Synthesis and characterization of cyclophosphazene based thiourethane ligands (using schiff

base and hydroxy chalcone) with transition metal complexes

Nur Iliyani Binti Mohd Ishak (21944)

Bachelor of Science with Honours

Resource Chemistry

2011

Faculty of Resource Science and Technology

I

ACKNOWLEDGEMENT

The special thank goes to my helpful supervisor, Miss Maya Asyikin Arif for her support that

she gave truly help the progression and smoothness of my Final Year Project. The cooperation is

much indeed appreciated. My grateful thanks also go to Dr. Zainab Ngaini who helped in

obtaining the 31

P NMR spectra and Dr. Tay Meng Guan who helped in synthesizing hydroxyl

chalcone.

Great deals appreciated go to all the technical staff and lab assistants for their proficient

assistance and providing the necessary laboratory facilities for analysis. Not forgetting my course

mates and postgraduates students for their patient and guidance to help me in completed the

Final Year Project.

Deepest thanks and appreciation to my parents, family, special mate of mine, and others for their

cooperation, encouragement, constructive suggestion and full of support for the project

completion from the beginning till the end. Also thanks to everyone that has been contributed by

supporting my work and helps myself during the Final Year Project progress till it is fully

completed.

Last but not least, my gratefully acknowledgement to the financial support from Unimas seed

grant.

II

DECLARATION

No portion of the work referred to in this dissertation has been submitted in support of an

application for another degree of qualification of this or any other university or institution of

higher learning. This declaration is to clarify that all of the submitted contents of this project are

original in its figure, excluding those, which have been admitted specifically in the references. I

hereby declare that this project is the work of my own excluded for the references document and

summaries that have been acknowledge.

Nur Iliyani Binti Mohd Ishak

Chemistry Department

Faculty Resource Science and Technology

Universiti Malaysia Sarawak

III

Table of Contents

Acknowledgement ........................................................................................................................... I

Declaration ...................................................................................................................................... II

Table of Contents ...........................................................................................................................III

List of Abbreviations ...................................................................................................................... V

List of Tables and Figures............................................................................................................. VI

Abstract ............................................................................................................................................1

Introduction ......................................................................................................................................2

Objectives .......................................................................................................................................3

Literature Review ............................................................................................................................4

Cyclophosphazene ........................................................................................................................4

Thiourethane.................................................................................................................................6

Chalcone .......................................................................................................................................7

Synthesis and Antimicrobial Activity of Some Chalcone Derivatives ................................9

Synthesis and Characterization of 4-hydroxy Chalcones by Aldol Condensation using

SOCl2/EtOH ........................................................................................................................10

Schiff base ..................................................................................................................................11

Application of Schiff base .................................................................................................12

Complexation of Schiff base ..............................................................................................14

Antimicrobial activity ................................................................................................................18

Materials and Method ....................................................................................................................20

Materials ............................................................................................................................20

Instrumentation ..................................................................................................................20

Synthesis of cyclophosphazene based-thiourethane ligands from hydroxy chalcone ......21

Synthesis of cyclophosphazene based-thiourethane ligands from Schiff base ..................23

IV

Synthesis of the transition metal (Cu, Pb and Zn) with cyclophosphazene-based

thiourethane ligand from hydroxy chalcone .....................................................................25

Synthesis of the transition metal (Cu, Pb and Zn) with cyclophosphazene-based

thiourethane ligand from Schiff base .................................................................................27

Antibacterial activity ..........................................................................................................29

Results and Discussion ..................................................................................................................30

Reactions of 2 with: Hydroxy Chalcones ...................................................................................30

Reactions of 3 with: Transition metal (Cu, Pb and Zn) .....................................................30

Characterization of Thiourethanes .....................................................................................31

Reactions of 2 with: Schiff bases ..............................................................................................44

Reactions of 3 with: Transition metal (Cu, Pb and Zn) .....................................................44

Characterization of Thiourethanes .....................................................................................45

Antibacterial activities ................................................................................................................58

Thiourethane ligand (hydroxy chalcone) ...............................................................................58

