Theoretical studies on bridged frustrated Lewis pair (FLP) mediated H2 activation and CO2 hydrogenation

DFT calculations show that H-2 and CO2 activation by bridged phosphane/borane frustrated Lewis pairs (FLPs) experience a one-step concerted mechanism with small reaction barriers. In addition, we found that some bridged FLPs can effectively catalyze the hydrogenation of carbon dioxide by two possible pathways: (1) CO2 hydrogenation can happen immediately after H-2 activation through a concerted pathway; (2) CO2 activation happens first and metathesis of H-2 and reductive elimination follow step-by-step. Based on our DFT calculations, bridged FLPs without electron donating substitutions on the B's adjacent carbon site possess a facile H-2 activation process, and bridged FLPs with longer carbon chains or bulky substitutes on the carbon chains possess lower reactivity than the bridged FLPs with shorter carbon chains and smaller substituents. Bridged FLPs with unsaturated carbon chains, such as Mes(2)PCHPhCB (C6F5)(2) and Mes(2)PCCH(3)CHB(C6F5)(2), are more prepared to react with H-2 than the others with saturated carbon chains. The same happens for CO2 activation, but most barriers for CO2 activation are higher than H-2 activation. However, for CO2 hydrogenation, our calculations demonstrate that quite stable intermediates are formed after H-2 activation, followed by high barriers for the concerted CO2 hydrogenation. The total barriers for the first kind of pathway with alkyl chain bridged FLPs are from 27-37 kcal mol(-1), while most of the other pathways have higher barriers. Mes(2)PCH(2)CH(2)B-(C6F5)(2) shows a smaller barrier for CO2 hydrogenation with CO2 activation first than the other pathway and other FLPs. Most of our calculation results are consistent with the experimental observations.