Photoinduced Reductive Elimination of H2from the Nitrogenase Dihydride (Janus) State Involves a FeMo-cofactor-H2Intermediate

N2reduction 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 E4(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 H2from this state. We are exploring a photochemical approach to obtaining atomic-level details of the reactivation process. We have shown that, when E4(4H) at cryogenic temperatures is subjected to 450 nm irradiation in an EPR cavity, it cleanly undergoes photoinduced reof H2to give a reactive doubly reduced intermediate, denoted E4(2H)*, which corresponds to the intermediate that would form if thermal dissociative reloss of H2preceded N2binding. Experiments reported here establish that photoinduced reprimarily occurs in two steps. Photolysis of E4(4H) generates an intermediate state that undergoes subsequent photoinduced conversion to [E4(2H)* + H2]. The experiments, supported by DFT calculations, indicate that the trapped intermediate is an H2complex on the ground adiabatic potential energy suface that connects E4(4H) with [E4(2H)* + H2]. We suggest that this complex, denoted E4(H2; 2H), is a thermally populated intermediate in the catalytically central reof H2by E4(4H) and that N2reacts with this complex to complete the activated conversion of [E4(4H) + N2] into [E4(2N2H) + H2].