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The standard enthalpies of formation at 298.15 K of the crystalline solids [Mo(pd)3], ΔHf⊖=–1323.9 ± 5.9 kJ mol–1, and [MoO2(pd)2](pd = pentane-2,4-dionate), ΔHf⊖=–1 365.5 ± 4.5 kJ mol–1, have been determined using solution reaction calorimetry. The mean Mo–O bond-enthalpy contributions involving the pentane-2,4-dionate groups have been assessed in the two compounds as 192.0 and 187.3 kJ mol–1, respectively.

Etude experimentale par spectrometrie EXAFS des oxydes stœchiometriques suivants: UO 2 , β-U 3 O 7 et α-U 3 O 8

Calculations, both at the SCF level and including correlation effects have been performed for Cr4+2, Mo4+2, [Cr2(CH38] 4−, and Cr2(O2CH)4 · 2H2O. For Mo4+2 the SCF ground state corresponds to the usual σ2 π4 δ2 quadruple bond description, whilst for Cr4+2 and the dichromium(II) complexes, the ground state for the metalmetal interaction is the σ2δ2σ*2 δ*2 no-bond configuration. Correlation effects, which allow bonding configurations of higher energy to mix into the ground state, are seen to provide a novel interpretation of the metalmetal interactions in these complexes.

The difference between the metal- metal δ and π orbital ionization energies for the complexes M 2 (O 2 CCF 3 ) 4 , M 2 (O 2 CCH 3 ) 4 , M 2 (C 3 H 5 ) 4 , and M 2 (MHP) 4 (where M = Cr or Mo and H{MHP} = 6-methyl-2-hydroxypyridine) increases, within each series, as the metal-metal bond length decreases. The variation in the metal-metal bond length is more marked for the Cr 4+ 2 complexes where the magnitude of the metal-metal ionization energies decrease as the Cr-Cr separation contracts, thus suggesting that the coulombic repulsion between these metal atoms is a prime factor which determines their separation in these complexes.

The title complex, [Cu2Cl(Him)4]Cl, has been prepared from copper(I) chloride and imidazole (Him) in acetonitrile solution. It crystallises in the trigonal space group P3221, with a= 8.546(1), c= 21.370(3)A, and Z= 3. The trigonally co-ordinated Cu atoms are joined by a Cl bridge [Cu–Cl 2.564(2)A, mean Cu–N 1.904(5)A, Cu–Cl–Cu 95.5(1)°, Cu ⋯ Cu 3.797(3)A]. The Cl– anion lies between the two hinged Cu co-ordination planes, with Cu ⋯ Cl 3.134(2)A. [Cu2Cl(Him)4]Cl reacts readily and irreversibly with dioxygen in solution; in the solid state, only surface reaction occurs, although the molecular structure is similar to the active centre proposed for hemocyanin proteins.

Der Titelrutheniumkomplex, der durch Reaktion von Chloro-tetraacetato-diruthenium mit dem Na-Salz des Liganden in MeOH bei Raumtemp. mit ca. 8% Ausb. erhalten wurde , kristallisiert mit Z=4 in der Raumgruppe P21 /n. Sein ESCA-Spektrum ist abgebildet.

Reaction of Mo2(O2CMe)4 with [Me3O][BF4] in MeCN solution at room temperature produces red crystals of [Mo2(O2CMe)2(NCMe)6][BF4]2, which have been characterized by X-ray crystallography and NMR spectroscopy. The central [Mo2(O2CMe)2]2+ core, with a quadruple MoMo bond of length 2.136(1) A, has two acetate bridges in cis positions. Octahedral coordination of each Mo atom is completed by two equatorial and one axial MeCN ligand, with the axial MoN much longer than the equatorial MoN. The MeCN ligands are labile in MeCN solution, giving complete scrambling on the NMR time-scale.

