14 Copper D. W. Smith Department of Chemistry, University of Waikato, Hamilton, New Zealand 1 Copper(I) chemistry Mononuclear species We begin with experimental and theoretical studies of the binding of copper(I) to ligands in the gas phase. Sequential bond dissociation enthalpies at 298K for [Cu(NH 3 )n]` (n\1–4) are found1a (by MS) to be respectively 237(15), 248(10), 46(6) and 45(6) kJ mol~1.Both free energy and entropy changes were measured for ligand exchange equilibria among [CuL 2 ]` species for 23 ligands L; histidine is found to be the most strongly-bound amino acid.1b A combination of experimental (MS) and theoretical (DFT) studies have been applied to the reaction of Cu` with formamide (a simple peptide model) in the gas phase;1c theoretical studies were extended to the isomers of formamide, formamidic acid and (aminohydroxy)carbene,1d and to the isomers of oxaziridine.1e High-level DFT calculations suggest that relativistic e§ects destabilize CuF(g) by only 2–5 kJ mol~1, compared with 50–70 kJ mol~1 for AuF.1f Mononuclear copper(I) compounds may be classified according to whether the co-ordination number of the metal is two, three or four. The structures of ‘higherorder’ cyanocuprates(I) CuCN·2LiR have aroused much debate: 15N NMR spectra in thf indicate cyclic structures with linear CuC 2 co-ordination,2a while the crystal structure of [MLi(Me 5 dien)(thf)N2 (l-CN)][CuBu5 2 ] reveals a Gilman cuprate anion.2b Methyl radicals react with Cu(CO)`(aq) to give the intermediate [CuMe(CO)]`, which undergoes CO insertion into the Cu–Me bond to give acetaldehyde.2c The reaction of CuX (X\Cl, Br) with pyrrolidine in MeCN gives [CuL 2 ]X [L\1-aza-2- (1-pyrrolidyl)] without the intermediate isolation of L; the copper atoms have linear CuN 2 co-ordination.2d Similar co-ordination is found in [Li(dme) 3 ]- [CuMNRCBu5––C(H)RN2 ] (R\SiMe 3 ).2e The dppf derivatives L\Fe[g5- C 5 H 4 P(E)Ph 2 ] 2 (E\S, Se) form two-co-ordinate [CuL][BF 4 ] (CuE 2 ) and four-coordinate [CuL 2 ][BF 4 ] (CuE 2 P 2 ), as well as polymeric three-co-ordinate species [Cu 2 L 3 ]n[BF 4 ] 2n.2f,g In a tetraarylstibonium salt, the [CuI 2 ]~ anions are considerably distorted from linearity with short (2.73Å) Cu–Cu distances.2h Two independent anions are found in [Pd(S 2 CNEt 2 )(dppe)][CuCl 2 ]; one is crystallographically centrosymmetric while the other has a Cl–Cu–Cl angle of 173°.2i The complex [NEt 4 ]- Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 189–211 189[CuL 2 ] (HL\adamantane thiol) is the first two-co-ordinate copper(I) complex with an aliphatic thiolate.2j Of three nominally three-co-ordinate monomeric copper(I) complexes characterised this year, none has the ideal planar triangular geometry.A sterically-hindered tridentate ligand 1-(2-hydroxy-3,5-di-tert-butylbenzyl)-5-isopropyl-1,5-diazacyclooctane (HL) gives a complex [CuL] where the co-ordination is T-shaped, with a very short Cu–O bond (1.88Å) to a phenolate oxygen atom, and two bonds (1.98 and 2.28Å) to tertiary amino nitrogen atoms, so that the co-ordination is best described as (2]1); the reactivity of the complex towards dioxygen resembles that of galactose oxidase.3a Similar dissymmetric bonding (in this case to two nitrogen atoms which are equivalent in the free ligand) is found in [CuL(tmphen)] (HL\2,6-diphenylthiophenol).3b In [CuL(PPh 3 ) 2 ] (HL\orotic acid), the carboxylate group is monodentate.However, the copper atom lies 0.56Å out of the OP 2 plane, forming a weak (2.28Å) bond with an exocyclic oxygen atom in a neighbouring unit cell, so that the highly insoluble compound is better viewed as a polymer with distorted tetrahedral (3]1) co-ordination. 3c Looking now at unequivocally four-co-ordinate copper(I) complexes, a good correlation is found between 63Cu NMR chemical shifts and CO stretching frequencies in [CuL(CO)] complexes (L\Tp and derivatives).4a The nuclease activity of [Cu(phen) 2 ]`, the mechanism of which has been studied by kinetic isotope e§ects,4b continues to stimulate work on bis(diimine)copper(I) complexes. Examples include the complexes [CuL 2 ][BF 4 ] [L\N-alkyl-(2-pyridyl)methanimine] which catalyse atom-transfer polymerisation of methyl methacrylate.4c The complex [CuL 2 ][BF 4 ] [L\3-chloro-6-(3,5-dimethylpyrazol-1-yl)pyridazine] exhibits the most distorted CuN 4 tetrahedron of any [Cu(diimine) 2 ]` known so far; a useful parameterisation scheme for assessment of the degree of flattening of the tetrahedron is proposed.4d Apart from the acuteness (79.9°) of the N–Cu–N angle, the structure of [CuI(phen)(PPh 3 )] is as expected.4e The electronic excited states and redox properties of diiminecopper(I) complexes have possibilities in the field of solar energy harvesting; spectroelectrochemical studies are reported for a series of complexes [CuL(PPh 3 ) 2 ]- [BF 4 ] (L\substituted dipyrido[3,2-a: 2@,3@-c]phenazine),4f and for analogous complexes with binaphthyridine4g and 2,3-bis(2-quinolyl)quinoxaline4h derivatives.X-Ray structural data for the complexes [Cu(NCMe) 2 (PPh 3 ) 2 ]X (X\BF 4 , PF 6 , ClO 4 ) have been complemented by solid-state 31P NMR studies of the two independent phosphine ligands.4i In the presence of chloride, the reaction of copper(I) with dppe in non-polar solvents gives the tumouricide [MCuCl(dppe)N2 (l-dppe)]; in polar solvents [Cu(dppe) 2 ]Cl is obtained.4j Likewise CuClP 3 or CuP 4 co-ordination geometries can be obtained with 2,3-bis(diphenylphosphino)maleic anhydride.4k N-[2-(1-Naphthyl) ethyl]-1-aza-4,8-dithiacyclodecane (L) forms the first copper(I)-g2-naphthyl complex [CuL][PF 6 ].4l The relatively rare CuOS 2 P co-ordination is found in [CuL(PPh 3 )][ClO 4 ] (L\1-oxa-4,7-dithiacyclononane),4m while the more common S 2 P 2 donor set occurs in [NEt 4 ][CuMS 2 C 2 (CN) 2N(PPh 3 ) 2 ].4n Bi-, oligo-, poly- and hetero-nuclear and supramolecular species Dicopper(I) species.This year has seen much discussion of ‘cuprophilicity’, relatively short Cu–Cu contacts in bi-, oligo- and poly-nuclear copper(I) complexes which Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 189–211 190may or may not indicate some kind of bonding. The copper–copper distance is often around 2.5–2.6Å, compared with 2.56Å in the metal, and even longer distances of 2.8–2.9Å have been attributed to cuprophilicity.DFT calculations on [CuL][CuCl 2 ] [L\1,1@-bis(2-pyridyl)octamethylferrocene], which has an unsupported Cu–Cu distance of 2.81Å, show that the interaction is purely electrostatic.5a However, ab initio calculations on [MCuCl(NH 3 )N2 ] led to an optimal eclipsed structure, with intermolecularNH · · ·Cl hydrogen bonds and significant bonding electron density between the copper atoms even at a distance of 3.17Å.5b Similar calculations on transannular Cu–Cu interactions in [MCu(l-PPh 2 CH 2 SPh)N2 ]2` show that the inclusion of correlation e§ects results in a contraction of the Cu–Cu distance from 3.6 to 3.1Å.5c Similar calculations on four- and eight-membered rings likewise lead to a cuprophilic e§ect as a consequence of electron correlation.5d DFT studies of compounds containing the shortest Cu–Cu contacts (2.35–2.45Å) reveal no significant covalency, and the closeness of the metal atoms in bis(pyrimidine)dicopper(I) compounds, for example, is attributed to a combination of strong Cu–Nbonding and the very short bite distances of the ligands.5e This paragraph deals with new dicopper(I) compounds having short Cu–Cu contacts in which cuprophilicity might be invoked.An unsupported Cu–Cu distance of 2.70Å is found in [MCuLN2 ][ClO 4 ] 2 (L\4-isopropylideneaminobenzo-2-thia-1,3- diazole), where the copper atoms have linear two-co-ordination and a head-to-head arrangement.6a In [MCu(l-L) 2N(OCMe 2 )][PF 6 ] 2 (L\1,8-naphthyridine) one copper atom is two- and the other three-co-ordinate, with d(Cu–Cu)\2.53Å.6b The copper –copper distance in [MCu(tu) 2 (l-tu)N2 ]2` depends on the anion, and can be signifi- cantly shortened by intermolecular hydrogen bonding.6c The Cu–Cu distance shortens from 3.61Å in [MCu(NCMe)(l-L)N2 ][ClO 4 ] 2 [L\2-(diphenylphosphino)-6-(pyrazol- 1-yl)pyridine] to 2.52Å in [MCu(l-L)N2 (l-g1-C 2 Ph)][ClO 4 ], but this is doubtless dictated by the acetylide bridge.6d Among other dicopper(I) complexes, straightforward halogen bridging with Cu 2 X 2 rings occurs in [MCu(l-Cl)LN2 ] [L\Ph 2 PCH 2 CH(Et)OPPh 2 ,7a 4-benzoylpyridine7b] and [MCu(l-Br)L(PPh 3 )N2 ] (L\quinoline).7c An azoaromatic radical anion X~ (X\ 5,5@-dichloro-2,2@-azopyrimidine) has been stabilised in [MCu(PPh 3 )N2 (l-X)][PF 6 ].7d In [MCu(l-L)(PPh 3 ) 2N2 ] [HL\HON––C(CN) 2 ] double CuNCCNCu bridges are formed. 