Effects of ortho-Methyl Substituents on the Rate of Dissociation of Imidazole Ligands in Tetraarylporphyrinatocobalt(III) Complexes

Article abstract of DOI:10.1246/bcsj.68.197

Dissociation rates of the imidazoles in ⁺ and ⁺, where L is an unhindered imidazole such as 1-methylimidazole or a hindered imidazole such as 2-alkylimidazole, have been determined in chloroform-d solution by the 1H NMR saturation transfer method.The rate constants in the tmp complexes were smaller than those of the corresponding tpp complexes in every imidazole ligand examined.The largest difference between these two systems was observed in bis(1,2-dimethylimidazole) complexes and the ratio of the rate constants, k(tpp)/k(tmp), reached as much as 2500 at 90 deg C.While the rate constants greatly increased in the tpp system on going from the unhindered imidazole to the hindered one, the increase was rather small in the tmp system.These results were interpreted in tems of the deformation of the porphyrinatocobalt core: Since the two hidered ligands in the tmp complexes are fixed perpendicularly in the cavities created by the deformed porphyrin ring, the distance between the 2-alkyl and the deformed porphyrinatocobalt core would increase, which results in weakening the steric repulsion and slowing down the dissociation rate.The attractive interaction between the ortho-methyls and the imidazole ? system would also contribute to the stability of the tmp complexes.