CHAPTER 1

INTRODUCTION

 

 

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CHAPTER 1 INTRODUCTION

Section I

1. Introduction

1.1. Metals and Metal Compounds

Metals are endowed with unique characteristics and offer good usage potential. They

have the ability to form alloys with other metals increasing the metallic properties like

malleability, ductility, tensile strength, making them corrosion-resistant. Their

complexing skills, oxidation-reduction properties, unparalleled catalytic properties,

durability, magnetic properties, high melting points, etc. are crucial for sustainable

industrial performance. Metals and their compounds like alloys are required in almost

every facet of our day-to-day life. They are important constituents in many of our key

modes of transportation. These elements play different roles in technologies that have

revolutionized the way of communication. Most of the components of modern wind

turbines and nuclear reactors rely on their alloys. Therefore, they also play a crucial role

in the production of renewable energy.

The use of metals or their compounds in medicine has been historical and can be traced

back for thousands of years. 1Copper sulfate and alum were among the many substances

used by the ancient Egyptians to prepare potions, due to their produced sterilizing effect.

Aqueous suspensions of gold flakes known as Goldschlager or Geldwasser to metals like

mercury have also been used in medicinal preparations. However, gold compounds were

subsequently found to be ineffective in the treatment of pulmonary tuberculosis, hence

arsenic containing compound like Salvarsan was used in combination with mapharsen for

treating syphilis.

These pharmaceuticals, particularly those involving arsenic, could have severe side

effects and no doubt this contributed to a common perception that metals are generally

toxic and not well suited to use in pharmaceuticals. Current research has aroused greater

interest in the medicinal use of metal compounds containing platinum and technetium.

Which as lead to the development of compounds like cisplatin containing platinum along

with technetium in addition to thallium, gallium and indium used for diagnostic

medicinal purpose. Metals with magnetic properties, has access to their use in

pharmaceuticals. In addition to the continued use of gold drugs to treat rheumatoid

 

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arthritis, lithium is now used to treat depression, platinum to treat certain types of cancer,

bismuth to treat stomach ulcers, vanadium to treat some cases of diabetes, iron and its

compounds to treat anaemia, and to control blood pressure, cobalt in vitamin B12 to treat

pernicious anaemia and certain radioactive metals to alleviate the pain of bone cancer.

1.1.1. Properties of metal

The special chemical property of metals for which nature has exploited them in biological

system is that they carry a positive charge in aqueous media, which distinguishes them

from most organic species. Bulk metals and trace metals broadly share similar chemical

properties but differ in the sizes and charges of the forms in which they exist in aqueous

media. The bulk metal ions are chemically promiscuous and generally do not form stable

complexes with any particular entity and prefer to be rapidly exchanged.

In trace metal ions the chemical reactivity is more limited and they remain in the same

positively charged state throughout their time in the body and the trace metals also show

more varied chemical reactivity with the exception of zinc which can change the

magnitude of its positive charge under the right conditions. In order to be useful in

medicine, chemical compounds need to meet a variety of criteria. The most obvious

requirement is that they must exhibit a medically beneficial effect with minimal toxic

side effects.

1.1.2. Chemistry of Metal Compound Formation

Metal ions are divided into two, viz class (a) and class (b), depending on whether they

prefer to bind to (a) the first elements in groups V, VI, and VII, i.e., N, O, F, or (b) the

second or later members of groups V, VI and VII. That is, complexes of class (a) metals

follow the stability sequence: F- > Cl- > Br- > I- ; O >> S > Se > Te; N >> P > As > Sb >

B. On the other hand class (b) metals tend to follow the reverse order. The elements N, O,

and F are the hardest (or least polarizable) atoms in groups V, VI, and VII respectively.

It can be stated therefore that class (a) metal ions prefer the hardest atom of a group,

whereas class (b) metal ions prefer the softest atom of a group.

