Equatorial Coordination Research Articles

Structural and Solution Characterization of Mononuclear Vanadium(IV) Complexes That Help To Elucidate the Active Site Structure of the Reduced Vanadium Haloperoxidases.

The complexes [VO(H(2)O)ada] (1), [VO(H(2)O)Hheida] (2), and [VO(H(2)O)aeida] (3) (H(2)ada, N-(carbamoylmethyl)iminodiacetic acid; H(3)heida, N-(2-hydroxyethyl)iminodiacetic acid; H(2)aeida, N-(2-aminoethyl)iminodiacetic acid) were synthesized and crystallographically characterized. Crystallographic parameters for 1.2H(2)O: monoclinic, space group P2(1)/c (No. 14), a = 7.327(2) Å, b = 23.386(7) Å, c = 7.258(3) Å, alpha = 90 degrees, beta = 110.95(2) degrees, gamma = 90 degrees, V = 1204.6(7) Å(3), Z = 4, R1 = 0.0353, and wR(2)() = 0.0848. Crystallographic parameters for 2.H(2)O: orthorhombic, space group Pbca (No. 61), a = 10.512(2) Å, b = 11.727(2) Å, c = 16.719(5) Å, alpha = 90 degrees, beta = 90 degrees, gamma = 90 degrees, V = 2060.6(8) Å(3), Z = 8, R1 = 0.0297, and wR(2)() = 0.0758. Crystallographic parameters for 3: monoclinic, space group P2(1)/c (No. 14), a = 6.785(1) Å, b = 9.714(2) Å, c = 14.959(2) Å, alpha = 90 degrees, beta = 95.12(1) degrees, gamma = 90 degrees, V = 982.2(3) Å(3), Z = 4, R1 = 0.0298, and wR(2)() = 0.0762. In each structure, the tetradentate ligand is disposed so that the tertiary nitrogen is bound trans to the vanadyl oxo, and the rest of the donors occupy equatorial coordination positions. In solution, the structural integrity of these compounds is maintained as observed by UV/visible and EPR spectroscopies, and axial ligation by nitrogen is inferred on the basis of ESEEM spectroscopy. The implications of this study with respect to understanding the coordination environment of VO(2+) in the reduced, inactive form of vanadium bromoperoxidase (VBrPO) are discussed, and it is proposed that significant changes in the coordination environment of vanadium in VBrPO occur upon its reduction, which may provide a plausible explanation for its irreversible inactivation.

The metal-binding site of imidazole glycerol phosphate dehydratase; EPR and ENDOR studies of the oxo-vanadyl enzyme

The apo protein of imidazole glycerol phosphate dehydratase (IGPD) from Saccharomyces cerevisiae combines stoichiometrically with certain specific divalent metal cations to assemble the catalytically active form comprising 24 protein subunits and tightly bound metal. VO2+ ions react similarly but, uniquely, result in a metallo-protein (VO-IGPD) with neither catalytic activity nor the ability to bind to the reaction intermediate analogue, 2-hydroxy-3-(1,2,4-triazol-1-yl) propylphosphonate. Since VO2+ apparently assembles the quaternary structure correctly, it is used in the present study as a spin probe to investigate the metal centre coordination environment by electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopy. At neutral pH, the EPR spectrum of VO-IGPD reveals at least three distinct VO2+ sub-spectra with one predominant at low pH. The spin Hamiltonian parameters for some of the sub-spectra are consistent with 51V having nitrogen in the inner-sphere equatorial coordination environment from, most probably, multiple coordinating histidines. Further evidence for inner-sphere nitrogen ligands is obtained from ENDOR spectroscopy. The spectra of the low rf region show signals from interactions with 14N which are consistent with couplings to the imino nitrogen of coordinated histidine residues. In addition a number of proton ENDOR line pairs are resolved. Of the few that disappear upon exchange of the protein into D2O, one most likely originates from the exchangeable proton of the N-H group of a coordinated histidine imidazole. 1H-ENDOR line pairs from non-exchangeable protons with splittings of approximately 3 MHz can be attributed to imidazole carbon protons. Thus, most of the couplings observed by ENDOR are consistent with being from the imidazole heterocycle of one or more histidine ligands.

