C-C Coupling of Methane Mediated by Atomic Gold Cations under Multiple-Collision Conditions

The reactivity of atomic metal cations toward CH4 has been extensively investigated over the past decades. Closed-shell metal cations in electronically ground states are usually inert with CH4 under thermal collision conditions because of the extremely high stability of methane. With the elevation of collision energies, closed-shell atomic gold cations (Au+) have been reported to react with CH4 under single-collision conditions to produce AuCH2+, AuH+, and AuCH3+ species. Further investigations found that the ion-source-generated AuCH2+ cations can react with CH4 to synthesize C-C coupling products. These previous studies suggested that new products for the reaction of Au+ with CH4 can be identified under multiple-collision conditions with sufficient collision energies. However, the reported ion-molecule reactions involving methane were usually performed under single- or multiple-collision conditions with thermal collision energies. In this study, a new reactor composed of a drift tube and ion funnel is constructed and coupled with a homemade reflectron time-of-flight mass spectrometer. Laser-ablation-generated Au+ ions are injected into the reactor and drift 120 mm to react with methane seeded in the helium drift gas. The reaction products and unreacted Au+ ions are focused through the ion funnel and accumulate through a linear ion trap and are then detected by a mass spectrometer. In the reactor, the pressure is approximately 100 Pa, and the electric field between the drift tube and ion funnel can regulate the collision energies between ions and molecules. The reaction of the closed-shell atomic Au+ cation with CH4 is investigated, and the C-C coupling product AuC2H4+ is observed under multiple-collision conditions with elevated collision energies. Density functional theory calculations are performed to understand the mechanism of the coupling reaction (Au+ + 2CH(4) -> AuC2H4+ + 2H(2)). Two pathways involving Au-CH2 and Au-CH3 species can separately mediate the C-C coupling process. The activation of the second C-H bond in each process requires additional energy to overcome the relatively high barrier (2.07 and 2.29 eV). Ion-trajectory simulations under multiple-collision conditions are then conducted to determine the collisional energy distribution in the reactor. These simulations confirmed that the electric fields between the drift tube and ion funnel could supply sufficient center-of-mass kinetic energies to facilitate the C-C coupling process to form AuC2H4+. The following catalytic cycle could then be postulated: AuC2H4+ + CH4 (Delta) under right arrow AuCH4+ + C2H4, AuCH4+ + CH4 (Delta) under right arrow AuC2H4+ + 2H(2), and 2CH(4) (Au+, Delta) under right arrow C2H4 + 2H(2). Thus, this study enriches the chemistry of both gold and methane.