Oxidation of Mn2+aq by HSO5- to manganese(IV) in the presence of molybdate ion in acetate buffer involves the formation of the soluble heteropolymolybdate anion [MnMo9O32]6-. This was crystallized as K6[MnMo9O32].6H2O from the reaction mixture in the rhombohedra1 space group R32, a 15.569(1), c 12.432(1) � , and the structure was determined by X-ray diffraction methods, refined to a residual of 0.028 for 1032 independent 'observed' reflections. Unlike the oxidation reaction in the absence of molybdate , which displays classical autocatalytic kinetics and generates manganese dioxide, no MnO2 is formed with Mn /Mo ≤ 1:12, the red heteropolymolybdate being the only isolated product. The kinetics of the oxidation were examined at 40�C over the pH range 4.0-5.3, and non-linear dependences on [HSO5-] and l/[H+] observed. The influence of variation of the manganese-to-molybdenum ratio between 1 : 12 and 1 : 50 on the observed rate constant was very small at pH 4.54, a result supporting the view that the manganese exists initially as the known [MnMo6O24H6]4- or a close analogue in solution. A rate expression of the form -d[Mn11dt = ko [HSO5-]+ k1 [HSO5-]2 where k0 = 0.021 dm3 mol-1 s-1 and k1 = 0.041 dm6 mol-1s-1 was observed at pH 4.54. Cyclic voltammetry identified the oxidation of the manganese(II) cluster as an irreversible process which occurs at +1.035 V (v. Ag/ AgCl ) at pH 3.95, becoming progressively more negative with increasing pH (+0.84 V at pH 5.3), the variation presumably related to variation in protonation of the cluster, which also governs the non-linear dependence of the chemical oxidation on [H+].
Selection from a pool of self-assembling lipid replicators