Reduction of tris(benzene-1,2-dithiolate)molybdenum(VI) by hydroxide ions in dry tetrahydrofuran solution

Tris(benzene-1,2-dithiolate)molybdenum(VI) reacts rapidly and quantitatively with tetrabutylammonium hydroxide to yield the corresponding Mo(V) and Mo(IV) complexes and hydrogen peroxide; the reaction has been executed in dry tetrahydrofuran where the reaction rate shows a fair dependence on complex and OH- concentrations.     

Unusual oxidation of phosphines employing water as the oxygen atom source and tris(benzene-1,2-dithiolate)molybdenum(VI) as the oxidant. A functional molybdenum hydroxylase analogue system

The kinetics of the reaction of Mo(VI)(S2C6H4)3 with organic phosphines to produce the anionic Mo(V) complex, Mo(V)(S2C6H4)3-, and phosphine oxide have been investigated. Reaction rates, monitored by UV-vis stopped-flow spectrophotometry, were studied in THF/H2O media as a function of the concentration of phosphine, molybdenum complex, pH, and water concentration. The reaction exhibits pH-dependent phosphine saturation kinetics and is first-order in complex concentration. The water concentration strongly enhances the reaction rate, which is consistent with the formation of Mo(VI)(S2C6H4)3(H2O) adduct as a crucial intermediate. The observed pH dependence of the reaction rate would arise from the distribution between acid and basic forms of this adduct. Apparently, the electrophilic attack by the phosphine at the oxygen requires the coordinated water to be in the unprotonated hydroxide form, Mo(VI)(S2C6H4)3(HO)-. This is followed by the concerted abstraction of 2e-, H+ by the Mo(VI) center to give Mo(IV)(S2C6H4)3(2-), H+, and the corresponding phosphine oxide. However, this Mo(IV) complex product is oxidized rapidly to Mo(V)(S2C6H4)3- via comproportionation with unreacted Mo(VI)(S2C6H4)3. The Mo(V) complex thus formed can be oxidized to the starting Mo(VI) complex upon admission of O2. Consequently, Mo(VI)(S2C6H4)3 is a catalyst for the autoxidation of phosphines in the presence of water. Additionally, there was a detectable variation in the reactivity for a series of tertiary phosphines. The rate of Mo(VI) complex reduction increases as does the phosphine basicity: (p-CH3C6H4)3P > (C6H5)3P > (p-ClC6H4)3P. Oxygen isotope tracing confirms that water rather than dioxygen is the source of the oxygen atom which is transferred to the phosphine. Such reactivity parallels oxidase activity of xanthine enzyme with phosphine as oxygen atom acceptor and Mo(VI)(S2C6H4)3 as electron acceptor.     

Silylation, sulfidation, and benzene-1,2-dithiolate complexation reactions of oxo- and oxosulfidomolybdates(VI) and -tungstates(VI)

The synthesis and structures of two types of molecules are presented: [MVIO3 - nSn(OSiR2R')]1- (M = Mo, n = 0-3; M = W, n = 3) and [MVIO2(OSiR2R')(bdt)]1- (M = Mo, W; bdt = benzene-1,2-dithiolate). For both types, R2R' are Me3, Pri3, Ph3, Me2But and Ph2But. The complete series of oxo/sulfido/silyloxo molybdenum complexes has been prepared. Complexes with n = 0 are readily prepared by the silylation of Ag2MoO4 and sustain mono- or disulfidation with Ph3SiSH to form a species with n = 1 and n = 2, respectively. Complexes with n = 3 are accessible by the silylation of [MOS3]2-. Structures of the representative series members [MoO3(OSiPh2But)]1-, [MoO2S(OSiPh3)]1-, [MoOS2(OSiPri3)]1-, [MoS3(OSiPh2But)]1-, and also [WS3(OSiMe2But)]1-, all with tetrahedral stereochemistry, are presented. Benzene-1,2-dithiolate complexes are prepared by the reaction of [MoO3(OSiR2R')]1-with the dithiol or by the silylation of previously reported [MO3(bdt)]2-. The structures of [MoO2(OSiPh2But)(bdt)]1- and [WO2(OSiPri3)(bdt)]1- conform to square-pyramidal stereochemistry with an oxo ligand in the apical position. The role of these complexes in the preparation of site analogues of the xanthine oxidoreductase enzyme family is noted. The sulfidation reactions reported here point to the utility of Ph3SiSH and Pri3SiSH as reagents for MoVI-based oxo-for-sulfido conversions.     

