Structurally characterised vanadium(v)-substituted Keggin-type heteropolysulfates [SVM11O40]3− (M = Mo, W): voltammetric and spectroscopic studies related to the V(v)/V(iv) redox couple

Abstract

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 ⁵¹V 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−.