CHAPTER 1

Introduction

Chapter-1

1

INTRODUCTION

Schiff bases continue to occupy an important position as ligand in metal coordination

chemistry, even almost a century since their discovery [1–7]. Schiff bases have played

a key role as chelating ligands in main group and transition metal coordination

chemistry due to their ease of synthesis, stability under a variety of oxidative and

reductive conditions and their structural versatility associated with their diverse

applications [8–13]. Their instant and enduring popularity undoubtedly stems from

the ease of their synthesis, puzzling versatility and wide ranging complexing ability

once formed. The literature clearly shows that the study of this diverse ligand system

is linked with many of the key advances made in inorganic chemistry. The schiff

bases played a seminal role not only in the development of modern coordination

chemistry but also displayed important roles in the development of inorganic

biochemistry, catalysis, medical imaging, optical materials and thin films. Transition

metal compounds containing Schiff base ligands have also been of great interest for

many years [14]. In Schiff base metal complexes the environment at the coordination

center can be modified by attaching different substituents to the ligand which

provides a useful range of steric and electronic properties essential for the fine-tuning

of structure and reactivity [15]. Schiff base complexes of transition metals have been

used as drugs and they possess a wide variety of biological activity against bacteria,

fungi and certain types of tumors [16–22].

It is difficult to cover in this chapter the literature on Schiff base metal complexes

which embraces very wide and diversified subjects comprising vast areas of

organometallic compounds and various aspects of bioinorganic chemistry. Therefore,

Chapter-1

2

the introduction part is limited to a brief discussion on the Schiff bases, their metal

complexes and general applications of Schiff base complexes.

Schiff bases named after Hugo Schiff [23] are formed when any primary amine reacts

with an aldehyde or a ketone under specific conditions. Structurally, a Schiff base

(also known as imine or azomethine) is a nitrogen analogue of an aldehyde or ketone

in which the carbonyl group (-C=O) has been replaced by an imine or azomethine

group.

The common structural feature of these compounds is the azomethine group with a

general formula, RHC=N-R, (R and R are alkyl, aryl, cyclo alkyl or heterocyclic

groups) which may be variously substituted. Presence of a lone pair of electron in an

sp2 hybridised orbital of nitrogen atom of the azomethine group is of considerable

chemical importance and impart excellent chelating ability especially when used in

combination with one or more donor atom(s) close to the azomethine group.

Examples of a few compounds are given in Fig.1. This chelating ability of the Schiff

bases combined with the ease of preparation and flexibility in varying the chemical

environment about the C=N group makes it an interesting ligand in coordination

chemistry.

Chapter-1

3

NH

N

R HNN

HN

H

NH

H

HN

N

H

H2N

1 2

3

Fig.1

Chapter-1

4

There are several reaction pathways to synthesize Schiff bases [9, 24-27]. The most

common is an acid catalyzed condensation reaction of amine and aldehyde or ketone

under refluxing conditions. In this case, the first step is the attack of nucleophilic

nitrogen atom of amine on the carbonyl carbon, resulting in a normally unstable

carbinolamine intermediate which undergoes dehydration to form carbon-nitrogen

double bond called as imine and often referred to as Schiff bases (Scheme 1) [28,

29]. All steps involved in this reaction sequence are reversible. Therefore, Schiff base

condensation under thermodynamically controlled conditions can be used for

generating dynamic combinatorial libraries if several different amines or carbonyl

compounds are used as starting compounds simultaneously [30].

NH2 + H+C O + N+ OH

H

H

-H+N

H

OH

N

H

OH+H+

N

H

O+

H

H-H2O

+H2ON

H +

N-H+

Scheme 1

Out of many factors affecting the condensation reaction, the pH of the solution plays

an important role, as amine is basic, it is mostly protonated in acidic conditions and

thus cannot function as a nucleophile and the reaction cannot proceed. Furthermore, in

very basic reaction conditions, the reaction is hindered as sufficient protons are not

available to catalyze the elimination of the carbinolamine hydroxyl group [31].

