[Fe₂(SR)₂(μ-CO)(CNMe)₆]²⁺ and Analogues: A New Class of Diiron Dithiolates as Structural Models for the H_o_x^Air State of the Fe-Only Hydrogenase

Low-temperature oxidation of Fe2(S2CnH2n)(CNMe)6-x(CO)x (n = 2, 3; x = 2, 3) affords a family
of mixed carbonyl-isocyanides of the type [Fe2(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(S2CnH2n)(μ-CO)(CNMe)6](PF6)2 (n = 2,3) established crystallographically and spectroscopically, are quite similar,
with Fe---Fe 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 Fe---Fe distance when MeNC (vs CO)
is bridging. In the dicarbonyl species, [Fe2(S2C2H4)(μ-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 Fe2(S2C2H4)(CNMe)3(CO)3
under 1 atm CO gives the deep pink tricarbonyl [Fe2(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.