Kinetic and mechanistic studies on the oxidation of hydroxylamine by a tri-bridged manganese(IV, IV) dimer in weakly acidic media

The complex ion, [Mn-2(IV)(mu-O)(2)(mu-MeCO2)(bipy)(2)(H2O)(2)](3+) (1(3+)) (bipy = 2,2'-bipyridine) and its aqua derivatives [Mn-2(IV)(mu-O)(2)(bipy)(2)(H2O)(4)](4+) (2(4+)) and [Mn-2(IV)(mu-O)(2)(mu-MeCO2)(bipy)(H2O)(4)](3+) (3(3+)) coexists in rapid equilibria in aqueous buffer in the presence of excess of bipy and MeCO2- in the range pH 4.00-5.30. The solutions are reasonably stable up to pH 5.50 and react quantitatively with hydroxylamine to produce manganese(II) and N2O. The reactions follow simple first-order kinetics in the presence of excess hydroxylamine. There is UV-vis spectral evidence for the intermediate Mn-III,Mn-IV complex, [(bipy)(2)Mn-III(mu-O)(2)Mn-IV(bipy)(2)](3+), in the presence of excess bipy and deficit hydroxylamine which supports that 1(3+) and its hydrolytic derivatives are reduced by one-electron steps. Increased extent of aquation at the manganese(IV) centre leads to increased kinetic activity in the order: 1(3+) < 2(4+) < 3(3+). The rate of reduction increased with an increase in the concentration of hydroxylamine but decreased with increase in c(bipy) and c(OAc). The mild oxidising character of the complex ion along with major structural changes associated with one-electron oxidation of hydroxylamine disfavours an outer-sphere pathway. The overall first-order rate constants decrease linearly with increased mol% of D2O suggesting proton-coupled electron transfer pathways.