Recent Progress in Dinitrogen Activation by Gas-Phase Metal Species

Activation of CH4, NH3, and N2 by Tantalum Ions, Clusters and Their Oxides: What Can Be Learnt from Studies of Ions in the Gas Phase

The emission control of harmful compounds and greenhouse gases and the development of alternative, sustainable fuel sources is a major focus in current research. A solution for this problem lies in the development of efficient catalytic materials. Here, gas phase model systems represent prominent examples for obtaining fundamental insights on reaction properties of prospective catalytic systems. In this work, we review results from studies of tantalum clusters and their oxides in the gas phase and discuss insights with a potential relevance for applied systems. We focus on reactions that are essential for sustainable chemistry in the future. In detail, we address the activation of methane, which may enable the transformation of a greenhouse gas to a chemical feedstock, and we discuss the activation of NH3, which may function as an alternative energy carrier whose unwanted emission needs to be curbed in future applications. Finally, we consider the activation of N2 as a third reaction, since reducing the high energy demand of ammonia synthesis still bears significant challenges. While tantalum may be an interesting catalytic material, the discussed studies may also serve as benchmark for investigations of other materials.

Conversion of Carbon Dioxide into a Series of CBxOy- Compounds Mediated by LaB3,4O2- Anions: Synergy of the Electron Transfer and Lewis Pair Mechanisms to Construct B-C Bonds

Converting CO2 into value-added products containing B-C bonds is a great challenge, especially for multiple B-C bonds, which are versatile building blocks for organoborane chemistry. In the condensed phase, the B-C bond is typically formed through transition metal-catalyzed direct borylation of hydrocarbons via C-H bond activation or transition metal-catalyzed insertion of carbenes into B-H bonds. However, excessive amounts of powerful boryl reagents are required, and products containing B-C bonds are complex. Herein, a novel method to construct multiple B-C bonds at room temperature is proposed by the gas-phase reactions of CO2 with LaBmOn- (m = 1-4, n = 1 or 2). Mass spectrometry and density functional theory calculations are applied to investigate these reactions, and a series of new compounds, CB2O2-, CB3O3-, and CB3O2-, which possess B-C bonds, are generated in the reactions of LaB3,4O2- with CO2. When the number of B atoms in the clusters is reduced to 2 or 1, there is only CO-releasing channel, and no CBxOy- compounds are released. Two major factors are responsible for this quite intriguing reactivity: (1) Synergy of electron transfer and boron-boron Lewis acid-base pair mechanisms facilitates the rupture of C═O double bond in CO2. (2) The boron sites in the clusters can efficiently capture the newly formed CO units in the course of reactions, favoring the formation of B-C bonds. This finding may provide fundamental insights into the CO2 transformation driven by clusters containing lanthanide atoms and how to efficiently build B-C bonds under room temperature.

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

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.

Observation of Continuum State Dissociation Enables the Determination of N2 Bond Dissociation Energy to Spectroscopic Accuracy

Nitrogen (N) is one of the most fundamental elements of life. Precise determination of the bond dissociation energy (BDE) of the corresponding homonuclear diatomic molecule N2 is not only important for calculating the enthalpies of formation for any N-containing molecules but also provides the best benchmark for evaluating theoretical computational methods. Thus, it has attracted many experimental and theoretical studies, while controversies still exist. Here, we report the observation of continuum state dissociation of N2 into the channel N(2D5/2,3/2)+N(2D5/2,3/2) for the first time by using the vacuum ultraviolet (VUV)-pump-VUV-probe time-sliced velocity-mapped imaging setup. The quantum-state-resolved images enable the direct visualization of the dissociation onsets corresponding to each of the correlated spin-orbit fine-structure channels within a few tenths of wavenumber. The BDEs of 14N2 and 15N2 are directly determined to be 78691.8 ± 0.3 cm-1 and 78731.5 ± 0.3 cm-1, respectively, which should represent the most accurate BDE of N2 thus far.

