Conversion of Methane at Room Temperature Mediated by the Ta-Ta σ-Bond

Metal-metal bonds constitute an important type of reactive centers for chemical transformation; however, the availability of active metal-metal bonds being capable of converting methane under mild conditions, the holy grail in catalysis, remains a serious challenge. Herein, benefiting from the systematic investigation of 36 metal clusters of tantalum by using mass spectrometric experiments complemented with quantum chemical calculations, the dehydrogenation of methane at room temperature was successfully achieved by 18 cluster species featuring σ-bonding electrons localized in single naked Ta-Ta centers. In sharp contrast, the other 18 remaining clusters, either without naked Ta-Ta σ-bond or with σ-bonding electrons delocalized over multiple Ta-Ta centers only exhibit molecular CH4-adsorption reactivity or inertness. Mechanistic studies revealed that changing cluster geometric configurations and tuning the number of simple inorganic ligands (e.g., oxygen) could flexibly manipulate the presence or absence of such a reactive Ta-Ta σ-bond. The discovery of Ta-Ta σ-type bond being able to exhibit outstanding activity toward methane conversion not only overturns the traditional recognition that only the metal-metal π- or δ-bonds of early transition metals could participate in bond activation but also opens up a new access to design of promising metal catalysts with dual-atom as reactive sites for chemical transformations.

Ion-molecule reactions of mass-selected ions

Gas-phase reactions of mass-selected ions with neutrals covers a very broad area of fundamental and applied mass spectrometry (MS). Oftentimes, ion-molecule reactions (IMR) can serve as a viable alternative to collision-induced dissociation and other ion dissociation techniques when using tandem MS. This review focuses on the literature pertaining applications of IMR since 2013. During the past decade considerable efforts have been made in analytical applications of IMR, including advances in one of the major techniques for characterization of unsaturated fatty acids and lipids, ozone-induced dissociation, and the development of a new technique for sequencing of large ions, hydrogen atom attachment/abstraction dissociation. Many advances have also been made in identifying gas-phase chemistry specific to a functional group in organic and biological compounds, which are useful in structure elucidation of analytes and differentiation of isomers/isobars. With "soft" ionization techniques like electrospray ionization having become mainstream for quite some time now, the efforts in the area of metal ion catalysis have firmly moved into exploring chemistry of ligated metal complexes in their "natural" oxidation states allowing to model individual steps of mechanisms in homogeneous catalysis, especially in combination with high-level DFT calculations. Finally, IMR continue to contribute to the body of knowledge in the area of chemistry of interstellar processes.

On the performance of the M-C (M = Fe, Ru, Os) unit toward methane activation

Gas-phase reactions of [MC]+ (M = Os and Ru) with methane at ambient temperature have been studied by using quadrupole-ion trap (Q-IT) mass spectrometry combined with quantum chemical calculations. Theoretical calculations reveal the influence of electronic signatures and that it is the energy gap of the associated frontier molecular orbitals that dominates the ability of the cluster in the initial H3C-H bond breaking. By extension, a theoretical consideration upon changing the ligand from carbide to carbyne and eventually to carbene reveals that the reactivities of the M-complex (M = Os, Ru and Fe) are determined by the energy gap of the involved orbitals. In addition, a few factors like the dipole moment, spin density and charge distributions influence the orbital energy gap to different extents. Thus, altering the local structure of the active center to modulate the orbital distribution may be a possible means of regulation of the activity.

Direct Conversion of Methane to Propylene

Nonoxidative coupling of methane exhibits promising prospect in that it affords value-added hydrocarbons and hydrogen with high atom economy. However, challenge remains in direct, selective conversion of methane to more valuable hydrocarbons like olefins. The current work presents a catalyst with well-dispersed Ta atoms anchored by graphitic C3N4-supported phthalocyanine. Such a catalyst is able to convert methane selectively to ethylene and propylene at a relatively low temperature (350 °C). The conception of the active center and construction of the catalyst have been described, and the origins of the catalytic performance are discussed.

