Hydrogen Activation by Frustrated and Not So Frustrated Lewis Pairs Based on Pyramidal Lewis Acid 9-Boratriptycene: A Computational Study.

Structural features and reactivity of frustrated Lewis pairs (FLPs) formed by pyramidal group 13 Lewis acids based on 9-bora and 9-alatriptycene and bulky phosphines P Bu, PPh, and PCy are considered at the M06-2X/def2-TZVP level of theory. Classic FLP is formed only in the B(CMe)CH/P Bu system, while both FLP and donor-acceptor (DA) complex are observed in the B(CF)CF/P Bu system. Formation of DA complexes was observed in other systems; the B(CH)CH·P Bu complex features an elongated DA bond and can be considered a "latent" FLP. Transition states and reaction pathways for molecular hydrogen activation have been obtained. Processes of heterolytic hydrogen splitting are energetically more favored in solution compared to the gas phase, while activation energies in the gas phase and in solution are close. The alternative processes of hydrogenation of B-C or Al-C bonds in the source pyramidal Lewis acids in the absence of a Lewis base are exergonic but have larger activation energies than those for heterolytic hydrogen splitting. The tuning of Lewis acidity of 9-boratriptycene by changing the substituents allows one to control its reactivity with respect to hydrogen activation. Interestingly, the most promising system from the practical point of view is the DA complex B(CH)CH·P Bu, which is predicted to provide both low activation energy and thermodynamic reversibility of the heterolytic hydrogen splitting process. It appears that such "not so frustrated" or "latent" FLPs are the best candidates for reversible heterolytic hydrogen splitting.