Dinitrogen Fixation and Reduction by Ta3N3H0,1â Cluster Anions at

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Dinitrogen Fixation and Reduction by Ta3N3H0,1- Cluster Anions at Room Temperature: Hydrogen-Assisted Enhancement of Reactivity Yue Zhao, Jia-Tong Cui, Ming Wang, David Yubero Valdivielso, André Fielicke, Lian-Rui Hu, Xin Cheng, Qing-Yu Liu, Zi-Yu Li, Sheng-Gui He, and Jia-Bi Ma J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b03168 • Publication Date (Web): 17 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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Journal of the American Chemical Society

Dinitrogen Fixation and Reduction by Ta3N3H0,1− Cluster Anions at Room Temperature: Hydrogen-Assisted Enhancement of Reactivity Yue Zhao,[a] Jia-Tong Cui,[a] Ming Wang,[a] David Yubero Valdivielso,[b] André Fielicke,*[b] LianRui Hu,*[c] Xin Cheng,[d] Qing-Yu Liu,[d] Zi-Yu Li,[d] Sheng-Gui He,[d] Jia-Bi Ma*[a] aKey

Laboratory of Cluster Science of Ministry of Education, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, 100081, Beijing, China bInstitute

for Optics and Atomic Physics, Technische Universität Berlin, 10623 and Fritz-Haber-Institut der MaxPlanck-Gesellschaft Faradayweg 4-6, 14195 Berlin, Germany cSchool

of Science, Xihua University, 610039, Chengdu

dState

Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, 100190, Beijing ABSTRACT: Dinitrogen activation and reduction is one of the most challenging and important subjects in chemistry. Herein, we report the N2 binding and reduction at the well-defined Ta3N3H− and Ta3N3− gas-phase clusters by using mass spectrometry (MS), anion photoelectron spectroscopy (PES), and quantum-chemical calculations. The PES and calculation results show clear evidence that N2 can be adsorbed and completely activated by Ta3N3H− and Ta3N3− clusters, yielding to the products Ta3N5H− and Ta3N5−, but the reactivity of Ta3N3H− is five times higher than that of the dehydrogenated Ta3N3− clusters. The Detailed mechanistic investigations further indicate that a dissociative mechanism dominates the N2 activation reactions mediated by Ta3N3H− and Ta3N3−; two and three Ta atoms are active sites and also electron donors for the N2 reduction, respectively. Although the hydrogen atom in Ta3N3H− is not directly involved in the reaction, its very presence modifies the charge distribution and the geometry of Ta3N3H−, which is crucial to increase the reactivity. The mechanisms revealed in this gas-phase study stress the fundamental rules for N2 activation and important role of transition metals as active sites as well as the new significant role of metal hydride bonds in the process of N2 reduction, which provides molecular-level insights into the rational design of tantalum nitride-based catalysts for N2 fixation and activation or NH3 synthesis.

1.

INTRODUCTION

Dinitrogen activation and reduction are one of the most challenging and important subjects in chemistry, due to the robust N≡N triple bond (9.8 eV), the large HOMOLUMO gap (10.8 eV), and the non-polarity of N2. These features render the molecule inert to most reagents. The reduction of N2 to NH3 in industry requires extreme conditions to weaken and break the strong NN bond. The long-standing goal of elucidating mechanisms of the reactions involving dinitrogen has motivated numerous experimental and theoretical investigations of the interactions between various reagents and dinitrogen.1-4 To overcome this challenge, chemists have typically relied on transition-metal compounds to provide electrons to weaken or even cleave the N≡N bond to open reactivity.57 In the Haber-Bosch process (1/2N + 3/2H → NH ), the N 2 2 3 ≡N bond is broken on Fe catalysts before any hydrogen additions occur,5 and dissociative adsorption of N2 comprises the rate-limiting step.8,9 In addition to the Fe (or Ru) catalysts employed to break the NN bond in the Haber-

