CHAPTER I

CHAPTER I

INTRODUCTION

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1

1.1.INTRODUCTION

The first complexes to be the subject of any great study were the cobalt

ammines. As early as 1798 Tassaert observed that a solution of a cobalt(II) salt in

aqueous ammonia becomes brown on exposure to air, the colour changing to wine red

on boiling. Some decades later, Fremy showed that the cobalt had been oxidized to

cobalt(III) and that the new salt is associated with up to six ammonia molecules, for

e.g., CoCl3.6NH3. This compound stimulated interest because it was difficult to

understand how the two compounds CoCl3 and NH3 each with its valency satisfied,

could combined together to form a stable compound. Many more complexes of

ammonia were prepared subsequently. The reactions of nickel chloride and copper

sulphate solutions with an excess of ammonia were found to give deeply coloured

solutions from which the purple NiCl2.6NH3 and the deep blue CuSO4.4NH3.H2O

could be isolated. However, it was with cobalt(III) and platinum(II) that a wide

variety of ammonia complexes (ammines) could be prepared, and it was the study of

the properties of these series of ammines that was to shed much light on the structures

of coordination compounds.

While their structures were unknown, these new compounds were often named

according to their colour. Thus the orange-yellow CoCl3.6NH3 was called

luteocobaltic chloride, the purple CoCl3.5NH3 purpureocobaltic chloride and the

green isomer of CoCl3.4NH3 praseocobaltic chloride. Other complexes were named

after their discoverers, for e.g. Magnus green salt PtCl2.2NH3 (now formulated as

[Pt(NH3)4)][PtCl4]) and Reinecks‟s salt Cr(SCN)3.NH4SCN.2NH3 (now formulated as

NH4[Cr(NH3)2(NCS4)]. Around 1830 Zeise found that ethylene could form

compounds with platinum salts, and he was able to isolate complexes of formulae

PtCl2.C2H4

and PtCl2.KCl.C2H4. These compounds were first organometallic

compounds of the transition elements to be prepared; their structures have been

elucidated only fairly recently and the potassium compound (now formulated as

K[PtCl3(C2H4)]) is still often called Zeise‟s salt.

Coordination chemistry has an old history originating in the discovery of

[Co(NH3)6]Cl3 by Tassaert in 1798 [1]. But it was Werner who systematized the

subject by propounding his coordination theory in 1893 [2]. Werner‟s basic ideas on

the stereochemistry of metal complexes, mechanisms of isomerization and

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2

racemization, etc. remain unchallenged even today despite all the monumental

developments which have taken places since his days and during the last five decades

in particular, owing to the advent of sophisticated physico-chemical techniques of

high precision and capability [3, 4]. These have considerably enriched our

understanding of the nature of the metal-ligand bond, structure and stereochemistry of

metal complexes, their stabilities and liabilities and other properties. Even metals

which were earlier thought to be non-complex formers are now known to form quite

stable complexes with special types of ligands, such as the alkali metal complexes

formed by crown ethers and cryptates [5-8].

The growth of coordination chemistry has been three dimensional,

encompassing breadth, depth, and applications. The ongoing respect for the evolving

science is apparent in the five Noble prizes that have been impinged heavily on the

subject (A. Werner, 1913; M. Eigen, 1967; Wilkinson & Fischer, 1973; H. Taube,

1983; Cram, Lehn and Pedersen 1987). The first (Werner) and last (Cram, Lehn &

Pedersen) in the list recognized the old and the new realms of coordination chemistry

specifically [9].

In the real sense, coordination chemistry is an area that has many fields, like

transition metal chemistry, organo-metallic chemistry, homogeneous catalysis,

bioinorganic chemistry, medicinal chemistry etc. an equally important, it is a

foundational to other growing fields for e.g. Solid-state chemistry, extended and

mesoscopic materials, photonic materials, model for solid surfaces, separation

science, molecular electronics, machines and devices. The enormous extensions of the

field reflect its fundamental nature; the principles are so basic that they have

immediate applications as some undreamt of new substances serendipitously appear in

chemistry (e.g., dihydrogen complexes, metal derivatives of fullerenes, metal

containing liquid crystals). The spawning of new fields is an inevitable consequence

of the foundational position of coordination chemistry in the chemical sciences.

The chemistry of coordination compounds with heterocyclic ligands

containing oxygen and nitrogen as donor atoms has attracted increasing attention in

recent years. It is well known that such ligands coordinate to a metal atom in different

ways in different media. Transition metal ions are essential in many biological

systems in nature [10]. These metal complexes with bidentate and tetradentate ligands

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3

containing both hard and soft donor groups have been used extensively in

coordination and organometallic chemistry [11]. The chelating properties of Schiff

bases display manifold applications in medicine, industry, and agriculture [12]. The

transition metal complexes of Schiff bases have exhibited fungicidal and bactericidal

activities including regulating the growth of plants [13]. It is known that chelation of

metal ions with organic ligands acts synergistically to increase its effect [14]. The

coordination chemistry of the square planar palladium(II) and platinum(II) complexes

of nitrogen and sulfur/oxygen donor ligands have gained enormous importance

because of their antitumour [15], anticancer [16], and catalytic activities [17].

Antimicrobial aspects [18] and antifertility activity [19] of coordination compounds of

palladium (II) and platinum (II) have also been reported in recent years.

