STUDIES ON ORGANOSILICON(IV) DERIVATIVES OF AMINO ACIDS

Abstract

Silicon is the most abundant element in the earth's crust after oxy gen. It shows a variety of chemical properties and lies at the heart of much modern technology. The Si-C bonds are almost as strong as C-C bonds. The first organosilicon compound SiEt4 was synthesized by C. Friedel and J.M. Crafts in 1863, but the extensive development of the field was due to F.S. Kipping in the first decades of this century. The silicones are a group of organosilicon polymers. They have a wide variety of com mercial uses as fluids, oils, elastomers (rubbers) and resins. Silicone liq uids are used as dielectric insulating media, hydraulic oils, and compress ible fluids for liquid spring. Pure methylsilicone oils are good lubricants of light loads. Other uses are as heat transfer media in heating baths and as components in car polish, sun-tan lotion, lipstick, and other cos metic formulations. Their low surface tension leads to their extensive use as antifoams in textile dyeing, fermentation processes, and sewage disposal. Organosilicon polymers have found many uses in the electron ics industry due to their low-dielectric constant and water repelling prop erties. The organosilicon polymers have been used in lithography, microlithography, X-ray lithography, multilevel lithography and photo lithography. Amino acids, the bifunctional compounds containing both an amino and a carboxylic acid group, are the building blocks of proteins. They serve as precursors of many other important biomolecules, such as hormones, purines, pyrimidines porphyrins, and some vitamins. They play an important role in the biosynthesis of proteins and are the bio logical precursors of a number of peptides, many of which, such as the (ii) hormones oxytocin, vasopressin, and bradykinin, have intense biologi cal activity. Knowledge of specific or selective bonding of metal and organometal species to donor sites in biological structures and also in simple biologically relevant oligofunctional molecules is still incomplete. Although organosilicon compounds find many practical applications, their coordination by biological molecules is not well understood. Being potentially polydentate ligands, amino acids and N-substituted amino acids may also present interesting structural possibilities when substi tuted with di- and tri-organosilicon groups at oxygen and nitrogen since organosilicon compounds adopt higher coordination whenever favourable conditions exist. The chemistry of metal complexes of amino acids has developed not only from the inorganic point of view, but also because of possible biological interest. Schiff bases, first prepared by Schiff, are the condensation prod uct of primary amine and aldehyde or ketone. They are the most im portant nitrogen donors and are used as ligands towards a wide variety of metal ions. The chemistry of Schiff bases has attracted a consider able attention as many of these have been used as anticancer, antitumour, antitubercular and as analytical agents. Some Schiff bases possess anticataract, anthelminthic, antiinflammatory, antipyretic, anal gesic, cardiotonic and diuretic properties. Pyridoxyl (Vitamin B6 aldehyde)- amino acid Schiff bases are believed to be intermediate in biologi cally important amination processes. Furthermore, metal ions catalyse transamination reactions involving vitamin B6. These concurrent results seem to indicate that metal complexes of Schiff bases derived from vari ous amino acids are formed as intermediates in transamination reactions (iii) involving vitamin 3$. In view of this, transition metal complexes of the Schiff bases in which amino groups are provided by amino acids have received considerable attention during the last a few decades due to their possible use as potential N-pyridoxylideneamino acid system. Although organotin(IV) and organosilicon(IV) complexes of Schiff bases derived from various amines have received much attention during the last few decades, comparatively little attention has been paid to the Schiff base systems derived from amino acids. Looking into the wide applications of organosilicon compounds, and biological relevance of the amino acids and Schiff bases derived from amino acids, it was thought worthwhile to synthesize and characterize new organosilicon(IV) complexes of biologically important ligands such as amino acids, N-substituted amino acids and the Schiff bases derived from amino acids and to study their biological activities. For the sake of convenience the work embodied in the thesis is presented in the following chapters. The first chapter of the thesis 'INTRODUCTION' describes the important applications of organosilicon compounds, amino acids and Shiff bases, and a critical review of the