome c increase along the series, L\NH 3 , py, isn, to an extent less than expected for a driving-force change of 0.3 eV, suggesting41 that the electron transfer step per se may not be rate-limiting.Volume profiles for reversible intramolecular electron-transfer in two cytochrome c’s modified, respectively, at His-33 (horse heart) and His-39 (Candida krusei) by trans-[Ru(NH 3 ) 4 L] (L\NH 3 , py, isn, 3,5-lut) are asymmetric,42 an e§ect associated with increased electronic coupling arising from pressure-dependent changes in non-bonding interactions.These and related processes are the subject of further theoretical and modelling studies.23,43–49 Protein folding associated with electron transfer has been reviewed.50 Distance-dependence51–58 and other aspects59–62 of long-range electron transfer mediated by DNA have been studied, including further work on non-covalently- Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 536bound donors and acceptors.63–68 *-[Ru(phen) 2 (dppz)]2` and *-[Rh(phi) 2 (5- Xphen)]3` (X\amidoglutaryl) show low site-selectivity when individually intercalated with aDNAdecamer duplex, though together each binds specifically towards the end of the duplex, with a four base-pair separation.The cleavage of DNAand RNA by metal complexes has been reviewed69,70 and the role of complexes of iron,71 cobalt,72 nickel,73 platinum,74 copper,71,75–79 ruthenium68,80 (and its inhibition by Mn2`81) and rhodium82 described.Studies of electron- and energy-transfer across M(l-L)M@ bridges83–102 have included [Ru(terpy)(l-L)Ru(terpy)]5` (L\L1, L2, L3)84 [Ru(bipy) 2 (l-L4)Ru- (bipy) 2 ]5`,86 [Mmer-Ru(NH 3 ) 3 (bipy)N2 (l-L5)]3` (R\H, Me, Cl)85 [MRu(NH 3 ) 5N2Ml- (C 5 H 4 N-2)(C 2 H 2 )n(C 5 H 4 N-2)N]5`,92 [Ru(bipy) 2 (l-tppa)Os(bipy) 2 ]4`,93 [Ru(terpy)- (l-tpypyz)RhCl 3 ]2`,96 [MRu(terpy)N2 (l-L6)(MLn)]5` MMLn \[M@(bipy) 2 ] (M@\Cu, Os), [Ni(H 2 O) 4 ]N,97 [Ru(bipy) 2 (l-L7)Cu(phen)(H 2 O)]3`,87 [Ru(Bu5 2 bipy) 3~nM(l- L)Pt(Bu5 2 bipy)Nn]2` (H 2 L\1,10-phenanthroline-5,6-diol),94 [Ru(bipy) 2 (l- L8)Mn(H 2 O) 4 ]5`,88 [RuIII(NH 3 ) 5M(NC)MIII(N 4 )(CN)NRuII(NH 3 ) 5 ]6` (M\Cr, Co, Rh; N 4 \a tetraazamacrocycle),89 [Fe(CN) 4 (l-L)Ru(bipy)(l-L@)Rh(terpy)LA]4` (L\L@\2,3-bpp or bipym, LA\N-methyl-4,4@-bipyridinium),90 [MFe(CN) 4N2 (l-g4- bmtz)]3~,91 and zinc porphyrin units linked covalently by 2,6-disubstituted 4,4@- diphenylethylene bridges98 and other groups.100 Long-range electron transfer in other donor (D)–spacer (S)–acceptor (A) systems continues to be actively investigated.Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 537Electron transfer in [RuII(bipy) 2Mphen-(S)(A)N]2` linked covalently to a 2,4-dinitrophenylamino moiety (A) via amino acid bridges (S)103 involves a through-bond, r-hole tunneling process without involvement of p-orbitals of aromatic substituents in the amino acid.Intramolecular and intervalence electron transfer Activated electron-transfer between two adjacent metal centres in the (crystalline) mixed-valence complex [M 3 O(O 2 CBu5) 6 (py) 3 ] (M\Fe) occurs at a rate lower than the infrared timescale at room temperature.104 For M\Mn, localised valence states are seen.Related studies of the [Fe 3 S 4 ]0 cluster in a 7Fe8S ferredoxin from Bacillus schlegelii have been reported.105 Changes in intramolecular electron-exchange coupling on protonation of l-O in [FeIII 2 O(O 2 CBu5) 2 Tp 2 ] I account106 for di§erences in Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 538bimolecular electron-transfer kinetics involving I or [IH]` and [Ru*(4,4@-Me 2 - bipy) 3 ]2`. Energy-transfer rates have been measured within complexes formed from [Ru(Bu5 2 bipy) 2 (X-bipy)]2` and [Os(Bu5 2 bipy) 2 (Y-bipy)]2`, linked by double (X/Y\adenine/thymine moieties) or triple (X/Y\cytosine/guanine moieties) hydrogen bonds.107 A related hydrogen-bonded complex between a zinc–porphyrin and 1,8: 4,5-naphthalenetetracarboxylic diimide has also been studied.108 Intervalence transfer in dinuclear cyano-bridged complexes of FeI–FeII, FeI–MnII 109,110 RuII–RuIII 111 and PtII–FeIII,112 and in pyz-bridged complexes of OsII–OsIII 113 and RuII–RuIII 114 have also been investigated.Dipole-moment changes arising from intervalence transitions in [MII(CN) 5 (l-CN)M@III(NH 3 ) 5 ]~ (M\Fe, Ru; M@\Ru, Os) lead to estimates of the charge-transfer distances which are only 50–65% of the geometric separation between the metal centres acting as donor and acceptor.115 Other workers have questioned these conclusions116 and reported additional data on [RuIII(NH 3 ) 5 (l-NC)MLn]m` [MLn \M@II(PPh 3 ) 2 Cp (M@\Ru, Os), n\3; Cr0(CO) 5 , n\2].Reviews117,118 and new studies are reported (some119–127 in special volumes of Coord. Chem. Rev.) on the dynamics (including on the fs timescale9,10) of photochemical and photophysical processes, including those devoted to carbonyl–a-diimine complexes of MnI,119,128–130 ReI,119–121,128,131–142 RuII 120,121,136 and OsII,134 other mononuclear M(a-diimine) complexes, M\Fe,143 RuII,122,123,126,144–161 OsII 162 and PtII,127,163 (some with electron-donor133,140,164 and acceptor103 moieties substituted in the a-diimine and electron-acceptors134–136 bound to the metal) and catecholate complexes of CoIII 165,166 and CrIII.167 The dpypyz-localised MLCT state in [Ru*(bipy) 2 (dpypyz)]2` increases the basicity of the non-bonded nitrogens su¶- ciently to complex to PtIV and RhIII.126,154 The delayed photoejection of an electron from [Cu(NH 3 ) 3 ]` involves168 a dinuclear exciplex, [Cu 2 (NH 3 ) 5 ]2`. Photoreduction of [PtII(N 3 ) 2 (dppp)] first gives [PtI(N 3 )(dppp)] and then [Pt0(dppp)] via intramolecular electron transfer.169 Outer-sphere electron-transfer and self-exchange reactions A two-term rate law has been established170 for the reaction, 2 [RuII(NH 3 ) 5 - (isn)]2`]I 2 ]2 [RuIII(NH 3 ) 5 (isn)]3`]2 I~, associated with outer-sphere electron transfer to form I 2 ~ or I 2 ~]I~, respectively.The self-exchange rate constant for I 2 /I 2 ~ was estimated to be 5]102M~1 s~1. Outer-sphere (and Br~-inhibited) bromine oxidation of [RuII(NH 3 ) 5 (pyzH)]3` is 500-fold slower than for [RuII(NH 3 ) 5 - (pyz)]2`.171 Self-exchange rate constants for the couples Br 2 /Br 2 ~,171 [Fe(CN) 5 (NO 2 )]3~@4~,172 [CuL 2 ]2`@` (L\2,9-Me 2 -4,7-Ph 2 phen173 and L\L9),174 and [W(CO) 5 L]0@~· 175 have been reported.Sowrey, MacDonald and Cannon176 have used NMR line-broadening to measure self-exchange kinetics for the couple [Fe 3 O(O 2 CBu5) 6 (py) 3 ]`@0 in dichloromethane.Ferrocene derivatives are included in a wider study177 of self-exchange kinetics involving organic substrates. Outer-sphere electron transfer is proposed for the two-step two-electron reduction of [NiIV(L10)]2`by nitrite and the single-step two-electron reduction of [NiIV(L11)]2`by nitrite at pHp6.0,178,179 for reduction of [IrCl 6 ]2~ in alkali,180 and for the oxidations of [Mo(CN) 8 ]4~ by [Mn(cdta)]~.181 Sykes and co-workers182,183 have studied the outer-sphere reactions of single- and double-cube complexes, e.g.[M 3 InS 4 (H 2 - Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 539O) 12 ]5` (M\Mo, W) and [Mo 6 ES 8 (H 2 O) 18 ]8` (E\As, In) by [Co(pydca) 2 ]~. Helicate complexes184 of copper(I), [CuI 2 (L12) 2 ]2`, may be reversibly oxidised to [CuII 2 (L12) 2 ]4` via a mixed-valence intermediate. Li and Ho§man185 have challenged the assignment186 of inverted Marcus behaviour for the bimolecular reductive quenching of [Ru*(a-diimine) 3 ]2` by phenolate. (See also refs. 187–190.) Rates of [e(H 2 O)n]~ and [CO 2 ]~· reduction of [Ru(bipy) 2 (g2-L)]2` (L\2,2@: 3@,2A: 6A,2@@@- quaterpyridine),191 have been measured, with rates of protonation for an unbound N in the product, [Ru(bipy) 2 (g2-L·)]2` also reported.The ligand-centred radical anion resulting from pulse-radiolytic reduction of [Ru(menbipy) 3 ]2` reduces [Co(acac) 3 ] with an enantioselectivity of 2.7, favouring * over ".192 In water, enantioselectivity in the quenching of the excited state of rac-[TbIII(pydca) 3 ]3~ by *-([)-[Ru(phen) 3 ]2` increases193 with applied pressure.Diastereomeric outer-sphere ion association between [Co(dien) 2 ]3` and [CoL(ox) 2 ]2~ (L\gly, b-ala),194 and between [Fe(4,4@- Me 2 bipy) 3 ]2` and [P(C 6 Cl 4 O 2 -O,O@) 3 ]~ 195 has been studied. Stereoselective electron transfer between rac-[CoIIILX]` (L\alamp, promp, X\H 2 O,196 py, Him, pyz197) and optically active complexes of iron(II) takes place via an inner-sphere mechanism with a carboxylato moiety on L acting as the bridge.Stereoselection in metal complex –protein binding,198 energy-199 and electron-200 transfer is noted. Inner-sphere electron and atom transfer [FeIII 2 (l-O)(phen) 4 (H 2 O) 2 ]4` II oxidises [S 2 O 3 ]2~ in the presence of [phenH]` by an inner-sphere process201 to give [S 2 O 3 ]~·, [Fe(phen) 3 ]2` (an inhibitor of the reaction) and [Fe(phen) 3 ]3` with [S 2 O 3 ]~· undergoing a self-reaction to give [S 4 O 6 ]2~.In the absence of phen, [Fe(S 2 O 3 ) 2 ]~ and [Fe(S 2 O 3 ) 3 ]3~ are involved.202 A similar mechan- Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 540ism (via NHOH· to N 2 O) occurs with II and NH 2 OH.203 Inner-sphere processes are also suggested204 from the kinetics of N 2 H 4 oxidation of [MnIV 2 (l-O) 2 (l- O 2 CMe)(bipy) 2 (H 2 O) 2 ]3`.Reduction of trans-[Pt(CN) 4 Cl 2 ]2~ by [Pt(NH 3 ) 4 ]2` in chloride-ion solution follows two parallel pathways,205 both involving chloridebridged transition states, [(H 2 O)PtII(l-Cl)PtIVCl] and [ClPtII(l-Cl)PtIVCl]. Hindmarsh, House and van Eldik206 have reviewed redox processes involving platinum(II) and platinum(IV) complexes.The rate of PtIV reduction and cytotoxicity are qualitatively associated for a series of anticancer PtIV complexes.207 Atom-transfer is the dominant primary process127,208 in the quenching of the triplet state [3Pt 2 (l- H 2 pop) 4 ]4~ by [Co(CN) 5 X]3~ (X\Cl, Br, I) in aqueous acid, giving aquacobalt(II) complexes and [PtIII(l-H 2 pop) 4 X 2 ]4~. Inner-sphere mechanisms are proposed for N-bromosuccinimide oxidation of [CoII(nta)(H 2 O) 2 ]~ 209 [and its iron(II)-catalysis210] and of [CrIII(hedta)(H 2 O)].211 Oxidation of dithiophosphates, [P(OR) 2 S 2 ]~, L, by copper(II) proceeds212 via [CuIIL 2 ].Rate di§erences, M\Mo[W, for the oxygen-atom transfers, [MO 2 (mnt) 2 ]2~]P(OR)nR@3~n ][MO(mnt) 2 ]2~] P(O)(OR)nR@3~n, are associated213 with di§erences in the ease of reduction of MVI to MIV and the strength of the M––O bonds.Transfers of O from MoVI to P,214 MnV to P,215 FeIV to P,216 RuIV to P,217 and from NV or NIII to MoV 218 and ClI, IIII or NV to RuIII 219 are also reported. ReVII and SII engage in reversible oxygen-atom transfer.220 Substituent e§ects and rate correlations for RR@S to RR@SO oxygenation by oxochromium( V)221 point to sulfide to CrV outer-sphere electron transfer as being ratedetermining.The OsVI-to-P N transfer between [OsNCl 2 (terpy)]` and PPh 3 222 involves the intermediate [OsIV(NPPh 3 )Cl 2 (terpy)]`. Steric e§ects dominate rates of –– NRtransfers223 in imide–imine metathesis reactions involving [Mo(––NR) 2 Cl 2 (dme)].Miscellaneous redox reactions In basic solution, NO reduction of [CuL 2 (H 2 O)]2` to HNO 2 and [CuL 2 ]` is faster for L\Me 2 phen than for L\phen,224 and involves NO binding inhibited by OH~ or the bu§er conjugate base. Disproportionation of [Cu(NO)(TpM4,H)] III to [Cu(NO 2 )(TpM4,H)] and N 2 O involves225 rate-determining attack of NO on III.Dinitrosyl intermediates are proposed226 in similar reactions of a manganese tropocoronand, [Mn(NO)(L13)]. Kinetics have been studied for reactions involving NO, NH 3 and CuII,227 N 2 O and [Co(tpp)]2~,228 N 2 and NbIII,229 NH 2 OH, HNO 3 and TcVII 230 or PuIV,231 NH 3 and MnVII,232 and N 2 H 4 233 or NH 2 OH234 and RhIV. The couple [CuII]/[CuIII]` is responsible for the accelerating e§ect of trace quantities of copper on the processes, 3I~]CrVI]1.5 I 2 ]CrIII,235 and 2 I~]2VV]I 2 ]2VIV.236 Photolysis of [FeX(H 2 O) 5 ]2` (X\Cl, OH) at j\347nm leads to [Fe(H 2 O) 6 ]2` and Cl·, [Cl 2 ]~·, [ClOH]~· and OH·.237 Related studies238,239 on the photoreduction of [IrCl 6 ]2~ are reported.Chromium(II)- Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 541catalyzed aquation of cis-[Cr(N 3 )(bipy) 2 (H 2 O)]2`,240 reduction in the rate of pyridine exchange on one-electron reduction of [Fe 3 O(O 2 CCBu5) 6 (py) 3 ],176 RuIII-catalyzed oxidation of [Fe(CN) 6 ]2~ by [IO 6 ]5~,241 reaction of OBr~242 or OCl~243 with NpIV and PuIV, of NCS~ and TcVII,244 H 3 PO 2 and TlIII,245 PuV disproportionation,246 RuIII-catalyzed oxidation of SeIV by MnIII,247 CrVI 248 and MnVII 249 oxidation of AsIII and FeIII 250 or CrVI 251 oxidation of PIII have all been described.Geochemically- and environmentally-relevant studies of the oxidation of iron(II),252–255 reduction of chromium(VI),255 iron(III),256 iron(IV),257 manganese(III)258 and mercury(II)259 and of the autoxidation of sulfur(IV)260–266 have appeared.Only in very acidic droplets is chromium concluded260 to have any role in atmospheric aqueous-phase SIV autoxidation. In a revised mechanism of manganese-catalyzed autoxidation of [HSO 3 ]~ 261 [MnIII(OH)(H 2 O)n(l-O)MnII(H 2 O)n]2` initiates a chain reaction by complex formation with [HSO 3 ]~ to produce chain-propagating [SO 3 ]~· several orders of magnitude faster than iron(III).The synergistic e§ect of iron(III) is associated with an increase in [MnIII] and of the catalytically-active dimer. [Fe(H 2 O) 4 (l-SO 3 )(l-OH)Fe(H 2 O) 4 ]3` forms in the reaction between sulfite and excess iron(III).262 Zinc(II) catalyzes the oxidation of RSH by CrVI 267 in acetate bu§er with [CrO 3 (SR)]~ formation from zinc–thiolate and acetatochromate species being ratedetermining.L-Cysteine, glutathione and DL-penicillamine (RSH) oxidations by [Cr(OO)(H 2 O) 5 ]2` proceed268 via a thiol complex, with [Cr(OOH)(H 2 O) 5 ]2` and [CrO(H 2 O) 5 ]2` as intermediates. O 2 participates in related processes involving RSH and [CrVO(ehba) 2 ]~.269 Propagation in the oxidation of AcrH 2 by H 2 CrO 4 involves270 reaction of AcrH 2 and a CrV species to give (by H· abstraction) chaincarrying AcrH· and CrO2` in competition with H~ transfer giving Cr3` and AcrH`.Other oxidants have also been studied.271 Other sulfur- (and selenium-272) centred oxidations have been described involving [FeO 4 ]2~,273,274 [Fe 2 (CN) 10 ]4~,275,276 tungstocobaltate(III),277,278 [Co(ox) 3 ]3~,279 [Co(OCrO 3 )(NH 3 ) 5 ]`,280 and complexes of NiIV,281 VV,282 FeIII,282,283 ReV 284 and CeIV.285 Ferrate(VI) oxidation of cyanide yields cyanate and then nitrite.286 Oxidation of H 2 ox by [CrVO(ehba) 2 ]~ involves287 mixed-valence CrV–CrVI complexes.Several studies relevant to water oxidation, both on photosystem II288–290 and model systems,291–294 have also appeared. Reactions of oxygen-containing oxidants and reductants Mechanistic studies continue to focus on a better understanding of enzymatic processes involving binding and activation of oxygen-containing oxidants with oxidases,20,32,295–298 oxygenases,299 superoxide dismutases,80,300 peroxidases301–304 and catalases and related model systems.305–307 [FeII(tpp)], generated in situ in MeOH, and O 2 react to gives308 [O 2 ]~· and [FeIII(tpp)]` via slowly-dissociating [FeII(O 2 )(tpp)] rather than by outer-sphere electron transfer.p-Nitroperbenzoic acid oxidizes [FeIII(tdcpp)] to either [FeV–– O] or [FeIV–O·] in the presence of methanol or [FeIV(––O)(tdcpp`·)] in its absence.309 The rate constant for formation of [MoVIO(O 2 )(tmp)] from [MoVIO(tmp)] and O 2 is ca. 105-fold smaller310 than those for porphyrins of iron(II). Peroxocomplexes are key intermediates311,312 in reactions of binuclear dicopper complexes, [CuI 2 (L14)(NCMe) 2 ]2`311 and [CuI 2 (L15)]2`312 in Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 542which L14 and L15 contain a benzene moiety capable of hydroxylation linking two chelating groups. An intramolecularly-bridged ‘‘side-on’’ l-g2:g2 peroxo intermediate was detected312 using resonance Raman spectroscopy.Aliphatic hydroxylation, observed for [CuI(L16)]` IV and O 2 , proceeds via rate-determining formation of a l-g2: g2 peroxodicopper(II) intermediate from IV and a monomeric superoxocopper( II) species.313 (See also refs. 314–316.) In the presence of excess oxygen, primary rather than tertiary C–Hof an N-isopropyl moiety of (mononucleating) Pr* 3 [9]aneN 3 (L) in [(CuIIL) 2 (l-g2: g2-O 2 )] is oxygenated, via317 a novel [MCuII(L[H)N2 (l-1,1- OO) 2 ] intermediate.The e§ect of L and R on the reactivity of a series of alkylperoxocobalt( III) complexes, [CoIII(OOR)L], with alkanes has been studied.318 (Reviews are noted for other metal-complex catalyzed oxygenations and hydroxylations of organic substrates299,319–323 not otherwise surveyed.) O 2 and the diiron complex, [FeII 2 (l-L17)(l-O 2 CR)(O 2 CR)(N) 2 ] (N\pyridine- or imidazole-derived ligand) react in non-polar solvents according to kinetics324 that are first order in each reagent. (See also ref. 325.) Monomeric and dimeric peroxoiron(III) complexes are proposed326 as intermediates in O 2 oxidation of [FeII([15]aneN 4 )(H 2 O)n]2`. Solvent dissociation is not rate-determining in the reversible binding of O 2 at the vacant site of some new cyclidene complexes of cobalt(II),327 with the rate of binding being a§ected primarily by the size of the lacuna andO 2 -dissociation rates by the axial base.The rate law328 for the oxidation of [CuI(fum)] by O 2 involves only reaction via [Cu(O 2 )]` formed from, and reacting with [Cu(H 2 O)n]` to give [CuII(H 2 O)n]2` and H 2 O 2 .Related studies329 on [NiIII(edta)]~show that ligand oxidation is first order inO 2 and second order in the complex. [P 2 O 8 ]4~ formation from [H 2 PO 2 ]~ and O 2 , in the presence of [CrO 4 ]~ 330 and [Ru*(bipy) 3 ]2`331 has been described. In acid, ozone oxidation of [Mn(H 2 O) 6 ]2` proceeds (without OH· formation) via oxygen-atom transfer to give [MnIVO(H 2 O)n]2`332 which itself reacts rapidly with excess manganese(II) to give manganese(III).The terminal label in O 3 isotopomers scrambles unimolecularly only at elevated temperatures.333 In the absence of added H 2 O 2 , surface-catalysed formation ofH 2 O 2 controls334 the initiation ofO 3 decompo- Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 543sition in aqueous acid.The kinetics of the liquid-phase reaction between O 3 and H 2 O 2 335 and SnHR 3 336 have been reported. [O 3 ]~· is a proposed intermediate337 arising from the photolysis of strongly alkaline oxygenated [S 2 O 8 ]2~ solutions. The hitherto unexamined equilibrium O 2 ]HO 2 ~]OH~H2[O 2 ]~·]H 2 O, has been studied.338 Rate contants have been reported339 for the copper-catalyzed dismutation of [O 2 ]~·, a process believed to be an important environmental sink for superoxide formed on photooxidation of organic matter dissolved in surface waters.Superoxide dismutation by copper–zinc enzymes has been reviewed300 and studied for model systems involving complexes of iron,340,341 manganese341,342 and copper.343,344 In the presence of sulfate, [HO 2 ]· oxidizes [Ni([14]aneN 4 )]2` to [NiIII([14]- aneN 4 )(SO 4 ) 2 ]~.345 Reaction of H 2 O 2 with [CrO 4 ]2~ gives rise to the chromium(V) complexes [Cr(g2- O 2 ) 3 (OH)]2~, [CrO(g2-O 2 ) 2 (H 2 O)]~ and [CrVO(g2-O 2 )(H 2 O)n]` as well as the known [Cr(g2-O 2 ) 4 ]3~.346 Related studies on dioxovanadium complexes have also been reported.347 Complexes with singlet O 2 co-ordinated to vanadium(V) have been proposed348 in oxidations involving H 2 O 2 , vanadium(V) and acetic acid.Reaction of H 2 O 2 with MnIII,332 NpVI 349,350 and NpVII,349 [S 2 O 8 ]2~ oxidation of PuVI,351 NpVI,352 UIV 353 and mixed-valence dimeric complexes of ruthenium,354,355 and CoII to CoIII oxidation of [CoIIW 12 O 40 ]6~ by [HSO 5 ]~ 356 have been reported. Catalysis by sulfito-bound cobalt(III) of the self-decomposition of peroxynitrite, 2 [ONOO]~ ]O 2 ]2 [NO 2 ]~,357 a related process involving [FeIII(TMPyP)],358 [MnIII(TMpyP)],306 catalysis of perborate oxidations by [Fe(CN) 6 ]3~ 359,360 or [MO 4 ]2~ (M\Mo, W)361 peracetic acid decomposition catalysed by cobalt(II) and vanadium(V)362 and the disproportionation of H 2 O 2 by manganese,363–370 dinuclear manganese–copper371 and iron372 complexes, by [WO 4 ]2~ 373 and calcium hydroxide374 have been described. The [Fe(H 2 O) 6 ]2`-catalyzed chain reaction, [Rh(H)L]2` V]Bu5OOH]H`][RhL]3`]MeH]Me 2 CO (L\[14]aneN 4 , Me6 [14]- aneN 4 ), involves375,376 rapid hydrogen-atom abstraction from V by methyl radical.Espenson and coworkers and others377–379 report further studies of [ReMeO 3 ]- catalyzed oxidations of arenes,380 alcohols381 (and isomerisation of allylic alcohols382), alkenes,377–379,383,384 hydrazones385 and silyl enol ethers.386 Other non-metal redox reactions The redox chemistry of peroxynitrite, which plays an important role in host defence against invading pathogens, continues to receive intense study.306,357,358,387–393 In neutral solution containing [HCO 3 ]~ and [NO 2 ]~, [ONOO]~ and CO 2 form an adduct,[ONOOCO 2 ]~.This homolyses to generate [CO 3 ]~· which oxidises nitrite to Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 544[NO 2 ]· (which subsequently hydrolyses to [NO 3 ]~ and [NO 2 ]~).387 Carbonateradical production from the same adduct (and subsequent reaction with a series of substrates to give superoxide) has been proposed independently388 in the formation of [O 2 NOO]~ from [ONOO]~.Oxidation of [ONOO]~ by [CO 3 ]·~, [N3 ]·~ and [ClO 2 ]· has also been studied.389 In carbonate- and oxygen-free water, [ONOOH] gives390 ca. 10% [OH]· in addition to nitric acid (formed by collapse of the radical pair [OH·/NO 2 ·]). An oxygen-atom shift from O to N is the preferred394 mechanism from kinetic isotope e§ect studies of the HONO 2 to HNO 3 isomerization.While homolysis is considered unlikely,395 decomposition of [O 2 NOO]~ has been reported to involve, 396 in part, homolysis to give NO 2 · and [O 2 ]·~. Further studies of the oxidation of nitrous acid by H 2 O 2 397,398 and of nitrite by oxygen399 and detailed studies of the thermal decomposition of nitramide400–403 have also appeared.Stanbury404 has reviewed the oxidation chemistry of aqueous hydrazine. New studies of the oxidation by iodine of N 2 H 4 405 and of NH 2 OH406 over wide pH ranges have been reported. At pH\1, I 2 reacts directly with [N 2 H 5 ]`. A proposed I 2 N 2 H 4 adduct loses I~ and H` (general base-assisted) in the rate-determining step at higher pH. A series of pulseradiolysis studies of aminyl and other inorganic nitrogen radicals has appeared.407–409 [NH 2 ]· and O 2 react more rapidly than previously reported with equilibrium formation of [NH 2 O 2 ]· which protonates and then isomerizes, giving NO which then reacts to give [ONOO]~.407 The self-reaction of [NH 2 ]· and its reaction with NH 2 NH 2 have also been studied.408 Iodide is oxidised by N-chloro compounds (for a general review see ref. 410) via rate-determining transfer of Cl` to I~411 in a process subject to general-acid catalysis. Rates of oxidation of aqueous arsenic(III) to arsenic(V) are relevant to the persistence412 of arsenic(III) entering the environment from geothermal sources. [(H 2 N) 2 CS], [(H 2 N)(HN––)CSO 2 H] and [(H 2 N)(HN–– )CSO 3 H] all give [SO 4 ]2~ on oxidation with oxyhalogen species.413 Oxidation of hypotaurine (H 2 NCH 2 CH 2 SO 2 H) and taurine414 (and formaldehyde415) by [ClO 2 ]~ and of cysteine by [BrO 3 ]~ 416 has also been studied.N-Bromination without C–S cleavage occcurs in the reaction of acidic bromate and taurine.417 Thiosulfate reacts with chlorine dioxide with irreversible formation of [S 2 O 3 ClO 2 ]·2~, which then forms [S 4 O 6 ]2~ and [ClO 2 ]~ via [S 4 O 6 ]·3~.418 Ab initio calculations on [HSO 3 ]~ and SO 2 oxidation by H 2 O 2 ,419 [SO 3 ]2~ oxidation by [BrO 3 ]~,420 S-nitrosothiol ascorbate reactions,421 and formation of chlorine-atom adducts with R 2 S422 have been described.In studies relevant to the chemistry of the troposphere, rate constants for the equilibrium, Cl·]Cl~a[Cl 2 ]~·423 for the reactions of Cl· and [Cl 2 ]~· with water (in two independent studies423,424) and for [Cl 2 ]~· and [OH]· 424 have been reported. Oxidation of elemental mercury by aqueous [OCl]~/HOCl (using chloramine as a reservoir) may also have implications for atmospheric chemistry.425 Oxidation of [ClO 2 ]~ by HOBr to produce ClO 2 and [ClO 3 ]~ involves426 a steady-state intermediate ([HOBrOClO]~or [HOBrClO 2 ]~) which gives [BrOClO] or [BrClO 2 ] by a general-acid catalyzed reaction, the latter reacting with OH~ (or H 2 O) to give [ClO 3 ]~ and Br~ and with [ClO 2 ]~ to give 2ClO 2 and Br~.The disproportionation of chlorous acid in strong acid427 and the role of chloride ion in the reaction of chlorous and hypochlorous acids428 are important industrially. ClOO and Cl formation and OClO reformation in the photochemistry of chlorine dioxide has been studied Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 545in aqueous and acetonitrile solution.429–431 Bromate formation, of serious environmental concern in the ozonation of drinking water, arises from the oxidation of bromine atoms by O 3 .432 The kinetics have been reported433 of reaction of [e(H 2 O)n]~ and [BrO 3 ]· (generated from [SO 4 ]~· and [Br- O 3 ]~) and of [BrO 3 ]· dissociation to [BrO]·.Further studies have appeared dealing with geminate recombination following triiodide photodissociation434 and dissociation of iodine encapsulated in cyclodextrin.11 Oscillating reactions and chemical chaos The activation energy for the oscillation of a wholly inorganic Belousov–Zhabotinskii (B-Z) reaction based on435 [BrO 3 ]~–[H 2 PO 2 ]~–MnII–[Fe(phen) 3 ]2`–H 2 SO 4 has been estimated from the variation with temperature of the oscillating period.Related B-Z systems with amino acids or peptides,435,436 organic acid–ketone mixtures,437–440 cyclohexanedione441,442 (a related metal-free 1,4-dihydroxybenzene –acidic bromate system exhibits Landolt-type dynamics443), bromomalonic acid,444 vanillin,445 pyrogallol446 or gallic acid447,448 as organic substrates have also been described.Oxidation of bromomalonic acid by CeIV 449 proceeds via two pathways, one at a low [CeIV]: [bromomalonic acid] (leading to bromoethenetricarboxylic acid, CO 2 and Br~) and one when this ratio is high (leading to complete oxidation to CO 2 ).A ‘‘Radicalator’’ model simulates observations in a B-Z system, controlled by the concentration of radicals (such as the malonyl radical) rather than by bromide, in which Hacac introduces an induction period before oscillations start.450 Photoinduced bifurcations of the B-Z reaction in a continuous-flow stirred tank reactor (CSTR) above a critical level of irradiation,451 stochastic resonance on illumination of a B-Z medium unable to support sustained wave propagation,452,453 light- flux and flow-rate control in a model photosensitive system,454,455 transient response to pulsed visible light,456,457 modulation of behaviour under continuous illumination, 458 and periodic and chaotic chemiluminescence459,460 in ruthenium(II)-catalysed B-Z reactions and the e§ect of an external emf461,462 of oxygen,463–465 and of oxygen and surface-to-volume ratio on chemical oscillations in mm-sized droplets466 have all been described.Pulsed-visible light responses467 and other phenomena468 of the [Fe(CN) 6 ]4~–H 2 O 2 –[SO 3 ]2~ system have also been examined. Chemical waves have been studied in catalytic membranes,469 polymer gels,470,471 beads of ion-exchange resin,472,473 and capillaries in which surface tension e§ects are suppressed.474 Redox indicators475 and a platinum indicator electrode476 reveal the otherwise invisible spatial patterns of uncatalyzed bromate oscillators.Microelectrodes also reveal macroscopic concentration gradients (arising from a coupling of chemical reaction with micromixing) in the most turbulent zone of a B-Z reaction in a CSTR.477 Electrode geometry e§ects have been noted478 for the oscillatory behaviour observed in [IO 3 ]~ alkaline reduction.Chemically-coupled Marangoni instabilities induced by surface tension gradients have also been proposed.479 Mixing-e§ects on the kinetics of the ferrous-ion reduction of nitrite in a CSTR have been studied.480 Spatial and temporal inhomogeneities in the coupling of two identical B-Z oscillators481 and growth dynamics of Turing patterns in a ClO 2 –I 2 –malonic acid–polyvinyl alcohol reaction –di§usion system482 have been reported.The suppression of oscillations in the Bray–Liebhafsky (BL) reaction, the [IO 3 ]~-catalyzed disproportionation ofH 2 O 2 ,483 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 546by the removal of iodine vapour from the reaction solution, confirms the importance of iodine interphase transport.484,485 Iodine autoxidation in acidic solution486 is also relevant to an understanding of the BL system. The 1H NMR signal for solvent water in the BL reaction displays a periodic chemical shift and line splitting.487 The role of bulk water was also revealed in related studies using D 2 O.488 pH and potential oscillations have been studied for the H 2 O 2 –[SO 3 ]2~–[CO 3 ]~–H 2 SO 4 system.489 Studies of the Briggs–Rauschler reaction, [IO 3 ]~–H 2 O 2 –organic substrate–Mn@ (n`1)`,490–492 reveal photoinduced chaos linked to photoautocatalysis of HIO 2 ,491 and suppression of oscillations by Br~.492 The autocatalytic exothermic oxidation of thiourea by chlorite493 also displays a range of non-linear behaviour. 