Reactivity of Tantalum Carbide Cluster Anions TaCnâ (n = 1â4) with

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A: Molecular Structure, Quantum Chemistry, and General Theory n-

Reactivity of Tantalum Carbide Cluster Anions TaC (n = 1-4) with Dinitrogen Li-Hui Mou, Qing-Yu Liu, Ting Zhang, Zi-Yu Li, and Sheng-Gui He J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b01329 • Publication Date (Web): 16 Mar 2018 Downloaded from http://pubs.acs.org on March 17, 2018

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The Journal of Physical Chemistry

Reactivity of Tantalum Carbide Cluster Anions TaCn− (n = 1−4) with Dinitrogen Li-Hui Mou†,‡ Qing-Yu Liu,† Ting Zhang,†,‡ Zi-Yu Li,*,† and Sheng-Gui He*,†,‡

†

Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural

Chemistry of Unstable and Stable Species, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

ACS Paragon Plus Environment 1

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Dinitrogen activation/fixation is one of the most important and challenging subjects in synthetic as well as theoretical chemistry. In this study, the adsorption reactions of N2 onto TaCn− (n = 1−4) cluster anions have been investigated by means of mass spectrometry in conjunction with density functional theory calculations. Following the experimental results that only TaC4− was observed to adsorb N2, theoretical calculations predicted that TaC4− reaction system (TaC4− + N2 → TaC4N2−) has a negligible barrier on the approach of N2 molecule while insurmountable barriers are located on the reaction pathways of TaC1-3−/N2 reaction systems. The natural bond orbital analysis and molecular orbital analysis indicate that the more positive charge on the metal center of TaC1-4− would facilitate the initial approach of the nonpolar N2 molecule and the appropriate frontier orbital of TaC1-4− with proper symmetry (π-type 5d orbital) which can match up well with the π* antibonding orbital of the N2 molecule with less σ repulsion and more possibility for π back-donation would be helpful for the formation of the stable encounter complexes. This study reveals the fundamental rules and key factors governing the N2 adsorption.


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1. INTRODUCTION Dinitrogen is the almost exclusive nitrogen source for biomolecules, fertilizers, medicines and other chemicals, hence fixing and converting the abundant dinitrogen molecules into other useful nitrogenous compounds are of great importance in many areas.1−3 However, the highly strong N≡N bond (bond energy of 9.75 eV), the remarkably high ionization potential (15.84 eV) and the exceptionally large HOMO–LUMO gap (10.82 eV) render most of the abiotic nitrogen fixation processes energy-intensive.4 It is worth noting that the Haber-Bosch process which requires a high temperature (400 °C) and pressure (15-25 MPa) remains the most common and the only largescale industrial process for ammonia production.5−7 Understanding N2 activation/fixation at an atomic, molecular and electronic state level is very important for future development of ambient process of N2 transformation. Gas-phase clusters are regarded as an ideal model to mimic active sites of condensed/surface catalysts because of the relatively well-defined structures, size-dependent properties, and atomic connectivities.8−12 Over the years, the cluster studies on nitrogen activation and fixation under mild conditions were proved to be a challenging and elusive project for researchers. Extensive investigations have been carried out to find apt catalysts and elucidate the efficient processes of N2 activation and transformation.13−16 It is found that an active reagent needs to be electron-rich or have electron-donating ligands to increase the electron density on the metal center, as thus more than one valence electron occupies (or has easy access to) the frontier orbitals which have the matched symmetry and energy to interact efficiently with the π* antibonding orbital of N2.17−20



