From C-H Bond Insertion to Hydrogen Atom Transfer: Tuning the Reaction Mechanisms of Methane Activation by the Oxidation of Ta₂⁺

The activation of methane under mild conditions is a challenging but rewarding goal; the underlying key parameters, however, remain elusive. In this study on isolated tantalum Ta2+ compounds exposed to methane in a ring-electrode ion trap, strong changes in the reactivity are observed depending on the compound's degree of oxidation. While the general reaction behavior is presented for species ranging from Ta2+ to Ta2O6+ based on experimental kinetic studies, we focus in more detail on the dehydrogenation reactions occurring on Ta2O2+ and the hydrogen atom transfer (HAT) on Ta2O5+, for which density functional theory calculations were performed. In the first part, we elucidate the role of Ta–C–Ta bridging motifs in product structures as driving forces for the dehydrogenation of methane on Ta2O2+; in the second part, we investigate the origins of the HAT – a hitherto unknown reaction scheme for binary tantalum oxides. For the latter, we show that the reactivity originates from the spin density on oxygen atoms, which is a typical characteristic of the reaction on other metal oxides. This reflects a change in the reactivity from oxidized metallic systems to metal oxides and demonstrates that chemical modifications of tantalum compounds can achieve different methane activation schemes.