Supramolecular System Of Polyoxometalates: Molecular Recognization,Induced Chirality And Chiral Control

Polyoxometalates(POMs)as a class of inorganic nanoclusters are composed by oxygens bridging with early transition metal atoms with unique chemical and pysical properties(such as br?nsted acid,strong oxidization,topologies and excellent catalytic properties).With the development of POMs' chemistry,chiral POMs have been attracted much attention due to the special properties in asymmetric catalysis,molecular recognization,nonlinear optics etc.The traditional chiral POMs can be prepared directly by hydro-thermal method with chiral crystal structure,however the chirality of POMs is not stable due to the quick recemization in solvent and even in solid state.Therefore,suitable method for developing POMs with robust chirality is a very important research topic.In few examples,the chiral molecules are introduced into the framework of POMs to induce the chirality,the metal ion Hf and Zr are used to combine the inorganic clusters and chiral organic units by coordination behaviour together with electrostatic interactions.Inspired by induced chirality,chiral POMs are yield by using chiral ligands and solvents into the synthetic process of POMs,but the unstable strcture caused by recemization is still the biggest problem.Further,chiral organic molecules are applied for covalently modification of POMs,the resulting hybrids exhibit robust chirality due to the covalent linking,but few POMs can be modified by this method and with complex synthetic procedure.In some other cases,chiral surfactants are used to encapsule achiral POMs through the strong electrostatic interaction and the chirality of POMs is successfully induced by this approach.In most cases,electrostatic complex has poor solubility in water as well as the elaborate synthesis and high cost for the chiral surfactants.Therefore,the construction of stable chiral POMs in aqueous solution is still a severe challenge.In this dissertation,we design a multi-component supramolecular system for realizing the induced chirality of POMs and chiral transfer,and the environmental response of chirality can be obtained by controlling the intensity of supramolecular interactions.The redox process of POMs can be regulated by electrochemical method with the shuttle changes of heteropolyblue,the switch corresponding to chiral electrochromism is realized.Subsequently,the induced chirality reversion of POMs is studied by controlling the position between POMs and cyclodextrins.First,we synthesize an organic-inorganic hybrid POM to combine other two components through supramolecular interactions in aqueous solution,and the induced chirality and chiral transfer of the hybrid are realized in this system.The hybrid POM(TBA-Azo-POM)is formed by using azobenzene derivative to single-side modify the Anderson type of POMs,the structure of TBA-Azo-POM is confirmed and characterized through x-ray single crystal diffraction,NMR spectra and elemental analysis.TBA as the counterions cause the poor solubility of hybrid in water,Na-Azo-POM with good water solubility is obtained by using the sodium ions to replace the TBA.Cyclodextrins(CDs)and cationic dyes(MB)are conbined with Azo-POM through the host-guest and electrostatic interactions in the aqueous solution.1D NMR,2D NOSEY NMR and UV-Vis spectra are applied to confirm the model of combination.The indued chirality of azobenze which is located into the chiral cavity of CDs is performed in the circular dichroism spectrum(CDS),and the induced chirality of MB is also yield by chiral transfer between Azo-POM and MB via electrostatic interaction,so the supramolecular interactions play important roles in the system of chiral induction.As we known that supramolecular interactions are dynamic and environment responsive,the decrease of electrostatic interaction together with increase of host-guest interaction by adding NaCl are caused the intensity changes of induced chirality.The dissociation between CDs and Azo-POM is triggered by improving the temperature of the aqueous solution,the intensity of induced chirality is decreased without the chiral source.In this section,the induced chirality and chiral transter are realized by recognition between Azo group and CDs.In addition,the responsively induced chirality by environmental stimulate is obtained by adjusting the supramolecular interactions.The research results provide a new method for the development of chiral POMs.Second,a simpler supramolecular system is developed for general POMs without organic modification to realize the induced chiraility,and the switch of chiral heteropolyblue by the electrochemical redox is obtained for the first time.Adamantanamine hydrochloride(AdH)as the cationic guest are applied to combine the ?-CD and POMs through both host-guest and electrostatic interactions in aqueous solution.The hydrophobic part of AdH is located into the cavity of ?-CD,the hydrophilic head of AdH locates outside of ?-CD,which can great improve the solubility of complex ?-CD-POM compared with ?-CD.PMo11 V and PMo12 are used for interacting with ?-CD-POM via electrostatic interaction,and the obviously induced chirality of POMs in LMCT band is obsevered in CDS.When adding dilute NaOH solution into the system,the decreasing intensity of induced chirality is cuased through the weaken electrostatic interaction by deprotonation of AdH.We can monitor the changes of pH to evaluate the process of deprotonation,and UV-Vis test is applied to confirm the intact structure of POMs.Electrochemical redox can regulate the heteropolyblue state of POMs with any structural change,the POMs in reduction state can yield the induced chirality in IVCT band.The chiral heteropolybule is disappeared in oxidation state of POMs by high voltage applied.And also we can control the reduction degree of POMs,the chiral heteropolybule in multi-electron reduction state of POMs is observed for the first time.The successful construction of chiral switch can be used in chiral sensor and asymmetric catalysis in further research work.Third,the induced chirality reversion of PMo12 is observed by controlling the position between CDs and POMs for the first time.CDs interact with POMs via hydrogen bond,the induced chirality of PMo12 by CDs can not be observed in low concentration of CDs.When improving the concentration of CDs,PMo12 has much opportunity to combine with CDs,the signals of induced chirality of PMo12 by ?-CD exhibits inversive compared with ?,?-CD.The PMo12 located in the small side of cavity is confirmed by the obvious chemical shift of H6 in the margin of small side via 1H NMR spectra.With the increase size of cavity,PMo12 partly moves into the cavity of ?-CD.Because the different positional location between CDs and PMo12,the intensity of induced chirality by ?-CD is weaker than ?-CD and exhibits inversive Cotton effect by ?-CD.The phenomena are match with the rule of induced chirality from CDs to organic guest.When adding anionic guest into the system,the guest locates into the cavity of ?,?-CD from the large side,which pushes PMo12 outside of the cavity increasing the distance between PMo12 and CDs.The enhanced intensity and inversion of induced chirality are observed via CDS.Meanwhile,the interaction between PMo12 and ?-CD is stronger than ?,?-CD,which is confirm by the strong intensity of induced peak at 310 nm in UV-Vis spectra.?-CD can be used to replace the ?,?-CD in the CDsPMo12 complexs,and the induced chirality of PMo12 chages due to the association with different CDs.The rule of induced chirality of CDs is applied into the clusters system for the first time and the research result will greatly promote the development of chiral POM system.In short,in order to construction of indued chirality of POMs,the multi-component systems are developed to realize the induced chirality by CDs through supramolecular interacitons.Chiral transfer and controllable chirality are firstly obtained in this aqueous system.Then the chiral electrochromism is switched by electrochemical redox by shuttling variation of heteropolyblue.In addition,the inversion chirality of PMo12 is regulated by the controllable position between CDs and PMo12.The present results not only provide the method for chiral POMs' system expecially in aqueous solution toward chiral functional application,but also enlarge the theory of induced chirality from CDs to guest including the inorganic clusters.