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 (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.

Gallium(III) Tetraphenylporphyrinates Containing Hydrosulfide and Thiolate Ligands: Structural Models for Sulfur-Bound Iron(III) Hemes.

Gallium(III) tetraphenylporphyrinates (TPP) containing anionic sulfur ligands have been prepared and characterized in the solid state and solution. The complexes serve as structural models for iron(III) heme sites containing sulfur coordination that otherwise prove challenging to synthesize due to the propensity for reduction to iron(II). The compounds prepared include the first well-characterized example of a trivalent metalloporphyrinate containing a terminal hydrosulfide ligand, [Ga(SH)(TPP)], as well as [Ga(SEt)(TPP)], [Ga(SPh)(TPP)], and [Ga(SSi(i)Pr3)(TPP)]. The stability of these compounds toward reduction has permitted an investigation of their solid-state structures and electrochemistry. The structural features and reaction chemistry of the complexes in relation to their iron(III) analogs is discussed.

NBu4SH provides a convenient source of HS(-) soluble in organic solution for H2S and anion-binding research.

Hydrogen sulfide (H2S) has gained significant interest within the scientific community due to its expanding roles in different (patho)physiological processes. Despite this importance, the chemical mechanisms by which H2S exerts its action remain under-scrutinized. Biomimetic investigations in organic solution offer the potential to clarify these mechanisms and to delineate the differential reactivity between H2S and HS(-). However, such studies are hampered by the lack of readily-available sources of HS(-) that are soluble in organic solution. Here we present a simple method for preparing analytically pure tetrabutylammonium hydrosulfide (NBu4SH), which we anticipate will be of significant utility to researchers in the H2S and anion-binding communities.

Synthesis, characterization, and atropisomerism of iron complexes containing the tetrakis(2-chloro-6-fluorophenyl)porphyrinate ligand.

Iron complexes of the meso-(tetraaryl)porphyrin, 5,10,15,20-tetrakis(2-chloro-6-fluorophenyl)porphine (H2ClFTPP) are reported. This unique ligand affords the opportunity to study atropisomerism in a porphyrin system containing similar substituents in the 2 and 6 positions of the meso-aryl rings. The atropisomerism displayed by the iron porphyrinates is observed to be a function of both the oxidation state of the metal and the nature of the axial ligand. In the case of iron(iii) porphyrinates, a single atropisomer is favored, whereas with the iron(ii) porphyrinate a statistical distribution of all possible atropisomers is observed. Variable temperature studies with the iron(ii) porphyrinate demonstrate that the distribution of atropisomers is maintained even at elevated temperatures. The results are discussed in the context of atropisomerism with other meso-(tetraaryl)porphyrins.

Studies of iron(III) porphyrinates containing silanethiolate ligands.

The chemistry of several iron(III) porphyrinates containing silanethiolate ligands is described. The complexes are prepared by protonolysis reactions of silanethiols with the iron(III) precursors, [Fe(OMe)(TPP)] and [Fe(OH)(H2O)(TMP)] (TPP = dianion of meso-tetraphenylporphine; TMP = dianion of meso-tetramesitylporphine). Each of the compounds has been fully characterized in solution and the solid state. The stability of the silanethiolate complexes versus other iron(III) porphyrinate complexes containing sulfur-based ligands allows for an examination of their reactivity with several biologically relevant small molecules including H2S, NO, and 1-methylimidazole. Electrochemically, the silanethiolate complexes display a quasi-reversible one-electron oxidation event at potentials higher than that observed for an analogous arenethiolate complex. The behavior of these complexes versus other sulfur-ligated iron(III) porphyrinates is discussed.