Mediated Ammonia Synthesis from N2 and H2 at Ambient Temperature

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 various 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 almost 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 following 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). Structurally properly characterized favorable pathway as well as selected structural parameters of gas-phase clusters have been chosen as prototypical models to key species (Fig. 2) reveals insight into the mechanism of the probe the active sites in (including but not limited to) hetero- + Ta -mediated activation of the N≡N triple bond at a molecular geneous catalysis aimed at a better understanding of the intrinsic 2 level. A Fortran-based genetic algorithm (96) to generate initial factors that govern reactivity patterns in the condensed phase + guessstructuresofTaN followed by optimizations at the (89, 90). In addition, it has been shown that fundamental ques- 2 2 level of the Becke-3–Lee–Yang–Par functional including the tions can be addressed when complementing the experimental def2–triple-zeta valence basis set with one set of polarization findings by quantum chemical (QC) calculations (91, 92). Given the enormous importance of the Haber–Bosch process, the study of metal clusters capable of activating dinitrogen so as to synthesize ammonia represents a worthy undertaking (39), not to 1.68 mention the fundamental problems being addressed. Herein, we 1.09 TaN: 247 N2 describe our findings on the unexpected, mechanistically unique 1.66 TaN 2.68 TaN+: ammonia synthesis from its elements at ambient temperature 1.29 + + a b 1.92 + mediated by the cationic tantalum dimer Ta2 in the highly diluted 2[Ta2]+ 2.19 1.96 TaN a gas phase using advanced MS complemented by QC calculations. a b 1.99 1.90 2.35 b + 1.95 Results and Discussion Ta2 (C1) a b N2 2.38 + + 0.0 1/2 (Cs) Ta2 Cleaves the N≡N Triple Bond to Form Ta2N2 . The spectra in Fig. 1 have been obtained by using Fourier transform ion cy- 1 -55 2 2/3 clotron resonance (FT-ICR) MS (details are in Experimental -93 -104 -94 Details) and show the results of the reactions of mass-selected 1.32 b + a m z = 14 1.82 2.08 1.90 a 2.56 a Ta2 ions ( / 362) (refs. 93 and 94 have details) with N2, 1.57 1.99 1.86 15 14 15 a b N2, and a 1:1 mixture of N2 and N2. To properly ther- b a b + 2.50 a b malize the precursor ion Ta2 , it was allowed to interact with 2.37 − (Cs) 2.69 b × 6 (C2v) pulsed-in argon (ca. 2 10 mbar) before reacting with molec- (D2h) ular nitrogen. A temperature of 298 K for the thermalized clusters 3 was assumed. Spectra resulting from the reactions with back- Ta N -453 ground impurities as well as with argon, serving as an inert sub- strate, have been recorded as well (Fig. 1A).

As displayed in Fig. 1A, when only argon was admitted to the + Fig. 2. Simplified PES (ΔH ) for the reactions of Ta with N . The cal- ion cyclotron resonance (ICR) cell, a signal B with Δm =+16 298K 2 2 + culations were done at the B3LYP/def2-QZVPP//B3LYP/def2-TZVP level of relative to the precursor ion Ta2 appears; this corresponds to + theory. Key ground-state structures with selected geometric parameters are the product ion Ta2O generated by reactions with background also provided. Charges are omitted for the sake of clarity, bond lengths are −1 gases. On leaking N2 into the ICR cell, in Fig. 1B, a new signal C given in Å, and relative energies are in kJ mol .

