Ta +-mediated ammonia synthesis from N and H at 2 2 2 INAUGURAL ARTICLE ambient temperature Caiyun Genga, Jilai Lia,b,1, Thomas Weiskea, and Helmut Schwarza,1 aInstitut für Chemie, Technische Universität Berlin, 10623 Berlin, Germany; and bInstitute of Theoretical Chemistry, Jilin University, Changchun 130023, People’s Republic of China This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected in 2018. Contributed by Helmut Schwarz, September 20, 2018 (sent for review August 24, 2018; reviewed by R. Graham Cooks and Markus Reiher) + In a full catalytic cycle, bare Ta2 in the highly diluted gas phase is oriented external electric fields (53, 54) followed by insertion of, able to mediate the formation of ammonia in a Haber–Bosch-like for example, a nitrogen atom in the C–C bonds of alkanes has process starting from N2 and H2 at ambient temperature. This find- been reported as well (55, 56). Another promising approach is ing is the result of extensive quantum chemical calculations sup- based on the electrochemical cleavage of N2 to produce ammonia ported by experiments using Fourier transform ion cyclotron (11, 31, 32, 51, 57–60). There are indications that the cooperative + resonance MS. The planar Ta N , consisting of a four-membered 2 2 activation of N2 by several transition metal atoms holds promise ring of alternating Ta and N atoms, proved to be a key intermedi- as well (28, 34–39, 45, 61–64). Furthermore, the reactivity of ate. It is formed in a highly exothermic process either by the re- μ = + ditantalum complexes of the type ([NPN]Ta( -H))2N2 ([NPN] action of Ta2 with N2 from the educt side or with two molecules + PhP(CH2SiMe2NPh)2) with dinitrogen (65) has also been ex- of NH3 from the product side. In the thermal reaction of Ta2 with tensively investigated in the past (28, 34, 61–71). ≡ N2, the N N triple bond of dinitrogen is entirely broken. A detailed Mechanistically, the catalytic activity of the transition metals is analysis of the frontier orbitals involved in the rate-determining based on the interplay of vacant and filled d-orbitals during step shows that this unexpected reaction is accomplished by the multielectron rearrangements along the reaction coordinate. interplay of vacant and doubly occupied d-orbitals, which serve as Here, the vacant orbitals of the metal center receive electrons CHEMISTRY both electron acceptors and electron donors during the cleavage from N and simultaneously weaken (or cleave) the triple bond of the triple bond of N≡N by the ditantalum center. The ability of 2 + of dinitrogen by donating electron density from the filled d-orbitals Ta2 to serve as a multipurpose tool is further shown by splitting into the antibonding π*-orbitals of N2 (49, 50). According to the the single bond of H2 in a less exothermic reaction as well. The insight into the microscopic mechanisms obtained may provide conceptual framework outlined by Fryzuk and coworkers (67), it guidance for the rational design of polymetallic catalysts to bring was recognized that the ability of Ta compounds to store two – about ammonia formation by the activation of molecular nitrogen electrons in a Ta Ta bond is a prerequisite for subsequent re- and hydrogen at ambient conditions. ductive transformations and is of paramount importance to split the N≡N triple bond completely. gas-phase catalysis | ammonia synthesis | dinitrogen activation | While the impressive progress made in recent decades is un- hydrogen activation | quantum chemical calculation deniable, a deep and comprehensive understanding of the vari- ous mechanistic details related to either fixation or activation of he direct use of molecular nitrogen with its thermodynami- N2 to ammonia is far from being complete. This also applies to a Tcally stable and kinetically inert triple bond as one of the very consistent description of the elementary steps involved, with the few commodities that are freely available worldwide and in al- most unlimited quantities is essential for life on Earth (1–3). Significance Nature utilizes nitrogen-binding enzymes, the nitrogenases, to catalyze the conversion of nitrogen to ammonia at ambient A combined experimental/computational approach provides conditions (4, 5). In contrast, its industrial production still relies deep mechanistic insight into an unprecedented cluster- on the highly energy-demanding Haber–Bosch process to bring mediated N−H coupling mimicking the industrially extremely important ammonia synthesis from N and H (the “Haber– about the challenging chemical marriage of N2 and H2 to form 2 2 Bosch” process) at room temperature. Crucial steps