Nitrogen adsorption on Nb₂C₆H₄⁺ cations: the important role of benzyne (ortho-C₆H₄)

Abstract

N₂ adsorption is a prerequisite for activation and transformation. Time-of-flight mass spectrometry experiments show that the Nb₂C₆H₄⁺ cation, resulting from the gas-phase reaction of Nb₂⁺ with C₆H₆, is more favorable for N₂ adsorption than Nb⁺ and Nb₂⁺ cations. Density functional theory calculations reveal the effect of the ortho-C₆H₄ ligand on N₂ adsorption. In Nb₂C₆H₄⁺, interactions between the Nb-4d and C-2p orbitals enable the Nb₂⁺ cation to form coordination bonds with the ortho-C₆H₄ ligand. Although the ortho-C₆H₄ ligand in Nb₂C₆H₄⁺ is not directly involved in the reaction, its presence increases the polarity of the cluster and brings the highest occupied molecular orbital (HOMO) closer to the lowest occupied molecular orbital (LUMO) of N₂, thereby increasing the N₂ adsorption energy, which effectively facilitates N₂ adsorption and activation. This study provides fundamental insights into the mechanisms of N₂ adsorption in “transition metal–organic ligand” systems.

---

1. Introduction

Dinitrogen (N₂) is the most significant and abundant source of various N-containing compounds.^1–5 However, the transformation of N₂ at room temperature remains quite challenging due to its chemical inertness, the high HOMO–LUMO energy gap, large triple bond energy (945 kJ mol⁻¹) and lack of a permanent dipole.^6,7 In nature, nodule bacteria efficiently perform biological nitrogen fixation at ambient temperatures.⁸ In the condensed phase, enhancing N₂ chemisorption capability on catalyst surfaces is critical for N₂ reduction.^9,10 Recently, metal–organic complexes, such as metal nitrogen heterocyclic compounds and multi-replacement metallocenes, have been reported to adsorb N₂ and break N≡N bonds under mild conditions.^1,11–17 These unsaturated organic ligands act as electron donors to facilitate N₂ adsorption. For example, a diketiminate-supported iron system that progressively activates benzene and N₂ to produce aniline derivatives has been reported.¹⁸ Despite significant progress in N₂ adsorption, the detailed reaction mechanisms and key influencing factors remain unclear. Furthermore, transition metal benzyne (TM-benzyne) complexes usually serve as reactive intermediates in organic synthetic chemistry.^19–22 It is interesting to explore the reactivity of TM-benzyne toward N₂.

Gas-phase studies on “isolated” reactants provide an ideal venue for investigating reaction mechanisms and kinetics at a strictly molecular level.^7,23–26 Recently, some encouraging developments in N₂ activation have been reported in the gas phase studies,^27–31 and the ligand effect significantly affects cluster reactivity. Gas-phase clusters usually contain metal atoms coupled with inorganic ligands, such as oxygen, boron, hydrogen, carbon, sulfur, and so on.^29,32–35 For example, we recently reported that in NbB₃O₂⁻, the B₃O₂ ligand and the single Nb center can form a dual active site to promote N₂ activation and transformation.²⁹ In the NbH₂⁻/CO₂/N₂ system, the NbN₂⁻ anion is formed by the reaction of NbH₂⁻ with N₂, and the N≡N triple bond is cleaved with the formation of the C–N bond in the reaction of NbN₂⁻ with CO₂.³³ Notably, only limited studies are about N₂ absorption and activation mediated by gas-phase ions containing organic ligands.^36–39 He and coworkers recently reported that Fe₂VC(C₆H₆)⁻ can feasibly break the N≡N triple bond.³⁹ Considering the high reactivity of TM-benzyne in the condensed-phase reaction, it is crucial to synthesize isolable TM-benzyne ions and investigate their reactivity toward N₂. Meanwhile, there is an urgent need to identify the interaction mechanism between metal atoms and organic ligands, which may influence the N₂ adsorption rate.

In the present work, the gas-phase TM-benzyne cation Nb₂C₆H₄⁺ is designed and synthesized through the reaction of Nb₂⁺ and C₆H₆. We further investigated its reaction with N₂. The presence of the ortho-C₆H₄ ligand increases the adsorption rate of N₂ on Nb₂C₆H₄⁺ compared to bare Nb₂⁺. It is ascribed to the greater polarity and higher HOMO energy level in Nb₂C₆H₄⁺, which favors N₂ adsorption. Exploring typical clusters possessing metal atoms and organic ligands in the gas phase may provide a novel understanding for N₂ adsorption.

