Six-Coordinate and Five-Coordinate FeII(CN)2(CO) x Thiolate Complexes (x = 1, 2):  Synthetic Advances for Iron Sites of [NiFe] Hydrogenases

The dicyanodicarbonyliron(II) thiolate complexes trans,cis-[(CN)2(CO)2Fe(S,S−C−R)]- (R = OEt (2), N(Et)2 (3)) were prepared by the reaction of [Na][S−C(S)−R] and [Fe(CN)2(CO)3(Br)]- (1). Complex 1 was obtained from oxidative addition of cyanogen bromide to [Fe(CN)(CO)4]-. In a similar fashion, reaction of complex 1 with [Na][S,O−C5H4N], and [Na][S,N−C5H4] produced the six-coordinate trans,cis-[(CN)2(CO)2Fe(S,O−C5H4N)]- (6) and trans,cis-[(CN)2(CO)2Fe(S,N−C5H4)]- (7) individually. Photolysis of tetrahydrofuran (THF) solution of complexes 2, 3, and 7 under CO led to formation of the coordinatively unsaturated iron(II) dicyanocarbonyl thiolate compounds [(CN)2(CO)Fe(S,S−C−R)]- (R = OEt (4), N(Et)2 (5)) and [(CN)2(CO)Fe(S,N−C5H4)]- (8), respectively. The IR νCN stretching frequencies and patterns of complexes 4, 5, and 8 have unambiguously identified two CN- ligands occupying cis positions. In addition, density functional theory calculations suggest that the architecture of five-coordinate complexes 4, 5, and 8 with a vacant site trans to the CO ligand and two CN- ligands occupying cis positions serves as a conformational preference. Complexes 2, 3, and 7 were reobtained when the THF solution of complexes 4, 5, and 8 were exposed to CO atmosphere at 25 °C individually. Obviously, CO ligand can be reversibly bound to the FeII site in these model compounds. Isotopic shift experiments demonstrated the lability of carbonyl ligands of complexes 2, 3, 4, 5, 7, and 8. Complexes [(CN)2(CO)Fe(S,S−C−R)]- and NiA/NiC states [NiFe] hydrogenases from D. gigas exhibit a similar one-band pattern in the νCO region and two-band pattern in the νCN region individually, but in different positions, which may be accounted for by the distinct electronic effects betwen [S,S−C−R]- and cysteine ligands. Also, the facile formations of five-coordinate complexes 4, 5, and 8 imply that the strong σ-donor, weak π-acceptor CN- ligands play a key role in creating/stabilizing five-coordinate iron(II) [(CN)2(CO)Fe(S,S−C−R)]- complexes with a vacant coordination site trans to the CO ligand.