were NH3 (3, 6–8), which consumes ca. 1–2% of the world’s energy – identified for both the forward reactions (i.e., the activation of production (8 11). In addition, presently about 1.5 tons of the + greenhouse gas carbon dioxide are produced per ton of ammonia N2) and the backward process (i.e., the Ta2 -mediated de- composition of NH3). The central intermediate for either path (9). To slow down global warming (12), it would, therefore, be + corresponds to Ta2N2 , a four-membered ring with alternating sensible, in addition to numerous other measures, to find a ’ process for producing ammonia on an industrial scale from the Ta and N atoms. The root cause of tantalum s ability to bring about nitrogen fixation and its coupling with H under mild molecular feedstock nitrogen and hydrogen in an economically 2 conditions has been identified by state-of-the-art quantum viable and environmentally benign way. chemical calculations. The greatest obstacle to the production of ammonia from N2 ≡ corresponds to the cleavage of the N N triple bond, which with a Author contributions: J.L. and H.S. designed research; C.G. and J.L. performed research; −1 bond energy of 945 kJ mol (13), constitutes one of the stron- C.G., J.L. and T.W. analyzed data; and J.L., T.W. and H.S. wrote the paper. gest chemical bonds. While some progress has been made on the Reviewers: R.G.C., Purdue University; and M.R., Swiss Federal Institute of Technology. daunting road to artificial nitrogen activation, the number of The authors declare no conflict of interest. well-defined complexes that bind N2 and ultimately, lead to a Published under the PNAS license. ≡ complete cleavage of the N N triple bond is rather limited so 1To whom correspondence may be addressed. Email: [email protected] or helmut.schwarz@ far. For instance, complexes with single (14–32) and multiple tu-berlin.de. – – (15, 33 39) transition metal centers, small metal clusters (40 This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. 48), and also, main group compounds (49–52) have been found 1073/pnas.1814610115/-/DCSupplemental. to be able to split dinitrogen. The activation of N–N bonds by www.pnas.org/cgi/doi/10.1073/pnas.1814610 115 PNAS Latest Articles | 1of8 Downloaded by guest on September 27, 2021 + A A with Δm =+28 appears, which has been identified as Ta2N2 ABAr 14N2 15 (Eq. 1). By using isotope-labeled N2, signal C from Fig. 1B is C = Ta2N2+ shifted by two mass units on the mass scale and shows up as peak 15 + + + D C 2 A = Ta2 in Fig. 1 (Ta2 N2 ) (Eq. ). If Ta2 is exposed to a 1:1 + + B = Ta O+ 14 15 14 15 2 mixture of N2 and N2, signals for both Ta2 N2 and Ta2 N2 are observed, but there are none containing both nitrogen iso- 14 15 + 3 4 C topes Ta2 N N (Eqs. and ). Mass-selected and properly B 14 + 15 B thermalized Ta2 N2 ,whenexposedto N2, does not react fol- lowing one of the degenerate exchange reactions 3 and 4: 340 362 420 340 362 420 + + A A Ta2 + N2 → Ta2N2 [1] CD15N2 + + 15 → 15 + [2] D = Ta15N2+ Ta2 N2 Ta2 N2 14 + 15 15 + 14 Ta2 N2 + N2↛Ta2 N2 + N2 [3] D CD + B B 14 + 15 14 15 14 15 Ta2 N2 + N2↛Ta2 N N + N N. [4] 340 362 420 340 362 420 + The rate constant k(Ta2 /N2) for reaction 1 is estimated to − − − + 14 × 12 3 1 1 Fig. 1. Mass spectra for the thermal reactions of Ta2 with Ar (A), N2 (B), 5.1 10 cm molecule s ; this corresponds to a collision 15 14 15 × −7 N2 (C), and a 1:1 mixture of N2 and N2 (D) at a pressure of ca. 2.0 10 efficiency of ϕ = 0.8%. Owing to the uncertainty in the de- mbar after a reaction time of 2 s. All x axes are scaled in m/z, and the y axes termination of the absolute N2 pressure, an error of ±30% is are normalized relative ion abundances. associated with these measurements. In addition to the labeling experiments, the elementary compositions of the charged parti- cles have been confirmed by exact mass measurements. Since exception of the elegant elucidation of the mechanism of the 14N/15N kinetic isotope effects (KIEs) are expected to be quite Haber–Bosch process by Ertl and coworkers (7, 8, 72, 73). small (95) and as it was not possible to reproducibly adjust the As has been shown time and again, gas-phase experiments pressure in the ICR cell to the required accuracy to obtain mean- provide an ideal arena for tackling many challenging mechanistic ingful data, we have renounced the determination of the 14N/15N issues at a strictly molecular level, such as investigating the de- + KIE for the formation of Ta N . tailed course of chemical reactions, including those that are in- 2 2 The simplified 2D potential energy surface (PES) of the most dustrially relevant (74–88).