2of8 | www.pnas.org/cgi/doi/10.1073/pnas.1814610115 Geng et al. Downloaded by guest on September 27, 2021 functions (B3LYP/def2-TZVP) could only identify the intermediates considered the following. If it were 1 that had been generated, a

1–3 as the most stable species. (Additional computational details are degenerate exchange of the N2 unit, according to Fig. 3, should INAUGURAL ARTICLE provided in SI Appendix.) be possible, as all relevant species are located well below the + Dinitrogen approaches the positively charged Ta2 , which in entrance asymptote. However, this computational finding is in 2 its ground state, is a doublet ( Δg) (97), through the known side- conflict with the experimental results (Eq. 3). Thus, the experi- μ η1 η2 – + on/end-on binding mode ( - : -N2) (34, 63, 65 67) to form mentally generated Ta2N2 does not have structure 1. Further- −1 intermediate 1 (−93 kJ mol , Cs symmetry) in a barrier-free more, 2 is also not likely to be long lived along the reaction process. The end-on bonded nitrogen atom (Na)in1 has an coordinate. As an isolated species, it cannot dissipate its internal − Na−Taa bond length of only 1.82 Å, while Tab binds side on to Na energy of ca. 104 kJ mol 1 and will rather easily surmount (2.08 Å) and end on to Nb (1.90 Å). The N–N and Ta–Ta bond transition state 2/3 to form the global minimum 3. For this ion, in distances are elongated by 0.23 and 0.31 Å, respectively, com- agreement with the experiments (Eq. 4), extensive calculations + pared with the isolated reactants; thus, on interacting with Ta2 , (SI Appendix, Fig. S1) reveal that, for the exchange reactions ≡ the N N triple bond is already weakened, but the N2 unit as with N2, prohibitive barriers are encountered. Finally, collisional + such is still intact. These geometric properties closely resemble activation (101–104) of Ta2N2 with argon leads to the loss of N2 μ those reported for the crystal structure of ([NPN]Ta( -H))2N2 only at rather high excitation energies [E(coll.,CM) > 4.8 eV]. This b b + (65). During the next step, atom N , while still connected to Ta , indicates that strong chemical bonds must exist between Ta2 + approaches Taa, eventually binding to Taa via transition state 1/2 and N . Although the direct dissociation of 3 to TaN /TaN by − 2 (−55 kJ mol 1). This leads to the formation of intermediate 2 cycloreversion may have an entropic advantage, obviously this −1 (−104 kJ mol ) having C2v symmetry; 2 displays a double side- process cannot compete with the energetically favored multistep μ η2 η2 “ ” + on ( - : -N2) butterfly geometry with an already significantly dissociation back to the starting reactants (3 →→→Ta2 /N2). – ̊ + elongated N N bond distance amounting to 1.57 A, clearly in- Thus, the cleavage of the N≡N triple bond of N2 by Ta2 forms dicating additional activation of the N2 molecule. Finally, the 3 and proceeds via the rate-limiting transition state 1/2. As the remaining bonding interaction between Na and Nb is disrupted rate-limiting transition state 1/2 is located below the entrance − entirely by passing through the low barrier of 2/3 (−94 kJ mol 1), asymptote, activation of N is accessible at ambient temperature. − 2 thus giving rise to the global minimum 3 (−453 kJ mol 1); 3 as well as its neutral counterpart (98) exhibit a slightly distorted Establishing a Working Hypothesis—a Brief Detour. Recently, D + planar square with 2h symmetry. It has a cyclic structure con- Arakawa et al. (107) have shown that Ta2N2 canalsobe CHEMISTRY a b + sisting of alternating Ta and N atoms. N and N in 3 are 2.56 Å generatedinthegasphasebythereactionofTa2 with NH3 + apart from each other, indicating that the direct interaction be- at 298 K (Eq. 5). In our experiments, we found that Ta2 + tween the two N atoms is negligibly small; this is confirmed by a reacts with N2 to form Ta2N2 as well (Eq. 1). Combining the Mayer bond order (99, 100) of less than 0.1. The energy re- two reactions (Eq. 6) represents the reverse of the Haber– quirements for dissociating 3 into various couples of, for exam- Bosch ammonia synthesis (Eq. 7). If the structures of the key + + + + + ple, TaN /TaN, Ta2N /N, TaN2 /Ta, and Ta /TaN2 are located intermediates Ta2N2 generated in reactions 1 and 5 are − 247, 288, 416, and 409 kJ mol 1, respectively, above the entrance identical, the principal of microscopic reversibility requires level. To enable these reactions, external energy must be sup- that there must be a way by which N2 and H2,mediatedby + plied, for instance, by collisional activation (101–104). Without Ta2 ,canformNH3 at ambient temperature (Eq. 7), since the external energy supply, intermediate 3 can only return to the reactions 1, 5, and 7 are exothermic (108): reactants, or its lifetime can be increased by IR photon emission + + (105) or collisional cooling (106). Ta2 + 2NH3 → Ta2N2 + 3H2 [5] + To further substantiate the claim that, in Ta2N2 , the binding interaction between the two nitrogen atoms originally tied to- 2NH3 → N2 + 3H2 [6] gether by a triple bond has indeed completely disappeared, we