μ-Cyano-bis(2,2':6',2''-terpyridine-N,N',N'')dicopper(II) Perchlorate Acetonitrile, [Cu2(CN)(C15H11N3)2](ClO4)3.CH3CN

The [Cu2(terpy)2(CN)]3+ cation (terpy is 2,2′:6′,2′′-terpyridine) is non-centrosymmetric and contains two CuII centres bridged by a disordered cyano group. The overall geometry around both metallic centres can be described as a distorted octahedron. The equatorial coordination plane is occupied by the N atoms of the tridentate terpy ligand and by an atom (C* or N*) of the cyanide bridge. The axial positions for one Cu atom are occupied by O atoms of two perchlorate anions while, for the other Cu atom, a perchlorate ion and an acetonitrile molecule lie in trans positions.

Stereoisomerism and Equilibrium Properties of Oxygen-Carrying Cobalt(II) Complexes of Histamine and Its Derivatives: A New Approach to an Old System.

The complex formation equilibria between pH 2 and 11.2 in Co(II)-L-O(2) ternary systems (L = histamine, glycylhistamine, and sarcosylhistamine) have been studied by pH-metric, spectrophotometric, and (1)H-NMR spectroscopic methods. In contrast to earlier findings, we detected several protonation states of oxygen-carrying complexes in the pH range 7-11.2. The active species in oxygen uptake is the CoL(2) complex already suggested in the Co(II)-histamine-O(2) system; however, in the case of pseudopeptides, both CoLH(-)(1) and CoL(2)H(-)(1) complexes can take up oxygen. The (1)H-NMR study revealed stereoisomerism of oxygenated species in the same protonation states, undergoing slow ligand exchange. In the case of the Co(2)(Hist)(4)(OH)(O(2)) complex, among the 20 possible geometrical isomers, at least six can be distinguished by their imidazole proton signals. The most abundant isomers have axial-equatorial imidazole coordination, presumably on steric grounds. The stability constants determined by pH studies for the above systems proved that {4N}-coordinated species have a greater affinity for oxygen uptake than the {3N}-coordinated ones. They also showed the stabilizing effect of the deprotonated amide nitrogen in the oxygenated complexes of pseudopeptides.

Potentiometric and spectroscopic studies on the ternary complexes of copper(II) with dipeptides and nucleobases

Ternary complexes of copper(II) with dipeptides (GlyGly, GlyAsp, and MetVal) and nucleobases (MeC, MeA, MeUH, MeTH, EtGH, uridine, and thymidine) or derivatives of amino acids (Ac-Lys and Ac-Hm) were studied by potentiometric, UV-visible, and EPR spectroscopic methods. It was found that all ligands form stable adducts with the CuAH −1 species of dipeptides, and the ternary complexes formed by the coordination of imidazole nitrogen donor atom are the most favored in the physiological pH range. The formation of a dimeric species Cu 2A 2H −2B was detected in the case of EtGH supporting that both N1 and N7 nitrogens of purine take part in metal ion coordination. The data of spectroscopic measurements supported the equatorial coordination of nitrogen donors in CuABH −1 ternary complexes.

An NMR and single-crystal X-ray diffraction structural study of Ru II [12]aneS 4 polypyridyl complexes

The synthesis of a series of Ru II complexes with the thioether ligand [12]aneS 4 (1,4,7,10-tetrathiocyclododecane) and various polypyridyl ligands was carried out using [Ru(DMSO) 4CL 2] (DMSO- = dimethylsulphoxide) as a starting material. The first synthetic step involved the introduction of the thioether ligand and the isolation of the compound [Ru([12]aneS 4)Cl 2]. The polypyridyl ligands were then exchanged to give a series of complexes [Ru([12]aneS 4)(X)] 2+ [X = 1,10 phenanthroline (phen), 2,2′-bipyridyl (bpy) and 4,7′-diphenyl-1, 10-phenanthroline (dip)]. Crystal structures of the metal complexes [Ru([12]aneS 4)(bpy)]Cl 2·4H 2O, [Ru([12]aneS 4)(phen)]Cl 2·4.25H 2O and [Ru([12]aneS 4)(dip)]Cl 2·2H 2O all show the Ru centre in a distorted cis-octahedral environment, in which the equatorial coordination plane is formed by two S-donor atoms of the macrocyclic ligand [12]aneS 4 and two N-donor atoms of a polypyridyl ligand : bpy, phen or dip. The hexa coordination is completed via the remaining sulfur atoms of the macrocycle. The distortion of the octahedral geometry is associated with the small cavity size of macrocyclic ligand and small bite angle of the bidentate polypyridyl ligands [77.9(3)° bpy, 79.6(3) phen and 77.4(4)° dip]. The small cavity size of [12]aneS 4 precludes the sulfur atoms of the macrocycle achieving axial positions of an ideal octahedron leading to angles, S axRuS ax, of 162.5(2), 161.9(1) and 162.0(1)° for the bpy, phen and dip complexes respectively. A 1H and 13C NMR study of the complexes [Ru([12]aneS 4)(phen)] 2+, [Ru([12]aneS 4)(bpy)] 2+ and [Ru([12]aneS 4)(dip)] 2+ was carried out. The 1H spectra exhibit broad lines for the H-2 and H-9 protons of the polypyridyl ligand and the CH 2 protons of the [12]aneS 4 ligand. A variable temperature study indicated that exchange processes were taking place in solution, most probably involving cleavage of the RuN bond as a result of the strain on the octahedral environment of the small cavity size of the [12]aneS 4 macrocycle.

