Synthesis,Structure And Property Of Wells-Dawson Type Metal-Oxygen Clusters

As the two most important subclasses of POMs,Keggin and Wells-Dawson polyanions have attracted much attention in the synthesis of organic-inorganic hybrid materials due to their unique geometric structure and properties.Compared with the Keggin type,the Wells-Dawson have the characteristics of large volume,high negative charge and more potential coordination sites for metal centers,which is conducive to the formation of more complex compounds.In this paper,Wells-Dawson polyanions are used as inorganic building blocks to construct organic-inorganic hybrid materials by changing the types of transition metals and organic ligands.The influence of reaction temperature,pH value and molar ratio of reactants were also investigated.The catalytic performance of the compound was preliminarily studied,and the influence of compound structure on property was explored,which provided theoretical basis for the design and synthesis of follow-up functional materials.In this paper,17 compounds based on Wells-Dawson polyanions were synthesized by hydrothermal synthesis.The structure of the compound was characterized by X-ray single crystal diffraction,elemental analysis,IR,UV,PXRD and TG.The electrochemical,photocatalytic and the catalytic performance in epoxidation of styrene were also studied.1.Wells-Dawson type polyoxometalates modified by transition metal-single ligand complexes.Seven new compounds（1-7）were synthesized by introducing different metal ions(Cu²⁺,Ag⁺,Zn²⁺)and organic nitrogen-containing ligand to the base reaction system of Wells-Dawson type polyanions.To compare the catalytic performance of these seven compounds in the catalytic epoxidation of styrene,two supramolecular compounds（8 and 9）containing only polyacid anions and ligands were also synthesized.In compound 1,{P₂W₁₈}acts as a bidentate inorganic ligand connected two copper ions,forming a bisupported coordination polymer.Compound 2is a 1-D chain structure constructed by{P₂W₁₈}polyanions with copper and 2,2’-bpy organic complexes.Compounds 3-6 were all composed of{P₂W₁₈}and silver/2,2’-bpy complexes.However,compound 3 contains two different types of silver complexes,forming a novel supramolecular structure.Compounds 4-6 were dimer constructed by{P₂W₁₈}building modules and silver complexes.The structure of compound 5 is similar to that of compound 4,in which the silver complexes are divided into five categories,but the two silver complexes in compound 4 are combined by sharing one terminal oxygen from{P₂W₁₈},while compound 5 is combined by Ag（3）and two nitrogen atoms,two terminal oxygen from two{P₂W₁₈}.Compound 6 contains four different kinds of silver complexes,which form a unique brick-wall 2-D layer structure by intermolecular force between the silver complexes and{P₂W₁₈}multi-anions.The structure of compound 7 is similar to that of compound 6,except the metal ions were replaced by Zn²⁺,which presenting a 3-D supramolecular structure with a circular cross-section channel.Compounds 8 and 9 are supramolecular structures based on{P₂W₁₈}polyanions and N-organic ligands.Their catalytic performance in epoxidation of styrene was studied.2.Wells-Dawson type polyoxometalates modified by Cu-mixed organic ligand complexes.Cu²⁺,N-organic ligand and carboxyl ligand were introduced into the Wells-Dawson polyanions and 7 new compounds with novel structure（10-16）were obtained.Compounds 10 and 11 are isomorphic,which are one-dimensional chain structures formed by transition metal ligands as bridges connecting{P₂W₁₈}.In the asymmetric units,there is a pyridine molecule formed by pyridine carboxylic acid through in-situ decarboxylation.There is a transition metal mixed ligand with four copper centers in compound 12,through which the adjacent{P₂W₁₈}multiple anions are connected to form a 1-D extended structure.The structure of compound 13 is similar to that of compound 12,but the transition metal mixed ligand is dicopper.Compound 14 is a one-dimensional extended structure formed by{P₂W₁₈}polyanions and hexa-copper clusters.Although compounds 15 and 16 were generated in the same reactor,their structures were quite different.There are two different types of transition metal mixed ligands in compound 15,and it was also the first compound having Cu…πinteractions.Compound 16 is a metal-oxygen cluster composed of{P₂W₁₈}and[Cu₂（2,2’-bpy）₂（hnic）]²⁺.3.Wells-Dawson type polyoxometalates modified by benzoic acid or 2,6-pyridine dicarboxylic acid ligand.It is difficult to construct TMMC composed of benzoic acid and 2,6-pyridine dicarboxylic acid as they can transformed to pyridine in situ decarboxylation reaction.In our work,three new compounds（17-19）were synthesized by introducing transition metal mixed ligands composed of benzoic acid or 2,6-pyridine dicarboxylic acid into the Wells-Dawson polyoxometalates by controlling the reaction condition.Compound 17 is the first compound constructed by transition metal mixture ligand based on benzoic acid.The transition metal mixed complexes in compounds 18 and 19 were all composed of 2,6-pyridine dicarboxylic acids.In compound 18,there are relatively complex C-H···O hydrogen bonds,which make compound 18 presenting a novel 3-D structure.TMMC and TMC in compound19 are charge supplemental units and space filling units.