[Fe_2(SR)_2(μ-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

摘要

Low-temperature oxidation of Fe_2(S_2C_nH_(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_2C_nH_(2n))(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 [Fe_2(S_2C_nH_(2n))-(μ-CO)(CNMe)_6](PF_6)_2 (n = 2,3) established crystallographically and spectroscopically, are quite similar, with Fe---Fe distances of ca. 2.5 A, although the μ-CO is unsymmetrical in the propanedithiolate derivative. Isomeric forms of [Fe_2(S_2C_3H_6)(CO)(CNMe)_6](PF_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_2C_2H_4)(μ-CO)(CO)(CNMe)_5](PF_6)_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_2C_2H_4)(CNMe)_3(CO)_3 under 1 atm CO gives the deep pink tricarbonyl [Fe_2(S_2C_2H_4)(CO)_3(CNMe)_4](PF_6)_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.