Reversible Photoinduced Reductive Elimination of H2 from the Nitrogenase Dihydride State, the E(4)(4H) Janus Intermediate.

We recently demonstrated that N2 reduction by nitrogenase involves the obligatory release of one H2 per N2 reduced. These studies focus on the E4(4H) "Janus intermediate", which has accumulated four reducing equivalents as two [Fe-H-Fe] bridging hydrides. E4(4H) is poised to bind and reduce N2 through reductive elimination (re) of the two hydrides as H2, coupled to the binding/reduction of N2. To obtain atomic-level details of the re activation process, we carried out in situ 450 nm photolysis of E4(4H) in an EPR cavity at temperatures below 20 K. ENDOR and EPR measurements show that photolysis generates a new FeMo-co state, denoted E4(2H)*, through the photoinduced re of the two bridging hydrides of E4(4H) as H2. During cryoannealing at temperatures above 175 K, E4(2H)* reverts to E4(4H) through the oxidative addition (oa) of the H2. The photolysis quantum yield is temperature invariant at liquid helium temperatures and shows a rather large kinetic isotope effect, KIE = 10. These observations imply that photoinduced release of H2 involves a barrier to the combination of the two nascent H atoms, in contrast to a barrierless process for monometallic inorganic complexes, and further suggest that H2 formation involves nuclear tunneling through that barrier. The oa recombination of E4(2H)* with the liberated H2 offers compelling evidence for the Janus intermediate as the point at which H2 is necessarily lost during N2 reduction; this mechanistically coupled loss must be gated by N2 addition that drives the re/oa equilibrium toward reductive elimination of H2 with N2 binding/reduction.