7e The discrete dimer [MCu(dmphen)(NCMe)N2 (l-4,4@-bipy)][BF 4 ] 2 points the way towards more complex 4,4@-bipy-bridged systems.7f Oligomeric species. A linear tricopper(I) array appears in [Cu 3 (l-L) 3 (NCMe)]- [ClO 4 ] 3 (L\7-diphenylphosphino-2,4-dimethyl-1,8-naphthyridine), with the MeCN ligand attached to a terminal copper atom.8a Most tricopper clusters have cyclic structures; a simple Cu 3 Cl 3 ring occurs in [Cu 3 (l3 -Cl)(l-Cl) 2 (l-L)] [L\2,5- bis(diphenylphosphino)thiophene].8b A Cu3 Br 2 C ring is found in [MCu 3 (l-Br) 2 (l- mes*)(SMe 2 ) 3N].8c Bridging pyrazoles enable (CuNN) 3 ring formation in [MCuLN3 ] [HL\2-M3(5)-pyrazolylN-6-methylpyridine].8d Turning now to tetracopper(I) complexes, a planar Cu 4 array is found in [MCu(l-L)N4 ] [L\ferrocenyltrisM(methylthio) methylNborate], where each S 3 ligand furnishes two terminal Cu–S bonds and the four bridging sulfur atoms are alternately above and below the Cu 4 plane.8e Another planar Cu 4 complex is [MCu(C 6 Me 5 )N4 ]; the Cu–Cu distance is 2.41Å and three-centre two-electron Cu–C–Cu bonds are postulated.8f An electrochemical synthesis with a Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 189–211 191copper anode, a platinum cathode and 1,3-thiazolidine-2-thione (L) in [NBu 4 ][BF 4 ]– toluene yields [MCuLN4 ][BF 4 ] 4 in which the Cu 4 core is an ‘open-butterfly’, with S 2 N co-ordination of the spinal copper atoms and S 3 N at the wingtips.8g Tetrahedral Cu 4 cores are found in [MCu(l3 -L)N4 ] [HL\SiMe 2 (PHCy) 2 ], the first tetranuclear copper( I) silylphosphido complex, prepared by an unexpected transphosphination/silylation reaction,8h and in [Cu 4 (l-tu) 6 (tu)][SO 4 ] 2 ·H 2 O, with one four-co-ordinate and three three-co-ordinate copper atoms.8i All the copper atoms have tetrahedral coordination in [Cu 5 (l2 -I) 3 (l3 -I) 2 L 2 ] (L\tetraethylthiuram monosulfide); each L is S 2 bidentate, with one sulfur atom on each L bridging two copper atoms.8j In [Cu 8 (l8 - Se)Ml4 -Se 2 P(OPr*) 2N6 ] an interstitial selenium atom sits at the centre of the Cu 8 cube, with a diselenophosphate bridging each face.8k Among high-nuclearity copper(I) clusters, the linear bidentate ligands bis(diphenylphosphino)acetylene and 1,4- bis(diphenylphosphino)benzene stabilise respectively Cu 16 and Cu 25 compounds by forming intramolecular bridges8l while [Cu 32 Se 16 (PPh 3 ) 12 ], [Cu 52 Se 26 (PPh 3 ) 16 ] and [Cu 72 Se 36 (PPh 3 ) 20 ] have been obtained by the reaction of Se(SiMe 3 ) 2 with copper(I) acetate in the presence of PPh 3 .8m Polymeric species.Beginning with chain structures, the vibrational spectrum of CuCN suggests a linear chain structure, similar to those found for AgCN and AuCN by powder neutron di§raction measurements.9a The chain polymer [MCu(CN)N4 L]n (L\2,2@-biquinoline) has alternate two- and three-co-ordinate copper(I) atoms.9b The polymeric anion in [ML]n[Cu 2 I 3 ]n (M\K, Rb, Cs: L\15-crown-5) is best described as catena-[M[Cu(l-I)Cu](l3 -I) 2Nn]n~, with an ‘up, up, down, down’ pattern for the doubly-bridging iodine atoms;9c with the cations [NEt 4 ]` and [K(18-crown-6)]` the pattern is ‘up, down, up, down’.9d In [Cu 2 Cl 2 L] (L\dipyrido[1,2-a: 2@,3@-d]- imidazole) the chain has alternate three- and four-co-ordinate copper atoms with both l and l3 bridging chlorine atoms.9e An electrochemical method in en provides a ‘bench-top’ synthesis for the chain polymer KCu 7~xS 4 (0\x\0.34) which has useful electrical properties for x[0.9f The compound BaDyCuTe 3 contains infinite chains of CuTe 3 5~ tetrahedra.9g Turning now to 2-D systems, [MCu(l-I)N2 L]n (L\1,2,5,6- tetrathiacyclooctane) consists of familiar Cu 2 I 2 chains linked into sheets by bridging L molecules.9h In [Cu(CM-TTF)]n[ClO 4 ]n each copper atom is tetrahedrally bonded to two methylsulfanyl sulfur atoms and two cyano nitrogen atoms from three tridentate CM-TTF molecules, leading to a 2-D net structure.9i Two distinct layers are present in KCuCeTe 4 , which might be rendered as [Kn]n`[MCuTeNn]n~[MCeTe 3Nn] with CuTe 4 tetrahedra in an anti-PbO layer;9j a similar [MCuTeNn]n~ layer can be discerned in Rb 2 Cu 3 CeTe 5 .9k Similar co-ordination of copper atoms occurs in CuTh 2 Te 6 , where Cu` ions link [MTh 2 Te 6N]n]n~ double chains into layers.9l The air-stable polymer [Cu 2 (g2-O 2 CCH–– CHCO 2 )]n, prepared by hydrothermal synthesis, is a rare example of trigonal planar copper(I) in an extended solid structure.9m Another hydrothermallyprepared 2-D polymer is [MCu(bipy)N2 (l-CN)] 2n[Cu 5 (CN) 7 ]n, which features anionic nets of alternating fused rows of MCu(CN)N6 and MCu(CN)N8 rings.9n 2-D oxide networks can be constructed by bridging oxomolybdate clusters such as [Mo 8 O 26 ]4~ with [Cu(4,4@-bipy)]` units.9o The co-ordination of the copper atoms in Cu 3 ClTeS 3 (sphalerite-type) is CuS 3 Cl.9p The remaining compounds in this section all have 3-D channelled structures.In [MCu(l-NCS)N2 (pyz)]n each copper atom is trigonally bonded to one nitrogen and two sulfur atoms of bridging thiocyanates, Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 189–211 192giving rise to honeycomb sheets with fused ten-membered rings linked by pyrazine bridges.9q In contrast, RbCuSb 2 Se 4 ·H 2 O contains CuSe 4 tetrahedra.9r Large van der Waals channels occur in CuAlCl 4 (corner-sharedMCl 4 tetrahedra), which adsorbs CO and ethylene reversibly.9s The hydrogen bonding capabilities of the cations in [NH 2 Et 2 ] 2 [CuCl 4 ][AlCl 4 ] give rise to the first reported ‘anti-zeotype’ structure.9t Supramolecular species.Here we cover copper(I) complexes whose assembly or topology is deemed to endow supramolecular status, although some species covered elsewhere might have been included.Tetrahedral CuN 4 cores are of special interest. A new double helicate cation has been established in [MCuLN2 ][PF 6 ] 2 , where L is a tetradentate ligand comprising two 3-(2-pyridyl)pyrazole units linked by an o- CH 2 C 6 H 4 CH 2 spacer.10a The self-assembled chiral metallophane [MCuLN2 ][BF 4 ] 2 [L\2-(2,2@-bipyridyl)-3-(2-pyridyl)pyrazine] is shown by 1H and 13C NMR spectra to be stable in CD 3 CN solution.10b Imine-bridged oligobipyridine ligands are shown by UV and NMR spectroscopy to form double helicates with copper(I).10c Double helical ‘twisted ring figure-of-eight loops’ characterise the topology of a series of dicopper(I) complexes with macrocyclic ligands having two N 2 S 2 donor sites, spaced by p-xylylene groups; in MeCN there is a dynamic equilibrium between the enantiomers, involving rupture of copper–ligand bonds and intermediate co-ordination of solvent.10d Another double helicate is [MCu(O 2 PX 2 )N2 ] [X\3-(2-pyridyl)pyrazol-1- yl], but in the analogous complex with OSPX 2 ~ co-ordination of the sulfur atoms prevents helication.10e The reaction of [Cu(NCMe) 4 ]` with 1,2,4,5-tetracyanobenzene (L) led to topologically-distinct polymers [Cu 2 L 3 ][PF 6 ] 2 depending on the solvent.10f In self-assembled [CuL 2 ][ClO 4 ] [L\2-Mbis(methylsulfanyl)methylene Npropane-1,3-dinitrile] each copper centre is bonded to nitrogen atoms from four di§erent L