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The compounds [(η5-C5H5)Co{SC(H)CRS}] [R = phenyl (1), pyridin-3-yl (2) or pyrazin-2-yl (3)] have been synthesized and characterized by elemental analysis, 1H NMR, mass spectrometry and X-ray crystallography. The variation in the UV/Vis and redox properties of these compounds alone and upon acidification has been investigated. In CH2Cl2 solution each compound undergoes a reversible one-electron reduction, and the EPR spectrum of each monoanion has been recorded. In the presence of a 5:1 excess of trifluoroacetic acid the one-electron reduction of both 2 and 3 occurs at a less negative potential. The information obtained has been interpreted with the aid of DFT calculations for [1]y, [2]y and [3]y (y = 0 or –1) and the monoprotonated forms [2H]z and [3H]z (z = +1 or 0), and this has provided insight into the nature of the redox-active orbitals in these complexes. The HOMOs and LUMOs of these species are delocalized and each involves contributions from cobalt, ene-1,2-dithiolate and R orbitals. The information from the experimental and theoretical investigations is used to suggest that, for the pyran ring-opened form of the molybdenum cofactor of oxygen-atom-transfer enzymes, the pterin may participate in the redox reactions involved in the catalytic cycle.

He II photoelectron spectral data for Ti(NO3)4(g) and Cu(NO3)2(g) are interpreted with the aid of ab initio molecular orbital calculations and show that the metal ligand bonding involves those orbitals which correlate with the 4e m.o.'s of the ligands; the ionization energies are consistent with the nitrato-groups of Ti(NO3)4(g) having a smaller overall negative charge than those of Cu(NO3)2(g).

The development of the coordination chemistry of dithiolene ligands is summarised, together with a consideration of the electronic structure of complexes of these ‘non-innocent’ ligands. This information provides a context for a consideration of the role of dithiolenes in natural systems, i.e . as the ligand that binds molybdenum (or tungsten) at the catalytic centre of an extensive series of enzymes. These enzymes catalyse the transfer of an oxygen atom to or from the substrate: e.g . the sulfite oxidases catalyse the conversion of sulfite to sulfate and the nitrate reductases catalyse the conversion of nitrate to nitrite. The nature of the catalytic centres of several of these enzymes has been determined and each involves one or two ‘molybdopterin’ (MPT) cofactors bound to a mononuclear metal centre via their dithiolene group. The biosynthesis of MPT is described and, given its nature, possible roles for this moiety in the function of the oxotransferase enzymes are discussed. The review concludes with a consideration of the coordination chemistry that has been stimulated by the present knowledge of the nature and function of the catalytic centres of these enzymes.

Two pro-ligands ((R)LH) comprised of an o,p-di-tert-butyl-substituted phenol covalently bonded to a benzimidazole ((Bz)LH) or a 4,5-di-p-methoxyphenyl substituted imidazole ((PhOMe)LH), have been structurally characterised. Each possesses an intramolecular O-H[dot dot dot]N hydrogen bond between the phenolic O-H group and an imidazole nitrogen atom and (1)H NMR studies show that this bond is retained in solution. Each (R)LH undergoes an electrochemically reversible, one-electron, oxidation to form the [(R)LH] (+) radical cation that is considered to be stabilised by an intramolecular O...H-N hydrogen bond. The (R)LH pro-ligands react with M(BF(4))(2).H(2)O (M = Cu or Zn) in the presence of Et(3)N to form the corresponding [M((R)L)(2)] compound. [Cu((Bz)L)(2)] (), [Cu((PhOMe)L)(2)] (), [Zn((Bz)L)(2)] and [Zn((PhOMe)L)(2)] have been isolated and the structures of .4MeCN, .2MeOH, .2MeCN and .2MeCN determined by X-ray crystallography. In each compound the metal possesses an N(2)O(2)-coordination sphere: in .4MeCN and .2MeOH the {CuN(2)O(2)} centre has a distorted square planar geometry; in .2MeCN and .2MeCN the {ZnN(2)O(2)} centre has a distorted tetrahedral geometry. The X-band EPR spectra of both and , in CH(2)Cl(2)-DMF (9 : 1) solution at 77 K, are consistent with the presence of a Cu(ii) complex having the structure identified by X-ray crystallography. Electrochemical studies have shown that each undergo two, one-electron, oxidations; the potentials of these processes and the UV/vis and EPR properties of the products indicate that each oxidation is ligand-based. The first oxidation produces [M(II)((R)L)((R)L )](+), comprising a M(ii) centre bound to a phenoxide ((R)L) and a phenoxyl radical ((R)L ) ligand; these cations have been generated electrochemically and, for R = PhOMe, chemically by oxidation with Ag[BF(4)]. The second oxidation produces [M(II)((R)L )(2)](2+). The information obtained from these investigations shows that a suitable pro-ligand design allows a relatively inert phenoxyl radical to be generated, stabilised by either a hydrogen bond, as in [(R)LH] (+) (R = Bz or PhOMe), or by coordination to a metal, as in [M(II)((R)L)((R)L )](+) (M = Cu or Zn; R = Bz or PhOMe). Coordination to a metal is more effective than hydrogen bonding in stabilising a phenoxyl radical and Cu(ii) is slightly more effective than Zn(II) in this respect.

The synthesis is described of a bis-ene-1,2-dithiolate pro-ligand, designed to model the stereochemical situation in the cofactor of the tungsten enzyme ferredoxin aldehyde oxidoreductase from Pyrococcus furiosus . Each masked ene-1,2-dithiolate unit is mounted on a pyrano[2,3- b ]tetrahydroquinoxaline tricycle, comparable to the pyrano[2,3- g ]tetrahydropteridines found in all molybdoenzymes and tungsten analogues. Hydrolytic release of the bis-ligand was confirmed by its entrapment as a double (η 5 -C 5 H 5 )Co complex.

