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

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 (K_f) 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 K_f 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.