[Fe2(SR)2(μ-CO)(CNMe)6]2+ and analogues: A new class of diiron dithiolates as structural models for the Hoxair state of the Fe-only hydrogenase

Low-temperature oxidation of Fe2(S2C nH2n)(CNMe)6-x(CO)x (n = 2, 3; x = 2, 3) affords a family of mixed carbonyl-isocyanides of the type [Fe 2(S2CnH2n)(CO)x(CNMe) 7-x]2+. The degree of substitution is controlled by the RNC/Fe ratio, as well as the degree of initial substitution at iron, with tricarbonyl derivatives favoring more highly carbonylated products. The structures of the monocarbonyl derivatives [Fe2(S2C nH2n)(μ-CO)(CNMe)6](PF6) 2 (n = 2,3) established crystallographically and spectroscopically, are quite similar, with FeFe distances of ca. 2.5 Å, although the μ-CO is unsymmetrical in the propanedithiolate derivative, Isomeric forms of [Fe2(S2C3H6)(CO)(CNMe) 6](PF6)2 were characterized where the CO is bridging or terminal, the greatest structural difference being the 0.1 Å elongation of the FeFe distance when MeNC (vs CO) is bridging. In the dicarbonyl species, [Fe2(S2C2H 4)(μ-CO)(CO)(CNMe)5](PF6)2, the terminal CO ligand is situated at one of the basal sites, not trans to the FeFe vector. Oxidation of Fe2(S2C 2H4)(CNMe)3(CO)3 under 1 atm CO gives the deep pink tricarbonyl [Fe2(S2C2H 4)(CO)3(CNMe)4](PF6)2. DFT calculations show that a bridging CO or MeNC establishes a 3-center, 2-electron bond within the two Fe(II) centers, which would otherwise be nonbonding.