Substitution Reactions of Tetracarboxyphenylporphinatocobaltate(III) with Thiocyanate and Pyridine as a Function of pH

The substitution reactions of tetracarboxyphenylporphinatocobaltate(III) (CoTCPP) with thiocyanate and with pyridine have been studied as a function of pH at 25 °C and an ionic strength of 0.5 M. While the mechanistic pathway proposed previously for the thiocyanate and pyridine substitution reactions of tetrakis(4-N-methylpyridyl)porphinecobalt(III) (CoTMpyP) is generally applicable to CoTCPP as well, some important differences are noted. Where comparisons between the two metalloporphyrins can be made, the change from a positively charged (CoTMpyP) to a negatively charged (CoTCPP) periphery of the macrocycle enhances the respective rate constants by one to three orders of magnitude while the equilibrium constants are altered by factors considerably less than an order of magnitude. These results are discussed with respect to the influence of the electron-withdrawing or -donating properties of the macrocyclic substituents on the electron density at the metal center. © 1976, American Chemical Society. All rights reserved