2. Methods

2.1 Experimental methods

Interactions of ions and reactant gases are investigated using a homemade time-of-flight mass spectrometer (TOF-MS) equipped with a laser ablation ion source, a quadrupole mass filter (QMF), and a linear ion trap (LIT) reactor. Only a concise description of the apparatus is presented here, since details of the experiment have been previously covered in the earlier work.^40–42 The Nb_1,2⁺ cations are generated by laser ablation of a niobium disk target (99.999%) in helium carrier gas at a backing pressure of 3.0 standard atmospheres. A 532 nm laser with a repetition rate of 10 Hz as well as 5–8 mJ/pulse energy is employed. The Nb_1,2⁺ cations are mass-selected by QMF, and then reacted with N₂ seeded in the cooling gas (He) first. The reaction between Nb₂⁺ and diluted benzene vapor seeded in the cooling gas (Ar) generates the Nb₂C₆H₄⁺ cluster. N₂ is then introduced into the LIT to investigate the N₂ adsorption reaction. Finally, the LIT releases reactant and product ions, which are subsequently detected by reflection TOF-MS. The previous study showed that cations were thermally stabilized around (or just approaching) 298.15 K prior to various reactions.⁴²

In our experiments, the concentrations of reactant molecules (C₆H₆ or N₂) are much larger than that of reactant cations (Nb⁺, Nb₂⁺, or Nb₂C₆H₄⁺). Thus, the pseudo-first-order reaction model (k₁) is assumed in each reaction. The following equation is applied to calculate the rate constant:⁴²

in which I_R is the reactant intensity after the reaction and I_T equals the sum of both the reactant and product intensities. In addition, k, t_R, and T correspond to the Boltzmann constant, reaction time, and temperature, respectively. P_e is the effective gas pressure of the reactant in the LIT. Finally, the least squares are used to derive the rate constant of each reaction.⁴³ The detailed method is described in ref. 43. The errors of t_R (±5%), T (±2%), and P_e (±20%) are also taken into account to estimate the error bars.

2.2 Computational methods

All density functional theory (DFT) calculations are carried out in the Gaussian 09 D.01 program package.⁴⁴ The bond dissociation energies of Nb–Nb, Nb–C, Nb–N, N≡N, C–C, C–H, C≡N, H–H and N–H bonds are predicted by 20 tested methods, with the M06L functional agreeing well with the experimental data (Table S1, ESI†), and M06L is more adaptable for the group VB transition metal complexes^45–47 and selected in the present work. Meanwhile, the density functional theory dispersion correction (DFT-D3) is included.⁴⁸ The def2-TZVPP basis set is used for the Nb atom,^49,50 and the 6-311+G** basis sets^51–53 are chosen for the H, C, and N atoms. Each initial structure is optimized for various possible spin multiplicities. Flexible scanning and intrinsic reaction coordinates are used to achieve the optimal geometry of intermediates (Is) and transition states (TSs). Calculations of vibrational frequencies are used to make sure that Is and TSs have zero and only one imaginary frequency, respectively. In each reported energy (ΔH_0K in eV), the zero-point vibration correction is taken into account. The rate constant of a reaction without an energy barrier is computed by variational transition state theory.⁵⁴ NBO 6.0 is used for natural population analysis, which includes natural population analysis (NPA) charge, adaptive natural density partitioning (AdNDP) and percentage of orbital contribution.⁵⁵ For the simulated density of states (DOS), the vertical detachment energies (VDEs) of the related clusters have been calculated using the same structures as their initial cations, corresponding to the energy difference between the cluster's Hartree–Fock energies and neutral species. Each peak is the result of the cation capturing an electron. Multiwfn 3.8 is applied to obtain localized orbital locator-π (LOL-π), electrostatic potential (ESP), interaction region indicator (IRI), DOS, and frontier molecular orbital.^56–60

3. Results

3.1 Experimental results

As shown in Fig. 1, the Nb⁺, Nb₂⁺, and Nb₂C₆H₄⁺ cations generated by laser ablation are mass-selected and subsequently react with N₂ in the LIT reactor. Note that Nb⁺ and Nb₂⁺ cations show relatively low reactivity toward N₂, Fig. 1b and d, reflected by the low intensities of NbN₂⁺ and Nb₂N₂⁺ even at relatively high N₂ pressures (up to 1.1 Pa and 0.8 Pa) and rather long reaction times (up to 13.4 ms and 14.9 ms). Interestingly, when the organic ligand ortho-C₆H₄ is attached on the Nb₂⁺ cation, the adsorption rate is increased dramatically. The Nb₂C₆H₄⁺ clusters can be produced by the mass-selected Nb₂⁺ cations (Fig. 1c) reacting with C₆H₆ molecules. As the majority of Nb₂⁺ is converted to Nb₂C₆H₄⁺ (Fig. 1e, reaction (1)), N₂ is pulsed into the LIT reactor. After the reaction with 1.3 Pa N₂ for approximately 5.0 ms, a significant peak corresponding to Nb₂C₆H₄N₂⁺ is observed in Fig. 1f, suggesting reaction (2). In the mass spectra, the presence of NbO⁺, Nb₂C₆H₄O⁺, and Nb₂C₆H₄C₆H₆O⁺ peaks arises from reactions of residual water impurities with Nb⁺, Nb₂C₆H₄⁺, and Nb₂C₆H₄C₆H₆⁺, respectively.

