Diferrous Cyanides as Models for the Fe-only Hydrogenases

The first systematic study of diferrous dicyano dithiolates is described. Oxidation of
[Fe2(S2C2H4)(CN)2(CO)4]2- in the presence of cyanide and tertiary phosphines and of Fe2(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 Hoxair state of the Fe-only hydrogenases. With
PPh3 as the trapping ligand, we obtained an unsymmetrical isomer of Fe2(S2C2H4)(μ-CO)(CN)2(PPh3)2(CO)2, as confirmed crystallographically. This diferrous cyanide features the semibridging CO-ligand, with
Fe-μC bond lengths of 2.15 and 1.85 Å. Four isomers of Fe2(S2C2H4)(μ-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 Fe2(S2C2H4)(μ-CO)(CN)2(PMe3)2(CO)2 have cyanide
trans to μ-CO. Oxidative decarbonylation also afforded the new tetracyanide [Fe2(S2C2H4)(μ-CO)(CN)4(CO)2]2-. Insights into the oxidative decarbonylation mechanism of these syntheses come from the
spectroscopic characterization of the tetracarbonyl [Fe2(S2C2H4)(μ-CO)(CN)3(CO)3]-. This species reacts
with PEt3 to produce the stable adduct [Fe2(S2C2H4)(μ-CO)(CN)3(CO)2(PEt3)]-.