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Chapter 1

Chapter 1

Introduction

1. INTRODUCTION

Coordination chemistry, the science concerned with the interaction of

organic / inorganic ligands with metal centers has remained one of the most

active research areas in inorganic chemistry.

The study of coordination chemistry in the modern day perspective was

initiated due to the significant contributions of two notable scientists; Alfred

Werner and Sorphus Mads Jorgenson. The pioneering contribution of

Werner to the study of coordination chemistry was well recognized by the

Nobel Prize awarded to him in the year 1913. Many of the basic ideas

related to the stereochemistry of metal complexes and the mechanism of

isomerization given by Werner has remained unchanged even today. The

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

metal complexes are now better understood with the help of advanced

sophisticated physicochemical techniques of high precision and capability.

Subsequent to the pioneering work by Werner and Jorgenson, the research in

chemistry has progressed a lot. The stepping stone provided by Werner and

Jorgenson has led the modern inorganic chemistry in the direction of

becoming truly a multidisciplinary subject in the present day context [1].

Coordination chemistry is found to be applicable in the fields of wide

diversity such as dyes, colors, nuclear fuels, catalysis, photography,

Chapter 1

toxicology, bioinorganic chemistry, medicine, ceramics, materials science

and toxicology [2-4].

Progress of the industries dealing with organic chemicals, pharmaceuticals,

petrochemicals and plastics depends a lot on the findings in the field of

coordination chemistry.

Nature makes extensive use of coordination compounds and the study of

such compounds is becoming increasingly popular in biology as well as in

chemistry. Inclusion of a variety of ligands in complexes has enabled their

applications as biocides, catalysts, NMR shift reagents and DNA binders [5].

The study of interaction of small molecules and metal complexes with DNA

is an active area of research at the interface of chemistry and biology [6].

Many of the biologically active compounds are transition metal complexes

and even the simpler types of metal complexes have served as model

compounds in investigating natural processes. The living system is also

partially supported by coordination compounds. Hemoglobin, an iron

complex, carries oxygen to the animal cells [1, 7].

In coordination chemistry, the search for molecular magnets with specific

properties is a challenging field for a synthetic chemist. The suitable designs

of new compounds with the improved features of magnetic properties are of

great interest for technological applications [8, 9].

Development of sophisticated analytical instruments and a synthesis of a

wide variety of coordination compounds have encouraged many researchers

to revisit the chemical reactions. This study has enabled the inorganic

chemists to make significant progress in the modification of the concept of

chemical bonding.

Chapter 1

1. 1 Ligand

In coordination chemistry, a ligand is an atom, an ion or a molecule attached

to a central atom / ion which are usually a metal to form a coordination

complex [10].

The atoms and molecules used as ligands are capable of functioning as

donors of electron-pair in the bond formed with the central metal atom / ion.

Ligands contain at least one donor atom with a lone pair of electrons used to

form covalent bonds with the central metal ion or atom with which they are

attached. Thus, the ligands act as Lewis bases and the central metals act as

Lewis acids. The central metal usually has a positive charge which is

stabilized by donation of electrons from the ligands. The nature of metal –

ligand bonding can be covalent or ionic. Ligands can be anions, cations or

neutral molecules. Examples of common ligands are the neutral molecules

like water (H2O), ammonia (NH3) and carbon monoxide (CO) and the anions

like cyanide (CN-), chloride (Cl

-), and hydroxide (OH

-). Cations like NO

+

and N2H5+ are also capable of acting as ligands [11].

Ligands are classified in different ways based on their charge, size, identity

of the coordinating atom(s), and the number of electrons donated to the

metal [12].

Ligands that bind the central atom through only one site are known as

monodentate ligands. Monodentate ligands are sometimes referred to as "one

toothed" because they bind to the central metal atom at one point. Some

examples of monodentate ligands are chloride ions (referred to as chloro

when it is a ligand), water (referred to as aquo / aqua when it is a ligand),

Chapter 1

hydroxide ions (referred to as hydroxo when it is a ligand) and ammonia

(referred to as ammine when it is a ligand).

When ligands have two donor atoms which allow them to bind the central

metal atom or ion at two points are called bidentate ligands. Common

examples of bidentate ligands are ethylenediamine (en), and the oxalate ion

(ox) as shown in figure 1.

Figure 1: Bidentate ligands.

Some ligand molecules containing many atoms capable of donating lone pair

of electrons are able to bind the metal ion through multiple sites. These

ligands are called polydentate ligands [13].

Diethylene triamine (dien) is an example of a tridentate ligand. Tridentate

ligands are also known as ‘scorpionate ligands’. EDTA

(ethylenediaminetetraacetate) has six sites available for bonding. Hence, it is

an example of hexadentate ligand as shown in figure 2.

Chapter 1

Figure 2: Polydentate ligands.

Most of the complexes of polydentate ligands are chelated complexes; they

tend to be more stable than the complexes of monodentate ligands as it is

necessary to break all of the bonds to the central atom for the ligand to be

displaced [14].

1. 2 Schiff base ligands

The condensation of primary amines with aldehyde and ketones gives

compound with a functional group that contain a carbon-nitrogen double

bond with the nitrogen connected to an alkyl group called Schiff bases, since

their synthesis reported by Hugo Schiff (1864) [15].

The general formula in the broadest sense of the Schiff base is R1R2C=NR3,

where R is an organic side chain. The Schiff base is synonymous with

azomethine. The presence of a lone pair of electron in sp2 hybridized orbital

of nitrogen atom of azomethine group is of considerable chemical

importance [16, 17]. This lone pair imparts an excellent chelating ability

especially when used in combination with one or more donor atoms close to

Chapter 1

azomethine group. So, Schiff bases are capable of forming coordinate

covalent bonds with many of the metal ions through azomethine and

phenolic group or other group present on the ligand [18, 19].

Chromatographic purification of Schiff bases on silica gel is not

recommended as they undergo hydrolysis. Examples of few such ligands are

shown in figure 3. A large number of Schiff base complexes have been

extensively investigated for their catalytic and biological activity [20].

Figure 3: Schiff base ligands.

1. 3 Chelation

Chelation is a process in which a polydentate ligand bonds to a metal ion

forming a ring. The complexes involving a ring in which metal ion is an

Chapter 1

integral part is called a chelate and the polydentate ligand is referred to as a

chelating agent.

Chelates are generally more stable than complexes derived from

monodentate ligands. This enhanced stability known as chelate effect is

usually accredited to effects of entropy which favors the displacement of

many ligands by one polydentate ligand [12]. When the chelating ligand

forms a large ring that at least partially surrounds the central atom and bonds

to it leaving the central atom at the centre of a large ring, the complexes are

called macrocyclic complexes [21]. These complexes are generally more

rigid and involve higher ligand denticity. These complexes are more inert.

Hemoglobin is an example of macrocyclic complex. Hemoglobin is a

complex with a porphyrin macrocycle and an iron atom at the center bound

to four nitrogen atoms of the tetrapyrrole macrocycle. A very stable

dimethylglyoximate complex of nickel is a synthetic macrocycle derived

from the anion of dimethylglyoxime ligand [22].

Ethylenediaminetetraacetic acid (EDTA), with four carboxylic acid groups

along with two amine groups can form six bonds with a metal ion and is a

popular hexadentate ligand in coordination chemistry. Under suitable

conditions, EDTA forms chelates with a wide variety of metal ions. These

chelates contain five 5-membered rings in their structure. The

hexacoordinated complexes from EDTA are octahedral in geometry as

shown in the figure 4.

Chapter 1

Figure 4: Chelated complex of EDTA.

1. 4 Transition metal complexes

The transition metals are known to form a wide variety of complexes with

different ligands. The formation of M-L bond requires the metal to possess

vacant orbitals known as secondary valency in coordination chemistry.

These orbitals must have suitable orientation in the space with reasonably

low energy. Since, the transition metal ions generally meet these

requirements best, they form complexes readily. Almost all the transition

elements form coordination complexes due to the availability of their vacant

d-orbitals [10].

Following characteristics make the transition metals particularly interesting

to coordination chemist:

Variable oxidation states (electron transfer properties),

Chapter 1

Spectral and magnetic features, ligand field effects, unpaired d-

electrons,

Formation of chelated complexes,

Coordination geometries (tetrahedral, octahedral, square planar,

pyramidal etc.)

Most M2+

and higher oxidation states are borderline or hard acids and

generally prefers borderline or hard base such as O- and N-donor

groups, whereas lower oxidation states bind soft bases.

