Experimental and Theoretical Study of N₂ Adsorption on Hydrogenated Y₂C₄H⁻ and Dehydrogenated Y₂C₄⁻ Cluster Anions at Room Temperature

Abstract

The adsorption of atmospheric dinitrogen (N₂) on transition metal sites is an important topic in chemistry, which is regarded as the prerequisite for the activation of robust N≡N bonds in biological and industrial fields. Metal hydride bonds play an important part in the adsorption of N₂, while the role of hydrogen has not been comprehensively studied. Herein, we report the N₂ adsorption on the well-defined Y₂C₄H_0,1⁻ cluster anions under mild conditions by using mass spectrometry and density functional theory calculations. The mass spectrometry results reveal that the reactivity of N₂ adsorption on Y₂C₄H⁻ is 50 times higher than that on Y₂C₄⁻ clusters. Further analysis reveals the important role of the H atom: (1) the presence of the H atom modifies the charge distribution of the Y₂C₄H⁻ anion; (2) the approach of N₂ to Y₂C₄H⁻ is more favorable kinetically compared to that to Y₂C₄⁻; and (3) a natural charge analysis shows that two Y atoms and one Y atom are the major electron donors in the Y₂C₄⁻ and Y₂C₄H⁻ anion clusters, respectively. This work provides new clues to the rational design of TM-based catalysts by efficiently doping hydrogen atoms to modulate the reactivity towards N₂.

1. Introduction

More than 99% of the global nitrogen exists in the shape of gaseous dinitrogen (N₂) in the atmosphere, yet most organisms can only metabolize nitrogen-containing substances such as NH₃ rather than N₂ directly. Although N₂ is the main nitrogen source for most natural and artificial nitrogen-containing compounds, the high bond dissociation energy (9.75 eV) and the large HOMO–LUMO gap (10.8 eV) render its adsorption and activation an enormous challenge in chemistry [1,2,3,4]. Scientists regularly rely on transition metal (TM) centers to catalyze the nitrogen conversion processes [5,6,7]. The initial and critical step in the complicated reduction of dinitrogen is the adsorption of N₂ molecules at the TM center [8,9]. The fixation of nitrogen in industry is carried out at metal-based (Fe⁻ or Ru⁻) catalysts under extremely high temperatures (300–500 °C) and high pressures (100–300 atm), involving the disadvantages of large energy consumption and greenhouse gas emission [10,11,12]. Thus, it is vital to develop mild, energy-saving, and environment−friendly catalytic systems for N₂ fixation at ambient conditions. The activation of nitrogen by transition metal compounds with the involvement of hydrogen atoms is of particular interest, while the most common feature of N₂ hydrogenative cleavage is the participation of metal hydride bonds [13,14,15]. A literature survey [13] shows that metal hydride bonds have several important roles: (1) as a hydrogen source; (2) as an electron source for N₂ reduction; (3) as a powerful reducing agent for the removal of activated nitrogen atoms; and so on.

As an ideal model of condensed-phase systems, gas-phase clusters can study chemical reactions and reveal related mechanisms at the strictly molecular level by simulating active sites. [16,17,18,19]. Several theoretical and experimental studies have reported the reactivity of metal species with nitrogen, however, only a few metal species such as, Sc₂ [20], Ta₂⁺ [21], V₃C₄⁻ [22], Ta₂C₄⁻ [23], NbH₂⁻ [24], Ta₃N₃H_0,1⁻ [25], Sc₃NH₂⁺ [26], FeTaC₂⁻ [27], and AuNbBO⁻ [28] have been characterized to cleave the N≡N triple bond completely. It can be seen that for the studies on N₂ adsorption in the gas phase, there are few metal species, and they mainly focus on the early transition metals. In the previous work, we found that a suitable number of hydrogen atoms has an influence on the reactivity of transition metal-containing clusters with N₂ [24,25,26,29,30]. Sc₃NH₂⁺ [26] can effectively realize the activation of N₂ by H₂, which is based on the regulation of N₂ reduction by two H atoms. Ta₃N₃H_0.1⁻ is an example that highlights the importance of the assisted reactivity of a single hydrogen atom, and the reactivity of Ta₃N₃H⁻ is higher by a factor of five compared with that of Ta₃N₃⁻ due to the hydrogen atom changing the charge distribution and geometry [25]. How can hydrogen atoms be efficiently doped to modulate the reactivity of TM-containing systems towards N₂ at the molecular scale? Considering the previous exploration of the Sc systems and the fact that Sc and Y belong to the same group, Y₂C₄⁻ and Y₂C₄H⁻ cluster anions were synthesized, and the reactivity towards N₂ was investigated by mass spectrometry and DFT calculations, to answer this question. This work clearly revealed that Y₂C₄H_0,1⁻ anions can adsorb N₂, and the hydrogen atom greatly enhances the reactivity of Y₂C₄H⁻ towards N₂.

