Mechanistic Foundations of the Sequential Activation of Methane by Ta+: Oxidative Addition, Ring-Opening σ-Bond Metathesis, and C-C Bond Formation.

Abstract

The kinetics of Ta⁺ + CH₄ and related reactions TaC_nH_m⁺ + CH₄ (n = 2-4, m = n, 2n, 3n) are measured from 300-600 K using a selected-ion flow tube apparatus. Complicated kinetics are analyzed through a novel bootstrapping methodology, and rate constants for 38 unimolecular, bimolecular, and ternary processes are reported at each of the four temperatures. As has been well-established, Ta⁺ efficiently dehydrogenates methane through a non-spin-conserved process. Sequential chemistry leads to the dehydrogenation of up to four methane molecules per tantalum center through the competing processes of TaC_nH_m⁺ + CH₄ → TaC_n+1H_m+2⁺ + H₂ (dehydrogenation) and TaC_n+1H_m+4⁺ (association). Supported by density functional theory calculations, the distinct mechanisms and product structures of the sequential reactions are derived. The activation energy for oxidative insertion of Ta into a C-H bond is well-predicted by a simple heuristic: whether or not the reactant tantalum atom possesses unbound valence electrons of opposite spin. TaCH₂⁺ is predicted to have a small activation energy for oxidative insertion but can only proceed to dehydrogenation of methane via carbon-carbon bond formation, enabled by three separate intersystem crossing events. The product is determined to be the tantalapropene dihydride cation, not the more intuitive tantalapropane cation, via comparison of measured and calculated thermal dissociation rates. The TaC₂H₄⁺ tantalapropene dihydride has a prohibitive barrier to oxidative insertion. It proceeds instead through a ring-opening insertion of the entire tantalapropene moiety into a C-H bond via σ-bond metathesis; the unbroken metallacycle bond acts as a tether, preventing the activated products from separating and allowing for further isomerization, leading to dehydrogenation. This and subsequent dehydrogenation processes occur without carbon-carbon bond formation; no evidence of a tantalabutane or larger metallacycle is found.