PHYSICO-CHEMICAL STUDIES ON THE METAL COMPLEXES OF SOME SULPHUR CONTAINING LIGANDS

Abstract

Ever since the introduction of organic reagents for various analytical purposes, a large number of compounds have been described to this end1. But it must be admitted that comparat ively few of these are of real value. The search for new sensitive and specific or selective reagents has greatly expanded during the last two decades or so, owing to the increas ing number of new metals used in Industry and to the increasing importance of trace determinations in various spheres of every day life. It is not always indicated how new reagents are discovered, and in such instances one may conclude that it is the chance observation of the reactions of a new compound. Certainly some important discoveries have been made in this way, for example, dimethylglyoxime, and in later times, sodium tetraphenylboron. Nevertheless, some systematisation is Imperative in the develop ment of new reagents. By this means efforts could be channeled in the right direction, and there would be less effort wasted in the development of reagents which are of little value. 2 Researchers in this direction owe a great debt to Feigl who, from his great knowledge of chemical reactions, long ago brought some systematisation into the haphazard development of new reagents. He indicated that certain organic groupings have an affinity for particular ion si and studied the effect of substi tution on analytical functional groups and showed how new reagents night be developed. He also made the first successful attempt to develop a new reagent from the known properties of another. Because rhodanine is a selective precipitant for silver, he introduced a ehromophorie group into the rhodanine 3 molecule and so obtained p-dimethylaminobenzalrhodanine , a very selective colour reagent for silver which Is now used widely for the determination of trace silver. The work of Merritt and talker4 is a notable example of the development of a new reagent from simple theoretical considerations. Although 8-hydroxyquinoline is widely used, its great disadvantage is its lack of selectivity. So they concluded that a suitable increase in molecular size would hinder the reaction of aluminium because of spatial effects. Accordingly, they prepared 8-hydroxyquinaldine which was found to have the desired property; it precipitated zinc and magnesium but not aluminium. One of the most important organic reagents of recent times is sodium tetraphenylboron used for the precipitation of potassium. This compound was first prepared by »Vittig and his collaborators5 without any analytical purpose in mind, but after Its analytical properties had been noted the reagent was studied by many others6'7. Besides potassium, It precipitates also caesium and rubidium within the alkali metal group. Most Interest ing, however, are the reactions of some of its derivatives. The triphenylcyanoboron derivative precipitates only caesium in the alkali metal group8; on the other hand tetra-p-tolylboron g precipitates all the alkali metals except lithium . 3 Probably the most systematic studies were conducted on 10 «. 1.10-phenanthroline by Saiith . From an examination of the properties of various substituted derivatives and from available data on electron distribution, he was able to predict that 4i7-dlphenyl-l«l0-phenanthroline should have the most sensitive colour reaction with iron(II), as was borne out by its later synthesis* Examination of these substituted l!10-phenanthrolines also showed that iron(II) could not react when substituents were in the 2:9-posltion, but the compound so produced then became selective for copper(I), and indeed contained the same analy tical functional groups as 2.2,-biquinoline which had for sometime been known as a specific reagent for copper(I) • Two other outstanding reagents in recent times were developed by Walter and Freiser, J2-(o-hydroxyphenyl) benzoxazole and 2-(o~hydroxyphenyl) benzimidazole j. Both these compounds were prepared with the expectation that they would have interesting chelating properties! they were found to be very selective reagents for cadmium and mercury(II), respectively. Because of their high selectivity, later investigators have since attempted to develop colorimetric reagents by the introduction of chromophoric and water-soluble 14 groupings but without success • From the foregoing examples it can be seen that, to discover useful new organic reagents, physico-chemical studies can be centered around any of the following variety of approaches: 4 (1) By the Introduction of a chromophoric or watersoluble group into a selective precipitant. (11) By the study of substituted derivatives of wellknown reagents. (Ill) By the synthesis of compounds likely to have select ive reactions because of their structural configuration. (iv) By the detailed study of functional groups which have not hitherto been used for analytical purposes. Scanty details of these approaches are found in some publications on the nitrogen- or oxygen-containing Uganda * . Lately some sulphur-containing ligands like xanthates, dithiocarbamates, thioamldes, thloethers, thiols, and thioaeids etc. have been 17 studied but without much reference to the above-outlined approaches (except the work of Salter and Freiser). The present Investiga tions were carried out mainly to bring about the furtherance of these Ideas together with a view to look into the structural and analytical behaviour of the metal complexes of some important member(s) of the following series of compounds! 1. Thiazoles and Benzothlazoles, 2. Dithiocarbamates, 3. Thiocarbazldes. The compounds were chosen irrespective of the fact whether sulphur atom In their molecules takes part In coordination or not, and hence the title of the thesis. 1. Thiazoles and Benzothlazoles; Thiazoles are five-membered heterocyclic compounds contain ing sulphur and nitrogen atoms in the 1,3-positions* *foen benzene ring is fused at position 4 and 5, the compounds are termed as benzothlazoles. 