Infrared photodissociation spectroscopy of mass-selected [TaO3(CO2)n]+ (n = 2-5) complexes in the gas phase

We report a joint experimental and theoretical study on the structures of gas-phase [TaO₃(CO₂)_n]⁺ (n = 2-5) ion-molecule complexes. Infrared photodissociation spectra of mass-selected [TaO₃(CO₂)_n]⁺ complexes were recorded in the frequency region from 2200 to 2450 cm^-1 and assigned through comparing with the simulated infrared spectra of energetically low-lying structures derived from quantum chemical calculations. With the increasing number of attached CO₂ molecules, the larger clusters show significantly enhanced fragmentation efficiency and a strong band appears at around 2350 cm^-1 near the free CO₂ antisymmetric stretching vibration band, indicating only a small perturbation of CO₂ molecules on the secondary solvation sphere while higher frequency bands corresponding to the core structure remain largely unaffected. A core structure [TaO₃(CO₂)₃]⁺ is identified to which subsequent CO₂ ligands are weakly attached and the most favorable cluster growth path is verified to proceed on the triplet potential energy surface higher in energy than that of ground states. Theoretical exploration reveals a two-state reactivity (TSR) scenario in which the energetically favored triplet transition state crosses over the singlet ground state to form a TaO₃⁺ core ion, providing new information on the cluster formation correlated with the reactivity of tantalum metal oxides towards CO₂.

Activity Origin of the Nickel Cluster on TiC Support for Nonoxidative Methane Conversion

Designing an active and selective catalyst for nonoxidative conversion of methane under mild conditions is critical for natural gas utilization as a chemical feedstock. Here, we demonstrate that the origin of the selective nonoxidative conversion of methane by the titanium carbide supported nickel cluster arises from the formation of a nickel carbide site under the reaction conditions, which could stabilize the CH_x intermediate to facilitate the C-C coupling, but further coking is rather limited. The reaction mechanism reveals that the C₂ products can be formed via a key -CH_x-CH₃ intermediate. In addition, we demonstrate that boration of the nickel cluster site can improve the methane conversion toward C₂ products. That higher activity and selectivity from the moderate rise in d orbital energy levels can therefore be considered as a descriptor of the catalyst effectiveness. These findings provide an understanding of the dynamic behavior of the single nickel cluster toward methane conversion to C₂ products and guidance for their future rational design.

Transition metal oxide complexes as molecular catalysts for selective methane to methanol transformation: any prospects or time to retire?

Transition metal oxides have been extensively used in the literature for the conversion of methane to methanol. Despite the progress made over the past decades, no method with satisfactory performance or economic viability has been detected. The main bottleneck is that the produced methanol oxidizes further due to its weaker C-H bond than that of methane. Every improvement in the efficiency of a catalyst to activate methane leads to reduction of the selectivity towards methanol. Is it therefore prudent to keep studying (both theoretically and experimentally) metal oxides as catalysts for the quantitative conversion of methane to methanol? This perspective focuses on molecular metal oxide complexes and suggests strategies to bypass the current bottlenecks with higher weight on the computational chemistry side. We first discuss the electronic structure of metal oxides, followed by assessing the role of the ligands in the reactivity of the catalysts. For better selectivity, we propose that metal oxide anionic complexes should be explored further, while hydrophylic cavities in the vicinity of the metal oxide can perturb the transition-state structure for methanol increasing appreciably the activation barrier for methanol. We also emphasize that computational studies should target the activation reaction of methanol (and not only methane), the study of complete catalytic cycles (including the recombination and oxidation steps), and the use of molecular oxygen as an oxidant. The titled chemical conversion is an excellent challenge for theory and we believe that computational studies should lead the field in the future. It is finally shown that bottom-up approaches offer a systematic way for exploration of the chemical space and should still be applied in parallel with the recently popular machine learning techniques. To answer the question of the title, we believe that metal oxides should still be considered provided that we change our focus and perform more systematic investigations on the activation of methanol.