Class (a) metal ions have a low polarizability while class (b) metal ions are highly

polarizable. Thus, class (a) metal ions may be identified as hard Lewis acids while class

(b) metal ions are soft Lewis acids. In the case of soft acids, the presence of d electrons

plays a critical role in the bonding interactions. These d electrons may be donated to

 

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appropriate ligands to yield π -bonds. Consequently the most likely ligands are those

which contain empty d orbitals and can accept d electrons from the metal. Based on this

concept of hard and soft acid base (HSAB) principle the metals form complexes with

different ligands. These metal ions and their complexes are important in a large variety of

natural and industrial processes. Hence it is essential to understand the interaction taking

place in the metal-ligand exchange reaction.

1.2. Metal Complex – Coordination Compounds

The chemistry of complexes or coordination compounds has originated in 1700’s with the

discovery of Prussian Blue by a colour maker Diesbach, now known as Iron(III)

hexacyanoferrate(II) a compound of Iron [Fe4[Fe(CN)6]3]. A later investigation of the

products of oxidation of ammoniacal cobalt solution by Tassaert was followed by efforts

on the studies of the complex or compounds of Cr, Co, Ni, Fe and Pt. The key

breakthrough occurred when Alfred Werner proposed in 1893 that Co(III) bears six

ligands in an octahedral geometry in the complex [Co(NH3)6]Cl3.

Coordination compounds are those that contain at least one dative bond. Such compounds

are also termed as Lewis acid-base adducts. Since an essential feature of such compounds

is a species that has a vacant available orbital and a species that has an electron pair,

which may be donated (lone pair of electrons), metal complexes are thus defined as ‘a

compound formed from a Lewis acid and a Bronsted base’, a Lewis acid being an

electron pair acceptor and a Bronsted base a proton acceptor. Simple example is the

interaction of the Lewis acid metal centre in Ni(ClO4)2 with the Bronsted base ammonia

leading to the formation of a coordination compound2-4

Such coordination compounds must be differentiated from complex salts or double salts,

which are the result of co-crystallization of two different compounds like copper sulfate

pentahydrate and ammonium sulfate. Metal chelate’s are an extremely important class of

coordination compounds as they occur in a variety of formation reactions and have

interesting features of metal-ligand bonds. They also have wide range of application in

analytical chemistry and have been found to play a vital role in various biochemical

processes ranging from photosynthesis to oxygen transport in living systems and different

geochemical processes.

 

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1.3. Types of Ligand

A huge variety of ligands appear in coordination complexes and may have different

elements that may function as donor atoms towards metal ions, but the most commonly

encountered are nitrogen, phosphorus, oxygen, sulfur and the halides. Bidentate ligands

may be classified according to the number of atoms in the ligand which separate the

donor atoms and hence the size of the chelate ring formed with the metal ion. Thus 1,1-

ligands form a four-membered chelate ring when bound to a metal ion 1,2-ligands a five

membered ring and so on.

Cyclic compounds which contain donor atoms that are oriented so that they can bind to a

metal ion and which are large enough to encircle it are known as macrocyclic proligands.

Bicyclic proligands are also known which can completely encapsulate a metal ion for

example cryptand, sepulchrate. Sometimes ligands can bind to more than one metal ion

in a bridging arrangement, for example in [W2Cl9]3- while, certain polydentate ligands are

particularly good at linking together several metal ions and are referred to as

polynucleating ligands.

Majority of the metal chelates are compounds that contain organic molecules (or groups)

as ligands. Chelates containing Schiff base C=N functional group in the ligands are fairly

common, and show a wide range of applications5.

1.3.1. Schiff bases

Schiff base was first reported by Hugo Schiff in 1864 and have been studied since 5

decades as they serve as excellent models for the study of keto-enol tautomerism both in

solution and in solid state. They have a prominent role to play in the study of metal

complexes and their interactions. This is due to the several advantages in their structure

and the change in properties that are obtained in their metal complexes.

