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

Oxidation of CoII by HSO5– to CoIII 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 CoII to CoIII, 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[CoIII]/dt=kox[Co2+][HSO5–]/[H+][HMoO4–]3. A value for kox 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 ‘HSO52–’(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 SO42–.