ORGANOTIN DERIVATIVES OF SOME HETEROCYCLIC LIGANDS

Abstract

Tin is unsurpassed by any other metal in the multiplicity of applications of its organometallic compounds. The first organotin compound wasmadein 1849 but largescale industrial, synthetic andbiological applications of organotin(IV) compounds have developed since 1950. Organotin compounds are versatile catalysts and synthetic reagents used for either selective reactions or multistep syntheses. Organotin compounds have been commercially used as stabilizers for polyvinyl chloride (PVC), ship paint additives, insecticides and fungicides, and as surface-modifying agents, etc. Further, organotin(IV) derivatives of ligands with hetero (N, Sor O) donor atoms have been explored to exhibit antimicrobial activity against a wide spectrum of pathogenic bacteria and fungi. Anew dimension in this field has been added with the emergence of these compounds as potential metallopharmaceuticals among the class ofmetal-based non-platinum chemotherapeutics. Therefore, several organotin(IV) derivatives with hetero (N, S or O) donor ligands, viz. carboxylic acids, pyrazoles, oxinates and heterocyclic thioamides, etc., and with biomolecules, viz. amino acids, peptides and DNA fragments, etc., have been tested for their antitumour potential. The chemotherapeutic significance of these compounds has been explored further by studying their interaction with some clinically recommended drugs, viz. antibiotics, antibacterial drugs and sulpha drugs, etc. Recently, the organotin compounds have also been explored as single source precursors to produce nanoscale semiconducting materials, viz. tin(IV) oxide and tin sulfides, by their pyrolysis. The coordination chemistry of heterocyclic thiones is of immense interest because they can (i) mimic cysteine sulfur coordination in metalloenzymes, and (ii) show electronic and structural properties of the active sites in copper blue proteins involving S, N-coordination. Among the conjugated heterocyclic systems, mercaptothiadiazoles are important compounds because of their analytical, industrial ® and biological applications. One of the most interesting properties of mercaptothiadiazoles is the existence of their thiol and thione tautomeric forms. 1,2,4-Triazoles constitute an interesting class of ligands as they represent a hybrid of pyrazole and imidazole with regard to the arrangement of ring nitrogens. Further, they show a versatile coordinating ability, especially when the triazole nucleus is substituted withadditional donor groups. Although, triazoles have theirnon-biological origin, but their 2,4-bridging form is similar to 1,3-imidazolate bridging found in enzymes. In view of the wide range of applications of organotin(IV) compounds and the versatile coordinating behavior of heterocyclic thiones, it was considered significant to synthesize, characterize, and study the solution behavior and biological activity of new organotin(IV) derivatives of these heterocyclic ligands. Further, it is important to study their thermal behavior followed by the surface morphology determination ofthe resulted residue in order to explore the synthesized compounds as single source precursors for the preparation ofnanoscale semiconducting tin(IV) oxide and tin sulfides. In order to maintain the clarity in the presentation, the work embodied in the thesis is systematically divided into the following chapters. First chapter of the thesis presents the general introduction and an overview ofsome important applications oforganotin(IV) compounds, thiadiazoles and triazoles. A critical and comprehensive review of the available literature on the organotin(IV) compounds ofheterocyclic thionates and metal complexes ofthiadiazole- and triazolethionates with special reference to their synthetic methods, spectral characterization, solution studies andtheir biological activity, if any. In addition to this, the applications of nanosacle tin sulfides and tin(IV) oxide and important methods employed for the preparation of these nanoscale materials have also been included. Second chapter incorporates thedetails ofmake, purity and other specifications of thematerials andequipments used in the present study. Forthe spectroscopic