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Drawing inspiration from the nitrogenase enzyme which carries Fe, Mo, and V at its active site,21−23 a lot of inorganic metal clusters, such as main-group and transition-element bare atoms,24−26 dimers,27−29 transition metal oxides,30−34 and other metal complexes, have been explored and demonstrated to cleave the N≡N triple bond to form nitride compounds or to generate ammonia catalytically. The most remarkable work among them was the catalytic ruthenium studied by Aika and co-workers, 35 making ruthenium-based ammonia synthesis catalyst become second-generation catalyst which showed high activity at low temperature and pressure.36, 37 It has been shown that, as anticipated according to the Dewar−Chatt−Duncanson model,38 the 4d75s1 electron configuration of Ru leads to less σ repulsion and more possibility for π back-donation, thus making it more reactive. Nevertheless, the rare metal properties of ruthenium greatly limit its potential as a catalyst. Transition metal carbides and nitrides (TMC/TMN), due to high melting point, long life span, and

great

anti-toxicity,

along

with

the electronic surface

properties and

catalytic

activities resembling those of precious metals such as ruthenium and platinum, are known as potential substitutes for noble metal catalysts.39−41 Hopefully, study on the reactions of TMC/TMN clusters with dinitrogen may help to find new catalysts for dinitrogen activation and transformation under ambient conditions, and reveal the fundamental rules and key factors including the electronic structures and the ligand effect on the metal centers that governing the reactivity toward the activation of dinitrogen, which may provide important basis in practice for the replacement of


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noble metal catalysts by TMC/TMN materials. However, the investigations on the activation of dinitrogen by TMC/TMN clusters remain quite limited.27−30,43 In this work, the adsorption of N2 onto anionic TaCn− (n = 1−4) clusters was investigated via experimental studies in combination with density functional theory (DFT) calculations. Specifically, this work included characterization of N2 adsorption onto TaCn− (n = 1−4) clusters, determination of adsorption rate constants, prediction of the structures of TaCn− (n = 1−4) and exploration of reaction mechanisms, which would provide reference for the further research on dinitrogen activation by metal carbide clusters.

2. METHODS 2.1. Experimental methods A reflectron time-of-flight mass spectrometer (TOF-MS)44 in conjunction with a laser ablation cluster source, a quadrupole mass filter (QMF)45 and a linear ion trap (LIT) reactor46 was used to study the adsorption of N2 onto TaCn− (n = 1−4) clusters. The tantalum carbide cluster anions were generated by laser ablation of a rotating and translating tantalum metal disk in the presence of 0.5% CH4 seeded in a He (99.99% purity) carrier gas with a backing pressure of about 6.5 standard atmospheres. In particular, a 532 nm (second harmonic of Nd3+: yttrium aluminum garnet) laser with an energy of 5−8 mJ/pulse and a repetition rate of 10 Hz was used. The cluster anions of interest (TaC1-4−) were mass-selected by the QMF, where appropriate combinations of



DC (direct current) and RF (radio frequency) potentials were applied to the quadrupole rods. The mass-selected cluster ions entered into the LIT reactor formed by a set of hexapole rods and two cap electrodes. Slightly before the cluster ions enter into the trap, a pulse of helium gas is delivered through a second valve into the trap so that the clusters can be thermalized. After confining the cluster ions in the trap for about 0.9 ms, the reactant gas molecules (N2) are delivered through a third pulsed valve to interact with the clusters. It has been demonstrated that the ions were thermalized to room temperature (298 K) before reactions.47−49 After interacting for about 7 ms, the reactant and product ions ejected from the LIT were transferred into the TOF−MS for mass and intensity measurements. 2.2 Computational methods The DFT calculations using Gaussian 09 program package50 were carried out to investigate the structures of TaCn− (n = 1−4) as well as the adsorption mechanisms with N2. With the aug-ccPVQZ basis set for C as well as N atoms, and the effective core potential combined with aug-ccPVQZ for Ta atom, the bond dissociation enthalpies (D0) of C2, Ta-N, N2, C-N molecules, adiabatic ionization energy of Ta atom, and adiabatic electron detachment energy of TaC− were calculated by various functionals and compared with the experimental data (Table S1). The B3LYP functional was found to be the best, and it was proved to have reliable predictions on the geometries and vibrational frequencies of early transition metal complexes.32, 51-54 To obtain reliable relative energies of the transition states (TSs), the single-point energy calculations at the high level quantum chemistry method of coupled-cluster method with single, double, and perturbative triple