The organic ligand of the cofactor of the oxomolybdoenzymes has been synthesised in the protected and masked form, 5. The key steps in the conversion of the previously prepared 3, a protected 5-(2-amino-4-oxopteridin-6-yl)-4-(1,2-dihydroxyethyl)-1,3-dithiol-2-one were: formation of the pyran ring by reaction with a chloroformate giving a protected 8-amino-4-hydroxymethyl-6-(alkyloxycarbonyl)-5a,6,9,10-tetrahydro-[1,3]dithiolo[4′,5′:4,5]pyrano[3,2-g]pteridine-2,10-dione, 10, cyanoborohydride reduction of the 11,11a-imine, protection at N-11, and finally conversion of the alcohol into a protected phosphate giving 5.

L-N(omega),N(omega)-dimethylarginine dimethylaminohydrolase-1 (DDAH-1) is a Zn(II)-containing enzyme that, through hydrolysis of side-chain methylated l-arginines, regulates the activity of nitric-oxide synthase. Herein we report the structural and functional properties of the Zn(II)-binding site in DDAH-1 from bovine brain. Activity measurements of the native and metal-free enzyme have revealed that the endogenously bound Zn(II) inhibits the enzyme. Native DDAH-1 could be fully or partially activated using various concentrations of phosphate, imidazole, histidine, and histamine, a process that is paralleled by the release of Zn(II). The slow activation of the enzyme by the bulky complexing agents EDTA and 1,10-phenantroline suggests that the Zn(II)-binding site is partially buried in the protein structure. The apparent Zn(II)-dissociation constant of 4.2 nm, determined by 19F NMR using the chelator 5F-BAPTA (1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid), lies in the range of intracellular free Zn(II) concentrations. These results suggest a regulatory role for the Zn(II)-binding site. The coordination environment of the Zn(II) in DDAH-1 has been examined by Zn K-edge x-ray absorption spectroscopy. The extended x-ray absorption fine structure observed is consistent with Zn(II) being coordinated by 2 S and 2 N (or O) atoms. The biological implications of these findings are discussed.

Reactions of [MoO2(acetylacetonate)2] with the proligands (N-hydroxyimino)diacetic acid (H3hidpa), R,R-2,2′-(N-hydroxyimino)dipropionic acid (R,R-H3hidpa) or R,S-2,2′-(N-hydroxyimino)dibutyric acid (R,S-H3hidba) yielded the compounds [PPh4][Δ,Λ-Mo(hida)2]·CH2Cl21, [H5O2][Δ-Mo(R,R-hidpa)2] 2, [PPh4][Mo(R,S-hidba)2]·2H2O 3a and Na[Δ,Λ-Mo(R,S-hidba)2]·¼iPr2O 3b, respectively. Reactions of H3hida with a methanolic solution of [PPh4][MoOCl4(H2O)] in the presence of NaOH (ca. pH 8) provided an alternative synthesis for 1. The complex of 1 when transferred into CH2Cl2 using [PPh4]Br yielded brown block-like crystals from a CH2Cl2–EtOH solution, however, 2 and 3b were crystallised from H2O and MeCN solutions with [H5O2]+ and [Na]+ counter cations, respectively. X-Ray crystallography confirmed the same distinctive eight-co-ordinate geometry of the complex anions of 1, 2 and 3b as identified for Amavadin, the form in which vanadium(IV) is bound in Amanita muscaria mushrooms. EPR and UV/vis spectra recorded for 1, 2 and 3a are consistent with the presence of molybdenum(V). Cyclic voltammetric studies using a glassy carbon working electrode in CH2Cl2 for 1 exhibited a reversible MoVI/MoV and a quasi-reversible MoV/MoIV redox couple at E1/2 = +0.96 and −0.99 V (vs. a saturated calomel electrode), respectively. Complex 3a also displayed a reversible MoVI/MoV redox couple at E1/2 = +0.77 V, whereas the MoV/MoIV couple was irreversible (Epc = −1.28 V). Additional electrochemical studies with 2 recorded a reversible MoVI/MoV redox couple in Me2SO (E1/2 = +0.77 V), however in H2O this one-electron oxidation process is irreversible.

DMSO reductase (DMSOR) from Rhodobacter capsulatus, well-characterised as a molybdoenzyme, will bind tungsten. Protein crystallography has shown that tungsten in W-DMSOR is ligated by the dithiolene group of the two pyranopterins, the oxygen atom of Ser147 plus another oxygen atom, and is located in a very similar site to that of molybdenum in Mo-DMSOR. These conclusions are consistent with W L(III)-edge X-ray absorption, EPR and UV/visible spectroscopic data. W-DMSOR is significantly more active than Mo-DMSOR in catalysing the reduction of DMSO but, in contrast to the latter, shows no significant ability to catalyse the oxidation of DMS.

Nitrous oxide reductase is the terminal component of a respiratory chain that utilizes N2O in lieu of oxygen. It is a homodimer carrying in each subunit the electron transfer site, CuA, and the substrate-reducing catalytic centre, CuZ. Spectroscopic data have provided robust evidence for CuA as a binuclear, mixed-valence metal site. To provide further structural information on the CuA centre of N2O reductase, site directed mutagenesis and Cu K-edge X-ray absorption spectroscopic investigation have been undertaken. Candidate amino acids as ligands for the CuA centre of the enzyme from Pseudomonas stutzeri ATCC14405 were substituted by evolutionary conserved residues or amino acids similar to the wild-type residues. The mutations identified the amino acids His583, Cys618, Cys622 and Met629 as ligands of Cu1, and Cys618, Cys622 and His626 as the minimal set of ligands for Cu2 of the CuA centre. Other amino acid substitutions indicated His494 as a likely ligand of CuZ, and an indirect role for Asp580, compatible with a docking function for the electron donor. Cu binding and spectroscopic properties of recombinant N2O reductase proteins point at intersubunit or interdomain interaction of CuA and CuZ. Cu K-edge X-ray absorption spectra have been recorded to investigate the local environment of the Cu centres in N2O reductase. Cu K-edge Extended X-ray Absorption Fine Structure (EXAFS) for binuclear Cu chemical systems show clear evidence for Cu backscattering at approximately 2.5 A. The Cu K-edge EXAFS of the CuA centre of N2O reductase is very similar to that of the CuA centre of cytochrome c oxidase and the optimum simulation of the experimental data involves backscattering from a histidine group with Cu-N of 1.92 A, two sulfur atoms at 2.24 A and a Cu atom at 2. 43 A, and allows for the presence of a further light atom (oxygen or nitrogen) at 2.05 A. The interpretation of the CuA EXAFS is in line with ligands assigned by site-directed mutagenesis. By a difference spectrum approach, using the Cu K-edge EXAFS of the holoenzyme and that of the CuA-only form, histidine was identified as a major contributor to the backscattering. A structural model for the CuA centre of N2O reductase has been generated on the basis of the atomic coordinates for the homologous domain of cytochrome c oxidase and incorporating our current results and previous spectroscopic data.

