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

摘要

The dioxo-bridged Mn~III Mn~IV complexes [Mn~III` Mn~IV(mu -O)2L4]3+(1~3+, L = bipy = 2,2'-bipyridine; 2~3+, L = phen = l,10-phelnanthroline) can be stabilizeld 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 [Mn~III MnIV (mu-O)2L3(H2O)2]3+ (Ia3+ for L=bipy, 2a~3+ for L=phen) in rapid equilibrium. The solutions are novel oxidants and quantitatively oxidize NO2 to NO3-, S2O~2-_3 to S4O~2-_6,, H2O2 to O2 and N2HS+ to N2. Simple first-order kinetics is observed except in reactions of the bipy complex with NO2, H2O2 and in the reaction of the phen complex with N2H_5~+. Reaction of the bipy complex with NO_2~- produces the one- electron reduced intermediate la~2+ while H2O2 gives the two-electron reduced intermediate [MnIII(bipy)2(OH)(H2O)]2+. It is concluded that (a) the aqua complexes are kinetically more active than their parents, (b) the phen complexes react slower than the bipy-complexes, (c) Mn2+ has no catalytic role, but intermediate manganese complexes and radical species may act as autocatalysts, (d) outer-sphere one-electron transfer occurs generally, but 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.