A DFT study of hydrogen and methane activation by B(C₆F₅)₃/P(t-Bu)₃ and Al(C₆F₅)₃/P(t-Bu)₃ frustrated Lewis pairs

This contribution presents a computational study aimed at understanding factors affecting barriers associated with the activation of the H–H bond in molecular hydrogen and the H–CH₃ bond in methane mediated by intermolecular Frustrated Lewis Pairs (FLPs). The classical phosphine P(t-Bu)₃ Lewis base in conjunction with two Lewis acids, B(C₆F₅)₃ and Al(C₆F₅)₃, were used as representative models of intermolecular FLPs. DFT calculations were performed using the dispersion corrected ωB97x-D functional, including toluene as a solvent through the PCM-SMD implicit solvent scheme. The results show that, in all cases, the activation barrier is larger for methane than for hydrogen. We conclude that the observed increase in the barrier for methane activation is due primarily to a larger distortion in methane compared to hydrogen to reach the transition state. Second, a large distortion of the Lewis acid to attain a better interaction with the σ-bond in methane was observed. Finally, we found that, for both hydrogen and methane activation, a considerable reduction in the free energy activation barrier is observed when the Lewis acid Al(C₆F₅)₃ is used. From the results extracted in this study, we propose the use of alanes acids as good candidates for methane activation.