Regulating Keto–Enol Tautomerism of β-Ketoenamine Covalent–Organic Frameworks for Photocatalytic Oxidative Coupling of Amines

Abstract

Covalent–organic frameworks (COFs), as promising metal-free porous catalysts, have become a hot research topic in the field of (photo)catalysis. Herein, a series of β-ketoenamine COFs are synthesized using different amino and aldehyde monomers, and these COFs exhibit diverse characteristics in terms of crystallinity, chemical structure, microscopic morphology, and optoelectronic properties. TpBD-(CH3)2 synthesized from 2,4,6-triformylphloroglucinol and o-tolidine exhibits an extended π-conjugated structure and higher crystallinity as well as a more suitable redox potential, which greatly enhances the photocatalytic activity of benzylamine oxidation. Besides, the increased number of hydroxyl groups on the trialdehyde-based monomer contributes to more ketoamines in the TpTAB skeleton, which is favorable for photocatalysis. Meanwhile, β-ketoenamine COFs can rapidly increase their own temperature through internal structural evolution under light irradiation, thus boosting photocatalytic oxidative coupling of amines with high efficiency. This work could provide some guidance for the rational design of functional β-ketoenamine COFs for advanced photocatalytic organic transformations.