ABSTRACT
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.
Subject(s)
Coordination Complexes/chemistry , Tungsten Compounds/chemistry , Vanadium/chemistry , Crystallography, X-Ray , Models, MolecularABSTRACT
Structures of the n-tetrabutylammonium salts of [SVM11O40](3-) (M = Mo, W) have been determined by X-ray crystallography and exhibit 3D networks with the V atom disordered over several sites. The cyclic voltammetric behavior of SVM11 in neutral and acidified acetonitrile solutions also has been investigated with respect to the V(V)/V(IV) couple. Results have been interpreted in conjunction with data provided by (51)V NMR spectroscopy on the oxidized V(V) form and by EPR spectroscopy on the reduced V(IV) form. Based on mechanistic details inferred from these studies, simulations of the cyclic voltammograms have been undertaken and results compared with experimental data in acidic media (two protonated forms) in order to provide estimates of equilibrium and kinetic parameters. For the V(V)/V(IV) couple in the series [XVM11O40](n-) (X = Si,Ge,P,As,S; M = Mo,W), the reversible potentials in neutral acetonitrile linearly depend on the total charge of the vanadium-substituted polyoxometalates, similar to the dependence previously reported for the non-substituted parent Keggin polyoxometalates [XM12O40](m-).
ABSTRACT
The one electron electrochemical reduction of vanadium(V) substituted-Keggin type polyoxometalate anions [XV(V)M(11)O(40)](4-) (XV(V)M(11)) where X = P, As; M = Mo or W has been investigated in CH(3)CN as a function of acid concentration. EPR studies confirm that the product is the highly basic and hence readily protonated V(IV), (XV(IV)M(11)), species. In the absence of acid or in the presence of a large concentration excess, the V(V/IV) redox couple gives one well defined chemically and electrochemically reversible process under conditions of cyclic voltammetry. In contrast, three well resolved V(V/IV) processes are observed in the presence of a moderate concentration of acid in the case of XV(V)Mo(11) but only two with XV(V)W(11). NMR and EPR spectra have been obtained as a function of acid concentration when the polyoxometalate anions are in the V(V) and V(IV) oxidation levels respectively. All voltammetric and spectroscopic data indicate that protonation reactions are coupled to the V(V) and V(IV) redox chemistry but that the reduced V(IV) state is much more basic than the V(V) one and that (XV(IV)Mo(11)) is more basic than (XV(IV)W(11)). Digital simulations of voltammograms for reduction of XV(V)Mo(11) and XV(V)W(11) have been undertaken in CH(3)CN as a function of acid concentration in order to define the thermodynamics and kinetics associated with the V(V/IV) process and the equilibrium constants that accompany the coupled acid-base chemistry. EPR spectra allow an estimation of the relative concentrations of protonated species present in frozen glasses derived from one-electron bulk electrolysis and also allow inferences to be drawn on their nature. This study provides a far more detailed analysis of the coupling of proton and electron transfer than previously available.
