Nitrogen Molecule Adsorption on Cationic Tantalum Clusters and Rhodium Clusters and Desorption from Their Nitride Clusters Studied by Thermal Desorption Spectrometry

Effect of External Electric Field on Nitrogen Activation on a Trimetal Cluster

Efficient nitrogen (N2) fixation and activation under mild conditions are crucial for modern society. External electric fields (Felectric) can significantly affect N2 activation. In this work, the effect of Felectric on N2 activation by Nb3 clusters supported in a sumanene bowl was studied by density functional theory calculations. Four typical systems at different stages of N-N activation were studied, including two intermediates and two transition states. The impact of Felectric on various properties related to N2 activation was investigated, including the N-N bond length, overlap population density of states (OPDOS), total energy of the system, adsorption energy of N2, decomposition of energy changes, and electron transfer. The sumanene not only functions as a support and protective substrate, but also serves as a donor or acceptor under different Felectric conditions. Negative Felectric is beneficial to N-N bond activation because it promotes electron transfer to the N-N region and improves the d-π* orbital hybridization between metals and N2 in the activation process. Positive Felectric improves d-π* orbital hybridization only when the N-N is nearly dissociated. The microscopic mechanism of Felectric's effects provides insight into N2 activation and theoretical guidance for the design of catalytic reaction conditions for nitrogen reduction reactions (NRR).

Toward Designing Reactive Metal Clusters for Dinitrogen Activation: A Guideline Based on N2 Initial Adsorption

Gas-phase metal clusters are ideal models to explore transition-metal-mediated N2 activation mechanism. However, the effective design and search of reactive clusters in N2 activation are currently hindered by the lack of clear guidelines. Inspired by the Sabatier principle, we discovered in this work that N2 initial adsorption energy (ΔEads) is an important parameter to control the N2 activation reactivity of metal clusters in the gas phase. This mechanistic insight obtained from high-level calculations rationalizes the N2 activation reactivity of many previously reported metal clusters when combined with the known factor determining the N≡N cleavage process. Furthermore, based on this guideline of ΔEads, we successfully designed several new reactive clusters for cleaving N≡N triple bond under mild conditions, including FeV2S2-, TaV2C2-, and TaV2C3-, the high N2 activation reactivity of which has been fully corroborated in our gas phase experiments employing mass spectrometry with collision-induced dissociation. The importance of ΔEads revealed in this work not only reshapes our understanding of N2 activation reactions in the gas phase but also could have implication for other N2 activation processes in the condensed phase. The more general establishment of this new perspective on N2 activation reactivity warrants future experimental and computational studies.

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.

Hydrogen Storage Capacity of Single-Nb-Atom-Doped Al Clusters in the Gas Phase Revealed by Thermal Desorption Spectrometry

Hydrogen is a promising energy resource as a substitute for fossil fuels, and metal alloy hydrides are considered to be good candidates as hydrogen storage materials. In the hydrogen storage processes, hydrogen desorption is as important as hydrogen adsorption. In order to understand the hydrogen desorption features of those clusters, here, single-Nb-atom-doped Al clusters were prepared in the gas phase and their reaction with hydrogen was investigated using thermal desorption spectrometry (TDS). On average, six to eight H atoms were adsorbed in Al_nNb⁺ (n = 4-18) clusters, and most H atoms were released upon heating of the clusters to 800 K. Two types of desorption features of Al_nNb⁺ clusters were found, which related to the flexibility of the clusters. This study demonstrated the potential of Nb-doped Al alloy as an efficient hydrogen storage material with high storage capacity, thermal stability at room temperature, and hydrogen desorption ability upon moderate heating.

Dinitrogen Activation in the Gas Phase: Spectroscopic Characterization of C-N Coupling in the V3 C+ +N2 Reaction

We report on cluster-mediated C-N bond formation in the gas phase using N₂ as a nitrogen source. The V₃ C⁺ +N₂ reaction is studied by a combination of ion-trap mass spectrometry with infrared photodissociation (IRPD) spectroscopy and complemented by electronic structure calculations. The proposed reaction mechanism is spectroscopically validated by identifying the structures of the reactant and product ions. V₃ C⁺ exhibits a pyramidal structure of C₁ -symmetry. N₂ activation is initiated by adsorption in an end-on fashion at a vanadium site, followed by spontaneous cleavage of the N≡N triple bond and subsequent C-N coupling. The IRPD spectrum of the metal nitride product [NV₃ (C=N)]⁺ exhibits characteristic C=N double bond (1530 cm^-1 ) and V-N single bond (770, 541 and 522 cm^-1 ) stretching bands.