3 Substitution High pressure kinetic approaches to the study of the interactions of small molecules with transition metal centres494 and the chemistry of ruthenium–polyaminocarboxylate complexes495 have been reviewed. Six-co-ordination Wilkins has produced a short overview496 of substitution processes in labile octahedral metal complexes.The forward and back rate constants (25 °C; pH 3.5) for the equilibrium, [Cr(H 2 O) 6 ]2`]bipyma[Cr(bipym)(H 2 O) 4 ]2`, are497 1.6]108M~1s~1 and 4.3]104 s~1, respectively. Complexation of [M(H 2 O) 6 ]n` VI (M\Fe; n\3) by 2-acetylcyclohexanone,498 2,6-dimethylheptane-3,5-dione,499 acetoacetamide500 [via the amide-tautomer; copper(II) and dioxouranium(VI) also studied] and a 3-hydroxy-4-pyridinone,501 of VI (M\Al; n\3) by dihydroxychalcones, 502 amino acids,503 b-diketones,504 and of VI (M\Ga; n\3) by substituted 8- hydroxyquinoline ligands505 have all been reported. Formation and dissociation of [Mn(bipy)(H 2 O) 4 ]2` at pH[6 involves506 only the unprotonated ligand. 1: 1 complex- formation of [Mg(H 2 O) 6 ]2` with monoprotonated methyl thymol blue is more rapid than that with the neutral ligand, ascribed507 to ligand intramolecular hydrogen- bonding.Reversible complexation between [Co(H 2 O) 6 ]2` and 2-(2- aminoethyl)benzimidazole and related ligands has also been reported.508 Binuclear complexation between VI (M\Co; n\2) occurs with both [CoIII(tetren)(Hpyca)]3` and [CoIII(pyca)(tetren)]2` (via an I$ mechanism) whereas forM\Ni (n\2) reaction only proceeds with the deprotonated form.509 Dissociation of [MCoIII(pyca)(tetren) NM(H 2 O) 4 ]4` is acid catalysed for M\Co but not for M\Ni.Bimetallic intermediates are also formed510 in the transfer of a tetradentate ligand from [MIICl 2 L] [L\bis(2-picolyl)-1,3-dithiopropane] to CuII. Studies involving ternarycomplex formation between [CoIII(pyca)(NH 3 ) 5 ]2` and [NiL(H 2 O) 6~n]m,511 and between [CoIII(nsa)(en) 2 (NH 3 )]2` and [Ni(H 2 O) 6 ]2` or [NiL(H 2 O) 4 ]2`,512 have also been reported. The substitution, [Ru(H 2 O) 6 ]2`]L][Ru(H 2 O) 5 L]2`]H 2 O (L\CH 2 –– CH 2 ) is confirmed513 to be I$.The ligand exchange, [Ru(H 2 O) 5 L]2` ]L@][Ru(H 2 O) 5 L@]2`]L (L\L@\CH 2 –– CH 2 , dmso, CO) involves rate-determining loss of axialH 2 Oand the intermediacy of trans-[Ru(H 2 O) 4 LL@]2`.Slower loss of equatorial H 2 O leads to cis-[Ru(H 2 O) 4 LL@]2`. Substitution of adenosine, L, in Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 547Fig. 1 Heterotrinuclear dichromium monorhodium aqua ion.518 [Rh(OH)(H 2 O) 5 ]2` to give [Rh(OH)(H 2 O) 3 L]2` is514 an I! process in the pH range 3.0–4.3.An initially-formed ion-pair between [CrIII(OH)(H 2 O) 5 ]2` and [ox]2~ gives515 [CrIII(ox)(OH)(H 2 O) 3 ] via an I! process. Anation of [CrIII(ox)(H 2 O) 4 ]` by glycine516 has also been studied. Substitution by dma of terminal H 2 O in [CrIII 3 (l3 - O)(l-O 2 CR) 6 (H 2 O) 3 ]` is dissociatively activated517 with changes in rate reflecting the influence of the r-donor character of RCO 2 on the strength of the cis-Cr–OH 2 bond.Acid cleavage of [RhCr 2 (l-OH) 4 (H 2 O) 9 ]5` (Fig. 1) giving [M(H 2 O) 6 ]3` (M\Rh, Cr) in a 1: 2 ratio follows518 [H`]-dependent and [H`]-independent (I$) pathways. Bridge-cleavage studies have also been reported for [MCr(phen) 2N2 (l-OH)a(l-O) 2~a]n` (a\2, n\4; a\1, n\3; a\0, n\2),519 [MCo(Hbig) 2N2 (l-OH) 2 ]4`520 and [O(tmpa)VIV(l-O)VVO(tmpa)]3`.521 Anations of [Cr(acac) 2 (H 2 O) 2 ]` and [Cr(N 3 )(acac) 2 (H 2 O)] are522 substantially more facile for N 3 ~ than for SCN~.Acid-catalysed aquations of [Cr(N 3 )(acac) 2 (H 2 O)] and [Cr(N 3 ) 2 (acac) 2 ]~ and anations of [Co(salen)(H 2 O) 2 ]`523 and of [Co(Hbig) 2 (H 2 O) 2 ]3`524 have been reported. Forward and reverse rate constants, k& and k$, for aqua substitution in [Fe(CN) 5 (H 2 O)]3~ by S- and N-heterocycles, 525 4- and 3-pyOR526 (R\H, Me), 3-methyl and 3-phenylsydnone527 have been measured.For 4-pyOH k& is [103-fold slower than for L\4-pyOMe, ascribed to tautomerism in the former. The sydnones are also relatively unreactive, arising from their mesoionic character rather than from a change in mechanism.The aqua-substitutions [IrCl 5 (H 2 O)]2~]MeCN,528 [Ru(NH 3 ) 5 (H 2 O)]2`]2-pyX (X\Cl, F)529 and [Ru(edta)(H 2 O)]~]2-mercaptobenzoic acid,530 cysteine, HSCH 2 CH 2 OH, glutathione531 and 4-sulfanylpyridine,532 have all been studied. For 4-pySH, both Nand S-bound species are formed with second-order kinetics.532 The N-co-ordinated species isomerizes slowly to the thermodynamically-favoured S-bound isomer by a dissociative process. Chloride-loss from cobalt(III) complexes continues to be studied, particularly by House and co-workers.533–536 House has concluded533 that no correlation exists between ground-state d C0–C- and the solvolysis reaction rate for chloropentaaminecobalt( III) complexes.The loss of the first Cl~ from trans-(R,S)(S,R)- [MCl 2 (223-tet)]` VII (M\Co, Cr) gives trans-aquo, chloro-complexes.534 However, forM\Co further aquation leads to trans-to-cis-b isomerization and, forM\Cr to Cr–N cleavage.Hg2`-assisted chloride-loss from VII, M\Co, gives trans-[M(223- tet)(H 2 O) 2 ]3` followed by trans-to-cis-b isomerisation. The kinetics of chloride loss from cis-a- and cis-b-[MCl 2 (amp)]` (M\Co, Cr),535 trans-[CrCl 2 (Me 8 [14]ane- Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 548N 4 )]`537, cis-[RuClL(dppe) 2 ]`538 and from cis,trans-[RuCl 2 (Me 2 NCH 2 CH 2 PPh 2 ) 2 ],539 aquation of [Co(diox) 2 (NO 2 )X]n,540 [Cr(NCS)L]~, [CrL(NCSHg)]` (L\R-pdtrp, edtrp),541 and [Co(ebb)L@]2` (L@\Gly, Ala),542 and cis- [Co(CO 3 )([15]aneN 4 )]`,543 anation of [Cr(ox) 2 (H 2 O) 2 ]~ by CO 2 ,544 [Co(cyclen)( NH 3 )(H 2 O)]3` by N 3 ~545 and of [RuI(NH 3 ) 5 ]2` by SCN~546 and the substitution of X in [PtIVMe 3 X(N–N)] (N–N\phen, bipy) by Y\SCN~, N 3 ~547 have also been described.The relative rates of Hg2`-assisted removal of Cl~ from [CoCl(N) 5 ]2` [(N) 5 \tetren, (tacn)(diamine), mer-(dien)(diamine) and related complexes] suggest536 that the trajectory of the incoming H 2 O is a major factor influencing the interchange mechanism.trans-OSO 2 -O (L) in [CrL(salen)L@]n has little labilising e§ect on the loss of L@\NCS, N 3 , py, Him.548 Similar rate constants for bipy dissociation from [Fe(bipy) 3 ]2` and from [Fe(bipy) 2 (dmf) 2 ]3` suggest that the latter is also a complex of low-spin t 2' 6 iron(II).549 Complexation with 1,4,7-tris(2,2@-bipyridin-5-ylmethyl)- 1,4,7-triazacyclononane was also studied.Rate constants for the processes, [NiX(bipy) 2 (dmf)]`]bipy][Ni(bipy) 3 ]2`]X~ and [NiX 2 (bipy) 2 ]]bipy] [Ni(bipy) 3 ]2`]2X~ (X\Cl, NCS) are significantly smaller than for [Ni(bipy) 2 (dmf) 2 ]2`]bipy][Ni(bipy) 3 ]2`.550 Ternary complex formation between [Cr(nta)(H 2 O) 2 ] and Mordant Orange 3551 or 3-phenylazo-5-sulfosalicylic acid,552 [Ni([12]aneN 4 )(H 2 O) 2 ]2` and phen, en or Gly,553 and between [FeL(NCMe) 2 ]2` ML\N,N@-dimethyl-2,11-diaza-[3.3](2,6)pyridinophaneN and dbsq554 and the formation of [Al(edta)F]2~ in an I! process555 have been studied.Axial dmso,556,557 CO,558 1-Meim,559H 2 O,560 NO,561 andONO561 substitutions in six-co-ordinate phthalocyanine556,558 and porphyrin557,559–561 complexes of iron(II),556–558 chromium(III),559,560 and ruthenium(II)561 have been analysed kinetically. The resolution of the sequential replacement of dmso of [Fe(pc)(dmso) 2 ] by pyor Him into two observable processes has been confirmed.556 Formation of [Fe(pc)(CN) 2 ]2~ from [Fe(pc)(CO)(dmso)] does not occur via formation of [Fe(pc)(CN)(CO)]~. The rate of cyanide binding558 is governed by the rate of CO loss, with dmso-lability reduced by the trans-CO.Base hydrolysis Syn(N),anti(O)-, syn(O),anti(N)- and syn(N),syn(O) forms of [CoM(S)-AlaON(cyclen)]2` equilibrate in base more rapidly than they hydrolyse to give [(S)-AlaO]~ and cis- [Co(OH) 2 (cyclen)]`562 via rate-determining loss of [AlaO]~ from ring-opened [CoMg1-(S)-AlaON(OH)(cyclen[H)]. OH~-catalyzed hydrolyses of [CoX(NH 3 )(cyclen)] n` have also been studied.545 The proportions of trans-[CoX(NH 3 ) 4 (NH 2 Me)]2` VIII (X\NO 2 , ONO) formed in competition between nitrite and water in the base hydrolysis of VIII (X\Cl, Br, NO 3 ) di§er563 from those observed in similar competition studies with [CoX(NH 2 R) 5 ]2` (R\H, Me).Isomerisation of the nitrito to the nitro forms of VIII in base is an intramolecular conjugate-base process564 occurring with full stereochemical retention.Base hydrolysis of [Co(OSO 2 -O)(tetren)]` is faster than for [Co(OSO 2 -O)(Meim)(en) 2 ]`, the latter only displaying isomerization (in excess sulfite) to [Co(SO 3 -S) 2 (en) 2 ]~.565 Isomerization in base566 of trans-meso- to trans-primary, rac-[CoMeL(H 2 O)]2` (L\5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca- 4,11-diene) involves inversion at a secondaryN–H.Base hydrolysis (by a D CB pathway) of ab(R)-[Co(tetren)(dmf)]3` is more rapid567 than that of the Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 549aa-isomer (and occurs without hydrolysis of dmf). Equilibrium constants have been measured568 for 1: 1 complex formation between OH~ and [Pt(N–N) 2 ]2` (N–N\bipy, bipyz, bipym, bipdz). Solvent and medium e§ects have been described for the base hydrolyses of [CoL(tetren)]2` ML\O 2 C(CH 2 )nNH 2 , n\1, 2569 or O 2 CC 6 H 4 OH-p570,571N and of [Fe(N–N) 3 ]2` (N–N\1,2-diazabutadiene or Schi§ base ligands).572,573 Four-co-ordination [Pd(H 2 O) 4 ]2` reacts reversibly with both [HSO 4 ]~ and [SO 4 ]2~ whereas [Pd(OH)(H 2 O) 3 ]` reacts predominantly with [HSO 4 ]~.574 Biphasic kinetics575 are observed for parallel formation of [Pd(O-O 2 CCH––CHCO 2 H)(H 2 O) 3 ]` IX from [Pd(H 2 O) 4 ]2` and H 2 mal and [Hmal]~.Ring closure gives a product also formed by irreversible attack of free ligand via the olefin moiety on IX. Complex formation between [Pd(H 2 O) 4 ]2` and (S)-carboxymethyl-L-cysteine576 and thioglycolic acid577 and between tu and [PtL(H 2 O) 2 ]2` (L\en, phen)578 has also been described.Bicarbonate- inhibited and bicarbonate-promoted reactions account for the complex kinetics observed579 for reactions of [PdX(dien)]` with [HCO 3 ]~. Hydrogen-bonding e§ects have been noted580 on rates of substitution of bipy by en in [Pd(bipy)(R 4 en)]2` (R\H, Me, Et, Ph).In chloroform, the rate of dmso exchange in [PtMe(dpa)(dmso)]` is little a§ected581 by either ion-pair formation or ligand deprotonation. In methanol, the complex shows little nucleophilic discrimination in reactions with a series of charged nucleophiles. The use of PtII (and more recently PtIV 582,583) complexes in anti-tumour therapy584 continues to be a major motivation for mechanistic and related structural work, including studies of interactions with DNA and synthetic oligonucleotides.585–590 Chloride loss from cis-[PtCl 2 (NH 3 )(CyNH 2 )]591 Ma metabolite of the orally-active PtIV anti-cancer drug, cis,trans,cis-[PtIVCl 2 (OAc) 2 (NH 3 )(CyNH 2 )]N, from cis- [PtCl 2 (NH 3 )(n-Mepy)] (n\2, 3),592 and displacements of Cl~ in [PtII 2 Cl 2 (hdta)]2~ by inosine593 have been described.The rates of substitution of Cl~ by H 2 O, and of H 2 O by 5@-GMP cis to 2-Mepy in cis-[PtCl 2 (NH 3 )(2-Mepy)] are ca. 2–12-fold slower than for the analogous processes with the corresponding 3-Mepy complex.594 The kinetics of the reaction of cis-[PtCl 2 (NH 3 )(n-Mepy)] and glutathione,594 of L-cysteine595 and cysteine-derived ligands596 with [Pt(en)(H 2 O) 2 ]2`,595,596 and [Pd(en)(H 2 O) 2 ]2`,596 of L-MetH with cisplatin597 and with [PtCl(15N-dien)]`598 have also been measured.[Pt(dien)(L-MetH-S]2` X, dominant at neutral pH, forms [Pt(L-Met-N)(dien)]` at pH[8. In acid, [Pt(dienH-N,N@)(L-Met-S,N)]2` is produced (half-life of days at pH 4.0) which slowly forms X. Hydrolysis for both Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 550[PtCl(dien)]` and [PtCl(NH 3 ) 3 ]` (and fast reverse Cl~ anation for the aqua complex formed) has been characterised kinetically.599 Rate constants for the equilibrium, [PtCl(terpy)]` XI]L a[PtL(terpy)]`]Cl~ (L\NH 3 , pyridine derivatives)600 reveal the e§ect of basicity, p-acceptor character and steric e§ects of L.When L\thiol substituted with OH, CO 2 H, NH 3 `,601 the e§ect of hydrogen-bonding with the departing Cl~ is seen. Compound XI reacts with cysteine and thioglycolic acid according to second-order kinetics,602 whereas methionine is unreactive under identical conditions. Competitive binding603 between XI and the sulfur-containing peptide, Ac-Gly-Met, and 5@-GMP has also been studied. Pt–S cleavage in [Pt(terpy)MSCH 2 CH 2 N(CH 2 CH 2 NH 2 ) 2N]` at pH 7–8, leading to XI, is accelerated by MCl 2 in the order M\Zn[Cu[Ni.604 Rapid intramolecular disproportionation for dinuclear [MPt(en)ClN2M(l-L-S) 2N]2` [L\C 2 H 4 (NMeCSNMe 2 ) 2 ] yields [PtCl 2 (en)] and [Pt(en)L]2`.605 Bridge-cleavage of [PtII 2 Me 4 (l-SMe 2 ) 2 ] byMe 2 S in CH 2 Cl 2 gives [PtIIMe 2 (SMe 2 ) 2 ] without intermediate formation.606 Bulky tmtu in [PtCl(tmtu)L]` (L\en, dach) decreases both the rate of Cl~ hydrolysis and binding of 5@-GMP and the nucleopeptide r(GpG)605 compared with related reactions for cisplatin.The rate ratio of the two steps (ring opening and ligand loss) in cbcda removal in the acid hydrolysis of carboplatin, cis-[Pt(cbdca)(NH 3 ) 2 ], is greater at lower acidity.607 Displacement of cbdca from the carboplatin analogues, [Pd(cbdca)L] (L\en, bipy), by tu, tmtu, I~ or 5@-IMP occurs consecutively by associative processes.608 Equilibria relevant to the role of platinum complexes as anti-cancer therapeutic agents involving acid–base609 and outer-sphere complexation (‘‘supermacrochelate’’ formation) of platinum(II)–nucleotide complexes, such as cis- [Pt(dGMP) 2 (NH 3 ) 2 ]2~,610 cis-[Pt(dGuo)(dGMP)(NH 3 ) 2 ],611 cis- [Pt(dCMP)(NH 3 ) 2 ],612 or [Pt(5@-GTP) 2 (en)]n~ (formally n\8)613 have been described. At pH 5 and room temperature, [MPt(en)(5@-GTP) 2NLa(H 2 O)n]5~, in which La3` is bound by the two triphosphate residues, is more stable than [La(edta)- (H 2 O)n]~.613 Temperature dependent line-broadening of the 1H NMR spectrum (at pH 7) is ascribed to atropisomerization of the * and " head-to-tail forms, trapped by La3` complexation, and not to La3` exchange.Chelate-ring conformational e§ects on atropisomerization rates in [Pt(G) 2 L] (L\S,R,S,R-, S,R,R,R- and R,R,R,R-bmap, G\9-substituted guanine) have been reported.614 Mixtures of the A-frame complexes [Ni 2 (C––CH 2 )X 2 (dppm) 2 ] (X\NCS, Cl) redistribute X according to an I! process.615 The uncatalyzed cis–trans isomerization of [Pd(C 6 F 5 ) 2 (tht) 2 ] involves a non-rate-determining dissociation of tht and rate-limiting topomerization of the three-co-ordinate intermediate.616 cis-[PdRI(PPh 3 ) 2 ] (R\C 6 Cl 2 F 3 ) converts617 to the trans-isomer by four pathways, two of which are initial direct and solvent-assisted replacement of PPh 3 giving cis-[MPdRI(PPh 3 )N(l- I)MPdR(PPh 3 ) 2N] followed by rearrangement to the trans-isomer and cleavage by PPh 3 .Isomerization also occurs via pseudo-rotations in [PdRI(thf)(PPh 3 ) 2 ]. Ringclosure for cis-[PtPh 2 (CO)MPh 2 P(CH 2 )nPPh 2N], to give [PtPh 2MPh 2 P(CH 2 )nPPh 2N] is rapid for n\2, 3 but observable for n\1, 4.618 The doubling of the observed rate of reaction of an excess of (R)-1,1@-bi-2-naphthol (H 2 binol) with [Pt(O 2 CO)M(S,S)- chiraphosN] to give619 [Pt(binol)M(S,S)-chiraphosN] compared with that for (S)- H 2 binol is ascribed to di§erences in stereochemical interactions between ligands across the square plane of co-ordination.Replacement of chloride in trans- [AuIIIX 2 Cl 2 ]`@~ (X\CN~ XII, NH 3 XIII), by the thione moiety in 4-thiouridine Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 5515@-monophosphate or 4-thio-2@-deoxyuridine are associative interchange processes [with reactivities of the gold complexes some 106-fold greater than the analogous platinum(II) complexes]. For XIII, the rates are similar for the two ligands, indicating620 that the outer-sphere electrostatic association with the phosphate group adjacent to the preferred binding site does not further accelerate an already rapid process.For XII, the nucleotide reacts significantly more slowly than the thiodeoxyuridine. In related reactions with thione-containing single-strand 16-omer oligonucleotides, XIII reacts more quickly than with the corresponding nucleoside whereas XII reacts more slowly,621 the e§ect being primarily reflected in di§erences in *St.The acceleration of the substitutions, [Fe 4 S 4 X 4 ]2~]EtSH][Fe 4 S 4 X 3 - (HSEt)]~]X~ (X\Cl, PhS) with increasing [lutH`] arises from protonation of the cluster core, probably at l3 -S. Core diprotonation further labilises the complex to thiolate exchange whereas, for [Fe 4 S 4 Cl 4 ]2~, chloride substitution is inhibited.622 At very high [lutH`], a kinetically characterized623 thiol dissociation from triprotonated [FeS 2 (SH) 2 (SPh) 2 (HSPh)] to give [FeS 2 (SH) 2 (SPh) 2 ] may lead to reduction of H` to H 2 or of added C 2 H 2 to C 2 H 4 .Intermediate formation and rates of assembly of the [Fe 4 S 4 ]-centre have been studied in the reaction of the apo form of the high potential iron protein from Chromatium vinosum with iron(II) and thiol.624 Five-, seven- and higher-co-ordination Rapid formation of an isolable 2: 1 *R-thepc12 (L18):EuIII intermediate, in which L18 is bound only by the OH moieties of eight pendant CH 2 CH(Ph)OH groups, is followed by a much slower first-order process to give the eight-co-ordinate 1: 1 complex.625 Rate constants for the "a* [Eu(pydca) 3 ]3~ interconversion in a racemic mixture have been obtained626 using time-resolved chiroptical luminescence.The transition state for formation of [FeII(CO)(PhBzXy)]2` from [FeII(PhBzXy)]2`, Fig. 2, involves almost complete association of CO into the cyclidene cavity and is followed by a change in spin state at the six-co-ordinate iron(II).627 CO loss involves a late transition state, with a change of spin state, movement of FeII out of the ligand plane and partial CO dissociation.CO photodissociation from a fully reduced cytochrome bo 3 628 involves CO transfer from Fe to Cu B . Rates of O 2 and CO binding to a series of capped iron porphyrin complexes629 and of H 2 O rebinding to ferrous sperm whale myoglobin630 have also been reported. Pyridine for water substitution in trigonal bipyramidal [Cu(Me 3 tren)(H 2 O)]2` follows an I! mechanism whereas the analogous process for [Cu(Me 6 tren)(H 2 O)]2` is more complex, involving two parallel competing paths, one a dechelation reaction.631 The log of the second-order rate constant for the I! axial substitution of SbPh 3 in square pyramidal [CoIIIL 2 (SbPh 3 )] (L\4,5-dichloro-1,2-benzosemiquinonediiminato),632 is a linear function of the nucleophilic reactivity of the entering ligand.The kinetics have been reported for axial co-ordination of im, Meim to [Fe(tmtp)Cl] in acetone,633 concerted coordination of two py to the axial sites of photoexcited [Ni(oep)],634 for the reaction ofNO 2 , NO,O 2 and py with [MnII(tpp)] following photodissociation of [MnIII(tpp)(ONO)] in toluene, 635 for NO][MIIMmeso-tetra(p-X-phenyl)porphyrinN] (M\Co, Mn; X\H, NO 2 , OMe),636 NO][CoPc]637 and for NO dissociation from [MII(tpp)(NO)] (M\Fe, Co).638 [ReIII(CN) 7 ]4~ is the ultimate product, formed via [ReVO 2 (CN) 4 ]3~, Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 552Fig. 2 Iron(II) cyclidene complex [FeII(PhBxXy)]2`.627 from [ReV 2 O 3 (CN) 8 ]4~ and CN~ after a fast pre-equilibration.639 Thermal reactions of photogenerated [M(CN) 7 (OH)]4~ with 5-NO 2 phen (M\W)640 and triethanolamine (M\Mo, W)641 have also been studied.Mechanisms of reactions of other high co-ordination number lanthanide and actinide complexes are discussed in the following two sections. Ligand exchange The rate constant for water exchange on [Al(OH)(H 2 O) 5 ]2`642 is 104-fold greater than that for [Al(H 2 O) 6 ]3`, a process also studied computationally.643 Water lability for [AlL(H 2 O) 4 ]`, where L are geochemically important ligands,644 is greater for methylmalonate than for [ox]2~. Limiting dissociative exchange for water cis and trans to the inert bridgingOHin [Rh(H 2 O) 4 (l-OH) 2 Rh(H 2 O) 4 ]4` occurs with similar rates.645 Exchange646 of H 2 O in p- and t-[CoXL(H 2 17O)]3` (L\tren, cyclen, NMecyclen; X\NH 3 , H 2 O) and for OH~ in [Co(OH)(tren)X]n` (X\NH 3 , n\2; X\OH~, n\1) is much slower than for [Co(p-OH)(tren)(t-H 2 O)]2` and [Co(OH)(cyclen)(H 2 O)]2`. Ligand–OH~ exchange proceeds via an aqua–conjugatebase complex, [Co(NR 2 )(H 2 O)]2` in equilibrium with [Co(NHR 2 )(OH)]2`.Associative water exchange in [PtMC 6 H 3 (CH 2 NMe 2 ) 2 -2,6N(H 2 O)]` is 107-fold faster647 than for [Pd(H 2 O) 4 ]2`. The faster exchange for H 2 O approximately trans to l-oxo in [Mo 3 (l3 -S)(l3 -O) 3 (H 2 O) 9 ]4` compared with that forH 2 Oapproximately trans to l-S arises648 from a pathway involving [Mo 3 (l3 -O) 3 (l3 -S)(OH)(H 2 O) 8 ]4`.Ab initio calculations have been made for water exchange,643,649–651 density functional theory applied to intramolecular oxygen exchange in [UO 2 (OH) 4 ]2~ 652 and molecular dynamics simulations for water exchange on Li` and Na`.653 Calculation649 replicates the experimentally observed D process for water exchange in [M(H 2 O) 6 ]3` (M\Al, Ga) whereas forM\In, an A or I! process is preferred.(New experimentally estimated lower limits for water exchange for [In(H 2 O)n]3`, [Lu(H 2 O)n]3` and [Zn(H 2 O)n]2` of 1]107 s~1, 1]107 s~1 and 5]107 s~1 respectively, were also reported.649) The structure of [MII(H 2 O) 7 ]2` (M\first-row transition metal), a putative intermediate or transition state for water exchange, is computed650,651 to be a pentagonal bipyramid distorted in the equatorial plane.Associative processes are feasible for water exchange for MII with less than seven d electrons, with cis-attack Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 553Fig. 3 Two diastereoisomeric representations (H 2 O molecules omitted) of [Eu(dotem)(H 2 O)]3`.660 predicted for VII (d3), FeII (d6) and CoII (d7) and trans-attack for CrII (d4).Solvent exchange for [Mn(NCR) 6 ]2` becomes less associative with increasing bulkiness of R.654 Reactivity towards dissociatively activated cyanide exchange in [MOX(CN) 4 ]n~ (M\WIV,MoIV, ReV, TcV, OsVI, X\O2~, OH~, H 2 O,CN~, HCN) follows the order MIV[MV[MVI.655 Oxygen exchange in [H 5 IO 6 ],656 ligand exchange in [VO(His) 2 ]657 and [GdIII(ATP) 2 (H 2 O) 2 ]5~ 658 have also been studied.The search for improved contrast agents for magnetic resonance imaging659 is dependent on understanding and control of water exchange rates.660–664 In a signifi- cant advance, rate constants for the dissociative exchange of water in the square antiprismatic M, Fig 3(a), and twisted antiprismatic m, Fig. 3(b), diastereoisomers660 of [Eu(Ln)(H 2 O)]3` (Ln\dotam) have been found to di§er by a factor of ca. 200. Water exchange has also been studied for [Gd(ttaha)(H 2 O) 2 ]n`,661 neutral [Gd(dtpabmea)( H 2 O)]662 and [Gd 3 (taci[3H) 2 (H 2 O) 6 ]3`.663 Related studies of proton exchange and water proton relaxivity have also been reported.663–675 Inter- and intra-molecular exchange of L and of F~ in [UVIO 2 LF 3 ]n~ (L\[ox]2~, [CO 3 ]2~, [pyca]~) has been studied further.676 Reactions of co-ordinated ligands and linkage isomerism [CoL(NH 3 ) 5 ]3` (L\carboxylate O-bonded phthalamic acid) slowly solvolyses in aqueous acid, whereas the amido N-bonded linkage isomer undergoes amido-N to amido-O as well as amido-N to carboxylato rearrangement in addition to solvolysis. 677 In excess sulfite, cis-[CoL(OSO 2 -O)(en) 2 ]` (L\Hbim, Meim) isomerizes565 by loss of L and formation of trans-[Co(SO 3 -S) 2 (en) 2 ]~.cis-Labilisation by O-bound sulfite and steric acceleration were noted. The keto-form of [CoMO 2 CC(O)MeN(NH 3 ) 5 ]2` gives [CoMO 2 CC(SO 3 )(OH)MeN(NH 3 ) 5 ]` on reaction with [SO 3 ]2~ and [HSO 3 ]~ rather than ligand substitution.678 Chen and Shepherd show that complexes formed between [RuII(hedta)(H 2 O)] and L\pyz,679 2,3- Me 2 pyz,680 pym,681 pydz,682 6-azauridine683 are fluxional, with the Ru centre migrating between various N-bonded and j2-bonded sites.[ReBr(CO) 3 L] (L\bbip, bmbip, bdmbip, btmbip,684 or mcpt, mcpmt, mmtt, bmtt685), [MReBr(CO) 3N2 L] [L\bis(terpy) ligands],686 fac-[PtXMe 3 L] (L\bip687 or mcpt685), [M(C 6 F 5 ) 2 (mcpt)] (M\Pd, Pt),685 [Ru(edta)L]~ (L\benztriazole)688 and trans- [Pt(o-tol)(PEt 3 ) 2 (N 3 Ar 2 )]`,689 [PdMeX(bppy)]690 and tris(pyrazolyl)triazine pallad- Annu. Rep.Prog. Chem., Sect. A, 1999, 95, 535–591 554ium complexes691 Mbut not [W(CO) 5 (pyz)]692N display similar metallotropic processes. Syn-anti isomerization of four-co-ordinate [Pd(g3-C 5 H 9 )(j2N,S-bmtpy)]` proceeds via an associative process involving attack by the non-co-ordinated sulfur at Pd.693 The linkage isomerization [W(CO) 5MjP-PPh 2 CH 2 CH(PPh 2 ) 2N] a[W(CO) 5MjP-PPh 2 CH(PPh 2 )CH 2 PPh 2N] is also associative, being 104-fold faster694 than [W(CO) 5MjP-PPh 2 CH 2 CH 2 P(p-tol) 2N]][W(CO) 5MjP-(ptol) 2 CH 2 CH 2 PPh 2N].