The Assembly Of Chiral Metal-organic Framework Materials And Their Application In Chiral Separation And Recognition

Chirality is the most common phenomenon in the nature and an essential attribute oforganismal survival. All kinds of biochemical reaction processes keep close to chiral separationand recognition. In recent years, with the rapid development of nanometer material, biologicalchemistry and crystal engineering, etc., the research of chirality has turned to the area ofsupramolecular chiral molecules. Metal-organic framework (MOFs) materials become one of thehighlights in agro-scientific research of chemical subject because of their novel structures, highspecific surface area and porosity, tunable pore size, good thermostability, easy modification,moderate growing conditions and their potential practical value. Although the research of MOFsmaterials grew rapidly, the development of chiral MOF (CMOF) materials is relatively slow.Chiral MOF materials have attracted more and more attention due to their potential applicationsin chiral separation and identification, nonlinear optics, asymmetric catalysis and chiral sensor.In this thesis, we synthesized three novel chiral MOF materials with chiral amino acids or“green” monosaccharides as starting materials. The structures of chiral MOFs were characterized,and the chiral MOFs were applied to enantioseparation and recognition, and synthesis ofhomochiral inorganic nanomaterials with chirality. The main content includes the following fourparts:1. Using the enantiopure schiff base ligand [(N-(4-Pyridylmethyl)-L-leucine·HBr)] and ZnAc2asmaterials, we synthesized a non-interpenetrated three-dimensional CMOF-1,{[ZnLBr]·H2O}n.The well-defned rod crystal of CMOF-1was characterized by SEM with about12μm in lengthand4μm in width. The solid-state circular dichroism (CD) spectrum of CMOF-1and its isomer,exhibited an obvious opposite cotton effect at240nm, which suggested the homochirality ofCMOF-1. The thermogravimetric analyses (TGA) indicated that the framework of CMOF-1wasthermally stable up to270°C. Particle size distribution of evacuated CMOF-1indicated themean particle size was around7μm. Evacuated CMOF-1was further used as a chiral stationary phase (CSP) for high-performance liquid chromatography to enantioseparate racemic drugs.(±)-ibuprofen,(±)-1-phenyl-1-propanol,(±)-1-phenylethylamine and (±)-benzoin weresuccessfully resolved and baseline separated on the CSP. The result showed the excellentperformances of the CSP in enantioseparation of drugs. The CMOF-1can be regarded as a novelmolecular sieve-like material with a chiral separation function based on the relative sizes of thechiral channels and the resolved molecules.2. For the frst time, helical CdS nanotubes was assembled using a convenient technique, inwhich {[CdLBr]·H2O}n(CMOF-2) as the template prepared by hydro-thermal method.Combined with characterization of sample SEM images, we know its formation mechanism.Firstly, after thioacetamide (TAA) and cadmium nitrate tetrahydrate were simultaneously addedto the solution of the ligand [N-(4-pyridylmethyl)-L-leucine·HBr] in ethanol and trace amountsof water, little CMOF-2crystal rods are generated instantaneously from initial coordinationreaction of Cd2+and ligands due to the rapid nucleation rate of the CMOF-2and the lowerconcentration of S2-from the relatively lagged hydrolysis of TAA. With the continuoushydrolysis of TAA, CdS starts to nucleate and aggregate quickly on the surface of the littleCMOF-2rod crystals to form the CdS nanotubes with well-defined helical morphology.Thereinto, the abundant Cd2+, contribute to the thicker wall in the method, not only stem fromthe crystal but also from the excess Cd2+in the solution. When the concentration of Cd2+ions inthe solution decrease to a certain degree, the corrosion of the opening CMOF-2proceed fasterfrom the inner surface with excess S2-ions in the solution, ultimately results in an openinginorganic helical CdS nanotube. The FTIR spectra