2. Results and Discussion

The time-of-flight (TOF) mass spectra of laser ablation-generated, further mass-elected Y₂C₄⁻ and Y₂C₄H⁻ cluster anions reacting with N₂ under thermal collision conditions in a linear ion trap (LIT) reactor are shown in Figure 1. The mass spectra for the generation of Y₂C₄H_0,1⁻ clusters has been given (Supplementary Figure S1). Upon the interactions of Y₂C₄⁻ and Y₂C₄H⁻ with N₂, two adsorbed complexes that are assigned as Y₂C₄N₂⁻ and Y₂C₄HN₂⁻ are observed (Figure 1b,d), suggesting the following channels in Equations (1) and (2):

Y₂C₄⁻ + N₂ → Y₂C₄N₂⁻

(1)

Y₂C₄H⁻ + N₂ → Y₂C₄HN₂⁻

(2)

Compared with Y₂C₄⁻, Y₂C₄H⁻ shows a higher reactivity towards N₂ under the same reaction conditions in Figure 1f. Besides the major products, two weak peaks in Figure 1 are assigned to Y₂C₄OH⁻ and Y₂C₄O₂H⁻, generated from the reaction of Y₂C₄H_0,1⁻ anions with water impurities in the LIT. The pseudo-first-order rate constants (k₁) for the reactions one and two are estimated to be (3.7 ± 0.8) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ and (6.2 ± 1.3) × 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹, which are based on a least-square fitting procedure, corresponding to reaction efficiencies (Φ) [31,32] of 0.6% and 0.01%, respectively. Additionally, the signal dependence of product Y₂C₄H_0,1N₂⁻ ions on N₂ pressures was obtained, which are derived and fitted with the mass spectrometry experimental data (Supplementary Figure S2).

BPW91 calculations are performed to investigate the structures of reactant Y₂C₄H_0,1⁻ anion clusters (Supplementary Figure S3), as well as the reaction mechanisms between Y₂C₄H_0,1⁻ and N₂. The lowest-energy isomer of Y₂C₄⁻ (doublet, ²IA1, Supplementary Figure S3), which is 0.08 eV lower than its quartet isomer, is a C_s−symmetric six−membered ring, with the Y-Y bond as the symmetry axis and two C₂ ligands bonded to the two Y atoms. Moreover, the most stable isomer of Y₂C₄H⁻ (¹IA2) has a hydrogen atom binding to the Y1 atom in the six-membered ring, similar to the Y₂C₄⁻ (²IA1), and it is 0.07 eV lower than the triplet state in energy (Supplementary Figure S3). Since the energies of the isomers are very close, their reaction paths are calculated. The results show that, in the reaction coordinates, the energies of the doublet and singlet stationary points and the products in the Y₂C₄⁻/N₂ and Y₂C₄H⁻/N₂ systems are lower than those of the corresponding quartet and triplet analogues, respectively (Supplementary Figure S4). Enthalpy and Gibbs free energies along with electronic and zero-point correction energies are added (Supplementary Table S1). The concentration of dinitrogen adducts in the gas phase is relatively low, so it is difficult to collect and continue to measure Raman spectra. Currently, it is difficult to characterize structures due to technical and instrumental limitations. Infrared multiple photon dissociation may be applied to reveal such types of anions. We have added the calculated infrared spectra (Supplementary Figure S5), and the vibrational frequencies may be used for future experimental identification of these clusters.

The potential energy surfaces (PESs) of the most favorable reaction pathways are given in Figure 2. The N₂ molecule is initially captured by the Y1 atom in both Y₂C₄⁻ and Y₂C₄H⁻ to form the end-on-coordinated complexes ²I1 and ¹I4. Notably, ²I1 (−0.71 eV) in Figure 2a is as stable as ¹I4 (−0.70 eV) in Figure 2b, suggesting that the N₂−adsorbed intermediates ²I1 and ¹I4 are not the final products in the Y₂C₄^–/N₂ and Y₂C₄H^–/N₂ systems. As for the Y₂C₄⁻/N₂ system, the coordination mode of N₂ is further changed from η¹ in ²I1 to η² in ²I2 via ²TS1. During this process, the N-N bond length is elongated from 110 pm in free N₂ to 119 pm in ²I2. Subsequently, the adsorbed N₂ unit is anchored by two Y atoms via ²TS2, forming a Y-N-N-Y bridge; at the same time, a longer N-N bond of 123 pm is generated in ²P1. Note that the rupture of the N-N bonds encounters a high energy barrier (²TS3, +2.46 eV with respect to the separated reactants), so that further activation of N₂ is hampered in this system.

