摘要:

DALTON PERSPECTIVE J. Chem. Soc., Dalton Trans., 1999, 1897–1909 1897 Electrochemical molecular recognition: pathways between complexation and signalling Paul D. Beer,*a Philip A. Gale *a and George Z. Chen *b a Department of Chemistry, University of Oxford, Inorganic Chemistry Laboratory, South Parks Road, Oxford, UK OX1 3QR b Department of Materials Science and Metallurgy, University of Cambridge, Pembroke St., Cambridge, UK CB2 3QZ Received 22nd February 1999, Accepted 26th March 1999 This perspective examines the mechanisms by which electrochemical recognition of various charged and neutral guest species by redox-active receptor molecules takes place.Particular emphasis is given to the intramolecular signalling pathway employed in each case. These include electrostatic interactions (through space or through conjugated bonds), conformational changes in the redox centre, participation of the redox centre in complexation and guest interference to an already present communication pathway in the host. 1 Introduction Electrochemical molecular recognition is a fast expanding research area at the interface of electrochemistry and supramolecular chemistry. The aim of this area of chemistry is the development of highly selective and sensitive electrochemical sensors for charged or neutral target guest species. Two strategies have been applied to the electrochemical detection of host–guest complex formation; extraction of a charged guest into a membrane and detection of the resultant membrane potential, or detection of a perturbation of the host’s properties on complex formation.The former method comprises an ion selective electrode (ISE) 1 in which a membrane containing the receptor separates an internal solution and a further reference electrode. The half potentials of both the reference electrodes are known thus enabling calculation of the membrane potential that is a function of analyte concentration.Alternatively, the membrane potential may be measured by a solid state device known as a Chemically Modified Field EVect Professor Paul D. Beer was born in Totnes, Devon. In 1979 he obtained a first class honours degree in chemistry from King’s College London, and remained there to undertake research in the field of organophosphorus chemistry under the supervision of Dr. C. D. Hall. In 1982 he received a Ph.D. and a Royal Society postdoctoral fellowship enabled him to conduct research in supramolecular chemistry with Professor J.-M.Lehn at the Université Louis Pasteur, Strasbourg, France. After a demonstratorship at the University of Exeter in 1983, in 1984 he took up a New Blood Lectureship at the University of Birmingham. In 1990 he moved to the University of Oxford, where he is also a tutorial fellow at Wadham College. He became a professor of chemistry in 1998. He was awarded in 1987 the RSC Meldola medal, in 1993 the UNESCO Javed Husain prize and in 1994 the RSC Corday-Morgan medal.His research interests cover many aspects of charged and neutral guest co-ordination chemistry, including the synthesis and co-ordination properties of redox- and photo-responsive receptors designed selectively to recognise and sense biological and environmentally important guest species. Dr. Philip A. Gale was born in Liverpool. In 1992 he graduated with a B.A. (Hons.) in chemistry from Wadham College, Oxford. He remained in Oxford (moving to Linacre College) to undertake research in calixarene chemistry under the supervision of Professor P.D. Beer, graduating in 1995 with an M.A. and a D.Phil. In October 1995 he joined Professor Jonathan L. Sessler’s research group at the University of Texas at Austin as a Fulbright postdoctoral fellow where he studied the anion co-ordination properties of calixpyrrole macrocycles. In October 1997 he took up a Royal Society University Research Fellowship at the Inorganic Chemistry Laboratory, Oxford.His research interests include the synthesis of new self-assembling receptor molecules. He is the author or co-author of over forty publications including a patent, several review articles and an Oxford Chemistry Primer (Supramolecular chemistry, co-authored with Paul Beer and David Smith). Dr. George Zheng Chen was born in Jiangxi Province, P.R. China. He obtained a Diploma in Chemistry from Jiujiang Teacher Training College (P.R. China) in January 1981.Supervised by Professors Q. Zhang and W. J. Albery respectively, he was awarded the degrees of M.Sc. by Fujian Teachers University in January 1985 and of Ph.D. by the University of London in June 1992, and also the Diploma of Imperial College of Science, Technology and Medicine. Since then he has carried out his postdoctoral work in the Universities of Oxford (associated with Professor P. D. Beer), Leeds and Cambridge (associated with Professor D. J. Fray). His research experience covers Physical, Inorganic and Materials Chemistry.He is the co-author of thirty publications and two patents. Philip A. Gale Paul D. Beer George Zheng Chen1898 J. Chem. Soc., Dalton Trans., 1999, 1897–1909 Transistor (CHEMFET).2 This is similar to the ISE except that the membrane is attached to the gate of a field eVect transistor. The second strategy is the subject of this review and is shown schematically in Fig. 1. The receptor has a binding site and a ‘reporter group’ in close proximity.The reporter group is chosen to have well behaved electrochemical properties (e.g. ferrocene) that are perturbed upon guest complexation. Such systems can be described by the scheme of squares shown (Scheme 1) where H, G and HG represent the host, guest and complex species respectively, subscripts “ox” and “red” indicate that the corresponding molecules or parameters are in oxidised and reduced states, E8 is the formal potential of the electron transfer reaction and K is the stability constant.The stability constant K of a host/guest (1 : 1) complex is defined by the equilibrium shown in Scheme 1. The four reactions in Scheme 1 constitute a closed route, therefore the total Gibbs free energy change in this cycle is zero. If we consider equilibrium to be approached via a clockwise route starting from Hox, the above statement can then be mathematically expressed as in eqn. (1). Rearranging (1) leads SDG = DGH 1 DGred 1 DGHG 1 DGox = 0 nF(E 2 E8H) 2 RT ln (Kred) 1 nF(E8HG 2 E) 1 RT ln (Kox) = 0 (1) to eqn.(2). Eqn. (2) links, in a simple way, the thermodynamicnF( E8HG 2 E8H) = RT ln (Kred/Kox) (2) ally important stability constants Kox and Kred of a complex in diVerent oxidation states with experimentally measurable redox potentials E8H and E8HG. Obviously, receptors designed electrochemically to recognise guest species must be able to respond to the binding of the guest with a significant change in their redox potentials. In practice, gauged by experimental error, the mini- Fig. 1 The presence of a guests species G triggers an electrochemical response in a host molecule. Scheme 1 The scheme of one square for guest binding and electron transfer. Hox Hred HGox HGred Kox Kred +G +e +e E °HG E °H +G mum measurable potential diVerence is for example about ±5 mV in voltammetric methods. To achieve a significant perturbation, the receptor should be able to couple its complexation to the redox reaction via one or a combination of several coupling pathways.In other words, the receptor must have a structure such that upon guest binding the electron transfer to or from the receptor’s redox centre is achieved at a significantly diVerent potential from that of the receptor in the absence of the guest species. The quotient Kred/Kox is a theoretically useful parameter because it allows not only the calculation of Kred if Kox is known and vice versa, but also the evaluation of the eVect of electron transfer on the complexation.Some authors have termed it the binding enhancement factor (BEF) 3 when considering some reducible and cation-binding receptors as molecular switches. However, guest binding by a redox active molecular receptor is not always enhanced upon electron transfer (either oxidation or reduction). For example, in the simplest case where the guest and electron transfer reactions are coupled by purely through space electrostatic interaction, oxidation (electron withdrawn from the receptor) will weaken cation binding but enhance anion binding, whilst the opposite is true for reduction (electron insertion to the receptor).For this reason, we have proposed that a better description for this quantity would be the reaction coupling eYciency (RCE).18b Regardless of the direction, the further away the quotient is from unity, the more eYcient is the coupling between the guest binding and electron transfer reactions, and therefore the greater the intramolecular signal transferred between the bound guest and the redox centre. Although demonstrating the importance of the RCE, eqn.(2) does not indicate how a desired RCE may be achieved. The presence of an intramolecular signalling pathway linking the binding site and redox-active centre is essential to produce an eVective redox-active sensor molecule. These pathways include (Fig. 2): (a) through space electrostatic interaction between the redox centre(s) and the complexed guest molecule; (b) through bond communication provided typically by conjugated chemical bond linkage between the redox centre(s) and the binding cavity; (c) additional direct co-ordination bond formation between the redox centre and the complexed guest molecule; (d) conformationally induced perturbation of the redox centre(s) caused by the complexation of a guest molecule; and (e) interference by the guest species in communication between two redox-active centres.Echegoyen and co-workers 4 have recently published an extensive review of the electrochemical properties of a wide range of supramolecular systems. The purpose of this perspective is not to provide a comprehensive review of supramolecular electrochemistry but rather to illustrate the role each of the binding-redox pathways can play in electrochemical molecular recognition processes. This will be achieved by highlighting selected redox-responsive receptor examples from the literature. 2 The through space electrostatic interaction mechanism Should an electrostatic interaction be the only intramolecular interaction as a result of guest binding, the change in energy in a mole of the complex upon electron transfer can be expressed by eqn. (3) according to electrostatics, where NA is the Avogadro W = NAQguestDQredox/4pe0ed (3) constant, Qguest the eVective charge on the bound guest, DQredox the change in eVective charge on the redox centre, e0 the vacuum permittivity, e the relative permittivity of the medium, and d the distance between the redox centre and the bound guest.Combining eqn. (3) with (2) will allow the establishment of a direct relation between the complexation induced potential change (E8HG 2E8H ) with the guest’s eVective charge. UsingJ. Chem. Soc., Dalton Trans., 1999, 1897–1909 1899 the relation DG = RT ln K, we find that the right hand side of eqn.(2) is the diVerence in the Gibbs free energy between the reduced and oxidised complexes, i.e. eqn. (4). RT ln (Kred/Kox) = DGred 2 DGox (4) Since the only energy diVerence between the reduced and oxidised complexes is the electrostatic energy represented by eqn. (3), eqn. (2) can now be rewritten as eqn. (5). nF(E8HG 2 E8H) = NAQguestDQredox/4pe0ed (5) Fig. 2 The five mechanisms for coupling complexation and redox reactions. Fig. 3 A plot of the inverse Fe–N distances in four ferrocene amines against the diVerences of the redox couples of ligands in their free and protonated forms.In fact, Plenio and Diodone 5 have applied this approximation to more than twenty ferrocene nitrogen compounds in which a proton may be regarded as the guest cation. Fig. 3 presents the linear correlation between the anodic shift in potential and the distance between the proton and ferrocene. The accuracy of the approximation is satisfactorily high. Anodic shifts are observed in these systems because, once protonated, the ferrocene compound becomes harder to oxidise and therefore the ferrocene–ferrocenium couple shifts to a higher potential.In earlier work, Gokel and co-workers 3 studied the electrochemical properties of the ferrocene cryptand molecule 1 by cyclic voltammetry. Gokel found a linear correlation between DE1/2 and the charge/(radius)2 of several Group 1 and 2 cationic guests added to a solution of the receptor (Fig. 4).Addition of substoichiometric amounts of NaClO4 to the electrochemical solution of the ligand caused the appearance of a new set of waves at 0.402 V vs. SSCE [Fig. 5(a) and (b)]. The currents for the new redox couple increase linearly with the concentration of Na1 ion until a single equivalent is added. At this point the waves corresponding to “free” ligand have disappeared and the cyclic voltammogram corresponds to the oxidation of the pure complex. This two wave behaviour 6,7 has been rationalised in terms of the high stability constant of the 1?Na1 complex.Slow decomplexation kinetics were discounted on the basis that no change in the cyclic voltammogram of an electrochemical solution of compound 1 in the presence of 0.5 equivalent Na1 cations was observed when the scan rate was varied between 0.02 and 5 V s21. Electrochemical metal cation recognition studies of compounds 2 and 3 reveal that these receptors exhibit Li1 selectivity.8 Upon addition of sodium or lithium cations to electrochemical solutions of 2 or 3 there are significant anodic Fig. 4 A plot of DE1/2 vs. charge/(radius)2 ratio for the complexation of Na1, K1 and Ca21. N N O O O O Fe 1 O O N O O O N O N O N O N O N O Fe Fe 2 31900 J. Chem. Soc., Dalton Trans., 1999, 1897–1909 shifts of the redox potentials of the receptors (Table 1). Interestingly, addition of cations to 3 caused the electrochemical behaviour to become reversible at room temperature. The highly selective co-ordination of lithium cations by 3 can be used to detect Li1 electrochemically in the presence of large excesses of these other Group 1 cations.Echegoyen and co-workers 9 have recently reported that the electrochemical properties of crown-ether modified fullerenes are perturbed by the presence of alkali metal cations. Compounds 4, 5 and 6 were synthesized by a Bingel macrocyclisation of C60 with a bis-malonate containing a dibenzo- 18-crown-6 tether. The electrochemical properties of these materials were studied in ion-selective electrodes as well as in solution.Fig. 6 shows a cyclic voltammogram for 4 in the absence and presence of ten equivalents of KPF6. The first reduction processes occurring in the fullerene shift anodically by 90 mV upon addition of potassium due to the through space electrostatic influence of the bound cation. The redox potentials of all three receptors in the absence and presence of potassium cations are shown in Table 2.Fig. 5 Voltammetric response of acetonitrile solutions of 1.0 mmol dm23 1: (a) stationary glassy carbon electrode (0.08 cm2), scan rate 100 mV s21; (b) same conditions as (a) with 0.5 equivalent NaClO4 added; (c) same conditions as (a) with 0.5 equivalent Ag1 added. (Reprinted with permission from ref. 3, Copyright 1992, American Chemical Society.) Table 1 Anodic shifts in the redox couples of compounds 2 and 3 upon addition of Li1 or Na1 cations DE(Li1)/mV DE(Na1)/mV Compound 2 1100 170 Compound 3 1140 1100 The detection of two diVerent cations simultaneously by a single redox-active receptor has recently been achieved.Beer et al.10,11 found that receptor 7, which contains two metal binding sites, could be used to sense the presence of barium, magnesium, or a mixture of barium and magnesium via a through space interaction. Unique three peak voltammetric Fig. 6 Cyclic voltammograms for (±)-4 recorded at 100 mV s21 on a glassy carbon mini-electrode: solid line, solution of free (±)-4 with one equivalent of cryptand 222 (4,7,13,16,21,24-hexaoxa-1,10-bicyclo- [8.8.8]hexacosane) present; dotted line, solution of (±)-4 with ten equivalents of KPF6.(Reprinted with permission from ref. 9, Copyright 1998, Wiley-VCH.) O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O 4 5 6J. Chem. Soc., Dalton Trans., 1999, 1897–1909 1901 features are observed for 7 in the presence of both Mg21 and Ba21, which correspond to the three complexes 7?2Ba21, 7?2Mg21 and 7?Ba21?Mg21 anodically shifted from free 7 by 150, 395 and 275 mV respectively. Electrochemical recognition is not limited to Group 1 or 2 metal cations.Various groups have produced receptors that are capable of recognising transition metals.12–14 In a recent example, Plenio and Aberle 15 have reported the synthesis of a ferrocene-bridged cyclam 8 that exhibits large changes in redox properties in the presence of transition metal cations.The crystal structure of the Ni(CF3SO3)2 complex (Fig. 7) reveals that the nickel is bound at a distance of 3.854(8) Å from the Fe atom. The electrochemical properties of 8 in the presence of various transition metal cations (Table 3) reveal large perturbations of the ferrocene–ferrocenium, mediated by through space electrostatic interactions although Plenio does not completely discount the possibility that there may be a direct co-ordination pathway between the iron atom of the ferrocene moiety and the transition metal guest.A number of systems that are capable of sensing transition N O O O O O O O O N O O Ba2+ Fe Mg2+ 7•Mg2+•Ba2+ N Fe NH N HN 8 Fig. 7 Crystal structure of the Ni(CF3SO3)2 complex of receptor 8. (Reprinted with permission from ref. 15, Copyright 1998, Royal Society of Chemistry.) Table 2 Redox potentials (V vs. Fc–Fc1) for compounds (±)-4–(±)-6 a 1 cryptand 222 b 1 KPF6 c Compound (±)-4 (±)-5 (±)-6 E1/2 1 21.04 (70) 21.02 (79) 21.05 (83) E1/2 2 21.51 d 21.48 d 21.47 d E1/2 1 20.95 (79) 20.97 (75) 21.01 (77) E1/2 2 21.36 (104) 21.43 d 21.46 d a In MeCN–CH2Cl2 (1 : 1) in the presence of 0.1 M n-Bu4NPF6; Fc–Fc1 was used as the internal reference, and measurements were made with a glassy carbon working mini-electrode.Values for DEpp are given in parentheses. b One equivalent. c Ten equivalents. d Only the cathodic potential is given. metals in aqueous environments have been reported.12,14 Beer and Smith 16 have produced a series of acyclic ferrocene receptor species (9–12) and studied the eVects of adding transition metal cations (as hydrated perchlorate or tetrafluoroborate salts) to electrochemical solutions of the receptors. The metal ions may either co-ordinate to the receptors, or alternatively act as acids and protonate them.Protonation causes a shift in the ferrocene oxidation potential (addition of HBF4 caused a shift of approximately 160 mV with each receptor).Species that are ineYcient receptors (e.g. 11 with a large donor bite angle) are more likely to be protonated rather than co-ordinate to the transition metal cations (i.e. a large shift of approximately 160 mV will be observed). The results are presented in Table 4. Receptors 10 and 11 are protonated on addition of any of the metal cations whereas 12 complexes transition metal cations with anodic shifts up to 115 mV with NiII. Receptor 12 is capable of co-ordinating to the transition metal via the pyridine nitrogen atom (that is not present in receptor 11).The electrochemical detection of anionic species is a particular challenge. Anion binding has generally proved to be more challenging (with respect to cations) due to their lower charge to radius ratio, pH sensitivity and range of geometries. Chemically, anions are utilised in many chemical reactions acting as nucleophiles (CN2), bases (2OR) and redox active centres (S2O8 22).The environmental impact of anionic pollutants such as excess nitrates from agricultural fertilisers leads to eutrophication of rivers. Surprisingly then, specific ligands that have NH HN N Me NH O O HN NH NH NH N NH Fe Fe Fe Fe Fe Fe Fe Fe 9 10 11 12 Table 3 Electrochemical data of compound 8 and transition metal complexes as determined by cyclic voltammetry in MeCN using n-Bu4NPF6 as supporting electrolyte vs. cobaltocene (E1/2 = 20.94 V) or ferrocene (E1/2 = 10.40 V) reference Compound 88 ?Co(CF3SO3)2 8?Ni(CF3SO3)2 8?Cu(CF3SO3)2 8?Zn(CF3SO3)2 E1/2/V (FeII–FeIII) 10.33 10.69 10.71 10.74 10.80 DE1/2/mV — 1360 1380 1410 14701902 J.Chem. Soc., Dalton Trans., 1999, 1897–1909 the capability of optically and/or electrochemically detecting anions are still rare.17,18 Redox-active thiourea or guanidinium hydrogen bond donating receptors containing ferrocene moieties 13–15 have been synthesized by Beer and co-workers.19 Thiourea containing compound 13 was found not to interact with anions (due to an intramolecular NH ? ? ? OC hydrogen bond forcing the molecule into an unfavourable conformation).This bond can not form in 14 and 15 and it was found that both these compounds bind anions. Cathodic shifts of up to 125 mV in the ferrocenium– ferrocene redox couple of 15 with H2PO4 2 anions were observed in DMSO, presumably mediated via through space interactions. Compound 15 is capable of recognising P2O7 42 anions in methanol–water (a highly competitive solvent mixture) giving a cathodic shift of 70 mV in the ferrocene redox wave.Beer, Martínez-Mañez et al.20 have recently achieved the selective electrochemical recognition of sulfate over phosphate and phosphate over sulfate using polyaza ferrocene macrocyclic receptors in aqueous solution.20 Receptors 16–19 can, through an electrochemical response, selectively detect at certain pH values sulfate and phosphate in the presence of competing anions in an aqueous environment.Maximum selective redox cathodic potential shifts of 54 and 50 mV were observed for sulfate and phosphate, using receptors 17 and 19 at pH 4 and 7 respectively. 3 The through bond electrostatic interaction mechanism One of the earliest examples of electrochemical recognition of cations was reported by Saji 7 in 1986. He showed that the ferro- Fe N NH O S Fe NH NH S Fe NH NH NH2 I- 13 14 15 H Table 4 Electrochemical shifts (mV) of the ferrocene redox couple in acetonitrile on the addition of metal ion salts as hydrated perchlorates/ tetrafluoroborates (n.i.