Thiourethane ligand (Schiff base) ..........................................................................................60

Minimal inhibition concentrations of ligands and complexes ...............................................62

Conclusion .....................................................................................................................................63

References ......................................................................................................................................64

Appendixes ...................................................................................................................................67

V

List of Abbreviations

s = singlet

m = multiplet

d = doublet

LMCT = ligand metal charge transfer

MIC = minimum inhibition concentrations

VI

List of Tables and Figures

List of Tables

Table 1: Physical data of the thiourethane ligands and their transition metal complexes and molar

conductance values for the transition metal complexes with thiourethane ligands ......................31

Table 2: Main IR data of thiourethane ligand and their complexes (cm-1

) ...................................32

Table 3: Electronic spectral data of the thiourethane ligand and their transition metal complexes

........................................................................................................................................................36

Table 4: 1H NMR data of the thiourethane ligand and their complexes (δ, ppm) ........................38

Table 5: 13

C NMR data of the thiourethane ligand and their complexes (δ, ppm) .......................40

Table 6: Physical data of the thiourethane ligands and their transition metal complexes and molar

conductance values for the transition metal complexes with thiourethane ligands .......................45

Table 7: Main IR data of thiourethane ligand and their complexes (cm-1

) ...................................46

Table 8: Electronic spectral data of the thiourethane ligand and their transition metal complexes

........................................................................................................................................................50

Table 9: 1

H NMR data of the thiourethane ligand and their complexes (δ, ppm) .........................52

Table 10: 13

C NMR data of the thiourethane ligand and their complexes (δ, ppm) .....................54

Table 11: Growth inhibition of E.coli ATCC 8399 by thiourethane ligand based on compound 3

........................................................................................................................................................58

Table 12: Growth inhibition of E.coli ATCC 8399 by thiourethane ligand based on compound 4

........................................................................................................................................................60

List of Figures

Figure 1: Structure of cyclic compound ..........................................................................................5

Figure 2: Formation of thiourethane ...............................................................................................6

Figure 3: Thiourethane derivatives .................................................................................................7

VII

Figure 4: Chalcone ..........................................................................................................................7

Figure 5: Anion delocalization of the aldehydic component ..........................................................8

Figure 6: Synthetic diagram of 4-hydroxy substituted chalcones .................................................10

Figure 7: Schiff base ......................................................................................................................11

Figure 8: Complexation of Schiff base .........................................................................................14

Figure 9: Design of the simple unsubstituted parent complexes ..................................................15

Figure 10: The thione and thiol forms of the ligands ....................................................................16

Figure 11: Preparation of 1-(substitutedphenyl)-3-(substitutedphenyl)-2-propen-1-ones ...........19

Figure 12: Formation of thiourethane derivatives based on hydroxy chalcone .............................22

Figure 13: Formation of thiourethane derivatives based on Schiff base .......................................24

Figure 14: Formation of metal bonding to thiourethane ligand based on hydroxy chalcone .......26

Figure 15: Formation of metal bonding to thiourethane ligand based on Schiff base ...................28

Figure 16: IR Spectrum of ligand and its complexes .....................................................................34

Figure 17: Electronic spectrum of compound 5 .............................................................................37

Figure 18: 1H NMR spectrum of compound 5 ...............................................................................39

Figure 19: 13

C NMR spectrum of compound 5 ..............................................................................41

Figure 20: 31

P NMR spectrum of compound 3 ..............................................................................43

Figure 21: IR spectrum of ligand and its complexes .....................................................................48

Figure 22: Electronic spectrum of compound 8 .............................................................................51

Figure 23: 1H NMR Spectrum of compound 9 ..............................................................................53

Figure 24: 13

C NMR Spectrum of compound 4 .............................................................................55

VIII

Figure 25: 31

P NMR spectrum of compound 4 ..............................................................................57

Figure 26: Graph of Growth inhibition of E.coli ATCC 8399 by thiourethane ligand based on

compound 3 (ln Nt vs. time) .........................................................................................................59

