Metalated-bipyridine-based porous hybrid polymers with POSS-derived Si–OH groups for synergistic catalytic CO2 fixation

Abstract

Herein, we construct a new series of N-heterocyclic ligand bipyridine-based porous hybrid polymers (denoted Bpy-PHPs) from the Heck reaction of a rigid building unit octavinylsilsesquioxane (VPOSS) and 5,5′-dibromo-2,2′-bipyridine. Surprisingly, the typical sample Bpy-PHP-4 was found to be a metal-/halogen-free heterogeneous catalyst in the cycloaddition reaction of CO₂ with a few epoxides under atmospheric pressure. After coordination with ZnBr₂, the resultant ZnBr₂@Bpy-PHP-4 afforded largely enhanced heterogeneous catalytic activities upon the conversion of carbon dioxide (CO₂) and various epoxides into cyclic carbonates without using any co-catalysts under mild conditions. The moderate catalytic activities of Bpy-PHP-4 may be due to the presence of hydrogen bond donors (HBDs), i.e., polyhedral oligomeric silsesquioxane (POSS)-derived Si–OH groups and N active sites from Bpy linkers. In comparison, the high catalytic efficiency of ZnBr₂@Bpy-PHP-4 should be attributed to the synergistic catalysis of Si–OH groups, N active atoms, and Bpy-coordinated ZnBr₂. Moreover, the catalyst ZnBr₂@Bpy-PHP-4 can be easily recovered and reused ten times without any significant loss of catalytic activities. This work affords an efficient metal-based porous hybrid polymer heterogeneous catalyst for the cycloaddition reaction of CO₂ and epoxides under mild and co-catalyst-free conditions.