Oxomolybdenum tetrathiolates with sterically encumbering ligands: modeling the effect of a protein matrix on electronic structure and reduction potentials

The effect of sterically encumbering ligands on the electronic structure of oxomolybdenum tetrathiolate complexes was determined using a combination of electronic absorption and magnetic circular dichroism spectroscopies, complimented by DFT bonding calculations, to understand geometric and electronic structure contributions to reduction potentials. These complexes are rudimentary models for a redox-active metalloenzyme active site in a protein matrix and allow for detailed spectroscopic probing of specific oxomolybdenum-thiolate interactions that are directly relevant to Mo-S(cysteine) bonding in pyranopterin molybdenum enzymes. Data are presented for three para-substituted oxomolybdenum tetrathiolate complexes ([PPh4][MoO(p-SPhCONHCH3)4], [PPh4][MoO(p-SPhCONHC(CH2O(CH2)2CN)3)4], and [PPh4][MoO(p-SPhCONHC(CH2O(CH2)2COOCH2CH3)3)4]). The Mo(V/IV) reduction potentials of the complexes in DMF are -1213, -1251, and -1247 mV, respectively. The remarkably similar electronic absorption and magnetic circular dichroism spectra of these complexes establish that the observed reduction potential differences are not a result of significant changes in the electronic structure of the [MoOS4]- cores as a function of the larger ligand size. We provide evidence that these reduction potential differences result from the driving force for a substantial reorganization of the O-Mo-S-C dihedral angle upon reduction, which decreases electron donation from the thiolate sulfurs to the reduced molybdenum center. The energy barrier to favorable O-Mo-S-C geometries results in a reorganizational energy increase, relative to [MoO(SPh)4](-/2-), that correlates with ligand size. The inherent flexible nature of oxomolybdenum-thiolate bonds indicate that thiolate ligand geometry, which controls Mo-S covalency, could affect the redox processes of monooxomolybdenum centers in pyranopterin molybdenum enzymes.     

Sulfur K-edge spectroscopic investigation of second coordination sphere effects in oxomolybdenum-thiolates: relationship to molybdenum-cysteine covalency and electron transfer in sulfite oxidase

Second-coordination sphere effects such as hydrogen bonding and steric constraints that provide for specific geometric configurations play a critical role in tuning the electronic structure of metalloenzyme active sites and thus have a significant effect on their catalytic efficiency. Crystallographic characterization of vertebrate and plant sulfite oxidase (SO) suggests that an average O(oxo)-Mo-S(Cys)-C dihedral angle of approximately 77 degrees exists at the active site of these enzymes. This angle is slightly more acute (approximately 72 degrees) in the bacterial sulfite dehydrogenase (SDH) from Starkeya novella. Here we report the synthesis, crystallographic, and electronic structural characterization of Tp*MoO(mba) (where Tp* = (3,5-dimethyltrispyrazol-1-yl)borate; mba = 2-mercaptobenzyl alcohol), the first oxomolybdenum monothiolate to possess an O(ax)-Mo-S(thiolate)-C dihedral angle of approximately 90 degrees . Sulfur X-ray absorption spectroscopy clearly shows that O(ax)-Mo-S(thiolate)-C dihedral angles near 90 degrees effectively eliminate covalency contributions to the Mo(xy) redox orbital from the thiolate sulfur. Sulfur K-pre-edge X-ray absorption spectroscopy intensity ratios for the spin-allowed S(1s) --> Sv(p) + Mo(xy) and S(1s) --> Sv(p) + Mo(xz,yz) transitions have been calibrated by a direct comparison of theory with experiment to yield thiolate Sv(p) orbital contributions, c(j)(2), to the Mo(xy) redox orbital and the Mo(xz,yz) orbital set. Furthermore, these intensity ratios are related to a second coordination sphere structural parameter, the O(oxo)-Mo-S(thiolate)-C dihedral angle. The relationship between Mo-S(thiolate) and Mo-S(dithiolene) covalency in oxomolydenum systems is discussed, particularly with respect to electron-transfer regeneration in SO.     

