Nitrogen reduction and functionalization by a multimetallic uranium nitride complex

Abstract

Molecular nitrogen (N2) is cheap and widely available, but its unreactive nature is a challenge when attempting to functionalize it under mild conditions with other widely available substrates (such as carbon monoxide, CO) to produce value-added compounds. Biological N2 fixation can do this, but the industrial Haber–Bosch process for ammonia production operates under harsh conditions (450 degrees Celsius and 300 bar), even though both processes are thought to involve multimetallic catalytic sites1,2. And although molecular complexes capable of binding and even reducing N2 under mild conditions are known, with co-operativity between metal centres considered crucial for the N2 reduction step1,2,3,4,5,6,7,8,9,10,11,12,13,14, the multimetallic species involved are usually not well defined, and further transformation of N2-binding complexes to achieve N–H or N–C bond formation is rare2,6,8,10,15,16. Haber noted17, before an iron-based catalyst was adopted for the industrial Haber–Bosch process, that uranium and uranium nitride materials are very effective heterogeneous catalysts for ammonia production from N2. However, few examples of uranium complexes binding N2 are known18,19,20,21,22, and soluble uranium complexes capable of transforming N2 into ammonia or organonitrogen compounds have not yet been identified. Here we report the four-electron reduction of N2 under ambient conditions by a fully characterized complex with two Uiii ions and three K+ centres held together by a nitride group and a flexible metalloligand framework. The addition of H2 and/or protons, or CO to the resulting complex results in the complete cleavage of N2 with concomitant N2 functionalization through N–H or N–C bond-forming reactions. These observations establish that a molecular uranium complex can promote the stoichiometric transformation of N2 into NH3 or cyanate, and that a flexible, electron-rich, multimetallic, nitride-bridged core unit is a promising starting point for the design of molecular complexes capable of cleaving and functionalizing N2 under mild conditions.

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Figure 1: Molecular structures of the nitrido and hydrazido complexes.
Figure 2: N2 reduction and functionalization reactions effected by 1.
Figure 3: Molecular structures of 3 and 4.
Figure 4: Molecular structures of 5 and 6.

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Acknowledgements

We acknowledge support from the Swiss National Science Foundation (200021-157158 and 200021-162430) and from the Ecole Polytechnique Fédérale de Lausanne (EPFL). We thank E. Solari for carrying out the elemental analyses, and D. Kubicky (of the L. Emsley group) for assistance with EPR measurements, M. Prevot (of the K. Sivula group) for assistance with Raman and B. Rozmyslowicz (of the J. Luterbacher group) for assistance with IR and GC-MS measurements. We thank F. F. Tirani for her contribution to the X-ray single crystal structure analyses.

Author information

M.F. carried out the synthetic experiments and analysed the experimental data; L.C. carried out preliminary synthetic experiments, including the isolation of the dinitrogen complex. R.S. performed the X-ray single crystal structure analyses; I.Z. carried out and analysed the magnetic measurements; M.M. and M.F. wrote the manuscript. M.M. originated the central idea, coordinated the work and analysed the experimental data.

Correspondence to Marinella Mazzanti.

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Extended data figures and tables

Extended Data Figure 1 EPR and magnetic data.

a, X-band (9.40 GHz) EPR spectrum of 2 in toluene/hexane glass at 10 K (red line, experiment; black line, fit to two 5f1 ions). b, Plot of measured magnetic susceptibility χM for 2 versus temperature. The measurements were repeated twice.

Extended Data Figure 2 Protonation of complex 2.

1H NMR spectra (400 MHz, THF-d8, 298 K) for the successive addition of 1 equiv. PyHCl (up to 7 equiv.) to 2. Bottom trace, before addition of PyHCl. Symbols on peaks show assignments (see key at top); f1, chemical shift.

Extended Data Figure 3 Formation of ammonia from the addition of H+ to 2 and to 15N-2.

Shown are 1H NMR spectra (400 MHz, DMSO-d6, 298 K) of: a, the white precipitate formed after addition of excess PyHCl (20 equiv.) to 2; b, the white precipitate formed after addition of excess PyHCl (20 equiv.) to 15N-2; c, NH4Cl formed after addition of HCl(Et2O) to 2 (2 equiv. of dimethyl sulfone added for quantitative determination). In each panel, symbols on peaks show assignments (see key at top).

