Binuclear manganese compounds of potential biological significance. Part 2. Mechanistic study of hydrogen peroxide disproportionation by dimanganese complexes: the two oxygen atoms of the peroxide end up in a dioxo intermediate.

The dimanganese(II,II) complexes 1a [Mn2(L)(OAc)2(CH3OH)](ClO4) and 1b [Mn2(L)(OBz)2(H2O)](ClO4), where HL is the unsymmetrical phenol ligand 2-(bis-(2-pyridylmethyl)aminomethyl)-6-((2-pyridylmethyl)(benzyl)aminomethyl)-4-methylphenol, react with hydrogen peroxide in acetonitrile solution. The disproportionation reaction was monitored by electrospray ionization mass spectrometry (ESI-MS) and EPR and UV−visible spectroscopies. Extensive EPR studies have shown that a species (2) exhibiting a 16-line spectrum at g ∼ 2 persists during catalysis. ESI-MS experiments conducted similarly during catalysis associate 2a with a peak at 729 (791 for 2b) corresponding to the formula [MnIIIMnIV(L)(O)2(OAc)]+ ([MnIIIMnIV(L)(O)2(OBz)]+ for 2b). At the end of the reaction, it is partly replaced by a species (3) possessing a broad unfeatured signal at g ∼ 2. ESI-MS associates 3a with a peak at 713 (775 for 3b) corresponding to the formula [MnIIMnIII(L)(O)(OAc)]+ ([MnIIMnIII(L)(O)(OBz)]+ for 3b). In the presence of H218O, th...