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excitations55 [CCSD(T)] were performed at the B3LYP optimized geometries. The low-lying structures of TaCn− (n = 1−4) cluster anions were obtained by optimizing the initial guess geometries generated by placing all atoms at random. The optimized structures with the lowest energies were considered as the species to adsorb N2 in the experiment. The relaxed potential energy surface scans were used to obtain the initial structures of reaction intermediates (IMs) and TSs along the reaction pathways. Vibrational frequency calculations were carried out to check that the IMs and TSs have zero and only one imaginary frequency, respectively. The TSs were optimized using the Berny algorithm and then verified to connect two appropriate local minima by intrinsic reaction coordinate (IRC) calculations. The natural bond orbital (NBO) analysis and molecular orbital analysis were also employed to interpret the experimental observation. Specifically, the molecular orbital analysis was based on the NBO orbitals. Note that the unrestricted Kohn-Sham calculations were performed for all species in this work since some intermediates were found to have biradicaloid electronic structures which can be either triplets or open-shell (broken-symmetry) singlets.56-57

3. RESULTS AND DISCUSSION Experimental Results. The TOF mass spectra for the interactions of laser ablation generated, mass-selected, confined and thermalized TaCn− (n = 1−4) cluster anions with N2 are shown in Figure 1. Note that the weak signals marked with asterisk shown in Figure 1b-1d are due to the



reaction with a trace amount of water impurity in the LIT. Upon the interaction of TaC1-3− with N2, no product peaks appear, as shown in Figure 1a-1c. While for TaC4−/N2 system in Figure 1d, an obvious product peak marked with arrow corresponding to TaC4N2− is produced. And a high resolution TOF mass spectra of the reaction between TaC4− and N2 is shown in Figure S1 in the Supporting Information. From the experiment, we can conclude that only TaC4− was capable of adsorbing N2 under thermal conditions.

Figure 1. Time-of-flight mass spectra for reactions of mass-selected (a) TaC−, (b) TaC2−, (c) TaC3−, and (d) TaC4− with N2. The reactant gas pressure is shown and the reaction time is 7.0 ms. The strong peak marked with arrow in (d) can be assigned as N2 adsorption product, and weak signals marked with asterisk are due to the residual water in the gas handling system.


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Equation (1)46 was used to determine the pseudo-first-order rate constant (k1) of the reaction between N2 and TaC4− cluster, in which IR is the signal intensity of the reactant cluster ions, IT is the total ion intensity including product ions, kB is the Boltzmann constant, T is the temperature (298 K), tR is the reaction time and P is the effective pressure of the reactant gas in the ion trap reactor. ln

I I

R T

  k1

P t k BT R

(1)

The rate constant k1 for TaC4− + N2 → TaC4N2− was estimated to be 3.25 × 10−14 cm3 molecule−1 s−1, according to equation (1).

Structures and Energetics of TaCn− (n = 1−4). The DFT optimized structures with the lowest energies and a few low-lying isomeric structures of TaCn− (n = 1−4) are presented in Figure 2. The DFT calculations predicted that the most stable structures of TaC− (1IS1), TaC3− (1IS4) and TaC4− (1IS8) have the singlet electronic states, whereas TaC2− (3IS2) has the triplet ground state. In addition, the ground states of TaC2−, TaC3− and TaC4− were predicted to have the cyclic structures (C2v), and yet, TaC− is a linear structure (C∞v).



Figure 2. DFT calculated isomeric structures of (a) TaC−, (b) TaC2−, (c) TaC3− and (d) TaC4−. The relative energies in electronvolts are listed under each structure. The superscripts indicate the spin multiplicities. The bond lengths in picometer are given for the most stable structures. Reaction Mechanisms. The potential energy curves for N2 adsorption onto TaCn− (n = 1−4) are given in Figure 3, and the relative energies of IMs and TSs with respect to the separate reactants are listed in Table 1. It is obvious that the transition state for the N2 adsorption onto TaC4− is much lower in energy than that of other reaction systems (TaC1-3−/N2), which is consistent with the only adsorption behavior of TaC4− observed experimentally. Moreover, additional DFT calculations for IMs and TSs under other functionals levels are shown in Table S2.