molecules, each L bridging two copper atoms to give a square grid arrangement.10g The reaction of [Cu(CN) 4 ]3~ with Me 3 SnCl and 4,4@-bipy gives [MCu[l-CNSn(Me 3 )NC]N2 (4,4@-bipy)] where the co-ordination is tetrahedral CuC 3 N; in each dimeric unit the copper atoms are bridged by cyano carbon atoms, and further polymerisation is achieved via both CNSnNC and 4,4@-bipy bridges to give a remarkable supramolecular structure.10h The chiral ligand L obtained by condensation of trans-cyclohexane-1,2-diamine with two molecules of 6-R-2-pyridinecarbaldehyde (R\H, Br) forms [MCuLN2 ]2`, whose self-assembly exhibits ligand self-recognition by virtue of chirality; the reaction of the metal ion with a racemic mixture of the ligands forms only homochiral complex ions.10i Heterometallic species.All the heteronuclear species containing copper(I) reported this year involve 4d or 5d elements as the hetero-metal.In [AgCu(SCN) 2 (py) 4 ], Cu(py) 3 units are linked via CuNCSAg bridges into infinite chains of thiocyanatebridged Ag(py) units. However, [AgCu(NCS) 2 (py) 3 ] contains ten-membered rings linked into infinite chains by 1,3-l and 1,1,3-l3 NCS groups (tetrahedral CuN 4 co-ordination) while [Ag 2 Cu(NCS) 3 (py) 3 ] has a 2-D structure; in all these compounds, the copper atoms have tetrahedral CuN 4 co-ordination, the softer Ag` ions having first claim on the sulfur donor atoms.11a Copper(I) thiomolybdate/tungstate complexes continue to attract attention.The solid state reaction of [NH 4 ] 2 [MS 4 ] (M\Mo, W) with [Cu(NCMe) 4 ][PF 6 ] and dppm produced [MCu(l-dppm)N4 (l-S 4 M)][PF 6 ] 2 , in Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 189–211 193which each sulfur atom bridges two copper atoms and the M atom, while in CH 2 Cl 2 the product was [MCu(dppm)N3 (l-S 4 M)][PF 6 ], where one sulfur atom bridges three copper atoms and M and the other sulfur atoms bridge only one copper and M.11b M(V) (M\Mo, W) is found in the clusters [NEt 4 ] 2 [MCuM(S)(O)- (SC 2 H 4 S)N6 (l6 -S 2 )], in which each sulfur atom of the central S 2 2~ ion is bonded to three copper atoms.11c Each of a series of four clusters [NBu 4 ] 4 [Cu 10 (l-S 3 EM) 3 (l- S 4 M)(l3 -S) 2 (l4 -S)] (M\Mo, W; E\S, O) contains one incomplete cubane Cu 3 MS 3 E, one trigonal prism-type Cu 3 MS 4 and two butterfly-type Cu 3 MS 3 E fragments, bridged by three sulfur atoms.11d The cuboidal clusters [Mo 2 WCuS 4 ]n`(aq) (n\4,5) have been studied by UV spectroscopy and cyclic voltammetry, completing the series [MxM 3~xCuS 4 ]n`(aq).11e Perhaps the most unexpected copper–tungsten cluster is [MCp*W(l-S) 3 Cu 2N3 (l3 -S) 2 ]; the copper–tungsten distances (2.67–2.68Å) are significantly shorter than the usual 2.80–2.82Å.11f The reaction between [Cu(CN)(AsPh 3 )(bipy)] and [RuCl 2 (bipy) 2 ] gives a product which has been characterised spectroscopically as [Cu(AsPh 3 ) 2 (l-CN)RuCl(bipy) 2 ][PF 6 ].11g The Cu–Re acetylide complex [MCu(dppm) 3NMl3 -g1-C 2 C 6 H 4 C 2 -p-Re(CO) 3 (bipy)N2 ][PF 6 ] and derivatives show rich photochemical and electrochemical properties.11h Both cis- and trans-[PtCl 2 (PPh 3 ) 2 ] react with Se(SiMe 3 ) 2 and CuCl in thf to give [MCuClN2MPt(PPh 3 ) 2N2 (l3 -Se) 2 ], in which the four metal atoms form a parallelogram with each selenium atom bridging one copper and both platinum atoms; the coordination about the copper atom is almost linear, and there is no reason to postulate Cu–Pt bonding at distances of 2.92 and 3.05Å.11i Photochemical and photophysical properties of copper(I) complexes Progress continues in the photochemistry of [Cu(phen) 2 ]` and related species; a concise review has appeared.12a The bulky Pr* groups in [Cu(2,9-Pr* 2 phen) 2 ]` inhibit structural relaxation in the photoexcited state, allowing reductive quenching by ferrocene derivatives; the phenyl groups in 2,3,6,7-tetraphenyl-1,4,5,8-tetraazaphenanthrene are even more e§ective.12b The photochemical and electrochemical e§ects of bulky electron-withdrawing substituents have been examined in the case of [CuL 2 ]` [L\2,9-bis(trifluoromethyl)-1,10-phenanthroline].12c For L\(2,3-b)-pineno-1,10- phenanthroline, both D and K stereoisomers of the chiral complex [CuL 2 ]` have been prepared.However, no enantioselective quenching was observed with D- and K- [Ru(bipy) 3 ]2` as donors.12d Photophysical studies of [CuL(PPh 3 ) 2 ]`, where L is a diimine ligand containing a thia-, selena- or tellura-crown ring, have established their potential as luminescence probes for soft cations.12e Complexes of the type CuXL (X\halogen, L\nitrogen donor) often exhibit oligomer/polymer isomerism, as well as rich photochemistry.Exposure of [CuIL]n (L\4-methylpyridine) to toluene liquid or vapour leads to the disappearance of the room temperature blue emission of the polymer and the appearance of the yellow emission characteristic of the oligomer [MCu(l3 -I)LN4 ]; exposure of the latter to n-pentane liquid or vapour reverses the process!12f The volatile cluster [MCu(l-L)N4 ][L\N(SiMe 3 ) 2 ], containing a square planar Cu 4 N 4 core, is phosphorescent both in solution and in the solid state at room temperature.12g The cluster [MCu(py) 2N2MCu(SCN)(py)N2 (l-S 4 W)] is among the best optical limiting materials yet discovered.12h Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 189–211 1942 Copper(I)–copper(II) chemistry Here we cover mixed-valence copper(I)–copper(II) compounds (other than oxides which are dealt with in the next section), as well as redox systems in which the two states are readily interconverted.Mixed-valence copper(I)–copper(II) compounds Welook first at compounds in which the two oxidation states are clearly distinguished. As well as discrete [CuBr 4 ]2~ ions, [LH] 4n[CuIIBr 4 ]n[CuI(l-Br) 2 ] 2n (L\4- aminopyridine) contains the first example of infinite chains of CuIBr 4 tetrahedra sharing edges.13a In contrast, [CuIICl(bipy) 2 ] 2n[CuICl 2 ] 2n·nC 6 O 2 (OH) 4 contains linear [CuCl 2 ]~ ions; hydrogen bonding via the tetrahydroxoquinone molecules holds together an infinite chain structure.13b Ab initio calculations have been performed on the electron-transfer matrix element in a model for the chain polymer [NEt 4 ]n[CuI(l- Cl) 2 CuII(l-Cl) 2 ]n.13c An electrochemical synthesis from copper metal, thiosalicylic acid (H 2 L) and triphenylphosphine in MeCN leads to the formation of [MCuI(PPh 3 ) 2N2 CuII(l-L) 2 ], where one copper(I) has S 2 P 2 and the other O 2 P 2 coordination. 13d The tellurite Ba 2 Cu 4 Te 4 O 11 Cl 4 contains oxide layers with square planar CuIIO 4 co-ordination and chloride layers with CuICl 4 tetrahedra.13e The pillared, layered compound Na 2 CuI 6 CuII 9 L 6 (OH) 2 ·H 2 O (L\1-hydroxyethylidenediphosphonate) is the first mixed-valence copper phosphonate; the copper(II) atoms are square planar while the copper(I) atoms have linear two-coordination. 13f UV irradiation of [MCu(OAc) 2 (H 2 O)N2 ] in HOAc–MeOH generates a CuI–CuII mixed valence species; electrochemical studies are consistent with two distinct sites.13g We turn now to delocalised systems where distinct copper(I) and copper(II) sites cannot be discerned (Class III in the Robin–Day scheme). The anion in [PPh 3 Me] 2 - [Cu 2 Br 5 ] has D 3) symmetry, a rare case of confacial tetrahedra for copper, containing a (Cu1.5`) 2 pair; the Cu–Cu distance is only 2.36Å, butDFT calculations indicate that the interaction is very weak.14a In [Cu 2 (l-pym)Mo 3 O 10 ]n, where equivalent CuO 3 N tetrahedra link chains of edge-sharing MoO 6 polyhedra with pym molecules bridging copper centres to give a 3-D structure, bond valence sums point to Cu1.5` [cf.ref. 9(o)].14b Models for the Cu A centre in cytochrome c oxidase and NO reductase continue to generate activity, both from synthetic14c and spectroscopic/theoretical approaches.14d The remainder of this section is devoted to copper(I)–copper(II) redox systems, beginning with bis(diimine) complexes.The kinetics of electron exchange for [CuL 2 ]`@2` (L\2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) have been studied in MeCN;15a the 4 and 7 substituents are apparently responsible for the e§ectiveness of [CuL 2 ]` in bridging pairs of DNA duplexes.15b A flexible bis(diimine) ligand L forms copper(II) complexes [CuL]X (X\[CuCl 4 ], [PF 6 ] 2 ) with square planar CuN 4 co-ordination; these undergo reversible reduction to copper(I) species with tetrahedral co-ordination. 