Fig. 1 Time-of-flight (TOF) mass spectra for the reactions of mass-selected Nb⁺ (a) and Nb₂⁺ (c) with He for 11.4 ms; Nb⁺ (b) and Nb₂⁺ (d) with N₂ for 13.4 ms and 14.9 ms, respectively; for the reaction of (e) the generated intermediate product Nb₂C₆H₄⁺ with N₂ (f) for 5.0 ms. The effective reactant gas pressures are shown. In panel (b), the signal magnitude is amplified by a factor of 40 (m/z = 121).

The pseudo-first-order rate constant (k₁) values of reactions (1) and (2) are estimated to be (1.27 ± 0.26) × 10⁻⁹ cm³ molecule⁻¹ s⁻¹ and (2.60 ± 0.53) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹, respectively, corresponding to the reaction efficiencies (Φ)^61,62 of 126.7% and 0.04%. The results for Φ are consistent for both the surface charge capture and the average dipole orientation models. The rate for the Nb₂C₆H₄⁺/N₂ system is 10 times larger than that of the Nb₂N₂⁺/N₂ [(2.64 ± 0.59) × 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹], and the presence of the C₆H₄ ligand indeed increases the adsorption rate of N₂. The signal dependence of product ions on N₂ pressures and C₆H₆ are derived and fitted to the experimental data (Fig. 2 and Fig. S1, ESI†).

Fig. 2 Variations of the relative intensities of the reactant and product cations in the reactions of Nb₂C₆H₄⁺ (a) and Nb₂⁺ (b) with N₂ with respect to the N₂ pressures for 5 ms and 14.9 ms, respectively. The solid lines are fitted to the experimental data points by using the equations derived from the approximation of the pseudo-first-order reaction model.

3.2 Computational results

DFT calculations are performed to obtain the lowest-energy isomers of Nb₂C₆H₆⁺ (Fig. S2a, ESI†) and Nb₂C₆H₄⁺ (Fig. S3a, ESI†), and the dehydrogenation mechanism for reaction (1) is shown in Fig. 3a. Nb₂⁺ has a quartet spin multiplicity.⁶³ A spin crossover from the quartet potential energy surface (PES) to the doublet PES potentially takes place during the formation of I1 (Nb₂C₆H₆⁺, ΔH_0K = −2.61 eV, with respect to the separate reactants). The first hydrogen (H1) transfers from the C₆H₆ unit in I1 to the Nb atom in Nb₂C₆H₆⁺viaTS1 to generate I2. Then I3 is formed by rotating H1 to the top of the linear Nb–Nb bond through TS2. The relative position of the Nb–Nb unit and C₆H₆ has been changed slightly. In the steps of I3 → I4 → I5, the Nb1–Nb2–H1 unit is further rotated relative to the C₆H₆ unit. It is emphasized that steps I3 → I4 → I5 are crucial to this reaction process because the direct C–H2 (second hydrogen) bond-breaking in I2 is difficult. From I5, the H2 migrates from the C₆H₆ unit to the Nb1 atom, leading to the formation of intermediate I6. ViaTS6, a planar H2–Nb1–Nb2–H1 unit in I7 is generated by rotating the H2 atom. Two hydrogen atoms bonded with Nb1 are combined to form one H₂ unit in I8. Finally, Nb₂C₆H₄⁺ and H₂ are generated. Thus, the dehydrogenation reaction of Nb₂C₆H₆⁺ cations is thermodynamically and kinetically favourable. As shown in Fig. 3b, the C2–C3 and Nb–Nb bonds in Nb₂C₆H₄⁺ are elongated by approximately 20 pm and 47 pm, respectively, in comparison to the separate structures of C₆H₄ and Nb₂⁺. In addition, we have considered benzyne (ortho-C₆H₄) and its isomers (meta-C₆H₄ and para-C₆H₄) in the Nb₂C₆H₄⁺ structures (Fig. S3, ESI†), and Nb₂C₆H₄⁺ given in Fig. 3a and b is the ground state. The dehydrogenation reaction of Nb₂⁺ with C₆H₆ has been previously reported, but the detailed reaction mechanism was not given.^64,65

Fig. 3 M06L-calculated potential energy surfaces (PESs) for the (a) Nb₂⁺ + C₆H₆, and (c) Nb₂C₆H₄⁺ + N₂ reactions, and the spin multiplicities are doublet. The zero-point vibration-corrected energies (ΔH_0K in eV) of the reaction intermediates, transition states, and products relative to the separated reactants are given. (b) Top (upper panel) and side (bottom panel) views of the optimized global minimum structure of the Nb₂C₆H₄⁺ cluster. The blue arrow represents the vector direction of dipole moment. The superscripts indicate the spin multiplicities. The bond lengths are given in pm. The values of unpaired spin density distributions are given in parentheses.

