KINETICS AND MECHANISM OF HYDRAZINE OXIDATION 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(mu-O)(2) (bipy)(2)(H2O)(4)](+4) {2}(4+) and [Mn-2(mu-O)(2)(mu-MeCO2)(bipy)(H2O) (4)](+3) {3}(3+) coexists in rapid equilibria in aqueous buffers (MeCO 2-+bipy; pH, 4.5-5.5). The solutions are reasonably stable and react w ith hydrazine to produce manganese(II) and N-2. The reactions follow s imple first-order kinetics in the presence of excess hydrazine. The ra te determining steps are one-electron reductions of {2}(4+) and {3}(3) to {2}(3+) and {3}(2+) respectively. The 1e-reduced products rapidly produce [(bipy)(2)Mn-III(mu-O)(2)-Mn-IV(bipy)(2)](3+) in the presence of excess bipy and deficit hydrazine. Kinetic activity of {1}(3+) is insignificant compared to {2}(4+) and {3}(3+). Rate of reduction incre ased with increase in the concentration of hydrazine and ethanoate, bu t it decreased with increase in [bipy]. Increase in rate at higher pH indicates inner-sphere coordination and deprotonation of N2H5+. The mi ld oxidising character of {2}(4+) and {3}(3+) along with major structu ral changes associated with one-electron oxidation of hydrazine disfav ours an outer-sphere pathway. (C) 1998 Elsevier Science Ltd. All right s reserved.