[Fe2(SR)2(mu-CO)(CNMe)6]2+ and analogues: a new class of diiron dithiolates as structural models for the H(ox)Air state of the fe-only hydrogenase.

Low-temperature oxidation of Fe(2)(S(2)C(n)H(2n)(CNMe)(6-x)(CO)x (n = 2, 3; x = 2, 3) affords a family of mixed carbonyl-isocyanides of the type Fe(2)(S(2)C(n)H(2n)(CO)x(CNMe)(7-x). 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 Fe(2)(S(2)C(n)H(2n))(mu-CO)(CNMe)(6)(2) (n = 2,3) established crystallographically and spectroscopically, are quite similar, with Fe---Fe distances of ca. 2.5 A, although the mu-CO is unsymmetrical in the propanedithiolate derivative. Isomeric forms of Fe(2)(S(2)C(3)H(6))(CO)(CNMe)(6)(2) were characterized where the CO is bridging or terminal, the greatest structural difference being the 0.1 A elongation of the Fe---Fe distance when MeNC (vs CO) is bridging. In the dicarbonyl species, Fe(2)(S(2)C(2)H(4))(mu-CO)(CO)(CNMe)(5)(2), the terminal CO ligand is situated at one of the basal sites, not trans to the Fe---Fe vector. Oxidation of Fe(2)(S(2)C(2)H(4))(CNMe)(3)(CO)(3) under 1 atm CO gives the deep pink tricarbonyl Fe(2)(S(2)C(2)H(4))(CO)(3)(CNMe)(4)(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.