Hydrogen Activation by Biomimetic [NiFe]-Hydrogenase Model Containing Protected Cyanide Cofactors

Described are experiments demonstrating incorporation of cyanide cofactors and hydride substrate into [NiFe]-hydrogenase (H2ase) active site models. Complexes of the type (CO)2(CN)2Fe­(pdt)­Ni­(dxpe) (dxpe = dppe, 1; dxpe = dcpe, 2) bind the Lewis acid B­(C6F5)3 (BArF3) to give the adducts (CO)2(CNBArF3)2­Fe­(pdt)­Ni­(dxpe), (1(BArF3)2, 2(BArF3)2). Upon decarbonylation using amine oxides, these adducts react with H2 to give hydrido derivatives [(CO)­(CNBArF3)2­Fe­(H)­(pdt)­Ni­(dxpe)]− (dxpe = dppe, [H3(BArF3)2]−; dxpe = dcpe, [H4(BArF3)2]−). Crystallographic analysis shows that Et4N­[H3(BArF3)2] generally resembles the active site of the enzyme in the reduced, hydride-containing states (Ni–C/R). The Fe–H···Ni center is unsymmetrical with rFe–H = 1.51(3) Å and rNi–H = 1.71(3) Å. Both crystallographic and 19F NMR analyses show that the CNBArF3– ligands occupy basal and apical sites. Unlike cationic Ni–Fe hydrides, [H3(BArF3)2]− and [H4(BArF3)2]− oxidize at mild potentials, near the Fc+/0 couple. Electrochemical measurements indicate that in the presence of base, [H3(BArF3)2]− catalyzes the oxidation of H2. NMR evidence indicates dihydrogen bonding between these anionic hydrides and R3NH+ salts, which is relevant to the mechanism of hydrogenogenesis. In the case of Et4N­[H3(BArF3)2], strong acids such as HCl induce H2 release to give the chloride Et4N­[(CO)­(CNBArF3)2­Fe­(Cl)­(pdt)­Ni­(dppe)].