Activation of Hydrogen Peroxide for Oxidation by Copper(II) Complexes

Copper(II) complexes of the general formula [LCu(H2O)4]n+ (where L is a bidentate ligand andn = 1 or 2) activate hydrogen peroxide for the oxidation of quinaldine blue, an oxidation indicator. The copper(II) complexes of tri- and tetradentate ligands are shown to be inactive, as are the bis-complexes of bidentate ligands. The proposed mechanism for peroxide activation involves the formation of a copper(II)-hydroperoxide complex, which then rapidly oxidized the substrate. Comparison of reaction rates with different ligand systems, and different ligand-to-metal ratios, lead to the conclusion that the most active complexes are those in which two equatorial coordination positions are occupied by easily displaced water to form the active catalyst. Rate studies are performed which give an experimental rate law which is first order in copper(II) complex, zero order in substrate, and variable order in peroxide. These kinetics are predicated by the rate law derived from our proposed mechanism. The variable order in peroxide can be explained in terms of Michaelis-Menten-type kinetics, as linear Lineweaver-Burk plots of (rate−1) vs ([O2H−]−1) are obtained from our experimental data. This is consistent with our proposed mechanism, as the derived rate law can be rearranged into the Michaelis-Menton equation.

Coordination of uranyl(VI) carbonate species in aqueous solutions

There are very few examples in nature for U(VI) compounds with carbonate ligands other than the well known tricarbonates. Especially examples of U(VI) dicarbonato compounds are nearly completely missing. Even in aqueous solutions, the dicarbonato complex was found as a species of minorimportance only. On the basis of structural data on the ligands H2O and carbonate as well as the available data on U(VI) coordination compounds, steric requirements of equatorial coordination are studied for aqueous solution species. A pentagonally coordinated monocarbonato species [UO2CO3(H2O)3] is found as the most likely coordination. For the dicarbonato species, hexagonally coordinated [UO2(CO3)2(H2O)2] with D2h symmetry is found as most probable structure. Possible causes of the instability of U(VI) dicarbonato species are discussed.

Tetrakis[μ-(3-bromobenzoato)-μ-(2-dimethylaminoethanolato)-copper(II)

A cubane-type Cu 4 O 4 core has been found in the title tetrameric copper(II) complex {tetrakis(μ-3-bromobenzoato)-1κO: 2κO'; 2κO: 3 κO'; 3 κO: 4κO';-4κO:1κO'-tetrakis[μ-2-(dimethylamino)ethanolato] -1κN,1:2:3κ 3 O;2κN,2:3:4κ 3 O;3κN,3:4:1κ 3 O;4κN,4:1:2κ 3 O-tetracopper(II), [Cu 4 (C 7 H 4 BrO 2 ) 4 (C 4 H 10 NO) 4 ]}. The short Cu-O ethanolato bonds form an eight-membered ring folded in a boat-like conformation. The Cu...Cu distances vary between 3.121(2) and 3.796(3)A. Each Cu atom has a distorted octahedral environment. Two ethanolato O atoms, a carboxy O atom and an amino N atom form the equatorial coordination plane, with Cu-O bond lengths in the range 1.914(12)-1.954(14)A and Cu-N bond lengths in the range 2.011(12)-2.057 (13)A. The axial sites are occupied by an ethanolato O atom and an O atom of the carboxylate group, with Cu-O distances in the range 2.443 (8)-2.877 (9) A.

