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A fundamental understanding of the exact structural characteristics and reaction mechanisms of interface active sites is vital to engineering an energetic metal–support boundary in heterogeneous catalysis. Herein, benefiting from a newly developed high-temperature ion trap reactor, the reverse water–gas shift (RWGS) (CO2 + H2 → CO + H2O) catalyzed by a series of compositionally and structurally well-defined RhnVO3,4− (n = 3–7) clusters were identified under variable temperatures (298–773 K). It is discovered that the Rh5–7VO3,4− clusters can function more effectively to drive RWGS at relatively low temperatures. The experimentally observed size-dependent catalytic behavior was rationalized by quantum-chemical calculations; the framework of RhnVO3,4− is constructed by depositing the Rhn clusters on the VO3,4 "support", and a sandwiched base–acid–base [Rhout−–Rhin+–VO3,4−; Rhout and Rhin represent the outer and inner Rh atoms, respectively] feature in Rh5–7VO3,4− governs the adsorption and activation of reactants as well as the facile desorption of the products. In contrast, isolated Rh5–7− clusters without the electronic modification of the VO3,4 "support" can only catalyze RWGS under relatively high-temperature conditions. [ABSTRACT FROM AUTHOR]

Cost-effective and readily accessible 3d transition metals (TMs) have been considered as promising candidates for alkane activation while 3d TMs especially the early TMs are usually not very reactive with light alkanes. In this study, the reactivity of Vn+ and VnO+ (n = 1–9) cluster cations towards ethane under thermal collision conditions has been investigated using mass spectrometry and density functional theory calculations. Among Vn+ (n = 1–9) clusters, only V3–5+ can react with C2H6 to generate dehydrogenation products and the reaction rate constants are below 10−13 cm3 molecule−1 s−1. In contrast, the reaction rate constants for all VnO+ (n = 1–9) with C2H6 significantly increase by about 2–4 orders of magnitude. Theoretical analysis evidences that the addition of ligand O affects the charge distribution of the metal centers, resulting in a significant increase in the cluster reactivity. The analysis of frontier orbitals indicates that the agostic interaction determines the size-dependent reactivity of VnO+ cluster cations. This study provides a novel approach for improving the reactivity of early 3d TMs. [ABSTRACT FROM AUTHOR]

The key to optimizing energy-consuming catalytic conversions lies in acquiring a fundamental understanding of the nature of the active sites and the mechanisms of elementary steps at an atomically precise level, while it is challenging to capture the crucial step that determines the overall temperature of a real-life catalytic reaction. Herein, benefiting from a newly-developed high-temperature ion trap reactor, the reverse water-gas shift (CO2 + H2 → CO + H2O) reaction catalyzed by the Rhn− (n = 3–11) clusters was investigated under variable temperatures (298–783 K) and the critical temperature that each elementary step (Rhn− + CO2 and RhnO− + H2) requires to take place was identified. The Rh4− cluster strikingly surpasses other Rhn− clusters to drive the catalysis at a mild starting temperature (∼440 K). This finding represents the first example that a specifically sized cluster catalyst that works under an optimum condition can be accurately filtered by using state-of-the-art mass spectrometric experiments and rationalized by quantum-chemical calculations. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The increasingly stringent emission regulations make it urgent to get a fundamental understanding on the catalytic behaviors of rhodium, the most efficient component in three-way catalysts (TWCs) to convert NOx into N2. Herein, the catalytic conversion of NO and CO into N2 and CO2 by gas-phase rhodium-aluminum oxide species (RhAlO0–3 and RhAl2O1–4) was identified by mass spectrometry and quantum chemical calculations. This finding represents a significant step in the field of gas-phase catalysis, given that only elementary reactions for this conversion are commonly observed because it is extremely challenging to rupture the strong N–O bond and couple the N–N bond as well as drive the resulting product to oxidize CO. It emphasized that a polarized Rh–Al bond in gas-phase species is indispensable and the RhAlO0,1 and RhAl2O1,2 species with direct Rh–Al bonding can be involved in catalytic NO reduction by CO. [ABSTRACT FROM AUTHOR]

