Syntheses, Structures And Properties Of Polyoxometalates Decorated By Multinuclear Transition Metal Clusters

The primary objective of this paper is to design and synthesize novel polyoxometalate-based compounds decorated by multinuclear metal clusters. 18 new compounds have been synthesized under hydrothermal conditions by choosing different types of nitrogen-containing organic ligands. These compounds have been structurally characterized by IR spectra, single crystal X-ray analysis, thermogravimetric analysis, X-ray powder diffraction and elemental analysis. The synthesis factors, which influence the structures of the compounds, such as conformation differences of ligands, are discussed. At the same time, the fluorescence, electrochemical and photocatalytic properties of some compounds are also studied.1. By utilizing 2,2?-biimidazole（H₂biim） as a bidentate ligand and the classical polyoxometalates as inorganic building units, seven unreported organic-inorganic hybrids were prepared under hydrothermal conditions.Cu₃（H₂biim）5（PMo1 2O₄ 0）·2H₂O（1）Cu₁0（H₂biim）10（PW₁2O₄0）2（OH）4·5H₂O（2）{[Ag₇（H₂biim）5][PW11O39]}·Cl·H3O（3）{[Ag₂（H₂biim）2][Ag_phnz]（PW₁2O₄0）}·H₂O（4）Ag₄（H₂biim）3（H₂biim-NO₂）（Si W₁2O₄0）·3H₂O（5）[Ag₆（H₂biim）6（OH）2（H₂O）（HPW₁2O₄0）2]·4H₂O（6）[Ag₄（H₂biim）4][K2（H₂O）3（P2W₁8O₆2）]·H₂O（7）In compounds 2-7, H₂ biim ligand adopts a bidentate coordination mode to generate multinuclear clusters with Cu⁺/Ag⁺, whereas it shows a unique bidentate chelating coordination mode in compound 1. Compound 1 shows a mono-capped bi-supporting Keggin cluster composed of a [PMo₁2O₄0]3- anion, a [Cu（H₂biim）]+ and two [Cu_H2biim2]+ complexes. Although ligand H₂ biim exhibits the simialr coordination mode in compounds 2-7, the structures of six compounds are significantly different owing to the properties of transition metal ions and polyoxometalates. Compound 2 is a 3D framework consisting of two kinds of CuH₂ biim chains which are linked together through polyanions. Compound 3 displays a 2D network featuring dimerized monolacunary Keggin anions {PW11O39}2 which are in-situ formed during the process of preparation. Taking double ligands synthetic route, phnz ligand is introduced to extend the structure under the similar reaction condtions to that of comound 3. However, only a 3D supramolecular compound 4 is obtained. When using polyanions [Si W₁2O₄0]4- with the different charges, a new 1D chainlike Keggin-based hybrid compound 5 is prepared. Interestingly, some of H₂ biim ligands are in situ functionalized by nityl groups during the Ag_-catalyzed reaction. Under the similar synthetic conditions to compound 5, we can obtain {PW₁2O₄0}-based compound 6 which has three different dinuclear Ag^-H₂ biim units. However the structure of H₂ biim does not change. A 3D framework of compound 7 is got by using Wells-Dawson anion with larger size and charge, in which there are potassium-Dawson inorganic 2D sheets and dinuclear silver-H₂ biim units.2. We got five novel unreported compounds by using 5-（4-imidazol-1-yl-phenyl）-2H-tetrazole（HL） ligand.[Cu₄（HL）4（Si W₁2O₄0）]·6H₂O（8）[Cu₆.5（HL）2（L）4（H₂O）（H1.5Si W₁2O₄0）]·3H₂O（9）[Ag₆（HL）4（H₂O）6（P2W₁8O₆2）]·4H₂O（10）（H₂L）2[Ag₂（HL）2（β-Mo₈O₂6）]（11）[Cu₄（HL）4（θ-Mo₈O₂6）]·5H₂O（12）Compounds 8 and 9 show the different structures when we choose [Si W₁2O₄0]4-as a building unit, and adjust the kinds of copper salts or p H. Tetranuclear copper clusters can be both found in compounds 8 and 9. In 8, Keggin polyanions [Si W₁2O₄0]4-（Si W₁2） link adjacent [Cu₄（HL）4]n chains to generate a 2D network. In 9, a cationic（2, 3, 11）-connected 3D framework is induced by Si W₁2 as the template. Compound 10 is a 3D structure based on infinite organic-metal chains {Ag₆（HL）4}n and Wells-Dawson anions with larger size. Compounds 11 and 12 have been synthesized by employing octamolybdate cluster as building block. Compound 11 consists of discrete [H₂L]+ cations and infinite one-dimensional（1D） anionic chains [Ag₂（HL）2（β-Mo₈O₂6）]2- constructed by [β-Mo₈O₂6]4- bridged via dinuclear clusters {Ag₂（HL）2}2+. Compound 12 displays a 2D network generated by 1D cationic tetranuclear copper cluster chains {Cu₄（HL）4}n and [θ-Mo₈O₂6]4- clusters.3. With utilization of 3,3’,5,5’-tetramethyl-4,4’-bipyrazole（H₂X） organic ligand, four new polyoxometalate-based hybrids were synthesized.[Cu₄（H₂X）4（Si W₁2O₄0）]（13）[Ag_H3X2]（PW1 2O₄ 0）（14）[Ag₂（H3X）2]（β-Mo₈O₂ 6）（15）[Cu₂（H₂X）2（Mo6O19）]·2H₂O（16）As a tetradentate ligand, the main skeleton of H₂ X is the isomer of H₂ biim. However, due to the presence of methyl groups, the coordination ability of H₂ X is greatly reduced. Only two coordination modes can be found. One is bidentate bridging mode, the other is monodentate mode. No multinuclear Cu⁺/Ag⁺ clusters can be formed in compounds 13-16. Compound 13 is a 2D network. Compound 14 is a supramolecular compound which is composed of [Ag_H3X2]3+ complexes and polyoxometalates. Compound 15 is a 2D layer which is constructed from [β-Mo₈O₂6]4- clusters linked together by [Ag_H3X]2+ complexes. Compound 16 contains four constituents: 1D cationic chain [Cu_H2X]n n+, 1D ladder-like chain [Cu₂（H₂X）2（Mo6O19）]n, discrete isopolymolybdate [Mo6O19]2- and water molecules.4. Two Keggin-based hybrids consisting of binuclear clusters can be obtained by using organic ligands 4,4’-bipyridine（bipy） and 1, 10-phenanthroline（phen）.[Co₂（Hbipy）2（bipy）（OH）2（H₂O）2]（Si W₁2O₄0）·4H₂O（17）[Fe₂Cl（phen）4]（PW₁2O₄0）·H₂O（18）Compound 17 is a 2D network, in which two adjacent Co²⁺ ions are joined together by two hydroxyl groups to form a binuclear cluster. Compound 18 is a supramolecular compound which is composed of binuclear clusters [Fe₂Cl（phen）4]2+ bridged by Cl-, [PW₁2O₄0]3- and water molecules.