The adaptive natural density partitioning (AdNDP) method is further applied to determine the bonding character of Nb₂C₆H₄⁺. Two types of bonds exist in the Nb₂C₆H₄⁺ cluster: the two-center two-electron (2c–2e) bond in Fig. S4 (ESI†) and the three-center two-electron (3c–2e) bond in Fig. 4a. There are two delocalized 3c–2e π-bonds located on the Nb–Nb–C (Nb–Nb–C2 and Nb–Nb–C3) and Nb–C–C (Nb–C1–C2, Nb–C3–C4, and Nb–C5–C6) units. The Nb2 atom carries most of the unpaired spin density (0.91 e) of the Nb₂C₆H₄⁺ cluster, as shown in Fig. 3b, which provides an ideal site for N₂ adsorption (I9 in Fig. 3c).

Fig. 4 (a) Adaptive natural density partitioning (AdNDP, unit: e) bonding analysis for three-center two-electron (3c–2e) bonds in the Nb₂C₆H₄⁺ cluster. ON stands for the occupation number. The red and blue colours represent the positive value and negative value, respectively. (b) Two-dimensional localized orbital locator-π (LOL-π, unit: eV) analysis of the ortho-C₆H₄. A higher LOL-π value indicates a more localized distribution of π-electrons, and the colour depth increases with π-electron density. (c) Electrostatic potentials (ESPs, unit: eV) of the ortho-C₆H₄. The maximum and minimum values of the electrostatic potential are shown by the blue and orange spheres, respectively. (d) Interaction region indicator (IRI, unit: eV) of Nb₂C₆H₄⁺. The isosurface map is 0.55. A stronger chemical strength is indicated by a lower value.

Nb₂C₆H₄N₂⁺ is observed in the reactions of Nb₂C₆H₄⁺ and N₂, and the calculated PES is given in Fig. 3c. The reaction pathway commences with the formation of encounter complex I9 (−0.92 eV), in which the incoming N₂ molecule is adsorbed at the Nb2 atom via an η¹-N₂ coordination mode. Subsequently, the Nb1–N2 bond is formed in I10 (−1.56 eV) by changing the ∠NbNbN from 119° in I9 to 50° in I10. The most stable structure for Nb₂C₆H₄N₂⁺ is P1 (−1.98 eV), and the N₂ unit is in a side-on end-on (η¹:η²) coordination mode. The further N–N bond breaking is kinetically impeded by an intrinsic energy barrier TS11. Along the reaction coordinate (Nb₂C₆H₄⁺ + N₂ →→ P1) in Fig. 3b, the N≡N bond length is significantly elongated from 109 pm in the free N₂ molecule to 121 pm in Nb₂C₆H₄N₂⁺, suggesting that the N≡N triple bond is activated.

4. Discussion

Based on the experimental results shown in Fig. 1b, Nb⁺ is almost inert toward N₂, and this may be due to the fast dissociation rate of N₂ desorption from the encounter complex (6.0 × 10⁹ s⁻¹ predicted by variational transition state theory). The dual-metal cation Nb₂⁺ greatly enhances the N₂ absorption rate, but this rate is still one order of magnitude smaller than that of N₂/Nb₂C₆H₄⁺. This interesting phenomenon gives rise to several questions: (1) What is the distinctive characteristic of the ortho-C₆H₄ ligand? (2) What is the interaction between Nb₂⁺ and the ortho-C₆H₄ ligand? (3) Why does the ortho-C₆H₄ ligand facilitate N₂ absorption?

4.1 Characteristic of the ortho-C₆H₄ ligand and interaction between Nb₂⁺ and ortho-C₆H₄

Inspired by the conjugated π-bond in C₆H₆, the localized orbital locator-π (LOL-π) calculation is employed to visualize the distribution of π-electrons in the ortho-C₆H₄ ligand, as shown in Fig. 4b. The deeper colour in the red region indicates a higher distribution of π-electrons. When hydrogen is abstracted from C₆H₆, the C–C bond decreases from 139 pm in C₆H₆ to 125 pm in ortho-C₆H₄, leading to the increased π-electrons accumulation on this bond (Fig. 4b). Note that other C–C bonds in ortho-C₆H₄ are just as strong as those in C₆H₆. Consequently, the intense π-electrons create the negative electrostatic potential region, as depicted in Fig. 4c. The region with the lowest electrostatic potential (−0.65 eV) is situated on the C–C bond, which favors Nb₂⁺ bonding.

