Mechanism of the Ta⁺-Mediated Activation of the C−H Bond in Methane

Abstract

The mechanistic details of the C−H bond activation brought about by a Ta + ion, which was observed in the gas phase, have been computationally investigated using approximate density functional theory. In accord with the experimental information, the overall reaction sequence Ta + + CH 4 → TaCH 2 + + H 2 is computed to be exothermic by ca. 14 kcal mol - 1 and the Ta−C binding energy in the tantalum carbene cation is computed as 119 kcal mol - 1, in satisfying agreement with the experimental lower bound for this quantity of 111 kcal mol - 1 . The bond activation process itself consists of several steps, commencing with the formation of an electrostatically bound encounter complex, followed by the insertion of the Ta + into a C−H bond and the generation of a H 2 TaCH 2 + dihydrido species. Finally, this latter intermediate rearranges into a productlike complex between molecular hydrogen and a tantalum carbene cation, which concludes the sequence by subsequent loss of H 2 . Interestingly, the reaction does not proceed adiabatically on one potential energy surface; rather, there are several changes in multiplicity along the reaction coordinate. The structures of singlet TaCH 2 + and related intermediates where a carbene substructure is involved show several peculiarities, which are rationalized by an analysis of the relevant molecular orbital and the electron density.