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

The nature of the species formed in solutions of MoO42– and WO42–(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)42–, MO(O2)32–, M2O3(O2)4(H2O)22– and, at lower pH, complexes containing the MO(O2)2+ unit. Vibrational assignments are proposed for K2M2O3(O2)4(H2O)2·2H2O (M = Mo or W) using 18O and 2H substitution. Parallel studies on the oxidatin of alcohols and alkenes by MO42– in excess of H2O2 from pH 7 to 0.5 suggest that M2O3(O2)4(H2O)22– 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 PPh42M2O3(O2)4 are briefly reported.