INTERACTION OF CARBAMATES WITH METAL-IONS AND METAL-LIGAND COMPLEXES

Abstract

Although carbamic acid (NH2COOH) itself is unstable in acidic media and decomposes rapidly to its constituents, viz. ammonia and carbon dioxide, it provides the basis of a useful and systematic naming system for a group of compounds i.e., carbamates. The salts and esters of N-substituted carbamic acid are called N-substituted carbamates. They are comparatively much more stable and are used as effective, broad spectrum peticides. The carbamates are represented by a general formula as shown below: 0 1 /\ 2 RHN OR The R1 and R2 may be any combination of alkyl or aryl groups. Depending on the nature of R1 group in the structure, three main classes of carbamate pesticides are known. (a) Carbamate insecticide; where R1 is methyl group. (b) Carbamate herbicide; where R1 is an aromatic moiety and (c) Carbamate fungicide; where R1 is a benzimidazole moiety. The carbamates do exhibit a tendency of bioaccumulation, but to a lesser extent than the organophosphates. They exhibit anti-cholinesterase activity and hence are neurotoxic should they gain entry into the human body through food or water. They are routinely used in the crop protection and in controlling household and garden pests, so the identification and conversion of carbamates to harmless residues bears much importance. The reactivity of simple carbamates (ifOR2 is not a good leaving group) is more akin to amides than to esters. Various soft metal ions exhibit good affinity to complexation with such N/O donor ligands. Thus the coordination tendency of carbamates has been exploited in the present work for their detection and decomposition using transition metal-ions or transition metal-ligand complexes (MLC). Amide group of carbamate in itself does not have a good ligating site, and generation of a stronger ligating site often required use of catalytic amounts of a base. In presence of a base carbamates readily solvolyze, and therefore the most interactions have been studied in solutions and without using any acid or base. Detection methods for carbamates both have been used with and without involving metal-ligand complexes using fluorimetry and RPLC-UV methods respectively. The present study is based on nine carbamate compounds, viz. aldicarb (IUPAC name: 2-methyl-2-(methylthio)propionaldehyde-omethylcarbamoyloxime), benomyl (IUPAC name: Methyl[l-[(butylamino)carbonyl]-lHbenzimidazol- 2-yl]carbamate), carbendazim (IUPAC name: MethylN-(lHbenzoimidazol- 2-yl)carbamate), carbaryl (IUPAC name: 1-naphthyl methylcarbamate), carbofuran (IUPAC name: 2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarbamate), ethiofencarb (IUPAC name: a-ethylthio-o-tolylmethylcarbarnate), methomyl (IUPAC name: N-Methylcarbamicacid[l-(methylthio)ethylideneamino]ester), thiophanate ethyl (IUPAC name: diethyl-4,4'-(o-phenylene)bis(3-thioallophanate)) and thiophanate methyl (IUPAC name: dimethyl-4,4'-(o-phenylene)bis(3-thioallophanate)). For decomposition studies only thiophanate methyl has been used with metal ions. The thesis has been divided into seven chapters. Chapter one provides general introduction to the work for easy understanding of the subject and review of the literature. This chapter is divided into five parts, the first of which deals with pesticides, their types, necessity and uses. An introductory account of different types of pesticides is also included therein along with a survey of their environmental impact. Among the chemical pesticides currently in use, carbamates are the subject matter of this study. In the second part definition, history, chemistry, mode of action, importance and uses of carbamate pesticides have discussed. The third part discusses environmental pollution (soil, water and air), bioaccumulation in foodstuffs and soils, natural ways of decomposition of carbamate pesticides and their metabolism. Carbamates are neurotoxic and may affect non-target living organisms as well. This part therefore also deals with risk factors, WHO recommendations and toxicity limit for a number of carbamates. As they are toxic in nature, their detection and decomposition of non-desired presence is also necessary and is the main objective of this thesis. The fourth part therefore is devoted to the available methods for the detection of the carbamate pesticides, comparison of different methods of detection and methodologies of decomposition of their undesired