5 % \ S Thlazole 5 Benzothiazole During the course of investigations the major portion of the efforts were devoted to these compounds. This was rendered necessary because of the scarcity of evidence on the bonding pattern of the complexes of these compounds. The system provides a unique opportunity to study the effect of substi tution in the ring on the basicity of the donor atom which, in turn, affects the complexing properties of the molecule. Such studies give a clue in the development of selective reagents for the determination and separation of various elements of current interest. The series also offers a system whereby an idea can be had of the relative affinity of palladium(II) for sulphur,oxygen, and nitrogen as palladium(II) gives colour reaction with mercapto-, hydroxy-, and amlno-derivatives of benzothiazole. Detailed spectrophotometric investigations have been carried out on the palladium(ll) complexes of a number of amino- and hydroxyderivatives of thiazoles and benzothlazoles. These investigations led to the development of 2-hydroxy-4-methylthiazole which provides a selective reagent for the spectrophotometric determina tion of palladium(II). This is in conformation with an earlier observation that five-membered rings containing nitrogen and sulphur and having a reactive group may result in the discovery of more selective reagents for the photometric determination of palladlum(II)18. Hitherto, 2-mercaptobenzothiazole has been reported as a 19 potential reagent for gravimetric and spectrophotometric 20 determination of a number of elements * The reagent has also 1 21 22 been described for extraction purposes * *. However, its use is limited by the susceptibility, of the reagent and the complexes it forms, to oxidation and the large number of interferences it inherits due to the presence of the reactive mercapto group. Therefore, preliminary studies began with the hydroxy- and amino- derivatives of thiazoles and benzothlazoles. On the basis of Qualitative behaviour detailed investigations were carried out on amino- derivatives for their possible use as analytical reagents for mercury(II). The results were much better than the conventional methods based primarily on the use of sulphur-coordinating reagents. Investigations began with 2-aminobensothiazole. Though the reactions of this reagent were selective in nature than those given by most of the other exist ing reagents, it was found to have a little tendency to give lower results due, probably, to the small solubility of the precipitate in alcohol (in which the reagent is soluble)* Moreover, interferences due to palladium(II) and vanadium(V) had to be masked. As a next step benzene ring was defused from the thiazole body and, instead, -phenyl and -methyl groups ware, respectively, substituted at position 4 in the thiazole body. Amongst these, 2-amlno-4-phenylthiazole was found to be waterinsoluble though It gave reactions more selective than those given by 2-amlnobenzothlazole* On the other hand, 2-amino-4- methylthlazole was found to be much better than the former two as it was water-soluble and, therefore, it gave better results with smaller number of interferences. The two reagents were also used for extraction purposes. Compared with other existing reagents, these two reagents have the general advantage of better selectivity coupled with their ability to provide a system whereby mercury(II) can be separated from elements like Ag(I) and Au(III) using extraction technique. To investigate the bonding pattern in these complexes It was intended to carry out infra-red spectral studies in conjunction with pH-netrie studies. Hydrolysis of the metal ions involved in the latter studies prevented any conclusions to be drawn from pH-metric studies* Therefore, structural aspects have been studied by using only the infra-red absorption characteris tics of these complexes. 2. Dlthlocarbamatesg In recent years large amount of work has been carried out 23*24 on the use of dithiocarbamates as analytical reagents * . Although the early work showed the properties of both mono- and di- substituted dithiocarbamates, more recent systematic studies liave shown very little interest in mono- substituted dithiocarba mates. However, mono- derivatives have shown behaviour differing from their di- analogues in their stability in acidic media and 25 their reaction with metals . These differences are a result of the ability of mono- derivatives to lose a proton from the nitrogen atom in solutions of high pfrT°. The complexing properties of dithiocarbamates are directly connected with the presence of two donor sulphur atoms, which primarily determine the nature of metals that can be bound, and the strength of the complexes formed! where R.R'sH, alkyl, aryl* To Investigate the analytical utility of mono-substituted dithiocarbamates, a typical member, viz.. potassium benzenesulphonyl dithlocarbamate was syntheslsed and put to detailed Investigations. On the basis of qualitative behaviour, its cobalt(II) complex was found suitable for many analytical purposes. By using various masking agents the compound can be used for the colorimetric determination of cobalt(H). The cobalt(II) complex can be titrated against EDTA. The complex can also be extracted into amyl acetate and amyl alcohol. However, as anticipated, the reactions of mono- substituted dithiocarbamates do not seem to be much selective than those given by di-substituted analogues. 3. Thlocarbazides: Thiocarbazide (Wg.NH.C.Nrf.NHg) is an N-amino S derivative of thiosemlcarbazide (NHg.CNH.N4g) which, in turn, S is an N-amino derivative of thiourea (NHg.Q.NHg). Both thiourea and thiosemicarbaxide react with a number of cations and anions to give reactions of analytical importance. It was, therefore, 9 conceived that a study of the analytical behaviour of various metal complexes of thlocarbazide may lead to a better analytical reagent than the former two. The study also deserves a high degree of merit after recalling the fact that thlocarbazide Is the precursor of the well-known extraction reagent, dlthlzone (CgHg.N.N.CNH.NH.CgHg). S Like thiourea and thiosemlcarbazide, thlocarbazide also reacts with a number of bivalent metal ions without yielding 26 a proton — — a fact which prevented the envisaged comparative pH-metric study of these complexes. Thiourea is mainly used as a suppressant in the extraction studies. This Is because its complexes, In general, do not extract into various solvents27. On the other hand, complexes of thiosemlcarbazide have a tendency 27 28 to extract into some organic solvents * . This is, probably, due to the presence of one more amldo group In this compound. Therefore, it was contemplated to investigate thlocarbazide, which contains two additional amldo groups than thiourea, for extraction purposes. Its complexes with a number of bivalent metal Ions were studied In this context. It was found that, in general, extraction of bivalent metal complexes of thlocarbazide does not depend upon pH. This corroborates the earlier observation of the Incapability of these complexes to yield protons . Using this reagent some separations of interest have been carried out. Notable amongst these is the mutual separation of zinc, cadmium, and mercury(II) -*— all members of the same sub-group of the periodic table.