Covalent-organic frameworks with keto-enol tautomerism for efficient photocatalytic oxidative coupling of amines to imines under visible light

Photocatalytic oxidation of organic molecules into highly value-added products is an innovative and challenging research which has gradually attracted remarkable attention of scientists. In this work, it is demonstrated that the COF-TpPa with keto-enol tautomerism equilibrium structure shows excellent performance (yield>99% after 8 h) in the selective photocatalytic oxidative coupling of amines to imines under visible light irradiation. It is revealed that three kinds of reactive oxygen species (superoxide radical, hydroxyl radical and singlet oxygen) participate in this photocatalytic oxidation reaction. In addition, hydrogen protons cleaved from the benzyl are proven to be reduced to hydrogen in the conduction band of COF-TpPa in anaerobic atmosphere, accompanied with the formation of imines. The direct hydrogen evolution from amine provides an effective way to extract clean energy from organic molecule as well as the production of value-added chemicals. As a contrast, COF-LZU1 with similar structure and chemical composition to COF-TpPa but without keto-enol tautomerism exhibits worse optical properties and photocatalytic performance. It is also demonstrated that keto-enol tautomerism favors the adsorption of benzylamine based on the characterization results and theoretical calculations.