Preparative and structural studies on iron(ii)–thiolate cyanocarbonyls: relevance to the [NiFe]/[Fe]-hydrogenases

Abstract

A number of thermally stable iron(II)–thiolate cyanocarbonyl complexes, cis,cis-[Fe(CN)₂(CO)₂(CS₃-S,S)]²⁻ (1), mer-[Fe(CO)₂(CN)₃(NCCH₃)]⁻ (2) mer-[Fe(CO)₃(CN)(CS₃-S,S)]⁻ (3), cis-[Fe(CO)₂(CN)(S(CH₂)₂S(CH₂)₂S-S,S,S)]⁻ (4), [Fe(CO)₂(CN)₃Br]²⁻ (5), mer-[Fe(CO)₂(CN)₃(m-SC₆H₄Br)]²⁻ (6) and mer-[Fe(CO)₂(CN)₃(SPh)]²⁻ (7) were isolated and characterized by IR and X-ray diffraction analysis. The extrusion of one strong σ-donor CN⁻ ligand instead of CO from the iron(II) center of the thermally stable complexes [Fe^II(CO)₂(CN)₃Br]²⁻ (5) containing less electron-donating bromide reflects the electron-rich character of the mononuclear [Fe^II(CN)₂(CO)₂(CS₃-S,S)]²⁻ (1) when ligated by by the bidentate thiolate, and the combination of one cyanide, two carbonyls and a tridentate thiolate provides the stable complex 4 as a result of the reaction of complex 5 and chelating ligand [S(CH₂)₂S(CH₂)₂S]²⁻. The preference of the sixth ligand coordinated to the unsaturated [Fe^II(CO)(CN)₂(CS₃-S,S)]²⁻ Fe(II) center, the iron-site architecture of the bimetallic Ni–Fe active-site of [NiFe] hydrogenases, is a strong π-acceptor CO group. Scrutiny of the coordination chemistry of iron(II)–thiolate cyanocarbonyl species [Fe^II(CO)_x(CN)_y(SR)_z]ⁿ⁻ reveals that certain combinations of thiolate, cyanide and carbonyl ligands (3 ≤ y + z ≤ 4) bound to Fe(II) are stable and this could point the way to understand the reasons for Nature's choice of combinations of these ligands in hydrogenases.