The ability to activate methane differs appreciably for different transition metals, and it is attractive to find the most suitable metal for the direct conversion of methane to value-added chemicals. Herein, we performed a comparative study on the reactions of CH4 with Au2+, AuRh+ and Rh2+ cations by mass-spectrometry based experiments and DFT-based theoretical analysis. Different reactivity has been found for these cations: Au2+ has the lowest reactivity, and it can activate methane but only produce H–Au2–CH3+ without H2 release; Rh2+ has the highest reactivity, and it can produce both carbene-type Rh2–CH2+ and carbyne-type H–Rh2–CH+ with H2 release; AuRh+ also has high reactivity to produce only AuRh–CH2+ with H2, avoiding the excessive dehydrogenation of CH4. Our theoretical results demonstrate that Rh is responsible for the high reactivity, while Au leads to selectivity, which may be caused by the unique intrinsic bonding properties of the metals. [ABSTRACT FROM AUTHOR]

Investigation on the reactivity of atomic clusters represents an important approach to discover new species to activate and transform methane, the most stable alkane molecule. While a few types of transition metal species have been found to be capable of cleaving the C–H bond of methane, methane activation by the transition metal boride species has not been explored yet. This study reports that vanadium boride cluster cations VBn+ (n = 3–6) can dehydrogenate methane under thermal collision conditions. The mechanistic details of the efficient reactions have been elucidated by quantum chemistry calculations on the VB3+ reaction system. Compared to the non-polar bare B3 cluster, the B3 moiety in VB3+ can be polarized by the V+ cation and thus its reactivity toward methane can be much enhanced. This study provides new insights into the rational design of boron-based catalysts for methane activation. [ABSTRACT FROM AUTHOR]

For many decades, astronomers have searched for biological molecules, including amino acids, in the interstellar medium; this endeavor is important for investigating the hypothesis of the origin of life from space. The space environment is complex and atomic species, such as nitrogen and oxygen atoms, are widely distributed. In this work, the reactions of eight typical deprotonated amino acids (glycine, alanine, cysteine, proline, aspartic acid, histidine, tyrosine, and tryptophan) with ground state nitrogen and oxygen atoms are studied by experiment and theory. These amino acid anions do not react with nitrogen atoms. However, the reactions of these ions with oxygen atoms show an intriguing variety of ionic products and the reaction rate constants are of the order of 10−10 cm3 s−1. Density functional calculations provide detailed mechanisms of the reactions, and demonstrate that spin conversion is essential for some processes. Our study provides important data and insights for understanding the kinetic and dynamic behavior of amino acids in space environments. [ABSTRACT FROM AUTHOR]

The reactivity of gas-phase cluster anions Ta3N2− with C2H4 under thermal collision conditions was studied by mass spectrometry in conjunction with density functional theory calculations. The full dehydrogenation of the C2H4 molecule was observed, with the formation of two dihydrogen molecules. Interestingly, the two carbon atoms originating from the first C2H4 molecule are used to construct another cluster Ta3N2C2−, which can activate one more C2H4 releasing one H2 molecule. Therefore, three dihydrogen molecules are liberated from two ethene molecules in the overall reaction. The full dehydrogenation of C2H4 by gas-phase anions as well as the structure and reactivity of M–N–C (M: transition metal) cluster is reported for the first time. The properties of Ta3N2− and Ta3N2C2− elucidated herein are of use in providing fundamental information that is necessary to tailor the design of new and effective catalysts by applying the related materials. [ABSTRACT FROM AUTHOR]

The study of gas phase ion–molecule reactions by state-of-the-art mass spectrometric experiments in conjunction with quantum chemistry calculations offers an opportunity to clarify the elementary steps and mechanistic details of bond activation and conversion processes. In the past few decades, a considerable number of publications have been devoted to the ion–molecule reactions of metal clusters, the experimentally and theoretically tractable models for the active phase of condensed phase systems. The focus of this perspective concerns progress on activation and transformation of important inorganic and organic molecules by negatively charged metal clusters. The metal cluster anions cover bare metal clusters as well as ligated systems with oxygen, carbon, and nitrogen, among others. The following important issues have been summarized and discussed: (i) dependence of chemical reactivity and selectivity on cluster structures and sizes, metals and metal oxidation states, odd–even electron numbers, etc. and (ii) effects of doping, ligation, and pre-adsorption on the reactivity of metal clusters toward rather inert molecules. [ABSTRACT FROM AUTHOR]

