Four organic-inorganic compounds based on polyoxometalates: crystal structures and catalytic epoxidation of styrene

Four compounds based on polyoxometalates, [Cu(4-bpo)(H2O)][Cu-2((2)-Cl)(4-bpo)(2)(H2O)][SiW12O40][N(CH3)(4)](2)H2O (1), [Cu(4-bpo)](4)[P2W18O62][N(CH3)(4)](2)6H(2)O (2), [Cu-2((2)-OH)(4-bpo)(2)(Hina)(H2O)(2)](2)[P2W18O62]4H(2)O (3), and [Cu-2(Hina)(4)(H2O)(2)][H2P2W18O62](Hina)11H(2)O (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 {Cu-2((2)-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 {Cu-2((2)-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 {Cu-2(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.