Based on the natural population analysis (NPA) charges of the Nb₂C₆H₄⁺ cation, the electron (0.72 e) transfers from the Nb₂⁺ unit to the ortho-C₆H₄ ligand. The interaction region indicator (IRI) analysis (Fig. 4d) reveals the formation of a strong chemical bond (green region) between Nb and C atoms. The bonding character and mechanism are further examined using the density of states (DOS) and the frontier orbital analyses, as shown in Fig. 5 and Fig. S6 (ESI†). In Fig. 5a, the 4d-electrons of Nb dominate the DOS around the HOMO. The Nb-4d and C-2p orbitals have strong interaction in the Nb–C bond, as demonstrated by the β-HOMO orbital, as well as the HOMO−1, HOMO−2, and HOMO−3 in both α and β orbitals.

Fig. 5 Total density of states (TDOS, black line) for (a) Nb₂C₆H₄⁺ and (b) Nb₂C₆H₄N₂⁺. Projected density of states (PDOS) of Nb (red line) and N₂ (purple line) are also given. In each panel, the HOMO position is indicated by the blue dotted line. The HOMO–LUMO energy gaps are also given. The orbital insets are shown to illustrate the bond interactions between the Nb₂C₆H₄⁺ cluster and N₂. The corresponding orbital contributions for Nb, C, and N are also presented. (c) The HOMO and LUMO energy levels of Nb₂⁺, Nb₂C₆H₄⁺, and N₂, respectively.

Notably, several significant changes are induced when the Nb₂⁺ bonds with ortho-C₆H₄. (1) The unpaired spin density (UPSD) distribution is adjusted. In ⁴Nb₂⁺, the UPSD is evenly distributed over each Nb atom (1.50 e), whereas the UPSD is localized on one Nb atom (0.91 e) in ²Nb₂C₆H₄⁺. (2) A polar Nb₂C₆H₄⁺ cluster is formed. The dipole moment is increased from 0 D for Nb₂⁺ to 3.5 D for Nb₂C₆H₄⁺. The vector direction in Nb₂C₆H₄⁺ (the blue arrow in Nb₂C₆H₄⁺, Fig. 3b) is along an axis perpendicular to the Nb–Nb bond, lying in the Nb1–O–Nb2 plane. Large dipole moment favors electron transfer and N₂ adsorption. (3) The HOMO energy level is raised from −15.87 eV in Nb₂⁺ to −14.42 eV in Nb₂C₆H₄⁺ (Fig. 5c), which is closer to the LUMO of N₂ (−1.76 eV).

4.2 Interaction between Nb₂C₆H₄⁺ and N₂

In Nb₂C₆H₄N₂⁺ and Nb₂N₂⁺, N₂ molecules are stably adsorbed on the Nb atoms in terms of side-on end-on (η¹:η²) coordination mode. The adsorption energy of N₂ on Nb₂C₆H₄⁺ (−1.98 eV) is larger than that of Nb₂⁺/N₂ (−1.01 eV), which is consistent with experimental observations.

As shown in Fig. 5a and Fig. S6a (ESI†), the HOMO of Nb₂C₆H₄⁺ is closer to the LUMO in N₂, compared with that of the Nb₂⁺ cation, resulting in a higher reactivity towards N₂. The frontier orbital analyses for Nb₂N₂⁺ and Nb₂C₆H₄N₂⁺ cations are performed in Fig. 5b and Fig. S6b (ESI†). As electron donors, Nb atoms directly interact with N₂, and the obvious orbital interaction between Nb-4d and N-2p is shown in Fig. 5b (e.g., the α-HOMO−1 and α-HOMO−3) and Fig. S6b (ESI,†e.g., the α-HOMO−3 and α-HOMO−6). The back-donating electrons from the Nb-4d orbital to the antibonding π*-orbital of N₂ activate the N≡N triple bond. The key factor in enhancing the N₂ adsorption rate stems from the addition of the ortho-C₆H₄ ligand, forming a polar Nb₂C₆H₄⁺ cluster. The positive-N1 (0.14 e) and negative-N2 (−0.15 e) charges in I9 indicate that N₂ has been slightly polarized. A strong interaction occurs between the polarized N₂ and the polar Nb₂C₆H₄⁺. In the condensed-phase system, increasing the polarity of the solid surface can increase the adsorption capacity of polar gas molecules.^66,67 Therefore, increasing the polarity of gas-phase clusters favors non-polar N₂ adsorption.

