Reactions of cobalt(II) protoporphyrin IX dimethyl ester, [COIIP], and [CoIIIP(Cl)] in co-ordinating aliphatic alcohols

Abstract

Chlorocobalt(III) protoporphyrin IX dimethyl ester, [Co^IIIP(Cl)], releases its chloride immediately on dissolution in methanol, and equilibrates as shown below. Species (2) predominates if no acid [graphic omitted] or alkali is added. [Co^IIP] dissolved in methanol, in the presence of air, undergoes oxidation yielding (2). The identities of the products obtained in both ways were established by comparison of absorption spectra and rates of replacements of their axial ligands. Kinetics of the reactions of (2) and various ligands L in methanol (L = pyridine, substituted pyridine, or nicotinamide) show that methanol is replaced first, and methoxide second, both in a dissociative manner. The replacements of methanol by L attain a limiting rate which does not depend on L [k_obs.(25 °C)= 57.8 s^–1]. The existence of a limiting rate here reveals that a D mechanism [S_Nl(lim.)] operates, whereas an I_d mechanism is excluded. This case appears to be a unique example of a fully established D mechanism in a polar, co-ordinating solvent. Values of k_obs. for the slower replacement of methoxide by L show a linear dependence on [L], with small differences in slopes for different entering ligands. This excludes a bimolecular mechanism, and suggests that the replacements of methoxide by L take place via the complex conjugate acid in a dissociative manner; the kinetic results do not allow a distinction between D and I_d mechanisms.