Polyoxometalate Supramolecular Complexes: Chiral Induction And Asymmetric Catalysis

Chirality as a general natural phenomenon always attracts great attention, becauseof the interest for both the mysterious origin and the numerous potential applicationsincluding drugs, catalysts, and hopefully, optical devices. To create functional chiralstructures and applicable materials, diverse organic and inorganic artificialcompounds have been prepared. However, it is not always an easy task in thepreparation of chiral molecules and assemblies by covalent synthesis due to thearduousness and limit in detailed chemical procedures. Fortunately, the inducedchirality through intermolecular interaction points to a convenient approach forcreating chiral materials from achiral molecules or components.Polyoxometalates (POMs) are discrete, molecularly defined, nanoscale inorganicmetal-oxide clusters with structural variety and interesting functional propertiesleading to great potential applications in catalysis, medicine, optics, electrics, andmagnetics. The functional properties also promote the process of chiral POM research.The chiral POM structures can lead to chiral functional material including asymmetriccatalysis, molecular recognition, and nonlinear optical material. To develop thefunctional properties of chiral POMs, an effective method to prepare chiral structure isneeded. Generally, POM has high symmetry, and it is not easy to form the chiralstructure. At the same time, POM with chiral structure is unstable and racemization always happens in solution. Stable chiral POM structures can be prepared throughcovalent modification with chiral organic molecules. However, covalent modificationis only suitable for POM with modification sites. Thus, it is not a general method. Forthe POM without modification sites it does not work. At the same time, the reactionconditions for the organic covalent modified synthesis are often strict, and theseparation of reaction products is difficult.Among the strategies of organic modification, a general and simple approachcurrently used is to replace the counterions of POMs by organic cations, resulting inorganic cation-encapsulated POMs, which can lead to increased stability of innerPOMs, improved solubility and machinability. The composition of POMsupramolecular complexes obtained by this method is defined. In supramolecularcomplexes, one can realize integrating the functions and synergy effects between theorganic part and inorganic part, and even get a new feature.In this dissertation, function integration and synergy effect between the organicpart and the POM part in the supramolecular complex are well developed. We use thechiral organic cations to encapsulate POMs through electrostatic interaction, andconstruct the chiral POM supramolecular complexes in solution. Throughencapsulating POMs with photochromic properties, we realize the chiral switch ofPOMs in the supramolecular complexes. Through encapsulating POM with catalyticactivities, we realize the asymmetric catalysis of POMs in the supramolecularcomplexes.First, we develop a general and simple method for the preparation of chiral POMstructures. In order to explore the principle of chiral induction, we design and use thechiral organic cations with two stereocenters R-or S-BPEA to encapsulate a series ofPOMs with different structures and different transition absorption bands in theaqueous solution, and construct chiral supramolecular complexes. We use IR, NMRspectra and elemental analysis to character the structure of the complex. These dataprove the replacement of counterions of POMs by R-or S-BPEA in the encapsulationstep. To study the optical activity of the complexes in solution, we use CD spectra to investigate the obtained POM complexes. Due to the induction of chiral organiccations, not only the POMs with chiral structure show optical activity, but also theachiral POMs show induced CD effect. At the same time, we observe the induced CDsignals in the absorption bands corresponding to LMCT transitions, d-d transitionsand the IVCT transitions in different complexes. To study the influence of differentchiral organic cations on the optical activity of the POMs, we design and synthesizethe chiral organic cations with single chiral center. To study the influence of differentdistance between the chiral center and the POM surface on the optical activity of thePOM, we design and synthesize the chiral organic cations with different spacersbetween the chiral center and ammonium group. We use CD spectra to examine theoptical activities of the corresponding complexes, and we found that the chiral organiccations have great influence on the induced CD of the POMs. The induced CD of thePOMs also has a direct relationship with the spacer length between the chiral centerand ammonium group. As the spacer length increases, the induced CD of the POMsbecomes weak and eventually disappears. In this section, we use the chiral organiccations to encapsulate POMs through electrostatic interaction and construct chiralPOM structures. This is a simple, stable, and generally applicable method forintroducing chirality to POM structures, which is favorable for the functional chirality.Second, we realize chiral photochromism of POMs in the chiral supramolecularcomplexes. We design and use the chiral organic