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)].