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.

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.

A comparative study on the reactivity of ditantalum deuteride cluster anions Ta2D2- and Ta2D4- toward N2.

The activation and transformation of molecular nitrogen (N2) by metal hydride species has attracted widespread attention due to its critical role in nitrogen fixation. Herein, the reactions between tantalum deuteride cluster anions Ta2D2,4- and N2 were investigated experimentally and theoretically. An unprecedented reaction channel of the liberation of a single D atom was observed and much superior reactivity was identified for Ta2D4-. Theoretical investigations indicate that the releasing of D atoms benefits from the completely dissociative adsorption of N2 on the dinuclear metal centres. The extra D atoms in Ta2D4- compared to Ta2D2- are helpful to create sufficient electron density at the adsorption site and modify the symmetry of active orbitals to facilitate a further reduction of N2. This comparative study provides a molecular-level insight to understand the high structure-modulating capability of the additional hydride ligands in polyhydride species in the adsorption and activation of nitrogen molecules.

Reverse water-gas shift reaction catalyzed by diatomic rhodium anions.

The reverse water-gas shift (RWGS, CO2 + H2 → CO + H2O, ΔH298 = +0.44 eV) reaction mediated by the diatomic anion Rh2- was successfully constructed. The generation of a gas-phase H2O molecule and ion product [Rh2(CO)ads]- was identified unambiguously at room temperature and the only elementary step that requires extra energy to complete the catalysis is the desorption of CO from [Rh2(CO)ads]-. This experimentally identified Rh2- anion represents the first gas-phase species that can drive the RWGS reaction because it is challenging to design effective routes to yield H2O from CO2 and H2. The reactions were performed by using our newly developed double ion trap reactors and characterized by mass spectrometry, photoelectron spectroscopy, and high-level quantum-chemical calculations. We found that the order that the reactants (CO2 or D2) were fed into the reactor did not have a pronounced impact on the reactivity and the final product distribution (D2O and Rh2CO-). The atomically precise insights into the key steps to guide the reaction toward the RWGS direction were provided.

Size-dependent reactivity of rhodium deuteride cluster anions Rh3Dn - (n = 0-3) toward dinitrogen: The prominent role of σ donation.

Nitrogen (N2) fixation is a challenging task for chemists. Adsorption of N2 on transition metal (TM) sites has been identified as a prerequisite for activating the very stable N≡N triple bond in both industrial and biological processes. The importance of π back-donation (filled orbitals of TM → π* orbitals of N2) between metal sites and N2 has been well elucidated while the role of another classic orbital interaction, namely σ donation (σ orbitals of N2 → empty orbitals of TM), remains ambiguous. Herein, the size-dependent reactivity of trinuclear rhodium deuteride cluster anions Rh3Dn - (n = 0-3) toward N2 adsorption in the gas phase was investigated experimentally and theoretically. A reverse relationship that higher electron-donating ability of clusters corresponds to lower N2 adsorption reactivity was experimentally observed, which is uncommon in N2 activation by gas-phase species. Theoretical analysis revealed that the σ donation rather than the π back-donation plays a predominant role in the adsorption complexes Rh3DnN2 - and the enhanced reactivity upon D addition is ascribed to the lowered energy levels of active orbitals in Rh3Dn - as n increases. This study provides the first experimental evidence to declare the important role of σ donation and new clues for the design of reactive metal species in nitrogen fixation.

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.

A Facile N≡N Bond Cleavage by the Trinuclear Metal Center in Vanadium Carbide Cluster Anions V3C4.

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.

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 Ta3Sn (n = 0-3) could adsorb N2, while Ta3S4 was inert to N2. The DFT calculations revealed that the complete cleavage of the N≡N bond on the trinuclear metal center in the Ta3S0-3/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 N2.

Side-on-End-on Coordination of Dinitrogen on a Polynuclear Vanadium Nitride Cluster Anion [V5 N5 ].

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).

C-N Coupling in N2 Fixation by the Ditantalum Carbide Cluster Anions Ta2C4.

The construction of C-N bonds by the direct incorporation of dinitrogen (N2) instead of ammonia (NH3) into active species is particularly desirable but has been rarely reported. Herein, a ditantalum carbide cluster anion (Ta2C4-) capable of cleaving the N≡N bond and constructing a C-N bond under mild conditions has been identified using mass spectrometry, photoelectron imaging spectroscopy, and quantum-chemical calculations. The photoelectron spectrum of Ta2C4N2- is remarkably different from that of Ta2C4- and matches the simulated spectrum of the Ta2C4N2- species with an end-on-bonded CN unit. The formation of the C-N bond has also been supported by the CN- fragment observed in the collision-induced dissociation of Ta2C4N2-. The exceptional reactivity of Ta2C4- is ascribed to the low-valent metal center serving as an electron reservoir. This study provides a non-NH3 route to construct C-N bonds by incorporating N2 into carbide compounds to produce nitrogenous species.

