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

Metatungstate salts alpha-[R4N](5)H[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](5)H[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](4)H-8[H2W12O40] in which one of the oxidised W-3(VI) trinuclear caps of metatungstate is reduced to a W-3(IV) trinuclear cap. [Bu4N](3)H-9[H2W12O40] and related salts were generated by phase transfer. The anion [H-2{W-3(IV)(OH2)(3)}(W9O34)-O-VI(OH)(3)](3-) is obtained by dissolving [Bu4N](3)H-9[H2W12O40] in dry CD3CN. The distribution of the eleven protons present in this anion is mapped by H-1 and W-183 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-3(IV) trinuclear cap and one of the oxidised W-3(VI) trinuclear caps. Three WOW links appear to be converted into longer W(OH)W links to accommodate the significant shortening (approximate to0.7 Angstrom) in W . . .W separation anticipated to occur upon reduction.