available literature on orgnosilicon(IV) complexes of Schiff bases. Second chapter incorporates the details of make, purity and other specifications of materials and equipments used in the present study. The experimental details of antimicrobial activities of the complexes have also been included. Third chapter describes synthesis and characterization of some organosilicon(IV) complexes of the formulae Me2Si(AA)2 and R3Si(AA) where AA = monoanion of the amino acids, viz. glycine [HGly], L-leucine lL-LeuHl, L-methionine [L-MethHl, DL-ct-alanine [DL-oc-AlaH], L-va- (iv) line [L-ValH] and D-phenylalanine [D-PheH]; R = Me or Ph. The com plexes have been characterized by elemental analyses, molar conduc tance, electronic, infrared, lH NMR and 13C NMR spectral studies. A distorted trigonal-bipyramidal amino bridged structure for the derivatives RsSi(AA) and a distorted octahedral structure with trans methyl groups for Me2Si(AA)2 around silicon atom have been proposed as supported from their spectral studies. The intermolecular hydrogen bonding be tween molecules may be responsible for their associated structure and for their very low solubility in common organic solvents. Thermal stud ies of two complexes, viz. !Me3Si(L-Leu)] and [Me2Si(L-Leu)2] have also been carried, out using TG, DTG, and DTA techniques. The complexes, found soluble in DMSO, have been tested in vitro against various bac teria, viz. Escherichia coli, Pseudomonas putida - 2252, Aeromonas formicans, Staphylococcus aureus - 740 and fungi, viz. Aspergillus niger ORS-4, Aureobasidium pullulans - 1991, Verticillium dahliae - 2063 and Penicillium notatum - 1348. Fourth chapter concerns with the synthesis and characterization of some organosilicon(IV) complexes of the formulae, Me2SiL2 and R3SiL (where L = monoanion of N-benzoylamino acids, viz. hippuric acid [HL1], N-benzoyl-DL-alanine [HL2], N-benzoyl-DL-valine [HL3] and N-benzoyl-DLleucine [HL ]; R = Me or Ph).The possible structures of the complexes have been proposed on the basis of the physico-chemical studies as used in third chapter. For penta-coordinate complexes of the type RsSiL, a distorted trigonal-bipyramidal structure with unidentate carboxyl oxygen (0-C=0) and amido carbonyl oxygen (0=C-NH) from the adjacent mol ecule in trans axial positions and three oganic groups in equatorial po sitions, and for six-coordinate complexes of the type Me2Si(L)2, a dis torted octahedral geometry with mutually trans silicon-carbon bonds has (v) been tentatively proposed.Thermal studies of some complexes, viz. Me2Si(L1)2, Ph3SiL\ Me3SiL2, Ph3SiL2, Me2Si(L3)2, Ph3SiL3, Me3SiL4 and Ph3SiL have been carried out using TG, DTG, and DTA techniques. The in vitro antimicrobial acitivity of the tri- and di-methylsilicon(IV) com plexes has also been screened against bacteria and fungi as mentioned in the third chapter. Fifth chapter of the thesis deals with the synthesis and struc tural features of some new organosilicon(IV) complexes of the formu lae, Me2SiL(I) and R3SiHL(I) (where L(I) and HL(I) = di- and monoanions of the Schiff bases derived from condensation of 2-hydroxy-lnaphthaldehyde with glycine [H2L-1(I)], L-leucine [H2L-2(I)], L-methionine [H2L-3(I)1, DL-a-alanine [H2L-4(I)], L-valine [H2L-5(I)], R = Me or Ph). Trimethyl- and triphenylsilicon(IV) derivatives of Schiff bases derived from condensation of 2-methoxybenzaldehyde [HL(II)] or acetophenone [HL(III)] with glycine [HL-1(H)/HL-1(IH)], L-leucine [HL-2(II)]/HL-2(III)], L-methionine [HL-3(II)/HL-3(III)], DL-a-alanine [HL-4(II)/HL-4(III)] and Lvaline [HL-5(II)/HL-5(III)] have also been prepared and characterized. For all the five-coordinate triorganosilicon(IV) complexes of the types, R3SiHL(I), R3SiL(II) and R3SiL(III), a trigonal-bipyramidal geometry around silicon has been proposed where the donor atoms of the bidentate ligands, viz. oxygen and nitrogen, are not in axial positions due to steric properties. For the penta-coordinate complexes of the type, Me2SiL(I), a trigonal-bipyramidal structure having two cis methyl groups in the equatorial positions, two axial oxygen and one equatorial nitro gen of the tridentate ligands has been tentatively proposed. The results of antimicrobial activity of the complexes against the bacteria and fungi as mentioned in third chapter along with the strains Rhizobium meliloti - 100 and Penicillium chrysogenum - 1348 are also discussed. (vi) The organosilicon(IV) derivatives of Schiff bases derived from amino acids are found to be more active than the organosilicon(IV) de rivatives of N-benzoylamino acids which in turn are found to be more active than the organosilicon(IV) derivatives of amino acids indicating that the bigger biomolecules on coordination with organosilicon(IV) moi eties enhance the biological activity.