Synthesis and application of macrocyclic compounds for metal cation sensors
Date
2018-07-03
Authors
Valiyaveettil, Suresh
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Abstract
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.
Description
Keywords
Metal complexes, Alkali metals, Crown ethers