Design of New Chiral Ligands Based on 4,5-Dihydrothiazole and the Properties of Their Complexes

Author:Abaid Ullah Malik

Supervisor:shu mou hai


Degree Year:2018





One of the most active current areas of chemical research is centered on how to synthesize chiral compounds in a selective manner,rather than as mixtures of mirror-image forms(enantiomers)with different three-dimensional structures(stereochemistries).The inspiration is pointed out by the nature by knowing the fact that different enantiomers of a given biomolecule can exhibits dramatically different biological activities,and enzymes have therefore evolved to catalyze reactions with exquisite selectivity for the formation of one enantiomeric form over the other enantiomer.For example one enantiomer of thalidomide is a causative agent of physiological disorder and malfunctioning while the other is a well-known sedative agent.This structure-activity relationship inspired the material chemists to peep carefully into the phenomenon of chirality,and off course strive to the synthesis of enantiomerically pure materials.Considering the applications in various fields such as medicines,drug delivery,enantiopurification,energy storage but not least in the asymmetric catalysis,and drawing inspiration from these natural catalysts,the synthesis of new inorganic,organic and hybrid(combination of organic and inorganic)chiral materials with desired architecture is a challenging research topic.In this work,various new chiral ligands based on dihydrothiazole have been designed and synthesized,and for the first time,these liagnds have been applied in the preparation of chiral complexes.Chiral organic cages based on helicene have been designed and synthesized for the first time.All these compounds have been characterized by using diversified analytical techniques,and exhibited the properties such as photochromic transformation,the photo-degradation of the organic dyes,optic properties and enantioselectively absorption and separation of organic racemates.The main results are listed as following.1.New enantiopure ligands,(R)-/(S)-2-(4-pyridyl)-4,5-dihydrothiazole-4-carboxylic acids(HLR,HLS)were synthesized,subsequently their reactions with Cd(ClO4)2.6H2O resulted in the formation of complexes[CdL2R(H2O)2]×3H2O and[CdL2S(H2O)2]×3H2O.The structure of this complex was determined by single crystal X-ray diffraction analysis.The complexes show instant colour change upon irradiation with UV light.The photochromic mechanism is investigated by ESR and UV-Vis spectroscopic techniques.The results indicate that the photochromic transformation is originated from the formation of the free radical.2.We have synthesized the enantiomeric ligands H2LRR and H2LSS and utilized these chiral precursors in the construction of a pair of enantiomeric homochiral Ni6 macrocyclic rings 2RR and 2SS and an enantiopure pair of coordination polymers 3RR and 3SS.Ni6macrocyclic rings were successfully applied as photo-catalyst in the photo-degradation of various organic dyes and found efficient for the number of selected dyes.Further research on these materials continued to discover more applications.In coordination polymers 3RRR and 3SS,water cluster in a well-organized helical geometry has been investigated in single crystal X-ray diffraction analysis.It has been observed that the helices of water clusters are arranged in a chiral way influenced by the chirality of chiral channels due to chiral induction effect.3.We have synthesized a pair of enatiomerically pure ligands,R-/S-2-(4-carboxyphenyl)-4,5-dihydrothiazole-4-carboxylic acid(H2LR and H2LS),by reacting 4-cyanobenzoic acid with D and L-2-amino-3-mercaptopropanoic acid(D and L-cysteine amino acids)respectively.These chiral ligands have consequently been utilized to construct the two enantiomeric pairs of copper(II),nickel(II)isostructural helical chains,a zinc(II)coordination polymer with two dimensional sheets from the chiral ligand,and a coordination polymer by solvothermal method in which the dihydrothiazole ring is oxidized to thiazole ring with loss of chirality.Chiroptical properties have also been investigated.4.A pair of new enantiomeric compounds,(R)-/(S)2,2’,2’’-(benzene-1,3,5-triyl)tris(4,5-dihydrothiazole-4-carboxylic acid)(H3LRRR,H3LSSS)were synthesized.Photo triggered phochromic transformation has been investigated to elaborate the electronic transformation by electron paramagnetic resonance spectroscopy(EPR),photoluminescence(PL)and UV-Vis spectroscopic techniques.The results indicate that the photochromic transformation is caused by the formation of the free cation radical.The results are also supported by computational analysis using DFT calculations by Guassian software package.Chiroptical properties has also been investigated.5.Two cobalt(Ⅱ)complexes from ligand2,6-bis-{3(2-pyridyl)-1H-pyrazole-1-yl}pyridine(L1)and a cobalt(II)complex from 4,6-bis-{3(2-pyridyl)-1H-pyrazole-1-yl}pyrimidine(L2)are synthesized and their structures are determined by single crystal X-ray diffraction analysis with a good control of geometry.Magnetic and electrical properties are also under investigation.6.A pair of enatiomerically pure covalent organic cages P,P,P-and M,M,M-3 using helicene as its backbone are synthesized via imine condensation.This pair of cages shows triple-helix supramolecular structure,determined by its inherent chirality of helicene moieties along with induced chirality caused by its helical structure.Besides,it also exhibits mild to moderate enantioselectivity towards various racemates while using as chiral adsorbent and the same may be applied in construction of more sophisticated architecture in supramolecular chemistry due to available exo-functionality.The synthesized cage is novel in structure with excellent chiroptical properties due the presence of helicene and significant induced chirality in solid and sotution caused by formation of triple helix in the crystal structure.In this escort we have developed various new chiral compounds which still need to be applied in the future,hence an opening for future researchers to meet the current challenges.We believe that this work will prove to be a good contribution for materializing the synthesis of predesigned new chiral compounds of interests by different strategies that can serve as a promising tool for organic chemists.The investigation of structure-activity relationships in various fields invisioned a new opening for the future research.