CLUSTER-EXPANSION OF A HIGH-VALENT OXOMANGANESE DIMER IN AQUEOUS-SOLUTION

Upon dissolution in water, high-valent oxomanganese dimer, [Mn2III,IV (O)2(bpy)4]3+ (1, bpy = 2,2'-dipyridyl), partially disproportionates i n aqueous solution to give Mn2+ and a higher valent species in equilib rium with 1. This equilibrium is buffer dependent and 1 is regenerated in quantitative yield in bpy buffer (pH = 4.5). 1 has previously been found to be in a pH-dependent equilibrium with the trinuclear species , [Mn3IV(O)4(bpy)4(H2O)2]4+ (2), in aqueous solution (Sarneski J. E.; Thorp H. H.; Brudvig G. W.; Crabtree R. H.; Schulte G. K., J. Am. Chem . Soc., 1990, 112, 7255). We now find that acidification of 1 yields a n intermediate on a msec time scale, which in turn generates the final products, 2 and Mn2+, on a considerably slower time scale. The kineti cs of these changes have been investigated by stopped-flow UV-Vis spec trophotometry. The rate of formation of the intermediate (k(obs) = 9.9 +/- 1.5 s-1 at 298 K, pH = 2.5, and 2 mM concentration of 1) is indep endent of added Mn2+ but shows a marked dependence on changes in the c oncentrations of 1, bpy or H+. The kinetics suggest that the rate-dete rmining step is loss of bpy. This is supported by the observation that there is no effect of pH from pH 1.4-3.1 on the observed kinetics of an analogous complex, [Mn2III,IV(O)2(phen)4]3+, (3, phen = 1,10-phenan throline). We propose that the opening of the bpy chelate initiates th e conversion of 1 to 2 and, on the basis of these data, the intermedia te is suggested to be [Mn2III,IV(O)2(bpy)3(H2O)2]3+. The slow conversi on of the intermediate to 2 shows a second-order dependence on the con centration of 1 and is independent of added Mn2+. The second order rat e constant for this reaction is 0.015 +/- 0.002 M-1 s-1 at 298 K, pH = 2.5. Also, there is an increase in this rate upon addition of an oxid ant, (NH4)2Ce(IV)(NO3)6. Thus, we conclude that the rate-determining r eaction of the slower step involves a redox reaction of two intermedia te dimeric species.