Partially reduced-linked covalent organic frameworks: A new approach for heterogeneous catalysis and naked-eye HCl sensing

Compared to the traditionally made imine-linked covalent organic frameworks (COFs), amine-linked COFs provided by in-situ reduction of the linkages represent significant potential for advanced applications, including catalysis and sensing. The current paper deals with preparing a partially reduced COF with secondary amine groups (IR-COF) via in-situ reduction of imine linkages as a dual-functional material adopted for heterogeneous catalysis and acid-sensing. This COF was synthesized using formic acid as a reductant and catalyst in a sol-vothermal reaction between 2,4,6-tris-(4-formyl-phenoxy)-1,3,5-triazine and p-phenylenediamine. The IR-COF was used as a basic organocatalyst for the efficient synthesis of tetrahydro-4H-chromenes through the Knoevenagel-Michael cyclocondensation. Excellent yields, short reaction times, easy workup, absence of toxic organic solvents, easy recovery, and reusability of the catalyst are outstanding aspects of this catalytic method, making it convenient and useful for fine chemical synthesis. Moreover, the immediate naked-eye color change of IR-COF upon exposure to the hydrochloric acid by protonating the secondary amine groups was investigated. The (time-dependent) density functional theory (TD)DFT PBE/DNP+ method was used to investigate the mechanism of color change during the sensing. The appearance of a new absorption band at higher wavelengths upon protonation is proved to be due to intermolecular or interlayer electron transitions.

Automated summary

This paper presents a study on the synthesis and applications of an amine-linked COF (IR-COF) with excellent catalytic performance and acid sensing capabilities. The IR-COF can efficiently catalyze the synthesis of tetrahydro-4H-chromenes and exhibits immediate color change upon exposure to hydrochloric acid.