studies, (iii) viz. multinuclear NMR and "9Sn Mossbauer of the synthesized organotin(IV) derivatives, the compounds were sent to various Institutes/University in India/Abroad. Some of the synthesized compounds were sent to L. L. R. M. Medical College, Meerut (India) for their antimicrobial screening. The specifications of the instruments and details of the procedures used to carry out these studies have been included. The data obtained for the biological studies have been compiled and discussed in Chapter 5. The details of procedures and equipments used for solution and thermal studies of the synthesized compounds have also been described. Third chapter deals with synthesis and characterization of some new di- and triorganotin(IV) derivatives of the general formula, RnSnLm (where n = 2, m = 2, R = Me, «-Bu, H-Oct and Ph; n - 3, m = 1,R = Me, n-Bu, «-Pr and Ph; HL - 5-amino-1,3,4- thiadiazole-2-thiol (HL-1)). The probablestructures of the synthesized derivatives have been discussed on the basis of UV, infrared (IR), far-infrared (far-IR), multinuclear ('H, ,3C and 119Sn) magnetic resonance and "9Sn Mossbauer spectroscopic studies. On the basis ofspectral studies, the structure proposed for P^SntL-l^ (R = Me and «-Bu) is a sifcew-trapezoidal-bipyramidal and for R2Sn(L-l)2 (R = Ph and n-Oct) is a cisoctahedral, in which the ligand acts as monoanionic bidentate coordinating through ring N and exocyclic S. However, a distorted trigonal-bipyramidal structure for RSn(L-l) (R • tj-Bu and n-?x) and a distorted tetrahedral structure for R3Sn(L-l) (R = Me and Ph) have been proposed in the solid-state. The X-ray crystallographic study of Me Sn(L-l) shows that two molecules are present in an asymmetric unit. In both the molecules, tin atom is four-coordinated with a distorted tetrahedral geometry with three Sn-C and one Sn-Sexo bond. In the crystals of Me3Sn(L-l), extensive intermolecular hydrogen bonding is present which constructs ID chains. In addition to this, intermolecular non-bonding S-S and N-N interactions are also present, which hold ID chains together. Thermogravimetric (TG), derivative thermogravimetric (DTG) and differential thermal analysis (DTA) ofthe synthesized derivatives have been carried (iv) out, which provide asimple route to prepare nanoscale SnS and Sn02 in nitrogen and air atmosphere, respectively, in the temperature range~600"700 °C. The residues obtained by thermal decomposition ofall these synthesized compounds are SnS or Sn in nitrogen and Sn02 in air, which have been characterized by IR, far-IR, powder X-ray diffraction analysis. The surface morphology of as obtained SnS and Sn02 has been studied by scanning electron microscopy (SEM). Their crystallite average size, determined by Scherrer equation using X-ray line broadening, is in the range of ~20-129 nm. Mathematical analysis of TGA data shows that the first decomposition step of Ph.SnCL-l),, n-Pr3Sn(L-l) and Ph3Sn(L-l) in both air and nitrogen follows first order kinetics for which kinetic and thermodynamic parameters, viz. E\ A, S\ 0* and H* have also been calculated using two different methods such as Horowitz and Metzger and Coats and Redfern methods. Fourth chapter of the thesis includes the synthesis and results ofspectroscopic investigations of some new di- and triorganotin(IV) compounds of general formula RSnL (n =2, m=2, R- Me, ,-Bu, n-Oct and Ph; n=3, m=1, R=Me, »-Pr and n m n-Bu and HL =3-amino-l,2,4-triazole-5-thiol (HL-2)). As good single crystals were not obtained, alarge number ofexperimental techniques, viz. UV, IR, far-IR, multinuclear ('H, 13C and U9Sn) NMR and u9Sn Mossbauer spectroscopic studies, were used to accomplish adefinitive characterization and determination of their most probable structures. In these compounds triazole acts as amonoanionic bidentate ligand, coordinating through Sexo and Nnn, Based on IR and "9Sn Mossbauer spectroscopic studies ahighly distorted cfe-trigpnal bipyramidal structure for R3Sn(L-2) and adistorted stew-trapezoidal bipyramidal structure for R2Sn(L-2)2 have been suggested. However, •H, 13C and '19SnNMR spectral studies revealed that weak bonding between tin and ring Nis further weakened in the solution leading to pseudotetrahedral/ tetrahedral