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.

Mutual functionalization of dinitrogen and methane mediated by heteronuclear metal cluster anions CoTaC2. Chem Sci 2022;13:9366-9372. [PMID: 36093004 PMCID: PMC9384824 DOI: 10.1039/d2sc02416k] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The direct coupling of dinitrogen (N2) and methane (CH4) to construct the N-C bond is a fascinating but challenging approach for the energy-saving synthesis of N-containing organic compounds. Herein we identified a likely reaction pathway for N-C coupling from N2 and CH4 mediated by heteronuclear metal cluster anions CoTaC2 -, which starts with the dissociative adsorption of N2 on CoTaC2 - to generate a Ta δ+-Nt δ- (terminal-nitrogen) Lewis acid-base pair (LABP), followed by the further activation of CH4 by CoTaC2N2 - to construct the N-C bond. The N[triple bond, length as m-dash]N cleavage by CoTaC2 - affording two N atoms with strong charge buffering ability plays a key part, which facilitates the H3C-H cleavage via the LABP mechanism and the N-C formation via a CH3 migration mechanism. A novel Nt triggering strategy to couple N2 and CH4 molecules using metal clusters was accordingly proposed, which provides a new idea for the direct synthesis of N-containing compounds.

Reaction of Ta3 - Clusters with Molecular Nitrogen: A Mechanism Investigation

Because of the inertness of molecular nitrogen, its practicable activation under mild conditions is a fundamental challenge. Ta3 - is an exceptionally small cluster that reacts with N2 at room temperature, leading finally to Ta3N2 -; Ta3N2 - also could react with N2 at room temperature, leading finally to Ta3N4 -, a product of interest in its own right because of its potential as a nitrogen fixation medium. The mechanisms of the Ta3 -- and Ta3N2 --mediated activation of the N≡N triple bond have been investigated. Our extensive computations elucidate mechanisms for the ready reactions, leading to stepwise cleavage of the N≡N bond. Initial isomeric N2/Ta3 - complexes, N≡N elongation, undergo a N≡N split over generally low barriers in a highly exothermic process. The nitrogen-atom or molecular exchange reactions found in the Ta3N2 -/N2 system may be of paramount importance in both applied and fundamental studies.

Recent Progress in Dinitrogen Activation by Gas-Phase Metal Species

Understanding the mechanisms to activate and functionalize dinitrogen (N₂) is of great importance for the rational design of nitrogen-fixation catalysts. Reactions of gas-phase species with N₂ are being actively studied to understand the bond activation and formation processes at a strictly molecular level. This Perspective provides an overview of the recent progress in combined experimental and theoretical studies on the activation and functionalization of N₂ by gas-phase metal species. New mechanistic insights into N₂ molecular adsorption, N≡N cleavage, and N-X (X = C, B, and H) formation have been introduced, in which the new reaction channels of ejecting neutral metal fragments and the coupling reactions of N₂ with other molecules are highlighted. Finally, the current challenges and outlooks of N₂ activation in the gas phase are discussed as well.

15 N/14N isotopic exchange in the dissociative adsorption of N2 on tantalum nitride cluster anions Ta3N3−

Adsorption and activation of dinitrogen (N2) is an indispensable process in nitrogen fixation. Metal nitride species continue to attract attention as a promising catalyst for ammonia synthesis. However, the detailed mechanisms at a molecular level between reactive nitride species and N2 remain unclear at elevated temperature, which is important to understand the temperature effect and narrow the gap between the gas phase system and condensed phase system. Herein, the 14N/15N isotopic exchange in the reaction between tantalum nitride cluster anions Ta314N3− and 15N2 leading to the regeneration of 14N2/14N15N was observed at elevated temperature (393−593 K) using mass spectrometry. With the aid of theoretical calculations, the exchange mechanism and the effect of temperature to promote the dissociation of N2 on Ta3N3− were elucidated. A comparison experiment for Ta314N4−/15N2 couple indicated that only desorption of 15N2 from Ta314N415N2− took place at elevated temperature. The different exchange behavior can be well understood by the fact that nitrogen vacancy is a requisite for the dinitrogen activation over metal nitride species. This study may shed light on understanding the role of nitrogen vacancy in nitride species for ammonia synthesis and provide clues in designing effective catalysts for nitrogen fixation.

