Reversible Photoinduced Reductive Elimination of H2 from the Nitrogenase Dihydride State, the E4(4H) Janus Intermediate

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• 作者：Dmitriy Lukoyanov ; Nimesh Khadka ; Zhi-Yong Yang ; Dennis R. Dean ; Lance C. Seefeldt ; Brian M. Hoffman
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：February 3, 2016
• 年：2016
• 卷：138
• 期：4
• 页码：1320-1327
• 全文大小：467K
• ISSN：1520-5126

文摘

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.