On the Activation of Methane and Carbon Dioxide by [HTaO](+) and [TaOH](+) in the Gas Phase: A Mechanistic Study

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.

On the origin of reactivity variation upon sequential ligation: the [Re(Cl)x]+/CH4 (x = 1-3) couples

The potential of [ReCl_x]⁺ (x = 1-3) in activating methane has been explored by using a combination of gas-phase experiments and high-level quantum calculations. When the number of Cl ligands increases, the reactivity towards methane activation varies accordingly. While [ReCl_x]⁺ (x = 1-2) are able to dehydrogenate methane by a three-state reactivity scenario, [ReCl₃]⁺ shows inertness towards methane at ambient conditions. Furthermore, the product ion [ClRe(H)CH]⁺ of the [ReCl]⁺/CH₄ couple could continue to activate methane and liberate molecular dihydrogen but another product ion [Cl₂ReCH₂]⁺ is unreactive with methane. Obviously, the nature and the number of ligands make a difference to the reactivity towards methane activation. The associated reaction mechanism and the electron origins for the rather different reactivities are discussed in detail. Finally and more importantly, instructive information concerning the rational design of Re-catalysts for methane conversion is obtained.

Reaction Monitoring and Structural Characterisation of Coordination Driven Self-Assembled Systems by Ion Mobility-Mass Spectrometry

Nature creates exquisite molecular assemblies, required for the molecular-level functions of life, via self-assembly. Understanding and harnessing these complex processes presents an immense opportunity for the design and fabrication of advanced functional materials. However, the significant industrial potential of self-assembly to fabricate highly functional materials is hampered by a lack of knowledge of critical reaction intermediates, mechanisms, and kinetics. As we move beyond the covalent synthetic regime, into the domain of non-covalent interactions occupied by self-assembly, harnessing and embracing complexity is a must, and non-targeted analyses of dynamic systems are becoming increasingly important. Coordination driven self-assembly is an important subtype of self-assembly that presents several wicked analytical challenges. These challenges are "wicked" due the very complexity desired confounding the analysis of products, intermediates, and pathways, therefore limiting reaction optimisation, tuning, and ultimately, utility. Ion Mobility-Mass Spectrometry solves many of the most challenging analytical problems in separating and analysing the structure of both simple and complex species formed via coordination driven self-assembly. Thus, due to the emerging importance of ion mobility mass spectrometry as an analytical technique tackling complex systems, this review highlights exciting recent applications. These include equilibrium monitoring, structural and dynamic analysis of previously analytically inaccessible complex interlinked structures and the process of self-sorting. The vast and largely untapped potential of ion mobility mass spectrometry to coordination driven self-assembly is yet to be fully realised. Therefore, we also propose where current analytical approaches can be built upon to allow for greater insight into the complexity and structural dynamics involved in self-assembly.

Rhodium chemistry: A gas phase cluster study

Due to the extraordinary catalytic activity in redox reactions, the noble metal, rhodium, has substantial industrial and laboratory applications in the production of value-added chemicals, synthesis of biomedicine, removal of automotive exhaust gas, and so on. The main drawback of rhodium catalysts is its high-cost, so it is of great importance to maximize the atomic efficiency of the precious metal by recognizing the structure-activity relationship of catalytically active sites and clarifying the root cause of the exceptional performance. This Perspective concerns the significant progress on the fundamental understanding of rhodium chemistry at a strictly molecular level by the joint experimental and computational study of the reactivity of isolated Rh-based gas phase clusters that can serve as ideal models for the active sites of condensed-phase catalysts. The substrates cover the important organic and inorganic molecules including CH₄, CO, NO, N₂, and H₂. The electronic origin for the reactivity evolution of bare Rh_x ^q clusters as a function of size is revealed. The doping effect and support effect as well as the synergistic effect among heteroatoms on the reactivity and product selectivity of Rh-containing species are discussed. The ingenious employment of diverse experimental techniques to assist the Rh₁- and Rh₂-doped clusters in catalyzing the challenging endothermic reactions is also emphasized. It turns out that the chemical behavior of Rh identified from the gas phase cluster study parallels the performance of condensed-phase rhodium catalysts. The mechanistic aspects derived from Rh-based cluster systems may provide new clues for the design of better performing rhodium catalysts including the single Rh atom catalysts.

Consecutive methane activation mediated by single metal boride cluster anions NbB4. Phys Chem Chem Phys 2021;23:12592-12599. [PMID: 34047332 DOI: 10.1039/d1cp01418h]

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.

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.

Coupling of Methane and Carbon Dioxide Mediated by Diatomic Copper Boride Cations

The use of CH₄ and CO₂ to produce value-added chemicals via direct C-C coupling is a challenging chemistry problem because of the inertness of these two molecules. Herein, mass spectrometric experiments and high-level quantum-chemical calculations have identified the first diatomic species (CuB⁺ ) that can couple CH₄ with CO₂ under thermal collision conditions to produce ketene (H₂ C=C=O), an important intermediate in synthetic chemistry. The order to feed the reactants (CH₄ and CO₂ ) is important and CH₄ should be firstly fed to produce the C₂ product. Molecular-level mechanisms including control of product selectivity have been revealed for coupling of CH₄ with CO₂ under mild conditions.

Ligand-induced decarbonylation in diphosphine-ligated palladium acetates [CH3CO2Pd((PR2)2CH2)]+ (R = Me and Ph)

Isotope labelling, IR spectroscopy and DFT calculations reveal a novel ligand-induced decarbonylation reaction.

Sequential Gas-Phase Activation of Carbon Dioxide and Methane by [Re(CO)2]+: The Sequence of Events Matters!

The potential of carbonyl rhenium complexes in activating and coupling carbon dioxide and methane has been explored by using a combination of gas-phase experiments (FT-ICR mass spectrometry) and high-level quantum chemical calculations. While the complexes [Re(CO)_x]⁺ (x = 0, 1, 3) are thermally unreactive toward CO₂, [Re(CO)₂]⁺ abstracts one oxygen atom from this substrate spontaneously at ambient conditions. Based on ¹³C and ¹⁸O labeling experiments, the newly generated CO ligand is preferentially eliminated, and two mechanistic scenarios are considered to account for this unexpected finding. The oxo complex [ORe(CO)₂]⁺ reacts further with CH₄ to produce the dihydridomethylene complex [ORe(CO)(CH₂)(H)₂]⁺. However, coupling of the CO and CH₂ ligands to form CH₂═C═O does not take place. Further, the complexes [Re(CO)_x(CH₂)]⁺ (x = 1, 2), generated in the thermal reaction of [Re(CO)_x]⁺ (x = 1, 2) with CH₄, are inert toward CO₂. Mechanistic insight on the origin of this remarkable reactivity pattern has been derived from detailed quantum chemical calculations.