Electronic structure of neutral and monoanionic tris(benzene-1,2-dithiolato)metal complexes of molybdenum and tungsten

The reaction of 3 equiv of the ligand 2-mercapto-3,5-di-tert-butylaniline, H2[LN,S], or 3,5-di-tert-butyl-1,2-benzenedithiol, H2[LS,S], with 1 equiv of [MoO2(acac)2] or WCl6 (acac=acetonylacetate(1-)) in methanol or CCl4 afforded the diamagnetic neutral complexes [MoV(LN,S)2(L*N,S)]0 (1), [MoV(LS,S)2(L*S,S)] (2), and [WV(LS,S)2(L*S,S)] (3), where (L*N,S)- and (L*S,S)- represent monoanionic pi-radical ligands (Srad=1/2), which are the one-electron oxidized forms of the corresponding closed-shell dianions (LN,S)2- and (LS,S)2-. Complexes 1-3 are trigonal-prismatic members of the electron-transfer series [ML3]z (z=0, 1-, 2-). Reaction of 2 and 3 with [N(n-Bu)4](SH) in CH2Cl2 under anaerobic conditions afforded paramagnetic crystalline [N(n-Bu)4][MoV(LS,S)3] (4) and [N(n-Bu)4][WV(LS,S)3] (5). Complexes 1-5 have been characterized by X-ray crystallography. S K-edge X-ray absorption and infrared spectroscopy prove that a pi-radical ligand (L*S,S)- is present in neutral 2 and 3, whereas the monoanions [MV(LS,S)3]- contain only closed-shell dianionic ligands. These neutral species have previously been incorrectly described as [MVI(L)3]0 complexes with a MoVI or WVI (d0) central metal ion; they are, in fact MV (d1) (M=Mo, W) species: [MoV(LS,S)2(L*S,S)] and [WV(LS,S)2(L*S,S)] with a diamagnetic ground state St=0, which is generated by intramolecular, antiferromagnetic coupling between the MV (d1) central ion (SM=1/2) and a ligand pi radical (L*S,S)- (Srad=1/2).     

Reduction of hexachloroiridate (IV) by benzene-1,2-diol in aqueous perchloric acid

The kinetics of the reaction between benzene-1,2-diol(catechol) and hexachloroiridate (IV) have been measured in aqueous acidic perchlorate solutions by the stopped-flow method. The reaction is second order overall, and first order in each reactant. A reverse reaction also occurs, but it is much slower than the forward process. Observed rate constants are dependent on acidity, but the variation can be attributed to activity rather than mechanistic effects. The reaction appears to proceed predominantly by an outer sphere electron transfer mechanism, yielding o-benzoquinone and hexachloroiridate (III), although monoaquopentachloroiridate (III) is formed also at the higher [catechol]/[IrCl₆^2?]ratios.     

Kinetics of the reaction of U(VI) with benzene-1,2-diphosphonic acid

The kinetics of the reaction between dioxouranium(VI) and benzene-1,2-diphosphonic acid (BzDPA) has been investigated by stopped-flow spectrophotometry. The rate of reaction of uranyl ions with Arsenazo III (2,7-bis(2,2'-arsonophenylazo)-1,8-dihydroxynaphthalene-3,6-disulfonic acid) in 50:50 methanol-water solutions was also determined. Both formation and dissociation rate constants for the 1:1 complex between uranyl-BzDPA in acidic solutions were resolved. To gain insight into the effect of solvation on the progress of the reaction, the system was studied in triply distilled water, in 50:50 methanol-water, 80:20 methanol-water and in 50:50 tert-butanol-water as a function of temperature at pH 1.0. The rates of complex formation and dissociation reactions decrease as methanol substitutes for water in the medium and further decrease as tert-butanol replaces methanol as co-solvent. Activation parameters are most consistent with an associative process governing the progress of both complex formation and dissociation reactions. Introduction of alcoholic co-solvents results in notably more negative activation entropies for both complex formation and dissociation reactions, while the activation enthalpies are only slightly reduced in the mixed methanol-water medium. These results are compared with the kinetic features of other U(VI) systems.     