Bosch process, tantalum nitride core-containing complexes are also capable of activating N210-14 and catalytically yielding NH3.15-17 tantalum hydrides are found to be essential to the NH3 generation.16,17 Both of transition metal (TM) TM–N bonds (or nitrides) and M–H bonds are closely related to the active centers, which activate N2.18 Nitrides and hydrides are extensively examined as reactants or intermediates in N2 fixation schemes; there is a pressing need to uncover their major roles and the fundamental mechanisms to guide rational development of appropriate catalytic materials. Therefore, the activation of N2 by transition metal nitrides in the presence and absence of H atoms is particularly of interest; however, this kind of studies on the fixation and activation of N2 at the molecular level are still scarce.19 The activation of the N≡ N bond in N2 is still underdeveloped.20 Gas-phase studies on “isolated” reactants provide an ideal arena for revealing experimentally and theoretically the mechanisms of chemical processes, such as N2 activation and reduction, at a strictly molecular level.21-32 Several

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theoretical studies about the reactions of metal clusters with N2 have been reported,33-39 such as Sc2,36 Ti2,35 V2-13,38 Ta1-4,6,37,39 Nb1-4, Mo1-4,33 and Ru11,15,21.34 There are also experimental reports about reactivity of gas-phase transition metal atoms and transition-metal-containing clusters toward N2.40-55 In addition to Os,42 Sc2,36 Ti2,35 Ta2+,56 and Ta2C4−55, most of transition metal atoms and clusters M adsorb one or several N2 molecules with the formation of physisorbed M(NN)n complexes, and the extent of N2 activation is relatively low, compared to the triple N≡N bond in the free N2 molecule. Literature about the N2 activation reactions mediated by cluster anions containing more than one metal atoms is very limited. Considering the fact that tantalum is applied in N2 catalytic activation reactions as discussed above, Ta3N3H0,1− cluster anions were synthesized and the reactions toward N2 were investigated experimentally and theoretically as models. 2.

METHODS

2.1. Experimental Methods. Reactivity Studies. The Ta3N3H0,1− ions are generated by laser ablation of a tantalum disk in the presence of about 0.1% NH3 seeded in a He carrier gas with a backing pressure of 4 atm. The ions of interest are mass-selected by a quadrupole mass filter (QMF) and enter into a linear ion trap (LIT) reactor, where they are confined and thermalized by collisions with a pulse of He gas and then interacted with a pulse of N2 for a period of time. The instantaneous gas pressure of He in the reactor is around 1.6 Pa. It has been proved that the clusters are thermalized to (or close to) room temperature before reactions in the previous works.57,58 The ions ejected from the LIT are detected by a reflectron time-of-flight mass spectrometer (TOF-MS). The method to derive the rate constants are described in detail in Ref. 59.59 Photoelectron Spectroscopy (PES). Similar to the reactivity studies, the cluster ions are produced by laser ablation from a tantalum rod in the presence of either 0.025 % NH3 or 0.5 % N2 in He. The ions are orthogonally extracted from the molecular beam into a linear time-offlight (TOF) mass spectrometer and mass analyzed. At about 80% of the field-free flight path they are irradiated with a laser pulse that is timed such to correspond to the TOF of the chosen anion. The kinetic energy distribution of the detached electrons is determined using velocity map imaging. Details of the experimental setup have been given elsewhere.60 While the total mass resolution of the linear TOF mass spectrometer m/Δm is on the order of 800, it is only 500 at the point of interaction with the detachment laser. Therefore, the signals of Ta3N3− and Ta3N3H− formed when using NH3 as N-source can not be fully resolved. To overcome this problem the PES of Ta3N3− is measured using N2 as N-source and a weighted spectrum is subtracted from the one of the Ta3N3−/Ta3N3H− mixture to obtain the spectrum of Ta3N3H−. The energy resolution of the VMI spectrometer is about 5% and its energy scale has been calibrated here using several transitions of Ir−. The PESs of

