KINETICS OF THE SUBSTITUTION REACTIONS OF A Cr(III)-PORPHYRIN

Abstract

Substitution reactions of low-spin ‘d⁶’ Co(III) cobalamins (derivatives of vitamin B12) are usually rapid.¹ The vitamin B12 model compounds² such as dimethylglyoximates of Co(III) are kinetically inert as are most other Co(III) complexes.^3,4 In order to understand the cause for this, Fleischer et al.,⁵ carried out some experiments on Co(III)-porphyrins. The substitution reactions of Co(III)-hematoporphyrins were found to be rapid although not as fast as similar reactions in the aquocobalamin system. In furtherance of that study, we have carried out some experiments on meso-tetra-(p-sulfonatophenyl)porphinato chromium(III).⁶ The substitution reactions of this Cr(III)-porphyrin also were rapid unlike the normal inert Cr(III) complexes. The detailed results are presented in this paper with discussion concerning the basis for the porphyrin molecule labilizing the substitution reactions in the usually inert Co(III) and Cr(III) ions.