Diverse polyoxometalate-based cobalt complexes for catalyzing olefin epoxidation reaction at room temperature: regulation of active sites by polyoxometalate templates

Enhancing the catalytic activity of catalysts is a core objective in their design and synthesis processes, and the accessibility of active sites is one of the crucial factors determining catalyst activity. Polyoxometalate-based metal-organic complexes (POMOCs) with well-defined structures, which combine the advantages of POMs and MOCs, may offer the possibility to construct catalysts with highly accessible active sites. In this study, a series of POMOCs were successfully designed and synthesized using different POM templates, including [CoII1.5(L)1.5(PMo12O40)(H2O)4]·3H2O (Co-PMo12), [CoII1.5(L)1.5(PW12O40)(H2O)4]·3H2O (Co-PW12), [CoII2(L)2-(SiW12O40)(H2O)4]·11H2O (Co-SiW12), and H[CoII2.5(L)3-(P2W18O62)(H2O)8]·10H2O (Co-P2W18), which were characterized by Fourier transform infrared spectroscopy, powder X-ray diffraction, and single crystal X-ray diffraction. The differences in catalytic activity among the four POMOCs for olefin epoxidation were attributed to the distinct accessibility of Co(II) sites upon thermal activation. Among them, Co-P2W18 achieved a remarkable 99% yield of 1,2-epoxycyclooctane within 3 h at room temperature using O2 as the oxidant, owing to its highly accessible unsaturated Co(II) sites. Co-P2W18 exhibits significantly superior catalytic activity for the cyclooctene epoxidation reaction compared to most reported catalysts. Additionally, the reaction mechanism was investigated using density functional theory.