Cerium–vanadium oxide cluster cations CeVO5+ were generated by laser ablation, mass-selected using a quadrupole mass filter, thermalized through collisions with helium atoms, and then reacted with ethene molecules in a linear ion trap reactor. The cluster reactions have been characterized by time-of-flight mass spectrometry and density functional theory calculations. The CeVO5+ cluster has a closed-shell electronic structure and contains a peroxide (O22−) unit. The cluster bonded O22− species is reactive enough to oxidize a C2H4 molecule to generate C2H4O2 that can be an acetic acid molecule. Atomic oxygen radicals (O−Ṗ), superoxide radicals (O2−Ṗ), and peroxides are the three common reactive oxygen species. The reactivity of cluster bonded O−Ṗ and O2−Ṗ radicals has been widely studied while the O22− species were generally thought to be much less reactive or inert toward small molecules under thermal collision conditions. This work is among the first to report the reactivity of the peroxide unit on transition metal oxide clusters with hydrocarbon molecules, to the best of our knowledge. [ABSTRACT FROM AUTHOR]

We investigated the interaction of TiO+ with water by conducting infrared photodissociation spectroscopy and density functional theory calculations on TiO(H2O)Ar+ and TiO(H2O)5–7+ clusters. The studies show that TiO(H2O)Ar+ has two isomers, Ti(OH)2Ar+ and (H2O)–TiOAr+, coexisting in our experiments. The structure of TiO(H2O)5+ is characterized by attaching four water molecules to a Ti(OH)2+ core with their O atoms interacting with the Ti atom directly. With the increasing number of water molecules, the additional water molecules start to form hydrogen bonds with the inner shell water molecules and the OH groups of Ti(OH)2+ instead of coordinating directly with the Ti atom. Therefore, the structures of TiO(H2O)6+ and TiO(H2O)7+ clusters are evolved from that of TiO(H2O)5+ by adding the sixth and seventh water molecules to the second solvent-shell. Our results demonstrate that a Ti(OH)2+ type of product is dominant when TiO+ interacts with water, especially when more water molecules are involved. [ABSTRACT FROM AUTHOR]

Aluminum oxide cluster anions are produced by laser ablation and reacted with n-butane in a fast flow reactor. A reflectron time-of-flight mass spectrometer is used to detect the cluster distribution before and after the reactions. Aluminum oxide clusters Al2O4,6− and Al3O7− can react with n-C4H10 to produce Al2O4,6H− and Al3O7H−, respectively, while cluster Al3O6− reacts with n-C4H10 to produce both the Al3O6H− and Al3O6H2−. The theoretical calculations are performed to study the structures and bonding properties of clusters Al2O4,6− and Al3O6,7− as well as the reaction mechanism of Al2O4− + n-C4H10. The calculated results show that the mononuclear oxygen-centred radicals (O−Ṗ) on Al2O4,6− and Al3O7−, and oxygen-centred biradical on Al3O6− are the active sites responsible for the observed hydrogen atom abstraction reactivity. Furthermore, mechanism investigation of the O−Ṗ generation in Al3O7− upon O2 molecule adsorption on un-reactive Al3O5− is performed by theoretical calculations. [ABSTRACT FROM AUTHOR]

We introduce chemical structures and reactivity of oxygen-centred radicals (O−) over transition metal oxide (TMO) clusters based on mass spectrometric and density functional theory studies. Two main issues will be discussed: (1) the compositions of TMO clusters that have the bonding characteristics of (or contain) the O− radicals; and (2) the dependences (cluster structures, sizes, charge states, metal types, etc.) of the chemical reactivity and selectivity for the O− radicals over TMO clusters. One of the goals of cluster chemistry is to understand the elementary reactions involved with complex heterogeneous catalysis. The study of the O− containing TMO clusters permits rather detailed descriptions for how mono-nuclear oxygen-centred radicals may exist and react with small molecules over TMO based catalysts. [ABSTRACT FROM AUTHOR]