[Co(O 2 CO)L]n` (L\tripodal tetradentate ligands, 5-dptma, i-dtma, aeida) hydrolyse in aqueous acid via [H`]-dependent and [H`]-independent processes, one involving the inert cis-[Co(O 2 CO)(H 2 O)L]n` formed from the intermediate cis-[Co(O 2 COH)(H 2 O)L](n`1)`.695 [Co(O 2 CO)(Meim) 4 ]` hydrolysis has also been studied.696 Rhodium(III)-amine-catalyzed methylglyoxal–lactate transformation involves697 a 1,2-hydride shift in a chelated substrate.Mechanistic reports have appeared related to RNA/DNA cleavage,698–707 nerve gas disposal,708–711 and prodrug and drug behaviour.712,713 More specifically, reports have appeared exploring metal-complex-promoted hydrolysis,689,700,701,703, 704,708–712,714–744 dephosphorylation730,745 or transesterification746–749 of carboxylic esters,712,715,718,725,726,729,733,740,746 peptides,709, 719,720,722,724,727,732,743 phosphate mono-,705,709,715,730,736,744 di-700,701,703,704,716,717,731,734,737,738,747, 748 or tri-esters,708,710,711,714,723, 726,727,729,735,741,742,744,749 phosphorothiotes,728 phosphonate mono-708 or di-esters710,721 by chiral palladacycles,715 mono-709 or di-nuclear738 cobalt(III), nickel(II)–terpy–poly(ethylenimine),748 copper(II)705,708,710, 714,717,721–723,726,728,729,745 complexes (including a dinuclear cis-diaqua-calix[4]- arene compound734 and a trinuclear ATP-AMP complex730), alkaline earth complexes of thiol-pendant crown ethers,746 magnesium(II),735 zinc(II),728 cadmium(II),728 palladium(II)-aqua,719,724,732,743 -thiolato,720 platinum(II),712 or zinc(II)-azamacrocycle714,717,726,733,740 complexes, mixed zinc(II)–lead(II)-compartmental ligand,741,742 cerium(IV),737 europium(III),747,749 gadolinium,728 and other lanthanide( III)718,736,739 ions, complexes of lanthanum(III) Mwith the bu§er Bis-Tris [2,2- bis(hydroxymethyl)-2,2@,2A-nitrilotriethanol]N,716 and of zirconium(IV).731 Catalytic activities718 for lanthanide ions in the hydrolysis of methyl and ethyl esters decrease along the series: CeIII, NdIII[SmIII[EuIII[GdIII[CeIV[PrIII[DyIII[TbIII[ ErIII[HoIII, TmIII[LaIII, LuIII, YbIII.Hydrolysis of phenyl acetates in the presence of [Zn([9]aneN 3 )(H 2 O)n]2` does not involve nucleophilic attack by a co-ordinated water or OH~, but water or OH~ attack on a co-ordinated ester.740 The catalysis by [Zn(OH)(tren)]` and [Cd(OH)(Me 6 tren)]` but not [Zn(OH)(Me 6 tren)]` of the hydration of acetaldehyde is ascribed to steric inhibition of the displacement of MeCH(OH) 2 by water.750 A nonadecapeptide (being the C-terminal segment of the protein myohemerythrin) which binds the cis-[Pd(en)(H 2 O)]2` moiety to His-5 and His-9 is regioselectively cleaved via the His-5 residue at the Val-3-Pro-4 amide bond.719 While K M values from the saturation kinetics seen for the hydrolysis of [P(O)(OAr) 2 O]~ (Ar\C 6 H 4 NO 2 -4), in the presence of Ln3` are similar,700 k#!5 increases 60–70-fold for La3` to Er3` and decreases for Yb3` and Lu3` (correlated with Ln ion diameter). Copper(II)-promoted hydrolysis of nucleoside 5@-triphosphates to nucleoside 5@-diphosphates and inorganic phosphate is further facilitated730 by the antiviral 9-[2-(phosphonomethoxy)ethyl]adenine.The role of weak aromatic-ring stacking interactions745 in influencing such processes has been discussed.Lanthanides more e¶ciently cleave 3@-AMP and 3@-GMP (to adenosine and guanosine respectively) Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 555compared with the 5@-mononucleotides.736 Zinc(II) has been implicated751 as a cofactor in the 1017-fold enzymatic acceleration of the decarboxylation of orotidine 5@- monophosphate (t 1@2 for the spontaneous process\7.8]107 years).Heterodinuclear iron(III)–zinc(II) complexes are responsible for the synergy701 between iron and zinc in the hydrolysis of the RNA-model phosphate diester, adenylyl(3@-5@)adenosine. Hydrolysis of the latter, accelerated 105-fold by [Co(trien)(H 2 O) 2 ]3`, displays703 a maximum at pH 7 and a D 2 O solvent isotope e§ect of 2.0.Adenylyl(3@-5@)adenosine hydrolysis, catalysed by lanthanum(III) or neodymium(III), may be further significantly accelerated by the addition of hydrogen peroxide.704 However, the accelerated phosphate mono- and di-ester hydrolysis observed706 in the presence of H 2 O 2 and [Cr 3 O(OAc) 6 (H 2 O) 3 ]` is not the mechanism by which this combination cleavesDNA (which is believed to involve hydroxyl radicals).Cleavage of a plasmid DNA can be accelerated by dinuclear macrocyclic complexes of lanthanides.699 Hydrolytic scission of phosphodiester linkages in a DNA 22-mer by cerium(IV) has been demonstrated.702 Methanol-loss from the phosphonate monoester, [PhP(O)(OMe)O]~, bonded as a bridging ligand in a dinuclear cobalt(III) complex (along with bridging peroxide), is accelerated ca. 1011–1012-fold compared with the non-co-ordinated substrate.752 Peroxide was shown not to be involved in intramolecular nucleophilic catalysis; rather, intramolecular catalysis by co-ordinatedOH~ is observed. The half-life for the hydrolysis of [P(O)(MeO) 2 (OH)] (calculated at pH 7) is reduced by CeIV from 8454 years to 22 min.737 While [Fe(S 2 MoS 2 )(bipy) 2 ] is a more active catalyst than [Ni(S 2 MoS 2 )(bipy) 2 ] in the hydrolysis of MeCN to MeC(O)NH 2 and MeC(O)OH,753 the Ni complex is the more selective towards acetamide formation.Base hydrolysis of O-bonded dmf in [Co(NH 3 ) 5 (dmf)]3` gives the formato ligand 104-fold faster than the free ligand.722 Aminoguanidinium hydrolysis,754 and urea755,756 and acetonitrile755 alcoholysis have been studied. Kaminskaia and Kostic� show756 that cis-[Pd(en)(H 2 O) 2 ]2` XIV accelerates 105-fold the conversion of urea and MeOH into methyl carbamate and ammonia, by initial urea binding to PdII, direct methanolysis of O- and N-bound urea, formation of N-bound carbamic acid, the methanolysis of the latter and the fast dissociation of NH 2 C(O)OMe.Intramolecular alcoholysis in cis-[PdL(H 2 O) 2 ]2` (L\2,6-dithiaoctane-1,8-diol) is 100-fold faster than for intermolecular alcoholysis by XIV. Amides of a-amino acids718 are more readily hydrolysed by cerium(IV) than by lanthanide(III) ions. Hydrolysis of azomethine moieties in trinuclear cobalt(II) complexes757 and of benzidine-Schi§ bases by MII, M\Fe, Co, Ni, Cu,758 has also been studied.The rate constant for reaction of OH~ with co-ordinated NO in [RuII(NH 3 ) 5 (l-NC)Os(CN) 4 (NO)] increases759 105-fold on one-electron oxidation of RuII. Metal-ion complexation with macrocycles The predominantly dissociative process for Li` exchange in [Li(C221)]` in methanol between 5 and 25 °C changes to predominantly associative between [30 and [60 °C.760 Tl` and hexacyclen behave similarly,761 whereas for hexamethylhexacyclen the process is dissociative over the entire temperature range.In MeCN–MeNO 2 mixtures, Li` exchange in [Li(C222)]` is bimolecular over the entire temperature range studied.762 Intra- and inter-molecular exchange kinetics for *[M(L18)]` Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 556(M`\Li`, Na`, K`, Cs`) in dmf763 and the solvent dependence of ligand exchange in [Ba(18-crown-6)]2` have also been described.764 Formation of a 1: 1 complex between Cs` and 5,11,17,23-tetra-p-tert-butyl-25,26,27,28-tetramethoxycalix[4]arene proceeds via initial complexation of its partial cone (pc) conformer followed by a change to the 1,3-alternate (alt,3) conformer, from which Cs` cannot directly escape.Exchange between the two Cs` binding sites in the latter involves interconversion to the pc complex.765 An intramolecular racemisation process has been characterised for the binuclear complexes [Lu 2 L(dmf) 5 ] (L\p-nitro- or p-tert-butyl-calix[8]- arenes).766 Acid dissociation kinetics for [CuL]2` (L\L19 767 and L20 768) have been described. In studies on ultra-rigid ligands,769 a lower limit for the half-life for the dissociation of the cross-bridged L21 from [Cu(L21)]2` in 1MHClO 4 at 40 °C is[6 years, the complex being ca. 100-fold less labile than even the square-planar red isomer of [Cu(tet-A)]2`. Substitution of CuII by NiII in [Cu(amben)] in dmf–[NR 4 ]X follows complex kinetics, with the overall reactivity trend order X\[ClO 4 ]~ \Br~\SCN~@Cl~ linked770 to the tendency to form nickel complexes with reduced co-ordination number. [FeIII(salmH)(H 2 O) 2 ]2` forms from the ligand and [Fe(H 2 O) 6 ]3` or [Fe(OH)(H 2 O) 5 ]2` by I! and I$ mechanisms, respectively.771 Iron(III) transfer from ferrioxamine B ([Fe(Hdfb)]`) to edta, catalyzed by [ox]2~,772 proceeds by four parallel paths involving [Fe(ox)(H 2 dfb)], [Fe(ox) 2 (H 3 dfb)]~ and [Fe(ox) 3 ]3~ which react with [H 2 edta]2~, with the observed dependency on [H`] ascribed to fast protonation of both ferrioxamine B and [ox]2~.Albrecht-Gary and Crumbliss773 have reviewed mechanisms of iron(III)–siderophore complexation. Removal of Zn2` from Zn 7 –metallothionein by nta follows biphasic kinetics in contrast to a hybrid partially silver-substituted Zn 4 Ag 6 –metallothionein.774 This di§erence arises from di§erences in reactivity of the metal bound to the C-terminal a- and the N-terminal b-domains.Step-by-step ring closure accounts for the three stages revealed for the complexation of [Ln(H 2 O)n]3` (Ln\Eu, Gd, Tb) with arsenazo III775 and par or pan.776 Di§erences in pressure dependence of the first stage for the three metals reflect changes in co-ordination number along the lanthanide series.775,776 The reduction by a-, b- or c-cyclodextrins of the second-order complex formation rate between M2` (M\Co, Ni, Cu, Zn) and 5-Br-paps is ascribed777 to the consequences of binding between ligand and cyclodextrin.The kinetics of formation of [Ce(dota)]~ Annu. Rep. Prog. Chem., Sect.A, 1999, 95, 535–591 557studied at higher pH than hitherto,778 suggest the intermediacy of [Ce(H 2 dota)]` and [Ce(Hdota)] and rate-determining general-base-catalyzed deprotonation of [Ce(Hdota)] (H 2 O-assisted at pH\7.5 and OH~-assisted at higher pH) followed by rearrangement. Related studies on the formation and dissociation of cerium(III) complexes of tetraazadioxa macrocyclic tetracarboxylates779 and of europium(III) complexes of hepta- and octa-dentate tetraaza macrocycles with a range of aryl, hydroxyethyl, carbamoylmethyl and carboxamidoarylalkyl pendant groups749 have also been described.In contrast to the acid-promoted dissociation of the [Eu- (H 2 O)n]3`–edta complex, the dissociation of edtp from its complex with europium(III) is inhibited by protonation.780 Stopped-flow EXAFS7 has been used to establish the structure of the heterobinuclear intermediate formed during CoII for HgII exchange in the homobinuclear [Hg 2 (tpps)]2~.[Cu(H 2 tpp)]2`, a ‘‘sitting-atop’’ complex formed from copper(II) and H 2 tpp in MeCN, deprotonates to [Cu(tpp)] with second-order kinetics on subsequent addition of py.781 [Cu(tpp)] formation in the presence of py was also characterised kinetically.The formation of [Zn(tpp)] from the reaction between H 2 tpp and a mixture of ZnII, CdII and HgII has been described782 in terms of similar intermediates. PbII 783 and CdII,784 respectively, catalyze the reaction of 3,8,13,18-tetramethyl- 21H,23H-porphine-2,7,12,17-tetrapropionic acid with CoII 783 and MnII.784 Kinetics for the reactions of HgII with the water soluble [H 2 tppsBr 8 ]4~,785 of ZnII, CdII and HgII with H 2 tmtp786 and of ZnII with TMPyP2~ 787 have also been described. Kinetics and mechanisms involving metal-containing complex assemblages are now receiving more attention.In benzene, an initially formed 1: 1 complex788 between 18-crown-6 and [Zn(tol) 2 ] more slowly transforms to a threaded rotaxane,789 with formation and dethreading both requiring the presence of non-complexed [Zn(tol) 2 ].Reaction of L22 and FeCl 2 in ethylene glycol at 170 °C yids two helicate complexes, 790 a triple helix XV and a circular double helix XVI, with the latter accumulating at longer reaction times. The rates of formation of both species increases with reagent concentration, with XV disappearing790 via a double exponential decay, suggesting that there may be a number of pathways leading to XVI.Racemisation Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 558kinetics791 for the bi- and tri-nuclear helicates, [Ti 2 L23 3 ]4~ and [Ti 3 L24 3 ]6~ point to a stepwise non-dissociative mechanism. Fast equilibration between two enantiomeric forms of helical dicopper(I) complexes of p-cyclophane-linked bis(N 2 S 2 ) chelate ligands involve both solvent-dependent and -independent processes.792 Main-group reactions The lower activation barriers for B–N bond-breaking in boranes and boronates containing C 6 H 3 (CH 2 NMe 2 ) 2 -2,6 compared with those containing C 6 H 4 (CH 2 NMe 2 )-2 supports793 an S N 2 process for B–N dissociation.Hydrolysis kinetics for NH 2 CN (and its conversion to dicyanodiamide), (NH 2 ) 2 CNCN,794,795 [NH 4 ]SCN and [NH 4 ]OCN796 have been studied at high temperature (400–575 K) and pressure (275 bar). Rate-determining decomposition of [NH 4 ]SCN proceeds via initial formation of [NH 2 CSO]~ and OCS as undetected intermediates, with the latter rapidly giving CO 2 and H 2 S.[NH 4 ]OCN reacts ca. 3]103 faster at 543K than [NH 4 ]SCN. Isomerization of [NH 4 ]SCN to thiourea in solution797 and the thermal decomposition of solid [NH 4 ][CO 2 NH 2 ]798 have also been described. Liebig and Wohler’s classic conversion of [NH 4 ]OCN to urea (in the solid state) has been reinvestigated.799 The kinetics of formation of a silicate cubic octamer in aqueous tetramethylammonium silicate (pH 13.2–13.6) show800 that the cation participates directly in the formation and stabilization of cage polysilicates rather than in the templating role suggested for molecular-sieve formation.The e§ects of ammonium ions on [SiF 6 ]2~ hydrolysis have been examined801 and substitution processes in fluorocomplexes of the Group III–V elements reviewed.802 Rate constants for Si exchange between [SiO 4 ]4~ and [Si 2 O 7 ]6~ in highly alkaline KOH solution have been reported. 803 [Si 3 O 10 ]8~ and [Si 3 O 8 ]4~ show comparable reactivity in forming or breaking the Si–O–Si moiety. The condensation reactions of [SiH 3 OH] and [SiH(OH) 3 ] have been treated theoretically.804 The [1.1.0]bicyclobutane analogue, [Si 4 (SiBu5Me 2 ) 6 ], rearranges805 to the corresponding cyclobutene analogue via 1,2- silyl migration and not by skeletal isomerization.Study of the addition reactions of silenes806–808 suggests that Me 2 Si–– CHRmoieties are formed as reactive intermediates on irradiation808 of [Me 3 SiSiMe 2 (CH–– CH 2 )] and related compounds in hexane solution. Steric e§ects are responsible809 for the 109–1012-fold slower rate of addition of phenol to [mes 2 Si––Simes 2 ] than that of alcohol addition to [MePhSi––SiMePh].[HN(NO 2 ) 2 ] decomposition in aqueous HNO 3 proceeds401,402 via the mixed anhydride, N 4 O 6 . de Ja� ger and Heyns810 propose that hydrolysis of sodium polyphosphate in water, pH\0, involves nucleophilic attack of water on protonated terminal phosphate.The hydrolysis of [O 3 PSe]3~ has also been reported811 and that of N 2 O 5 812 and SO 3 813 studied theoretically.Rate-determining proton loss from the nitrosated intermediate, [RC(O)NH 2 ·NO]`, is confirmed814 for the reaction of primary organic amides with nitrous acid, with the e§ect of variations in R suggesting that reactivity is controlled by the basicity of the amide favouring the formation of the intermediate rather than loss of a proton.S-Nitrosothiols, RSNO, react with [20- fold excess of RSH to give RSSR with clean first-order kinetics which are una§ected by Cu2` or edta.815 Nitrosation of 2-mercaptopyridine has also been studied.816 Kinetics of [(H 2 N)(HN––)CSSC(NH 2 ) 2 ]` decomposition, catalyzed by [SCN]~, involves817 nucleophilic displacement of thiourea and formation of [(H 2 N)(HN––)CSSCN].In Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 559studies stimulated by its immunomodulatory properties, the reaction of ammonium trichloro(dioxoethylene-O,O@)tellurate(IV) with cysteine is shown818 to be first order in [Te] and second order in [RSH]. Exchange between [MBu5Ga(l3 -Te)N4 ] and the element, E\S, Se, is first order in complex, to give [MBu5Ga(l3 -E)N4 ], and is faster for S than for Se.819 All mixed Te–E cubanes were observed.Understanding the aqueous chemistry of ClO 2 is important in understanding its role in the atmosphere. Under photolysis at 400 nm, aqueous ClO 2 su§ers 93^3% dissociation to ClO and O (which may recombine in the solvent cage) and 7^3% to Cl and O 2 . As photon energy is raised, cage escape of O increasingly occurs.820 Further studies of the femtosecond dissociation of [I 3 ]~ in solution have been reported. 12,13 Iodine–magnesium exchange in the system PhCH 2 CHI 2 and Pr*MgBr occurs via kinetically characterized821 [PhCH 2 C(I)HIPr*]MgBr. 4 Organometallics r-Bonded organotransition-metal compounds Further studies have addressed cobalt–carbon bond cleavage in coenzyme B 12 (5@- deoxy-5@-adenosylcobalamin, AdoCbl),822–826 modified cobinamides,827–829 methylcobalamin (MeCbl)824,830 and model831–833 systems as well as processes834–836 mediated by B 12 and related systems.Co–C homolysis for AdoCbl, weakly bound to ribonucleotide triphosphate reductase,823 is accelerated 1.6]109-fold at 37 °C (associated with a much reduced *Ht and very similar *St) compared with the nonenzymatic thermal homolysis. 27% of the initially photoexcited MeCbl homolyses830 on the sub-ps timescale, with the remainder giving a metastable cob(III)alamin photoproduct which deactivates within 1.2 ns. On Co–C photohomolysis826 in AdoCbl (also studied by time-resolved photoacoustic calorimetry824) 76^4% geminate radical pairs recombine (biphasically) and 24^4% give solvent-separated radicals.The much reduced rates831 of Co–C thermal homolysis (in the presence of a spin trap) for [CoIIIR(salen)] [R\(CH 2 )n, n\3, 4, bridging cobalt and ligand intramolecularly] compared with non-bridged analogues are linked to in-cage radical recombination.Cage e§ects in these systems have been reviewed.827 Photolytic metal–alkyl bond homolysis121,128 of [ReR(CO) 3 (N–N)] (R\alkyl, benzyl; N–N\4,4@-Me 2 bipy, Pr* 2 dab) occurs131,837 via a triplet-state precursor.CO loss and isomerization occurs838 on irradiation of trans,cis-[RuRI(CO) 2 (4,4@-Me 2 bipy)] (R\Me) whereas Ru–C homolysis occurs for R\CH 2 Ph. The kinetics of Co–C homolysis in [CoRL] (L\tpp, tmp, R\CMe 2 CN; L\tap, R\CMe 2 CO 2 Me, CHMePh) have been investigated832 by 1H NMR line-broadening.Oxidation of [PtMe 2 (N–N)] XVII (N–N\R 2 dab) in MeCN to a mixture of [PtIIMe(NCMe)(N–N)]` and fac- [PtIVMe 3 (NCMe)(N–N)]` occurs839 via methyl transfer involving [PtIIIMe 2 (N–N)]`·. Reduction of XVII has also been reported.840 [RhII(por)] react with alkyl halides via S N 2 reactions of [RhI(por)]~ formed by disproportionation rather than via radical processes involving the rhodium(II) species.841 Structure-activity e§ects in the alkylation of [Co(dmgH) 2 (PBu 3 )] by alkyl chlorides have been reported.842 [SnBu 3 ]~ reacts with primary butyl halides as a nucleophile843 and sec- and tert-butyl halides by electron transfer.844 New data at pH 7–10 reveal845 that [ReMeO 3 (OH)n]n~ (n\1, 2) are involved in the base hydrolysis of [ReMeO 3 ] to CH 4 and [ReO 4 ]~.Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 560Photolytic ring-opening of [FeR(CO) 2 Cp] (R\2,2-dimethylcyclopropylmethyl) by a concerted four-centre rearrangement is preferred846 to a radical process. Thermolysis of [MIV(CH 2 R)(N 3 N F )] MM\Mo, W; 3 N F ]3~\ [(C 6 F 5 NCH 2 CH 2 ) 3 N]3~; R\Me, Pr/, SiMe 3 , CMe 3N is a first-order process847 leading to [MVI(N 3 N F )(–– – CR)] and H 2 .a-Hydrogen exchange has been directly observed848 between [W(CH 2 Bu5)(SiBu5Ph 2 )(––CHBu5) 2 ] and [W(CH 2 Bu5) 2 (SiBu5Ph 2 )(–– – CHBu5)]. [Ta 2 (CH 2 R) 4 Cl 2 (––CHR) 2 ] (R\SiMe 3 ) eliminates RMe with first-order kinetics to give849 the unstable intermediate [Ta 2 (CH 2 R) 3 Cl 2 (–– CHR)(–– – CR)]. The absence of deuterium incorporation into CHPh of [RuDX(PBu5 2 Me) 2 (––C––CHPh)] (X\Cl, Br) suggests850 that [RuX(PBu5 2 Me) 2 (–– – CCHDPh)] is not readily accessible.Calculations suggest that while [RuHCl(PH 3 ) 2 (––C–– CH 2 )] and [RuHCl(PH 3 ) 2 (–– – CCH 3 )] are close in energy, a very high activation barrier exists between them. (See also refs. 851, 852. Migratory insertion processes of these and related compounds are described in the next section.) Ph 2 S is 4–5 times more reactive than cyclooctene towards [Fe(CH 2 )(CO) 2 Cp]`, formed reversibly by loss of Ph 2 S from [Fe(CH 2 SPh 2 )(CO) 2 Cp]`.853 Isomerisation of [MHCl(CH 2 )(CO)(PBu5 2 Me) 2 ] (M\Ru, Os) Mfrom [MHCl(CO)(PBu5 2 Me) 2 ] and CH 2 N 2N to [M(CH 3 )Cl(CO)(PBu5 2 Me) 2 ] requires854 phosphine dissociation.The rate of the reaction [Ti(g2-N 2 CHR)Cp* 2 ]]PhCH––CH 2 ] [TiMCH(R)CH(Ph)CH 2NCp* 2 ] (R\C 6 H 4 X-4) via a complexed carbene intermediate855,856 is insensitive to X. The role of ruthenium–carbene complexes in olefin metathesis,857 reactions of carbene complexes of Cr,858–863 W,860,864 and of allenylidene complexes of Mo and W865 have been studied. The e§ect has been described866 of phosphine, L, on the rates of equilibrium approach between the two isomers of [Os 3 (l-g2-CH––CH 2 )(l-H)(CO) 9 L].A l3 -CH 2 -bound intermediate is proposed867 in CH 2 exchange between the two RhCo(l-CH 2 ) sites in [(CoCp) 2 (RhCp*)(l-CH 2 )(CO) 2 ]. Related studies868 have been described for [Ru 2 (l- CH 2 )(CO) 2 (NCMe)Cp 2 ]. Theoretical studies rule out [WVI(HC–– – CH)] and [WVI(C––CH 2 )]869 as being of importance in acetylene polymerisation processes.The isomerization [Ti(g3-C 3 H 5 )(fv)Cp*] to [TiM(E)-CH––CHMeN(fv)Cp*] proceeds870 via [Ti(g2-CH 2 ––C––CH 2 )Cp* 2 ], [Ti(g1-CMe––CH 2 )(fv)Cp*] and [Ti(g2-CH–– – CMe)Cp* 2 ]. (See also ref. 871.) The [AuR(tht)]-catalyzed isomerization of trans- to cis-[PdR 2 (tht) 2 ] involves872 associative aryl-exchange between palladium(II) and gold(I) and ratedetermining tht loss from palladium.The interchange of agostic and non-agostic methyls and exchange of hydrogens within the agostic methyl groups in the b-agostic complex [PdMCH(CH 2 -l-H)(CH 3 )N(RN––CR@CR@––NR)]`873 have similar activation barriers. Carbonyl-insertion and alkyl-migration reactions [FeMe(CO) 2 Cp]874 and [FeMC(O)MeN(CO) 2 L] (L\Cp, ind)875 lose CO to give the corresponding monosolvento complex on photolysis.Using time-resolved infrared spectroscopy,876 the *Vt for trapping of the intermediates with CO (or other ligands) to give [FeMe(CO) 2 L] is more negative than that for the corresponding methylmigration reaction.875 When L\ind, ring-slippage is not favoured.Added salts, NaI, NaPF 6 or CaI 2 , increase877 the rate of the migratory-insertion reaction, [FePr*(CO) 2 L]]L@][FeMC(O)Pr*N(CO)L@L] for L@\PPh 2 CH 2 (aza-15-crown-5), Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 561L\ind, whereas there is no e§ect for L\Cp. The methyl group in [CrMe(CO) 3 (g5- ind)] migrates first to C-1 and then irreversibly, via a trimethylenemethane-type transition state,878 to C-3 of the ind moiety in [Cr(CO) 3 (g6-Meind)].Iridiumcatalyzed methanol carbonylation879 involves CO migratory insertion that is faster for [IrMeI 2 (CO) 3 ] than for [IrMeI 3 (CO) 2 ]~. Phosphite-induced CO insertion of the latter is inhibited by iodide ion and involves880 initial [IrMeI 2 (CO) 2MP(OR) 3N] formation. Rhodium-catalyzed carbonylation881,882 of CH 2 I 2 to malonate esters by [RhI(CO)(PEt 3 ) 2 ] involves loss of I~ from [RhMC(O)CH 2 INI 2 (CO)(PEt 3 ) 2 ], ROH addition to the resulting ketene complex [Rh(CH 2 ––C–– O)I 2 (CO)(PEt 3 ) 2 ]` then CO insertion to give [RhMC(O)CH 2 CO 2 RNI 2 (CO)(PEt 3 ) 2 ]. Insertion of norbornadiene into the Pd–acyl bond of [PdMC(O)MeN(L 3 )]` (L 3 \a tridentate nitrogen ligand883 or Cl with a bidentate nitrogen ligand884) involves partial dissociation of the chelated ligand.Migratory insertion in [PdMe(CO)(dippe)]` to form [PdMC(O)MeN(CO)- (dippe)]`885 and in [CuIIMe(CO)(H 2 O)n]2` to [CuIIMC(O)MeN(H 2 O)n]2`886 have been studied. Insertions of CH 2 ––CHCO 2 Me into PtII–Me,887 alkenes into Zr–C,888 alkynes into RhI–C,889 PtII–Si,890 ArNC into NiII–C,891 CO 2 into Ni–O,892 Ni–C,893 and into O–O in [RhCl(O 2 )(PEt 2 Ph) 2 ],894 and O 2 into CoIII–C895 have all been described.Theoretical studies have been made of CO 2 insertion into RhIII–H,896 of SnCl 2 into PtII–Cl,897 of CH 2 –– CH 2 into [M(C 2 H 5 )L] (M\ScIII, YIII, LaIII, LuIII, CeIV, ThIV, VV,898), TiIV–C,898–900 Zr–C,898,900 Hf–C,898,900 Pd–C,901–903 PtII–H904–906 or PtII–Si906 and of related metal-catalyzed olefin polymerization900,907–910 processes.Experimental studies of iron(II)-911 or palladium(II)-complex912,913 catalyzed ethylene oligomerization, palladium(II)-complex catalyzed copolymerization of ethylene with CO914,915 or methyl acrylate,916 norbornene with CO,917 styrene918 or p-methylstyrene919 with CO have been reported. Shaw has proposed920 a new mechanism for Heck reactions.Rates of Co to N migration of R in [CoR(tpp)]`, formed on oneelectron oxidation of [CoR(tpp)] by [Fe(phen) 3 ]3`, di§er921 106-fold for R\Ph and Bu. (See also ref. 922.) Migration of R from Fe to Nin [FeIVR(oep)]` is faster the more electron-donating is R.923 Values of the activation parameters for the thermolysis of [Rh(13CH 2 CH 2 Ph)(bocp)] to give [RhMCH(13CH 3 )PhN(bocp)] (and other data) support924 a stepwise cis b-hydride elimination/olefin Rh–H insertion mechanism.An elimination/insertion mechanism is also proposed925 for the formation of [RhIIIEt(acac)] from protonation of [RhI(CH 2 ––CH 2 ) 2 (acac)]. [RuHX(H 2 )(PBu5 2 Me) 2 ] (X\Cl, I) reacts with PhC–– – CD to give [RuDX(––C––CHPh)(PBu5 2 Me) 2 ] via alkyne insertion into Ru–H and a-H migration in [RuC(D)–– C(H)Ph] to give [RuD(––C––CHPh)].926 [RuHClL 2 ] (L\PPr* 3 ) reversibly binds EtOCD–– CH 2 , with D incorporation into RuH and P–Me.Slower formation of [RuHClM–– CMe(OMe)NL 2 ] follows.927 [IrEt(CH–– CD 2 )(PMe 3 )Tp@] reacts with D` to give two rotameric forms of [IrEt(––CHCD 3 )(PMe 3 )Tp@]` XVIII which then gives [IrH(CHMe––CHCD 3 )(PMe 3 )Tp@] (without D incorporation at vinylic sites).In rate comparisons with [IrH(––CHMe)(PMe 3 )Tp@]` XIX it is concluded928 that migratory insertion of ––CHMe into Ir–H of XIX is faster than into Ir–Et of XVIII. Related protonation processes of [Rh(Me 3 [9]aneN 3 )(CH––CH 2 ) 3 ] have been described.929 Ligand-displacement reactions of metal carbonyl and other low-valent compounds At high [L] (L\Pr 2 dab, 4,4@-Me 2 bipy), when a competing reaction to give [Cr(CO) 6 ] Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 562is unimportant, the thermal ring-closure reaction of [Cr(CO) 5 L] is dissociatively activated930 with positive values of *Vt. Rates of ring-opening have been measured for [W(CO) 4 L] (L\Bu5SCH 2 CHRSBu5, R\H, Me).931 [M(CO) 4 (phen)] (M\Cr, Mo, W) gives [M(CO) 3 (PR 3 )(phen)] by a dissociative process932 on ligand-field (LF) photolysis whereas charge-transfer photolysis may lead to associative substitution, depending on M and R.Photochemical substitution of [W(CO) 4 (en)] to [W(CO) 4 (NCMe)(en)] is more e¶cient via the LF singlet state than the lowest lying LF triplet state.933 Wavelength and solvent dependence for the photoreaction [Ru(g6- arene) 2 ]2`]3 solv][Ru(g6-arene)(solv) 3 ]2`]arene934 has also been reported.Photochemistry of [Cr(CO) 6 ]14,15 and [Fe(CO) 5 ]16–18 has been studied on the fs timescale. The role of LF and CT excited states in M–CO photodissociation935,936 (also discussed in previous sections) and other aspects of organometallic photochemistry937 –939 have been reviewed.*St for displacement by CO of solvent in [Cr(CO) 5 (RH)] becomes less negative as the alkane chain length increases,940 whereas *Ht remains unchanged. cis- And trans-[Cr(CO) 4MP(OPr*) 3NL] XX (L\ClPh) formed transiently in the presence of P(OPr*) 3 , each react to cis- and trans-XX [L\P(OPr*) 3 ]. The cis isomer very rapidly converts to the trans isomer non-dissociatively.941 Related studies have been reported on [Nb(CO) 3 (g2-C 2 H 4 )Cp] (CO-for-ethylene substitution in supercritical ethylene as solvent),942 [Cr(CO) 5 L] (displacement by alkenes of X-bonded L\PhX, X\Cl, Br,943,944 and of arenes, L\C 6 H 6~nMen 945), [Mo(CO) 2 (alkane)(g6-arene)],940 [W(CO)L(PhC–– – CMe)Tp@]` (exchange of L\Et 2 O by MeCN;946 E/Z isomerisation for L\g1-Me 2 CO947), [Fe(CH 3 )(CO)(solv)Cp],874,875 [M(CO) 4 (g2-CF 3 C–– – CCF 3 )] (M\Fe, Ru, Os) (substitution of CO by PR 3 948), [Mn(CO) 4 (cod)]` (replacement of cod by MeCN949), and [Mn(CO) 3 (g5-C 5 H 4 CH 2 CH 2 X)] (displacement of CO by X\Br, Cl950).Intermolecular CO exchange rates have been measured for [W(CO) 4 L]2~ [L\orotate (L25), dihydroorotate].951 Calculations suggest863 that alkynes react associatively with chromium carbene complexes, [Cr(CO) 5M–– C(OH)(C 2 H 3 )N], in the Do� tz reaction, rather than by an initial CO dissociation.Broadening and coalescence of v(CO) IR bands in [Fe(CO) 3 (g4-cod)] Mand low-temperature studies on stereoselectively labelled [Fe(CO) 2 (13CO)(g4-cod)]N provide estimates952 for a rate constant for CO site exchange at 293K of 1.54]1012 s~1.Isomerization of cis-[Rh(CO) 2 I 4 ]~ to the favoured trans complex involves CO dissociation. 