Activation and Transformation of Methane on Boron-Doped Cobalt Oxide Cluster Cations CoBO2.

The cleavage of inert C-H bonds in methane at room temperature and the subsequent conversion into value-added products are quite challenging. Herein, the reactivity of boron-doped cobalt oxide cluster cations CoBO2+ toward methane under thermal collision conditions was studied by mass spectrometry experiments and quantum-chemical calculations. In this reaction, one H atom and the CH3 unit of methane were transformed separately to generate the product metaboric acid (HBO2) and one CoCH3+ ion, respectively. Theoretical calculations strongly suggest that a catalytic cycle can be completed by the recovery of CoBO2+ through the reaction of CoCH3+ with sodium perborate (NaBO3), and this reaction generates sodium methoxide (CH3ONa) as the other value-added product. This study shows that boron-doped cobalt oxide species are highly reactive to facilitate thermal methane transformation and may open a way to develop more effective approaches for methane (CH4) activation and conversion under mild conditions.

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

N2 adsorption is a prerequisite for activation and transformation. Time-of-flight mass spectrometry experiments show that the Nb2C6H4+ cation, resulting from the gas-phase reaction of Nb2+ with C6H6, is more favorable for N2 adsorption than Nb+ and Nb2+ cations. Density functional theory calculations reveal the effect of the ortho-C6H4 ligand on N2 adsorption. In Nb2C6H4+, interactions between the Nb-4d and C-2p orbitals enable the Nb2+ cation to form coordination bonds with the ortho-C6H4 ligand. Although the ortho-C6H4 ligand in Nb2C6H4+ is not directly involved in the reaction, its presence increases the polarity of the cluster and brings the highest occupied molecular orbital (HOMO) closer to the lowest occupied molecular orbital (LUMO) of N2, thereby increasing the N2 adsorption energy, which effectively facilitates N2 adsorption and activation. This study provides fundamental insights into the mechanisms of N2 adsorption in "transition metal-organic ligand" systems.

Consecutive Reduction of Five Carbon Dioxide Molecules by Gas-Phase Niobium Carbide Cluster Anions Nb3C4-: Unusual Mechanism for Enhanced Reactivity by the Carbon Ligands.

Studying the cleavage of the C═O bond during CO2 activation at room temperature is highly significant for comprehending the CO2 conversion processes. Herein, mass spectrometry experiments and density functional theory calculations indicate that the niobium carbide anions Nb3C4- can continuously convert five CO2 molecules to CO under thermal collision conditions, while the other clusters with less carbon ligands Nb3C1-3- reduce fewer CO2 molecules. Size-dependent reactivity of Nb3C1-4- cluster anions toward CO2 is observed. Interestingly, the carbon atoms in Nb3C4- not only act as highly active adsorption sites for CO2 but also serve as electron donors to reduce CO2. The stored electrons are released through a carbon-carbon coupling process. Our findings on the role of carbon ligands in enhancing transition metal carbide reactivity can offer new insights for designing active sites on catalysts with both high activity and selectivity.

Manipulating Reactivity of Ir(CH2)0-2+ Cations toward Dinitrogen at Room Temperature: A Unique Dependence on the Organic Ligand Structures.

Nitrogen (N2) activation at room temperature has long been a great challenge. Therefore, the rational design of reactive species to adsorb N2, which is a prerequisite for cleavage of the strong N≡N triple bond in industrial and biological processes, is highly desirable and meaningful. Herein, the N2 adsorption process is controlled by regulating the types and numbers of organic ligands, and the organic ligands are produced through the reactions of Ir+ with methane and ethane. CH4 molecules dissociate on the Ir+ cations to form Ir(CH2)1,2+. The reaction of Ir+ with C2H6 can generate HIrC2H3+, which is different from the structure of Ir(CH2)2+ obtained from Ir+/CH4. The reactivity order of N2 adsorption is Ir(CH2)2+ > HIrC2H3+ ≫ HIrCH+ ≈ Ir+ (almost inert under similar reaction conditions), indicating that different organic ligand structures affect reactivity dramatically. The main reason for this interesting reactivity difference is that the lowest unoccupied molecular orbital (LUMO) level of Ir(CH2)2+ is much closer to the highest occupied molecular orbital (HOMO) level of N2 than those of the other three systems. This study provides new insights into the adsorption of N2 on metal-organic ligand species, in which the organic ligand dominates the reactivity, and it discovers new clues in designing effective transition metal carbine species for N2 activation.