A series of vanadium and phosphorus heteronuclear oxide cluster cations (VxPyOz+) are prepared by laser ablation and the reactions of V3PO10+and V4O10+with methane in a fast flow reactor under the same conditions are studied. A time of flight mass spectrometer is used to detect the cluster distribution before and after reactions. In addition to previously identified reaction of V4O10++ CH4→ V4O10H++ CH3, the observation of hydrogen atom pickup cluster V3PO10H+suggests the reaction: V3PO10++ CH4→ V3PO10H++ CH3. The rate of the reaction of V4O10+with CH4is approximately 2.5 times faster than that of V3PO10+with CH4. Density functional theory (DFT) calculations predict that structure of V3PO10+is topologically similar to that of V4O10+, as well as that of P4O10+, which is very similar to V4O10+in terms of methane activation in previous studies. The facile methane activation by the homo- and hetero-nuclear oxide clusters can all be attributed to the presence of an oxygen-centered radical (O) in these clusters. Further theoretical study indicates that the O radical (or spin density of the cluster) can transfer within the high symmetry V4O10+and P4O10+clusters quite easily, and CH4molecule further enhances the rate of intra-cluster spin density transfer. In contrast, the intra-cluster spin density transfer within low symmetry V3PO10+is thermodynamically forbidden. The experimentally observed reactivity difference is consistent with the theoretical consideration of the intra-cluster spin density transfer. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Cost-effective and readily accessible 3d transition metals (TMs) have been considered as promising candidates for alkane activation while 3d TMs especially the early TMs are usually not very reactive with light alkanes. In this study, the reactivity of V n + and V n O + ( n = 1-9) cluster cations towards ethane under thermal collision conditions has been investigated using mass spectrometry and density functional theory calculations. Among V n + ( n = 1-9) clusters, only V 3-5 + can react with C 2 H 6 to generate dehydrogenation products and the reaction rate constants are below 10 -13 cm 3 molecule -1 s -1 . In contrast, the reaction rate constants for all V n O + ( n = 1-9) with C 2 H 6 significantly increase by about 2-4 orders of magnitude. Theoretical analysis evidences that the addition of ligand O affects the charge distribution of the metal centers, resulting in a significant increase in the cluster reactivity. The analysis of frontier orbitals indicates that the agostic interaction determines the size-dependent reactivity of V n O + cluster cations. This study provides a novel approach for improving the reactivity of early 3d TMs.

A fundamental understanding of the exact structural characteristics and reaction mechanisms of interface active sites is vital to engineering an energetic metal-support boundary in heterogeneous catalysis. Herein, benefiting from a newly developed high-temperature ion trap reactor, the reverse water-gas shift (RWGS) (CO 2 + H 2 → CO + H 2 O) catalyzed by a series of compositionally and structurally well-defined Rh n VO 3,4 - ( n = 3-7) clusters were identified under variable temperatures (298-773 K). It is discovered that the Rh 5-7 VO 3,4 - clusters can function more effectively to drive RWGS at relatively low temperatures. The experimentally observed size-dependent catalytic behavior was rationalized by quantum-chemical calculations; the framework of Rh n VO 3,4 - is constructed by depositing the Rh n clusters on the VO 3,4 "support", and a sandwiched base-acid-base [Rh out - -Rh in + -VO 3,4 - ; Rh out and Rh in represent the outer and inner Rh atoms, respectively] feature in Rh 5-7 VO 3,4 - governs the adsorption and activation of reactants as well as the facile desorption of the products. In contrast, isolated Rh 5-7 - clusters without the electronic modification of the VO 3,4 "support" can only catalyze RWGS under relatively high-temperature conditions.

The key to optimizing energy-consuming catalytic conversions lies in acquiring a fundamental understanding of the nature of the active sites and the mechanisms of elementary steps at an atomically precise level, while it is challenging to capture the crucial step that determines the overall temperature of a real-life catalytic reaction. Herein, benefiting from a newly-developed high-temperature ion trap reactor, the reverse water-gas shift (CO 2 + H 2 → CO + H 2 O) reaction catalyzed by the Rh n - ( n = 3-11) clusters was investigated under variable temperatures (298-783 K) and the critical temperature that each elementary step (Rh n - + CO 2 and Rh n O - + H 2 ) requires to take place was identified. The Rh 4 - cluster strikingly surpasses other Rh n - clusters to drive the catalysis at a mild starting temperature (∼440 K). This finding represents the first example that a specifically sized cluster catalyst that works under an optimum condition can be accurately filtered by using state-of-the-art mass spectrometric experiments and rationalized by quantum-chemical calculations.

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 V 4 N 5 - 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 V 4 N 5 CO - 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 (V 3 N 4 )VN - cluster and the V 3 N 4 moiety resembles a support. This finding strongly suggests that a novel mode for facile CO dissociation was identified in a gas-phase cluster study.

