Methane C−H Bond Activation by Neutral Lanthanide and Thorium Atoms in the Gas Phase: A Theoretical Prediction

Abstract

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 2D([fn]s2d1) as compared to a two non-f1S([fn+1]s2d0) electronic configuration. Among all the lanthanides, only 2D([fn]s2d1)La, Ce, Gd, and Lu may react exergonically with methane; the lowest activation barrier is calculated for La and Ce atoms (ΔG⧧ = 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 [4fn+1]s2d0 reactivity of the Pr, Nb, and Tb−Tm lanthanides as a configuration mixing with the [4fn]s2d1 electronic configuration may be quite effective. The most interesting result obtained in this work is for the insertion of the [5f 0]7s26d2 thorium into the methane C−H bond, where an essentially barrierless (ΔG⧧ = 0.3 kcal·mol-1) and considerably exergonic (ΔG = − 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]6s25d2 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.