A CO 2 -Masked Carbene Functionalized Covalent Organic Framework for Highly Efficient Carbon Dioxide Conversion.

Free N-heterocyclic carbenes (NHCs) are generally prepared by treatment of imidazolium precursors with strong alkali reagents, which usually produces inactive NHC dimers. This treatment would destroy porous supports and thus make supported NHC catalysts difficult to recovery and reuse. Herein, we report the first stable CO 2 -masked N-heterocyclic carbenes (NHCs) grafted on a porous crystalline covalent organic framework (COF). The stable NHC-CO 2 moieties in the COF-NHC-CO 2 could be transformed in situ into isolated NHCs by heating, which exhibit superior catalytic performances in hydrosilylation and N-formylation reactions with CO 2 . The NHC sites can reversibly form NHC-CO 2 and thus can be easily recycled and reused while maintaining excellent catalytic activity. Density functional theory calculations revealed that NHC sites can be fully exposed after removal of CO 2 -masks and rapidly react with silanes, which endows COF-NHC with high catalytic activity.