Chapter-1

5

Metal complexes of the Schiff bases are generally prepared by treating metal salts

with Schiff base ligands under suitable experimental conditions. However, for some

catalytic application the Schiff base metal complexes are prepared in situ in the

reaction system. Cozzi in his review has outlined five synthetic routes that are

commonly employed for the preparation of Schiff base metal complexes as depicted

in Scheme 2 [9].

R2

R2

N

R1

R1

R1

R1

OR2

R2

R1

R1

O

R1

R1

M

N

N

Xn-2

1) Y = H; M(OR)n

2) Y = H; M(NR2)n

3) Y = H; MRn

R = Alkyl, Aryl

4) Y = H; M(OAc)n

5) Y = Na, K; MXn

X = Cl, Br

OY

OYN

X = OR, NR2, R, OAc,Cl, Br

Scheme 2: Preparation of Schiff base complexes

Route 1 involves the use of metal alkoxides [M(OR)n]. Alkoxides of early transition

metals (M = Ti, Zr) are commercially available and easy to handle. The use of other

alkoxide derivatives is not easy, particularly in the case of highly moisture-sensitive

derivatives of lanthanides. Metal amides, M(NMe2)4 (M = Ti, Zr) are also employed

as the precursors in the preparation of Schiff base metal complexes (Route 2). The

reaction occurs via the elimination of the acidic phenolic proton of the Schiff bases

Chapter-1

6

through the formation of volatile NHMe2. Other synthetic routes include treatment of

metal alkyl complexes with Schiff bases (Route 3) or treatment of the Schiff bases

with the corresponding metal acetates under reflux conditions (Route 4). The

synthetic scheme presented in Route 5 which is quite effective in obtaining salen-type

metal complexes consists of a two-step reaction involving the deprotonation of the

Schiff bases followed by reaction with metal halides. Deprotonation of the acidic

phenolic hydrogen can be effectively done by using NaH or KH in coordinating

solvents and the excess sodium or potassium hydride can be eliminated by filtration.

The deprotonation step is normally rapid at room temperature, but heating the reaction

mixture to reflux does not cause decomposition.

The term salen was originally used only to describe the tetradentate Schiff bases

derived from salicylaldehyde and ethylenediamine, the term salen-type is now used in

the literature to describe the class of (O, N, N, O) tetradentate bis-Schiff base ligands.

Stereogenic centers or other elements of chirality (planes, axes) can be introduced in

the synthetic design of Schiff bases (Fig. 2). As various achiral and chiral

salicylaldehyde and diamine derivatives are commercially or synthetically available, a

wide variety of metal-N2O2-Schiff base complexes have been synthesized as well.

These complexes adopt mostly octahedral configuration, albeit with a few exceptions

(vide infra), and they usually carry two ancillary ligands [32, 33]. Tetradentate

Schiff-base ligands are dianionic and the ancillary ligands are anionic or neutral,

depending on the valency of the central metal ion. When the complex bears a non-

coordinating counter ion such as PF6- and ClO4

-, a neutral ligand such as water or

solvent is coordinated instead to the metal ion. The geometry of the N2O2-Schiff-base

ligand depends on the structure of the diamine unit and the nature of the ancillary

ligand and that of the central metal ion.

Chapter-1

7

The chemistry of multimetallic complexes is nowadays a broad and interdisciplinary

research field. Dinucleating metal complexes have been attractive area of research, in

view of their significance as biomimetic catalysts in the process of oxygenation [34,

35]. Discoveries of dinuclear cores at the active sites of some metalloproteins have

aroused interest in the investigation of multimetallic systems [36]. The multidentate

binucleating Schiff bases with in built spacers can take up two same or different metal

ions. Various mono- and dialdehyde/ketones have been employed to condense with

amines or amino acid to explore multidentate binucleating Schiff bases to design a

variety of binuclear transition metal complexes [37].

Tetradentate Schiff base ligands containing imine nitrogen and alcoholic or phenolic

oxygen donor atoms can be utilized to synthesize trinuclear complexes. The internal

N2O2 cavity of the tetradentate Schiff base is a suitable complexation point for

divalent metal ions. However, the Schiff base complexes can act as a bidentate

chelating ligand for further coordination as a result of the interaction with other metal

ions to form homo and hetero-trinuclear complexes [38]. The ability of these

complexes to act as host molecules to capture guest metal ions by coordinating

through cis oxygen atoms leads to the generation of trinuclear metal complexes [39].

