Highly air- and moisture-sensitive N-heterocyclic carbenes (NHCs) are often prepared in situ by treatment of imidazolium precursors with strong alkali reagents. This synthetic method can lead to formation of inactive NHC dimers and to the destruction of porous supports. Moreover, it is often difficult to recover and reuse NHCs or their precursors.

Rong Cao, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou and colleagues have synthesized a CO₂-masked NHC-functionalized covalent organic framework (COF) for highly efficient catalytic CO₂ conversion. These are the first stable CO₂-masked N-heterocyclic carbenes (NHCs) grafted on a porous crystalline COF.

The imidazole-functionalized Im-COF-1 was synthesized by condensing 4,7-bis(4-formylbenzyl)-1-methyl-1H-benzimidazole and 4,4′,4”,4”’-(ethene-1,1,2,2-tetrayl)-tetraaniline. The researchers then reacted the imidazole-functionalized Im-COF-1 with dimethyl carbonate at 120 °C for 24 h, forming NHC-CO₂-COF.

The well designed porous COF-NHC-CO₂ shows superior activity and selectivity in the hydrosilylation of CO₂ and N-formylation of amines with CO₂ to produce methanol and formamides, respectively.

On application of heat, the CO₂ moiety was released in situ to form free NHC single active sites. The NHC sites were highly active in catalytic hydrosilylation and N-formylation reactions with CO₂. Moreover, exposure to a CO₂ atmosphere regenerated the NHC-CO₂ moieties.

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• A CO₂‐Masked Carbene Functionalized Covalent Organic Framework for Highly Efficient Carbon Dioxide Conversion,
  Chang He, Duan-Hui Si, Yuan-Biao Huang, Rong Cao,
  Angew. Chem. Int. Ed. 2022.
  https://doi.org/10.1002/anie.202207478