The intramolecular hydrogen bonds between the (O) and the (N) atoms play an important

role in the formation of metal complexes. Most of the metals form 1:1 complexes with

Schiff base ligands. The first row transition metal complexes of schiff base have been

reported to exhibit fungicidal, bactericidal, antiviral and antitubercular activity. Schiff

base is prepared by condensing carbonyl compounds and amines in different reaction

medium with elimination of water molecules and dehydrating agent normally favor the

formation of schiff base.

 

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When the aldehyde is a salicylaldehyde derivative and amine is an aniline derivative, the

condensation produces interesting N, O Schiff base compounds also known as Anil’s.

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

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

groups which may be variously substituted. Schiff bases play an important role in

coordination chemistry as they easily form stable complexes with most transition metal

ions. In organic synthesis, Schiff base reactions are useful in making carbon-nitrogen

bonds6.

1.3.2. Chemistry of Schiff bases

A schiff base with an o-hydroxyl phenolic group behaves as a flexidentate ligand and

commonly coordinates through the O atom of the deprotonated phenolic group and the N

atom of azomethine group. Presence of a lone pair of electrons in a sp2 hybridised orbital

of nitrogen atom of the azomethine group is of considerable chemical importance. It

imparts an excellent chelating ability especially when used in combination with one or

more donor atoms close to the azomethine group. This chelating ability of the Schiff

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

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

chemistry.

From a synthetic perspective, imines are important in the syntheses of complicated

amines where a schiff base can be reduced with hydrogen to give the amine. Importance

of Schiff base structure from a biological perspective is transamination. Transaminases

are found in mitochondria and cytosole of eukaryotic cells. Imines also play a key

function in the chemistry of vision. Schiff bases that contain aryl substituents are

substantially more stable and more readily synthesized, while those which contain alkyl

substituents are relatively unstable. Schiff bases of aliphatic aldehydes are relatively

unstable and readily polymerizable, while those of aromatic aldehydes having effective

conjugation are more stable. Schiff bases are generally bidentate, tridentate, tetradentate

or polydentate ligands. 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 are

 

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used, e.g., in optical and electrochemical sensors, as well as in various chromatographic

methods, to enable detection of enhanced selectivity and sensitivity7-9.

Among the organic reagents actually used, Schiff bases possess excellent characteristics,

structural similarities with natural biological substances. Relatively simple preparation

procedures and synthetic flexibility enables varied design of suitable structural

properties10,11. 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 solvent12. The bidentate class of Schiff base

ligand is reported in this thesis.

1.3.3. Schiff base Metal Complexes

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

Schiff base ligands under suitable experimental conditions. Series of structurally varying

complexes of transition metal ion comprising of schiff base ligands have been obtained

and proven to show excellent catalytic activity in various reactions at high temperature, in

the presence of moisture. Some of these types of complexes are also effective as

stereospecific catalysts for oxidation, reduction, hydrolysis, biocidal activity and other

transformations of organic and inorganic chemistry. Schiff base ligands assisted by metal

ions provide highly organized supramolecular metal complexes possessing binding sites

and cavities for various cations, anions and organic molecules, which has applications in

homogeneous and heterogeneous catalysis.

Schiff base transition metal ion complexes comprises of vast areas of organometallic

compounds and attract particular interest due to their biological activity as in

radiopharmaceuticals. They have been effectively exploited as synthetic model system

for biological macromolecules that provides insight for metal-containing sites in

metalloproteins/enzymes. Many schiff bases as well as their complexes are known to

show photochromism and thermochromism in the solid state by proton transfer from the

hydroxyl (O) to the imine (N) atoms. These compounds find application in various areas

 

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of material science like measurement of radiation intensity, display systems and optical

memory.

Transition metal complexes are used as drugs to treat several human diseases like

carcinomas, lymphomas, infection control, anti-inflammatory, diabetes, and neurological

disorders. Many Gold(III), Platinum(II), Ruthenium(II, III, IV), Iron(II) and

Vanadium(IV) complexes for anti-cancer, anti-HIV, and as enzyme inhibitors for

potential therapeutic applications were reported.