In Fig. 6, the natural population analysis charges of Nb, and N atoms, as well as the ortho-C₆H₄ ligand along the reaction coordinates of the Nb₂C₆H₄⁺/N₂ system are carried out. In the course of R →→ P1, two Nb atoms and the ortho-C₆H₄ ligand act as an electron reservoir, transferring 0.53 e to N2. In the formation of encounter complex I9 (R → I9), the N1 atom releases negative charges (ΔQ = 0.14 e) to the Nb2 and N2 atoms, forming the Nb2–N2 bond. Then negative charges flow from two Nb atoms to the N2 unit (ΔQ = 0.52 e), leading to the formation of the Nb1–N2 bond (I9 → I10). In the last step for the formation of the Nb1–N1 bond (I10 → P1), the N2 atom donates 0.16 e to the Nb1 and N1 atoms. In a word, although the ortho-C₆H₄ ligand in Nb₂C₆H₄⁺ seems like a spectator and does not contribute electrons to the reaction, its presence is significant in increasing polarity, raising the HOMO energy level, and increasing the N₂ adsorption energy. These behaviors facilitate N₂ adsorption. In addition, Nb₂C₆H₄N₂⁺ has more vibrational degrees of freedom than Nb₂N₂⁺ does, and therefore the former has more chance to be stabilized in the experiment.

Fig. 6 Natural population analysis (NPA) charges on the Nb, N atoms, and ortho-C₆H₄ ligand along reaction coordinates of N₂ activation on the Nb₂C₆H₄⁺ cation.

5. Conclusions

In summary, the gas-phase reaction of Nb₂⁺ with C₆H₆ is identified, which releases one H₂ molecule. The N₂ adsorption activity mediated by three different cations, that is, Nb⁺, Nb₂⁺, and Nb₂C₆H₄⁺, under thermal collision conditions has been investigated using mass spectrometry experiments and theoretical calculations. The experimental results indicate that the reaction rate constant of Nb₂C₆H₄⁺/N₂ is 10 times faster than that of Nb₂⁺/N₂, suggesting that organic ligand ortho-C₆H₄ facilitates N₂ adsorption, while the reaction rate of Nb⁺/N₂ is quite low under similar reaction conditions. Using DFT calculations, the electron transfer, chemical bonding, and orbital interaction mechanisms have been investigated between Nb₂⁺ and ortho-C₆H₄. In Nb₂C₆H₄⁺, the enhanced polarity as well as the raised HOMO level facilitate N₂ adsorption. The present gas-phase study provides molecular level insight into the mechanism of N₂ adsorption mediated by metal active sites with organic ligands.

Author contributions

Feng-Xiang Zhang: conceptualization, data curation, formal analysis, investigation, methodology, software, validation, visualization, writing – original draft and writing – review and editing. Yi-Heng Zhang and Ming Wang: data curation, formal analysis, methodology and validation. Jia-Bi Ma: conceptualization, formal analysis, funding acquisition, project administration, resources, supervision and writing – review and editing.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22222301), the Beijing Natural Science Foundation (2222023), the National Key Research and Development Program of China (2021YFA1601800), and the BIT Research and Innovation Promoting Project (2022YCXY029).