Metal coordination complexes have a wide diversity of technological and

industrial applications from catalysis to anticancer drugs [23].

1. 5 Applications of metal complexes

Metal complexes plays essential role in pharmaceutical, petrochemical,

plastic and agricultural industries. Particularly, the Schiff base complexes

are widely applicable in various biological systems, catalysts, polymers,

dyes as well as enzymatic agents [20].

I. Catalysis

Many Schiff base metal complexes show excellent catalytic activity in

various reactions at temperatures above 100oC in the presence of moisture.

These types of applications have been reported both in homogeneous and

heterogeneous catalysis [24]. The Schiff base metal complexes are capable

of catalyzing reactions like hydrolysis, oxygenation, electro-reduction and

decomposition [25]. The production of commercially important branched

and linear polyethylenes is increased by the catalytic activity of Schiff base

metal complexes in polymerization of olefins [26]. The transition metals like

Chapter 1

tungsten, molybdenum and ruthenium in the presence of alkylating agents

such as R4Sn or RAlCl2 catalyzed the ring opening polymerization of

cycloalkenes at high temperature without any control on the molecular

weight of polymers [27]. Schiff base metal complexes showed important

applications in the reduction of ketones to alcohols and alkylation of allylic

substrates [28]. The oxidation of hydrocarbon using Schiff base metal

complexes has been a field of academic and industrial interest to analyze the

catalytic activity of various metal complexes. Schiff base metal complexes

also show catalytic activity in the carbonylation of alcohols and alkenes at

low pressure to produce α-arylpropionic acid and their esters [29].

In addition to monometallic, the bimetallic Schiff base complexes also show

catalytic activity in carbonylation reaction. The Heck reaction, an

industrially useful process to synthesize fine chemicals and pharmaceuticals

has been catalyzed successfully using Schiff base complexes [30]. The

Schiff base complexes of nickel(II) and copper(II) ions have increased

enantioselectivity in the alkylation of enolates [31]. The Schiff base

complexes of copper(II) and manganese(III) moderately improved the

enantiomeric synthesis of aziridines and amides with chiral

metalloporphirins [32]. The isomerization of norbornadiene to quadricyclane

was considerably catalyzed using diimine complexes of rhodium. These

types of interconversions were useful for the storage of solar energy [33].

The aluminium-salen Schiff base complexes when used in catalyzing the

addition of hydrogen cyanide to N-allylbenzaldimine showed enantiomeric

excess [34]. The optically active cyanohydrins are extensively used in the

synthesis of drugs and pesticides. These cyanohydrins were synthesized

successfully reacting trimethyl silylcyanide with aldehydes in the presence

Chapter 1

of transition metal Schiff base complexes [35]. Schiff base complexes are

also useful in desymmetrization of meso compounds with significant yield

and enantiomeric excess [36]. The homogeneous chiral lanthanum(III)

Schiff base complexes showed catalytic activity in asymmetric Diels–Alder

reactions [37]. These studies point out that the Schiff base metal complexes

are potential catalysts to influence the yield and selectivity in chemical

transformations.

Dinuclear Schiff base complexes of transition metal ions are more efficient

catalysts than mononuclear complexes probably due to the synergic effect of

two metal ions. A variety of homo and hetero dinuclear Schiff base

complexes of transition metal ions are used as catalysts in various reactions

[38]. The dinuclear Schiff base complexes of copper (II) are used as

catalysts in oxygen activation [39]. Dinuclear Schiff base complexes of

copper (II) ions prepared from 2-hydroxyisopthalaldehyde and 2-(pyridine-

2-yl)ethanamine as shown in figure 5(b), are used successfully in

hydroxylation of phenol [40]. The homo and hetero dinuclear complexes of

transition metal ions are effective catalysts in direct oxygenation of

unfunctionalized hydrocarbons and phenols. The dinuclear complexes

formed from the palladium ion as shown in figure 5(a) are also useful in

oxidation reactions.

Chapter 1

(a) Dinuclear palladium complex (b) Dinuclear copper complex

Figure 5: Dinuclear complexes used as catalyst.

II. Materials science

The magnetic properties of solids are very important and attempts to

understand them have provided a deep insight into the fundamental structure

of many coordination complexes [41]. Nowadays, magnetic materials are

being used in biomedical applications like contrast agent in magnetic

resonance imaging (MRI), targeted drug delivery and magnetic separation of

cell, DNA, protein etc.

Magnets are also used for storage of information in various devices like

tapes, hard discs and other memory devices [42]. Reduction the size of

particles can permit the size reduction of several hardwares used in computer

related applications. A type of magnetism known as superparamagnetism is

observed in some ferromagnetic and ferrimagnetic nanoparticles. Under the

influence of temperature, the magnetization in some small nanoparticles is

known to flip the directions randomly. The time gap between two such

consecutive flips is called Neel relaxation time [9]. In absence of the

external magnetic field if the time used to measure magnetization is greater

Chapter 1

than the Neel relaxation time, the average magnetization appears as zero and

the material is said to be in superparamagnetic state. Under such conditions,

the behavior of the materials in external magnetic field becomes similar to a

paramagnetic material with a very high magnetic susceptibility [43, 44]. The

usage of such small nanoparticles has facilitated the size reduction in

hardware. However, there is a lower limit to the size of the particle provided

by superparamagnetic size below which the information cannot be

permanently stored due to the free fluctuation of the magnetization.

Recently, it has been discovered that molecules containing transition metal

ions show a slow relaxation of magnetization at low temperature and

behaving like superparamagnets [45]. This is a very interesting property,

because they give rise to the magnetic hysteresis effect and become bistable.

Thus, there is a possibility to open a storage of information in a single

molecule.

Slow relaxation of magnetism in some higher nuclearity polynuclear

complexes is known to occur via quantum tunneling mechanism [46, 47].

When the magnetization returns to the ground state it does not follow the

rules laid down by the classical mechanics which involves going over the

energy barrier (Figure 6, route 1) [48]. A single molecular magnet (SMM)

with the spin state of Ms = 10 at the ground state when placed in the

magnetic field so that the spin state becomes Ms = -10. When the field is

removed, it does not return to Ms = 10 by going from Ms = -10 → Ms = -9

→ Ms = -8 …. Ms = 0 →….Ms = 9 → Ms = 10. Instead, the spins states are

tunneled from one spin state to another so that it does not go through all the

levels (route 1, route 2, route 3). This occurs only if the two spin states are

equal in energy or may be observed by steps in the hysteresis curve. The

Chapter 1

quantum tunneling mechanism enables the use of these complexes in the

study of quantum effects or in the quantum computing as quantum bits [49].

The polynuclear complexes are equivalent to binary units in classical

computing [50]. The advantage of the quantum computers is that they have

the potential to perform the same functions as conventional computers but

on a smaller scale.

Figure 6: Quantum tunneling in single molecular magnets.

( Route 1: No quantum tunnelling. Route 2: Thermally assisted quantum

tunnelling. Route 3: Pure quantum tunnelling.)

The magnetic properties of poly- and dinuclear transition metal complexes

are characterized by (a) the exchange coupling between the transition metal

cations, given by the exchange integral J (direct exchange, super exchange,

double exchange), (b) the electronic g-factor, describing the splitting of

degenerate electronic levels in an external magnetic field, and (c) the zero-

field splitting parameter D which contains information on local geometrical

distortions and spin orbit interactions.

Chapter 1

The quantum tunneling is observed generally in the inorganic metal

complexes having more than one metal centers. The magnetic interaction

between two nonequivalent paramagnetic centers may lead to situations

which cannot be encountered in species containing only one kind of metal

center. The polynuclear complexes of transition metals can act as single

molecular magnet. Especially in the polynuclear complexes, there is

increasing interest in dinuclear complexes, which contain ligand structures

capable of holding two metal centers in close proximity. These dinuclear

complexes have formed the basis for a detailed understanding of magnetic

exchange interactions and provide fundamental insights into the geometrical

and electronic properties of the bridged group [51, 52]. The field of

molecular magnetism has developed rapidly in the last two decades and

particular emphasis has been placed on hetero bimetallic complexes. In

physicochemical aspects these dinuclear complexes have noteworthy

significance as new inorganic materials. They show various magnetic

properties with antiferromagnetic coupling depending upon the bridge angle

and degree of distortion. The dinuclear complexes also show redox

properties based on the macrocyclic ring size and show redox driven

translocation mechanism [53].