The Coordination of N,N‐Diethyl‐N′‐p‐tolylthiourea to Polynuclear Rhenium Carbonyl Complexes

AbstractThe reactions of Re2(CO)9(NCMe) and Re3(CO)10‐(CH3CN)2(μ‐H)3 with S=C(NEt2)N(H)(p‐tolyl) have yielded the new compounds Re2(CO)9[S=C(NEt2)N(H)(p‐tolyl)] (1) and Re3(CO)10(μ‐SC(NEt2)N(H)(p‐tolyl)](μ‐H)3 (2) in 89% and 98% yields, respectively. Compounds 1 and 2 were characterized by single‐crystal X‐ray diffraction analyses. Compound 1 contains an S‐coordinated S = C(NEt2)N(H)(p‐tolyl) ligand terminally coordinated in an equatorial coordination site on one of the rhenium atoms. Compound 2 contains the first example of an S‐coordinated bridging thiourea grouping. When solutions of 1 were heated to 97°C, it was transformed to the new compound {Re(CO)3[μ‐SC(N‐p‐tolyl)(NEt2)]}2 (3) plus Re2(CO)10. Compound 3 is a centro‐symmetrical dimer of the unit Re(CO)3[SC(N‐p‐tolyl)(NEt2)] in which the sulfur atom bridges the two rhenium atoms. The tolyl‐substituted nitrogen atoms are coordinated to the rhenium atoms to form two Re–S–C–N rings.

Synthesis of a Heteroditopic Cryptand Capable of Imposing a Distorted Coordination Geometry onto Cu(II): Crystal Structures of the Cryptand (L), [Cu(L)(CN)](picrate), and [Cu(L)(NCS)](picrate) and Spectroscopic Studies of the Cu(II) Complexes.

The synthesis of a new macrobicyclic cryptand (L) with heteroditopic receptor sites has been achieved in good yields by the [1 + 1] Schiff base condensation of tris(2-aminoethyl)amine (tren) with the tripodal trialdehyde, tris{[2-(3-(oxomethyl)phenyl)oxy]ethyl}amine at 5 degrees C temperature. The crystal structure of L (P2(1)/c, a = 10.756 (5) Å, b = 27.407(9) Å, c = 12.000(2) Å, beta = 116.22(3) degrees, Z = 4, R = 0.060, R(w) = 0.058) shows a pseudo-3-fold symmetry axis passing through the two bridgehead nitrogens. This symmetry is maintained in chloroform solution also, as indicated from its (1)H-NMR spectral data. The cryptand readily forms inclusion complexes with the Cu(II) ion at the tren end of the cavity. The tetracoordinated Cu(II) cryptate (1) thus formed with Cu(picrate)(2) exhibits a very small A(II) value (60 x 10(-)(4) cm(-)(1)) in its EPR spectrum and low-energy ligand field bands in its electronic spectrum in MeCN at room temperature. The bound Cu(II) ion readily accepts the anions CN(-), SCN(-), or N(3)(-), forming distorted trigonal bipyramidal complexes (2-4). The crystal structure of [Cu(L)(CN)](picrate) (2) (P2(1)/C, a = 13.099(1) Å, b = 11.847(8) Å, c = 25.844(7) Å, beta = 91.22(1) degrees, Z = 4, R = 0.056, R(w) = 0.054) has been determined. The equatorial coordination is provided by the three secondary amino N atoms of the tren unit in L, while the two axial positions are occupied by the bridgehead N of the tren unit and the C atom of the cyanide group. One of the equatorial Cu-N bond distances is 2.339(6) Å, which is longer than normal values. The crystal structure of [Cu(L)(NCS)](picrate) (3) (C2/c, a = 47.889(10) Å, b = 10.467(5) Å, c = 16.922(2) Å, beta = 93.90(2) degrees, Z = 8, R = 0.054, R(w) = 0.055) shows the coordination geometry around the Cu(II) ion to be very similar to that in the case of 2. The electronic spectral and EPR spectral data obtained on 2-4 are characteristic of trigonal bipyramidal Cu(II) complexes. The three meta-substituted benzene rings present in L makes the donor atom somewhat rigid in nature which enforces a distorted geometry around the Cu(II) ion.

