Understanding the reactivity of geminal P/B and P/Al frustrated Lewis pairs in CO2 addition and H2 activation

Abstract

Understanding CO₂ addition and H₂ 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 CO₂ conversion. The distortion–interaction energy decomposition and natural bond orbital were used to gain insights into the reactivity of CO₂ addition and H₂ activation. Remarkably, the orbital interactions in the reactions of CO₂ and H₂ 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 H₂ activation, the distortion energy is significantly lower for CO₂ addition. Thus, geminal P/Z FLPs are more reactive toward CO₂ than toward H₂. Overall, the reactivity and thermodynamic stability for the reactions of geminal P/Z FLPs with CO₂ and H₂ are not affected by the type of bridging carbon (sp³-C or sp²-C) but mainly influenced by the substituents on P and Z (B or Al). An insight into the thermodynamic stability of the H₂ 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^tBu₃/B(C₆F₅)₃.