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 N<sub>2sub> reduction by nitrogenase involves the obligatory release of one H<sub>2sub> per N<sub>2sub> reduced. These studies focus on the E<sub>4sub>(4H) “Janus intermediate”, which has accumulated four reducing equivalents as two [Fe-H-Fe] bridging hydrides. E<sub>4sub>(4H) is poised to bind and reduce N<sub>2sub> through reductive elimination (re) of the two hydrides as H<sub>2sub>, coupled to the binding/reduction of N<sub>2sub>. To obtain atomic-level details of the re activation process, we carried out in situ 450 nm photolysis of E<sub>4sub>(4H) in an EPR cavity at temperatures below 20 K. ENDOR and EPR measurements show that photolysis generates a new FeMo-co state, denoted E<sub>4sub>(2H)*, through the photoinduced re of the two bridging hydrides of E<sub>4sub>(4H) as H<sub>2sub>. During cryoannealing at temperatures above 175 K, E<sub>4sub>(2H)* reverts to E<sub>4sub>(4H) through the oxidative addition (oa) of the H<sub>2sub>. 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 H<sub>2sub> 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 H<sub>2sub> formation involves nuclear tunneling through that barrier. The oa recombination of E<sub>4sub>(2H)* with the liberated H<sub>2sub> offers compelling evidence for the Janus intermediate as the point at which H<sub>2sub> is necessarily lost during N<sub>2sub> reduction; this mechanistically coupled loss must be gated by N<sub>2sub> addition that drives the re/oa equilibrium toward reductive elimination of H<sub>2sub> with N<sub>2sub> binding/reduction.