Voltammetric behavior of 1- and 4-[S2VVW17O62]5− in acidified acetonitrile

Abstract

Data derived from a voltammetric and spectroscopic study of the V^V/IV couple associated with the initial reduction of the Wells–Dawson-type mono vanadium-substituted polyoxometalates, 1- and 4-[S₂V^VW₁₇O₆₂]⁵⁻ in CH₃CN as a function of CF₃SO₃H acid concentration have been obtained. ⁵¹V NMR (V^V component) and EPR (V^IV component) spectra were measured in CH₃CN in the presence and absence of an acid. These data showed a small fraction of the 1-isomer in the 4-[S₂V^VW₁₇O₆₂]⁵⁻ sample and that protonation could occur at both redox levels for both isomers. On the basis of the mechanism postulated from the voltammetric and spectroscopic data, simulations of cyclic voltammograms were undertaken for the reduction of the isomerically pure 1-[S₂V^VW₁₇O₆₂]⁵⁻ isomer over a wide acid concentration range, and the results were compared with experimental data. Cyclic voltammograms of the V^V/IV couple derived from the reduction of 1- and 4-[X₂V^VW₁₇O₆₂]⁷⁻ (X = P, As) were also obtained in CH₃CN and the results were compared with those for 1- and 4-[S₂V^VW₁₇O₆₂]⁵⁻. Reversible potentials for the V^V/IV couple are dependent on the anion charge of the polyoxometalate. Analysis of cyclic voltammograms obtained for 1- and 4-[S₂V^VW₁₇O₆₂]⁵⁻ in acetonitrile, acetone, dimethyl sulfoxide, dimethyl formamide and nitromethane showed that these V^V/IV reversible potentials are also dependent on the acceptor numbers and the polarity index (E^N_T) values of the organic solvents.