Mechanism of Reduction of Ferric Porphyrins by Sulfide: Identification of a Low Spin Fe-SH Intermediate.

The reaction of Fe porphyrin complexes bearing distal hydrogen bonding residues with sulfide/hydrosulfide is kinetically monitored to reveal the presence of an intermediate and a k/k of 3.0. This intermediate is trapped at low temperatures and investigated with resonance Raman and electron paramagnetic resonance spectroscopy. The results, corroborated by density functional theory calculations, indicate that this species is a six-coordinate low spin hydrosulfide bound ferric porphyrin. The homolytic cleavage of the Fe-SH bond resulting in the formation of a ferrous porphyrin and hydrosulfide radical (trapped with 5,5-dimethyl-1-pyrrilone-N-oxide) is found to be the overall rate-determining step of the reaction.