= not investigated). In each case the electrochemical response corresponds to either protonation (p) or complexation of the metal ion (c) Metal salt (DE/mV) Compound 9 10 11 12 Free E1/2/V 0.05 a 0.027 0.040 0.030 NiII 58c 73c 175p 115c CuII 98c 160p 169p 105c ZnII 162p 168p n.i. 75c CaII n.i. 172p 160p 158p PbII n.i. 165p n.i. n.i. a Broad oxidation peak.All E1/2 values (quoted for the ferrocene wave) are relative to Ag1–Ag in MeCN and are accurate to ±5 mV. cene crown ether molecule 20 could be used as an electrochemical sensor for alkali metal cations via a mixture of through space and through bond interactions (two of the co-ordinating oxygen atoms are attached to the ferrocene cyclopentadienyl ring). Initially on addition of sodium cations to an electrochemical solution of the ligand, two distinct CV waves were observed, corresponding to the uncomplexed and complexed compound 20 (Fig. 8). The wave at the higher positive potential corresponds to the solution complexed species. The oxidised ferrocene crown ether has a lower binding constant with sodium than the unoxidised receptor due to an electrostatic repulsion of the ferrocenium positive charge and the guest alkali metal cation. For sodium and lithium cations the RCEs (K(1)/K(1) 1) were 740 and 72 respectively. This repulsion can be used to switch oV cation binding and was utilised by Saji and Kinoshita 21 to transport alkali metal cations across liquid membranes containing 20 as a carrier (Scheme 2).In 1990 we reported the synthesis of new redox responsive crown ether molecules 21a and 21b that contain a conjugated link between the crown ether unit and a ferrocene redox active centre.22 The electrochemical behaviour of these species was investigated and also the electrochemical behaviour of their analogues with a saturated link between the ferrocene unit and the crown ether.The changes in the cyclic voltammograms of compound 21a upon addition of magnesium cations are shown in Fig. 9. The metal cation induced anodic shifts of 21a and 21b and also the saturated analogue 22 are shown in Table 5. These results show that significant anodic shifts in the ferrocene oxidation wave result if cations are added to the conjugated receptor systems N N N N Fe Fe Fe Fe 16 Fe NH HN Fe HN NH 17 Fe Fe HN HN HN HN 18 Fe NH NH HN HN 19 O O O O O Fe 20J.Chem. Soc., Dalton Trans., 1999, 1897–1909 1903 where the p-electron system links the heteroatoms of the ionophore to the redox centre. Much smaller shifts are observed for the saturated analogue suggesting a through bond mode of coupling as the primary mechanism of electrochemical recognition in compounds 21a and 21b. Fig. 8 Cyclic voltammograms for 0.2 mmol dm23 compound 20 (in the presence of 0.1 mol dm23 n-Bu4NPF6 in CH2Cl2) in the absence of NaClO4 (a) and in the presence of 1 mmol dm23 NaClO4 (partially precipitated) in the course of stirring a solution for (b) 5 min and (c) 1 h.Scan rate 40 mV s21. (Reprinted with permission from ref. 7, Copyright 1986, Chemical Society of Japan.) Scheme 2 Transport of alkali metal cations across a liquid membrane using compound 20 as a carrier. Table 5 The electrochemical anodic shifts of the ferrocene oxidation wave of compounds 21a, 21b and 22 upon addition of 4 equivalents of cation Compound DE(Na1)/mV DE(K1)/mV DE(Mg21)/mV 21a 50 20 100 21b 65 20 110 22 <5 <5 <20 Hall and Chu 23 have used CV to investigate the co-ordination of alkaline earth and lanthanide metal cations by a series of ferrocene cryptands such as compound 23.They noted that large anodic shifts of the ferrocenoyl redox couple are produced Fig. 9 Cyclic voltammograms for 3 mmol dm23 compound 21 (in the presence of 0.2 mol dm23 n-Bu4NBF4 in CH2Cl2): (a) in the absence of Mg21 and in the presence of (b) 0.75 equivalent Mg21, (c) 1.5 equivalents Mg21.Scan rate 100 mV s21. (Reprinted with permission from ref. 22, Copyright 1990, American Chemical Society.) O N O O O O N O O O O N O O O Fe Fe Fe 21a 22b 221904 J. Chem. Soc., Dalton Trans., 1999, 1897–1909 with these metal cations and that there exists a broad linear correlation between the DE1/2 value and the charge : radius ratio of the cationic guest species (Fig. 10). It has been proposed by Hall that this behaviour is indicative of a through bond interaction (i.e. the cations are co-ordinating to the carbonyl group of the amide). There may also be a through space contribution to the electrochemical shift. Alkali metal cations gave only small (<20 mV) anodic shifts with this cryptand. Whilst examining the co-ordination properties of new diand tri-aza crown ether ligands containing multiple ferrocene moieties we discovered using 1H and 13C NMR titration studies that these systems form selective 1 : 1 stoichiometric complexes with ammonium cations.24 Significant one-wave anodic shifts of the ferrocene redox couple were observed using CV on addition of ammonium to solutions of compounds 24 to 27, however ligand 28 showed no response, suggesting that the amine nitrogen donor atoms are a prerequisite for ammonium binding O O N N O O O O Fe 23 (Table 6).Substantial anodic shifts of 220 mV were observed with ligand 26 presumably due to a combination of through space interactions and N–H1 ? ? ?O]] C hydrogen bonds as illustrated in Fig. 11. Compound 27 is also capable of the electrochemical recognition of a cation and an anion (i.e. ion pair recognition).25 The through bond mechanism is also eVective in communicating anion co-ordination events to redox-active centres. The Fig. 10 Plot of DE1/2 vs. charge : radius ratio for the complexation of compound 23 with various metal cations.N O O N O O NH NH O O O N O N N O O O O O N O N N O N O O N O O O N O N N O NH O HN O HN O Fe Fe Fe 24 Fe Fe 25 Fe Fe Fe 26 Fe Fe 27 Fe Fe Fe 28J. Chem. Soc., Dalton Trans., 1999, 1897–1909 1905 tripodal receptors 29 and 30 that contain redox-active positively charged cobaltocenium moieties were synthesized in 65 and 60% yields.17 The addition of tetrabutylammonium chloride to deuteriated acetonitrile 1H NMR solutions of 29 and 30 resulted in remarkable shifts of the respective protons of both receptors.Of particular note were the substantial downfield shifts of the amide protons (Dd = 1.28 ppm for 29 and 1.52 ppm for 30) on addition of one equivalent of chloride. These results suggest that a significant –CO–NH ? ? ? Cl2 hydrogen bonding interaction is contributing to the overall anion complexation process. Subsequent 1H NMR titration curves suggesting 1 : 1 stoichiometry with anion complexes of 29 and 30 were found in all cases. Negligible shifts were observed under identical experimental conditions with cobaltocenium hexafluorophosphate ester derivative 31. However, the simple monoamidesubstituted cobaltocenium compound 32 did exhibit some significant solution interactions with halide anions (Table 7) highlighting the importance of the (CONH) amide group in anion binding.The cathodic shifts of the redox potentials of compounds 29, 30 and 32 on addition of halide anions are due to the stabilisation of the cobaltocenium cation by the bound anion which causes the redox couple to shift to a more negative potential.Receptor 31 that does not contain the amide CONH group showed no electrochemical response to the addition of anions ruling out the possibility of the cathodic shift being caused by ion-pairing eVects. Similar anion induced cathodic perturbations with various ferrocene amide and ruthenium(II) 4,49- diamide bipyridyl systems have also been noted.26,27 Compounds 33 and 34 contain multiple redox centres (a redox-active ruthenium bipyridyl moiety in addition to a ferrocene or cobaltocenium group).In the presence of chloride anions the amide substituted bipyridyl reduction wave is shifted cathodically by 40 mV for 33 and 90 mV for 34 whereas the ferrocene–ferrocenium redox couple is shifted cathodically by Fig. 11 Proposed lariat co-ordination of NH4 1 with compound 26. O N O N NR O N H H H H O O HN NH Fe Fe (CH2)3 (CH2)3 Table 6 Electrochemical data and ammonium cation dependence for compounds 24 to 28 Compound Ea/V DEp d/mV DE(NH4 1) e/mV DE(K1) e/mV DE(CH3NH3 1) e/mV DE(PhCH2NH3 1) e/ mV 24 10.43 b 90 30 20 —— 25 10.41 c 90 50 40 —— 26 10.62 b 100 220 50 <10 <10 27 10.54 c 100 170 85 <10 <10 28 10.67 b 80 <10 <10 <10 <10 a Solutions were ca. 2 × 1023 mol dm23 in compound, and potentials were determined with reference to the SCE. b Three-electron reversible oxidation process. c Two-electron reversible process.d Separation between anodic and cathodic peak potentials; values for ferrocene under identical conditions ranged from 80 to 90 mV. e Shift in respective ferrocenyl oxidation potential produced by presence of guest cation (2 equivalents) added as their thiocyanate salts for potassium and ammonium, and their picrate salts for methylammonium and phenethylammonium. 60 mV for 33 and the cobaltocene–cobaltocenium couple by 110 mV for 34.28 The cathodic shift is therefore observed for both redox centres in these receptor species. 4 The conformational change mechanism Guest induced conformational changes in redox-active molecules may provide a mechanism to induce perturbations in electrochemical behaviour.29 The bipyridinium bis(benzo crown) 35 is one example.30 Earlier work on ferrocene bis(crown ethers) has shown that the amide bond linkage is insulating 22 therefore any perturbation of the redox behaviour of 35 will not be caused by through bond interactions.It was confirmed by NMR, UV/Vis techniques and X-ray crystallography (Fig. 12) that this molecule binds Group 1, 2 metal and ammonium cations forming 1: 1 intramolecular sandwich complexes with Ba21, K1 and NH4 1 and 2 : 1 complexes with Mg21 and Na1 with a cation in each crown ether moiety. The formation of a 1: 1 sandwich complex forces a significant twist of the HN O N O NH O HN O N O NH O N Co Co Co Co Co Co H H 29 30 [PF6 – ]3 [PF6 – ]3 Co O OCH3 PF6 – Co O NH PF6 – 31 32 Table 7 Electrochemical data for compounds 29–32 Compound E1/2 a/V DE(F2) c/mV DE(Cl2) c/mV DE(Br2) c/mV 29 20.74 b 55 d 30 — 30 20.75 b 60 40 — 31 20.45 <5 <5 <5 32 20.74 — 30 40 a Obtained in MeCN solution containing 0.2 mol dm23 n-Bu4NBF4 as supporting electrolyte.Solutions were ca. 2 × 1023 mol dm23 in ligand, and potentials were determined with reference to the SCE. b Three electron reduction process as determined by coulometric experiments.c Cathodic shift in reduction potential produced by the presence of anions (4 equivalents) added as their ammonium or butylammonium salts.1906 J. Chem. Soc., Dalton Trans., 1999, 1897–1909 4,49-bipyridinium redox moiety. However formation of the Na1 complex does not induce such a dramatic change in the conformation for the 4,49-bipyridinium unit. Upon addition of Ba21 cations the 21/11 bipyridinium redox couple was found to shift anodically by 45 mV and the 11/0 couple shift cathodically by 10 mV.Potassium ion and NH4 1 produce similar eVects (Table 8). However addition of Na1 cations caused a small cathodic shift of the 21/11 couple and an Fig. 12 Crystal structure of the barium complex of receptor 35. N N N N N N Ru O O HN NH NH HN O O Fe 2+ [PF6 – ]2•2H2O N N N N N N Ru O O HN NH NH HN O O Co+ 2+ [PF6 – ]2•3H2O 33 34 N N O O N N H H O O O O O O O O O O Me Me 2PF6 – 35 Table 8 Electrochemical data for compound 35 in acetonitrile containing 0.2 mol dm23 n-Bu4NBF4 as supporting electrolyte (4 equivalents cation salt added) Redox couple E1/2/V DEp/mV DE(Ba21)/mV DE(K1)/mV DE(NH4 1)/mV DE(Na1)/mV 21/11 20.73 70 45 10 10 210 11/0 20.87 70 210 240 240 30 anodic perturbation of the 11/0 couple.These results therefore support the proposal that the conformational change pathway for coupling the complexation and redox reactions is operating in this case. Interestingly the sulfur linked bis(crown) ligand 36 shows an unprecedented cathodic potential shift (60 mV) upon addition of K1 cations to the electrochemical solution.31 It is believed to be a conformational process that causes the anomalous shift of the ferrocene–ferrocenium redox couple and not a through space or through bond interaction as these would produce the expected anodic potential shift of the ferrocene redox couple. In the presence of K1 the receptor forms a 1 : 1 ‘sandwich’ complex with the cation with the metal ion bound between the two crown ether moieties (this does not occur with the smaller Na1 cations).The origin of the eVect may therefore be a redirection of the lone pairs of the sulfur donor atoms towards the iron centre upon complexation caused by ‘sandwich’ formation. This would increase the electron density on the iron causing a cathodic shift (rather than the expected anodic shift) of the redox potential of the ferrocene–ferrocenium couple. 5 The direct co-ordination mechanism Direct co-ordination between the redox centre and guest leads to generally large perturbations in the electrochemical behaviour of the host.Reducible redox-active nitrobenzene macrocyclic polyether systems have been prepared by a number of groups in particular by Gokel and co-workers 32,33 who were the first to demonstrate the electrochemical recognition of a sodium cation by such a system. For example the introduction of sodium cations to an electrochemical solution of compound 37 causes the evolution of a new wave on the cyclic voltammogram corresponding to solution complexed species.The redox active nitro group is directly co-ordinated to the sodium cation [Scheme 3(a)]. However the addition of sodium cations to electrochemical solutions of compound 38 has very little eVect on the cyclic voltammogram, presumably because the position of the nitro group on the aromatic ring allows no interaction between the sodium cation bound in the macrocycle and the nitro group [Scheme 3(b)].Therefore the eVects of simple through space interactions between the bound cation and nitro aromatic group can be dismissed in both these cases and the pathway for the coupling between the complexation and redox reactions with Na1 and 37 is a direct co-ordination route. Interestingly Gokel has demonstrated the existence of a ‘direct co-ordination’ coupling pathway between ferrocene cryptand receptor 1 and silver cations. Complexation studies carried out with 1 (as well as other ferrocene cryptand type species) by X-ray crystallography, FAB mass spectral analysis, NMR and UV/vis spectroscopy reveal that compound 1 has an unusual aYnity for Ag1 cations.3 X-Ray crystallographic determination of the structures of free 1, sodium and silver complexes were carried out and it was found that the Ag ? ? ? Fe distance in the silver complex of 1 is only 3.37 Å whereas the Na ? ? ?Fe distance in the sodium complex is 4.39 Å.This evidence together with the FAB MS and UV spectroscopic data suggests that there may be a co-ordination interaction between the silver cation and the iron present in the ferrocene moiety.Fe S S O O O O O O O O O O 36J. Chem. Soc., Dalton Trans., 1999, 1897–1909 1907 The CV of compound 1 is strongly aVected by addition of Ag1 cations. The behaviour is similar to that observed on addition of sodium cations however the magnitude of the DE1/2 value is much larger than that observed with Na1 (Table 9).The DE1/2 value with Ag1 is larger even than that with Ca21. This is inconsistent with the relatively small charge-to-size ratio of the Ag1 cation. The fact that the Ag1 ion exerts a much larger eVect on the half-wave potential of the ferrocenyl group than would be predicted in terms of its charge-to-size ratio suggests that the bound cation resides closer to the ferrocenyl subunit than the other cations studied. UV/vis Studies suggest this may be due to the ferrocene group acting as a donor to the Ag1 cation.Electrochemical experiments were also conducted in an aqueous environment and it was found Scheme 3 (a) A direct co-ordination pathway is possible between the binding site of compound 37 and the redox active nitro aromatic moiety whilst it is not available in 38 (b). O O N O O O2N N O O O N O O O O N O O N O O O O2N N O O Na+ Na+ Na+ Na+ 37 38 (a) (b) 37.Na+ 38.Na+ O O O O O O n 39: n = 1 40: n = 2 41: n = 3 42: n = 4 Table 9 Electrochemical data a for compound 1 in the absence and presence of several cations Cation none Li1 Na1 K1 Ca21 Ag1 Amount/ equivalents 0 0.5 0.5 0.5 0.5 0.5 E8 0.216 0.210 0.214 0.224 0.214 0.214 E8ox 0.402 0.348 0.488 c 0.496 DE8 0.188 0.124 0.274 0.282 RCE(K/K1) b 3 × 104 4 × 103 2 × 105 2 × 105 a E8 and E8ox are the apparent half-wave potentials of free 1 and the specified metal ion complex respectively.The values are given in V vs. SSCE; DE8 is the diVerence between these two values.b K and K1 represent the metal ion binding constants of the reduced and oxidised forms of the ligand respectively. The RCEs given were obtained by optimising the fit of experimental and simulated voltammograms. c This redox couple exhibited a marked degree of electrochemical irreversibility. that compound 1 can selectively recognise silver cations in water. Another particularly elegant example of this type of redox/ co-ordination coupling are the quinone crown ether species 39– 42 synthesized by Cooper and co-workers.34 A number of different size crown ethers were synthesized and the shift of the first reduction potential found for each compound in the presence of excess of alkali metal tosylate.The shifts were all between 60 and 70 mV for 39 but the larger crowns displayed larger shifts (Table 10). In contrast to the expected order of the magnitudes of the shifts from ion pairing eVects alone, K1 with compound 40 yields the largest potential shift followed by Rb1 > Na1 > Cs1 > Li1.Quinone groups have also been incorporated into ionophoric calixarene skeletons in order to produce amperometric cation sensors. One example, a calixdiquinone ligand 44 that is bridged by a crown ether like a polyglycol strand was synthesized by oxidation of 43 with thallium trifluoroacetate (Scheme 4).35 The crystal structure of the potassium perchlorate complex of this receptor is shown in Fig. 13. In this case the quinone groups are co-ordinated directly to the added metal cation which is also bound to the phenolic groups at the lower rim of the calixarene and to the crown ether oxygen atoms.The electrochemical responses of 44 upon addition of cations are shown in Table 11. Particularly notable is the anodic shift of 555 mV observed on addition of barium cations (which to the best of our knowledge is the largest shift so far observed for any redox-active receptor on addition of Group 2 metal cations). Cyclic voltammograms of receptor 44 in the absence and presence of sodium cations are shown in Fig. 14. 6 The interference mechanism Beer et al.36 have used a 1,3,4-tris(ferrocene) substituted calix- [5]arene 45 as a neutral guest sensor. Electrochemical studies of the behaviour of 45 have been carried out using cyclic and square wave voltammetric techniques. The receptor itself undergoes two quasi-reversible oxidations at Ep1 = 1350 and Ep2 = 1450 mV referenced to Ag–Ag1.Rotating disk electrode electrochemistry was used to resolve the two oxidation processes and it was found that one ferrocene was oxidised at 350 mV while the other two were oxidised at the larger anodic potential of 450 mV (Fig. 15). Scheme 4 Synthesis of receptor 44. TFA = Trifluoroacetic acid. O O O O O O O O O O O HO O O O Tl(OCOCF3)3 TFA OH 44 43 Table 10 Anodic shifts (mV) in the formal reduction potentials of compounds 39–42 upon addition of alkali metal cations Compound DE(Li1) DE(Na1) DE(K1) DE(Rb1) DE(Cs1) 39 66 68 68 67 60 40 56 130 162 138 117 41 38 68 106 114 132 42 33 67 74 87 911908 J. Chem.Soc., Dalton Trans., 1999, 1897–1909 Electrochemical investigations on the eVects of addition of potential neutral guests (DMF, DMSO, ethanol) to an electrochemical solution of compound 45 in CH2Cl2 show an interesting eVect. Addition of polar solvents such as DMF causes the two redox couples to merge. Similar eVects were also observed on addition of DMSO or ethanol.However on addition of Fig. 13 Crystal structure of the potassium perchlorate complex of receptor 44. (Reprinted with permission from ref. 35, Copyright 1997, American Chemical Society.) O O O Fe Fe Fe O OH OH O O 45 Table 11 Reduction potentials of compound 44 and the anodic shifts in the presence of 1.0 or 2.0 equivalents of diVerent cationic species a Epc/V (vs. Ag–Ag1) E1/2(free)/V DE(K1) b/mV DE(Na1) b/mV DE(Ba21) b/mV DE(NH4 1) d/mV DE(nBuNH3 1) d/mV 21.155 210 255 555 405 355 21.930 250 290 c c c a Obtained by both cyclic (100 mV s21) and square wave (10 Hz, Osteryoung-type) voltammetry in acetonitrile solution containing 0.1 mol dm23 NBu4BF4 as supporting electrolyte.Solutions were ca. 1 × 1023 mol dm23 in compound with reference to a Ag–Ag1 electrode (330 ± 10 mV vs. SCE) at 21 ± 1 8C. b Anodic shift of the reduction waves of (57) in the presence of 1.0 equivalent of the respective cationic species added as their perchlorate or hexafluorophosphate salts.c The second reduction wave of compound 44 became obscrure or disappeared in the presence of more than one equivalent of the respective cations. d Anodic shift in the presence of 2.0 equivalents of the respective cations. toluene no shifts in the redox couples were observed (Fig. 16). The relative permittivity of CH2Cl2 is 8.9. Addition of DMF (e = 36.7) causes the peaks to merge however the addition of toluene (e = 2.4) causes little change in the cyclic voltammogram.It may therefore be deduced that the splitting of the peak in CH2Cl2 alone is at least partially due to the interaction between the ferrocene moieties. The decrease in the interaction upon addition of the higher relative permittivity solvent implies that the polar solvent is interposing itself between the ferrocene Fig. 14 Cyclic voltammograms of receptor 44 (1.0 × 1023 mol dm23) in acetonitrile in the absence (a) and the presence of 0.3 equivalent (b) and 1.0 equivalent (c) of sodium cations added as the perchlorate salt.Supporting electrolyte 0.1 mol dm23 n-Bu4NBF4. Scan rate: 100 mV s21. Glassy carbon working electrode. Fig. 15 Computer fit of the Nernst equation to the rotating disk electrode electrochemistry at 121 rpm of compound 45 (5 × 1024 mol dm23) in CH2Cl2 with n-Bu4NBF4 (0.1 mol dm23) as supporting electrolyte [Reprinted with permission from ref. 36(a), Copyright 1995, Royal Society of Chemistry.]J.Chem. Soc., Dalton Trans., 1999, 1897–1909 1909 moieties (Fig. 17). As the relative permittivity of the interposed solvent increases, the shielding between the ferrocene centres increases due to the guest’s higher polarisability. The inclusion of the more highly polarisable guest therefore interferes with the electrochemical interactions present in the host so producing an electrochemical response. 7 Conclusion This perspective has covered recent advances in the electrochemical recognition of cations, anions and neutral guest species by redox-active receptor molecules.The mechanisms of complexation–redox coupling via through bond, through space and direct co-ordination have been highlighted in each case. Over recent years there have been few papers reporting examples of receptors employing the conformational change mechanism. The interference mechanism also remains to be exploited. This is clearly an area of electrochemical molecular recognition that is full of opportunity for the interested chemist.The electrochemical recognition of ion pairs 25 and neutral guests 37 is another area that remains to be fully explored. We hope to see the emergence, over the next few years, of ‘real-world’ devices based on the concepts illustrated here. Acknowledgements We thank the EPSRC, the Royal Society, Kodak Limited, British Petroleum, Serpentix, MediSense and the Ministry of Defence Fig. 16 The two redox processes become more equivalent upon addition of DMF to the solution of compound 45 (5 × 1024 mol dm23) in CH2Cl2 with n-Bu4NBF4 (0.1 mol dm23) as supporting electrolyte.Fig. 17 Insertion of polar guest species into the lower rim of compound 45 causing a decrease in the interaction between the ferrocene groups. Fc+ Fc+ Fc+ d+ d- d+ d- d+ d- Solvent Insertion in lower rim for financial support. Special thanks go to Dr M. G. B. Drew (University of Reading) for his many crystal structure determinations. References 1 D.R. Crow, Principles and Applications of Electrochemistry, Blackie, London, 1994. 2 P. L. H. M. Cobben, R. J. M. Egberink, J. G. Bomer, P. Bergveld, W. Verboom and D. N. Reinhoudt, J. Am. Chem. Soc., 1992, 114, 10573. 3 J. C. Medina, T. T. Goodnow, M. T. Rojas, J. L. Atwood, B. C. Lynn, A. E. Kaifer and G. W. Gokel, J. Am. Chem. Soc., 1992, 114, 10583. 4 P. L. Boulas, M. Gomez-Kaifer and L. Echegoyen, Angew. Chem., Int. Ed. Engl., 1998, 37, 216. 5 H. Plenio and R. Diodone, J. Organomet. Chem., 1995, 492, 73. 6 G. Charlot, J. Badoz-Lambling and B. Trémillion, in Electrochemical Reactions, Elsevier, Amsterdam, 1962. 7 T. Saji, Chem. Lett., 1986, 275. 8 H. Plenio and R. Diodone, Inorg. Chem., 1995, 34, 3964. 9 J.-P. Bourgeois, L. Echegoyen, M. Fibboli, E. Pretsch and F. Diederich, Angew. Chem., Int. Ed. Engl., 1998, 37, 2118. 10 P. D. Beer, Z. Chen and A. J. Pilgrim, J. Chem. Soc., Faraday Trans., 1995, 4331. 11 P. D. Beer, Z. Chen and A. J. Pilgrim, J. Electroanal. Chem., 1998, 444, 209. 12 P. D. Beer, Z. Chen, M. G. B. Drew, J. E. Kingston, M. I. Ogden and P. Spencer, J. Chem. Soc., Chem. Commun., 1993, 1046. 13 M. E. Padilla-Tosta, R. Martínez-Máñez, T. Pardo, J. Soto and M. J. L. Tendero, Chem. Commun., 1997, 887. 14 J. M. Lloris, R. Martínez-Máñez, T. Pardo, J. Soto and M. E. Padilla-Tosta, Chem. Commun., 1998, 837. 15 H. Plenio and C. Aberle, Chem. Commun., 1998, 2697. 16 P. D. Beer and D. K. Smith, J. Chem. Soc., Dalton Trans., 1998, 417. 17 P. D. Beer, C. Hazlewood, D. Hesek, J. Hodacova and S. E. Stokes, J. Chem. Soc., Dalton Trans, 1993, 1327. 18 (a) P. D. Beer, Chem. Commun., 1996, 689; (b) P. D. Beer, P. A. Gale and Z. Chen, Adv. Phys. Org. Chem., 1998, 31, 1. 19 P. D. Beer, M. G. B. Drew and D. K. Smith, J. Organomet. Chem., 1997, 543, 259. 20 P. D. Beer, J. Cadman, J. M. Lloris, R. Martínez-Mañez, M. E. Padilla-Tosta, T. Pardo, D. K. Smith and J. Soto, J. Chem. Soc., Dalton Trans., 1999, 127. 21 T. Saji and I. Kinoshita, J. Chem. Soc., Chem. Commun., 1986, 716. 22 P. D. Beer, C. Blackburn, J. F. McAleer and H. Sikanyika, Inorg. Chem., 1990, 29, 378. 23 C. D. Hall and S. Y. F. Chu, J. Organomet. Chem., 1995, 498, 221. 24 P. D. Beer, D. B. Crowe, M. I. Ogden, M. G. B. Drew and B. Main, J. Chem. Soc., Dalton Trans., 1993, 2107. 25 P. D. Beer, Z. Chen and M. I. Ogden, J. Chem. Soc., Faraday Trans., 1995, 295. 26 P. D. Beer, Z. Chen, A. J. Goulden, A. Graydon, S. E. Stokes and T. Wear, J. Chem. Soc., Chem. Commun., 1993, 1834. 27 P. D. Beer, A. R. Graydon, A. O. M. Johnson and D. K. Smith, Inorg. Chem., 1997, 36, 2112. 28 P. D. Beer, F. Szemes, V. Balzani, C. M. Salà, M. G. B. Drew, S. W. Dent and M. Maestri, J. Am. Chem. Soc., 1997, 119, 11864. 29 A. Gourdon, New J. Chem., 1992, 16, 953. 30 P. D. Beer, Z. Chen, A. Grieve and J. Haggitt, J. Chem. Soc., Chem. Commun., 1994, 2413. 31 P. D. Beer, J. P. Danks, D. Hesek and J. F. McAleer, J. Chem. Soc., Chem. Commun., 1993, 1735. 32 A. Kaifer, L. Echegoyen, D. A. Custowski, D. M. Croli and G. W. Gokel, J. Am. Chem. Soc., 1983, 105, 7168. 33 A. Kaifer, D. A. Custowski, L. Echegoyen, V. J. Gatto, R. A. Schultz, T. P. Cleary, C. R. Morgan, D. M. Goli, A. M. Rios and G. W. Gokel, J. Am. Chem. Soc., 1985, 107, 1958. 34 M. Delgado, J. R. E. Wolf, J. R. Hartman, G. McCaVerty, R. Yagbasan, S. C. Rawle, D. J. Watkin and S. R. Cooper, J. Am. Chem. Soc., 1992, 114, 8983. 35 P. D. Beer, P. A. Gale, Z. Chen, M. G. B. Drew, J. A. Heath, M. I. Ogden and H. R. Powell, Inorg. Chem., 1997, 36, 5880. 36 (a) P. D. Beer, Z. Chen, M. G. B. Drew and P. A. Gale, J. Chem. Soc., Chem. Commun., 1995, 1851., (b) P. D. Beer, P. A. Gale, Z. Chen and M. G. B. Drew, Supramol. Chem., 1996, 7, 241. 37 J. D. Carr, L. Lambert, D. E. Hibbs, M. B. Hursthouse, K. M. A. Malik and J. H. R. Tucker, Chem. Commun., 1998, 1649. Paper 9/01462D

