Functionalization Of Chiral Polyoxometalates: Asymmetric Resolution And Chirality Transfer

Polyoxometalates(POMs) as a family of functional inorganic clusters exhibit abundant chemical compositions and tropological structures, which have potentials in many fields such as catalysis, optical materials, electrics and even medicine. Due to the specificity and importance, chemists start to learn about chirality in-depth and get more and more comprehensive understanding. Recently, introducing chirality factor into the preparation process of POMs and further constructing functional systems based on chiral POMs becomes a very promising research topic in POM chemistry. For the purpose of exploring the functionality and application of chiral POMs in chiral catalysis, recognition, nonlinear optics, and biology, several methods have been developed to prepare chrial POMs, mainly including spontaneous resolution, direct synthesis, covalent modification, and electrostatic encapsulation. However, these methods possess obvious disadvantages and problems in different aspects. For example, the methods of spontaneous resolution and covalent modification are just limited to the POM clusters with intrinsic chiral framework and modifiable site respectively, thus leading to the bad generality. Direct synthesis always involves large and systematic experiments, which is almost unpredictable to obtain the chiral POMs. Electrostatic encapsulation has been proved to be a more general approach to realize the chiral modification of most POMs, but the employed organic chiral cations inevitably require complicated synthetic procedure and/or high acquisition costs in current state. Overall, the synthetic chemistry of chiral POMs is still in its infancy because of their pretty limited sort and number. On the other hand, many chiral POM structures are not that stable in solution state, which further greatly hinder the development of functional applications of chiral POMs.Utilizing the organic chiral cations with hydroxyl group at their end to encapsulate POMs produces chiral POM supramolecular complexes, and in combination with a sol-gel process, the chiral POM complexes could be covalently immobilized into a matrix. In addition, by choosing the stable chiral POMs as chiral source and fully employing their synergistic interaction with other components, novel functional features and applications could be expected in the hybrid systems.In this dissertation, we combine the electrostatic encapsulation and sol-gel method to immobilize the chiral POM complexes into the silica matrix, promoting the stability of chiral microenvironment in the complex, and then we investigate the enantioselectivity of these catalysts in the asymmetric resolution of secondary alcohols. Next, through the co-assembly of chiral and achiral POMs in the aqueous solution and on the surface of gold nanoparticle, we realize the unprecedented chirality transfer from chiral to achiral POMs. Subsequently, we introduce the inherent chiral POMs into the synthesis of nanoparticle, and confirm that the chiral POMs could serve as inorganic chiral ligand for the stabilization and chiral induction to the nanoparticle.First, we get a effective method for the stabilization of chiral microenvironment in the chiral POM complexes. For this purpose, we design and synthesize a chiral cation bearing hydroxyl group at the end of alkyl chain to encapsulate POM with efficient catalytic activity. Through IR, UV-Vis, NMR spectra, elemental analysis, and thermogravimetic analysis, we characterize the structure of chiral POM complexes in detail and confirm the successful exchange of chiral cations. Then CD spectra are employed to figure out the optical activity of the POM complexes, it is found that the POM exhibits induced chirality. The HR-TEM results show highly disordered assembly structure of the chiral POM complexes in organic solution. To study the influence of immobilization on the enantioselectivity in the catalytic reaction of the chiral POM compelxes, we realize the immobilization of chiral POM complexes in the silica matrix via a sol-gel method and successfully obtain a series of catalysts with variable loading content of chiral POM complexes. XRD and HR-TEM are perfomed to know the microstructure of the complex in the immobilized catalysts, and the results prove that a morphology change from uniformly dispersion to formation of nanocrystalline domains occurs as the loading content of chrial POM complexes increasing. We choose the asymmetric resolution of secondary alcohols as model reaction to investigate the enantioselectivity, and a 89% ee value is obtained under the optimized reaction condition. Compared with the pure chiral POM complexes in solution, the immobilized chiral catalysts give rise to higher enantioselectivity. Moreover, the increase of loading content of chiral POM complexes in the silica matrix leads to the correspondingly promotion of ee values. In this section, we