structure. TG, DTG and DTA of the synthesized derivatives (except «-Bu3Sn(L-2) and (v) n-Pr3Sn(L-2)) have also been carried out in nitrogen and air atmosphere. The residues, obtained by thermal decomposition of the synthesized compounds in nitrogen and air, have been characterized as SnS/Sn and Sn02, respectively, byIR, far-IR, X-ray powder diffraction analysis. The crystallite average size of the residues (SnS/Sn02) has been determined by Scherrer equation andfound in the range of- 6-63 nm. Theparticle size of the residues obtained bythepyrolysis of «-Bu2Sn(L-2)2 and Ph2Sn(L-2)2 in nitrogen, is determined by transmission electron microscopy (TEM) and found in the range - 2-55 nm. The surface morphology of the residues has been investigated by SEM. Kinetic and thermodynamic parameters for the decomposition of n-Bu2Sn(L-2)2 and Ph2Sn(L-2)2 in air have also been calculated and discussed. Fifth chapter enumerates the synthesis and characterization of some new diand triorganotin(IV) derivatives ofgeneral formula R„SnLm (where n=2, m- 2; R= Me, h-Bu, and Ph; n - 3, m= 1; R= Me, «-Pr, «-Bu and Ph and HL - 4-amino-3- mefhyl-l,2,4-triazole-5-thiol (HL-3); and 4-amino-3-ethyl-l,2,4-triazole-5-thiol (HL-4)). Based on IR and ll9Sn Mossbauer spectroscopic studies, a distorted skewtrapezoidal- bipyramidal and distorted trigonal-bipyramidal geometries have been proposed for R2Sn(L-3)2/R2Sn(L-4)2 and R3Sn(L-3)/ R3Sn(L-4), respectively, in the solid state, in which the ligands are coordinating through Sexo and Nring. Multinuclear ('H, l3C and 119Sn) magnetic resonance studies of these compounds have also been carried out. TG, DTG and DTA ofthe synthesized derivatives (except «-Bu3SnL and n- Pr SnL) have been carried out in nitrogen and air atmosphere, which suggest the formation ofSnS (or Sn) in nitrogen and Sn02 (or mixture ofSn02 and SnO) in air. The residues thus obtained were characterized by IR, far-IR, powder X-ray diffraction analysis. The crystallite average size ofthe residues (SnS/Sn02) determined by Scherrer equation is in the range of-12-50 nm. The particle size of the residues, obtained by the pyrolysis of/j-Bu2Sn(L-3)2 in nitrogen and n-Bu2Sn(L-4)2 in air, has been determined (vi) by TEM and found in the range - 3.0-60.0 nm. The surface morphology and the grain size of the residues obtained by the pyrolysis of «-Bu2Sn(L-3/L-4)2, Ph2Sn(L-3)2 and RSn(L-3/L-4) (R=Me and Ph) in air and nitrogen have been determined by SEM. The SEM images show spherical and almost uniform boundaries of the grains with grain size in the range - 60-105 nm. In addition to this, in vitro antimicrobial screening results of HL-1, HL-2, HL-3 and HL-4 and some of their synthesized organotin(IV) derivatives together with standard drugs such as fluconazole and ciprofloxacin against the bacteria, Staphylococcus aureas and Escherichia coli, and against some fungi, viz. Aspergillus fumigatus, Candida albicans, Candida albicans (ATCC 10231), Candida krusei (G03) and Candida glabrata (H05), have also been included and discussed. Sixth chapter of the thesis incorporates the equilibrium (pH-metric) studies of the interaction of Me2Sn(IV)2+ and Me3Sn(IV)+ with the ligands: 5-amino-3H-1,3,4- thiadiazole-2-thiol (HL-1), 3-amino-l,2,4-triazole-5-thiol (HL-2), 4-amino-3-R-1,2,4- triazole-5-thiol (R=CH3 (HL-3) and C2H5 (HL-4)) in aqueous solution (ionic strength =0.1MKN03, 298 ±0.1 K) performed on Orion 960 plus autotitroprocessor equipped with Orion Ross flow combined glass electrode. The stability constants of the species formed in solution have been calculated by SCOGS program using the potentiometric titration data. The concentration distribution of the various complex species in solution has been evaluated as afunction of pH. The species exist at physiological pH (7.0) are Me2SnL(OH) (- 70-85 %) and Me2Sn(0H)2 (~ 15-30 %) for dimethyltin(IV) systems, whereas Me3SnL(OH) (- 73-95 %) exists along with Me3SnL (- 8-24 %) and Me Sn(OH) (~ 1-3 %) for Me3Sn(IV)+ systems. Significant amounts of the hydroxo species are also formed at pH >8.0. The 'H, 13C and "9Sn NMR studies have also been carried out at different pH in order to characterize the possible geometry of the proposed complex species in aqueous solution.