Diverse Polyoxometalate-Based Cobalt Complexes for Catalyzing Olefin Epoxidation at Room Temperature: Regulation of Active Sites by Polyoxometalate Templates
Enhancing catalytic activity is a core objective in catalyst design, with active site accessibility being a critical determinant. Polyoxometalate-based metal-organic complexes (POMOCs), combining advantages of POMs and MOCs, offer potential for constructing catalysts with highly accessible active sites. In this study, a series of POMOCs were synthesized using different POM templates: [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). These were characterized by FT-IR, PXRD, and single-crystal X-ray diffraction. Catalytic activity differences for olefin epoxidation were attributed to distinct accessibility of Co(II) sites upon thermal activation. Notably, Co-P2W18 achieved 99% yield of 1,2-epoxycyclooctane within 3 hours at room temperature using O2 as oxidant, owing to highly accessible unsaturated Co(II) sites. This performance is superior to most reported catalysts. The reaction mechanism was investigated using density functional theory. The catalyst exhibited excellent stability over five cycles, with FT-IR, PXRD, and XPS confirming structural and oxidation state integrity. This work highlights the potential of POMOCs in designing catalysts with highly accessible active sites for enhanced catalytic activity.