Conversion of Dinitrogen and Oxygen to Nitric Oxide Mediated by Triatomic Yttrium Cations: Reversible N-N Bond Switching.

The direct coupling of dinitrogen (N2) and oxygen (O2) to produce value-added chemicals such as nitric acid (HNO3) at room temperature is fascinating but quite challenging because of the inertness of N2 molecules. Herein, an interesting reaction pathway is proposed for a direct conversion of N2 and O2 mediated by all-metal Y3+ cations. This reaction pattern begins with the N≡N triple bond cleavage by Y3+ to generate a dinitride cation Y2N2+, and the electrons that lead to N2 activation in this process mainly originate from Y atoms. In the following consecutive reactions with two O2 molecules, the electrons stored in the N atoms are gradually released to reduce O2 through re-formation and re-fracture of the N-N bonds, with concomitant release of two NO molecules. Therefore, the reversible N-N bond switching acts as an efficient electron reservoir to drive the oxidation of the reduced N atoms, leading to the formation of NO molecules. This method of producing NO by direct coupling N2 and O2 molecules, which is the reversible N-N bond switching, may provide a new strategy for the direct synthesis of HNO3, etc.

Two Carbon Dioxide Molecules Consecutively Reduced by Metal-Free B2O2- Anions.

Compared with transition metals, nonmetallic elements have always been considered to have low reactivity toward carbon dioxide. However, in recent years, main-group compounds such as boron-based species have gradually attracted increasing attention due to their prospective applications in different kinds of reactions. Herein, we report that metal-free anions B2O2- can promote two CO2 reductions, producing the oxygen-rich product B2O4-. In most of the reported CO2 reduction reactions mediated by transition-metal-containing clusters, transition metals usually provide electrons for the activation of CO2; one oxygen atom in CO2 is transferred to metal atoms, and CO is released from the metal atoms. In sharp contrast, B atoms are electron donors in the current systems and the formed CO is liberated directly from the activated CO2 unit. The synthesis of novel-metal-free gas-phase clusters and investigation of their reactivity toward carbon dioxide as well as reaction mechanisms can provide a fundamental basis in practice for the rational design of active sites on metal-free catalysts.

Plasma-Assisted Coupling Reactions of Dinitrogen and Carbon Dioxide Mediated by Monometallic YB1-4 - ⋠Anions: Carbon-Nitrogen Bond Formation.

Transition-metal catalyzed coupling to form C-N bonds is significant in chemical science. However, the inert nature of N2 and CO2 renders their coupling quite challenging. Herein, we report the activation of dinitrogen in the mild plasma atmosphere by the gas-phase monometallic YB1-4 - anions and further coupling of CO2 to form C-N bonds by using mass spectrometry and theoretical calculation. The observed product anions are NCNBO- and N(BO)2 - , accompanied by the formation of neutral products YO and YB0-2 NC, respectively. We can tune the reactivity and the type of products by manipulating the number of B atoms. The B atoms in YB1-4 N2 - act as electron donors in CO2 reduction reactions, and the carbon atom originating from CO2 serves as an electron reservoir. This is the first example of gas-phase monometallic anions, which are capable to realize the functionalization of N2 with CO2 through C-N bond formation and N-N and C-O bond cleavage.

Plasma-promoted reactions of the heterobimetallic anions CuNb- with dinitrogen and subsequent reactions with carbon dioxide: formation of C-N bonds.

The activation and functionalization of dinitrogen with carbon dioxide into useful chemicals containing C-N bonds are significant research projects but highly challenging. Herein, we report that N2 molecules are dissociated by heterobimetallic CuNb- anions assisted by surface plasma radiation, leading to the formation of CuNbN2- anions; the CuNbN2- anions can further react with CO2 to generate products NCO- with one C-N bond and NbO2NCN- with two C-N bonds under thermal collision conditions. For the activation of dinitrogen, the plasma atmosphere is conducive to the dissociation of the NN bond, which renders the coupling reactions of N2 and CO2 molecules easier to proceed. This is the first report of coupling of N2 and CO2 to generate C-N bonds by making good use of the plasma effect to assist in the activation of N2 molecules. This new strategy with the assistance of plasma provides a practicable route to construct C-N bonds by directly using N2 and CO2 at room temperature.

