NMR, Voltammetric, and Photoelectrochemical Studies on the Dark and Light-Catalyzed Reactions of α-[S2Mo18O62]4- with Aryl- and Alkylphosphines

A lacunary tungstomolybdophosphate as an electronic pendulum: The "blue" electron under examination

The photoreduction of a Keggin type lacunary tungstomolybdophosphate, α-(Bu₄N)₄[H₃PW₉Mo₂O₃₉], in acetonitrile, led to the formation of a monoreduced lacunary heteropoly anion, or a one electron reduced "heteropoly blue" species, whereby the added "blue" electron was captured by the molybdenum atoms. The magnetic properties and behavior of the "blue" electron were studied by a modified Evans nuclear magnetic resonance method (small downshift of the ³¹P signal) and variable-temperature electron paramagnetic resonance (g = 1.936 for Mo^V). The intermolecular exchange of the "blue" electron was limited by a geometrical factor, which requires the contact between Mo caps to exchange it between the heteropoly couple. The intramolecular exchange of the "blue" electron between Mo atoms was rather fast (5.3 × 10⁹ s^-1), with a rate of more than six orders of magnitude larger than the intermolecular exchange rate. Density functional theory was used to determine the most prevalent protonation sites in the mixed lacunary isomers with the aim of studying the intramolecular electron transfer pathway in the isolated [H₄PW₉Mo₂O₃₉]^4- species. The singly occupied molecular orbital (SOMO) is essentially localized in one of the two nonequivalent molybdenum sites. The kinetics of the intramolecular electron exchange equilibrium Mo^V + Mo^VI → Mo^VI + Mo^V between the two molybdenum atoms bridged by an oxygen atom was found to be fast in agreement with the experimental result. The transition state is of mixed-valence type, with the SOMO delocalized over the Mo-O-Mo group. Spectroscopic parameters were found to be in fair agreement with experimental results.

Oxygen atom transfer with organofunctionalized polyoxovanadium clusters: O-atom vacancy formation with tertiary phosphanes and deoxygenation of styrene oxide

We report a rare example of oxygen atom transfer (OAT) from a polyoxometalate cluster to a series of tertiary phosphanes followed by OAT from styrene oxide to the reduced scaffold, resulting in the formation of styrene.

We report a rare example of oxygen atom transfer (OAT) from a polyoxometalate cluster to a series of tertiary phosphanes. Addition of PR₃ (PR₃ = PMe₃, PMe₂Ph, PMePh₂, PPh₃) to a neutral methoxide-bridged polyoxovanadate-alkoxide (POV-alkoxide) cluster, [V₆O₇(OMe)₁₂]⁰, results in isolation of a reduced structure with phosphine oxide datively coordinated to a site-differentiated V^III ion. A positive correlation between the steric and electronic properties of the phosphane and the reaction rate was observed. Further investigation of the steric influence of the alkoxy-bridged clusters on OAT was probed through the use of POV clusters with bridging alkoxide ligands of varying chain length ([V₆O₇(OR′)₁₂]; R′ = Et, ⁿPr). These investigations expose that steric hinderance of the vanadyl moieties has significant influence on the rate of OAT. Finally, we report the reactivity of the reduced POV-alkoxide clusters with styrene oxide, resulting in the deoxygenation of the substrate to generate styrene. This result is the first example of epoxide deoxygenation using homometallic polyoxometalate clusters, demonstrating the potential for mono-vacant Lindqvist clusters to catalyze the removal of oxygen atoms from organic substrates.

Synthesis and characterization of novel Wells-Dawson-type mono vanadium(V)-substituted tungsto-polyoxometalate isomers: 1- and 4-[S2VW17O62](5-)

Two vanadium(V)-substituted tungsto-polyoxometalate isomers, 1- and 4-[S2VW17O62](5-), were prepared as their tetra-alkyl ammonium salts from a W(VI)-H2SO4-V(V) reaction mixture in aqueous CH3CN solution. X-ray crystallographic structural analysis revealed that both isomers have a Wells-Dawson-type structure with a higher occupancy of vanadium at polar sites and belt sites for 1- and 4-[S2VW17O62](5-), respectively. The isomers were also characterized by elemental analysis, infrared, Raman, UV-vis, and (51)V NMR spectroscopies as well as voltammetry, and the data obtained were compared with that derived from [S2W18O62](4-). Significantly, the reversible potentials for the vanadium(V/IV) couple for both 1- and 4-[S2VW17O62](5-) in CH3CN (0.1 M n-Bu4NPF6) are considerably more positive than the tungstate reduction process exhibited by the [S2W18O62](4-) framework, implying that the presence of vanadium should be useful in catalytic reactions. The one-electron-reduced [S2V(IV)W17O62](6-) forms of both isomers were prepared in solution by controlled potential bulk electrolysis and characterized by voltammetry and EPR spectroscopy.

Polyoxometalate water oxidation catalysts and the production of green fuel

In the last five years and currently, research on solar fuels has been intense and no sub-area in this field has been more active than the development of water oxidation catalysts (WOCs). In this timeframe, a new class of molecular water oxidation catalysts based on polyoxometalates have been reported that combine the advantages of homogeneous and heterogeneous catalysts. This review addresses central issues in green energy generation, the challenges in water oxidation catalyst development, and the possible uses of polyoxometalates in green energy science.