We have used the model compound approach to comment on the structural hypotheses that in reduced bromoperoxidase (BPO) the vanadium is coordinated by at least one imidazole-type nitrogen donor and possibly a phenolate-type oxygen donor. We report here the synthesis and spectroscopic characterization of representative examples of three classes of oxovanadium(IV)—imidazole complexes (i) [VO(acac) 2 (imid)], where acac = 2,4-pentanedionate and imid = N- or 2-substituted imidazole. (ii) [VO(imid) 4 Cl] + , which is stabilized in solution in the presence of excess imid. (iii) [VO(ON) 2 ], where (ON) = , n = 0 or 1, and E = O, S, NH, NMe. It is the complexes of type (iii) above which most closely resemble the non-axial nature of ESR spectrum of the reduced BPO enzymes.

The free-living, nitrogen-fixing bacterium Azotobacter chroococcum has recently been shown to contain two nitrogenase systems1. One of these is the well-characterized molybdenum-containing enzyme2,3 and the other, which is encoded by different genes, contains vanadium. Both enzyme systems consist of two proteins: an iron protein and a molybdenum- or vanadium-containing protein which also contains iron. The vanadium-containing nitrogenase protein, Ac1*, has a relative molecular mass ~-210,000 and contains 2 vanadium atoms, 23 iron atoms, and 20 acid-labile sulphide ions per mole4. X-ray absorption spectroscopy has been extensively used to probe the environment of molybdenum in the MoFe protein5. The molybdenum is closely associated with iron and sulphur in an iron-molybdenum cofactor, FeMoco, which is probably the N2-binding site of the enzyme6. We report here vanadium X-ray absorption data of the VFe protein, the first such report for any vanadium-containing protein, and compare them with those from a recently synthesized VFeS cluster compound7. The results suggest that the vanadium in this protein is present in a vanadium-containing cofactor analogous to FeMoco. We conclude that substitution of vanadium for molybdenum probably accounts for the different substrate specificities of the two nitrogenases.

Cristallisation dans le groupe P2 1 /c (Z=4; Affinement jusqu'a R=0,074). L'atome de cuivre est coordine a trois soufres selon un arrangement trigonal plan (Cu-S=2,250 (9) A)

The results of ab initio molecular orbital calculations for [CrOfs]2− and polarised single crystal electronic spectra of [MoOCl3(Op(NMe2)3)2] and Ph4As[MoOCl4(H2O)] are presented. These data are consistent with the electronic transitions of the MO3+ moieties, O2pπ → Mdxy and Mdxy → Mddxy,dyz being the lowest energy transitions in the spectra of their respective complexes, both these transitions being of low intensity.

Complete thiolato-ligand exchange has been demonstrated for [Fe6Mo2S8(SR)9]3–(R = alkyl) complexes and used to prepare the water-soluble 2-hydroxyethanethiolato-derivative which has been structurally characterised as its [Et4N]+ salt; the complex is stable in buffered aqueous solution, in the presence of an excess of 2-hydroxyethanethiol, and in the absence of dioxygen.

Resolution des structures par diffraction RX de Mo 2 (mhp) 4 , Mo 2 (mhp) 4 •CH 3 OH et de Rh 2 (mhp) 4 •CH 2 Cl 2

The reaction of cis-[MoO2(C5H10NO)2] (C5H10NOH = 1-piperidinol) with benzene 1,2-diol(catechol) in CHCl3 leads to the formation of [MoO(C5H10NO)2(C6H4O2)], which has been characterized by X-ray crystallography, NMR spectroscopy and cyclic voltammetry.

The structure and function of the nitrogenase enzymes is a topic which continues to attract considerable attention. A significant contribution to the understanding of the role of molybdenum in these enzymes has been the measurement and inter pretation of the extended X-ray absorption fine structure (EXAFS) associated with the molybdenum K-edge of the molybdenum iron proteins from Azotobacter vinelandii and Clostridium pasteurianum and the FeMo-cofactor of the former protein.2,3 The environment of the molybdenum is very similar in each of these three systems and is suggested to consist of three or four sulfur atoms at a distance of 2.35A, one or two sulfur atoms at 2.49A, and two or three iron atoms at 2.72A. A structurally attractive possibility is that the molybdenum is present in an {MoFe3S4} cubane-like cluster. We have, therefore, directed our efforts to the synthesis and characterization of compounds containing such clusters.