Crystal structure, magnetic properties and spectroscopic studies of bis[1,3-bis(pyrazol-3-yl)triazenido]dicopper(II) trihydrate: a dinuclear complex with an asymmetric double pyrazolate bridge

The crystal and molecular structure of [{Cu(bpzt)}2]·3H2O [H2bpzt = 1,3-bis(pyrazol-3-yl)triazene] was determined by X-ray diffraction methods. The complex has a quasi-planar dinuclear structure with an asymmetric double pyrazolate bridge. The asymmetry results from the presence, on the C3 of each bridging pyrazolate, of a pyrazolyltriazenide chelating arm which completes the equatorial CuN4 co-ordination. The centrosymmetric dinuclear units have intramolecular Cu ⋯ Cu distances of 3.852(5) and 3.839(4)A respectively. The units are stacked in columns along the b axis. The cell also contains crossing channels filled with water molecules. The complex was characterized by spectroscopic and magnetic measurements. Magnetic susceptibility data show antiferromagnetic behaviour with an exchange parameter |J|= 151 cm–1 significantly decreased with respect to symmetric doubly bridged complexes (|J|=≈200 cm–1). The X-band powder EPR spectrum recorded at 77 K is typical of a triplet spin state lying above the singlet ground state. The results obtained for this complex are compared to recent literature data on other planar doubly bridged pyrazolate dicopper(II) complexes. A correlation, valid for symmetric as well as asymmetric systems, is described between the exchange parameter J and the deviation of the Cu–N–N bridge angle from the optimum value corresponding to the highest |J|.

Complexes With Biologically Active Ligands. Part 2. Preparation of Copper(II) Complexes of Positively-Charged Derivatives of Aminoglutethimide

Cu(II) complexes of 1-[4-(3-ethyl-piperidine-2,6-dione)-3-yl]-phenyl-2,4,6-trisubstituted pyridinium perchlorates, containing alkyl, aryl and combinations of these two types of moieties in their molecule were synthesized and characterized by elemental analysis, spectroscopy, magnetic, thermogravimetric and conductimetric measurements. In these complexes, Cu(II) ions are in octahedral geometry with four water molecules occupying the equatorial coordination sites and the two organic ligands in deprotonated state the remaining axial ones. The donor atom of these ligands is constituted by the ionized nitrogen of the glutarimide moiety. The new derivatives possess weak inhibitory activity towards the zinc enzyme carbonic anhydrase.

Reactions of the linear triosmium cluster [Os3H(CO)11(η2-C6F5NNNC6F5)] with [Os3(CO)12–n(NCMe)n](n= 1 or 2): crystal structures of [Os4(µ-H)(CO)14(η2-C6F5NNNC6F5)] and [Os5(µ-H)(CO)17(η2-C6F5NNNC6F5)

Reaction of the linear triosmium carbonyl cluster [Os3H(CO)11(η2-C6F5NNNC6F5)] with [Os3(CO)11(NCMe)] in hexane at 60 °C under vacuum afforded the ‘spiked’ tetraosmium cluster [Os4(µ-H)(CO)14(η2-C6F5NNNC6F5)]1. Reaction with [Os3(CO)10(NCMe)2] in CH2Cl2 at room temperature gave the ‘spiked’ hexaosmium cluster [Os6(µ-H)(CO)21(NCMe)(η2-C6F5NNNC6F5)]2. Cluster 2 is reactive and converts into the ‘spiked’ pentaosmium cluster [Os5(µ-H)(CO)17(η2-C6F5NNNC6F5)]3 and a known cluster [Os5(CO)16] respectively when heated with and without C6F5NNNHC6F5. Clusters 1 and 3 were characterized by single-crystal X-ray crystallography. The structure of 1 consists of a triangular unit with a Os(CO)3(η2-C6F5NNNC6F5) portion ‘spiked’ equatorially to it. The two nitrogen atoms in the triazenide ligand occupy an axial and an equatorial position in the Os(CO)3(η2-C6F5NNNC6F5) group. The hydride ligand bridges, in a cis manner, the Os–Os edge where the Os(CO)3(η2-C6F5NNNC6F5) portion is attached. The structure of 3 adopts a ‘4 + 1 spiked’ geometry hitherto unknown. Its metal core consists of a planar ‘kite-like’ Os4 unit with the Os(CO)3(η2-C6F5NNNC6F5) portion ‘spiked’ to one of the equatorial co-ordination sites of an osmium atom. The two nitrogen atoms in the triazenide ligand and the hydride are bonded as in cluster 1. Based upon 13C NMR studies, the structure of cluster 2 in solution was also deduced to have a ‘spiked’ feature with the linear triosmium fragment Os3(CO)11(η2-C6F5NNNC6F5) bound to one atom in the osmium triangle at an eqatorial co-ordination site. The transformation of cluster 2 and the formation of cluster 1 are also briefly discussed.

