| Rare earth?RE?containing polyoxometalates?POMs?have become one of the important areas of inorganic chemistry due to their structural diversity and promising applications in the fields of catalysis,medicine,magnetism,optics,electricity.Polyoxoniobates?PONbs?are an important of subclass of POMs,which have much higher charge:size ratios than the Group VI POMs,and have potential applications in nuclear waste disposal,alkaline catalytic degradation of pollutant and photocatalytic decomposition water.Niobium usually exits as Nb5+and there are no such readily soluble NbO43-salts in the aqueous solution,which impede the progress of the synthesis of PONbs.Nb-precursors(alkali metal salts of[Nb6O19]8-)are limited to alkaline conditions with a pH higher than 10.5,in which RE3+ions tend to hydrolyze into RE-hydroxides,so the incompatibility of RE3+ions and Nb-precursors in the same system is large challenge in the synthesis of RE containing PONbs?RCPONbs?.Thus far,there are only four isostructural RCPONbs connected by RE3+and Al3+have been reported.The thesis developed a new strategy to generate RCPONbs soluble Nb?V?species by introducing of the organic acid to regulate the Nb-precursor(K7HNb6O19·13H2O)in acidic or neutral solution.At the same time,the carboxyl group from organic acid can induce the oxophilic RE3+ions to form metastable soluble intermediates instead of amorphous oxide precipitation.Therefore,the compatibility of precursor and rare earth ions in the same solution was realized,which raised the possibility of further reaction.Using a conventional aqueous solution method,three series of RCPONbs with different structures and properties were obtained from K7HNb6O19·13H2O and RE3+ions by adjusting the media reaction solution,stoichiometric ratio of reactants,reaction temperature,system acidity,count cationic type,etc.The main contents are as follows:?.Threeteen RCPONbs were obtained by a conventional aqueous solution method in the presence of organic acid?Compound 1?13?.?CN3H6?7K3H17{Ln3?H2O?9Nb48O138?H2O?6}·40H2O?Ln=Ce?1?,Y?2?,Nd?3?,Sm?4?,Eu?5?,Gd?6?,Tb?7?,Dy?8?,Ho?9?,Er?10?,Tm?11?,Yb?12?,Lu?13??.Single-crystal X-ray analysis indicates that Compound 1?13 were iso-structures.The polyoxoanion structure unit{RE3?H2O?9[Nb48O138?H2O?6]}27?is constructed by two fragments{Nb24O72}joined together with three RE3+ion pillars,and displays an interesting triangular cylindrical entity.The ESI-MS spectrum showed that the polyoxoanion is intact clusters{RE3?H2O?9[Nb48O138?H2O?6]}in the solution.At the same time,the fluorescence properties of some compound were studied.?.Three RCPONbs were obtained in the presence of citric acid with the formula Cs7H17{La4?H2O?14[Nb24O69?H2O?3]2}·16H2O?14?,Cs7H17{Ce4?H2O?14[Nb24O69?H2O?3]2}·16H2O?15?and Cs7H17{Pr4?H2O?14[Nb24O69?H2O?3]2}·16H2O?16?,respectively.Compound 14?16 were iso-structures and constructed from the{Nb24O72}secondary building units?SBUs?linked by bridging{RE?H2O?n}groups into the 3D extended framework with{RE4(Nb24O69)2?H2O?20}as its asymmetric unit.The asymmetric unit are composed with two{Nb24O72}SBUs and four{RE?H2O?n}groups,where three{RE?H2O?3}3+fragments pillared two{Nb24O72}SBUs resulting in an interesting triangular cylindrical entity,and a{RE?H2O?5}3+fragment supporting on one of{Nb24O72}SBUs.Compound 14?16 are the first example of pure inorganic covalently connected 3D polyoxoniobate based open-frameworks.?.Three RCPONbs were obtained in organic phosphonic acid solution with the formula?CN3H6?17K7H36{La4?H2O?10Nb96O276?H2O?12}·76H2O?17?,?CN3H6?17K7H36{Ce4?H2O?10Nb96O276?H2O?12}·76H2O?18?and?CN3H6?17K7H36{Pr4?H2O?10Nb96O276?H2O?12}·76H2O?19?,respectively.Single-crystal X-ray analyses showed that 17?19 were isostructural and composed of four{Nb24O72}SBUs and four{RE?H2O?n}3+groups forming giant cuboctahedral molecular cage.To the best of our knowledge,these compounds contain the largest number of Nb centers in a discrete PONbs and the polyoxoanions are the largest PONbs reported thus far.The work will promote the major progress in synthesis of RCPONbs,offer the possibility for finding polyoxoniobate materials with potential application value,and lay a better foundation for realizing the application in photo,electric materials and catalysis. |
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