The binding of ligands to the iron-molybdenum cofactor (FeMoco) from Klebsiella pneumoniae nitrogenase has been studied by XANES and EXAFS at the Fe. Mo and Se k-edges and by EPR. Ligands investigated include the anions derived from thiophenol, 2-bromophenylthiol, phenylselenol, a trithiolate ligand, 1, and the cyanate and thiocyanate anions. Evidence has been obtained directly that phenylselenol, and indirectly that thiophenol, 2-bromophenylthiol and ligand, 1, are bound by iron. For FeMoco plus phenylselenol, an iron-selenium distance of 2.36 A was determined from the Fe k-edge data and 2.38 A from the Se k-edge data. The results suggest that one selenol (and by analogy thiophenol and 2-bromophenylthiol) is bound in a non-bridging mode by FeMoco. Binding of ligand 1 led to a large splitting of the Fe shell at the Mo k-edge. At the Fe k-edge, binding of ligand 1 led to large decreases in the intensity of the iron-iron contacts. No evidence was found for cyanate or thiocyanate binding to Fe.

The synthesis and the structure of the new ligand bis(1-methyl-4,5-diphenylimidaz-2-oyl)carbinol (bimOH) is reported; bimOH, C33H28N4O, 496.61, triclinic P1¯, a = 10.020(9), b = 14.848(8), c = 9.57(1) A, α = 104.98(5), β = 96.86(8), γ = 76.95(6)°, V = 1337(2) A3, Z = 2, R = 0.067, Rw = 0.075, bimOH reacts with CuII forming Cu(bimOH)22+ and salts have been synthesised with the counter-anions CuCl42, CuBr42, BF4 and PF6. The structures of Cu(bimOH)2CuCl4 · H2O and Cu(bimOH)2PF62 · 2EtOH have been determined. Cu(bimOH)2CuCl4 · H2O. C66H56N8Cl4Cu2O2.5, 1270.12, monoclinic C2/c, a = 27.584(9), b = 18.50(2), c = 12.677(5) A, β = 91.71(3)°, V = 6467(9) A, Z = 4, R = 0.069, Rw = 0.082: Cu(bimOH)2PF62 · 2EtOH, C70H68N8CuF12O4P2, 1436.2, orthorhombic Pbca, a = 23.928(5), b = 23.570(6), c = 11.587(9) A, V = 6535(8) A3, Z = 4, R = 0.084, Rw = 0.093. In both of these compounds, the CuIIN4 centres possess an almost square-planar geometry. The powder EPR spectra of these compounds are characteristic of magnetically dilute CuIIN4 square-planar centres: g = ∼2.21, gv, = ∼2.05, A = ∼200 × 10−4 cm1. The cyclic voltammograms of these CuII compounds display a one-electron reversible process attributed to the Cu(bimOH)22 1/4 couple at a potential of ca. +0.36 V in CH2Cl2 versus SCE. Reaction of bimOH with Zn(BF4)2 · xH2O yields Zn(bimOH)2BF42: C66H56N8B2F8O2Zn, 1176.18, triclinic P1¯, a = 14.861(4), b = 16.408(4), c = 14.336(2) A, α = 112.85(2), β = 98.72(2), γ = 105.75(2)°, V = 2967(1) A3, Z = 2, R = 0.106, Rw = 0.117; and the ZnIIN4 centre possesses a geometry reasonably close to tetrahedral and this is related to the geometry proposed for the CuIN4 centre with the bimOH ligand. The implications of these results for the Cu centres in proteins are discussed.

The coupling of 2-iodoquinoxaline to 4-(2,2-dimethyl-1,3-dioxolan-4-yl)-5-(tri-n-butylstannyl)-1,3-dithiol-2-one5 gave 4-(2,2-dimethyl-1,3-dioxolan-4-yl)-5-(quinoxalin-2-yl)-1,3-dithiol-2-one 4.

A detailed study of the soft X-ray photochemistry occurring in some nickel dithiocarbamate and xanthate complexes has shown that soft X-ray induced photoreduction of [NiIV(S2CNEt2)3][BF4] results in a NiII square-planar species and the rate is dependent on photon flux, photon energy and ligand set, but is essentially independent of temperature. Also, it has been demonstrated that [PPh4][NiII(S2COEt)3] is photoisomerised to a NiII square-planar species on exposure to soft X-rays. The implication for experiments on third generation synchrotron sources is discussed briefly.

The reaction of [WO2(CN)4]4– with a series of dimethylamino- or thione-protected asymmetric dithiolenes (ene-1,2-dithiolates) has been used to synthesise [WO(dithiolene)2]2– complexes (dithiolene = –SC(H)C(R)S–, R = phenyl, pyridin-2-yl, pyridin-3-yl, pyridin-4-yl or quinoxalin-2-yl), which have been characterised by elemental analysis, spectroscopy and electrochemistry. The prototypical compound [PPh4]2[WO(sdt)2]·EtOH [sdt = styrenedithiolate, –SC(H)C(Ph)S–], crystallises in space group P21/c with a = 13.242(4), b = 31.894(8), c = 14.589(5) A, β = 113.80(2)°, and Z = 4. The WOS4 moiety is square-based pyramidal with the O atom at the apex [WO 1.724(7) A, W–Sav 2.37 A, O–W–Sav 108.7°] and contains a cis geometry of the phenyl groups, although 1H NMR studies indicate that both cis and trans isomers are present in the isolated solid. The physical properties of all of the [WO(dithiolene)2]2– complexes are consistent with a retention of the WIVOS4 centre and each system undergoes a reversible electrochemical one-electron oxidation to the corresponding WV system. The variation in the dithiolene 1H NMR resonances, ν(WO) and ν(CC) (dithiolene) IR stretching frequencies, and the E½ values for the WV–WIV couple can be rationalised by a consideration of the nature of the aryl substituent. These data are compared to those of the analogous MoIV complexes and to those of the catalytic centres of the tungstoenzymes and their oxomolybdoenzyme counterparts.