1.4. Mixed Ligand Metal Complexes

The metal ion reacts with different forms of ligands, organic and inorganic, occurring in

the environment or reaction medium forming different types of complexes. These may

involve similar ligands, mixed metals or a single metal complexed with two or more

different types of ligands. All metal complexes occurring in nature are mixed ligand

complexes. In many synthetic metal complexes traditional ligands such as Phenanthroline

and bipyridyl ligands are common as mixed ligands, since their metal chelate’s have

enhanced activity ranging from biological to catalytic.

Various amino acid schiff base mixed ligand transition metal ion complexes show high

anti microbial activity compared to the corresponding ligand, metal salt or bis-complex of

the same ligand. Transition metals exhibit different oxidation states and can interact with

a number of negatively charged molecules. Mixed ligand metal complexes are those in

which metal ion is attached to two different ligands through coordinate bond and / or

covalent bond.

1.4.1. Literature Review- Schiff base Mixed Ligand Metal Complexes

A detailed literature review shows that various schiff base mixed ligand metal complexes

have been prepared and studied for different applications ranging from catalysis,

epoxidation reaction to antimicrobial activities. Nickel schiff base mixed ligand

complexes containing nitrogen and oxygen donor atoms have been reported by Thaker et

al13. NiLL1 (HL = salicylaldehyde, HL1 = 2-hydroxyacetophenone (I), 2-

hydroxynaphthaldehyde (II); HL = II, HL1 = I) & NiL2L3 (HL2= salicylideneamine, HL3

= 1-(2-hydroxyphenyl)ethylideneamine (III), 1-(2-hydroxynaphthyl)methylideneamine

(IV); HL2 = III, HL3 = IV) reacted in 1:1 molar ratios with o- and p-phenylenediamine to

give NiL4 (H2L4 = tetradentate di-Schiff bases).

 

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NiL4 was characterized by elemental analysis, TLC, electrochemical, conductivity,

magnetic, thermogravimetric analysis, IR and electronic spectral methods. Refluxing

ethylenediamine was unable to displace o-phenylenediamine from its Schiff base

complex with salicylaldehyde and I. Template preparation of Schiff base complexes

were carried out by Thaker et al14. with 2-hydroxynaphthaldehyde and coordinated

ethylenediamine or propylenediamine (pn) in the mixed ligand complexes [MLQ] (M =

Cu, Ni; L = en, pn; H2Q = catechol, 2,3-dihydroxynaphthalene).

OH groups of the Schiff base N,N'-alkylenebis(2-hydroxy-1-naphthylmethyleneimine)

remained uncoordinated, as confirmed by molar conductance, magnetic and spectral

studies. Nine ternary complexes of copper(II) Na[CuLL'], where L = pyrocatechol, 2,3-

dihydroxynaphthalene or pyrogallol and L' = salicylaldehyde, 2-hydroxynaphthaldehyde

or 2-hydroxyacetophenone, were synthesized, as well as six binuclear mixed-ligand

copper(II) complexes Na2[LCu-X-CuL], where X is a bridging binucleating Schiff base,

derived by the condensation of salicylaldehyde or 2-hydroxyacetophenone with p-

phenylenediamine, the complexes were characterized by elemental analysis, spectral

analysis, and magnetic susceptibility measurements by Bhattacharya et al15 they

confirmed the antiferromagnetic interaction existing between the two paramagnetic

Cu(II) centers.

Kureshy et.al 16 have reported mixed ligand complex Na[RuL(PPh3)N3(H2O)] from a

chiral Schiff bases (H2L = chiral Schiff bases derived from salicylaldehyde, chloro- and

methoxysalicylaldehyde and L-histidine). [RuL(PPh3)(IM)(H2O)] (IM = imidazole) and

[RuL(PPh3)(bpy)] were synthesized, complexes were characterized by microanalytical,

IR, UV-visible {1H}, 13C{1H} and 31P{1H} NMR spectroscopy, conductance

measurement, optical rotation, CD spectral studies and electrochemistry. Gupta et al 17synthesized a tridentate ligand L from salicylaldehyde and 1-dimethylamino-2-

propylamine.