Control of oxo-molybdenum reduction and ionization potentials by dithiolate donors

The compounds (L-N3)MoO(qdt) and (L-N3)MoO(tdt) [(L-N3) = hydrotris(3,5-dimethyl-1-pyrazolyl)borate; tdt = toluene-3,4-dithiolate; qdt = quinoxaline-2,3-dithiolate] have been studied by cyclic voltammetry and photoelectron, magnetic circular dichroism, and electronic absorption spectroscopies, and the experimental data have been interpreted in the context of ab initio molecular orbital calculations on a variety of dithiolate dianion ligands. The PES data reveal very substantial differences between (L-N3)MoO(qdt) and (L-N3)MoO(tdt) in that the first ionization (originating from the Mo dxy orbital) for (L-N3)MoO(qdt) is about 0.8 eV to deeper binding energy than that of (L-N3)MoO(tdt). This stabilizing effect is also reflected in the solution reduction potentials, where (L-N3)MoO(qdt) is approximately 220 mV easier to reduce than (L-N3)MoO(tdt). A direct correlation between the relative donating ability of a given dithiolate ligand and the reduction potential of the (L-N3)MoO(dithiolate) complex has been observed, and a linear relationship exists between the calculated Mulliken charge on the S atoms of the dithiolate dianion and the Mo reduction potential. The study confirms previously communicated work (Helton, M. E.; Kirk, M. L. Inorg. Chem. 1999, 38, 4384-4385) that suggests that anisotropic covalency contributions involving only the out-of-plane S orbitals of the coordinated dithiolate control the Mo reduction potential by modulating the effective nuclear charge of the metal, and this has direct relevance to understanding the mechanism of ferricyanide inhibition in sulfite oxidase. Furthermore, these results indicate that partially oxidized pyranopterins may play a role in facilitating electron and/or atom transfer in certain pyranopterin tungsten enzymes which catalyze formal oxygen atom transfer reactions at considerably lower potentials.     

Electronic spectral studies of molybdenyl complexes. 2. MCD spectroscopy of [MoOS4]- centers

Magnetic circular dichroism (MCD) and absorption spectroscopies have been used to probe the electronic structure of [PPh4][MoO(p-SC6H4X)4] (X = H, Cl, OMe) and [PPh4][MoO(edt)2] complexes (edt = ethane-1,2-dithiolate). The results of density functional calculations (DFT) on [MoO(SMe)4]- and [MoO(edt)2]- model complexes were used to provide a framework for the interpretation of the spectra. Our analysis shows that the lowest energy transitions in [MoVOS4] chromophores (S4 = sulfur donor ligand) result from S-->Mo charge transfer transitions from S valence orbitals that lie close to the ligand field manifold. The energies of these transitions are strongly dependent on the orientation of the S lone-pair orbitals with respect to the Mo atom that is determined by the geometry of the ligand backbone. Thus, the lowest energy transition in the MCD spectrum of [PPh4][MoO(p-SC6H4X)4] (X = H) occurs at 14,800 cm-1, while that in [PPh4][MoO(edt)2] occurs at 11,900 cm-1. The identification of three bands in the absorption spectrum of [PPh4][MoO(edt)2] arising from LMCT from S pseudo-sigma combinations to the singly occupied Mo 4d orbital in the xy plane suggests that there is considerable covalency in the ground-state electronic structures of [MoOS4] complexes. DFT calculations on [MoO(SMe)4]- reveal that the singly occupied HOMO is 53% Mo 4dxy and 35% S p for the equilibrium C4 geometry. For [MoO(edt)2]- the steric constraints imposed by the edt ligands result in the S pi orbitals being of similar energy to the Mo 4d manifold. Significant S pseudo-sigma and pi donation may also weaken the Mo identical to O bond in [MoOS4] centers, a requirement for facile active site regeneration in the catalytic cycle of the DMSO reductases. The strong dependence of the energies of the bands in the absorption and MCD spectra of [PPh4][MoO(p-SC6H4X)4] (X = H, Cl, OMe) and [PPh4][MoO(edt)2] on the ligand geometry suggests that the structural features of the active sites of the DMSO reductases may result in an electronic structure that is optimized for facile oxygen atom transfer.     

