Synthesis and application of macrocyclic compounds for metal cation sensors




Valiyaveettil, Suresh

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This thesis comprises three chapters united by a single theme: development of alkali metal cation sensors based on ion complexing macrocycles. In part 1, benzo-18-crown-6 and cryptand 2.2.2B were immobilised on polyacrylic acid backbone through an amide linkage. The benzo-18-crown-6 and 2.2.2B were functionalised using the Friedal-Crafts acylation reaction with ω-amino acids. The spacer between the polymer backbone and the crown ether was varied by using co-amino acids with varying numbers of methylene groups [special characters omitted]. Attempts to use co-amino acids with an intermediate spacer length [special characters omitted] failed due to formation of a cyclic imine. The amino crown ethers were immobilised on a poly(acryloyl chloride). Polymers 2a, 5ad and 6a failed to give self supporting membranes but a polymer blend with PVC/Plasticizer was employed for membrane fabrication. Ion Selective Electrodes (ISEs) and Coated Wire Electrodes (CWEs) were made from polymer blend membranes and their response to alkali metal cations was tested. The ISEs made with mobile carriers were active, while those prepared from immobilised carriers were inactive. The reverse was the case with CWEs. This dichotomy existed in all cases. The selectivity of the ionophores among the alkali metals was unaffected by linkage to the polymer backbone. However, the alkali metal/alkaline earth metal selectivity was enhanced. The effect of plasticizer and hydrophilic additives on electrode response was insignificant. The spacer length had considerable influence: the longer the spacer, the better the electrode response of the CWEs. In part 2, the mass transport of ions across the polymer blend membrane under a temperature gradient was investigated. The immobilised polymers prepared in part 1 were used here to fabricate membranes from polymer blends with NOMEX. In thermodialysis experiments, a low level of ion transport was detected. These preliminary experiments led to a rediscovery of membrane distillation. The scope of this latter process with hydrophobic membranes was explored in detail. Part 3 was devoted to the design and synthesis of water soluble photoionophores. Three series of molecules were synthesised: captands, bis crown ether compounds and phenol derivatives of tartaro crown ether carboxylic adds. Captand molecules were synthesised by a capping reaction of crown ether tetraacid chloride 14 with 1,3-bis(aminomwthyl) benzene, 1,4- bis(aminomethyl) benzene and 2,2’-bis(aminomethyl) biphenyl. Crystals of meta- and para xylene capped molecules were grown and their structures solved to establish the conformation of the molecules. Fluorescence quenching studies of these molecules were done in 0.3% methanol:water (v/v). Quenching due to alkali metal ions was insignificant ( < 20%) while copper and mercury cations quenched the emission significantly ( > 90%). Stern-Volmer analysis showed an upward curvature indicating association between the ligand and the cations [special characters omitted] cations, but dynamic and static components of the quenching could not be separated. Potentiometric titration with a potassium selective electrodes was carried out to obtain the stability constants for these ligands with potassium ion. The bis crown ethers 28 and 29, designed to increase water solubility, were prepared by the reaction of anhydride 27 with 9,10-bis(ammomethyl) anthracene and 1,2-bis(aminomethyl) benzene. The pKa values of the ligands and their stability constants with alkali and alkaline earth metal ions were determined by potentiometric titration. Fluorescence quenching studies were done in aqueous buffer at pH 10. These compounds also failed to give an emission quenching in the presence of alkali or alkaline earth metal cations, but both copper and mercury cations showed a significant amount of quenching. Stability constants were derived from emission quenching studies for [special characters omitted]. Chromoionophores, phenol derivatives of tartaro crown ethers, were synthesised from the reaction of crown ether anhydrides and 2-aminophenol. The structure of the compound 31 was assigned as the syn isomer based on nmr data in comparison to literature reports. Absorption studies were carried out in water. The absorption spectra of compound 30 were perturbed by alkali metal as well as alkaline earth metal ions, while the absorption spectrum of compound 31 showed no response to varying cation concentration. The lack of response from compound 31 was attributed to the competitive binding of cations among syn carboxylic groups away from the syn phenolic groups.



Metal complexes, Alkali metals, Crown ethers