SYNTHESIS AND CHARACTERISATION OF POLYAZA-MACROCYCLES AND THEIR ANALYTICAL APPLICATIONS

Abstract

The co-ordination chemistry of various types of ligands has been a fascinating area of current interest to the inorganic chemists all over the world. Recognition of the importance of complexes containing macrocyclic ligands has led to a considerable effort being invested in developing reliable inexpensive synthetic routes for these compounds. The development of the field of macrocyclic chemistry has also been an important factor in spurring the growth of interest in complexes of macrocyclic compounds since it has been recognized that many complexes containing synthetic macrocyclic ligands may serve as models for biologically important species such as haemoglobin, cytochromes or chlorophyll, porphyrins, etc., which contain metal ions in macrocyclic ligand environments. Thesemacrocyclic complexes performvital functions in various biological processes. The significant role played by metal ions in living systems depends on their confinement in an approximately planar, tetradentate and totally enclosed framework. Macrocyclic ligands are noted for their remarkable selectivity towards soft and heavy metal ions (Na+, Ag+, Cu2+, Ni2+, Cd2+, Cr3+, Al3+, Hg2+, etc.). The strong and selective interaction of macrocycles with specific metal ions make these ligands suitable candidates for use as reagents in separating metal ions which are otherwise difficult to separate. The use of macrocycles to separate metal ions which are present in very low concentration from other more concentrated metal ions, and the separation of metal ions which do not differ in their ionic radii considerably are of particular interest. However, all macrocycles are not useful for preparing membrane electrodes. The membrane of a macrocycle is likely to function as a good ion-selective electrode (ISE) provided the macrocycle shows (i) high complexation or extraction selectivity for a particular metal cation, (ii) enough conformational flexibility for rapid ion exchange, (iii) high lipophilicity to remain in the membrane and (iv) moderate molecular (i) weight to allow high mobility. Macrocycles occupy an intermediate positions between crown ethers and cryptands because they are stronger cation binders than crown-ethers and more flexible than cryptands.Therefore, they offer great promise in broad areas of metal sensing processes such as ion selective electrodes. The armed macrocycles having intermediate stability constants have been developed recently as they exhibit kinetically fast complexation properties of crown-ethers and three dimensional binding characteristics ofcryptands. Probably the most intriguing characteristic of macrocyclic compounds is their ability to selectively bind certain cations in preference to other which may be present in solution. Indeed, much of the current work in this field involves seeking of new ligands which will be specific for particular cations. Macrocyclic rings are quite strained hence are subjected to great reactivity. The mechanism of ring opening and subsequent dissociation of the complexes are of considerable interest. The dissociation rate constants of these complexes led to the discovery of the extraordinary kinetic stability of these types of complexes. In order to provide insight into the ways in which the macrocyclic ligand might impart unusual properties to the metal complexes, it was considered desirable to study the kinetics and mechanisms of the reactions of the macrocylic complexes in different solvents with different acids at various temperatures. The electronic, infra-red spectroscopy and nuclear magnetic resonance spectroscopy remain the most widely used tools in the study of macrocyclic complexes and these above mentioned aspects adds to our existing knowledge on macrocyclic complexes. For the sake of convenience the work embodied in the thesis is presented in the following chapters : The First Chapter of the thesis 'General Introduction' presents an up-to-date survey of literature on the use and role of macrocyclic compounds, which presents a review of all the relevant work done till date. The problem of present research has (ii) been posed in the context of the cited work. The basic principles, experimental details, reagent and materials are described in the Second Chapter of the thesis. Methodology employed for kinetic studies and in the preparation of membranes, measurement of electrode potentials, determination of selectivity and other functional properties of the membrane using the macrocycle is also presented in the the same chapter. The Third Chapter of the thesis deals with the synthesis and characterization of mono and bicyclic multidentate macrocycles with different ring sizes and varying degree of unsaturation. Metal-template and non-template methods have been used to synthesize the macrocycles. UV-visible spectroscopy, infra-red spectroscopy, nuclear magnetic resonance spectroscopy and elemental analysis has been made. The Fourth Chapter contains a detailed kinetic study of copper complexes. Under kinetic studies copper(II) complex of tetraaza cyclotetradecane macrocycle undergo rapid dissociation in acidic solutions.A plot of Kobs vs hydrogen ions concentration shows good first-order dependence. A general mechanism proposed for the aciddissociation of linear polyamine complexes may be adopted for this complex. Relevant activation parameters has also been calculated. Analysis of product was done on the basis of comparison of spectral data and Rf values which show that the product is the protonated ligand. In view of the importance of ISEs, in diverse fields, tremendous efforts by various workers have been made for the development of large number of sensors for various ions. However, only limited number of sensors have been found to be sufficiently selective and thus made commercially available. Most of the sensors reported in literature suffer from poor sensitivity, selectivity and long response time. Hence further efforts are needed to have better electrodes. This appears possible with the increasing availability of new materials which show selectivity for specific metals and therefore, can be used as electroactive component in the membrane to be used (iii) as ions sensors. Efforts in this direction were initiated using some of these macrocycles as membrane materials, the results of which are given in the Fifth Chapter of the thesis. Adioxa, dimethyl substituted fourteen membered macrocycle based membrane using PVC as binder have been explored for Zn2+ and Mg2+ ions, while apolystyrene based membrane of phenyl substituted macrocycle is used for the determination of Cr +ions. Cr3+ and Al3+ selective electrode of octamethyl substituted macrocycle have been fabricated using PVC as binder. These electrodes functions well over a wide concentration range (1CT5 - 10"6M) within a Nernstian slope. It also exhibits a fast response time and good selectivity over a number of cations. All the membrane sensors developed in the laboratory have also been used in partially non-aqueous medium and also as an indicator electrode in the potentiometric titration of determinand ions. Practical utility ofthe membranes have also been judged in presence of various surfactants.