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

摘要

A number of thermally stable iron(II)-thiolate cyanocarbonyl complexes, cis,cis-[Fe(CN)_2(CO)_2(CS_3-S,S)]~(2-), mer-[Fe(CO)_2(CN)_3(NCCH_3)]~- (2) mer-[Fe(CO)_3(CN)(CS_3-S,S)]~- (3), cis-[Fe(CO)_2(CN)(S(CH_2)_2S(CH_2)_2S-S,S,S)]~- (4), [Fe(CO)_2(CN)_3Br]~(2-) (5), mer-[Fe(CO)_2(CN)_3(m-SC6H_4Br)]~(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 -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(CS_3-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(CH_2)_2S(CH_2)_2S]~(2-). The preference of the sixth ligand coordinated to the unsaturated [FeII(CO)(CN)_2(CS_3-S,S)]~(2-)> Fe(II) center, the iron-site architecture of the bimetallic Ni-Fe active-site of [NiFe] hydrogenases, is a strong -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 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.