Q-band single-crystal and powder EPR spectra at multiple frequencies (X, Q and 180 GHz) of V(IV)-doped Ca[Ti(hida)2]·6H2O and Mo(V)-doped [PPh4][Nb(hida)2] and [NEt4][Ta(R,R-hidpa)2] (H3hida=2,2′-(hydroxyimino)diacetic acid; H3hidpa 2,2′-(hydroxyimino)dipropionic acid) are reported. The Ti{V} system has the spin-Hamiltonian parameters g11=1.910, g22=1.987, g33=1.990, A11=176.4 G, A22=53.5 G and A33=46.3 G with non-coincidence between the principal axes of the g- and A-matrices. The Euler angles for the non-concidence are α=11.5°, χ=4.63° and γ=346.0°. The triclinic EPR symmetry is consistent with the low (C1) point symmetry of the [Ti{V}(hida)2]2- anion in the solid state. The small angles of non-coincidence between the principal axes of the g and A matrices are also evident from the Q-band powder spectrum, but not the X-band spectrum. High-frequency (34–180 GHz) EPR measurements on powders of both the Nb{Mo} and Ta{Mo} systems reveal the presence of two magnetically distinct Mo centers in each case. The Nb{Mo} system has g-values of g11(a)=1.9765, g11(b)=1.9755, g22(a)=1.9675, g22(b)=1.9665, g33(a)=1.8870 and g33(b)=1.8840, while the Ta{Mo} system has g11(a)=1.976, g11(b)=1.974, g22(a)=1.970, g22(b)=1.967, g33(a)=1.894 and g33(b)=1.892.

A combination of vanadium K- and L-edge XAFS has been used to characterise a series of monomeric oxovanadium(IV), monomeric dioxovanadium(V) and dimeric oxovanadium(V) complexes with oxyoxime and oxyoximate ligands. The K- and L-edge spectra confirm the presence of VV in the dimeric species and the L-edge spectra have been used to discriminate between six-co-ordinate V–N–O–V bridged oxovanadium(V) dimers and seven-co-ordinate phenolate bridged oxovanadium(V) dimers containing η2-N–O groups.

Ab initio calculations have been performed on 7,8-dihydro-2-aminopteridin-4(3H)-one and on 39 dihydropteridine tautomers of it leading to an assessment of their relative energies and thence the likelihood of their involvement in the operation of the cofactor of the oxomolybdoenzymes, Moco. The effect of a polar environment on the tautomer energetics has been explored using a continuum model. At the most advanced level of calculation, and in the presence of water, the 7,8-dihydro-3H-(T1) and 5,6-dihydro-3H- (T4) tautomers are essentially isoenergetic, with 7,8-dihydro-6H- (T13) and 5,8-dihydro-3H- (T3) tautomers only 8.0 and 15.5 kJ mol-1 more energetic than T1. Any of these four tautomers can reasonably be implicated in the mode of action of Moco. The ortho quinonoid tautomer (T2) is of considerably higher energy and seems much less likely to be implicated.

The new pro-ligand meso-2,2′-(hydroxyimino)dibutyric acid (R,S-H3hidba) has been synthesised from hydroxylammonium chloride and 2-bromobutyric acid. Reaction of H3hidba with [VO(acac)2] (acac = acetylacetonate) yielded the complex [V(R,S-hidba)2]2– 1, which was crystallised in the presence of Ca2+ ions from H2O as blue tabular crystals. X-Ray crystallography confirmed the same distinctive eight-co-ordinate structure of the complex anion as identified for Amavadin, the form in which vanadium(IV) is bound in Amanita muscaria mushrooms. The crystal structure of 1 contains a network of linked Ca and V centres where the asymmetric unit consists of a Ca2V2 box-like configuration. The Ca and V metal ions are bridged to one another via a series of unidentate and bidentate carboxylate groups from H3hidba, extending throughout the lattice framework. The novel interaction between Ca2+ ions and Amavadin-style complexes has been further illustrated in the crystal structure of [Ca(H2O)5][V(hida)2]·H2O (H3hida = N-hydroxyiminodiacetic acid) 2. The unit cell packing arrangement observed for 2 differs from 1, comprising helical chains formed by alternate Ca and V units linked only by unidentate carboxylate groups. Cyclic voltammetric studies of 1 exhibited a reversible VV–VIV redox couple in H2O (E½ = +0.43 V, vs. saturated calomel electrode), this oxidation potential is considerably lower in organic solvents (e.g. Me2SO, E½ = –0.07 V). The chemical oxidation of 1 in aqueous medium by ammonium ceric nitrate produced a dark red solution which was transferred into CH2Cl2 using [PPh4]Br. From this solution [PPh4][V(R,S-hidba)2] 3 was isolated and studied using 1H, 13C and 51V NMR spectroscopy. The cyclic voltammogram of 3 also displays a reversible VV–VIV redox couple in CH2Cl2 (E½ = –0.09 V).

