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Ammonia synthesis by photocatalytic hydrogenation of a N2-derived molybdenum nitride

Abstract

Although metal complexes are known to split dinitrogen at ambient temperature and pressure, the synthesis of ammonia from these compounds with H2 as the terminal reductant is rarely achieved. Here we report a photocatalytic ammonia synthesis from a N2-derived terminal molybdenum nitride by using H2 as the terminal reductant. An iridium hydride photocatalyst mediates the reaction on irradiation with blue light. A molybdenum pentahydride was identified as the principal metal product to arise after ammonia release. Conversion of the molybdenum pentahydride back to the terminal molybdenum nitride was accomplished in three steps and completed a synthetic cycle for NH3 formation from N2 and H2. Mechanistic investigations support a pathway that involves photoexcitation of the iridium hydride and a subsequent energy transfer rather than electron transfer. Deuterium labelling confirmed H2 as the source of the N–H bonds. This photodriven, proton-coupled electron transfer allows the use of H2 as the terminal reductant for the catalytic formation of NH3 from N2 using metal catalysts.

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Fig. 1: Ammonia synthesis from well-defined metal complexes.
Fig. 2: Catalytic hydrogenation of the N2-derived molybdenum nitride.
Fig. 3: Identification of molybdenum products.
Fig. 4: Identification of photoactive species during catalysis.
Fig. 5: Evaluation of electron transfer pathways.
Fig. 6: Ultraviolet irradiation to regenerate the molybdenum dinitrogen complex.
Fig. 7: Synthetic cycle for ammonia synthesis from N2 and H2 mediated by well-defined molybdenum and iridium complexes.

Data availability

The data that support the findings of this study are included with the article and Supplementary Information. Crystallographic data for the structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre, under deposition numbers CCDC 2068569 ([(depe)2Mo(O)(F)][BF4]) and CCDC 2068568 ([(depe)2Mo(H)2(MeCN)2][BF4]2). Copies of the data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures.

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Acknowledgements

This research was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Catalysis Science Program, under award DE-SC0006498 (P.J.C., S.K., Y.P. and J.K.). S.K. thanks Samsung Scholarship for partial financial support.

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Authors

Contributions

S.K. and P.J.C. designed the experiments and prepared the manuscript. S.K. conducted the majority of the experiments. Y.P. contributed to the preparation of iridium hydrides, the detection of ND3 and density functional theory (DFT) computational studies. J.K. contributed reproducibility experiments during revision. T.P.P. collected and solved the crystal structures using data from X-ray diffraction.

Corresponding author

Correspondence to Paul J. Chirik.

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Nature Synthesis thanks Hideki Masuda and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Peter Seavill, in collaboration with the Nature Synthesis team.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–48, Tables 1 and 2, discussions and experimental details.

Supplementary Data 1

Crystallographic data of [(depe)2Mo(H)2(MeCN)2][BF4]2, CCDC 2068568.

Supplementary Data 2

Crystallographic data of [(depe)2Mo(O)(F)][BF4], CCDC 2068569.

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Kim, S., Park, Y., Kim, J. et al. Ammonia synthesis by photocatalytic hydrogenation of a N2-derived molybdenum nitride. Nat. Synth 1, 297–303 (2022). https://doi.org/10.1038/s44160-022-00044-1

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