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 PtBu3, PPh3, and PCy3 are considered at the M06-2X/def2TZVP level of theory. Classic FLP is formed only in the B(C6Me4)3CH/PtBu3 system, while both FLP and donor-acceptor (DA) complex are observed in the B(C6F4)3CF/PtBu3 system. Formation of DA complexes was observed in other systems; the B(C6H4)3CH center dot PtBu3 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(C6H4)3CH center dot PtBu3, 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.

Automated summary

This study investigates the structural features and reactivity of frustrated Lewis pairs (FLPs) composed of pyramidal group 13 Lewis acids and bulky phosphines. Different systems were analyzed, and it was found that classic FLPs and donor-acceptor (DA) complexes can be formed. The activation energy and reaction pathways for hydrogen activation were also studied, and it was observed that heterolytic hydrogen splitting is favorable in solution. The Lewis acidity of the Lewis acids can be tuned by changing the substituents, which allows for control over their reactivity with hydrogen. The DA complex B(C6H4)3CH center dot PtBu3 shows promise for reversible heterolytic hydrogen splitting due to its low activation energy and thermodynamic reversibility.