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

Low-temperature oxidation of Fe-2(S2CnH2n)(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)(7-2)](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 [Fe-2(S2CnH2n)-(mu-CO)(CNMe)(6)](PF6)(2) (n = 2,3) established crystallographically and spectroscopically, are quite similar, with Fe---Fe distances of ca.2.5 Angstrom, although the mu-CO is unsymmetrical in the propanedithiolate derivative. Isomeric forms of [Fe-2(S2C3H6)(CO)(CNMe)(6)](PF6)(2) were characterized where the CO is bridging or terminal, the greatest structural difference being the 0.1 Angstrom elongation of the Fe---Fe distance when MeNC (vs CO) is bridging. In the dicarbonyl species, [Fe-2(S2C2H4)(mu-CO)(CO)(CNMe)(5)](PF6)(2), the terminal CO ligand is situated at one of the basal sites, not trans to the Fe---Fe vector. Oxidation of Fe-2(S2C2H4)(CNMe)(3)(CO)(3) under 1 atm CO gives the deep pink tricarbonyl [Fe-2(S2C2H4)(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.