The reaction of N-hydroxy-2,2′-iminodipropionic acid with [PPh 4 ][ReOBr 4 ], [ReOCl 3 (PPh 3 ) 2 ], [ReOCl 2 (OEt)(PPh 3 ) 2 ] or [PPh 4 ][MoOCl 4 (H 2 O)] has produced four novel complexes: [ReOBr 3 (C 6 H 8 NO 5 )] - , [ReOBr 3 (C 3 H 3 NO 2 )] - , [ReOCl 2 (R,S-idpa)] - (H 3 idpa = 2,2′-iminodipropionic acid) and [MoO 2 Cl 2 (C 3 H 4 NO 3 )] - . All have been successfully crystallised as their [PPh 4 ] + salts which have been characterised by single-crystal X-ray diffraction. The anions [ReOBr 3 (C 6 H 8 NO 5 )] - and [ReOBr 3 (C 3 H 3 NO 2 )] - involve tetragonally distorted octahedral Re V OBr 3 (N,O) centres with the bromides cis to the oxo-group; [ReOCl 2 (R,S-idpa)] - involves a tetragonally distorted octahedral Re V OCl 2 (O,N,O) centre with the 2Cl cis and O trans to the oxo-group; [MoO 2 Cl 2 (C 3 H 4 NO 3 )] - involves a skew-trapezoidal Mo VI O 2 Cl 2 (N,O) centre with essentially mutual cis oxo-groups and trans chloro-groups. The dimensions of each of these co-ordination spheres are generally typical of related complexes; however, [ReOBr 3 (C 3 H 3 NO 2 )] - involves an unusually short Re–O oxo bond of 1.45(4) A. The organic moieties are considered to arise from rearrangement and/or fragmentation of N-hydroxy-2,2′-iminodipropionic acid, presumably under the influence of the metal centre.

The compound [TiO(acac)2] (acac = acetylacetonate) reacts with N-hydroxyiminodiacetic acid (H3hida) or (R,R)-2,2′-(hydroxyimino)dipropionic acid (R,R-H3hidpa) to yield [Ti(hida)2]2– or [Ti(R,R-hidpa)2]2–, respectively. The anion [Ti(R,R-hidpa)2]2– has been obtained from H2O in the presence of Ca2+ as [Ca(H2O)4][Ca(H2O)5]3[Δ-Ti(R,R-hidpa)2]4·2H2O 3 which crystallises in the space group P21 (no. 4) with a = 14.258(2), b = 16.516(3), c = 19.169(3) A, β = 110.907(9)°; least-squares refinement of 6082 reflections and 1126 variables converged with agreement factors of R = 0.069 and R′ = 0.054. The anions possess the co-ordination geometry identified for other Amavadin-style complexes, viz. eight-co-ordination achieved by the ligation of two mutually trans and staggered η2-NO groups and four unidentate carboxylate oxygen atoms; Ti–N(NO) 2.04(1)–2.10(1), Ti–O(NO) 1.962(9)–2.001(9), Ti–O(CO2) 2.024(9)–2.100(9), N–O 1.38(1)–1.44(1) A. The lattice of compound 3 comprises of parallel zigzag strands of alternating [Δ-Ti(R,R-hidpa)2]2– and [Ca(H2O)x]2+ (x = 4 or 5) linked interstrand by CaII binding to carboxylato-oxygen atoms and intra- and inter-strand by hydrogen bonding from the H2O molecules to the oxygen atoms of the anions. The anions are obtained exclusively in the Δ-helical form and this has enabled 1H and 13C NMR studies to follow epimerisation at the TiIV centre to produce a mixture of the Δ- and Λ-helical forms. Carbon-13 NMR spectroscopy has been used to investigate the interaction of the carboxylato groups of [Ti(hida)2]2– with solvents.

The nature of bonding in a series of complexes [CoL 2 ] n + [L = the tripodal ligand tris(2-pyridyl)methane or tris(2-pyridyl)phosphine, n = 1–3] has been investigated by single-crystal X-ray diffraction, X-ray absorption and electronic spectroscopy and density functional theory ab initio calculations. The structural studies reveal that the cobalt ions each exist in a distorted octahedral geometry defined by six N-donor atoms; the cations are all centrosymmetric. In both series of complexes the bond lengths Co I –N ≈ Co II –N > Co III –N. Data from the various studies indicate that the ‘cobalt(I)’ state of the complex [Co{X(C 5 H 4 N-2) 3 } 2 ] + (X = CH or P) is better described by the d 8 cobalt(I)–ligand formulation rather than as d 7 cobalt(II)–ligand radical.

Cobalt(III) complexes are reported in which (a) a dihydroquinoxalinylethenedithiolate ligand models the pyrazine ring oxidation level in Moco, and (b) a pteridinylethenedithiolate models the pteridine ligand in Moco.

The first observation of soft X-ray induced excited spin state trapping has been made for [Fe(phen)2(NCS)2] and [Fe(phen)2(NCSe)2] (phen = 1,10-phenanthroline) at ca. 45 K, analogous to light induced excited spin state trapping but at much higher energy. Soft X-ray photochemistry has also been observed in the high- and low-spin states of [Fe(phen)2(NCS)2] and [Fe(phen)2(NCSe)2], resulting in the formation of a thermally irreversible low-spin state. The rate of formation of this low-spin state is greater from low than from high spin, and is also greater in [Fe(phen)2(NCSe)2] than in [Fe(phen)2(NCS)2].

A synthetic strategy is reported for a general route to asymmetric dithiolenes. This has been used for the generation of [MoO(dithiolene)2]2– complexes [dithiolene = –SC(H)CS–, R = phenyl, pyridin-2-yl, pyridin-3-yl, pyridin-4-yl, quinoxalin-2-yl or 2-(dimethylaminomethyleneamino)-3-methyl-4-oxopteridin-6-yl], which have been characterised by analysis, spectroscopy and electrochemistry. The prototypical compound [PPh4]2[MoO(sdt)2]·EtOH, where sdt = α,β-styrenedithiolate (R = phenyl), crystallises in the space group P21/c with a = 13.217(4), b = 31.820(8), c = 14.534(7) A, β = 113.8(2)° and Z = 4. The MoOS4 moiety is square-based pyramidal with the O atom at the apex [MoO, 1.700(5) A] and contains a cis geometry of the phenyl groups. The physical properties of all the compounds are consistent with a retention of the MoOS4 centre and the variation in the dithiolene 1H NMR resonances, infrared ν(MoO) and ν(CC) stretching frequencies and the E½ values for the MoV–MoIV couple are rationalised by a consideration of the nature of the R substituent. The complexes with the pterin [2-amino-4(1H)-pteridinone] substituent have a particular relevance to the Mo centre in oxomolybdoenzymes, and are the closest structural models to date for these enzymes containing two molybdopterin ligands per metal.

