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 H-2 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 [(NiMnBr)-Mn-II-Br-I] ((dppe)Ni-II(m-pdt)(m-Br)MnI(CO)(3)) heterobinuclear mimic to track the reaction processes. Our results reveal that [(NiMnBr)-Mn-II-Br-I] is reduced to form [(NiMnH)-Mn-II-H-I] ((dppe) Ni-II(m-pdt)(m-H)Mn-I(CO)(3)) in the presence of a proton (H+). IR-SEC, NMR, and X-ray crystal-structure analyses identify that the key [(NiMnH)-Mn-II-H-I] intermediate interacts with a strong acid for H2 evolution via Mn ([(NiMnII)-Mn-II/IH]), whereas in the presence of a weak acid, further 1e(-) reduction at the Ni site affords [(NiMnH)-Mn-I-H-I](-)-R species to react with H+. The full 2e( -)/2H(+) H-2 evolution picture of the heterobinuclear mimic relying on different metal centers presents a unique mechanistic understanding of hydrogenase mimics.