N2+3H2 → 2NH3. [7]

1.16 ⇄ 1 2.86 1 Crucial Intermediates Along the N2 NH3 Reaction Coordinates. Next, we consider computationally (Fig. 4) the elementary steps 0.0 N2 2.14 0.0 + 2.14 N2 associated with the Ta2 -mediated reactions in Eq. 7.Itisim- 2.86 1.16 portant to stress that these processes turned out to be mechanistically rather complex, and as they do not form the main target of 3.1/2 3.1/2 our investigations, they are not discussed here in detail. Further- -32 -32 more, we are well aware that most likely not all conceivable in- 3.1 3.1 termediates and sideways have been included that could be -47 -47 involved in the ammonia synthesis from molecular nitrogen and 1.13 + hydrogen mediated by Ta2 . However, the intermediates and 2.34 transition states, shown in Fig. 4 (details are in SI Appendix, Fig. S6), already form a feasible road map to the formation of NH3 out 2.34 + of N and H mediated by Ta at room temperature. 1.13 2 2 2 3.2 As the key step, a reaction with N2 takes place followed by the -96 sequential uptake of two H2 molecules. After liberation of the first NH3 molecule and subsequent uptake of a third H2 mole- + cule, the second NH3 molecule is released, and Ta2 is regen- 1.89 1.95 erated to be ready for another catalytic cycle. It is quite impressive to note how stable some of the intermediates are! For 1.29 − example, the global minimum (8, −690 kJ mol 1) is arrived at + + Fig. 3. Simplified PES (ΔH298K) for the degenerate exchange reactions of 1 after the uptake of two H2 molecules by Ta2N2 .Ta2N2H4 , with N2. Details are in Fig. 2. after several intramolecular isomerizations, finally splits off one

Geng et al. PNAS Latest Articles | 3of8 Downloaded by guest on September 27, 2021 Ta2+ [Ta2,N2]+ [Ta2,N2,H2]+ [Ta2,N2,H4]+ [Ta2,N,H]+ [Ta2,N,H3]+ Ta2+

+ +N +H +H NH3 +H NH3 0 2 2 2 2 2NH3 -55 1/2 + -94 2/3 -101 N2 1 1.34 -148 -93 2 18/19 + -104 2.30 3H2 1.38 2.29 -275 16/17 19 -337 -249 14 -346 -368 17/18 15/16 -397 17 18 12/13 15 -348 -365 -352 -468 -482 -479 -479 4/5 -498 9/10 -490 11/12 3 8/9 10/11 4 6/7 -526 -453 16 -468 7/8 13 -473 11 -493 7 -510 -528 1.78 10 12 1.76 9 -577 -583 5 1.89 6 -613 (C ) -618 2v -639 1.78

2.41 8 14 0.76 -690 1.02

+ + + Fig. 4. Simplified PES (ΔH298K) for the reactions of Ta2 with one molecule of N2 and three molecules of H2 and the reverse process Ta2 + 2NH3 → Ta2N2 + 3H2. Details are in Fig. 2.