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Figure 3. Partial potential energy curves calculated by DFT for the interaction of TaC−(1IS1), TaC2−(3IS2), TaC3−(1IS4), and TaC4−(1IS8) with N2. The superscripts indicate the spin multiplicities.

For TaC−, when a N2 molecule approaches to the TaC− (1IS1) cluster, the N2 is slightly trapped by the Ta atom to form the intermediate 1I1 with a binding energy of0.04 eV. However, the adsorption is impeded by a high energy barrier (1TS) of 0.17 eV (0.24 eV at [CCSD(T)] level). Furthermore, the unrestricted Kohn-Sham calculations were also taken into consideration. It is proved that the encounter complex I2 is more stable in the triplet state (3I2) with a binding energy of 0.54 eV (relative to 1IS1 and N2) than that in open-shell singlet (broken-symmetry) state (1I2, 0.37 eV). This means that a spin conversion58-60 occurs during the formation of the stable intermediate 3I2. The crossing point (CP) from 1TS1 to 3I2 is located higher than the separate reactant in energy. More details can be found in Figure S2 in the Supporting Information. However,



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Table 1. DFT calculated zero-point vibration corrected energies (∆H0 in eV) of adsorption intermediates (IMs) and transition states (TSs) with respect to the separate reactants [N2 and TaCn− (n = 1−4)].

Clusters 1

3

1

1

IMs

−

TSs

IMs

1I1

1TS1

1I2[BS]

3I2

-0.04

0.17 (0.24)

-0.37

-0.54

3I3

3TS2

3I4

-0.03

0.14 (0.25)

-1.06

1I5

1TS3

1I6

3I6

-0.01

0.27 (0.51)

-0.17

-0.62

1I7

1TS4

1I8[BS]

3I8

-0.02

0.08 (0.05)

-0.62

-0.79

IS1 (TaC )

−

IS2 (TaC2 )

−

IS4 (TaC3 )

−

IS8 (TaC4 )

Energies in round brackets are at the [CCSD(T)] level. The superscripts indicate the spin multiplicities. [BS] denotes the broken-symmetry state. the high energies of the TS and CP lead to the dissociation rather than the adsorption in the experiment (TaC− + N2 → [TaCN2−]* → TaC− + N2).

For TaC2−, when a N2 molecule meets the TaC2− (3IS2) cluster, the N2 is scarcely trapped by the Ta atom, resulting in the formation of 3I3 with a binding energy of only 0.03 eV. Even though the next intermediate 3I4 has a high binding energy of 1.06 eV, the barrier (3TS2) of 0.14 eV (0.25 eV at [CCSD(T)] level) opposing the conversion from 3I3 to 3I4 is difficult to be overcome, which gives rise to no adsorption in experiment (TaC2− + N2 → [TaC2N2−]* → TaC2− + N2). Analogously,


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the N2 molecule is loosely bound to the TaC3− (1IS4) cluster and the intermediate 1I5 with a binding energy of 0.01 eV is generated. Nevertheless, the adsorption is opposed by a large energy barrier (1TS3) of 0.27 eV (0.51 eV at [CCSD(T)] level). As the same to TaC−/N2 system, a spin conversion occurs from TS3 to form a more stable intermediate 3I6 with the crossing point located upon the separate reactant, as seen in Figure S2 in the Supporting Information. Therefore, the 1I5 prefers to dissociate (TaC3− + N2 → [TaC3N2−]* → TaC3− + N2) instead of forming the stable intermediate neither in singlet (1I6) nor in triplet (3I6).

