Time-Resolved EPR Study of H2 Reductive Elimination from the Photoexcited Nitrogenase Janus E4(4H) Intermediate

Nitrogenase is activated for N-2 reduction through the accumulation of four reducing equivalents at the active-site FeMo-cofactor (FeMo-co: Fe7S9MoC; homo-citrate) to form the key Janus intermediate, denoted E-4(4H), whose lowest-energy structure contains two [Fe-H-Fe] bridging hydrides and two protons bound to the sulfurs that also bridge the Fe pairs. In the critical step of catalysis, a H-2 complex transiently produced by reductive elimination (re) of the hydrides of E-4(4H), denoted E-4(H-2;2H), undergoes H-2 displacement by N-2, which then undergoes the otherwise energetically unfavorable cleavage of the N N triple bond. In pursuing the study of the re activation process, we have employed a photochemical approach to obtaining its atomic-level details. Continuous 450 nm irradiation of the ground state of the dihydride Janus intermediate, denoted E-4(4H)(a), in an EPR cavity at cryogenic temperatures causes photoinduced re of H-2 to generate E-4(H-2;2H). We here extend this photochemical approach with time -resolved EPR studies of the photolysis process on the ns time scale. These studies reveal an additional intermediate in the catalytic reductive elimination process, an isomer of the E-4(4H) FeMo-co metal-ion core that is formed prior to E-4(H-2;2H) and is thought to be created by breaking an Fe-SH bond, thus further integrating the calculational and structural studies into the experimentally determined mechanism by which nitrogenase is activated to cleave the N N triple bond.