ISSN:1477-9226    

DOI:10.1039/a901462d

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1999, 1911–1912 1911 Control of intramolecular acetate–allenylidene coupling by spectator co-ligand �-acidity Karsten J. Harlow, Anthony F. Hill* and Thomas Welton * Department of Chemistry, Imperial College of Science Technology and Medicine, South Kensington, London, UK SW7 2AY. E-mail: a.hill@ic.ac.uk Received 15th March 1999, Accepted 7th May 1999 The reactions of [RuHX(PPh3)3] (X = Cl, O2CMe) and [MHCl(CO)(PPh3)3] (M = Ru, Os) with 1,1-diphenylprop- 2-yn-1-ol provide convenient access to alkynyl, alkenyl, propenylidene, and acetoxyallenyl complexes of divalent ruthenium and osmium, including [RuCl2(]] CHCH]] CPh2)(PPh3)2] and the complexes [Ru(C]] ] CCPh2OH)- (O2CMe)(CA)2(PPh3)2] (A = NCMe3, O), protonation (HPF6) of which provides [Ru(O2CMe)(]] C]] C]] CPh2)- (CNCMe3)2(PPh3)2]PF6 or the metallacycle [Ru{k2C,OC(]] C]] CPh2)O2CMe}(CO)2(PPh3)2]PF6, respectively.There is currently enormous interest in the chemistry of alkylidene complexes of divalent ruthenium.1 This is inspired primarily by Grubbs’ ground-breaking discovery of highly eVective and remarkably tolerant alkene metathesis catalysts of the form [RuCl2(]] CHR)(PR93)2] (R = Ph, CH]] CPh2; R9 = Ph, Cy)2 which are currently enjoying increasingly wide application in a variety of synthetically useful C–C bond-forming processes. 3 We have recently shown that [RuCl2(PPh3)3] reacts with 1,1-diphenylprop-2-yn-1-ol 1 to provide the coordinatively unsaturated allenylidene complex [RuCl2(]] C]] C]] CPh2)(PPh3) 2].4a This complex may be easily converted to [RuCl2- (]] C]] C]] CPh2)(PCy3)2] which serves as a conveniently accessible alternative to Grubbs’ catalysts for the ring-closure alkene metathesis of a,w-dienes and dienynes.4b The reactions of propargylic alcohols with metal hydride complexes however, take a diVerent course, viz.hydrometallation of the alkyne to provide g-hydroxyvinyl complexes which have been shown to be particularly prone to dehydroxylation, providing either s-butadienyl 5 or propenylidene 6,7 complexes depending, respectively, on the presence or absence of protons d to the metal.In search of alternative routes to coordinatively unsaturated alkylidenes of ruthenium and osmium, we have investigated the reactions of the complexes [MHCl(CO)(PPh3)3] (M = Ru 2a, Os 2b), [RuHCl(PPh3)3] 3, and [RuH(O2CMe)(PPh3)3] 4 with 1. The results which include convenient routes to alkenyl, alkynyl, allenylidene, propenylidene and acetoxyallenyl complexes are reported herein.The g-hydroxyvinyl complex [Ru(CH]] CHCPh2OH)Cl(CO)- (PPh3)2] 5 forms in high yield from the reaction of 2a with 1 (Scheme 1).† Treating 5 with Cl2PPh3 results in the high yield conversion to the propenylidene complex [RuCl2(]] CHCH]] CPh2)(CO)(PPh3)2] 6a.†,‡ The analogous osmium complex 6b † may be similarly obtained in 75% yield directly from 2b, 1 and Cl2PPh3. The complexes 6 may be viewed as analogues of the benzylidene complexes [MCl2(]] CHR)(CO)(PPh3)2] long since described by Roper.1a,b,9 The coordinatively unsaturated, carbonyl-free complex [RuCl2(]] CHCH]] CPh2)(PPh3)2] 7 was shown by Grubbs to result from the reaction of [RuCl2(PPh3)3] with 3,3-diphenylcyclopropene2a but required the non-trivial preparation and handling of 3,3-diphenylcyclopropene. We find that the reaction of 3 with 1 in acetonitrile followed by acid (HCl) work-up provides 7 conveniently and in high yield (83%).† The presumed g-hydroxyvinyl intermediate 8 in this sequence (Scheme 1) has not been fully characterised due to its sensitivity, however carbonylation (1 atmosphere) provides the air stable adduct [Ru(CH]] CHCPh2OH)Cl(CO)(NCMe)(PPh3)2] 9a, which is an isomer (CO trans to vinyl) of 9b (MeCN trans to vinyl) obtained from 5 and acetonitrile.The acetate complex 4 reacts with 1 via a quite diVerent sequence, to ultimately provide the alkynyl complex mer- [Ru(C]] ] CCPh2OH)(O2CMe)(PPh3)3] 10 (Scheme 2).† The mechanism presumably involves alkyne hydrometallation, as above, followed by oxidative addition of a second alkyne C–H bond to provide [RuH(C]] ] CCPh2OH)(CH]] CHCPh2OH)- (O2CMe)(PPh3)2] which undergoes reductive elimination of alkene and re-coordination of phosphine to provide 10.The facility of the proposed sequence is consistent with the increase in basicity of the acetate ligand in 4 relative to the chloride in 3, favouring the involvement of tetravalent ruthenium intermediates.The formulation of 10 rests firmly on spectroscopic and FAB-MS data with the mer stereochemistry at ruthenium following unequivocally from 13C-{1H} and 31P-{1H} NMR data.† Both the acetate chelation and the phosphine coordination in 10 are labile. Thus treating 10 with carbon monoxide (1 atmosphere, 25 8C) results in clean conversion to [Ru(C]] ] CCPh2- OH)(O2CMe)(CO)2(PPh3)2] 11. Similarly, addition of two equivalents of 1,1-dimethylethyl isocyanide leads to formation of [Ru(C]] ] CCPh2OH)(O2CMe)(CNCMe3)2(PPh3)2] 12, whilst excess isocyanide provides the cationic complex mer-[Ru(C]] ] CCPh2OH)(CNCMe3)3(PPh3)2]1 131, readily isolated as the tetrafluoborate salt [13]BF4.By analogy with the dehydroxylation of g-hydroxyvinyl ligands, the g-hydroxyalkynyl ligands in 11 and 12 are also prone to dehydroxylation although the final products diVer depending on the nature (p-acidity) of the co- Scheme 1 M Cl Cl CO PPh3 PPh3 Ph Ph Ru Cl CO PPh3 PPh3 Ph OH Ph Ru Cl CO PPh3 PPh3 Ph OH Ph MeCN Ru Cl NCMe PPh3 PPh3 Ph OH Ph MeCN Ru Cl NCMe PPh3 PPh3 Ph OH Ph OC Ru Cl Cl PPh3 PPh3 Ph Ph (i) HCºCCPh2OH 1 (ii) Cl2PPh3 1 MeCN Cl2PPh3 [RuHCl(PPh3)3] 3 1, MeCN CO HCl 5 M = Ru 6a M = Os 6b 7 8 9a 9b [MHCl(CO)(PPh3)3] M = Ru 2a M = Os 2b1912 J.Chem. Soc., Dalton Trans., 1999, 1911–1912 ligands. Thus the reaction of 12 with HPF6 provides an allenylidene complex viz.[Ru(O2CMe)(]] C]] C]] CPh2)(CNCMe3)2- (PPh3)2]PF6 ([14]PF6). Amongst the spectroscopic data for 141, the intense infrared absorption at 1970 cm21 is characteristic of the allenylidene ligand. The protonation of 11 with HPF6 however takes a diVerent course although an allenylidene complex akin to 141 is clearly involved. The product obtained is formulated as the metallacyclic complex [Ru{k2C,O-C(]] C]] CPh2)O2CMe}(CO)2(PPh3)2]- PF6 [15]PF6) on the basis of spectroscopic data.† We have recently observed the formation of a related metallacycle (A, Scheme 2) derived from the intermolecular coupling of an allenylidene ligand with dithiocarbamate,10 whilst Roper has shown that the coupling of methylene and acetate ligands provides the metallacycle B.11 Complex 151 may therefore be usefully viewed as a hybrid of A and B.The reason for the dichotomy in products arising from the protonation of 11 and 12 may be understood by considering the p-acidity of the coligands CO and CNCMe3. By far the majority of allenylidene complexes of Group 8 metals involve strong donor co-ligands coordinated trans to the allenylidene,1c a feature which may be expected to deactivate the allenylidene towards nucleophilic attack.Whilst the isocyanide ligands in 12 and 141 are only modest p-acids, the carbonyl ligand coordinated trans to the allenylidene in the carbonyl analogue of 141 may be expected to strongly activate the allenylidene towards attack by the internal acetate nucleophile.Acknowledgements We wish to thank the Engineering and Physical Sciences Research Council (U.K.) for the award of a studentship (to K. J. H.). A. F. H. gratefully acknowledges the award of a Senior Research Fellowship by The Royal Society and The Leverhulme Trust. Ruthenium salts were generously provided by Johnson Matthey Chemicals Ltd. Notes and references † Selected data for new complexes (satisfactory microanalytical and/or FAB-MS data obtained); IR (Nujol, cm21), NMR (CDCl3, 25 8C, ppm) 1H (270), 31P (109), 13C (68 MHz). 5: yield 97%. IR: 3573 (OH), 1917 (CO). NMR 1H: d 5.40 [d, 1 H, RuCH]] CH; J(HH) = 12.9 Hz], 6.94– 7.45 [m, 41 H, Ph 1 RuCH (obscured)]. 31P-{1H}: d 33.2. 13C{1H}: Scheme 2 R = CMe3. Ru O C PPh3 PPh3 PPh3 C C Ph OH Ph O C Me Ru RNC C O2CMe PPh3 PPh3 C C Ph OH Ph RNC Ru OC C O2CMPPh3 PPh3 C C Ph OH Ph OC Ru RNC C O2CMe PPh3 PPh3 C C Ph Ph RNC Ru OC C O PPh3 PPh3 C C Ph Ph OC O C Me (Ph3P)(OC)(Me2NCS2)Ru C S C C Ph Ph S C NMe2 (Ph3P)2(OC)(Ph)Ru H2 C O O C Me 10 B11 A10 HPF6 12 11 14+ 15+ 1 HPF6 CO CNR [RuH(O2CCH3)(PPh3)3] 4 d 80.0 [CPh2OH], 139.7 [RuCH]] CH], 144.6 [RuCH]] CH], 202.3 [t, CO; J(PC) = 14.3 Hz].This complex was also crystallographically characterised. 12 6a: yield 95%. IR: 1955 (CO). NMR 1H: d 8.01 [d, 1 H, Ru]] CHCH; J(HH) = 13.8], 15.93 [d, 1 H, Ru]] CH; J(HH) = 13.9 Hz]. 31P- {1H}: d 16.7. 13C-{1H}: d 146.9 [Ru]] CHCH], 154.2 []] CPh2], 199.0 [t, CO; J(PC) = 13.4], 322.1 [t, Ru=CH; J(PC) = 10.7 Hz]. 6b: yield 75%. IR 1932 (CO). NMR 1H: d 17.50 [dt, 1 H, Os]] CHCH; J(HH) = 13.5; J(PH) = 2.0 Hz] (OsCH]] CH obscured by Ph resonances). 31P-{1H}: d 28.0. 13C-{1H}: d 151.2 [Os]] CHCH], 152.4 []] CPh2], 177.6 [t, CO; J(PC) = 9.7 Hz], 278.1 [m, Os]] CH]. 7: yield 83%. NMR 1H: d 8.20 [d, 1 H, Ru]] CHCH; J(HH) = 9.9], 17.74 [dt, 1 H, Ru]] CH; J(HH) = 9.9; J(PH) = 9.6 Hz]. 31P-{1H}: d 28.9. These data correspond to those previously reported.2a 9a: yield 75%.IR: 3564 (OH), 2283 (CN), 1949 (CO). NMR 1H: d 0.82 [s, 3 H, CH3], 5.32 [d, 1 H, RuCH]] CH; J(HH) = 17.8], 7.59 [d, 1 H, RuCH; J(HH) = 18.5 Hz]. 31P-{1H}: d 29.3. 13C-{1H}: d 2.6 [CH3], 80.2 [CPh2OH], 119.6 [NC], 136.4 [t, RuCH]] CH; J(PC) = 4.3], 153.2 [t, RuCH; J(PC) = 15.1], 198.9 [t, CO; J(PC) = 10.3 Hz]. 9b: yield 86%. IR: 3564 (OH), 1944 (CO). NMR 1H: d 1.60 [s, 3 H, CH3], 5.48 [dt, 1 H, RuCH]] CH; J(HH) = 15.9; J(PH) = 2.0], 7.40 [d, 1 H, RuCH, J(HH) = 15.9 Hz]. 31P-{1H}: d 27.3. 10: yield 71%. IR: 3558 (OH), 2057(C]] ] C), 1531 (CO2). NMR 1H: d 0.92 [s, 3H, CH3]. 31P-{1H}: d 35.5 [d, 2 PA, J(PAPB) = 26.8], 50.9 [t, 1 PB, J(PAPB) = 26.8 Hz]. 13C-{1H}: d 24.3 [O2CCH3], 76.7 [CPh2OH], 110.5 [dt, RuC]] ] C; J(PaxC) ª J(PeqC) = 17.3], 118.3 [RuC]] ] C], 185.1 [CO2]. 11: yield 88%. IR: 3579, 3561 (OH), 2121(C]] ] C), 2051, 1978 (CO). NMR 1H: d 1.20 [s, 3 H, CH3]. 31P-{1H}: d 31.4. 13C-{1H}: d 22.8 [CH3], 75.0 [CPh2OH], 106.8 [t, RuC]] ] C; J(PC) = 20.0], 116.2 [t, RuC]] ] C; J(PC) = 2.4], 176.2 [CO2], 194.3 [t, CO; J(PC) = 9.2], 198.5 [t, CO; J(PC) = 11.9 Hz]. 12: yield 87%. IR: 3567 (OH), 2150 (CN), 2105 (CN), 2073 (C]] ] C), 1606 (CO2). NMR 1H: d 0.81, 0.89 [s × 2, 9 H × 2, CNC(CH3)3], 1.25 [s, 3 H, O2CCH3]. 31P-{1H}: d 38.3. 13C-{1H}: d 24.5 [O2CCH3], 29.8, 30.6 [CNC(CH3)3], 55.6, 56.1 [CNC(CH3)3], 75.1 [CPh2OH], 115.2 [RuC]] ] C], 176.3 [CO2]. [13]BF4: yield 65%. IR: 3563 (OH), 2194 (CN), 2150 (CN), 2111 (C]] ] C).NMR 1H: d 0.81 [s, 9 H, C(CH3)3], 0.93 [s, 18 H, C(CH3)3]. 31P-{1H}: d 34.8. [14]PF6: yield 79%. IR: 2184 (CN), 2148 (CN), 1970 (C]] C]] C), 1587 (CO2). NMR 1H: d 0.96 [s, 9 H, C(CH3)3], 1.08 [s, 9 H, C(CH3)3], 1.11 [s, 3 H, O2CCH3]. 31P-{1H}: d 34.3. [15]PF6: yield 88%. IR: 2071 (CO), 2003 (CO), 1598 (C]] C]] C). NMR 1H: 1.32 [s, 3H, O2CCH3]. 31P-{1H}: d 22.4. 13C-{1H}: d 18.4 [O2CCH3], 118.6 []] CPh2], 147.4 [t, RuC(OCO), J(PC) = 15.1], 183.6 [O2CCH3], 192.0 [t, CO; J(PC) = 9.7], 198.7 [t, CO; J(PC) = 11.3], 201.8 [t, RuC]] C, J(PC) = 4.9 Hz].‡ Whilst Cl2PPh3 was found to be the most convenient dehydroxylating agent,8 similar yields were obtained using anhydrous HCl, OSCl2 or PhSeCl and the complexes [Ru(CH]] CHCR2OH)Cl(CO)(PPh3)2] (CR2 = cyclo-C6H10, CMe2, C13H8), obtained from 2a and the appropriate propargylic alcohol. 1 For reviews on the chemistry of alkylidenes of Group 8 metals see (a) M. A. Gallop and W.R. Roper, Adv. Organomet. Chem., 1986, 25, 121; (b) W. R. Roper, J. Organomet. Chem., 1986, 300, 167; (c) A. F. Hill, in Comprehensive Organometallic Chemistry II, ed. E. W. Abel, F. G. A. Stone and G. Wilkinson, Pergamon, Oxford, 1995, vol. 7. 2 (a) S. T. Nguyen, R. H. Grubbs and J. W. Ziller, J. Am. Chem. Soc., 1993, 115, 9858; (b) P. Schwab, M. B. France, J. W. Ziller and R. H. Grubbs, Angew. Chem., Int. Ed. Engl., 1995, 34, 2039; (c) E. L. Dias, S. T. Nguyen and R. H. Grubbs, J.Am. Chem. Soc., 1997, 119, 3887. 3 A. Fürtsner, Top. Organomet. Chem., 1998, 1, 37. 4 (a) K. J. Harlow, A. F. Hill and J. D. E. T. Wilton-Ely, J. Chem. Soc., Dalton Trans., 1999, 285; (b) A. Fürstner, A. F. Hill, M. Liebl and J. D. E. T. Wilton-Ely, Chem. Commun., 1999, 601. 5 M. C. J. Harris and A. F. Hill, J. Organomet. Chem., 1992, 438, 209. 6 (a) M. A. Esteruelas, F. J. Lahoz, E. Oñate, L. A. Oro and B. Zeier, Organometallics, 1994, 13, 4258; (b) M. A. Esteruelas, F. J. Lahoz, E. Oñate, L. A. Oro and B. Zeier, B., ibid., 1994, 13, 1662. 7 K. J. Harlow, A. F. Hill, T. Welton, A. J. P. White and D. J. Williams, Organometallics, 1998, 17, 1916. 8 S. Anderson, D. J. Cook and A. F. Hill, J. Organomet. Chem., 1993, 463, C3. 9 G. R. Clark, K. Marsden, W. R. Roper and L. J. Wright, J. Am. Chem. Soc., 1980, 102, 6570. 10 B. Buriez, K. J. Harlow, A. F. Hill, T. Welton, A. J. P. White, D. J. Williams and J. D. E. T. Wilton-Ely, J. Organomet. Chem., 1999, 578, 264. 11 D. S. Bohle, G. R. Clark, C. E. F. Rickard, W. R. Roper, W. E. B. Shepard and L. J. Wright, J. Chem. Soc., Chem. Commun., 1987, 563; D. S. Bohle, G. R. Clark, C. E. F. Rickard, W. R. Roper and L. J. Wright, J. Organomet. Chem., 1989, 358, 411. 12 A. J. P. White and D. J. Williams, unpublished work. Communication 9/02021G

