Disproportionation of hydrogen peroxide in the presence of Mn(III) complexes with various porphyrins and acid anions

The kinetics of homogeneous decomposition of H2O2 in the presence of Mn(III) complexes with octaethylporphine or meso-phenyloctaethylporphines and acid anions Cl−, AcO−, and SCN− was studied by volumetry. The ionic-molecular mechanism of the transformation, involving reversible coordination of the H2O2 molecule, its irreversible decomposition with the release of H2O and removal of two electrons from the metal porphyrin, reversible coordination of the second peroxide molecule in the form of HO2−, and slow irreversible reduction of the catalyst with the release of O2 and H2O was substantiated by electronic absorption spectra. The catalytic activity of Mn(III) porphyrin complexes is independent of the acid anion present as extra ligand, but depends on the structure of the porphyrin ligand, which can be used for controlling the catalytic activity. Unsymmetrical (chloro)(monophenyloctaethylporphinato)managanese(III) is the most active; it increases the rate of O2 evolution by a factor of 2 at the peroxide: catalyst molar ratio of (3 × 105): 1.