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

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 sigma-donor CN- ligand instead of CO from the iron(II) center of the thermally stable complexes [FeII(CO)2(CN)3Br]2-(5) containing less electron-donating bromide reflects the electron-rich character of the mononuclear [FeII(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 [FeII(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 pi-acceptor CO group. Scrutiny of the coordination chemistry of iron(II)-thiolate cyanocarbonyl species [FeII(CO)x(CN)y(SR)z]n- reveals that certain combinations of thiolate, cyanide and carbonyl ligands (3 < or = y+z > or = 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.