ISSN:1477-9226    

DOI:10.1039/a902021g

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1999, 1913–1914 1913 Selective oxidation of cyclohexane to cyclohexanol catalyzed by a Ï-hydroxo diiron(II) complex and tert-butylhydroperoxide Jean-Marc Vincent, Stéphane Béarnais-Barbry, Céline Pierre and Jean-Baptiste Verlhac* Laboratoire de Chimie Organique et Organométallique (UMR 5802) Université Bordeaux I, 351 cours de la Libération, 33405 Talence Cedex, France. E-mail: j-b.verlhac@lcoo.u-bordeaux.fr Received 22nd March 1999, Accepted 13th May 1999 A new Ï-hydroxo diiron(II) complex [Fe2L(OH)]31 obtained with a dinucleating macrocyclic ligand catalyzes the selective oxidation of cyclohexane into cyclohexanol (ª85%) using the controlled addition of tert-butylhydroperoxide.Functional modeling of the soluble methane monooxygenase (MMO) enzyme,1 which contains a dinuclear non-heme iron center, has provided new catalysts for the oxidation of alkanes using hydroperoxide oxidants. The most eYcient models are m-oxo dinuclear iron(III) complexes with bidentate (bipyridine, bpy) or tetradentate [tris(2-pyridylmethyl)amine, TPA] pyridine-type and exchangeable m-acetato or terminal aqua ligands.2 tert-Butylhydroperoxide (TBHP), widely used in oxidation reactions, has proved to be the most useful oxidant in association with these catalysts.Mixtures of alcohol, ketone and dialkyl peroxide are obtained in agreement with autoxidation reactions involving alkoxyl- or alkyl-radicals and O2.Here, we report that a m-hydroxo diiron(II) complex of a dinucleating macrocyclic ligand, a good MMO model, is an eYcient catalyst for cyclohexane oxidation with TBHP as oxidant. Moreover, selective oxidation of cyclohexane to cyclohexanol was achieved using a controlled addition of TBHP via a syringe-pump as shown recently by Que and co-workers.3 The macrocyclic ligand 1,4,10,13-tetrakis(2-pyridyl)methyl- 1,4,10,13-tetraaza-7,16-dioxacyclooctadecane L (Fig. 1), with four pendant 2-pyridylmethyl arms, was synthesized according to a previously reported procedure.4 The iron complex [Fe2L(OH)][BF4]3 1 was prepared by adding a deoxygenated methanolic solution (10 ml) of the ligand L (1.6 mmol) to a degassed methanolic solution of Fig. 1 Schematic representation of the dinucleating ligand L. N O N N N O N N N N Fe(BF4)2?6H2O (3.2 mmol). Dropwise addition of deoxygenated diethyl ether allows the precipitation of the complex as a pale yellow powder in 80% yield.Complex 1 can be further recrystallized by slow diVusion of diethyl ether into an acetonitrile solution of the complex. Elemental analysis † and electrospray mass spectroscopy support the proposed structure with a hydroxo bridged diiron core. Electrospray ionization mass spectra shown an ion cluster at m/z 927.2, the mass and isotope patterns of which are consistent with the [{Fe2L(OH)}(BF4)2]1 ion. The UV-visible spectrum of 1 is in agreement with the presence of an iron(II) complex.5 We speculate that the structure of 1 is related to that previously reported for the Mn(II) analogue [Mn2L(OH)]- [ClO4]3 in which the manganese atoms are hexa-coordinated with the ether function completing the coordination sphere.4 Interestingly, the complex is poorly oxygen sensitive even in acetonitrile solution, as checked by UV-visible spectroscopy.This is also in agreement with hexa-coordinated iron(II) lacking binding sites for O2 coordination. We tested the ability of this novel iron(II) complex to catalyze the oxidation of cyclohexane with TBHP as oxidant.Oxidation reactions were carried out in acetonitrile at 25 8C using conventional Schlenk techniques to ensure very eYcient deoxygenation when required. Cyclohexane oxidation results are reported in Table 1. In a typical reaction 0.5 mmol of catalyst was reacted with 0.5 mmol TBHP and 5 mmol cyclohexane in 5 ml acetonitrile. A nearly equimolar mixture of cyclohexanol and cyclohexanone was obtained in 16% yield, corresponding to 160 turnovers in less than 20 minutes.It has to be noted that 85% of the TBHP was consumed (checked by GC titration) revealing the high ‘catalase-like’ activity of 1 leading to the production of O2 in the reaction mixture. Increasing the catalyst concentration (5 mmol) led to a 46% yield of oxidation products in 2 minutes. These results are similar to those obtained with the best systems reported so far.2 Dialkylperoxide is also formed but in smaller amounts than was observed by Que et al.with the iron(III) TPA complexes.2 Addition of another aliquot of TBHP at the end of the reaction did not increase the amount of product suggesting inactivation of the catalyst. When 10 equiv. of dilute TBHP (50 mmol in 2 ml MeCN) Table 1 Product distribution in the oxidation of cyclohexane catalyzed by 1 and TBHP Products c Reaction Cat a Oxb CyO CyOH CyOOtBu CyBr time/min Yields d (%) 0.5 5555 500 500 50 (sp) 50 (sp) e 50 (sp) f 26 64 14 0.4 25 54 14 4 0.5 4 24 200 ———— 18.2 20 5 60 60 60 16 46 36 24 39 a mmol of catalyst.b mmol of TBHP, (sp) when added with a syringe-pump. c mmol of product. d Total yield based on oxidant. The ketone yields are molar yields multiplied by 2 since 2 equivalents of TBHP are required to make one equivalent of ketone. e Solutions not degassed. f 250 mmol of CCl3 were added.1914 J. Chem. Soc., Dalton Trans., 1999, 1913–1914 were added to a solution of 1 with a syringe-pump over a 1 hour period, selective oxidation (ª85%) of cyclohexane into cyclohexanol occurred in 37% yield.Under the same conditions but in non-degassed solution no selectivity was observed. A selective oxidation of alkane to alcohol could be assigned to a metal centered oxidation reaction expected from a genuine monooxygenase mimic. However, MacFaul and co-workers have clearly shown by using the 2-methyl-1-phenylprop-2-yl hydroperoxide (MPPH) that alcohol oxidation selectivity can be due to freely diVusing alkoxyl radicals.6 The tert-alkoxyl radical formed after homolysis of the MPPH O–O bond undergoes b-scission (kb ª 2 × 108 s21) too quickly for it to abstract a hydrogen atom from a saturated hydrocarbon.When MPPH (10 equiv. added with a syringe-pump and diluted in 2 ml Me3CN) is used, no oxidation products are detected, showing that the hydrogen abstracting species with TBHP [eqn. (2)] is the tert-butoxyl radical produced from the homolysis of the FeO–OBut bond [eqn.(1)]. FeOOBut æÆ FeO? 1 ButO? (1) ButO? 1 CyH æÆ ButOH 1 Cy? (2) Cy? 1 O2 æÆ CyOO? æÆ alcohol and ketone (3) Cy? 1 FeO? æÆ FeOCy æÆ alcohol (4) Cy = cyclohexyl Preliminary, low temperature UV-visible and ESR studies have shown that a transient iron(III) alkylperoxo species is formed in the early stages of the reaction. A blue intermediate, stabilized at 240 8C and generated by the addition of 50 equiv. TBHP in an acetonitrile solution of 1, displays a broad and intense absorption band at 600 nm.This species has a rhombic ESR signal centered at g = 2 (2.15, 1.94), characteristic of low spin iron(III) complexes. This strongly suggests the participation of an iron(III) alkylperoxo intermediate as previously found with the iron–bpy and iron–TPA catalysts.7 Addition of a small amount of CCl3Br (50 equiv., 250 mmol) to a cyclohexane oxidation reaction gave mainly cyclohexyl bromide demonstrating that freely diVusing cycloalkyl radicals are formed during the reaction.These radicals can either: (i) be trapped by O2 when a large excess of TBHP is used, to produce cyclohexyl peroxy radicals [eqn. (3)] leading to mixtures of alcohol and ketone or (ii) react with FeO? when the TBHP concentration is very low to produce an iron alkoxy species. The latter pathway leads to alcohol selectively [eqn. (4)]. Complex 1 represents one of the few examples of a MMO mimic able to selectively oxidize cyclohexane to cyclohexanol via the well-disguised free radical chemistry recently evidenced by MacFaul et al.6 for the iron–TPA catalysts developed by Que and co-workers.3 We are currently testing the ability of the diiron(II) complex to perform hydrocarbon oxidations in the presence of other oxidants such as hydrogen peroxide.Acknowledgements We are indebted to the CNRS and the Bordeaux 1 University for financial support. We thank Dr. J.-M. Bassat for providing the ESR spectrum of the peroxo intermediate.Notes and references † Analytical and spectroscopic data for complex 1: Found: C, 41.73; H, 5.21; N, 10.60; Fe, 9.51; B, 3.22. Calc. for C38H59N8F12Fe2O6B3?2CH3- OH?H2O: C, 41.65; H, 5.38; N, 10.22; Fe, 10.19; B, 2.96%. lmax/nm (Me3CN) 365 (e/dm3 mol21 cm21 1230). 1H NMR (250 MHz in CD3CN) : the spectrum of complex 1 displays broad resonances ranging from d 240 to 150 in agreement with high spin iron(II) atoms. By comparison with the diiron(II)–TPA complex described by Que et al.,5 the resonances observed at d 41 and 43 are tentatively attributed to the b-protons of the pyridine ring.A minor species (<10%) is also detected in solution and is assigned to OH ligand exchange by residual water molecules. 1 B. J. Wallar and J. D. Lipscomb, Chem. Rev., 1996, 96, 2625; A. C. Rosenweig, P. Nordlund, P. Takahara, C. A. Frederick and S. J. Lippard, J. Chem. Biol., 1995, 2, 409. 2 J. B. Vincent, J. C.HuVman, G. Christou, M. A. Nanny, D. N. Hendrickson, R. H. Fong and R. H. Fish, J. Am. Chem. Soc., 1988, 110, 6898; R. A. Leising, J. Kim, M. A. Pérez and L. Que, jun., J. Am. Chem. Soc., 1993, 115, 9524; S. Ménage, J.-M. Vincent, C. Lambeaux, G. Chottard, A. Grand and M. Fontecave, Inorg. Chem., 1993, 32, 4766; J.-M Vincent, S. Ménage, C. Lambeaux and M. Fontecave, Tetrahedron Lett., 1994, 35, 6287; A. Rabion, S. Chen, J. Wang, R. M. Buchanan, J.-L. Séris and R. H. Fish, J. Am. Chem. Soc., 1995, 117, 12356. 3 J. Kim, R. G. Harrison, C. Kim and L. Que, jun., J. Am. Chem. Soc., 1996, 118, 4373. 4 D. Tétard, A. Rabion, J.-B. Verlhac and J. J. Guilhem, J. Chem. Soc., Chem. Commun., 1995, 531. 5 S. Ménage, Y. Zang, M. P. Hendrich and L. Que, jun., J. Am. Chem. Soc., 1992, 114, 7786. 6 P. A. MacFaul, K. U. Ingold, D. M. Wayner and L. Que, jun., J. Am. Chem. Soc., 1997, 119, 10594; P. A. MacFaul, I. W. C. Arends, K. U. Ingold and D. M. Wayner, J. Chem. Soc., Perkin Trans. 2, 1997, 135. 7 S. Ménage, E. C. Wilkinson, L. Que, jun. and M. Fontecave, Angew. Chem., Int. Ed. Engl., 1995, 34, 203; J. Kim, E. Larka, E. C. Wilkinson and L. Que, jun., Angew. Chem., Int. Ed. Engl., 1995, 34, 2048. Communication 9/02225B

ISSN:1477-9226    

DOI:10.1039/a902225b

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1999, 1915–1916 1915 A three-dimensional network coordination polymer, (terephthalato)(pyridine)cadmium, with blue fluorescent emission Hoong-Kun Fun,*a S. Shnamuga Sundara Raj,a Ren-Gen Xiong,*b Jing-Lin Zuo,b Zhi Yu b and Xiao-Zeng You b a X-Ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, 11800 Penang. E-mail: hkfun@usm.my b Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing, 210093, P.R. China. E-mail: xyz@netra.nju.edu.cn Received 16th March 1999, Accepted 30th April 1999 The first terephthalato (TPT) bis-tridentate bridging cadmium coordination polymer, [(TPT)(py)Cd] 1, with blue fluorescent emission, was synthesized by a hydrothermal reaction between Cd(ClO4)2?6H2O, 1,4-dicyanobenzene and pyridine (py). The terephthalato (TPT) ligand has been extensively studied in the field of molecular magnetism on account of its promising application to areas of technology such as magnetic recording 1 due to TPT exhibiting a variety of chelating abilities as manifested by the formation of dinuclear,2 tetranuclear,3 one- and two-dimensional copper(II) systems,4 as well as three-dimensional manganese(II) coordination polymers.5 More recently, coordinatively unsaturated metal centers in the extended porous frameworks of Zn3(TPT)3?6MeOH and Zn(TPT)(DMF)(H2O) have been addressed and show a clear preference for the inclusion of alcohols and gas sorption properties.6 However, it is noteworthy that in these compounds TPT adopts a m4-bridging mode of coordination with the metal atom and no m6-bridging TPT ligands have so far been found.To our surprise, we have successfully obtained a novel threedimensional condensed polymer, [Cd(TPT)(py)]n 1, with an unprecedented m6-TPT-bridging mode under hydrothermal synthesis conditions using 1,4-dicyanobenzene as starting material. Moreover, 1 also displays a very strong blue fluorescent emission in the solid state.Herein, we report the synthesis, structure and fluorescence properties of 1 which represents the first example of a metal coordination polymer with m6-TPT-coordination. Polymer 1 was synthesized under hydrothermal reaction conditions from Cd(ClO4)2?6H2O and 1,4-dicyanobenzene.† In this reaction, 1,4-dicyanobenzene was hydrolyzed to form TPT and is considered to be the precursor of TPT. The situation is quite similar to the reaction system of Fe(ClO4)3 and 4-pyridinecarbaldehyde in which 4-pyridinecarbaldehyde is the precursor of 4-pyridinecarboxylic acid.7 The presence of a carboxylate group in 1 was confirmed by the very strong peaks at 1568, 1535 and 1386 cm21, in the IR spectrum.No peaks at about 2100 cm21 were found, indicating that the cyano group no longer exists in 1. It should be emphasized that with the replacement of 1,4-dicyanobenzene by 1,4-benzenedicarboxylic acid, we failed to get compound 1, even if the same reaction conditions were used.This shows that the use of the precursor is crucial for the synthesis of the novel condensed metal coordination polymer. The three-dimensional polymeric structure of 1 was revealed by an X-ray single crystal diVraction investigation.‡ The local coordination environment around the Cd(II) ion in 1 can best be described as approximately pentagonal bipyramidal with the two bidentate chelating carboxylates and one N atom of pyridine in the equatorial plane and with two monodentate carboxylates in the axial positions, as shown in Fig. 1.Each TPT in 1 adopts a m6-bridging mode to connect with 4 Cd(II) ions (see Fig. 2) to form a three-dimensional network structure. Though the TPT carboxylates in 1 can be considered as being in a common bridging mode (bis-tridentate),8 to our knowledge, all of the carboxylate groups in the known compounds containing TPT take part in coordination to the metal ion with a syn-syn bridging mode.Furthermore, the seven-coordination of the Cd(II) ion is shown by the following. Firstly, two oxygen atoms of the two diVerent TPT carboxylate anions coordinate with the Cd(II) ion almost equally in a bidentate chelating mode. The Cd–O distances (2.307–2.448 Å) are quite similar to normal Cd–OCO distances (2.251–2.879 Å).9 Secondly, the Cd–O distance (2.497 Å) in the apical positions is slightly longer than those in the equatorial position, indicating that it shows some pendant oxygen characteristics.However, both of these carboxylates are bound asymmetrically with one Cd–O bond length somewhat longer than the other.8 Thirdly, the Cd–N (2.281 Å) distance also has a typical bond length (2.32–2.39 Å).10 It can be seen from a perspective view of 1 down the b-axis , as shown in Fig. 2 (pyridine omitted for clarity) that the TPT anion acts as bis-tridentate ligand to the two coplanar Cd(II) ions and of two Cd(II) ions in a diVerent layer, thus resulting in the formation of a very regular three-dimensional network structure with channel dimensions of 11.23 × 11.23 Å.As a result, molecules such as 4- and 3-methylpyridine, imidazole etc. can be included in this channel. {The seventh coordinated ligand is labile, this situation is very similar to that of [Cu3(TMA)2- (H2O)3]n (TMA = 1,3,5-benzenetricarboxylic acid) in which the lability of the aqua ligands allows their replacement by other groups.11} To study the thermal stability of compound 1 Fig. 1 An ORTEP12 view of the coordination polymer [(TPT)(py)Cd] 1. Hydrogen atoms are omitted for clarity.1916 J. Chem. Soc., Dalton Trans., 1999, 1915–1916 thermogravimetric analysis (TGA) was performed on the polycrystalline sample, indicating that one strikingly clean weight loss step occurred at 220 8C (20.8% loss), corresponding to the removal of one pyridine molecule per formula unit (22.23% calculated).Most important is the fact that no weight loss was recorded between the temperatures 220 and 408 8C, probably suggesting the formation of a stable phase formulated as Cd(TPT) (see Fig. 2). The most important feature of the structure of 1 is that its three-dimensionally condensed polymeric structure leads to significant enhancement of fluorescent intensity, almost 100 times larger than that of the free ligand, probably due to the symmetry decrease of TPT (seriously twisted) in 1 compared with the free ligand.The emission of 1, lmax = 464 nm (Fig. 3), is neither MLCT (metal-to-ligand charge transfer) nor LMCT (ligand-to-metal charge transfer) in nature, and can probably be assigned to the intraligand fluorescent emission since a very weakly similar emission (dmax at 466 nm) is also observed for the TPT acid. On the other hand, the diVuse reflectance spectrum of 1 is dominated by an intraligand p–p* transition at 298 nm. Owing to the blue fluorescence emission of 1 it may be used as Fig. 2 A perspective view of 1 along the b axis (pyridine omitted for clarity). Fig. 3 Fluorescence emission spectrum of 1 in the solid state at room temperature. an advanced material for blue-light emitting diode devices. This condensed polymeric material may be an excellent candidate for highly thermally stable and solvent-resistant blue fluorescent material because 1 is almost insoluble in most common solvents such as ethanol, chloroform, ethyl acetate, acetone, acetonitrile, benzene and water.In conclusion, we have synthesized an unprecedented and novel m6-TPT metal coordination polymer using the precursor of TPT, 1,4-dicyanobenzene, under hydrothermal hydrolysis conditions. This opens up a new synthetic route for novel metal coordination polymers with promising photo-electronic properties. Acknowledgements This work was supported by a grant for a key research project from the State Science and Technology Commission and the National Nature Science Foundation of China.The authors would like to thank the Malaysian Government and Universiti Sains Malaysia for research grant R & D No. 190-9609-2801. SSSR thanks the Univerisiti Sains Malaysia for a Visiting Post-Doctoral Fellowship. Notes and references † Preparation of Cd (TPT)(py) 1. Hydrothermal treatment of Cd- (ClO4)2?6H2O (1.2 mmol), 1,4-cyanobenzene (1 mmol), pyridine (1 ml) and water (10 ml) for 1 day at 140 8C yielded a yellow prismatic crystalline product (only one pure phase).The yield of 1 was almost quantitative based on 1,4-cyanobenzene (Found: C, 43.65; H, 2.56; N, 4.01. (Calc.: C, 43.87; H, 2.53; N, 3.94%). IR(KBr, cm21): 1710vw, 1605vw, 1568vs, 1535vs, 1487m, 1449m, 1386vs, 1296vw, 1221m, 1143m, 1067m, 1038m, 1015m, 890m, 838s, 750s, 706s, 630m, 528m. ‡ Crystal data for 1. C13H9NO4Cd, monoclinic, C2/c, a = 16.9875(4), b = 9.9995(2), c = 7.8476(1) Å, b = 104.584(1)8, V = 1290.09(4) Å3, Z = 4, M = 355.6, Dc = 1.83 Mg m23, R1 = 0.026, wR2 = 0.059 (2075 reflections), T = 293 K, m = 1.70 mm21.CCDC reference number 186/ 1446. See http://www.rsc org/suppdata/dt/1999/1915/ for crystallographic files in .cif format. 1 K. S. Burger, P. Chaudhuri, K. Wieghardt and B. Nuber, Chem. Eur. J., 1995, 1, 586. 2 C. E. Xanthopoulos, M. P. Sigalas, G. A. Katsoulos, C. A. Tsipis, A. Terzis , M. MenTzafos and A. Hountas, Inorg. Chem., 1993, 32, 5433. 3 E. G. Bakalbassis, A. P. Bozopoulos, J. Mrozinski, P. J. Kentzeperis and C. A. Tsipsi, Inorg. Chem., 1988, 27, 529. 4 E. Bakalbassis, P. Bergerat, O. Kahn, S. Jeannin, Y. Jeannin, Y. Dromzee and M. Guillot, Inorg. Chem., 1992, 31, 625. 5 C. S. Hong and Y. Do, Inorg. Chem., 1997, 36, 5684; C. S. Hong and Y. Do, Inorg. Chem., 1998, 37, 4470. 6 H. Li, C. E. Davis, T. L. Groy, D. G. Kelly and O. M. Yaghi, J. Am. Chem. Soc., 1998, 120, 2186; H. Li, M. Eddaoudi, T. L. Groy and O. M. Yaghi, J. Am. Chem. Soc., 1998, 120, 8571. 7 R.-G. Xiong, S. R. Wilson and W. Lin, J. Chem. Soc., Dalton Trans., 1998, 4089. 8 R. L. Rardin, W. B. Tolman an S. J. Lippard, New J. Chem., 1991, 15, 417. 9 W. Clegg, J. T. Cressey, A. McCamley and B. P. Straughan, Acta Crystallogr., Sect. C, 1995, 51, 234. 10 M. Fujita, Y. J. Kwon, M. Miyazawa and K. Ogura, J. Chem. Soc., Chem. Commun., 1994, 1977; S. D. Huang and R.-G. Xiong, Polyhedron, 1997, 16, 3929. 11 S. S.-Y. Chui, S. M.-F. Lo, J. P. H. Charmant, A. G. Orpen and I. D. Williams, Science, 1999, 283, 1148. 12 C. K. Johnson, ORTEP, Report ORNL-5138, Oak Ridge National Laboratory, Oak Ridge, TN, 1976. Communication 9/02056J

ISSN:1477-9226    

DOI:10.1039/a902056j

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年代:1999

数据来源: RSC

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DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1999, 1917–1919 1917 Observation of triple-quantum eVects in the HMQC spectra of substituted derivatives of Rh6(CO)16 † Brian T. Heaton,a Jonathan A. Iggo,*a Ivan S. Podkorytov,*b Daniel J. Smawfield,a Sergey P. Tunik c and Robin Whymana a Department of Chemistry, University of Liverpool, PO Box 147, Liverpool, UK L69 7ZD. E-mail: iggo@liv.ac.uk b S.V. Lebedev Central Synthetic Rubber Research Institute, Gapsalskaya 1, St. Petersburg, 198035, Russia c Department of Chemistry, St.Petersburg University, Universitetskii pr., 2, St. Petersburg, 198904, Russia Received 15th March 1999, Accepted 5th May 1999 Triple-rhodium quantum effects in the HMQC spectra of [Rh6(CO)15L] [L 5 MeCN, I, PBun 3, P(OPh)3, P(4-XC6H4)3; X 5 H, OMe, F] modulate the intensity and position of the correlations in the rhodium dimension; cross peaks are displaced from the true chemical shift, additional cross peaks are seen, and the intensity of the coherences varies as a function of the mixing time and coupling constant, going to zero at the conventional value of 1/(2J).Inverse detected multiple quantum coherence (HMQC) experiments1 are now routinely used for the detection of insensitive nuclei such as 15N and 13C using 1H as the detector nucleus. More recently there has been a small number of reports using e.g. 31P as the detector nucleus to study metal nuclides such as 57Fe and 183W in inorganic systems.2–4 In all these experiments the low natural abundance of the insensitive nucleus I ensures that only single-quantum transitions of the I spin are important.Extension of the HMQC experiment to metal cluster compounds in which the insensitive (i.e. metal) nucleus has high natural abundance is not straightforward since the detector nucleus may now couple to several I spins. Multiple-quantum transitions, in which the assembly of metal spins acts as a unit, must now be considered.Although the eVect of multiplequantum transitions of the I spins is well understood,5 until now, it has been of little practical significance since, as mentioned above, studies using HMQC have concentrated on nuclei where multiple metal-quantum eVects cannot be observed because of the low natural abundance of the I spin. Ruegger and Moskau have recently presented a set of rules for the interpretation of 31P detected 195Pt–31P HMQC spectra (acquired without phase cycling to allow observation of resonances from multiple-quantum coherences normally suppressed by the phase cycling) of some platinum-phosphine systems in which the detector phosphorus nucleus can couple to two 195Pt.6 We now report that where the detector nucleus can couple to three metal spins, e.g.when a face-bridging carbonyl is used to detect 103Rh in rhodium carbonyl cluster compounds, not only are the correlations displaced from the “true” chemical shift in the rhodium dimension but also may be missing altogether as a result of modulation of the spectral intensity by the multiple metal-quantum transitions.Simple experimental modifications are described to overcome these eVects. The eVect of the HMQC pulse sequence, Fig. 1, on a facebridging carbonyl coupled to three rhodium atoms can be described using the product operator formalism.7,8 At the † Supplementary data available: the HMQC pulse program and phase cycling used in the experiments. For direct electronic access see http:// www.rsc.org/suppdata/dt/1999/1917/, otherwise available from BLDSC (No.SUP 57547, 2 pp.) or the RSC Library. See Instructions for Authors, 1999, Issue 1 (http://www.rsc.org/dalton). beginning of the evolution period t1, just after the first (p/2)– 103Rh pulse, the state of the four-spin face-bridging 13C103Rh3 system is given by eqn. (1) s = (0Rh) 1 (1Rh) 1 (2Rh) 1 (3Rh) (1) where (0Rh) = C(p/2 2 WCd2)cos3pJd2 (2) (1Rh) = 2C(p 2 WCd2) × {Rh1(p/2) 1 Rh2(p/2) 1 Rh3(p/2)}cos2pJd2sinpJd2 (3) (2Rh) = 4C(3p/2 2 WCd2) × (4) {Rh1(p/2)Rh2(p/2) 1 Rh1(p/2)Rh3(p/2) 1 Rh2(p/2)Rh3(p/2)} × cospJd2sin2pJd2 (3Rh) = 8C(2p 2 WCd2) × (5) Rh1(p/2)Rh2(p/2)Rh3(p/2)sin3pJd2 and C(f) and Rhi(f) denote the carbon and rhodium spin operators respectively where, for arbitrary phase f, and S is S(f) = Sxcosf 1 Sysinf (6) either C or Rhi.The chemical shift of carbon is WC, J is the coupling constant and d2 is the delay used in the pulse sequence for polarization transfer, Fig. 1.Spin operators belonging to the three diVerent rhodium atoms are numbered by the subscripts 1, 2, and 3. It is convenient to refer to the operators (0Rh), (1Rh), (2Rh) and (3Rh) as zero-, one-, two-, and three-rhodium spin coherences, respectively since we are interested principally in the order of the coherence with respect to rhodium. Fig. 1 Pulse sequence used for obtaining inverse detected HMQC 2D spectra.1918 J. Chem. Soc., Dalton Trans., 1999, 1917–1919 The zero-rhodium spin coherence (0Rh) contains no rhodium spin operators so cannot be used to correlate the carbon and rhodium chemical shifts.Magnetization arising from this term is eliminated by the phase cycling used in the standard HMQC experiment. The terms (1Rh), (2Rh) and (3Rh) evolve under the action of the rhodium chemical shifts, WRh1, WRh2 and WRh3 and can be used to correlate carbon and rhodium shifts. Cross peaks resulting from (2Rh), through zero- and double-rhodium quantum coherences, are suppressed by the standard phase cycling whilst (1Rh) gives cross peaks at the expected rhodium chemical shifts through single-rhodium quantum transitions.These cross peaks appear as 1:1:1:1 quartets in the carbon dimension, F2. The magnitude of each component of the quartets is given by eqn. (7) |(1/16)cos2pJd2sinpJd2| (7) and reaches a maximum of ÷3/72 when d2 = (1/pJ)arctg(÷2/2) ª 1/(5J) (8) Thus the maximal magnitude of these one-rhodium spin cross peaks is a factor of ca. 10 weaker than the maximal amplitude of those arising from a terminal Rh(CO) moiety (which, as can be shown, is equal to 1/4). Importantly, the maximum does not occur at the conventional mixing delay of 1/(2J), when the intensity of these cross peaks equals zero, but at a delay d2 that is ca. 2.5 times shorter. The evolution of the three-rhodium spin coherence (3Rh) under the action of the rhodium chemical shifts produces four cross peaks in the 2D spectrum, three single-quantum peaks and one triple-quantum.All the cross peaks appear as 1:3:3:1 quartets centred at WC in the carbon dimension, however in the rhodium dimension the three single-quantum peaks occur at 2WRh1 1 WRh2 1 WRh3, WRh1 2 WRh2 1 WRh3 and WRh1 1 WRh2 2 WRh3 whilst the triple quantum peak occurs at 2WRh1 2 WRh2 2 WRh3. The magnitude of the single- and triple-quantum peaks (the height of the inner quartet lines) is |(3/64)sin3pJd2| (9) and reaches a maximum of 3/64 at the conventional value of d2 = 1/(2J).The structure of, and labelling scheme for, [Rh6(CO)15{P(4- FC6H4)3}] are shown in Fig. 2.9 Figs. 3 and 4 show respectively the inverse detected HMQC 13C-{103Rh} spectra of this cluster recorded under standard conditions using mixing times of 1/(2J), J = 28 Hz, a typical value for 103Rh coupling to a facebridging 13CO, and 1/(5J).10 The only “correct” correlation (i.e. single-spin–single-quantum rhodium) seen in Fig. 3 is the correlation C(1)–Rh(B). All other “correct” correlations are too weak to be observed. All the remaining cross peaks seen are due to three-rhodium spin operators (triple-quantum and Fig. 2 Structure of and labelling scheme for [Rh6(CO)15{P(4-FC6- H4)3}]. single-quantum with respect to rhodium). Two of the threespin –single-quantum correlations are accidentally located at the expected coordinates of the ‘correct’ correlations C(2)– Rh(D) and C(3)–Rh(D) because the cluster contains two equivalent atoms Rh(B) and Rh(C).These “unexpected” correlations are reproduced in the simulated spectrum, Fig. 5. By contrast in Fig. 4, using the unconventional 1/(5J) mixing delay, cross peaks are observed at the ‘correct’ places even though the delay is suYciently far removed from the conventional value that no correlations might have been expected to be observed. These single-quantum (with respect to rhodium) transitions are produced by one-rhodium spin operators and are observed due to their intensities reaching a maximum at 1/(5J) whilst the intensity of correlations due to multiple-rhodium spin transitions is close to zero for this delay.Fortuitously, for rhodium Fig. 3 Experimental inverse detected HMQC 13C-{103Rh} NMR spectrum of [Rh6(CO)15{P(4-FC6H4)3}] (bridging region) obtained with a conventional delay d2 = 1/(2J) = 17.9 ms, J = 28 Hz. The “expected” correlations to the face-bridging carbonyls are weak or entirely absent and correlations due to three-rhodium spin coherences are seen.The projections in the rhodium and carbon dimensions and a 1D 13C spectrum are also shown. Fig. 4 Experimental inverse detected HMQC 13C-{103Rh} NMR spectrum of [Rh6(CO)15{P(4-FC6H4)3}] (face-bridging region) obtained with a non-conventional delay d2 = 1/(5J) = 7.14 ms, J = 28 Hz. Strong correlations to the bridging carbonyls are seen at the “correct” rhodium chemical shifts although the delay d2 used is “unconventional”.J.Chem. Soc., Dalton Trans., 1999, 1917–1919 1919 carbonyl clusters, couplings to the terminal carbonyls (70 Hz) are approximately 2.5 times greater than those to face-bridging carbonyls (28 Hz) allowing the one-rhodium spin correlations to both face-bridging and terminal carbonyls to be observed in a single experiment. These eVects are not limited to [Rh3(m-CO)] fragments but are expected whenever the detector nucleus is coupled to several metal spins that can act as a unit.Clearly care must be taken in the application and interpretation of HMQC experiments to the detection of insensitive nuclei in which the coupling of the detector nucleus to several insensitive spins can occur. In Fig. 5 Simulated HMQC spectrum for C(1)O for the case of a conventional delay d2 = 1/(2J) = 17.9 ms. J[Rh(A)–P] = 140 Hz, J[C(1)– Rh(A)] = 20 Hz, J[C(1)–Rh(B)] = 29 Hz, J[C(1)–Rh(C)] = 31 Hz. particular the spin system must be carefully analysed to determine the optimal value of the mixing delay to ensure that single metal spin transitions are observed.Acknowledgements The authors thank INTAS/RFBR, EPSRC and the University of Liverpool Research Development Fund for funding this work. D. J. S. thanks EPSRC for a studentship and B. T. H. thanks the Leverhulme Foundation for the award of a Research Fellowship. References 1 A. Bax, R. H. GriVey and B. L. Hawkins, J. Magn. Reson., 1983, 55, 301. 2 L. Carlton, Magn. Reson. Chem., 1997, 35, 153. 3 F. Lianza, A. Macchioni, P. Pregosin and H. Ruegger, Inorg. Chem., 1994, 33, 4999. 4 S. J. Berners-Price, R. J. Bowen, P. J. Harvey, P. C. Healy and G. A. Koutsantonis, J. Chem. Soc., Dalton Trans., 1998, 1743. 5 D. Nanz and W. von Philipsborn, J. Magn. Reson., 1991, 92, 560. 6 H. Ruegger and D. Moskau, Magn. Reson. Chem., 1991, 29, S 11. 7 O. W. Sorensen, G. W. Eich, M. H. Levitt, G. Bodenhausen and R. R. Ernst, Prog. Nucl. Magn. Reson. Spectrosc., 1983, 16, 163. 8 R. R. Ernst, G. Bodenhausen and A. Wokaun, Principles of Nuclear Magnetic Resonance in One and Two Dimensions, Oxford University Press, London, 1987. 9 S. P. Tunik, I. S. Podkorytov, B. T. Heaton, J. A. Iggo and J. V. Sampanthar, J. Organomet. Chem., 1998, 550, 222; S. P. Tunik, unpublished work. 10 S. Allevi, S. Bordoni, C. P. Clavering, B. T. Heaton, J. A. Iggo, C. Seregni and L. Garlaschelli, Organometallics, 1989, 8, 385. Communication 9/03513C