+ −1 molecule of NH3. The remaining intermediate 14,Ta2NH ,is the catalytic cycle shown in Fig. 4 amounts to −101 kJ mol and − able to catch another dihydrogen molecule. After a series of compares well with the experimental one (−91.8 kJ mol 1) (108). To test experimentally the QC predictions of this catalytic cycle, additional hydrogen migrations toward the NH group, a second + NH3 is formed and finally, liberated. Note that the generation thermal reactions of Ta2 with NH3 were examined. As can be seen + + + from Fig. 5A,Ta2 on reacting with NH3 gives rise to Ta2NH as of the terminal products Ta2 and NH3 corresponds to the + + E rate-determining step for the whole Ta N hydrogenation– the main product (signal ), with Ta2N2 also emerging as a weak 2 2 C denitrogenation reactions; however and importantly, as it lies signal ; the reaction efficiency amounts to ca. 90%. This confirms the results already obtained by Arakawa et al. (107). Next, when below the reaction entrance, a “Haber–Bosch” process transpires + Ta NH is mass selected and further reacted with another at room temperature. The theoretically obtained value for the 2 heat of formation of two NH3 molecules resulting at the end of

H2 1.03 Ta2+ 1.83 2.22 0.0 A E ABNH3 NH3 6.1 6.1/2 -25 -23 A = Ta2+ 0.79 C = Ta2N2+ 2.07 E = Ta2NH+ F = Ta2NH2+

E F C C 1.90 340 362 377 420 340 377 420 (C2v) 6.2 + Fig. 5. Mass spectra for the thermal reactions of (A)Ta2 with NH3 at an NH3 -134 −9 + pressure of ca. 2.0 × 10 mbar after a reaction time of 2 s and (B)Ta2NH with −9 + NH3 at an NH3 pressure of ca. 2.8 × 10 mbar after a reaction time of 1 s. The Fig. 6. Simplified PES (ΔH298K) for the reactions of Ta2 with H2. Details are x axes are scaled in m/z,andthey axes are normalized relative ion abundances. in Fig. 2.

4of8 | www.pnas.org/cgi/doi/10.1073/pnas.1814610115 Geng et al. Downloaded by guest on September 27, 2021 INAUGURAL ARTICLE CHEMISTRY

Fig. 7. Schematic orbital diagrams based on a frontier orbital analysis. Only representative orbitals are shown. The structures are oriented such that the x axis

is along the Ta–Ta vector and that the y axis is confined to the Ta2N2 plane. The πv- and πv*-orbitals of the dinitrogen molecule are vertical (v) to this plane, whereas the πp- and πp*-orbitals are lying within this plane. The magenta borders refer to π-backdonation; the green ones represent metal centers that accept electron density from N2. The more intense the color, the more electron density is transferred.

+ + molecule of ammonia, the only product obtained is Ta2N2 (ϕ ≈ Brief View on the Reaction of Ta2 with H2. Although less exothermic 0.95), thus connecting 3 with 14 (Fig. 4). Starting the reaction from than in the reaction with N2, as shown computationally (Fig. 6), + the product side has the benefit that part of the excess energy Ta2 also is able to react with H2, and the rate-determining step − gained in the course of the exothermic reaction steps can be dis- corresponds to transition state 6.1/2 (−23 kJ mol 1). In a concerted, sipated by the loss of neutral hydrogen molecules, thus increasing almost barrier-free way, the C2v-symmetric, butterfly-shaped − the lifetime of the remaining charged counterparts. We have also intermediate 6.2 (−134 kJ mol 1) is generated, which strongly re- + performed numerous experiments aimed at obtaining additional sembles structure 2 in the Ta2N2 system; 6.2 is able to react further + SI Appendix details for the forward reaction (i.e., the system Ta2 /N2/H2). with another molecule of H2 ( ,Fig.S2). In contrast to However, these experiments were not conclusive, most likely the Haber–Bosch process where H2 poisoning constitutes an im- + due to limited sensitivity and a lack of a sufficiently long life- portant issue (109), in this system, for all clusters Ta2(H2)x (x = 1, time of the intermediates. Nevertheless, the combined experimental/ 2, 3) investigated, N2 is able to displace molecular hydrogen and computational findings show the existence of a catalytic room thus, suppress H2 poisoning of the catalyst as shown in SI Appendix, temperature cycle in the conversion of N2/H2 to NH3. Figs. S3 and S5.