Nature of the oxomolybdenum-thiolate pi-bond: implications for Mo-S bonding in sulfite oxidase and xanthine oxidase

The electronic structure of cis,trans-(L-N(2)S(2))MoO(X) (where L-N(2)S(2) = N,N'-dimethyl-N,N'-bis(2-mercaptophenyl)ethylenediamine and X = Cl, SCH(2)C(6)H(5), SC(6)H(4)-OCH(3), or SC(6)H(4)CF(3)) has been probed by electronic absorption, magnetic circular dichroism, and resonance Raman spectroscopies to determine the nature of oxomolybdenum-thiolate bonding in complexes possessing three equatorial sulfur ligands. One of the phenyl mercaptide sulfur donors of the tetradentate L-N(2)S(2) chelating ligand, denoted S(180), coordinates to molybdenum in the equatorial plane such that the OMo-S(180)-C(phenyl) dihedral angle is approximately 180 degrees, resulting in a highly covalent pi-bonding interaction between an S(180) p orbital and the molybdenum d(xy) orbital. This highly covalent bonding scheme is the origin of an intense low-energy S --> Mo d(xy) bonding-to-antibonding LMCT transition (E(max) approximately 16000 cm(-)(1), epsilon approximately 4000 M(-)(1) cm(-)(1)). Spectroscopically calibrated bonding calculations performed at the DFT level of theory reveal that S(180) contributes approximately 22% to the HOMO, which is predominantly a pi antibonding molecular orbital between Mo d(xy) and the S(180) p orbital oriented in the same plane. The second sulfur donor of the L-N(2)S(2) ligand is essentially nonbonding with Mo d(xy) due to an OMo-S-C(phenyl) dihedral angle of approximately 90 degrees. Because the formal Mo d(xy) orbital is the electroactive or redox orbital, these Mo d(xy)-S 3p interactions are important with respect to defining key covalency contributions to the reduction potential in monooxomolybdenum thiolates, including the one- and two-electron reduced forms of sulfite oxidase. Interestingly, the highly covalent Mo-S(180) pi bonding interaction observed in these complexes is analogous to the well-known Cu-S(Cys) pi bond in type 1 blue copper proteins, which display electronic absorption and resonance Raman spectra that are remarkably similar to these monooxomolybdenum thiolate complexes. Finally, the presence of a covalent Mo-S pi interaction oriented orthogonal to the MOO bond is discussed with respect to electron-transfer regeneration in sulfite oxidase and Mo=S(sulfido) bonding in xanthine oxidase.     

Dioxo-molybdenum(VI) and mono-oxo-molybdenum(IV) complexes supported by new aliphatic dithiolene ligands: new models with weakened Mo=O bond characters for the arsenite oxidase active site

The cis-dioxo-molybdenum(VI) complexes, [MoO2(L(H))2]2- (1b), [MoO2(L(S))(2)]2- (2b), and [MoO2(L(O))2]2- (3b) (L(H) = cyclohexene-1,2-dithiolate, L(S) = 2,3-dihydro-2H-thiopyran-4,5-dithiolate, and L(O) = 2,3-dihydro-2H-pyran-4,5-dithiolate), with new aliphatic dithiolene ligands were prepared and investigated by infrared (IR) and UV-vis spectroscopic and electrochemical methods. The mono-oxo-molybdenum(IV) complexes, [MoO(L(H))2]2- (1a), [MoO(L(S))2]2- (2a), and [MoO(L(O))2]2- (3a), were further characterized by X-ray crystal structural determinations. The IR and resonance Raman spectroscopic studies suggested that these cis-dioxo molybdenum(VI) complexes (1b-3b) had weaker Mo=O bonds than the common Mo(VI)O2 complexes. Complexes 1b-3b also exhibited strong absorption bands in the visible regions assigned as charge-transfer bands from the dithiolene ligands to the cis-MoO2 cores. Because the oxygen atoms of the cis-Mo(VI)O2 cores are relatively nucleophilic, these complexes were unstable in protic solvents and protonation might occur to produce Mo(VI)O(OH), as observed with the oxidized state of arsenite oxidase.     

