Methane activation by metal-free Lewis acid centers only--a computational design and mechanism study

In the present computational study by using the density functional theory (DFT) method, we found that silylboranes, which have metal-free Lewis acid centers only, can break the C-H bond of the exceedingly unreactive methane. The study shows that, unlike the activation mechanism of small molecules by the frustrated Lewis pairs (FLPs), the Lewis acidic boron center plays a key role in breaking the C-H bond of methane. Detailed analyses indicate that in the transition state the C-H bond is substantially activated by the empty 2p orbital of boron (2p(B)) primarily due to the orbital interaction between the C-H σ-bonding orbital and 2p(B). On the other hand, the orbital interaction between the C-H σ-anti-bonding orbital and the B-Si σ-bonding orbital also contributes to the activation but plays a minor role. A statistical method was used to find the relationship between the reactivity of 57 silylboranes and their electronic properties. The results indicate that the boron center does have more prominent effect on the reactivity, especially the occupancy (n(2p)(B)) and energy (ε(2p)(B)) of 2p(B), where lowering n(2p)(B) and ε(2p)(B) will increase the reactivity of the silylboranes. Based on the activation mechanism and taking kinetic and thermodynamic possibilities, as well as the possible side reactions, into consideration, three silylboranes suitable for methane activation under mild experimental conditions were designed. The analogous line of thought can be used as a hint for further experimental realizations, even under ambient conditions. This strategy can also be expected to be transplanted to more extensive C-H activation of hydrocarbons.