Synthesis,structure,and Catalytic Performances Of Polyoxometalate-materials Containing Imidazole Ligands

Polyoxometalates(POMs)are a fascinating and important class of metal-oxygen cluster complexes that mainly composed of early transition-metals(TM=Mo(?),W(?),V(?),etc.)bridged by oxygen atoms.POMs have been favored by researchers as a class of promising smart material candidates not only due to multitudinous architectures but also many advantages such as catalytic,magnetic,and photochromic properties.Synthesizing organic-inorganic hybrid POMs materials and studying the structure-activity relationship have important reference value for the directional synthesis of POMs materials with novel structures and excellent performances.Solvothermal method and hydrothermal method had been used in this synthetic process.Imidazole nitrogen-containing ligand 4,4'-bis(imidazolyl)biphenyl(bibp),transition metal ions and silicotungstic acid(H4Si W12O40)had been chosen as reactants.We obtained four new POM hybrid compounds through adjusting the reaction conditions:[Co(bibp)₂(DMF)(DMA)]_2n·[Si W₁₂O₄₀]_n·2n(DMA)(1)[Cd(bibp)₂(DMF)(DMA)]_2n·[Si W₁₂O₄₀]_n·2n(DMA)(2)[Cu(bibp)(Hbibp)₂(H₂O)]_n·n[Si W₁₂O₄₀]·n(bibp)(3)[Co(bibp)(DMF)₄]_2n·n[Si W₁₂O₄₀](4)Compounds'structures and catalytic performances had been studied by single crystal X-ray diffraction analysis,infrared spectroscopy,thermogravimetric analysis,X-ray powder diffraction analysis,electrochemical analysis and photocatalytic analysis.Structural analysis shows that compound 1 is composed of transition metal Co(?),bibp unit,solvent molecule DMF,DMA and polyacid anion[Si W₁₂O₄₀]^4-.Co(?)coordinated with bibp to form a layered structure of grid as a unit,each grid is embedded with a[Si W₁₂O₄₀]^4-through hydrogen bonding,and the polyacid anion is sandwiched by the grid layer structure of two orientations facing each other,a sandwich configuration two-dimensional layered structure is formed.Compounds 1 and 2 are isostructural.The difference is that the central metal ion of compound 2 was Cd(II).Compound 3 is composed of transition metal Cu(II),bibp unit,inorganic water molecule and[Si W₁₂O₄₀]^4-.Cu(?)and bibp coordinate to form a"fishbone"long chain.Each"fishbone"half-frame structure is equipped with a[Si W₁₂O₄₀]^4-through hydrogen bonding,forming a one-dimensional chain structure.Compound 4 is composed of transition metal Co(II),bibp unit,DMF solvent molecule and[Si W₁₂O₄₀]⁴.Co(?)and bibp coordinate to form a"wave-shaped"long chain.Each arc-shaped chain structure is equipped with a[Si W₁₂O₄₀]^4-through hydrogen bonding.A one-dimensional chain structure is formed.There are differences in the types of hydrogen bond donors between the host framework and the guest polyacid of the four compounds.The hydrogen bond donors of compounds 1,2,and 4all come from the carbon atoms of the benzene ring and the imidazole ring in the bibp unit,while the hydrogen bond donors of compound 3 only come from the carbon atoms of the imidazole ring in the bibp unit.Electrochemical studies had shown that compounds 1,2,and 4 all have redox processes with W atoms as the active center.Compounds 1 and 2 have electrocatalytic reduction properties for the pollutants Cr(?),nitrite and bromate.Compound 1 has higher electrocatalytic efficiency(CAT)for nitrite and bromate.Photocatalytic studies had shown that compounds 1,2,and 4 have good photocatalytic degradation effects on Rhodamine B.The structure-activity relationships of the compounds had been discussed,it was believed that when the hydrogen bond donors come from the carbon atoms on the imidazole rings and the benzene rings,the polyacid anion in the compounds exhibits better electrochemical behavior.The types of coordinating metal ions in the host frameworks of isomorphic compounds might cause differences in electrocatalytic efficiencies.