Chapter-1

8

N N

OH HO

N N

OH HO

N N

OH HO

N,N'-bis(Salicylidene) - orthophenylenediamine N,N'-bis(salicylidene) -ethylenediamine

N,N'-bis(salicylidene-1,1'-binaphthyl-2,2'-diamine

Fig. 2

Chapter-1

9

The Schiff base complexes do have a number of applications which are discussed

briefly.

The synthesis of the well known Schiff base complex, N,N’- bis(3,5-di-tert-

butylsalicylidene)-1,2-cyclohexanediaminomanganese (III)chloride is presented in

Scheme 3 [40, 41]. This manganese complex is known as Jacobsen’s catalyst. The

Schiff base can be successfully prepared by the condensation between trans- 1,2-

diaminocyclohexane and 3,5-di-tert-butyl-2-hydroxybenzaldehyde and finally

Jacobsen’s catalyst can be prepared from the ligand by treatment with manganese(II)

acetate followed by oxidation with air.

OH

O

t-Bu

t-Bu

H2N NH2

OH

t-Bu

t-Bu HO t-Bu

N

t-Bu

t-Bu

t-Bu t-Bu

N

t-Bu

Mn

O O

Cl-

NaCl, H2O

K2CO3, H2O, EtOH

N

N

+

Mn(OAc) 3 .4H2O

Scheme 3: Synthesis of Jacobsen’s catalyst

Chapter-1

10

Bhunia et al. synthesized a new hybrid catalyst by tethering a nickel(II) Schiff base

complex via post-synthesis modification of mesoporous silica, MCM-41. The Schiff-

base has been derived from salicylaldehyde and 3-aminopropyltriethoxysilane (3-

APTES) which is chemically anchored on MCM-41 via silicon alkoxide route. The

activity of the catalyst has been assessed in the epoxidation of olefins using tert-butyl-

hydroperoxide (tert-BuOOH) as oxidant in heterogeneous condition. The catalyst can

be recycled and reused several times without significant loss of activity [42].

Flourescent chemosensors have been developed to be a useful tool to sense in vitro

and in vivo biologically important species such as metal ions and anions because of

the simplicity and high sensitivity of fluorescence assays [43]. A typical fluorescent

chemosensor contains a receptor (the recognition site) linked to a fluorophore (the

signal source) which translates the recognition event into fluorescence signal [44].

Therefore, an ideal fluorescent chemosensor must meet two basic requirements:

firstly, the receptor must have the strongest affinity with the relevant target (binding-

selectivity). Secondly, on the basis of good binding-selectivity the fluorescence signal

should avoid environmental interference (signal-selectivity) such as photobleaching,

sensor molecule concentration, the environment around the sensor molecule (pH,

polarity, temperature, etc.) and stability under illumination. According to the well-

known fluorophore–spacer–receptor scaffold, the receptor is the central processing

unit (CPU) of a chemosensor. Although the ultimate aim for a fluorescent

chemosensor is to image the target of interest in a biological setting, a thorough

understanding of the available constructs can help to elucidate and improve the design

of chemosensors. Zn(II) ion has attracted a great deal of attention ascribing to the

biological significance of zinc. Zinc is the second most abundant transition metal ion

in the human body after iron. Zn(II) is now recognized as one of the most important

Chapter-1

11

cations in catalytic centers and structural cofactors of many Zn(II) containing

enzymes and DNA-binding proteins (e.g., transcription factors). Zinc is believed to be

an essential factor in many biological processes such as brain function and pathology,

gene transcription, immune function and mammalian reproduction [45] as well as

some pathological processes such as Alzheimer’s disease, epilepsy, ischemic stroke

and infantile diarrhea [46]. Although most Zn(II) is tightly bound to enzymes and

proteins, free zinc pools exist in some tissues such as the brain, intestine, pancreas,

and retina. Because Zn(II) is spectroscopically silent due to its d10 electron

configuration, many fluorescent chemosensors for the detection of Zn(II) have been

studied intensively.