Lanthanide Complexes Containing a Terminal Ln═O Oxo Bond: Revealing Higher Stability of Tetravalent Praseodymium versus Terbium

We report on the reactivity of gas-phase lanthanide-oxide nitrate complexes, [Ln(O)(NO₃)₃]^- (denoted LnO²⁺), produced via elimination of NO₂^• from trivalent [Ln^III(NO₃)₄]^- (Ln = Ce, Pr, Nd, Sm, Tb, Dy). These complexes feature a Ln^III-O^• oxyl, a Ln^IV═O oxo, or an intermediate Ln^III/IV oxyl/oxo bond, depending on the accessibility of the tetravalent Ln^IV state. Hydrogen atom abstraction reactivity of the LnO²⁺ complexes to form unambiguously trivalent [Ln^III(OH)(NO₃)₃]^- reveals the nature of the oxide bond. The result of slower reactivity of PrO²⁺ versus TbO²⁺ is considered to indicate higher stability of the tetravalent praseodymium-oxo, Pr^IV═O, versus Tb^IV═O. This is the first report of Pr^IV as more stable than Tb^IV, which is discussed with respect to ionization potentials, standard electrode potentials, atomic promotion energies, and oxo bond covalency via 4f- and/or 5d-orbital participation.

Regioselective Bond-Forming and Hydrolysis Reactions of Doubly Charged Vanadium Oxide Anions in the Gas Phase

The gas-phase reactivity of vanadium-containing dianions, NaV3O92− and its hydrated form H2NaV3O102−, were probed towards sulphur dioxide at room temperature by ion-molecule reaction (IMR) experiments in the collision cell of an ion trap mass spectrometer. The sequential addition of two SO2 molecules to the NaV3O92− dianion leads to the breakage of the stable V3O9 backbone, resulting in a charge separation process with the formation of new V-O and S-O bonds. On the contrary, the H2NaV3O102− hydroxide species reacts with SO2, promoting regioselective hydrolysis and bond-forming processes, the latter similar to that observed for the NaV3O92− reactant anion. Kinetic analysis shows that these reactions are fast and efficient with rate constants of the 10−9 (±30) cm3 s−1 molecule−1 order of magnitude.

CO2-Activation by size-selected tantalum cluster cations (Ta1-16+): thermalization governing reaction selectivity

The reactions of tantalum cluster cations of different sizes toward carbon dioxide are studied in an ion trap under multi-collisional conditions. For all sizes studied, consecutive reactions with several CO₂ molecules are observed. This reveals two different pathways, namely oxide formation and the pickup of an entire molecule. Supported by calculations of the thermochemistry of Ta_nO⁺ formation upon reaction with CO₂, changes in the branching ratios at a particular cluster size are related to heat effects due to the vibrational heat capacity of the clusters and the exothermicity of the reaction.

Room-Temperature Methane Activation Mediated by Free Tantalum Cluster Cations: Size-by-Size Reactivity

The energetics of small cationic tantalum clusters and their gas-phase adsorption and dehydrogenation reaction pathways with methane are investigated with ion-trap experiments and spin-density-functional-theory calculations. Ta_n⁺ clusters are exposed to methane under multicollision conditions in a cryogenic ring electrode ion-trap. The cluster size affects the reaction efficiency and the number of consecutively dehydrogenated methane molecules. Small clusters (n = 1-4) dehydrogenate CH₄ and concurrently eliminate H₂, while larger clusters (n > 4) demonstrate only molecular adsorption of methane. Unique behavior is found for the Ta⁺ cation, which dehydrogenates consecutively up to four CH₄ molecules and is predicted theoretically to promote formation of a [Ta(CH₂-CH₂-CH₂)(CH₂)]⁺ product, exhibiting C-C coupled groups. Underlying mechanisms, including reaction-enhancing couplings between potential energy surfaces of different spin-multiplicities, are uncovered.

