Reaction kinetics and mechanism of formation of [H4Co2Mo10O38]6– by peroxomonosulfate oxidation of CoII in the presence of molybdate

Abstract

Oxidation of Co^II by HSO₅^– to Co^III in weakly acidic solution, in the presence of molybdate, resulted in the formation of (primarily) the soluble dicobalt species [H₄Co₂Mo₁₀O₃₈]^6–. The kinetics of formation of this species was examined at 15–35 °C over the range pH 4.0–5.5 and found to exhibit three separate kinetically observable steps, oxidation of Co^II to Co^III, followed by a ligand-breakdown reaction and a slow ligand-replacement step which produces the observed product(s). The first stage followed the expanded rate expression +d[Co^III]/dt=k_ox[Co²⁺][HSO₅^–]/[H⁺][HMoO₄^–]³. A value for k_ox of 7.05(4)× 10^–14 mol³ dm^–9 s^–1 at 25 °C was calculated using reported formation constants describing the speciation of [MoO₄]^2–, [Mo₇O₂₄]^6–, and their protonated forms in solution. This rate expression may be accounted for by a mechanism arising from a series of pre-equilibria involving the loss of three [HMoO₄]^– units and a H⁺ from a cobalt(II) heteropolymolybdate, most likely [H₆CoMo₆O₂₄]^4–, which then allows the one-electron oxidation of Co²⁺ to Co³⁺ by HSO₅^– to occur following co-ordination of the latter. In keeping with this proposal, [NH₄]₄[H₆CoMo₆O₂₄]· 4H₂O was crystallized from an acidic (pH 4.5) solution containing Co²⁺(aq) and molybdate, and its structure determined by X-ray diffraction methods. The heteropolymetalate anion exhibits a standard Anderson structure with six octahedral molybdate edge-sharing units surrounding the central cobalt, and all metal atoms effectively in a common plane. The second observable kinetic step shows no dependences on [Co³⁺], [oxidant], pH or [molybdate], and is interpreted as a ligand-breakdown reaction, involving loss of an OH˙ radical from the co-ordinated ‘HSO₅^2–’(radical) present following the actual one-electron oxidation step. Dimerization is then assumed to occur, followed by slow [H₄Co₂Mo₁₀O₃₈]^6– formation stemming from further reaction of the immediate product of the dimerization step, involving loss of co-ordinated SO₄^2–.