Steric effects in the reactions of zinc(II) and iron(III)‘capped’ porphyrins

Abstract

Equilibrium constants were measured for the reactions of a capped zinc porphyrin with 1-methylimidazole (mim) and of metal imidazolate complexes of Cu^II and Ni^II and for zinc 5,10,15,20-tetra (o-methoxyphenyl)porphyrinole with mim in toluene at 20, 25, 30, and 35 °C. Enthalpy and entropy values obtained from a least-squares fit of a plot of In K vs. 1/T indicate that the effect of the capping benzene on the thermodynamics of the axial ligation reaction is entropic rather than enthalpic. Monoadducts of iron(III) porphyrins can be prepared using the sterically hindered ‘capped’ porphyrin. The axial adduct with the copper(II) imidazolate complex has the same binuclear core, Fe^IIICu^II, as the resting form of the mammalian enzyme cytochrome c oxidase.