presence. Since colorimetric method is not suitable due to low sensitivity there exists pressing need of their trace level detection. High polarity and boiling point of carbamates makes Gas Chromatographic analysis unsuitable. Therefore most of this part is devoted to HPLC/RPLC analysis, bioenzymatic detection and fluorimetric analysis. In the fifth and the last part of the introduction, the new idea of interaction with metal-ions and/or metal-ligand complex moieties have been introduced and established by general discussion about the chemistry involved. This method has the dual potential of either being used for the detection of carbamates or for their decomposition. The importance of such a study has also has been highlighted. Most carbamates cannot be conveniently synthesized under normal laboratory conditions due to their highly toxic nature. Pure products are available from Fluka and Aldrich in small quantities but at a very high cost. Thus for reasons of safety and economy purification of the commercially available carbamate pesticide formulations was undertaken. The separation and purification of five commercially available carbamate pesticides namely benomyl, carbaryl, carbofuran, methomyl and thiophanate methyl are described in chapter two. They have been separated from known and unknown impurities by extraction process and have been purified by using column chromatography. Repeated crystallization was done to obtain compounds of desired purity. The recrystallization was effected generally in a mixture of solvents. However for crystallization solvent evaporation technique (using single solvent) and vapour diffusion method (using a pair of solvents) have also been used. The purity of the compound thus obtained was checked by comparing IR, UV-vis, NMR and RPLC-UV data of isolated compounds with that of standard products purchased from Fluka-Aldrich. The quantitative percentage purity of all the isolated compounds was determined by the RPLC-UV method and by the UV-vis spectroscopic analysis. It was found that carbofuran, thiophanate methyl and methomyl were obtained in more than 98% purity while benomyl and carbaryl were obtained with more than 95% purity. The percentage purity after every step of purification was also investigated by RPLC-UV and UV-vis spectroscopic method of analysis for the development of the purification technique. In order to minimize the error of analytical procedure five or more replicates were run and final results were averaged. The optimization of analytical method and study of recovery from sprayed food products constitutes chapter three. The most suitable method of carbamate pesticide analysis as reported in literature is RPLC-UV method. The third chapter deals with the RPLC-UV method for the simultaneous determination of seven carbamates: aldicarb, benomyl, carbofuran, thiofencarb, methomyl, thiophanate ethyl and thiophanate ethyl in mixed samples. It was found that at a single wavelength of 235 nm in 3:7 CH3CN/H20 mixture, each carbamate absorbs well. Their calibration plots were obtained and gave high quality straight lines for each of above carbamates. Standard deviations in the data and the limit of detection have also been discussed. A very low detection limit was obtained in each case ranging between 5-25 ng/lOul. General equations for the calibration plot of aldicarb, benomyl, carbofuran, ethiofencarb, methomyl, thiophanate ethyl and thiophanate methyl have been obtained to a high degree of precision from the peak area ofthe eluate. The method was then used for the recovery analysis of the above mentioned seven compounds from grains, fruits and vegetable residues like Gram, wheats, lentil, fenugreek, soyabeen and apple sprayed with these pesticides and stored over 24 hours. The carbamate pesticide formulations commonly available in the Indian market are based on benomyl, carbofuran, methomyl and thiophante methyl only and hence, residue analyses of these only were carried out. Chapter four deals with the metal assisted decomposition of thiophanate methyl carbamate studied through UV-vis spectroscopy. In this context effect of different metalion concentration, temperature and pH of the medium have been studied. Copper(II) was found to catalyze to the maximum the decomposition of thiophanate methyl. Nickel(II) also catalyzed decomposition of thiophanate methyl significantly, while iron(III) and cobalt(II) required alkaline medium to effect