15c The crystal structures of [Cu(dpphen) 2 ][PF 6 ] and [Cu(dpphen) 2 ][ClO 4 ] 2 have been compared, in the context of the room-temperature luminescence of [Cu(dpphen) 2 ]`; the structural change on oxidation is significant but less than for Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 189–211 195unhindered analogues where copper(II) tends to adopt five-co-ordination.15d Copper( I)–copper(II) complexes with 1,1@-bis(bipy) ester-bridged derivatives of ferrocene exhibit interesting electrochemistry; the dimer [MCuLN2 ][BF 4 ] 2 has a helical structure. 15e A terpy derivative having two imino nitrogen atoms as well as the three pyridine groups forms a helicate [MCuLN2 ]2` whose 15N NMR spectrum shows a dynamic fluctuation in which the non-co-ordinated and co-ordinated imino nitrogen atoms are interchanging; all five nitrogen atoms are co-ordinated on oxidation to copper(II), and the reversible redox properties o§er possibilities for the design of catalytically-active copper helicates.15f The couple [Cu(NCS)L]–[Cu(NCS)L]` [L\(S)-N,N@-bisM(2-quinolyl)methylN-1-(2-quinolyl)ethylamine] acts as a redox switch via the distinct CD spectra of the chiral ligand.15g The complex [MCu(NCMe)N2 (l-L)]n [L\3,4-bis(dicyanomethylene)cyclobutane-1,2-dione dianion] has a complex electrochemistry, involving the dianion, the radical anion and the neutral dione as well as CuI–CuII.15h The photochemical reduction of copper(II) dicarboxylate complexes has been the subject of a thorough investigation;15i one of the ligands studied, fumarate, has interesting e§ects on the kinetics of the oxidation of Cu`(aq) by dioxygen, elucidated by pulse radiolysis studies.15j This paragraph is devoted to copper(I)–(II) redox systems of biological importance.Beginning with the blue (type I) copper proteins, X-ray MCD measurements on plastocyanin o§er promise that this technique will be valuable as an electronic structure probe in metalloproteins.16a Further work on the resonance Raman spectra of plastocyanin from various sources emphasises the sensitivity of this technique to the copper environment;16b ‘chromophore-in-protein’ modelling successfully reproduces some of the complex features of these spectra.16c Type I copper sites (CysHis 2 Met) have been constructed in the hydrophobic core of thioredoxin; it was necessary, however, to introduce an exogenous azide ligand to exclude water from the coordination sphere.16d Among studies of other mononuclear systems, XAS/EXAFS studies on amine oxidases show that the oxidised form probably contains five-coordinate copper(II) (three histidine nitrogen atoms and two water molecules) while the reduced form has three-co-ordinate copper(I); the latter presumably reacts with dioxygen in the enzyme.16e The toxicity of cyanide is believed to involve the bridging of the iron and copper atoms at the haem a 3 /Cu B site in cytochrome c oxidase; model studies on [(oep)Fe(NC)Cu(tmpa) 3 ]n` with FeIII–CuII, FeIII–CuI and FeII–CuI have been reported.16f Dopamine b-hydroxylase contains two copper sites Cu A (three histidines and one water) and Cu B (two histidines, one water, one other ligand and a weakly-bound methionine sulfur atom), more than 4Å apart; a ligand L having one tertiary amino and two pyridine nitrogen donor atoms, plus a thiol sulfur atom, has been used in modelling studies of the Cu B site.16g The same laboratory has investigated the mechanism of dioxygen activation of dopamine b-hydroxylase by appeal to a binuclear model system, using a ligand derived from 2-aminoindane with two pendant pyridyl groups attached to the amino nitrogen atom; the CuI 2 form reacts with dioxygen to give the bridged peroxo complex CuIIOOCuII, which then transfers an oxygen atom to the ligand, resulting in a dicopper(II) complex with bridging phenolate and hydroxide.16h Kinetic studies of a similar reaction, but with a Schi§ base ligand in which each copper has an imino nitrogen donor in place of the amino nitrogen in ref. 16(h), provide evidence for an intermediate peroxo complex which cannot be detected spectroscopically.16i With the bis[2-(2-pyridyl)ethyl]amine ligands more commonly Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 189–211 196used in such studies, the l-g2:g2 side-on peroxo-bridged intermediate can be observed by resonance Raman spectroscopy, and this can be used to monitor the kinetics.16j There is both spectroscopic and chemical evidence for the bis(l-alkylperoxo) intermediate.16k With a di§erent but related ligand, the mechanism of hydroxylation is believed to involve intramolecular C–H bond activation with a bis(l- oxo)dicopper(III) intermediate.16l The l-1,1-hydroxoperoxo-bridged dimer has also been studied both spectroscopically and theoretically.16m 3 Copper–oxygen chemistry Here we cover oxides, mixed oxides and precursors thereto, and copper exchanged silicates, etc., without regard for the (often indeterminate) oxidation state of copper.Oxides and oxocuprates Copper(I) oxide catalyses the photolysis of water by visible light.17a The adsorption of CO on Cu 2 O has been studied by He II UPS, supported by SCF-Xa calculations.17b Turning to superconducting oxocuprates, the high-pressure synthesis of high T C systems has been reviewed.17c X-Ray absorption spectroscopy has been used to investigate the hole distribution in Y(Ba 2~ySry)Cu 3 O 6`d and (Cd 0.5 Pb 0.5 )Sr 2 (CaxY 1~x)Cu 2 O 7 .17d,e FT-IR spectroscopy, supported by MM calculations, has elucidated the ordering of amine monolayers adsorbed on YBa 2 Cu 3 O 7 .17f Electron energy-loss and XPS spectra show that the adsorption of NO on LaBaSrCu 2 O 6~d produces holes in oxygen 2p states; the resulting NO~ ions abstract oxygen from the lattice to form nitrite.17g In chlorooxocuprates, p-type superconductivity occurs when the apical sites are completely occupied by chlorine atoms.17h The intercalation of a superionic conducting Ag–I layer into the superconducting Bi 2 Sr 2 Can~1 CunOy lattice leads to hybrid systems having both high electronic and ionic conductivities.17iWe now look at non-superconducting lanthanoid oxocuprates, whose structures and preparations are not without relevance to the superconductor industry.High pressure stabilises the perovskite structure; the ambient-pressure phase of La 4 Cu 3 MoO 12 is transformed into a perovskite under a pressure of 6 GPa, with a change in the co-ordination geometry about copper from trigonal bipyramidal CuO 5 to elongated-octahedral CuO 6 .17j The e§ect of pressure (up to 36 GPa) on the structure of Nd 2 CuO 4 has been studied using synchrotron radiation.17k Among lanthanide oxocuprates, La 4 BaCu 5 O 13`d is unique in exhibiting metallic behaviour down almost to absolute zero without superconductivity; metallic conductivity is preserved on partial substitution of copper by nickel, but with iron or cobalt transitions to insulating phases are observed.17l Di§raction data alone can give only an approximate structure, with averaged cation distributions, of the perovskite Gd 2 Ba 2 CaCu 2 Ti 3 O 14 , which contains two CuO 2 layers.However, the use of EXAFS data for all five metals in the refinement (combining the determination of long-range order and of local environments) gives a more accurate structure.17m The catalytic activity of (CeO 2 ) 1~y(La 2 CuO 4 )y for the reduction of NO by CO is