The presence of two metals in the same molecule largely affects both the

physical properties and the reactivity of the complexes. This is either due to

the significant modifications in the individual properties of two metals are in

the development of new characteristics not observed in mononuclear

complexes. The physical properties of these dinuclear metal complexes

(redox properties, florescence etc.) vary to a great extent [54]. In the case of

polymetallic complexes, it is possible to study the effects of chemical,

Chapter 1

electrochemical or photochemical modification of one metallic center on the

properties of the other. The reciprocal interaction between the active centers

often brings amazing results. A great interest for transition metal complexes

arises not only from the above mentioned particularities but also from the

wide range of arrangements, which can be created due to the huge flexibility

in their structures [55]. Besides the availability of a large number of

transition metals it is often sufficient through simple reactions to vary either

the nature of the auxiliary ligands bound to the metals or the oxidation state

of the metals themselves in order to induce particular chemical and physical

properties in the system [56].

III. Biology

The metal complexes exhibit a lot of bioactivities such as antimicrobial

activity, antiviral activity, antifungal activity, analgesics, HIV protease and

anticonvulsants [20].

a. Antimicrobial activity

Schiff bases and their complexes exhibit antimicrobial activity according to

types of metals and ligands. Schiff bases prepared from indane-1, 3-dione-2-

imine-N-acetic acid and 2-imino-N-2-propionic acid and ninhydrin, glycine

/L-alanine and their metal(II) chelates shows unique geometries and good

antimicrobial activities against E. coli, P. mirabilis, S. aureus and P.

faecalis. Various metal complexes with aniline exhibit different behavior

with different types of bacteria. Some aldimines, pyrazines, aminoacids

derived Schiff base and heterocyclic ketones derived Schiff bases and their

complexes also exhibit antimicrobial activity [57]. Schiff bases and metal

complexes derived from benzimidazole, toluidinones, furaldehydes,

Chapter 1

thiazoles, pyridine, pyrazolone, hydrazine and imidazolinones exhibits

antibacterial activity. Salicylidine derivatives and metal complexes exhibit

antibacterial activity against S. typhi, S. aureus, B. subtilis and S. flexneri

[58, 59].

b. Antifungal activity

Thiazole and benzothiazole Schiff bases show effective antifungal activity

[60]. Presence of methoxy, halogen and napthyl groups enhance fungicidal

activity towards Curvularia. Furfurglidene nictoinamide Schiff base

exhibits antifungal activity against A. niger, Alternaria solani and

Collectotricum capsici. Pyrandione Schiff bases exhibits physiological

activity against A. niger. Some Schiff bases of quinazolinones show

antifungal activity against Candida albicans, Trichophyton rubrum, T.

mentagrophytes, A. niger and Micosporum gypseum [61, 62].

Schiff bases and their metal complexes formed between furan or

furylglycoxal with different amines show antifungal activity against

Helminthosporium gramineum (causing stripe disease in barely),

Syncephalostrum racemosus (causing fruit rot in tomato) and C. capsici

(causing die back disease in chillies). Moreover, hydrazine and

carbothioamide as well as their metal complexes exhibit antifungal activity

against A. alternata and H. graminicum. Molybdenum and manganese

complexes control disease (caused by A.alternata) in brinjal crop. Schiff

bases derived from benzothiazole or phenyl-azo-thiazole and its metal

complexes show microbiological activity against A. niger and A. alternata.

The salicyladmine Schiff base complex of ruthenium(II), thalium(I)

complexes with benzothiazolines, copper(II) complexes of the

benzoylpyridine Schiff base show antifungal activities [63].

Chapter 1

Oxovanadium(IV) complexes with triazole exhibits antifungal activity [64].

As(III), Sb(III), and Bi(III) complexes with o- tolylammonium di-

thiocarbamate are antifungal against A. niger and A. alternata. Some novel

Schiff bases derived from cephalexin and their metal complexes show

antifungal activities. Schiff bases derived from salicylaldehydes and

boronate esters exhibited antifungal activities against A. niger and A. flaves.

The Schiff base prepared from salicylaldehyde and di-methyl

thiophosphoramide and their complexes with Cu(II), Ni(II), and Zn(II) are

effective chemicals to kill Tetranychus bimaculatus [65, 66].

c. Antiviral activity

Schiff bases of gossypol shows high antiviral activity. Silver complexes in

oxidation state (I) exhibited inhibition against Cucumber mosaic virus;

glycine salicylaldehyde Schiff base Ag (I), gave effective results up to

74.7% towards C. mosaic virus [67].

d. Anti – tumor and cytotoxic activity

Salicylidiene anthranilic acid possesses antiulcer activity and complexation

behaviour with copper complexes, which exhibit an increase in antiulcer

activity. Some Schiff bases and their metal complexes containing Cu, Ni, Zn

and Co were prepared from salicylaldehyde, 2,4 dihydroxy- benzaldehyde,

glycine and L-alanine possess antitumor activity and their order of reactivity

with metal complexes is Ni˃Cu˃Zn˃Co [68].

The mononuclear complex of platinum cisplatin as shown in figure 7(A) is

currently used in the treatment of testicular, ovarian, head and neck, neck

and cervical cancers. However, a large percentage of human cancers have a

natural resistance to treatment with cisplatin. Nowadays, there is good

Chapter 1

development in the multinuclear platinum complexes shown in figure 7(B),

that can used as anticancer drugs. Multinuclear platinum complexes contain

two or more linked platinum centers that can each covalently bind to DNA.

Hence these are capable of forming a completely different range of DNA

adducts compared to cisplatin. These multinuclear complexes have great

potential as new anticancer agents [69].

(A): Cisplatin (B): Dinuclear linked cisplatin complex.

Figure 7: Anticancer platinum complexes.

Transition metals have an important place within medicinal biochemistry.

There is significant progress in the utilization of transition metal complexes

as drugs to take care of several human diseases. Transition metals exhibit

different oxidation states and can interact with a number of negative charged

molecules. This activity of transition metals has started the development of

metal based drugs showing potential pharmacological applications [70].

IV. Medical therapy

The carbon monoxide (CO) and nitric oxide (NO) affects the biological

cycle can be appropriately described by the general observation of

Chapter 1

Paracelsus in the early 16th

century that “the dose makes the poison”. These

diatomic molecules are odorless, colorless gases that were long known only

as toxic poisons until the discovery in the 1980s of NO as the endothelium-

derived relaxing factor [71]. The nitric oxide (NO) is now known as an

important signaling molecule that impacts a wide range of physiological

responses, including blood pressure regulation, neurotransmission, immune

response and cell death. The information on the biological roles of CO is

about 15 years behind that of NO. But, it appears to play a role in some of

the same pathways as NO in addition to other important processes.

The concentration dictates the biological effect in both the cases, with low

concentrations being critical for signaling events and high concentrations

being toxic. These gas molecules also have the common importance in the

transition metal coordination chemistry which increases their biological

impact. Also, their biosynthesis is metal-dependent, and in many cases their

biological targets are metal centers, typically heme iron. These reactions

notify that transition metal complexes in biological environments can

liberate and sequester NO and CO by breaking or forming metal-nitrosyl

(M-NO) and metal-carbonyl (M-CO) bonds. Therefore, it is possible to

develop artificial small molecule coordination complexes that can detect NO

and CO in order to study their biological roles and also act as therapeutic

agents [3].

2. POLYTOPIC LIGANDS

Dinuclear and polynuclear transition metal complexes bridged by

polyatomic ligands have gained much concentration in the recent years

towards synthesis, characterization and applications. Transition metal

Chapter 1

complexes of such type have become subject of methodical investigation

due to their applications in the study of intermetallic interactions, magnetic

exchange, catalysis and binuclear metal reactivity. Polynuclear complexes

are interesting both from the magnetochemical point of view as well as

potential model of biological systems [72].

The behavior of single metal ions embraced by a monodentate as well as the

chelating ligand environment has been studied extensively till date [19, 73].

In such systems individual metal centers are usually too far from each other

which restricts the possibility of spin-spin coupling between paramagnetic

metal ions. Polynucleating ligands, on the other hand, have structural

attributes that combine separate coordination pockets. They are continuously

arranged, metal ions are bound at a smaller distance and can be linked

directly by endogenous or exogenous ligand fragments which can lead to

spin communication between metals [74].