ISSN:1477-9226    

DOI:10.1039/a903513c

出版商:RSC    

年代:1999

数据来源: RSC

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DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1999, 1921–1922 1921 A metallated primary arsine; synthesis and structure of [PhAsHLi?2thf ]• Michael A. Beswick, Yvonne G. Lawson, Paul R. Raithby, Jody A. Wood and Dominic S. Wright * Department of Chemistry, Lensfield Road, Cambridge, UK CB2 1EW. E-mail: dsw1000@cus.cam.ac.uk Received 31st March 1999, Accepted 14th May 1999 The lithium primary arsenide [PhAsHLi?2thf]•, obtained by reaction of nBuLi with PhAsH2 in toluene–thf, crystallises in the form of helical polymers; the structure of the right-hand enantiomorph has been obtained.Amido alkali metal compounds have been intensively investigated in the past few decades and the key factors governing their preferred structures, whether existing as uncomplexed oligomers [R2NM]n or Lewis base solvated species [R2NM?xL]n (M = Li–Cs), have been uncovered.1 Although the structural patterns found in related alkali metal complexes containing Group 15 anions of the type R2E2 (E = P, As, Sb, Bi) appear to be similar to those found for the amido complexes, far fewer complexes have been characterised in the solid state.2 Recent attention in this area has focused on the structures adopted by alkali metal complexes containing primary amido and phosphido groups (RNH23 and RPH24 anions).However, to date only a handful of these species have been structurally characterised and no examples have been reported for the heavier Group 15 elements (As–Bi).Our interest in species containing REH2 anions stems from their use as precursors in the synthesis of heterometallic imido and phosphinidene cage complexes,5 such as [{Sb(PCy)3}2Li6?6Me2NH] {obtained from the metallation reaction of [CyPHLi] with Sb(NMe2)3}.6 During recent attempts to extend this work to related systems containing [E(AsR)3]32 (E = As–Bi), we had cause to investigate the structure of the lithiate of PhAsH2. The reaction of PhAsH2 in toluene with nBuLi yields a yellow precipitate at room temperature which was dissolved in a minimum of thf.Storage at –25 8C gives colourless crystalline rods of [PhAsHLi?2thf]• 1 suitable for X-ray crystallography.† The low-temperature [180(2) K] structure of 1‡ shows that the complex is composed of PhAsHLi?2thf monomer units linked together by Li–As–Li bridges into infinite helical polymer chains (Fig. 1). This structural arrangement is similar to that occurring in the primary phosphido lithium complex [(Mes)PHLi?2thf]• (Mes = 2,4,6-Me3C6H2) 7 and in the related secondary phosphido complexes [Ph2PLi?nL]• (nL= Et2O, 2thf) and [Cy2- PLi?thf ]•.8 A noteworthy feature of 1 is the presence exclusively of only the right-hand helical chain in the solid state structure.Although there is little doubt that the other (left-hand) form is also present in samples of 1, the crystallisation of separate enantiomorphs is rare for polymeric alkali metal complexes of this type. Generally, as in the cases of [(Mes)PHLi?2thf]• 7 and [LiNPri 2]•,9 co-crystallisation of both helical elements occurs and, to our knowledge, only for [Cy2PLi?thf]• 8 has a chiral structure been identified in the solid state (however, unlike 1 it did not prove possible to assign the absolute configuration in this case).Although a number of other As–Li bonded complexes have been structurally characterised,10,11 all of these have molecular (rather than polymeric) arrangements in the solid state as a result of high degrees of Lewis base solvation of the Li1 cations and/or the presence of sterically demanding substituents. Complex 1 is the first example of an alkali metal primary arsenide to be structurally characterised.In the chains of 1 the As–Li bonds in the Li–As–Li bridges are of equal lengths within statistical error [mean 2.70 Å] and are in the range of values previously observed for other As–Li bonded complexes (2.46–2.76 Å).10 The Li1 cations adopt a distorted tetrahedral geometry, with the most marked distortion occurring in the As–Li–As angle [116(1)8].The As centres also exhibit a large distortion in the chain Li–As–Li angle [143.4(5)8; cf. 103.3(7) and 91.0(7)8 for the C–As–Li angles]. The pattern of expanded Li–As–Li and As–Li–As angles in 1 is similar to that observed in the polymeric phosphides [(Mes)PHLi?2thf]• [Li–P–Li 130.0(2), P–Li–P 122.3(2)8]7 and [Ph2PLi?2thf]• [Li–P–Li 135.0(5), P–Li–P 123.1(8)8],8 in which bis solvation of Li1 by thf is also present.However, the almost linear Li–P–Li bridge (1778) observed in the structure of [H2PLi?2thf]• 12 suggests that (where the Lewis base and extent of solvation are comparable) the steric bulk of the organic substituents has the major influence over the P geometries. On this basis, the large Li–As–Li angle in 1 is probably steric in origin rather than providing evidence for any fundamental change in hybridisation of the As centre.In conclusion, the first alkali metal primary arsenide [PhAsHLi?2thf]• exhibits an interesting helical structure in the solid state. The structure of this species provides a rare insight into the nature of aggregation for an arsenide with very low steric demands, and suggests a link with the structural patterns found for related polymeric phosphorus complexes. Fig. 1 Polymeric structure of 1. H atoms and the disorder of the thf molecules have been omitted for clarity. Key bond lengths (Å) and angles (8): As(1)–Li(1) 2.70(3), As(1)–Li(1A) 2.69(3), As(1)–C(5) 1.95(2), Li–O (average) 1.94; Li(1)–As(1)–Li(1A) 143.4(5), As(1A)– Li(1)–As(1) 116(1), C(5)–As(1)–Li(1) 103.3(7), C(5)–As(1)–Li(1A) 91.0(7).1922 J. Chem.Soc., Dalton Trans., 1999, 1921–1922 Acknowledgements We thank the EPSRC (Y. G. L., J. A. W.) and the Leverhulme Trust (M. A. B.) for financial support. Notes and references † Synthesis of 1. PhAsH2 was prepared by the reduction of phenylarsonic acid with Hg–Zn amalgam and HCl.Using half the quantity of Hg–Zn amalgam as that suggested in the literature 13 leads to more reproducible yields (ca. 40–50%). 1H NMR (125 8C, d6-benzene, 250 MHz): d 7.34 (2H, m, o-C–H), 7.07 (3H, m, p- and m-C–H), 3.52 (2H, s, AsH2). To a solution of PhAsH2 (0.91 ml, 8.0 mmol) in toluene (20 ml) at 278 8C was added nBuLi (5.3 ml, 1.5 mol dm23 in hexanes, 8.0 mmol). A yellow precipitate was formed which was dissolved by addition of thf (10 ml).The solution was concentrated under vacuum until precipitation commenced. The solid was dissolved by heating and storage at 225 8C gave colourless crystalline rods (1.48 g, 61% on the basis of the empirical formula [PhAsHLi?2thf ]). Isolation of 1 under vacuum (1021 atm) leads to loss of some of the coordinated thf (ca. 0.75 equiv.). 1H NMR (125 8C, d6-benzene, 250 MHz): d 7.29 (2H, d, o-C–H), 7.07 (2H, t, m-C–H), 6.93 (1H, t, p-C–H), 3.62 (5H, m, thf CH2O) and 1.36 (5H, m, thf CH2), 2.07 (1H, s, As–H) {Found: C, 52.5; H 6.8.Calc. for [PhAsHLi?xthf] (x = 1.25): C, 52.8; H, 6.4%}. ‡ Crystal data for 1. C14H22AsLiO2, M = 304.18, monoclinic, space group P21, Z = 2, a = 8.150(2), b = 8.564(2), c = 11.278(2) Å, b = 101.87(3)8, V = 770.3(3) Å3, m(Mo-Ka) = 1.311 mm21, T = 180(2) K. Data were collected on an Siemens-Stoe AED diVractometer. Of a total of 2438 reflections collected, 2004 were independent (Rint = 0.051). Final R1 = 0.104 [I>2s(I)] and wR2 = 0.244 (all data). The high residual R value is the result of poor crystal quality (resulting from very high air-sensitivity) and from the extensive disorder in both of the thf molecules, which could only be modelled approximately.C(12)–C(14) and O(1) of one thf molecule, and C(9) and C(10) of the other were modelled over two sites with ca. 50 : 50 occupancy. CCDC reference number 186/1464. See http:/www.rsc.org/suppdata/dt/1999/1921/ for crystallographic files in .cif format. 1 K. Gregory, P. v. R. Schleyer and R. Snaith, Adv. Inorg. Chem., 1994, 37, 47; R. E. Mulvey, Chem. Rev., 1991, 20, 167. 2 F. Pauer and P. P. Power, Lithium Chemistry, A Theoretical and Experimental Overview, eds. A.-M. Sapse and P. v. R. Schleyer, Wiley, New York, 1995, ch. 9, pp. 295; M. A. Beswick and D. S. Wright, Comprehensive Organometallic Chemistry II, eds. E. W. Abel, F. G. A. Stone and G. Wilkinson, Pergamon, 1995, vol. 1, ch. 1, p. 1. 3 R. E. Mulvey, Chem. Soc.Rev., 1998, 27, 339. 4 J. D. Smith, Angew. Chem., 1998, 110, 2181; Angew. Chem., Int. Ed., 1998, 37, 2071. 5 M. A. Beswick and D. S. Wright, Coord. Chem. Rev., 1998, 176, 373. 6 M. A. Beswick, J. M. Goodman, C. N. Harmer, A. D. Hopkins, M. A. Paver, P. R. Raithby, A. E. H. Wheatley and D. S. Wright, Chem. Commun., 1997, 1897. 7 E.-M. Hey and F. Weller, J. Chem. Soc., Chem. Commun., 1992, 220. 8 R. A. Bartlett, M. M. Olmstead and P. P. Power, Inorg. Chem., 1986, 25, 1243. 9 N. D. R. Barnett, R. E. Mulvey, W. Clegg and P. A. O’Neil, J. Am. Chem. Soc., 1991, 113, 8187. 10 For As–Li bonded complexes, see (a) G. Becker and C. Whitthauer, Z. Anorg. Allg. Chem., 1982, 492, 28; (b) A. M. Arif, R. A. Jones and K. B. Kidd, J. Chem. Soc., Chem. Commun., 1986, 1440; (c) R. A. Barlett, H. V. R. Dias, H. Hope, B. D. Murray, M. M. Olmstead and P. P. Power, J. Am. Chem. Soc., 1986, 108, 6921; (d ) M. Driess and H. Pritzkow, Angew. Chem., 1992, 104, 350; Angew. Chem., Int. Ed. Engl., 1992, 31, 316; (e) L. Zsolnai, G. Huttner and M. Driess, Angew. Chem., 1993, 105, 1549; Angew. Chem., Int. Ed. Engl., 1993, 32, 1439; ( f ) L. J. Jones, A. T. McPhail and R. L. Wells, J. Coord. Chem., 1995, 34, 119; (g) M. Driess, H. Pritzkow, S. Martin, S. Rell, D. Fenske and G. Baum, Angew. Chem., 1996, 35, 986; Angew. Chem., Int. Ed. Engl., 1996, 108, 1064; (h) M. Driess, K. Merz, H. Pritzkow and R. Janoschek, Angew. Chem., 1996, 108, 2688; Angew. Chem., Int. Ed. Engl., 1996, 35, 2507. 11 For other As–M bonded complexes, see A. Belforte, F. Calderazzo, A. Morvillo, G. Pelizzi and D. Vitali, Inorg. Chem., 1984, 23, 1504. 12 R. A. Jones, S. U. Koschmieder and C. M. Nunn, Inorg. Chem., 1987, 26, 3610; G. Becker, H. M. Hartmann and W. Z. Schwarz, Z. Anorg. Allg. Chem., 1989, 577, 9. 13 C. S. Palmer and R. Adams, J. Am. Chem. Soc., 1922, 44, 1356. Communication 9/02590A

ISSN:1477-9226    

DOI:10.1039/a902590a

出版商:RSC    

年代:1999

数据来源: RSC

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DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1999, 1923–1924 1923 The first copper(I) complex containing a cyanato ligand. Synthesis and structural characterization of [Cu(pyz)(Ï-NCO)]n (pyz 5 pyrazine) Mohamed A. S. Goher*a and Franz A. Mautner b a Department of Chemistry, Faculty of Science, Kuwait University, PO Box 5969 Safat, 13060 Kuwait. E-mail: Goher@kuco1.kuniv.edu.kw b Institut fur Physikalische und Theoretische Chemie, Technische Universitat Graz, A-8010 Graz, Austria Received 27th April 1999, Accepted 13th May 1999 The first copper(I) complex containing a cyanato ligand, [Cu(pyz)(Ï-1,1-NCO)]n (pyz 5 pyrazine), has been synthesized and characterized.The cyanate ion, NCO2 is known to co-ordinate to metals in both terminal and bridging modes. As a bridging ligand it can link a pair of metal centres in end-on (m-1,1-OCN, m-1,1-NCO) or end-to-end (m-1,3-NCO) bonded fashion (see below). Bi- and poly-nuclear copper cyanate systems are of considerable interest due to the broad range of their structural and magnetic properties,1 and as synthetic models for the natural copper proteins and their derivatives.2 However, the vast majority of studies have focused on copper(II) cyanato complexes.1 On the other hand, while a lot of eVort has been devoted to the study of copper(I) complexes of other pseudo-halides,3 to the best of our knowledge, there is as yet no known example of a copper(I) complex containing a cyanato ligand of any type, terminal or bridging. Moreover, the preparation of the copper(I) cyanate salt itself has been described only once in the early literature, more than four decades ago.4 We report here the first copper(I) complex [Cu(pyz)(m-1,1-NCO)]n (pyz = pyrazine) containing an end-on cyanato bridge bound by the nitrogen atom.The reaction between copper(I) cyanate and pyrazine aVorded a deep red 1 : 1 complex. That copper(I) cyanate has never been used in co-ordination chemistry may be due to its insolubility and/or instability in common solvents.Signifi- cantly, a concentrated solution of sodium or potassium cyanate can be used to dissolve polymeric CuI(NCO), giving rise to a solution suYciently stable to air oxidation. Using this procedure allowed us to grow single crystals of [Cu(NCO)L]n †‡ by adding pyrazine in EtOH to the mixture, thus avoiding having to isolate any CuII impurities. This synthesis was found to be reproducible. The complex is suYciently stable against air-oxidation when well-dried.It is insoluble in many common solvents, e.g. water, methanol, ethanol, acetone, benzene, etc., but soluble in DMF or DMSO giving rise to non-conducting solutions. In deuteriated DMF or DMSO, however, the complex is not suYciently soluble to allow NMR measurements. The IR spectrum of [Cu(pyz)(m-1,1-NCO)]n shows characteristic asymmetric and symmetric NCO stretching vibrations at 2245, 2225 and 1330, 1316 cm21, respectively.These values are at substantially higher frequencies than the free-ion values,5 and on the basis of earlier studies involving cyanate complexes,6 this is consistent with N-bonding rather than O-bonding. In earlier N C O M M O C N M M N M C O M m–1, 1–NCO m–1, 1–OCN m–1, 3–NCO studies,7 it has been observed that dNCO is split by at most a few wavenumbers when the cyanate ion is terminal, while it typically shows a splitting of 30–50 cm21 when the ion is bridging. The appearance of two peaks at 628 and 597 cm21 in the spectrum of the present complex is, therefore, consistent with m-1,1- cyanato bridge bound by the nitrogen atom.Additionally, the spectrum shows, as the most significant feature, a single band at 448 cm21 (pyrazine) which corresponds to a symmetrical bridging pyrazine with a relatively weak interaction with the metal centre, its value being in the low frequency range for bridging pyrazines.8 A structure determination has shown the complex [Cu(pyz)- (NCO)]n to possess a polymeric structure.Each distorted tetrahedrally bonded copper atom links two nitrogen atoms from two pyrazine ligands and the other two sites are occupied by nitrogen atoms from two cyanato ligands (Fig. 1). Thus the cyanate ion behaves as a m-1,1 bridge bound via its nitrogen atom as inferred from the IR spectroscopy. The Cu(I)–m-1,1- bridging cyanato sublattice [located at the mirror planes] forms chains along the a-axis. These chains are further connected via m-N,N9 bridging pyrazine molecules to form a 3-D network (Fig. 2). This structure diVers from those of the corresponding polymeric [Cu(pyz)X]n (X = NCS or N3) complexes.9 In the Fig. 1 Co-ordination diagram (40% probability ellipsoids) with atom labelling scheme of polymeric [Cu(pyz)(NCO)]n. The cyanato group acts as a m-N,N and the pyrazine molecules as m-N,N9 bridging ligands. Hydrogen atoms are omitted. Selected bond lengths (Å) and angles (8): Cu(1) ? ? ? Cu(1D)[Cu(1C)] 3.333(1), Cu(1) ? ? ? Cu(1A) 6.899(2), Cu(1)– N(1) 2.101(3), Cu(1)–N(1C) 2.035(3), Cu(1)–N(2)[N(2B)] 2.056(2); N(1)–Cu(1)–N(1C) 123.31(11), N(1)–Cu(1)–N(2)[N(2B)] 104.95(8), N(1C)–Cu(1)–N(2)[N(2B)] 104.87(8), N(2)–Cu(1)–N(2B) 114.37(13), Cu(1C) ? ? ? Cu(1) ? ? ? Cu(1D) 124.65(4); symmetry codes: A 2x, 1 2 y, 1 2 z; B x, 1/2 2 y, z; C 21/2 1 x, y, 3/2 2 z; D 1/2 1 x, y, 3/2 2 z; E 2x, 21/2 1 y, 1 2 z.1924 J.Chem. Soc., Dalton Trans., 1999, 1923–1924 Fig. 2 Packing views of [Cu(pyz)NCO]n along the main axes of the unit cell: (a) along the a-axis; (b) along the b-axis; (c) along the c-axis. Atoms are represented as spheres with arbitrary size and hydrogen atoms are omitted for clarity. The Cu(I)–m-N,N bridging cyanato sublattice [located at the mirror planes] forms chains along the a-axis. These chains are further connected via the m-N,N9 bridging pyrazine molecules to form a 3-D network. latter complexes the m-1,3 bridging mode of the thiocyanate or azido ligand, along with the m-N,N9 pyrazine resulted in two perpendicular 1-D zigzag chains crossing at the copper centres giving rise to a 2-D network structure.That the cyanate ion behaves as a m-1,1 bridge means that a 2-D structure would not be formed and that therefore the 3-D network is the expected structure in this case. The structural motif observed in the complex [Cu(pyz)- (m-NCO)]n represents both the first example of copper(I) complexes containing a cyanato ligand and also a new 3-D network. The preparation of this compound illustrates a new and potentially versatile approach to the construction of uncharged inorganic co-ordination networks and we are currently pursuing this methodology towards the synthesis of such new materials.Acknowledgements Financial support by the Kuwait University Administration (Project SC 097) and The Department of Chemistry General Facility Projects (Analab) are gratefully acknowledged.The authors thank Professor Kratky and Dr Belaj (Graz University) for the use of the four-circle diVractometer. Notes and references † Preparation of [Cu(pyz)(m-1,1-NCO)]n. To an aqueous suspension of Cu(NCO) (2 mmol) a saturated aqueous solution of KNCO was added until a clear solution was obtained and any impurities filtered oV. Pyrazine (4 mmol) dissolved in EtOH (ca. 10 ml) was then added and the final mixture allowed to stand over several days to yield large deep red crystals of the complex, along with an unknown pale red powder.Yield, ca. 20% [Found (calc.): C, 32.1 (32.34); H, 2.3 (2.17); N, 22.3 (22.62); Cu, 34.6 (34.23%)]. The complex was stored over ca. three months in a normal vial at room temperature without any sign of oxidation. IR (KBr disc): n1 (nCN) 2245s, 2225vs; n3 (nCO) 1330m, 1316m; dNCO 628m, 597m; n16b (pyrazine) 448 cm21. Electronic spectrum (solid Nujol mull): 420 (br), 500 nm (CuIÆLCT). ‡ Crystal data for [Cu(pyz)(NCO)]n: C5H4CuN3O, M = 185.65, orthorhombic, space group Pnma (no. 62), a = 5.904(2), b = 11.593(3), c = 9.038(4) Å, U = 618.6(4) Å3, Z = 4, F(000) = 368, Dc = 1.993 g cm23, m(Mo-Ka) = 3.447 mm21, T = 295(2) K. 573 unique reflections (Rint = 0.0308) were collected. At final convergence R1 [520 data with I > 2s(I)] = 0.0261, wR2 (all 573 data) = 0.0655 for 53 parameters. CCDC reference number 186/1462. See http://www.rsc.org/suppdata/ dt/1999/1923/ for crystallographic files in .cif format. 1 See for example, T. Otieno, S. J. Retting, R. C. Thompson and J. Trotter, Inorg. Chem., 1993, 32, 4384. 2 See for example, V. McKee, M. Zvagulis, J. V. Dagdigian, M. G. Patch and C. A. Reed, J. Am. Chem. Soc., 1984, 106, 4765. 3 See for example, (a) S.-M. Kuang, Z.-Z. Zhang, Q.-G. Wang and T. C. W. Mak, J. Chem. Soc., Dalton Trans., 1997, 4477; (b) M. A. S. Goher, R.-J. Wang and T. C. W. Mak, J. Coord. Chem., 1996, 38, 151 and refs. therein. 4 E. Soederbarcy, Acta Chem. Scand., 1975, 11, 1622. 5 A. Maki and J. C. Decius, J. Chem. Phys., 1959, 31, 772. 6 (a) D. Forster and D. M. L. Goodgame, J. Chem. Soc., 1965, 1286; (b) A. Sabatini and I. Bertini, Inorg. Chem., 1965, 4, 959. 7 (a) R. A. Baily, S. L. Kozak, T. W. Michelsen and W. N. Mills, Coord. Chem. Rev., 1971, 6, 407; (b) J. Nelson and S. M. Nelson, J. Chem. Soc. A, 1969, 1597. 8 T. Otieno, S. J. Retting, R. C. Thompson and J. Trottor, Can. J. Chem., 1990, 68, 1901. 9 M. A. S. Goher and F. A. Mautner, Polyhedron, in the press. Communication 9/03332G