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the products Ta3N3HN2− and Ta3N3N2− are carried out with another separate instrument equipped with a laser ablation cluster source, a fast flow reactor, a tandem timeof-flight mass spectrometer, and a photoelectron imaging spectrometer.61 The laser ablation generated Ta3N3H0,1− anions were expanded and reacted with N2 in the fast flow reactor (length ∼60 mm) for about 60 μs. The instantaneous total effective pressure in the fast flow reactor was estimated to be around 38 Pa at T = 298 K. The mass spectra for the generation of TaxNy− and the reactions with N2 in the fast flow reactor are given in Figure S1, SI. 2.2. Theoretical Methods. All density functional theory (DFT) calculations were performed using the Gaussian 0962 program package employing the hybrid B3LYP exchange– correlation functional.63-65 For all the reaction pathways, 6311+G(d) basis sets66 were selected for N and H atoms; the Stuttgart relativistic small core basis set and an effective core potential67 augmented with two f-type and one g-type polarization functions for Ta [ζ(f) = 0.210, 0.697; ζ(g) = 0.472] as recommended by Martin and Sundermann.68 The reaction mechanism calculations involve geometry optimization of reaction intermediates and transition states (TSs). Vibrational frequency calculations were performed to ensure that the intermediates and TSs have zero and only one imaginary frequency, respectively. The intrinsic reaction coordinate (IRC) calculations69,70 were carried out to make sure that a TS connects two appropriate minima. The reported energies (∆H0K in eV) are corrected with zero-point vibrations. Natural bond orbital (NBO) analysis was performed using NBO 6.071 implemented in Gaussian 09, and the program Multiwfn72 is employed to analyze orbital compositions by natural atomic orbital method. The dispersion effect,73 which can be explicitly estimated from the DFT-D3 correction of the complexes,74 is included in our systems. To determine reliable relative energies of two lowest-lying electronic structures (I1', I2' and I3', I4') of the Ta3N3H0,1− cluster, single point energy calculations with high-level quantum chemistry method of CCSD(T)-F1275,76 were performed at the DFT optimized geometries. The CCSD(T)-F12 approach used throughout this work employs the CCSD(T)-F12b method with the diagonal fixed amplitude 3C(FIX) Ansatz.77,78 The Molpro program package79 was used in the CCSD(T)-F12 calculations in which the reference orbitals were from the Hartree−Fock calculations. For Ta, N, and H atoms, aug-cc-pwCVDZPP/aug-cc-pVDZ (abbreviated as AwDZ) and aug-ccpwCVTZ-PP/aug-cc-pVTZ (abbreviated as AwTZ) basis sets80 were employed in CCSD(T)-F12 calculations, and two-point complete basis set (CBS) limit extrapolation from AwDZ and AwTZ data was conducted by using the Schwenke’s two-point CBS limit extrapolation scheme,81 with CCSD-F12b correlation energy and perturbative triples contribution (T) extrapolated with equation Ecorr,n = Ecorr,CBS + A/npow separately. The optimal parameter “pow” in CBS limit extrapolations was chosen to be 2.48307 for CCSD-F12b and 2.7903 for (T) contribution, which was determined by Hill et al. for ADZ-ATZ extrapolation.82 The


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Hartree−Fock CBS limit in the CCSD(T)-F12b/CBS value was not obtained from extrapolation but was approximated by the SCF+CABS-singles energy83 calculated with the largest basis set (AwTZ) used in CCSD(T)-F12b calculations. Without taking the default setting of Molpro, we did not scale (T) energy by a factor of MP2-F12/MP2 in all CCSD(T)-F12b calculations. In CCSD(T)-F12b calculations, without further specification, density fitting (DF) of the Fock and exchange matrices employed the auxiliary basis sets of def2AQZVPP/JKFIT84,85 for Ta and aug-cc-pVTZ/JKFIT86,87 for the rest of the atoms, while DF of the other two-electron integrals and resolution of the identity (RI) approximation adopted aug-cc-pVTZ-PP/MP2FIT88 for Ta and aug-ccpVTZ/MP2FIT89 for the rest of the atoms. The value of the geminal Slater exponent in these CCSD(T)-F12b calculations was chosen to be 1.4 a0−1, the same as the previous computational practices.90-93 T1 diagnostic calculations for CCSD(T)-F12 have also been done to test the multiconfigurational character of the Ta3N3H0,1− clusters under study. All tested T1 diagnostic values are below 0.05, which according to the common experience means that the multiconfigurational characters are not serious (see Table S1, SI). 3.