Adsorption Forms of NO on Iridium-Doped Rhodium Clusters in the Gas Phase Revealed by Infrared Multiple Photon Dissociation Spectroscopy

The adsorption of an NO molecule on a cationic iridium-doped rhodium cluster, Rh₅Ir⁺, was investigated by infrared multiple photon dissociation spectroscopy (IRMPD) of Rh₅IrNO⁺·Ar_p complexes in the 300-2000 cm^-1 spectral range, where the Ar atoms acted as a messenger signaling IR absorption. Complementary density functional theory (DFT) calculations predicted two near-isoenergetic structures as the putative global minimum: one with NO adsorbed in molecular form in the on-top configuration on the Ir atom in Rh₅Ir⁺, and one where NO is dissociated with the O atom bound to the Ir atom in the on-top configuration and the N atom on a hollow site formed by three Rh atoms. A comparison between the experimental IRMPD spectrum of Rh₅IrNO⁺ and calculated spectra indicated that NO mainly adsorbs molecularly on Rh₅Ir⁺, but evidence was also found for structures with dissociatively adsorbed NO. The estimated fraction of Rh₅IrNO⁺ structures with dissociatively adsorbed NO is approximately 10%, which was higher than that found for Rh₆⁺, but lower than that for Ir₆⁺. The DFT calculations indicated the existence of an energy barrier in the NO dissociation pathway that is exothermic with respect to the reactants, which was considered to prevent NO from dissociating readily on Rh₅Ir⁺. The height of the barrier is lower than that for NO dissociation over Rh₆⁺, which is attributed to the higher binding energy of atomic O to the Ir atom in Rh₅Ir⁺ than to a Rh atom in Rh₆⁺.

Decomposition of nitric oxide by rhodium cluster cations at high temperatures

Decomposition reactions of NO molecules on gas-phase Rh_n⁺ (n = 6-9) clusters were investigated by gas-phase thermal desorption spectrometry and density functional theory calculations. We found that NO adsorbs on the clusters, forming Rh_nN_xO_x⁺ at room temperature. Upon heating, NO desorption was observed below 800 K. Above 800 K, while for n = 7 and 8, each of Rh₇N₃O₃⁺, Rh₇N₄O₄⁺, and Rh₈N₃O₃⁺ was found to release an N₂ molecule, no N₂ formation was clearly observed for Rh_6,9N_xO_y⁺. We considered that both Rh₇N₃O₃⁺ and Rh₈N₃O₃⁺ have at least two dissociated NO molecules, while Rh₆N_xO_x⁺ (x = 1-3) has one or less. Our computational results for Rh₈N₃O₃⁺ suggested that the formation of an N-N bond in the Rh₈N₃O₃⁺ structure must overcome an energy barrier of ∼2 eV, which is the highest among the suggested possible reaction pathways. These findings suggested that the size-dependent activity of NO decomposition is governed primarily by how NO molecules are adsorbed on Rh_n⁺ clusters, i.e. whether two or more N atoms from dissociated NO molecules exist in the NO adsorbed clusters, and secondly, by the readiness of the N-N bond formation.

Facile N≡N Bond Cleavage by Anionic Trimetallic Clusters V3-x Tax C4 - (x=0-3): A DFT Study

Activation of N₂ on anionic trimetallic V_3-x Ta_x C₄ ^- (x=0-3) clusters was theoretically studied employing density functional theory. For all studied clusters, initial adsorption of N₂ (end-on) on one of the metal atoms (denoted as Site 1) is transferred to an of end-on: side-on: side-on coordination on three metal atoms, prior to N₂ dissociation. The whole reaction is exothermic and has no global energy barriers, indicating that the dissociation of N₂ is facile under mild conditions. The reaction process can be divided into two processes: N₂ transfer (TRF) and N-N dissociation (DIS). For V-series clusters, which has a V atom on Site 1, the rate-determining step is DIS, while for Ta-series clusters with a Ta on Site 1, TRF may be the rate-determining step or has energy barriers similar to those of DIS. The overall energy barriers for heteronuclear V₂ TaC₄ ^- and VTa₂ C₄ ^- clusters are lower than those for homonuclear V₃ C₄ ^- and Ta₃ C₄ ^- , showing that the doping effect is beneficial for the activation and dissociation of N₂ . In particular, V-Ta₂ C₄ ^- has low energy barriers in both TRF and DIS, and it has the highest N₂ adsorption energy and a high reaction heat release. Therefore, a trimetallic heteronuclear V-series cluster, V-Ta₂ C₄ ^- , is suggested to have high reactivity to N₂ activation, and may serve as a prototype for designing related catalysts at a molecular level.