Notes and references

1. Q. Zhuo, X. Zhou, T. Shima and Z. Hou, Angew. Chem., Int. Ed., 2023, 62, e202218606.
2. T. T. Liu, D. D. Zhai, B. T. Guan and Z. J. Shi, Chem. Soc. Rev., 2022, 51, 3846–3861.
3. L. J. Wu, Q. Wang, J. Guo, J. Wei, P. Chen and Z. Xi, Angew. Chem., Int. Ed., 2023, 62, e202219298.
4. T. Itabashi, K. Arashiba, A. Egi, H. Tanaka, K. Sugiyama, S. Suginome, S. Kuriyama, K. Yoshizawa and Y. Nishibayashi, Nat. Commun., 2022, 13, 6161.
5. X. Shi, Q. Wang, C. Qin, L. J. Wu, Y. Chen, G. X. Wang, Y. Cai, W. Gao, T. He, J. Wei, J. Guo, P. Chen and Z. Xi, Natl. Sci. Rev., 2022, 9, nwac168.
6. R. J. Burford and M. D. Fryzuk, Nat. Rev. Chem., 2017, 1, 1–13.
7. L.-H. Mou, Z.-Y. Li and S.-G. He, J. Phys. Chem. Lett., 2022, 13, 4159–4169.
8. S. Jiang, M.-F. Jardinaud, J. Gao, Y. Pecrix, J. Wen, K. Mysore, P. Xu, S.-C. Carmen, Y. Ruan, M. Zhu, F. Li, E. Wang, P. Poole, P. Gamas and J. D. Murray, Science, 2021, 374, 625–628.
9. M. Wang, J. Ma, Z. Shang, L. Fu, H. Zhang, M. B. Li and K. Lu, J. Mater. Chem. A, 2023, 11, 3871–3887.
10. X. Gao, L. An, D. Qu, W. Jiang, Y. Chai, S. Sun, X. Liu and Z. Sun, Sci. Bull., 2019, 64, 918–925.
11. M. Reiners, D. Baabe, K. Munster, M. K. Zaretzke, M. Freytag, P. G. Jones, Y. Coppel, S. Bontemps, I. D. Rosal, L. Maron and M. D. Walter, Nat. Chem., 2020, 12, 740–746.
12. P. Garrido-Barros, J. Derosa, M. J. Chalkley and J. C. Peters, Nature, 2022, 609, 71–76.
13. H. J. Himmel and M. Reiher, Angew. Chem., Int. Ed., 2006, 45, 6264–6288.
14. X. Xin, I. Douair, Y. Zhao, S. Wang, L. Maron and C. Zhu, J. Am. Chem. Soc., 2020, 142, 15004–15011.
15. Z. J. Lv, Z. Huang, W. X. Zhang and Z. Xi, J. Am. Chem. Soc., 2019, 141, 8773–8777.
16. Y. Ohki, K. Munakata, Y. Matsuoka, R. Hara, M. Kachi, K. Uchida, M. Tada, R. E. Cramer, W. M. C. Sameera, T. Takayama, Y. Sakai, S. Kuriyama, Y. Nishibayashi and K. Tanifuji, Nature, 2022, 607, 86–90.
17. C. J. P. J. Talarmin, Nature, 1985, 317, 652–653.
18. S. F. McWilliams, D. L. J. Broere, C. J. V. Halliday, S. M. Bhutto, Q. M. Brandon and P. L. Holland, Nature, 2020, 584, 221–226.
19. S. L. Buchwald, B. T. Watson, R. T. Lum and W. A. Nugent, J. Am. Chem. Soc., 1987, 109, 7137–7141.
20. S. L. Buchwald and R. B. Nielsen, Chem. Rev., 1988, 88, 1047–1058.
21. S. J. McLain, R. R. Schrock, P. R. Sharp, M. R. Churchill and W. J. Youngs, J. Am. Chem. Soc., 1979, 101, 263–265.
22. X. Chen, Z. Xiong, M. Yang and Y. Gong, Chem. Commun., 2022, 58, 7018.
23. B. Yuan, Z. Liu, X.-N. Wu and S. Zhou, Sci. China: Chem., 2022, 65, 1720–1724.
24. Y.-Q. Ding, F. Ying, Y. Li, J. Xie and J.-B. Ma, Inorg. Chem., 2023, 62, 6102–6108.
25. X. Sun, S. Zhou, L. Yue, C. Guo, M. Schlangen and H. Schwarz, Angew. Chem., Int. Ed., 2019, 58, 3635–3639.
26. R. A. J. O’Hair and G. N. Khairallah, J. Cluster Sci., 2004, 15, 331–363.
27. Y. Zhao, J.-T. Cui, M. Wang, D. Y. Valdivielso, A. Fielicke, L.-R. Hu, X. Cheng, Q.-Y. Liu, Z.-Y. Li, S.-G. He and J.-B. Ma, J. Am. Chem. Soc., 2019, 141, 12592–12600.
28. Y.-Q. Ding, Z.-Y. Chen, Z.-Y. Li, X. Cheng, M. Wang and J.-B. Ma, J. Phys. Chem. A, 2022, 126, 1511–1517.
29. M. Wang, H.-Y. Zhou, A. M. Messinis, L.-Y. Chu, Y. Li and J.-B. Ma, J. Phys. Chem. Lett., 2021, 12, 6313–6319.
30. Y.-Q. Ding, Y. Li, F. Ying, M. Wang and J.-B. Ma, J. Phys. Chem. Lett., 2022, 13, 492–497.
