Understanding the reactivity of geminal P/B and P/Al frustrated Lewis pairs in CO 2 addition and H 2 activation.

Understanding CO ₂ addition and H ₂ activation by geminal P/Z (Z = B, Al) frustrated Lewis pairs (FLPs) is crucial for the development of transition metal-free catalysts for hydrogenation of unsaturated compounds and CO ₂ conversion. The distortion-interaction energy decomposition and natural bond orbital were used to gain insights into the reactivity of CO ₂ addition and H ₂ activation. Remarkably, the orbital interactions in the reactions of CO ₂ and H ₂ with geminal P/B FLPs are stronger than those in the reactions with more reactive geminal P/Al FLPs, which contain a stronger Lewis acid. The fact that geminal P/B FLPs react with a higher energy barrier than geminal P/Al FLPs is actually due to the unfavorable core repulsion contribution to the interaction energy and the higher distortion energy found for the geminal P/B FLPs. In addition, we revealed that although the interaction energy is higher for H ₂ activation, the distortion energy is significantly lower for CO ₂ addition. Thus, geminal P/Z FLPs are more reactive toward CO ₂ than toward H ₂ . Overall, the reactivity and thermodynamic stability for the reactions of geminal P/Z FLPs with CO ₂ and H ₂ are not affected by the type of bridging carbon (sp ³ -C or sp ² -C) but mainly influenced by the substituents on P and Z (B or Al). An insight into the thermodynamic stability of the H ₂ activation product is gained by quantifying the hydride affinity, the proton affinity, and the stability of the zwitterionic product relative to the prototypical Lewis acid-base pair, P ^t Bu ₃ /B(C ₆ F ₅ ) ₃ .