Fabrication of Polyoxometalate-Based Metal-Organic Frameworks Integrating Paddlewheel Rh₂(OAc)₄ for Visible-Light-Driven Oxidative Coupling of Amines

Four isomorphic polyoxometalate-based metal-organicframeworks (POMOFs) were derived by assembling Rh-2(OAc)(4) and polyoxometalates, and the synergistic effect betweenthem enhanced the activities of POMOFs for the photocatalytic oxidativecoupling of amines and alkene epoxidation. The development of visible-light-responsive,environmentally friendly,and reusable photocatalysts for organic oxidation reactions is ofvital significance. Herein, four polyoxometalate-based metal-organicframeworks (POMOFs) were synthesized and systematically characterizedby assembling the paddlewheel complex Rh-2(OAc)(4) and various polyoxometalates (POMs). Single-crystal X-ray diffractionanalysis revealed that the four POMOFs were isomorphic and possessedrare structural features among the POMOFs, with POMs as nodes andRh(2)(OAc)(4) as linkers. As expected, the activitiesof the four POMOFs for the photocatalytic oxidative coupling of benzylaminewere better than that of Rh-2(OAc)(4) or POMs individually,which was ascribed to the synergistic effect between them, and theintrinsic reasons for the difference in the activity were explainedvia electrochemical measurements. In particular, the product imineyield reached 96.1% with NaRh-SiW ( 12 ) as the catalyst and a turnover number and a turnover frequencyof 480.5 and 120.5 h(-1), respectively, while theproduct yield remained as high as 92% after three repetitions, evidencingits high stability. Moreover, the higher activities of the four POMOFsfor the selective epoxidation of various alkenes reaffirm the synergisticeffect between Rh-2(OAc)(4) and POMs.