Coordination compounds exhibit different characteristic properties, which

depend on the metal ion to which ligands are bound, the nature of the metal, as well as

the type of ligand. These metal complexes have found extensive applications in

various fields such as biology, medicine etc. The nature of a coordination compound

depends on the metal ion and the donor atoms, as well as on the structure of the ligand

and the metal–ligand interaction [20]. With increasing knowledge of the properties of

functional groups, as well as the nature of donor atoms and the central metal ion,

ligands with more selective chelating groups, i.e., imines or azomethines which are

more commonly known as Schiff bases are used for complex formation studies. It is

reported that the rapidly developing field of bioinorganic chemistry is centered on the

presence of coordination compounds in living systems [21].

One of the most important problem in coordination chemistry has been the

nature and strength of metal-ligand bond. Normally the metal ion does not form bonds

of equal strength with two different donor atoms. Similarly, a particular donor atom

does not form bonds of the same strength with different metal ions [22].

Donor Atoms

Fundamentally any part of molecule that happens to be more basic than the C-

H portions is potential electron donors. Amino acids, peptides, proteins, hormones,

nucleoproteins, nucleic acids, carboxylic acids, carbohydrates, lipids, simple anions

and even the solvent water contain some electron donor elements such as nitrogen,

oxygen, fluorine, phosphorous, sulphur, chlorine, bromine and iodine. Quite recently

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4

even carbon has been shown to form bonds to metal ion [23]. The common electron

donor groups employed in drugs are indicated in below Figure. The actual donor

groups selected by the metal depend upon many factors that are conveniently

discussed using the “hard and soft acids and bases” theory.

Common electron donor groups

Schiff Base and Metal Complexes

With ever increasing knowledge of properties of functional groups and nature

of donor atoms and the central metal ion, ligands with more selective chelating group

called anils, imines and azomethines or best known as Schiff bases are being used for

the complex formation studies. They have general structure RC=NR‟ where R and R‟

are alkyl, aryl or heterocyclic compounds. Extensive investigation in the field of

Schiff bases have been reported by Bayer [24]. Their preparation, chemical and

physical properties have been described by Layer [25], Dwyer and Mellor [26]. It

concern primarily with the stereochemistry of Schiff base complexes as well as aspect

of behavior of such complexes in solution, has also been published [27].

Schiff bases have recently gained much interest due to their delocalized π-

orbitals, flexible behaviour, multifunctional ligating sites etc., Also the Schiff base

complexes have great biological importance especially in the field of model

compounds studies. They impart to the catalyst a good tolerance towards organic

functionalities, air and moisture, in this way widening the area of application.

Schiff base ligands have played an important role in the development of

coordination chemistry since the late nineteenth century. The finding that metal

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5

complexes of these ligands are ubiquitous is a reflection of their facile synthesis, wide

application and accessibility of diverse structural modifications [28]. Schiff base

metal complexes have been known since the mid nineteenth century [29] and even

before the general preparation of the Schiff base ligands themselves. Schiff base metal

complexes have occupied a central place in the development of coordination

chemistry after the work of Jorgensen and Werner [30]. However, there was no

comprehensive, systematic study until the preparative work of Pfeiffer and associates

[31]. Pfeiffer et al., [32] reported a series of complexes derived from Schiff bases of

salicylaldehyde and its substituted analogues. In the past two decades, the properties

of Schiff base metal complexes stimulated much interest for their contributions to

single molecule-based magnetism, material science, catalysis of many reactions like

carbonylation, hydroformylation, oxidation, reduction and epoxidation, their

industrial applications and complexing ability towards some toxic metals. The high

affinity for the chelation of the Schiff bases towards the transition metal ions is

utilized in preparing their solid complexes [33]. Schiff base complexes containing

nitrogen and oxygen as donor atoms play an important role in biological systems and

represent models for metalloproteins and metalloenzymes that catalyze the reduction

of nitrogen and oxygen [34].

In recent years, it has been the trend to synthesize the ligand molecule having

definite frame work of donor atoms and to stitch them to different metal ions. Among

the coordination compounds, those derived from Schiff bases are of particular

interest. Schiff base ligands have been in the chemistry literature for over 150 years.

The literature survey clearly reveals that the study of this diverse ligand system is

linked with many of the key advances made in the field of inorganic chemistry.

Schiff base ligands are a class of polyatomic systems containing one or more

C=N bond, with at least one aryl group attached to either C or N. The ligands are

generally accessible by easy methods and the synthesis of these ligands was first

reported by Schiff [35]. The most common method of obtaining Schiff base [35] is

straight forward as indicated in the condensation reaction between a primary amine

with aldehyde or ketone.

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6

Other methods of synthesis of Schiff bases are widely reviewed by Dayagi and

Degani [36]. Schiff bases can be characterized by IR, NMR, UV-Vis and mass

spectral methods. The C=N stretching frequencies of the ligands generally occur in

the region 1680 and 1603 cm-1

when H, alkyl or aromatic groups are bound to C and

N atoms. The natures of different substituents on these atoms determine the position

of the stretching frequencies in the above range. For e.g., aryl groups on C and N

atoms cause a shift of the frequency towards the lower side of the range. A summary

of typical IR frequencies for various Schiff base ligands are available in the literature

[37]. NMR spectroscopy is very useful in understanding structural features of ligands

in solution, in particular keto-enol tautomerism found in these ligands (2, 3).

