Reactions between TaxOy- (x = 1-5, y = 0-7) cluster anions and C1-C3 alkanes

Abstract

Light alkanes, such as methane, ethane, and propane, are pivotal feedstocks in the chemical industry. However, their activation and transformation remain challenging due to the intrinsic chemical inertness of the C(sp3)-H bonds, particularly under mild reaction conditions. Gas-phase metal clusters serve as an ideal platform for probing mechanisms of alkane activation at a molecular level. Herein, we systematically investigated the reactions of gas-phase tantalum oxide anions (TaxOy-, x = 1-5, y = 1-7) with CH4, C2H6, and C3H8 combining mass spectrometry experiments and density functional theory (DFT) calculations. Among the 27 mass-selected clusters, the Ta2O4- cluster has exceptional reactivity, converting ethane and propane to free alkenes with rate constants significantly higher than other reactive clusters. In contrast, most clusters, including Ta2O1-3- and Ta2O5-, have negligible reactivity or conventional dehydrogenation pathways to form free H2. The DFT calculations revealed that the unique reactivity of Ta2O4- originates from its distinct electronic and geometric structures. In the reaction of Ta2O4- with C2H6, the flexible geometric structures of the critical transition state (TS) result in a high number of vibrational states above the TS, which leads to alkene rather than dihydrogen formation. A novel strategy to tune metal cluster reactivity and product selectivity toward alkanes is thus provided.