Microporous and mesoporous materials, such as zeolites, activated carbon, silica and metal organic frameworks (MOFs) are widely used in catalysis, gas adsorption, storage and separations. [1] The recent development of polymerbased microporous materials may provide new opportunities in hydrogen storage and heterogeneous catalysis, as organic materials have certain advantages over other materials. [2] Microporous polymeric materials possess unique surface properties that can be tailored to facilitate chemoselective adsorption, separation and catalysis. [2, 3] Although hard and soft templates have been widely used in the synthesis of porous materials, [4] a bottom-up approach facilitates the tailoring of porous materials with tailored porous structure and surface chemistry. [2, 3,5] Cooper and co-workers recently reported the synthesis of conjugated microporous poly(arylene ethynylA C H T U N G T R E N N U N G ene) (PAE) by using palladium-catalyzed Sonogashira-Hagihara crossing-coupling reaction. [6] The PAE networks bridge the gap between covalent organic frameworks (COFs) [2f,g,k,l] and polymers with intrinsic microporosity (PIMs) [2a, 3] or hypercrosslinked polymers (HCPs). [2c,d] Microporous polymers have also been applied in a wide range of applications, such as, gas storage and adsorption, [3a] gas separation membrane, [7] and heterogeneous catalysis. [8] We are interested in the design and synthesis of novel covalent organic frameworks with different functional groups and controlled pore size so that the properties of porous polymers can be designed for the desired applications. Herein we report the synthesis and catalytic application of microporous polyisocyanurate (PICU) composed of rigid carbon and nitrogen networks. Although macroporous poly-A C H T U N G T R E N N U N G isocyanurates with foam-like structure have been studied, [9,10] microporous and mesoporous polyisocyanurates have not been reported. In this study, novel microporous PICUs were derived by cyclotrimerization of diisocyanate using N-heteroA C H T U N G T R E N N U N G cyclic carbene (NHC) as catalyst. The unique microporous PICU demonstrated excellent potential in reactions such as selective oxidation.
NHCs have been widely used as organocatalysts in many important transformations. [11] Recently, it was found that NHCs can efficiently catalyze cyclotrimerization of isocyanates to form a planar six-membered heterocyclic ring structure. [12] This reaction was adapted in this study to prepare porous polymer networks by replacing simple isocyanates with diisocyanates. Scheme 1 illustrates that a porous C,N organic framework could be woven by NHC organocatalysts. When rigid aryl diisocyanates were used, rigid polymer frameworks were achieved. Diisocyanate B was employed in this study, and various NHCs were examined in the synthesis of PICU networks. It was found that the NHCs of 1,3-bismesitylimidazol-2-ylidene (IMes) and 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) did not work for this synthesis. NHCs of 1,3-bis-mesityl-4,5-dihydroimidazol-2-ylidene (SIMes) and 1,3-bis-(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene (SIPr) showed low activities. In contrast, NHCs with more flexible substituents, 1,3-bis-tertbutyl-4,5-dihydroimidazol-2-ylidene (SItBu) displayed very high activities for this synthesis.
Typically, porous PICU was synthesized by dissolving 1 mmol of monomer in 5 mL of N,N'-dimethylformamide (DMF) in a pressure flask, and 0.02 mmol of SItBu NHC was added. The reaction flask was closed and heated to 80 8C for 24 h. The reaction can be conducted at temperatures ranging from 25 8C to 150 8C; a longer period of time would be needed for the completion of the reaction at lower temperatures. The polymer product was collected by filtration, washed, and dried in a vacuum oven. In all syntheses, quantitative yields were obtained. Owing to the low solubility of the starting materials and product, the reaction was essentially conducted under a heterogeneous condition. Unlike conventional heterogeneous catalysts, the NHC catalysts in this case might migrate onto the polymer network and initiate the next catalytic cycle. Photoacoustic Fourier [a] Dr.