Lithium-Assisted Dinitrogen Reduction Mediated by Nb2LiNO1-4- Cluster Anions: Electron Donors or Structural Units.

Alkali atoms are usually used as promoters to significantly increase the catalytic activity of transition-metal catalysts in a wide range of reactions such as dinitrogen conversion reactions. However, the role of alkali metal atoms remains controversial. Herein, a series of quaternary cluster anions containing lithium atoms Nb2LiNO1-4- have been synthesized and reacted with N2 at room temperature. The detailed experimental and theoretical investigations indicate that Nb2LiNO- is capable to cleave the N≡N bond and the Li atoms in Nb2LiNO1,2- act as electron donors in the N2 reduction reaction. With the increase in the number of oxygen atoms, the reactivity toward N2 is reduced from adsorption via a side-on end-on mode in Nb2LiNO2- to the inertness of Nb2LiNO4-. In Nb2LiNO3,4- anions, the Li atoms are bonded with oxygen atoms, acting as structural units to stabilize structures. Therefore, the roles of alkali atoms are able to change with different chemical environments of active sites. For the first time, we reveal how the number of ligands (oxygen atoms herein) can be used to finely regulate the reactivity toward N2.

Experimental and Theoretical Study of N2 Adsorption on Hydrogenated Y2C4H- and Dehydrogenated Y2C4- Cluster Anions at Room Temperature.

The adsorption of atmospheric dinitrogen (N2) on transition metal sites is an important topic in chemistry, which is regarded as the prerequisite for the activation of robust N≡N bonds in biological and industrial fields. Metal hydride bonds play an important part in the adsorption of N2, while the role of hydrogen has not been comprehensively studied. Herein, we report the N2 adsorption on the well-defined Y2C4H0,1- cluster anions under mild conditions by using mass spectrometry and density functional theory calculations. The mass spectrometry results reveal that the reactivity of N2 adsorption on Y2C4H- is 50 times higher than that on Y2C4- clusters. Further analysis reveals the important role of the H atom: (1) the presence of the H atom modifies the charge distribution of the Y2C4H- anion; (2) the approach of N2 to Y2C4H- is more favorable kinetically compared to that to Y2C4-; and (3) a natural charge analysis shows that two Y atoms and one Y atom are the major electron donors in the Y2C4- and Y2C4H- anion clusters, respectively. This work provides new clues to the rational design of TM-based catalysts by efficiently doping hydrogen atoms to modulate the reactivity towards N2.

Conversion of carbon dioxide to a novel molecule NCNBO- mediated by NbBN2- anions at room temperature.

The activation of carbon dioxide (CO2) mediated by NbBN2- cluster anions under the conditions of thermal collision has been investigated by time-of-flight mass spectrometry combined with density functional theory calculations. Two CO double bonds in the CO2 molecule are completely broken and two C-N bonds are further generated to form the novel molecule NCNBO-. To the best of our knowledge, this new molecule is synthesized and reported for the first time. In addition, one oxygen atom transfer channel produces another product, NbBN2O-. Both of the Nb and B atoms in NbBN2- donate electrons to reduce CO2, and the carbon atom originating from CO2 serves as an electron reservoir. The reaction of NbB- with N2 was also investigated theoretically, and the formation of NbBN2- from this reaction is thermodynamically and kinetically quite favorable, indicating that NCNBO- might be produced from the coupling of N2 and CO2 mediated by NbB- anions.

Consecutive methane activation mediated by single metal boride cluster anions NbB4.

Cleavage of all C-H bonds in two methane molecules by gas-phase cluster ions at room temperature is a challenging task. Herein, mass spectrometry and quantum chemical calculations have been used to identify one single metal boride cluster anions NbB4- that can activate eight C-H bonds in two methane molecules and release one H2 molecule each time under thermal collision conditions. In these consecutive reactions, the loaded Nb atoms and the support B4 units play different roles but act synergistically to activate CH4, which is responsible for the interesting reactivity of NbB4-. Moreover, there are some mechanistic differences in these two reactions, including the mechanisms for the first C-H bond activation steps, dihydrogen desorption sites, and major electron donors. This study shows that non-noble metal boride species are reactive enough to facilitate thermal C-H bond cleavages, and boron-based materials may be one kind of potential support material facilitating surface hydrogen transport.