of the final product reflected that there wereno residual ligands and CMOF-2crystals in the nanotubes. The correspondingenergy-dispersive X-ray (EDX) analyses further indicated that the ratio of Cd and S is1.07:1,which is consistent with the stoichiometric composition of CdS. CD spectra of helicalnanotubes templated by CMOF-2suggested the homochirality of the product. The preparedhelical CdS nanotubes were remarkably sensitive to D/L-aspartic acid (Asp) and can be used asa potential sensor for enantioselective recognition of D/L-Asp.3. For the frst time, we synthesized helical Ag nanoparticles formed onto a chiralmetal organic framework (CMOF-2@h-Ag NPs) by one-pot process at room temperature.CMOF-2,{[CdLBr]·H2O}n(CMOF-2) was prepared by the reaction of the ligand, [N-(4-Pyridylmethyl)-L-leucine·HNO3](l-HL·HNO3), potassium bromide and cadmium nitratetetrahydrate under ultrasonic irradiation at room temperature. Due to the weak coordinationbonds between central Cd (II) ions and Br (I) ions as ligand on the helical chains of CMOF-2,the Br (I) ions combine with Ag(I) positive ions from solutions into AgBr on the surface ofCMOF-2. Subsequent photoreduction results in highly ordered Ag NPs. Controllingthe concentrations of Br (I) ions during synthesis of CMOF-2crystals to obtainCMOF-2@h-Ag NPs is very important. If CMOF-2crystals were synthesized using thebromine-containing ligand,[N-(4-Pyridylmethyl)-L-leucine·HBr] and cadmium nitrate as rawmaterials, we only found irregular Ag NPs aggregation. If CMOF-2crystals were evacuated,larger Ag clusters distributed uniformly throughout CMOF-2crystals. A characteristicabsorption band in UV-vis spectra of CMOF-2@h-Ag NPs appears around410nm, suggestingthe formation of spherical Ag NPs. This material still retain the rod-shape characters ofCMOF-2crystals. The Ag NPs have an averaged diameter about15nm and narrow distribution,and exhibits polycrystalline nature. CD spectrum of CMOF-2@h-Ag NPs shows new signals at413nm and710nm in visible spectrum, meaning that the formed helical Ag NPs are chiral.Energy dispersive X-ray spectroscopy (EDX) measurements revealed that about1.5wt%Agwas loaded. PXRD of CMOF-2@h-Ag NPs is consistent with to that of the original CMOF-2template, indicating that the material still retains highly crystalline microstructures of CMOF-2after Ag NPs formation. This material, as surface-enhanced Raman scattering (SERS) substratecan effciently recognize D/L-cysteine enantiomers.4. The size and shape of the chiral MOFs play a key role in asymmetric catalysis, chiralrecognition and chiral separation. Micropores (<2nm) are in almost all of the reported chiralMOFs. To achieve larger pores in chiral MOFs, longer ligands are usully used, but this routesuffers from problems such as high cost. Herein we report, for the first time, the facile assemblyof homochiral hierarchical porous CMOG-3with micro-meso-macro ternary pores in a rapidand convenient sol-gel process from nontoxic and cheap calcium ions as metallic nodes and“green” D-saccharic acid potassium as organic linkers. The single crystals of CMOF-3,[Ca2(C6H8O8)2·4H2O·2H2O]nprepared by hydrothermal method, have neither channels norpores. However, CMOG-3retains the crystallinity of CMOF-3although the significantdifferences of the morphologies between the two. The as-prepared CMOG-3using DMF as solvent takes a continuous porous3D network skeleton and channels composed of a monolayerof interconnected nanoparticles Micropores (<2nm), mesopores (2-50nm) and macropores (>50nm) were formed among these particles. Varying the selection of common organic solvents,the pore sizes of CMOG-3also were changed accordingly. The results suggested that the sizesof hierarchically porosity in these materials can easily be tuned. The materials were used as achiral stationary phase (CSP) for high-performance liquid chromatography to enantioseparateracemic drugs.(±)-ibuprofen and (±)-1-phenylethylamine were successfully resolved andbaseline separated on the CSP.