Figure 27: Graph of Growth inhibition of E.coli ATCC 8399 by thiourethane ligand based on

compound 4 (ln Nt vs. time) .........................................................................................................61

Figure 28: Graph of minimum inhibitory concentrations of ligands and its complexes determined

by extrapolating the concentration at the zero growth rate of E. coli (μ=o) ..................................62

1

ABSTRACT

Synthesis and characterization of cyclophosphazene with hydroxy chalcone and schiff base in

forming thiourethane ligand has been reported in this thesis. The ligands appear to act as

hexadentate species which has six lone pair electrons form coordinate bonds with the metal

ion forming tetrahedral or square planar structure. The ratio between metal: ligand is 3:1 for

all the transition metal complexes in this study. However the complexation of Schiff base did

not occur. Newly synthesized compounds have been fully characterized by FTIR, UV-

Visible, 1H NMR,

13C NMR and

31P NMR. From the results, it is shown that the metal

complexes exhibited better antibacterial activity compared to their ligands.

Key words: Cyclophosphazene, hydroxy chalcone, Schiff base, thiourethane, antibacterial

ABSTRAK

Sintesis dan pencirian siklofosfazin bersama hidroksi kalkon dan schiff base dalam

pembentukan ligan tiouretana dilaporkan dalam tesis ini. Ligan yang terhasil bertindak

sebagai spesies hexadentat yang mempunyai enam pasangan elektron terpencil yang

membentuk ikatan koordinat dengan ion logam peralihan untuk menghasilkan struktur

tetrahedral atau square planar. Nisbah di antara logam:ligan ialah 3:1 untuk kompleks logam

peralihan. Walaubagaimanapun, kompleks Schiff Base tidak berlaku. Sebatian baru yang

terhasil telah dicirikan menggunakan spektroskopi inframerah (FTIR),ultralembayung-boleh

Nampak (UV-Vis) dan resonans magnet nuclear 1H,

13C dan

31P. Hasil kajian menunjukkan

kompleks logam mempamerkan aktiviti antibakteria yang lebih berkesan berbanding ligan.

Kata kunci:Siklofosfazin, hidroksi kalkon, Schiff base, tiouretana, aktiviti antibakteria

2

1.0 Introduction

Transition metals represent the d block element which includes groups 3 - 12 on the

periodic table. Their d shells are in process of filling. This property of transition metals

resulted in the foundation of coordination complexes. Metal complexes or coordination

compounds structure consisting of a central metal atom, bonded to a surrounding array of

molecules or anions (Shazia et al., 2010).

Phosphazenes are cyclic or linear molecules that contain a framework of alternating

phosphorus and nitrogen atoms with two substituent groups attached to each phosphorus atom.

Although the chemistry of the cyclic phosphazenes has been concerned mainly with the

substitution by various nucleophiles at the ring phosphorus, there have been constant attempts

to explore the organometallic chemistry of phosphazenes. Phosphazenes possess skeletal

nitrogen atoms as a potential donating site for transition metals (Yangha et al., 1999).

Thiourethane and thiourea linkages are characterized by hindered rotation about the C-

N bonds caused by resonance effects (Allcock et al., 1991).

Chalcones are precursor compounds for flavonoids biosynthesis in plants, and they can

also be synthesized in laboratory. Chalcones possess a broad spectrum of biological activities

including antioxidative, antibacterial, anthelmintic, amoebicidal, antiulcer, antiviral,

insecticidal, antiprotozoal, anticancer, cytotoxic and immunosuppressive (Jamal et al., 2009).

Schiff bases are aldehyde- or ketone-like compounds in which the carbonyl group is

replaced by an imine or azomethine group. The imine group present in such compounds has

been shown to be critical to their biological activities. Schiff bases are some of the most

widely used organic compounds (Cleiton, et al., 2011).

3

The main objectives of this research are:

1. To synthesize cyclophosphazene-based thiourethane ligands.

2. To synthesize several transition metal complexes with cyclophosphazene thiourethane

ligands.

3. To characterize cyclophosphazene thiourethane ligands and their complexes by UV-

visible, FTIR, 1H NMR,

13C NMR and

31P NMR spectroscopy.

