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-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.

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.

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.

Coupling of N2 and O2 in the Gas Phase to Synthesize Nitric Oxide at Room Temperature: A Zeldovich-Like Strategy.

Dinitrogen (N2) activation and its chemical transformations are some of the most challenging topics in chemistry. Herein, we report that heteronuclear metal anions AuNbBO- can mediate the direct coupling of N2 and O2 to generate NO molecules. N2 first forms the nondissociative adsorption product AuNbBON2- on AuNbBO-. In the following reactions with two O2 molecules, two NO molecules are gradually released, with the formation of AuNbBO2N- and AuNbBO3-. In the reaction with the first O2, the generated nitrene radical (N••-) originating from the dissociated N2, induces the activation of O2. Subsequently, the second O2 is anchored and forms a superoxide radical (O2•-); this radical attacks the other N atom to form an N-O bond, releasing the second NO. The N••- and O2•- radicals play key roles in the reactions. The mechanism adopted in this direct oxidation of N2 by O2 to NO can be labeled as a Zeldovich-like mechanism.

Dinitrogen Activation by Dihydrogen and Quaternary Cluster Anions AuNbBO-: Nb- and B-Mediated N2 Activation and Au-Assisted Nitrogen Transfer.

Nitrogen fixation and hydrogenation are important issues in chemistry and industry. Herein, we used mass spectrometry and quantum chemical calculations to identify the quaternary AuNbBO- anions that can efficiently activate N2 and H2 to form imido (or amido) units in the products AuNbBON2H2- under thermal collision conditions. In these reactions, Nb and B atoms work in synergy to dissociate N2 and the Au atom acts as a reducing agent, which facilitates the removal of one activated N atom for the following hydrogenation process; generation of three-centered, two-electron bonds facilitates N2 activation and N transformation. This study shows that the noble metal-assisted early transition metal boronyl cluster is highly reactive to facilitate thermal N-N and H-H bond cleavage, and NH (or NH2) and NBO units, which are important intermediates in N2 hydrogenation reactions, are formed. These findings may provide insight into the design of new catalysts for the synthesis of NH3 from N2 and H2.

Room-Temperature Dinitrogen and Carbon Dioxide Activation to Form Nitrogen-Carbon Bonds by Quaternary Cluster Anions: Gold-Assisted Enhancement of Reactivity.

Coupling conversion of CO2 and N2 molecules under mild conditions to form useful N-C bond-containing products has attracted significant attention. However, the activation and direct coupling of such very inert molecules are quite challenging. Herein, we determined that this coupling reaction can be realized by AuNbBO- quaternary anions at room temperature. The well-defined AuNbBO- anions can cleave the N≡N bond in N2 and two C═O bonds in CO2 to form a novel product NCNBO-. To the best of our knowledge, the NCNBO- anion has been experimentally synthesized for the first time by coupling N2 and CO2. Comparative studies with Nb2BO-/N2 and NbBO-/N2 systems further indicate that the presence of a Au atom in AuNbBO- is indispensable for this N2 and CO2 coupling reaction, because the Au atom can decrease the active orbital energies of AuNbBO- anions to facilitate the π-back-donating interaction between AuNbBO- and N2.

Dinitrogen and Carbon Dioxide Activation to Form C-N Bonds at Room Temperature: A New Mechanism Revealed by Experimental and Theoretical Studies.

In light of the current energy requirements, the conversion of CO2 and N2 into useful C-N bond-containing products under mild conditions has become an area of intense research. However, the inert nature of N2 and CO2 renders their coupling extremely challenging. Herein, nitrogen and carbon atoms originating from N2 and CO2, respectively, are fixed sequentially by NbH2- anions in the gas phase at room temperature. Isocyanate and NbO2CN- anions were formed under thermal collision conditions, thus achieving the formation of new C-N bonds directly from simple N2 and CO2. The anion structures and reaction details were studied by mass spectrometry, photoelectron spectroscopy, and quantum chemical calculations. A novel N2 activation mode (metal-ligand activation, MLA) and a related mechanism for constructing C-N bonds mediated by a single non-noble metal atom are proposed. In this MLA mode, the C atom originating from CO2 serves as an electron reservoir to accept and donate electrons.

Nb2BN2- cluster anions reduce four carbon dioxide molecules: reactivity enhancement by ligands.

The thermal gas-phase reactions of Nb2BN2- cluster anions with carbon dioxide have been explored by using the art of time-of-flight mass spectrometry and density functional theory calculations. Four CO2 molecules can be consecutively reduced by Nb2BN2-, resulting in the formation of Nb2BN2O1-4- anions and the release of one CO molecule each time. To illustrate the role of ligands in Nb2BN2-, the reactivities of Nb2N2- and Nb2B- toward CO2 were also investigated; two and three CO2 molecules are activated, respectively, and the rate constants are slower than that of Nb2BN2-/CO2 systems. This comparison indicates that metal-metal multiple bonds and appropriate ligands, such as B, are important factors for CO2 reduction. The synergy between a transition metal atom (Nb) and a main-group atom (B) in CO2 reduction mediated by gas-phase clusters is revealed for the first time. To the best of our knowledge, Nb2BN2- anions are gas-phase clusters that reduce the largest number of CO2 molecules. A fundamental understanding of the efficient reduction of carbon dioxide molecules may shed light on the rational design of active sites on supported transition metal/boron nitride catalysts.