It undergoes partial hydrolysis to regenerate salicylaldehyde in methanolic KOH solution,

reaction of Mn(II) acetate with this solution, followed by addition of NaSCN, produces a

dark brown complex [Mn(L)(NCS){o-(CHO)C6H4O}] (1), which was

crystallographically characterized. Coordination of Mn was Jahn-Teller distorted

octahedral by L, salicylaldehyde, and thiocyanate, coordinated aldehyde group reacts

 

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with 2-aminoethanol and with 2-aminophenol to give mononuclear neutral Mn(III)

complexes 2 and 3, respectively with mixed Schiff-base ligands, [Mn(L)(L')].

Complexes 1 and 2 are high-spin, while 3 have been reported to be low-spin. Sureshan et

al18 synthesized mixed ligand complexes of the type LFeACl.nH2O and LMnA.H2O

where L is the mannich base obtained by hydrogenation of Schiff base ligands of

salicylaldehyde and amino acids, and A is the bipyridyl or orthophenanthroline.

These complexes were characterized by elemental analysis, spectral, magnetic, FAB

mass spectral and electrochemical studies, further used as catalysts for epoxidation of

different olefins like cyclohexene, cis-cyclooctene and norbornene using iodosylbenzene

as the oxidant. It was found that Mn (II) complexes act as better catalysts compared to Fe

(III) complexes. Tumer et al19 prepared mixed-ligand complexes of copper(II) with 1,10-

phenanthroline and various salicylaldehyde-aniline Schiff bases (HL) and characterized

by elemental analysis, electronic and IR spectra, magnetic moment and molar

conductance data. Schiff bases behave as bidentate ligands, and the mixed-ligand

copper(II) complexes [CuL(phen)]Cl and [Cu2L(phen)Cl2]Cl are mono- and binuclear,

respectively. Conductance data for all the complexes are consistent with those expected

for an electrolyte.

Antimicrobial activities of some of the ligands and complexes were tested against

Bacillus megaterium and Candida tropicalis. 1H and 13C NMR spectra were recorded to

solve the solution structure of the ligands. Thermal properties of all complexes were

studied by the DTA and TGA techniques. Mukherjee et al20 prepared three cubane

copper(II) clusters, [Cu4(HL')4] (1), [Cu4L2(OH)2] (2), and [Cu4L2(OMe)2] (3), from two

pentadentate Schiff-base ligands N,N'-(2-hydroxypropane-1,3-

diyl)bis(acetylacetoneimine) (H3L') and N,N'-(2-hydroxypropane-1,3-

diyl)bis(salicylaldimine) (H3L), further structurally characterized by x-ray

crystallography, and by their variable-tempearture magnetic properties studies.

Complex 1 has a metal-to-ligand stoichiometry of 1:1 and the structure consists of a

tetranuclear core with metal centers linked by a µ3-alkoxo oxygen atom to form a cubic

arrangement of the metal and oxygen atoms. Each ligand displays tridentate binding

mode. New ternary Cu(II) complexes [CuLnB](ClO4) (1-3), where HLn is the NSO

donor Schiff base derived from condensation of 2-mercaptoethylamine hydrochloride

 

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with salicylaldehyde (HL1) or 2-hydroxy-3-methoxybenzaldehyde (HL2) and (B: NN-

donor heterocyclic base= 2,2'-bipyridine (bpy,1), 1,10-phenanthroline (phen,2) or 2,9-

dimethyl-1,10-phenanthroline (dmp, 3)), were prepared, structurally characterized by x-

ray crystallography and their DNA cleavage activity was studied.

