SYNTHESIS AND ANALYTICAL APPLICATIONS OF SOME POLYDENTATE MACROCYCLES AND THEIR COMPLEXES

Abstract

The field ofcoordination chemistry of macrocyclic compounds has undergone a spectacular growth due to the synthesis of a great number and variety of synthetic macrocycles that behave as coordinating ligands for metal ions. Macrocyclic compounds with a wide range of ring sizes have the ability to accommodate metal ions of suitable size and form inclusion complexes. The coordination chemistry of metals reveals that they possess very high affinity for macrocycles as these offer different type of donor atoms, ionic charges, coordination number and vivid geometry of the resultant complex. Macrocyclic complexes have close resemblance in their structure with some of the naturally occurring species viz. the iron-porphyrin core in haemoglobin, the cobalt-corrin of vitamin B12 and the magnesium-hydroporphyrin in chlorophyll and these may serve as models for biologically important species which contain certain metal ions in macrocyclic ligand environment. In the era of rapid industrialization and technological advances, it is imperative to watch keenly for the contamination of the environment and changes in its vital composition from toxic metal wastes emanating out of industries. Even the various metabolic disorders are accompanied by alterations in the concentration of one or more trace elements in the body. The strong and selective interaction of macrocycles with specific metal ions make these ligands suitable candidates for use as reagents in separating metal ions present in low concentration from other more concentrated metal ions and also in the separation of metal ions which do not significantly differ in their ionic radii. Polyaza and polythia macrocycles occupy an intermediate position between crown ethers and cryptands because they are stronger cation binders than crown ethers (ii) and more flexible than cryptands. Thus, they offer great scope in the field of ionselective electrodes for metal sensing. The current work in the area of ion-selective electrodes involves design and synthesis of new ligands specific for aparticular metal ion. The properties of macrocycles viz. high complexation capability or extraction selectivity for a particular metal ion, enough conformational rigidity for rapid ion exchange, high lipophilicity to remain in the membrane and moderate molecular weight to allow high mobility, make them suitable candidates as ionophores in the construction of ion-selective electrodes. The present work is aimed to design and synthesise new polyaza/thia macrocyclic ligands and their complexes and for their use as electroactive component in the preparation of membranes for determination of various metal ions. The study on macrocycles has been carried out with the aim to add further data to our existing knowledge on macrocyclic complexes. For the sake of convenience, simplicity and clarity, the work embodied in the thesis has been organized as follows: The First Chapter of the thesis 'General Introduction and Theory' presents an up-to-date review of all the literature on the use and role of existing macrocyclic compounds and metal-selective electrodes. It gives an insight into the definition of macrocycles, methods of synthesis and reactions shown by their complexes. It also gives an introduction to the ion-selective electrodes, their principle, response mechanism, classification and delves into the terms used in the study of ion-selective electrode. The problem ofpresent research activities has also been posed in the context ofthe cited work. The Second Chapter, 'Materials &Methodology' mainly encompasses the reagents and materials, physical measurements, methodology adopted and experimental (iii) details. The methods employed in the preparation of macrocyclic ligands and complexes; preparation of polyvinyl chloride) and polystyrene based membranes, equilibration of membrane, potential measurements, determination of selectivity and other functional properties of membranes prepared using the synthesized macrocyclic ligands have been described in the aforesaid chapter. The Third Chapter of the thesis 'Syntheses & Characterization' deals with synthesis and characterization ofmacrocyclic ligands to be used as membrane materials in the preparation of ion-selective electrodes and macrocyclic complexes with some transition