The recognition that molybdenum is present in the nitrogenase enzymes as an extraetable cofactor, containing iron, molybdenum, and sulfur in the approximate ratios 7:1:6,1-4 stimulated much interest in the synthesis and study of compounds containing these three elements. This article describes those compounds presently known to contain an Fe-Mo-S framework; their physical properties are then compared with those of the iron-molybdenum cofactor, with particular emphasis being given to systems containing Fe3MoS4 cubane-like clusters.

Vanadium is well recognised as an essential trace element, is widely distributed throughout the lithosphere and biosphere, and is present in all mammalian tissues at concentrations of ≤ 10μM (Chasteen, 1983). However, only in a few instances can a precise biological role be assigned to this element. It is a potent inhibitor of phosphatases, Na,K-ATPases, and a variety of other important enzymes, doubtless because of the similarity of AO4 3- (for A=V or P) and their derivatives. Amavadin, an oxovanadium(IV) complex with two molecules of N-(1-carboxyethyl)-N-hydroxyalanine, has been isolated from the mushroom Amanita muscaria (Kneifel and Bayer, 1986) and may function as an electron-transfer catalyst (Nawi and Riechel, 1987). Two enzyme systems were shown to be dependent upon vanadium: nitrogenases from Azotobacter (Robson et al., 1986) and bromoperoxidases from marine algae (de Boer et al., 1986; de Boer et al., 1986). Herein we describe the results of vanadium K-edge X-ray absorption spectroscopic studies for a member of each of these two types of vanadium-dependent enzymes.

The structure of the title compound has been established by three-dimensional X-ray crystal-structure analysis from diffractometer data. Crystals are monoclinic, space group P21/c, with a= 8·65(1), b= 17·64(1), c= 8·38(1)A, β= 93·20(10)°, Z= 2. The structure was solved by the heavy-atom approach and molecular parameters were refined by full-matrix least-squares calculations to R 0·043 for 2137 independent reflections. The copper atom co-ordination in the centrosymmetric cation is distorted octahedral with Cu–N 2·012(2), 2·049(2), and 2·593(3)A. In the crystal the nitrate ions bridge the complex cations through N–H ⋯ O hydrogen bonds, with N ⋯ O 2·815, 2·897, 3·073, and 3·086 A. The single-crystal e.s.r. and polarised electronic spectra are also reported and the electronic structure of the chromophore discussed with reference to these and the crystallographic data.

The compound [(CH2CH)2Sn(O2CCF3)2], (I), has been prepared and characterised by elemental analysis, and 1H, 19F, i.r., and mass spectra. Salt-elimination reactions of compound (I) with Na[Mn(CO)5], Na2[Fe(CO)4], and Na[Co(CO)4] afford [(CH2CH)2Sn(O2CCF3)2{Mn(CO)5}], (II), [(CH2CH)2Sn{Mn(CO)5}2], (III), [{(CH2CH)2Sn[Fe(CO)4]}2], (IV), and [(CH2CH)2Sn{Co(CO)4}2], (V).

In the presence of excess of nitrite ion the complex [MoOCl3(OPPh3)2] is completely oxidised to MoVI in CH2Cl2. The reaction proceeds by three observable stages. Data for the first stage correspond to co-ordination of nitrite at molybdenum(V), via a limiting SN1 mechanism, involving loss of a triphenylphosphine oxide molecule. At 25 °C, k1= 51·8 ± 2·1 s–1, ΔH1‡= 12·6 ± 0·3 kcal moh–1, and ΔS1‡=–8·2 ± 1·0 cal K–1 mol–1. The data for the second stage of reaction are taken to indicate chelation and/or isomerisation of co-ordinated nitrito-ligand into a position cis to the oxo-group, followed by rapid non-rate-determining electron transfer. The third and final stage of reaction appears to involve co-ordination of the available nitrite to the primary molybdenum(VI) product, and subsequent rearrangement of these molybdenum(VI) products.