The activation and transformation of molecular nitrogen (N 2 ) by metal hydride species has attracted widespread attention due to its critical role in nitrogen fixation. Herein, the reactions between tantalum deuteride cluster anions Ta 2 D 2,4 - and N 2 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 Ta 2 D 4 - . Theoretical investigations indicate that the releasing of D atoms benefits from the completely dissociative adsorption of N 2 on the dinuclear metal centres. The extra D atoms in Ta 2 D 4 - compared to Ta 2 D 2 - are helpful to create sufficient electron density at the adsorption site and modify the symmetry of active orbitals to facilitate a further reduction of N 2 . 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.

Noble metals dispersed on transition-metal carbides exhibit extraordinary activity in CO 2 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 CO 2 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 RhTaC 2 - clusters with CO 2 was studied. The experimental results identified that RhTaC 2 - can reduce four CO 2 molecules consecutively and generate the product RhTaC 2 O 4 - . The pivotal roles of Rh-Ta synergy and the C 2 ligand in driving CO 2 reduction were rationalized by theoretical calculations. The presence of an attached CO unit on the product RhTaC 2 O 4 - was evidenced by the collision-induced dissociation experiment, providing a fundamental strategy to alleviate carbon deposition under a CO 2 atmosphere at elevated temperatures.

The direct coupling of dinitrogen (N 2 ) and methane (CH 4 ) 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 N 2 and CH 4 mediated by heteronuclear metal cluster anions CoTaC 2 - , which starts with the dissociative adsorption of N 2 on CoTaC 2 - to generate a Ta δ + -N t δ - (terminal-nitrogen) Lewis acid-base pair (LABP), followed by the further activation of CH 4 by CoTaC 2 N 2 - to construct the N-C bond. The N[triple bond, length as m-dash]N cleavage by CoTaC 2 - affording two N atoms with strong charge buffering ability plays a key part, which facilitates the H 3 C-H cleavage via the LABP mechanism and the N-C formation via a CH 3 migration mechanism. A novel N t triggering strategy to couple N 2 and CH 4 molecules using metal clusters was accordingly proposed, which provides a new idea for the direct synthesis of N-containing compounds., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The ability to activate methane differs appreciably for different transition metals, and it is attractive to find the most suitable metal for the direct conversion of methane to value-added chemicals. Herein, we performed a comparative study on the reactions of CH 4 with Au 2 + , AuRh + and Rh 2 + cations by mass-spectrometry based experiments and DFT-based theoretical analysis. Different reactivity has been found for these cations: Au 2 + has the lowest reactivity, and it can activate methane but only produce H-Au 2 -CH 3 + without H 2 release; Rh 2 + has the highest reactivity, and it can produce both carbene-type Rh 2 -CH 2 + and carbyne-type H-Rh 2 -CH + with H 2 release; AuRh + also has high reactivity to produce only AuRh-CH 2 + with H 2 , avoiding the excessive dehydrogenation of CH 4 . Our theoretical results demonstrate that Rh is responsible for the high reactivity, while Au leads to selectivity, which may be caused by the unique intrinsic bonding properties of the metals.

Investigation on the reactivity of atomic clusters represents an important approach to discover new species to activate and transform methane, the most stable alkane molecule. While a few types of transition metal species have been found to be capable of cleaving the C-H bond of methane, methane activation by the transition metal boride species has not been explored yet. This study reports that vanadium boride cluster cations VB n + (n = 3-6) can dehydrogenate methane under thermal collision conditions. The mechanistic details of the efficient reactions have been elucidated by quantum chemistry calculations on the VB 3 + reaction system. Compared to the non-polar bare B 3 cluster, the B 3 moiety in VB 3 + can be polarized by the V + cation and thus its reactivity toward methane can be much enhanced. This study provides new insights into the rational design of boron-based catalysts for methane activation.

The reactivity of gas-phase cluster anions Ta 3 N 2 - with C 2 H 4 under thermal collision conditions was studied by mass spectrometry in conjunction with density functional theory calculations. The full dehydrogenation of the C 2 H 4 molecule was observed, with the formation of two dihydrogen molecules. Interestingly, the two carbon atoms originating from the first C 2 H 4 molecule are used to construct another cluster Ta 3 N 2 C 2 - , which can activate one more C 2 H 4 releasing one H 2 molecule. Therefore, three dihydrogen molecules are liberated from two ethene molecules in the overall reaction. The full dehydrogenation of C 2 H 4 by gas-phase anions as well as the structure and reactivity of M-N-C (M: transition metal) cluster is reported for the first time. The properties of Ta 3 N 2 - and Ta 3 N 2 C 2 - elucidated herein are of use in providing fundamental information that is necessary to tailor the design of new and effective catalysts by applying the related materials.

