Three POM-based coordination polymers: hydrothermal synthesis, characterization, and catalytic activity in epoxidation of styrene

Abstract

Three polyoxometalates (POM)-based coordination polymers, [Cu^II₂(C₅H₅NCOO)₂(4-bpo)₂(H₂O)₂]SiW₁₂O₄₀·H₂O (1), [Cu^I₄(4-bpo)₆]SiW₁₂O₄₀·3H₂O (2), and [Cu^I₄(3-bpo)₄]SiW₁₂O₄₀·3H₂O (3), were hydrothermally synthesized from the mixture of H₄SiW₁₂O₄₀, copper salts, and 3-bpo (2,5-bis(3-pyridyl)-1,3,4-oxadiazole) or 4-bpo (2,5-bis(4-pyridyl)-1,3,4-oxadiazole) and characterized by elemental analysis, IR spectroscopy, TG analysis, and single crystal X-ray diffraction. The 4-bpo molecules are connected into an infinite double-chain in compound 1 as a bidentate ligand. Compound 2 contains large hexagonal prism shaped molecular cages defined by copper ions and 4-bpo molecules which wrap Keggin ions inside. In compound 3 {Cu₂(3-bpo)₂} molecular rings connect with Keggin ions to form a 1D chain. By inspection of the structures, the geometry and the coordination mode of bpo ligands play important roles in the formation of 1–3. The catalytic performance of 1–3 was investigated for epoxidation of styrene using tert-butyl hydroperoxide (TBHP) as the oxidant. Compounds 1–3 present promising activity as heterogeneous catalysts while compound 1 exhibits the best yield of styrene epoxide.