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Molybdenum-catalysed ammonia production with samarium diiodide and alcohols or water

Abstract

The production of ammonia from nitrogen gas is one of the most important industrial processes, owing to the use of ammonia as a raw material for nitrogen fertilizers. Currently, the main method of ammonia production is the Haber–Bosch process, which operates under very high temperatures and pressures and is therefore very energy-intensive1. The transition-metal-catalysed reduction of nitrogen gas2,3,4,5,6 is an alternative method for the formation of 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. Here we show that the combination of samarium(ii) diiodide (SmI2) with alcohols or water enables the fixation of nitrogen to be catalysed 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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Fig. 1: Catalytic reduction of nitrogen gas to ammonia in the presence of molybdenum complexes.
Fig. 2: Stoichiometric reactions of ammonia formation in reaction system B.
Fig. 3: Samarium species in the catalytic reaction.
Fig. 4: Typical turnover frequencies and chemical overpotentials.

Data availability

Crystallographic data for the reported structures have been deposited at the Cambridge Crystallographic Data Centre, under deposition numbers: CCDC 185998 (5), 1857999 (6), 1858000 (7a) and 1858001 (7b·2C4H8O). All other data supporting the findings of this study are available within the paper or from the corresponding author upon reasonable request.

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Acknowledgements

This project is supported by CREST, JST (JPMJCR1541). We thank Grants-in-Aid for Scientific Research (numbers JP17H01201, JP15H05798 and JP18K19093) from JSPS and MEXT. Y.A. is a recipient of the JSPS Predoctoral Fellowships for Young Scientists. We also thank J. C. Peters and S. Schneider for helpful discussions.

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Nature thanks Robert Flowers and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Authors and Affiliations

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Contributions

Y.N. directed and conceived this project. Y.A., K.A. and K.N. conducted the experimental work including X-ray analysis. All authors discussed the results and wrote the manuscript.

Corresponding author

Correspondence to Yoshiaki Nishibayashi.

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Competing interests

Y.A., K.N. and Y.N. have filed a patent based on the work described here (Japanese patent application number 2018-036967).

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Extended data figures and tables

Extended Data Fig. 1 Reactions via direct nitrogen cleavage pathway.

a, A reaction pathway via direct cleavage of the nitrogen–nitrogen triple bond. b, Synthesis of the molybdenum oxo complex (5) from 1a and water, and reduction of 5 to 4 via direct nitrogen cleavage of the nitrogen–nitrogen triple bond. SmI2(THF)2 was used as the source of SmI2.aYield based on NMR.

Extended Data Fig. 2 1H NMR spectra of catalytic reduction of dinitrogen to ammonia under 15N2.

ac, 1H NMR (DMSO-d6) spectra of product from catalytic reaction with 1c under 15N2 and [ColH]OTf (a), authentic sample of the mixture of 14NH4Cl and [ColH]OTf (b) and authentic sample of the mixture of 15NH4Cl and [ColH]OTf (c).

Extended Data Fig. 3 Kinetic study of stoichiometric reactions.

A kinetic study of the stoichiometric reaction was carried out by monitoring the formation of 4 by UV–vis spectroscopy at room temperature. A typical procedure is described below. A THF solution containing 1a (1.2 μmol) was added into a quartz glass cell (1 cm × 1 cm × 4 cm) with a septum in a nitrogen-filled glove box. The THF solution of SmI2 (3.0 μmol) was added to the quartz glass cell using a syringe with stirring. The total amount of solution was adjusted to be 3.0 ml. The spectra were measured every 0.4 s, and the rate was determined from the time profile of the initial 100 s. The reaction rate (kobs, in abs s−1) was determined from the formation rate of the absorbance of 4 at 828 nm. a, Typical time profile of the formation of 4 observed at 828 nm with SmI2 (1.0 mM) and 1a (0.4 mM) in THF at room temperature. b, Rate of formation of 4 at various concentrations of 1a. c, Rate dependence of the formation of 4 on concentration of 1a in THF. d, UV–vis absorption spectra between 380 and 1,100 nm of 1a (0.48 mM in THF; blue line), 4 (0.60 mM in THF; red line), and SmI2 (1.2 mM in THF; green line).

Extended Data Table 1 Catalytic reactions with various molybdenum complexes as catalysts
Extended Data Table 2 Catalytic reactions in various solvents using typical experimental procedure
Extended Data Table 3 Catalytic reactions using water as a proton source
Extended Data Table 4 Catalytic reactions using larger amounts of reducing reagent and proton source using 1a or 9 as catalysts
Extended Data Table 5 Determination of the turnover frequencies of catalytic reactions
Extended Data Table 6 Catalytic reduction by using hydrazine as a substrate instead of dinitrogen gas

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Ashida, Y., Arashiba, K., Nakajima, K. et al. Molybdenum-catalysed ammonia production with samarium diiodide and alcohols or water. Nature 568, 536–540 (2019). https://doi.org/10.1038/s41586-019-1134-2

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