Molybdenum oxo and imido complexes of beta-diketiminate ligands: synthesis and structural aspects

Treatment of [MoO2(eta2-Pz)2] (Pz = 3,5-di-tert-butylpyrazolate) with the diketiminate ligand NacNacH (NacNac = CH[C(Me)NAr]2-, Ar = 2,6-Me2C6H3) at 55 degrees C leads under reduction of the metal to the formation of the dimeric molybdenum(V) compound [{MoO2(NacNac)}2] (1). The compound was characterized by spectroscopic means and by X-ray crystal structure analysis. The dimer consists of a [Mo2O4]2+ core with a short Mo-Mo bond (2.5591(5) A) and one coordinated diketiminate ligand on each metal atom. The reaction of [MoO2(eta2-Pz)2] with NacNacH in benzene at room temperature leads to a mixture of 1 and the monomeric molybdenum(VI) compound [MoO2(NacNac)(eta2-Pz)] (2). From such solutions, yellow crystals of 2 suitable for X-ray structural analysis were obtained revealing the coordination of one bidentate NacNac and one eta2-coordinate Pz ligand. This renders the two oxo groups inequivalent. Further high oxidation state molybdenum compounds containing the NacNac ligand were obtained by the reaction of [Mo(NAr)2Cl2(dme)] (Ar = 2,6-Me2C6H3) and [Mo(N-t-Bu)2Cl2(dme)] (dme = dimethoxyethane) with 1 equiv of the potassium salt NacNacK forming [Mo(NAr)2Cl(NacNac)] (3) and [Mo(N-t-Bu)2Cl(NacNac)] (4), respectively, in good yields. The X-ray structure analysis of 3 revealed a penta-coordinate compound where the geometry is best described as trigonal-bipyramidal.     

Probing the electronic structure of [MoOS(4)](-) centers using anionic photoelectron spectroscopy

Using photodetachment photoelectron spectroscopy (PES) in the gas phase, we investigated the electronic structure and chemical bonding of six anionic [Mo(V)O](3+) complexes, [MoOX(4)](-) (where X = Cl (1), SPh (2), and SPh-p-Cl (3)), [MoO(edt)(2)](-) (4), [MoO(bdt)(2)](-) (5), and [MoO(bdtCl(2))(2)](-) (6) (where edt = ethane-1,2-dithiolate, bdt = benzene-1,2-dithiolate, and bdtCl(2) = 3,6-dichlorobenzene-1,2-dithiolate). The gas-phase PES data revealed a wealth of new electronic structure information about the [Mo(V)O](3+) complexes. The energy separations between the highest occupied molecular orbital (HOMO) and HOMO-1 were observed to be dependent on the O-Mo-S-C(alpha) dihedral angles and ligand types, being relatively large for the monodentate ligands, 1.32 eV for Cl and 0.78 eV for SPh and SPhCl, compared to those of the bidentate dithiolate complexes, 0.47 eV for edt and 0.44 eV for bdt and bdtCl(2). The threshold PES feature in all six species is shown to have the same origin and is due to detaching the single unpaired electron in the HOMO, mainly of Mo 4d character. This result is consistent with previous theoretical calculations and is verified by comparison with the PES spectra of two d(0) complexes, [VO(bdt)(2)](-) and [VO(bdtCl(2))(2)](-). The observed PES features are interpreted on the basis of theoretical calculations and previous spectroscopic studies in the condensed phase.     

N-heterocycle chelated oxomolybdenum(VI and V) complexes with bidentate citrate

A 1,10-phenanthroline (phen) chelated molybdenum(VI) citrate, [(MoO2)2O(H2cit)(phen)(H2O)2] x H2O (1) (H4cit = citric acid), is isolated from the reaction of citric acid, ammonium molybdate and phen in acidic media (pH 0.5-1.0). A citrato oxomolybdenum(V) complex, [(MoO)2O(H2cit)2(bpy)2] x 4H2O (2), is synthesized by the reduction of citrato molybdate with hydrazine hydrochloride in the presence of 2,2'-bipyridine (bpy), and a monomeric molybdenum(VI) citrate [MoO2(H2cit)(bpy)] x H2O (6) is also isolated and characterized structurally. The citrate ligand in the three neutral compounds uses the alpha-alkoxy and alpha-carboxy groups to chelate as a bidentate leaving the two beta-carboxylic acid groups free, that is different from the tridentate chelated mode in the citrato molybdate(VI and V) complexes. 1 and in solution show obvious dissociation based on 13C NMR studies.     

