Mechanistic insights into consecutive 2e− and 2H+ reactions of hydrogenase mimic

The [NiFe] heterobinuclear hydrogenase active site in nature is responsible for a typical 2e− and 2H+ transformation with extraordinarily efficient H2 evolution. However, the difficulty in isolating the [NiFe] catalytic center and the sensitivity to air pose great challenges to unravelling the detailed 2e−/2H+ process. Herein, we design and synthesize a [NiIIMnIBr] ((dppe)NiII(μ-pdt)(μ-Br)MnI(CO)3) heterobinuclear mimic to track the reaction processes. Our results reveal that [NiIIMnIBr] is reduced to form [NiIIMnIH] ((dppe)NiII(μ-pdt)(μ-H)MnI(CO)3) in the presence of a proton (H+). IR-SEC, NMR, and X-ray crystal-structure analyses identify that the key [NiIIMnIH] intermediate interacts with a strong acid for H2 evolution via Mn ([NiIIMnII/IH]), whereas in the presence of a weak acid, further 1e− reduction at the Ni site affords [NiIMnIH]−-R species to react with H+. The full 2e−/2H+ H2 evolution picture of the heterobinuclear mimic relying on different metal centers presents a unique mechanistic understanding of hydrogenase mimics.