Dinitrogen Activation by Heteronuclear Metal Carbide Cluster Anions FeTaC2-: A 5d Early and 3d Late Transition Metal Strategy

Cleavage of the strong N≡N bond has long been a great challenge for energy-efficient dinitrogen (N₂) fixation; thus a reasonable design of reactive species to activate N₂ under mild conditions is highly desirable and meaningful. Herein a novel N₂ activation strategy of combining 5d early (E) and 3d late (L) transition metals (TMs) is proposed, which is verified by the facile and complete N≡N cleavage via the polarized Fe-Ta bond in gas-phase cluster FeTaC₂^-. The efficient N≡N cleavage benefits from an electronic-level design of highly strengthened donor-acceptor interactions, in which the 5d-ETM (Ta) mainly pushes electrons from occupied 5d-orbitals to N₂ π*-orbitals while the 3d-LTM (Fe) simultaneously pulls electrons from N₂ σ/π-orbitals to its unoccupied 3d-orbitals. Through employing 5d-ETM and 3d-LTM to play their respective roles, this work provides a new and versatile idea for activating the inert N≡N bond and inspires relevant design of TM-based catalysts.

Nitrogen Activation and Transformation on Monometallic Niobium Boron Oxide Cluster Anions at Room Temperature: A Dual-Site Mechanism

Dinitrogen activation and transformation at room temperature is a goal that has been long sought after. Despite that, it remains underdeveloped due to being a challenging research area and the need for a better mechanistic understanding. Herein, we report that well-defined NbB₃O₂^- gas-phase clusters can activate one N₂ molecule and generate the products B₃N₂O^- and B₃N₂O₂^-, as applying mass spectrometry and theoretical calculations. This unusual N₂ activation reaction results from the different functions of the Nb and B₃O₂ moieties in NbB₃O₂^-. Theoretical calculations suggest that a catalytic cycle can be completed by the recovery of NbB₃O₂^-, which is achieved through the reactions of Nb and NbO with B₃O₂^- and B₃O^-, respectively. This is the first example of N₂ efficient transformation at a monometallic cluster, and this method for generating dual active sites by designing proper ligands may open the way toward the development of more effective N₂ fixation and functionalization methodologies.

Non-Dissociative Activation of Chemisorbed Dinitrogen on One or Two Vanadium Atoms Supported by a Mo6 S8 Cluster

Adsorption of N₂ on Mo₆ S₈ ^q _V_x clusters (x=0, 1, 2; q=0, ±1) were systematically studied by density functional theory calculations with dispersion corrections. It was found that the N₂ can be chemisorbed and undergo non-dissociative activation on single or double metal atoms. The adsorption and activation are influenced by metal types (V or Mo), N₂ coordination modes and charge states of the clusters. Particularly, anionic Mo₆ S₈ ^- _V₂ clusters have remarkable ability to fix and activate N₂ . In Mo₆ S₈ ^- _V₂ , two V atoms prefer to adsorb on two adjacent S-Mo-S hollow sites, leading to the formation of a supported V…V unit. The N₂ is bridged side-on coordinated with these two V atoms with high adsorption energy and significant charge transfer. The bond order, bond length and vibration frequency of the adsorbed N₂ are close to those of a N-N single bond.