attributed mainly to Cu2` at the surface, but the bulk plays a part via oxygen exchange with the surface.17n Four new Ba–Cu–Ir oxides have been predicted and duly characterised.17o Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 189–211 197Precursors This paragraph covers studies of volatile complexes which decompose on heating to give thin films of copper metal, copper oxides or mixed oxides. The liquid copper(I) complex [CuLMP(OMe) 3N] (HL\tert-butyl-3-oxobutanoic acid) is probably the best of several [CuLL@] complexes studied as precursors for theCVDof copper metal.18a In [Ba 4 CuI 6 (l4 -O)(l-OCEt 3 ) 12 ] the central oxygen atom is surrounded by four barium atoms, with each MCu(OCEt 3 ) 2N unit (linear CuO 2 ) bridging an edge of the Ba 4 tetrahedron; in [BaCu 6 (l-OCEt 3 ) 8 ] two MCu 3 (OR) 4N moieties are co-ordinated to a central barium atom having an unusually low co-ordination number of four.Interesting though these structures may be, neither compound has much potential as a CVD precursor.18b Much more promising are polynuclear Ln–Sr/Ba–CuII complexes with bifunctional ligands such as 2-hydroxypyridine and 1,3-bis(dimethylamino)propan-2- ol.18c The 2-D solids [MCu(l-O 2 CCH 2 CO 2 ) 2 (H 2 O) 4 ] (M\Sr, Ba), where the copper atoms have elongated-octahedral co-ordination, are not volatile but mixed oxides MCuO 2 can be obtained by ceramic methods.18d Copper-exchanged silicates and related species A new Cu` site in calcined Cu-ZSM-5, described as defective (AlOd~)Cu`, has been identified by FT-IR spectra of adsorbed CO.19a DFT analyses have been extensively employed to investigate the binding of small molecules to copper in zeolites; examples include studies of the energetics of water adsorption to H-ZSM-5 and Cu-ZSM-5,19b comparison of the binding ofNO 2 to Cu` in the gas phase with that in zeolites,19c and the decomposition of NO in copper-exchanged zeolites.19d A general expression has been derived for the rate constant for NO decomposition in these systems.19e Experimental work has not been altogether forsaken; the first observation (by NMR spectroscopy) of complexation of a neutral organic free radical (cyclohexadienyl) with a diamagnetic metal ion in a zeolite has been reported for Cu-ZSM-5.19f Copperexchanged silicates other than ZSM-5 are also of interest; an example is MFIferrisilicate, where activity towards the reduction of NO with ammonia is independent of the exchange level.19g The adsorption of water, ammonia, methanol and ethylene on CuH-SAPO-35, a small-pore silicoaluminophosphate similar to the zeolite levyne, has been studied by EPR spectroscopy.19h Alkali metal cations increase the yield of benzaldehyde in the gas-phase oxidation of benzyl alcohol catalysed by copperexchanged Y-type zeolite.19i Most of the aforementioned systems are prepared by treating the zeolite with Cu2`(aq), followed by calcining which results in at least partial reduction to copper(I).Highly-exchanged CuI–mordenite has been prepared by treating the zeolite with CuCl vapour; this material adsorbs CO at room temperature, with formation (according to EXAFS and FT-IR spectra) of CuI(CO)n adducts: at 77 K, tricarbonyl species can be detected.19j Hyperfine sublevel correlation spectroscopy has been applied to [Cu(py) 4 ]2` in the molecular sieves Cu-Na-Y-zeolite and MCM-41; in the former the planes of the py rings are perpendicular to the CuN 4 , while in the latter the py planes are parallel to the equatorial plane.19k The complex [Cu(salen)] encapsulated in Na-Y zeolite changes from green to red on treatment with MeCN; the red form catalyses organic oxidations while the green form is inactive.19l Finally, the first example of a copper-exchanged silicon-free aluminophosphate molecular sieve has been reported.19m Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 189–211 1984 Copper(II) chemistry Mononuclear species Complexes with N-donor ligands.This section covers copper(II) complexes where the primary ligands are deemed to be nitrogen donors, although the focus of interest may lie in other ligands. Wefollow the sequence: amines, imines; nitrogen heterocycles; phen and bipy complexes; porphyrins and phthalocyanins. Both the isomers [CuL 2 ][NO 3 ] 2 ·2H 2 O(red) and [CuL 2 (H 2 O)][NO 3 ] 2 ·H 2 O(blue) exhibit thermochromism (L\N1-isopropyl-2-methylpropane-1,2-diamine).20a The conformations of six-membered rings in 174 copper(II) diamine complexes in the Cambridge Structure Database have been analysed; 167 have chair and 5 boat conformations, with one representative each of the d- and k-twist boat conformations. 20b The cation in [CuL(Me 5 dien)][ClO 4 ] (HL\valine) has trigonal bipyramidal co-ordination geometry, which is unusual for a copper(II) complex with a chelating amino acid.20c An EPR study of [CuL(en)][ClO 4 ] 2 [L\NH 2 (CH 2 ) 3 NH(CH 2 ) 3 NH 2 ] is perhaps the most detailed yet published for a trigonal bipyramidal CuN 5 complex.20d The e§ects of methyl substituents on the structural, spectroscopic, thermodyamic and kinetic properties of [CuL(H 2 O)]2` (L\Mentren; n\3,6) have been studied.20e The bulky ligand in [CuCl 2 L] [L\([)-sparteine] enforces a distorted tetrahedral co-ordination geometry about the copper atom.20f The kinetics of substitution of nickel for copper in [CuL] [H 2 L\N,N@-ethylenebis(2-aminobenzaldimine)], studied in dmf, show that the rate is strongly dependent on the anion X in the order ClO 4 ~\Br~\NCS~@Cl~; the mechanism apparently involves the reaction of [CuL] with [NiX 2 (dmf) 2 ] to form a binuclear intermediate.20g This paragraph covers copper(II) complexes with heterocyclic N-donors, apart from bipy and phen derivatives.Solutions of copper(II) chloride in pyridine, methylpyridines or dimethylpyridines act as homogeneous catalysts for the water–gas shift reaction.21a Ferromagnetic coupling between the unpaired spins on CuII and 4-NOpy [\4-(Noxyl- tert-butylamino)pyridine] in [Cu(facac) 2 (4-NOpy) 2 ] leads to an S\3 2 ground state.However, [Cu(facac) 2 (3-NOpy) 2 ] behaves as if the spins on the radical ligands are strongly antiferromagnetically coupled, and the complex is simply an S\1 2 paramagnet. 21b From solutions of copper(II) nitrite and the tridentate ligand 2,6-bis[(3,5- dimethyl)pyrazol-1-yl]pyridine (L) in MeCN–H 2 O a compound can be crystallised containing neutral square pyramidal [Cu(ONO) 2 L] molecules; the unit cell also contains discrete [Cu(ONO) 4 ]2~ ions and nitrite-bridged dimeric units.21c Co-ordinated hydroperoxide has been established structurally and spectroscopically in [Cu(OOH)L][ClO 4 ], where L is a tripodal pyridylamine.21d The first example of hexafluorosilicate acting as a bidentate chelating ligand is found in [CuL(MeOH)]- [Cu(F 2 SiF 4 )L][BF 4 ] 2 , where L is a tetradentateN 4 ligand containing two pyridyl and two pyrazolyl rings; the Cu–F distances (2.20 and 2.36Å) are intermediate between the ‘short’ and ‘long’ bonds found in elongated-octahedral fluoro-complexes of copper( II).10a Several new copper(II) complexes with 3-chloro-6-(pyrazol-1-yl)pyridazine (L) have been characterised; intramolecular NH· · ·N hydrogen bonding between L and L@ (\2-cyanoguanidine) and the presence of both co-ordinated and non-coordinated tetrafluoroborate are features of [Cu(FBF 3 )L(L@)(H 2 O)][BF 4 ].21e By Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 189–211 199adding bulky (mesityl) substituents in the 3,3A positions of 2,6-di(pyrazol-1-yl)pyridine, a compressed octahedral structure with a (dz2)1 ground state can be sterically imposed in [CuL 2 ]X 2 (X\BF 4 , ClO 4 ).21f Pulsed EPR studies of polyimidazole copper(II) complexes show how the mode of co-ordination of histidine to copper (via Nd or Ne) in proteins can be determined.21g This paragraph deals with copper(II) complexes having bipy, phen and their derivatives as primary ligands.Trigonal