metal ions. The synthesized ligandsand complexes are of different ring sizes and varying degree of unsaturation. Firstly, the syntheses of macrocyclic ligands: 6,7:14,15-dibenzo-5, 8,13,16-tetraoxo-1,4,9,12-tetrathiacyclohexadeca-6,14-diene(Lj); 7,8:16,17-dibenzo-6,9,15,18-tetraoxo-1,5,10,14-tetrathiacyclooctadeca-7,16-diene(L2); 5,7:12,14-dibenzo-2,3,9,10-tetraoxo-1,4,8,11 -tetraazacyclotetradeca-6,13-diene(L3); 6, 8:14,16-dibenzo-4,12-dimethyl-2,10-diphenyl-1,5,9,13-tetraazacyclohexadeca-1,4,7,9, 12,15-hexaene(L4);8,16-dimethyl-6,14-diphenyl-2,3,4:10,11,12-dipyridine-1,3,5,9,11, 13-hexaazacyclohexadeca-3,5,8,ll,13,16-hexaene (L5) and 5,6:11,12-dibenzo-2,3,8,9- tetraphenyl-l,4,7,10-tetraazacyclododeca-l,3,5,7,9,ll-hexaene (L6) are given. These macrocyclic ligands have been used as ionophore for ion-selective membrane electrodes in a subsequent chapter. The description of synthesis and characterization of macrocyclic complexes has also been included. The present work reports five series of macrocyclic complexes. The first macrocyclic series is based on a sixteen membered hexadentate macrocyclic ligand 2,3,4:9,10,1 l-dipyridine-l,3,4,8,10,12-hexaaza cyclotetradeca-3,10-diene with Cu(II), Co(II), Ni(II) metal ions; the second series is based on a tetradentate macrocyclic ligand 5,ll-dimethyl-2,3,8,9-tetraoxo-l,4,7,10- (iv) tetraazacyclododecane with Ni(II), Cr(III), Mn(II), Cu(II) metal ions; the third series is based on macrocyclic ligand L3 with Co(II), Ni(II), Mn(II), Cr(III), Zn(II) metal ions; the fourth series is based on ligand L4 with Cr(III), Cu(II), Ni(II), Mn(II), Zn(II) metal ions and the fifth series is based on hexaazamacrocycle L5 with Co(II), Ni(II), Cu(II), Mn(II) metal ions. The ligands and the complexes have been characterized by elemental analysis, molar conductance, magnetic susceptibility measurements, *H NMR, IR, UV-Vis. and mass spectral studies. The synthesized ligands have been used as electroactive component in an inert matrix viz. polyvinyl chloride) or polystyrene, for the preparation of membrane to be used as ion sensorfor T1(I), Ag(I), Sr(II), Zn(II), La(III) and Ni(II) ionsand the results are given in the Fourth Chapter of the thesis. Membrane of Li in PVC matrix was successfully used in the construction of Tl(I)-selective electrode which exhibited a working concentration range of 2.2 x 10"6 - 1.0 x 10"1 Mwith a slope of 58.2 ± 0.1 mV/decade ofactivity and a detection limit of 1.5 x 10"6MT1(I) ions; membrane ofL2 was used in the construction of polystyrene based Ag(I)-selective electrode which gave a working concentration range of 1.2 x 10"6 - 1.0 x 10"1 Mand a Nernstian slope of 58.4 ± 0.1 mV per decade ofactivity with a detection limit of 1.0 x 10"6MAg(I) ions; membrane of L3 in PVC matrix gave the best response to Sr(II) ions in the concentration range of3.9 x 10"6-1.0 x 10"1Mwith a slope of29.0 ±0.1 mV/decade of activity and a detection limit of 2.8 x lO^M Sr(II) ions; PVC based membrane of L4 responded well to Zn(II) ions over the concentration range 2.8 x 10"6-1.0 x 10"1 Mwith a Nernstian slope of 28.5 ± 0.2mV/decade of activity and a detection limit of 2.2 x 10"6 MZn(II) ions; membrane of L5 in PVC matrix was used in the preparation of La(III)- selective membrane electrode that exhibited a Nernstian response to La(III) ion in the concentration range 7.9 x 10"7 - 1.0 x 10"1 Mwith a slope of 19.8 ± 0.2 mV/decade of activity and a detection limit of5.6 x 10"7 M; electrochemical sensor for Ni(II) ions was prepared using L6 as electroactive component in PVC matrix. The Ni(II)-selective electrode exhibited a liner working concentration range of3.9 x 10"6-1.0 x 10"1 Mwith a Nernstian slope of 29.5 ± 0.2 mV perdecade of activity and a detection limit of 2.9 x 10"6 - 1.0 x 10*1 M. The detailed study of their working concentration range, slope, response time, lifetime, working pH range, performance in non-aqueous media and determination of selectivity over various other ions have been reported in this chapter. All the membrane sensors developed have been used as indicator electrodes in the potentiometric titration of respective metal ions. The sensors for T1(I) and Sr(II) were checked for their performance in the presence of surfactants whereas the practical utility of the La(III), Ni(II), Zn(II) and Ag(I) sensors has beentestedby carrying out the determination ofthese metal ions in real samples.