structural information of reactants and product clusters. As shown in Figure 2a, although broad bands are present for both Ta3N3− and Ta3N3H− due to unresolved vibrational substructures, vertical detachment energies (VDEs) can be obtained at 355 nm excitation, which are 2.07 eV and 1.87 eV for the former and latter clusters, respectively. The Ta3N5H0,1− formed by the interactions of Ta3N3H0,1− with N2 in the fast flow reactor were mass-selected for PES study. The VDE of the product Ta3N5H− (2.69 eV, Figure 2b2) is significantly blue-shifted relative to that of Ta3N3H− (1.87 eV), suggesting that N2 is dissociative adsorption in Ta3N5H−. The PES spectrum of the N2 adsorbate complex on Ta3N3− (Figure 2b1) shows that the VDE of Ta3N5− is 2.50 eV. The blue shift of VDE of products relative to reactant clusters is higher for Ta3N3H−+N2 system (Δ=0.82 eV) than for Ta3N3−+N2 system (Δ=0.43 eV).

RESULTS AND DISCUSSION

Cluster Reactivity: Ta3N3H0,1− cluster anions were generated through laser ablation, further mass-selected, thermalized, and then reacted with reactants in a linear ion trap (LIT) reactor. Quite interesting results were obtained. The time-of-flight (TOF) mass spectra for interactions of mass-selected Ta3N3H0,1− cluster anions with reactants N2 are shown in Figure 1a-d, and adsorption complexes are observed, suggesting the following reactions: Ta3N3H− + N2 → Ta3N5H−

(1)

Ta3N3− + N2 → Ta3N5−

(2)

The isotopic labeling experiments with 15N2 (Figure S1c-f) were also performed. Interestingly, Ta3N3H− shows a higher reactivity toward N2 than Ta3N3− does. In addition to the major products, two peaks assigned to Ta3N3H0,1O− and Ta3N5H0,1O− are also present in Figure 1, which originate from the reactions with water impurities in the LIT. The pseudo-first-order rate constants (k1) for the reactions of Ta3N3H− and Ta3N3− with N2 were estimated on the basis of a least-square fitting procedures (Figure S2),59 and are (3.2±0.7) × 10-12 cm3 molecule-1 s-1 as well as (6.4±1.3) × 10-13 cm3 molecule-1 s-1, corresponding to reaction efficiencies (Φ) of 0.5 % and 0.1 %, respectively.94,95 Both of Ta3N3− and Ta3N3H− clusters have no unreactive components according to the kinetic fits (Figure S2, SI). To obtain the accurate relative rate constants, both of Ta3N3− and Ta3N3H− clusters were coexisting and reacted with N2 (Figure 1e and f), and all of Ta3N3H− generates Ta3N5H−. Structure Characterization: Anion photoelectron spectroscopy (PES) was applied in an attempt to get more

Figure 1. Time-of-flight (TOF) mass spectra for the reactions of (a) mass-selected Ta3N3− with (b) N2 for 11 ms, (c) Ta3N3H− with (d) N2 for 8 ms, and (e) the coexisting Ta3N3− and Ta3N3H− clusters with (f) N2 for 6 ms. The effective reactant gas pressures are shown. Structures and Reaction Mechanisms: Quantumchemical calculations were performed to study the structures of Ta3N3− and Ta3N3H− clusters, and the mechanistic details of the reactions. As predicted by B3LYP-D3, which was adopted for other TaxNy− systems,96 the energy differences of the two isomers for Ta3N3H− is within 0.11 eV, as shown in Figure 2a, and other calculated structures are given in Figure S3, SI. Since the accuracy of the B3LYP functional is not better than ± 0.1 eV, the singlepoint energies of isomers for Ta3N3H0,1− were recalculated by the high-level CCSD(T)-F12. Based on the CCSD(T)-F12 calculations, the lowest-energy structure of Ta3N3− (R1 in Figure 2c1) is a doublet and contains one Ta–Ta bond; the