Schiff bases have exhibited higher coordination number and from kinetics and

thermodynamic point of view, they are important class of compounds, resulting in an

enormous number of publications and literature review, ranging from pure synthetic

work to physico-chemical and biochemically relevant studies of metal complexes and

found wide range of applications.

Schiff bases are generally bidentate, tridentate, tetradentate or polydentate

ligands capable of forming very stable complexes with transition metals. They can

only act as coordinating ligands if they bear a functional group, usually the hydroxyl,

sufficiently near the site of condensation in such a way that a five or six membered

ring can be formed when reacting with a metal ion. Schiff bases derived from

aromatic amines and aromatic aldehydes have a wide variety of applications in many

fields, e.g., biological, inorganic and analytical chemistry [38, 39]. Schiff bases are

used in optical and electrochemical sensors, as well as in various chromatographic

methods, to enable detection of enhanced selectivity and sensitivity [40-42]. 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 [43, 44]. Schiff bases are also effective corrosion inhibitors and could be

adsorbed on the surface of metals [45].

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7

Schiff bases offer opportunities for inducing substrate chirality, tuning the

metal centered electronic factor, enhancing solubility, either performing homogenous

or heterogeneous catalyses, include diversified subjects comprising their various

aspects in bio-coordination and bio-inorganic chemistry. 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) using complex

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

utilizing the variation in their spectroscopic characteristics following changes in pH

and solvent [46]. Schiff bases are widely studied in coordination chemistry as they

easily form stable complexes with most transition metal ions [47, 48]. Many

biologically important Schiff bases have been reported in the literature possessing,

antimicrobial, anticonvulsant, antitumor and anti HIV activities [49-54] which may be

due to the presence of azomethine linkage [55-57]. The applications of Schiff bases

and their importance in the field of coordination chemistry have generated a great deal

of interest in the synthesis of new metal complexes.

Schiff base transition metal complexes are one of the most adaptable and

thoroughly studies systems [58, 59]. They are of both stereochemical and

magnetochemical interest due to their preparative accessibility, diversity and

structural variability [60]. Metal complexes play an essential role in agriculture,

pharmaceutical and industrial chemistry [61]. Heteronuclear Schiff base complexes

have found applications as magnetic materials, catalysts and in the field of bio-

engineering [62, 63].

Transition metal complexes have attracted attentions of inorganic, metallo-

organic as well as bio-inorganic chemists because of their extensive applications in

wide ranging areas from materials to biological sciences [64]. It is well known that N

and S atoms play a key role in the coordination of metals at the active sites of

numerous metallobiomolecules [65]. Schiff base metal complexes have been widely

studied because they serve as models for biologically important species and find

applications in biomimetic catalytic reactions. Chelating ligands containing N, S and

O donor atoms show broad spectrum of biological activity and are of special interest

because of the variety of ways in which they are bonded to metal ions. It is known

that the existence of metal ions bonded to biologically active compounds may

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8

enhance their activities [66, 67]. Transition metal complexes have emerged as

potential building blocks for nonlinear optical materials due to the various excited

states present in these systems as well as due to their ability to tailor metal-organic-

ligand interactions [68-70]. Inorganic complexes can be used in foot printing studies,

as sequence specific DNA binding agents, as diagnostic agents in medicinal

applications, and for genomic research.

Transition metal complexes derived from Schiff bases have occupied a central

role in the development of coordination chemistry. The azomethine group > C=N of

the Schiff base forming a stable metal complexes by coordinating through nitrogen

atom. Large number of Schiff bases has been investigated by many workers as

chelating agents. Most of them are prepared by the condensation of salicyldehyde

with aromatic amines and aliphatic amines. Calvin and Berkelow [71] have

synthesized nearly twelve anils by condensing salicyldehyde with substituted anilines

and other aromatic amines.

Azomethines and their complexing capabilities have been enlightened in many

review articles [72-75]. Hydrazones are the special group of compounds of Schiff

bases. They are characterized by the presence of >C=N-N< group. The presence of

two inter-linked nitrogen atoms separates from imines, oximes, etc. Many hydrazones

and their metal complexes have biological and pharmaceutical activities such as

anticancer, antitumor and antioxidative activities as well as inhibition of lipid

peroxidation etc. [76-78]. Many drugs may possess modified toxicological and

pharmacological properties when administered in the form of complexes. The most

widely studied metal in this respect is Copper(II), which proved to be beneficial in

diseases such as tuberculosis, gastric ulcers and rheumatoid arthritis [79, 80].

Hydrazides represent a very interesting class of compounds because of their

wide applications in pharmaceutical, analytical and industrial aspects, e.g., as

antibacterial, antifungal, anti-inflammatory, antitubercular, anti-HIV, anti-

degenerative activities and herbicides [81-86]. Numerous hydrazide derivatives of

Schiff base ligands and their transition metal complexes have been investigated by

various physico-chemical techniques [87-90].

The basic strength of C=N group is not sufficient to obtain stable complexes

by coordination of the imino nitrogen atom to the metal ion. Hence, the presence of at

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9

least one other group is required to stabilize metal-nitrogen bond [91]. This is evident

from the literature review that synthesis of different types of potential Schiff bases on

the basis of their donor atoms set has been attempted. Based on the donating sites,

further Schiff bases are classified as monodentate, bidentate, tridentate, tetradentate

and polydentate ligands containing O, N and S donor atoms. Such type of donor site

ligands have been tried for their complexation and the structures were deduced with

the aid of analytical, physico-chemical and spectral data.

