Studies on the redox turnover of polyoxometalates using potentiometric chemical sensors

Abstract

Redox reactions of Keggin-type polyoxometalates (POM), [PV_xMo_12−xO₄₀]^(3+x)− (x = 1, 2), have been studied for the first time employing an array of potentiometric chemical sensors. Variations in the concentrations of vanadyl ions (VO²⁺) during the redox reactions of POMs were followed in the real-time. Differences in the vanadyl concentration profiles observed during re-oxidation of di- and mono-substituted POMs allow us to infer principally different re-oxidation mechanisms for these compounds. At the beginning of the oxidation of [PV^IV₂Mo₁₀O₄₀]⁷⁻ under air an instant release of VO²⁺ (ca. 7%) from the coordination sphere of POM to the outside solution was detected. VO²⁺ concentration decreased rapidly while oxidation proceeded indicating return of vanadyl back to the coordination sphere of [PV^V₂Mo₁₀O₄₀]⁵⁻. This finding indicates the participation of VO²⁺ outside Keggin structure in the re-oxidation mechanism of di-substituted POM. No release of VO²⁺ was detected during re-oxidation of [PV^IVMo₁₁O₄₀]⁵⁻ by air oxygen in the presence of laccase (this POM is not oxidisable by air at low temperature and atmospheric pressure). Biocatalysis of [PV^IVMo₁₁O₄₀]⁵⁻ oxidation may include activation of the molecular oxygen by the prosthetic group of laccase thus allowing the oxidation of V(IV) inside the Keggin structure of POM, i.e., without vanadyl release into the outside solution.