ISSN:1477-9226    

DOI:10.1039/a903332g

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1999, 1925–1927 1925 A new class of macrocycle capable of binding exogenous metals: synthesis, structure, magnetic and electrochemical properties of a Cu(II) trinuclear complex based upon 1,4,8,11-tetraazacyclotetradecane- 2,3-dione [exoO2]cyclam † Leroy Cronin, Andrew R. Mount, Simon Parsons and Neil Robertson* Department of Chemistry, University of Edinburgh, King’s Buildings, West Mains Road, Edinburgh, UK EH9 3JJ Received 8th April 1999, Accepted 12th May 1999 Synthesis of the new macrocycle 1,4,8,11-tetraazacyclotetradecane- 2,3-dione [exoO2]cyclam and its complexation with copper(II) salts results in the formation of a complex comprising two macrocycles each coordinated to a copper(II) ion via the tetraaza groups and linked together through a copper(II) ion coordinated to the exo-cis oxygen donors of each macrocycle.The macrocycle known as cyclam and its derivatives have been the subject of an immense amount of interest to coordination chemists due to their ability to complex many of the transition metals.1–3 Cyclam-based complexes have been used in a wide range of studies from bioinorganic 1 systems to catalytic 2 systems and as sensors;3 a search on the Cambridge Crystallographic Data Base 4 revealed over 500 structures of cyclam and cyclam derivatives complexed with transition metals.In this communication the first example in a new class of macrocycle based on cyclam is presented.The new macrocycle, in addition to its ability to coordinate metals via the N4 donor set of the ring, can also bind exogenous metal ions via the exo-cis oxygen atoms which form part of the oxamide groups present in the macrocycle. Oxamide-based ligands such as H2oxpn have also been of great interest due to their ability to form polymetallic 5–7 and heterobimetallic 8–13 systems. Bis complexes containing copper/copper 5,7 and copper/nickel 8,12 have been found to be very strongly antiferromagnetically coupled through the oximidate bridges.However, bis complexes containing copper/ gadolinium are ferromagnetically coupled.10,11 These types of features have been exploited in the design of model magnetic systems and the formation of extended structures which utilise the fact the oxpn can adopt either a cis or trans conformation.14 Although this flexibility can give rise to a rich variety of complexes and extended structures, it allows much less control over the final type of complex obtained.14 We have extended this approach by producing a macrocyclic analogue of H2oxpn, in which the exo-cis conformation of the oxygen donors is enforced (see Scheme 1).Such a system allows the controlled formation of complexes via the stepwise complexation of the macrocyclic and the exo donors. Therefore, this approach could be used to synthesise new ion sensors, heterobimetallic systems and model magnetic systems in a more controlled fashion, taking advantage of the macrocyclic eVect.The new macrocycle, [exoO2]cyclam 1 was synthesised very simply ‡ in one step by the condensation reaction of dimethyloxalate and the tetraamine N,N9-bis(3-aminopropyl)ethylenediamine in refluxing ethanol. Although initially the synthesis was hampered by formation of higher [2 1 2] and [3 1 3] adducts and polymer, the macrocycle can be synthesised in over 60% yield if the reaction is performed at high dilution over a period of 32 h.The trinuclear complex can be synthesised § by † Supplementary data available: Magnetic and electrochemical data. Available from BLDSC (No. SUP 57551, 5 pp.). See Instructions for Authors, 1999, Issue 1 (http://www.rsc.org/dalton). the step-wise addition of copper(II) perchlorate in methanol to a solution of the macrocycle in methanol–water (50 : 50 v/v). (CAUTION: Perchlorate salts are potentially explosive and should therefore be handled with appropriate care.) It has been shown that the synthesis of the trimetallic complex 3 proceeds after the formation of a monomeric complex of the macrocycle with copper (2).Complex 2 § has been independently isolated presumably with the copper bound in the N4 coordination environment of the macrocyclic ring. Dark red single crystals of 3 suitable for single crystal X-ray diVraction studies were obtained in 53% yield by leaving the reaction mixture standing for 4 weeks. The crystal structure ¶ of the complex (Fig. 1) shows a central copper(II) ion ligated by the two exo oxygen groups of two [exoO2]cyclam units. These cyclam units are also coordinated to copper(II) ions. The central copper is located on a centre of symmetry and is ligated by an O4 donor set in a square planar coordination environment. The oxygen atoms of the two associated perchlorate anions are weakly interacting with the central the copper(II) and are positioned in the remaining apical sites above and below the copper(II) ion at a distance of 2.827(5) Å.The copper(II) ions ligated by the macrocycles have a square pyramidal coordination geometry with a N4 donor set which forms the square base of the pyramid. The apex of the pyramid is completed by a water molecule, however there is also a perchlorate (this is present in the adjacent asymmetric unit interacting with the central copper(II) of another trimer unit) positioned in the sixth vacant coordination site with a weak copper–oxygen interaction of 2.983(6) Å.Examination of the coordination environment of the three coppers reveals that they are all co-planar, with maximum r.m.s. deviation of 0.072, in a plane consisting of Cu(1), O(1), O(2), C(1), C(2), N(1), N(2) and Cu(2). Furthermore, the sum of the angles around C(1), C(2), N(1) and N(2) is 360.0(3)8 indicating Scheme 1 2ClO4- 2+ cyclam [Cu{Cu[ exoO2]cyclam}2][ClO4]2 3 {Cu[ exoO2]cyclam} 2 [ exoO2]cyclam 1 O O N N N N H H H H O O N N N N H H Cu O O N N N N H H Cu Cu Cu N N N H N H O O O O N N NH2 NH2 H H N N N N H H H H H2 oxpn1926 J.Chem. Soc., Dalton Trans., 1999, 1925–1927 that these atoms are trigonal planar. This implies that Cu(1)– O–C–N–Cu(2) is a delocalised unit as a result of the conjugation of electrons from the oxygen atoms and the lone pairs on the nitrogen atoms. This is further supported by the average O–C and C–N bond lengths [1.282(5) and 1.286(5) Å] which are shorter than expected for single O–C or C–N bonds [ca. 1.481 and 1.462 Å]. The magnetic properties of the trimer were investigated over the temperature range 1.8–300 K using a dried powder sample of 3 on a Quantum Design SQUID magnetometer. The data were found to be modelled accurately using an expression derived by Kahn et al.5b which was used to fit data from an analogous copper(II)-based trimer. The best fit of the data gave an exchange parameter, J1 = 2364.2 cm21, for the antiferromagnetic exchange between adjacent copper(II) ions whereas the exchange parameter for the coupling between terminal copper(II) ions (J2) was set to zero.This is consistent with Kahn’s analysis 5b and an intermolecular exchange parameter was determined, J3 = 20.79 cm21. This may be attributed to the weak interaction of the copper(II) ions of the trimer through the perchlorate anions. The cyclic voltammogram obtained for a 1 mmol dm23 solution of complex 3 at a platinum electrode in acetonitrile (with [NBu4]1PF6 2 electrolyte, 0.1 M) shows two peaks at 21.2 and 20.67 V with respect to the ferrocene–ferrocenium couple.The peak at 21.2 V is an electrochemically irreversible reduction which then gives rise to an associated oxidation peak at 20.67 V. This peak has an approximately Lorentzian profile indicative of an electrode stripping process. Thus reduction of the trimer appears to result in the adsorption of an electroactive product onto the electrode which can be removed by reoxidation. Integration of the time vs.current curves associated with these peaks shows that twice as much current is passed in the reduction process than the oxidation process at all sweep rates. Preliminary electrochemical studies on 2 suggest reduction of the macrocyclic copper ions in 3 is unlikely in the range of study. Sweeping to higher potentials reveals no further signifi- cant oxidation peaks other than a very weak feature at 20.35 V which can be attributed to a very small amount of free ligandbased oxidation.The single reduction peak observed for 3 at all sweep rates between 10 and 200 mV s21, plus the electroinactivity of 2, combined with the charge passed during reduction suggest that only the central copper ion is reduced and deposited on the electrode in a two-electron process. In support it should be noted that the reoxidation potential is similar to that found for the oxidation of a copper electrode under the same conditions.This would suggest that the reoxidation generates a soluble copper(I) species and further studies are underway to fully characterise these processes. There are now several examples of macrocyclic ligands which incorporate another ligand/binding unit for either a metal or organic unit but these are all connected via either aromatic spacer or aliphatic linking groups.15 The system we describe in this paper however, provides the opportunity to allow direct Fig. 1 Molecular structure of 3.The two perchlorate anions are omitted for clarity. Selected bond lengths (Å) and angles (8): Cu(1)–O(1) 1.933(3), Cu(1)–O(2) 1.933(3), Cu(2)–N(1) 1.961(3), Cu(2)–N(2) 1.957(3), Cu(2)–N(3) 2.007(4), Cu(2)–N(4) 2.007(4), Cu(2)–O(3) 2.545(4); O(2)–Cu(1)–O(1) 86.24(11), O(2)9–Cu(1)–O(1) 93.76, O(1)9– Cu(1)–O(1) 180.0, N(2)–Cu(2)–N(1) 84.28(13), N(2)–Cu(2)–N(3) 95.06(14), N(1)–Cu(2)–N(3) 164.59(17), N(2)–Cu(2)–N(4) 175.02(16), N(1)–Cu(2)–N(4) 96.62(15). communication from the metal ion bound in the macrocycle to the metal bound to the exo oxygen groups, because the groups can form a planar delocalised system.Further studies will exploit this feature by examining the possibility of using 1 as a basis to produce multicentre redox species and model magnetic systems. Furthermore, as a result of the electrochemical studies, further work will also examine the possibility of using 1 in the electrowinning of copper(II) and other redox active metal ions from solution by deposition onto an electrode surface.Acknowledgements We thank the Royal Society of Edinburgh/BP for a research Fellowship (N. R.) and the Leverhulme trust for financial support. We thank Andrew Harrison, University of Edinburgh for help with the SQUID measurements. Notes and references ‡ The new macrocycle, [exoO2]cyclam 1 was synthesised by adding solutions of dimethyloxalate (2.12 g, 18.1 mmol) in ethanol (350 cm3) and the tetraamine, N,N9-bis(3-aminopropyl)ethylenediamine (3.14 g, 18.0 mmol) in ethanol (350 cm3) to a solution of refluxing ethanol (50 cm3) dropwise via a peristaltic pump over a period of 32 h. After 32 h addition was complete and the solution was cooled and filtered.The solution was then reduced to dryness and the white product taken up into hot propan-2-ol (100 cm3). Filtering the hot solution and then reduction to dryness gave the desired product in 64% yield (2.5 g, 11.0 mmol), mp 141–142 8C (Found: C, 51.53; H, 8.85; N, 23.87.Calc. for C10H20N4O2?0.25H2O: C, 51.62; H, 8.88; N, 24.07%). 1H NMR (CD3OD, 200 MHz): d 1.99 (q, 4 H, 3JHH = 7.7, OCNHCH2CH2), 2.99 (t, 4H, 3JHH = 7.1, OCNHCH2CH2CH2), 3.06 (t, 4H, 3JHH = 7.5 Hz, OCNHCH2), 3.13 (s, 4H, NHCH2CH2NH). IR (cm21, KBr): 3300s, 3550–3000w, 2929m, 2875m, 1728w, 1655m, 1652s, 1521m, 1466m, 1438m, 1364w, 1288w, 1112m, 1073w, 767m, 569w. Positive ion mass spectrum (nitrobenzyl alcohol matrix): m/z 229 (MH1). § Crystals of [Cu{Cu[exoO2]cyclam}2][ClO4]2 3 were obtained by the slow addition of copper(II) perchlorate hexahydrate (0.081 g, 0.22 mmol) in methanol (ca. 10 cm3) over a peroid of 2 h to a stirred solution of 1 (0.033 g, 0.15 mmol) in water (2 cm3).(CAUTION: Perchlorate salts are potentially explosive.) After the addition was complete the colour of the solution had changed to dark red and on standing for 4 weeks yielded deep red crystals (0.065 g, 0.077 mmol, 53% yield), mp >160 8C (decomp.) (Found: C, 27.72; H, 4.49; N, 12.81.Calc. for C20H36N8O12Cu3Cl2?2H2O: C, 27.36; H, 4.58; N, 12.72%). IR (cm21, KBr): 3630–3000m, 3434m, 3251m, 2937m, 2872m, 1625s (br), 1433s, 1398w, 1352m, 1342m, 1316m, 1259w, 1174m, 1092s (br), 1015m, 991w, 939w, 886w, 816w, 626m, 541w, 502w. Positive ion electrospray mass spectrum (from methanol and water): m/z 742.5 (M1 2 ClO4 2). Compound 3 can also be synthesised by adding compound 2 (0.05 g, 0.17 mmol) to copper(II) perchlorate hexahydrate (0.032 g, 0.089 mmol). Compound 2 was synthesised by the addition of potassium hydroxide (0.29 g, 5.2 mmol) in water (0.5 cm3) to solution of [exoO2]- cyclam 1 (0.59 g, 2.58) in methanol (200 cm3); this was followed by the addition of hexane (40 cm3) and the slow addition of copper(II) chloride (0.34 g, 2.58 mmol) in methanol (50 cm3) to the solution over a period of 3 h.After this time, a purple precipitate had formed and was isolated by filtration. The filtrate was dried in vacuo to yield the product as a fine purple powder (0.25 g, 0.859 mmol, 33.3% yield), mp >160 8C (decomp.) (Found: C, 41.19; H, 6.73; N, 18.92.Calc. for C10H18N4O2Cu: C, 41.49; H, 6.27; N, 19.35%). IR (cm21, KBr): 3630– 3000m, 3179m, 3099m, 2894m, 2860m, 1606s, 1576s, 1478w, 1446m, 1384m, 1358m, 1341w, 1324m, 1175w, 1132w, 1109w, 1096w, 1079m, 1060w, 1028m, 1006w, 936w, 900w, 877m, 794w. Positive ion electrospray mass spectrum (from methanol and water): m/z 291 (MH1). ¶ Crystal data for 3: C20H40N8O14Cl2Cu3?2H2O, red lath, crystal dimensions 0.39 × 0.19 × 0.10 mm, monoclinic, P21/n, a = 8.6599(17), b = 16.907(4), c = 12.784(3) Å, b = 106.21(2)8, U = 1797.4(10) Å3, m = 3.986 mm21, Z = 2.Data were collected at 220 K on a Stoe Stadi-4B diVractometer using graphite-monochromated Cu-Ka radiation, l = 1.54184 Å. A total of 3746 reflections were collected in the range 8.9 £ 2q £ 140.148 and the 3171 independent reflections were used in the structural analysis after an absorption correction was applied on the basis of y-scans (Tmax = 0.926, Tmin = 0.653).The structure was solved using direct methods with SIR9216 and refinement on F 2 using SHELXL-97.16 The structure converged satisfactorily to R1 = 0.066 and wR2 = 0.1532 on all data and R1 = 0.055 and wR2 = 0.146 for the observed data [for 2553F > 4s(F)]. Goodness-of-fit = 1.048 on all F 2J. Chem. Soc., Dalton Trans., 1999, 1925–1927 1927 (3171); 235 parameters; 10 restraints; residuals in the final map = 10.794/20.761 e Å23.CCDC reference number 186/1461. See http:// www.rsc.org/suppdata/dt/1999/1925/ for crystallographic files in .cif format. 1 M. Borel, M. F. Moreau, A. Veyre and J. C. Madelmont, J. Labelled Compd. Radiopharm., 1998, 41, 755; S. Carotti, A. Guerri, T. Mazzei, L. Messori, E. Mini and P. Orioli, Inorg. Chim. Acta, 1998, 281, 90; S. R. Zhu, W. D. Chen, H. K. Lin, X. C. Yin, F. P. Kou, M. R. Lin and Y.T. Chen, Polyhedron, 1997, 16, 3285. 2 G. Pozzi, M. Cavazzini, S. Quici and S. Fontana, Tetrahedron Lett., 1997, 38, 7605; R. W. Hay, J. A. Crayston, T. J. Cromie, P. Lightfoot and D. C. L. deAlwis, Polyhedron, 1997, 16, 20; 3557. 3 Y. Katayama, S. Takahashi and M. Maeda, Anal. Chim. Acta, 1998, 365, 159. 4 D. A. Fletcher, R. F. McMeeking and D. J. Parkin, J. Chem. Inf. Comput. Sci., 1996, 36, 746. 5 (a) Y. Journaux, J. Sletten and O. Kahn, Inorg. Chem., 1985, 24, 4063; (b) Y.Journaux, J. Sletten and O. Kahn, Inorg. Chem., 1986, 25, 439; (c) R. Viet, J.-J. Girerd, O. Kahn, F. Robert and Y. Jeannin, Inorg. Chem., 1986, 25, 4175. 6 F. Lloret, J. Sletten, R. Ruiz, M. Julve and J. Faus, Inorg. Chem., 1992, 31, 3778. 7 F. Lloret, M. Julve, J. Faus, R. Ruiz, I. Castro, M. Mollar and M. Philoche-Levisalles, Inorg. Chem., 1992, 31, 785; F. Lloret, M. Julve, J. A. Real, J. Faus, R. Ruiz, I. Castro, M. Mollar and C. Bois, Inorg. Chem., 1992, 31, 2956. 8 A. Escuer, R. Vicente, J. Ribas, R. Costa and X. Solans, Inorg. Chem., 1992, 31, 2627. 9 C. Mathonière, O. Kahn, J.-D. Daran, H. Hilbig and F. H. Köhler, Inorg. Chem., 1993, 32, 4057. 10 C. Benelli, A. C. Fabretti and A. Giusti, J. Chem. Soc., Dalton Trans., 1993, 409. 11 J. Sanz, R. Ruiz, A. Gleizes, F. Lloret, J. Faus, M. Julve, J. Borrás- Almenar and Y. Journaux, Inorg. Chem., 1996, 35, 7384. 12 J. Ribas, C. Diaz, R. Costa, J. Tercero, X. Solans, M. Font-Bardía and H. Stoeckli-Evans, Inorg. Chem., 1998, 37, 233. 13 T. Sanada, T. Suzuki and S. Kaizaki, J. Chem. Soc., Dalton. Trans., 1998, 959. 14 J. Sanz, B. Cervera, R. Ruiz, C. Bois, J. Faus, F. Lloret and M. Julve, J. Chem. Soc., Dalton Trans., 1996, 1359; Z.-N. Chen, H.-X. Zhang, K.-B. Yu, K.-C. Zheng, H. Cai and B.-S. Kang, J. Chem. Soc., Dalton Trans., 1998, 1133. 15 N. D. Lowe and C. D. Garner, J. Chem. Soc., Dalton Trans., 1993, 3333. 16 L. J. Farrugia, WinGX, Windows Program for Crystal Structure Analysis, University of Glasgow, UK, 1998; P. T. Beurskens, G. Beurskens, W. P. Bosman, R. de Gelder, S. Garcia-Granda, R. O. Gould, R. Israel and J. M. M. Smits, DIRDIF 96 Program System, Technical Report of the Crystallographic Laboratory, University of Nijmegen, The Netherlands, 1992; G. M. Sheldrick, SHELXL-97, Program for Crystal Structure Refinement (Release 97-2), University of Göttingen, Germany, 1997; A. L. Spek, Acta Crystallogr., Sect. A, 1990, 46, C34. Communication 9/02792K

