KINETICS AND MECHANISM OF THE REDUCTION OF MANGANESE PORPHYRINS - REACTION AT THE PERIPHERY OF THE PORPHYRIN RING

The reduction of [tetrakis(N-methyl-4-pyridinium)porphinato]manganese(III) and [tetrakis(4-carboxyphenyl)-porphinato]manganese(III), Mn(III)TMPyP and Mn(III)TCPP, by cobalt(II) sepulchrate and sodium dithionite has been studied. The reduction of MnTMPyP by both reducing agents is strongly dependent on the ionic strength of the medium, and the reduction with Co(sep)2+ is also catalyzed by chloride ion. The kinetic data are consistent with attack of reducing agent at the periphery of the porphyrin ring. For MnTCPP, the reduction by Co(sep)2+ is biphasic, indicating two parallel reaction pathways-attack of Co(sep)2+ at the periphery of the ring and at the axial waters. The data for the reaction of MnTCPP with dithionite are consistent with rate-determining formation of a radical anion followed by rapid intramolecular reduction of manganese(III) to manganese(II).