Oxygen-17 NMR study of aqueous peroxotungstates

Abstract

Aqueous peroxotungstates have been studied from pH 0.5 to 9.0, over a range of peroxide concentrations. Although equilibria are not always established, many anions can be identified by ¹⁷O NMR because the ratio of δ_O(W anion)/δ_O(known Mo anion) is consistently 79 ± 3%. They are [WO₃(HO₂)]⁻; [WO(OH)(O₂)₂]⁻; [WO(OH₂)(O₂)₂]⁰; [W₂O₃(O₂)₄]²⁻; [W₂O₃(OH)(O₂)₄]³⁻; [W₄O₁₂(O₂)₂]⁴⁻; [W₇O₂₃(O₂)]⁶⁻ and [W₇O₂₂(O₂)₂]⁶⁻. Their pK_a values, where measurable, are about 2 units lower than the corresponding peroxomolybdates, e.g. 0.0 for [WO(OH₂)(O₂)₂]⁰ and 8.0 for [W₂O₃(O₂)₄]²⁻. Other peroxotungstate species are also present but can only be broadly identified. These include Keggin structures with relatively low peroxo content, and a very unsymmetrical anion appearing at pH ca. 7 that bears no obvious structural relationship to any species previously reported. The main product from the reaction of powdered W metal with 30% aqueous peroxide is provisionally identified as the symmetrical anion [W₆O₁₃(OH)₂(OH₂)₂(O₂)₅]²⁻, although other minor species are also formed, probably having fewer peroxo substituents.