SOLUTION EQUILIBRIA AND REDOX REACTIVITIES OF A DIOXO-BRIDGED MANGANESE COMPLEX

The dinuclear complex [(bipy)2Mn(III)O2Mn(IV)(bipy)2]3+ (bipy = 2,2'-bipyridyl) 1 was stabilised in bipy-Hbipy+ buffer where it coexists with the diaqua derivative [(H2O)2Mn(III)O2Mn(IV)(bipy)]3+ 2 (equilibrium constant K(H)). Reduction of 1 by NO2- has been found to be kinetically insignificant, but that of 2 is rapid, quantitative and follows the sequence Mn(III)Mn(IV) k1 --> (Mn(III))2 k2 --> Mn(II)Mn(III) fast --> 2Mn(II). In the presence of an excess of NO2-, first-order kinetics was observed at 830 nm, but a biphasic profile at 525 nm. At 30.0-degrees-C and I = 1.0 mol dm-3, k1K1H and K2K2H are 34.9 +/- 0.8 and 3.3 +/- 0.2 dm3 mol-1 s-1 respectively. Added bipy retards, while Mn2+ accelerates, the reaction. The solution equilibria are much more complex for 1 in HNO3 and H3PO4, but the known solution chemistry of 1 in such different media may be correlated by a unified reaction scheme.