ground state of Ta3N3H− (R2) has Cs symmetry and one three-coordinated N atom, which is lower in energy than R2' by 0.26 eV. Natural bond orbital analysis shows that Ta–Ta bond in Ta3N3−, as well as Ta–H and Ta–Ta bond orders in Ta3N3H− amount to ≈1, ≈1 and ≈2, respectively; other bond orders are given in Table S2. The B3LYPcalculated VDE values of the CCSD(T)-F12 ground states, that are 2.08 eV and 1.83 eV, respectively, agree well with the experimental values. The VDEs of several low-lying isomers were also calculated (Figure S3) and the simulated density of states (DOS)[97] spectra were given (please check Figure S4A and B in SI for more details). The simulated spectrum of R1 is in reasonable agreement with the observed photoelectron spectrum bands (Figure 2a1), although the relative intensities of the experimental peaks have not been reproduced by theory, indicating that R1 is the major component in the generated Ta3N3− anions. In addition to R1, R1’ and the excited quartet isomers of R1 and R1’ may also exist, which cannot be ruled out based on the DOS and PES spectra. For Ta3N3H−, the DOS spectra suggest that R2, R2’, and their singlet isomers may coexist in the cluster beam. The existences of some other isomers can be excluded. Note that these possible coexistent species of Ta3N3H0,1− are with higher energies (Figures 2c and S3), thus their populations should be much smaller than R1 and R2. The MS spectra show that the protonated Ta3N3H− cluster reacts with N2 is five times faster than Ta3N3−, as can be seen clearly from the relative intensities of Ta3N3−/Ta3N5− versus Ta3N3H−/Ta3N5H− in Figure 1e and f, and the reaction pathways calculated at B3LYP-D3 level for the Ta3N3H0,1−/N2 systems are given in Figure 3. The N2 molecules are initially adsorbed on Ta atoms in η2 coordination at both Ta3N3H− (I1) and Ta3N3− (I3), which are the most stable adduct complexes; other adducts with different adsorption modes were given in Figure S5, SI. Notably, I3 (-1.42 eV) in Figure 3b is more stable than I1 (1.10 eV) in Figure 3a, which cannot explain the rate differences between Ta3N3H−/N2 and Ta3N3−/N2 systems, and further steps of N2 activation are expected. In the reaction of Ta3N3H− with N2, by surmounting TS1, N2 is anchored by two Ta atoms, and in I2 the N–N bond is elongated from 110 pm in the free N2 to 134 pm, forming a Ta–N–N–Ta bridge. Via TS2, NN bond is completely broken, forming one terminal nitrogen (Nt) atom. In P1, one unpaired electron is located in one of the N 2p orbitals and the N–Ta bond order amounts to ≈2, indicating the presence of an atomic nitrogen radical anion (N•‒). Even if N2 is firstly adsorbed on Ta3N3H− in an end-on (η1-N2) fashion, the transformation between η2-N2 (I1) and η1-N2 modes involves a barrier which lies at an energy of -0.27 eV (Figure S6). As shown in Figure 3b, the coordination mode of N2(ad) is changed from η2 (I3) to a side-on end-on (η1:η2) mode (I4) through TS3. Via TS4, a three-coordinated N atom shows up in I5; then the product Ta3N5− (P2), in which the adsorbed N2 molecule has been completed activated, forms

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by surmounting TS5. The three-coordinated N atom is important for N2 activation. If this key atom does not present in the electronic structures along the reaction coordinates, the NN unit in Ta3N5− cannot be completely activated (Figure S7, SI), and this pathway is less favorable than that in Figure 3b. Note that the pathway in Figure 3b originates from η2-N2 mode, and other adsorption modes can be easily isomerized to this η2-N2 mode (I3) or I4 (Figure S8). On the basis of Rice−Ramsperger−Kassel−Marcus (RRKM)98 calculations, the rates for the most energy-demanding steps of I2→TS2, and I5→TS5 (kconversion) were estimated to be 7.8 × 107 s-1 and 6.3 × 107 s-1, respectively. For these processes, the values of kconversion are much larger than the collision rate (kcollision) that a cluster experiences with the buffer gas He in the LIT reactor (around 4 × 105 s-1), indicating that the lifetimes of I2 and I5 are short; therefore, P1 and P2 dominate the populations in the final products. To further understand the low efficiencies of the investigated reactions, the potential-energy curve to form I1 and I3 were carefully followed. As shown in Figure S9, small approaching barriers exist in the shallow entrance channels when N2 approaches Ta3N3H0,1−, which explains the observed low reaction efficiencies. A comparison of the HOMO energies of Ta3N3H− and Ta3N3− reveals that the HOMO of Ta3N3H− is pushed higher in energy through bonding of one hydrogen atom, facilitating electron detachment (the smaller VDE of Ta3N3H− compared to that of Ta3N3−, Figure 2a) and N2 adsorption. Furthermore, the pathway shown in Figure 3a is thermodynamically and kinetically more favorable than that in Figure 3b, and thus Ta3N3H− anions are more reactive toward N2 than Ta3N3−, which is consistent with the experimental results.