31. L.-Y. Chu, Y.-Q. Ding, M. Wang and J.-B. Ma, Phys. Chem. Chem. Phys., 2022, 24, 14333–14338.
32. M. Gao, Y.-Q. Ding and J.-B. Ma, Int. J. Mol. Sci., 2022, 23, 6976.
33. M. Wang, L.-Y. Chu, Z.-Y. Li, A. M. Messinis, Y.-Q. Ding, L. Hu and J.-B. Ma, J. Phys. Chem. Lett., 2021, 12, 3490–3496.
34. G.-D. Jiang, Z.-Y. Li, L.-H. Mou and S.-G. He, J. Phys. Chem. Lett., 2021, 12, 9269–9274.
35. L.-H. Mou, G.-D. Jiang, Z.-Y. Li and S.-G. He, Chin. J. Chem. Phys., 2020, 33, 507–520.
36. V. I. Baranov, G. Javahery and D. K. Bohme, Chem. Phys. Lett., 1995, 239, 339–343.
37. R. K. Milburn, V. Baranov, A. C. Hopkinson and D. K. Bohme, J. Phys. Chem. A, 1999, 103, 6373–6382.
38. D. Caraiman and D. K. Bohme, Int. J. Mass Spectrom., 2003, 223–224, 411–425.
39. X. Cheng, Z.-Y. Li, G.-D. Jiang, X.-X. Liu, Q.-Y. Liu and S.-G. He, J. Phys. Chem. Lett., 2023, 14, 6431–6436.
40. Y. Zhao, J.-C. Hu, J.-T. Cui, L.-L. Xu and J.-B. Ma, Chem. – Eur. J., 2018, 24, 5920–5926.
41. H.-Y. Zhou, M. Wang, Y.-Q. Ding and J.-B. Ma, Dalton Trans., 2020, 49, 14081.
42. Z. Yuan, Z.-Y. Li, Z.-X. Zhou, Q.-Y. Liu, Y.-X. Zhao and S.-G. He, J. Phys. Chem. C, 2014, 118, 14967–14976.
43. Z.-Y. Li, Z. Yuan, X.-N. Li, Y.-X. Zhao and S.-G. He, J. Am. Chem. Soc., 2014, 136, 14307–14313.
44. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford CT, 2013.
45. Y. Zhao and D. G. Truhlar, J. Chem. Phys., 2006, 125, 194101.
46. Y. Li, M. Wang, Y.-Q. Ding, C.-Y. Zhao and J.-B. Ma, Phys. Chem. Chem. Phys., 2021, 23, 12592–12599.
47. Q. Chen, Y.-X. Zhao, L.-X. Jiang, H.-F. Li, J.-J. Chen, T. Zhang, Q.-Y. Liu and S.-G. He, Phys. Chem. Chem. Phys., 2018, 20, 4641–4645.
48. S. Grimme, J. Antony, S. Ehrlich and H. Krieg, J. Chem. Phys., 2010, 132, 154104.
49. B. Attila, A. M. Steven and Z. Z. Marek, J. Phys. Chem. A, 1998, 102, 6340–6347.
50. F. Weigend and R. Ahlrichs, Phys. Chem. Chem. Phys., 2005, 7, 3297–3305.
51. R. Krishnan, J. S. Binkley, R. Seeger and J. A. Pople, J. Chem. Phys., 1980, 72, 650–654.
52. T. Clark, J. Chandrasekhar, G. W. Spitznagel and P. V. R. Schleyer, J. Comput. Chem., 1983, 4, 294–301.
53. R. Ditchfield, W. J. Hehre and J. A. Pople, J. Chem. Phys., 1971, 54, 724–728.
54. J. L. Bao and D. G. Truhlar, Chem. Soc. Rev., 2017, 46, 7548–7596.
55. E. D. Glendening, C. R. Landis and F. Weinhold, J. Comput. Chem., 2013, 34, 1429–1437.
56. T. Lu and F. Chen, J. Comput. Chem., 2012, 33, 580–592.
57. T. Lu and Q. Chen, Theor. Chem. Acc., 2020, 139, 25.
58. T. Lu and S. Manzetti, Struct. Chem., 2014, 25, 1521–1533.
59. T. Lu and Q. Chen, Chem.: Methods, 2021, 1, 231.
60. Z. Liu, T. Lu and Q. Chen, Carbon, 2020, 165, 461–467.
61. G. Gioumousis and D. P. Stevenson, J. Chem. Phys., 1958, 29, 294–299.
62. G. Kummerlöwe and M. K. Beyer, Int. J. Mass Spectrom., 2005, 244, 84–90.
63. K. Balasubramanian, J. Chem. Phys., 1995, 114, 10375–10388.
64. C. Berg, T. Schindler, G. Niedner-Schatteburg and V. E. Bondybey, J. Chem. Phys., 1995, 102, 4870–4884.
65. S. Roszak, D. Majumdara and K. Balasubramaniana, J. Phys. Chem. A, 1999, 103, 5801–5806.
66. F. Mohajer and M. N. Shahrak, Heat Mass Transfer, 2019, 55, 2017–2023.
67. M. R. A. Tourrette, R. H. R. Valentin, M. Boissière, J. M. Devoisselle, F. D. Renzo and F. Quignard, Carbohydr. Polym., 2011, 85, 44–53.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cp05524h

---