Kinetics and mechanism of hydrazine oxidation by a tri-bridged manganese(IV,IV) dimer in weakly acidic media

Abstract

The complex ion [ Mn 2 IV (μ-O) 2(μ-MeCO 2)(bipy) 2(H 2O) 2] +3 { 1{ 3+ (bipy = 2,2′-bipyridine) and its aqua derivatives, [Mn 2(μ-O) 2(bipy) 2(H 2O) 4] +4 { 2{ 4+ and [Mn 2(μ-O) 2(μ-MeCO 2)(bipy)(H 2O) 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 with hydrazine to produce manganese(II) and N 2. The reactions follow simple first-order kinetics in the presence of excess hydrazine. The rate determining steps are one-electron reductions of { 2{ 4+ and { 3{ 3+ to (bipy) 2]{ 2{ 3+ and { 3{ 2+ respectively. The le-reduced products rapidly produce [(bipy) 2Mn III(μ-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 increased with increase in the concentration of hydrazine and ethanoate, but it decreased with increase in [bipy]. Increase in rate at higher pH indicates inner-sphere coordination and deprotonation of N 2H 5 +. The mild oxidising character of { 2{ 4+ and { 3{ 3+ along with major structural cha associated with one-electron oxidation of hydrazine disfavours an outer-sphere pathway.