For TaC4−, as seen in Figure 4a, along with the approaching of N2 molecule toward TaC4− (1IS8), the encounter complex 1I7 with the weak interaction and small binding energy of 0.02 eV is formed, and then a negligible barrier (1TS4) with the energy of 0.08 eV (0.05 eV at [CCSD(T)] level) can be easily overcome with the internal and kinetic energies of the reactants to form a more stable intermediate I8. It is worth noting that this intermediate is more stable in triplet (3I8) with the binding energy of 0.79 eV than that in the open-shell singlet (1I8 [BS]) with the binding energy of 0.62 eV. Therefore, a spin conversion occurs before the formation of the stable intermediate (3I8). As shown in Figure 4b, the crossing point (CP) takes place along 1TS4 to 3I8 with an energy lower than the separate reactants by 0.01 eV. It is noteworthy that the N−N bond is lengthened from 109 pm in the free N2 molecule to 114 pm in TaC4N2− (3I8). The negligible transition state and the low energy of the crossing point well support the experimentally observed N2 adsorption onto TaC4− under thermal conditions.



Figure 4. (a) DFT calculated structures of adsorption intermediates and transition states for the interaction of TaC4− with N2. (b) Partial potential energy curves (PECs) calculated by DFT for spin conversion occurring in 1TS4 → 3I8. The filled circle line is the relaxed PEC obtained by IRC calculations starting from the separate reactants (1IS8 + N2) to 1I8. The optimized geometries (singlet) from the filled circle line were used for single-point energy calculations of the triplet (3IS8 + N2, empty circle line). The energy of the crossing point (CP) relative to the separate reactants is given.


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NBO Analysis. Adsorption of N2 onto TaCn− (n = 1−4) cluster anions was then investigated by charge analysis using the NBO method.61 The natural charges on Ta and N atoms as well as the metal valence electron population on Ta atom are presented in Table 2. NBO charge rather than Mulliken charge was adopted in this study because of its greater reliability on the large basis set for Ta atom.62 It can be seen that the natural charge on Ta center is much higher in 1IS8 than those found in 1IS1, 3IS2 and 1IS4 structures, which can contribute to an easier approach of N2 to TaC4− in the adsorption process. In addition, the natural electron configurations calculated by NBO analysis clearly exhibit the variations in electronic occupation of the s and d atomic orbitals of the Ta atom in TaCn− (n = 1−4) clusters. Concretely, a 6s1.315d3.01 electronic configuration is observed in 1IS8, whereas more s and/or less d occupations are found in 1IS1, 3IS2 and 1IS4. This difference may be taken as an indication of the higher reactivity of TaC4− (as compared to TaC1-3−) in N2 adsorption process, where the presence of electrons in s orbitals gives rise to a higher electrostatic repulsion for the closed-shell system than the electrons in d orbitals.17 The reason for this is, in the entrance channel of the reaction, the spherical symmetry of s orbitals provides a higher electrostatic repulsion to the apolar closed-shell dinitrogen, which can be intuitively reflected by Figure 3. Since the s orbital is almost fully occupied, the repulsive interaction arises when the dinitrogen molecules meet TaC1-3− clusters, resulting in an inefficient reaction due to the introduction of an activation barrier. To be more convinced, the analysis of 3IS8 and 3I8 were also performed. It is clear that more positive charges and less s occupation on Ta atom in 3IS8 than those in 1IS8 lead to the



Table 2. Natural Charges on Ta and N atoms and valence electron population on the Ta atom analyzed by natural bond orbital (NBO) analysis.

species

natural charge Ta

N

metal natural electron configuration

IS1 (TaC−)

-0.10

6s( 1.70)5d( 3.16)

3

IS2 (TaC2−)

0.08

6s( 1.61)5d( 2.94)

1

IS4 (TaC3−)

0.23

6s( 1.48)5d( 2.96)

1

IS8 (TaC4−)

0.42

6s( 1.31)5d( 3.01)

3

IS8 (TaC4−)

0.58

6s( 0.69)5d( 3.29)

I8(TaC4N2−)

