Theoretical Investigation of the Steric Effects on Alkyne Semihydrogenation Catalyzed by Frustrated Lewis Pairs

Frustrated Lewis pairs (FLPs) have received increasing attention for offering a distinctive pathway for the activation and conversion of small molecules. Here, we employ density functional calculations to investigate the electronic and steric effects of FLPs and alkynes on the activity toward alkyne semihydrogenation, a crucial reaction in the production of pharmaceuticals and polymers. We investigated the steric effect of FLPs by replacing the LA-LB linker (d) of the model catalyst FLP, 1-NMe2-2-B(C6F5)2-C6H4 (d1), with various linkers. Additionally, we studied the steric effect of alkynes by varying the substrate from acetylene to but-2-yne, hex-3-yne, 2,5-dimethylhex-3-yne, 2,2,5,5-tetramethylhex-3-yne, and 1,2-diphenylethyne. Our computational results suggest that the overall activity of alkyne and alkene hydrogenation is influenced by both electronic and steric effects when using FLPs with varied LA-LB linkers. To achieve better activity, one could increase the steric hindrance of the LA neighboring environment and reduce the electron density of the LB site of FLPs. Structure-activity relationships for each elementary step involving alkyne semihydrogenation were identified in this work too. In contrast, when the reaction is catalyzed by the same FLP, the activity is solely governed by the steric hindrance of the alkynes. The overall activity exhibits a volcano-shaped trend as a function of the buried volume of the alkynes. FLP (d1) shows the highest activity toward hex-3-yne, as predicted by the volcano correlation, while it exhibits lower activity toward alkynes with either less or greater steric hindrance compared to hex-3-yne.