Helical Self-assembly And Chirality Inversion Of Covalently Modified Anderson-type Polyoxometalates

Supramolecular assemblies based on intermolecular forces have the characteristics of a wide variety of building blocks and rich connection modes,which can be used to develop supramolecular materials with different functional properties.As a special kind of supramolecular assemblies,helical supramolecular assemblies have asymmetric assembly structures,which have attracted extensive attention in the field of stereoselective catalysis and biology.How to select appropriate building blocks and forces to achieve the preparation of helical supramolecular assemblies with different structural and functional properties has always been one of the important research contents in the field of supramolecular chemistry.Chiral properties characterized by asymmetry play an important role in supramolecular helical assembly.Under the induction of supramolecular forces,building motifs containing chiral groups are more conducive to the formation of helical assemblies.As a kind of inorganic nano clusters with definite composition and structure,polyoxometalates have the characteristics of gaining and losing multiple charges while maintaining the same structure,thus showing potential applications in many fields.The introduction of polyoxometalates as building blocks into supramolecular assembly can not only bring rich structural changes,but also make the assembly have more functional properties.Through covalent modification,organic compounds with different functional groups can also be modified to polyoxometalates,thus expanding their application as supramolecular assembly building units.Using this method,organic compounds containing chiral sites can be covalently grafted onto polyoxometalates,and organic-inorganic hybrids with specific chiral properties can be obtained.Through the supramolecular assembly strategy,it is expected to obtain asymmetric helical assembly structures.By changing the supramolecular force,the structure and chirality of the assembly can be regulated.Based on the above discussion,this paper has carried out the following research work.(1)Synthesis of chiral organic compounds with double covalently modified Mn~Ⅲcentered and single covalently modified All~Ⅲcentered Anderson type polyoxometalates.For the double covalently modified structure,R-type,S-type and racemic 2-phenylpropionic acid were first reacted with amino terminated trihydroxy compounds,and then covalently grafted onto Anderson type polyoxometalates in the center of Mn~Ⅲ.For the single-sided covalently modified structure,R-type,S-type and racemic 2-phenylpropionic acid were respectively linked to the amino terminal trihydroxy ligand modified Al~Ⅲcentered Anderson type polyoxometalates by amidation reaction.For the above synthesized compounds,the cations were replaced bysodiumions,tetrabutylammoniumcations(TBA)and dioctadecyldimethylammonium cations(DODA)through the method of ion replacement,so as to obtain 18 organic-inorganic hybrid compounds with different metal centers,chiral configurations and counterions.The obtained compounds were characterized in detail by elemental analysis,IR,NMR and TGA,and their structures were determined,which laid a foundation for further supramolecular assembly research.(2)Supramolecular assembly and chiral regulation of covalently modified Anderson type polyoxometalates.The experimental results show that the complexes with sodium ion and TBA as counterions have weak intermolecular force and are not easy to form ordered supramolecular assemblies,while the complexes with DODA as counterions have appropriate hydrophilic hydrophobic ratio and can obtain ordered assembly structures.In the mixed solution of tetrahydrofuran and methanol containing chiral Mn~Ⅲcentered Anderson type polyoxometalate complex,with the increase of methanol volume,helical assemblies with different pitches can be obtained,while the corresponding racemes have no similar phenomenon,indicating that chiral components play an important role in the assembly.When the amount of methanol in the mixed solvent continues to increase,the helical direction of the supramolecular assembly will reverse.The circular dichroism spectrum showed that the chiral signal of the assembly also reversed in this process,and the gradual change process was consistent with the helical structure,indicating that the two were closely related.The powder X-ray diffraction results show that the layered structure of the assembly remains unchanged throughout the transformation process.For the Al~Ⅲcentered Anderson type polyoxometalate complexes containing chiral components,they also show a similar structural transition in the mixed solvent of tetrahydrofuran and water.The change of helix direction and chirality of supramolecular assembly results from the change of force between alkyl chains caused by solvent environment and the chirality of polyoxometalate complexes.Through the above research work,supramolecular complexes based on polyoxometalates were successfully prepared.Through solvent regulation and other means,controllable helical assembly and chiral regulation of the complexes were realized,which laid a foundation for their further application in the fields of chiral catalysis and chiral recognition.