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Abstract
Four compounds based on polyoxometalates, [Cu(4-bpo)(H2O)][Cu2(μ2-Cl)(4-bpo)2(H2O)][SiW12O40][N(CH3)4]2·H2O (1), [Cu(4-bpo)]4[P2W18O62][N(CH3)4]2·6H2O (2), [Cu2(μ2-OH)(4-bpo)2(Hina)(H2O)2]2[P2W18O62]·4H2O (3), and [Cu2(Hina)4(H2O)2][H2P2W18O62](Hina)·11H2O (4) (4-bpo=2,5-bis(4-pyridyl)-1,3,4-oxadiazole, ina=isonicotinic acid), have been hydrothermally synthesized and characterized by elemental analysis, IR, and single-crystal X-ray diffraction. The 3-D framework of 1 is composed by Keggin-type polyoxoanions {SiW12} and two types of infinite chains, {Cu(4-bpo)(H2O)}n and {Cu2(μ2-Cl)(4-bpo)2(H2O)}n, through hydrogen bonds. Compound 2 has a 3-D rigid framework which is fabricated by Wells–Dawson type polyoxoanions {P2W18} and Cu-(4-bpo) chains through covalent bonds. Compound 3 contains an infinite {Cu2(μ2-OH)(4-bpo)2(Hina)(H2O)2}n double-chain and {P2W18} polyoxoanions immobilized in the voids between the chains. Compound 4 exhibits a 3-D supramolecular network directed by hydrogen bonds between {P2W18} polyoxoanions and the double paddle-wheel {Cu2(Hina)4(H2O)2}. Compounds 1–4 were tested as heterogeneous catalysts for the epoxidation of styrene using tert-butyl hydroperoxide (TBHP) as oxidant. The compounds show catalytic activity with 2 giving the highest yield of styrene oxide.
Four organic–inorganic hybrid compounds were hydrothermally synthesized based on polyoxometalates, copper salts, and 2,5-bis(4-pyridyl)-1,3,4-oxadiazole or isonicotinic acid ligands. Among those four compounds, compound 2, [Cu(4-bpo)]4[P2W18O62][N(CH3)4]2·6H2O, showed the best catalytic activity for heterogeneous epoxidation of styrene to styrene oxide using tert-butyl hydroperoxide as oxidant.
Funding
This work was supported by the National Natural Science Foundation [grant number 21371035], [grant number 21173050]; and the National Science Foundation for Young Scientists of China [grant number B010204].