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.

Oxidation of isoprene by titanium oxide cluster cations in the gas phase.

The heterogeneous oxidation of isoprene (C5H8) by metal-oxide particles, such as the typical mineral aerosols TiO2, plays an important role in the isoprene atmospheric chemistry. However, the underlying mechanism of C5H8 oxidation remains elusive owing to the complexities of aerosol surfaces and reaction channels. Herein, we report the gas-phase reactions of TixOy+ (x = 1-7, y = 1-14) cations with isoprene by using mass spectrometry and density functional theory (DFT) calculations. Five types of reaction channels were observed: association, hydrogen atom transfer (HAT), C-C bond cleavage, combined oxygen atom transfer (OAT) and HAT and combined OAT and C-C bond cleavage. It is noteworthy that formaldehyde is known as the major oxidation product of isoprene/hydroxyl radicals in the atmosphere. In addition, CO has not been observed in the reactions of isoprene with gas-phase ions. Therefore, the reaction mechanisms of CH2O and CO generation observed in Ti2O5+/C5H8 and Ti4O8+/C5H8 systems were further investigated by DFT calculations, and the calculated results are in agreement with the experimental observations. In these two reactions, both Ti and O atoms can be the adsorption sites for C5H8. The reaction channels and mechanistic information gained in these gas-phase model reactions may offer fundamental insights relevant to the corresponding oxidation processes over titanium oxide aerosols in the atmosphere.

Dinitrogen Fixation and Reduction by Ta3N3H0,1- Cluster Anions at Room Temperature: Hydrogen-Assisted Enhancement of Reactivity.

Dinitrogen activation and reduction is one of the most challenging and important subjects in chemistry. Herein, we report the N2 binding and reduction at the well-defined Ta3N3H- and Ta3N3- gas-phase clusters by using mass spectrometry (MS), anion photoelectron spectroscopy (PES), and quantum-chemical calculations. The PES and calculation results show clear evidence that N2 can be adsorbed and completely activated by Ta3N3H- and Ta3N3- clusters, yielding to the products Ta3N5H- and Ta3N5-, but the reactivity of Ta3N3H- is five times higher than that of the dehydrogenated Ta3N3- clusters. The detailed mechanistic investigations further indicate that a dissociative mechanism dominates the N2 activation reactions mediated by Ta3N3H- and Ta3N3-; two and three Ta atoms are active sites and also electron donors for the N2 reduction, respectively. Although the hydrogen atom in Ta3N3H- is not directly involved in the reaction, its very presence modifies the charge distribution and the geometry of Ta3N3H-, which is crucial to increase the reactivity. The mechanisms revealed in this gas-phase study stress the fundamental rules for N2 activation and the important role of transition metals as active sites as well as the new significant role of metal hydride bonds in the process of N2 reduction, which provides molecular-level insights into the rational design of tantalum nitride-based catalysts for N2 fixation and activation or NH3 synthesis.

Efficient Liberation of Ammonia from Thermal Reaction of ScNH+ Cations and Water.

Ammonia synthesis by using water as a hydrogen source is a challenging task. Laser-ablation-generated ScNH+ cations have been mass-selected using a quadrupole mass filter and reacted with H2O in a linear ion trap reactor under thermal collision conditions. Through mass spectrometry in conjunction with density functional theory calculations, we found that ammonia is released as the product in the reaction of ScNH+ with H2O, and this reaction is with high efficiency and selectivity, and the rate constant for the reaction is (1.14 ± 0.23) × 10-10 cm3 molecule-1 s-1, corresponding to the reaction efficiency of 15%. Metal imido complexes (*MNH) are one of the important intermediates in the currently reported NH3 synthetic reactions. The gas-phase ScNH+ cation can be a simplified model of *MNH over catalysts of NH3 synthesis, and the facile proton transfer mechanism obtained in this model system may offer fundamental mechanistic insights into how to design catalysts for ammonia production by using water as the hydrogen source under ambient conditions.