Complexes show distorted square-pyramidal (4 + 1) CuN3OS coordination geometry in

which the NSO-donor Schiff base is bonded at the basal plane and the NN-donor

heterocyclic base displays axial-equatorial mode of bonding21 Panchal et al22

synthesized, characterized, and studied antibacterial activities of the mixed-ligand

complexes [ML(SB)H2O], where M = Mn(II), Co(II), Ni(II), Cu(II), Zn(II), and Cd(II),

SB1 = bis(benzylidene)ethylenediamine (benen) or SB2 = bis(1-

phenylethylidene)ethylenediamine (acphen) and KHL = potassium salt of

salicylideneglycine (salgly). Their structures were elucidated from elemental and

thermogravimetric analyses, magnetic measurements, and reflectance and IR spectra, are

in accordance with an octahedral environment around the central metal ion.

Mixed-ligand complexes of Mn(II), Co(II), Ni(II), and Cu(II) are paramagnetic while the

Zn(II) and Cd(II) complexes are diamagnetic, mixed-ligand complexes were tested

against bacteria and the results are compared with a standard drug (tetracycline) and

control (DMSO). The same group also prepared23 some mixed-ligand complexes,

[M(salgly)(L)(H2O)], of the transition metal ions Mn(II), Co(II), Ni(II), Cu(II), Zn(II),

and Cd(II) with the potassium salt of salicylideneglycine (Ksalgly) and 2,2'-

bipyridylamine or di(benzylidene)-1,8-diaminonaphthalene.

Coordination behavior of the mixed-ligand complexes was characterized for magnetic

measurements from elemental analyses, IR, and electronic spectra. Mixed-ligand

complexes of Mn(II), Co(II), Ni(II), Cu(II) are paramagnetic, while the Zn(II) and Cd(II)

complexes are diamagnetic. All of these mixed-ligand complexes are colored crystalline

solids. Octahedral geometries were assigned to all of the synthesized mixed-ligand

complexes. Antibacterial activities of the ligands, the metal chlorides, the mixed-ligand

complexes, the standard drug (tetracycline), and control (DMSO) were tested on the

pathogenic bacteria.

Maurya et al24 prepared some mixed-ligand ternary complexes of Cu(II), Ni(II), Co(II),

Zn(II), Sm(III), Th(IV) and UO2(VI) with the Schiff base derived from salicylaldehyde

 

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and the sulfa drug sulfabenzamide, [N-(salicylidene)sulfabenzamide] (LH) and the

heterocyclic base 1,10-phenanthroline (phen) were synthesized and characterized by IR,

NMR, diffuse reflectance spectra, and thermal, magnetic and molar conductance

measurements. Thermal analyses indicate lattice and coordinated H2O molecules in the

complexes, which was also supported by the IR spectral data. The coordination by the

azomethine N was inferred by the upfield shifting of the -CH:N- signal in the NMR

spectra and the shift of (C:N) to lower wave number in the IR spectra upon

complexation.

The Schiff base ligand (LH) acts as a monobasic bidentate ligand in complex formation.

The presence of anion, viz., CH3COO- or NO3- in the coordination sphere was also

inferred by the IR spectral data and conductance measurements. The general formula of

the complexes are [M(L)(phen)(OAc)(H2O)], where M = Cu(II), Ni(II), Co(II), Zn(II) or

UO2(VI), [Sm(L)(phen)(OAc)2(H2O)2].H2O and [Th(L)(phen)(NO3)3(H2O)].4H2O, where

AcOH = HOAc and LH = [N-(salicylidene)sulfabenzamide]. Mixed ligand complexes

CuL2.L' (HL = glycine and L' = Schiff bases of salicylaldehyde and anthranilic acid

(SalH.AnthA), 2-hydroxy-1-naphthaldehyde and anthranilic acid (2HIN.AnthA),

salicylaldehyde and ethylenediamine (SalH.EDA) and 2-hydroxy-1-naphthaldehyde and

ethylenediamine (2HIN.EDA)) were synthesized and characterized.

The IR spectral data of the complexes indicate H-bonding which may be one of the

stabilizing factors. The IR spectra also indicate the coordination of Cu(II) metal chelate

with the O atom of -COOH or -CHO group of the ligand. UV spectra and magnetic

moment values suggest octahedral geometry for the complexes. Low values of molar

conductance show their nonelectrolytic nature25, bis-Schiff-base N2O2 donor dibasic

ligand 4,6-bis(1-(ethylimino)ethyl)resorcinol (H2L) was synthesized by the reaction of

4,6-diacetylresorcinol with ethylamine in 1:2 molar ratio.

