Synthesis And Properties Of Polyoxometalate-based Complexes Constructed From N-heterocycle Azole Ligands

In this paper,ten POM-based complexes have been obtained by the combination of N-heterocycle azole ligand,polyoxometalate（POM）,cobalt or copper ion under hydrothermal/solvothermal condition,which have been characterized by the following methods:FT-IR spectra（IR）,thermogravimetric analysis（TGA）,single crystal X-ray diffraction analysis（XRD）and powder X-ray diffraction analysis（PXRD）.Further,the electrochemical,photocatalytic degradation properties towards organic dyes of these compounds have been also studied.1.Considering the transformation in situ of ligands from nitrile to carboxyl group under hydrothermal condition,3,5-di-[1,2,4]triazol-1-yl-benzonitrile（DTCN）and 3,5-di（1H-imidazol-1-yl）benzonitrile（DICN）are chosen as original ligands here,respectively,and five POM-based hybrids have been synthesized through adjusting the temperature,metal ions and polyoxoanions.[Cu₄^ⅠCu₂^Ⅱ（OH）₂（H₂O）₄（DTCA）₄(HPMo₁₀^VIMo₂^VO₃₉)]·2H₂O（1）[Cu₄（OH）₂（H₂O）₂（DTCA）₂(SiW₁₂O₄₀)]·11H₂O（2）{[Cu₂Cl（H₂O）（HDTCA）₂[CrMo₆（OH）₆O₁₈)]}·4H₂O（3）{[Cu（HDICA）₂[CrMo₆（OH）₇O₁₇)]}·13H₂O（4）{[Co₂（H₂O）₂（HDTCA）₂[CrMo₆（OH）₆O₁₈)]}·4H₂O（5）Compounds 1-5 are obtained under hydrothermal condition,where the nitrile group of ligand is transformed into the carboxyl group in situ.When the reaction temperature is 170°C,the oxygen atoms of the carboxyl groups coordinate with metal centers in the structures of 1 and 2,but which are free in 3-5 at 120°C.That is to say,the reaction tempetature displays an important role in influencing the coordination mode of carboxyl group.Meanwhile,the different POMs lead to the diverse linking modes and structures.In the 3D structure of 1,the{PMo₁₂}polyoxoanions are connected via the shared oxygen atoms into a 1D POM-based inorganic chain,further being linked by the 2D metal-organic layers into a 3D framework.The 3D structure of 2 is constructed from{SiW₁₂}polyoxoanions and metal-organic chains.Compounds 3-5 show 2D layer structures containing{CrMO₆}polyoxoanion.Compounds 1,3 and 4 display bifunctional electrocatalytic properties,and 2 has a photocatalytic activity for the degradation of organic dyse MB andRhB.2.Choosing 3,5-di（2-methyl-imidazol-1-yl）benzonitrile（DMICN）as organic ligand,three POM-based complexes 6-8 are prepared under hydrothermal/solvothermal condition in the presence of Cu²⁺ion,{SiW₁₂}or{P₂W₁₈}polyoxoanions.[Cu₂（H₂O）₂（DMICN）₄(SiW₁₂O₄₀)]·2DMA·13H₂O（6）[Cu₂（H₂O）₂（DMICN）₄(SiW₁₂O₄₀)]·2DMA·19H₂O（7）[Cu₂^ⅠCu₂^Ⅱ（H₂O）₂（HDMICA）₄(P₂W₁₈O₆₂)]·5H₂O（8）All three compounds have 3D supramolecular structures.Compounds 6 and 7are obtained under solvothermal condition by changing reaction temperature,which are constructed from{SiW₁₂}polyoxoanions and metal-organic chains.Compound 8 is prepared under hydrothermal condition in the presence of{P₂W₁₈}polyoxoanion.The results reveal that the reaction temperature has an important influence on the transformation in situ of the ligand from nitrile to carboxyl group.Compound 8 shows bifunctional electrocatalytic properties.3.The tridentate N-heterocycle ligand,1,3,5-tris（triazol-1-ylmethyl）-2,4,6-trimethyl benzene（ttb）,is introduced here,and complexes 9 and 10 have been synthesized.[Co₂（ttb）₂（H₂O）₄(H₂P₂W₁₈O₆₂)]·10H₂O（9）[Cu₂（ttb）₂（H₂O）₄](SiW₁₂O₄₀)（10）In the 3D supramolecular structures of 9 and 10,the ttb ligand shows the trans-configuration and tri-dentate linking mode,which are connected by metal centers into two kinds of metal-organic motifs,namely,a 1D metal-organic belt[Cu₂（ttb）₂（H₂O）₄]_n⁴ⁿ⁺in 9,further extended into a 2D layer by[P₂W₁₈O₆₂]^4-anions,but a 2D cationic layer[Cu₂（ttb）₂（H₂O）₄]_n⁴ⁿ⁺in 10.Compound 9 exhibits multifunctional catalytic properties,including the electrocatalytic reduction of H₂O₂,the electrocatalytic oxidation of AA and photocatalytic activity for the degradation of MB,but 10 shows only the electrocatalytic activity for the reduction of H₂O₂.