Photoinduced Reductive Elimination of H 2 from the Nitrogenase Dihydride (Janus) State Involves a FeMo-cofactor-H 2 Intermediate.

N ₂ reduction by nitrogenase involves the accumulation of four reducing equivalents at the active site FeMo-cofactor to form a state with two [Fe-H-Fe] bridging hydrides (denoted E ₄ (4H), the Janus intermediate), and we recently demonstrated that the enzyme is activated to cleave the N≡N triple bond by the reductive elimination (re) of H ₂ from this state. We are exploring a photochemical approach to obtaining atomic-level details of the re activation process. We have shown that, when E ₄ (4H) at cryogenic temperatures is subjected to 450 nm irradiation in an EPR cavity, it cleanly undergoes photoinduced re of H ₂ to give a reactive doubly reduced intermediate, denoted E ₄ (2H)*, which corresponds to the intermediate that would form if thermal dissociative re loss of H ₂ preceded N ₂ binding. Experiments reported here establish that photoinduced re primarily occurs in two steps. Photolysis of E ₄ (4H) generates an intermediate state that undergoes subsequent photoinduced conversion to [E ₄ (2H)* + H ₂ ]. The experiments, supported by DFT calculations, indicate that the trapped intermediate is an H ₂ complex on the ground adiabatic potential energy suface that connects E ₄ (4H) with [E ₄ (2H)* + H ₂ ]. We suggest that this complex, denoted E ₄ (H ₂ ; 2H), is a thermally populated intermediate in the catalytically central re of H ₂ by E ₄ (4H) and that N ₂ reacts with this complex to complete the activated conversion of [E ₄ (4H) + N ₂ ] into [E ₄ (2N2H) + H ₂ ].