The ligand was characterized using IR, UV-visible, 1H NMR and mass spectroscopy. 1H

NMR spectrum of the ligand shows phenolic coordinating groups. 12 new mixed ligand

complexes of the Schiff-base ligand (H2L) and (L') where (L') = deprotonated 8-

hydroxyquinoline (8-HQ), 2,2'-bipyridine (2,2'-Bipy) and 1,10-phenanthroline (1,10-

Phen) with different metal ions such as Co(II), Ni(II), Cu(II) and UO2(VI) were

synthesized. Elemental analyses, IR, UV-visible, ESR and thermal analysis, as well as

 

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conductance and magnetic susceptibility measurements, were used to elucidate the

structures of the newly prepared metal complexes. Complexes were isolated as binuclear

and confirmed by ESR spectra.

Thermal studies for some complexes show that the final product is the metal oxide. TGA

was used as a tool to detect that the water molecules associated with the complexes are

either coordinated or crystalline. An octahedral geometry is suggested for the Co(II),

Ni(II), Cu(II) (for 2,2'-Bipy and 1,10-Phen), and UO2(VI) complexes, square planar for

Cu(II) (for 8-HQ) complexes and their calculated ligand field parameters agree with their

proposed geometries26. IR spectral studies for mixed ligand complexes of Co(II), Ni(II)

and Cu(II) from Schiff base and amino acid were shown27.

The IR spectra of Schiff base and its metal complexes with Co(II), Ni(II) and Cu(II)

divalent metal ion were recorded. The IR spectra obtained indicated the presence of

>C=N, -SH, -NO2, -NH2, -COOH, M-O, M-N, M-S and -COO-. The mixed-ligand

complexes [M(SB)2acphen] (M = Mn, Co, Ni, Cu, and Cd; HSB=3,5-

dibromosalicylideneaniline; and acphen= bis(acetophenone) ethylenediimine) were

prepared and characterized by elemental analyses, magnetic measurements, TG, and IR

and electronic absorption spectroscopy. All the mixed-ligand complexes exhibit an

octahedral geometry.

The mixed-ligand complexes show antimicrobial activities against bacteria, yeast, and

fungi28. The complexation behavior of mixed complexes of mefloquine hydrochloride

and chloroquine phosphate with Cobalt(II), Nickel(II) and Iron(III) were studied; the

complexes were prepared29 using template methods, and chelates of 1:1:1 stoichiometries

were formed. The nature of the bonding of the mixed ligands (mefloquine and

chloroquine) and structure of the isolated metal complexes were proposed on the basis of

their physical and spectroscopic characterization (conductivity measurement, electronic,

atomic absorption spectroscopy, magnetic measurements, elemental analysis and infra-

red spectroscopy).

The complexes in general show 4- and 6-coordinate geometry. The conductivity

measurements revealed that all the complexes are non-electrolytes. There was an

indication that the mixed ligands were covalently bonded to the metals. In vivo evaluation

of the biological studies of the mixed antimalarial metal complexes and free ligands

 

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showed greater activity against some of the micro-organisms, when compared to the

parent compounds. Toxicological studies revealed that mefloquine, chloroquine and

Ni(Mef)(CQ)Cl2 may have affected the plasma membrane integrity of the cells and were

toxic to the tissues, while the mixed metal complexes of mefloquine and chloroquine

(Co(Mef)(CQ)Cl2 and Fe(Mef)(CQ)Cl3) would be a better therapeutic drug for malaria.

Bidentate schiff base having nitrogen and oxygen atom was prepared30 by condensation

of the p-amino-2,3-dimethyl-1-phenyl-3-pyrozoline-5-one with salicylaldehyde in

methanol.

