PHYSICO-CHEMICAL STUDIES ON SYNTHETIC MACROCYCLES AND THEIR ANALYTICAL APPLICATIONS

Abstract

Coordination compounds containing macrocyclic ligands have been known and studied since the beginning of this century; however, until quite recently, the number and variety of these compounds was limited. The nitrogen donor synthetic macrocyclic complexes haveclose resemblance in theirstructure with the naturally occurring macrocyclic complexes like chlorophyll, vitamin B12, haemoglobin, porphyrins etc. and can thus be considered as models for them on the following two accounts : (i) Presence of four nitrogen donor sites confined to a single four-fold or a slightly distorted four-fold plane in a ringed structure, (ii) Extensive conjugation of the ligand appropriate for metal-ligand bonding. Thesemacrocyclic complexes performvital functions in various biological proo^ses. The significant role played by metal ions in living systems depends on their confinement in anapproximately planar, tetradentate, totally enclosed framework. Synthetic macrocycles have, in principle, the potential to work as selective and sensitive extractants for soft and heavy metal ions (Ag\ Cu2\ Cd2+ and Hg2+). These carrier ligands may be tailored to ion selectives suitable for practical applications as components in membrane electrodes has generated widespread interest in developing new ion-selective electrodes specially for alkali and alkaline earth metals. Macrocyclic compounds contain a cavity in which a cation can be encapsulated. The complex thus formed are of great analytical interest. The macrocycle can also be used to separate metal ions which are present in very low concentration from other more concentrated metal ions. During the last two decades, many macrocycles have beensynthesized which provide different donor atoms, ring sizes and ligand geometries. However, all macrocycles are not useful for preparing membrane electrodes. The membrane of a macrocycle is likely to function as a good ISE provided the macrocycle shows(i) high complexation or extraction selectivity for a particular metal cation (ii) enough (i) conformational flexibility for rapid ion-exchange(iii) high lipophilicity to remain in the membrane and (iv) moderate molecular weight to allow high mobility. Polyaza macrocycles occupy an intermediate position between crown ethers and cryptands since they are stronger cation binders than crown ethers and more flexible than cryptands. Therefore, they offer great promises in broad areas of metal sensing processes such as ion-selective electrodes (ISEs). Probably the most intriguing characteristic of macrocyclic compoundsis their ability to selectively bindcertain cations in preference to others whichmaybe 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. Inorder to havea better understanding of the effect of the macrocycle structure on the rate of acid dissociation, the kinetics of these reactions were studied in different solvents with acids at various temperatures. Inspite of the prodigious development of other physical methods, electrochemistry, electronic and absorption spectroscopy and nuclear magnetic resonance(NMR) spectroscopy remain the most widely used tools in the study of macrocyclic complexes. In the present studies an attempt has been made to investigate the above mentioned aspects to add to our existing knowledge on macrocyclic complexes. With this aim in view, the following studies on the chemistry of macrocycles has been carried out. For the sake of simplicity and clarity, the thesis has been organised as follows : The First Chapter presents a review of all the relevant work reported in the literature forming a part of introduction on the subject. Special attention has been given to the use and role of macrocyclic compounds. The basic principles, experimental details, reagents and materials are described in the Second Chapter of the thesis. Methodology employed in the preparation of (ii) membranes, measurement of electrode potentials, determination of selectivity and other functional properties of the membrane using the macrocycles is also presented in the same chapter. The Third Chapter of the thesis deals with the synthesis and characterisation of methyl and phenyl substituted tetra and hexaaza macrocycles with different ring sizes and varying degree of unsaturation. Metal-template and non-template methods have been used to synthesise the macrocycles. For characterisation, use of different techniques like electronic and absorption spectroscopy, infrared spectroscopy, nuclear magnetic resonance spectroscopy and elemental analysis has been made. Detailed kinetic studies of the aciddissociation reactions of Co(II) and Ni(II) complexes of a tetraaza cyclotetradentate macrocycle has been included in the Fourth Chapter. Both the complexes undergo rapid dissociation in acidic solutions. A plot of kobs vs hydrogen ion concentration shows good first-order dependence. The acid-dissociated path is consistent with the following kinetic scheme. [ML]2+ + H+ _Jl_> [MLH]3+ (MLH]3+ _Jl_> M2+ + fLHl2+ A general mechanism proposed for the acid-dissociation of linear polyamine complexes may be adopted for these complexes. The dissociation mechanism involves some angular expansion of the bond angles in the chelate rings. This is followed by a cleavage of the metal nitrogen bond, which is assumed to be rate determining. Breaking of subsequent metal nitrogen bonds resulting in complete dissociation is a very rapid process. Alarge negative values of entropysuggests formation of an"activated" intermediate species. Relevant activation parametershas also been calculated. Analysis of product was done on the basis of comparison of spectral data and Rf values which shows that the product is the protonated ligand. In view of the importance of ISEs in diverse fields, extensive efforts have been initiated during the last two decades to develop electrodes for various ions and a number (iii) of ISEs are now commercially available mainly for H\ NH4f, alkali and alkaline earth metals. These electrodes consists of a solid and liquid membrane which when placed across the two appropriate solutions, allow the transport of a particular ionic species. As a result of the restricted transport of the ions, a potential difference across the membrane is developed which is used for the determination of ionic concentration. Various materials tried or are being tried for this purpose include inorganic and organic ion exchangers, metal chelates, crown ethers, cryptands, calixarenes etc.. With the availability of highly selective materials, the possibility ofdeveloping ion-selective electrode systems has opened up. 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. A PVC based membrane of methyl substituted tetraaza fourteen membered macrocycle have been explored for Ni2+ ions. These electrodes functions well over a wide concentration range (106 M) within a Nernstian slope. It also exhibits a fast response time and good selectivity over a number of cations. Ag+ and Zn2+ selective electrode of hexamethyl substituted macrocycle have been fabricated using polystyrene as binder. All the membrane sensors developed in the laboratory have also been used in partially non-aqueous medium and also as indicator electrode in the potentiometric titration of determinand ions. Practical utility of the membranes have also then judged in presence of various surfactants.