Synthesis, characterization, and binding affinity of hydrosulfide complexes of synthetic iron(II) porphyrinates

Abstract

The binding and reactivity of the hydrosulfide ion (HS−) to iron(II) porphyrinates has been examined for several synthetic meso-tetraphenylporphine (TPP) derivatives. In all cases, HS− coordinates to the iron centers in a 1:1 stoichiometry with formation constants (Kf) that reflect the electronic characteristics of the porphyrinate ligands. In the case of the F8TPP ligand (F8TPP=dianion of 5,10,15,20-tetrakis(2,6-difluorophenyl)porphine), an intermediate complex proposed as the hydrosulfide bridged dimer, (Bu4N)[Fe2(μ-SH)(F8TPP)2], was identified by NMR spectroscopy en route to formation of (Bu4N)[Fe(SH)(F8TPP)]. A robust procedure is reported for the synthesis and isolation of the parent hydrosulfide adduct, (Bu4N)[Fe(SH)(TPP)], which has permitted a detailed examination of its spectroscopy and chemical reactivity. Electrochemical measurements demonstrate that [Fe(SH)(TPP)]− is oxidized reversibly at a potential of −0.832V (vs ferrocene/ferrocenium) consistent with other iron porphyrinates containing sulfur-based ligands. Despite this fact, chemical oxidation of (Bu4N)[Fe(SH)(TPP)] with ferrocenium tetrafluoroborate produced only [Fe(TPP)] indicating that the putative iron(III) hydrosulfide adduct, [Fe(SH)(TPP)], decomposes rapidly. Treatment of (Bu4N)[Fe(SH)(TPP)] with other biologically relevant molecules such as NO and 1,2-dimethylimidazole resulted in simple displacement of the HS− ligand as governed by the relative Kf values of the added ligands. The solid-state structure of one hydrosulfide adduct, (Bu4N)[Fe(SH)(F8TPP)], was determined by X-ray crystallography and found to display the expected five-coordinate geometry about iron with an Fe-S distance of 2.323(1) Å. The relevance of the hydrosulfide chemistry with synthetic iron porphyrinates is discussed in terms of the possible reactivity for H2S and its derivatives at heme sites in biology.