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NH3 formation from N2 and H2 mediated by molecular tri-iron complexes

Nature Chemistry volume 12, pages 740–746 (2020)Cite this article

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Abstract

Living systems carry out the reduction of N2 to ammonia (NH3) through a series of protonation and electron transfer steps under ambient conditions using the enzyme nitrogenase. In the chemical industry, the Haber–Bosch process hydrogenates N2 but requires high temperatures and pressures. Both processes rely on iron-based catalysts, but molecular iron complexes that promote the formation of NH3 on addition of H2 to N2 have remained difficult to devise. Here, we isolate the tri(iron)bis(nitrido) complex [(Cp′Fe)3(μ3-N)2] (in which Cp′ = η5-1,2,4-(Me3C)3C5H2), which is prepared by reduction of [Cp′Fe(μ-I)]2 under an N2 atmosphere and comprises three iron centres bridged by two μ3-nitrido ligands. In solution, this complex reacts with H2 at ambient temperature (22 °C) and low pressure (1 or 4 bar) to form NH3. In the solid state, it is converted into the tri(iron)bis(imido) species, [(Cp′Fe)3(μ3-NH)2], by addition of H2 (10 bar) through an unusual solid–gas, single-crystal-to-single-crystal transformation. In solution, [(Cp′Fe)3(μ3-NH)2] further reacts with H2 or H+ to form NH3.

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Fig. 1: Synthesis and reactivity of compound 1 towards H2 in solution and in the solid state, producing compounds 2, 3 and NH3.
Fig. 2: Molecular structures of compound 1 and compound 2.
Fig. 3: Conversion of complex 1 into complex 2 upon different H2 exposure times followed by Mössbauer spectroscopy.
Fig. 4: Complete conversion of 15N-labelled 1 into 2 as monitored by 15N Hahn-echo MAS NMR spectra.
Fig. 5: Computed enthalpy profile at room temperature for the reaction of complex 1 with H2 to yield compound 2.

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Data availability

The authors declare that all of the data supporting the findings of this study are available within the paper and the Supplementary Information, and also from the corresponding authors upon reasonable request. This includes experimental details, NMR studies in solution, solid-state NMR studies, crystallographic details, X-band EPR studies, solid-state magnetic susceptibility studies, zero-field 57Fe Mössbauer studies, and computational details. Crystallographic data for the structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre under deposition numbers 1939746 (1) and 1939747 (2). Copies of the data can be obtained free of charge at https://www.ccdc.cam.ac.uk/structures.

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Acknowledgements

We thank P. Schweyen for EPR data collection on complex 1, and M. Bröring and F. Jochen Litterst for providing access to the SQUID magnetometer and 57Fe Mössbauer spectrometer. We acknowledge the help of S. Schneider and J. Abbenseth in the studies of solution 15N and 2H NMR. This work was supported by the Emmy Noether (WA 2513/2) and Heisenberg (WA 2513/6-8) programs of the Deutsche Forschungsgemeinschaft (DFG) (for M.D.W.); a fellowship for experienced researchers of the Humboldt Foundation and the Chinese Academy of Science (L.M.); a computational grant from CalMip; and CNRS (S.B.). This contribution is dedicated to the memory of Richard A. Andersen.

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Authors and Affiliations

  1. Technische Universität Braunschweig, Institut für Anorganische und Analytische Chemie, Braunschweig, Germany

    Matthias Reiners, Dirk Baabe, Katharina Münster, Marc-Kevin Zaretzke, Matthias Freytag, Peter G. Jones & Marc D. Walter

  2. CNRS, LCC (Laboratoire de Chimie de Coordination), Université de Toulouse, UPS, INPT, Toulouse, France

    Yannick Coppel & Sébastien Bontemps

  3. Université de Toulouse, INSA-UPS-LPCNO and CNRS-LPCNO, Toulouse, France

    Iker del Rosal & Laurent Maron

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  1. Matthias Reiners
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Contributions

M.R., D.B., Y.C., S.B. and M.D.W. conceived the experiments; M.R., D.B., K.M., M.-K.Z., M.F., P.G.J., Y.C., S.B. and M.D.W. designed and performed experiments and analysed the data; I.d.R. and L.M. performed computational studies; M.R., D.B., S.B., L.M. and M.D.W. wrote the manuscript.

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Correspondence to Marc D. Walter.

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Supplementary Information (download PDF )

Supplementary Figs. 1–26, Supplementary Tables 1–5 and supplementary references 1–14.

Supplementary Data 1

Crystallographic data (CIF) for complex 1

Supplementary Data 2

Crystallographic data (CIF) for complex 1

Supplementary Data 3 (download ZIP )

Cartesian coordinates for optimized computed structures as .xyz files

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Reiners, M., Baabe, D., Münster, K. et al. NH3 formation from N2 and H2 mediated by molecular tri-iron complexes. Nat. Chem. 12, 740–746 (2020). https://doi.org/10.1038/s41557-020-0483-7

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