Polyoxometalate-Based Metal-Organic Complexes:Syntheses,Structures And Their Multiple Functionalities

The primary objective of this thesis is to design and synthesize new organic-inorganic hybrid compounds based on polyoxometalates?POMs?and metal organic moieties.10 new compounds have been obtained using hydrothermal method by choosing different types of N-heterocyclic ligands.These compounds have been structurally characterized by IR spectra,single crystal X-ray diffraction analysis,thermo-gravimetric analysis,X-ray powder diffraction and elemental analysis.The synthesis factors influencing the structure of the compounds,such as conformation differences of N-heterocyclic ligands,and pH values are discussed.Furthermore,photo-catalytic,electrochemical,bi-functional electro-catalytic and proton conduction properties of some compounds have also been studied.1.By utilizing a new chelating ligand 2-?pyridin-2-yl?-1H-benzo[d]imidazole?PBI?and classical polyoxometalates as inorganic building units,four unreported organic-inorganic hybrid compounds were obtained under hydrothermal conditions.[Cu?PBI?₂?H₂O?][{Cu?PBI??OH??H₂O?}{PW₁₂O₄₀}]·5H₂O?1?[Cu₂?PBI?₃?H₂O?₃][SiW₁₂O₄₀]·5H₂O?2?[Cu?PBI?₂?H₂O?][{Cu?PBI?₂?H₂O?}{SiW₁₂O₄₀}]·4H₂O?3?[Cu₄?HPO₄??PO₄??H₂O?₂?PBI?₄][PMo₁₂O₄₀]·H₂O?4?In compounds 1-3,the Keggin-type POM units are decorated by the{Cu/PBI}metal-organic fragments,together with the extra isolated cationic{Cu/PBI}moieties,which further stack into a 3-D supra-molecular framework with porous structural features.Compound 4 consists of a rare cationic cluster[Cu₄?HPO₄??PO₄??H₂O?₂?PBI?₄]⁴⁺and Keggin-type POM anions,forming a 3-D supra-molecular framework with a limited interspace for the entry of guest molecules.The photo-catalytic dye degradation of RhB and MB contaminants of these four materials were screened.All four compounds showed good photo-catalytic properties for MB degradation,while RhB degradation activity was very low under visible light.It is speculated that the smaller MB molecular size compared to RhB molecule can significantly affect the dye adsorption capacity of compounds 1-4,resulting in this apparent difference.The cyclic voltammetry was carried out by using carbon paste electrodes?CPEs?.The electrochemical properties of the title compound-containing n-CPEs?n=1-4?in 1M H₂SO₄+Na₂SO₄ solution?pH=2?were investigated.Tests have been shown that the four compounds are electro-catalytically active in the reduction of nitrite.2.We got one novel organic-inorganic hybrid by using 2-?pyridin-2-yl?-1H-benzo[d]imidazole?PBI?ligandbyhydrothermalmethod.[Cu₄?HPO₄??PO₄??H₂O?₂?PBI?₄][PW₁₂O₄₀]·H₂O?5?Compound 5 was hydrothermally synthesized at low pH value.X-ray structural analyses reveal that it is a novel inorganic-organic hybrid material based on the Keggin-type polytungstate and tetra-copper phosphate core butterfly structure and has a 2-D structural feature.Compound 5 has an impressive photo-catalytic MB dye degradation activity and excellent stability and recyclability.In addition,the electro-chemical behavior of 5-CPE were measured by cyclic voltammetry.The electrochemical properties of the solutions in 1M H₂SO₄+Na₂SO₄ solution?pH=2?were investigated.Experiments show that compound 5 is electro-catalytically active in the reduction of H₂O₂ and oxidation of ascorbic acid.It exhibits an excellent photo-catalytic response and better bi-functional electro-catalytic activity which can be used as a novel electrochemical sensor.3.With utilization of a saturated Keggin polyoxometalate?POMs?and two Y-shaped tib/ditb organic ligands,four new polyoxometalate-based hybrids were synthesized.[Cu₂?tib?₂?H₂O?₂(?-SiW₁₂O₄₀)].4H₂O?6??H₂ditb?₂[SiW₁₂O₄₀].2H₂O?7?Ag₂?Hditb?₂[SiW₁₂O₄₀].4H₂O?8?[Co₂?ditb?₃?H₂O?₃][SiW₁₂O₄₀].6H₂O?9?In compound 6 there is extensive?....?stacking interactions between the parallel tib imidazole rings from adjacent 1-D chains,which are further,interconnected via?....?stacking interactions to form 2-D layer.The extensive water molecules can further extend the overall framework to a 3-D supra-molecular structure via hydrogen bonding.Compound 7 is a simple 2D supra-molecular structure and intermolecular hydrogen bonding was found amongst the Keggin polyanions while the ditb ligands are sandwiched between the Keggin clusters.In compound 8,the organic bridging ligand is coordinated to the adjacent two Ag centers into a 1D chain.These 1D chains are further connected with[SiW₁₂O₄₀]^4-units via Ag center,forming 2D networks.These 2D networks are packing parallel to generate a 3D structure.In compound9,the ligand act as two-connection-nodes and linked the adjacent two Co centers into 2D nets.These adjacent 2D nets are further arranged parallel to form into a 3D structure.The guest Keggin-type POM anions and lattice water molecules are located between layers and further stacking leads to a 3D supra-molecular framework.In addition,the electro-chemical behavior of 6-9-CPEs were measured by cyclic voltammetry in 1M H₂SO₄+Na₂SO₄ solution?pH=2?.Tests have shown that compound 6 displayed a better performance for electro-catalytic reduction of NO₂than H₂O₂ whereas excellent electro-catalytic activities for the oxidation of biological molecules ascorbic acid and L-cysteine.The former is ascribed to W-centers while the latter bi-functional electro-catalytic response is attributed to Cu-centers.Photo-catalytic experiments conducted for compounds 7-9 under visible light illumination indicate that all the three compounds are photoactive for the degradation of MB and MO.The excellent dye degradation activity of compound 8is attributed to Ag ions and fine channels in the crystal structure.As for compound9,it have better bi-functional electro-catalytic activity as compared to 7 and 8 for the reduction of H₂O₂ and the oxidation of ascorbic acid,which is attributed to W^VI/W^v and Co^II/Co^I centers.In fact,compounds 7,8 and 9 have the same Keggin POM units but different metal source namely Ag and Co.The differences in electro-catalytic performances are due to different crystal structures and different metal source present in the clusters.4-Compound 10 is a new type of 3D lanthanide silicotungstic acid working as a proton conductor and was synthesized hydrothermally.La_0.67?H₂O?La?H₂O?₆[{La?H₂O?₃(SiW₁₁O₃₉)}]·7H₂O?10?Compound 10 has athree-dimensional structure built from lanthanide-substituted polyoxoanion[{La?H₂O?₃(SiW₁₁O₃₉)}]^5-building blocks with a large number of channels filled with free water molecules.Compound 10 shows a high proton conductivity of 1.26×10^-33 Scm^-1 at 90?under 98%RH.The activation energy of compound 10calculated from the Arrhenius plots of the proton conductivity is 0.57eV,which indicates the vehicle mechanism of proton transfer.