Quick scanning X-ray absorption fine structure (QXAFS) spectroscopy is applied to an examination of the pyrolytic decomposition of nickel formate dihydrate. Experimental QXAFS data reveal good quality edge and extended XAFS spectra being obtained in ca. 4 and 30 s, respectively. Comparison of data with normal (step/count) techniques reveals no adverse effect on the structural parameters derived using quick scanning. Temperature-programmed and isothermal decomposition studies are consistent with a pyrolysis mechanism involving rapid initial dehydration of the dihydrate followed by subsequent decomposition of the anion, via a nickel oxide intermediate, to yield a finely divided nickel metal (mean particle diameter ca. 12 A). These data confirm the nature of the principle intermediate and final product, both previously uncertain.

Reactions of M(OEt) 5 (M = Nb or Ta) with N-hydroxyiminodiacetic acid (H 3 hida) yielded the complexes [M(hida) 2 ] - (M = Nb 1 or Ta 2). Single-crystal X-ray diffraction revealed the same novel eight-co-ordinated structure identified for Amavadin, the form in which vanadium(IV) is bound in Amanita muscaria mushrooms. This type of structure gives rise to chirality at the metal, where 1 and 2 consist of an equimolar mixture of the Δ and Λ helical forms in the crystal lattice. The complexes [Nb(R,R-hidpa) 2 ] - 3a and [Nb(R,S-hidpa) 2 ] - 3b [H 3 hidpa = 2,2′-(hydroxyimino)dipropionic acid] were synthesized via the route used for 1 and 2, and X-ray crystallography confirmed the presence of the same Amavadin-style geometry. Cyclic voltammetry in CH 2 Cl 2 showed that at 254 K 1 and 3a exhibit quasi-reversible reductions at E ½ = -1.22 and -1.35 V (vs. saturated calomel electrode), respectively, whereas 2 displays an irreversible reduction (-1.92 V).

The new diimidazole ligand, bis (1-methyl-4,5-diphenylimidazol-2-yl)ketone (BIMDPK), has been synthesised, characterised and shown to form four-coordinate bis(diimidazole) CuII and CuI complexes in the salts [Cu(bimdpk)2]-[BF4]2 and [Cu(bimdpk)2][PF6], the structures of which have been determined by X-ray crystallography. The cations of these salts have a very similar geometry with CuII–Nav = 1.949 and CuI–Nav = 1.999 A; the N-Cu-N interbond angles are constrained by 1) the bite angle of the BIMDPK ligand to 94 ± 2° and 2) the interligand steric interactions, which lead to the dihedral angle of the intraligand CuN2 planes of 68.2° for CuII and 74.9° for CuI–that is, a CuN4 geometry intermediate between tetrahedral and square planar. The X-band EPR spectrum for the powdered CuII compound is typical of an approximately D2 CuN4 centre possessing a dxy ground state (gx = 2.080, gy = 2.075, gz = 2.291; Az = 112.3 × 10−4 cm−1). The UV/vis spectra are dominated by charge-transfer bands, and both the CuII and CuI systems are intensely coloured. The EPR and electronic spectra indicate that these cations have a very similar structure in the solid state and in solution; the potential of the [Cu(bimdpk)2]2+/[Cu(bimdpk)2]+ couple is 0.59 V vs. SCE in MeCN and 0.80 V vs. SCE in CH2Cl2, and the electron self-exchange constant in MeCN is 1.9 × 104M−1S−1. Comparisons are made between the properties of the [Cu(bimdpk)2]2+/+ centres and related Cu centres in chemical and biological systems; the results of this study reinforce the view that a [Cu(His)4] centre should not be precluded from consideration in biological electron transport.

Structural models for the hydroxo(oxo)iron core of ferritin include extended mineral structures and cluster systems such as [Fe11O6(OH)6(O2CPh)15](= Fe11) and two clusters that crystallize in the same lattice (compd. 1), namely, [Fe17(?3-O)4(?3-OH)6(?2-OH)10(heidi)8(H2O)12]3+ and [Fe19(?3-O)6(?3-OH)6(?2-OH)8(heidi)10(H2O)12]1+. The suitability of these systems as models for the structure of the core of ferritin has been tested by comparing their Fe K-edge EXAFS and X-ray crystallog. results with the Fe K-edge EXAFS data on horse spleen ferritin. The interpretative procedure for the EXAFS anal. was optimized by using the X-ray crystallog. data for compd. 1 as a basis. This protocol was then used to interpret the Fe K-edge EXAFS spectra of ?- and ?-Fe(O)OH and to reinterpret that previously recorded for horse spleen ferritin. The published Fe K-edge EXAFS data on Fe11 were also considered. The Fe���Fe distances provide a clear indication of the nature of the hydroxo(oxo)iron assembly. It was found that the iron-iron interactions are the most reliable guide. Clusters are shown to be more appropriate than infinite lattices as structural models for the core of ferritin. [on SciFinder(R)]

Bond distances, vibrational frequencies, gas-phase energetics, and proton affinities for various thioarsenite molecules and ions are predicted from molecular orbital theory and used to interpret EXAFS and Raman spectra of dissolved thioarsenites in undersaturated, alkaline 1 M NaHS solutions. From MO predictions, Raman peaks at 325 and 412 cm(-1) are assigned to AsS(SH)(2)(-) and a peak at 382 cm(-1) to AsS2(SH)(2-). At alkaline pH, As-S distances in dissolved thioarsenites are 2.21-2.23 Angstrom and no statistically significant As-As interactions are recorded, consistent with predominance of the monomers, AsS(SH)(2)(-) and AsS2(SH)(2-). Estimated proton affinities suggest that thioarsenites with a negative charge greater than 2 are unstable in water. In seeming contradiction to this spectroscopic evidence, a new analysis of published solubility studies reinforces previous inferences that the trimer, As3S4(SH)(2)(-), is the predominant thioarsenite in systems saturated with As2S3. Previously proposed dimeric species of the form, HxAs2S4x-2, are rejected based on predicted thermodynamic properties. Dimer plus tetramer combinations also are rejected. Estimated free energies for AsS(OH)(SH)(-) and AsS(SH)(2)(-) are presented. We reconcile the spectroscopic and solubility evidence by showing that in undersaturated solutions monomers can become thermodynamically favored over oligomers. This pattern should be looked for in other sulfide systems as well. Sulfidic natural waters are in many cases undersaturated with respect to As2S3 phases, so monomeric thioarsenites could be more important in nature than the trimers that have been characterized in saturated solutions. EXAFS spectra show that amorphous As2S3 resembles orpiment in the first shell around As, but that higher shells are disordered. Disorder may be caused by occasional realgar-like, As-As bonds, consistent with the observation that amorphous As2S3 is slightly S deficient.

