Syntheses,Structures And Properties Of Polyoxoniobate/Polyoxotantalate In Acidic Conditions

Polyoxometalate clusters,a class of multinuclear clusters formed by the linkage of front transition metal and oxygen atom,is received extensive attention from chemists at home and abroad in recent years.Generally,polyoxometalates can be divided into three broad families,namely isopolyanions,heteropolyanions and mixed-addendum polyanions.Compared with the common VIB polyoxometalates?Mo,W,V?,the development of polyoxoniobates/polyoxotantlates is still relatively lagging,mainly due to only one soluble precursor is available for reaction and the precursors{M₆O₁₉}?M=Nb/Ta?only stable in alkaline environment?pH>9?.The singularity of the precursor and the narrow pH range determine the synthesis of these compounds under alkaline conditions.In recent years,we found that{M₆O₁₉}exists in various peroxy forms in the presence of H₂O₂,which provides a new idea for the synthesis of such compounds.In this thesis,we intend to research the reaction,synthesis,identification and properties of{M₆O₁₉}under acidic conditions,including two aspects mainly:mixed-addendum polyoxometalates and heteropolyoxometalates,as follows:1.Four mixed-addendum oligomers based on Well-Dawson type{P₂W₁₂Nb₆}unit were obtained successfully:H₂₆[{P₄W₂₄Nb₁₂O₁₂₂}₂{Ni?H₂O?₄}₄{Nb₄O₄?OH?₆}]×27H₂O?1?K₄[Co?H₂O?₆][H₈{P₂W₁₂Nb₆O₆₁}₂{Co₃?H₂O?₁₅}]×21.5H₂O?2?K₄?NH₄?₁₄[H₁₀{P₄W₂₄Nb₁₂O₁₂₂}₂{Co₆?H₂O?₂₄}]×34H₂O?3?K₄?NH₄?₁₂[{(P₄W₂₄Nb₁₂O₁₂₂)₂}{Dy₈?H₂O?₄₆}]×33H₂O?4?The structure analysis shows that the compound 1 and 3 are tetrameric.In addition to the different metal ion species,the four{P₂W₁₂Nb₆}units surround{Nb₄O₆}core in central to forming a dense structure in 1.Compound 3 is composed of two{P₄W₂₄Nb₁₂}units,which is connected by the mode of"rotation misplace";Compound 2 is consisted of three{CoO₆}octahedrons linking of two{P₂W₁₂Nb₆}units;The asymmetric unit of compound 4 is a tetrameric,which containing eight Dy³⁺,and further connected to a one-dimensional chain by a Dy-O-W bond.2.Two Lindqvist-like peroxoheteropolyoxoniobate have been obtained successfully:K₃[HTe₂Nb₄?O₂?₄O₁₀?OH?₈]×5H₂O?5?K₃?NH₄?[Te₂Nb₄?O₂?₄O₁₀?OH?₈]×4H₂O?6?The two compounds have the same structure that is dinuclear-substituted Lindqvist-type polyoxoniobate clusters,in which two Te atoms have the hexacoordinate square bipyramidal configuration.Photocatalytic dye degradation experiments show that compound 5 has a good catalytic effect on methyl violet dye.3.A Lindqvist-like peroxoheteropolyoxotantalate have been obtained successfully:K₃[HTe₂Ta₄?O₂?₄O₁₀?OH?₈]×5H₂O?7?This compound is the first peroxo heteropolyoxotantalate containing Te atoms.The study of its solution behavior shows that the compound can be stably present in the acidity range of 2.70-6.6,and it is stable for at least 15 h.