In this introductory section of the thesis, a brief review of Schiff base ligands

will be presented using illustrative examples from some of the above mentioned

classes of ligands.

Monodentate Schiff bases

It has been claimed that the basic strength of C=N group is not sufficient to

obtain stable complexes by coordination of the azomethine nitrogen atom to a metal

ion [91]. Hence the presence of at least one other group is required to stabilize metal-

nitrogen bond. Aryl groups attached to either O or N generally stabilize the ligands by

resonance. For e.g. monoamine Schiff bases such as N-(o-

hydroxybenzaldehydo)aniline BAAN derivatives (4) are well known in the literature

[37].

The ligand (4) variously substituted in the phenyl rings, has been the most

extensively studied ligand by NMR and X-ray analysis. 1H and

13C NMR spectra of

some BAAN derivatives have been reported. The azomethine proton was observed in

the region 8.41-8.5 ppm. These ligands exhibit solution spectral properties different

from their corresponding isoelectronic trans-azobenzene and trans-stilbene. These

differences are attributed to the non-planar structure of N-(o-

hydroxybenzaldehydo)aniline BAAN derivatives in solution. Since the spectral

properties are sensitive function of the molecular conformation, systematic studies on

the solid state structure have been carried out. Furthermore, since the original work of

Burgi and Dunitz [92], many workers have thought of utilizing the BAAN derivatives

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10

in crystal engineering because of their thermochromism and photochromism

properties. The geometrical parameters defining the overall conformation of these

Schiff bases and the geometry of C=N moiety have been described in the literature.

On the other hand, it has been reported recently [93] that the Schiff base

(5 and 5a) being monodentate in nature, is able to act as a ligand in the Pd complex,

because of the suggested interaction Pd---H, which may stabilize the coordination of

the ligand to the metal.

Bidendate Schiff bases

Bidentate Schiff bases have been amongst the widely used ligands in

preparing metal complexes. A brief review of potential bidentate ligands according to

their donor atom set is described in the following subsections.

N, N donor atom set

Schiff bases having two nitrogen atom donors may be derived either from the

condensation of dialdehydes or diketones with two molecules of an amine or from

reaction of diamines with aldehydes or ketones. The typical example may be

bis(benzylidene)-1,2-diphenylethylenediamine (6). The structure and coordination

behaviour of these ligands have been extensively studied [94].

Another ligand is derived from 2-pyrrolecarboxaldehyde and ammonia acts as

a monoanionic ligands and forms a complex (7) with copper(II) ion.

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11

Recently, a series of condensation products of the ketoximes (8) with amines

have been prepared and the species formed were investigated by X-ray, NMR and IR

studies [95].

N, O donor atom set

A large group of bidentate Schiff bases having N, O donor atom set have been

utilized as ligands for metals, since oxygen is often present as an OH group, these

ligands generally act as chelating monoanions. But, it must be pointed out that

Sacconi et al., [96] have shown that the hydroxyl oxygen atoms under certain

circumstances may bridge two metal atoms. In this case the Schiff bases should be

considered as tridentate ligands which favour the formation of binuclear complexes.

Ekk Sinn [97]

has reported some Cu-complexes of N-(ethyl-2-

hydroxybenzylidenimine) (9) in which the hydroxyl oxygen is coordinated to two

copper atoms, the ligand as a whole behaved in a tridentate fashion bonding through

imine nitrogen and phenoxide bridging.

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12

The copper(II) complexes of such ligands have shown dimeric structures and

these complexes have shown some antiferromagnetic exchange behaviour between

the two paramagnetic centres [96, 97].

On the other hand, there are few examples of coordinated neutral N, O

bidentate Schiff bases derived from -diketones and 2-hydroxy aldehydes. The most

studied bidentate Schiff bases containing N, O donor set are those derived from

substituted salicylaldehyde derivatives and amines (10). Large variety of transition

metal complexes of these ligands are known in the chemistry literature [98, 99].

Kandil et al., [100] have reported a new bidentate Schiff base ligand

containing heterocyclic moiety, 3-(2-furylidene)hydrazino-5,6-diphenyl-1,2,4-triazine

(11) and its copper(II) and cobalt(II) complexes. The ligand behaved in a bidentate

fashion coordinating through azomethine nitrogen and triazine nitrogen

Alaaddin et al., [101] have synthesized two new Schiff base ligands containing

cyclobutane and thiazole rings. 4-(1-methyl-1-mesitylcyclobutane-3-yl)-2-(2,4-

dihydroxy benzilidenehydrazino)thiazole (12) and 4-(1-methyl-1-mesitylcyclobutane-

3-yl)-2-(2-hydroxy-3-methoxybenzylidenehydrazine)thiazole (13) and their

mononuclear complexes of cobalt(II), copper(II), nickel(II) and zinc(II). The

complexes were characterized by microanalysis, IR, UV-visible, NMR and magnetic

susceptibility measurements.

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Singh et al., [102] have synthesized 2-furoyl hydrazones of 2-acetyl thiophene

and 2-acetyl furan (14) and (15) and their Cu(II), Co(II), Ni(II), Zn(II) and Mn(II)

complexes. Later on they have also synthesized Fe(III) complexes with the same

ligand [103].

Aurora et al., [104] have synthesized N-(2-furanylmethylene)-3-

aminodibenzofuran (16) and their Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II)

complexes.

