Voltammetric Studies on the Reduction of Polyoxometalate Anions in Ionic Liquids

The electrochemical reduction of tetrabutylammonium salts of isostructural pairs of polyoxometalates [Bu4N]2[M6O19], [Bu4N]4[(alpha)-SiM12O40], and [Bu4N]4[(alpha)-S2M18O62] (M = Mo or W) has been investigated at glassy carbon electrodes in dissolved and surface-confined states in ionic liquids and other media. In the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6], between two and six reversible one-electron-transfer processes were detected. Detailed studies on the process [(alpha)-S2W18O62]4-/5- in a range of ionic liquids, water, and conventional organic solvents (containing 0.1 M electrolyte) suggest that the polarity of the medium plays a key role in the determination of the reversible potential. Reduction processes involving very highly charged [(alpha)-S2W18O62]8-/9-/10- species are strongly influenced by the purity of the medium.