The study of gas phase ion-molecule reactions by state-of-the-art mass spectrometric experiments in conjunction with quantum chemistry calculations offers an opportunity to clarify the elementary steps and mechanistic details of bond activation and conversion processes. In the past few decades, a considerable number of publications have been devoted to the ion-molecule reactions of metal clusters, the experimentally and theoretically tractable models for the active phase of condensed phase systems. The focus of this perspective concerns progress on activation and transformation of important inorganic and organic molecules by negatively charged metal clusters. The metal cluster anions cover bare metal clusters as well as ligated systems with oxygen, carbon, and nitrogen, among others. The following important issues have been summarized and discussed: (i) dependence of chemical reactivity and selectivity on cluster structures and sizes, metals and metal oxidation states, odd-even electron numbers, etc. and (ii) effects of doping, ligation, and pre-adsorption on the reactivity of metal clusters toward rather inert molecules.

Cerium-vanadium oxide cluster cations CeVO5(+) were generated by laser ablation, mass-selected using a quadrupole mass filter, thermalized through collisions with helium atoms, and then reacted with ethene molecules in a linear ion trap reactor. The cluster reactions have been characterized by time-of-flight mass spectrometry and density functional theory calculations. The CeVO5(+) cluster has a closed-shell electronic structure and contains a peroxide (O2(2-)) unit. The cluster bonded O2(2-) species is reactive enough to oxidize a C2H4 molecule to generate C2H4O2 that can be an acetic acid molecule. Atomic oxygen radicals (O(-)˙), superoxide radicals (O2(-)˙), and peroxides are the three common reactive oxygen species. The reactivity of cluster bonded O(-)˙ and O2(-)˙ radicals has been widely studied while the O2(2-) species were generally thought to be much less reactive or inert toward small molecules under thermal collision conditions. This work is among the first to report the reactivity of the peroxide unit on transition metal oxide clusters with hydrocarbon molecules, to the best of our knowledge.

We investigated the interaction of TiO(+) with water by conducting infrared photodissociation spectroscopy and density functional theory calculations on TiO(H2O)Ar(+) and TiO(H2O)5-7(+) clusters. The studies show that TiO(H2O)Ar(+) has two isomers, Ti(OH)2Ar(+) and (H2O)-TiOAr(+), coexisting in our experiments. The structure of TiO(H2O)5(+) is characterized by attaching four water molecules to a Ti(OH)2(+) core with their O atoms interacting with the Ti atom directly. With the increasing number of water molecules, the additional water molecules start to form hydrogen bonds with the inner shell water molecules and the OH groups of Ti(OH)2(+) instead of coordinating directly with the Ti atom. Therefore, the structures of TiO(H2O)6(+) and TiO(H2O)7(+) clusters are evolved from that of TiO(H2O)5(+) by adding the sixth and seventh water molecules to the second solvent-shell. Our results demonstrate that a Ti(OH)2(+) type of product is dominant when TiO(+) interacts with water, especially when more water molecules are involved.

Aluminum oxide cluster anions are produced by laser ablation and reacted with n-butane in a fast flow reactor. A reflectron time-of-flight mass spectrometer is used to detect the cluster distribution before and after the reactions. Aluminum oxide clusters Al₂O4,6⁻ and Al₃O₇⁻ can react with n-C₄H₁₀ to produce Al₂O4,6H⁻ and Al₃O₇⁻, respectively, while cluster Al₃O₆⁻ reacts with n-C₄H₁₀ to produce both the Al₃O₆H⁻ and Al₃O₆H₂⁻. The theoretical calculations are performed to study the structures and bonding properties of clusters Al₂O4,6⁻ and Al₃O6,7⁻ as well as the reaction mechanism of Al₂O₄⁻ + n-C₄H₁₀. The calculated results show that the mononuclear oxygen-centred radicals (O⁻˙) on Al₂O4,6⁻ and Al₃O₇⁻, and oxygen-centred biradical on Al₃O₆⁻ are the active sites responsible for the observed hydrogen atom abstraction reactivity. Furthermore, mechanism investigation of the O⁻˙ generation in Al₃O₇⁻ upon O₂ molecule adsorption on un-reactive Al₃O₅⁻ is performed by theoretical calculations.