Polytopic ligands can be designed in such a way that the synthesis becomes

convenient. Such ligands range from ditopic to hexatopic displaying

different geometry and length. They may also contain a variety of functional

groups. Such a rich library of ligands gives an opportunity to systematically

study the relationship between the geometry of the ligand and the

functionality of the product obtained. The increasing interest for hybrid

ligands (hard and soft type) has induced new chemical and physical

properties in their complexes. Some examples of the polytopic ligands with

hard coordination functions containing oxygen, nitrogen as donor atoms are

discussed here [75].

Chapter 1

2. 1 Diketones

Classical β-diketones and related ligands have been studied for more than a

century. Their ability to give rise to rich and interesting coordination

chemistry is well recognized. They act under appropriate conditions as

nonnegative O2-chelating donors, capable of stabilizing mononuclear or

polynuclear complexes. In particular, their keto-enol tautomerism has been

studied in solution by IR and NMR spectroscopy and in the solid state by X-

ray single crystal diffraction as shown in figure 8 [76].

Figure 8: Keto-enol tautomerism in diketones.

There is a growing amount of literature which especially deals with the

possible applications of these complexes as components of molecular

devices or as precursors in the formation of new materials. The application

of the complexes as phosphors for lighting and high efficiency

electroluminescent devices for light emitting diodes, contrast agents for

medical magnetic resonance imaging, NMR shift reagents, luminescent

probes for proteins and amino acids, magnetically addressable liquid

crystals, magnetic alloys for refrigeration and superconducting materials

give good reason for the efforts made to control the metallic sites and to

selectively introduce specific metal ions into organized assemblies [77].

Chapter 1

Here, the increasing of carbonyl groups with the consequent formation of tri-

or tetraketones and bis-β-diketones as shown in the figure 9 allows the

formation of well-defined homo- and /or heteropolynuclear complexes [78].

Figure 9: Poly-diketones.

These complexes have unusual physico-chemical properties, arising due to

the coordination of equal or different metal ions in close connection and

interacting with each other through the carbonyl bridges. Moreover, these

ligands generally coordinate in the equatorial plane of metal ions like

copper(II), nickel(II), cobalt(II).

Furthermore, the insertion of phenol groups at the end or between the

carbonyl moieties generates an interesting series of β-ketophenolate systems

as shown in figure 10, capable of securing up to eight metal ions in close

proximity. The metal ion assembly is mainly governed by the presence or the

release of the phenol protons [79].

Chapter 1

Figure 10: Keto-phenol derivatives

2. 2 Dicarboxylates

The dicarboxylate anion is capable of generating homo- and

heteropolynuclear complexes. The wide structural diversity of the oxalato-

bridged complexes (binuclear species to high-nuclearity compounds and to

two and three dimensional coordination polymers) is due to the outstanding

flexibility of the oxalato ligand as shown in figure 11 [80]. The oxalate

ligand is the simplest dicarboxylate. The oxalato bridge can powerfully

mediate the exchange interactions between the paramagnetic metal ions

shows interesting magnetic properties.

Chapter 1

Figure 11: Modes of coordinations of dicarboxylates.

Numerous oxalato-bridged homopolynuclear complexes have been

synthesized and characterized till date. These compounds are generally

obtained through the reaction between a cationic complex having potentially

free coordination sites and the oxalate anion [81].

2. 3 Phenolic ligands

In 1970, Robson defined a class of ligands able to simultaneously bind two

or more metal ions thus forming di- or polynuclear metal complexes [82].

From amongst the many different types of dinucleating ligands and

Chapter 1

polynucleating ligands, the phenol-based molecules have involved the

attention of a great number of researchers. This is due to the key role played

by the phenolic group which has many useful electronic and structural

characteristics such as:

(i) Charge as a function of pH: The charge in such complexes ranges

from 0 at low pH to −1 at high pH values;

(ii) Bridging capability: The phenolic donor atom is capable of binding

two metal centres in close proximity;

(iii) Effect of aromatic ring: The benzene ring present allows a great

synthetic flexibility.

Regarding the first point, the acid–base properties of the phenol in aqueous

solution depends on the ligand topology in which it is inserted; in fact,

although the phenol loses its acidic proton at pH > 10 giving the phenolate

H−1L− anionic species, this process can occur also at lower pH values when

the phenol takes part of an amino-phenolic ligand. Often, it is the H−1L

species involved in the second point, which is the phenolate oxygen atom to

bridge two metal centres [83].

I. Ligands containing one phenolic unit:

The ligand with one phenol unit has appeared as an important class of

acyclic ligands capable of binding two metal ions close to each other in

addition to bridging abilities of the phenolate oxygen atom towards metal

ions. Some of the phenolic ligands containing amine or polyamine side arms

as shown in figure 12. The basicity behavior and coordination properties in

aqueous solution of these types of ligands are reported [84].

Chapter 1

Figure 12: Ligand containing one phenolic unit.

Some of the phenolic ligands bearing the imine and pyridine side arms as

shown in figure 13.

Figure 13: Phenolic ligand with imine and pyridine side arms.

Chapter 1

II. Ligands containing two phenolic units:

The ligands containing two phenolic ligands can form the dinuclear as well

as polynuclear complexes. The symmetrical biphenolic N5O2 donor ligand

(2,6-bis(2-hydroxyphenyl) pyridine) forming µ-phenoxo bridge complexes,

whereas some tetradentate amino diphenolic ligands such as (N,N’-bis[(2

hydroxyphenyl] methyl)-1,3-propylendiamine) forming complexes without a

bridge is shown in figure 14 [85].

Figure 14: Ligands containing two phenolic units.

Some ligands containing one phenolic unit with reactive group (aldehyde)

can react with diamines to form the Schiff base ligands with polynucleating

atoms. Bis (o-vanillin) benzidine prepared by the Schiff base reaction of o-

vanillin and benzidine is shown in figure 15. These types of ligands

generally used for the formation of homonuclear complexes.

Figure 15: Phenolic Schiff base ligand.

Chapter 1

Different oximes and metal oximates are established to be versatile ligands

in polynuclear complexes due to their ambidentate character with potential

for oxygen and nitrogen coordination. Since 1905, when Tschugaeff first

introduced dimethylglyoxime as a reagent for detection of nickel, oximes as

ligands have played an important role for the long-term progress in

coordination chemistry. Due to the flexibility in bonding, oximes are

excellent bridging units in modular synthesis [86]. Modular preparation with

oximate ligands enables the synthesis of linear symmetrical and

asymmetrical cores containing two different metal ions [87]. Some oximes

which have been used for synthesizing polynuclear complexes are listed

below as shown in figure 16.

Figure 16: Different types of oxime ligands.

Chapter 1

2. 4 Bridging ligands

Bridging ligands have received much interest recently due to their ability to

couple metal centers in a covalent manner resulting in polymetallic

complexes that frequently possess new and interesting properties. Bridging

ligands of the type discussed here bind to each metal through one or more

donor atoms forming a coordinate covalent σ-bond. Due to the presence of

π-systems on the ligands, commonly π- backbonding or less commonly π-

bonding, can lead to enhanced stability of the metal-ligand bonds. The

denticity of a ligand is commonly used to describe its bonding [88].

Herein, monodentate bridging ligands include the class of ligands that bind

to each metal center through only one donor atom. These monodentate

bridging ligands are historically significant, thus some of their chemistry is

also included in the discussion. The monodentate bridging ligands

containing primarily nitrogen-donor atoms such as cyanide, linked pyridines

and fused pyrazine ring systems as shown in figure 17. The fused pyrazine

have two donor atoms but the each donor atom can bridge only one metal

centre [89].

Figure 17: Monodentate bridging ligand.

Bidentate bridging ligands encompass the broad and rapidly expanding field

of ligands which bind to each metal center through two donor atoms. Such

bidentate ligands have generated a lot of recent research in this field and lead

Chapter 1

to the study of a wide assortment of supramolecular complexes using these

ligands as the connectors. Bidentate bridging ligands bind to each metal by

two donor atoms. This gives added stability through the chelating effect.

Ligands such as 1,10-phenanthroline and 2,2′ bipyridine as shown in figure

18, are the bidentate bridging ligands [90].

Figure 18: Bidentate bridging ligands.

Tridentate bridging ligands include bridging ligands that bind to multiple

metal centers using three donor atoms per metal center as shown in figure

19.

Figure 19: Tridentate bridging ligand.

