Synthesis, Crystal Structures, And Catalytic Properties Of Compounds Based On Oxomolybdate Isomers

In recent years, polyoxomolybdate (POM) anions have been widely employed as inorganic building blocks to construct organic-inorganic hybrid materis with the structural diversity and excellent physical property. Octamolybdates are one of important building blocks due to their rich isomeric chemistry. In this paper, utilizing flexible monodentate ligand dpmea (N,N-diethyl-N-(pyridin-2-ylmethyl)ethanaminium), tridentate ligand mebpa (bis(2-pyridylmethyl)methylamine), quadridendate ligand dpoen (2-(pyridin-2-ylmethoxy)-N-(pyridin-2-ylmethyl)ethanamine), quinquidentate ligand tpoen (2-(pyridin-2-ylmethoxy)-N,N-bis(pyridin-2-ylmethyl)ethanamine) and sexadentate ligand tpdoen(2-(pyridin-2-ylmethoxy)-N-(2-(pyridin-2-ylmethoxy)ethyl)-N-(pyridin-2-ylmethyl)ethanamine), eight new compounds based on octamolybdate isomers and Cu/Ni ions have been synthesized under hydrothermal conditions. All these compounds have been structurally characterized by elemental analyses, IR, TG and X-ray single crystal diffractions.Two new organic-inorganic hybrid compounds based onβ- andγ-octamolybdate isomers have been prepared by using MoO3, CuSO4?5H₂O, mebpa, H₂O with the same molar ratio 2:1.5:1:2500, [{Cu(mebpa)}2β-Mo₈O₂₆] 1 has been obtained at 120 oC, while, [{Cu(H₂O)2(mebpa)}]2[γ-Mo₈O₂₆] 2 has been obtained in the range of 140-180 oC. Compound 1 is constructed fromβ-[Mo₈O₂₆]4- anions with {Cu(mebpa)}2+ fragments covalently attached via terminal oxygen atoms into an 1D ribbon-like chain. Compound 2 is a discrete structure which is constructed fromγ-Mo₈O₂₆ anions and {Cu(H₂O)2(mebpa)}22+ fragments. Their successful isolation demonstrate that the tempreture is one of the crucial factors to influence the crystallization or formation of octamolybdate isomers. Six new organic-inorganic hybrid compounds based on octamolybdate isomers have been prepared by using [Bu4N]4[α-Mo₈O₂₆] as precursors at 120 and 160 oC and pH≈4.5 under hydrothermal conditions: [{Cu(tpdoen)}2][α-Mo₈O₂₆]·3H₂O 3, [{Cu(dpoen)}2α-Mo₈O₂₆] 4, [{Cu(dpmea)}2β-Mo₈O₂₆] 5, [{Ni(tpoen)}2β-Mo₈O₂₆]·5H₂O 6, [{Cu(tpoen)}2δ-Mo₈O₂₆]·2H₂O 7, [{Cu(tpoen)}2δ-Mo₈O₂₆]·3H₂O 8. The compound 3 is a discrete struture which consists ofα-[Mo₈O₂₆]4- anions and Cu(tpdoen)2+ cations. The compound 4 containsα-Mo₈O₂₆ and Cu(tpoen)2+ fragments connected to form a 2D network. Compound 5 is aβ-octamolybdate-bisupported structure, which contains Mo₈O₂₆ and Cu(dpmea)2+ fragments; Compounds 6, 7 and 8 exhibit octamolybdate-bisupported structures. For compound 6 Mo₈O₂₆ isβ-form, but for compounds 7 and 8,δ-form. Compounds 7 and 8 are linkage isomers. Their successful isolation demonstrated that the complex fragments are also another crucial factor for the formation or crystallization of the octamolybdate isomersThe electrochemical behaviors of compounds 1-8 have been studied on solid bulk-modified carbon paste electrodes of compounds 1-8 (1-MCPE-8-MCPE) and the electrocatalytic properties of compounds 1 and 2 were investigated using the 1-MCPE and 2-MCPE. They display electrocatalytic activity toward to reduction of nitrite. In addition, the catalytic activities of [{Cu(H₂O)2(mebpa)}]2[γ-Mo₈O₂₆], [{Cu(o-phen)}2θ-Mo₈O₂₆] and [Bu4N]4[α-Mo₈O₂₆] have been investigated by the oxidation reaction of benzaldehyde to benzoic acid using hydrogen peroxide (H₂O2) as oxidant.