Spinu et al., [105] have synthesized N-(2-thienylmethylidene)-2-

aminopyridine (17) and their Fe(II), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II)

complexes. The complexes have been characterized by elemental analysis, IR, 1H

NMR, electronic spectra and magnetic susceptibility measurements. These results

suggest a distorted octahedral geometry for the Fe(II), Co(II), Ni(II) and Cu(II)

complexes and a tetrahedral geometry for the Zn(II) and Cd(II) complexes.

The Co(II), Mn(II) and Zn(II) complexes of bidentate Schiff base (18) derived

from aniline and salicylaldehyde have been reported by Rehman [106] and others.

These complexes have been characterized by elemental analysis and spectral

techniques. The results obtained showed that the complexes have octahedral

geometry.

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14

Cu(II) complexes of bidentate Schiff base 4-methoxy-2-(1H-benzimidazol-2-

yl)-phenol and its methyl / chloro / nitro derivatives (19) have been reported by

Tavman et al., [107].

Synthesis, physico-chemical investigations and biological studies on Mn(II),

Co(II), Ni(II), Cu(II) and Zn(II) complexes with p-amino acetophenone isonicotinoyl

hydrazone have been reported by Singh et al., [108]. The results obtained by spectral

and other physico-chemical techniques showed that the complexes have octahedral

geometry (20).

Recently, Mruthyunjayaswamy et al., [109] have synthesized 3-chloro-N'-(3-

methoxybenzylidene)benzo[b]thiophene-2-carbohydrazide (21) and their Cu(II),

Co(II), Ni(II), Zn(II), Cd(II) and Hg(II) complexes. The complexes have been

characterized by elemental analysis, IR, 1H NMR, electronic spectra and magnetic

susceptibility measurements. These results suggest all the complexes were octahedral

geometry with slight distortion in Cu(II) complex.

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15

Tridentate Schiff bases

There are large number of tridentate Schiff bases containing N2O, N2S, NO2

and NSO donor sets [110]. These may be generally considered as derived from the

bidentate analogues by the addition of another donor group. It must be pointed out

that the oxygen donor atom of such ligands may often act as bridging between two

metal centres giving polynuclear complexes. Some typical tridentate ligands. (22, 23)

are given below.

The tridentate Schiff bases containing two active centres and a donor centre

form complexes with transition metals which are generally dimeric in nature.

Copper(II) and oxovanadium(IV) complexes with such ligands are some examples

reported in the literature [111]. The structures of such complexes have been proposed

on the basis of spectrochemical and magnetic susceptibility measurement studies. The

octahedral complexes with such type of tridentate ligands have been reported in the

literature [111].The neutral tridentate ligands are shown to form octahedral complexes

with transition metals where two ligands encompass the metal ion in an octahedral

array [112, 113].

The Co(II), Ni(II), Cu(II) and Cd(II) complexes of tridentate Schiff base (24)

have been reported by Nawar et al., [114]. These complexes have been characterized

by elemental analysis, IR, 1H NMR, mass, electronic spectra and magnetic

susceptibility measurements, spectrophotometric and potentiometric studies.

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16

Naik et al., [115] have reported the synthesis, spectroscopic and thermal

studies of Co(II), Ni(II) and Cu(II) complexes of the hydrazone (25) derived from 2-

benzimidazolyl mercaptoaceto hydrazide and o-hydroxy aromatic aldehydes.

The Co(II), Ni(II), Cu(II) and Cd(II) complexes of tridentate Schiff base, 4-[2-

(aminomethyl)pyridylisonitroacetyl)diphenylether (26) have been reported by Coskun

and Yilmaz [116].

Gudasi et al., [117, 118] have reported Co(II), Ni(II), Cu(II), Zn(II), Cd(II),

Oxovanadium(IV) and Ln(III) complexes with 2-(3-coumarinyl)imidazole[1-2a]

pyridine (CIP), (27) which were found to possess good antifungal and antibacterial

activity.

Sanjay Annarao [119] have reported the synthesis of 3-acetyl coumarine

semicarbazone (28) and 3-acetyl coumarine thiosemicarbazone (29) and their Zn(II),

Cd(II) and Hg(II) complexes. Based on the elemental, spectral, magnetic and

solubility studies, the Co(II), Ni(II) and Cu(II) complexes are predicted to possess

octahedral geometry and Zn(II), Cd(II) and Hg(II) complexes tetrahedral geometry.

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Bamfield et al., [120] and Chakravarty et al., [121] have reported penta

coordinated Fe(II), Co(II), Ni(II) and Zn(II) complexes containing tridentate ligands.

Monovalent tridentate ligands containing ONN, ONS and ONO donor sets have been

synthesized in recent years and metal complexes have been studied in detail. A simple

example of a Schiff base containing ONN sequence may be found in 2-

aldehydopyridine-o-aminophenol (30) [122].

Revankar et al., [123] have synthesized a series of Co(II), Ni(II), Cu(II) and

Zn(II) complexes of quinoline-thiosemicarbazone Schiff base (31) with ONS donor

atoms. The ligand and complexes were characterized by elemental analysis and

various spectral studies. Based on these studies, all the complexes have been assigned

octahedral geometry except the Cu(II) complexes which possess square pyramidal

structure. Further, the Schiff base has exhibited good antimicrobial activity and the

complexes have shown higher activity than the ligand.

Mruthyunjayaswamy et al., [124] have synthesized Co(II), Ni(II) and Cu(II)

complexes Schiff bases derived from 3-Phenyl-5-substituted indole-2-

carboxyhydrazones of salicylaldehyde (32).