0.63

1

3

-0.21

barrierless process of N2 adsorption on 3IS8 and a larger binding energy for 3I8 than 1I8. Furthermore, electrons transferring from the Ta atom to N2 molecule in the formation of the stable intermediate 3I8 is also observed. NBO bond order analysis was also performed to understand the activation of N2. The result reveals that the bond order of Ta−N is 1 in TaC4N2− (3I8), while that of N−N bond decreases from 3 in the free N2 molecule to 2 in the adsorption complex TaC4N2− (3I8). With the fact that the cleavage of N−N bond does not occur, it can be concluded that the N2 is chemically adsorbed and nondissociatively adsorbed in TaC4N2−. The NBO analysis indicate that the natural charge and electronic configuration which can be tuned by the different Cn ligands in TaCn− (n = 1−4) are the key factors governing the N2 adsorption and activation.

Molecular Orbital Analysis. The DFT calculations of reaction pathways for TaCn− (n = 1−4) and N2 indicate that TaC4− reaction system has a lower TS than those of TaC1-3− reaction systems.


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To further understand the adsorption mechanism, the molecular orbital analysis was carried out for the separate reactants [TaCn− (n = 1−4) and N2] and reaction intermediates.63 It turns out that the nature of active orbitals plays an important role in the reactivity of TaCn− (n = 1−4) clusters toward N2. Firstly, the π−type orbitals of Ta atom including 5dxz, 5dxy, 5dx2-y2 and 5dyz make a great contribution to the binding of N2 molecule. As shown in Figure 5a, the lowest unoccupied molecular orbital (LUMO) of N2 is π* antibonding type. For TaC4− (3IS8), the α−HOMO is mainly composed of the Ta 5dxz orbital (68.91%), which has the matched symmetry with the π* antibonding orbital of N2, thus leading to an effective orbital overlap together with electron transfer from the Ta atom primarily to N2 molecule followed by the π back-donation. Similar orbital interactions can also be identified in the stable intermediate 3I4 in TaC2−/N2 reaction systems in Figure 5b. Secondly, it is revealed that the composition and relative position of 5d and 6s orbitals among the frontier orbitals have a great influence on N2 adsorption process especially on the energy height of transition state. Take 3IS8, 3IS2 and 1IS8 as an example and the difference in detail is shown in Figure 5c. Among the frontier orbitals, the α−HOMO of 3IS8 has a high composition of π−type Ta 5d orbital (5dxz, 68.91%), which has a favorable orbital overlap with π* antibonding orbital of N2. In contrast, it is the α−HOMO−2 of 3IS2 that is dominated by Ta 5dxy orbital (94.98%), while the α−HOMO is composed of 5dyz orbital with a less percentage of 58.06%. Therefore, the α−HOMO−2 has priority over the α−HOMO to interact with the π* antibonding orbital of N2, yet leading to more σ repulsion and an energetically unfavorable TS, which is consistent with the reaction pathway that the adsorption of N2 onto TaC2− is hampered by the



Figure 5. Orbital overlaps between triplet TaC4− (3IS8), triplet TaC2− (3IS2) and N2 are shown in (a) and (b) respectively. Comparison on frontier orbitals of 3IS8, 3IS2 and 1IS8 are presented in (c). The related orbital compositions are given. intrinsic energy barrier. For 1IS8, not very high composition of Ta 6s and π−type 5d in frontier orbitals (HOMO: Ta 6s 27.96%, 5dz2 9.94%; HOMO-1: Ta 6s 17.85%, 5dx2-y2 35.08%) result in a small TS and relatively low binding energy of 1I8 respectively. The above rules can also be used to explain the high energies of barriers and more stable triplet intermediates in both TaC− and TaC3− systems, as is shown in Figure S4. Specifically, the percentage of 6s orbital in the HOMO, HOMO−1 in 1IS1 as well as the HOMO in 1IS4 is 28.76%, 56.60% and 41.87% respectively, which is relatively high and results in large σ repulsion during N2 approaching and an unfavorable transition state (Figure S4a and 4d). While for 3IS1 and 3IS4, more 5d and less 6s compositions in the frontier orbitals than the corresponding singlet states are identified, thus leading to effective orbital overlaps and more stable triplet intermediates (Figure S4b and 4c). Therefore, the orbital


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analysis reveals that the composition and relative position of 5d and 6s orbitals on the Ta center which can be regulated by the Cn ligands of TaCn− (n = 1−4) clusters play vital roles in N2 adsorption and activation.