The reaction of Y₂C₄H⁻/N₂ (Figure 2b) follows the similar mechanism. The complex is coordinated laterally to form a Y-N-N-Y bridge like ²P1 by overcoming a negligible barrier ¹TS4, and the activation energy (ΔE_a, i.e., the energy difference between the encounter complex and the transition state) is lower than that of ²I2 → ²TS2 (ΔE_a = 0.23 eV) in Y₂C₄⁻. In the step of ¹I4 → ¹P2, an elongation of the N–N bond from 115 to 121 pm occurs. Further cleavage of N–N is also hindered due to the positive energy barrier of 4.89 eV (¹TS5). In addition, another adsorption of N₂ on the Y2 atom (Supplementary Figure S6) that is not bonded with the hydrogen atom can be eventually trapped in ¹P2 by generating the η²-mode intermediate ¹I7. In conclusion, the reactions of Y₂C₄H⁻ and Y₂C₄⁻ with N₂ result in the formation of bridging adsorption products ²P1 and ¹P2, and the adsorbed N₂ molecules are in the η¹:η² mode. As shown in Figure 3, the potential energy curves reveal that the adsorption process of Y₂C₄H⁻/N₂ is more favorable kinetically compared to that of Y₂C₄⁻/N₂, since it is barrier−free for Y₂C₄H⁻/N₂. A small barrier exists in the shallow entrance channels when N₂ approaches Y₂C₄⁻, which further explains the experimental observed low reaction rate constant for the dehydrogenated Y₂C₄⁻/N₂.

Frontier orbital analysis shows that the immobilization of the N₂ ligand, as well as the formation of ²P1 and ¹P2, involve d-electrons transfer from the single-occupied molecular orbital-1 (SOMO-1) of Y₂C₄⁻ and the HOMO orbital of Y₂C₄H⁻ to the antibonding π*-orbitals of N₂ (Supplementary Figure S7). The presence of hydrogen atoms enhances the reactivity of the cluster cations toward N₂ since it changes the charge distribution. As shown in Figure 4a, the Y1 linked to the hydrogen atom on the Y₂C₄H⁻ cluster has more negative charges compared to Y₂C₄⁻, and it promotes π-back-donation. Note that the energy differences between the transition states and the separated reactants, which is the apparent barrier (ΔE^‡), matters in gas−phase studies. The apparent barrier for Y₂C₄H⁻/N₂ (ΔE^‡ = −0.70 eV) is lower than that of Y₂C₄⁻/N₂ (ΔE^‡ = −0.48 eV), and the energy of ¹P2 is lower than that of ²P1 (−1.61 eV vs. −1.35 eV). According to the Rice-Ramsperger-Kassel-Marcus (RRKM) theory [33], the internal conversion rate of I4 → TS4 (8.49 × 10¹¹ s⁻¹) is 32 times larger than that of I2 → TS2 (2.65 × 10¹⁰ s⁻¹). These theoretical results are consistent with the experiments.

To further improve the understanding of Y₂C₄H_0,1⁻/N₂ systems, NBO analysis along reaction coordinates was performed (Figure 4b,c). The charge details were added (Supplementary Table S2). In the adsorption processes IA1 → I1 and IA2 → I4 of Y₂C₄H_0,1⁻/N₂, the yttrium atoms transfer 0.37 e and 0.29 e to the N1 atom, respectively, leading to the formation of the Y-N1 bonds, while two N2 atoms in Y2C4⁻ and Y₂C₄H⁻ only increase by 0.11 e. In the subsequent steps I2 → P1 and I4 → P2 for the formation of the N2-Y2 bonds, more electrons are stored in the two nitrogen atoms, resulting in the gradual elongation of the N-N bonds. Overall, the electrons required for the N₂ adsorption by the Y₂C₄⁻ and Y₂C₄H⁻ clusters are mainly provided by Y atoms with total transferred amounts of 0.88 e and 0.78 e, respectively. Differently, two and one Y atoms are the electron donors in Y₂C₄⁻ and Y₂C₄H⁻, respectively. The active Y1 atom in Y₂C₄⁻ (IA1) has more 5s electron occupancies (5s^1.10 4d^1.03), which causes an unfavorable approach and a high σ-repulsion on the N₂ molecule. When one hydrogen atom on the Y₂C₄⁻ (²IA1) cluster bonds to form Y₂C₄H⁻ (¹IA2), the natural charge on the Y1 increases from 0.79 e to 1.48 e; at the same time, more 4d and less 5s electron occupancies are located (5s^0.38 4d^1.12), which can make N₂ more accessible to the Y₂C₄H⁻ cluster anions. The values of bond orders of Y-Y bond in Y₂C₄H_0,1⁻ anions are an important indicator for the ability of storing electrons, which increases from 0.55 in Y₂C₄⁻ (²IA1) to 0.66 in Y₂C₄H⁻ (¹IA2). Therefore, although hydrogen appears to be a bystander in N₂ adsorption, its presence indeed stores more electrons in the Y-Y bond and facilitates N₂ adsorption. It can be concluded that the hydrogen atom in the Y₂C₄H⁻ cluster significantly affects the charge distribution and electronic structure, and a suitable number of hydrogen atoms can enhance the reactivity towards N₂.