bipyramidal co-ordination geometry is found in [Cu(NCO)(bipy) 2 ][C(CN) 3 ]; attempts to prepare the phen analogue result in [Cu(NCO) 2 (phen)].22a A chiral bipy derivative (L), having asymmetric substituents in the 3 and 3@ positions, forms the complexes [CuX 2 L 2 ] (X\Cl, triflate); the triflate catalyses the asymmetric cyclopropanation of alkenes.22b The complexes [Cu(phen) 2 Br]X (X\Br, ClO 4 , NO 3 , PF 6 ) have co-ordination geometries described as ‘square based pyramidal distorted trigonal bipyramidal’, while with X\BPh 4 the co-ordination is ‘extreme trigonal bipyramidal square based pyramidal’; as with the chloro-analogues, a common structural pathway between the regular five-co-ordinate geometries can be traced.22c The EPR spectrum of [Cu(HL)(phen) 2 ]·0.5phen·7H 2 O [H 3 L\1,3,5-triazine-2,4,6-(1H,3H,5H)trione] can be interpreted in terms of a mixed dz»–dx»~y» ground state, consistent with the co-ordination geometry which is intermediate between trigonal bipyramidal and square pyramidal; although binuclear units (via hydrogen bonding) can be discerned in the structure, there is no evidence of magnetic coupling down to 4.2 K.22d Elongated octahedral co-ordination, with long bonds to water and one phen nitrogen atom, is found in [CuMONC(CN) 2N(phen) 2 (H 2 O)].22e In [Cu(dmphen) 2 (H 2 O)][CF 3 SO 3 ] 2 the equatorial Cu–O distance in the distorted trigonal bipyramidal co-ordination polyhedron is considerably shorter (2.08Å) than in [Cu(phen) 2 (H 2 O)][BF 4 ] 2 ; the kinetics of NO reduction by the dmphen complex point to formation of an inner-sphere NO complex. 22f A conformational transition from B (right-handed) to Z (left-handed) DNA is induced22g by [Cu(5,6-Me 2 phen)]2`. The unsymmetrical chelation of dafone in complexes such as [CuBr 2 (dafone) 2 ] can be rationalised in terms of the unusually large bite (ca. 3.0Å, compared with ca. 2.6Å for bipy and phen).22h Measurements of the temperature dependence of the Soret bandwidth combined with resonance Raman spectroscopy have been applied in the study of the coupling of solvent motion to vibrations involving the metal atom in [M(oep)]; such coupling was found to be almost negligible for M\Cu, weak for Co and strong for Ni and Pd.23a The ‘sitting-atop’ complex [Cu(H 2 tpp)]2` has been established in acetonitrile; 1H NMR spectra show that the copper atom is co-ordinated to two trans pyrrolenine nitrogen atoms with two pyrrole nitrogens remaining protonated.23b The complex [CuL] with the new ligand L\tetrakis(1,2,5-thiadiazole)porphyrazine should be of interest to workers in the field of conducting materials.23c Eight tetraazomacrocycles with co-ordinated metal ions can be attached to the benzenoid rings in [Cu(pc)]; their e§ect on the CuII EPR parameters is small, and can be correlated with the electronegativities of the metal centres.23d Complexes with O- or N,O-donor ligands.Beginning with aqua-complexes, MS studies show that when [Cu(OH)(H 2 O) 4 ]` reacts with D 2 O in the gas phase, ligand exchange but not hydrogen/deuterium exchange takes place.24a Irradiation of[NH 4 ] 2 - [Cu(H 2 O) 6 ][SO 4 ] 2 , containing about5% deuterium, at theN–Dstretching frequency Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 189–211 200induces switching between the di§erent structures of the deuteriated and nondeuteriated Tutton salt, via coupling of the O–D modes to the Jahn–Teller distortion. 24b ESEEM spectra of the same salt are particularly sensitive to the vibronic dynamics which produce the Jahn–Teller distortion.24c Aqueous solutions containing 0.5 mol kg~1 CuCl 2 and 5 mol kg~1 NaCl induce the condensation of amino acids to peptides; neutron di§raction studies and Monte Carlo simulations show that [CuCl(H 2 O) 5 ]` is the dominant species.24d This paragraph deals with mononuclear copper(II) carboxylate complexes. Discrete cations with square planar CuO 4 co-ordination are found in [CuL 4 ][ClO 4 ] 2 and in Na[CuL 4 ][ClO 4 ] 3 (L\trimethylammonioacetate),25a while square pyramidal coordination is found in [CuClL 2 (H 2 O) 2 ][ClO 4 ] (L\4-pyridinioacetate).25b In [CuL 2 (tn) 2 ] (HL\2-, 3- or 4-aminobenzoic acid) the monodentate L groups form long Cu–O bonds in the elongated octahedral co-ordination; in the 4-aminobenzoate complex, there are two molecules of water in the lattice which form hydrogen bonds with the amino groups.25c Elongated octahedral co-ordination also occurs in [CuL 2 (L@) 2 ] (HL\nonanoic acid, L@\2-aminoethanol); the monodentate carboxylates and amino nitrogen atoms furnish the equatorial ligands, while the alcohol oxygen atoms form long (2.48Å) axial bonds.25d The only other studies of copper(II) complexes with purely O-donor ligands involve dionates.The square planar complexes [CuL 2 ] (HL\5-alkoxytropolone) exhibit both enantiotropic and mesophase behaviour.25e The square pyramidal complex [CuL 2 (H 2 O)] (HL\4-tert-butylacetyl- 3-methyl-1-phenylpyrazol-5-one) is of interest on account of its intricate hydrogen bonding network, the ability of other ligands to displace the apical water and the possibilities a§orded by the bulky neopentyl group.25f Looking now at complexes with amino acids and peptides, the structures of copper( II) complexes with glycine in aqueous solution have been studied by XAS.In the mono- and bis-species, the glycinate is bidentate with water molecules completing elongated octahedral co-ordination; the tris-species is square pyramidal, the apical position being occupied by an amino nitrogen atom.26a In [CuLL@] (H 2 L\glycylglycine, L@\isocytosine) L is tridentate N 2 O bonding with the pyrimidine contributing a nitrogen donor atom.26b The complexes [CuL]` and [CuClL@]`, where HLand L@ are bis(2-pyridylmethyl) derivatives of glycylglycine and glycylglycylglycine respectively, perform site-specific single-strand DNA scission.26c Among numerous papers on copper(II) complexes with Schi§ bases and related ligands, we mention four of special interest.In complexes [CuL], where H 2 L is obtained by condensation of 2-(2-pyridylmethyl)propane-1,3-diamine with substituted salicylaldehydes, the pendant arm is enganged in neither inter- nor intra-molecular bonding.27a The EPR spectrum of [CuClL] [L\(o-O)C 6 H 4 CH––NC(Me)(CH 2 OH) 2 ] indicates an S\1 ground state, the molecules interacting via extended hydrogenbonded networks.27b Copper(II) complexes with chiral Schi§ bases catalyse the peroxidation of PhMeS to PhMeSO, with a modest enantiomeric excess.27c The smectogenic properties of complexes [CuL 2 ] [L\4-alkoxy-N-(4-ethoxyphenyl)salicylaldiminate] have been examined.27d Among complexes with miscellaneous O- and N,O-donor ligands,DFT calculations have been performed on 18 possible structures for [Cu(NO 2 ) 2 ]; the most stable is found to be the planar, D 2) structure [Cu(g2-O 2 N) 2 ].28a The fungal enzyme galactose oxidase (GOase), whose active site contains a square-pyramidally co-ordinated cop- Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 189–211 201per(II) atom with an O-bonded tyrosyl radical in the equatorial plane, is EPR-silent; this suggests antiferromagnetic coupling between Cu2` and the tyrosyl radical which is surprising since in model systems examined to date similar coupling is always ferromagnetic.Studies of the model complexes [CuXL] MX\Cl, R 2 acac (R\Ph, Bu); HL\N 3 Oligand based on [9]aneN 3N show that antiferromagnetic vs. ferromagnetic coupling depends on the dihedral angle between the equatorial plane and the plane of the phenyl ring of the radical ligand.28b In another EPR-silent GOase model, the nitrogen atoms in the N 3 O 2 donor set are furnished by TpP) and the oxygen atoms by a salicylaldehyde derivative; its electronic spectrum is remarkably similar to that of the active enzyme.28c Another GOase model contains square-pyramidally bonded copper( II) with a long (2.57Å) apical bond