4. To study coordination behavior of cyclophosphazene-based thiourethane ligands.

5. To evaluate the antibacterial activity of the synthesized compounds.

4

2.0 Literature review

2.1 Cyclophosphazene

Cyclic phosphazenes are a unique class of inorganic–organic compound that offer a

high degree of tailor ability in synthesis procedures. With the proper selection of chemical

structure and substituents, it is possible to prepare thermally and hydrolytically stable

compounds, including fluids with low pour points and good oxidative and thermal stability.

Most of the research and development on cyclic phosphazenes has centered on the alkoxy- and

aryloxycyclophosphazenes as lubricants, or as additives for lubricating oils and greases (Liu,

2004).

Cyclophosphazenes have been considered for use as hydraulic fluids and lubricants,

since 1960s. Most of the synthetic development of this class has involved fluoroalkoxyl and

phenol pendant groups on cyclotrimers and cyclotetramers. These compounds exhibited high

spontaneous temperatures and high thermal and chemical stability as well as very low vapor

pressure. Thus, they could be potential candidates as high temperature lubricants (Weimin et

al., 2002).

Cyclophosphazenes are inorganic heterocyclic rings. These are among the oldest

inorganic rings that have been studied. Cyclophosphazenes are made up of a valence

unsaturated skeleton containing the [N=PR2] repeat unit. The ring is composed of alternate

phosphorus and nitrogen atoms. Within these rings the phosphorus center is pentavalent and

tetracoordinate, while the nitrogen is trivalent and dicoordinate. Thus, each phosphorus is

connected to two adjacent ring nitrogen atoms as well as two exocyclic substituents. In

contrast, each nitrogen atom is attached to two adjacent phosphorus atoms. The nitrogen atom

5

does not have any exocyclic substituents. The smallest ring that is possible is a four-membered

ring and higher rings with an increment of two atoms (phosphorus and nitrogen) are possible.

Indeed, cyclophosphazenes form a regular and homologous series (Gleria and Jaeger, 2004).

Cyclophosphazenes have many advantages as useful luminescent materials for

electroluminescent devices. For example, substituted cyclic phosphazenes are very stable and

do not breakdown under very aggressive chemical conditions. Also the functional groups are

projected above and below the cyclophosphazene plane thus producing a rigid spherical core

from which to grow the dendrons of interest. Hence there has recently been considerable

interest in fluorescent compounds based on cyclic phosphazene cores or cyclolinear polymers

with the cyclotriphosphazene units for the development of electroluminescent devices. In

phosphazene chemistry, there are many examples of organic or inorganic side group bearing

cyclophosphazenes or polyphosphazenes. However, there are limited examples of

cyclophosphazenes and polyphosphazenes or organic polymers that bear organic group

substituted cyclophosphazene ring as side group (Bunyemin et al., 2011).

R =

PhO

PhO

PhO

OPh

OPh

Figure 1: Structure of cyclic compound

6

2.2 Thiourethane

Isothiocyanates (R-NCS) have the ability to undergo addition reactions with alcohols

and amines to form thiourethanes and thioureas. The present invention relates to a

thiourethane compound which is used for various coating materials, UV-curing or

thermosetting coating materials, molding materials, adhesives, inks, resists, optical materials,

photo-shaping materials, printing board materials, resist materials and etc. They are also

indicated that thiourethane or thiourea derivatives are formed when react with alcohols or

amines, although these species were not well-characterized (Allcock et al., 1991).

R

R'OH

R OR'

Figure 2: Formation of thiourethane

Thiourethane compounds, compared with thiokol materials, exhibit enhanced strength,

elasticity and adhesion. They have good low temperature properties and are resistant to

corrosive media. They can be prepared from a wide range of initial components by means of

diverse curing procedures (A.D. Elchueva et al., 2000). Thiourethane formation in compounds

based on polyoxyalkylenethiols and polyoxyalkylene isocyanates in the presence of catalysts

were considered (Elchueva et al., 2002).