Sensitization of photo-reduction of the polyoxometalate anions [S(2)M(18)O(62)](4-) (M = Mo, W) in the visible spectral region by the [Ru(bpy)(3)](2+) cation

Voltammetric, photo-physical and photo-electrochemical properties of the Dawson polyoxometalate anions alpha-[S(2)M(18)O(62)](4-) (M = Mo, W) are presented, both in the presence and absence of a series of [Ru(II)L(n)](+/2+) cations [L(n) = (bpy)(3), (bpy)(2)(Im)(2), (bpy)(2)(dpq), (bpy)(2)(box) and (biq)(2)(box)]. Electrochemical processes for both the anion and Ru(II/III) couples were detected in solutions of the salts [Ru(II)L(n)](2)[S(2)M(18)O(62)] in dimethylformamide (0.1 M Bu(4)NPF(6)) by both cyclic and hydrodynamic voltammetries. Responses were also detected when the solid salts were adhered to the surface of a glassy carbon electrode in contact with an electrolyte in which they are insoluble (CH(3)CN; 0.1M Bu(4)NPF(6)). Photolysis experiments were performed on solutions of the salts [R(4)N](4)[S(2)M(18)O(62)] (R = n-butyl or n-hexyl) and [Ru(II)L(n)](2)[S(2)M(18)O(62)] at 355 and 420 nm in dimethylformamide and acetonitrile in the presence and absence of benzyl alcohol (10% v/v). When associated with [Ru(bpy)(3)](2+), the molybdate anion exhibited a large increase in the quantum yield for photo-reduction at 420 nm. The quantum yield for the tungstate analogue was lower but the experiments again provided clear evidence for sensitization of the photo-reduction reaction in the visible spectral region. The origin of this sensitization is ascribed to the new optical transition observed around 480 nm in static ion clusters {[Ru(bpy)(3)][S(2)M(18)O(62)]}(2-) and {[Ru(bpy)(3)](2)[S(2)M(18)O(62)]} present in solution. Measurable photocurrents resulted from irradiation of solutions of the anions with white light in the presence of the electron donor dimethylformamide. Evidence is also presented for possible quencher-fluorophore interactions in the presence of certain [Ru(II)L(n)](+) cations.

Electrochemical investigation of photooxidation processes promoted by sulfo-polyoxometalates: coupling of photochemical and electrochemical processes into an effective catalytic cycle

Oxidative photocurrents measured upon irradiation by a 7-W visible light (wavelength 312-700 nm) demonstrated that the sulfo-polyoxometalate anion clusters [S2W18O62]4- (1a), [S2Mo18O62]4- (1b), and [SMo12O40]2- (2) may be activated photochemically to oxidize the organic substrates benzyl alcohol, ethanol, and (-)-menthol. In the case of catalytic photooxidation of benzyl alcohol to benzaldehyde in the presence of 1a, quantitative electrochemical methods have identified pathways for the oxidation of reduced forms of 1 generated during the catalysis. More generally, the oxidation pathways P(n+2)- + 2H+ <==> Pn- + H2 and 2P(n+2)- + O2 + 4H+ <==> 2Pn- + 2H2O have been evaluated by monitoring acidified acetonitrile solutions of the 2e(-)-reduced clusters by rotating disk electrode voltammetry under anaerobic and aerobic conditions, respectively. Neither of the reduced forms 1b(2e-) nor 2(2e-) reacted under these conditions. In contrast, 1a(2e-) was oxidized via both pathways, consistent with its more negative redox potential, with the rate of oxidation by air-oxygen being significantly faster than that by H+. The present work demonstrated that the crucial step necessary to oxidize reduced catalyst in photocatalytic reactions involving the anions studied may be achieved or accelerated by application of an external potential more positive than the first redox potential of the polyoxometalate anion. Voltammetric analysis revealed that this in situ electrolytic regeneration of the reduced catalyst is an option that leads to a viable photoelectrocatalytic pathway, even when the H+ and O2 pathways are not available.

Synthesis and redox characterization of the polyoxo anion, gamma*-[S2W18O62]4-: a unique fast oxidation pathway determines the characteristic reversible electrochemical behavior of polyoxometalate anions in acidic media

The synthesis and characterization of (Bu4N)4[S2W18O62].1.23MeCN.0.27H2O are reported. It crystallizes in the monoclinic space group C2/c with a = 22.389(6) A, b = 22.104(3) A, c = 25.505(5) A, beta = 95.690(15) degrees, V = 12560(5) A3, and Z = 4. The anion exists as the gamma* isomer, the second example of this isomer type to be characterized structurally. The equivalent molybdenum salt occurs as the alpha isomer. gamma*-[S2W18O62]4- in MeCN solution displayed four electrochemically reversible one-electron redox processes at E1/2 values of -0.24, -0.62, -1.18, and -1.57 V versus the Fc+/Fc couple. Upon addition of acid in MeCN/H2O (95/5 v/v), the two most cathodic processes converted to an overall two-electron process at -0.71 V. The total data suggested that this process actually comprises two one-electron transfer processes, occurring at different potentials, with associated proton-transfer reactions. The interpretation is supported by simulation of the effect of acid titration upon the cyclic voltammetry. While multiple pathways for correlated reduction and protonation are present in both the molybdenum and tungsten systems, only a single fast oxidation pathway is available. As the reduced forms of [S2W18O62]4- are much weaker bases than those of [S2Mo18O62]4-, the individual oxidation pathways are not the same. However, their existence determines the highly reversible electrochemical behavior that is characteristic of these anions, and that of polyoxometalate systems in general.