Kinetics of oxidation of thiosulfate ion with [Mn(IV)(2)(mu-O)(2)(mu-MeCO2)(H2O)(2)(bipy)(2)](3+) (bipy=2,2'-bipyridine) and its hydrolytic derivatives

The complex [Mn-2(IV)(mu-O-2)(2)(mu-MeCO2)(H2O)(2)(bipy)(2)](3+) 1 (bipy = 2,2'-bipyridine) has been found to co-exist in rapid equilibrium with [Mn2O2(H2O)(4)(bipy)(2)](4+) 2 and [Mn2O2(MeCO2)(H2O)(4)(bipy)](3+) 3 in aqueous buffer containing and excess of bipy and MeCO2- in the range pH 4.0-5.5. Equilibrium constants (mol dm(-3)) for 1 reversible arrow 2 + MeCO2- and 1 reversible arrow 3 + bipy are 10(-2) and 10(-3), respectively. Water molecules co-ordinated to the manganese(IV) centres are stable at least up to pH 5.5. Reduction of the binuclear manganese(IV) species thiosulfate ion proceeded via [(bipy)(2)(MnO2MnIV)-O-III(bipy)(2)](3+) 4, in the presence of an excess of bipy. An excess of thiosulfate reduced complex 4 further and ultimately produced Mn-II. Rate constants for reduction of 1-3 to 4, as determined under second-order conditions, increase with increasing number of water molecules at the manganese centre. A plausible mechanism is proposed and the relevance of the results to photosystem II discussed.