953 Thermal mer to fac isomerization of [MnBr(CO) 3 (N–N)] (N–N\4,4@- Me 2 bipy) in thf involves954 partial dissociation of Br~ whereas for N–N\Rdab, Mn–N cleavage is preferred. Magnetization transfer has been employed955 in a study of the stereodynamics of fac-[ReX(CO) 3 (L26)].Intramolecular CO exchange956 in the metal–metal bonded [RuI 2 (CO) 6 (dppe) 2 ]2` involves pairwise bridging carbonyls. Cleavage of the Mn–Mn bond in Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 563[Cp@(CO) 2 MnMn(CO) 5 ]~ by L\PR 3 gives957 [Mn(CO) 2 LCp@] and [Mn(CO) 5 ]~ with [N(PPh 3 ) 2 ]` as counter-ion and [Mn(CO) 3 Cp@] and [Mn(CO) 4 L]~ with Na`.Calculations and experiment958 suggest that photochemically generated [MFe(CO)Cp*N2 ] is a triplet species with terminal CO which relaxes to a singlet CO-bridged ground-state. [(CO) 5 MnMn(CO) 3 ] has been proposed959 as a possible structure for the intermediate, [Mn 2 (CO) 8 ], formed on photolysis of [Mn 2 (CO) 10 ]. Intramolecular scrambling of CO (with negative *Vt)960 is faster for [Rh 4 (CO) 12 ] than for [IrRh 3 (CO) 12 ] which has apical Ir.Limiting rate constants for the dissociative displacement of L from [Rh 6 (CO) 15 L] by P(OPh) 3 961 span a range of ca. 105 along the series L\dmso, MeCN, thf, EtOH, with evidence that the [Rh 6 (CO) 15 ] intermediate may be stabilised by co-ordination of the solvent CHCl 3 or additional bridging by CO.Isokinetic relationships in reactions of [Ru 5 C(CO) 14MP(OPh) 3N] with P-donors have been studied.962 Phosphines with Tolman cone angles, h, p143° form monosubstituted products from [Os 3 (CO) 9 (l-C 4 Ph 4 )] via associative adducts,963 whereas for hq145° a single bimolecular step leads to both mono- and di-substitution. Joerg, Drago and Sales964 have expanded their database of phosphine basicity parameters, though their approach has been criticised by Giering, Prock and co-workers.965 Arene exchange in triple-decker [(CoCp*) 2Mg4: g4-areneN] proceeds via an equilibrium involving [CoCp*(g6-arene)] and the 14-electron [Co(solv)Cp*].966,967 Similarity of rate parameters suggests968,969 that [RuL(g4-C 5 H 4 O)Cp]` (L\MeCN, py, tu) react with PR 3 to give 1,2-disubstituted ruthenocenes Mvia [Ru(PR 3 )(g3-C 5 H 5 )(g4-C 5 H 4 O)]` involved in a pre-equilibrium between g5- and g3- forms or via [RuL(g3-C 5 H 4 O-2-PR 3 )Cp]` depending on Rand LN.Hapticity changes are also observed in reactions of [Mo(CO) 2 L 2 (g5-ind)]0,` with MeCN (g5 to g3)970 and [Ru(g4-cod)(g6-C 10 H 8 )] with phosphines or phosphites (g6 to g4).971 A dissociative mechanism dominates972 in the endo–exo isomerization of [ML 3Mg4- C 6 Me 4 (CH 2 ) 2 -oN] (L\PMe 3 ) in the presence of L when M\Ru, whereas an addition process, giving r-bonded [ML 4Mg2-C 6 Me 4 (CH 2 ) 2 -oN] dominates forM\Os.Redox reactions Zavarine and Kubiak have reviewed the redox chemistry of 19-electron [W(CO) 5 - L]·~.175 Rate constants for the cis–trans isomerization of 16-electron [ReIII(NCR) 2 (dppe) 2 ]3` (R\aryl) formed by two one-electron oxidations of the rhenium(I) precursor, increase973 with the electron-withdrawing character of the nitrile. The rate constant for axial–equatorial phosphine-related stereo-isomerization in [Ru 3 H 3 (l3 -COMe)(CO) 6 (PPh 3 ) 3 ]n` is ca. 104-fold faster974 for the 47-electron n\1 species than the 48-electron n\0 species.Electron-transfer catalysis accelerates975 the substitution, [FeI(CO) 2 Cp]]PPh 3 ][Fe(CO) 2 (PPh 3 )Cp]`]I~ via more labile 17- and 19-electron intermediates even though the redox process which propagates the catalytic chain is endergonic. The Fe–X relative bond dissociation energy increases as a consequence of oxidation of [FeX(dppe)Cp*]976 along the series F@Cl\Br\I.Facile formation of aldehyde, RCHO, and [Fe(SnR@3 )(CO) 2 Cp] from R@3 SnH and [FeMC(O)RN(CO) 2 Cp] (R\Me, CH 2 CH 2 Ph) is initiated977 by an electron-transfer step. The 17-electron monocation formed from [MoHL 2 (PMe 3 )Cp] XXI (L\PMe 3 ) and Ag` (or Fc`) disproportionates following a second-order rate law,978 a process accelerated by a competing deprotonation by excess XXI.XXI Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 564(L\CO) gives [Mo(CO) 2 (NCMe)(PMe 3 )Cp]`, H 2 and Ag0 via a hydride-bridged adduct, [MoM(l-H)Ag(NCMe)nN(CO) 2 (PMe 3 )Cp]`.979 Unusually stable [RhIIH(CO)(PPh 3 ) 3 ]` displays980 very rapid electron self-exchange with its rhodium( I) precursor. Further oxidation to [RhIIIH(CO)(PPh 3 ) 3 ]` leads to rapid proton loss and reformation of the rhodium(I) parent.[PdIICl 2MPPh 2 C 5 H 4 ) 2 FeIIN] is formed photooxidatively981 in CCl 4 from [Pd0MPPh 2 C 5 H 4 ) 2 FeIIN] 2 via initial oxidation of iron(II) to iron(III) in the redox-active ligand,982 intramolecular electron transfer and disproportionation of a palladium(I) intermediate. Arylpalladium(I) transients are proposed983 in the reduction of [PdAr(PPh 3 ) 2 (solv)]` present in equilibrium with [PdArX(PPh 3 ) 2 ].Redox processes have been reported for cis- and trans- [Re(CO) 2 (dppe) 2 ]`,984 [Re(CO) 3 L(bipy)]` [L\PR 3 , P(OR) 3 ],985 (relevant to photochemical reduction of CO 2 985,986) [MoF(CO) 2 (dppe) 2 ]`,987 [CoI(g3- C 4 H 7 )Cp],988 [Rh(1,n-cod)Cpt] (n\3, 5)989 cis-[Mo(SAr) 2 (CNBu5) 4 ],990 [WX(CO) 2 LCp] (X\Cl, Br, I, L\CO; X\I, L\PCy 3 )991 [UCl(g-C 5 H 4 R) 3 ],992 and (the previously discussed) [FeIIIR(oep)]923 and [CoIIIR(tpp)] (R\CH 2 Ph, Bu).921,922 Homolysis of the Rh–In bond in [Rh(tpp)In(oep)] has beennvestigated. 993 Di§erences in cage-recombination e¶ciencies for the radicals [M(CO) 3 Cp@]· (M\Mo, W) formed from photolysis of [MM(CO) 3 Cp@N2 ] arise994 from di§erences in metal–metal bond energy or from spin–orbit coupling rather than from di§erences in radical mass.Reactions of the spin-triplet [MoCl(PMe 3 ) 2 Cp*] with CO and N 2 have been reviewed.995 Photolysis of [Os 3 (CO) 10 (a-diimine)] leads to zwitterion-forming heterolysis in co-ordinating solvents or to biradical-forming homolysis in non-co-ordinating solvents.996 Successive one-electron oxidations of [MRu(l-AsPh 2 )(CO)CpN2 ] give Ru–Ru bonded species.997 Oxidative addition and reductive elimination [PtIIMg1:g1-(CH 2 ) 4N(bipy)] oxidatively adds EtI ca. 2.5-fold more rapidly than [PtII(CH 3 ) 2 (bipy)].998 Variation with X of overall rates of oxidative addition of PhI in mixtures of [Pd(dba) 2 ] and P(C 6 H 4 X-4) 3 is associated999 with e§ects on the intrinsic reactivity and equilibrium concentration of Pd0 complexes.Oxidative addition of the disulfide in [W(CO) 3 (phen)(MeSSMe)] is slow and kinetically first-order at high [Me 2 S 2 ] and second-order at low [Me 2 S 2 ], consistent1000 with formation of [MW(CO) 3 (phen)N2 (MeSSMe)] prior to S–S cleavage. Aubart and Bergman1001 suggest that diaryl disulfides react with [Cp 2 Ta(l-CH 2 ) 2 CoCp] giving [Cp 2 Ta(l- CH 2 ) 2 Co(SR)Cp] with modest charge separation in the transition state.Stereoselectivity in the oxidative addition of RX to [Rh(CO)Mg1:g5-R 2 P(CH 2 )n(ind)N] (n\2–4) is governed by the spacer length, n.1002 Di§erent stereoselectivies for Ph–H and Cy–H have been observed1003 in photochemical C–H addition at Ir in the pro-chiral ligandcontaining [IrH 2Mg1:g5-Me 2 PCH 2 CMe 2 (C 5 H 3 Bu5)N].[IrMe(C 2 H 4 ) 2 Tp] reacts with benzene to give [MIrMePh 2 TpNnN 2 ] (n\1, 2).1004 The 10–20-fold greater rate of the exchange,1005 [PtR(O 2 CCF 3 )- (dmpe)]`]C 6 D 6H[Pt(C 6 D 5 )(O 2 CCF 3 )(dmpe)]`]RD, for R\C 6 H 5 compared with R\CH 3 supports rate-determining aryl C–D addition to the PtII centre. Spin saturation transfer experiments point1006 to scrambling of the four hydrogens of the OsHMe moiety in [OsIVHMe(dmpm)Cp*]` between [100 and [120 °C [via Os(CH 4 ), a species favoured over OsH 2 (–– CH 2 ) or OsHCH 2 ~]H`] via a di§erent Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 565transition state from that involved in reductive elimination seen above [95 °C. Diastereomeric and isotopic scrambling in [IrRH(PMe 3 )Cp*] [R\(RS),(SR)- and (RR),(SS)-2,2-dimethylcyclopropyl] involves1007 Ir–(r-alkane) intermediates. Cyclohexane activation by [Rh(CO) 2 Cp] proceeds via a single intermediate,1008 the g5 species [Rh(CO)LCp] XXII (L\c-C 6 H 12 ).The complex XXII (L\c-C 5 H 10 ) has been directly observed byNMRspectroscopy.1009 [M(CO) 2 (solv)Cp] (solv\SiHEt 3 ) bonded either via Si–H or Et,1010 formed on photolysis of [M(CO) 3 Cp] (M\Mn, Re),1011 reacts either via Si–H cleavage (on a timescale of 5 ps) or C–H cleavage (230 ns).Successive Si–H and C–H reductive-elimination oxidative-addition1012 leads to deuterium scrambling involving RhH, SiH, RhCH 2 and o-Me groups in fac- [RhHMSi(H)mes(C 6 H 2 Me 2 CH 2 )N(PMe 3 ) 3 ] formed from [RhMe(PMe 3 ) 4 ] and SiD 2 mes 2 .The p CO -dependence of product ratios and kinetic isotope e§ects for CyH dehydrogenation in the presence of [RhCl(CO)(PMe 3 ) 2 ] point to1013 C–H activation via [RhCl(PMe 3 ) 2 ]. C–C rather than C–F cleavage is preferred in the reaction of L27 (R\CF 3 ) with [MRhCl(C 2 H 4 )N2 ].1014 For R\C 2 H 5 ,1015 Ar–CH 2 is cleaved in preference to CH 2 –CH 3 . For R\OMe, Rh inserts into aryl–O at room temperature. 1016 When R\Me, an g2-RhH–CH 2 interaction results in a highly acidic proton in a complex1017 of rhodium(I). Related cleavages1018,1019 have been rendered catalytic. An unusual C–N cleavage in [Co(CNCH 2 Ph)(PMe 3 )Cp] to give [Co(CH 2 Ph)(CN)(PMe 3 )Cp] has also been studied.1020 The unusual thermolysis of [WH 3 (OCH 2 Ph)(PMe 3 ) 4 ] to [WH 2 (CO)(PMe 3 ) 4 ] and PhH occurs1021 viaH 2 elimination. Niu and Hall,1022 being unable to find a r-bond metathesis pathway using DFT methods, conclude that C–H activation in [IrMe(PMe 3 )Cp]` proceeds only via oxidative addition, in contrast to earlier suggestions.Other computational and theoretical studies have addressed activation of C–H,1023–1036 C–F,1035 C–C,1027 Si–H,1027,1037 H–H,1030,1036,1038–1042 O–H1043 and B–B1044,1045 activation by W0,1040 RhI,1023,1031,1035 IrI,1031,1038,1041 MnI and ReI,1010 Fe0,1038 Ru0,1024,1038 Os0,1024,1035 Pd0,1025,1026,1036,1043,1044 Pt0,1024,1027,1036,1042–1044 NiII,1041 PdII,1028,1031 PtII,1031 OsII,1029,1030,1034 and HgII.1032,1033 RH addition in [PtMe(RH)L 2 ]` (R\Me, Et) and RH elimination from [PtHMe 2 L 2 ]` or [PtMe 3 L 2 ]` (L\NH 3 , PH3 ),1045 CH 4 elimination from [WHMeCp 2 ] and [WHMeM(g5-C 5 H 4 ) 2 CH 2N],1046H–C 2 H 5 addition to [IrMe(PH 3 )Cp]`,1047 the rearrangement [MCl(PH 3 ) 2 (HC–– – CPh)] to [MCl(PH 3 ) 2 (––C––CHPh)] (M\Rh, Ir),1048 intermediate formation in the reaction of RH with [Rh(CO) 2 Tp*],1049 and C–S vs.C–H activation in thiophene by [Rh(PMe 3 )Cp*]1050 have been studied theoretically.Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 566Fig. 4 Cyclometallated platinum(II) alkyl complex involving a2-(diisopropylphosphino) isodurene.1059 Rates of intramolecular concerted RSR@ loss from [PdR(SR@)(P–P)] (P–P\a chelating diphosphine)1051 depend on the state of hybridisation at C bound to Pd (sp2[spAsp3), are faster for electron-donating R@ and -withdrawing R and increase with diphosphine bite angle.Reductive elimination of RCN from [PdR(CN)(P–P)] (R\CH 2 SiMe 3 ) is 104-fold faster for P–P\diop (bite angle ca. 100°) compared with dppe (bite angle ca. 85°).1052 Multiple H–D exchange prior to methane loss accounts for CH 4 isotopomer formation from [PtMe(OEt 2 -d 10 )(tmen)] decomposition.1053 Reductive elimination of RH from [IrIIIHRMg2-Si(H)(mes)CH 2 CH 2 PPh 2N(PMe 3 ) 2 ] (R\Me, Et) in methanol proceeds1054 via the silylene-bound [IrHMg2- –– Si(mes)CH 2 CH 2 PPh 2N(PMe 3 ) 2 ] and subsequent cyclometallation.Si–H addition of SiPhH 3 to Ta–– N in [TaMeM––N(C 6 H 3 Me) 2 N(SiMe 3 )NCp*] and elimination of SiMe 3 H from [TaHMeM(PhSiH 2 )N(C 6 H 3 Me) 2 N(SiMe 3 )NCp*]1055 and SiMe 4 loss from [W(NO)(CH 2 SiMe 3 )(CPh––CH 2 )Cp*] to give [W(NO)(CPh–– – CH)Cp*]1056 have been studied kinetically.Pt–H and Si–H exchange in cis-[PtH(SiHmes 2 )(PCy 3 ) 2 ] by reductive elimination of SiH 2 mes 2 1057 rather than by PtH 2 (–– Simes 2 ) formation. Hydrogen migration from Pt to Si in the process [PtHMSi(SBu5) 2 (OTf)N(PEt 3 ) 2 ] ][PtH(NCMe)MSiH(SBu5) 2N(PEt 3 ) 2 ]` 1058 involves [PtHM––SiH(SBu5) 2N(PEt 3 ) 2 ]` as an intermediate.The cyclometallated complex cis-[PtR(g1-P)(g2-P[H)] (Fig. 4, R\Me) reacts with SiR 3 H to give SiR 3 Me and (Fig. 4, R\H).1059 Mechanisms of loss of PhH from [IrHClPh(CO)(PPr* 3 ) 2 ],1060 of SnHPh 3 from [RhH(SnPh 3 )(NCBPh 3 )(dmap)(PPh 3 )],1061 of EMePh 3 (E\Si, Ge) from cis- [PtMe(EPh 3 )(PMe 2 Ph) 2 ],1062 and of RONp from [PdR(ONp)(P–P)],1063 have all been described.New proposals for the Heck920,1064 and Stille1065 reactions have appeared. Negative Hammett o values for cyclopalladation in [Pd(L28)(NCMe)]2` point to1066 electrophilic attack of PdII on ortho-benzyl C. Rate constants for cyclopalladation of [Pd(L28)(NCMe)]2`follow the order dmf\dmso@py, which, with other data, establish1067 the importance for C–H cleavage of solvent assistance at the ortho proton.Hydrogen and hydrido complexes No evidence for a r-H 2 adduct was seen in a kinetic study of the equilibrium [RhI(bipy) 2 ]`]H 2a[RhIIIH 2 (bipy) 2 ]`.1068 Self exchange between [RhIIIH 2 (bipy) 2 ]` and [RhI(bipy) 2 ]` proceeds through a symmetric bis(hydridobridged) intermediate. [Ru(H 2 O) 5 (H 2 )]2` has been characterised.1069 Highly siteselective H–D scrambling and diastereomer interconversion for [IrH 2 (Me- DuPhos)(cod)]` formed by H 2 oxidative addition does not involve H 2 reductive Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 567elimination.1070 Para-hydrogen induced polarisation has been used to study H 2 addition to [TaH(L)Cp* 2 ] (L\benzyne)1071 or [RhI(CO)(PMe 3 ) 2 ],1072 H 2 exchange with [IrH 3 (CO) 3~x(PPh 3 )x] (x\1–3)1073 or [PtH 2 (P–P)] (P–P\unsymmetrical diphosphine)1074 and hydrogenations.1075–1077 Ammonia is formed1078,1079 on treatment of cis-[W(N 2 ) 2 (PMe 2 Ph) 4 ] and [RuCl(H 2 )(dppp) 2 ]` with H 2 at 1 atmosphere. Second-order rate constants, k 2 , for 20 single-step hydride transfers, MH][CPh 3 ][BF 4 ]][M(FBF 3 )]]CHPh 3 , span a range of 107,1080 M\[Mn(CO) 5 ]\cis-[Mn(CO) 4 (PCy 3 )]\[Re(CO) 5 ]; [W(CO) 3 Cp] \[Mo(CO) 3 Cp]\[W(NO) 2 Cp]\trans-[Mo(CO) 2 (PCy 3 )Cp] [trans-XXIII].k 2 is 103-fold smaller for cis-XXIII than for trans-XXIII (at [55 °C). The substitution, trans-[FeH(H 2 )(dppe)]`]L]trans-[FeHL(dppe)]`]H 2 (L\MeCN, dmso) involves1081 rate-determining attack of L on an intermediate with monodentate dppe followed byH 2 displacement on chelate ring closure.Inverse kinetic isotope e§ects for the common first-order reaction of HX and DX (X\BF 4 , Br, Cl, CF 3 CO 2 ) with cis-[FeH 2 L 2 ] [L\dppe,1082 (L 2 )\pp31083] to give trans-1083 or cis-[FeH(H 2 )- L 2 ]` 1082 suggest a late transition state. Related studies on trans- [W(N 2 )(NCR)(dppe) 2 ]1084 and [Ni(dppe) 2 ]1085 have been described.cis- [Re(H 2 )(CO) 4 (PR 3 )]` is su¶ciently acidic to protonate Pr* 2 O.1086 Exchange of thiolate in cis,cis,trans-[RuH(SR)(CO) 2 (PPh 3 ) 2 ] with R@SH involves phosphine dissociation, R@SH co-ordination, intramolecular proton-transfer to SR and loss of RSH.1087 Protonation of trans-[MH(SPh)(dppe) 2 ] (M\Ru, Os) gives1088 trans- [MH(HSPh)(dppe) 2 ]` with one equivalent of H` Mwith no evidence for the formation of [M(H 2 )(SPh)(dppe) 2 ]`N and trans-[Os(H 2 )(HSPh)(dppe) 2 ]2` in excess acid. Protonation at Re1089 of [Re(CO) 2 (MeC–– – CMe)Cp*] precedes H`-migration to alkyne, the resulting 1-metallacyclopropene rearranging to [Re(CO) 3 (g3-exo,anti- MeHCCHCH 2 )Cp*]`.Corresponding processes for [Re(CO) 2 (g2-MeC–– – CMe)Cp] (giving both g3-allyl and g2-allene products)1090 and for isomers of [Mo(g3- C 3 H 5 )(g4-C 4 H 6 )Cp]1091 have also been reported.Theoretical and computational studies have examinedH 2 addition toW0,1039 IrI,1036,1040 Fe0,1038 Ru0,1038 OsII,1030 NiII,1041 Pd0,1036 and Pt0,1036 as well as aspects of stability,1092 reactivity and dynamics1093–1100 or bonding.1101–1104 Relevant experimental studies of reactivity and dynamics1086,1098,1105–1119 and bonding1108,1120 are noted.Cooper and Caulton report1121 that variation in L causes *Ht and *St for exchange between chemically inequivalent hydride sites in [IrH 2 LL@2 ]n` to be varied over a wide range. Solvent and other medium e§ects Studies of solvent,10,111,118,149,150,524,528,562,570,571,573,761,762,807,1067 electrolyte and counter-ion,181,572,801,802,877,957 encapsulation114 and surfactant726,729 e§ects and reactions in supercritical media794–796,942 have been discussed in earlier sections.E§ects of mixed solvents on redox reactions between [Co(en) 2 L]n` (LH\HSCH 2 CH 2 NH 2 , n\2;1122,1123 LH 2 \HOSCH 2 CO 2 H, n\11124) and [S 2 O 8 ]2~ 1122,1124 or [IO 4 ]~,1123,1124 between [Fe(CN) 2 (bipy) 2 ]1125 or [Fe(CN) 3 (terpy)]~1126 and [S 2 O 8 ]2~, [Fe(solv)(dppe)Cp]` and HSR,1127 on solvolytic reactions of [CoBr(Hedta)]~,1128 [CoCl(CN) 5 ]3~,1129 cis- and trans- [CoCl(NO 2 (en) 2 ]` 1130 and on anation of cis-[Co(H 2 O) 2 (big) 2 ]` by glutamic acid1131 have been reported.Ionic strength e§ects have been treated using the mean Annu. Rep.Prog. Chem., Sect. A, 1999, 95, 535–591 568spherical approximation1132 and studied experimentally for [Fe(CN) 6 ]3~ oxidation of ascorbic acid,1133 photoaquation of [Co(CN) 5 (SO 3 )]4~ in the presence of polyammonium macrocycles, e.g. M[32]aneN 8 H 8N8`,1134 ester interchange in non- and weakly- polar solvents1135 and oxidative quenching of [Ru*(bipy) 3 ]2`.1136 Copper(II) dodecyl sulfate produces1137 a 106-fold acceleration in a Diels–Alder reaction by a combination of Lewis acid and micellar catalysis. Surfactants, micelles and microemulsions a§ect nickel(II)1138 and palladium(II)1139,1140 complexation, [Fe(Ph 2 phen) 3 ]2` aquation,1141 electron-transfer quenching of [RuII*(phen) 2M4,7-(~SO 3 C 6 H 4 ) 2 - phenN],1142–1144 and oxidation of ferrocene derivatives.1145,1146 Complexation between 5-alkoxymethyl-8-quinolinoland NiII or ZnII at liquid–liquid interfaces has been studied kinetically.1147 Studies relevant to catalytic processes in novel media include investigations of the reactions [Nb(CO) 3 (g2-C 2 H 4 )Cp]]CO][Nb(CO) 4 Cp] ]C 2 H 4 in supercritical (sc) ethylene942 and [Mn(CO) 2 (g2-HSiEt 3 )Cp*] ]H 2 ][Mn(CO) 2 (g2-H 2 )Cp*]]SiHEt 3 in scCO 2 .1148 Palladium catalysts, solubilised into water using sulfonated dppp, rapidly copolymerise ethylene and COin the presence of excess TsOH.1149 Hydroformylations in fluorous media1150 and in sc CO 2 1151 have also been studied.Dec-1-ene reacts with CO and H 2 in toluene–C 6 F 13 CF 3 in the presence of [RhH(CO)L 3 ] (L\P[CH 2 CH 2 (CF 2 ) 5 CF 3 ] 3 ) though the catalyst is an order of magnitude less active than for L\PPh 3 .Palladiumcatalyzed C–C bond formation processes,1152 molybdenum-catalyzed alkene oxidation1153 –1155 and Friedel–Crafts alkylations1156 have also been studied in sc CO 2 . Photolysis of [M(CO) 6 ] (M\Mo, W) in a polyethylene matrix has permitted1157 the study of the formation and reactions of [M(CO) 5 (g2-H 2 )] and cis-[M(CO) 4 (g2-H 2 ) 2 ].Cage-e§ects on ion-pair recombinations in viscous media have been studied.994,1158 Magnetic field e§ects on redox processes have been the subject of several reports. 1159–1162 Di§erent isotope e§ects, k H‘O /k D‘O , have been reported for solutionand surface-catalyzed proton-coupled electron transfer, [RuIII(OH)(bipy)(terpy)]2` ][RuII(bipy)(terpy)(H 2 O)]2`.1163 The need to understand critical heterogeneous processes in the atmosphere has stimulated further studies of the dynamics of reactions of HOBr,1164 HNO 3 ,1165,1166 and XNO 2 (X\Cl, Br)1167 with solid alkali metal halides, of ClONO 2 , HCl and HOCl1168–1171 or N 2 O 5 , ClNO 2 and HNO 3 1172,1173 on water ice, of CH 3 SO 3 H on water droplets,1174 ofN 2 O 5 and XNO 2 on salt solution droplets1175,1176 and of ClNO 2 , NO 2 and HONOon a sulfuric acid aerosol.1177–1180 References 1 A.Drljaca, C. 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ISSN:0260-1818    

DOI:10.1039/a804900i

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

28 Radiochemistry David S. Urch Chemistry Unit, IPES, Brunel University, Uxbridge, Middlesex, UK UB8 3PH 1 Introduction Most research activity in radiochemistry is now concentrated in nuclear medicine, in association with the nuclear power industry and with broad environmental concern about ‘radioactivity’ and ‘radiation’. This review will therefore concentrate on recent radiochemical work in these areas but will exclude a detailed consideration of radiation chemistry and the inorganic chemistry of elements that happen to be radioactive.The International Atomic Energy Agency continues to host, to sponsor and to organise conferences, symposia and work-shops on topics which include much radiochemistry, ‘Modern trends in radiopharmaceuticals for diagnosis and therapy’,1 ‘Therapeutic applications of radiopharmaceuticals’,2 and ‘In vitro radionuclide techniques in medical diagnosis’.3 Other international conferences continue to flourish, the 23rd International Symposium on Radioactive Isotopes in Clinical Medicine and Research, 4 the 7th World Congress of Nuclear Medicine and Biology,5 the 4th International Conference on Methods and Applications of Nuclear Chemistry,6 and the 2nd International Conference on Isotopes,7 as well as more parochial events such as the 2nd Russian Conference on Radiochemistry,8 the 2nd Conference on Nuclear Science and Engineering in Australia,9 the 2nd Regional Mediterranean Congress on Radiation Protection,10 the 9th ‘Tihany’ Conference on Radiation Chemistry,11 and the 13th (Czech) Radiochemical Conference.12 In this review the first section will deal with ways in which specific isotopes can be made or purified whilst the second will consider recent advances in the ways in which radioactive isotopes can be incorporated into labelled compounds. The third section will consider environmental aspects of radiochemistry and the last section miscellaneous topics such as ‘hot-atom’ chemistry. 2 Isotope production Light elements (Z\50) Optimum conditions for the release of tritium from neutron irradiated lithium silicate have been reported,13 as have improved techniques for collecting 11C as labelled Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 593–609 593carbon dioxide14 and 13N (from [16O(p16.5MeV,a)13N]) as labelled ammonia.15 Proton spallation in potassium chloride16 has been used to produce high specific activity 32Si, whilst the bombardment of iron with a particles produces17 isotopes of manganese (52 and 56), cobalt (55, 56 and 58) and nickel 56 and 57).Improvements in generators for both 52.Mn (from 52Fe)18 and for 68Ga (from 68Ge)19 have been described and so has a method20 for the separation of 79Se from fission products. Fission (from low-enriched uranium)21 is also a source of 99Mo, but alternative methods for production, such as neutron irradiation of molybdenum trioxide,22 have been reviewed,23 such is the world wide demand in nuclear medicine for this isotope as a source of technetium-99m.A new tandem cation–anion exchange column has been reported24 which enhances the concentration of 99.Tc in the eluent.A method for the preparation of the lighter isotope of technetium, 97Tc, which might be useful in environmental studies, has been described;25 95Mo is irradiated with a particles to form 97Ru which then b-decays to technetium. The needs of nuclear medicine have stimulated interest in the preparation of high purity 90Y26,27 and 105Rh.27 The latter isotope is also produced28 along with 101,106Rh, 103,104,105,106,110.,112Ag and 104,105,107,109,111.Cd when a palladium target is irradiated with a particles.Irradiation of silver29 (with deutrons) leads to the formation of 109Cd and the irradiation of cadmium30 produces 111In. Heavier elements The decay modes of 149Tb(17% a, 4%b`) and short half-life (q1@2 , 4.15 h) make this isotope uniquely interesting for nuclear medicine since it combines the potential for diagnosis by positron emission with therapy using a decay.It can be made31 by bombarding neodymium with carbon nuclei [142Nd(12C,5n)149Dy (decay) 149Tb]. Techniques for the preparation of other ‘nuclear medicine’ isotopes, 153Sm (from the neutron irradiation of 152Sm),32 163Ho (from irradiated erbium)33 and 166Ho (from neutron irradiation of 164Dy followed by the decay of 166Dy)34 have all been reported. 178.2Hf is quite a di§erent isotope, of interest in nuclear physics because of its high spin state and long half life (q1@2 , 31 y). It has now been extracted35 from a tantalum target that was irradiated in 1981. Rather more usual is the report36 of a generator based on 178W for the extraction of 178Ta. Following the succes-fou of technetium- 99m as the isotope for nuclear medicine a great deal of e§ort has been expended during the past year or two to produce the rhenium isotopes 186 and 188, since they can be used in similar ways to 99.Tc. 188Re can now be obtained from generators24,37 based on the b decay of 188W; this isotope is produced38,39 following double neutron capture by naturally occurring 186W.The lighter isotope 186Re can be made either by the neutron irradiation39,40 of natural rhenium [185Re(n,c)186Re] or by the proton irradiation39 of tungsten [186W(p,n)186Re]. The irradiation of natural iridium with protons and of natural osmium with a particles can lead to the formation41 of platinum isotopes, 188, 189 and 191 whilst the heavier isotope 199Pt can be made42 by neutron irradiation of natural platinum itself.The a emitting isotope 213Bi is now being tested for cancer chemotherapy and a generator43 based on 225Ac has been developed. 225Ac, which has a half-life of only 10 days, results from the decay of 229Th (via 225Ra). A detailed account has been given44 of the extraction and purification of 229Th (from the decay of 233U) and of the preparation of 225Ac.An alternative route to 229Th, by the Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 593–609 594neutron irradiation of 227Ra, was also considered,45 however the 228Th which is also produced gives rise to serious contamination. A method46 for the extraction of the heavier isotope of thorium, 235Th, following the neutron irradiation of 238U has been described.Extraction procedures have also been given for the isolation47 of 233Pa following fast neutron irradiation of 232Th and of 234U from 238Pu.48 Macroscopic (well, sort of) chemistry has proved possible49 with about 30 atoms of seaborgium made by the fusion reaction [248Cm(22Ne, 4 or 5 n)265 03 266Sg]. 3 Labelled compounds Reviews of progress in nuclear medicine have emphasised the advantages of the use of short lived positron emitting isotopes for diagnosis1 and for the potential of certain a-emitting isotopes50 for cancer therapy.It is one thing, however, to identify a suitable isotope because of its nuclear characteristics, quite another to get that isotope into such a form that it can be administered clinically. That is where the radiochemist comes in and this section will consider recent advances in labelling procedures. Tritium (3H) The simplest way of producing tritium labelled compounds is by exposure to tritium gas, but this can lead to the indiscriminate labelling of a range of decomposition products as well as the target molecule.Much greater specificity and regioselectivity can be achieved by the use of appropriate rhodium51 or iridium52 based catalysts.An obvious and widely used way of introducing tritium at specific sites in a molecule is to use tritium gas as a reducing agent (as in the preparation of tritium labelled derivatives of inositol53), and again catalysts are usually employed (e.g. 2,3[3H]-n-butyl alcohol,54 and [3H]dihydrorotenone55), variations on the palladium on charcoal theme being popular in the reduction of triple bonds,56 and in the preparation of [3H]azaline B acetate57 and [b-4,5-3H]cholestan-3-one.58 Another way of ensuring specificity in the location of a tritium label is to use a catalyst (again Pd–C is e§ective) to bring about tritium (from tritium gas)–halogen (usually bromine) exchange; recent examples of this procedure have been the labelling of quinolinone,59 naltrindole60 and enkephalin61 derivatives, of fluxetine,62 and of (R,S)-2-amino-2-(3-hydroxy-5-methylisoxazol-4- yl)acetic acid.63 Selective labelling can also be achieved by the reduction of carbonyl groups using sodium [3H]tetrahydridoborate. This technique has been used in the preparation of [12-3H]calanolide A,64 of a prokinetic motilide (ABT-229) labelled in the macrolactone ring,65 of labelled moth pheromones,66 and of tritiated derivatives of sphinganine- 1-phosphate,67 ribofuranosyl–imidazole68 and glutamic acid.69 Very high specific activities can be realised by the use of almost wholly tritiated tetrahydridoborate, NaB[3H] 4 (4.12 TBq mmol~1).A new simple synthesis of this reagent has been reported70 via lithium tritide which is made by reacting lithium metal with tritium gas.If wholly tritiated sodium tetrahydridoborate is allowed to react with glacial acetic acid, sodium triacetoxyborotritide is formed,71 which is even more selective than sodium tetrahydridoborate and can be used to reduce aldehydes in the presence of ketones, indole double bonds, lactones, etc. The preparation of N-tritioacetoxy- Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 593–609 595phthalimide has been described;72 a new reagent which can be used to introduce a very high specific activity tritium labelled acetoxy group into peptides. Carbon (11C) Improved procedures have been reported73 for the semi-automated, direct production of [11C] carbon dioxide by the proton bombardment of a pressurised cell filled with 2% oxygen, 98% nitrogen (see also ref. 14). This reagent can then be used to make [11C]methyl iodide,73 or reduced to [11C]carbon monoxide74 or used directly in reactions with Grignard reagents.75 Labelled methyl iodide can be used to prepare compounds such as N-[11C]methyl-1-(1,3-benzodioxol-5-yl)-2-butanamine,76 3-[M1- [11C]methyl-2(R or S)-pyrrolidinylNmethoxy]pyridine,77 N-[2-[11C]- (dimethylamino)ethyl]acridine-4-carboxamide78 and labelled nicotinic cholinergic agonists,79 all of which have applications in nuclear medicine.Medical applications have also stimulated the preparation of [11C]-labelled acids75 ([1-11C]-2-octynioc, [1-11C]-2-decynoic and [1-11C]-3-(R,S)-methyloctanoic) from [11C]carbon dioxide. A new development to facilitate the reaction of [11C]carbon dioxide with a Grignard reagent is to coat the inside of a polypropylene tube with the Grignard compound and then pass the radioactive gas down the tube.80 The complex so formed can then be decomposed in a variety of ways.If dilute acid is used following methylmagnesium bromide then [11C]acetate is made81 or if a solution of thionyl chloride is used after cyclohexylmagnesium chloride then cyclohexyl[11C]carbonyl chloride results.80 The on-line production of the useful reagent [11C]cyanogen bromide, has been described. 