A General Base-free Route toward the Building of Metal N-Heterocyclic Carbenes into Covalent Organic Frameworks for CO2 Conversion

Covalent organic frameworks (COFs) would be a promising platform to immobilize metal N-heterocyclic carbenes (M-NHCs) for efficient CO2 conversion, but they are seldom reported. Herein, we develop a facile base-free strategy to prepare M-NHCs (M = Ir, Au, Pd) decorated M-NHC-COFs (Ir-NHC-COF, Au-NHC-COF, and Pd-NHC-COF) via the reactions of the imidazolium carboxylate functionalized NHC-based COF (NHC-CO2-COF) with the corresponding metal complexes under mild conditions. This facile synthesis method for the generation of the desired M-NHC single sites avoids the handling of the free carbenes and the use of strong bases. The obtained Ir-NHC-COF catalyst showed efficient catalytic activity and recyclability in the CO2 hydrogenation process, attaining a noteworthy turnover frequency of up to 17244 h(-1), which placed it among the top-performing heterogeneous catalysts for this reaction. This study presents a universal and straightforward approach for attaching functional M-NHC single sites onto COFs, offering combined advantages and acting as highly efficient catalysts for CO2 hydrogenation reactions. This general strategy would be widely applicable to prepare other M-NHC-functionalized porous materials with covalently bound M-NHC single sites for various applications in the future.