Carbide Dihydrides: Carbonaceous Species Identified in Ta4 + -Mediated Methane Dehydrogenation

The products of methane dehydrogenation by gas‐phase Ta₄⁺ clusters are structurally characterized using infrared multiple photon dissociation (IRMPD) spectroscopy in conjunction with quantum chemical calculations. The obtained spectra of [4Ta,C,2H]⁺ reveal a dominance of vibrational bands of a H₂Ta₄C⁺ carbide dihydride structure over those indicative for a HTa₄CH⁺ carbyne hydride one, as is unambiguously verified by studies employing various methane isotopologues. Because methane dehydrogenation by metal cations M⁺ typically leads to the formation of either MCH₂⁺ carbene or HMCH⁺ carbyne hydride structures, the observation of a H₂MC⁺ carbide dihydride structure implies that it is imperative to consider this often‐neglected class of carbonaceous intermediates in the reaction of metals with hydrocarbons.

Phenyl-Group Exchange in Triphenylphosphine Mediated by Cationic Gold-Platinum Complexes-A Gas-Phase Mimetic Approach

The PPh₃ ligands in the heterodinuclear AuPt complex [(Ph₃P)AuPt(PPh₃)₃][BAr₄^F] (BAr₄^F = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) exhibit a high fluxionality on the AuPt core. Fast intramolecular and slow intermolecular processes for the reversible exchange of the PPh₃ ligands have been identified. When [(Ph₃P)AuPt(PPh₃)₃][BAr₄^F] is heated in solution, the formation of benzene is observed, and a trinuclear, cationic AuPt₂ complex is generated. This process is preceded by reversible phenyl-group exchange between the PPh₃ ligands present in the reaction mixture as elucidated by deuterium-labeling studies. Both the elimination of benzene and the preceding reversible phenyl-group exchange have originally been observed in mass-spectrometry-based CID experiments (CID = Collision-Induced Dissociation). While CID of mass-selected [Au,Pt,(PPh₃)₄]⁺ results exclusively in the loss of PPh₃, the resulting cation [Au,Pt,(PPh₃)₃]⁺ selectively eliminates C₆H₆. Thus, the dissociation of a PPh₃ ligand from [Au,Pt,(PPh₃)₃]⁺ is energetically not able to compete with processes which result in C-H- and C-P-bond cleavage. In both media, the heterobimetallic nature of the employed complexes is the key for the observed reactivity. Only the intimate interplay of the gas-phase investigations, studies in solution, and thorough DFT computations allowed for the elucidation of the mechanistic details of the reactivity of [(Ph₃P)AuPt(PPh₃)₃][BAr₄^F].

Catalytic Non-Oxidative Coupling of Methane on Ta8O2

Mass-selected Ta₈O₂⁺ cluster ions catalyze the transformation of methane in a gas-phase ion trap experiment via nonoxidative coupling into ethane and H₂, which is a prospective reaction for the generation of valuable chemicals on an industrial scale. Systematic variation of the reaction conditions and the isotopic labeling of methane by deuterium allow for an unambiguous identification of a catalytic cycle. Comparison with the proposed catalytic cycle for tantalum-doped silica catalysts reveals surprising similarities as the mechanism of the C-C coupling step, but also peculiar differences like the mechanism of the eventual formation of molecular hydrogen and ethane. Therefore, this work not only supplies insights into the mechanisms of methane coupling reactions but also illustrates how the study of trapped ionic catalysts can contribute to the understanding of reactions, which are otherwise difficult to study.

The decisive role of 4f-covalency in the structural direction and oxidation state of XPrO compounds (X: group 13 to 17 elements)