82 Oxygen (15O) An improved method for the (necessarily rapid) reduction of [15O]oxygen (made by the deutron bombardment of nitrogen containing 1% oxygen) to water using a palladium catalyst at 1500 °C has been reported.83 Fluorine (18F) Nucleophilic substitution reactions using [18F]~ continue to be widely used to e§ect the incorporation of radiofluorine into many molecules, either by halogen exchange ([18F]haloperidol)84 or by the use of reagents such as kryptofix2.2.2 (1-amino-3-[18F]- fluoromethyl-5-methyladamantane)85 or by aromatic substitution (3,4-dihydroxy-5- nitro-2@-[18F]fluorobenzophenone).86 Other procedures use [18F]fluorine gas, to prepare labelled fluoronucleoside analogues,87 or [18F]acetylhypofluorite, to synthesise 6-[18F]fluoro-L-dopa.88 Details have been given of a rapid method for making carrier free [18F]fluoroacetate89 and two methods for the labelling of oligodeoxynucleotides using either N-(4-[18F]fluorobenzyl)-2-bromoacetamide90 or succinimido-4-[18F]- fluorobenzoate,91 have been reported.Phosphorus (32P) Photoactive analogs of farnesyl pyrophosphate labelled with radiophosphorus can be prepared92 using [32P]phosphoric acid. Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 593–609 596Copper (64Cu) A series of copper complexes based on bis(thiosemicarbazone) or bis(salicylaldimine) ligands and labelled with [64Cu] has been prepared93 in order to test in vivo stability.Gallium (67Ga) The labelling of peptides with radiogallium can best be achieved by the use of a ligand which can not only chelate gallium but also bond (conjugate) to the peptide. A new complex of 67Ga with 1,4,7–triazacyclononane-1-succinic acid-4,7-diacetic acid has been prepared94 which links e§ectively to model compounds. Octreotide has also been labelled95 with 67Ga.Bromine (76 and 77Br) Nucleophilic halogen exchange was used to synthesise 5[77Br], 7-dibromo-4-oxo-1,4- dihydroquinoline-2-carboxylic acid96 from the corresponding 5-iodo compound, whilst compounds such as E-1-azabicyclo[2.2.2]oct-3-yl a-(1-[76Br]bromo-1,1- propen-3-yl)-a-hydroxy-a-phenylacetate97 and related compounds98 were prepared by electrophilic substitution of a strategically placed tributylstannyl group.Technetium (99mTc) Technetium-99m continues to be one of the most popular isotopes in nuclear medicine; a review99 of radiochemical procedures and clinical applications in Russia gives a typical picture. Current research is proceeding on two broad fronts, the optimisation of methods for the preparation of widely used compounds and the detailed characterisation of existing and new complexes.Thus three papers report100–102 improved ways of making the [99.Tc]-methoxyisobutylisocyanide complex and a couple more discuss103,104 the optimal procedures for the preparation of the compound of technetium( V) with dimercaptosuccinic acid. In these and other reports the aim is often to produce a direct labelling method, as in the preparation of derivatives of 1-aminoethylenediphosphonate acid,105 of pyridoxylamino acids,106 or of (4R)-1,3- thiazolidine-4-carboxylic acid.107 The other main strand of research concentrates on the identification of structural features in technetium compounds which can make them suitable for specific radiopharmaceutical applications.A recent paper reports108 on a detailed study of such compounds in which the technetium nitrogen triple bond is present.But it is the technetium–sulfur bond which is most often found to be important in complex formation. A series of recent papers has described109 the preparation of many complexes of pentavalent technetium in which the [TcO]3` unit is present and in which the technetium can accept co-ordination from four other ligand atoms thus completing an irregular trigonal bipyramid.Polydentate ligands such as C 2 H 5 N(CH 2 CH 2 SH) 2 can occupy three sites via the two sulfurs and the nitrogen. The fourth site can then be filled with another ligand (e.g. octanethiol or a substituted thiophenol). The biological activity and reactivity of these complexes has been evaluated.Other studies have Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 593–609 597investigated110 the labelling of tetrapeptides with technetium. Invariably these compounds contain cysteine. In a similar study, but of rather longer polypeptides, it was found111 that co-ordination of the [TcO]2` unit was restricted to the cysteine-glycinecysteine sequence. A much simpler complex can be readily made112 by complexing technetium directly with N-acetylcysteine.Although so readily formed, the Tc–S bond is succeptable to oxidation and it has been shown recently113 that the thiol group, in mercaptoacetyltriglycine, can be replaced by a hydroxyl to give a more stable and structurally similar complex. Labelling e¶ciency has also been investigated114 by studying the competitive kinetics of complex formation using a series of ligands with 1S, 3N or 2S, 2N or 3S, 1N as the four co-ordinating atoms. Rhodium (105Rh) The possibility of incorporating rhodium-105 into radiopharmaceuticals has been investigated by the preparation of suitable bifunctional ligands in which rhodium(III) is encapsulated in thiomacrocyclic ligands115 or tetradentate thioethers.116 Indium (111In) Bifunctional ligands have been made117 based on sterylamine or phosphatidylethanolamine which can co-ordinate to indium and also conjugate with systems having a high a¶nity for tumor cells. Iodine (123I, 125I, 131I) The simplest way of labelling a compound with radioiodine is by iodine isotope exchange, starting with radioactive sodium iodide. Optimum conditions118,119 for the preparation of labelled m-iodobenzylguanidine, using this method, as well as the kinetics of the exchange process,120 have been investigated.In a similar study of the radioiodination of amitriptyline,121 in which a range of catalysts was considered, a triphenylphosphine iridium complex was found to be the most e§ective. The preparation of long-chain fatty acid derivatives, such as x-p-[125I]iodophenylpentadecanoic acid122 has been accomplished by both [125I] for iodine and by [125I] for bromine exchange. Optimum conditions for the latter reaction have also been established for making 16-[131I]iodohexadecanoic acid123 and 7-chloro-5-[123I]iodokynurenic acid.124 This type of exchange was found to give better yields than radioiododestannylation in the preparation of [123I]iodomidazenil and related compounds.125 Both methods were used to produce a range of [123I] labelled ligands in a study of peripheral benzodiazepine receptors;126 the iodine–bromine exchange was e¶ciently catalysed by copper(I) compounds.The replacement of a trialkyl (often tributyl) substituted tin group by radioiodine (radioiododestannylation) in the presence of a mild oxidising agent such as chloramine-T has been successfully used to prepare, [123I]iodo analogues of (R- and S-)bretazenil,127 1-(2-hydroxyethyl)-4-(4-[123I]iodophenoxymethyl) piperidine128 and 4-(2-(4-azidophenyl)-5-(3-[125I]iodophenyl)-1Himidazol- 4-yl)pyridine.129 A similar procedure, but involving the removal of a chloro- Annu.Rep. Prog.Chem., Sect. A, 1999, 95, 593–609 598mercury group, was found to be the most e§ective way of making 5-[125I]iodo-2@- deoxyuridine.130 Samarium (153Sm) The complex [153Sm]samarium ethylenediamine tetramethylene phosphonate has been prepared131 and used in clinical trials for the relief of pain associated with skeletal metastases. Radiopharmaceuticals have also been made from [153Sm]samarium hydroxide132 and by the incorporation of samarium-153 into hydroxyapatite.131,133 Holmium (166Ho) Optimum procedures for the preparation of similar complexes, based on ethylenediamine tetramethylene phosphonate, have been reported134,135 for holmium- 166 and for their incorporation into hydroxyapatite.136 This holmium isotope has also been used to make a radioactive holmium–chitosan complex137 and to label monoclonal antibodies.138 Rhenium (186Re, 188Re) Generators, and other sources of rhenium radioisotopes, usually produce rhenium as the perrhennate anion which must be reduced before other labelled species can be made.It has been found139 that stannous chloride is an e§ective reducing agent and that the required Re(V) can be stabilised as the glucoheptonate.140 Direct labelling of antibodies, such as IgG,139,141 can then be achieved following the reduction of disulfide linkages with ascorbic acid.The same technique has been used to label small cyclic peptides142 (e.g. RC-160) and bleomycin.143,144 In other labelling procedures145 rhenium(VII) is reduced with SnCl 2 in the presence of critic- and tartaric-acids and the disulfide links in the monoclonal antibodies are reduced with bisulfite.A rather more sophisticated approach, which reduces the possibility of compromising the reactivity of the labelled moeity, is to synthesise a bifunctional ligand, which can both coordinate to the rhenium ion and also be conjugated to a protein; a range of derivatives of mercapto-acetyl-glycyl-glycyl-glycine has been shown146 to be e§ective in labelling many antibodies. Much recent work has been directed towards optimising the procedures for the preparation of Re(V) compounds of dimercaptosuccinic acid147–151 and 1-hydroxyethylene diphosphonic acid.150,151 Other routes to rhenium labelled compounds have utilised the tendency of rhenium to form strong multiple bonds with imido groups,152 so that labelled peptides and steroids can be made from hydrazine and phosphiniminimido precursors, and for the [Re(CO) 3 ]` cation to form complexes with proteins153 (e.g.histamine). Lead (203Pb) Monoclonal antibodies conjugated with bifunctional ligands can be labelled with lead-203 to produce154 radiopharmaceuticals with tumor localization potential. Annu. Rep. Prog. Chem., Sect.A, 1999, 95, 593–609 599Bismuth (213Bi) A similar procedure, using diethylenetriaminepentaacetic acid as the bifunctional compound,155 has permitted the development of a-emitting labelled proteins for cancer therapy. Astatine (211At) The same ligand has been used to label human IgC with astatine.156 Polymers157 and methylene blue have also been ‘tagged’.158 Automation, microwaves Automation of radiochemical procedures can often result in a reduction of synthesis time and can certainly lead to reduced doses to laboratory personnel.Specific examples have been described for work with 11C159 and 18F.160 The use of a microwave oven has been shown161 to facilitate the preparation of certain technetium complexes. Stability of labelled compounds Whilst much e§ort is expended in the synthesis of new and ever more complex compounds it is a pleasure to be able to report that comparable e§ort is now being put into a study of the radiochemical integrity of labelled compounds as a function of time.For tritium compounds the autoradiolysis of tritiated thymidine can be used as a bench-mark for decomposition,162 labelled water has been shown163 to be the main volatile product.A review of the self-radiolysis of carbon-14 labelled compounds has been published164 and detailed accounts given of methods to determine the radiochemical purity of technetium165 and iodine (123I and 131I)166 compounds. The stability of polypeptides labelled with either 131I or 188Re has been shown167 to be enhanced by the presence of ascorbic acid. 4 Environment Current radiochemical research that deals with the environmental impact of radioactive substances will be considered in this section. Much of the concern is related to the nuclear power industry and to the disposal and containment of its waste products.168 Also since many of the actinide by-products are very long-lived a detailed understanding of the geochemistry169 of these elements is essential.Solid matrices Various di§erent matrices continue to be proposed for the long-term containment of radionuclides, oxalates perhaps,170 or maybe a perovskite,171 zircon or monazite structure172 might be suitable, and a phosphate lattice, it has been shown,173 makes an Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 593–609 600excellent host for Th4` and Np4` ions (but for how long? the cynics ask).A detailed study of borosilicate glasses has established174 an optimum composition for the immobilisation of radioactive caesium and strontium. Other work has investigated175 the e§ect of various metals, their oxides and hydroxides on the rates at which radionuclides might be leached from their glassy matrix. And what other processes might take place with time? the build up of gases,176 for example, and the e§ect of irradiation upon the rocks that would surround an underground store,177 have both been the subject of recent papers.Solutions The behaviour of radionuclides in solution is of interest either because they are already there or because they might eventually get there (having escaped from their permanent long-term store where they had been immobilised).Thus recent studies have investigated what happens when dilute nitric acid solutions of thorium and plutonium pass through aluminosilicate rocks178 or the leaching of americium-241 from low-level waste.179 The solution chemistry of uranium(IV) oxide has been extensively studied, as a function of salt concentration and of pH,180 as a function of redox species181 and electrochemically.182 Uranium in ground water of old uranium mining areas183 or modern disposal sites,184 is usually present as a complexed uranyl species, its mobility being controlled by the vaguaries of the concentrations of other ions, calcium, sulfate and especially carbonate.It has been found that organophosphorus compounds185 or poly-2-ethylhexylphosphonitrilic acid186 can form complexes with itinerant actinides and so inhibit their migration.Omnipresent humic acid also forms complexes with many radionuclides and those with americium,187,188 neptunium189 and technetium190 have been studied. Levels of natural radioactivity in water can sometimes pose health problems. In Iran, bacteria (M G F-48) have been found191 to preferentially remove radium from solution. Radon too is soluble in water, but much more soluble192 in aromatic solvents. Adsorption Clay minerals, especially illites, have a great capacity for adsorbing ions from solution.A recent study193 has looked in detail at the di§erent types of site present in such minerals and established the optimum conditions for the extraction of caesium-137 from solution.The adsorption of some of the radionuclides released following the Chernobyl incident, strontium-90,194 plutonium and americium,195 has been studied as a function of soil structure, pH, humus content etc. Technetium ions too can be adsorbed on mineral surfaces,196 iron and sulfur containing species being especially active.197 Other investigations have shown how uranium and thorium ions can be adsorbed onto soil sediments198 and how uranium adsorption in particular varies with ionic strength.199 Rather more specific have been the reports of the adsorption of uranium on montmorillonite,200 of neptunyl cations on ferric oxide,201 of caesium on hexacyanoferrates202 and of stronium on zirconia.203 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 593–609 601Other Air-born radioactive contamination following the Chernobyl incident has been shown204 to depend on a variety of environmental factors, but in particular the prevelence of forest fires which can facilitate the transport of caesium (137Cs) and strontium (90Sr) as aerosols. Mixed waste can be treated by the Molten Salt Oxidation process by heating with fused sodium carbonate.Organic material is oxidised away and metals, which may include radioactive ones, accumulate in the residue. Whilst this is clearly a very practical way of dealing with potentially toxic waste, it does pose the problem of what to do then with the contaminated carbonate.205 Another recent report has described206 how natural phosphorites may be ‘decontaminated’ by the removal of the uranium which they contain, using polyalkyl phosphazine. This has the twofold (or maybe just onefold, depending on your point of view) advantage of generating a new source of phosphate fertilizer and also of uranium. 5 Miscellaneous ‘Hot-atom’ chemistry Many di§erent types of nuclear reaction have been considered in a general review207 of the ways in which recoil atoms can be used to label fullerenes.When recoil tritium (made by neutron irradiation of either 3He or 6Li) comes to rest in graphite, a considerable amount is bound within the lattice but some is released, on heating, as labelled methane.208 EPR has been used to examine the initially generated paramagnetic centres. The recoil chemistry of 7Be, following the [12C(c,a)7Be] reaction, has been studied both in a series of b-diketonate complexes209 and also with fullerenes.210 In the latter case 11C is also produced so the reactions of both recoil species can be investigated.When recoil 11C is generated in a 5% hydrogen, 95% nitrogen mixture [11C]methane is formed211 in good yield whilst the recoil 13Nproduced by the proton irradiation of ultra-pure water ends up as [13N]ammonia.212 Conventional ‘Szilard–Chalmers’ studies have been made of the reactions of recoil arsenic-76 (following the neutron irradiation of arsenobetaine and arsenocholine),213 of recoil holmium- 166, [165Ho(n,c)166Ho],214 of recoil rhenium-186, and -188 in rhenium metal and oxide,215 and of recoil ruthenium-97 and -103 (made by c,n reactions) in solid ruthenocene complexes.216 Further studies have been made to elucidate the role of charged species, in the reactions of recoil bromine that results from the n,c reaction, by allowing the reaction to take place in an electric field.When dibromomethane is irradiated217 the addition of amines appears to neutralize any potentially reactive ions. Electrostatic fields have also been used218 to collect the charged 216Po (ThA) ions that result from the decay of thoron.The subsequent decay of ThA endows the daughter 212Pb with considerable recoil energy, giving rise to a lead labelled surface layer. Radioactive sources The method described above can be used to make a 212Pb source; other techniques Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 593–609 602involve the condensation of metal vapour on a suitable substrate and the electrochemical reduction of a solution which will leave the radionuclide bound in a polymeric film.The former procedure was used to make an 243Cm a-source for the X-ray emission analysis of Martian rocks219 and the latter used to make uranium and neptunium sources220 in a polyacrylonitrile layer. Other reference sources reported recently are for thorium,221 uranium and plutonium.222 The preparation of a 147Pmsource,223 for use as a b-ray thickness gauge and of potassium hexacyano[57Fe]ferrate(II and III) sources224 for use in Mo� ssbauer spectroscopy have also been described.Perturbed angular correlation (PAC) A very few isotopes, when they decay by cascade c-emission exhibit an angular correlation between one c-ray and the next.This correlation angle can be a§ected by the chemical environment of the decaying atom. Thus a study of such ‘perturbed angular correlation’ can be used to deduce chemical and structural information. 111Cd is such an isotope and is formed by the b-decay of 111Ag. Thioethers have been used to complex Ag(I) and also to conjugate to various antibodies.225 When such labelled antibodies exhibit tumor selectivity the PAC technique can provide a unique diagnostic tool.Other Polyacrylic acid can apparently absorb a considerable amount of water, so that if a solution of acrylic acid in tritiated water is c-irradiated the subsequent polymerization will lead to a gel;226 convenient for the storage of tritiated water.References 1 International Symposium on Modern Trends in Radiopharmaceuticals for Diagnosis and Therapy, International Atomic Energy Agency, Vienna, Austria, 1998. 2 International Seminar on Therapeutic Applications of Radiopharmaceuticals, International Atomic Energy Agency, Vienna, Austria, 1998. 3 In vitro Radionuclide Techniques in Medical Diagnosis, International Atomic Energy Agency, Vienna, Austria, 1998. 4 23rd International Symposium on Radioactive Isotopes in Clinical Medicine and Research, Eur. J. Nucl. 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Kawazoe, Kakuriken Kenkyu Hokoku (Tohoku Daigatu), 1997, 30, 69. 211 J. I. Sachinidis, H. J. Tochon-Danguy, J. Mu, J. G. Chan and W. J. McKay, Aust. N.Z. J. Med., 1998, 28, 359. 212 K. Suzuki, M. Sasaki, Y. Yoshida, T. Haradahira and O. Inoue, Proceedings of the 7th International Symposium on Advanced Nuclear Energy Research, Japan Atomic Energy Research Institute, Tokyo, Japan, 1997, p. 318. 213 Z. Slejkovec, A. R. Byrne, J. T. van Elteren and J. J. M. de Goeij, Proceedings of the 13th Radiochemical Conference, Czech Chemical Society, Prague, Czech Republic, 1998, p. 337. 214 S. K. Zeisler and K. Weber, J. Radioanal. Nucl. Chem., 1998, 227, 105. 215 W. Jia and G. J. Ehrhardt, Radiochim. Acta, 1997, 79, 131. 216 T. Sekine, I. Yamaguchi and H. Kudo, Kakuriken Kenkyu Hokoku (Tohoku Daigaku), 1997, 30, 23. Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 593–609 608217 M. R. Zaman, Pak. J. Sci. Ind. Res., 1997, 40, 12. 218 C. Jech, Proceedings of the 13th Radiochemical Conference, Czech Chemical Society, Prague, Czech Republic, 1998, p. 221; C. Jech, J. Kubasta and A. Gosman, J. Radioanal. Nucl. Chem., 1998, 230, 281. 219 V.M. Radchenko, V. D. Gavrilov, M. A. Rajbinin, B. M. Andrejchikov, G. Venke, B. N. Korchuganov, R. Rider and T. Ekonomoy, Proceedings of the 13th Radiochemical Conference, Czech Chemical Society, Prague, Czech Republic, 1998, p. 339. 220 L. Martinot, P. Faack, M. Krausch, M. Mertens, C. Calberg, R. Jerome, L. Lopes, J. Guillaume, G. Ghitti, J. Marien, J. Riga and J. Schrijnemackers, Radiochim. Acta, 1996, 75, 111. 221 K. Raptis, F. Hendrickx, P. De Bievre and K. Mayer, Fresenius’ J. Anal. Chem., 1998, 361, 400. 222 G. Kueppers, J. Radioanal. Nucl. Chem., 1998, 230, 167. 223 P. S. Balasubramanian, J. Radioanal. Nucl. Chem., 1998, 229, 157. 224 S. Ganguli, S. Das and M. Bhattacharya, J. Radioanal. Nucl. Chem., 1998, 232, 229. 225 B. Ctortecka, W. Troeger, T. Butz, R. Alberto, D. Angst and P. A. Schubiger, Proceedings of the 2nd International Conference on Isotopes, ed. C. J. Hardy, Australian Nuclear Association Inc., Sutherland, NSW, Australia, 1997, p. 135. 226 C. I. Postolache, C. C. Ponta, A. Dragomir and I. Mihalcea, National Seminar on Public Information on Peaceful Uses of Atomic Energy, National Agency for Atomic Energy, RO–70168 Bucharest, Romania, 1998, vol. 2, p. 227. Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 593–609 609

ISSN:0260-1818    

DOI:10.1039/a804889d

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