Tridentate bridging ligands bind two or more metal centers through three

donor atoms per metal center. The pyridine based bridging ligands and

phenanthroline based bridging ligamds are tridentate bridging ligands. The

function of such bridging ligands is also to prevent undesired

oligomerization processes. The bridging ligands including di-, tri- and tetra-

Chapter 1

amines, 2,2′-bipyridine, 1,10-phenanthroline, 2,2′,6′,2′′-terpyridine have

been used as end-cap ligands [75].

3. METAL ORGANIC LIGANDS (MOLS)

In the last twenty five years, complexes of transition metals with higher

nuclearity has attracted the attention of many coordination chemists. This

class of compounds is found in the literature with several names such as

oligomeric complexes, polynuclear complexes, cages and clusters [91]. A

central question for the synthesis of polynuclear complexes is that ‘Are there

any general methodologies for the template synthesis of polynuclear

complexes ?’ In general the answer is ‘No’. In contrast to the predictive

synthetic schemes applied in organic chemistry, the reaction in polynuclear

transition metal chemistry cannot be described as operator controlled. In

order to enable the tailor making of polynuclear complexes, the greatest

concern is the development of synthetic routes that can provide metal

complexes with high nuclearity, particularly heterometallic systems in a

controlled manner [92]. Over the last 15 - 20 years several groups (Escuer

and Vicente, Decurtins, Christou, Thompson, Verdaguer etc.) have

commendably attempted the development of route to the synthesis of such

compounds.

From amongst the variety of methodologies applied to synthesize

polymetallic coordination compounds, the use of ‘metalloligands’, i.e. metal

complexes as ligands, in which the ligands already bounds to one metal have

free coordination sites that can bind a second metal of the same or a different

kind has proven to be very successful [93]. Different complexes containing

one or more metal centers are able to react further with other complexes

Chapter 1

through available appropriate donor atoms. One should begin with

monometallic precursors which can then be assembled by chemical reaction

to form polymetallic complexes. These types of complex precursors

considered as ligand named as metal organic ligands (MOLs) [94, 95]. The

well recognized approach ‘metal complexes as ligands’ involving

multinucleating ligands offers many potential advantages over the self-

assembly route and it enables greater control over the route of the reaction

and the products [96]. The MOL approach allows the synthesis of larger

molecules by reaction of mono or binuclear complexes of appropriate

ligands. These approach containing polynucleating ligands proceeds step by

step and provide a route to gain control of the nuclearity in addition to the

preparation of species containing different metal ions, i.e. heterometallic

complexes. Thus, in this manner, it is possible to build polymetallic systems

in a stepwise fashion having both homo and hetero-variants [97].

3.1 Homopolynuclear complexes

The homopolynuclear complexes synthesized by polytopic ligands allows

the coordination of metal ions through the reactive groups having donor

atoms such as nitrogen, oxygen or sulfur. This self-assembly approach

allows the rational preparation of linear double- and triple-stranded

oligonuclear complexes from mono- to di- and trinuclear species depending

on the molecular symmetry, coordination mode of the polytopic ligands and

the preferred coordination geometry of the metal ions as shown in figure 20

[98].

Chapter 1

Figure 20: Homopolynuclear complexes.

The polynuclear complexes are prepared for different purposes. The pursuit

of the molecular magnetic materials requires an intelligent design of the

ligand which can organize the paramagnetic metal ions in a desired topology

and efficiently transmit electron exchange interactions between the metal

ions. This basic principle is remarkably supported by the work on the

magnetic properties of oxamato – bridged multimetallic coordination

compounds which was initiated by Kahn and coworkers in the late 1980’s

and then extended to the oxamidato – bridged analogues by Journaux and

Lioret in the late 1990’s. In this work, as discussed earlier, aliphatic or

aromatic group substituted bis(oxamato)- and related bis(oxamidato) –

copper(II) complexes were used as ligands, referred to as metal-organic

ligands (MOLs) for the preparation of multimetallic compounds [9, 94, 95].

Chapter 1

The study of polynuclear complexes was initiated using the ligands like

oxalates which formed the homopolynuclear complexes. After 1990’s, a

great number of oxalate-bridged homopolynuclear complexes have been

synthesized and characterized till date [99]. These compounds are generally

obtained during the reaction between cationic complexes having potentially

free coordination sites and the oxalate anion. These types of polynuclear

complexes have general formula M(C2O4).nH2O where, M is a divalent

metal ion. These types of homopolynuclear compounds can be formed using

the polytopic ligands [100]. But, as discussed earlier, one of the challenges

in the field of polynuclear self – assembly of molecular organic – inorganic

material is the controlled and on the spot generation of well defined

molecular structures [101]. A high degree of structural organization can be

achieved through the multiple one- to three- binding of transition metals

[102]. Therefore, it is important to design the organic ligand together with

the proper choice of the metal ion, since the overall topology of the network

is influenced by match or mismatch of coordination of the linking metal ions

in addition to the binding properties of the bridging ligands [103, 104].

The oxalate – type ligands can be modified to bis – chelating coordination

ability which makes them a versatile choice for interconnecting metal ions

with different spins. The self – assembly of inorganic structures

encompassing bridging oxalate-type units have produced different types of

frameworks, which include 1D-chains, 2D-layer and 3D-networks bearing

the same types of metal ions [105].

Chapter 1

3.2 Heteropolynuclear complexes

The synthesis of heterometallic complexes is known to be complicated. A

one-pot procedure involving the reaction of two different metal ions with the

ligand anion produces a mixture of compounds [106]. In order to overcome

this difficulty, an alternative synthetic strategy (MOL) has been developed

which consists the use of mononuclear complexes as ligands towards the

second metal ion.

Plenty of work on the well – recognized ‘metal complex as ligands’

approach has been done till date [51]. F. Lioret and coworkers reported the

homo- and heteroleptic anionic complexes of chromium(III) with one or

more potentially bridging groups such as [Cr(salen)(C2O4)]-

,[Cr(acac)2(C2O4)]- and [Cr(C2O4)3]

3- which are particularly useful designing

heterometallic complexes with interesting magnetic properties [107].

M.C. Dul and coworkers explained the molecular–programmed self-

assembly of metal-organic ligands with the free carbonyl oxygen which

allows self-assembled mono-,di-,and trinuclear square planar or octahedral

complexes to be used as MOLs toward other metal ions (M(II) = Cu, Ni, Co,

Mn, Fe, Zn) as shown in figure 21. This molecular self-assembly approach

allows the rational preparation of polynuclear complexes as well as

coordination polymers of varying nuclearity, topology, and dimensionality,

depending on the coordination mode of MOLs and coordinated metal ions

[72].

Chapter 1

Figure 21: Heteropolynuclear complexes.

H. Adams and coworkers synthesized the homodinuclear complexes from

ligand (L) by the reaction of sodium 2,6-diformyl-4-methylphenolate and

1,3-diamine propane as shown in figure 22 [108]. The Schiff base complex

[Na2(L)] formed readily underwent transmetallation by Zn(ClO4)2.6H2O to

produce Zn2(L) (ClO4)2.2H2O.

Figure 22: Polydentate ligand used for polynuclear complexes.

Whereas, the heterodinuclear complexes [MAMB(L)]2+

, where MA = Ni2+

, MB

= Mn2+

, Fe2+

, Co2+

, Cu2+

, Zn2+

were prepared by two step synthesis, based on

Chapter 1

the reaction of metal acetates with mononuclear nickel complex. In the first

step, the mononuclear nickel complex was formed, one of the coordination

compartments was blocked by protonation of inner penalty groups. In the

second step acetate ions deprotonated the phenolate groups and made it

available for the further coordination. So, by the modification of the

synthetic method the homodinuclear and heterodinuclear complexes from

same Schiff base ligands can be formed.

E.Q. Gao and coworkers also used ‘complex as ligand’ approach with an

intention to prepare polynuclear species. Many oxamido-bridged

polynuclear complexes were prepared by using mononuclear Cu(II)

complexes of some N,N- bis(coordinating group substituted) oxamides such

as [Cu(oxpn)] and [Cu(obze)]2 as ligands as shown in figure 23. The

diamines and dioxalates are used for the preparation of the mononuclear

complexes. The mononuclear complexes with free coordinating sites were

used as MOLs for the preparation of the dinuclear complexes [109].

Figure 23: Mononuclear complexes used as MOLs.

