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Nitrogen fixation under ambient conditions

By Tetsuo Satoh |

The transition-metal-catalyzed reduction of nitrogen is an alternative to the traditional energy-intensive Haber-Bosch process for producing ammonia. In these reaction systems, metallocenes or potassium graphite are typically used as the reducing reagent, and conjugate acids of pyridines or related compounds are used as a proton source. To develop a next-generation nitrogen-fixation system, these reagents should be low cost, readily available and environmentally friendly. Back in 2010, professor Yoshiaki Nishibayshi and colleagues at the University of Tokyo (www.t.u-tokyo.ac.jp/soe/index.html) developed a molybdenum-nitrogen complex catalyst having a PNP (phosphorus-nitrogen-phosphorus)-type pincer ligand that produces 23 molecules of NH3 per catalyst molecule. But the catalytic activity was rather low due to the decomposition of the catalyst system during the reaction.

Now, the research teams of Nishibayshi and Kazunari Yoshizawa at Kyushu University have designed a new PNP-type pincer ligand, which combines samarium (II) diiodide (SmI2) with alcohols or water. This new catalyst system enables the fixation of nitrogen by molybdenum complexes under ambient conditions. Up to 4,350 equivalents of ammonia can be produced (based on the molybdenum catalyst), with a turnover frequency of around 117 per minute. The amount of ammonia produced and its rate of formation are one and two orders of magnitude larger, respectively, than those achieved in artificial reaction systems reported so far, and the formation rate approaches that observed with nitrogenase enzymes. The high reactivity is achieved by a proton-coupled electron-transfer process that is enabled by weakening of the O–H bonds of alcohols and water coordinated to SmI2. Although the current reaction is not suitable for use on an industrial scale, this work demonstrates an opportunity for further research into catalytic nitrogen fixation.

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