Both catalyzed and uncatalyzed thiourethane formation reactions are described by a

second-order kinetic equation respect to concentrations of thiol and isocyanates groups. The

efficiency of thiourethane cross-linking is affected by the structure of oligothiol (Elchueva et

al., 2002).

7

Besides that, thiourethane or thiourea derivatives are formed when cyclic phosphazene

react with alcohols and amines, although these species were not well-characterized (Allcock et

al., 1991).

NHCSOR

NHCSOR

NHCSOR

ROSCHN

ROSCHN

ROSCHN

Figure 3: Thiourethane derivatives

2.3 Chalcone

Chalcones are 1, 3-diphenyl-2-propene-1-one, in which two aromatic rings are linked

by a three carbon α, β- unsaturated carbonyl system as:

Figure 4: Chalcone

These are abundant in edible plants and are considered to be precursors of flavonoids

and isoflavonoids. Chalcones possess conjugated double bonds and a completely delocalized π

electron system on both benzene rings. Molecules possessing such system have relatively low

redox potentials and have a greater probability of undergoing electron transfer reactions (Patil

et al., 2009).

8

Chalcones are products of condensation of simple or substituted aromatic with simple

or substituted acetophenones in presence of alkali. Chalcones constitute an important group of

natural products and some of them possess a wide range of biological activities such as

antimicrobial, anticancer, antitubercular, and antiviral. Chalcones represent an important class

of natural compounds with a variety of biological activities. Recent studies on biological

evaluation of chalcones revealed some to be antibacterial, antifungal, anticancer, anti-

inflammatory, antitubercular, anti hyperglycemic and anti malarial agents (Jayapal and

Sreedhar, 2010).

Chalcones are very common in natural products chemistry. Some derivatives are used

as sweeteners, drugs and sunscreen agents. Chalcones are also well-known intermediates in

the synthesis of various heterocyclic compounds (Jayapal and Sreedhar, 2010).

The reaction of 4-hydroxy acetophenones with different aromatic aldehydes to form

chalcones (1-3), and the success of the entire various synthesized compound were assigned on

for basis of elemental analysis, IR and mass spectral data. The presence of hydroxyl

substituents in the aromatic aldehyde hinders the basic catalyze aldol reaction. The reaction

behind that is the fact that the basic catalysts decrease the activity the aldehyde component

because of delocalization of the anion (Figure 5). It is necessary to use protective group for the

preparation of the hydroxyl chalcones under basic conditions. SOCl2 is used as a convenient

alternative to the gaseous HCl in the aldol condensation (Jayapal and Sreedhar, 2010).

HO

Figure 5: Anion delocalization of the aldehydic component

9

Chalcones, originally isolated from natural plant sources, considered as precursors of

flavonoids and isoflavonoids are abundant in edible plants. Chemically, chalcones are open-

chain flavonoids in which two aromatic rings are joined by a three carbon, -unsaturated

carbonyl system (1, 3-diphenyl-2- propen-1-ones). Being a majority subgroup of the flavonoid

family, chalcones have been reported to have a variety of biological activities, including

antiviral and anticancer, anti-microbial, anti-inflammatory, anti-ulcer and spasmolytic and

antiproliferative activities. Hence, chalcones is considered as a class with important

therapeutic potentials (Than et al., 2008).

2.4 Synthesis and Antimicrobial Activity of Some Chalcone Derivatives

In an effort to develop antimicrobial agents, a series of chalcones were prepared by

Claisen-Schmidt condensation of appropriate acetophenones with appropriate aromatic

aldehydes in the presence of aqueous solution of potassium hydroxide and ethanol at room

temperature. The synthesized compounds were characterized by means of their IR, 1H-NMR

spectral data and elemental analysis. All the compounds were tested for their antibacterial and

antifungal activities by the cup plate method (Prasad et al., 2007).

Compounds with electron releasing groups such as methoxy and hydroxyl showed

better antibacterial activity than the others not having such groups. Compounds having

pharmacophores such as, chloro, dichloro and fluoro groups have exhibited more antifungal

activity on all the three fungi than the others. These results suggest that the chalcone

derivatives have excellent scope for further development as commercial antimicrobial agents.