cations with two stereocenters R-and S-BPEA to encapsulate [PMo11VO40]4-(PMo11V) with photochromic propertiesthrough electrostatic interaction, and construct POM supramolecular complexes R-and S-PMo11V with novel chiral photochromic properties.1H NMR and IR spectraindicate that the frame structure of POM and chiral organic cations are well kept inthe complex, and the strong electrostatic interaction exists between the organic cationand the POM. Based on the elemental analysis and TG results, all the counterions ofPMo11V are replaced by R-BPEA or S-BPEA cations. CD spectra prove that theachiral POM in the complexes exhibits chiroptical activity in the absorption bandcorresponding to the LMCT transitions. We propose that the induced CD in the PMo11V clusters is sourced from the chirality transfer from the R-or S-BPEA to thetransition through the electrostatic interaction between POM and chiral organiccations. By changing the solvent polarity and the temperature of the system, we cancontrol the chirality transfer based on electrostatic interactions effectively. The UVirradiation experiments, UV-Vis and CD spectra confirm the POM supermolecularcomplexes show obvious chiral photochromic properties. After photo-reduction,R-PMo11V is reduced to heteropoly blue. For the first time, we observe CD signals inthe absorption band region corresponding to the IVCT transition of the reduced POMcomplex. In air, the heteropoly blue returns to the initial state and the CD signalsdisappear. The31P and1H NMR results confirm that in the complex the PMo11V isstable and the chiral organic cations are not oxidized during the reversiblephotochromic process. The cyclic voltammetric (CV) curves and XPS data confirmthe photo-reduction take place in vanadium atoms instead of molybdenum atoms. Acontrol experiment of R-PMo12proves that the localization of reduction electron hasimportant influence on chiral photochromism of the POM. By the addition of redoxreagents, we achieve redox controlled chiral switch of the POM. For the first time, werealize the reversible control of the chiroptical activity of POM in the supramolecularcomplex, which is meaningful for making chiral POM materials and devices.Third, we realize asymmetric catalysis of POMs in the chiral supramolecularcomplex. We design and use the chiral organic cations with two stereocenters R-andS-BPEA to encapsulate a sandwich-type [WZn3(H2O)2(ZnW9O34)2]12-(Zn5W19) withexcellent catalytic activity through electrostatic interaction, and construct POMsupramolecular complexes R-Zn5W19and S-Zn5W19with asymmetric catalyticactivity.1H NMR and IR spectra indicate that the frame structure of Zn5W19and chiralorganic cations are well kept in the complex, and the strong electrostatic interactionexists between the organic cation and the Zn5W19. Based on the elemental analysisand TG results, all the counterions of Zn5W19are replaced by R-BPEA or S-BPEAcations. CD spectra prove that, due to the induction of chiral organic cations, thePOMs in the complexes exhibit chiroptical activity in the absorption band corresponding to the Oâ†'W LMCT transitions. Using the complex as building block,we construct supramolecular assemblies in situ in the reaction solution of sulfideoxidation. The DLS, SEM and TEM data confirm the complexes in the reactionsolution form self-assemblies with regular spherical shape. XRD results confirm thecomplex just aggregates into a tight packed assembly with non-ordered structure inself-assemblies. We also investigate the aggregation state of the complexes indifferent reaction solvent. Experiments prove that the complex in stable assemblystate shows better stereoselectivity (72%ee) for the asymmetric catalysis ofthioanisole oxidation than in the monodispersed state. Under the optimized reactionconditions, the oxidation of a series of sulfides shows stereoselectivity. Detailedkinetic study of the catalytic process shows that the enantioselectivity of sulfideoxidation is derived from the combination of an ineffective asymmetric sulfoxidationand an effective kinetic resolution of the sulfoxide. The coverage density of chiralorganic cations enwrapped on the POM surface is proved to have an importantinfluence on the enantioselectivity. Therefore, the self-assemblies increase of thestereocenter coverage density on the POM surface and improve the chiral induction.Supramolecular assembly offers a strategically protocol for the direct construction ofselective and robust supramolecular asymmetric catalysts based on POMs.In a word, in this thesis we use chiral organic cations to encapsulate POMs withelectrostatic interaction, successfully construct chiral POM supramolecular structuresin the solution system, by means of the chiral induction based on supramolecularinteractions. The POMs show induced chiral activities in absorption bandscorresponding to different transitions. By using the synergetic effect between thechiral induction of organic components and the functional properties of inorganiccomponents in the chiral supramolecular complex, we realize the chiralphotochromism and asymmetric catalysis of POMs. Encapsulation by chiral organiccation through electrostatic interaction is a general method to prepare chiral structureof POMs. This method is meaningful for the development of chiral functionalmaterials and devices based on POMs.