Geng et al. PNAS Latest Articles | 5of8 Downloaded by guest on September 27, 2021 Closer Inspection of the Mechanism of the N≡N Triple-Bond Cleavage and on the other hand, weakens the N≡N triple bond further by by a Frontier Orbital Analysis. To obtain a more detailed mecha- π-backdonation of electron density from the metal center into ≡ nistic insight into the splitting of the N N triple bond as the key the antibonding orbitals of N2 (50). step of the whole reaction sequence, a frontier orbital analysis for the rate-determining step has been performed (110–117). Conclusion Fig. 7 shows the detailed evolution of the electronic structures As shown above, our working hypothesis about a tantalum- along the reaction coordinates. mediated coupling of N2 and H2 finally has been confirmed by According to Fig. 7, the multiple bonding in the cationic gas-phase experiments and QC calculations. Here, we describe + 2 7 tantalum dimer, having a doublet ground-state Ta2 (6s 5d ), the concept of an ammonia synthesis from molecular N and H σ σ σ 2 2 comprises two doubly occupied -bonds [ (6s-6s) and (dx2-dx2)], catalyzed by a bare metal cluster cation at ambient temperature π π π two doubly occupied -bonds [ (dxz-dxz) and (dxy-dxy)], and a (118). The key step consists of the complete rupture of the N≡N δ δ 1 singly occupied -bond [ (dyz-dyz)]. In , the approaching N2 triple bond, rendered possible by the interplay of vacant and + molecule lies in the same plane as the metal dimer, and the doubly occupied d-orbitals at the ditantalum center of Ta2 to + strong interaction between these two fragments mainly results form Ta N as the central intermediate. This combined exper- π 2 2 from the -backdonation from the metal centers to N2 (i.e., imental/computational study further improves our knowledge – π electron density relocates from the metal metal -bonding to the about mechanistic details of the catalytic action of transition vacant antibonding orbitals of the N2 ligand); as a consequence, metals and emphasizes the crucial role that electron-donating ≡ weakening of the N N triple bond occurs to a certain extent. In and -accepting orbitals play (49, 50). These findings might return, although relatively small, donation of electron density serve as a base to improve or even invent “real world” catalysts to – π – from the N N -bonds to the vacant metal metal orbitals can be save economic and ecologic resources in the future (73). found as well. On its way to 2, the N2 molecule gradually rotates perpendicular to the Ta–Ta axis while simultaneously adjusting Materials and Methods – 2 the metal metal orbitals. After conversion to , only a small Experimental Details. The ion/molecule reactions were performed with a −1 barrier of 10 kJ mol precludes the four atoms to be trapped in Spectrospin CMS 47X FT-ICR mass spectrometer equipped with an external ion 3 + the deep potential well to form . This process is accompanied by source as described elsewhere (119–121). In brief, Ta2 was generated by both π-backdonation from the metal dimer to the σ*- and laser ablation of a tantalum disk using a neodymium-doped yttrium alumi- π*-orbitals of N2 and partial donation of bonding electrons from num garnet (Nd:YAG) laser operating at 532 nm and seeded in helium; the latter serves as a cooling and carrier gas. Using a series of potentials and ion N2 to the metal dimer. It is this electronic reorganization that eventually leads to the complete rupture of the N≡N triple bond, lenses, the ions were transferred into the ICR cell, which is positioned in the which is accompanied by generating strong Ta–N bonds in 3. bore of a 7.05-T superconducting magnet. To properly thermalize the precursor + × −6 As shown in Fig. 7, the orbital components of the antibonding ion Ta2 , it was allowed to take a bath in pulsed-in argon (ca. 2 10 mbar) before its reaction with dinitrogen. After thermalization, the reactions of