Observation and mechanism of cryo N2 cleavage by a tantalum cluster

We explore the cryogenic kinetics of N2 adsorption to Ta4+ and the infrared signatures of [Ta4(N2)m]+ complexes, m = 1-5. This is accomplished by N2 exposure of isolated ions within a cryogenic ion trap. We find stepwise addition of numerous N2 molecules to the Ta4+ cluster. Interestingly, the infrared signatures of the [Ta4(N2)1]+ and [Ta4(N2)2]+ products are special: there are no NN stretching bands. This is consistent with cleavage of the first two adsorbed dinitrogen molecules. DFT calculations reveal intermediates and barriers along reaction paths of N2 cleavage in support of these experimental findings. We indicate the identified multidimensional path of N2 cleavage as an across edge-above surface (AEAS) mechanism: initially end-on coordinated N2 bends towards a neighboring Ta-atom which yields a second intermediate, with a μ2 bonded N2 across an edge of the Ta4+ tetrahedron core. Further rearrangement above a Ta-Ta-Ta surface of the Ta4+ tetrahedron results in a μ3 bonded N2 ligand. This intermediate relaxes swiftly by ultimate NN cleavage unfolding into the final dinitrido motif. Submerged activation barriers below the entrance channel confirm spontaneous cleavage of the first two dinitrogen molecules (-59 and -33 kJ mol-1, respectively), while cleavage of the third N2 ligand is kinetically hindered (+55 kJ mol-1). We recognize that substoichiometric N2 exposure allows for spontaneous activation by Ta4+, while higher N2 exposure causes self-poisoning.

Rhodium chemistry: A gas phase cluster study

Due to the extraordinary catalytic activity in redox reactions, the noble metal, rhodium, has substantial industrial and laboratory applications in the production of value-added chemicals, synthesis of biomedicine, removal of automotive exhaust gas, and so on. The main drawback of rhodium catalysts is its high-cost, so it is of great importance to maximize the atomic efficiency of the precious metal by recognizing the structure-activity relationship of catalytically active sites and clarifying the root cause of the exceptional performance. This Perspective concerns the significant progress on the fundamental understanding of rhodium chemistry at a strictly molecular level by the joint experimental and computational study of the reactivity of isolated Rh-based gas phase clusters that can serve as ideal models for the active sites of condensed-phase catalysts. The substrates cover the important organic and inorganic molecules including CH₄, CO, NO, N₂, and H₂. The electronic origin for the reactivity evolution of bare Rh_x ^q clusters as a function of size is revealed. The doping effect and support effect as well as the synergistic effect among heteroatoms on the reactivity and product selectivity of Rh-containing species are discussed. The ingenious employment of diverse experimental techniques to assist the Rh₁- and Rh₂-doped clusters in catalyzing the challenging endothermic reactions is also emphasized. It turns out that the chemical behavior of Rh identified from the gas phase cluster study parallels the performance of condensed-phase rhodium catalysts. The mechanistic aspects derived from Rh-based cluster systems may provide new clues for the design of better performing rhodium catalysts including the single Rh atom catalysts.

The sequential activation of H2 and N2 mediated by the gas-phase Sc3N+ clusters: Formation of amido unit

The activation and hydrogenation of nitrogen are central in industry and in nature. Through a combination of mass spectrometry and quantum chemical calculations, this work reports an interesting result that scandium nitride cations Sc₃N⁺ can activate sequentially H₂ and N₂, and an amido unit (NH₂) is formed based on density functional theory calculations, which is one of the inevitable intermediates in the N₂ reduction reactions. If the activation step is reversed, i.e., sequential activation of first N₂ and then H₂, the reactivity decreases dramatically. An association mechanism, prevalent in some homogeneous catalysis and enzymatic mechanisms, is adopted in these gas-phase H₂ and N₂ activation reactions mediated by Sc₃N⁺ cations. The mechanistic insights are important to understand the mechanism of the conversion of H₂ and N₂ to NH₃ synthesis under ambient conditions.

Dinitrogen Activation and Functionalization by Heteronuclear Metal Cluster Anions FeV2C2- at Room Temperature

It is of great importance to study the mechanisms to activate dinitrogen (N₂), the very inert molecule, under mild conditions. Gas-phase metal clusters are being actively generated to react with N₂ to identify new reaction types and mechanisms. Herein, an unprecedented, mechanistically unique metal atom (Fe or V) ejection in the thermal reaction of FeV₂C₂^- with N₂ has been identified using mass spectrometry, photoelectron imaging spectroscopy, and quantum chemistry calculations. Strong evidence suggests that the complete cleavage of the N≡N triple bond and subsequent functionalization of two N atoms via C-N coupling were achieved in this reaction. The complementary cooperation between V atoms with strong electron-donating ability and an Fe atom with large electron-withdrawing ability as well as the geometric flexibility of the Fe-V-V ring drives the whole reaction. The important role of C ligands in N≡N cleavage was also revealed. This study emphasizes the importance of heteronuclear metal systems for N₂ fixation.