Cryo IR spectroscopy and cryo kinetics of dinitrogen activation and cleavage by small tantalum cluster cations

We investigate small tantalum clusters Tan+, n = 2-4, for their capability to cleave N2 adsorption spontaneously. We utilize infrared photon dissociation (IR-PD) spectroscopy of isolated and size selected clusters under cryogenic conditions within a buffer gas filled ion trap, and we augment our experiments by quantum chemical simulations (at DFT level). All Tan+ clusters, n = 2-4, seem to cleave N2 efficiently. We confirm and extend a previous study under ambient conditions on Ta2+ cluster [Geng et al., Proc. Natl. Acad. Sci. U. S. A. 115, 11680-11687 (2018)]. Our cryo studies and the concomitant DFT simulations of the tantalum trimer Ta3+ suggest cleavage of the first and activation of the second and third N2 molecule across surmountable barriers and along much-involved multidimensional reaction paths. We unravel the underlying reaction processes and the intermediates involved. The study of the N2 adsorbate complexes of Ta4+ presented here extends our earlier study and previously published spectra from (4,m), m = 1-5 [Fries et al., Phys. Chem. Chem. Phys. 23(19), 11345-11354 (2021)], up to m = 12. We confirm the priory published double activation and nitride formation, succeeded by single side-on N2 coordination. Significant red shifts of IR-PD bands from these side-on coordinated μ2-κN:κN,N N2 ligands correlate with the degree of tilting towards the second coordinating Ta center. All subsequently attaching N2 adsorbates onto Ta4+ coordinate in an end-on fashion, and we find clear evidence for co-existence of end-on coordination isomers. The study of stepwise N2 adsorption revealed adsorption limits m(max) of [Tan(N2)m]+ which increase with n, and kinetic fits revealed significant N2 desorption rates upon higher N2 loads. The enhanced absolute rate constants of the very first adsorbate steps kabs(n,0) of the small Ta3+ and Ta4+ clusters independently suggest dissociative N2 adsorption and likely N2 cleavage into Ta nitrides.

Superior Reactivity of Molybdenum-Sulfur Cluster Anions Mo5S2- and Mo5S3- toward Dinitrogen

Inspired by the fact that Mo is a key element in biological nitrogenase, a series of gas-phase MoxSy- cluster anions are prepared and their reactivity toward N2 is investigated by the combination of mass spectrometry, photoelectron imaging spectroscopy, and density functional theory calculations. The Mo5S2- and Mo5S3- cluster anions show remarkable reactivity compared with the anionic species reported previously. The spectroscopic results in conjunction with theoretical analysis reveal that a facile cleavage of N≡N bonds takes place on Mo5S2- and Mo5S3-. The large dissociative adsorption energy of N2 and the favorable entrance channel for initial N2 approaching are proposed as two decisive factors for the superior reactivity of Mo5S2- and Mo5S3-. Besides, the modulation of S ligands on the reactivity of metal centers with N2 is proposed. The highly reactive metal-sulfur species may be obtained by the coordination of two to three sulfur atoms to bare metal clusters so that an appropriate combination of electronic structures and charge distributions can be achieved.

Activation of Dinitrogen Promoted by Adsorption of C6H6 on Fe2VC- Cluster Anions

The introduction of organic ligands is one of the effective strategies to improve the stability and reactivity of metal clusters. Herein, the enhanced reactivity of benzene-ligated cluster anions Fe₂VC(C₆H₆)^- with respect to naked Fe₂VC^- is identified. Structural characterization suggests that C₆H₆ is molecularly bound to the dual metal site in Fe₂VC(C₆H₆)^-. Mechanistic details reveal that the cleavage of N≡N is feasible in Fe₂VC(C₆H₆)^-/N₂ but hindered by an overall positive barrier in the Fe₂VC^-/N₂ system. Further analysis discloses that the ligated C₆H₆ regulates the compositions and energy levels of the active orbitals of the metal clusters. More importantly, C₆H₆ serves as an electron reservoir for the reduction of N₂ to lower the crucial energy barrier of N≡N splitting. This work demonstrates that the flexibility of C₆H₆ in terms of withdrawing and donating electrons is crucial to regulating the electronic structures of the metal cluster and enhancing the reactivity.