29 Bioinorganic chemistry J. D. Crane Department of Chemistry, University of Hull, Cottingham Road, Kingston-upon-Hull, UK HU6 7RX 1 Introduction A new reference text on spectroscopic methods in bioinorganic chemistry has been published.1 Specialist review articles have covered enzyme bioelectrochemistry in biomembrane-like films,2 organometallic derivatives of amino acids and peptides,3 and the biomedical chemistry of technetium and rhenium.4 The bond valence sum analysis of copper and iron metalloproteins, and of biologically relevant iron coordination compounds has been reported.5 The various roles of protein radicals in enzyme catalysis have been reviewed,6 as has the chemistry of the three classes of ribonucleotide reductase.7 An ab initio study of the oxidative damage to cysteine residues in proteins and the oxidative damage caused as a result of H· abstraction by the resulting thiyl radicals has been reported.8 The incorporation of a cysteine residue in a Thr-89-Cys mutant of bacteriorhodopsin in order to use the S–H group as an IR probe for hydrogen-bonding has also been described.9 The chemistry of RNA/DNA cleavage has been extensively reviewed.10 The DNA binding properties of several ruthenium(II) and rhodium(III) polypyridine luminescent probes have been investigated.11 A CD and NMR spectroscopy study has shown that the observed, fast, long-range electron transfer mediated byDNAis not simply a result of the metallointercalator probes binding cooperatively within close proximity of one another.12 The DNA binding of tetracationic porphyrin complexes of vanadyl(IV) and cobalt(III),13 dicationic polypyridyl platinum(II) complexes,14 and the nuclease properties of phenanthroline complexes of copper(I/II) have also been reported.15 2 Magnesium and calcium Three crystal structures of adenylate kinase from Bacillus stearothermophilus with bound substrate and magnesium(II) or manganese(II) have been reported.16 The role of the nucleophilic Ser-102 residue in the alkaline phosphatase from Escherichia coli has been investigated through crystal structure determinations of the Ser-102-Gly, Ser- Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 611–630 611102-Ala and Ser-102-Cys mutants with phosphate bound at the active site.17 The functional roles of the two magnesium(II) ions at the active site of DNA polymerase b have been investigated using inert chromium(III) nucleotide complexes.18 The stability constants for the binding of magnesium(II), zinc(II) and copper(II) to the nucleotide 2-deoxyguanosine-5-monophosphate (dGMP2~) and its platinum(II) complex cis- [Pt(NH 3 ) 2 (dGMP) 2 ]2~ have been determined; the platinum(II) centre had little influence upon metal ion binding to the phosphate group.19 Recent progress in the development of synthetic nucleases and related metalloenzymes has been reviewed.20 The structure and function of calcium binding proteins have been reviewed,21 as have the EF-hand proteins in particular.22 The thermodynamics of the binding of magnesium(II) and calcium(II) to calmodulin have also been described.23 The crystal structure of calsequesterin from rabbit muscle has been reported and a mechanism proposed for the binding and release of 40–50 calcium(II) ions for each contraction/ relaxation cycle.24 The role of the calcium(II) ion in the reoxidation of PQQH 2 to the coenzyme PQQ by dioxygen has been investigated using semi-empirical MO calculations.25 Biomineralisation processes have been investigated with a variety of artificial systems.The crystallisation of spherical calcite particles on colloidal gold particles coated with a monolayer of p-sulfanylphenol has been described,26 as has the deposition of aragonite and strontianite using similar monolayers on a gold(III) surface.27 The nucleation of calcium phosphate and calcite/aragonite with polymers,28 and the crystallisation of apatite and vaterite in polymer matrices,29 have also been described. 3 Vanadium The full speciation for the aqueous vanadate(V)–uridine (UrH 2 )–imidazole system has been determined.30 In the absence of imidazole the major species are dimeric complexes of general composition [V 2 Ur 2 ]2~,3~, whereas in the presence of imidazole monomeric mixed ligand species are formed.Model compounds for the binding of vanadium(III) and vanadyl(IV) to peptides,31 and vanadate(V) esters of chelating alcohols as models of phosphate esters,32 have been reported. The catalytic oxidation of thioethers using several vanadium halogenoperoxidases has been described;33 the bromoperoxidase from Ascophyllum nodosum oxidises phenylmethylsulfide to the (R)-sulfoxide with 85% ee.Several model complexes for vanadium halogenoperoxidases have also been reported.34 Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 611–630 6124 Manganese The structure and reactivity of manganese redox enzymes and relevant model compounds have been reviewed.35 The di§erences in reactivity for iron and manganese haems in peroxidases have been studied through the preparation of the manganese(III) complex of microperoxidase-8 via demetalation of the well-known iron(III) analogue.36 The active site of human manganese superoxide dismutase has been investigated with the Gln-143-Asn and Tyr-34-Phe mutants, both of which have been structurally characterised;37 the Gln-143 residue appears to control the redox balance of the manganese ion and the Tyr-34 residue is proposed as the proton source for the production of hydrogen peroxide.Inactivation of the wild type enzyme with peroxynitrite proceeds by exclusive nitration of Tyr-34 at the 3-position.38 Despite the fact that manganese and iron superoxide dismutase enzymes often have similar site structures the presence of the wrong metal ion usually produces little or no catalytic activity.The determination of the crystal structure of the iron-substituted manganese superoxidase dismutase from Escherichia coli has shown that the stereochemistry of the resting state di§ers significantly from the active manganese form.39 The inactivity of these metal-exchanged superoxide dismutases has been explained by the incorrect redox balance of the metal ion rather than competitive inhibition of superoxide binding by hydroxide.40 Simple model complexes for manganese superoxide dismutases have been described,41 as has a stable alkylperoxo complex of manganese( II).42 Spin-coupled binuclear manganese(II) sites have been identified by EPR spectroscopy in the aminopeptidase P from Escherichia coli and the dinitrogenase reductase activating glycohydrolase from Rhodospirillum rubrum.43 A dicobalt(II) functional model complex for the dimanganese(II) site of arginase has also been described.44 The oxidised dimanganese(III) form of manganese catalase from Thermus thermophilus exists as an equilibrium mixture of two forms, dependent upon pH;45 one displays weak antiferromagnetic coupling (low pH, J[[2 cm~1) whereas the other is strongly coupled (high pH, J[[100cm~1).Several model complexes for manganese catalase have also been studied.46 Dioxygen production by the oxygen evolving complex of photosystem II has been reviewed.47 A multiline EPR signal has been reported for the S 1 state of photosystem II.48 In calcium depleted photosystem II advancement past the S 2 state is inhibited and illumination produces a new state formally equivalent to S 3 .ESE-ENDOR spectroscopy of this state indicates that a tyrosyl radical (YZ ·) has been generated and is magnetically coupled to the manganese cluster.49 This redox-active tyrosine residue has been proposed as facilitating hydrogen abstraction during water oxidation.50 The reaction of the S 2 state of photosystem II with nitric oxide at low temperature generates a state with an EPR spectrum characteristic of a magnetically isolated, mixed-valence dimanganese(II/III) unit.51 The nature of the species responsible for the g\4.1 EPR signal of the S 2 state has been investigated by EPR spectroscopy and a SQUID magnetisation study, and an S\5 2 spin state is proposed.52ESEEM spectroscopy has shown that the manganese cluster of photosystem II is accessible to small alcohols, specifically methanol and ethanol, with limited access for propan-1-ol and none for propan-2-ol.53 Several multinuclear manganese model complexes for the oxygen-evolving complex of photosystem II have been decribed.54 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 611–630 6135 Iron haem biosites The autoxidation chemistry of oxygenated myoglobin and haemoglobin,55 the spectroscopy of metal ion reconstituted haemoglobins,56 and the chemistry of leghaemoglobin, 57 have all been reviewed.The majority of the protons of the haem region for several deoxy-myoglobins have been assigned by 1H NMR spectroscopy.58 The photodissociation and rebinding of the axial water molecule of the low temperature (20 K), low-spin form of sperm whale deoxy-myoglobin has been studied.59 The iron–ligand bond lengths in horse heart deoxy- and aquamet-myoglobin have been determined by XAFS spectroscopy.60 ResonanceRaman studies of the carbon monoxide adducts of haemoglobin with selectively deuteriated haems have helped to clarify the debate concerning the assignment of the d(Fe–CO) fundamental, favouring the weak band at[580 cm~1.61 Resonance Raman spectroscopy of the carbon monoxide adduct of the His-64-Leu mutant of sperm whale myoglobin has shown that the Fe–CO group is hydrogen-bonded to the distal His-64 residue in the wild type protein.62 Density functional theory has also been used to investigate the geometry and the ease of deformability of the Fe–COgroup in haem proteins.63 The reactivity of haemoglobin towards nitric oxide under physiological conditions has been described. 64 The iron–ligand bond lengths in the nitric oxide adducts of iron(II) and iron(III) horse heart myoglobin have been determined by XAFS spectroscopy and are in good agreement with model compounds.65 Haemoglobin I from Lucina pectinata is known to bind reversibly hydrogen sulfide rather than dioxygen; resonance Raman and 1H NMRspectroscopy have indicated that the haem is not firmly anchored in the haem pocket and that the resulting rocking freedom may facilitate substrate binding.66 A 13C NMR spectroscopy study of six-co-ordinate haem model complexes has shown that the chemical shift of the meso carbon atoms is a good indicator of the degree of porphyrin rußing.67 The chemistry of haem oxygenase has been reviewed.68 The role of the distal His residue in haem oxygenase has been studied by 1H NMR and EPR spectroscopy,69 and by the e§ect of mutation of this residue upon the products of haem degradation.70 The co-ordination chemistry and reactivity of verdohaems, which are observed as intermediates in haem degradation, have been investigated with model compounds.71 Barley grain peroxidase is reversibly deactivated at pH[5 and crystal structure determinations at several pH values have allowed the identification of the structural changes responsible for the change in activity.72 Resonance Raman and infrared spectroscopy have been used to investigate the role of the proximal His residues in the chloroperoxidase from Notomastus lobatus and the dehalogenoperoxidase from Amphitrite ornata.73 Resonance Raman spectroscopy has also been used to investigate the hydroxylation of benzene by horseradish peroxidase.74 An agostic mechanism for the oxidation of C–H bonds by cytochrome P450 has been suggested as an alternative to the rebound mechanism.75 This is not supported by subsequent studies with substrate probes, which indicate that there are two di§erent oxidants present in the P450 system, both of which can e§ect hydroxylation.76 Cytochrome P450 has been the subject of several theoretical investigations, one conclusion of which is that simultaneous delivery of two protons to the bound dioxygen is necessary to generate the ferryl Fe––O intermediate e¶ciently.77 The role of Asp-251 as a proton delivery group at the active site of cytochrome P450 has been investigated through determination of the Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 611–630 614crystal structure of the Asp-251-Asn mutant.78 Several model compounds for cytochrome P450 have also been described.79 Resonance Raman spectroscopy has been used to study the reduction of nitric oxide to nitrous oxide by cytochrome P450nor (nitric oxide reductase) from Fusarium oxysporum.80 A Cys-ligated, low-spin, iron(III) haem similar to cytochrome P450 has been found in the carbon monoxide sensing protein CooA from Rhodospirillum rubrum.81 The structure, spectroscopy and function of cytochrome c oxidase have been reviewed. 82 The binding of carbon monoxide at the haem a 3 /Cu B site of cytochrome bo 3 from Escherichia coli has been investigated with photoacoustic calorimetry.83 The magnetic coupling between haem a 3 and Cu B of cytochrome bo 3 has been quantified through the successful simulation of the EPR spectra of this site.84 This work indicates very weak coupling (J[1 cm~1) in contrast to the currently accepted description for this site (J[100 cm~1).Several model complexes for the haem a 3 /Cu B unit have been described.85 The crystal structure of turnip cytochrome f has been determined at 1.96Å resolution.86 6 Iron non-haem biosites The structure of the outer membrane receptor for the iron-bound form of the ferrichrome siderophore of Escherichia coli has been crystallographically characterised both with and without bound siderophore.87 The solution structure of the gallium(III) complex of the peptidic siderophore pyoverdin G4R from Pseudomonas putida G4R has been determined by 1H NMR spectroscopy.88 The co-ordination chemistry of the bis(catecholate) siderophores (amonabactins) from Areomonas hydrophilia and the cyclic dihydroxamate siderophores alcaligin and bisucaberin has been studied.89 Models for the co-ordination of tris(catecholate) siderophores have also been described. 90 The crystal structures of several forms of human transferrin have been determined which have enabled the identification of structural changes upon iron binding/release. 91 The role of second co-ordination sphere hydrogen-bonding on the ironbinding properties of human transferrin has been investigated with a series of mutant proteins.92 The reduction potentials for the iron centre in several forms of human transferrin at pH 5.8 have been determined.93 In all cases the potentials were more negative than [500mV (vs.NHE) and thus too negative for physiological reducing agents, suggesting that there is an additional process involved in iron release which significantly raises this potential. Various derivatives of the iron(II) and iron(III) forms Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 611–630 615of bleomycin and the interaction of its substituted hydroperoxocobalt(III) form with oligonucleotides have been studied with a range of spectroscopic techniques.94 Mononuclear iron(III) complexes as functional models for iron superoxide dismutase,95 and as spectroscopic models for the binding of peroxide at an iron(III) centre,96 have also been reported.A combined NIR and MCD spectroscopy study of a selection of structurally characterised, high-spin, iron(II) model complexes has allowed the correlation of spectroscopic properties and structural features, which may then be used to elucidate the structures of iron(II) sites in metalloproteins.97 Specifically, this approach has been used to investigate the binding of the a-ketoglutarate cosubstrate at the iron(II) centre in clavaminate synthase.98 The structure of the mononuclear iron(II) site of peptide deformylase from Escherichia coli and those of the nickel(II) and zinc(II) substituted forms, one with bound peptide reaction product, have been determined by X-ray crystallography.99 The axial Tyr-447 iron ligand in protocatechuate 3,4-dioxygenase dissociates from the metal centre upon binding of the substrate. In order to investigate this process the crystal structure and reactivity of the Tyr-447-His mutant has been determined; both substrate binding and product release were substantially slower than for the wild type enzyme.100 This ligand dissociation process has also been established for several other dioxygenases by XAFS spectroscopy.101 Several functional model complexes for catechol dioxygenases have been described.102 The crystal structure of rat tyrosine hydroxylase with bound cofactor analogue 7,8-dihydrobiopterin has been determined at 2.3Å resolution.103 The structure shows that the Phe-300 residue is autocatalytically hydroxylated at the 3-position, hydrogen-bonds through this group to the active site and p-stacks with the pterin cofactor.The crystal structure of the similar human phenylalanine hydroxylase has also been reported.104 The relationship between iron co-ordination environment and activity for soybean lipoxygenase-1 and human 15-lipoxygenase has been investigated by EPR and MCD spectroscopy on the wild type enzymes and the Asn-694-His mutant of the soybean enzyme, a mutation designed to produce a site structure more similar to that of the human enzyme.105 A structural and spectroscopic model for the hydroxyiron(III) form of soybean lipoxygenase-1 has also been described.106 The crystal structure of dark-inactivated nitrile hydratase with nitric oxide bound at the iron(III) centre has been determined.107 Two of the Cys ligands are modified to sulfenic and sulfinic acids respectively, and together with the Ser residue these surround the nitric oxide ligand with oxygen atoms, an arrangement that is postulated to assist in the photodissociation of the Fe–NO group required to generate the active enzyme.Several model compounds for nitrile hydratase, both with and without bound nitric oxide, have been described.108 Low temperature radiolytic reduction of the diamagnetic di-l-oxo-diiron(IV) form of methane monooxygenase (intermediate Q) has been shown by 57Fe Mo� ssbauer spectroscopy to generate a mixed-valence diiron(III/IV) form.109 In addition, the 57Fe Mo� ssbauer parameters for this species are very similar to those of the diiron(III/IV) form of ribonucleotide reductase (intermediate X), suggesting a strong similarity in structure and reaction chemistry for the two biosites.ENDOR spectroscopy of the diiron(II/III) form of methane monooxygenase has shown that methanol binds directly to a vacant site at the diiron core, and that methanol, water and dimethyl sulfoxide are able to bind simultaneously to the metal centres.110 A resonance Raman spectroscopy signature Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 611–630 616for the postulated di-l-oxo-diiron(IV) diamond core of intermediate Q has been identified for model complexes containing this structural unit.111 The reaction of methane with a di-l-oxo-diiron(IV) diamond core has also been studied theoretically. 112 XAFS spectroscopy of ribonucleotide reductase intermediate X from Escherichia coli has identified a short Fe–Fe distance of 2.5Å that is not present in the diiron(III) form.113 ENDOR spectroscopy of intermediate X generated from the reaction of the diiron(II) form with dioxygen has shown that the latter is incorporated as a single oxo bridge and a terminal water ligand. If there are additional bridging ligands they are carboxylates rather than oxo or hydroxo.114 Comparison of the site structures of methane monooxygenase from Methylococcus capsulatus and ribonucleotide reductase from Escherichia coli reveals only one non-equivalent amino acid residue; the counterpart of Glu-114 in the former is Asp-84 in the latter.The consequence of this di§erence has been investigated through the preparation of the Asp-84-Glu mutant of the ribonucleotide reductase.115 Unlike the wild type protein the reaction of the diiron(II) form of this mutant with dioxygen forms an observable peroxodiiron(III) intermediate similar to that known for methane monooxygenase.For the successful generation of intermediate X a reducing equivalent must be transferred to the diiron core and site-directed mutagenesis studies have indicated that the reductant is probably H· and that it is transferred via Trp-103 and Asp-266.116 In other mutagenesis studies the absence of the Tyr-122 residue (the normal residue oxidised to a tyrosyl radical by the diiron centre) in the Tyr-122-Phe–Glu-238-Ala mutant results in 3- hydroxylation of the Phe-208 residue, which then binds to the diiron core.117 Furthermore, in the Phe-208-Tyr mutant the Tyr-208 residue is co-ordinated to iron even in the diiron(II) form and upon reaction with dioxygen is hydroxylated to dopa (3,4- dihydroxyphenylalanine) which remains co-ordinated to the diiron(III) core.EPR and ENDOR spectroscopy have shown that the tyrosyl radical in mouse ribonucleotide reductase is probably directly hydrogen-bonded to the water ligand formed at the diiron core; it is postulated that this interaction helps to stabilise the radical.118 Functional diiron model compounds for methane monooxygenase and ribonucleotide reductase have been reviewed.119 The structure, spectroscopy and reactivity towards dioxygen of several new model complexes for these metalloenzymes have also been described.120 The alkene monooxygenase from Xanthobacter Py2 contains a diiron core similar to that found in toluene 4-monooxygenase.121 XAFS and 57Fe Mo� ssbauer spectroscopy of the diiron(III) site of stearoyl-ACP D9-desaturase has identified a mixture of two site structures: [Fe 2 (l-O)(l-O 2 CR 2 ) 2 ]2` with Fe–Fe 3.12Å and [Fe 2 (l-OH)(l- O 2 CR 2 ) 2 ]3` with Fe–Fe 3.41Å.122 The peroxodiiron(III) intermediate form of this biosite has also been characterised by optical and resonance Raman spectroscopy.123 The mixed metal iron–zinc form of uteroferrin with bound molybdate and tungstate anions has been studied by XAFS spectroscopy.124 A dioxygen binding model complex for haemerythrin has also been described.125 The mineralisation of ferritin has been reviewed.126 The formation of a peroxodiiron( III) intermediate during the ferroxidase reaction of ferritin mineralisation has been observed by 57Fe Mo� ssbauer spectroscopy.127 The structure and electronic properties of iron–sulfur proteins,128 and recent developments towards understanding the many biological roles for these proteins,129 have been reviewed.The crystal structure of the rubredoxin from Pyrococcus furiosus has Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 611–630 617been determined at 0.95Å resolution.130 Isotope-dependent (2H/1H) Fermi-contact e§ects on the 15N NMR chemical shifts for the iron(II) and iron(III) forms of the rubredoxin from Clostridium pasteurianum have been used to identify the peptide hydrogen-bonds to the Cys ligands.131 In addition, the origins of the observed contact shifts for 1H, 2H, 13C and 15N nuclei for both oxidation states of this rubredoxin have been investigated with density functional calculations.132 The solution structure of the iron(II) form has also been determined by 1H NMR spectroscopy.133 Sulfur K-edge XAFS spectroscopy has been used to determine the degree of Fe–S covalency in a range of iron(III) rubredoxins and model compounds, although there was no apparent correlation between the degree of covalency and the site structure.134 Rubredoxin-like biosites have been rationally designed into iron–sulfur free proteins by site-directed mutagenesis.135 The crystal structure of the Cys-42-Ser mutant of the iron(III) rubredoxin from Clostridium pasteurianum reveals an Fe–Oc(Ser-42) distance of 1.83Å and significant structural changes around the Ser-42 residue.136 An attempt to reduce this biosite to the iron(II) form with dithionite resulted in loss of iron and SO 2 addition to the Cys-6 and Cys-9 residues of the iron binding site.Rubredoxin model compounds which incorporate amide NH· · · S hydrogen-bonding have been reported.137 The crystal structure of the [2Fe-2S] ferredoxin adrenodoxin has been determined at 1.85Å resolution.138 A method for incorporating protein environment e§ects into density functional calculations on [2Fe-2S] ferredoxins has been described.139 The electronic and magnetic properties of [4Fe-4S] ferredoxins and their model compounds have been reviewed.140 The all-iron(II) form of the [4Fe-4S] cluster of the iron protein of Azotobacter vinelandii nitrogenasen studied by MCDspectroscopy; it exhibits transitions in the 450–1500nm range, very di§erent from iron(II) rubredoxins.141 The solution structure of the oxidised ferredoxin I from Desulfovibrio africanus has been determined by 1H NMR spectroscopy and compared with the crystal structure.142 The minimal protein ligand requirements for the incorporation of a stable [4Fe-4S] cluster has been investigated using a series of models with peptides containing sixteen amino acids.143 The structure and properties of high potential iron proteins (HiPIPs) have been reviewed.144 The crystal structure of the HiPIP from Chromatium purpuratum has been determined,145 and the kinetics of iron cluster assembly for the HiPIP from Chromatium vinosum have been studied by time-resolved fluorescence spectroscopy.146 The crystal structure of the [3Fe-4S]-containing ferredoxin I from Azotobacter vinelandii has been determined at 1.35Åresolution.147 The [3Fe-4S] and [4Fe-4S] forms of the iron–sulfur cluster of Pyrococcus furiosus ferredoxin have been studied by 57Fe ENDOR spectroscopy.148 A new type of [3Fe- 4S]` cluster with S\5 2 has been spectroscopically identified in the Ala-33-Cys mutant of this ferredoxin.149 The proton-coupled electron transfer behaviour of several [3Fe- 4S] clusters has been investigated by protein film voltammetry.150 The solution structure of the [3Fe-4S]][4Fe-4S] ferredoxin from Bacillus schlegelii has been studied by 1H and 2H NMR spectroscopy.151 The crystal structure of the iron-only hydrogenase from Clostridium pasteurianum has been determined at 1.8Å resolution.152 The active site consists of a [4Fe-4S] cluster bridged through a Cys residue to an unusual triply bridged diiron site.A structural model for this diiron site has also been reported.153 Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 611–630 6187 Cobalt The crystal structures of two di§erent forms of methylmalonyl-CoA mutase from Propionibacterium shermanii have been determined.154 The structure of an isomer of coenzyme B 12 in which the configuration of theN-glycosidic bond is inverted has been reported, and its reactivity compared with that of the normal coenzyme.155 The chemical environment of the cobalt nucleus has been investigated by single crystal 59Co NMR spectroscopy.156 A resonance Raman study of the binding of coenzyme B 12 to methylmalonyl-CoA mutase has shown that although the conformation of the corrin ring is significantly altered upon binding, the Co–C bond appears una§ected.157 The kinetics of Co–C bond cleavage in enzyme bound coenzymeB 12 have been studied by stopped flow spectrophotometry.158 The role of coenzyme B 12 in the interconversion of methylmalonyl-CoA and succinyl-CoA has been investigated with ultrafast radical clocks.159 The results indicate that the isomerisation does not occur by a radical mechanism, but by either an anionic or organocobalt route.The photolysis of methylcobalamin and coenzyme B 12 has been studied by ultrafast transient absorption spectroscopy,160 and time-resolved photoacoustic calorimetry.161 The crystal structure of trifluoromethylcobalamin has been reported.162 The possible sites for ion binding to cobalamins have been identified in the LiCl-containing crystal structures of azido- and chloro-cobalamin.163 The synthesis of the pentafluorothiophenolate derivative of cobalamin has also been described.164 Some methanogenic bacteria are known to use methanol as a source of methyl for the synthesis of methane and acetyl-CoA.This chemistry has been successfully modelled by the synthesis of a Co–CH 3 derivative from the reaction of methanol with a reduced cobalt(I) corrin complex in the presence of a Lewis acid.165 Cobaloxime model compounds for coenzyme B 12 and their evaluation by molecular modelling have also been described. 166 The crystal structure of the methionine aminopeptidase from Pyrococcus furiosus containing a dicobalt(II) active site has been determined at 1.75Å resolution.167 8 Nickel Recent developments in elucidating the structure and function of nickel metalloproteins have been reviewed.168 The crystal structure of urease from Bacillus pasteurii with the inhibitor 2-mercaptoethanol bound at the active site has been determined at 1.65Å resolution.169 Binuclear nickel(II) model complexes for urease which contain bridging deprotonated urea or carbamate ligands have been prepared and structurally characterised.170 Several other model compounds for urease have also been reported. 171 XAFS spectroscopy has shown that the nickel-binding protein NikA from Escherichia coli contains a six-co-ordinate nickel(II) ion with O/N-donor ligands at an average distance of [2.06Å.172 The solution structure of nickel peptide deformylase has been solved by 1H NMR spectroscopy.173 The structure and properties of the nickel–iron hydrogenases have been reviewed. 