Kinetics and mechanism of redox reactions of oxo-bridged complexes of higher-valent manganese

The dioxo-bridged (MnMnIV)-Mn-III complexes [(MnMnIV)-Mn-III(mu-O)(2)L-4](3 +)(1(3+), L = bipy 2,2'-bipyridine; 2(3+), L = phen = 1,10-phenanthroline) can be stabilized in aqueous solutions containing excess L in the range pH 4-6, Thus stabilized in solution, the parent complex coexists only with its aqua derivative [(MnMnIV)-Mn-III (mu-O)(2)L-3(H2O)(2)](3+) (1a(3+) for L = bipy; 2a(3+) for L = phen) in rapid equilibrium. The solutions are novel o xidants and quantitatively oxidize NO2- to NO3-, S2O32- to S4O62-, H2O2 to O-2 and N2H5+ to N-2 Simple first-order kinetics is observed except in reac tions of the bipy complex with NO2-, H2O2 and in the reaction of the phen c omplex with N2H5+ Reaction of the bipy complex with NO2- produces the one-e lectron reduced intermediate 1a(2+) while H2O2 gives the two-electron reduc ed intermediate [Mn-III(bipy)(2)(OH)(H2O)](2+). It is concluded that (a) th e aqua complexes are kinetically more active than their parents, (b) the ph en complexes react slower than the bipy-complexes, (c) Mn2+ has no catalyti c role, but intermediate manganese complexes and radical species may act as autocatalysts, (d) outer-sphere one-electron transfer occurs generally, bu t H2O2 prefers a two-electron pathway in its reactions with the dimers, (e) nuclearity is retained until one of the manganese in the dimers is reduced to the +2 state.