Extended Data Figure 4 Hydrogenation of 2 and formation of ammonia from the addition of H2/H+ to 2 and to 15N-2.

a, 1H NMR spectra (400 MHz, C7D8, 298 K) of the crude reaction mixture at different times after reaction of 2 with H2 (1 atm) at room temperature. Bottom trace, immediately after H2 addition. b, c, Quantitative 1H NMR spectra (400 MHz, DMSO-d6, 298 K) of: b, NH4Cl formed after addition of HCl(Et2O) to the residual solid of reaction between 2 and H2 (2 equiv. of dimethyl sulfone added); c, 15NH4Cl and NH4Cl formed in 2:1 ratio after addition of HCl(Et2O) to the residual solid from the reaction between complex 15N-2 and H2 (1 equiv. of dimethyl sulfone added). In each panel, symbols on peaks show assignments (see key at top).

Extended Data Figure 5 Summary of 1H NMR and 13C NMR data for 5.

a, 1H NMR spectrum (400 MHz, C6D6, 298 K) of the crude mixture 1 h after addition of 10 equiv. of CO to 2 to afford 5. b, 1H NMR spectrum (400 MHz, C6D6, 298 K) of crystals of 5. c, 13C NMR spectrum (100.6 MHz, D2O, 298 K) of the crude mixture after addition of 10 equiv. of CO to 2, showing the presence of 13C14N and O13C15N in a 1:2 ratio; inset shows the doublet nature of the peak assigned to O13C15N. In each panel, symbols on peaks show assignments (see key at top).

Extended Data Figure 6 Summary of 1H NMR and 13C NMR data for 6.

a, 1H NMR spectrum (400 MHz, C7D8, 298 K) of crystals of 6. b, 13C NMR spectrum (400 MHz, C7D8, 298 K) of crystals of 6. c, 13C NMR spectrum (400 MHz, D2O (DMSO as reference), 298 K) of the crude reaction mixture after addition of 10 equiv. of CO to complex 3. d, Mass spectrum of the gas evolved during the reaction of 15N-3 with CO, showing formation of 14N15N and 15N2 with m/z = 29 and 30, respectively. The peak at m/z = 28 arises from 14N2 and CO. 14N15N and 14N2 arise from nitride/N2 scrambling during the synthesis of 3. The inset details the measured abundance (intensity) for each peak. In ac, symbols on peaks show assignments (see key at top).

Extended Data Figure 7 Summary of 1H NMR data for 1, 2 and 3.

a, 1H NMR spectra (400 MHz, THF-d8, 298 K) of [Cs{[U(OSi(OtBu)3)3]2(μ-N)}] before (bottom) and after (top) addition of excess KC8 to yield to 1. b, 1H NMR spectra (400 MHz, C7D8, 298 K) of the crude mixture before (bottom) and after (top) addition of N2 to complex 1. c, 1H NMR spectrum (400 MHz, THF-d8, 298 K) of crystals of 2. d, 1H NMR spectrum (400 MHz, C7D8, 298 K) of crystals of complex 3. In each panel, symbols on peaks show assignments (see key at top).

Extended Data Figure 8 Additional reactivity of 3 and 6.

a, 1H NMR spectra (400 MHz, C7D8, 298 K) of the crude reaction mixture after reaction of 3 with H2 (1 atm) at room temperature for 1 week leading to a new product (top; bottom, immediately after introduction of H2). b, 1H NMR spectrum (400 MHz, C7D8, 298 K) of the crude reaction mixture after addition of 100 equiv. of CO to 3. The chemical shift of the only product (labelled with a blue dot) is 1.08 p.p.m., very close to the chemical shift of complex 5. c, 1H NMR spectrum (400 MHz, DMSO-d6, 298 K) of the white precipitate formed after addition of excess PyHCl (20 equiv.) to 15N-6. In each panel, symbols on peaks show assignments (see key at top).

Extended Data Table 1 Crystallographic data of 1, 2 and 3.toluene
Extended Data Table 2 Crystallographic data of 4, 5.THF and 6

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Falcone, M., Chatelain, L., Scopelliti, R. et al. Nitrogen reduction and functionalization by a multimetallic uranium nitride complex. Nature 547, 332–335 (2017). https://doi.org/10.1038/nature23279

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