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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−. [ABSTRACT FROM AUTHOR]

Gas-phase metal clusters are ideal models to explore transition-metal-mediated N 2 activation mechanism. However, the effective design and search of reactive clusters in N 2 activation are currently hindered by the lack of clear guidelines. Inspired by the Sabatier principle, we discovered in this work that N 2 initial adsorption energy (Δ E ads ) is an important parameter to control the N 2 activation reactivity of metal clusters in the gas phase. This mechanistic insight obtained from high-level calculations rationalizes the N 2 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 Δ E ads , we successfully designed several new reactive clusters for cleaving N≡N triple bond under mild conditions, including FeV 2 S 2 - , TaV 2 C 2 - , and TaV 2 C 3 - , the high N 2 activation reactivity of which has been fully corroborated in our gas phase experiments employing mass spectrometry with collision-induced dissociation. The importance of Δ E ads revealed in this work not only reshapes our understanding of N 2 activation reactions in the gas phase but also could have implication for other N 2 activation processes in the condensed phase. The more general establishment of this new perspective on N 2 activation reactivity warrants future experimental and computational studies.

Cleavage of the strong N≡N bond has long been a great challenge for energy-efficient dinitrogen (N 2 ) fixation; thus a reasonable design of reactive species to activate N 2 under mild conditions is highly desirable and meaningful. Herein a novel N 2 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 FeTaC 2 - . 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 N 2 π*-orbitals while the 3d-LTM (Fe) simultaneously pulls electrons from N 2 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.

Inspired by the fact that the active centers of natural nitrogenases are polynuclear iron-sulfur clusters, the reactivity of isolated iron-sulfur clusters toward N 2 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 Fe x S y - ( x = 1-8, y = 0- x ) were prepared and their reactivities toward N 2 were investigated systematically by mass spectrometry. Among the 44 investigated clusters, only Fe 5 S 2 - and Fe 5 S 3 - showed superior reactivity toward N 2 . Theoretical results revealed that N 2 binds molecularly to the iron sites of Fe 5 S 2,3 - 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 N 2 . This is the first example to disclose the significant contribution of the dual metal sites rather than the single metal atom to N 2 adsorption in the prevalent end-on binding mode.

It is of great importance to study the mechanisms to activate dinitrogen (N 2 ), the very inert molecule, under mild conditions. Gas-phase metal clusters are being actively generated to react with N 2 to identify new reaction types and mechanisms. Herein, an unprecedented, mechanistically unique metal atom (Fe or V) ejection in the thermal reaction of FeV 2 C 2 - with N 2 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 N 2 fixation.

Nitrogen (N 2 ) 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 N 2 has received considerable attention to gain fundamental insights into nitrogen fixation. Herein, neutral tantalum sulfide clusters have been prepared and their reactivity toward N 2 has been investigated by mass spectrometry in conjunction with density functional theory (DFT) calculations. The experimental results showed that Ta 3 S n ( n = 0-3) could adsorb N 2 , while Ta 3 S 4 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 Ta 3 S 0-3 /N 2 reaction systems was overall barrierless under thermal collision conditions. The sulfur ligands can facilitate the approaching of N 2 toward the metal center but weaken the electron-donating ability of the metal center. The inertness of Ta 3 S 4 is ascribed to the electron-deficient state of Ta 3 in Ta 3 S 4 and the least effective orbital interaction in the Ta 3 S 4 /N 2 couple. This study provides new insights into the ligand effect on the interaction of the metal clusters with N 2.

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 N 2 and subsequent C-N coupling on trinuclear carbide cluster anions V 3 C 4 - 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 N 2 bonded with the V 3 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 N 2 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 N 2 . Furthermore, carbon ligands also play a considerable role in enhancing the reactivity of the metal core toward N 2 . This study strongly suggests a new mechanism of N≡N bond cleavage by gas-phase metal clusters.

Oxidation of CO into CO 2 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 O 2 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 O 2 . The future directions and the challenges were also discussed.

The construction of C-N bonds by the direct incorporation of dinitrogen (N 2 ) instead of ammonia (NH 3 ) into active species is particularly desirable but has been rarely reported. Herein, a ditantalum carbide cluster anion (Ta 2 C 4 - ) 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 Ta 2 C 4 N 2 - is remarkably different from that of Ta 2 C 4 - and matches the simulated spectrum of the Ta 2 C 4 N 2 - 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 Ta 2 C 4 N 2 - . The exceptional reactivity of Ta 2 C 4 - is ascribed to the low-valent metal center serving as an electron reservoir. This study provides a non-NH 3 route to construct C-N bonds by incorporating N 2 into carbide compounds to produce nitrogenous species.

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 N 2 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 TaC 4 - was observed to adsorb N 2 , theoretical calculations predicted that the TaC 4 - reaction system (TaC 4 - + N 2 → TaC 4 N 2 - ) has a negligible barrier on the approach of N 2 molecule while insurmountable barriers are located on the reaction pathways of TaC 1-3 - /N 2 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 N 2 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 N 2 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 N 2 adsorption.