realize the stabilization of chiral microenvironment in the chiral POM complexes through immobilization into the matrix and thus promote their enantioselectivity, which provides an effective method for the development of efficient asymmetric catalysts based on POMs.Second, for the first time, we realize the chirality transfer between chiral and achiral POMs both in the aqueous solution and on the surface of gold nanoparticle. For the mixture aqueous solution containing both chiral and achiral POMs, induced CD signal is observed in the wavelength range attributed to just achiral POMs, which demonstrates chiral induction effect of chiral POMs. PMo10V2, PMo11 VIV, and PMo12 all exhibit induced optical activity, suggesting that the present approach is general at a certain degree. It is also found that the chirality transfer between POMs is obviously concentration-dependent, and only the high concentration of chiral POMs could lead to the occurrence of chirality transfer. Meanwhile, the addition of sodium chloride endows a higher efficiency of chiral induction while small amounts of acetone could results in completely disappearance of induced CD. Interestingly, we find that under the same condition, the dimethylamine cations rather than sodium ions balanced chiral POMs could induce achiral POMs to be chiral. Then we use UV-Vis, NMR spectra, and ITC titration to figure out the role of the dimethylamine cations in the chirality transfer process. Besides the tight association with chiral POM framework through electrostatic attraction and hydrogen boding, the dimethylamine cations could form charge transfer complex with the achiral POMs, which means that they serve as salt bridge to connect chiral and achiral POMs together. This unqiue structure shortens the inter-cluster distance between POMs, thus promoting the chirality transfer from chiral to achiral POMs. In addition, we co-assembly the chiral and achiral POMs on the surface of gold nanoparticle, and the efficiency of chirality transfer increases. The chirality transfer from chiral to achiral POMs provides a simple and effective strategy for the preparation of induced chiral POMs, which is meaning for the development of chiral POMs.Third, we further employ the chiral POMs as a kind of novel inorganic chiral ligand and realize the successful preparation of chiral nanoparticle. Sodium borohydride is used as the reducing agent to prepare the silver nanoparticle in aqueous solution, and meanwhile, the chiral POMs in-situ stabilize the nanoparticle. UV-Vis and XPS are carried out to conform the appearance of silver nanoparticle, while we study their morphology and size by using HR-TEM. A monolayer surrounding the silver nanoparticle with width of 1.4 ± 0.1 nm is clearly detected, which proves the hybrid structure of chiral POMs protected silver nanoparticle. The 31 P NMR experiences little change before and afer the preparation of nanoparticle, suggesting that the structure of chiral POMs remains intact. The mirror symmetric CD signals are observed in the absorption band corresponding to the silver nanoparticle, which confirms that the chiral POMs have the capability of inducing the inorganic silver nanoparticles. And it is also found that the intensity of induced chirality follows a nearly linear relationship with the amounts of chiral POMs added. To sdudy the interaction between chiral POMs and silver nanoparticle, we conduct a series of characterizations. XPS results suggest that it is the chiral POM framework rather than tartaric acid that directly interact with the nanoparticle surface, while the raman and HR-TEM measurements further illustrate the binding mode of chiral POMs on the nanoparticle surface. Based on the uncovered binding motif, we could give an understanding as follows. On one hand, the chiral POMs could stabilize the silver nanoparticles, on the other hand, they play a role of chiral template to induce the surface atoms to arrange in a asymmetric fashion and thus give rise to chiral surface, which should be responsible for the induced chirality. Overall, we fully take advantage of the structural feature of chiral POMs in this study and employ them as inorganic chiral ligand, which is of great importance and significance in the development of functional applications of chiral POMs.In a word, in this thesis we mainly study the construction of stable chiral POM systems and their functional applications. Through immobilization into the silica matrix, the stability of the chiral microenvronment in the chiral POM complexes is promoted, giving rise to efficient enantioselectivity. Then we choose the inherent chiral POMs as chiral source and realize chirality transfer to achiral POMs and nanoparticles by means of exploiting the interactions between them. The present study provides strategically protocol for the construction of chiral POM-related systems and significant guidance for the development of functional applications based on the chiral POMs.