PHYSICO-CHEMICAL STUDIES ON ORGANOTIN(IV) COMPLEXES OF SCHIFF BASES

Abstract

Schiff bases are basically one of the most important nitrogen donors used as ligands towards a wide variety of Lewis acids and are characterized by the presence of azomethine grouping (>C = N-), a readily available coordinating site. They behave as bi-, tri-,tetra- or polydentate ligands depending upon the number of other coordinating sites available in the molecule. Schiff bases are generally derived by the condensation of an aldehyde or ketone with a primary amine eliminating a water molecule. The reaction was first introduced by cSchiff' and the products are named as Schiff bases. They are also termed as anils and ketoanils if derived from aldehydes and ketones, respectively. These ligands exhibit a tautomeric equilibrium between the keto/thione and enol/thiolo forms. The chemistry of Schiff bases has attracted a considerable attention of chemists during last two decades as many of these have been used as anticancer, antitumour, antitubercular and as analytical agents. Their uses as additives in paints and synthetic fibre industries, and as stabilizers for photographic developers and in preparing special paper for electrophotography have also been repoi«ted in the literature. The enhancement of biological activity and suppression of toxicity by carrying out the complexation of biologically active compounds with metals is the major factor for the recent growing importance of Schiff base metal complexes, since several of these have been found to show a remarkable anticancerous, antitumour, antitubercular, antipyretic, bactericidal and fungicidal activit ies. Schiff bases containing amino groups provided by amino acids and their complexes with metal ions have also received a considerable attention due to their possible use as potential N-pyridoxylideneamino acid systems. The pyridoxal ( vitamin Be aldehyde)amino acid Schiff bases are believed to behave as intermediates in biologically important amination processes. The properties of Schiff bases can be greately modified by introducing organic substituents into the ligand molecule, thereby, inducing different stereochemistries in the resultant metal complexes. Organotin chemistry has been one of the strongest areas in the interdisciplinary organometallic field during the last four decades. The first organotin compound was prepared by lFrankland' in 1849. Organotins have become a leading commercial organometallics first through their use in poly(vinyl chloride) (PVC) stabilization, as rubber antioxidants, catalysts in the formation of urethans, Ziegler catalyst systems for olefin polymerization,and as additives for varnishes,and now as biocides. Today organotins are widely used as agricultural fungicides, miticides, industrial biocides, wood preserving agents, surface disinfectants,anthelmintic and marine antifouling agents. Tin metal has two stable oxidation states, viz. tin(II) and tin (IV), with contrasting chemistries, and a wide variety of structural types, which among tin(IV) derivatives alone encompass four-, five-,six-,seven-, and eight-coordination at tin in neutral,cationic and anionic species, with intra- and intermolecular association to give dimers and higher oligomers with one-, two-, and three- dimensional lattices in the solid state. iii Looking into the wide applicabilities of Schiff base metal complexes and organotin compounds , it was thought worthwhile to synthesize and characterize new organotin(IV) complexes of biologically important Schiff base ligands particularly of those having o_, N and S donor atoms for coordination. These complexes may have a great biological importance. For the sake of convenience the work embodied in the thesis is presented in seven chapters. CHAPTER-1 deals with brief introduction of Schiff bases including their historical bdckground, basicity, keto-enol