Porous aromatic frameworks (PAFs) as efficient supports for N-heterocyclic carbene catalysts

Porous polymeric aromatic frameworks (PAFs) have high porosity and surface area, as well as high physicochemical stability; such characteristics are important in the design of heterogeneous catalysts with high catalytic efficiency and recyclability. This paper presents the synthesis, characterization, post-functionalization and catalytic performance of the resulting modified PAFs based on tetra-phenyladamantane (PAF(Ad)), tetraphenylmethane (PAF(C)) and 9,9'-spirobisfluorene (PAF(spf)) nodes. The PAFs were obtained by the Suzuki-Miyaura cross-coupling under microwave heating, and were sequentially reacted with 1-(chloromethoxy) octane and 1-mesityl-1H-imidazole or 2-(1H-imidazol-1-yl) pyridine to yield the corresponding imidazolium chloride derivative (PAF-Im) which readily formed stable N-heterocyclic carbene (NHC)iridium and ruthenium complexes (PAF-(NHC)Ir, PAF-(NHC)Ru). The materials were characterized by solid-state NMR spectroscopy, FTIR spectroscopy, and textural analysis. The PAF-(NHC)M materials display excellent catalytic performance in the N-alkylation of amines with alcohols and transfer hydrogenation of ketones over multiple catalytic cycles.