Fe2 O+ Cation Mediated Propane Oxidation by Dioxygen in the Gas Phase.

The mass-selected Fe2 O+ cation mediated propane oxidation by O2 was investigated by mass spectrometry and density functional theory calculations. In the reaction of Fe2 O+ with C3 H8 , H2 was liberated by C-H bond activation to give Fe2 OC3 H6+ . Interestingly, when a mixture of C3 H8 /O2 was introduced into the reactor, an intense signal that corresponded to the Fe2 O2+ cation was present; the experiments indicated that O2 was activated in its reaction with Fe2 O(C3 H6 )+ to give Fe2 O2+ and C3 H6 O (acetone or propanal). A Langmuir-Hinshelwood-like mechanism was adopted in the propane oxidation reaction by O2 on gas-phase Fe2 O+ cations. In comparison with the absence of Fe2 O2+ in the reaction of Fe2 O+ with O2 , the ligand effect of C3 H6 on Fe2 OC3 H6+ is important in the oxygen activation reaction. The theoretical results are consistent with the experimental observations. The propane oxidation by O2 in the presence of Fe2 O+ might be applied as a model for alkane and O2 activations over iron oxide catalysts, and the mechanisms and kinetic data are useful for understanding corresponding heterogeneous reactions.

Direct hydroxylation of benzene to phenol mediated by nanosized vanadium oxide cluster ions at room temperature.

Gas-phase vanadium oxide cluster cations and anions are prepared by laser ablation. The small cluster ions (<1000 amu) are mass-selected using a quadrupole mass filter and reacted with benzene in a linear ion trap reactor; large clusters (>1000 amu) with no mass selection are reacted with C6H6 in a fast flow reactor. Rich product variety is encountered in these reactions, and the reaction channels for small cationic and anionic systems are different. For large clusters, the reactivity patterns of (V2O5) n+ (n = 6-25) and (V2O5) n O- (n = 6-24) cluster series are very similar to each other, indicating that the charge state has little influence on the oxidation of benzene. In sharp contrast to the dramatic changes of reactivity of small clusters, a weakly size dependent reaction behavior of large (V2O5)6-25+ and (V2O5)6-24O- clusters is observed. Therefore, the charge state and the size are not the major factors influencing the reactivity of nanosized vanadium oxide cluster ions toward C6H6, which is not common in cluster science. In the reactions with benzene, the small and large reactive vanadium oxide cations show similar reactivity of hydroxyl radicals (OH•) toward C6H6 at higher and lower temperatures, respectively; different numbers of vibrational degrees of freedom and the released energy during the formation of adduct complexes can explain this intriguing correlation. The reactions investigated herein might be used as the models of how to realize the partial oxidation of benzene to phenol in a single step, and the observed mechanisms are helpful to understand the corresponding heterogeneous reactions, such as those over vanadium oxide aerosols and vanadium oxide catalysts.

Liberation of three dihydrogens from two ethene molecules as mediated by the tantalum nitride anion cluster Ta3N2- at room temperature.

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.

Methane Activation Mediated by a Series of Cerium-Vanadium Bimetallic Oxide Cluster Cations: Tuning Reactivity by Doping.

The reactions of cerium-vanadium cluster cations Cex Vy Oz (+) with CH4 are investigated by time-of-flight mass spectrometry and density functional theory calculations. (CeO2 )m (V2 O5 )n (+) clusters (m=1,2, n=1-5; m=3, n=1-4) with dimensions up to nanosize can abstract one hydrogen atom from CH4 . The theoretical study indicates that there are two types of active species in (CeO2 )m (V2 O5 )n (+) , V[(Ot )2 ](.) and [(Ob )2 CeOt ](.) (Ot and Ob represent terminal and bridging oxygen atoms, respectively); the former is less reactive than the latter. The experimentally observed size-dependent reactivities can be rationalized by considering the different active species and mechanisms. Interestingly, the reactivity of the (CeO2 )m (V2 O5 )n (+) clusters falls between those of (CeO2 )2-4 (+) and (V2 O5 )1-5 (+) in terms of C-H bond activation, thus the nature of the active species and the cluster reactivity can be effectively tuned by doping.