Li et al. designed and synthesized a new Zn(II) chemosensor, 2-((2-hydroxynaphtha-

len-1-yl) methyleneamino)-3-(1H-indol-3-yl) propanoic acid (Fig. 3), which

possesses a complete solubility in aqueous environment, high selectivity to zinc ions

and easy synthesis. The author chose a naphthalene group as the fluorophore due to its

characteristic photophysical properties and the competitive stability in the

environment. In addition, they combine 2-hydroxy-1-naphthaldehyde with tryptophan,

which is inexpensive, water-soluble, and is expected to provide the binding site for

the target metal ion. The structure was confirmed by its spectroscopic data [47].

O

HO

N

OH

Fig.3

Chapter-1

12

Liu et al. synthesized fluorescent probe based on Schiff Base (L) Fig. 4 for the

detection of Zn (II) which has high selective response to Zn (II). The IR, electronic

spectra, selectivity, the effect of Zn (II) ion concentration, pH and solution of the

fluorescent probe (L) were studied. This Schiff base is a promising system for the

development of new fluorescent probe for the detection of Zn (II) cation [48].

N

OHH3C

HO

N

HOCH3

HO

Fig. 4

Luminescent coordination complexes have been one of the most active research area

due to their potential applications in fields of optoelectronic devices and chemical

sensors [49–51]. One of the most important considerations in organic light-emitting

diodes (OLEDs) is the design and synthesis of high emission efficiency molecules

through the modification of ligands and the selection of metal [52, 53]. Organic boron

compounds have attracted much attention due to their good luminescent properties

and electron-transporting properties. Xiaoming Liu et al. reported the structure and

EL characterizations of a boron complex with conjugated Schiff-base ligand, LBF2 [L

= ortho-C6H4(NC6H3Me2-2,6)(CH=NC6H3Me2-2,6)] (Fig.5) as emitter in polymer

OLEDs by simple spin-coating technology [54].

Chapter-1

13

N

N

B

F

F

Fig.5

Bhattacharjee and coworkers synthesized a series of novel photoluminescent hemi-

disclike distorted square planar Zn(II) Schiff base complexes containing 4-substituted

alkoxy chains on the side aromatic ring [Zn (4 -CnH2n+1O)2 salophen] (Fig. 6), n = 14,

16, 18 and salophen = N,N’-4-methyl phenylene bis (salicylideneiminato), their Zn(II)

complexes exhibiting green luminescence were accessed [55].

N

O

H3CO

N

O

OCH3

Zn

Fig. 6

The design and synthesis of organic chromophores as nonlinear optical (NLO)

materials have attracted much attention in recent years because of their potential

applications in optical communication, optical signal processing and transmission,

optical data acquisition and storage, optical computing and especially optical limiting

Chapter-1

14

effects utilized in the protection of optical sensors and human eyes from high-

intensity laser beams [56, 57]. Azomethines (known as Schiff-bases), having imine

group (-CH=N-), exhibit interest as materials for wide applications, partially as

corrosion inhibitors, catalyst carriers, thermo-stable materials and in biological

systems [58-60]. NLO properties of this group of materials have been also widely

investigated for many years [61, 62].

Tanga et al. synthesized a series of asymmetric donor–acceptor substituted salen-type

Schiff-bases with cyanide as an electronic acceptor (Fig. 7) and characterized by

proton nuclear magnetic resonance (1H NMR), Fourier transform infrared (FT-IR) and

ultra-violet and visible light (UV–vis) spectroscopy measurements. DFT methods

were used to optimize the geometries of these Schiff-base compounds. The results

would be helpful in the stimulation, screening and design of new Schiff-base ligands

which have powerful cyanide acceptor group and different substitute as NLO

materials [63].

N

N

HN

N

O N

O

NH

O M

O

Fig. 7

Chapter-1

15

Many Schiff bases are known to be medicinally important and used to design

medicinal compounds [64, 65]. It was observed that the biological activity of Schiff

bases either increase or decrease upon chelation with metal ions [66-68].

