Construction of Ultrastable Nonsubstituted Quinoline‐Bridged Covalent Organic Frameworks via Rhodium‐Catalyzed Dehydrogenative Annulation

Abstract

Exploring new routes to lock the dynamic C=N bonds in imine-linked covalent organic frameworks (COFs) is highly desired for enhancing their stability and functionality. Herein, a novel C=N bridge locking strategy via rhodium-catalyzed [4+2] annulation is developed to construct nonsubstituted quinoline-linked COFs (NQ-COFs). The notable feature of this strategy includes high C=N conversion efficiency, oxidant-free, and generality for synthesis of a variety of NQ-COFs with high chemical stability. Particularly, after postsynthetic modification, the crystallinity, topology, and porosity of pristine imine-linked COFs are well retained. When used as photocatalysts, NQ-COFs display better visible light absorption and carriers' separation efficiency due to enhanced in-plane π conjugation ability, as well as more facile generation of superoxide anion radicals than their counterparts, thus leading to efficient synthesis of 2,4,6-tris(aryl)pyridines, benzimidazole, and sulfoxide derivatives.

One-pot cascade construction of nonsubstituted quinoline-bridged covalent organic frameworks

“…24,25 For instance, C]N bonds in imine-linked COFs could be transformed to more robust amide linkages under an oxidizing condition with high conversion efficiency and crystallinity. [26][27][28][29] Similarly, locking of the imine linkage by other organic reactions could afford stable benzothiazole, 30,31 quinoline, [32][33][34][35] and benzoxazole linkages, 36 and C-N bonds. 37,38 Following these pioneering works, we recently reported a novel oxidant-free PSM method to construct ultrastable nonsubstituted quinolinelinked COFs (labelled as NQ-COFs) via rhodium-catalyzed [4 + 2] annulation.…”

“…37,38 Following these pioneering works, we recently reported a novel oxidant-free PSM method to construct ultrastable nonsubstituted quinolinelinked COFs (labelled as NQ-COFs) via rhodium-catalyzed [4 + 2] annulation. 34 However, these PSM strategies, including ours, still suffer from some drawbacks, such as collapse of the framework and decreased crystallinity during the solid-state transformation, which might hamper the separation and transfer of photoinduced electron-hole pairs when these PSM derived COFs are applied in photocatalytic processes. Therefore, although the NQ-COFs synthesized in our work have demonstrated their rst application in photoredox organic transformation, there is still much room to boost their photocatalytic activities.…”

Weakly Hydrophilic Imine-Linked Covalent Benzene–Acetylene Frameworks for Photocatalytic H2O2 Production in the Two-Phase System

“…The FT-IR signals of the aldehyde groups (1656 cm –1 ) and amino bands of the corresponding precursors almost disappeared after polymerization (Figures S11–S13), further verifying the formation of imine bonds via the condensation of aldehyde and primary amine . The 13 C NMR spectra of three COFs showed the characteristic signals of acetylene functionalities at ∼90 ppm and imine CN at ∼161 ppm. ,− The peak at ∼170 ppm was assigned to the carbon atoms in 1,3,5-triazine ring for COF-TAPT-BPDA and COF-TAPP-BPDA (Figure b). ,,− The 13 C NMR signal at ∼158 ppm for COF-TAPP-BPDA might correspond to the carbon atoms linked with pyridine N atoms . The NMR signals between 100 and 150 ppm were mainly attributed to carbon atoms of benzene rings on three COFs.…”

“…10,13,30−32 The 13 C NMR signal at ∼158 ppm for COF-TAPP-BPDA might correspond to the carbon atoms linked with pyridine N atoms. 33 The NMR signals between 100 and 150 ppm were mainly attributed to carbon atoms of benzene rings on three COFs.…”

General Strategy for Incorporation of Functional Group Handles into Covalent Organic Frameworks via the Ugi Reaction

“…Additional confirmation of the imine nitrogen transformation to the Ugi amide nitrogen was obtained using magic angle spinning (MAS) dynamic nuclear polarization (DNP) to enhance solid-state NMR sensitivity 53,54 and enable 1 H → 15 N cross-polarization MAS (CPMAS) SSNMR experiments. The DNP-enhanced 15 N CPMAS NMR spectra verify the nitrogen functional groups present in the frameworks before and after functionalization. We note that DNP has been used previously to enhance the sensitivity of solid-state NMR experiments on a variety of porous materials such as silica, 55,56 MOF, 57,58 COF, 59 polymers, 60 and zeolites.…”