Oxidation of [CoIIW12O40]6– to [CoIIIW12O40]5– by peroxomonosulfate in strong and weak acid solutions, an example of zero-order kinetics

Abstract

Oxidation of [Co^IIW₁₂O₄₀]^6– by HSO₅^– in both strong and weak acid solution (the latter in the presence of added [WO₄]^2–) gives [Co^IIIW₁₂O₄₀]^5–. The kinetics of oxidation was examined at 15–35 °C over the range [H⁺] = 0.05–0.625 mol dm^–3 and found to exhibit pseudo-zero-order kinetics, –d[Co^IIW₁₂O₄₀^6–]/dt = 2k_OX[HSO₅^–]/[H⁺] where k_ox = 1.03(7) × 10^–6 mol dm^–3 s^–1 at 25.0 °C. The rate expression may be accounted for by a mechanism arising from the deprotonation of HSO₅^– to give SO₅^2– as the oxidant species, enabling oxidation of two [Co^IIW₁₂O₄₀]^6– ions by an SO₅^2– ion, presumably through an outer-sphere electron-transfer mechanism given the stability of the polyoxotungstate framework under highly acidic conditions. Over the pH range 4.2–5.7 and 5–25 °C the reaction exhibits pseudo-first-order reaction kinetics which is independent of the oxidant concentration and involves breakdown of the polyoxotungstate framework as the limiting step, with removal of a W₃O₁₃ unit as a precursor to oxidation through the loss of three [HWO₄]^– ions following attack by water molecules. The reaction kinetics for the oxidation by HSO₅^– of the related Co^II-substituted Keggin ion [Co^II₂(H₂O)W₁₁O₃₉]^8– to give [Co^IIICo^II(H₂O)W₁₁O₃₉]^7– was also investigated, and likely involves initial oxidation of the framework Co^II to Co^III followed by fast electron transfer to give a central cobalt(III) heteroatom.