Figure 2. Photoelectron spectra of Ta3N3H0,1− (a) and the products of Ta3N5H0,1− (b) at a laser wavelength of 355 nm. (c) The B3LYP and the high-level CCSD(T)-F12 calculated


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energy differences for two isomers of Ta3N3H0,1−, as well as the B3LYP-calculated vertical detachment energies (VDEs) for Ta3N3− and Ta3N3H−. The peaks marked with asterisks in (b) are noises.

P2 (Table S3, SI), P2 dominates the population. Little quartet state of P2, the product of the quartet state R1’/N2 system (Figure S10c), may exist. In addition, the DOS spectra of I3 in Figure 3b has a low-energy feature, and the higher energy feature overlaps the peak in that of P2; thus, the observed spectrum for Ta3N5‒ is mainly contributed from P2 and, to a lesser extent, I3. More DFT calculations also indicate that in the reactions of R1’ and R2’ with N2, I3 and I2 are located on the potential energy surfaces (Figure S10), respectively, suggesting that P1 and P2 are the final products. Thus, the complete and facile dissociation of the adsorbed N2 molecules, a dissociative mechanism, happens on Ta3N3H− and Ta3N3−.

Figure 3. B3LYP-D3-calculated potential energy surfaces for the reactions of Ta3N3H− with N2 (a) Ta3N3− with N2 (b) at the B3LYP-D3/stuttgart+2f1g & 6-311+G* level of theory. Some bond lengths are given in pm. The zero-point vibration-corrected energies (∆H0K in eV) of the reaction intermediates, transition states, and products with respect to the separated reactants are given. The superscripts indicate the spin states. The calculated vertical detachment energies (VDEs) of some intermediates are given in square brackets (in eV). The calculated VDEs of the products Ta3N5H‒ (P1) and Ta3N5‒ (P2) at the B3LYP level are 2.82 eV and 2.64 eV, respectively, in reasonable agreement with the PES results (2.69 eV and 2.50 eV in Figure 2b, respectively). The simulated DOS spectra of the products Ta3N5H‒ (P1) and some crucial intermediates along the reactions with N2 (Figure S4D) show that P1 should be the product; I1 and I2 in Figure 3a, or the adduct complex with η1-N2 mode (IA6 in Figure S5) can be excluded. The tail of spectrum in Figure 2b2 should come from minor population of the singlet state of P1, originating from the reaction of the singlet state of R2 with N2 (Figure S10b). For Ta3N5‒, the DOS spectrum of P2 (Figure S4C) is in line with the experimental peak centered at 2.50 eV (Figure 2b1), and I5 in Figure 3b may contribute to the tail of the spectrum. Considering the fact that the lifetime of I5 is shorter than

Figure 4. (a) Electrostatic potentials of the Ta3N3− and Ta3N3H−. Charges on atoms of stationary points along reaction coordinates of N2 activation on Ta3N3− (b) and Ta3N3H− (c) clusters. Some atoms are labelled and shown in the right panel. The Role of the Hydrogen Atom: The frontier orbital analysis shows that back-donating d-electrons from the singly-occupied molecular orbital-1 (SOMO-1) and the SOMO-2 of Ta3N3H− as well as those two orbitals of Ta3N3− to the antibonding π*-orbitals of N2 contribute to the Ta– N bond formation, thus promoting the activation of the N2 ligands (Figure S11). The elongated Ta–Ta bond lengths in I1 (283 pm) and I3 (276 pm), compared to those in the free



