A mechanistic study of the reaction of iron (III) porphyries with imidazoles. Hydrogen bonding by the propionic acid side chains in hemin chloride

A kinetic study is reported for the reaction of [Fe(por)Cl][por = dianion of protoporphyrin IX (pp)(3,7,12,17-tetramethyl-8,13-divinylporphyrin-2,18-dipropionic acid) or the corresponding dimethyl ester (ppdme)] with imidazole (H-im) or 1-methylimidazole (1 Me-im) to give [Fe(por)(R-im)2]Cl (R = H or 1-Me). The reaction in acetone goes through a transient green intermediate, [Fe(por)(R-im)Cl], which was trapped at –78 °C and shown by visible spectroscopic, e.s.r., and conductivity measurements to be high-spin and six-co-ordinate. The reaction is accelerated by hydrogen-bond donors, including H-im, which assist the rate-determining chloride ionization from the intermediate. [Fe(pp)Cl] reacts more than ten times faster than [Fe(ppdme)Cl], and this is shown to be due to enhanced stability of the intermediate and a faster chloride ionization rate with [Fe(pp)Cl]. An explanation is offered that invokes a hydrogen-bonding interaction between an axial ligand and the propionic acid side chains, which were shown not to be deprotonated by imidazole in acetone.