The structure of the native zinc form of the DNA binding domain in the yeast transcriptional activator PPR1 was investigated by extended X-ray absorption fine structure (EXAFS). By carrying out the EXAFS measurements at 11k we were able to demonstrate explicitly the proximity of the two zinc ions ( ZnZn distance = 3.16 ± 0.03 A ) and the presence of bridging cysteine ligands. The results show that the six cysteine residues co-ordinate two zinc ions in a two-metal ion cluster. PPR1 is the first member of this class of protein for which such information has been obtained.

X-Ray absorption spectra at the chromium and cobalt K edges have been recorded for the metal-disordered compound [CrIII2CoIIO(MeCO2)6(py)3]·py (py = pyridine) and the associated extended X-ray absorption fine structure (EXAFS) analysed using curved-wave theory with the inclusion of multiple scattering for the co-ordinated pyridine and acetate groups; a corresponding study for the iron K-edge EXAFS of [Fe3O(MeCO2)6(py)3]·py was accomplished as a calibrant of the interpretive procedure. The dimensions Cr–µ3-O 1.87, Co–µ3-O 1.95, Cr–O (acetate) 1.98, Co–O (acetate) 2.09, Cr–N 2.26, Co–N 2.24, Cr ⋯ Cr 3.30 and Cr ⋯ Co 3.28 A were obtained from the analysis of the EXAFS, with good agreement between the values of the Cr ⋯ Co distance obtained independently from the analyses of the chromium and cobalt EXAFS. The metal–ligand and –metal distances are in good agreement with crystallographic data on related compounds. The difference in the Cr–µ3-O and Co–µ3-O distances indicates that the central (µ3) oxygen atom is slightly displaced (ca. 0.05 A) from the centre of the metal triangle, away from the CoII and towards the point midway between the two chromium(III) atoms. Such a structural change and the increased electron density of the central oxygen combine to produce a marked increase in the CrIII–O–CrIII superexchange and a reduced ligand field at the chromium(III) sites, for the Cr2CoO system as compared with CrIII3O analogues.

The first example of a Donor-spacer-Acceptor tryad, based upon a molybdenum-ene-1,2-dithiolate unit as the Donor and a naphthalene-diimide as the Acceptor, has been synthesized and its photophysical properties investigated. Synthesis required the preparation of a new pro-ligand containing a protected ene-1,2-dithiolate bound through a phenyl linkage to a naphthalenetetracarboxylicdiimide (NDI) group. Deprotection of this pro-ligand by base hydrolysis, followed by reaction with [Cp(2)MoCl(2)], produced the new dyad [Cp(2)Mo(SC(H)C(C(6)H(4)-NDI)S)] (2). Electrochemical studies showed that 2 can be reversibly oxidized to [2](+) and reduced to [2](-), [2](2-), and [2](3-). These studies, augmented by UV/vis, IR, and electron paramagnetic resonance (EPR) spectra of electrochemically generated [2](+) and [2](-), show that the highest occupied molecular orbital (HOMO) of 2 is ene-1,2-dithiolate-based and the lowest unoccupied molecular orbital (LUMO) is NDI-based; these conclusions are supported by density functional theory (DFT) calculations for the electronic ground state on a model of 2 which also showed that these two parts of the molecule are electronically distinct. The dynamics of the excited states of 2 in CH(2)Cl(2) solution were investigated by picosecond time-resolved IR spectroscopy following irradiation by a 400 nm ∼120 fs laser pulse. These investigations were complemented by an ultrafast transient absorption spectroscopic study from 420 to 760 nm of the nature of the excited states of 2 in CH(2)Cl(2) solution following irradiation by a 383 nm ∼120 fs laser pulse. These studies showed that irradiation of 2 at both 400 and 383 nm leads to the formation of the [(Cp)(2){Mo(dt)}(+)-Ph-{NDI}(-)] charge-separated state as a result of a cascade electron transfer initiated by the formation of an (1)NDI* excited state. (1)NDI* rapidly (ca. 0.2 ps) forms the local charge transfer state [Cp(2)Mo(dt)-{Ph}(+)-{NDI}(-)] which has a lifetime of about 1.7 ps and decays to produce the ground state and the charge-separated state [(Cp)(2){Mo(dt)}(+·)-Ph-{NDI}(-)]; the latter has an appreciable lifetime, about 15 ns in CH(2)Cl(2) at room temperature.

Iodine and molybdenum K-edge extended X-ray absorption fine structure of the compounds [I(NCMe) 2 ][MoF 6 ], [I(py) 2 ][MoF 6 ],[I(py) 2 ][NO 3 ] and [Ag(py) 4 ][MoF 6 ] provide good evidence for the discrete ions [IL 2 ] + , L=MeCN or py, having linear N-I-N skeletons with I-N distances in the range 2.16–2.22 A, and octahedral [MoF 6 ]- with an Mo-F distance of 1.78±0.02 A, both in the solid state and in acetonitrile solution.