Kinetics of reduction of transition metal ions by sodium tetrahydroborate. Reduction of molybdenum(VI) and tungsten(VI)

Summary The kinetics of reduction of molybdenum(VI) and tungsten(VI) ions by NaBH₄ in buffered aqueous solution have been investigated. The reaction rate depends upon the first powers of the concentrations of the reactants. The temperature was varied, and the activation parameters were evaluated. Chemical and spectral evidence for the formation of molybdenum(V) and tungsten(V), as the reaction products, is presented. Plausible mechanistic pathways for these reactions are suggested.     

Molecular and electronic structures of oxo-bis(benzene-1,2-dithiolato)chromate(V) monoanions. A combined experimental and density functional study

Two oxo-bis(benzene-1,2-dithiolato)chromate(V) complexes, namely, [CrO(L(Bu))2]1- and [CrO(L(Me))2]1-, have been synthesized and studied by UV-vis, EPR, magnetic circular dichroism (MCD), and X-ray absorption spectroscopy and by X-ray crystallography; their electro- and magnetochemistries are reported. H2L(Bu) represents the pro-ligand 3,5-di-tert-butylbenzene-1,2-dithiol, and H2L(Me) is the corresponding 4-methyl-benzene-1,2-dithiol. A structural feature of interest for both the complexes is the folding of the dithiolate ligands about the S-S vector providing Cs symmetry to the complexes. Geometry optimizations using all-electron density functional theory with scalar relativistic corrections at the second-order Douglas-Kroll-Hess (DKH2) and zeroth-order regular approximation (ZORA) levels result in excellent agreement with the experimentally determined structures and electronic and S K-edge X-ray absorption spectra. From DFT calculations, the Cs instead of C2v symmetry for the complexes is attributed to the strong S(3p) --> Cr(3d(x2-y2)) pi-donation in Cs geometry providing additional stability to the complexes.     

Synthesis and catalytic properties of molybdenum(VI) complexes with tris(3,5-dimethyl-1-pyrazolyl)methane

The complex [MoO(2)Cl{HC(3,5-Me(2)pz)(3)}]BF(4) (1) (HC(3,5-Me(2)pz)(3) = tris(3,5-dimethyl-1-pyrazolyl)methane) has been prepared and examined as a catalyst for epoxidation of olefins at 55 °C using tert-butyl hydroperoxide (TBHP) as the oxidant. For reaction of cis-cyclooctene, epoxycyclooctane is obtained quantitatively within 5 h when water is rigorously excluded from the reaction mixture. Increasing amounts of water in the reaction mixture lead to lower activities (without affecting product selectivity) and transformation of 1 into the trioxidomolybdenum(VI) complex [{HC(3,5-Me(2)pz)(3)}MoO(3)] (4). Complex 4 was isolated as a microcrystalline solid by refluxing a suspension of 1 in water. The powder X-ray diffraction pattern of 4 can be indexed in the orthorhombic Pnma system, with a = 16.7349(5) ?, b = 13.6380(4) ?, and c = 7.8513(3) ?. Treatment of 1 in dichloromethane with excess TBHP led to isolation of the symmetrical [Mo(2)O(4)(μ(2)-O){HC(3,5-Me(2)pz)(3)}(2)](BF(4))(2) (2) and unsymmetrical [Mo(2)O(3)(O(2))(2)(μ(2)-O)(H(2)O){HC(3,5-Me(2)pz)(3)}] (3) oxido-bridged dimers, which were characterized by single-crystal X-ray diffraction. Complex 2 displays the well-known (Mo(2)O(5))(2+) bridging structure where each dioxidomolybdenum(VI) center is coordinated to three N atoms of the organic ligand and one μ(2)-bridging O atom. The unusual complex 3 comprises dioxido and oxidodiperoxo molybdenum(VI) centers linked by a μ(2)-bridging O atom, with the former center being coordinated to the tridentate N-ligand. The dinuclear complexes exhibit a similar catalytic performance to that found for mononuclear 1. For complexes 1 and 2 use of the ionic liquids (ILs) 1-butyl-3-methylimidazolium tetrafluoroborate and N-butyl-3-methylpyridinium tetrafluoroborate as solvents allowed the complexes to be completely dissolved, and in each case the catalyst and IL could be recycled and reused without loss of activity.     