Vanadium oxide cluster anions (V(x)O(y)(-), x = 2-3; y = 3-7) are produced by laser ablation and reacted with water in a fast flow reactor. A time-of-flight mass spectrometer is used to detect the cluster distribution before and after the reactions. Reaction channels of molecular hydrogen elimination (for V(2,3)O(3)(-)), water association (for V(2)O(5)(-) and V(3)O(6,7)(-)) and the coexistence of both channels (for V(2)O(4)(-) and V(3)O(4,5)(-)) are observed. V(2)O(6)(-) and V(3)O(8)(-) are nearly inert toward water. Density functional theory (DFT) calculations are performed to study the reaction mechanism of V(2)O(3)(-) in different spin states with water and the results support the experimental observation. The reaction mechanism of V(2)O(3)(+) with water is also studied, which is in agreement with the experimental report in previous literature [Eur. J. Inorg. Chem., 2008, 4961] that molecular hydrogen elimination is a minor reaction channel for V(2)O(3)(+) + H(2)O. The influence of cluster charge states and oxidation states of vanadium atoms on the cluster reactivity are presented based on the experimental and theoretical studies.

The Sc(3)O(6)(-) cluster anions were produced by laser ablation and studied by reaction with n-butane in a fast flow reactor and by photoelectron spectroscopy. The reactivity experiments indicated that one Sc(3)O(6)(-) cluster can activate two n-butane molecules consecutively with rate constants on the order of 10(-10) cm(3) molecule(-1) s(-1) under near room-temperature conditions, suggesting that the even-electron system Sc(3)O(6)(-) has a highly reactive electronic structure. The photoelectron spectroscopy determined a high vertical detachment energy (VDE) of 5.63 ± 0.08 eV for the Sc(3)O(6)(-) cluster. Density functional computations indicated that the lowest energy isomer of Sc(3)O(6)(-) is an oxygen-centered biradical with a high VDE and is highly reactive toward n-butane, which is in good agreement with the experiments. The Sc(3)O(6)(-) cluster may serve as an ideal model system to provide insight into the real-life chemistry involved with the coupled O(-)˙···O(-)˙ dimers over the surfaces of metal oxide catalysts.

A series of vanadium and phosphorus heteronuclear oxide cluster cations (V(x)P(y)O(z)(+)) are prepared by laser ablation and the reactions of V(3)PO(10)˙(+) and V(4)O(10)˙(+) with methane in a fast flow reactor under the same conditions are studied. A time of flight mass spectrometer is used to detect the cluster distribution before and after reactions. In addition to previously identified reaction of V(4)O(10)˙(+) + CH(4)→ V(4)O(10)H(+) + CH(3)˙, the observation of hydrogen atom pickup cluster V(3)PO(10)H(+) suggests the reaction: V(3)PO(10)˙(+) + CH(4)→ V(3)PO(10)H(+) + CH(3)˙. The rate of the reaction of V(4)O(10)˙(+) with CH(4) is approximately 2.5 times faster than that of V(3)PO(10)˙(+) with CH(4). Density functional theory (DFT) calculations predict that structure of V(3)PO(10)˙(+) is topologically similar to that of V(4)O(10)˙(+), as well as that of P(4)O(10)˙(+), which is very similar to V(4)O(10)˙(+) in terms of methane activation in previous studies. The facile methane activation by the homo- and hetero-nuclear oxide clusters can all be attributed to the presence of an oxygen-centered radical (O˙) in these clusters. Further theoretical study indicates that the O˙ radical (or spin density of the cluster) can transfer within the high symmetry V(4)O(10)˙(+) and P(4)O(10)˙(+) clusters quite easily, and CH(4) molecule further enhances the rate of intra-cluster spin density transfer. In contrast, the intra-cluster spin density transfer within low symmetry V(3)PO(10)˙(+) is thermodynamically forbidden. The experimentally observed reactivity difference is consistent with the theoretical consideration of the intra-cluster spin density transfer.

