On the Mechanism of Irreversible Carbon Dioxide Binding with a Frustrated Lewis Pair: Solvent-Assisted Frustration and Transition-State Entropic Encouragement

The mechanism of irreversible carbon dioxide binding with a Lewis pair Mes(3)P:AlCl3 (Mes = 2,4,6-C6H2Me3) is computationally investigated to reveal that the steric congestion is not the driving force for the activation of CO2. Instead, we find that the specific solute-solvent interaction between the Lewis acid and a bromobenzene molecule lowers the effective binding energy of the Lewis pair. This solvation effect affects the reaction in a similar manner to the steric encumbering of conventional frustrated Lewis pairs. Additionally, the transition state toward the CO2 binding becomes extraordinarily flexible upon solvation. This flexibility encourages the adduct formation entropically and thus lowers the free-energy barrier of the reaction. We conclude that this combination of energy-barrier lowering through solvent-assisted frustration and the entropic encouragement generates a feasible activation route for CO2 under mild conditions.