An ethanolic solution of Schiff base and an ethanolic solution of 8-hydroxyquinoline

were reacted with aqueous solution of metal salts to give complexes with the general

formula [M(L)(Q)] where Q = 8-hydroxyquinoline, L = o-hydroxybenzylidene-1-phenyl-

2,3-dimethyl-4-amino-3-pyrazolin-5-one and M = Fe(II), Co(II), Ni(II) and Cu(II) ions.

The resulting product was found to be solid which have been characterized using FTIR

and UV Visible spectroscopy. Elemental analyses have been performed using C, H, N

and atomic absorption technique, the magnetic susceptibility and the conductivity have

also been measured.

The mixed ligand complexes of Co(II), Ni(II), Cu(II) and Zn(II) with Schiff bases N-(2-

hydroxy-1-naphthylidene)-4-methylaniline (L1H) and N-(2-hydroxybenzylidene)-2,3-

dimethylaniline (L2H) were synthesized and characterized31, using elemental analysis,

thermogravimetric analysis, magnetic moment measurements, conductivity

measurements, 1H NMR, IR, UV-visible and ESR spectral studies. The Schiff bases acts

as bidentate monobasic ligands, coordinating through deprotonated phenolic oxygen and

azomethine nitrogen atoms. The complexes were non-electrolytic in DMSO. The

presence of the two coordinated water molecules in these complexes was indicated by IR

spectra and thermogravimetric analysis of the complexes.

All the complexes exhibited octahedral geometry and from the analytical and spectral

data the stoichiometry of these complexes was found to be[M(L1)(L2)(H2O)2]. The

antimicrobial activities of the metal complexes are higher than the ligands. A novel Schiff

base mixed ligand chelates of Cu(II), Ni(II), Co(II) and Mn(II) complexes with two

newly synthesized Schiff base mixed ligands derived from salicylaldehyde and o-

phenylenediamine (H2L1), benzaldehyde and o-phenylenediamine (H2L2)was prepared32.

 

14

The ligands and their transition metal complexes were characterized on the basis of

various physico-chemical methods including elemental analysis, molar conductance,

infrared, electronic spectra, EPR and cyclic voltammetry.

The mixed ligand complexes are formed in the 1:1:1 (L1:L2:M) ratio as found from the

elemental analyses and found to have the formulae [ML1L2] where M= Cu(II), Ni(II),

Co(II) and Mn(II), L1= N, N′-Bis-(2-hydroxy-benzylidine)-benzene-1, 2-diamine, L2= N,

N′-Dibenzylidinebenzene-1, 2-diamine. The molar conductance data revealed that the

chelates are non-electrolytes. The IR spectral data suggest the involvement of azomethine

nitrogen in co-ordination to the central metal ion. The electronic spectral results indicate

that all the complexes had octahedral geometry. From the electrochemical studies of the

Schiff base complexes it was observed that the redox transitions were assigned to specific

redox-active sites of the molecule.

The antibacterial activities was carried out with comparison with known antibiotics.

Furthermore, the antioxidant activity of the Schiff base complexes was determined by

DPPH method in vitro. Co(II) complexes of Schiff base 2-amino-4-nitrophenol-N-

salicylidene with some amino acids were synthesized33. The Schiff base and its mixed

ligand complexes, in general, were non-hygroscopic and stable solids. Their structural

characterization included melting point, solubility, elemental analyses, conductivity

measurements, GC-MS, FTIR, NMR, UV-Visible spectroscopy, magnetic susceptibility,

and X-ray diffraction studies. The morphology of mixed ligand Co(II) complexes was

studied by scanning electron microscopy.

The compounds were subjected to simultaneous thermogravimetric analysis to study their

decomposition mechanism and thermal stability. The Schiff base and mixed ligand

complexes were preliminarily scanned against various strains of microbes and showed

very good antibacterial and antifungal activity at 12.5, 25, 50, 100 and 200 ppm,

compared to standard compounds amphiciline for anti bacterial activities and

Streptomycin for anti fungal activities.

1.5. