Substitution and Oxidation-Reduction Reactions of a Water-Soluble Porphyrin. Tetrakis(4-N-methylpyridyl)porphinecobalt(III)-Pyridine System

The substitution reactions of tetrakis(4-N-methylpyridyl)porphinecobalt(III)(CoTMpyP) with pyridine have been studied as a function of pH at 25° and an ionic strength of 0.5 M. The mechanistic pathway proposed for the thiocyanate substitution reactions is applicable to pyridine as well. A pyridine axial substituent has a smaller influence than does coordinated thiocyanate on the rate of further ligand substitution and on the basicity of the metalloporphyrin. The reductions by dithionite of the diaquo, aquohydroxo, and the bis(pyridine) complexes of CoTMpyP have been investigated. The more active form of the reducing agent is SO in all cases and the mechanism for the reduction reactions is outer sphere. The rates of reduction correlate with the ΔG° for the reactions. © 1975, American Chemical Society. All rights reserved. 8a −