Diferrous cyanides as models for the Fe-only hydrogenases

The first systematic study of diferrous dicyano dithiolates is described. Oxidation of [Fe-2(S2C2H4)(CN)(2)(CO)(4)](2-) in the presence of cyanide and tertiary phosphines and of Fe-2(S2C2H4)-(CO)(4)(PMe3)(2) in the presence of cyanide affords a series of diferrous cyanide derivatives that bear a stoichiometric, structural, and electronic relationship to the H-ox(air) state of the Fe-only hydrogenases. With PPh3 as the trapping ligand, we obtained an unsymmetrical isomer of Fe-2(S2C2H4)(mu-CO)(CN)(2)(PPh3)(2)-(CO)(2), as confirmed crystallographically. This diferrous cyanide features the semibridging CO-ligand, with Fe-mu C bond lengths of 2.15 and 1.85 angstrom. Four isomers of Fe-2(S2C2H4)(mu-CO)(CN)(2)(PMe3)(2)(CO)(2) were observed, the initial product again being unsymmetrical but more stable isomers being symmetrical. DFT calculations confirm that the most stable isomers of Fe-2(S2C2H4)(mu-CO)(CN)(2)(PMe3)(2)(CO)(2) have cyanide trans to mu-CO. Oxidative decarbonylation also afforded the new tetracyanide [Fe-2(S2C2H4)(mu-CO)(CN)(4)(CO)(2)](2-). Insights into the oxidative decarbonylation mechanism of these syntheses come from the spectroscopic characterization of the tetracarbonyl [Fe-2(S2C2H4)(mu-CO)(CN)(3)(CO)(3)](-). This species reacts with PEt3 to produce the stable adduct [Fe-2(S2C2H4)(mu-CO)(CN)(3)(CO)(2)(PEt3)](-).