Gas-Phase Synthesis of Metal Olefins: Plasma-Assisted Methane Dehydrogenation and C═C Bond Formation

Methane dehydrogenation and C-C coupling under mild conditions are very important but challenging in chemistry. Utilizing a customized time of flight mass spectrometer combined with a magnetron sputtering (MagS) cluster source, here, we have conducted a study on the reactions of methane with small silver and copper clusters simply by introducing methane in argon as the working gas for sputtering. Interestingly, a series of [M(CnH2n)]+ (M = Cu and Ag; n = 2-12) clusters were observed, indicating high-efficiency methane dehydrogenation in such a plasma-assisted chamber system. Density functional theory calculations find the lowest energy structures of the [M(CnH2n)]+ series pertaining to olefins indicative of both C-H bond activation of methane and C-C bond coupling. We analyzed the interactions involved in the [Cu(CnH2n)]+ and [Ag(CnH2n)]+ (n = 1-6) clusters and demonstrated the reaction coordinates for the "Cu+ + CH4" and "Ag+ + CH4." It is illustrated that the presence of a second methane molecule enables us to reduce the necessary energy of dehydrogenation, which concurs with the experimental observation of an absence of the metal carbine products Cu+CH2 and Ag+CH2, which are short-lived. Also, it is elucidated that the higher-lying excitation states of Cu+ and Ag+ ions enable more favorable dehydrogenation process and C═C bond formation, shedding light on the plasma assistance of the essence.