Double-Strand Cleavage of DNA by a Monofunctional Transition Metal Cleavage Agent

A copper-based transition metal complex has been designed which performs double-stranded cleavage of DNA in a nonrandom fashion. The complex, ((2S,8R)-5-amino-2,8-dibenzyl-5-methyl-3,7-diazanonanedioate)copper(II), presents an ammonium group on one side of the metal equatorial coordination plane to the DNA backbone phosphate groups, while the aromatic phenylalanine-derived side chains are constrained to the opposite side of the coordination plane toward the DNA groove. This structure was designed to bind at locations where phosphate groups are in proximity to accessible hydrophobic regions of the DNA. We have estimated single-strand break to double-strand break ratios for DNA strand scission by this complex under a variety of activation conditions, and they are substantially lower than that predicted by statistical models for a random DNA linearization process. This means that more double-strand breaks are produced per single strand break than can be accounted for by random coincident single-strand breaks. We have also investigated the formation of abasic sites, and found that at least as many abasic sites can be cleaved to linear DNA as are linearized in the initial cleavage reaction. We interpret this to mean that the complex binds both at the intact DNA surface for strand scission, and binds at nicked sites on the DNA (where the charged end groups of the nick are likely to be proximate to the accessible hydrophobic interior) for reactivation and complementary strand scission. Insofar as double-strand cleavage may be more potent biologically than single-strand cleavage as a source of lethal DNA lesions, the recognition characteristics of this complex may aid in the design of chemotherapeutic agents.

Two-Dimensional ESEEM Study of VO2+ Complexes with Imidazole and Histidine: Histidine Is a Polydentante Ligand

The oxovanadium cation, VO{sup 2+}, has been successfully employed as a spin probe in electron-nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM) studies of metalloprotein active sites. Here we demonstrate that the complex of VO{sup 2+} with histidine has been misinterpreted and that the dominant ESEEM pattern is produced by the {alpha}-amino group. Nevertheless, we show that histidine side chain ligands can be identified from either nitrogen hyperfine or quadrupole couplings. The application of 2D ESEEM to the [{sup 15}N]imidazole complex shows, in the same spectrum, both the coordinated and the remote ring nitrogens with very different isotropic hyperfine couplings. In the case of histidine ligation, equatorial coordination by both imine and amine nitrogen is demonstrated. The reproducible differences between both the hyperfine and the quadrupole constants of the amine and imine nitrogens are reflected in the shape and position of the lines in the 2D spectra and allow them to be distinguished in proteins. With both imine and amine coordination, the VO{sup 2+}/histidine complex cannot be considered an adequate model for coordination by the histidine side chain in proteins. The imidazole, as a monodentante ligand, is better for this role. 16 refs., 3 figs.

Copper (II)-thiolate complexes with novel tripodal- and tetrapodal-like benzimidazoles

The tripodal copper(II) thiolate complexes Cu(L1)(Cl)-2H2O and Cu(L1)(mim)(Cl) [H(L1) = N-(2-mercaptoethyl)-N,N-bis (pyrid-2′-ylmethyl)amine and mim = N-methylimidazole] have been isolated. Both contain monomeric copper(II) and display two ligand field bands and axial cryogenic e.p.r. spectra, suggesting a squarebased geometry. A copper(II) thiolate complex Cu(L2)(Cl)-H2O [H(L2) = N-(2-mercaptoethyl)-N,N′,N′-tris(benzimidazol-2′-ylmethyl)-1,2-ethanediamine] with a CuN5S chromophore has been also isolated. It exhibits only one ligand field band and an axial cryogenic e.p.r. spectrum, consistent with a distorted tetragonal coordination geometry. All the thiolates display intense S− → CuII charge transfer bands in the u.v. region, suggesting equatorial thiolate coordination. All of the complexes exhibit irreversible electrochemical behaviour.