Liquid-liquid extraction of molybdenum(VI) and uranium(VI) by LIX 622

The order of extraction of Mo(VI) from 1M acid solutions by 5% (v/v) LIX 622 (HL) in benzene is HCl>HNO₃>HClO₄>H₂SO₄, and extraction decreases with increasing concentration of HCl and H₂SO₄, and increases slightly with increasing concentration of HNO₃ and HClO₄. The extracted species is shown to be MoO₂L₂ as established by IR data of organic extracts and the extracted species in the solid form. Extraction is almost quantitative at and above 10% LIX 622, and is found to be independent of [Mo(VI)] in the range of 10^–4 to 10^–3 M. The diluents CCl₄, CHCl₃ and C₆H₆ are found to be superior to solvents of high dielectric constant for extraction of Mo(VI). Extraction of uranium(VI) by 10% (v/v) LIX 622 in benzene was found to increase with increasing equilibrium pH (3.0 to 6.0), and becomes quantitative at pH 5.9. Tributyl phosphate acts as a modifier up to 2% (v/v). Thorium(IV) is almost not extracted by LIX 622 or its mixture. Separation of Mo(VI) and U(VI) is feasible.     

The different kinetic and mechanistic behaviors of molybdenum and tungsten in the reduction of tris(benzene‐1,2‐dithiolato)Mo(VI) and W(VI) complexes by ascorbic acid in aqueous media

The mono‐electronic reduction of tris(benzene‐1,2‐dithiolato)Mo(VI) and W(VI) complexes (ML₃: M = Mo, W; L = S₂C₆H^2?₄, S₂C₆H₃CH^2?₃) to their anionic forms ML^?₃ by L (+)‐ascorbic acid (H₂A) has been studied in tetrahydrofurane (THF):water and THF:methanol by means of diode‐array, stopped‐flow, and mass spectrometry–electrospray ionization (MS‐ESI) spectroscopy. The kinetic study in methanol demonstrates that the reaction is first order in each reactant, the electron transfer being rate limiting. This fact was assessed by the absence of a primary saline effect and by the correlation observed between the activation free enthalpy (ΔG^≠) and the reduction potentials measured by cyclic voltamperometry. In aqueous media, Mo(VI)‐tris(dithiolenes) also reduce to their Mo(V) anionic forms. The reaction obeys the rate law ? d[ML₃]/dt = (k_S+k_A[H₂A]_T)[ML₃] (M = Mo), in agreement with a parallel kinetic scheme involving the reduction of complexes by ascorbic acid (k_A) and by interaction with the solvent (k_S). Unexpectedly, the W(VI) complexes were not reduced by excess hydrogen ascorbate in the presence of water. These compounds underwent an extremely rapid autoreduction, which initially yielded an oxo W(VI)‐dithiolene and [W(S₂C₆H₄)₃]^?, as assessed by the MS‐ESI spectra. This observation suggests that tungsten tris(dithiolenes) are capable of coordinating water efficiently, undergoing further reduction after ligand displacement. © 2011 Wiley Peiodicals, Inc. Int J Chem Kinet 43: 279–291, 2011     

Anderson-type heteropolyanions of molybdenum(VI) and tungsten(VI)