4. CONCLUSION The reactions of mass-selected TaCn− (n = 1−4) cluster anions with N2 have been investigated by mass spectrometry in conjunction with density functional calculations. Only TaC4− was observed to adsorb N2 experimentally. The theoretical calculations predicted that the ground states of TaC2−, TaC3− and TaC4− have the cyclic structures (C2v). Natural bond orbital analysis showed that a more positive natural charge and a less 6s electron occupancy of the Ta atom in TaC4− result in an easier approach of N2 in the adsorption process. In addition, the molecular orbital analysis indicated that the π−type 5d orbital of the Ta atom in TaC4−, which can overlap well with π* antibonding orbital of N2, is a pivotal factor for the strength of N2 adsorption. In a word, the Cn ligands can tune the natural charge, the electron configuration, the composition and the relative position of 5d/6s orbitals of Ta atom, thus being able to control the reactivity of TaCn− (n = 1−4) towards N2. This combined experimental/computational study provides an electronic and molecular level understanding of the fundamental mechanisms of dinitrogen activation by metal carbide species.



ASSOCIATED CONTENT Supporting Information Tests on various functionals, additional TOFMS, DFT calculations and molecular orbital analysis.

AUTHOR INFORMATION Corresponding Author *S.-G. He. E-mail: [email protected]. *Z.-Y. Li. E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 21603237 and 21325314)

REFERENCES (1) Bezdek, M. J.; Chirik, P. J. Expanding Boundaries: N2 Cleavage and Functionalization beyond Early Transition Metals. Angew. Chem. Int. Ed. 2016, 55, 7892–7896. (2) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. Advanced Inorganic Chemistry, 6th Edition; Wiley: New York, 1999.


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TOC Graphic


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graphic for TOC 135x57mm (150 x 150 DPI)

ACS Paragon Plus Environment


Figure 1. Time-of-flight mass spectra for reactions of mass-selected (a) TaC−, (b) TaC2−, (c) TaC3−, and (d) TaC4− with N2. The reactant gas pressure is shown and the reaction time is 7.0 ms. The strong peak marked with arrow in (d) can be assigned as N2 adsorption product, and weak signals marked with asterisk are due to the residual water in the gas handling system. 150x180mm (300 x 300 DPI)


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Figure 2. DFT calculated isomeric structures of (a) TaC−, (b) TaC2−, (c) TaC3− and (d) TaC4−. The relative energies in electronvolts are listed under each structure. The superscripts indicate the spin multiplicities. The bond lengths in picometer are given for the most stable structures. 129x111mm (300 x 300 DPI)



Figure 3. Partial potential energy curves calculated by DFT for the interaction of TaC−(1IS1), TaC2−(3IS2), TaC3−(1IS4), and TaC4−(1IS8) with N2. The superscripts indicate the spin multiplicities. 297x210mm (300 x 300 DPI)


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Figure 4. (a) DFT calculated structures of adsorption intermediates and transition states for the interaction of TaC4− with N2. (b) Partial potential energy curves (PECs) calculated by DFT for spin conversion occurring in 1TS4 → 3I8. The filled circle line is the relaxed PEC obtained by IRC calculations starting from the separate reactants (1IS8 + N2) to 1I8. The optimized geometries (singlet) from the filled circle line were used for single-point energy calculations of the triplet (3IS8 + N2, empty circle line). The energy of the crossing point (CP) relative to the separate reactants is given. 85x121mm (300 x 300 DPI)



Figure 5. Orbital overlaps between triplet TaC4− (3IS8), triplet TaC2− (3IS2) and N2 are shown in (a) and (b) respectively. Comparison on frontier orbitals of 3IS8, 3IS2 and 1IS8 are presented in (c). The related orbital compositions are given. 177x85mm (300 x 300 DPI)


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