Plasma-Assisted Chain Reactions of Rh3+ Clusters with Dinitrogen: N≡N Bond Dissociation

Dinitrogen activation is known as one of the most challenging subjects in chemistry. A number of well-defined metal complexes, nitrides, and clusters have been studied that show catalysis for dinitrogen activation. However, direct evidence of a complete cleavage of the N≡N triple bond at mild conditions is rather limited to date. Herein, we report a study on the dissociation of N₂ on small rhodium clusters assisted by surface plasma radiation. From mass spectrometry observation, a few rhodium nitride clusters with an odd number of nitrogen atoms are produced, such as the Rh₃N_2m-1⁺ (m = 1-5) series, indicative of N≡N bond dissociation in the mild plasma atmosphere. Interestingly, Rh₃N₇⁺ is identified with outstanding mass abundance among the Rh_nN_2m-1⁺ products, and its ground-state structure is in the form of Rh₃N(N₂)₃⁺ by capping a nitrogen atom on the top of Rh₃⁺ plane and hanging three N₂ molecules beneath the three Rh atoms respectively, giving rise to a C_3v symmetry and excellent stability. We demonstrate the catalysis of such a three-atom rhodium cluster and reveal a dinitrogen activation strategy by thermodynamics- and dynamics- favorable chain reactions of multiple N₂ molecules with two rhodium clusters under plasma atmosphere.

Reactivity of Neutral Tantalum Sulfide Clusters Ta3Sn (n = 0-4) with N2

Nitrogen (N₂) fixation is a challenging and vital issue in chemistry. Inspired by the fact that the active sites of nitrogenases are polynuclear metal sulfide clusters, the reactivity of gas-phase metal sulfide clusters toward N₂ has received considerable attention to gain fundamental insights into nitrogen fixation. Herein, neutral tantalum sulfide clusters have been prepared and their reactivity toward N₂ has been investigated by mass spectrometry in conjunction with density functional theory (DFT) calculations. The experimental results showed that Ta₃S_n (n = 0-3) could adsorb N₂, while Ta₃S₄ was inert to N_2. The DFT calculations revealed that the complete cleavage of the N≡N bond on the trinuclear metal center in the Ta₃S_0-3/N₂ reaction systems was overall barrierless under thermal collision conditions. The sulfur ligands can facilitate the approaching of N₂ toward the metal center but weaken the electron-donating ability of the metal center. The inertness of Ta₃S₄ is ascribed to the electron-deficient state of Ta₃ in Ta₃S₄ and the least effective orbital interaction in the Ta₃S₄/N₂ couple. This study provides new insights into the ligand effect on the interaction of the metal clusters with N_2.

A Facile N≡N Bond Cleavage by the Trinuclear Metal Center in Vanadium Carbide Cluster Anions V3C4. J Am Chem Soc 2020;142:10747-10754. [PMID: 32450693 DOI: 10.1021/jacs.0c02021]

Cleavage of the triple N≡N bond by metal clusters is of fundamental interest and practical importance in nitrogen fixation. Previous studies of N≡N bond cleavage by gas-phase metal clusters emphasized the importance of the dinuclear metal centers. Herein, the dissociative adsorption of N₂ and subsequent C-N coupling on trinuclear carbide cluster anions V₃C₄^- under thermal collision conditions have been characterized by employing mass spectrometry (collision induced dissociation), cryogenic photoelectron imaging spectroscopy, and quantum chemistry calculations. A theoretical analysis identified a crucial adsorption intermediate with N₂ bonded with the V₃ metal core in the end-on/side-on/side-on (ESS) mode, which most likely enables the facile cleavage of the N≡N bond. Such a vital N₂ coordination in the ESS mode is a result of symmetry-matched interactions between the occupied orbitals of the metal core and both of the two empty π* orbitals of N₂. Furthermore, carbon ligands also play a considerable role in enhancing the reactivity of the metal core toward N₂. This study strongly suggests a new mechanism of N≡N bond cleavage by gas-phase metal clusters.

