Thermal Dehydrogenation of Methane Enhanced by μ₂-Oxo Ligands in Tantalum Cluster Cations [Ta_xO]⁺, x = 4, 5

The gas-phase reactions of [Ta_xO_y]⁺ (x = 4, 5; y = 0, 1) nanoclusters with methane have been explored in a ring-electrode ion trap under multicollision conditions and theoretically with the use of first-principles quantum simulations. At room temperature, Ta₄ ⁺ dehydrogenates consecutively two methane molecules with the concurrent elimination of H₂, whereas Ta₅ ⁺ is found to be unreactive. Both of the corresponding mono-oxides, [Ta₄O]⁺ and [Ta₅O]⁺, demonstrate significantly increased reaction rates. Binding of the methane molecule to the tantalum clusters is found to occur through Pauli-repulsion-driven polarization of the electronic charge distribution in the metal cluster, induced by the closed-shell methane molecule. The subsequent dehydrogenation reaction is found to entail active participation of up to four tantalum atoms, whereas the doping oxygen atom does not form bonds to the methane molecule or the reaction intermediates and acts merely as a cluster-charge-polarizing ligand spectator. Clusters exhibiting such enhanced reactivity, influenced by oxo-ligand modification of the local electronic charge distribution, with consequent tuned local Lewis acid-base-pair balancing, may serve as potent models for active centers in small particle and surface metalorganic chemistry or heterogeneous nanocatalysis.