Ligand influences of the structures of molybdenum oxide networks

The influence of organonitrogen ligands on the network structure of molybdenum oxides was examined by preparing three new molybdenum oxide phases [MoO3(4,4'-bpy)0.5] (MOXI-8), [HxMoO3(4,4'-bpy)0.5] (MOXI-9), and [MoO3(triazole)0.5] (MOXI-32). The structure of [MoO3(4,4'-bpy)0.5) consists of layers of corner-sharing MoO5N octahedra, buttressed by bridging 4,4'-bipyridyl ligands into a three-dimensional covalently bonded organic-inorganic composite material. Partial reduction of [MoO3(4,4'-bpy)0.5] yields the mixed-valence material [HxMoO3(4,4'-bpy)0.5] (x approximately 0.5). The most apparent structural change upon reduction is found in the Mo-ligand bond lengths of the MoO5N octahedra, which exhibit the usual (2 + 2 + 2) pattern in [MoO3(4,4'-bpy)0.5] and a more regular (5 + 1) pattern in [HxMoO3(4,4'-bpy)0.5]. Substitution of triazole for 4,4'-bipyridine yields [MoO3(triazole)0.5], which retains the layer motif of corner-sharing MoO5N octahedra but with distinct sinusoidal ruffling in contrast to planar layers of [MoO3(4,4'-bpy)0.5] and [HxMoO3(4,4'-bpy)0.5]. The folding reflects the ligand constraints imposed by the triazole ligand that bridges adjacent Mo sites within a layer. MOXI-8, C5H4NMoO3: monoclinic P2(1)/c, a = 7.5727(6) A, b = 7.3675(7) A, c = 22.433(3) A, beta = 90.396(8) degrees, Z = 8. MOXI-9, C5H4.5NMoO3: monoclinic I2/m, a = 5.2644(4) A, b = 5.2642(4) A, c = 22.730(2) A, beta = 90.035(1) degrees, Z = 4. MOXI-32, C2H3N3Mo2O6: orthorhombic Pbcm, a = 3.9289(5) A, b = 13.850(2) A, c = 13.366(2) A, Z = 4.     

Dimeric dioxomolybdenum(VI) and oxomolybdenum(V) complexes with citrate at very low pH and neutral conditions

A novel dimeric dioxomolybdenum(VI) citrate complex, K[(MoO2)2-(OH)(H2cit)2].4H2O (1), with weak coordination of beta-carboxylic acid groups and the first structural example of an oxomolybdenum(V) citrate complex, (NH4)6[Mo2O4(cit)2].3H2O (2) (H4cit = citric acid), are isolated in a very acidic solution (pH 0.5-1.0) and neutral conditions (pH 7.0-8.0), respectively. Complex 1 displays strong double hydrogen bonds through beta-carboxyl and beta-carboxylic acid groups [2.621(9) A]. Transformations of the dimeric molybdenum(VI) citrate show that protonation of a carboxyl group will weaken the coordination of molybdenum(VI) citrate. There are obvious dissociations of molybdenum(VI/V) citrate complexes based on 13C NMR observations in solution.     