The equilibria [MoOCl3L2]⇌[MoOCl3L]+ L in CH2Cl2 solution at 25 °C have been studied by e.s.r. Spectroscopy. When L = Ph3PO, (Me2N)3PO, or thf, no [MoOCl3L] species have been detected, where as when L = Ph3PS, (Me2N)2CS, or Me2S, [MoOCl3L] predominates. The new complexes [MoOCl3L][L = Ph3PS, (Me2N)2CS, Ph3PSe, orPh3PO] and [MoOCl3{SC(Nme2)2}2] have been isolated. Similarly the equilibria [MoOCl4]–+ Ln–⇌[MoOCl4L](n+1)– have been studied [L = Ph3PO, (Me2N)PO, py, Et3N, thf, H2O, Ph3PS, and (Me2N)2CS, n= 0; L = Cl–, n= 1]. When L =(Me2N)3PO only [MoOC4L]– has been detected. With the other neutral donors the order of ability to form [MoOCl4L]– is py > Ph3PO ≈ H2O thf Et3N, whereas no six-co-ordinate species have been detected for L = Ph3PS or (Me2N)2CS. With L = Cl– the degree of formation of [MoOCl5]2–is low and dependent on both the nature of the cation and total molybdenum concentration. By comparison of the results in CH2Cl2, it is concluded that the molybdenum(V) species formed on dissolving [Et3N]2[MoOCl5] or MoOCl5 in concentrated aqueous HCl is [MoOCl4(OH2)]– and not [MoOCl5]2– as is generally supposed.

Preparative aromatic nitration, including recently developed methods, is reviewed. Previous attempts to use metal nitrates as nitrating agents are reviewed. The techniques developed for handling anhydrous titanium(IV) nitrate are described. Mechanistic and preparative investigations of titanium(IV) nitrates are described. Results and scope for future investigations are summarised.

Zinc is known to be associated with a large variety of proteins, and is present in enzymes in each of the six categories designed by the International Union of Biochemistry [1], In enzymes, zinc is often thought to play a catalytic role, i. e. to be essential for and directly involved in the catalysis, but also purely non-catalytic (i.e. structural or regulatory) roles have been implied, and in some cases its role is undefined [1]. Unlike manganese, iron, copper, or molybdenum, zinc exists in only one oxidation state, viz. Zn(II). Therefore, it is notably absent from the classes of dioxygen-binding proteins or electron-transfer proteins, and in the oxidoreductases where it is found either catalyses hydride transfer, as in alcohol dehydrogenase [2], or zinc assists another metal in its catalytic action, like copper in Cu, Zn-superoxide dismutase [3], rather than temporarily accommodating an electron, or having dioxygen as a ligand. The general concept of the catalytic role for zinc in enzymes appears to be the polarisation of molecules by virtue of its Lewis acidity. This may be the activation of water in its coordination sphere, e.g. in carbonic anhydrase to release a proton at physiological pH [4], or of a carbonyl function for nucleophilic attack on carbon, e.g. in carboxypeptidase [5], or of an alcohol for proton release and subsequent reactions, e.g. alcohol dehydrogenase [2]. The presence of a catalytic zinc site is typically signalled by the presence in its coordination sphere of a resting water molecule which may be activated or displaced.

The electronic ground state of [Ti(NO3)4] has been calculated using an ab initio SCF molecular-orbital (m.o.) procedure. A correlation is presented of the m.o.s of the complex with those of [NO3]– calculated in the same basis set and it is seen that the overlaps between the metal 3d and ligand 2pσ orbitals provide the major contributions to the metal–ligand bonding. The titanium configuration is calculated as 3d1.6 4s0.2 4p0.2 with the major loss of electron density from each nitrato-group being experienced by the terminal oxygen atom (ca. 0.3 e). The marked reactivity exhibited by anhydrous metal nitrates is discussed with reference to these results.

5-Aminolevulinic acid dehydratase (ALAD, 5-aminolevulinate hydrolyase, EC 4.2.1.24) catalyzes the synthesis of the pyrrole porphobilinogen from 2 molecules of 5- aminolevulinic acid. The enzyme from bovine liver has a molecular weight of 280.000 daltons and consists of eight subunits of 35.000 daltons each [1]. Lysine, hystidine and cysteine have been identified as amino acid constituents of the enzyme protein involved in the active site [2]. Two cysteine residues essential for the enzyme activity are rapidly oxidized by air, and the maintenance of the active state requires the presence of a thiol [3]. The isolated enzyme protein contains 2–6 g. atoms of zinc per octamer and it binds a maximum number of eight zinc ions per octamer with high affinity [4]. The function of zinc in ALAD is not clear. Several findings imply a direct catalytic function for zinc whilst other results support the assumption that zinc stabilizes the active structure of the enzyme protein, without directly interacting with the substrate.

