Towards the characterization of the mechanism of the sequential activation of four methane molecules by Ta+
Résumé
Mass spectrometry and quantum chemistry have been used to characterize the mechanism of the sequential dehydrogenation of four methane molecules by third row transition metal cation Ta+. A comprehensive study of the second reactive intermediate TaC2H4+ of this sequence is presented here. Mass-selected TaC2H4+ ions have been characterized by UV-visible photodissociation showing a sequential loss of H2 and C2H2. The appearance threshold of TaC2H2+ is much higher than the corresponding thermodynamic threshold. We show that this difference is probably due to a lack of low-energy electronically excited state for the TaC2H4+ ions formed by dehydrogenation of two methane molecules. Within the nine low-energy isomers characterized at the density functional level, the dihydridometallacyclopropene (Ta(H)2(C2H2)+) is the most stable. The very selective reactivity of TaC2H4+ with both D2 and CD4 strongly suggests that the Ta(H)2(C2H2)+ is the reactive species of the sequential dehydrogenation of four methane molecules, and that the two methane dehydrogenations proceed through two successive σ-bond metatheses involving Ta+---H and C---H σ-bonds. On the contrary, when TaC2H4+ ions are formed by dehydrogenation of ethane, reactions with D2 and CD4 are less selective. The photodissociation behaviors of the corresponding ions are also very different, and the appearance thresholds of TaC2H2+ and Ta+ are in agreement with the corresponding thermodynamic thresholds determined theoretically.