Vanadium(III) and -(V) alkolo complexes of the ligands hydridotripyrazolylborate, L, and hydridotris(3,5-dimethylpyrazolyl)borate, L * , have been prepared and characterized by X-ray diffraction, 1 H, 13 C, and 51 V NMR, electrochemistry, optical spectroscopy, and EXAFS. Two general classes of V(V) complexes have been synthesized with the general formulas LVO(OR) 2 and LVO(OR)Cl. The latter complexes contain the shortest V-OR bonds yet reported (1.706-1.754 A) and show that a vanadium-alkoxide bond could account for the short vanadium «light atom» interaction seen in the EXAFS of the enzyme, bromoperoxidase

The availability of synchrotron radiation has transformed the status of X-ray absorption spectroscopy (XAS) from a topic of relatively minor interest to one of major scientific importance and activity. The availability of a continuum of electromagnetic radiation with a high intensity throughout a large part of the X-ray region has permitted reliable XAS data to be obtained for a wide range of elements. XAS is ideally suited to probe the immediate environment of specific atoms in a material, irrespective of its physical state.

Copper and zinc bisulfide complexes in NaHS solutions were investigated by Extended X-ray Absorption Fine-Structure spectroscopy (EXAFS). A goal was to investigate the possibility that bisulfide complexes are multinuclear. Copper and zinc K-edge absorption spectra were recorded in fluorescence mode at room temperature and at 80 K. Lowering temperature improved signal to noise ratios, but did not significantly change measured metal-sulfur distances or coordination numbers. In equilibrium with S-absent assemblages in the Cu-S system, dissolved copper consisted of colorless Cu(I) species having Cu-S interatomic distances of 2.30-2.33 A (coord. no. 3 ± 1 ). Second shell Cu atoms were present at 2.72-2.75 A ( coord. no. 1.1). The presence of second shell Cu atoms, which was confirmed by duplicate experiments using different starting materials, demonstrates that the predominant complexes in these solutions are multinuclear. The Cu-Cu distance is shorter than in most known Cu-S cluster complexes and Cu-S minerals, but is in good agreement with known Cu-Cu distances in Cu4(RS)2−6 clusters. In equilibrium with a CuS + S assemblage, shorter Cu-S distances and no evidence of an ordered second shell were observed. In these polysulfide-rich solutions, different Cu complexes apparently predominate. Solutions saturated with sphalerite or amorphous ZnS precipitate yielded Zn-S and Zn-Zn distances similar to those in solid sphalerite, although the Zn-Zn coordination number was lower and second shell S was also present. In the Zn case, but not the Cu case, it is possible that the species being characterized by EXAFS were

Two cobalt complexes, 5 and 7, both containing 11,11a-dihydropyrano[2,3-b]quinoxaline nuclei, were synthesised as model substances for the molybdenum cofactor of the oxomolybdoenzymes. The organic proligands for the ene-1,2-dithiolates, from which these complexes were formed, were 1,3-dithiol-2-ones, the dianionic ligands being liberated by reaction with caesium hydroxide. The pyran ring in the tetracyclic 1,3-dithiol-2-one proligands, 4b and 6, was formed by ring closure of the side-chain alcohol in a 4-(1-hydroxyalkyl)-5-(quinoxalin-2-yl)-1,3-dithiol-2-one onto an aromatic quinoxaline via reaction with a chloroformate, generating 6-alkyloxycarbonyl-2-oxo-5a,6-dihydro-4H-[1,3]dithiolo[4′,5′:4,5]pyrano[2,3-b]quinoxalines which were then reduced with cyanoborohydride to give 6-alkyloxycarbonyl-5a,6,11,11a-tetrahydro-2-oxo-4H-[1,3]dithiolo[4′,5′:4,5]pyrano[2,3-b]quinoxalines – the proligands.

6-Alkenylpterin (6) was prepared from pteridine (3a) andR-glyceraldehyde acetonide via a Wittig condensation. Treatment of 6 with bromine and then diazabicycloundecane (DBU) produced the corresponding alkyne (8).

The extended X-ray absorption fine structure (EXAFS) associated with the iron K-edge has been measured and interpreted for ferritin and haemosiderin extracted from horse spleen, and haemosiderin extracted from the livers of humans with treated primary haemochromatosis, and from the spleens of humans with treated secondary haemochromatosis. For ferritin, the data are consistent with, on average, each iron atom being in an environment comprised of approx. six oxygen atoms at 1.93 +/- 0.02 A, approx. 1.5 iron atoms at 2.95 +/- 0.02 A and approx. 1.1 iron atoms at 3.39 +/- 0.02 A, with a further shell of oxygens at approx. 3.6 A. Iron in horse spleen haemosiderin is in an essentially identical local environment to that in horse spleen ferritin. In contrast, the EXAFS data for primary haemochromatosis haemosiderin indicate that the iron-oxide core is amorphous; only a single shell of approx. six oxygen atoms at approx. 1.94 +/- 0.02 A being apparent. Secondary haemochromatosis haemosiderin shows an ordered structure with approx. 1.4 iron atoms at both 2.97 +/- 0.02 and 3.34 +/- 0.02 A. This arrangement of iron atoms is similar to that in horse spleen haemosiderin, but the first oxygen shell is split with approx. 2.9 atoms at 1.90 +/- 0.02 A and approx. 2.7 at 2.03 +/- 0.02 A, indicative of substantial structural differences between secondary haemochromatosis haemosiderin and horse spleen haemosiderin.

A tetranuclear iron cluster is the principal component of the purple coatings produced by treating a mild steel surface with a salicylaldoxime corrosion inhibitor. This was shown by comparison of the spectroscopic data with those of the cluster [{Fe(salH)(HsalH)}4 ], which was obtained from FeCl3 and salicylaldoxime (H2 salH) and has a distorted tetrahedral arrangement of Fe(III) atoms coordinated by terminal (1-) and bridging (2-) salicylaldoximate ligands (the central core of the cluster is depicted).

Fly agaric accumulate vanadium in the form of amavadin, whose structure has been elucidated (see picture for the crystal structure). Amavadin contains one VIV center coordinated to two (S,S)-hidpa3- ligands (H3 hidpa=2,2'-(hydroxyimino)dipropionic acid) through one η2 -NO- group and two unidentate carboxylato groups from each ligand. The arrangement of the two η2 -NO- groups leads to a chiral vanadium center, which has been characterized in both the Λ and Δ forms. The carboxylato groups of the amavadin anions bind cations, for example, Ca2+ ions, and/or become involved in hydrogen bonding.