Cerium oxide cluster cations (Ce(m)O(n)(+), m = 2-16; n = 2m, 2m +/- 1 and 2m +/- 2) are prepared by laser ablation and reacted with carbon monoxide (CO) and small hydrocarbon molecules (CH(4), C(2)H(4), and C(2)H(6)) in a fast flow reactor. A time of flight mass spectrometer is used to detect the cluster distribution before and after the reactions. The observation of oxygen reduction and hydrogen pickup of Ce(m)O(2m)(+) clusters strongly suggests the following reactions: (1) Ce(m)O(2m)(+) + C(2)H(4) --> Ce(m)O(2m-1)(+) + C(2)H(4)O (m = 2-6); (2) Ce(m)O(2m)(+) + CO --> Ce(m)O(2m-1)(+) + CO(2) (m = 4-6); and (3) Ce(m)O(2m)(+) + CH(4)/C(2)H(6) --> Ce(m)O(2m)H(+) + CH(3)/C(2)H(5) (m = 2-4). Density functional theory (DFT) calculations are performed to study reaction mechanisms of Ce(2)O(4)(+) + X (X = CO, CH(4), C(2)H(4), and C(2)H(6)). The calculated results are in good agreement with the experimental observations. The structural and bonding properties of Ce(m)O(2m)(+) (m = 2-5) clusters are also investigated by the DFT calculations. The unpaired electron in each of the clusters is mainly distributed over one Ce atom (4f and 5p orbitals) and two O atoms (2p orbital) in a CeO(2) moiety, which can be considered as the active site in the cluster. To further understand the nature of the active sites in Ce(m)O(2m)(+) clusters, the fast flow reaction experiments are also carried out on zirconium oxide clusters Zr(m)O(n)(+), because both Zr ([Kr]4d(2)5s(2)) and Ce ([Xe]4f(1)5d(1)6s(2)) have the same number of valence electrons while the latter has one more f and one less d electrons. In addition to the oxygen transfer reactions such as Zr(m)O(2m)(+) + C(2)H(4) --> Zr(m)O(2m-1)(+) + C(2)H(4)O (m = 1-4) reported in the literature, hydrogen abstraction reactions Zr(m)O(2m)(+) + CH(4)/C(2)H(6) --> Zr(m)O(2m)H(+) + CH(3)/C(2)H(5) are also identified. The rate constants of CO oxidation as well as hydrogen abstraction by Ce(m)O(2m)(+) and Zr(m)O(2m)(+) are very different. The reactivity and selectivity of Ce(m)O(2m)(+) versus Zr(m)O(2m)(+) can be well rationalized based on the DFT calculations. The oxygen transfer and hydrogen abstraction reactions studied in this work are of widespread importance. The nature of the active sites of Ce(m)O(2m)(+) clusters is unique and may be considered in the use and design of cerium oxide based catalysts.

Stoichiometric early transition metal oxide cations (TiO(2))(1-5)(+), (ZrO(2))(1-4)(+), (HfO(2))(1-2)(+), (V(2)O(5))(1-5)(+), (Nb(2)O(5))(1-3)(+), (Ta(2)O(5))(1-2)(+), (MoO(3))(1-2)(+), (WO(3))(1-3)(+), and Re(2)O(7)(+) are able to activate the C-H bond of methane under near room temperature conditions.

The intermolecular interactions between two meso-tetraphenylporphyrin diacid H(4)TPPCl(2) monomers are investigated by density functional theory with the PBE1PBE functional and 6-31G* basis set. Structures of five stable isomers of (H(4)TPPCl(2))(2) are determined. It is found that the interaction (IMHB-1) of Cl with ortho H atoms in two phenyl groups of H(4)TPPCl(2) is unique in that it is the strongest interaction between two H(4)TPPCl(2) monomers. Natural bond orbital analysis is carried out to explain the subtle differences of hydrogen bondings in these isomers. To understand the interactions of water molecule with H(4)TPPCl(2) and (H(4)TPPCl(2))(2), structures of H(4)TPPCl(2) x H(2)O and nine isomers of (H(4)TPPCl(2))(2) x H(2)O are also determined. The binding energy of H(4)TPPCl(2) x H(2)O is 36.47 kJ mol(-1), less than that of the most stable structure of (H(4)TPPCl(2))(2) dimers, 41.59 kJ mol(-1). The dimers containing the special IMHB-1 interactions may be the elementary building blocks for the aggregation of H(4)TPPCl(2), which is supported by the crystal structure of H(4)TPPCl(2).H(2)O x 2CH(3)CN. Thus, study of the interactions between two or a small number of gas molecules can provide important information for understanding the main interactions and stable structures in related condensed-phase systems.