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18

The ligands and complexes were characterized by elemental analysis and

various spectroscopic techniques. The results reveled that all the complexes possess

octahedral geometry. The ligands and complexes were also screened for antimicrobial

activity against Staphylococcus aureus, Escherichia coli and Aspergillus niger

Singh et al., [125] have synthesized 2-furoylhydrazones of 2-acetylpyridine

(33) and their Cu(II), Ni(II), Co(II), Zn(II), Mn(II) and VO(IV) complexes, later they

have also synthesized Fe(II) complexes with the above said ligands [126].

Mruthyunjayaswamy et al., have synthesized three new Schiff base ligands 3-

(4-phenylthiozole-2-yl)-1-(2-hydroxy-1-iminomethylphenyl)urea (34) and 3-(4-

phenylthiozole-2-yl)-1-(2,4-dihydroxy/2-hydroxy-5-chloro-1-methylimino

methylphenyl)ureas (35a-b) and their Cu(II), Co(II) and Ni(II) complexes. The

complexes have been characterized by elemental analysis, conductance

measurements, magnetic susceptibility, electronic spectra, IR and TGA studies. The

studies reveal that the ligand has coordinated as tridentate chelating agent bonding

through o-hydroxy group, amide carbonyl function and azomethine nitrogen. All the

complexes have octahedral stereochemistry [127].

Recently, Vivekanad and Mruthyunjayaswamy [128, 129] have synthesized

Schiff bases 5-chloro-3-phenyl-N1-((2-thioxo-1,2,dihydroquinoline-3-yl)methylene)-

1H-indole-2-carbohydrazide (36) and 3-chloro-6-methoxy-N‟-((2-thioxo-1, 2-

dihydroquinolin-3-yl) methylene)benzo[b]thiophene-2-carboxyhydrazide (37) and

their metal complexes. These complexes were characterized by elemental analysis,

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19

conductance measurements, magnetic susceptibility, electronic spectra, IR and TGA

studies.

Mruthyunjayaswamy et al., have synthesized [130] the Schiff base 5-chloro-

N-[(2‟-dihydro-2‟-oxoquinolin-3‟-yl methylene)-3-phenyl-1H-indole-2-

carbohydrazide (38) and its Cu(II), Co(II), Ni(II), Zn(II), Cd(II) and Hg(II)

complexes. The ligand and complexes were characterized by elemental analysis,

conductance measurements, magnetic susceptibility, electronic spectra, IR and TGA

studies. The results reveled that Schiff base acts as tridentate (ONO) chelating agent

coordinate with metal ions through the oxygen atom of carbonyl group of amide

function attached to 2-position of indole, azomethine nitrogen and oxygen atom of the

enolized amide function of qunilinone moiety via deprotonation. Analytical, spectral

and magnetic studies revealed mononuclear nature of the complexes. The Cu(II),

Co(II) and Ni(II) complexes exhibited octahedral geometry whereas Zn(II), Cd(II)

and Hg(II) complexes exhibited tetrahedral geometry. The synthesized ligand and its

complexes were screened for antimicrobial, antioxidant and DNA cleavage activity.

Tetradentate Schiff bases

Tetradentate Schiff bases with N2O2 donor set have been widely studied for

their ability to coordinate with metal ions. The properties of complexes obtained by

these ligands are determined by an electronic nature of the ligands as well as by their

conformational behavior [131].

CHAPTER I

20

Dubsky and Sokol [132] have reported the reactions of salicylaldehyde with

diamines. These display tetradentate behavior by forming square-planar complexes

(39) with Ni(II) and Cu(II).

Sacconi and Bertini [133] have reported Cu(II) complexes of quadridentate

ligands derived from ethylene diamine and acetyl acetone. Similarly Morgan and

Mainsmith [134] have reported a green complex (40).

X-ray analysis of the structure of Copper(II) chelates revealed that acac rings

are considerably conjugated with possible preponderance of the structure derived

from ketimine form of the ligand. The chelate molecule is expected to be planar but

for some puckering in the central chelate ring, it is slightly concave towards the centre

[135]. The cobalt(II) complexes (41) of such bases have also been reported [136].

Dubsky and Sokol [137] have prepared Schiff bases by reacting

salicylaldehyde with diamines. These display tetradentate behaviour by forming

square planar complexes with Ni(II) and Cu(II). It has been established that most of

the Ni(II) complexes with tetradentate Schiff bases are square planar and monomeric

in nature. In an extensive IR study of Ni(II) complexes, Gluvchinsky et al., [138] have

demonstrated that Schiff bases obtained from salicylaldehyde and diamines

H2N(CH2)nNH2, when n =2 and 3 form monomeric square planar complexes but when

n = 5 and 6 they form trimeric complexes with octahedral and square planar Ni(II)

moieties. The most interesting observation is due to Tanaka and Kono [139] who

observed the impact of lengthening of hydrocarbon chain between the two azomethine

groups on the colours of the copper(II) complexes of the above ligand . With change

CHAPTER I

21

in number from 2-6, the colour of the complex changed from yellowish green to pale

blue. Similar observations have also been made in the case of Co(II) and Ni(II)

complexes. It has been found that the lengthening of the hydrocarbon chain has a

profound impact on the geometry of Cobalt(II) complexes [140].

