Methane C-H bond activation by neutral lanthanide and thorium atoms in the gas phase: A theoretical prediction

Density functional theory (DFT) and Hartree-Fock effective core potential calculations have been performed to investigate the reactivity of neutral f-block atoms toward methane C-H bond activation. The first step of the methane dehydrogenation process, which corresponds to an oxidative insertion, was studied for all lanthanide and actinide thorium atoms. The DFT/B3LYP-correlated results indicate more favorable kinetic and thermochemical conditions for the insertion of the lanthanides with a three non-f valence electron D-2([f(n)] s(2)d(1)) as compared to a two non-f(1)S([f(n+1)] s(2)d(0)) electronic configuration. Among all the lanthanides, only D-2([f(n)] s(2)d(1)) La, Ce, Gd, and Lu may react exergonically with methane; the lowest activation barrier is calculated for La and Ce atoms (Delta G double dagger = 25 kcal . mol(-1)). A semiquantitative analysis from a simple two-state model shows that an indirect participation of the 4f-orbitals is expected to modify the [4f(n+1)] s(2)d(0) reactivity of the Pr, Nb, and Tb-Tm lanthanides as a configuration mixing with the [4f(n)] s(2)d(1) electronic configuration may be quite effective. The most interesting result obtained in this work is for the insertion of the [5f(0)] 7s(2)6d(2) thorium into the methane C-H bond, where an essentially barrierless (Delta G double dagger = 0.3 kcal . mol(-1)) and considerably exergonic (Delta G = - 38 kcal . mol(-1)) reaction is predicted to occur. The performance of a Th neutral atom overshadows the catalytic power of the best of the lanthanides, Ce, in the [4f(0)] 6s(2)5d(2) electronic configuration. One of the most important factors for this effectiveness comes from the 5f-orbital radial overlap onto the 7s6d valence shell, which enhances the ability of thorium as a catalyst for methane C-H bond activation.