orbitals of N2 are dominant in π*(dxz/dxz)_πv*, π(dxz/dxz)_σ*, and + mass-selected Ta2 were studied by introducing isotopologues of dinitrogen δ(dyz/dyz)_πv*in3; once again, this clearly indicates the cleavage 15 ≡ (N2 and N2) and ammonia via leak valves at stationary pressures. A tem- of the N N triple bond. Furthermore, the components of all perature of 298 K for the thermalized clusters was assumed. Before the 15 three N2-based bonding orbitals greatly shrink in going from exchange reactions between unlabeled and N-labeled N2, the precursor 2 3 + −6 to , sharing their electrons now within the four-membered ion Ta2N2 was thermalized by pulsed-in argon (ca. 2 × 10 mbar). In the ring. In sharp contrast, for the cleavage of the single bond of a collision-induced dissociation (102–104) experiments, mass-selected, prop- + + dihydrogen molecule by Ta2 , the frontier orbital interaction erly thermalized Ta2N2 ions were reacted with argon. mainly concerns the π(dxy/dxy)-orbital of the metal dimer center and Computational Details. The unbalanced treatment of static and dynamic cor- the antibonding σ*(H–H)-orbital of H2 (SI Appendix, Fig. S8). An even deeper understanding of the intrinsic reactivity can be relation makes the transition metal chemistry hard to handle for any density obtained by a comparison of the “naked” cationic tantalum di- functional. We, therefore, carried out extensive investigations, even using μ multireference perturbation theory [like the n-electron valence state pertur- mer with the ligated ditantalum core in the ([NPN]Ta( -H))2N2 – – bation theory (NEVPT2)] (122 124) as well as a comparison of many density complex (65). The cationic Ta dimer is characterized by a Ta Ta functionals before deciding to base our conclusions on B3LYP calculations multiple bond with a Wiberg bond index (WBI) of 4.5. In con- (125–128). The geometry optimization was conducted at the B3LYP/def2-TZVP trast, in the ligated complex, the WBI amounts to only 1.7 due to level of theory. Subsequently, the electronic energies were refined by using significant binding interactions between the metal atoms and the B3LYP functional with def2–quadruple-zeta valence basis set and two sets the coordinating N and P atoms of the ligands and the two of polarization functions (B3LYP/def2-QZVPP//B3LYP/def2-TZVP) (129). bridging hydrido ligands. SI Appendix, Fig. S7 displays repre- Additional information with regard to the computational details can be sentative frontier orbitals dominating the metal atoms inside found in SI Appendix. the complex. Only two doubly occupied orbitals are left. As the metal–metal σ-bonding orbital is not a good electron donor, the ACKNOWLEDGMENTS. We thank the following fellow colleagues for π-backdonation is expected to be rather small. More importantly, helpful suggestions on the computational work: Prof. Dr. Frank Neese (Max Planck Institut für Kohlenforschung), Prof. Dr. Wenli Zou these orbitals are highly localized at one of the two metal cen- (Northwest University), and Dr. Jun Zhang (University of Illinois at ters, and their lobes are leaning toward the ligands; in addition, Urbana–Champaign). We thank the reviewers for their thorough review they are not regarded as potential electron acceptors, as an op- and appreciate the comments and suggestions. This research was timal overlap is difficult to achieve. Thus, the reactivity of the sponsored by the Deutsche Forschungsgemeinschaft, in particular the Cluster of Excellence “Unifying Concepts in Catalysis,” and the Fonds cationic ditantalum cluster toward dinitrogen can be attributed der Chemischen Industrie. The work at Jilin University was supported to the interplay of empty and doubly occupied d-orbitals at the by National Natural Science Foundation of China Grants 21473070 metal center, which on the one hand, accepts electrons from N2 and 21773085.

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