Nickel−Iron Dithiolato Hydrides Relevant to the [NiFe]-Hydrogenase Active Site

The ferrous dithiolato carbonyl Fe(S2C3H6)(CO)2(dppe) (1) condenses with NiCl2(dppe) to give [FeNi(pdt)(μ-Cl)(CO)(dppe)2]BF4 ([2Cl]+). The corresponding reaction of the ditosylate Ni(OTs)2(dppe) gave the dication [(CO)2(dppe)Fe(pdt)Ni(dppe)](OTs)2 ([2(CO)](OTs)2) (pdt = 1,3-propanedithiolate; dppe = 1,2-C2H4(PPh2)2; OTs− = CH3C6H4-4-SO3 −). Reduction of the bimetallic dicarbonyl with borohydride salts afforded impure, thermally stable hydride, [(CO)(dppe)Fe(pdt)(μ-H)Ni(dppe)]+ ([2H]+). A reliable route to NiFe(SR)2H species entailed protonation of (CO)3Fe(pdt)Ni(dppe) to give [(CO)3Fe(pdt)(μ-H)Ni(dppe)]+ ([3H]+). The iron−nickel dithiolato hydride, [3H]BF4, was characterized crystallographically: as anticipated by biophysical studies, the hydride ligand is bridging, the Fe center is octahedral, and the Ni center is pentacoordinate. Solutions of [3H]BF4 undergo substitution by dppe to give [2H]+. The hydride undergoes rapid deprotonation and is an electrocatalyst for hydrogen evolution from trifluoroacetic acid. Oxidation of 3 gives a mixed valence species [3]+, a potential model for the Ni−L state.