Catalytic CO Oxidation by Gas-Phase Metal Oxide Clusters.

Oxidation of CO into CO2 is a prototypical reaction in heterogeneous catalysis and is one of the extensively studied reactions in the gas phase to explore the underlying mechanisms of related catalysis. In this Feature Article, we present and discuss our recent advances in the fundamental understanding of catalytic CO oxidation by O2 mediated with heteronuclear metal oxide clusters (HMOCs) using state-of-the-art mass spectrometry and quantum chemistry calculations. The HMOCs can be considered as ideal models for active sites on mixed or oxide supported catalysts at a strictly molecular level. A concept of electronegativity-ladder effect was proposed to account for the enhanced reactivity of noble metal (NM) doped HMOCs, and then this effect was successfully extended in the design of NM-free HMOCs in catalytic CO oxidation by O2. The future directions and the challenges were also discussed.

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 TaC1-3-/N2 reaction systems. The natural bond orbital and molecular orbital analysis indicated that the more positive charge on the metal center of TaC1-4- would facilitate the initial approach of the nonpolar N2 molecule, and the appropriate frontier orbital of TaC1-4- with proper symmetry (π-type 5d orbital) which can match up well with the π* antibonding orbital of the N2 molecule with less σ repulsion and more possibility for π back-donation would be helpful for the formation of the stable encounter complexes. This study reveals the fundamental rules and key factors governing the N2 adsorption.

Machine Learning Descriptors for Data-Driven Catalysis Study.

Traditional trial-and-error experiments and theoretical simulations have difficulty optimizing catalytic processes and developing new, better-performing catalysts. Machine learning (ML) provides a promising approach for accelerating catalysis research due to its powerful learning and predictive abilities. The selection of appropriate input features (descriptors) plays a decisive role in improving the predictive accuracy of ML models and uncovering the key factors that influence catalytic activity and selectivity. This review introduces tactics for the utilization and extraction of catalytic descriptors in ML-assisted experimental and theoretical research. In addition to the effectiveness and advantages of various descriptors, their limitations are also discussed. Highlighted are both 1) newly developed spectral descriptors for catalytic performance prediction and 2) a novel research paradigm combining computational and experimental ML models through suitable intermediate descriptors. Current challenges and future perspectives on the application of descriptors and ML techniques to catalysis are also presented.

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.

Mutual functionalization of dinitrogen and methane mediated by heteronuclear metal cluster anions CoTaC2.

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.

Dual Iron Sites in Activation of N2 by Iron-Sulfur Cluster Anions Fe5S2- and Fe5S3.

Inspired by the fact that the active centers of natural nitrogenases are polynuclear iron-sulfur clusters, the reactivity of isolated iron-sulfur clusters toward N2 has received considerable attention to gain fundamental insights into the activation of the N≡N triple bond. Herein, a series of gas-phase iron-sulfur cluster anions FexSy- (x = 1-8, y = 0-x) were prepared and their reactivities toward N2 were investigated systematically by mass spectrometry. Among the 44 investigated clusters, only Fe5S2- and Fe5S3- showed superior reactivity toward N2. Theoretical results revealed that N2 binds molecularly to the iron sites of Fe5S2,3- in a common end-on coordination mode with an unprecedented back-donation interaction from the localized d-d bonding orbitals of Fe-Fe sites to the π* antibonding orbitals of N2. This is the first example to disclose the significant contribution of the dual metal sites rather than the single metal atom to N2 adsorption in the prevalent end-on binding mode.

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.

Size-Dependent Association of Cobalt Deuteride Cluster Anions Co3Dn- (n = 0-4) with Dinitrogen.

Dinitrogen (N2) activation by metal hydride species is of fundamental interest and practical importance while the role of hydrogen in N2 activation is not well studied. Herein, the structures of Co3Dn- (n = 0-4) clusters and their reactions with N2 have been studied by using a combined experimental and computational approach. The mass spectrometry experiments identified that the Co3Dn- (n = 2-4) clusters could adsorb N2 while the Co3Dn- (n = 0 and 1) clusters were inert. The photoelectron imaging spectroscopy indicated that the electron detachment energies of Co3D2-4- are smaller than those of Co3D0,1-, which characterized that it is easier to transfer electrons from Co3D2-4- than from Co3D0,1- to activate N2. The density functional theory calculations generally supported the experimental observations. Further analysis revealed that the H atoms in the Co3Hn- (n = 2-4) clusters generally result in higher energies of the Co 3d orbitals in comparison with the Co3Hn- (n = 0 and 1) systems. By forming chemical bonds with H atoms, the Co atoms of Co3H2-4- are less negatively charged with respect to the naked Co3- system, which leads to higher N2 binding energies of Co3H2-4N2- than that of Co3N2-.