Energetic Compensation Between H2 Activation and CO2 Hydrogenation to Formic Acid in Alkyl-Substituted B/N Frustrated Lewis Pairs

Efficient CO2 hydrogenation by frustrated Lewis pairs requires balancing H2 activation with subsequent delivery of proton and hydride equivalents to CO2. Here, we investigate this interplay in a systematic 2 × 2 series of intramolecular boron/nitrogen Lewis pairs bearing methyl and tert-butyl substituents at the reactive centers. Density functional theory calculations were used to characterize the free-energy landscapes for H2 activation and CO2 hydrogenation, followed by analysis using the energetic span model. At the M06-2X level, the free-energy cost of forming the hydrogenated Lewis pair decreases from 13.4 to 6.4 kcal mol−1 across the series, whereas the local barrier for subsequent CO2 hydrogenation increases from 13.5 to 19.9 kcal mol−1. These opposing trends largely compensate, yielding a narrow range of dominant standard-state energetic spans (26.3–27.7 kcal mol−1) despite differences between the reaction stages. Mechanistic analysis reveals a single-transition-state CO2-hydrogenation pathway for two systems and stepwise stationary-point pathways for the other two, with hydride transfer more advanced than proton transfer at the principal transition state. The results show that facilitating H2 activation does not necessarily lower the energetic requirements of the complete reaction sequence, highlighting the importance of balancing hydrogen uptake and delivery in molecular Lewis-pair design.