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Catalytic conversion of nitrogen to ammonia by an iron model complex

Abstract

The reduction of nitrogen (N2) to ammonia (NH3) is a requisite transformation for life1. Although it is widely appreciated that the iron-rich cofactors of nitrogenase enzymes facilitate this transformation2,3,4,5, how they do so remains poorly understood. A central element of debate has been the exact site or sites of N2 coordination and reduction6,7. In synthetic inorganic chemistry, an early emphasis was placed on molybdenum8 because it was thought to be an essential element of nitrogenases3 and because it had been established that well-defined molybdenum model complexes could mediate the stoichiometric conversion of N2 to NH3 (ref. 9). This chemical transformation can be performed in a catalytic fashion by two well-defined molecular systems that feature molybdenum centres10,11. However, it is now thought that iron is the only transition metal essential to all nitrogenases3, and recent biochemical and spectroscopic data have implicated iron instead of molybdenum as the site of N2 binding in the FeMo-cofactor12. Here we describe a tris(phosphine)borane-supported iron complex that catalyses the reduction of N2 to NH3 under mild conditions, and in which more than 40 per cent of the proton and reducing equivalents are delivered to N2. Our results indicate that a single iron site may be capable of stabilizing the various NxHy intermediates generated during catalytic NH3 formation. Geometric tunability at iron imparted by a flexible iron–boron interaction in our model system seems to be important for efficient catalysis13,14,15. We propose that the interstitial carbon atom recently assigned in the nitrogenase cofactor may have a similar role16,17, perhaps by enabling a single iron site to mediate the enzymatic catalysis through a flexible iron–carbon interaction18.

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Figure 1: Chemical line representations of the FeMo-cofactor of nitrogenase.
Figure 2: Stoichiometric (TPB)Fe–N2 model reactions.
Figure 3: Spectral data for ammonia analysis, and catalyst poisoning.

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Acknowledgements

This work was supported by the NIH (GM 070757) and the Gordon and Betty Moore Foundation. A. Takaoka is thanked for developing the calibration curves used for ammonia and hydrazine quantification. D. Rees and D. Newman are acknowledged for many discussions.

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J.S.A., J.R. and J.C.P. designed the study. J.S.A. and J.R. conducted the experiments. J.S.A., J.R. and J.C.P. interpreted the data. J.S.A., J.R. and J.C.P. wrote the manuscript.

Corresponding author

Correspondence to Jonas C. Peters.

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The authors declare no competing financial interests.

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Supplementary Information

This file contains Supplementary Text, Supplementary Tables 1-21, Supplementary Figures 1-7 and additional references. (PDF 615 kb)

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Anderson, J., Rittle, J. & Peters, J. Catalytic conversion of nitrogen to ammonia by an iron model complex. Nature 501, 84–87 (2013). https://doi.org/10.1038/nature12435

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