Zacharios et al., [141] have reported the reactions of salicylaldehyde with o-

phenylenediamine and studied the behavior of the resultant Schiff base in

complexation with Co(II), Ni(II) and Cu(II) metal ions . These display tetradentate

behavior by forming metal complexes (42) containing two six membered and one five

membered ring.

Henri et al., [142] have synthesized a tetradendate ligand (43) by reacting 2, 3-

diaminopyridine with o-vanillin and its Cu(II), Ni(II), Ru(II), Zn(II), and Fe(III)

complexes.

Dencinti et al., [143] have reported electrochemical properties of copper(II)

complexes with salen ligands (44). The Schiff base ligands studied gave cyclic the

voltammograms at 100 mVs-1

consists of a single cathodic peak potential ranging

from –1.6 to –2.0 V. No anodic wave occurred in the reverse scan. This behavior has

been observed for a wide range of scan rates from 25 to 100 mVs-1

. Hence such

reduction process should correspond to a totally irreversible electron transfer. The

cyclic voltammetric curves from the electrochemical reduction of the studied

CHAPTER I

22

copper(II) complexes exhibit a redox couple at potential in the range -0.91 to -1.28 V,

which can be attributed to the reduction of the metal center in the complex.

Some neutral tetradentate N2O2 type complexes of Co(II) have been reported

by Naeimi et al., [144]. The complexes have been synthesized using Schiff bases

formed by condensation of 5-nitro-salicylaldehyde with various diamines in alcohol.

The results obtained by spectral and other physico-chemical techniques showed that

the complexes have square-planar geometry (45).

Binuclear transition metal complexes

Synthesis of binuclear complexes in which a ligand structure maintains two

metal centers in close proximity but in different compartments separated by an

intervening group represents an important current objective in coordination chemistry.

These complexes serve as simple models for multi-metal-centered catalysts and

multielectron-transfer reagents [145-148]. The orientation of the metal centers, and

hence the nature of the metal–metal interactions, are controlled via appropriate

bridging ligands [149, 150].

Binuclear copper(II) complexes are of interest as models for investigating

intramolecular magnetic-exchange interactions between two metal centers in different

structural motifs, viz. the „„paddle-wheel‟‟ di-copper(II) tetracarboxylates,

symmetrically dibridged hydroxo or alkoxo species, and asymmetrically di-bridged

complexes with a (l-hydroxo/alkoxo)(l-carb-oxylato)dicopper(II) core [151-160].

Dicopper(II) complexes are also important as precursors in the chemistry of

CHAPTER I

23

supramolecular and discrete molecular high-nuclearity copper(II) complexes [161–

170].

Azza [171] have synthesized the Schiff base (46) by the condensation of

bicarboxyl compound 4, 6-diacetylresorcinol with 2-aminobenzenthiol in the 1:2

molar ratio and their binuclear Cu(II), Co(II), Ni(II), Zn(II) and Ru(III) complexes.

The results obtained by spectral and other physico-chemical techniques showed that

ligand acts as tetrabasic hexadentae ligand, bonding sites are azomethine nitrogen

atoms, sulfur atoms and phenolic oxygen atoms.

Kulkarni et al., [172] have synthesized the binuclear transition metal

complexes of bicompartmental SNO donor ligands (47 and 48). Ligand (47) act as

hexadentate tetrabasic for Co(II), Ni(II) and Cu(II) complexes and hexadentate

dibasic chelate for Zn(II) complex. On the other hand ligand (48) is hexadentate

tetrabasic for Cu(II) and dibasic for Co(II), Ni(II) and Zn(II) complexes. Both the

ligands provide SNO donating sites to each metal ion in the binuclear complexes.

Cu(II) and Zn(II) complexes of both ligands have square-planar and tetrahedral

geometry, respectively. Whereas Co(II) and Ni(II) complexes are of octahedral

geometry.

CHAPTER I

24

1.2.PRESENT WORK

The major objectives of the present work are

i. To develop methodology for the synthesis of novel bicompartmental and

heterocyclic Schiff base ligands derived from indole, benzo[b]thiophene,

coumarin moieties which have been utilized in the present work.

ii. To synthesize Cu(II), Co(II), Ni(II) and Zn(II) complexes derived from the above

ligands.

iii. To elucidate the structures of the synthesized ligands and their complexes by

making use of various physico-chemical and spectroscopic techniques. viz. IR, 1H

NMR, mass, UV-Vis., thermal, ESR and powder X-ray diffraction etc.

Electrochemical behaviour of some metal complexes has been studied by the

cyclic voltammetry.

iv. With respect to the significant applications of Schiff base ligands and their metal

complexes in medicinal field, appropriately termed as “Medicinal Inorganic

Chemistry”, Schiff bases and their metal complexes synthesized in the present

study have been evaluated for their antibacterial and antifungal activities. Also

some of the selected ligands and their metal complexes have been tested for their

DNA cleavage properties on isolated genomic DNA of various human pathogens

by agarose gel electrophoresis method and antioxidant activity by di(phenyl)-

(2,4,6-trinitrophenyl)iminoazanium (DPPH) method .

The above studies will be presented in the subsequent seven chapters (Chapter I-

VII). Each chapter begins with an introduction followed by discussion of results.

CHAPTER I: -This chapter deals with the overview of general discussion to Schiff

bases, mononuclear and dinuclear transition metal complex of

heterocyclic and bicompartmental Schiff base ligands.