ISSN:1477-9226    

DOI:10.1039/a902792k

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1999, 1929–1935 1929 Phenyl tris(3-tert-butylpyrazolyl)borato complexes of lithium and thallium, [PhTpBut]M (M 5 Li, Tl): a novel structure for a monomeric tris(pyrazolyl)boratothallium complex and a study of its stereochemical nonrigidity by 1H and 205Tl NMR spectroscopy Jennifer L. Kisko, Tony Hascall, Clare Kimblin and Gerard Parkin * Department of Chemistry, Columbia University, New York, NY 10027, USA Received 9th March 1999, Accepted 19th April 1999 The syntheses and structures of the phenyl substituted tris(3-tert-butylpyrazolyl)borato complexes, [PhTpBut]M (M = Li, Tl, H), are reported.In contrast to other monomeric [TpRR9]Tl derivatives, which exhibit symmetric tridentate coordination of the tris(pyrazolyl)borate ligand, [PhTpBut]Tl exhibits an unprecedented structure. Specifically, one of the tert-butylpyrazolyl groups is rotated by ca. 908 and the Tl interacts with the nitrogen attached directly to the boron via a p-orbital component of the aromatic p-system of the pyrazolyl nucleus.[PhTpBut]Tl is stereochemically nonrigid on the NMR spectroscopic timescale in solution at room temperature, but cooling to ca. 280 8C slows down the dynamic processes suYciently to allow “axial” and “equatorial” isomers to be identified, with the descriptors denoting the position of the pyrazolyl group relative to the boat configuration of the six-membered [BN4Tl] ring. Introduction The poly(pyrazolyl)borato ligand system is one of the most widely used in modern coordination chemistry.1 Alkali metal and thallium 2 derivatives, in particular, have played a prominent role in the development of this chemistry by virtue of their use as ligand transfer agents.Other than their use as reagents, however, these complexes have received relatively little attention. To date, the majority of studies using tris(pyrazolyl)- borato ligands have concentrated on [TpRR9] derivatives in which the fourth site on boron is a hydride, with comparatively fewer studies having been reported for those with an alkyl or aryl substituent, [RTpRR9].3 We are particularly interested in applying such [RTpRR9] ligands to situations in which the potential reactivity associated with the B–H bond may prove problematic. 4 In this paper, we report the syntheses and structures of the phenyl substituted tris(3-tert-butylpyrazolyl)borato complexes, [PhTpBut]M (M = Li, Tl, H).Significantly, [PhTpBut]Tl exhibits an unprecedented type of structure for monomeric tris(pyrazolyl)borato thallium derivatives.Results and discussion Syntheses of [PhTpBut]Li, [PhTpBut]Tl, and [PhTpBut]H Tris(pyrazolyl)borato ligands with alkyl or aryl substituents on boron, i.e. [RTpRR9], have been known for more than 30 years 5 and have been synthesized by a variety of methods, as illustrated in Scheme 1.6–10 However, the majority of these [RTpRR9] ligands are derivatives of unsubstituted pyrazole, with relatively few examples incorporating bulky substituents in the 3-positions.For example, methyl is the bulkiest pyrazolyl substituent reported in the literature to have been incorporated into the [RTpRR9] ligand system; specifically, Na[MeTpMe] was obtained via the reaction of MeB(OPri)2 with a mixture of Na[pzMe] and HpzMe.11 In view of the important role that bulky substituents in the 3-positions of [TpRR9] ligands has played in allowing isolation of certain reactive M–X functionalities,12 we are particularly interested in the construction of related boronsubstituted counterparts, [RTpRR9], with the notion that a more resistant ligand system will be obtained upon elimination of possible degradation reactions involving the B–H bond.Signifi- cantly, the phenyl tris(3-tert-butylpyrazolyl)borato ligand [PhTpBut] may be constructed by heating a mixture of Li[Ph- BH3] 13 and 3-tert-butylpyrazole (ca. 3 equivalents) at ca. 220 8C (Scheme 2).14 Subsequent metathesis of [PhTpBut]Li with Tl(O2CMe) yields the thallium derivative, [PhTpBut]Tl, from which the acid form [PhTpBut]H may be obtained by treatment with H2S (Scheme 2). In contrast to the hydrated derivatives [TpRR9]H?n(H2O) that have been prepared by other methods,15,16 the use of H2S permits the synthesis of an anhydrous material.17 Molecular structures of [PhTpBut]Li, [PhTpBut]Tl, and [PhTpBut]H The molecular structures of [PhTpBut]Li, [PhTpBut]Tl, and [PhTpBut]H, as determined by single crystal X-ray diVraction (Fig. 1–3), exhibit several interesting features. For example, the lithium complex is notable because there are no other structurally characterized [RTpRR9]Li (or even [TpRR9]Li) complexes listed in the Cambridge Structural Database,18 and it exists as a discrete mononuclear species with an uncommon trigonal coordination environment.19 The average Li–N bond length of Scheme 11930 J. Chem. Soc., Dalton Trans., 1999, 1929–1935 1.96 Å (Table 1) is comparable to, though marginally shorter than, the mean value of 2.08 Å for all complexes with Li–N bonds listed in the Cambridge Structural Database.20 It is also shorter than the mean Li–N bond length of 2.06 Å in the tris(3,5-dimethylpyrazolyl)methane derivative, [HC(pzMe2)3]- Li(h3-H3BH).21 Structurally characterized [TpRR9]H derivatives are also rare, and the only examples of which we are aware are the tetrakis- (pyrazolyl) complexes [pzTp]H?(H2O)16 and [(pzMe2)TpMe2]H,22 the latter of which was obtained as a decomposition product in the reaction of Cp2TiCl2 with [TpMe2]H. The structure of [PhTpBut]H, as illustrated in Fig. 3, diVers from that of the lithium derivative [PhTpBut]Li in that the pyrazolyl groups of [PhTpBut]H are not symmetrically disposed with C3v symmetry. The [PhTpBut]H hydrogen atom was both located and refined, and is localized on a single pyrazolyl nitrogen atom [N(12)] with a bond length of 0.84 Å; with a distance of 2.00 Å to the closest nitrogen atom [N(22)] on an adjacent pyrazolyl group, it is evident the N–H group does not participate in a significant hydrogen bonding interaction.In contrast to the paucity of structurally characterized [RTpRR9]Li and [RTpRR9]H complexes, structurally characterized thallium derivatives are numerous.23 Nevertheless, the structure of [PhTpBut]Tl (Fig. 2) is unique amongst [RTpRR9]Tl Fig. 1 Molecular structure of [PhTpBut]Li.Scheme 2 derivatives. Specifically, whereas all other monomeric [RTpRR9]- Tl complexes possess trigonally coordinated thallium, similar to that of the lithium derivative [PhTpBut]Li, the thallium center of [PhTpBut]Tl is principally coordinated to only two of the pyrazolyl groups: the third pyrazolyl group is eVectively rotated by ca. 908 about the B–N bond. Despite the fact that one of the pyrazolyl groups is orthogonal to its position in other [RTpRR9]Tl derivatives, it is evident that there is still an interaction with the thallium center.The interaction is, however, of a fundamentally diVerent nature to those in other [RTpRR9]Tl derivatives for two reasons: (i) the Tl interacts with the nitrogen atom attached directly to the boron, and (ii) the interaction is with the nitrogen p-orbital component of the aromatic psystem of the pyrazolyl nucleus. As a consequence, the unique Tl–N bond length [2.833(2) Å] is distinctly greater than those for the conventional bonds [2.528(3) Å and 2.585(3) Å], which are comparable to the average value in the counterpart without the phenyl substituent, [TpBut]Tl [2.59(1) Å].24 While examples of bidentate [h2-TpRR9]MX derivatives are known, as summarized in Table 2, the two coordinate nature of monomeric [PhTpBut]Tl is of particular interest because such a coordination motif is unknown for thallium in [TpRR9]Tl complexes.Since all other monomeric [TpRR9]Tl complexes with a hydrogen substituent on boron adopt a common trigonal (C3v) h3-coordination geometry, the unusual structure of [PhTpBut]Tl Fig. 2 Molecular structure of [PhTpBut]Tl. Table 1 Selected bond lengths (Å) and angles (8) for [PhTpBut]Li, [PhTpBut]Tl, and [PhTpBut]H [PhTpBut]Li [PhTpBut]Tl [PhTpBut]H M–N(12) M–N(22) M–N(3X)a B–N(11) B–N(21) B–N(31) B–C(41) N(12)–M–N(22) N(22)–M–N(3X)a N(12)–M–N(3X)a N(11)–B–N(21) N(21)–B–N(31) N(11)–B–N(31) C(41)–B–N(11) C(41)–B–N(21) C(41)–B–N(31) 1.977(3) 1.934(3) 1.979(3) 1.572(2) 1.571(2) 1.568(2) 1.615(2) 96.1(2) 97.6(2) 98.1(2) 109.3(2) 109.4(2) 104.5(2) 112.7(2) 107.0(2) 113.8(2) 2.585(3) 2.528(3) 2.833(2) 1.560(4) 1.554(4) 1.558(4) 1.610(4) 71.58(8) 69.35(7) 66.09(8) 110.1(2) 107.7(2) 104.9(2) 110.6(2) 111.1(2) 112.3(2) ——— 1.572(4) 1.554(3) 1.540(4) 1.603(4) ——— 107.6(2) 107.6(2) 106.0(2) 109.8(2) 111.6(2) 114.0(2) a X = 2 for [PhTpBut]Li and [PhTpBut]H; X = 1 for [PhTpBut]Tl.J.Chem. Soc., Dalton Trans., 1999, 1929–1935 1931 is most likely a consequence of the increased steric demands of the phenyl group.Thus, steric interactions between the 5-H substituents and the phenyl group would be expected to destabilize symmetric h3-coordination.25 Rotation about the B–N bond relieves such non-bonding interactions, but at the expense of the Tl–N interaction. The observed structure is, therefore, a compromise of maximizing Tl–N interactions and minimizing steric interactions with the phenyl substituent.Evidently, for [PhTpBut]Tl, the steric interactions must be the dominant component. In contrast, the conventional h3-coordination geometry of the lithium complex [PhTpBut]Li is presumably a result of the Li–N interactions providing the dominant influence. Such a diVerence between [PhTpBut]Tl and [PhTpBut]Li is not unreasonable in view of the fact that twocoordinate thallium complexes, and especially [BpRR9]Tl derivatives, 26 are well known. In contrast, two coordination is not common for lithium.27 The novel coordination geometry of [PhTpBut]Tl has a specific feature in common with polymeric {[FcTp]Tl}x,10 namely that one of the pyrazolyl groups in {[FcTp]Tl}x is also orthogonal to its conventional position; however, in this case the 2-N coordinates to the thallium center of another molecule, thereby resulting in a polymeric structure (Fig. 4). The Fig. 3 Molecular structure of [PhTpBut]H. Fig. 4 Schematic representation of the polymeric nature of the ferrocynyl derivative {[FcTp]Tl}x.“intermolecular” Tl–N bond length of 2.780(5) Å in {[FcTp]Tl}x is only slightly greater than the intramolecular bond lengths [2.638(5) and 2.676(5) Å], but is distinctly shorter than the value of 3.21 Å for the intramolecular interaction involving 1-N from the orthogonal pyrazolyl group (Fig. 4). The latter value for {[FcTp]Tl}x is significantly greater than the corresponding bond length in [PhTpBut]Tl [2.833(2) Å], which therefore provides a good indication that the unconventional interaction between thallium and the orthogonal pyrazolyl group in [PhTpBut]Tl is structurally significant. It is also important to note that for [PhTpBut]Tl the closest intermolecular Tl ? ? ? N interaction is 5.67 Å; thus, the orthogonal pyrazolyl group of [PhTpBut]Tl exhibits no significant interaction with adjacent molecules, and the strictly monomeric nature is presumably a consequence of the bulky tert-butyl substituents.28 Stereochemical nonrigidity of [PhTpBut]Tl Whereas the asymmetric nature of [PhTpBut]Tl in the solid state is certain, the question remains as to how the ligand coordinates in solution.At room temperature, the 1H NMR spectrum of [PhTpBut]Tl reveals only one set of resonances attributable to the tert-butylpyrazolyl moiety. Such an observation is consistent with at least two common possibilities for the solution structure: (i) [PhTpBut]Tl exists as a static symmetric C3v tridentate structure, or (ii) [PhTpBut]Tl exists with the asymmetric structure shown in Fig. 2, but the molecule is stereochemically nonrigid on the NMR spectroscopic timescale. In order to Table 2 Representative examples of structurally characterized complexes with bidentate [RTpRR9] ligands Compound Ref. [h2-TpBut]AlEt2 [h2-TpMe2]Ga(Me)X (X = Me, Cl) [h2-pzTp][pzTp]GaMe [h2-TpMe2][TpMe2]Sn [h2-TpMe2]V(O)Cl2 [h2-TpMe2]Rh(CNR)2 (R = 2,6-Me2C6H3, CH2CMe3) [h2-TpMe2]Rh(CO)(PMe3) [h2-Tp]2Pd [h2-Tp]Pt(Me)CO {[h2-TpMe2]PtMe2}{PPN} [h2-pzTp]Cu(PPh3)2 [h2-TpPh2]2Zn [h2-pzTp]2Pb [h2-pzTp]HgSMe [h2-Tp][Tp]2Yb T1 T2 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 T1 M.H. Chisholm, N. W. Eilerts and J. C. HuVman, Inorg. Chem., 1996, 35, 445. T2 D. L. Reger and Y. Ding, Organometallics, 1993, 12, 4485. T3 A. H. Cowley, R. L. Geerts, C. M. Nunn and C. J. Carrano, J. Organomet. Chem., 1988, 341, C27. T4 E. Kime-Hunt, K. Spartalian, M. DeRusha, C. M. Nunn and C. J. Carrano, Inorg. Chem., 1989, 28, 4392.T5 W. D. Jones and E. T. Hessel, Inorg. Chem., 1991, 30, 778. T6 V. Chauby, C. S. Le Berre, P. Kalck, J.-C, Daran and G. Commenges, Inorg. Chem., 1996, 35, 6354. T7 A. J. Canty, N. J. Minchin, L. M. Engelhardt, B. W. Skelton and A. H. White, J. Chem. Soc., Dalton Trans., 1986, 645. T8 P. E. Rush and J. D. Oliver, J. Chem. Soc., Chem. Commun., 1974, 996. T9 D. D. Wick and K. I. Goldberg, J. Am. Chem. Soc., 1997, 119, 10235. T10 P. Cecchi, B. Bovio, G. G.Lobbia, C. Pettinari and D. Leonesi, Polyhedron, 1995, 14, 2441. T11 F. Hartmann, W. Klaüi, A. Kremer-Aach, D. Mootz, A. Strerath and H. Z. Wunderlich, Z. Anorg. Allg. Chem., 1993, 619, 2071. T12 D. L. Reger, M. F. HuV, A. L. Rheingold and B. S. Haggerty, Inorg. Chem., 1992, 114, 579. T13 S. Aime, G. Digilio, R. Gobetto, P. Cecchi, G. G. Lobbia and M. Camalli, Polyhedron, 1994, 13, 2695. T14 M. V. R. Stainer and J. Takats, Inorg. Chem., 1982, 21, 4050.1932 J. Chem. Soc., Dalton Trans., 1999, 1929–1935 address these possibilities, variable temperature 1H NMR spectroscopic studies were conducted.Significantly, upon lowering the temperature to 280 8C, decoalescence was observed (Fig. 5), giving rise to three sets of resonances in the ratio ca. 2 : 2 : 1 (as most clearly seen by examination of the tert-butyl region). This pattern, however, is not consistent with the solid state structure of [PhTpBut]Tl, which would result in three sets of resonances in the ratio 1:1:1 corresponding to the three inequivalent tert-butylpyrazolyl groups (note that the two coordinated pyrazolyl groups of a static structure are not chemically equivalent, but are diastereotopically related).Alternatively, if the chemical shifts of the diastereotopically related pyrazolyl groups are coincidentally the same, or a mechanism exists which allows for their facile interconversion, then a 2 : 1 pattern of pyrazolyl groups would be expected. It is, therefore, evident that the experimentally observed 2:2:1 pattern is not consistent with the presence of a single chemical species at low temperature; the presence of a second species is thus required to generate the observed low temperature 1H NMR spectrum.Excellent support for the presence of a second species is provided by 205Tl NMR spectroscopy: specifically, at low temperature (ca. 270 8C), the 205Tl NMR spectrum exhibits two resonances (Fig. 6). Furthermore, these two signals coalesce upon warming to room temperature, clearly indicating that the two species are isomers of each other.The 2:2:1 distribution of pyrazolyl groups in the 1H NMR spectrum can, therefore, be rationalized in terms of two species, A3:0 and B2:1 (Scheme 3), which possess equivalent and inequivalent (in the ratio 2 : 1) sets of pyrazolyl groups, respectively. Thus, if the ratio of A3:0 to B2:1 is 2 : 3,29 the composite 1H NMR spectrum would show Fig. 5 Variable temperature 1H NMR spectra of [PhTpBut]Tl in toluene (* = residual protio solvent).pyrazolyl groups in the ratio 6 : [6:3], i.e. 2:2:1. Assuming this hypothesis to be correct, the important issue is concerned with identifying the nature of A3:0 and B2:1. At least two possible scenarios emerge. (a) Scenario I. A3:0 and B2:1 correspond to isomers related by interchange of the uncoordinated pyrazolyl and phenyl substituents (Scheme 3). These isomers may be described as “axial” and “equatorial”, with the descriptors denoting the position of the pyrazolyl group relative to the boat configuration of the six-membered [BN4Tl] ring.Of these two structures, the axial isomer may be identified as A3:0 and the equatorial isomer as B2:1. The rationale for these assignments is based on the notion that the axial isomer is more likely to exhibit equivalent pyrazolyl groups on the NMR spectroscopic timescale than is the equatorial isomer. Specifically, rotation about the B–N bond of the axial isomer allows access to a symmetric tridentate species, thereby enabling facile exchange of all three pyrazolyl groups. In contrast, rotation about the B–N bond in the equatorial isomer (B2:1) does not result in chemical exchange of the pyrazolyl groups, which therefore retain a 2 : 1 pattern.For this scenario, the interchange of axial (A3:0) and equatorial (B2:1) isomers corresponds to flipping the boat configuration of the six-membered [BN4Tl] ring.(b) Scenario II. A second possibility is that A3:0 corresponds to a symmetric h3-structure, and B2:1 corresponds to the h2- axial isomer as observed in the solid state.30 The principal diVerence between the structure corresponding to B2:1 in Scenario II and that of A3:0 in Scenario I is that in Scenario I it is highly fluxional. Of these two scenarios, we consider the first to be the more reasonable since it makes no assumptions concerning coincidental chemical shifts of the diastereotopic pyrazolyl groups for B2:1.Thus, as summarized in Scheme 3, the proposed identity of B2:1 is the equatorial bidentate isomer, while A3:0 is the axial isomer in rapid equilibrium with the tridentate species. Precedent for this suggestion is provided by Venanzi’s study of stereochemical nonrigidity within complexes of the types [TpRR9]Rh(CO)2 and [TpRR9]Rh(L2) (L2 = cyclooctadiene, nor- Fig. 6 Variable temperature 205Tl NMR spectra of [PhTpBut]Tl in toluene.J.Chem. Soc., Dalton Trans., 1999, 1929–1935 1933 bornadiene), each of which exhibits magnetically equivalent pyrazolyl groups at room temperature.31 IR spectroscopic studies on the carbonyl complexes demonstrated that the solutions consisted of three diVerent species: (i) two fourcoordinate complexes with bidentate [TpRR9] ligands, diVering according to whether the uncoordinated pyrazolyl group was in an axial or equatorial position, and (ii) a five-coordinate complex with tridentate [TpRR9] coordination.The relative amounts of these three species depended upon both the nature of the [TpRR9] ligand and the solvent. In one specific example, [Tp32Pri,4-Br]Rh(CO)2 was observed to decoalesce to a “symmetric” and asymmetric species. IR spectroscopy demonstrated that both species were four coordinate, so that the “symmetric” Scheme 3 species was proposed to be stereochemically nonrigid and bidentate rather than a complex with tridentate coordination. As a further illustration that magnetic equivalence of the three pyrazolyl groups in [h2-TpRR9]MX complexes does not require species with tridentate coordination to be present in detectable concentrations, Jones reported that the equivalence of the pyrazolyl groups in [h2-TpMe2]Rh(CNR) (R = 2,6-Me2C6H3, CH2CMe3) is a consequence of facile chemical exchange of coordinated and uncoordinated pyrazolyl groups, with a symmetric tridentate species having no detectable concentration as determined by IR spectroscopy.32 Conclusions In summary, the phenyl substituted tris(3-tert-butylpyrazolyl)- borato complexes, [PhTpBut]M (M = Li, Tl, H) have been synthesized and structurally characterized by X-ray diVraction.In contrast to other monomeric [TpRR9]Tl derivatives, which exhibit symmetric tridentate coordination of the tris(pyrazolyl)- borate ligand, [PhTpBut]Tl exhibits an unprecedented structure in which one of the tert-butylpyrazolyl groups is rotated by ca. 908 and the Tl interacts with the nitrogen attached directly to the boron via a p-orbital component of the aromatic p-system of the pyrazolyl nucleus. [PhTpBut]Tl is stereochemically nonrigid on the NMR spectroscopic timescale in solution at room temperature, equilibrating between isomers which diVer according to whether the uncoordinated pyrazolyl group is located axially or equatorially relative to the six-membered [BN4Tl] ring. Experimental General considerations All manipulations were performed using a combination of glovebox, high-vacuum or Schlenk techniques.33 Solvents were purified and degassed by standard procedures.Commercially available reagents were not further purified. All glassware was oven dried prior to use. NMR spectra were recorded on Bruker Avance 300wb DRX, Bruker Avance 400 DRX, and Bruker Avance 500 DMX spectrometers. 1H and 13C chemical shifts are reported in ppm relative to SiMe4 (d = 0) and were referenced internally with respect to the protio solvent impurity or the 13C resonances, respectively; 205Tl NMR spectra were recorded on a Bruker Avance 300wb DRX (173.393 Hz) instrument and are referenced relative to aqueous TlNO3 (extrapolated to infinite dilution; d = 0.00),34,35 using an external solution of aqueous Tl(O2CMe) as calibrant.All coupling constants are reported in Hz. C, H, and N elemental analyses were measured using a Perkin-Elmer 2400 CHN Elemental Analyzer. Synthesis of [PhTpBut]Li A rapidly stirred mixture of Li[PhBH3] (5.82 g, 59.5 mmol) and HpzBut(22.2 g, 0.18 mol) was heated at ca. 220 8C under N2 until evolution of H2 ceased (ca. 1 h). The product was allowed to cool to room temperature and extracted into pentane (ca. 200 mL). The mixture was filtered and the volatile components removed in vacuo giving [PhTpBut]Li as a white solid (27.0 g, 98%). The product may be crystallized from pentane at 218 8C over a period of several days (Calc. for [PhTpBut]Li: C, 69.8; H, 8.3; N, 18.1.Found: C, 69.6; H, 9.5; N, 18.9). NMR spectroscopic data are listed in Table 3. Synthesis of [PhTpBut]Tl A mixture of [PhTpBut]Li (9.5 g, 20 mmol) and Tl(O2CMe) (8.1 g, 31 mmol) in THF (50 mL) was stirred at room temperature for 1 day. After this period, the mixture was filtered and the volatile components were removed in vacuo giving [PhTpBut]Tl as an oV-white solid (5.97 g, 45%). Pure [PhTpBut]Tl was1934 J. Chem. Soc., Dalton Trans., 1999, 1929–1935 Table 3 NMR spectroscopic data for [ThTpBut]Li, [PhTpBut]Tl, and [PhTpBut]H Assignment [PhTpBut]Li a [PhTpBut]Tl [PhTpBut]H 1H (C6D6) C(CH3)3 C3N2H2 C6H5 NH 13C (C6D6) C(CH3)3 C(CH3)3 C3N2H2 BC6H5 1.31, s 5.91, d, 3JH–H = 2 7.96, br 7.24–7.55 — 31.0, q, 1JC–H = 126 32.1, s 100.3, d, 1JC–H = 173 136.5, d, 1JC–H = 181 161.8, s 128.0, d, 1JC–H = 158 (3C) 135.6, 1JC–H = 159 (2C) (Cipso not observed) 1.32, s 6.04, d, 3JH–H = 2 7.66, br 7.12–7.20 — 31.7, q, 1JC–H = 126 32.4, s 101.4, d, 1JC–H = 172 137.2, d, 1JC–H = 184 163.7, s 128.6, d, 1JC–H = 162 (3C) b 135.4, d, 1JC–H = 157 (2C) 148.5, br (Cipso) 1.24, s 5.94, d, 3JH–H = 2 7.29, d, 3JH–H = 2 7.20–7.25 Not observed 30.6, q, 1JC–H = 126 32.0, s 101.5, d, 1JC–H = 174 136.6, 1JC–H = 186 161.1, s 127.7, d, 1JC–H = 156 (3C) b 133.2, d, 1JC–H = 156 (2C) (Cipso not observed) a The NMR signals in the presence of additional tert-butylpyrazole are perturbed from the values listed here, presumably due to the formation of an adduct. b An HMQC experiment indicates that the meta and para resonances are coincident with each other.obtained as colorless crystals by repeated crystallization from pentane at 215 8C (Calc. for [PhTpBut]Tl: C, 49.0; H, 5.8; N, 12.7. Found: C, 49.3; H, 5.7; N, 13.4). NMR spectroscopic data are listed in Table 3. Synthesis of [PhTpBut]H A solution of [PhTpBut]Tl (220 mg, 0.33 mmol) in benzene was treated with H2S resulting in the formation of a black precipitate. The mixture was filtered and the volatile components were removed in vacuo giving [PhTpBut]H as a white powder, from which colorless crystals were obtained by crystallization from pentane (ca. 1 mL) at 215 8C (66 mg, 43%) (Calc. for [PhTpBut]H: C, 70.7; H, 8.6; N, 18.3. Found: C, 71.0; H, 8.4; N, 17.4). NMR spectroscopic data are listed in Table 3. Crystal structure determinations Crystal data, data collection and refinement parameters for [PhTpBut]Li, [PhTpBut]Tl, and [PhTpBut]H are summarized in Table 4.X-Ray diVraction data were collected on a Bruker P4 diVractometer equipped with a SMART CCD detector. The structures were solved using direct methods and standard Table 4 Crystal, intensity collection and refinement data. [PhTpBut]Li [PhTpBut]Tl [PhTpBut]H Lattice Formula Formula weight Space group a/Å b/Å c/Å a/8 b/8 g/8 V/Å3 ZT /K l/Å rcalc/g cm23 m(Mo-Ka)/mm21 q max/8 No. of data No. of parameters R1 a wR2 a S Monoclinic C27H38BN6Li 464.38 C2/c (no. 15) 18.6329(10) 16.1563(9) 18.7233(10) 90.454(1) 5636.3(5) 8 203 0.71073 1.095 0.065 28.3 6413 326 0.0518 0.1185 1.045 Monoclinic C27H38BN6Tl 661.81 P21/c (no. 14) 15.2499(7) 13.8902(7) 15.3951(7) 117.055(1) 2904.2(2) 4 203 0.71073 1.514 5.586 28.3 6600 326 0.0279 0.0585 1.044 Triclinic C27H39BN6 458.45 P1� (no. 2) 11.6639(8) 12.1658(8) 12.2480(8) 114.003(1) 101.409(1) 109.440(1) 1380.7(2) 2 173 0.71073 1.103 0.066 28.3 5727 321 0.0740 0.1610 1.070 a R1 = S|Fo| 2 |Fc|/S|Fo| for [I > 2s(I)]; wR2 = {S[w(Fo 2 2 Fc 2)2]/S [w(Fo 2)2]}1/2 for [I > 2s(I)].diVerence map techniques, and were refined by full-matrix least-squares procedures using SHELXTL.36 Hydrogen atoms on carbon were included in calculated positions. Systematic absences for [PhTpBut]Li were consistent with Cc (no. 9) and C2/c (no. 15), of which a satisfactory solution was obtained in the centrosymmetric alternative C2/c (no. 15). Systematic absences for [PhTpBut]Tl were consistent uniquely with P21/c (no. 14). Systematic absences for [PhTpBut]H were consistent with P1 (no. 1) and P1� (no. 2), of which a satisfactory solution was obtained in the centrosymmetric alternative, P1� (no. 2). CCDC reference number 186/1440. See http://www.rsc.org/suppdata/dt/1999/1929/ for crystallographic files in .cif format. Acknowledgements We thank the National Science Foundation (CHE 96-10497) for support of this research. Drs. JeVrey B. Bonanno and Peter Desrosiers are thanked for helpful suggestions concerning the variable temperature NMR studies, and Drs.Jun Ho Shin and Brian Bridgewater are thanked for helpful assistance. The referees are thanked for their helpful comments. Notes and references 1 For recent reviews, see: (a) S. Trofimenko, Chem. Rev., 1993, 93, 943; (b) G. Parkin, Adv. Inorg. Chem., 1995, 42, 291; (c) N. Kitajima and W. B. Tolman, Prog. Inorg. Chem., 1995, 43, 419; (d ) I. Santos and N. Marques, New. J. Chem., 1995, 19, 551; (e) D.L. Reger, Coord. Chem. Rev., 1996, 147, 571; ( f ) M. Etienne, Coord. Chem. Rev., 1997, 156, 201; ( g) P. K. Byers, A. J. Canty and R. T. Honeyman, Adv. Organomet. Chem., 1992, 34, 1. 2 For reviews of Tl[TpRR9] complexes, see: (a) C. Janiak, Main Group Met. Chem., 1998, 21, 33; (b) C. Janiak, Coord. Chem. Rev., 1997, 163, 107. 3 The abbreviations adopted here for tris(pyrazolyl)hydroborato ligands are based on those described by Trofimenko [ref. 1(a)]. Thus, the tris(pyrazolyl)hydroborato ligands are represented by the abbreviation Tp, with the 3- and 5-alkyl substituents listed respectively as superscripts. If the fourth substituent on boron is anything other than hydrogen, the substituent is listed as a prefix, e.g., [pzTp] and [RTp]. 4 See, for example: J. L. Kisko, T. Hascall and G. Parkin, J. Am. Chem. Soc., 1998, 120, 10561. 5 S. Trofimenko, J. Am. Chem. Soc., 1967, 89, 6288. 6 K. Nidenzu and S. Trofimenko, Top. Curr. Chem., 1986, 131, 1. 7 Na[RTp] (R = Pri,a Bun,b Ph,b,c p-C6H4Brc) have been prepared by reaction of RB(OH)2 with Napz in the presence of pzH. (a) D. L. Reger and M. E. Tarquini, Inorg. Chem., 1982, 21, 840; (b) ref. 5; (c) D. L. White and J. W. Faller, J. Am. Chem. Soc., 1982, 104, 1548. 8 [PhTp]Li has been prepared by reaction of Li[PhBH3] with pyrazole. See: F. A. Cotton, C. A. Murillo and B. R. Stults, Inorg. Chim. Acta, 1977, 22, 75.J. Chem. Soc., Dalton Trans., 1999, 1929–1935 1935 9 [pzH2][PhTp] was obtained by the reaction of PhBCl2 with excess pyrazole.See ref. 5. 10 The ferrocenyl [Fc = (C5H4)(C5H5)Fe] derivative, [FcTp]H has been obtained by reaction of FcBBr2 with pzH in the presence of Et3N. Subsequent reaction of [FcTp]H with TlOEt gives the thallium complex Tl[FcTp]. F. Jäkle, K. Polborn and M. Wagner, Chem. Ber., 1996, 129, 603. 11 U. E. Bucher, T. F. Fässler, M. Hunziker, R. Nesper, H. Rüegger and L. M. Venanzi, Gazz. Chim. Ital., 1995, 125, 181. 12 For example, terminal alkyl, hydride, hydroxide and chalcogenide moieties have been stabilized with such ligation.See, for example, ref. 1(b) and (a) M. C. Kuchta and G. Parkin, J. Am. Chem. Soc., 1995, 117, 12651; (b) M. C. Kuchta and G. Parkin, Inorg. Chem., 1997, 36, 2492; (c) M. C. Kuchta and G. Parkin, J. Chem. Soc., Dalton Trans., 1998, 2279. 13 B. Singaram, T. E. Cole and H. C. Brown, Organometallics, 1984, 3, 774. 14 If the reaction is carried out in benzene solvent at room temperature, the phenylbis(3-tert-butylpyrazolyl)borato species [Ph(H)BpBut]Li(ButpzH) may be isolated.J. L. Kisko and G. Parkin, unpublished work. 15 (a) S. Trofimenko, J. Am. Chem. Soc., 1967, 89, 3170; (b) R. A. Kresinski, J. Chem. Soc., Dalton Trans., 1999, 401. 16 C. López, R. M. Claramunt, C. Foces-Foces, F. H. Cano and J. Elguero, Rev. Roum. Chim., 1994, 9, 795. 17 Other protonated [TpRR9]H derivatives have also been prepared by this method. See: J. Blackwell, C.Lehr, Y. Sun, W. E. Piers, S. D. Pearce-Batchilder, M. J. Zaworotko and V. G. Young, Jr., Can J. Chem., 1997, 75, 702 and footnote 5 therein. 18 For structures of other alkali metal [TpRR9]M derivatives, see, for example: (a) C. M. Dowling, D. Leslie, M. H. Chisholm and G. Parkin, Main Group Chem., 1995, 1, 29; (b) C. Lopez, R. M. Claramunt, D. Sanz, C. Foces Foces, F. H. Cano, E. Faure, E. Cayon and J. Elguero, Inorg. Chim. Acta, 1990, 176, 195; (c) G. G. Lobbia, P. Cecchi, R.Spagna, M. Colapietro, A. PiVeri and C. Pettinari, J. Organomet. Chem., 1995, 485, 45; (d) K. Weis and H. Vahrenkamp, Inorg. Chem., 1997, 36, 5589; (e) H. V. R. Dias and H.-J. Kim, Organometallics, 1996, 15, 5374; ( f ) H. V. R. Dias, W. C. Jin, H. J. Kim and H.-L. Lu, Inorg. Chem., 1996, 35, 2317; ( g) H. V. R. Dias, H.-L. Lu, R. E. RatcliV and S. G. Bott, Inorg. Chem., 1995, 34, 1975. 19 Although three coordination of lithium is less common than its ubiquitous tetrahedral coordination, it is nevertheless well precedented.For example, ca. 20% of the lithium complexes listed in the Cambridge Structural Database have a coordination number of three for lithium. 20 CSD Version 5.16, 1999, 3D Search and Research Using the Cambridge Structural Database, F. H. Allen and O. Kennard, Chem. Des. Automat. News, 1993, 8, pp. 1, 31–37. 21 D. L. Reger, J. E. Collins, M. A. Matthews, A. L. Rheingold, L. M. Liable-Sands and I. A. Guzei, Inorg. Chem., 1997, 36, 6266. 22 D. C. Bradley, M. B. Hursthouse, J. Newton and N. P. C. Walker, J. Chem. Soc., Chem. Commun., 1984, 188. 23 For a compilation of data, see: R. Han, P. Ghosh, P. J. Desrosiers, S. Trofimenko and G. Parkin, J. Chem. Soc., Dalton Trans., 1997, 3713. 24 A. H. Cowley, R. L. Geerts, C. M. Nunn and S. Trofimenko, J. Organomet. Chem., 1989, 365, 19. 25 For other examples in which intraligand interactions influence the binding mode of poly(pyrazolyl)borate complexes, see: (a) Y.Sohrin, H. Kokusen and M. Matsui, Inorg. Chem., 1995, 34, 3928; (b) D. L. Reger, M. F. HuV, A. L. Rheingold and B. S. Haggerty, Inorg. Chem., 1992, 114, 579; (c) F. A. Cotton, B. A. Frenz and C. A. Murillo, J. Am. Chem. Soc., 1975, 97, 2118. 26 (a) C. Dowling, P. Ghosh and G. Parkin, Polyhedron, 1997, 16, 3469; (b) T. Fillebeen, T. Hascall and G. Parkin, Inorg. Chem., 1997, 36, 3787; (c) P. Ghosh, T. Hascall, C. Dowling and G. Parkin, J. Chem. Soc., Dalton Trans., 1998, 3355. 27 For example, only 7% of the structurally characterized lithium complexes listed in the Cambridge Structural Database are two coordinate. 28 Furthermore, it should be noted that the closest Tl ? ? ? Tl separation in [PhTpBut]Tl is 6.68 Åthat there is also no weak Tl ? ? ? Tl interaction of the type that has been suggested for other [TpRR9]Tl complexes. See, for example: (a) G. Ferguson, M. C. Jennings, F. J. Lalor and C. Shanahan, Acta. Crystallogr., Sect. C, 1991, 47, 2079; (b) A. L. Rheingold, L. M. Liable-Sands and S. Trofimenko, J. Chem. Soc., Chem. Commun., 1997, 1691; (c) C. Janiak, S. Temizdemir and T. G. Scharmann, Z. Anorg. Allg. Chem., 1998, 624, 755. 29 The 2: 3 ratio is comparable to the ratio of areas of the two 205Tl NMR spectroscopic signals (2 : 2.6 at 270 8C). In view of the potentially diVerent relaxation times for the diVerent thallium nuclei, the integral ratio is not expected to be an accurate measure of their relative concentrations. 30 The 2 : 1 ratio of pyrazolyl groups assumes that either the chemical shifts of the diasterotopic pyrazolyl groups are coincidentally the same, or a mechanism exists for their facile interconversion. Alternatively, B2:1 could correspond to the equatorial isomer of Scenario I. 31 U. E. Bucher, A. Currao, R. Nesper, H. Rüegger, L. M. Venanzi and E. Younger, Inorg. Chem., 1995, 34, 66. 32 W. D. Jones and E. T. Hessell, Inorg. Chem., 1991, 30, 778. 33 (a) J. P. McNally, V. S. Leong and N. J. Cooper, in Experimental Organometallic Chemistry, ed. A. L. Wayda and M. Y. Darensbourg, American Chemical Society, Washington, DC, 1987, ch. 2, pp. 6–23; (b) B. J. Burger and J. E. Bercaw, in Experimental Organometallic Chemistry, eds. A. L. Wayda and M. Y. Darensbourg, American Chemical Society, Washington, DC, 1987, ch. 4, pp. 79–98; (c) D. F. Shriver and M. A. Drezdzon, The Manipulation of Air- Sensitive Compounds, Wiley-Interscience, New York, 2nd edn., 1986. 34 J. J. Dechter and J. I. Zink, J. Am. Chem. Soc., 1975, 97, 2937. 35 Specifically, the resonance frequencies of three solutions of Tl(NO3) in H2O (1.0, 0.5 and 0.25 M) were extrapolated to zero concentration. 36 G. M. Sheldrick, SHELXTL, An Integrated System for Solving, Refining and Displaying Crystal Structures from DiVraction Data, University of Göttingen, 1981. Paper 9/01875A