The molybdenum-iron-sulphur cluster [Fe 6 Mo 2 S 8 (SCH 2 CH 2 OH) 9 ] 3− , which contains two Fe 3 MoS 4 cubane-like centres, is the best plausible analogue available to date for the molybdenum site of the nitrogenase enzymes. The iron-sulphur cluster [Fe 4 S 4 (S · CH 2 CH 2 OH) 4 ] 2− and the iron-selenium cluster [Fe 4 Se 4 (S · CH 2 CH 2 OH) 4 ] 2− are structural analogues of the ferredoxin Fe 4 S 4 active centre. All three clusters would replace ferredoxin and mediate electron transfer to Clostridium pasteurianum hydrogenase in a H 2 -evolving system with sodium dithionite as the electron donor. The clusters would not replace hydrogenases which themselves are unable to evolve H 2 from reduced ferredoxins. The molybdenum-iron-sulphur cluster would also replace ferredoxin in a chloroplast-ferredoxin-hydrogenase H 2 evolving system.

The kinetics of the redox reaction between [Et4N][NO3] and [MoOCl3(OPPh3)2] have been studied in CH2Cl2 solution at temperatures from 3·2 to 25 °C. This reaction proceeds in three observable steps when [NO3–] > [MoV]. The first step involes co-ordination of the nitrate ion via an SN1 (limiting) mechanism involving loss of a loss of a triphenylphosphine oxide molecule; at 25 °C, kobs.= 40 ± 1 s–1 and the corresponding activation parameters are ΔH‡= 9·7 ± 0·5 kcal mol–1 and ΔS‡=–18·4 ± 1·7 cal K–1 mol–1. The data obtained for the second and third steps strongly suggest that they involve intramolecular substitution at molybdenum(V) and inter-molecular substitution at molybdenum(VI) centres, respectively. Rapid non-rate-determining electron transfer from molybdenum(V) to nitrate is proposed to occur between these two latter steps, ke.t. 1 s–1 at 25 °C, giving a dioxomolybdenum(VI) complex and nitrogen dioxide. The kinetic results imply that a specific orientation of the nitrato-group with respect to the oxomolybdenum(V) centre is necessary prior to rapid electron transfer.

Le compose du titre cristallise dans le systeme quadratique, groupe P4 2 /n. Spectre RPE et mesure des moments magnetiques a la temperature ambiante

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.

The dimethylsulfoxide reductase (DMSOR) from Rhodobacter capsulatus is known to retain its three-dimensional structure and enzymatic activity upon substitution of molybdenum, the metal that occurs naturally at the active site, by tungsten. The redox properties of tungsten-substituted DMSOR (W-DMSOR) have been investigated by a dye-mediated reductive titration with the concentration of the W V state monitored by EPR spectroscopy. At pH 7.0, E m (W VI /W V ) is −194 mV and E m (W V /W IV ) is −134 mV. Each E m value of W-DMSOR is significantly lower (220 and 334 mV, respectively) than that of the corresponding couple of Mo-DMSOR. These redox potentials are consistent with the ability of Mo-DMSOR to catalyze both the reduction of DMSO to DMS and the back reaction, whereas W-DMSOR is very effective in catalyzing the forward reaction, but shows no ability to catalyze the oxidation of DMS to DMSO.

Toluene-p-sulphonic acid displaces one of the 6-methylpyridin-2-olato (mhp)ligands of[Rh2(mhp)4]; the product dimerises to form [{Rh2(mhp)3(p-MeC6H4SO2O)}2],in which each toluene-p-sulphonate ligand bridges a Rh–Rh bond, as demonstrated by a crystal structure determination.

An X-ray crystallographic study shows that the anion [{(PhSFe)3MoS4}2(SPh)3]3– contains two Fe3MoS4‘cubes’ linked across their molybdenum centres by three µ2-benzenethiolato-groups.