Engineering Ru(N^N)₃-modified covalent organic framework for photocatalytic olefin epoxidation

Photocatalytic olefin oxidation using air as an oxidant is an environmentally friendly method for producing epoxides. Covalent organic frameworks (COFs) have emerged as promising photocatalysts in various organic synthesis reactions. The combination of tri-(2-pyridinal aldehyde)-containing precursor with 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)trianiline generated (N^N)-I-COF. Via imine-to-quinoline transformation and coordination anchorage of Ru(N^N)₃ unit, Ru(N^N)₃@Q-COF was obtained with improved chemical stability, skeleton conjugation, and novel photochemical characteristics, and demonstrated excellent photocatalytic activity in olefin epoxidation with a wide range of substrates. The presence of H₂O played a crucial role in the formation of reactive oxygen species (ROSs), which in turn influenced the olefin oxidation process. The hydrophilicity of Ru(N^N)₃ facilitated the approach of H₂O and O₂ to the photogenerated charges, thereby promoting ROSs generation. The lipophilicity of Q-COF allowed for the absorption of olefin substrates, and its nano-channels increased encountering possibility between olefins and ROSs. Consequently, Ru(N^N)₃@Q-COF provided an intriguing platform for olefin photooxidation and could be recycled multiple times without any degradation in performance. This report revealed that the conversion of classical ROSs into less potent oxidants with rapid kinetic rates played a crucial role in achieving highly efficient and selective epoxidation of terminal olefins.