Further experiments were needed to elucidate their mechanism of action (Prasad et al., 2007).

10

2.5 Synthesis and Characterization of 4-hydroxy Chalcones by Aldol Condensation

using SOCl2/EtOH

It is a simple and effective method for the synthesis of chalcones by an acid catalyzed

aldol reaction. SOCl2 is used as a convenient alternative to the gaseous HCl in aldol

condensation. The HCl is generated in situ by the reaction of SOCl2 with absolute ethanol.

Chalcones are obtained in good to good to excellent yields. The purpose was to synthesize a

series of chalcones, starting from benzaldehyde and acetophenone or their substituted

derivatives using SOCl2/ Et OH as a catalyst. The synthesized chalcone derivatives were

undergone physicochemical characterization (Jayapal and Sreedhar, 2010).

a. R= 2-Chlorob. R= 4-Chloroc. R= 3-nitro

HOEtoH.r.t.

HO CHO

CH3 SOCl2

Figure 6: Synthetic diagram of 4-hydroxy substituted chalcones

11

2.6 Schiff base

A Schiff's base (azomethine or imine), named after Hugo Schiff is a functional group

with the general formula of R1R2C=N-R3. These bases can be synthesized from an aromatic

amine and a carbonyl compound (e.g. aldehydes, ketones) by nucleophilic addition, forming a

hemiaminal, and followed by a dehydration to generate an imine. An imine (Schiff's base)

particularly binds metal ions via the two donor atoms N and O. The steric and electronic effect

around the metal core can be finely tuned by an appropriate selection of electron withdrawing

or electron donating substituents, incorporated into the Schiff's bases (Mohammad et al.,

2008).

R'

Figure 7: Schiff base

Schiff base ligands have been in the research for over 150 years. Their instant and

enduring popularity undoubtedly stems from the ease with which they can be synthesized,

their puzzling versatility and their wide ranging complexing ability once formed (Shouvik et

al., 2009).

The condensation of an amine with an aldehyde or ketone, forming what is called a

Schiff base, is one of the oldest reactions in chemistry. Schiff base ligands coordinate to a

metal through the imine nitrogen and another group, usually oxygen, situated on the original

carbonyl compound (Prasanta et al., 2011).

12

When a diamine is combined with one equivalent of a β-diketone, a tetradentate Schiff

base is formed and when this tetradentate Schiff bases are made tetradentate unsymmetrical

Schiff base ligands and have four donor sites (Prasanta et al., 2011).

This makes the Schiff base ligands ideal for the equatorial coordination of transition

metals, leaving the two axial sites open for auxiliary ligands. They are very much like

porphyrins in this regard, but unlike porphyrins, these ligands are easy to prepare and

inexpensive (Prasanta et al., 2011).

2.7 Application of Schiff base

Schiff bases derived from aromatic amines and aromatic aldehydes have a wide variety

of applications in many fields such as biological, inorganic and analytical chemistry

(Mohamed et al., 2007). Application of many new analytical devices requires the presence of

organic reagents as essential compounds of the measuring system. They are used in optical

and electrochemical sensors, as well as in various chromatographic methods, to enable

detection of enhance selectivity and sensitivity (Mohamed et al., 2007).

Among the organic reagents actually used, Schiff bases possess excellent

characteristics, structural similarities with natural biological substances, relatively simple

preparation procedures and the synthetic flexibility that enables design of suitable structural

properties. Schiff bases are widely applicable in analytical determination, using reactions of

condensation of primary amines and carbonyl compounds in which the azomethine bond is

formed (determination of compounds with an amino or carbonyl group) and using complex

formation reactions (determination of amines, carbonyl compounds and metal ions (Mohamed

et al., 2007).

13

They are not only played a seminal role in the development of modern co-ordination

chemistry, but they can also be found at key points in the development of inorganic

biochemistry, catalysis, medical imaging, optical materials and thin films (Shouvik et al.,

2009).