The mononuclear complexes as given in figure 23 have free oxalate sites for

the further coordination. The mononuclear complexes with variation of

Chapter 1

different ligands with the substitutions give different types of dinuclear

complexes [110]. The dinuclear complexes prepared from the above method

are different from which they reported the dinuclear Cu(II)-M(II) (M=Cu,

Ni, Mn) complexes as shown in figure 24 [111].

Figure 24: Dinuclear complexes of MOLs

Some other types of metal organic ligands have been designed using a ligand

containing number of potential sites from which all cannot contribute in the

coordination, the monotopic complexes formed from these types of ligands

can act as precursors. The mononuclear complexes of transition metals

prepared from 6-amino-2-thiouracil containing the amine group as unutilized

functionality is shown in figure 25. These types of mononuclear complexes

can be modified for further reaction to form the polynuclear complexes

[112].

Chapter 1

M(II)-(6-amino-2-thiouracil)

Figure 25: Mononuclear with active sites.

From amongst many methods for the preparation of polynuclear complexes,

Y. Sunatsuki and coworkers implemented two methods. The first method (1)

in which, one kind of metal complex contains simultaneous donor and the

acceptor ability for the further reaction and construct the assembly structure.

The second method (2) in which, two kinds of metal complexes contain

simultaneous donor and the acceptor ability for the further reaction. The

second method is successfully used in the preparation of molecular magnetic

materials [113].

The representative example of first method is given as Cu(II) complexes

with multidentate Schiff-base ligands containing imidazole groups with both

donor and acceptor abilities in the formation of coordination bonding. Under

the acidic condition condensation of 4-formylimidazole and N,N-

dimethyldiethylenetriamine form compound 1 as shown in figure 26.

Chapter 1

Figure 26: Mononuclear MOL.

Compound 1 reacts with itself in the presence of the base to give imidazolate

bridge species 2 as shown in figure 27. Here the compound 1 acts as metal

organic ligand and react itself to form the homodinuclear complex [114].

Figure 27: Homodinuclear complex from MOLs.

Brewer and coworkers isolated a mono-deprotonated species 3 which is the

mononuclear complex with the free amine of the ring skeleton [115]. This

complex functions as monodentate ligand complex i.e. metal complex as

ligand and react with the other mononuclear complex,

tetraphenylporphyrinato Fe(III) to give linear trinuclear complex 4 [116].

Chapter 1

Figure 28: Heterotrinuclear complex from mononuclear MOLs.

The same concept was used to synthesize the trinuclear complex as shown in

figure 29. The mononuclear complex of Cu(II) ion of unsymmetrical

tetradentate ligands containing an imidazole group per molecule were

synthesized [117]. The mononuclear complex of Cu(II) was self – assembled

to the mononuclear complex of iron to form various polynuclear complexes

such as compound 5.

Figure 29: Heterotrinuclear complex.

Here, Y. Sunatsuki have used precursor complexes which act as MOLs and

form a variety of the heteropolynuclear complexes with metal variants [118].

A. Jana and coworkers reported the solvent dependent reaction of

mononuclear complexes of Cu(II) and N,N’-ethylenebis(3-

ethoxysalicylaldimine) with nickel(II) perchlorate in 1:1 ratio in acetone

produces the trinuclear copound [(CuIIL)2Ni

II(H2O)2](ClO4)2. On the other

Chapter 1

hand, reaction with same reactants in the same ratio with the methanol

produces the pentanuclear compound [(CuIIL)2Ni

II(H2O)2](ClO4)2

.2[(CuIIL(H2O)][103].

The new concept of the coordinated amine reacts with the free aldehyde

group of the mononuclear complex that form the trinuclear complexes by the

Schiff base reaction between the two mononuclear complexes was reported

by S.Deeplathaand coworkers. The copper(II) complex of 3,4-dihydroxy

benzaldehyde as the ligand system in which the aldehyde group could be

modified to attach additional nitrogen donor sites through Schiff base

condensation has been attempted by S. Deeplatha and coworkers [119]. They

formed the trinuclear complexes using the method (2); the two kinds of

metal complexes having reactive sites. In first step 2,2’-bipyridyl 3,4-

dihydroxo benzaldehyde monometallic cupric(II) complex was allowed to

react with diethylenetriamine complexes of Cu(II), Zn(II) and Ni(II) to

afford trinuclear complexes as shown in figure 30.

Figure 30: Heterotrinuclear complex.

Chapter 1

4. MAGNETIC INTERACTION IN POLYNUCLEAR

COMPLEXES

Magnetic properties of materials have been utilized over years in mechanical

applications such as electric motors, electric generators as well as in

communication technologies such as television and telephones. More

recently, magnetism has found extensive application in the area of data

storage and processing, from magnetic tape to floppy discs and computer

hard drives [120].

The most usable types of magnets are either ferro- or ferrimagnets, each of

which has a characteristic Curie temperature, below the Curie temperature it

shows its magnetic properties. Such magnets can be pure metals, alloys,

metal oxides or molecular magnets [121]. The spins on the magnetic centers

are interacting with their neighbors in the three dimensional lattice of a

conventional magnet and their spontaneous magnetization relies on the

alignment of a very large number of spin centres in the bulk material. The

bulk magnetic structure is divided into many magnetic domains within each

of which the spins are aligned in ferromagnetic materials. When the different

domains having different alignment directions such that their overall

magnetic moments cancels out, the materials are classified as

antiferromagnetic materials [122].

Traditional magnetic materials consist of two- and three-dimensional

structures of transition metal or lanthanide metal containing spin units.

These types of materials are generally prepared at very high temperatures

using metallurgical methods. In contrast to traditional magnetic materials,

molecular magnets are organic or inorganic / organic hybrid materials

consisting either metal with spin units or organic radical containing spin

Chapter 1

units [123]. These magnetic materials are conveniently prepared at low

temperatures. In addition to this, such materials can possess better optical

properties and the combination of magnetic properties with other properties

like mechanical, electrical and / or optical properties makes them easily

processable.

In the last two decades, an increasing interest in polynuclear complexes

containing ligand structures capable of holding two or more metal centers in

close proximity and act as single molecular magnets has been observed. The

field of molecular magnetism exhibited a rapid development in the last two

decades with a particular emphasis on heteropolymetallic complexes. The

magnetic interaction between two nonequivalent paramagnetic centers may

lead to situations which cannot be encountered in species containing only

one kind of center [124].

The occurrence of magnetic interactions in coordination compounds is

because of the incompletely filled d-orbitals in addition to the arrangement

of electrons. An electron acts as a tiny bar magnet with a negative charge. If

the magnetic fields of two or more electrons in the atoms of a molecule are

parallel then that molecule becomes magnetic.

Normally, the magnetic fields of electrons in atoms cancel each other and the

atom cluster appears as non-magnetic. But in some molecules, because of

some special type of bonding between its atoms (i.e. bridge between atoms),

the magnetic fields of two or more electrons in the molecule becomes

parallel and these types of molecules exhibit molecular magnetism. The field

strength of a molecular magnet depends on the number parallel electron

magnetic fields in the molecule [125, 126].

Chapter 1

4.1 Types of Magnetic behavior

The origin of magnetism lies in the orbital and spin motion of electrons and

their interactions with each other. Atoms, ions, molecules and solids having

unpaired electrons when brought under the influence of a magnetic field,

develop a net magnetization because of very strong interaction between the

moments of neighboring atoms [127, 128]. In some materials there is no

collective interaction between the atomic magnetic moments. The magnetic

behavior can be classified as follows:

I. Diamagnetism

Systems having only paired electrons are described as diamagnetic. Most

organic molecules are diamagnetic. In this case, magnetic susceptibility is

usually independent of the temperature and the strength of the applied field.

These substances when exposed to a magnetic field, a negative

magnetization is produced and the susceptibility will also negative so, they

will be weakly repelled by the field [129].

II. Paramagnetism

Systems having one or more unpaired electrons are called paramagnetic. The

paramagnetic substance possesses a net magnetic moment and gets attracted

to a magnetic field due to a partial alignment of the atomic magnetic

moments in the direction of the field as shown in figure 31. Such substances

have a net positive magnetization and positive susceptibility [130]. The

individual magnetic moments do not retain any magnetization in the absence

of an externally applied magnetic field [131].

Chapter 1

Figure 31: Spin alignments in paramagnets.