ISSN:1477-9226    

DOI:10.1039/a901875a

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1998, Pages 1935–1936 1935 The first two aqua-bridged dimagnesium(II) complexes: structural models for active sites in dimetallic hydrolases Bao-Hui Ye, Toby Mak, Ian D. Williams and Xiao-Yuan Li *,† Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Two novel complexes Mg2(m-H2O)(m-OAc)2(L)4(OAc)2 (L = imidazole or benzimidazole) have been synthesized and characterized; they contain the first example of an aquabridged dimagnesium core, and may have implications for the hydrolytic activity of dimetallic hydrolases. Closely assembled magnesium ions have been found at the active sites of several important hydrolase enzymes.1 These include inositol polyphosphate 1-phosphatase,2 inositol monophosphatase, 3 Escherichia coli DNA polymerase I,4 rat DNA polymerase,5 enolase6 and possibly phosphodiesterase.Although the carboxylate-bridged dimetallic unit has been the subject of extensive studies in mimetic bioinorganic chemistry, the dinuclear magnesium(II) model complex with carboxylate bridges has been very rarely reported.Lippard and co-workers have reported the first dinuclear MgII model complex with two carboxylate and one phosphodiester bridge.7 To the best of our knowledge, no dinuclear MgII complexes with aqua and carboxylate bridges have been reported to date. Our goal has been to synthesize dimetallic model complexes with aqua and carboxylate bridges employing monodentate N-donor ligands such as imidazole (Him), benzimidazole (Bzim) or their analogues which are good mimics of histidine side chains. These ligands not only co-ordinate the metal as N-donors but also provide non-co-ordinating NH groups which form hydrogen bonds with neighboring molecules. Such H bonding has been proposed to play a crucial role in the structural assembly and/or function of many biological systems.8 In this communication, we report the syntheses and structural characterization of two novel dinuclear MgII complexes with a (m-aqua)bis(mcarboxylate) core, Mg2(m-H2O)(m-OAc)2(L)4(OAc)2 (L = Him 1 or Bzim 2), which are the first examples of dinuclear MgII complexes connected by aqua and carboxylate bridges.The complexes 1 and 2 were synthesized by reaction of Him or Bzim with Mg(OAc)2?4H2O in 2.0 : 1 ratio, respectively, in methanol at room temperature.‡ Colorless single crystals suitable for X-ray crystallographic determination were obtained by diVusing diethyl ether into the methanol solution.The molecular structures of 1 and 2 were determined by X-ray crystallo- † E-Mail: CHXYLI@UST.HK ‡ Synthesis of 1: imidazole (0.136 g, 2.0 mmol) in methanol (5 ml) was added to a methanol solution (10 ml) containing Mg(OAc)2?4H2O (0.214 g, 1 mmol). The colorless solution was stirred at room temperature for 5 h, and filtered. The filtrate was reduced to 5 ml by rotatory evaporation under reduced pressure, the resulting solution was then added dropwise to diethyl ether.White precipitates were produced immediately, collected by filtration, and recrystallized from methanol– diethyl ether. Yield: 0.15 g, 52.3% (Found: C, 41.56; H, 5.02; N, 19.42. Calc. for 1, C20H30Mg2N8O9: C, 41.73; H, 5.21; N, 19.47%). A single crystal suitable for X-ray diVraction was obtained by diVusing diethyl ether into a methanol solution of 1. Synthesis of 2: benzimidazole (1.00 g, 8.48 mmol) in methanol (10 ml) was added to a methanol solution (10 ml) containing Mg(OAc)2?4H2O (0.909 g, 4.24 mmol) as described above.Yield: 0.77 g, 47% (Found: C, 55.74; H, 5.22; N, 14.17. Calc. for 2, C36H38Mg2N8O9: C, 55.70; H, 4.90; N, 14.44%). 13C NMR for 1 (solid): d 179.98, 178.62 (CO2 2); 137.53, 135.00 (C2, Him); 128.20, 127.23 (C4, Him); 116.64, 114.89 (C5, Him); 26.39, 24.16 (Me). 15N NMR for 1 (solid): d 215.67, 211.70 (N3, Him); 148.66, 146 (sh) (NH, Him).graphic diVraction.§ A dimagnesium(II) core was identified for each complex which is joined by an aqua and two carboxylate bridges. Each of the Mg ions is further co-ordinated by one additional monodentate carboxylate and two N-donor ligands, forming two equivalent but slightly distorted MgN2O4 octahedra joined at their shared vertex by the oxygen atom of the bridging water molecule in a face-to-face fashion (Fig. 1). The molecule lies on a crystallographically-imposed two-fold axis passing through the bridging aqua oxygen O(1).The hydrogen atoms of the m-H2O were revealed at the highest residual electron density peaks after all other atoms, including the nonwater hydrogens, and were included in the crystal structure elucidation. The bridging water ligands were also identified on the basis of the Mg]O (aqua) distances in 1 [2.144(4) Å] and in 2 [2.131(2) Å] which are longer than those in [Mg(H2O)6- (1-Mecyt)6]21 (average 2.061 Å),10a [Mg(H2O)4(1-Mecyt)2]21 (average 2.086 Å) (1-Mecyt = 1-methylcytosine),10a [Mg(H2O)4- (cyt)2]21 (average 2.080 Å) (cyt = cytosine),10a [Mg(OH2)6]21 (average 2.058 Å),10b [Mg(AMPH)2(OH2)2] (2.092 Å) [AMPH = aminomethyl(hydrogen)phosphonate],10c [Mg(L-Asp)(OH2)2] (average 2.082 Å) 10d and [Mg(L-Glu)(OH2)4] (average 2.076 Å),10e respectively, and considerably longer than the distance of Mg]OH (average 1.953 Å) in a Mg2(m-OH)2 core.10f The Mg? ? ? Mg distances in 1 [3.656(2) Å] and 2 [3.625(2) Å] are shorter than those found in enolase (4.2 Å),6 [Mg2(XDK)- (DPP)2(CH3OH)3(H2O)] [4.108(3) Å] [XDK = m-xylenediaminebis( Kemp’s triacid imide); DPP = diphenylphosphate] 7 and rat DNA polymerase (4 Å),5 and comparable to those found in E.coli DNA polymerase I (3.9 Å),4 inositol polyphosphate 1-phosphatase (3.88 Å),2 inositol monophosphatase (3.8 Å) 3 and fructose 1,6-bis(phosphatase) (3.7 Å).11 Acetate ligands display two co-ordination modes in both complexes, terminal monodentate and bridging bidentate, respectively.Several quite interesting structural features are noticeable in 1 and 2. First and most important, the terminal carboxylate groups display an interesting geometry. While the distances of C]O (free) [1.251(7) Å] and C]O (co-ordinating) [1.255(7) Å] in 1 are comparable statistically, their counterparts in 2 [1.273(4) for C]O (free) and 1.252(4) Å for C]O (coordinating)] are significantly diVerent. Both observations are contrary to the classical geometry of terminally bound monodentate carboxylate groups, in which the distance of C]O (free) is usually significantly shorter than that of C]O (coordinating). 12 A careful examination of the structures led us to believe that the elongated C]O (free) distance is the con- § Crystal data for 1 at 293 K: C20H30Mg2N8O9, M = 575.14, orthorhombic, space group Aba2, a = 8.698(3), b = 19.131(5), c = 16.852(4) Å, U = 2804.2(14) Å3, Z = 4, Dc = 1.362 g cm21, F(000) = 1208, m = 0.147 mm21. 1632 Independent reflections (Rint = 0.0336) were collected with Mo-Ka (l = 0.710 73 Å) radiation 2.13 < q < 27.498 and used in the refinement based on F2. The final R values: R1 = 0.051 and wR2 = 0.126; R for all data: R1 = 0.111, wR2 = 0.183. For 2 at 228 K: C36H38Mg2N8O9, M = 775.4, orthorhombic, space group Aba2, a = 18.884(1), b = 8.868(1), c = 22.904(2) Å, U = 3835.6(6) Å3, Z = 4, Dc = 1.343 g cm21, F(000) = 1624, m = 0.127 mm21. 2846 Independent reflections (Rint = 0.0180) were collected with Mo-Ka (l = 0.710 73 Å) radiation 2.16 < q < 30.08.The final R values: R1 = 0.047 and wR2 = 0.098; R for all data: R1 = 0.08, wR2 = 0.140. CCDC reference number 186/994.1936 J. Chem. Soc., Dalton Trans., 1998, Pages 1935–1936 sequence of a strong ‘pulling eVect’ on the O (free) atom by two H-bonds, an intramolecular one with the bridging H2O [O(1) ? ? ? O(31) distances in 1, 2.648(6) and 2, 2.621(4) Å] and an intermolecular one with the non-co-ordinating NH group from the adjacent molecule [O(31) ? ? ? N(23B) in 1, 2.763(6) and in 2, 2.738(5) Å], giving rise to what can be regarded as a ‘pseudo-bridging’ six-membered ring arrangement in the terminal carboxylate groups.The second interesting observation is that the bidentate acetate bridges are markedly asymmetric [Mg]O(2) 2.098(4) and 2.080(3) Å, Mg]O(3) 2.017(4) and 2.026(2) Å for 1 and 2, respectively], although the molecule as a whole possesses a two-fold symmetry.This can be attributed in part to an additional intermolecular H-bond formed between O(2) and a NH group from an adjacent molecule [O(2) ? ? ? N(13C) 2.834(6) in 1 and 2.836(5) Å in 2]. The significance of the observed framework of H-bonds in the formation and stabilization of 1 and 2 can therefore be envisioned. The water molecule in a bridging fashion between two magnesium centers is stabilized by the intramolecular H-bonds formed with the terminal carboxylates.This H-bond may play a crucial role in the fixation and activation of the bridging H2O in the hydrolysis of the substrate by dimetallic hydrolases.2–4,6,13 The IR spectrum of 1 shows that there are two instead of four carboxylate absorption bands at 1624 [nas(CO2 2)] and 1423 cm21 [ns(CO2 2)], respectively, reflecting the structural similarity between the terminal monodentate and the bridging bidentate carboxylates. A broad band centered at ca. 2370 cm21 was observed in the FT-IR spectrum of 1, and is attributed to the O]H stretching of the bridging water.The unusual low Fig. 1 An ORTEP drawing of complexes 1 (top) and 2 (bottom), showing 40% probability thermal ellipsoids with the atom-labeling scheme. Important interatomic distances (Å) and angles (8) of 1 and 2 (in square brackets): Mg ? ? ? Mg 3.656(2) [3.625(2)], O(1) ? ? ? O(31) 2.648(6) [2.621(4)], O(31) ? ? ? N(23B) 2.763(6) [2.738(5)], O(2) ? ? ? N(13C) 2.834(6) [2.836(5)], Mg]O(1) 2.144(4) [2.131(2)], Mg]O(2) 2.098(4) [2.080(3)], Mg]O(3) 2.017(4) [2.026(2)], Mg]O(30) 2.072(4) [2.084(3)], Mg]N(11) 2.176(5) [2.187(3)], Mg]N(21) 2.204(5) [2.227(3)], C(30)]O(30) 1.255(7) [1.252(4)], C(30)]O(31) 1.251(7) [1.273(4)], Mg(1)]O(1)]Mg(1A) 117.0(3) [116.5(2)] vibrational frequency of the O]H band stretching is consistent with H2O involvement in strong H bonding as revealed also by the crystal structure.Similar phenomena were also observed for water-bridged dicobalt(II) complexes.14 The solid-state 13C NMR spectrum of 1 exhibits clearly two sets of resonances for the acetate groups, in agreement with their two binding modes, terminal monodentate and bridging bidentate, in the complex.The solid-state 15N NMR spectrum of 1 also displays two resonances for each nitrogen atom of the co-ordinated Him ligands, consistent with the crystal structure that the two Him ligands on a metal ion are chemically non-equivalent, with one trans to the bridging OAc ligand and the other trans to the bridging H2O, despite the fact that the two MgN2O4 cores in 1 are equivalent to each other by a two-fold axis.In conclusion, we have reported the syntheses and structural characterization of two novel dimagnesium(II) complexes with (m-aqua)bis(m-carboxylate) cores and with monodentate Ndonor Him and Bzim ligands. To the best of our knowledge, 1 and 2 represent the first example of dinuclear MgII complexes with (m-aqua)bis(m-carboxylate) bridges.The terminal monodentate carboxylates display an unusual geometry in 1 and 2, suggesting the possible existence of isomers in the coordination chemistry of carboxylate side chains of amino acids. Finally, we believe that the formation of strong H-bonds between the bridging water and the ancillary carboxylate groups in 1 and 2 provides a good mimic for the active sites of dimetallic hydrolases, in which the terminal carboxylate stabilizes and activates the bridging or bound water co-substrate.2–4,6,13 Acknowledgements This work was supported by the Hong Kong Research Grant Council and Hong Kong University of Science and Technology (X.-Y.L.). References 1 D. E. Wilcox, Chem. Rev., 1996, 96, 2435; N. Sträter, W. N. Lipscomb, T. Klabunde and B. Krebs, Angew. Chem., Int. Ed. Engl., 1996, 35, 2024. 2 J. D. York, J. W. Ponder, Z.-W. Chen, F. S. Mathews and P. 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Acta, 1995, 232, 189; (d) H. Schmidbaur, G. Muller, J. Riede, G. Manninger and J. Helbig, Angew. Chem., Int. Ed. Engl., 1986, 25, 1013; (e) H. Schmidbaur, I. Bach, D. L. Wilkinson and G. Muller, Chem. Ber., 1989, 122, 1433; ( f ) P. Ghosh and G. Parkin, Inorg. Chem., 1996, 35, 1429. 11 H. Ke, C. M. Thrope, B. A. Seaton, F. Marcus and W. N. Lipscomb, Proc. Natl. Acad. Sci. USA, 1989, 86, 1475. 12 C. Oldham, in Comprehensive Coordination Chemistry, eds. G. Wilkinson, R. D. Gillard and J. A. McCleverty, Pergamon, Oxford, 1987, vol. 2, p. 435. 13 Z. F. Kanyo, L. R. Scolnick, D. E. Ash and D. W. Christianson, Nature (London), 1996, 383, 554. 14 U. Turpeinen, R. Hämäläinen and J. Reedijk, Polyhedron, 1987, 6, 1603. Received 9th April 1998; Communication 8/02691B

ISSN:1477-9226    

DOI:10.1039/a802691b

出版商:RSC    

年代:1998

数据来源: RSC