Metalloporphyrins. The nature of ligand bonding and the mechanism of replacements

Abstract

The imidazole ring is an essential component of many biological systems (hemoglobin and myoglobin, nucleic acids, vitamin B^, cytochromes, metalloenzymes, etc.). The nature of the bond between imidazole-nitrogen and the metal is therefore of biological interest. Imidazole is an electron-donating ligand in both a and n sense, much more electron donating than the majority of nitrogen heterocycles. As ligands, most imidazoles are good a donors and moderate n donors. They can also function as n acceptors if the imidazole ring has one or more electron-withdrawing substituents. Our recent study of bonding modes of pyridine and imidazole type ligands in the transition state of a Conl(protoporphyrin IX) model complex enabled us to conclude that stabilization of the reaction transition state is more sensitive to the change in the strength of n bonding than in that of a bonding. This observation is important for some metallo-enzymatic reactions. Mechanisms of replacements in porphyrin and in corrin rings are discussed