Recently, S. Hasnain et al. synthesized Schiff base N,N’

bis(salicylidene)thiosemicarbazide derived from thiosemicarbazide and

salicylaldehyde and its Mn(II), Co(II), Ni(II) and Cu(II) complexes which showed

potent antibacterial activity against Escherichia coli, Staphylococcus aureus, Bacillus

subtilis and antifungal activities against Aspergillus niger,Candida albicans, and

Aspergillus flavus (yeast) [69]. Schiff base derived from 2’-methyleacetoacetanilide

and 2-amino-3-hydroxypyridine and its Co(II), Ni(II), Cu(II) and Zn(II) complexes

have been screened for their antibacterial (Staphylococcus aureus, Escherichia coli,

Bacillus subtilis and Pseudomonas aeruginosa) and antifungal (Aspergillus niger,

Rhizopus stolonifer, Rhizoctonia bataicola and Candida albicans) activities and the

data revealed that the complexes have higher activity than the free ligand. The DNA

binding and cleavage properties of the complexes have also been investigated [70].

The binding and reaction of metal complexes with DNA have been the subjects of

intense investigation in relation to the development of new reagents for biotechnology

and medicine [71-78]. Numerous biological experiments have demonstrated that DNA

is the primary intracellular target of anticancer drugs due to the interaction between

small molecules and DNA which cause DNA damage in cancer cells blocking the

division of cancer cells and resulting in cell death [79, 80]. The interaction of

transition metal complexes containing multidentate aromatic ligands with DNA has

gained much attention. This is due to their possible application as new therapeutic

agents and their photochemical properties which make them potential probes of DNA

Chapter-1

16

structure and conformation [81-86]. The design of small complexes that bind and

react at specific sequences of DNA becomes important. A more complete

understanding of how to target DNA sites with specificity will lead not only to novel

chemotherapeutics but also to a greatly expand ability for chemists to probe DNA and

to develop highly sensitive diagnostic agents [81].

In order to develop new antitumor drugs which specifically target DNA, it is

necessary to understand the different binding modes which a complex is capable of

undertaking. Basically, metal complexes interact with the double helix DNA in either

a non-covalent or a covalent way. The former way includes three binding modes:

intercalation, groove binding and external static electronic effects. Among these

interactions, intercalation is one of the most important DNA binding modes as it

invariably leads to cellular degradation. It was reported that the intercalating ability

increases with the planarity of ligands [87, 88] (Additionally, the coordination

geometry and ligand donor atom type also play key roles in determining the binding

extent of complexes to DNA) [89, 90]. The metal ion type and its valence which are

responsible for the geometry of complexes also affect the intercalating ability of metal

complexes to DNA [91, 92].

Errors in gene expression can often cause diseases and play a secondary role in the

outcome and severity of human diseases [93]. So a more complete understanding of

DNA-drug binding is valuable in the rational design of DNA structural probe, DNA

footprinting, sequence-specific cleaving agents and potential anti-cancer drugs [94,

95].

Schiff bases are potential anticancer drugs, so they have been studied extensively

[96, 97]. The anticancer activity of the Schiff bases increase on complexation with

Chapter-1

17

metal ions [98]. Nickel(II) salen (where salen is 1,2-bis(salicylideneamino)ethane) in

the presence of monoperoxyphthalic acid or iodosyl benzene has been found to bring

about cleavage in plasmid DNA [99]. Griffin and coworkers have investigated DNA

binding/ cleaving properties of 26 salen complexes of Mn(III) in the presence of

terminal oxidants using DNA affinity cleaving techniques and found that DNA

cleaving efficiency varies with both the structure and stereochemistry of the bridging

substituent [100].