Lanthanide oxo compounds are of vital importance in lanthanide chemistry, as well as in environmental and materials sciences. Praseodymium, as an exceptional element in lanthanides which can form a +V formal oxidation state (OSf) besides the dominant +III among the 4f-block element, displays the significant participation of the Pr 4f orbitals in bonding interactions which is commonly crucial in stabilizing the high oxidation state of Pr in PrO2+ and NPrO species. Here, we report a systematic theoretical study on the structures and stabilities of a series of XPrO (X: B, Al, C, Si, N, P, As, O, S, F, Cl) compounds along with [XPrO]+ cation (X: O, S) and [X3PrO] complexes (X: F and Cl). This work reveals that Pr is able to achieve the lowest and highest OSf and the OSf exhibits periodic variation from +I in BOPr and AlOPr to +II in SiOPr to +III in CPrO, FPrO, ClPrO and AsPrO to +IV in OPrO and SPrO and even to +V in NPrO, [OPrO]+, [SPrO]+, F3PrO and Cl3PrO. We found that the molecular structures are correlated to the Pr oxidation state due to the highly important 4f orbital in the chemical bonding of the high oxidation state compounds. Thus, not only the electronegativity of the ligand but also the quasi-degenerate Pr valence 4f orbitals, namely energetic covalency, control the oxidation state and play a fundamental role in affecting the electronic structural stability of Pr(v) compounds as well. This work demonstrates the structurally directing role of the f-orbital in the formation of the linear structure and is constructive for achieving the higher oxidation state of a given element by tuning the ligand.

Sulphur dioxide cooperation in hydrolysis reactions of vanadium oxide and hydroxide cluster dianions

Gas-phase hydrolysis takes place due to the synergistic action of SO₂ and vanadium-containing dianions that succeeds in tightening hydrogen bonds.

Oriented external electric fields as mimics for probing the role of metal ions and ligands in the thermal gas-phase activation of methane

Unusual, if not unprecedented, effects of transition-metal ions and ligands are discovered when simple metal oxides or carbides activate methane in the gas phase in manners reminiscent of oriented external electric fields.

Electronic Effects on Room-Temperature, Gas-Phase C-H Bond Activations by Cluster Oxides and Metal Carbides: The Methane Challenge

This Perspective discusses a story of one molecule (methane), a few metal-oxide cationic clusters (MOCCs), dopants, metal-carbide cations, oriented-electric fields (OEFs), and a dizzying mechanistic landscape of methane activation! One mechanism is hydrogen atom transfer (HAT), which occurs whenever the MOCC possesses a localized oxyl radical (M-O^•). Whenever the radical is delocalized, e.g., in [MgO]_n^•+ the HAT barrier increases due to the penalty of radical localization. Adding a dopant (Ga₂O₃) to [MgO]₂^•+ localizes the radical and HAT transpires. Whenever the radical is located on the metal centers as in [Al₂O₂]^•+ the mechanism crosses over to proton-coupled electron transfer (PCET), wherein the positive Al center acts as a Lewis acid that coordinates the methane molecule, while one of the bridging oxygen atoms abstracts a proton, and the negatively charged CH₃ moiety relocates to the metal fragment. We provide a diagnostic plot of barriers vs reactants' distortion energies, which allows the chemist to distinguish HAT from PCET. Thus, doping of [MgO]₂^•+ by Al₂O₃ enables HAT and PCET to compete. Similarly, [ZnO]^•+ activates methane by PCET generating many products. Adding a CH₃CN ligand to form [(CH₃CN)ZnO]^•+ leads to a single HAT product. The CH₃CN dipole acts as an OEF that switches off PCET. [MC]⁺ cations (M = Au, Cu) act by different mechanisms, dictated by the M⁺-C bond covalence. For example, Cu⁺, which bonds the carbon atom mostly electrostatically, performs coupling of C to methane to yield ethylene, in a single almost barrier-free step, with an unprecedented atomic choreography catalyzed by the OEF of Cu⁺.

Consecutive reactions of small, free tantalum clusters with dioxygen controlled by relaxation dynamics

The reaction of small cationic tantalum clusters (Ta_n⁺, n = 4–8) with molecular oxygen is studied under multi-collision conditions in the gas phase, and the reaction kinetics are analyzed in order to elucidate underlying mechanisms.

From oxidative degradation to direct oxidation: size regimes in the consecutive reaction of cationic tantalum clusters with dioxygen

Cationic tantalum clusters (Ta_9–12⁺) are reacted with molecular oxygen under multi-collision conditions in the gas phase in order to analyze the reaction kinetics.