Synthesis And Assembly Of Polyoxometalates-cyclodextrins Inorganic-organic Composites

Based on supramolecular interactions,polyoxometalates(POMs)and cyclodextrins(CD)are assembled into POM-CD composites,which can exert their own characteristics and improve the stability of the composites.It is expected to have a wide range of applications in green catalysis,biology and medicine.POMs have diverse structures,but POM-CD inorganic-organic composite materials are relatively limited and the assembly rules between them are still unclear.There is still much space for research in this field.In this thesis,six POM-CD nanocomposites were synthesized under mild conditions.Two are isostructural POM-CD nanocomposites(compounds 1 and 2)synthesized by self-assembly of stable Keggin-type POM and CD.Another three POM-CD composites were synthesized in-situ(compounds 3-5).In addition,a POM-CD composite(compound 6)was synthesized with Dawson-type POM and CD.Firstly,we explored the assembly of stable Keggin-type POMs with cyclodextrins.Compounds 1 and 2 were composed of Keggin PW₁₂and?-CD with the assistance of transition metal ions,we obtained two examples of isostructures.In the composite structure,POM is embedded in the secondary face(large opening)of the CD to form the basic unit of POM@CD.Then,a“cage in cage”supercubic structure(POM@?-CD)₃₆{(POM@?-CD)₁₂(?)(POM@?-CD)₂₄}with a large hole(4.2nm)was further assembled.The self-assembly rules of the supramolecular system were investigated by high resolution mass spectrometry(ESI-MS),1D NMR,2D DOSY and CV.The study shows that the size of POM and CD,the stoichiometric ratio,the p H value of the solution and the transition metal ions have important effects on the formation of the supramolecular assembly.Based on the assembly rules of Keggin-type POMs and CDs,we further explored the assembly rules of the Na₂Mo O₄precursor and CD under in situ,and obtained compounds 3-5 under mild conditions.Single crystal structure analysis shows that 3and 4 are both supramolecular framework structures assembled by?-Mo₈O₂₆and?-CD,the difference is that 3 has 1.2 nm channel and 4 has only 0.68 nm channel.In addition,an in-situ reaction of transition metal ions with Na₂Mo O₄precursor and CD has been performed to obtain a Keggin-type Co Mo₁₂and?-CD assembled supramolecular framework(compound 5)with two different channels of 0.62 nm and0.57 nm.These studies indicate that the in-situ method can obtain more POM-CD complexes.At the same time,and the size of POM precursor and CD,the stoichiometric ratio,the p H value of the solution and the transition metal ions have important influences on the formation of supramolecular assembly.After that,we investigated the assembly rules of Dawson-type POMs and CDs.Under the same conditions,Dawson-type POM P₂W₁₈or lacunary POM P₂W₁₂and?-CD were used as building units,it was found that P₂W₁₈could not be assembled with?-CD to obtain a supramolecular framework structure,while P₂W₁₂and?-CD can assemble a similar“cage in cage”supercubic structure as compound 1 and 2(compound 6).The reason may be due to the fact that the larger P₂W₁₈and cyclodextrin are kinetically difficult to assemble,while the smaller lacunary POM P₂W₁₂facilitates the assembly.Meanwhile,the unstable lacunary POM slowly formed a stable configuration in the assembly process.We speculate that the thermodynamics affected the POM-CD complexes assembly in the system.The related mechanism remains to be studied.In addition,we also discussed how chiral CD induced the chiral arrangement of nonchiral P₂W₁₈.The above work has screened out suitable POM and CD building units and prepared a variety of new POM-CD composites.The structural characteristics of the composites and the self-assembly rules of building units were studied,which enriched the structural diversity of POM-CDs.The POM-CD composites synthesized in this work,opens the door to a new generation of hybrid materials,and establishes and develops new synthetic ideas and preparation methods for POM-CD composites.