The interaction of chromium(III) Schiff base complexes, [Cr(salen)(H2O)2]+ where

salen = N,N’-ethylenebis(salicylideneimine) and [Cr(salprn)(H2O)2]+ where salprn =

N,N’-propylenebis(salicylideneimine) (Fig.8) with calf thymus DNA (CT-DNA) has

been reported [101]. Chromium(III) complexes derived from chiral binaphthyl Schiff

base ligands (R- and S-2,2’-bis(salicylideneamino)1,10-binaphthyl) were also found

to interact with CT-DNA through groove binding [102]. Binuclear copper(II)

complexes having the Schiff base ligand, N,N’-bis(3,5-tert-butylsalicylidene-2-

hydroxy)-1,3-propanediamine, were found to be effective in the cleavage of plasmid

DNA without the addition of any external agents and in the presence of hydrogen

peroxide at pH = 7.2 and 37°C. DNA cleavage mechanism studies showed that these

complexes may be capable of promoting DNA cleavage through an oxidative DNA

damage pathway [103]

N

O

Cr

N

O

H2O

H2O

(CH2)n

n = 2, L = [Cr(salen)(H2O)2]+

Fig. 8 n = 3, L = [Cr(salprn)(H2O)2]+

Chapter-1

18

Wu and co-workers synthesized a binuclear complex, [(phen)Cu(l-

bipp)Cu(phen)](ClO4)4 (Fig. 9) (phen = 1, 10- phenanthroline and bipp = 2,9 – bis(2-

imadazo [4,5-f] [1, 10] phenanthroline) – 1, 10-phenanthroline). Photophysical and

viscometric studies have suggested that binuclear copper (II) complex strongly binds

to CT-DNA by intercalation of the two phenanthroline copper (II) terminals [104].

N N

N

NH

N

HN

N

N N

N

CuCu

NN N

N

4+

Fig. 9

Raman and co-workers reported DNA binding properties of the Cu(II) and

VO(IV) complexes derived from Schiff base ligand, dbdppo

(4-(3’,4’dimethoxybenzaldehydene)2-3-dimethyl-1-phenyl-3-pyrazolin-5-one or

hnbdppo (4-(3’-hydroxy-4’-nitrobenzaldehydene)2-3-dimethyl-1-phenyl-3-pyrazolin-

5-one with polypyridyl ligand(s) as co-ligand(s). The complexes bound to CT-DNA

by partial intercalation into the base pairs of DNA. Moreover, these complexes have

been found to promote the photocleavage of plasmid DNA pBR322 under irradiation

at 365 nm [105].

Chapter-1

19

Schiff base complexes of amino acids have gained importance not only from the

inorganic point of view but also because of their physiological and pharmacological

activities [106]. Schiff base complexes obtained from amino acids have found

applications in understanding many biochemical reactions in vivo. The poisoning role

of certain metal ions in living organisms was ascertained using these complexes by

determining the action of drugs [107]. Amino acids constitute the building blocks of

proteins and are chemical species indispensable for performing a huge number of

biological functions as exemplified by the role of enzymes [108]. It has been well

known that the amino acids are main component element of various proteins. They

generally play an important physiological role in life process. In view of special three-

dimensional geometric configuration, all amino acids also exhibit the perfect

identification and selection abilities to biological tissue [109]. A huge interest on

metal complexes of Schiff-bases derived from amino acids and salicylaldehyde has

emerged due to their structural, magnetic and electrochemical properties, as well as

their potential use as models for a number of important biological systems [110-112].

o-phthalaldehyde plays important role in amino acid assay. The amino acid residues

in various enzymes and biological fluids have been determined by using o-

phthalaldehyde [113]. It finds application in the clinical field as a high level

disinfectant [114]. In recent years, considerable interest has been developed in copper

complexes with Schiff base ligands as structural models for the active site of copper

proteins. In copper proteins a distorted metal ion environment of low symmetry with

mixed donor sets is present [115]. Zinc containing carboxylate-bridged complexes

[116] have varied structural motifs in hydrolytic metalloenzymes, such as

phosphatases and aminopeptidases. The cobalt complexes of tetradentate Schiff base

ligands have been widely used to mimic cobalamine (B12) coenzymes [117].

Chapter-1

20

Sakiyan and co-workers have synthesized the schiff base manganese(III) complexes

(Fig. 10) derived from amino acids and 2-hydroxy-1-naphthaldehyde. The

coordinating behaviour has been studied and it has been found that Schiff base binds

through the ONO donor set derived from the carboxyl, imino and phenoxy groups of

the ligands [118].

