Kinetics and mechanism of axial ligand substitution in iron(III) tetraphenylporphyrin complexes

Abstract

Axial imidazole ligands in [Fe(tpp)(R-im)₂]⁺[tpp = 5,10,15,20-tetraphenylporphyrin dianion, R-im = imidazole or N-methylimidazole) are rapidly replaced by PBu₃ to form [Fe(tpp)(R-im)(PBu₃)]⁺ and [Fe(tpp)(PBu₃)₂]⁺. Substitution of the first R-im is rate-limiting and follows a strictly dissociative pathway. The five-co-ordinate intermediate [Fe(tpp)(R-im)]⁺ reacts at similar rates with R-im and PBu₃. The kinetic trans effect of PBu₃ is much greater than that of R-im in these complexes, which may be of relevance to certain haemoprotein systems.