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

Abstract

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 t Bu 3 , PPh 3 , and PCy 3 are considered at the M06-2X/def2-TZVP level of theory. Classic FLP is formed only in the B(C 6 Me 4 ) 3 CH/P t Bu 3 system, while both FLP and donor−acceptor (DA) complex are observed in the B(C 6 F 4 ) 3 CF/P t Bu 3 system. Formation of DA complexes was observed in other systems; the B(C 6 H 4 ) 3 CH•P t Bu 3 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(C 6 H 4 ) 3 CH•P t Bu 3 , 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.

Association of Pyramidal Boron Lewis Superacids with Pyridines: Bending 2,4,6‐Collidine with the 10‐Sulfonium‐9‐boratriptycene

“…As 9‐boratriptycene 6 is a weaker Lewis acid (gas phase isodesmic‐corrected FIA=−476 kJ mol −1 ) than the 9‐boratriptycene‐10‐sulfonium Lewis superacid 9 (gas phase isodesmic‐corrected FIA=−854 kJ mol −1 with TMS as reference [16], see the experimental section), the Δ G 0 value for the association between collidine and 9‐boratriptycene should be much less negative than that for the association of collidine and 9‐boratriptycene‐10‐sulfonium 9 reported in Table 1 (−142 kJ mol −1 ). Thus using collidine or more bulky pyridines in combination with 9‐boratriptycene which can be generated in solution, could open the way to new types of FLP system, with potential interesting applications for the activation of small molecules, as could be other FLP systems based on 9‐boratriptycene Lewis acids and sterically hindered phosphines Lewis bases [17] …”

“…Thus using collidine or more bulky pyridines in combination with 9-boratriptycene which can be generated in solution, could open the way to new types of FLP system, with potential interesting applications for the activation of small molecules, as could be other FLP systems based on 9boratriptycene Lewis acids and sterically hindered phosphines Lewis bases. [17] Table 1. Reaction state functions of the complexation of 9sulfonium-10-boratriptycene with pyridine, 2,4,6-collidine and the fluoride anion.…”

The Field of Main Group Lewis Acids and Lewis Superacids: Important Basics and Recent Developments

“…Systematic computational study of complexes of 9-boratripticene and its derivatives with phosphines was performed in order to evaluate their activity toward heterolytic splitting of molecular hydrogen, compared to hydrogenation of BÀ C bond without LB. [136] The most promising system for the catalytic studies is the DA complex B(C 6 H 4 ) 3 CH • P t Bu 3 , which can be described as a "latent" FLP. Despite the quite high DA bond energy of 225 kJ mol À 1 , the dissociation of this complex is facilitated by steric repulsion, and the dissociation enthalpy is only 56 kJ mol À 1 .…”

“…The activation energy of the process is only 45-56 kJ mol À 1 , weakly depending on the medium. [136] Pyramidal boron-containing LA, recently studied computationally, [137] are predicted to form stable complexes with noble gases.…”

“…Systematic computational study of complexes of 9‐boratripticene and its derivatives with phosphines was performed in order to evaluate their activity toward heterolytic splitting of molecular hydrogen, compared to hydrogenation of B−C bond without LB [136] . The most promising system for the catalytic studies is the DA complex B(C 6 H 4 ) 3 CH ⋅ P t Bu 3 , which can be described as a “latent” FLP.…”