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.