to a phenolic oxygen atom.28d The first structurally-characterised copper(II) nitrone complex [Cu(facac) 2 L] (L\N-tertbutyl- 2-pyridylmethyleneamine N-oxide) contains two independent molecules in the unit cell, both having slightly elongated octahedral co-ordination geometry; in one molecule the axial atoms are one nitrone oxygen and one facac oxygen, while in the other the axial oxygen atoms are furnished by two facac ligands.28e Finally, a cryptand N 4 ligand L forms a complex [CuL(H 2 O)][pic] 2 in which the hydrogen atoms of the water molecule, the apical ligand in a square pyramidal arrangement, appear to be hydrogen bonded to benzene rings.28f Complexes with S- and Se-donor ligands.This year’s highlight is the characterisation of the first stable CuIIS 4 compound, [CuMPt 2 (dppe) 2 (l3 -S) 2N2 ]; the co-ordination geometry is distorted tetrahedral.29a Exact ferrodistortive ordering is found in the distorted square planar complex [CuLL@] (H 2 L\N-salicylideneglycine, L@\N,N@- dibutylthiourea).29b EPR spectra are useful in the characterisation of copper(II) complexes with thiourea derivatives (HL) of the types R 2 NC(S)NHC(O)R (which are S,O bidentate ligands) and R 2 NC(S)N–– C(NHR)(R@) (S,N bidentate); in chloroform–methanol [CuL 2 ], where L is an (S,O) ligand, consists of both cis and trans isomers but for (S,N) ligands only one isomer is present.29c In [CuL 2 ]·2H 2 O [HL\4-(benzimidazol- 2-yl)-3-thiabutanoic acid] the equatorial ligands in the elongated octahedron are cis-N 2 O 2 with distant (2.74–2.75Å) axial sulfur atoms; in the nickel analogue the sulfur atoms occupy cis positions.29d Two new acyclic ligands, each furnishing N 2 (pyridyl) N 2 (amido)S 2 (thioether) donor sets give complexes [CuL] and [CuL@], where the sulfur atoms are spaced by two or three methylene groups in L and L@ respectively; [CuL] has an axially-compressed octahedral structure (which, however, becomes axially-elongated in solution) while [CuL@] exhibits flattened tetrahedral CuN 4 coordination. 29e The tridentate N 2 S thiosemicarbazone HL formed from 2-formylpyridine forms dimeric copper(II) complexes but with a methyl group in the 6-position the monomeric species [CuXL(H 2 O)n] (n\0, 1; X\NCS, OAc, N 3 ) and [Cu(OSO 3 )L(H 2 O)] can be prepared.29f Bis(thiosemicarbazone)copper(II) complexes are of considerable medical interest for imaging hypoxic tissues; structure–activity relationships, including correlations involving the CuI–CuII redox potential, have been explored.29g In [Cu([9]aneNS 2 ) 2 ][PF 6 ] 2 the co-ordination geometry is a slightly compressed octahedron, with four relatively long Cu–S distances.29h The first copper( II) 1,2-diselenooxalate (L) complex [CuL 2 ]2~ has been identified in solution by EPR spectroscopy; it decomposes rapidly.29i Annu.Rep.Prog. Chem., Sect. A, 1999, 95, 189–211 202Bi-, oligo-, poly- and hetero-nuclear species Binuclear complexes. We begin with complexes where the metal centres have simple bridges (halide, alkoxide, etc.), through diatomic and polyatomic bridges to binucleating ligands where the co-ordination sites are remote from each other. The complex [Cu(Se 2 CNEt 2 ) 2 ] reacts with CuX 2 (X\Cl, Br,NO 3 ) in dmf, for example, to give both [CuX(Se 2 CNEt 2 )]n and the EPR-silent dimers [MCu(Se 2 CNEt 2 )(l-X)N2 ].30a The bidentate nitrate in [Cu(O 2 NO)(ONO 2 )(bipy)] is easily displaced by OH~ and N 3 ~ (X) to give [MCu(ONO) 2 (l-X)(bipy)N2 ].30b DFT calculations reproduce satisfactorily the ferromagnetic coupling constants found experimentally in end-on azidobridged copper(II) complexes.30c The spin distribution in the triplet ground state of [MCu(l-1,1-N 3 )L 2N2 ][ClO 4 ] 2 (L\p-tert-butylpyridine) has been investigated by polarised neutron di§raction; while DFT calculations reproduce qualitatively the spin density map, they overestimate the spin delocalisation towards the ligands.30d Zero field electron magnetic resonance spectrocopy has produced accurate spin- Hamiltonian parameters for the triplet ground state of [MCu(l-1,1-N 3 )(Me 5 dien)N2 ]- [BPh 4 ] 2 .30e The new chelating ligand L (2-methoxymethylamino-3-methylpyridine) was formed in situ from Cu(NO 3 ) 2 , 2-amino-3-methylpyridine, MeOH and O 2 , giving the complex [MCu(l-OMe)(ONO 2 )LN2 ].30f Another unexpected product is the dimethoxy-bridged complex [MCuBr(l-OMe)LN2 (MeOH)] [L\N-tert-butyl-N-M(2- pyridyl)methylideneNamine], prepared by the reaction of CuBr with two equivalents of L and two of phenol; in the absence of phenol (presumably the oxidising agent) [CuBrL] is the product.30g Four new bridging modes for carbonate have been established in dicopper(II) complexes with alkyl-substituted dien.30h Bis-bidentate sulfate is the bridging ligand in [MCu(HL)(H 2 O)N2 (l-O 2 SO 2 )] (H 2 L\pyruvic acid thiosemicarbazone).30i Many more complexes contain alkoxo- or phenoxo-bridged copper(II) atoms which form part of a much larger binucleating ligand. Several magnetically- diverse phenoxo-bridged dicopper(II) species (including both ferro- and antiferro- magnetic species) exhibit sharp hyperfine-shifted 1HNMRsignals; these provide probes of the active sites of, for example, hyperactive copper(II)-substituted aminopeptidase. 30j A dicopper(II) complex with a Schi§ base ligand exhibits, as expected, an oxime bridge, CuNOCu, but also (unexpectedly) a Cu–O–Cu bridge via a ketonic oxygen atom.30k A double oxime bridge in [MCu(l-L)(l-O 2 ClO 2 )N2 ] (HL\2,6-diformyl- 4-methylphenol dioxime) is supplemented by asymmetrically-bridging perchlorate ions, with long Cu–O bonds (2.51 and 2.76Å).30l Other binucleating ligands with fairly short inter-copper bridges include diazines,30m hydrazones,30n tetra-amino substituted diamines,30o and oxamidates.30p This paragraph is devoted to dicopper(II) complexes with carboxylate bridges.The familiar ‘paddlewheel’ structure, as in [MCu(l-OAc) 2 (H 2 O)N2 ], has been established in the superoxide dismutase mimetic [MCu(l-L) 2 (NCMe)N2 ] [HL\a-methyl-4-(2- thienylcarbonyl)phenylacetic acid],31a [MCuCl(l-L) 2N2 ]Cl 2 ·H 2 O (L\b-alanine),31b and [MCu(l-L) 2 (H 2 O)N2 ][NO 3 ] 2 [ClO 4 ] 2 (L\pyridinioacetate).31c Attempts to displace the co-ordinated MeOH by py in [MCu(l-L) 2 (MeOH)N2 ] (HL\4-chlorophenoxyisobutyric acid) led to a mononuclear complex [CuL 2 - (py) 2 ].31d The first example of two non-equivalent apical ligands in the paddlewheel dimer is found in [MCu(l-L)N2 L@(H 2 O)] (HL\flufenamic acid; L@\ca§eine).31e The dimers [MCuL 2 L@N2 ] (HL\diphenylacetic acid, L@\MeCN, Me 2 CO) have been Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 189–211 203detected as intermediates in the CuII-catalysed oxidation of carboxylic acids.31f Just two bridging carboxylate groups are present in [MCu(l-L)(bipy)N2 ][ClO 4 ] 2 (HL\ferrocenecarboxylic acid)31g and in [MCu(l-OAc)N2 (l-L)][ClO 4 ] where HL is an N 4 O binucleating ligand furnishing a phenolate bridge.31h Among dicopper(II) complexes with one bridging carboxylate [MCuLN2 (l-OAc)(l-OH)(l-OH 2 )][ClO 4 ] 2 (L\1,4- dimethyl[9]aneN 3 ) contains a rare example of a bridging water molecule.31i The reaction of one equivalent each of Cu(BF 4 ) 2 and [MCu(OAc) 2 (H 2 O)N2 ] with two equivalents of cis,cis-1,3,5-triaminocyclohexane and one equivalent of isophthalaldehyde led to the self-assembly of the EPR-silent complex [Cu 2 (l-OAc)(l-OH)(l-L)]- [BF 4 ] 2 [L\bis-1,3-(cis,cis-1,3,5-triaminocyclohexane)xylylidiene].31j However, ferromagnetic coupling, presumably due to accidental orthogonality of the magnetic orbitals, is present in [MCu(H 2 O)N2 Cl(l-OAc)(l-L)][ZnCl 4 ] where L is an N 5 binucleating ligand.31k Among dicopper(II) complexes where the copper sites are remote from each other, an unusual case of hydrate isomerism is found in [MCu(OAc)(H 2 O)N2 (l-L)][ClO 4 ] 2 and [MCu(OAc)N2 (l-L)][ClO 4 ] 2 ·2H 2 O where L is a binucleating (N 3 ) 