III. Ferromagnetism

The ferromagnetic substance are type of paramagnetic substances which

have magnetic interactions between neighboring paramagnetic centers such

that the individual moments are aligned parallel to each other as shown in

figure 32 and there will be a net increase in the magnetic moment. The

ferromagnetic substance is usually divided into domains to minimize its total

free energy leading to spontaneous magnetization arises in each domain

even in the absence of a magnetic field. Its magnetic susceptibility is

positive and very large. However, above its Curie temperature the interaction

is no longer strong enough to maintain the moments in alignment, and the

substances then behave as simple paramagnetic material [132].

Figure 32: Spin alignments in ferromagnets.

Chapter 1

IV. Antiferromagnetism

When the magnetic interactions between paramagnetic centers lead to an

antiparallel alignment of the moments, the paramagnetic substance is said to

exhibit antiferromagnetism as shown in figure 33. Because of the complete

compensation of the magnetic moments, the substance does not show any

spontaneous magnetization. Above its Neel temperature, an antiferromagnet

behaves as a paramagnet [132].

Figure 33: Spin alignments in antiferromagnets.

V. Ferrimagnetism

Substances where the magnetic moments are aligned antiparallel as shown in

figure 34, but there is an incomplete compensation of the spins are termed

ferrimagnets. This usually results either from unequal numbers of spins

being oriented in the two directions, or when the two types of spin centers

have different numbers of unpaired electrons. Ferrimagnets exhibit

spontaneous magnetization as in the case of ferrromagnets [74, 133].

Chapter 1

Figure 34: Spin alignments in ferrimagnets.

A detailed description of the magnetic properties and behavior of different

materials will be provided in chapter 3.

4.2 History of magnetism in polynuclear complexes

In 1704, a new blue pigment useful for paints and fabrics was discovered by

Berlin Draper. This discovery was reported anonymously in 1710 [134] and

the method was described in 1724 by Woodward and Brown [135, 136]. This

pigment, now called Prussian blue (or Berlin Blue) was the first synthetic

coordination compound. The history of its discovery and the various theories

about its nature and the origin of its bright color were reviewed recently in

the context of the development of chemical ideas in the 18th

and 19th

centuries [137, 138]. The crystal structure of Prussian Blue

FeIII

4[FeII(CN)6]3.14H2O was determined by Buser, Ludi and Gudel in 1972

[139].

In 1928, Davidson and Welo published the first magnetic investigation of the

parent species Prussian Blue. They measured the magnetic susceptibility at

three temperatures in the range of 200 to 300 K [140]. An extension of this

Chapter 1

early work on magnetic properties of Prussian Blue was done in 1940. In

1968, Prussian Blue was shown to be a ferromagnet with Tc = 5.6 K [141].

The polynuclear complexes exhibit many interesting magnetic properties

owing to bridging ligands as well as the intermetallic bonding. The magnetic

properties of dinuclear complexes have been studied over many years [142].

E.F. Hasty and coworkers reported the binuclear copper(II) Schiff base

compounds of salicylaldehyde with aromatic polyamines in 1978. The

binuclear complexes have the iminobenzene moiety bridging between the

two metal ions. The compound Cu2(sal-m-phenylinediamine) had an

antiferromagnetic interaction (J= -1.5 cm-1

). The compound with nickel

metal also showed an antiferromagnetic interaction [124].

O’ Conner and coworkers also reported a series of dinuclear complexes

based on new binucleating ligands derived from the condensation of

salicylaldehyde, pyridine-2-carboxaldehyde. The first example of

ferromagnetic coupling in dinuclear complexes was observed for compound

shown in figure 35, with J value of about +17 and +80 cm-1

[143].

Figure 35: Homodinuclear complex with ferromagnetism.

In 1987, Mallahi and coworkers reported the binuclear complexes of copper

with ligands having various exogenous donors (OH, N3, OCN). These

Chapter 1

dinuclear complexes exhibit strong antiferromagnetic nature with OH bridge

while the species with OCN bridge show a weak antiferromagnetic coupling.

The magnetic properties of the homonuclear complexes are studied

extensively since a long but, interest in magnetic properties of

heterodinuclear complexes has increased in the recent times. Many reviews

on the magnetic properties of dinuclear and polynuclear complexes of

transition metal complexes are well documented [99, 144]. The interest in

magnetic properties of coordination polymers and clusters is also emerging

these days. The magnetic properties of asymmetric heterodinuclear

complexes containing the µ-Oxo-bis(p-acetato)dimetal was reported in 1992

by Hotzelmann and coworkers. Bulk magnetic properties of all the

compounds have been studied in the temperature range 4.2 – 298 K. It was

found that the spins of the two paramagnetic metal ions are either

intramolecularly antiferromagnetically or ferromagnetically coupled [142].

The oxalate-bridged dinuclear complexes of Cr(III)-M(II) (M=Cu, Ni, Co,

Fe, Mn) complexes as shown in figure 36 were synthesized and the magnetic

properties were investigated by Ohba and coworkers. The magnetic

investigations of these compounds in the temperature range 4.2–300 K

reveal a ferromagnetic interactions between the Cr(III) and M(II) ions for all

the complexes [145].

Chapter 1

Cr(III)-M(II) (M=Cu, Ni, Co, Fe, Mn)

Figure 36: Heterodinuclear complex with ferromagnetism.

The polynuclear complexes of different transition metals and ligands have

generated interesting magnetic properties [146]. The dinuclear complexes of

nickel(II) containing triethylenetetramine and tricyanomethanide exhibits a

ferromagnetic coupling. But, the dinuclear complexes synthesized from

polytopic ligands like 2-[N-bis-(2-pyridylmethyl) aminomethyl]-4-methyl-6-

[N-(2 pyridylmethyl) aminomethyl]phenol as shown in figure 37 exhibited

the antiferromagnetic coupling [147].

Figure 37: Homodinuclear complex with antiferromagnetism.

Chapter 1

The synthesis, structure and magnetic properties of a dinuclear Co(II)

complex of a tridentate verdazyl radical was reported by M. T. Lemaire and

others. The magnetic susceptibility data of the dinuclear complexes

suggested ferromagnetic interactions at low and high temperatures with

J= +20 cm-1

. Rodriguez-Dieguez and coworkers reported the

antiferromagnetic interaction occurs in dinuclear complexes of Co(II) with

pyrimidine-2-carboxylato bis bidentate ligands [99, 148].

The dinuclear complexes of iron(II) and iron(III) are also synthesized and

well characterized. The dinuclear complexes of iron(II) with dicarboxylate

and 2,3-lutidine was reported by Sidorov and others. These homodinuclear

complexes of iron(II) are found to be antiferromagnetic as shown in figure

38 [149].

Figure 38: Homodinuclear complex with antiferromagnetism.

The dinuclear complexes of iron(II) with pyrimidine-2-dicarboxylato bis

bidentate ligands also exhibits the antiferromagnetism. Kil Sik Min and

coworkers reported the ferromagnetic interactions at low temperatures in

dinuclear complexes of Iron(II) with tris(2-pyridylmethyl)amine and 2,3,5,6-

tetrahydroxy-1,4-benzoquinonate [150].

The magnetic properties of mononuclear as well as dinuclear complexes of

manganese are found to be much interesting. Sujin Han reported a weak

Chapter 1

ferromagnetism in mononuclear manganese complex [151]. The structure

and magnetic properties of manganese(III) complexes with pentaanioc

pentadentate ligands including alkoxo, amido and phenoxo donors were

reported by Stoicescu and coworkers [152]. All the complexes exhibit the

antiferromagnetism at low temperatures with J value -3.6 cm-1

. Ribas and

coworkers reported the trinuclear complexes with oxo-manganese center

exhibits the antiferromagnetism at lower temperature [153].

Ferromagnetic CuII

4, CoII

4, NiII

6 azido complexes derived from di-2,6-(2

pyridylcarbonyl) pyridine was reported by Georgopoulou and others. The

polynuclear complexes prepared from all these three transition metals

exhibited the ferromagnetic interactions. Hence, the magnetic interaction is

different according to the transition metals and ligands used [154].

