Chiral Cavities with Amino Acid Derived Ligands and their Interactions with Chiral and Achiral Guests
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In this thesis we have attempted to explore the synthesis of small molecular weight complexes having guest binding site within a cavity. During the course we have explored two different sets of amino acid derived ligands (Figure 1) and systematically synthesized number of Cu(II) and Ni(II) complexes via two different approaches were we would able to form cavity capable of binding external guest either through metal coordination or H-bonding. We have used amino acid derivative ligands, as it is the easiest and cost effective way to incorporate chirality in the host due to availability of pure L-amino acids from the natural source. Further studies with external guest we were able to separate enantiomers from a racemic mixture with one of the host. This particular result constitutes a rare example of chiral separation through non-covalent interaction between a host and guest where host-guest adducts are structurally characterized. The thesis has been divided in to six chapters. Chapter wise summary of the work are given below. Figure 1 The chiral amino acid derived ligands used in this thesis. Chapter 1: Introduction This chapter summarizes a brief literature review on different types of organic or inorganic base macrocyclic receptors used for guest recognition particularly keeping the view of importance of chirality. Chapter 2: Ferrocene substitution in amino acids enhances the axial binding in Cu(II) biscomplexes and separates the hydrophobic and hydrophilic region in the lattice. This chapter describe the synthesis, characterization and electrochemical properties of a set of ferrocenylmethyl substituted L-amino acid ligands and their Cu(II) complexes. It was observed that the two ligands organize around Cu(II) in a C2 symmetry such that both amino acid residues stay on the same side of the Cu(II) coordination plane. Structural characterization of the complexes of L-methionine (Figure 2) and L-asparagine derived ligands showed axial coordination of weak thioether and amide respectively to Cu(II). We like to note that usually methionine thioether does not bind in synthetic Cu(II) complexes. Coordination of the thioether group of L-methionine to copper (2.791D) is shorter than observed in the electron transfer protein plastocyanin (2.9D). The characterization of bis-complexes with L-serine and L-threonine derivatives showed axial coordination of water with a shorter CuDO bond length compared to that observed in the corresponding amino acid complexes. The structures also revealed separation of the hydrophilic and hydrophobic regions due to amino acid and ferrocene respectively which resulted in the formation of interesting H-bonded networks (Figure 2). Figure 2. Molecular and lattice structure of Cu(II) complex of ferrocenylmethyl-L-methionine ligand. Chapter 3: Formation of a narrow chiral cavity in Cu(II) bis-complexes of ferrocenylmethyl Ltyrosine derivative and its inter action with achiral guests. The tendency of ferrocenylmethyl substituted amino acids to organize around Cu(II) in a C2 symmetry, as observed in Chapter 2, was utilized to form a narrow chiral cavity employing ferrocenylmethyl-L-tyrosine ligand. The structural characterization of [CuII(S-fTyr)2(MeCN)2] and subsequently other adducts prepared from it by substituting MeCN showed that planar heterocyclic N-donors as guests stabilized the cavity, while polar water molecules.....
Supervisor: Manabendra Roy