Deciphering the Proton Reduction Mechanism in a Biomimetic FeFe Hydrogenase: A DFT Study Revealing Metal-Dependent Intermediate Structures and Ligand-Driven Catalysis

Inspired by [MFe] (M = Fe/Ni) hydrogenases, both FeFe and NiFe complexes follow an analogous E(ECEC) mechanistic pathway for proton reduction, yet they exhibit distinct electronic and structural characteristics governed by the metal identity. The FeFe species adopts a triplet state {FeFe'}_CO⁺ with a CO ligand displaced toward the Fe' site, while its NiFe counterpart remains in a singlet ground state. Upon one- and two-electron reduction, the FeFe system undergoes Fe-S bond cleavage, stabilizing a terminal CO bound to the {Fe'Cp} fragment, in contrast to the NiFe system, which retains bridging thiolates and a bridging CO. Further reduction and protonation lead to a semibridging hydride in FeFe, whereas a terminal hydride is favored in NiFe. The comparable hydrogen evolution activity of both catalysts, despite their divergent intermediate structures, is attributable to the redox-active bipyridine unit within the supporting ligand. DFT mechanistic analysis reveals that the metal ion identity dictates key geometric features of the intermediates, while the noninnocent ligand mitigates inverted coordination preferences, thereby providing design principles for next-generation biomimetic catalysts.