N

O

O

OR

Mn3+

O N

RO

O

Fig. 10

R = H, CH3, CH(OH)CH3,

(Gly.), (Ala.), (Thr.), (His.), (Trp)

Offiong et al. reported novel Pd(II) and Pt(II) complexes of substituted o-

hydroxyacetophenone-glycine (Fig 11). The complexes were characterized by

conductivity measurements, I.R., electronic and 1H N.M.R. spectra. The spectral data

indicated that the ligands were monobasic bidentate, coordinating through imino

nitrogen and the carboxylate group. A four coordinate square planar configuration has

been proposed for all the complexes. The ligands as well as their Pd(II) and Pt(II)

complexes, exhibited potent cytotoxic activity against Ehrlich ascites tumour cells in

vitro but appeared to be more active in vivo [119].

N

N

HN

H

CH2

,

Chapter-1

21

Me

OH

NCH2CO2-

R

R = H, OMe, Cl, Br

Fig. 11

Boghaei and co-workers synthesized a series of new ternary zinc(II) complexes

[Zn(L)(phen)] (Fig.12) where phen = 1,10-phenanthroline and L = tridentate Schiff

base ligands derived from the condensation of amino acids (glycine, l-phenylalanine,

l-valine, l-alanine, and l-leucine) and sodium salicylaldehyde-5-sulfonate and sodium

3-methoxy-salicylaldehyde-5-sulfonate. The schiff base ligand act as tridentate ONO

moiety, coordinating to the metal through its phenolic oxygen, imine nitrogen,

carboxyl oxygen and nitrogens of 1,10-phenanthroline [120].

R1 = H, CH2Ph, CH(CH3)2, CH3, CH2CH(CH3)2

R2= H, OCH3

Fig. 12

Raso et al. reported X-ray crystal structures for two novel N-salicylidene

tryptophanato diaquocopper(II) isomers, [Cu(Sal-Trp)(H20)2], erythro (1) and threo

(2) derived from salicyaldehyde and tryptophan [121].

Dong et al. synthesized and characterized a new ternary copper(II) complex with

mixed ligands of an ONO-donor Schiff base derived from salicylaldehyde and L-

valine and 1,10-phenanthroline. The DNA-binding properties of the complex have

O

R2

N

Zn

O

OR1

N

NNaSO3

Chapter-1

22

been investigated by UV–visible, fluorescence, circular dichroism spectroscopies,

viscosity and thermal denaturation measurements [122].

Sharma and co-workers reported monomeric complexes of amino acid Schiff base

ligands (Fig. 13) derived from the condensation of furfuraldehyde and indole-3-

carbaldehyde with alanine, glycine, valine, isoleucine and tryptophan. The

coordination behaviour of Schiff bases to organosilicon (IV) has been investigated by

IR, 1H, I3C & 29Si NMR spectral studies. Schiff bases and their silicon complexes

have also been screened for their antifungal activity [123].

C

R

R'

N C

O

O

Si CH3

CH3

HC

C

R

R'

NC

O

O

CH

R"

R"

O N,R'

R = H

=

H, CH3, -CH(CH3)2, -CH(CH3)-CH2-CH3,

H

NH

CH2

R" =

Fig. 13

Gao and coworkers reported three o-Vanillin Schiff Bases (o-VSB: o-Vanillin-d-

Phenylalanine (o-VDP), o-Vanillin-l-Tyrosine (o-VLT) and o-Vanillin-l-Levodopa

(o-VLL)) (Fig. 14) with alanine constituent by direct reflux method in ethanol

solution and were used to study the interaction to bovine serum albumin (BSA)

molecules by fluorescence spectroscopy [109].

Chapter-1

23

OH

O

N

HC

OH

OCH3

OH

O

N

HC

OH

OCH3

HO

OH

O

N

HC

OH

OCH3

HO

HO

o-Vanillin-D-Phenylalanine o-Vanillin-L-Tryrosine o-Vanillin-L-Levodopa

Fig. 14

Abdallah and co-workers reported a new Schiff base ligand (Fig. 15) via condensation

of o-phthaldehyde and 2-aminophenol and its metal complexes with Cr(III), Mn(II),

Fe(II), Fe(III), Co(II), Ni(II), Cu(II) and Zn(II) ions. The parent Schiff base and its

metal complexes were assayed against two fungal and two bacterial species. The

parent Schiff base and four metal complexes inhibited the growth of the tested fungi

at different rates. A relative antifungal activities were reported as Ni(II) complex

Cr(III) complex parent schiff base Co(II) complex. However, only two metal

complexes Mn(II) and Fe(II) were tested for antibacterial activity. They showed weak

growth of inhibition against Esherichia coli and Staphylococcus aures [124].