Ta3N3H− (263 pm) and Ta3N3− (269 pm) are also indicators of electron injection from the Ta–Ta bonds to the N2 molecules. The root cause for their different reactivity stems from the modifications of the charge distribution and cluster geometries by the additional hydrogen atom in Ta3N3H−. Compared to Ta3N3−, the presence of the hydrogen atom renders the two Ta sites of the Ta3N3H− cluster less negatively charged (Figure 4a), and facilitates the π-back-donation. In addition, the apparent barriers of TS1 (ΔE‡=-0.82 eV) and TS3 (ΔE‡=-0.72 eV) are comparable, but the following N2(ad) activation step in Ta3N3H−/N2 is with much lower activation energy (ΔEa=0.28 eV for TS1) than that in Ta3N3−/N2 (0.70 eV for TS3); the value of kconversion(I1→TS1) is 30 times faster than kconversion(I3 → TS3), as given in Table S2. Note that in literature, strong adsorption of N2 over catalysts is believed to be important to obtain small activation energy.99 Our study indicates that when the adsorption modes of N2 are the same, facilitating the π-back-donation and weaken the N2 adsorption energy by modifying active sites are favorable for the following steps.

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activation. Relevance of the present tantalum nitride clusters to the organometallic Ta-nitride-hydride complexes is provided, as discussed below, which is quite important to understand how to modify active sites by adding H atoms.

According to the B3LYP-calculated natural charges on several atoms along the reaction coordinates, Figure 4b and c, the electrons required to reduce N2 come from the Ta atoms, and the total transferred amounts are 2e. Differently, two and three Ta atoms are the electron donors in Ta3N3H− and Ta3N3−, respectively, which further supports that two and three Ta atoms are the active sites in the corresponding N2 activation reactions. Due to the presence of the hydrogen atom in Ta3N3H−, there is one three-coordinated N atom, but no two-coordinated nitrogen bridge between the Ta5 and Ta6 atoms, in comparison with Ta3N3−. More electrons are stored in Ta5– Ta6 bond for Ta3N3H− than Ta1–Ta3 for Ta3N3−, as indicated by the different bond orders of these two bonds (1.7 vs. 1.3, respectively, Table S1). Therefore, although the hydrogen atom in Ta3N3H− is not directly involved in the reaction processes and seems like a spectator ligand, its very presence is crucial in the N2 fast reduction process: decreasing the adsorption energy and storing more electrons in Ta–Ta bond.

A Correlation with the Condensed Phase System: The complex ([NPN]Ta)2(μ-H)4 ([NPN] = PhP(CH2SiMe2NPh)2) has been extensively investigated; Ta–N, Ta–H, Ta–Ta, and P→Ta coordination bonds constitute the reactive core.13 In its reaction toward N2, the side-on end-on dinuclear bonding mode ([NPN]Ta(μ-H))2(η1:η2-N2) is formed,13,14 and an N–N bond distance and an N–N stretching frequency are 132 pm and 1165 cm-1, respectively, which is the strong activation according to the ability of the metal center(s) to reduce or ‘activate’ the N–N bond.12 Based on one conceptual framework outlined by Fryzuk et. al, two electrons stored in Ta–Ta bond is a prerequisite for N2 reductive transformation, and is of paramount important to broke the N ≡ N bond completely.14 In our gas-phase model systems Ta3N3H0,1−, Ta–N, Ta–H, and Ta–Ta bonds exist. Moreover, the η1:η2-N2 mode was also located along the reaction coordinates of Ta3N3H0,1− reacting with N2 (I2 and I4, I5 in Figure 3); the N–N bond distances and the N– N stretching frequencies (1113, 1190, and 1108 cm-1, respectively) in these intermediates, which reflect the extent of N2 activation, are similar with the those values reported in ([NPN]Ta(μ-H))2(μ-η1:η2-N2)14. Furthermore, the natural charge analysis indicates that the amounts of transferred charges from Ta3N3H− and Ta3N3− to N2 are 2e, but the electrons originate from two Ta atoms (Ta–Ta bond) and three Ta atoms, respectively. In Ta3N3H−/N2 model system, the part of this charge transfer picture is more similar with the one depicted in ([NPN]Ta(μ-H))2(μη1:η2-N2) complex. Differently, the η1:η2-adsorbed N2 unit in the gas-phase clusters can be activated completely to form Ta3N5H0,1−. However, may be due to the steric hindrance, saturated coordination environment of the Ta atoms and other factors, the formation of a dinitrogen complex ([NPN]Ta(μ-H))2(μ-η1:η2-N2) was obtained. Through the comparison of these two cases, the gas-phase model mimics the condensed-phase reaction to a certain extent.