Schiff base complexes have been amongst the most widely studied coordination

compounds in the past few years, since they are becoming increasingly important as

biochemical, analytical and antimicrobial reagents. It has been shown that Schiff base

complexes derived from 4-hydroxysalicylaldehyde and amines have strong anticancer activity,

e.g., against Ehrlich ascites carcinoma (EAC). It is well known that some drugs have greater

activity when administered as metal complexes than as free organic compounds. A large

number of reports are available on the chemistry and the activities of transition metal

complexes containing O, N and S, N donor atoms. The transition metal complexes having

oxygen and nitrogen donor Schiff bases possess unusual configuration, structural lability and

are sensitive to molecular environment (Aysegul et al., 2005).

Metal complexes play an essential role in agriculture, pharmaceutical and industrial

chemistry. Ligand, a metal surrounded by a cluster of ions or molecule, is used for preparation

of complex compounds named as Schiff bases which are condensation products of primary

amines and aldehydes or ketones. The uses of Schiff bases and their metal complexes as

catalyst, in various biological systems, polymers and dyes, besides some uses as antifertility,

enzymatic agents, in birth control, food packages and O2 detector (Shalin et al., 2008).

In the area of bioinorganic chemistry, interest in Schiff base complexes has centered on

the role of such complexes in providing synthetic models for the metal-containing sites in

metallo-proteins and metallo-enzymes. Indole and its derivatives are widely used in making

perfumes, dyes, agrochemicals and medicines (Josephus et al., 2010).

14

Schiff bases are used as pigments and dyes, catalysts, intermediates in organic

synthesis, and as polymer stabilisers. Schiff bases have also been shown to exhibit a broad

range of biological activities, including antifungal, antibacterial, antimalarial, antiproliferative,

anti-inflammatory, antiviral, and antipyretic properties (Cleiton et al., 2011).

2.8 Complexation of Schiff base

Transition metal complexes of Schiff bases derived from S-alkyl esters of

dithiocarbazic acid have generated considerable interest because of their interesting physico-

chemical properties and potentially useful biological activities. The methylpyruvate Schiff

base of S methyldithiocarbazate has recently been screened for its anticancer activity against a

variety of human tumor cell lines. The Schiff base has been found to exhibit promising activity

against leukaemia cells. In view of its potential biological activity and as part of our

continuing study of metal–dithiocarbazates, the preparation and X-ray crystal structure

determination of the bis-ligand nickel(II) complex of the methylpyruvate Schiff bases of S-

methyldithiocarbazate and the preparation and characterization of two new copper(II)

complexes of this ligand, together the X-ray crystal and molecular structures of the ligand and

its chlorocopper(II) complex have been reported (Mohammad et al., 2001).

H3CCH3

Figure 8: Complexation of schiff base

15

Transition metal compounds containing Schiff base ligands have also been of great

interest for many years. These compounds play an important role in the coordination

chemistry related to catalysis and enzymatic reactions, magnetism and molecular architectures.

Effects of transition metal ions acting as Lewis acid catalysts on organic reactions have

been extensively investigated. Although imines are labile and readily hydrolyzed, metal ions

stabilize imine bonds present on macrocyclic rings. The formation of a mono or di-Schiff base

complex is directed by the counter anion present in the system. A method to build mono- or

di-Schiff base complexes of nickel(II) exploiting the coordinating ability of the counter

anions. The bis-complex of the nickel(II) with the terdentate ligand is the sole product when

the counter anions practically do not have any coordinating ability, whereas ligands with good

coordinating ability such as SCN-favours the formation of nickel(II) complex with tetradentate

ligand with two trans axial positions coordinated by the thiocyanate ligands (Shouvik et al.,

2009).

Schiff base complexes have remained an important and popular area of research due to

their simple synthesis, versatility and diverse range of applications. However, it is

undoubtedly the range of donor sets (e.g., N2D2, where D = O, N, S) and the coordination

geometries afforded to metal centres by these multidentate species which has sustained their

widespread interest over the past 150 years (Michelle et al., 2004).

M = Ni, Cu, ZnD = O, N, S

D

M

D

Figure 9: Design of the simple unsubstituted parent complexes.