174 The mechanism of dihydrogen activation by the mixed metal site of the nickel–iron hydrogenases has been investigated with density functional theory; the dihydrogen is proposed to bind to the iron centre and undergo heterolytic cleavage to Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 611–630 619generate a bound, protonated Cys residue and a hydride on the iron centre which is then transferred to nickel.175 A series of model compounds based on [Fe(CN) 2 (CO)Cp]~ have been used to interpret the structure and IR spectroscopy of the iron centre at this biosite.176 The [Ni-4Fe-4S] cluster of carbon monoxide dehydrogenase from Clostridium thermoaceticum in both its nickel labile and nonlabile forms has been studied by XAFS, EPR and UV/vis spectroscopy.177 In the labile form the nickel(II) centre can be reduced to nickel(I) and is then able to bind carbon monoxide.The nickel(II) centre in the nonlabile form is square planar with two N/O-donor ligands at 1.87Å and two S-donor ligands at 2.20Å, and is probably no longer directly bridged to the [4Fe-4S] cluster. ENDOR spectroscopy of the reduced form of this biosite has indicated that HxO and a His residue are probably co-ordinated to the [4Fe-4S] cluster.178 Model complexes for the reduced nickel(I) centre in this metalloprotein have also been reported.179 9 Copper Recent developments concerning general copper biosites,180 and plastocyanin in particular,181 have been reviewed.The crystal structures of spinach plastocyanin, the azurin from Pseudomonas putida and a mutant of the plastocyanin from Cyanobacterium synechocystis have been reported.182 The copper site of plastocyanin has been studied by resonance Raman spectroscopy of various isotopically labelled derivatives, 183 and copper L-edge XMCD spectroscopy.184 The rate of intramolecular electron transfer for the Ru(bipy) 2 L–His-83 derivative of azurin increases slightly below 220 K.185 This unusual behaviour is rationalised as a near activationless electron- transfer process combined with the contraction of hydrogen-bonding distances along the electron transfer pathway.The solution structure of the complex of plastocyanin and cytochrome f has been determined by 1H NMR spectroscopy, and reveals a short copper–iron electron transfer distance of 10.9Å.186 The designed construction of a blue copper protein site in a host peptide has been described.187 The structure and spectroscopy of blue copper centres have also been modelled with several theoretical studies.188 The spectroscopy of cobalt(II) and nickel(II) substituted azurins has been described,189 and in particular the site of cobalt(II) binding to unfolded azurin has been elucidated by 1H NMR spectroscopy.190 XAFS spectroscopy has shown that the Menkes disease protein binds copper(I) with two Cys residues in a two-co-ordinate geometry.191 A model mononuclear copper(I) bis(thiolate) complex for this site has also been described.192 The crystal structures of several forms of the nitrite reductase from Alcaligenes xylosoxidans have been determined.193 The crystal structure of the copper amine oxidase from Hansenula polymorpha in the active conformation has been determined at 2.4Åresolution, revealing the position of the topaquinone redox cofactor relative to the copper centre.194 XAFS spectroscopy has been used to investigate the active site structure of the resting copper(II) and reduced copper(I) forms of several amine oxidases.195 Simple model compounds for the resting copper(II) form have also been reported.196 Mechanistic studies on the copper–cysteinyl–tyrosyl radical site of galactose oxidase have been reported.197 The wild-type enzyme and several mutant forms Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 611–630 620have also been studied by CD spectroscopy.198 The observed diamagnetic ground state for the copper(II)–cysteinyl–tyrosyl radical form of galactose oxidase has been explained by the particular orientation of the tyrosyl radical relative to the copper(II) centre and confirmed through the study of a series of model compounds.199 A structural and spectroscopic model for the putative mononuclear copper(II)–hydroperoxo species formed in galactose oxidase and related copper enzymes has been reported.200 Several model compounds for general mononuclear copper biosites have also been described.201 Recent developments concerning multinuclear copper sites in metalloproteins have been reviewed.202 The crystal structure of a functionally active subunit of octopus haemocyanin has been determined at 2.3Å resolution.203 The nature of the species formed in the binding of dioxygen to haemocyanin has been discussed in the light of model compound studies;204 oxyhaemocyanin displays catechol oxidase activity if the substrate is able to access the active site.205 Two catechol oxidases with dicopper active sites have been isolated from sweet potato and characterised by EPR, XAFS and UV/vis spectroscopy.206 Resonance Raman spectroscopy has been used to identify the metal-bound His residues in copper superoxide dismutase.207 The formation of a non-symmetrical copper(II) complex of ochratoxin A has been reported, and the relevance of this for the understanding of its observed toxicity and DNA cleavage activity is discussed.208 1HNMRspectroscopy has been used to study the active site of the dicopper-substituted, functionally active form of aminopeptidase from Aeromonas proteolytica.209 The crystal structure of the type II copper depleted laccase from Coprinus cinereus has been determined at 2.2Å resolution; the absence of the type II copper is found to perturb substantially the structures of the remaining type I and type III sites.210 ESEEM spectroscopy has been used to identify His co-ordination at the multinuclear copper site of particulate methane monooxygenase.211 The copper(II) sites of the multicopper Fet3 iron uptake protein from Saccharomyces cerevisiae have also been spectroscopically characterised.212 The chemistry of oxidative copper metalloenzymes and the similarity to recently reported model compounds have been reviewed.213 The dioxygen binding to and activation by di- and multi-copper biosites has been theoretically investigated.214 A number of functionally active model compounds have also been reported.215 Nitrous oxide reductase contains two binuclear bis(Cys) bridged copper sites, an electron-transfer Cu A site and the presumed active site Cu Z .MCD spectroscopy has indicated that the Cu Z site is a variant of a Cu A site and that the true active site may be a copper centre without Cys co-ordination.216 The Cu A sites of several proteins, and an engineered Cu A site in azurin, have been investigated by EPR, 1H NMR, UV/vis, MCD and XAFS spectroscopy.217 Spectroscopic models for Cu A sites have also been described.218 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 611–630 62110 Zinc Recent developments in understanding the structure and reactivity of zinc metalloenzymes have been reviewed.219 The role of the Tyr residue as a detachable ligand in the zinc protein astacin has been studied in the cobalt(II) and copper(II) substituted forms.220 Orotidine 5@-monophosphate decarboxylase is known to increase the rate of reaction of its substrate by a factor of 1017 compared with the uncatalysed reaction.The protein needs two zinc ions for activity and Lys-93 may act as a bridging carbamylated Lys ligand.221 The proton-transfer reactions of the zinc-bound intermediates in carbonic anhydrase have been investigated by ab initio calculations.222 The crystal structures of several forms of the alcohol dehydrogenases from Clostridium beijerinckii and Thermoanaerobacter brockii have been determined.223 The structural basis for the enantioselectivity of the secondary alcohol dehydrogenase from Thermoanaerobacter ethanolicus has been studied with reference to its Ser-39-Thr mutant. 224 The crystal structure of the dizinc enzyme b-lacamase from Bacillus cereus has been determined at 1.9Å resolution.225 The dicobalt(II) and mixed zinc(II)–cobalt(II) substituted forms have also been studied by UV/vis and 1H NMR spectroscopy.226 The zinc(II) ion e§ectively inhibits the oxidation of the ligand Cys residues in zinc finger proteins whereas substitution with nickel(II) appears to enhance oxidation to disulfide and thiosulfinate groups.227 The di¶culties of determining the active site structures of zinc enzymes with a mixture of S-donor and N/O-donor ligands by XAFS spectroscopy have been investigated with a study of crystallographically characterised model compounds.228 The mom gene activator proteinCOMbinds a single zinc(II) ion at a tetrakis(Cys) site with an a¶nity 105 times greater than for cobalt(II).229 Zinc(II) binding is even maintained at pH 4, the pH at which zinc(II) dissociates from zinc finger proteins.The role of a zinc ion in the activation of a bound Cys residue towards methylation has been studied for the DNA repair protein Ada and methionine synthase.230 Model systems for zinc-promoted Cys methylation,231 and general mononuclear zinc biosites,232 have been described.The speciation of cadmium(II) with the simplest plant phytochelatin PC 2 has been investigated in detail to elucidate the co-ordination chemistry of these heavy metal stress relief peptides.233 11 Molybdenum, tungsten and the nitrogenases The structure and reactivity of molybdenum and tungsten metalloenzymes containing pterin cofactors have been reviewed.234 The crystal structure of dimethyl sulfoxide reductase with the substrate bound at the active site has been determined at 1.9Å Annu.Rep. Prog. Chem., Sect.A, 1999, 95, 611–630 622resolution.235 The crystal structure of the similar enzyme trimethylamine N-oxide reductase has also been determined at 2.5Å resolution.236 XAFS spectroscopy of the selenium-containing formate dehydrogenaseH from Escherichia coli has revealed four S-donor ligands at 2.35Å, one O-donor ligand at 2.1Å and a Se-donor ligand at 2.62Å. However, there also appears to be a close Se–S interaction of 2.19Å suggesting the presence of an unusual seleno–sulfide ligand.237 Isotopic labelling studies have also shown that this enzyme does not display typical oxotransferase activity as formate oxidation occurs without any oxygen transfer.238 The active site structure of chicken liver sulfite reductase has been investigated by ESEEM spectroscopy at pH 9.5 and 7.0,239 and the binding of arsenate to human sulfite reductase has been studied by EPR and XAFS spectroscopy.240 Substrate binding and oxidation in xanthine oxidase has been the subject of density functional calculations.241 Several model compounds for oxotransferase biosites have also been reported.242 The crystal structures of the nitrogenase iron proteins from Azotobacter vinelandii and Clostridium pasteurianum have been determined.243 The two oxidation states of the P-clusters in nitrogenase have been investigated by XAFS spectroscopy.244 The binding of carbon monoxide to molybdenum nitrogenase has been studied by EPR spectroscopy and the binding of thiols and selenols has been studied by XAFS spectroscopy.245 The role of the R-homocitrate ligand of the molybdenum centre of the FeMo-cofactor of nitrogenase has been investigated.It is proposed that Rhomocitrate is uniquely able to hydrogen bond to the His-a-442 ligand and that, in addition, the bound carboxylate group can dissociate from the molybdenum centre to allow substrate access.246 Model systems for the proton transfer from bridging SH groups of [M 2 L 2 (l-SH) 3 ]` complexes to dinitrogen bound at a tungsten centre have been reported.247 References 1 Spectroscopic Methods in Bioinorganic Chemistry, ed.E. 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ISSN:0260-1818    

DOI:10.1039/a804901g

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

30 Inorganic pharmaceuticals P. J. Blower Nuclear Medicine Department, Kent and Canterbury Hospital, Ethelbert Road, Canterbury, Kent, UK CT1 3NG 1 Introduction Inorganic pharmaceuticals are taken to include compounds that contain elements other than carbon, hydrogen, nitrogen, oxygen, sulfur, phosphorus and halogens (except astatine), developed or evaluated for clinical or veterinary use to treat or diagnose disease.Radiopharmaceuticals and materials for controlled drug delivery are included, but materials for prosthetic devices are not. In most inorganic pharmaceuticals the ‘‘inorganic’’ component is a metal. Metallic elements provide radioisotopes suitable for radionuclide imaging and therapy1–3 (such as Tc, Re, In, and Cu) or neutron capture therapy (B, Gd).The last two years in particular have witnessed a significant increase in activity in the area of therapeutic radiopharmaceuticals, especially those containing rhenium, copper and yttrium isotopes (discussed below). Metals (Fe, Co, Cu) participate in radical chemistry with oxygen species leading to applications as radiosensitisers or free-radical scavengers. They o§er cytotoxicity (Pt, Au and others including first row transition metal complexes4 and early transition metal polyoxometalates5) for anti-tumour agents,6–8 anti-inflammatory properties (Au) for treatment of rheumatoid disease,6,9–11 and antimicrobial properties. The magnetic properties of metals may be exploited (Gd, Fe, Mn) in contrast agents for magnetic resonance imaging (MRI), in magnetically targeted drug delivery, or in localised magnetisation-induced hyperthermia treatment.The drug-chelating properties of metals (especially Cu) o§er opportunities for control of delivery and biodistribution of organic drugs, and enhancement of their activity.12 Other areas of development include anti-ulcer treatments (Bi),6 photodynamic therapy,13 and anti-viral agents.14 In this report, the foregoing summary serves to list the categories of applications of inorganic pharmaceuticals. Review articles and papers covering several metals are cited above, while in the following sections developments are categorised by the ‘‘inorganic’’ element concerned, in order of atomic number. 2 Boron The main attraction of boron continues to be the high neutron capture cross-section of the 11B nucleus, useful in neutron capture therapy.The chemical background has been Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 631–655 631reviewed15 and an in vitro method of screening boron compounds for radiobiological e§ects has been developed.16 New bioconjugates designed to deliver boron selectively to tumours include conjugates of carboranes with glycosides17 and intercalating agents (phenanthridium18), and liposomes loaded with boronophenylalanine.19 Some boron-containing compounds have also been evaluated for their inherent cytotoxic properties, including complexes of copper and zinc with boron-containing ligands,20 carboranes and polyhedral hydroborate salts.21 Finally, the potential of 11BNMRfor in vivo MRI using tissue-targeted boron compounds is now being evaluated, and di§erences in boron concentration in di§erent tissues could be demonstrated.22 3 Aluminium Aluminium compounds have long been used as antacids and as adjuvants in vaccine formulation.This area has been reviewed23 and the adsorption of aluminium salts (hydroxide, phosphate) on model vaccine proteins (lysozyme, ovalbumin) has been characterised.24 4 Scandium Scandium has found little use in pharmaceutics to date, but recently production and purification of the b-emitting radionuclide 47Sc, and its conjugation to antibodies using bifunctional chelators for radionuclide therapy, has been described.25 5 Titanium Titanocene dichloride is a cytotoxic complex being evaluated as an anti-cancer agent.26 Use of transition metals as enzyme inhibitors is being explored for possible applications in anti-microbials and other areas.Titanyl sulfate was found to be a potent inhibitor of the trypsin class of proteases and to prevent growth of a number of microorganisms.27 6 Vanadium The most important physiological response of vanadium is its insulin mimetic property. Vanadium complexes and their interactions with biomolecules are being studied in order to improve the currently minimal understanding of this action.28–31 A new class of vanadium-based spermicides comprising vanadocene dithiocarbamate complexes, [V(dtc)Cp 2 ]` is reported, and a crystal structure of the archetypal compound (the diethyldithiocarbamate derivative) has been obtained.32 The anticarcinogenic properties of vanadium salts have been investigated for many years.A vanadium(III) cysteine complex exerted a much more potent anti-carcinogenic and anti-tumour e§ect than either vanadyl sulfate or cysteine.33 Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 631–655 6327 Manganese Paramagnetic manganese compounds continue to be developed as contrast enhancing agents for MRI. Manganese complexes are typically unstable in vivo, leading to neurotoxicity as a result of manganese accumulation within specific regions of the brain.34 The need for stable complexes has prompted evaluation of manganese(III) porphyrin complexes, isolated in the form [MnCl(por)] (por\tetraphenylporphyrins with polyhydroxyamide substituents).These dissociate in water to give [Mn(H 2 O) 2 (por)]`, in which exchange of water molecules leads to enhanced proton relaxation rates.Unfortunately the complexes are too toxic for clinical use.35 Manganese( III) [as well as Gd(III), see below] interacts with phosphate-containing ligands including ATP, leading to possible use to enhance spin relaxation in 31P as well as 1H MRI.36 The ability of manganese to catalyse dismutation of the highly toxic superoxide radicals (HOO·) to hydrogen peroxide and dioxygen (superoxide dismutase- or SODlike activity) is of interest to minimise the toxic e§ects of radicals produced by radiation and inflammation/autoimmune disease.The in vivo anti-inflammatory e§ect of a manganese(III) complex with tetrakis(4-benzoic acid)porphyrin has been attributed to SOD-like activity of the complex and possible scavenging of peroxynitrite ions.37 The crystallographically characterised manganese(II) complex [Mn(ntb)(Hsal)]` also shows SOD-like activity in vitro.38 8 Iron Formulations with improved bioavailability and reduced toxicity for the supplementation of dietary iron are being sought.Phospholipid-encapsulated ferrous sulfate is an example.39 Other applications of iron are closely analogous to those of manganese in that they arise from magnetic properties and interactions with reactive oxygen species.Iron is believed to be an essential co-factor in the cytotoxic activity of the anti-cancer drug bleomycin, which catalyses production of reactive oxygen species in close proximity to DNA. The role of individual domains of the bleomycin molecule has been revisited.This has resulted in improved understanding of the modular design, including the nature and role of the iron-binding site,40 the pyrimidine group,41 the threonine side-chain,42 the bithiazole moiety,43 and the valeric acid linker.44 Iron complexes of salicylic acid45 and bis(salicyl)glycine46 participate in the generation of toxic radical species. In the latter case this induces DNA damage and lipid damage in the presence of sulfite, possibly via formation of the sulfuroxyl radical.It is suggested that such processes may contribute to the biological toxicity of iron. Iron-chelating agents such as pyridoxal isonicotinoyl hydrazone47 have been shown in vitro to protect against such damage induced by ‘‘free’’ (i.e. weakly chelated) iron.On the other hand, iron complexes may also protect against radical damage by demonstrating SOD-like activity, exemplified by a series of iron(III) complexes of pentaazamacrocyclic ligands [FeCl 2 L]~ Mwhich probably exist as [Fe(H 2 O) 2 L]` in aqueous solutionN.48 The magnetism of iron compounds can be exploited in drug targeting by combining drugs with magnetic materials such as metallic iron, iron oxides and ferrites, and then Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 631–655 633magnetically guiding the particles to the target site. This area has been reviewed.49 The magnetic properties of iron (synthetic iron oxides or the ferroprotein ferritin50) find application as contrast agents in MRI. Iron oxide particles conjugated to transferrin accumulate selectively in rat mammary carcinomas, leading to a significant (40%) localised decrease in MRI signal, thus allowing in vivo detection of tumours.51 Another developing application of magnetic iron-containing materials is the treatment of tumours, by using an alternating external magnetic field to induce hyperthermia in tissue to which magnetite particles, contained within cationic liposomes, have been delivered.52 Complexes of iron, especially ferrocene derivatives, have previously been shown to exhibit carcinostatic properties, and 4-ferrocenylbutanoic acid has been coupled reversibly (via amide bond formation) to water-soluble polymers bearing pendant amine groups, to give carcinostatic conjugates.53 The discovery of the biological importance ofNOhas led to renewed interest in its metal complexes, both as agents to lower tissue NO concentration in patients with toxic shock and to deliver it selectively to tumours so that, upon release by photolysis, it can sensitise the tissue to radiotherapy.Iron complexes being evaluated for this application include the Roussin salts [Fe 2 S 2 (NO) 4 ]2~ and [Fe 4 S 3 (NO) 7 ]~ and iron nitrosyl porphyrin complexes.54 9 Cobalt In previous years complexes of Co(III) containing mustard-like ligands have been described as part of a programme to design hypoxia-activated prodrugs for cancer treatment.The complexes are designed to release active nitrogen mustard groups upon reduction (selectively in hypoxic tissue) from kinetically inert Co(III) to labile Co(II). This programme has continued with the synthesis of dithiocarbamate complexes [Co(R) 2 L]` [L\N,N-bis(2-chloroethyl)ethylamine diamine or N,N@-bis(2-chloroethyl) ethylenediamine, R\diethyl-, dimethyl-, or pyrrolidine-dithiocarbamate].Although previous complexes have shown hypoxia-selective cytotoxicity, these complexes do not because the re-oxidation of the reduced species by O 2 is too slow to compete with release of the mustard.55 Use of the b`-emitting radionuclide 57Co for positron emission tomography (PET) applications has been reported.It is proposed that cobalt, administered as CoCl 2 , mimics the distribution of calcium, which is deposited at sites of tissue damage resulting from ischaemia. This o§ers the opportunity to image these sites in vivo, and imaging of ischaemic brain damage in stroke patients56,57 has been reported.Use of 57CoCl 2 to radiolabel lymphocytes for imaging inflammatory processes in vivo has also been reported.58 10 Nickel Antifungal activity of nickel complexes has been demonstrated by preliminary studies with crystallographically characterised complexes of glycoside-containing triamine ligands, which inhibit growth of the pathogenic yeast Candida albicans.59 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 631–655 63411 Copper The ability of copper complexes to interact with reactive oxygen species has led to possible uses in both sensitising tissues to, and protecting them from, radical damage. The Cu(II) complex of nitrilotriacetic acid increased the levels of reactive oxygen species in cancer cell culture suspensions, and induced DNA fragmentation and apoptosis.60 Likewise, DNA damage to human fibroblasts, induced by the combination of gallate and copper(II), was ascribed to reactive oxygen species.61 A copper complex of famotidine caused DNA damage in the presence of sulfite, possibly due to generation of sulfoxyl radicals.46 Protection against radical damage is the impetus for studies of the SOD-like activity of copper complexes of thiosemicarbazones and bis(thiosemicarbazones),62 cyclic peptides [cyclo-(Gly-His) 4 and cyclo-(Gly-His- Gly) 2 ],63 and bis(Schi§ base) ligands,64 and imidazolate-bridged dinuclear complex.65 Copper complexes also demonstrate inherent cytotoxic properties.Thiosemicarbazone ligands feature again here as they have previously, with thiosemicarbazone derivatives of 5-formyluracil66 and 2-formylpyridine67 each forming ternary complexes with Cu(II) which were subjected to in vitro cytotoxicity tests.The cytotoxic series of copper(I) bis(diphosphine) complexes is extended to include [Cu(dppe) 2 ]Cl, which was conspicuously missing from earlier studies.68 Bis(1,10-phenanthroline)copper69 and bis(acetato)bis(1-methyl-4,5-diphenylimidazole)copper(II)70 both induced DNA scission, leading to suggested applications as anti-tumour agents.Copper complexes of the phenothiazine drug trifluoperazine can inhibit the multi-drug resistance phenotype in both bacteria and mammalian lymphoma cells. The e§ect was greater than that seen with trifluoperazine itself and may reflect inhibition of both the induction of the gene expression and of the eßux pump itself.71 Potential applications of copper in drug delivery/release include the use of ternary complex formation to link immunogens, used for vaccination, to water-soluble polymers via a copper ion bridge.Bovine serum albumin, as a model immunogen, was more immunogenic in this form than alone when administered to mice.72 A prolongedrelease implant formulation for basic fibroblast growth factor was developed by ternary complex formation in which copper ions bridge between the amylopectin hydrogel matrix and the protein drug.73 Several copper radionuclides for both PET imaging and therapy, especially 64Cu (t 1@2 \12 h), are becoming more widely available.The radiotherapeutic e¶cacy and toxicity of the small somatostatin-receptor targeting peptide octreotide, labelled with 64Cu via the well-known macrocyclic bifunctional chelator teta, showed promise in tumour-bearing rats.74 67Cu-labelled monoclonal antibody Lym-1 showed a high therapeutic ratio in patients with non-Hodgkin’s lymphoma.75 The di¶culty in obtaining a reliable supply of 67Cu remains the main obstacle to routine adoption of this promising therapeutic approach.The redox properties of copper have been exploited in the development of PET imaging (and potentially therapeutic) agents selective for hypoxic tissue: two series of complexes based on tetradentate bis(thiosemicarbazone) and Schi§-base ligands show hypoxia selectivity in mammalian cells in vitro,76 and structure–activity relationships have been identified that link hypoxia selectivity, Cu(II)–Cu(I) redox potential and alkyl substitution pattern.77 Another approach suggested for targeted radionuclide therapy for hypoxic tumours combines the hypoxiaselective drug tirapazamine with copper radionuclides, although this report does not Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 631–655 635appear to contain any experimental data.78 Two new monocationic radiocopper complexes79,80 have been synthesised and evaluated in animals as myocardial imaging agents but are insu¶ciently lipophilic to show the required level of myocardium uptake. A series of macrocyclic chelators for linking copper to biomolecules has been evaluated to find those with the most appropriate e§ects on conjugate biodistribution. Among the series examined, those which were anionic by virtue of carboxyl group substitutents (teta) would prove most suitable because these were more e¶ciently cleared than cationic complexes.81 Understanding of the in vivo processing of copper ions is important to the design of copper radiopharmaceuticals. In mice injected with 64Cu2`, initial accumulation in liver (mostly bound to metallothionein) was followed by transport out of the liver into other tissues including tumours.82 12 Zinc Much of the use of zinc is concerned with supplementing body/tissue concentrations of Zn2` for incorporation into enzymes such as metalloproteinases and transcription factors.These are important in tissue-remodelling and cell division (wound healing, etc.).Recent studies have addressed e¶cacy of topical formulations (zinc ointments for skin disorders,83 zinc lozenges for reducing the duration of common colds,84 and oral zinc supplements85–87). Zinc has also been used in the preparation of controlledrelease forms of protein drugs including hirudin (anti-coagulant) and insulin, by complexing Zn2` with the proteins.88 The earlier discovery that a zinc complex with 3,5-dips, of unknown structure, has anti-convulsant activity has been followed up with a crystal structure determination of the dimethyl sulfoxide adduct [Zn(3,5- dips) 2 (dmso) 2 ].The latter was found to prevent seizures and to reduce radical generation by activated granulocytes.89 13 Gallium The main value of gallium stems from the properties of its radionuclides 67Ga (a c-emitter used for imaging lymphoma and inflammation) and 68Ga (a generatorproduced positron emitter).The similarity of gallium and iron has led in the past to imaging of iron transport processes such as transferrin receptor activity in tumours. Experiments addressing the mechanism of this targeting using gallium citrate (the structure of which has been determined90) broadly confirm the suggestion that transferrin receptors play an important role in the process.91 The iron analogy leads to the use of iron chelators (such as desferrioxamine92) both as bifunctional chelators to label targeting agents with gallium radionuclides, and as an aid to clearance of non-tumour bound gallium to improve tumour imaging.93 The radionuclides (particularly 68Ga) are of interest as labels for non-iron-related imaging agents also.This has led to new developments in bifunctional chelators for gallium (such as 1,4,7-triazacyclononane-1- succinic acid-4,7-diacetic acid94) and in production of the parent radionuclide 68Ge.95 Complexation and structural studies with chelators tris(2-mercaptobenzyl)amine and tris(2-hydroxybenzyl)amine96,97 have been reported.A chelate of 68Ga was used as a hapten in a pre-targeting approach, using a bi-specific antibody with one arm specific Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 631–655 636for a human colon carcinoma antigen and the other specific for one enantiomer of the gallium chelate, leading to improved PET detection of tumours.98 14 Selenium Selenium has long been of interest for possible cancer chemopreventive e§ects, largely assumed to be due to its role in the anti-oxidant enzyme glutathione peroxidase.Recent studies suggest that selenium compounds may increase oxidative stress as well as relieve it.99 Sodium selenite has been found to be an e§ective prophylactic treatment for erysipelas (a Streptococcal infection of the skin) in patients with secondary lymphoedema after cancer surgery.100 15 Rubidium The spin of the 87Rb nucleus (natural abundance 28%, I\3/2) stimulated interest in imaging rubidium distribution in myocardium (in which it is accumulated by mimicking potassium and hence acting as a substrate for the Na/K-ATPase pump) after intravenous administration.Preliminary experiments in isolated hearts suggest that di§erences in Rb` uptake in normal and ischaemic myocardium are su¶cient to detect by MRI, suggesting possible future applications as a diagnostic agent in cardiology. 