5. LITERATURE SURVEY

The polynuclear complexes of transition metals are synthesized and

extensively characterized till date. Many excellent papers have been

published during the last three decades on the preparation and properties of

polynuclear complexes and the most relevant results are reviewed here [155,

156]. The applications of the polynuclear complexes and dinuclear

complexes are also found in different fields. The preparation of the

polynuclear complexes with different methodologies are also well

documented [157, 158]. A brief literature survey according to the application

in different fields and the methodologies for the preparation of dinuclear

complexes is reported here. The dinuclear complexes with bridged and

aliphatic ligands started from the late ninteenth century. A good deal of work

was done on binuclear complexes in the late nineteenth century including,

Chapter 1

Fremy (1852), Gibbs (1876), Vortmann (1898), Jorgensen (1894) and

Mascetti (1900). A number of papers by Werner (1898-1918) greatly

expanded the scope of the field. The first mention of trinuclear complex was

in 1908 by the German chemist Weinland. In 1970, the first homodinuclear

complexes with macrocyclic ligands were prepared by Robson with template

reaction so, called Robson type complexes [82]. From that time Robson type

complexes have been synthesized and characterized. The preparation of

dinuclear iron(III) complexes with aromatic and aliphatic ligands were

reported by K. S. Murray in 1974 [156].

The dinuclear complexes with different metal ions prepared by ‘metal

complexes as ligand’ stretagy are the most successful. F. Lioret and

coworkers reported the homo- and heteroleptic anionic complexes of

chromium(III) with one or more potentially bridging groups such as

[Cr(salen)(C2O4)]-

,[Cr(acac)2(C2O4)]- and [Cr(C2O4)3]

3- are particularly

useful for the design of heterometallic complexes of interest in magnetism

[159]. M.C. Dul and coworkers explained the molecular–programmed self-

assembly of metal-organic ligands with the free carbonyl-oxygen which

allows self-assembled mono-, di-, and trinuclear square planar or octahedral

complexes to be used as MOLs toward other metal ions (M(II) = Cu, Ni, Co,

Mn, Fe, Zn) [72]. The heterodinuclear complexes [MAMB(L)]2+

, where MA =

Ni2+

, MB = Mn2+

, Fe2+

, Co2+

, Cu2+

, Zn2+

have been prepared by two step

synthesis, based on the reaction of metal acetates with mononuclear nickel

complex by J. Lisowski in 1999. Brewer and coworkers synthesized the

tinuclear complexes of unsymmetrical tetradentate ligands containing an

imidazole group per molecule by using the mononuclear precursor [115].

Chapter 1

A variety of homo and heterodinuclear Schiff base complexes of transition

metal ions were used as catalysts by B. Carbezon and coworkers. Ligtenbarg

and others reported the dinuclear complexes of copper(II) ions used

successfully in hydroxylation of phenol in 1998. A review on visible light

promoted reactions catalysed by tansition metal complexes by A. Inagaki

and coworkers [160]. Daniel Seidel reported the dinuclear copper complexes

as asymmetric catalyst. Simon-Manso and Kubiak reported the dinuclear

nickel complexes as catalyst for electrochemical reduction of carbondioxide

[161]. V. Lozan and coworker reported the dinuclear nickel(II) and

palladium(II) complexes in combination with co-catalyst as highly active

catalyst for the vinyl / addition polymerization of norbornene.

The synthesis and magnetic properties of dinuclear as well as polynuclear

complexes of oxamato-bridged coordination compounds was initiated by

Kahn and coworkers in the late 1980s and then extended to the

oxamidatobridged analogues by Journaux and Lioret in the late 1990s [94].

In this work, aliphatic or aromatic group substituted bis(oxamato)- and

related bis(oxamidato)-copper(II) complexes were used as ligands [162]. In

1987, Mallahi and coworkers reported the binuclear complexes of copper

with ligands with various exogeneous donors (OH, N3, OCN). The magnetic

properties of asymmetric heterodinuclear complexes containing the µ-Oxo-

bis(p-acetato)dimetal was reported in 1992 by Hotzelmann and coworkers.

O’ Conner and others have reported a series of dinuclear complexes based

on new binucleating ligands derived from the condensation of

salicylaldehyde, pyridine-2-carboxaldehyde [163]. This was the first

example of ferromagnetic coupling in dinuclear complexes observed for

compound with J value of about +17 and +80 cm-1

. The oxalate-bridged

Chapter 1

dinuclear complexes of Cr(III)-M(II) (M=Cu, Ni, Co, Fe, Mn) complexes

were synthesized and the magnetic properties were investigated by Ohba and

coworkers [145]. The synthesis, structure and magnetic properties of a

dinuclear Co(II) complex of a tridentate verdazyl radical was reported by M.

T. Lemaire and coworkers [92]. Rodriguez-Dieguez and others reported the

antiferromagnetic interaction occurring in dinuclear complexes of Co(II)

with pyrimidine-2-carboxylato bis bidentate ligands. Nishida and coworkers

have reported the copper (II) complex shows weak anti ferromagnetic

coupling [164]. The heptadentate polybenzimidazole ligand 2,6

bis[benzimidazoylmethyl) aminomethyl]-p-cresol and its copper(II)

complexes with pyridine and pyrazole bridges have been reported by McKee

and coworkers [165].

5.1 Present work

I. Scope and interest

The chemistry of metal complexes including ordinary complexes, chelates

and mixed ligand complexes has been extensively studied till date for their

roles as biological models as well as a wide range of physicochemical

properties. In order to fiddle with the properties of the complexes originating

due to metal ions, polynuclear complexes with inclusion of different metals

is imperative. Mixed-metal complexes are a class of compounds which can

have properties that are not present in ordinary complexes. Interest in these

kind of complexes stems from their bioinorganic relevance and magnetic

properties. Numerous homopolynuclear complexes have been prepared

using polydentate ligands in which some of the donor atoms are unable to

coordinate with the same metal ions due to steric factors and form the

Chapter 1

multinuclear complexes of the same metal ion. Synthetic routes leading to

heteropolynuclear complexes with pre-established structures and properties

has remained a challenge for synthetic chemists. A one - pot procedure for

template synthesis of heteromettallic complexes has not been well

established yet. This difficulty has been overcome to some extent using a

well-recognized approach ‘Complexes as Ligands’. We report here the

synthesis of dinuclear complexes via novel route the inter – complex

reaction is capable of opening a new era in the preparation of complexes

with a lot more variations. The magnetic interactions in these types of

polynuclear complexes are quite interesting and can be used as magnetic

materials.

II. Overview

The present work is to study the mixed metal complexes prepared by the

novel method i.e. ‘complexes as ligands’. A series of complexes were

synthesized by this method and then characterized by analytical techniques.

The work is divided into five chapters.

The first chapter of the thesis includes an extensive literature survey. This

chapter describes the general introduction and background of transition

metal complexes, polytopic ligands and polynuclear complexes, metal

organic ligands (MOL’s) as well as the magnetic interactions in metal

organic ligands based complexes.

Chapter 2: Synthesis and characterization of dinuclear complexes of

salicylaldehyde and 2–aminophenolwith Zn(II), Mn(II) and Fe(II).

This chapter will deal with the general information of the complexes of the

particular ligands. Synthetic method for the all the nine dinuclear complexes

Chapter 1

and various characterization techniques and methods employed to

characterize the synthesized compounds will be discussed here.

Physicochemical data and results from various techniques like infrared

spectra, TGA, electronic spectra, magnetic properties, 13

C NMR spectra, 1H

NMR spectra, mass spectra and powder X – ray diffraction data will also be

discussed. A brief explanation of absorption and emission spectrometry are

reported here.


salicylaldehyde and 2 – aminophenol with Co(II), Ni(II) and Cu(II).

This chapter contains general synthetic method for nine dinuclear complexes

of transition metals viz. Co(II), Ni(II), Cu(II). The physicochemical data and

results from various techniques like infrared spectra, thermogravimetric

analysis, electronic spectra, mass spectra and detailed study of magnetic

properties of all the synthesized compounds will be discussed. The

molecular magnetic theory and the methods used to study the magnetic

properties for the interpretations of the nature of compounds either

ferromagnetic or antiferromagnetic is described.


o–vanillin and 2–aminophenol with Z (II), Mn(II) and Fe(II).

This comprises general information on complexes of o–vanillin. The general

synthetic method for homo and hetero dinuclear complexes of transition

metals Zn(II), Mn(II) and Fe(II) with o-vanillin and 2-aminophenol. The

chapter includes study similar that of the second chapter carried out with a

new series. Theories of various methods of the mass spectrometry which can

be used for structure elucidation is described here.

Chapter 1


o–vanillin and 2–aminophenol with Co(II), Ni(II) and Cu(II).

In this chapter the general methods for the preparation of dinuclear

complexes and characterization data are discussed as like that of the third

chapter. The molecular mechanical methods used to identify the most stable

structure is described.

Chapter 1

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