Covalent organic frameworks (COFs) expand organic chemistry from discrete molecules into the realm of crystalline 2D and 3D extended structures. The nature of the covalent linkages dictates the crystallinity and the physical and chemical properties of the resulting material. In this context, chemically robust COFs that maintain high crystallinity and porosity are pursued through the development of linkage chemistry. In this abstract, we report the development of the new strategy of crystalizing unsubstituted olefin (-CH=CH-) linked COFs through Brønsted acid catalyzed Aldol condensation between activated methyl groups of 1,3,5-triazine and aryl aldehydes. The reaction of the Aldol condensation has been commonly employed in various fields, but direct application of the conventional conditions to the synthesis of COFs results in amorphous materials, indicating insufficient reversibility or the formation of side products. The chemical space of such reaction conditions in solvothermal synthesis was expanded through extensive screening. The target COFs were successfully crystallized, exemplified by COF-701. The formation of olefin linkage was unambiguously confirmed by FT-IR and solid-state 13C CP-MAS NMR spectroscopy on both unlabeled and 13C isotope-labeled samples. WAXS and sorption experiments established the crystalline and porous structure of COF-701 as a layered framework with hcb topology. The unsubstituted olefin linkage endows COF-701 with exceptionally high stability toward Brønsted acids and bases, organolithium reagents, and Lewis acids. To highlight the stability of this linkage we immobilized BF3 etherate, a strong Lewis acid, within the pores of COF-701. A benchmark Diels-Alder reaction was used to show that COF-701 retains its integrity in the presence of the BF3 species and that the immobilized Lewis acid, in turn, retains its catalytic activity. We anticipate that unsubstituted olefin-linked COFs will serve as a robust, functionalizable platform for a broad array of chemical transformations.

Porous crystalline olefin-linked covalent organic frameworks

ACS Fall 2020 Virtual Meeting

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