Side-on-End-on Coordination of Dinitrogen on a Polynuclear Vanadium Nitride Cluster Anion [V5 N5 ]. Chemistry 2019;25:16523-16527. [PMID: 31637740 DOI: 10.1002/chem.201904362]

The side-on-end-on coordination of N₂ can be very important to activate and functionalize this very stable molecule. However, such coordination has rarely been reported. This study reports a gas-phase species (a polynuclear vanadium nitride cluster anion [V₅ N₅ ]^- ) that can capture N₂ efficiently (12 %), and the quantum chemistry modelling suggests an unusual side-on-end-on coordination. The cluster anions were generated by laser ablation and the reaction with N₂ has been characterized by mass spectrometry, photoelectron imaging spectroscopy, and density functional theory calculations. The back-donation interactions between the localized d-d bonding orbitals on the low-coordinated dual metal (V) sites and the antibonding π* orbitals of N₂ are the driving forces to adsorb N₂ with a high binding energy (about 2.0 eV).

Adsorption Kinetics of Nitrogen Molecules on Size-Selected Silver Cluster Cations

We present adsorption processes of dinitrogen on size-selected silver cluster cations, Ag n + (n = 1–10), studied by kinetics measurement using an ion trap. The cluster ions showed sequential adsorption of N2 molecules when the ion trap was cooled down to 105 K, excluding n = 8 and 9 that were exceptionally inactive at this temperature. Termolecular rate coefficients of each adsorption step are determined by analyzing time-dependent changes in the reactant and product ion signals. The first-step rate coefficients were found to increase exponentially from n = 1 to 7 due to increased internal degrees of freedom at larger sizes, which are favorable for accommodating the adsorption energy in a free cluster. In contrast, the adsorption rate turned to decrease for n > 7 due to weaker binding of dinitrogen as revealed by density-functional-theory (DFT) calculation. Adsorption sites on Ag n + are further discussed on the basis of the maximum number of adsorbing N2 molecules observed in the experiment.

Reactivity of Tantalum Carbide Cluster Anions TaC n- ( n = 1-4) with Dinitrogen

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 TaC _n^- ( 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 the 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 TaC_1-3^-/N₂ reaction systems. The natural bond orbital and molecular orbital analysis indicated that the more positive charge on the metal center of TaC_1-4^- would facilitate the initial approach of the nonpolar N₂ molecule, and the appropriate frontier orbital of TaC_1-4^- 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.

An efficient laser vaporization source for chemically modified metal clusters characterized by thermodynamics and kinetics

A laser vaporization cluster source that has a room for cluster aggregation and a reactor volume, each equipped with a pulsed valve, is presented for the efficient gas-phase production of chemically modified metal clusters. The performance of the cluster source is evaluated through the production of Ta and Ta oxide cluster cations, Ta_xO_y⁺ (y ≥ 0). It is demonstrated that the cluster source produces Ta_xO_y⁺ over a wide mass range, the metal-to-oxygen ratio of which can easily be controlled by changing the pulse duration that influences the amount of reactant O₂ introduced into the cluster source. Reaction kinetic modeling shows that the generation of the oxides takes place under thermalized conditions at less than 300 K, whereas metal cluster cores are presumably created with excess heat. These characteristics are also advantageous to yield "reaction intermediates" of interest via reactions between clusters and reactive molecules in the cluster source, which may subsequently be mass selected for their reactivity measurements.

The role of electronegativity on the extent of nitridation of group 5 metals as revealed by reactions of tantalum cluster cations with ammonia molecules

The electronegativity of the metal (V > Ta) plays a key role in determining the composition of the metal nitrides.

Consecutive reactions of small, free tantalum clusters with dioxygen controlled by relaxation dynamics

The reaction of small cationic tantalum clusters (Ta_n⁺, n = 4–8) with molecular oxygen is studied under multi-collision conditions in the gas phase, and the reaction kinetics are analyzed in order to elucidate underlying mechanisms.

From oxidative degradation to direct oxidation: size regimes in the consecutive reaction of cationic tantalum clusters with dioxygen

Cationic tantalum clusters (Ta_9–12⁺) are reacted with molecular oxygen under multi-collision conditions in the gas phase in order to analyze the reaction kinetics.

Tantalum nitride coatings prepared by magnetron sputtering to improve the bioactivity and osteogenic activity for titanium alloy implants

TaN film has a positive effect on the biocompatibility and osteoinductive ability of Ti6Al4V-based implants.