Pulsed EPR investigations of systems modeling molybdenum enzymes: hyperfine and quadrupole parameters of oxo-17O in [Mo 17O(SPh)4]-

Ka band ESEEM spectroscopy was used to determine the hyperfine (hfi) and nuclear quadrupole (nqi) interaction parameters for the oxo-17O ligand in [Mo 17O(SPh)4]-, a spectroscopic model of the oxo-Mo(V) centers of enzymes. The isotropic hfi constant of 6.5 MHz found for the oxo-17O is much smaller than the values of approximately 20-40 MHz typical for the 17O nucleus of an equatorial OH(2) ligand in molybdenum enzymes. The 17O nqi parameter (e2qQ/h = 1.45 MHz, eta approximately = 0) is the first to be obtained for an oxo group in a metal complex. The parameters of the oxo-17O ligand, as well as other magnetic resonance parameters of [Mo 17O(SPh)4]- predicted by quasi-relativistic DFT calculations, were in good agreement with those obtained in experiment. From the electronic structure of the complex revealed by DFT, it follows that the SOMO is almost entirely molybdenum d(xy) and sulfur p, while the spin density on the oxo-17O is negative, determined by spin polarization mechanisms. The results of this work will enable direct experimental identification of the oxo ligand in a variety of chemical and biological systems.     

Electrodeposition mechanism of Ni-Mo-P alloy in the solution of ammoniac citrate

The induced codeposition mechanism of Mo, P and Ni from the solution of ammoniac citrate was studied by means of steady-state polarization, AC impedance and X-ray Photoelectron Spectroscopy (XPS). The result of electrochemical measurements proved that [NiCit(NH3)2]- is the electro-active species of nickel, though nickel ions exist mainly as [NiCit(NH3)3]- in ammoniac citrate. XPS experiments proved the existence of tetravalent molybdenum corresponding to MoO2 on the surface of some deposits. The intermediate product, MoO2, was probably reduced to Mo in the alloy deposit by atomic hydrogen adsorbed on the induced metal nickel. The reduction of H2PO2- occurs through two distinctive steps with PH3 as an intermediate, which subsequently reacts with atomic hydrogen to form P in the alloy deposit. The electrodeposition mechanism was proposed in this paper.     

Structural and functional analogue of the active site of polysulfide reductase from Wolinella succinogenes

Synthesis of [PPh4]2[Mo(SPh)2(S2C2(CN)2)2] (2) from [PPh4]2[MoO(S2C2(CN)2)2] (1) has been achieved to mimic the postulated [Mo(S)6] core of polysulfide reductase with two thiolates and two bis(ene-dithiolate) ligands. Compound 2 reacts with polysulfide to yield H2S, modeling the function of polysulfide reductase. The facile conversion of 2 back to 1 in moist solvent suggests that the interconversion of the [MoIV = O] and [MoIV - X] (X = O-Ser, S-Cys, Se-Cys) moieties might occur in the DMSO reductase class of enzymes under appropriate hydrophobic/hydrophilic conditions.     

Understanding the origin of metal-sulfur vibrations in an oxo-molybdenum dithiolene complex: relevance to sulfite oxidase

X-ray crystallography and resonance Raman (rR) spectroscopy have been used to further characterize (Tp*)MoO(qdt) (Tp* is hydrotris(3,5-dimethyl-1-pyrazolyl)borate and qdt is 2,3-quinoxalinedithiolene), which represents an important benchmark oxomolybdenum mono-dithiolene model system relevant to various pyranopterin Mo enzyme active sites, including sulfite oxidase. The compound (Tp*)MoO(qdt) crystallizes in the triclinic space group, P1, where a = 9.8424 (7) A, b = 11.2323 (8) A, c = 11.9408 (8) A, alpha = 92.7560 (10) degrees, beta = 98.9530 (10) degrees, and gamma = 104.1680 (10) degrees. The (Tp*)MoO(qdt) molecule exhibits the distorted six-coordinate geometry characteristic of related oxo-Mo(V) systems possessing a single coordinated dithiolene ligand. The first coordination sphere bond lengths and angles in (Tp*)MoO(qdt) are very similar to the corresponding structural parameters for (Tp*)MoO(bdt) (bdt is 1,2-benzenedithiolene). The relatively small inner-sphere structural variations observed between (Tp*)MoO(qdt) and (Tp*)MoO(bdt) strongly suggest that geometric effects are not a major contributor to the significant electronic structural differences reported for these two oxo-Mo(V) dithiolenes. Therefore, the large differences observed in the reduction potential and first ionization energy between the two molecules appear to derive primarily from differences in the effective nuclear charges of their respective sulfur donors. However, a subtle perturbation to Mo-S bonding is implied by the nonplanarity of the dithiolene chelate ring, which is defined by the fold angle. This angular distortion (theta = 29.5 degrees in (Tp*)MoO(qdt); 21.3 degrees in (Tp*)MoO(bdt)) observed between the MoS2 and S-C=C-S planes may contribute to the electronic structure of these oxo-Mo dithiolene systems by controlling the extent of S p-Mo d orbital overlap. In enzymes, the fold angle may be dynamically modulated by the pyranopterin, thereby functioning as a transducer of vibrational energy associated with protein conformational changes directly to the active site via changes in the fold angle. This process could effectively mediate charge redistribution at the active site during the course of atom- and electron-transfer processes. The rR spectrum shows bands at 348 and 407 cm(-1). From frequency analysis of the normal modes of the model, [(NH3)3MoO(qdt)]1+, using the Gaussian03 suite of programs, these bands are assigned as mixed-mode Mo-S vibrations of the five-membered Mo-ditholene core structure. Raman spectroscopy has also provided additional evidence for an in-plane pseudo-sigma dithiolene S-Mo d(xy) covalent bonding interaction in (Tp*)MoO(qdt) and related oxo-Mo-dithiolenes that has implications for electron-transfer regeneration of the active site in sulfite oxidase involving the pyranopterin dithiolene.     

