Studies on transition metal peroxo complexes. Part 8. The nature of peroxomolybdates and peroxotungstates in aqueous solution

Abstract

The nature of the species formed in solutions of [MoO4]2– and [WO4]2–(1–0.1 mol dm–3) in the presence of H2O2(5–0.5 mol dm–3) from pH 12 to acid has been studied by Raman and i.r. spectroscopy and, for molybdenum-containing species, by 95Mo n.m.r. spectroscopy. The polymerisation processes are far less complex in the presence of H2O2 than in its absence. The principal species are likely to be [M(O2)4]2–, [MO(O2)3]2–, [M2O3(O2)4(H2O)2]2– and, at lower pH, complexes containing the [MO(O2)]2+ unit. Vibrational assignments are proposed for K2[M2O3(O2)4(H2O)2]·2H2O (M = Mo or W) using 18O and 2H substitution. Parallel studies on the oxidatin of alcohols and alkenes by [MO4]2– in excess of H2O2 from pH 7 to 0.5 suggest that [M2O3(O2)4(H2O)2]2– is the most effective oxidising species. Stoicheiometric oxidations of primary alcohols to aldehydes, of secondary alcohols to ketones, and of cyclohexene to its epoxide by [PPh4]2[M2O3(O2)4] are briefly reported.