CHAPTER II: - It deals with the synthetic procedures of ligands (H2L1, H2L2, H2L3,

H2L4, HL5, HL6, HL7 and L8) and their copper(II), cobalt(II),

nickel(II) and zinc(II) complexes. It also describes different physico-

chemical and spectroscopic techniques used for the structural

elucidation of the ligands and their complexes synthesized.

CHAPTER I

25

Methodology carried out for the antimicrobial, DNA cleavage and

antioxidant activity screening has also been described in this chapter.

CHAPTER III: - This chapter describes the characterization of bicompartmental

Schiff bases ligands 5 -substituted-N'-(1 -(5 - 1 -(2 -(5 - substituted -

3a, 7a-dihydro- 1H-indole- 2 -carbonyl)hydrazono)ethyl)- 2, 4 -

dihydroxyphenyl)ethylidene)-1H-indole- 2 -carbohydrazide (H2L1

and H2L2) and their copper(II), nickel(II) and zinc(II) complexes. The

ligands have been characterized by elemental analysis, IR, 1H NMR

and ESI-mass spectral techniques whereas the complexes have been

characterized by elemental analysis, IR, 1H NMR, ESI-mass, electronic

spectra, conductivity measurements, magnetic susceptibility, ESR,

TGA and powder-XRD methods. Electrochemical behaviour of Cu(II)

complex of ligand H2L1 has been studied by the cyclic voltametry.

Antimicrobial activity evaluation of the synthesized ligands and their

complexes were carried out. DNA cleavage activity of ligand H2L1 and

its complexes and ligand H2L2 and its Cu(II) complexes has been

carried out and their results have been discussed.

CHAPTER IV: - This chapter explains the characterization of bicompartmental

Schiff bases ligands (N',N'',N',N'')-N',N''-(1,1'-(4,6-dihydroxy-1,3-

phenylene)bis(ethan-1-yl-1-ylidene))bis(3-chloro-6-substituted

benzo[b]thiophene-2-carbohydrazide) (H2L3 and H2L4) and their

copper(II), cobalt(II), nickel(II) and zinc(II) complexes. The ligands

have been characterized by elemental analysis, IR, 1H NMR and ESI-

mass spectral techniques whereas the complexes have been

characterized by elemental analysis, IR, 1H NMR, ESI-mass, electronic

spectra, conductivity measurements, magnetic susceptibility, ESR,

TGA and powder-XRD methods. Electrochemical behaviour of Cu(II)

complexes of both the ligand has been studied by the cyclic

voltametry. Antimicrobial screening of the synthesized ligands and

their complexes were carried out. DNA cleavage and antioxidant

activity of ligand H2L3 and its complexes were tested and their results

have been discussed.

CHAPTER I

26

CHAPTER V: - It deals with the characterization of Schiff bases ligands 5-

substituted-N'-((7-hydroxy-4-methyl-2-oxo-2H-chromen-8-

yl)methylene)-3-phenyl-1H-indole-2-carbohydrazide (HL5 and

HL6) and their copper(II), cobalt(II), nickel(II) and zinc(II) complexes.

The ligands have been characterized by elemental analysis, IR, 1H

NMR and ESI-mass spectral techniques whereas the complexes have

been characterized by elemental analysis, IR, 1H NMR, ESI-mass,

electronic spectra, conductivity measurements, magnetic susceptibility,

ESR, TGA and powder-XRD methods. Electrochemical behaviour of

Cu(II) complexes of both the ligand has been studied by the cyclic

voltametry. Antimicrobial screening of the synthesized ligands and

their complexes were carried out. DNA cleavage and antioxidant

activity of ligand HL5 and its complexes were tested and their results

have been discussed.

CHAPTER VI: - This chapter describes the characterization of Schiff base ligand 3-

chloro-N'-((7-hydroxy-4-methyl-2-oxo-2H-chromen-8-

yl)methylene)benzo[b]thiophene-2-carbohydrazide (HL7) and its

copper(II), cobalt(II), nickel(II) and zinc(II) complexes. The ligand has

been characterized by elemental analysis, IR, 1H NMR and ESI-mass

spectral techniques whereas the complexes were characterized by

elemental analysis, IR, 1H NMR, ESI-mass, electronic spectra,

conductivity measurements, magnetic susceptibility, ESR and TGA

methods. Electrochemical behaviour of Cu(II) complex has been

studied by the cyclic voltametry. Antimicrobial, DNA cleavage and

antioxidant activity screening of the synthesized ligand and its

complexes were carried out and their results have been explained.

CHAPTER VII: - This chapter deals with the characterization of Schiff base ligand

5-chloro-N'-(4-(dimethylamino)benzylidene)-3-phenyl-1H-indole-

2-carbohydrazide (L8) and its copper(II), cobalt(II), nickel(II) and

zinc(II) complexes. The ligand has been characterized by elemental

analysis, IR, 1H NMR and ESI-mass spectral techniques whereas the

complexes have been characterized by elemental analysis, IR, 1H

NMR, ESI-mass, electronic spectra, conductivity measurements,

CHAPTER I

27

magnetic susceptibility, ESR, TGA and powder-XRD methods.

Antimicrobial screening of the synthesized ligand and its complexes

were carried out. DNA cleavage and antioxidant activity of ligand L8

and its complexes were tested.

SUMMARY AND CONCLUSIONS: -At the end of the thesis overall summary of

the present work with general conclusions on the bonding modes of the

Schiff base ligands and their metal complexes along with a brief

account of the possible routes and scope for further work in this field

has been dealt with.

CHAPTER I

28

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