tautomerism, H-bonding and their applicabilities in various biological fields. A brief account of the acceptor behaviour of organotin halides and stereochemistries of the five- and sixcoordinated organotin complexes have also been discussed. CHAPTER-2 comprises a critical review of the available literature on metal complexes of Schiff bases particularly of those derived from semicarbazones, thiosemicarbazones, S-alkyldithiocarbazates, N-methyl-S-alkyldithiocarbazates and amino acids with special reference to their synthetic utility, structural behaviour, analytical and biological applications. Thermal studies on organotin compounds and their derivatives are also included. CHAPTER-3 incorporates the details of make, purity and other specifications of materials and equipments used in the present study. CHAPTER-4 concerns with the preparation and characteri zation of complexes of aryltin(]V) chlorides wi th various semicarbazones and thiosemicarbazones of furfuraldehyde, 2-methoxybenzaldehyde, 4-methoxybenzaldehyde, benzylmethyl ketone, benzil, 2-hydroxybenzaldehyde , 2-hydroxy-l-naphthaldehyde and 2-hydroxyacetophenone in 1:1 molar ratio.Semi- and thiosemicar bazones exhibit keto (thione) and enol (thiolo) tautomeric forms in equilibrium. The interaction of semi- and thiosemicarbazones in keto and thione forms, respectively, with triphenyltin chloride yields six-coordinated molar adducts of the general formula, Ph3SnClrL(L^semi- and thiosemicarbazones). However,the replacement reactions of diphenyltin dichloride and triphenyltin chloride with the sodium salts of semi- anrd thiosemicarbazones in the enol and thiolo forms, respectively Pled to the formation of stable non-electrolyte five-coordinated di- and triphenyltin(lV) semiand thiosemicarbazone complexes. All the reactions have been carried out under strictly anhydrous conditions. The possible structures of the complexes have been proposed on the basis of physico-chemical studies, viz. elemental analyses, molar conductance, infrared, far-infrared, ultraviolet and proton magnetic resonance spectral studies. A distorted octahedral structure has been proposed for all the six - coordinated complexes of the general formula, PhgSnCl.L, in which the ligands are bidentate and coordinating through the azomethine nitrogen and the ketonic oxygen/sulphur atoms. Whereas, the five - coordinated di- and triphenyltin(IV) semi- and thiosemicarbazone complexes have been proposed to exhibit a distorted trigonal bipyramidal geometry around the tin atom as supported from their spectral studies . The ligands having ONO/ONS donor sites are coordinated through the azomethine nitrogen, phenolic oxygen and enolic oxygen/thiolo sulphur atoms. CHAPTER-5 describes the synthesis and the results of spectroscopic investigations of organotin(IV) complexes with bidentate and tridentate thio Schiff bases having NS and ONS donor systems, derived from the condensation of S-benzyl- and N-methyl-S-benzyldithiocarbazate amines with various aldehydes and ketones, viz. furfuraldehyde, 2-methoxybenzaldehyde, 4-methoxybenzaldehyde, benzil, 2-hydroxybenzaldehyde, 2-hydroxy-l-naphthaldehyde and 2-hydroxyacetophenone. The ligands exist in the thioketo form in the solid state but in the solution they may remain as an equilibrium mixture of both,the thioketo and thiolo tautomeric forms. Stereochemistry cf these thio ligands (in thi olo form) is also quite interesting as only the (i -nitrogen coordinates to the metal atom, while the aL - nitrogen atom remains uncoordinated as in the case of the complexes of S-benzyldithiocarbazate Schiff bases. However, in N-methyl-Sbenzyldithiocarbazate Schiff bases, the substitution of hydrogen by methyl group on the ^-nitrogen makes the Schiff bases to coordinate in thioketo form only. The organotin(IV) complexes with S-benzyldithiocarbazate Schiff bases (in thiolo form) of the type, R SnL (where, n _3, n R-CH3 or CgH5;n =2,R=CgH5; L=mono- or dianion of the Schiff bases) have been synthesised by the replacement reactions of RnSnC14-n (where» n=3, R-CH3 or CgHg and n=2, R = CgHg) with sodium salts of Schiff bases in dry acetone : ethanol (1:1, V/V) mixture and characterized through various physico-chemical studies. All the complexes are moiiomeric, non-electrolytes and are stabte for 5-6 weeks. LR. spectral studies reflect that thio Schiff bases behave as bidentate (NS) cr tridentate (ONS) ligands and form rings through the thiolo sulphur, the azomethine nitrogen and the phenolic oxygen. On the basis of spectral studies, a distorted trigonal bipyramidal geometry has been proposed for R3Sn(NS) and Ph2Sn(ONS) complexes (where, RsCH"3 or CgHg , NS = bidentate Schiff vi bases and ONS = tridentate Schiff bases). S-debenzylation of S,S-dibenzyl-/?-N-(2-hydroxyphenyl)methylendithiocarbazate with diphenyltin dichloride is also studied. Non-electrolytic complexes of the type, R~SnL5 [where W R=CH3 or CgH5; L = monofunctional tridentate Schiff bases such as N-methylS-benzyl-(?-N- (2-hydroxyphenyl )methylendithiocarbazate, Nmethyl- S-benzyl-/?-N-(2-hydroxy-l-naphthyl)methylendithiocarbazate, N-methyl-S-benzyl-(?-N-(2-hydroxyphenylethylidene)dithiocarbazate] have been prepared by the interaction of sodium salts of Schiff bases with RgSnCl (where,R=CH3 or CgHg). Whereas, the adducts of the type,RnSnCl4_n.L'[ where, n=2, R, CgH5; n=3, R=CHg or CgHg and L = neutral bidentate ligands such as N-methyl- S- benzyl-/?-N- (2-furanyl/2-methoxyphenyl / 4-methoxyphenyl / benzoylphenyl) methylendithiocarbazates] have been synthesized by the interaction of RnSnCl4_n with the corresponding Schiff base. The UV, I.R. and XH NMR studies suggested a distorted octahedral geometry for all these complexes. The bidentate (NS) and tridentate (ONS) Schiff bases are coordinated through the azomethine nitrogen,the thioketo sulphur and the phenolic oxygen atoms. Synthesis and spectral studies on dibutyltin(IV) complexes of Schiff bases derived from amino acids have been presented in chapter-6. The dibutyltin(IV) complexes of the general formula, Bu2Sn(ONO) (where, ONO =dianionic tridentate Schiff bases derived from the condensation of o-hydroxybenzaldehyde, o-hydroxyacetophenone and pyruvic acid with glycine7 /^-alanine, 2-aminobutyric acid, 4-aminobutyric acid, L-valine, DL-methionine and phenylglycine) have been synthesized by the reaction of dibutyltin oxide and preformed Schiff bases in situ with simultaneous azeotropic removal of water. All these complexes are stable and cryVII stallized after prolonged standing under vacuum. A distorted trigonal bipyramidal geometry around the tin atom has been proposed for all these complexes. The ligands are coordinated through ONO donor set derived from the phenolic oxygen, the azomethine nitrogen and the carboxyl oxygen as indicated from the ultraviolet, infrared, far-infrared and proton magnetic resonance spectral studies of their metal complexes . CHAPTER-7 deals with the thermal studies of seven organotin(lV) Schiff base complexes, viz- chloro(benzoylphenyl)- semicarbazonetriphenyltin(IV),chloro(2-furany1)semicarbazonefcri - phenyltin(IV) ,(2-hydroxyphenyl >Tnethylsemicarbazonodiphenyltin (IV ), (4-methoxyphenyl)thiosemicarbazonotriphenyltin(IV), S-benzyl-^-N (2-hydroxyphenyl)methylendithiocarbazatodiphenyltin(IV), S-benzyl- /?-N-(benzoylphenyl)Tnethylendithiocarbazatotriphenyltin (IV ) and N-methyl-S-benzyl-^-N-(2-hydroxyphenylethylidene)dithiocarbazatotriphenyltin( IV) in the temperature range 27-750°c using TG, DTG and DTA techniques. The decomposition of the complexes usually takes place in more than one step, giving either Sn0o or SnS2 or a mixture of tin oxide and sulphur as an end product which has been characterized by X-ray analyses. A tentative thermal decomposition scheme for each complex has also been proposed.