Theory-Guided Construction of the Unsaturated V-N2 Site with Carbon Defects for Highly Selective Electrocatalytic Nitrogen Reduction

Renewable energy-driven, electrocatalytic nitrogen reduction reaction (NRR) is a promising strategy for ammonia synthesis. However, improving catalyst activity and selectivity under ambient conditions has long been challenging. In this work, we obtained the potential active V-N center through theoretical prediction and successfully constructed the associated V-N₂/N₃ structure on N-doped carbon materials. Surprisingly, such a catalyst exhibits excellent electrocatalytic NRR performance. The V-N₂ catalyst affords a remarkably high faradaic efficiency of 76.53% and an NH₃ yield rate of 31.41 μg_NH3 h^-1 mg_Cat.^-1 at -0.3 V vs RHE. The structural characterization and density functional theory (DFT) calculations verified that the high performance of the catalyst originates from the tuned d-band upon coordination with nitrogen, in line with the original design intention as derived theoretically. Indeed, the V-N₂ center with carbon defects enhances dinitrogen adsorption and charge transfer, thereby lowering the energy barriers to form the *NNH intermediates. Such a strategy as a rational design─controllable synthesis─theoretical verification may prove effective as well for other chemical processes.

Probing the binding and activation of small molecules by gas-phase transition metal clusters via IR spectroscopy

Isolated transition metal clusters have been established as useful models for extended metal surfaces or deposited metal particles, to improve the understanding of their surface chemistry and of catalytic reactions. For this objective, an important milestone has been the development of experimental methods for the size-specific structural characterization of clusters and cluster complexes in the gas phase. This review focusses on the characterization of molecular ligands, their binding and activation by small transition metal clusters, using cluster-size specific infrared action spectroscopy. A comprehensive overview and a critical discussion of the experimental data available to date is provided, reaching from the initial results obtained using line-tuneable CO₂ lasers to present-day studies applying infrared free electron lasers as well as other intense and broadly tuneable IR laser sources.

Gas-phase reactions driven by polarized metal-metal bonding in atomic clusters

Multimetallic catalysts exhibit great potential in the activation and catalytic transformation of small molecules. The polarized metal-metal bonds have been gradually recognized to account for the reactivity of multimetallic catalysts due to the synergistic effect of different metal centers. Gas-phase reactions on atomic clusters that compositionally resemble the active sites on related condensed-phase catalysts provide a widely accepted strategy to clarify the nature of polarized metal-metal bonds and the mechanistic details of elementary steps involved in the catalysis driven by this unique chemical bonding. This perspective review concerns the progress in the fundamental understanding of industrially and environmentally important reactions that are closely related to the polarized metal-metal bonds in clusters at a strictly molecular level. The following topics have been summarized and discussed: (1) catalytic CO oxidation with O₂, H₂O, and NO as oxidants (2) and the activation of other inert molecules (e.g., CH₄, CO₂, and N₂) mediated with clusters featuring polarized metal-metal bonding. It turns out that the findings in the gas phase parallel the catalytic behaviors of condensed-phase catalysts and the knowledge can prove to be essential in inspiring future design of promising catalysts.

Dinitrogen Activation and Addition to Unsaturated C-E (E=C, N, O, S) Bonds Mediated by Transition Metal Complexes

Dinitrogen (N₂ ) activation and functionalization is of fundamental interest and practical importance. This review focuses on N₂ activation and addition to unsaturated substrates, including carbon monoxide, carbon dioxide, heteroallenes, aldehydes, ketones, acid halides, nitriles, alkynes, and allenes, mediated by transition metal complexes, which afforded a variety of N-C bond formation products. Emphases are placed on the reaction modes and mechanisms. We hope that this work would stimulate further explorations in this challenging field.

Catalysis of dinitrogen activation and reduction by a single Fe13 cluster and its doped systems

Catalyzing N₂ reduction to ammonia under ambient conditions is known to be significant both in the fertilizer industry and life sciences. To unveil the synergy of multiple sites, here, we have studied the catalysis of ammonia synthesis using a typical Fe₁₃ cluster and its doped systems, Fe₁₂X (X = V, Cr, Mn, Co, Ni, Cu, Zn, Nb, Mo, Ru, and Rh). The energetics analysis showed that center substitution (X@Fe₁₂) was favored while doping single V, Cr, Co, and Mo atoms, whereas Mn, Ni, Cu, Zn, Nb, Ru, and Rh tended to form shell-doped structures (Fe₁₂X). Among all the 13 clusters, Fe₁₂Nb exhibited the lowest activation energy for N₂ dissociation; moreover, in the hydrogenation process, Fe₁₂Nb could convert N₂ to ammonia efficiently. We have fully illustrated the reaction dynamics and structural chemistry essence of these diverse 13-atom systems and propose Fe₁₂Nb as an ideal candidate for catalytic ammonia synthesis.