3. Methods

3.1. Experimental Methods

The metal carbide clusters were generated by laser ablation metal target (made of pure yttrium powder) (Jiangxi Ketai New Materials Co. Ltd, Jiangxi, China) seeded at 2‰ CH₄(Beijing Huatong Jingke Gas Chemical Co. Ltd, Beijing, China) in a helium carrier gas (backing pressure 4 atm). The pulsed laser is a 532 nm laser with 5–8 mJ/energy pulses and 10 Hz repetition rate (140 Baytech Drive, San Jose, CA, USA). Y₂C₄⁻ and Y₂C₄H⁻ anion clusters were mass-selected by a quadrupole mass filter (QMF) (China Academy of Engineering Physics, Mianyang, Sichuan, China) [34] and subsequently entered into a linear ion trap (LIT) reactor (homemade) [35]. After being confined and thermalized by the pulsed gas He for about 2 ms, they interacted with N₂ for about 6 ms and 14 ms, at room temperature, respectively. The anion clusters were ejected from the LIT and then detected by a reflection time-of-flight mass spectrometer (TOF-MS) [36]. The rate constants of the reactions between Y₂C₄H_0,1⁻ cluster anions and N₂ were described [37]. A schematic diagram of the experimental apparatus is shown in ref [34].

3.2. Computational Methods

All DFT [38] calculations were formed using the Gaussian 09 [39] program package to explore the structures of reactant clusters Y₂C₄H_0,1⁻ and the mechanistic details of Y₂C₄H_0,1⁻ with N₂. To give the best interpretation of the experimental data, we calculated the dissociation energies of the Y-Y Y-C, N-N and C-C (Supplementary Table S3) bonds using 20 methods. The results show that BPW91 functional [40,41,42] performs very well. For application of basis sets in reaction systems, the def2-TZVP [43] basis set was used for the Y atom, and the 6 − 311 + G * basis sets [44,45] were selected for the C, H, and N atoms, which were applied in other systems containing these elements [24,27,46]. The zero-point vibration corrected energies (ΔH_0K in eV) in unit of eV are reported. Vibrational frequency calculations must be performed for the geometric optimization of the reaction intermediates (IMs) and transition states (TSs) [47]. Intrinsic reaction coordinate [48] calculations were employed to ensure whether each TS was connected to two appropriate local minima. DFT-D3 correction for the complexes were contained in the system. Natural population analysis was performed using NBO 6.0 [49], and the orbital composition was analyzed by the method of natural atomic orbitals employing the Multiwfn program [50].

4. Conclusions

In summary, the reactions of Y₂C₄H⁻ and dehydrogenated Y₂C₄⁻ cluster anions with N₂ have been investigated experimentally and theoretically. The experimental results indicate that the reaction rate constant of Y₂C₄H⁻/N₂ is higher by a factor of 50 compared with that of Y₂C₄⁻/N₂. DFT calculations indicate that the differences are caused by the different charge distributions and the bonding of the additional hydrogen atom to the yttrium atom in the Y₂C₄H⁻ cluster, resulting in more 4d electron occupancies and thus more efficient π-back-donation bonding with N₂ molecules. The electron donor atoms of Y₂C₄⁻ and Y₂C₄H⁻ anion clusters are different, for Y₂C₄⁻, two Y atoms donate electrons, while only one Y atom donates electrons in Y₂C₄H⁻. Storing more electrons in the Y-Y bond is also an important influence of the hydrogen atom on the reactivity of Y₂C₄H⁻ to N₂. This study clearly reveals the significance of hydrogen-assisted reactions in N₂ adsorption processes. Attaching an appropriate number of hydrogen atoms on active sites can enhance the N₂ adsorption rates, providing a new strategic direction for the rational design of TM-based energy-efficient nitrogen fixation catalysts.