decomposition. Decomposition products were characterized using IR, UV-vis, thermal studies and magnetic susceptibility measurements. Mechanisticaly, the metal ions were found to attach with S,0-donor sites of thiophanate methyl. The thiophanate methyl acted as a dinegative anion ligand and therefore fitted better with divalent cations. Copper(II), nickel(II) and cobalt(II) formed square planar complexes. Mangnese(II) and zinc(II) ions also decompose, but with slower rate probably due to larger sizes of the cations. Decomposition by the trivalent metal ions, viz. iron(III) and chromium(III) were extremely slow. In this context effect of simultaneous presence of acids and bases was also investigated. In acidic medium rate of decomposition were very-very slow even in presence of high metal ion concentration. Though thiophanate methyl is unstable in basic aqueous medium, high rates of decomposition was found with divalent metal ions, this to be partly due to high rate of complex formation in basic medium. The rate of decomposition was found to increase with increase in temperature. Chapter five deals with the fluorimetric estimation of six carbamates using fluorescent probe [Fe(thpme)py2]Cl, where thpme represent dianionic ligand obtained after removal of two N-H protons from thiophanate methyl. The characterization of this complex was done using physico-chemical methods. The N-H peak which were observed in free-base thiophanate methyl at 10.1 and 11.3 ppm highly deshielded and represent respectively a pair of acidic protons were found to disappear in the complex. Also there appeared a signal at 5.6 ppm due to pair of protons. This suggested disappearance oftwo N-H protons and anisotropic deshielding by disappearance of anilinic proton and breaking of C=0 bond. C=0 and C=S groups were also found to disappear as suggested by IR spectroscopic measurement. The characterization ofthe above compound was also done by UV-vis, ESR and mass spectroscopic measurement. Subsequently this compound was used as a fluorescent probe for the estimation of carbamates such as aldicarb, benomyl, carbaryl, carbofuran, ethiofencarb, and methomyl. Our probe was excited at 274 and 280 nm and emission was observed at 595 and 610 nm respectively. The emission intensity was found to increase linearly with increase in the concentration ofthe carbamates and found to satisfy Bensi- Hildebrand equation. Using this probe picogram levels of carbamates were detected in solutions. It is reported that all these interaction have appeared due to axial coordination of carbamates to the probe. The emission life time of the probe excited at 280nm alone with different carbamates showed a marked change in presence of other additional carbamates and thus was also used in their fluorimetric determinations. Chapter six deals with the fluorimetric determination of the carbamates using teraazaporphyrin (TAP) complexes taken as the probe. For the purpose here nine TAP compounds of copper(II), i.e. [Cu(OMTTAP)] (where OMTTAP = octakis(methylthio)tetraazaporphyrin) and its bi- and trimetallic derivatives have been reported. Studies on fluorimetric estimation using bimetallic complexes formed by reaction with [Ru(bpy)2Cl2].2H20 and [Ru(phen)2Cl2].2H20 have shown an increasing trend of the emission intensities in presence of carbamates like aldicarb, benomyl, carbaryl, carbofuran, ethiofencarb, and methomyl, making it a probe for their estimation. The trinuclear complexes formed by the reaction of the above written dinuclear complexes again with [Ru(bpy)2Cl2].2H20, [Ru(phen)2Cl2].2H20 and [RuCp(Pph3)Cl] were also used as fluorometric probes in the determination of carbamates. As the trinuclear complexes were separated in two different cis- and trans- isomers, both of which were used as fluorimetric probes in the quantitative estimation of carbamates. The interaction were checked for molar ratios of 10:1 to 10:600 of the probexarbamate. The Bensi- Hildebrand equation was then applied to the results wherever applicable and the equilibrium constant for the probe-carbamate interaction have been determined. The life time measurements of these probes with and without the carbamates have also been measured and an attempt is made to establish a relation as life-time probe in the fluorimetric determination of carbamates. Chapter seven summarizes the results and gives future directions to continue development of better analytical tools for carbamate pesticides.