Facile CO bond cleavage on polynuclear vanadium nitride clusters V4N5. Phys Chem Chem Phys 2022;24:29765-29771. [PMID: 36458914 DOI: 10.1039/d2cp04304a] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Identifying the structural configurations of precursors for CO dissociation is fundamentally interesting and industrially important in the fields of, e.g., Fischer-Tropsch synthesis. Herein, we demonstrated that CO could be dissociated on polynuclear vanadium nitride V₄N₅^- clusters at room temperature, and a key intermediate, with CO in a N-assisted tilted bridge coordination where the C-O bond ruptures easily, was discovered. The reaction was characterized by mass spectrometry, photoelectron spectroscopy, and quantum-chemistry calculations, and the nature of the adsorbed CO on product V₄N₅CO^- was further characterized by a collision-induced dissociation experiment. Theoretical analysis evidences that CO dissociation is predominantly governed by the low-coordinated V and N atoms on the (V₃N₄)VN^- cluster and the V₃N₄ moiety resembles a support. This finding strongly suggests that a novel mode for facile CO dissociation was identified in a gas-phase cluster study.

Trimetallic clusters in the sumanene bowl for dinitrogen activation

It is of great importance to find catalysts for the nitrogen reduction reaction (NRR) with high stability and reactivity. A series of M₃ clusters (M = Ti, Zr, V, and Nb) supported on sumanene (C₂₁H₁₂) were designed as potential catalysts for the NRR by taking advantage of the high reactivity of trimetallic clusters and the unique geometric and electronic properties of sumanene, a bowl-like organic molecule. Detailed mechanisms of NN bond cleavage on C₂₁H₁₂-M₃ were investigated by DFT calculations and compared with those on bare M₃ clusters. M₃ in the sumanene bowl is very stable with large binding energies, which prohibits the cohesion of M₃ into M₆. In the bowl, M₃ has a (quasi-) equilateral triangle structure with lengthened M-M bonds, which is particularly beneficial to the N₂ transfer process on Ti₃ and V₃ clusters. The N-N bond can be dissociated by both M₃ and C₂₁H₁₂-M₃ clusters without the overall energy barriers. A blurring effect is found in which some geometric and electronic properties of different metal types become similar when M₃ is supported on the substrate. Our work demonstrates that sumanene is a suitable substrate to support M₃ in the activation of N₂ with enhanced stability and maintained a high level of reactivity compared to bare M₃.

Multiple CO2 reduction mediated by heteronuclear metal carbide cluster anions RhTaC2. Dalton Trans 2022;51:11491-11498. [PMID: 35833563 DOI: 10.1039/d2dt01612e] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Noble metals dispersed on transition-metal carbides exhibit extraordinary activity in CO₂ catalytic conversion and bimetallic carbides generated at the interface were proposed to contribute to the observed activity. Heteronuclear metal carbide clusters (HMCCs) that compositionally resemble the bimetallic carbides are suitable models to get a fundamental understanding of the reactivity of the related condensed-phase catalysts, while the reaction of HMCCs with CO₂ has not been touched in the gas phase. Herein, benefiting from the newly designed double ion trap reactors, the reaction of laser-ablation generated and mass-selected RhTaC₂^- clusters with CO₂ was studied. The experimental results identified that RhTaC₂^- can reduce four CO₂ molecules consecutively and generate the product RhTaC₂O₄^-. The pivotal roles of Rh-Ta synergy and the C₂ ligand in driving CO₂ reduction were rationalized by theoretical calculations. The presence of an attached CO unit on the product RhTaC₂O₄^- was evidenced by the collision-induced dissociation experiment, providing a fundamental strategy to alleviate carbon deposition under a CO₂ atmosphere at elevated temperatures.