The previously reported preparation of some Anderson-type molybdopolyanions containing divalent metal ions (Zn, Cu, Co or Mn) as a heteroatom has been reinvestigated. The molybdopolyanions of Zn(II) and Cu(II) were confirmed, although the Cu(II) polyanion was not stable and could not be recrystallized. On the other hand, the polyanions of Co(II) and Mn(II) could not be reproduced. Another type of heteropoly compound, [X(H₂O)_6-x(Mo₇O₂₄)]⁴⁻ [X = Cu(II), Co(II) or Mn(II)], was isolated as solids, which are not stable thermally. The mixed-type Anderson polyanions, [Ni(II)Mo_6-xW_x,O₂₄H₆]⁴⁻, which have been questioned as mixtures of species with different x values, were also reinvestigated using IR, UV absorption and MCD spectra. They are single species, but not mixtures, although some positional isomers may be present for the compounds where x = 2-4. The possibility of oxidation of the heteroatom with the Anderson structure maintained was examined. The oxidation of [Ni(II)Mo₆O₂₄H₆]⁴⁻ by the S₂O²⁻₈ ion in aqueous solution gave the Waugh-type [Ni(IV)Mo₉O₃₂]⁶⁻ polyanion, whereas the oxidation of [Ni(II)W₆O₂₄H₆]⁴⁻ gave no heteropoly compound.     

Spectrophotometric determination of molybdenum(VI)

Molybdenum(IV) gives a red colour with ammonium thiocyanate in 5-8M hydrochloric acid medium, the Sandell sensitivity index being 0.018 ppm Mo(VI)/cm(2). Molybdenum(VI) in 4-7M hydrochloric acid medium forms a red complex with ethyl xanthate and ammonium thiocyanate and this can be extracted into acetophenone. Beer's law is obeyed over the range of 1.2-13.8 ppm, and the Sandell indices at 370 and 470 nm are 0.0016 and 0.0068 ppm/cm(2) respectively. The colour is stable for 40 hr. Most cations do not interfere.     

Complexometric determination of molybdenum(VI)

In acidic medium molybdenum(VI) forms a stable complex on boiling with excess of DCTA and hydroxylamine hydrochloride. Molybdenum can then be determined by back-titration of the excess of DCTA either with zinc chloride at pH 5-5.5 or with thorium nitrate at pH 3-4.5, Xylenol Orange being used as indicator in both cases. A simple method for the determination of molybdenum in the presence of moderate amounts of tungsten is also described.     

New oxovanadium bis(1,2-dithiolate) compounds that mimic the hydrogen-bonding interactions at the active sites of mononuclear molybdenum enzymes

Reaction of VO(acac)(2) with 1,2-dithiols in the presence of triethylamine gives pentacoordinate oxovanadium complexes [HNEt(3)](2)[VO(bdt)(2)] (1), [HNEt(3)](2)[VO(tdt)(2)] (2), and [HNEt(3)](2)[VO(bdtCl(2))(2)] (3) (where H(2)bdt = 1,2-benzenedithiol, H(2)tdt = 3,4-toluenedithiol, and H(2)bdtCl(2) = 3,6-dichloro-1,2-benzenedithiol). Compounds 1-3 have been characterized by IR, UV/visible, EPR, and mass spectroscopies. The X-ray crystal stuctures of 1 and 2 show hydrogen-bonding interactions between the terminal oxo atom and triethylammonium counterions and between ligand sulfur atoms and the counterions. These interactions are comparable with those found at the active sites of mononuclear molybdenum enzymes.     

Adsorptive stripping voltammetry determination of molybdenum(VI) in water and soil

A differential pulse stripping voltammetry method for the trace determination of molybdenum(VI) in water and soil has been developed. In 0.048M oxalic acid and 6 x 10(-5)M Toluidine Blue (pH 1.8) solution, Mo(V), the reduction product of Mo(VI) in the sample solution, can form a ternary complex, which can be concentrated by adsorption on a static mercury drop electrode at -0.1 V (vs. Ag/AgCl). The adsorbed complex gives a well-defined cathodic stripping current peak at -0.30 V, which can be used for determining Mo(VI) in the range 5 x 10(-10)-7 x 10(-9)M, with a detection limit of 1 x 10(-10)M (4 min accumulation). The method is also selective. Most of the common ions do not interfere but Sn(IV) and large amounts of Cu(2+), Ag(+) and Au(3+) affect the determination.