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DOI: 10.1039/A804598D (Paper) Reference Section for: J. Chem. Soc., Dalton Trans., 1998, 3041-3048

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

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Annette L. Nolan, Robert C. Burns and Geoffrey A. Lawrance

Abstract

Oxidation of [CoIIW12O40]6– by HSO5– in both strong and weak acid solution (the latter in the presence of added [WO4]2–) gives [CoIIIW12O40]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[CoIIW12O406–]/dt = 2kOX[HSO5–]/[H+] where kox = 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 SO52– as the oxidant species, enabling oxidation of two [CoIIW12O40]6– ions by an SO52– 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 CoII-substituted Keggin ion [CoII2(H2O)W11O39]8– to give [CoIIICoII(H2O)W11O39]7– was also investigated, and likely involves initial oxidation of the framework CoII to CoIII followed by fast electron transfer to give a central cobalt(III) heteroatom.

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