Electrosynthesis and solution structure of six-electron reduced forms of metatungstate, [H2W12O40]6−

Abstract

Metatungstate salts α-[R₄N]₅H[H₂W₁₂O₄₀] (R = Pr or Bu) were produced by phase transfer methods. Two one-electron reduction processes (E_1/2 −1100, −1590 mV versus Ag–AgCl (saturated KCl in MeCN)) are seen for [Bu₄N]₅H[H₂W₁₂O₄₀] in MeCN (Bu₄NClO₄, 0.1 M) solution. They convert into a single two-electron process in MeCN–water (95∶5 v/v) upon the addition of acid. Controlled potential electrolysis in aqueous HCl at the two electron potentials resulted in isolation of the six-electron reduced salt [NH₄]₄H₈[H₂W₁₂O₄₀] in which one of the oxidised W^VI₃ trinuclear caps of metatungstate is reduced to a W^IV₃ trinuclear cap. [Bu₄N]₃H₉[H₂W₁₂O₄₀] and related salts were generated by phase transfer. The anion [H₂{W^IV₃(OH₂)₃}W^VI₉O₃₄(OH)₃]³⁻ is obtained by dissolving [Bu₄N]₃H₉[H₂W₁₂O₄₀] in dry CD₃CN. The distribution of the eleven protons present in this anion is mapped by ¹H and ¹⁸³W NMR, allowing assessment of the structural changes which accompany reduction. C_s point symmetry is observed and imposed by the association of the three surface hydroxyl protons with the reduced W^IV₃ trinuclear cap and one of the oxidised W^VI₃ trinuclear caps. Three WOW links appear to be converted into longer W(OH)W links to accommodate the significant shortening (≈0.7 Å) in W ⋯ W separation anticipated to occur upon reduction.