101,102 16 Strontium The b-emitting radioisotope 89Sr has been in use for many years for the relief of pain associated with bone metastases in cancer patients. It has also been shown that stable strontium administered to rats inhibits carcinogenesis.103 17 Yttrium The b-emitting radionuclide 90Y is one of the leading metallic radionuclides finding applications in targeted radionuclide therapy of cancer, attached via bifunctional chelators to targeting agents such as octreotide104–106 and other peptides which bind to somatostatin receptors and are internalised,104 and monoclonal antibodies.107–112 These conjugates are giving very encouraging results in preliminary clinical and animal evaluations.104,105 Although the widely used dota ligand104–106 forms a chelate with acceptable in vivo kinetic stability, and labelling conditions have been optmised,108 design of new bifunctional chelators has continued.Designs being evaluated include backbone-linked (rather then carboxyl group-linked) derivatives of diethylenetriamine pentaacetic acid,107,112,113 and linker groups that are cleavable in vivo.109,110 A modular delivery system is reported whereby a dota–adenine conjugate labelled with 90Y(or 111In) is injected after pre-adminsitration of a bi-specific antibody that recognises both a tumour antigen and the conjugate.114 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 631–655 63718 Molybdenum NMR experiments suggest that the anti-tumour activity of molybdocene dichloride [MoCl 2 Cp 2 ] is unlikely to be due to direct interactions with DNA, and other mechanisms must be sought.115 Tetrathiomolybdate [MoS 4 ]2~ reduces the excess copper burden in the livers of LEC rats (a model for Wilson’s disease) o§ering a means of treating Wilson’s disease and copper overload, probably by formation of aMo–S–Cu complex.116 19 Technetium Development of applications of 99.Tc in medical imaging agents continues to be a major part of the inorganic pharmaceuticals field and has been reviewed.117 The developments reported here are categorised into ‘‘technetium-essential’’ tracers (small molecules), ‘‘technetium-tagged’’ tracers (in which the targeting is largely dependent on the carrier molecule), and new technetium cores and chelators.A major area of investigation of Tc-essential tracers is the use of lipophilic cations such as [Tc(mibi) 6 ]` 118–123 and analogues,124,125 [TcO 2M(EtOCH 2 CH 2 ) 2 - PCH 2 CH 2 P(CH 2 CH 2 OEt) 2N2 ]`,120,126 and [Tc(PR 3 ) 2 L]`120,127,128 for imaging myocardium,122,128,120 tumours,119,120,124,126 and assessment of tumour multi-drug resistance.127,125,119 Progress in applications129,130 and tumour targeting mechanisms of the ‘‘pentavalent technetium dmsa complex’’ [TcO(dmsa) 2 ]~131 has been reported, leading to the suggestion that it may be a marker of low tissue pH.132,133 The range of tracers that cross the blood–brain barrier, for cerebral perfusion imaging, has been extended and the mechanisms of brain retention of existing agents [TcO(ecd)] (intracellular ester hydrolysis)134 and [TcO(L)] ML\Me 2 C[CH 2 NHCH(Me)C(Me)–– NOH]N (possible intracellular reaction with thiols)135,136 have been further investigated.New lipophilic complexes based on the TcO3` bis(aminoethane)thiol137 and TcO 2 ` cyclam138 cores have been reported.A long-standing problem with [99.TcO(L)] ML\Me 2 C[CH 2 NHCH(Me)C(Me)–– NOH]N is autoradiolysis, and reports of kit modifications to improve stability continue to appear.139–141 99.Tccomplexes target ischaemic tissue in humans, either because of the oxygen deficiency in these regions M99.Tc–[HON––C(Me)C(Me) 2 NH] 2 (CH 2 ) 4 142N or because of the resulting increased activity of sugar transporters (99.Tc–glucarate complex143).A kit for preparing 99.Tc–ethylenediamine tetrakis(methylenephosphonate) (edtmp) as a new bone imaging agent,144 and optimal conditions for preparation of the uncharged myocardial imaging agent [99.TcN(noet) 2 ], have been reported.145 Synthesis and biodistribution of a 99.Tc complex purportedly containing the disulfide of penicillamine has been reported.146 A number of technetium-tagged targeting agents have been reported, in which 99.Tc is conjugated to cyclic glycoprotein IIb/IIIa receptor antagonist,147 monoclonal antibodies,148–152 oligonucleotides,153,154 dopamine transporter ligands,155–157 peptides,158–165 nicotinamide,166 synthetic polymers,167 endogenous natural surfactant,168 biocytin and biotin.169,170 These have applications or potential applications in imaging tumours,149–154,158,160,161,165,170,171 neurodegenerative disease, 155–157 atherosclerosis,159 inflammation,162 hypoxia,166 blood pool,167 liver Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 631–655 638function169 and the alveolar delivery of aerosolised drugs.148,168 The chemists’ contribution is largely concerned with developing simple and e¶cient methods of linking 99.Tc to the carrier molecule.Progress has been made both in direct binding of 99.Tc to the carrier,148,149,151,158,159,171,172 and in bifunctional linker design. New bifunctional linkers include designs based on diaminodithiols,155–157,161,162 mercaptoacetyltriglycine and analogues,152–154,160,165,169,170 dtpa,167 monodentate thiol,166 cyclopentadienyl,164 hydrazinonicotinamide with various co-ligands,163,147 dithiocarbazate,173 and metallothionein.150 Advances in design of liposomes for delivery of encapsulated 99.Tc have been reported.174,175 Electron microscope studies of the carbon-particle based aerosol Technegas (used for lung ventilation imaging) shows that the 99.Tc label does not a§ect the ultrastructure of the particles.176 The development of novel technetium ‘‘core complex’’ types has continued in a number of directions. The ‘‘3]1’’ approach, developed over the last few years [comprising the 99TcO3` core, a monodentate thiolate ligand and a tridentate ligand E(CH 2 CH 2 SH) 2 where E\S or NR] has for the first time been successful at the carrier-free level with 99.Tc.177 The complexes are remarkably stable in vivo.178 However, they may be subject to intracellular trapping mechanisms by substitution of the monodentate thiolate, suggesting possible applications in brain imaging.179 Another developing theme is the use of chelating and non-chelating organohydrazine, isodiazene and related ligands, demonstrated by the prototype 99Tc complexes [TcCl 3 (N–– NPh 2 )(PPh 3 ) 2 ],180 [TcCl 2 (NO)(g2-HN–– NC 5 H 4 N)(PPh 3 )],181 [TcCl(NO)(g2-N–– NR)(PPh 3 ) 2 ] (R\trifluoromethylpyrimidine),181 [TcCl 3 (g1- N––NC 5 H 4 N)(g2-HN––NC 5 H 4 N)],182 and [Tc(pyS) 2 (g1-N–– NC 5 H 4 N)(g2-HN––NC 5 - H 4 N)],182 which serve as models for the co-ordination of 99.Tc by the hydrazinonicotinamide ligand used to link technetium to peptides. The well-known high kinetic stability of the technetium(I) and rhenium(I) tricarbonyl cores led to progressively milder routes to technetium carbonyl complexes suitable for radiopharmaceutical application, culminating in simple synthesis of the reactive tris(aqua) species [Tc(H 2 O) 3 (CO) 3 ]`.183 This can be easily coordinated to a range of tridentate chelators such as macrocyclic thioethers184 for linkage to carrier molecules.A technetium(I) carbonyl has also been used to synthesise a benzodiazepine complex [Tc(CO) 2 (PPh 3 ) 2 L] (HL\1,4-benzodiazepine-2-one or 1,2-benzodiazepine-2- thione).185 The interaction between TcO3` and cysteine (H 2 cys) has been further elucidated by demonstrating that in [TcO(cys) 2 ]~ the co-ordination sphere comprises the oxo-group, two thiolate sulfurs, two amine groups, and one carboxylate.186 New polydentate N/O ligand complexes of 99.Tc chelated by N,N@-bis(2-aminoethyl)propane- 1,3-diamine hexaacetic acid187 and N,N-bis(benzohydroxamamide) alkane-x,ydiamine (alkane\1,2-ethane or 1,3-propane)188 have been reported though not structurally characterised. 20 Ruthenium Anti-cancer activity of ruthenium complexes has been reviewed.189 Reactions of a Ru(III) complex [RuCl 2 (pdta)], that shows anti-cancer properties, with water and proteins have been investigated to help elucidate possible mechanisms of action.190 Interactions between ruthenium polypyridyl complexes and DNA have been inves- Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 631–655 639tigated.191 The ability of ruthenium porphyrin nitrosyl complexes to releaseNO upon photolysis o§ers the potential for tissue selective delivery of NO.54 Among a series of transition metal (Ru, Rh, Pt, Au, Cu) complexes of ctz, investigated as anti-trypanosome agents, the ruthenium complex [RuCl 2 (ctz) 2 ] proved most active and its mechanism of action has been investigated.192 21 Rhodium A series of Rh(III) chloride adducts with imidazole and NH 3 ligands, including trans- [RhCl 4 (im) 2 ]~ which has been structurally characterised, has been investigated for cytotoxicity following the observation that their ruthenium analogues have anticancer properties.193 The DNA-intercalating ability, and e§ects on helix structure, of a rhodium phenanthrene derivative have been elucidated by molecular dynamics calculations. 194 22 Palladium Binuclear palladium complexes [MPdCl(diamine)N2 L] (where L is a briging bidentate ligand) analogous to cytotoxic platinum complexes (see below) have been synthesised and screened for cytotoxic activity.195 23 Silver The principal pharmaceutical value of silver compounds is based on the long-established antimicrobial properties, exemplified by topical treatments such as silver sulfadiazene for prevention of infection of burns.Combination treatment with this compound together with other antimicrobials including cerium nitrate was found to o§er no improvement in anti-microbial e¶cacy in vitro.196 Antimicrobial co-ordination polymers of silver, [AgL]n and [Ag(PPh 3 ) 2 L]n (L\1,2,3- or 1,2,4-triazole) have been structurally characterised.197 Other new developments in antimicrobial use involve incorporation of silver into polymeric materials198 and surfaces of medical and surgical devices (wound dressings,199 catheters,200 hydroxyapatite and other coatings for prostheses201–204) for prolonged antimicrobial e§ect in vivo.Silver has some radioisotopes with attractive properties (e.g. 111Ag) for radionuclide therapy but the applications have been hampered by the challenge of finding a kinetically inert chelating system for bioconjugates synthesis. A macrocyclic thioether–amine ligand has been reported to complex silver with high stability205 o§ering a potential solution to this problem. The insolubility and high atomic weight of silver halides is exploited in the use of silver iodide particles as a radio-opaque contrast agent for radiography.206 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 631–655 64024 Indium All reported pharmaceutical uses of indium involve either radionuclide imaging or radionuclide therapy. Indium-111 is routinely used in labelling proteins and other macromolecules for imaging (e.g. 111In-labelled octreotide analogues). New examples (usually labelled with use of the diethylenetriaminepentaacetic acid anhydride bifunctional chelator) include conjugates of biotin for ‘‘pre-targeting’’,207,208 disulfide crosslinked anti-myosin F!" antibody fragment aggregates,209 and folate.210,211 An important area of investigation is the metabolic fate of the radiolabel during the intracellular catabolism of the carrier peptide or protein, since this a§ects the residence time of the radiolabel within the tissue.Several studies in this area indicate that the indium–dtpa chelate remains intact linked to the amino acid (usually lysine) to which it was originally bound, while the remainder of the protein is broken down around it. The chelate then shows prolonged intracellular retention in target tissue212,213 or kidney. 214 Interposing a cleavable group between the chelator and the protein can facilitate or enhance clearance of radioactivity from non-target tissues.215 As well as emitting gamma photons, 111In emits Auger electrons which are highly cytotoxic when emitted close to DNA. 111In-labelled octreotide is therefore being clinically evaluated for treatment as well as imaging of tumours.Also, when 111In is bound to the metal-binding site of bleomycin to irradiate the DNA, cytotoxicity is greater than for the bleomycin alone or ‘‘free’’ 111In.216 25 Tin Tin compounds are an important class of antifungal agents and new compounds under evaluation include [SnR 2 Cl(tscz)]217 and [SnR 3 X] (X\Schi§ base derivative of x-amino acids, linked to Sn via the carboxylate group).218 Related complexes have also been screened for anti-cancer activity.219,220 Tin (stannous salts) is also important as a reducing agent in 99.Tc radiopharmaceutical kits, and methods of protecting it against aerial oxidation,221,222 and analysing it as Sn(II) in the kits,223 have been reported. Tin also has radionuclides that are useful in their own right as therapeutic radiopharmaceuticals: 117.Sn–dtpa complex is being developed as a palliative treatment for painful bone metastases in cancer patients.224,225 26 Samarium Samarium is of interest for its therapeutic radionuclide 153Sm, which as a chelate with ethylenediaminetetrakis(methylenephosphonate) is a lead radiopharmaceutical for palliative treatment of bone metastases.226,227 A number of cyclic and acyclic nitrogen/ oxygen donor chelators have been evaluated for possible application in future therapeutic conjugates of 153Sm.228 27 Gadolinium The Gd3` ion is highly paramagnetic and hence gadolinium compounds have become Annu. Rep.Prog. Chem., Sect. A, 1999, 95, 631–655 641the most important class of paramagnetic contrast enhancing agents for magnetic resonance imaging.229 The prototypical complex is the dtpa chelate [Gd(dtpa)- (H 2 O)]2~ (‘‘gadopentetate’’), which is in routine clinical use230–232 for determination of various physiological parameters by dynamic MRI.New developments include incorporation of the dtpa complex into a variety of vehicles to modify biodistribution, including polymers of varying molecular weight,233,234 red blood cells,235 complexes that bind to albumin236 and lipid nanoparticles.237 E§orts are underway to increase the relaxivity of contrast agents to provide greater sensitivity, by modifying the spin properties [using di- and tri-nuclear complexes such as Fe(III)–Gd(III),238 V(IV)–Gd(III)239 and Gd(III) 3 240 complexes).An alternative approach is to increase the rate of either exchange of co-ordinated water molecules (e.g.by designing chelates with more than one co-ordinated water molecule241,242) or exchange of protons between co-ordinated and outer-sphere water molecules.243,244 Other approaches to contrast agents include porphyrin complexes incorporated into liposomes,245 and dota and dtpa conjugates with fluorescence agents for dual optical/MRI detection.246 Gadolinium is also of interest for its neutron capture cross section, which, like boron, o§ers the potential for neutron capture therapy of cancer if the gadolinium can be deposited within tumours in su¶cient quantity e.g.by means of microcapsules/ microspheres.247,248 28 Other lanthanides Radionuclides of holmium (166Ho), erbium (165Er) and lutetium (177Lu) are finding applications in diagnostic and therapeutic nuclear medicine.A complex of the b- emitter 166Ho with the modified biopolymer chitosan has been synthesised for intraarterial delivery to tumours.249 165Er decays by electron capture with emission of a low energy X-ray rendering it ideal for radioguided surgery, while 177Lu is a b-emitter with therapeutic potential.Both form chelates with edtmp that are taken up in bone metastases and some soft tissue metastases.227,250 Polyaminocarboxylate and polypyridyl ligands have been evaluated as bifunctional chelators for 177Lu for targeted delivery.228 29 Rhenium The use of b-emitting radionuclides 186Re and 188Re for targeted radionuclide therapy provide the main impetus for the pharmaceutical chemistry of rhenium.Much of the chemistry is done in parallel with technetium (see above). Recent developments in rhenium radiopharmaceutical chemistry have been reviewed.128 Interest in therapy with rhenium has heightened recently. This is reflected in improvements in calibration of radionuclide measuring devices for use with rhenium radionuclides.251,252 New applications outside oncology, e.g.use of 188Re (either as a solution of perrhenate253 or as electrodeposited rhenium metal254) in cardiology to prevent re-stenosis after balloon angioplasty, are being investigated. Developments in both ‘‘rhenium-tagged’’ and ‘‘rhenium-essential’’ vehicles have been reported. The rhenium-tagged class includes antibodies and antibody fragments with rhenium Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 631–655 642directly attached, as Re(V) bound via reduced disulfide bonds.255–257 Progress in understanding the co-ordination around rhenium ReO3` in this mode of labelling has been made through structural studies of rhenium complexes with the small peptides Gly-Gly-Cys and Cys-Gly.258 Rhenium-188 has been attached directly to microspheres for intra-tumoral injection259 and radiation synovectomy,260 again by reduction of perrhenate with stannous chloride.For coupling to biomolecules, development of new bifunctional chelating systems continues, including tridentate S-methyl-2- methyldithiocarbazate amino acid conjugates,173 and tetradentate ligands containing thiolate/amide/thioether donors261,262 for chelating ReO3`.The ‘‘3]1’’ approach described above for technetium has also been further developed with rhenium. It provides access to a wide range of uncharged and cationic263 complexes, with targeting moieties bound to rhenium through a monodentate thiol,264 for targeting dopamine transporters265 and steroid hormone receptors.266 Somewhat analogous is the use of tridentate phosphinodithiolate ligands to chelate the ReO3` and Re3` cores.267 A new bis(amido)bis(thiolate)N 2 S 2 chelating system incorporating an (uncoordinated) histidine group268 has been reported.Complexes containing 2-hydrazinopyridine ligands, analogous to those discussed above for technetium, have been characterised.182 A new bifunctional chelator for ReO3` has been developed comprising 1,3,5-triamino-1,3,5-trideoxyinositol with pendant arms, and used for antibody fragment labelling.269 Rhenium analogues of technetium-essential tracers, including [188ReO(dmsa)] 2 ~270–273 and hydroxyethylidine diphosphonate complexes270,271,274,275 are being developed and evaluated for palliative treatment of bone metastases and soft-tissue tumours.Rhenium oxo-complexes with diaminodithiolate ligand sets provided by cysteine,186 D-penicillamine methyl ester,276 and 2,9-dimethyl-4,7-diaza-2,9-decanedithiol277 have been characterised.The use of water-soluble phosphines as ancillary ligands in chelation of rhenium isotopes has been suggested.278 30 Platinum Anti-tumour properties of platinum complexes are the most important themes of inorganic pharmaceuticals when judged by numbers of papers published.The topics currently being developed may be divided into four main areas: in vivo studies of e¶cacy; mechanisms of biological activity; incorporation into various controlled release formulations; and new chemistry. Several clinical and animal studies of e¶cacy and side-e§ects of platinum drugs (cisplatin,26,279,280 carboplatin,281–284 nedaplatin285,286 and oxaliplatin287) in combination with other drugs, have been reported.The major area of activity in the understanding of mechanisms of cytotoxicity is the structural and kinetic study of interactions beween platinum and DNA and the resulting changes in DNA structure.288–298 As well as structural changes caused by direct binding, cisplatin induces superoxide radical formation and this e§ect is enhanced by hyperthermia, providing a possible rationale for the hyperthermic enhancement of cisplatin therapy in vivo.299 Cisplatin can also interfere with repair of DNA double strand breaks induced by radiation, suggesting a way in which radiotherapy and cisplatin treatment can augment each other.300 Toxicity mechanisms that are not necessarily DNA-related have also been inves- Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 631–655 643tigated. The kinetics of ligand substitution (relevant to intracellular delivery and binding to biomolecules) in platinum drugs have been examined,301–304 and the pK!’s of some known in vivo reaction products have been determined.305 The reaction of cisplatin with oxidised glutathione306 and methionine307 are both relevant since these interactions appear to contribute to toxicity.308 The distribution and speciation (protein binding, etc.) of platinum in the body after treatment with platinum drugs has been investigated,309 analytical methods for these studies have been compared310–312 and a tritium-labelled form of oxaliplatin has been reported to support such studies.313 Additional physiological e§ects of platinum drugs include enzyme inhibition properties (erythrocyte acetylcholinesterase314,315), apoptosis induction in certain cell lines,316,317 and enhanced immune system activation.318,319 Since platinum(IV) complexes have become established as a new class of anti-cancer drugs, discussion has arisen over whether they act as prodrugs for platinum(II) species, by intracellular reduction.320–322 The search for orally active platinum drugs has generated studies aimed at optimising gastrointestinal absorption.323 The stability of various platinum drug formulations (infusions of cisplatin324,325 and carboplatin,326,327 either alone or in combination with other drugs) has been determined.With platinum-based drugs firmly established, attention has also turned to development of new dosage forms o§ering improved drug release and delivery properties.Platinum complexes have been incorporated into water-soluble, biodegradable polymeric microsphere carriers for direct injection or implantation into tumours.328,329 Microsphere materials include polyacetamide,53 poly(L-lactic acid),280,330 poly(DLlactic- co-glycolic acid),331,332 albumin,333,334 dextran,335,336 hydroxypropylmethacrylamide337 and hydroxyethylcellulose.338 Implants made of apatite cement mixed with cisplatin have also been evaluated.339 Liposomes340,341 have been used to encapsulate platinum drugs, and the permeation of platinum drugs through various polymer membranes has been measured as a basis for controlled release devices.342 Approaches for targeting specific tissues include attaching the platinum complex to poly(organophosphazene) to which is also attached a b-galactosyl group,343 and incorporating cholylglycinate into the Pt co-ordination sphere.344,345 Both provide selectivity for liver.A novel method of increasing cisplatin uptake, and hence e§ectiveness, into tumour cells was application of electrical pulses (electrochemotherapy) to the tumour at the time of intra-tumoural injection.346 The need for cytotoxic platinum complexes that have reduced side-e§ects (e.g.neuro- and nephro-toxicity) and can circumvent platinum drug resistance has led to synthesis of new types of platinum complexes for cytotoxicity screening, including those that retain the cis-diammine Pt(II) core M[PtCl(NH 3 ) 2 (L)]` (L\procaine)347,348N. Others are based on Pt(II) 1,2- and 1,3-diamine chelates,349 including ester- (for intracellular trapping by hydrolysis350), phenylquinoline- (for DNA intercalation351), or cyclodextrin-functionalised derivatives,352 [PtCl(diamine) L]` (L\thiourea derivative353,354), and [Pt(diamine)(L) 2 ] (L\acyclovir).355 Other Pt(II) complexes reported include sterically hindered complexes [PtCl 2 - (NH 3 )L]356 and [PtCl 2 L 2 ]357 (L\sterically encumbered pyridine or pyrimidine), [PtX 2 (dmso) 2 ] (X\halide or carboxylate358), [Pt(L)(PMe 3 ) 2 ] (L\glycolate; phosphine analogue of nedaplatin359), [PtCl 2 L] (L\thiosemicarbazone derivative360), [PtCl 2 (H 2 C––CH 2 )(R)] (R\pyroxicam),361 and [PtBr 3 (R)]~ (R\benzothiazole).362 Binuclear,363–366 trinuclear367 and tetranuclear368 Pt(II) complexes have also been Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 631–655 644synthesised with the aim of enhancing cytotoxicity by cross-linking DNA strands. Despite the recent recognition of the potential for drugs based on Pt(IV), there are relatively few reports of new Pt(IV) complexes.They include [PtCl 2 (dach) 2 L 2 ] (L\carboxylate)369 and Pt(IV) complexes with carbohydrate ligands.370 31 Gold Gold complexes have been used in arthritis treament for many years and are now being evaluated for anti-cancer purposes as well. Crystallographic studies with antiarthritic drugs have revealed the solid-state structures of gold thiomalate (myochrysin) 371 and an analogue of auranofin, [Au(PR 3 )L] (L\2,3,4,6-tetra-O-acetyl-1- thio-b-glucopyranoside, R\1-ethyl-2-isopropylimidazole).372 The latter was designed to interact with hard metals via the phosphine imidazole groups.Square planar cis-dichlorogold(III) complexes analogous to platinum(II) anti-cancer drugs have shown good cytotoxicity in vitro against cisplatin resistant cell lines.373 Polyamine complexes [Au(en) 2 ]3`, [AuCl(L)]2` (L\1,5-diamino-3-azapentane), and [Au(cyclam)] 3` were also synthesised for evaluation as anti-tumour agents.374 A number of Au(I) phosphine and diphosphine complexes have also been evaluated in an in vitro cytotoxicity screen.375 The b-emitting gold radionuclide 199Au is of interest for targeted radionuclide therapy, and a series of Au(I) complexes with water-soluble monodentate and bidentate phosphines P(CH 2 OH) 3 , (HOCH 2 ) 2 PCH 2 CH 2 P(CH 2 OH) 2 and (HOCH 2 ) 2 P(C 6 H 4 )P(CH 2 OH) 2 has been reported.Only the latter ligand gave a complex [Au(diphos) 2 ]` that showed adequate in vivo stability.376 32 Lead Lead is of interest for its therapeutic radionuclide 212Pb and its imageable c-emitting analogue 203Pb.The former decays by b-emission to 212Bi, which itself then rapidly decays by a-emission. This combination o§ers excellent cell-killing potential if a chelate system capable of retaining the lead during the decay to 212Bi can be designed. The dota ligand provides such a chelator and can be used to conjugate the lead radionuclides to antibodies.377,378 33 Bismuth Anti-ulcer treatments containing bismuth have been used for many years.The complex equilibria and dynamics of these bismuth complexes (e.g. with citrate) in solution have been investigated by NMR using 13C-labelled citrate.379 A formulation containing both bismuth and the antibiotic ciprofloxacin has been described for evaluation as a treatment for the stomach ulcer-causing bacterium Helicobacter pylori.The compound is a salt, [H 2 cip] 2 [Bi 2 Cl 10 ] rather than a co-ordination compound.380 Bismuth has therapeutic a-emitting radionuclides 212Bi (see lead above) and 213Bi, and a Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 631–655 645biotin-conjugated chelate has been synthesised for use in an avidin–biotin pre-targeting approach to tumour radionuclide therapy.381 34 Astatine Astatine has no stable nuclides and its applications are exclusively concerned with potential use in targeted radiotherapy with a-particles (211At).382 Most work on astatine has focused on the formation of At–C bonds by analogy with iodine, leading to species in which the 211At-label is bound to an aromatic ring linked to the targeting moiety.These include a conjugate with biotin (for binding to avidin pre-targeted to tumours)383 and antibody conjugates.384,385 In vitro386 and theoretical387 studies of the radiobiology of a-particles emitted by 211At are now demonstrating their extremely potent cytotoxicity.Thyroid protection strategies are also being evolved to prevent thyroid accumulation of astatide ions released from these conjugates.These include blocking uptake with thiocyanate, perchlorate, or periodate, and attempting to chelate free astatine in vivo using thiol-based chelators cysteine, 2,3-dimercaptopropanesulfonic acid and meso-2,3-dimercaptosuccinic acid.388 35 Actinium Actinium is of interest for its a-emitting radionuclide 225Ac for potential use in targeted radiotherapy, and a practicable generator system has been devised for it.382 This application has not yet been realised even in model systems because of the expected di¶culty in designing chelating agents capable of withstanding the recoil energy of the a-decay.References 1 P. 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ISSN:0260-1818    

DOI:10.1039/a808038k

出版商:RSC    

年代:1999

数据来源: RSC