Constructing ambivalent imidazopyridinium-linked covalent organic frameworks

Covalent organic frameworks (COFs) are dynamic covalent porous organic materials synthesized from molecular organic building blocks. However, the chemical linkages used to construct COFs are limited by the dynamic bond formation needed to ensure crystallinity. Thus, there is a continual search for new, chemically stable linkages that tailor both the chemical properties and topologies of COFs. As opposed to electrophilic linkages used to construct COFs, nucleophilic linkages that can react with electron-deficient species are rare. Here we report the synthesis of picolinaldehyde-derived imine-linked COFs that can be transformed into imidazopyridinium-linked COFs (IP-COFs) with a Lieb-like lattice. IP-COFs serve as precursors to ambivalent N-heterocyclic carbenes that can dissociate disulfide bonds to form carbon-sulfur bonds. IP-COFs exhibit a vastly improved sulfur redox chemistry when used as cathode materials in lithium-sulfur batteries, as they achieve a rate performance of 540 mAh g-1 (10 C) and a high areal capacity of 6.2 mAh cm-2 with a high sulfur loading of 9 mg cm-2 and a low electrolyte-to-sulfur ratio of 6 mu l mg-1. In addition, the ionicity of the linkages enables the cleavage of IP-COFs into highly crystalline flakes with well-defined fringes, as resolved by atomic force microscopy and transmission electron microscopy. Making highly crystalline covalent organic frameworks (COFs) is challenging and existing linkages used for constructing COFs are limited in terms of variety and functionality. Now, crystalline COFs with imidazopyridinium linkages, which serve as precursors to N-heterocyclic carbenes, have been synthesized, expanding the reactivity of COFs by incorporating chemical ambivalence (electrophilicity and nucleophilicity) into the linkages. The COFs are also used as cathode materials in lithium-sulfur batteries.