2 ligand.32a Other types of binucleating ligands for copper(II) include amide-based cyclophanes,32b a pyrazole-bridged bis(N 2 S-macrocycle),32c a triaminopentabenzimidazole (which provides a tyrosinase model),32d a calix[4]arene,32e bis([9]aneN 3 ) macrocycles with 2- pyridylmethyl arms,32f a tetraoxime whose two compartments are linked by a C––C bond,32g and a metallacyclic oxime complex with two copper atoms occupying opposite positions in the ring.32h Oligonuclear complexes.The template reaction of Cu(CF 3 SO 3 ) 2 with 2- aminopyridine or 2-aminopyrimidine and triethylorthoformate in EtOH results in the complexes [Cu 3 L 4 ][CF 3 SO 3 ] 2 , where HL is a formamidine; the cations have a propeller-like structure about the linear Cu–Cu–Cu axis.33a Other linear Cu 3 arrays are found in [Cu 3 (l-OMe) 4 L 2 ][BF 4 ] 2 , where L is a stereochemically-rigidN 3 -trisubstituted derivative of 1,3,5-triaminocyclohexane,33b and [Cu 3 L 4 ][BF 4 ] 2 [HL\(2- hydroxyphenyl)bis(pyrazolyl)methane].33c An equilateral triangle of copper atoms occurs in a dodecaaza macrotetracyclic complex, prepared by the simple template condensation of tren with formaldehyde in the presence of Cu2`; an unusual l3 - oxygen atom sits above the centre of the Cu 3 plane.33d A polyamine alcohol H 3 L having three OH groups and six amine nitrogen atoms forms the complex [Cu 3 Cl 2 (HL)][ClO 4 ] 2 where two copper atoms are bridged by the two chlorine atoms and the others are bridged by alkoxo-oxygen atoms.33e In [MCuL(py)N3 ] (H 2 L\2,2@- dihydroxyazobenzene) the monomeric units are joined by long (2.52–2.77Å) Cu–O bonds.33f Quadruply-bridging phosphate is an unusual feature of [MCuL(NCMe)N4 (l4 -O 4 P)][PF 6 ] 5 , where L is a tridentate bipy derivative.33g In the ‘dimer of dimers’ [MCu 2 L[OPO(OH) 2 ][O 2 P(OH) 2 ]N2 ][NO 3 ] 2 [HL\bis(pyridine-2- aldehyde) thiocarbohydrazone] one phosphate ligand in each dimeric unit is monodentate and the other bidentate.33h Other Cu 4 species include a linear oximate-bridged array,33i a cyclic tetramer formed by deprotonation of the monomer,33j a number of l4 -oxo- and -peroxo-bridged systems33k and a double-helical complex with a bis(bidentate) Schi§ base ligand.33l Benzotriazole (LH) is a useful corrosion inhibitor for copper metal and its alloys; its l3 -bridging mode in complexes such as [Cu 5 L 6 L@4 ] (HL@\substituted butane-1,3-dione) may be involved in the formation of protective Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 189–211 204monolayer coatings on oxidised copper surfaces.33m Finally, an octanuclear complex with the immunosuppressant azathioprine is perceived as two Cu 4 units bridged by two azothioprine ligands.33n Polycopper(II) compounds.Reports of new polymeric copper(II) compounds (mostly chain structures with the usual bridging ligands) are legion; here we focus on polymers which qualify as products of crystal engineering. The chain polymer [Cu(l- O 4 C 4 )(H 2 O) 2 (dmf) 2 ]n lacks the zigzag motif characteristic of the Mn, Co, Ni and Zn analogues.34a Unlike the ferromagnetic cobalt and nickel analogues, the rutile-related [CuMl3 -N(CN) 2N2 ]n is a ‘near-paramagnet’.34b Copper(II) nitrate reacts with 1,2-bis(4- pyridyl)ethyne (L) in EtOH to give the 1-D ladder polymer [Cu(l-L)(l- ONO 2 )(O 2 NO)]n and a more complex product having a triply-penetrating chiral frame based on square planar centres.34c 2-D square grids, with large (11]11Å) square cavities characterise the structure of [4,4@-H 2 bipy]n[Cu(4,4@-bipy) 2 (H 2 O) 2 ]n- [ClO 4 ] 4n,34d while [Cu(4,4@-bipy)(pyz)(H 2 O) 2 ]n[PF 6 ] 2n has rectangular grids with smaller (11]7Å) cavities.34e However, instead of the expected square grid, [Cu(ONO 2 ) 2 L 2 ]n [L\1,2-bis(4-pyridyl)ethane] is the first example of a co-ordination polymer having the NbO 3-D network structure.34f Rugged hexagonal grids are the feature of [Cu 6 (l3 -OH) 2 (l-L) 3 (H 2 O) 2 ]nBr 4n [H 2 L\trans-N,N@-bis(2- aminoethyl)oxamide].34g Tetrakis-(4-cyanophenyl) and -(4-nitrophenyl) derivatives of copper(II) porphyrins form the basis for 2-D polymers having large cavities.34h The hydrothermal reaction of CuSO 4 ·5H 2 O with MoO 3 and ligands such as 1,2,4-triazole and 1,2-trans-(4-pyridyl)ethene produces 3-D organic–inorganic hybrid materials.34i,j Heteronuclear complexes.These are covered in increasing order of the atomic numbers of the heteroatoms present. The unsymmetrical tridentate Schi§ base HL obtained by condensation of 2-imidazolecarbaldehyde and histamine forms the Vshaped, imidazolate-bridged trinuclear complexes [Cu(facac)(l-L)M(facac) 2 (l- L)Cu(facac)] (M\Mn, Ni, Zn).35a A cytochrome c oxidase model in which an FeII atom is bonded to tpp and a CuI atom to tmpa, the tpp and tmpa being linked by a peptide bridge, reversibly binds dioxygen in l-1,2-peroxo fashion.35b The oxidised (FeIII–CuII) form of a similar model, with the copper attached to a tris(imidazolyl)methane moiety, has been prepared.35c An EPR study of cytochrome bo 3 indicates that the FeIII–CuII coupling is much weaker than previously proposed.35d Ageneral approach has been developed for the rational synthesis of linear FeIIICuIINiII, FeIIINiIICuII and CoIIICuIINiII complexes where two metal atoms are bound to a binucleating Schi§ base oxime and the third to Me 3 [9]aneN 3 .35e The tetradentate Schi§ base H 2 L, derived from imidazole-2-carbaldehyde and tn, forms [Cu(OClO 3 )(H 2 L)][ClO 4 ] which reacts with [Ni(facac) 2 ] under basic conditions to give an imidazolate-bridged NiIICuIINiII complex with a quartet ground state.35f The central copper(II) atom in [Cu(l-ONO) 2MNiL(dmf)N2 ] is bonded to two nitrite oxygen atoms and four more oxygen atoms from the Schi§ base L.35g In a tetranuclear [NiII(l-L)CuII] 2 system, where L is bis(3-aminopropyl)oxamide, the binuclear units are linked by thiocyanate bridges.35h Although spectroscopic and electrochemical measurements show no ground state coupling between the metal centres in [RuII(bipy) 2 (l-L)CuII(phen)(H 2 O)][PF 6 ] 3 (HL\a,x-diamino acid-substituted bipy), Annu.Rep. Prog. Chem., Sect.A, 1999, 95, 189–211 205their luminescence is significantly quenched compared with the parent RuII complexes. 35i The 2-D complex [NBu 4 ][RuIIICuII(l-ox) 3 ] is ferrimagnetic.35j While carboxylate- bridged CuII 2 MIII 2 (M\Ce, Gd) complexes obey the Curie–Weiss law,35k there is evidence of antiferromagnetic Cu–Pr exchange in [Pr 2 Cu 4 (l6 -O)(l-L) 3 (l- HL) 2 (facac) 4 ] (H2 L\2,2@-thiodiethanol).35l The reaction of copper powder with Pb(SCN) 2 and 2-dimethylaminoethanol (HL) in MeCN gave [MCu 2 Pb(l-SCN) 3 (l- L) 3N2 ]; the trinuclear units, within which the metal atoms are alkoxo-bridged, are dimerised via bridging thiocyanate groups.35m 5 Copper(III) chemistry The stability of [PPh 4 ][CuL]·MeCN [H 4 L\bis(methylamide) of N,N@-ophenylenebis( oxamic acid)] is attributed to its large crystal field stabilisation energy; electrochemical studies of this and related compounds show a correlation between the CuII–CuIII redox potentials and the absorption maxima in the d–d spectra of the copper(II) species.36a The template reaction of Cu(OH) 2 with oxalodihydrazide and formaldehyde produced a square planar copper(III) macrocyclic anion.36b The reaction between [NHEt 3 ] 2 [MnIVL 3 ] (H2 L\quinoxaline-2,3-dithiol) and [MCu(OAc) 2 (H 2 O)N2 ] in dmf led to [MnII(dmf) 4 (H 2 O) 2 ][CuIIIL 2 ] 2 .36c The product of the reaction between [CuL(NCMe)]` [L\bisM2-(2-pyridyl)ethylNmethylamine] and O 2 is a mixture of l-g2:g2-peroxodicopper(II) and bis-l-oxodicopper(III) species.36d The reaction of [CuL(NCMe)]` (L\N,N,N@,N@-tetramethylcyclohexane-1,2-diamine) with O 2 gives [MCuLN3 (l3 -O) 2 ][CF 3 SO 3 ] 3 which is best described as a localised CuIICuIICuIII mixed-valence species.36e References 1 (a) D.Walter and P. 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