CH

CH

N

N

HOHO

Fig. 15

Chapter-1

24

Neelakantan et al. synthesized metal complexes of Cu(II), Ni(II), Zn(II), Co(II),

Mn(II) and VO(II) species (Fig. 16) of a Schiff base derived from o-phthalaldehyde

and amino acids (glycine, l-alanine, l-phenylalanine). The [N2O2] Schiff base ligands

bind the metal ion via imine nitrogens and carboxylate oxygens. The metal complexes

were evaluated for their DNA cleaving activities with CT-DNA under aerobic

conditions. Cu(II) and VO(II) complexes show more pronounced activity in the

presence of the H2O2 oxidant [125].

C N

C NH

H

OR

O

OR

O

M OH2H2O

Fig. 16

Sinha et al. reported eight novel heterocyclic Schiff bases derived from the

condensation reactions of indole-3-carboxaldehyde with different L-amino acids

(histidine, glutamic acid, aspartic acid, leucine and valine). The radiolabeling, radio

imaging and antimicrobial activity have been studied [126].

Schiff base metal complexes including 1,8-diaminonaphthalene have been widely

studied because of their industrial, antifungal, antibacterial and biological applications

[127, 128]. Schiff bases derived from 1,8- diaminonaphthalene can be used to obtain

optical materials and conducting polymers [129-131].

R = H Glycine

R = CH3 Alanine

R = C6H5 Phenylalanine

Chapter-1

25

Krishnapriya and co-workers synthesized a new dicompartmental ligand N,N’- bis

(2-iminomethyl- 4-6 -(- 4 -methylpiperazin – 1 – yl methyl) 1- hydroxyl benzene] 1,8

naphthalene (Fig. 17) by the Schiff base condensation of the precursor compound, [2-

Formyl- 4- methyl – 6 [4- methyl- 6 [ 4- methylpiperazin - 1 – yl) methyl ] phenol

with 1,8 diaminonaphthalene and their complexes have been characterized by various

spectroscopic studies [132].

OH N

OH N

N

N

N

N

Fig. 17

Shakir et al. synthesized and characterized Co(II), Ni(II), Cu(II) and Zn(II)

complexes with Schiff base ligand, N,N’-bis – ( 2- pyridine carboxaldimine) – 1,8

diaminonaphthalene, (Fig.18) derived from 1,8 diaminonaphthalene and 2-pyridine

carboxaldehyde. The ligand is coordinated to metal ion in a tetradentate manner with

N4 donor sites. Absorption and fluorescence spectroscopic studies support that Cu(II)

complex exhibits significant binding to CT-DNA [133].

Chapter-1

26

N N

M

Cl

Cl

NN

Fig. 18

A novel tetradentate Cu(II) complex of the type, [CuL](NO3)2 by the interaction of

Schiff base ligand L, N,N-bis[(E)-thienylmethylidene]-1,8- diaminonaphthalene,

obtained by the condensation of thiophene-2-carboxaldehyde and 1,8

diaminonaphthalene. The ligand and its Cu(II) complex were characterized by

different spectroscopic studies. The DNA binding and DNA cleavage studies of

Cu(II) complex has shown considerable DNA cleavage and generated reactive

oxygen species such as superoxide anion. The results suggested a putative role of

Cu(II) complex similar to various anticancer drugs [134].

N N

SS

Fig. 19

Chapter-1

27

It goes without exaggeration to mention that the voluminous work has been reported

in the field of Schiff base chemistry and vivid applications have been explored by the

chemists worldwide therefore, it is impossible to contain the various aspects in this

field in few pages, enumerated in thousands of publications, articles and number of

reviews.

Chapter-1

28

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