Quite recently, the Ta2C4− cluster was reported to be able to construct a C–N bond in the reaction with N2.55 Several distinct differences between Ta2C4−/N2 and Ta3N3H0,1−/N2 exist: 1) Due to the different bonding characteristics of C and N atoms, two C2 ligands symmetrically bound to their respective Ta atoms in Ta2C4−, and its Ta–Ta distance (246 pm) is 20 pm shorter than those values in Ta3N3H0,1− (269 and 263 pm, respectively). The smaller ligands in Ta3N3H0,1− render the initially generated adduct complexes, I1 and I3, more stable (≥1.1 eV) than that in Ta2C4−/N2 system (0.53 eV). 2) In Ta2C4−, the formation of C–N triple bond is the emphasis, and the two C2 ligands engineer the metal centers favoring the N2 coordination and reduction. In this study, N2 reduction and the important role of hydrogen atom are the key points. 3) Metal hydriÞ bonds generally exist in kinds of transition metal systems (homogeneous, heterogeneous, and biological), capable of achieving N2

In homogeneous, heterogeneous and biological systems, transition metal hydrides (or hydride bonds), either terminal or bridging, are quite closely involved in N2 activation reaction, and the most recurrent feature in N2 hydrogenative cleavage appears to be the involvement of such bonds.6,15-17 These compounds can exist as subunits in the reactants,6,100,101 or as transient species during the preparation of NH3.102,103 It is highly desirable to reveal the crucial mechanistic roles of these metal hydride bonds at the reactive sites and related mechanisms. There are several key roles of metal hydride bonds from the literature survey:6 1) as a source of electrons for N2 reduction, 2) as a source of hydrogen, 3) as a strong reducing agent to remove activated nitrogen atoms, and so on. Much attention has been focused on the hydride atoms bonded with the reactive metal atoms, which directly participate to the N2 activation processes. The modifications of geometry


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and charge distribution of the active sites by H atoms constitute a rare example and a new function of metal hydride bonds has been identified. Thus, when design new catalysts for N2 activation or improve existing ones, it is quite necessary to modify the immediate atomic environment as well as the central atomic structure of the active site.

4.

CONCLUSION

The reactions of Ta3N3H0,1− clusters with N2 have been investigated both experimentally and computationally. For the N2 adsorption, the reactivity of Ta3N3H− is higher than that of Ta3N3−. The photoelectron spectroscopy results and DFT calculations support that the adsorbed N2 molecules at Ta3N3H− and Ta3N3− clusters are completely activated (a dissociative mechanism), and the former reaction is more favorable kinetically and thermodynamically than the latter one. These variations are induced by the different charge distributions and geometries as well as the higher HOMO energy caused by the presence of the additional hydrogen atom in Ta3N3H− cluster, and decreasing the adsorption energy of N2 and storing more electrons in Ta– Ta bond are two important effects of the H atom to the reactivity of Ta3N3H− toward N2. To the best of our knowledge, this is the first example of complete activation of N2 in well-defined gas-phase nitride cluster anions under room temperature, and this work also highlights the significance of hydrogen-assisted reactivity, which provides molecular-level insights into the rational design of synthetic catalysts for N2 fixation and activation as well as NH3 synthesis. ASSOCIATED CONTENT Supporting Information.

Details of additional experimental and theoretical results (spectra, data analysis, and DFT calculated structures and reaction mechanisms)

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] *[email protected]

ORCID André Fielicke: 0000-0003-0400-0932 Lianrui Hu: 0000-0002-4044-5640 Jiabi Ma: 0000-0002-7428-0231

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT Dedicated to Professor Helmut Schwarz on the occasion of his 75th birthday. We particularly appreciate Professor Helmut Schwarz and Dr. Bin Yang for valuable discussions, as well as an anonymous reviewer for persistent and insightful comments on the photoelectron spectra. This work was supported by National Key R&D Program of China (No.

2016YFC0203000), the National Natural Science Foundation of China (Nos. 21503015 and 21833011), and the Fundamental Research Funds for the Central Universities (No. 2017CX01008). AF thanks the Deutsche Forschungsgemeinschaft for support (Fi 893/3 and Fi 893/5).

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