Integrating TEMPO into a Metal–Organic Framework for Cooperative Photocatalysis: Selective Aerobic Oxidation of Sulfides

Metal–organic frameworks (MOFs) are promising visible light photocatalysts due to their adaptable building blocks. In principle, further optimization of the activity could be envisaged by integrating a redox mediator into a MOF to assemble cooperative photocatalysis. Considering the abundant mesopores and the accessible hydroxyl binding sites, PCN-222, consisting of Zr6(μ-OH)8 clusters and carboxyl porphyrin ligands, was synthesized to integrate with 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl (HOOC-TEMPO). As a proof of concept, the selective aerobic oxidation of sulfides was selected as the model reaction. Significantly, PCN-222 exhibited a cooperative impact with HOOC-TEMPO on selective aerobic oxidation of sulfides powered by 660 nm red light. In contrast, PCN-226, devoid of accessible hydroxyl sites, only had an inconspicuous cooperation with HOOC-TEMPO. The density functional theory calculations reveal that HOOC-TEMPO could be spontaneously adsorbed on the Zr6(μ-OH)8 clusters via the bidentate model. Due to the lack of binding sites of Zr6(μ-OH)8, TEMPO was proven to be much less effective than HOOC-TEMPO in promoting the oxidation of sulfides over PCN-222. The covalent integration of HOOC-TEMPO into PCN-222 is essential for the effectiveness and durance of cooperative photocatalysis for the selective aerobic oxidation of a wide range of organic sulfides. This work suggests that MOFs, a versatile platform, could be integrated with a redox mediator to enable smooth photocatalytic selective transformations.