Phase Transition of Hexaamminechromium(III) Pentachlorocuprate(II)

The crystal of [Cr(NH 3 ) 6 ][CuCl 5 ] shows a phase transition at 259.2 K. An X-ray diffraction study of the cubic and tetragonal phases has been carried out at 295 and 120K, respectively. Although the arrangement of the metal complexes remained unchanged through the phase transition, the trigonal bipyramidal [CuCl 5 ] 3- ion, which has D 3h symmetry in the cubic phase, deformed into approximate C 2v symmetry in the tetragonal phase, as observed in the isomorphous [Co(NH 3 ) 6 ][CuCl 5 ] crystal. Drastic changes were observed in the equatorial coordination plane. One of three equatorial Cu-Cl bonds was elongated and the Cl-Cu-Cl bond angle formed by the two shortened Cu-Cl bonds expanded to 143.6(1)°.

Factors controlling electron transfer reactions and stabilisation of uncommon oxidation states of chromium

Some experimental approaches to seek semi-quantitative understanding of factors controlling outer sphere electron transfer reactions of some transition metal complexes have been made. The relative importance of nuclear and electronic factors to outer sphere processes has been examined. By the manipulation of Franck-Condon or nuclear factors, it has now been possible to gain access into the chemistry of chromium in unusual oxidation states. An example of a reorganisation controlled electron transfer reaction involving Cr(IV)-Cr(III) system has been demonstrated. The bimolecular rate of reduction of diperoxoaquaethylenediamine chromium(IV) and diperoxodiethylenetriamine chromium(IV) is independent of the nature of th reductant employed viz. Fe2+ or VO2+ indicating that the generation of6 coordinate Cr(IV) species from7 coordinate of diperoxochromium(IV) reactant may be rate limiting. Similarly by increasing the barrier for the6 coordinate to4 coordinate structures through equatorial coordination of macrocyclic ligands, it has now been possible to detect through cyclic voltommogram the formation of relatively stable Cr(IV) species in the electrochemical oxidation of Cr(Me4[14] tetraene)(H2O) 2 3+ in aqueous sulphuric acid media. The kinetics and mechanism of the cerium(IV) and iodosyl benzene oxidation of Cr(salen)(H2O) 2 + and Cr(salprn)(H2O) 2 + have been investigated and kinetic and spectroscopic evidence for the formation of Cr(IV) transients and stable Cr(V) products has been presented. The relative importance of Franck-Condon factors in the oxidation of Cr(III) to Cr(IV) and Cr(V) states in different macrocyclic and multidentate ligand environments has been discussed.

Synthesis of monothiohydroxamic ligands and their lead complexes. Structures of N-methyl-3-pyridothiohydroxamic acid, bis( N-methyl-3-pyridothiohydroxamato) lead(II) and bis( N-cyclohexyl-phenylacetothiohydroxamato) lead(II)

An improved synthesis of thiohydroxamic acids is described. The reaction of carboxymethyl dithiophenylacetate with several substituted hydroxylamines in aqueous solution at neutral pH gives good yields of the resultant thiohydroxamic acids. The lead coordination chemistry of these compounds is being investigated as part of a program to develop lead-specific sequestering agents. The single crystal structure of bis( N-cyclohexyl-phenylacetothiohydroxamato) lead(II) shows that it is essentially trigonal bipyramidal, with the lead lone electron pair dominating the coordination geometry. The complex has crystallographic two-fold symmetry and the sulfur atoms occupy equatorial positions, with a PbO distance of 2.746 Å and PbO distance of 2.383 Å. The space group is P2 12 12 with a = 14.146, b = 18.003, c = 5.373 A ̊ , Z = 4 . The structure of bis( N-methyl-3-pyridothiohydroxamato) lead(II) shows a similar geometry but a different isomer. The approximate trigonal bipyramidal geometry has the equatorial coordination positions occupied by one oxygen and a sulfur. The PbS distances are 2.725 (equatorial) and 2.809 (axial) Å. The corresponding PbO distances are 2.497 and 2.349 Å. The space group is P2 12 12 1 with a = 7.512, b = 12.686, c = 21.583 A ̊ , Z = 4 . For comparison the crystal structure of the free ligand N-methyl-3-pyridothiohydroxamic acid has been determined. It is found in the E configuration (whereas the Z configuration is required for metal chelation). This represents the first such isomeric form for a thiohydroxamic acid, although both forms have been seen in hydroxamic acid crystal structures. The space groups is Pc, with a = 4.098, b = 8.999, c = 11.184 A ̊ , β = 100.48°, Z = 2 . The 207Pb NMR of a series of these complexes presents a convenient way to monitor the electronic environment of the lead nucleus; the resonances of these compounds occur in the range 1555-1493 ppm relative to PbMe 4.