非化学计量的混合价态氧化钼(VI,V)修饰微电极的电化学制备

在新鲜配制的Na2MoO4的弱酸性水溶液中, 通过循环电位扫描在碳纤维微电极表面可沉积一层均匀的蓝色氧化钼(VI, V)薄膜, 膜的厚度通过电量进行控制。在电沉积之前, 电极的阳极化处理不仅可以加快氧化钼的电沉积, 而且可以改善膜的伏安行为。溶液的pH值对膜的电沉积和伏安行为有极大的影响。膜的阴极过程被认为是产生青钼铜: HxMoO3 (0相似文献   

Solid-state coordination chemistry: influences of (M(terpyridyl))(M = Fe(III), Cu(II), Ni(II)) subunits on molybdenum oxide structures

The hydrothermal reactions of Na2MoO4 x 2H2O and 2,2':6',2"-terpyridine with appropriate salts of Fe(II), Cu(II), and Zn(II) yield a variety of mixed metal oxide phases. The Cu(II) system affords the molecular cluster [Cu(terpy)MoO4].3H2O (MOXI-40 x 3H2O), as well as a one-dimensional material [Cu(terpy)Mo2O7](MOXI-41) which is constructed from (Mo4O14)4- clusters linked through (Cu(terpy))2+ units. In constrast, the Zn(II) phase of stoichiometry identical to that of MOXI-41, [Zn(terpy)Mo2O7](MOXI-42), exhibits a one-dimensional structure characterized by a (Mo2O7)n2n- chain decorated with peripheral (Zn(terpy))2+ subunits. The iron species [(Fe(terpy))2Mo4O12](MOXI-43) is also one-dimensional but exhibits [(Fe(terpy))2(MoO4)2]2+ rings linked through (MoO4)2- tetrahedra. A persistent structural motif which appears in MOXI-40, MOXI-41, and MOXI-43 is the [(M(terpy))2(MoO4)2]n cluster with a cyclic )(M2Mo2O4) core. In general, the secondary metal sites M(II, III) are effective bridging groups between molybdate subunits of varying degrees of aggregation. Furthermore, the ligands passivate the bimetallic oxide from spatial extension in two or three dimensions and provide a routine entree into low-dimensional structural types of the molybdenum oxide family of materials.     

若干线型Mo-Fe-S簇合物红外光谱及其与结构关系的研究

对若干线型Mo一Fe一S簇合物[Cl2FeS2MoS2FeCl2][-2](1)、[S2MoS2FeCl2]^2^-(2)、[S2MoS2Fe(SPh)2][2-](3)、[S2MoS2FeS2Fe(SPh)2][3-](4)、[S2MoS2FeS2MoS2][3-](5)、Cl2FeS2FeCl2][2-](6)、[(PhS)2FeS2Fe(SPh)2][2-](7)的红外光谱进行了研究。通过比较它们的特征频率、结构参数和金属原子的氧化态,对νMo-St、νMo-SbνFe-Sb、νFe-SPh、νFe-Cl进行了归属。并对δS-Mo-S的归属作了初步探讨。文中讨论了MoS2Fe单元中Mo原子对νFe-Sb的影响, 通过振动频率与结构关系的研究揭示其内在联系及规律性。对两条途径的亲电诱导效应进行了讨论, 并提出一个能定性标志Fe→Mo电荷迁移大小的有用参数Δν值。     

Monodithiolene molybdenum(V, VI) complexes: a structural analogue of the oxidized active site of the sulfite oxidase enzyme family

The active sites of the xanthine oxidase and sulfite oxidase enzyme families contain one pterin-dithiolene cofactor ligand bound to a molybdenum atom. Consequently, monodithiolene molybdenum complexes have been sought by exploratory synthesis for structural and reactivity studies. Reaction of [MoO(S(2)C(2)Me(2))(2)](1-) or [MoO(bdt)(2)](1-) with PhSeCl results in removal of one dithiolate ligand and formation of [MoOCl(2)(S(2)C(2)Me(2))](1-) (1) or [MoOCl(2)(bdt)](1-) (2), which undergoes ligand substitution reactions to form other monodithiolene complexes [MoO(2-AdS)(2)(S(2)C(2)Me(2))](1-) (3), [MoO(SR)(2)(bdt)](1-) (R = 2-Ad (4), 2,4,6-Pr(i)(3)C(6)H(2) (5)), and [MoOCl(SC(6)H(2)-2,4,6-Pr(i)(3))(bdt)](1-) (6) (Ad = 2-adamantyl, bdt = benzene-1,2-dithiolate). These complexes have square pyramidal structures with apical oxo ligands, exhibit rhombic EPR spectra, and 3-5 are electrochemically reducible to Mo(IV)O species. Complexes 1-6 constitute the first examples of five-coordinate monodithiolene Mo(V)O complexes; 6 approaches the proposed structure of the high-pH form of sulfite oxidase. Treatment of [MoO(2)(OSiPh(3))(2)] with Li(2)(bdt) in THF affords [MoO(2)(OSiPh(3))(bdt)](1-) (8). Reaction of 8 with 2,4,6-Pr(i)(3)C(6)H(2)SH in acetonitrile gives [MoO(2)(SC(6)H(2)-2,4,6-Pr(i)(3))(bdt)](1-) (9, 55%). Complexes 8 and 9 are square pyramidal with apical and basal oxo ligands. With one dithiolene and one thiolate ligand of a square pyramidal Mo(VI)O(2)S(3) coordination unit, 9 closely resembles the oxidized sites in sulfite oxidase and assimilatory nitrate reductase as deduced from crystallography (sulfite oxidase) and Mo EXAFS. The complex is the first structural analogue of the active sites in fully oxidized members of the sulfite oxidase family. This work provides a starting point for the development of both structural and reactivity analogues of members of this family.     

Syntheses of hydrated molybdenum bronzes by reduction of MoO3 with NaBH4

Hydrated molybdenum bronzes have been prepared by reduction reaction of MoO3 with NaBH4 in ethanol and DMSO. The reduction reactions in both solvents occur smoothly; thus, the layered structure of MoO3 is maintained in the product. Divalent cation Ca2+ has been intercalated between the MoO3 layers, which leads to highly reduced molybdenum bronze (Mo5.26+). Solvated molybdenum bronze catalyzes the reduction reaction of DMSO by NaBH4, producing CH3SCH3. The structure model of hydrated sodium molybdenum bronze has also been reinvestigated by using the Rietveld analysis. The hydrated molybdenum bronze crystallizes in an orthorhombic structure, in which the structure of Mo octahedron layers is closely related to that in MoO3. However, the structure refinement reveals that the Mo octahedron in the MoO3 layers is axially distorted, which is different from that in MoO3 but similar to an isoelectron compound H0.33MoO3.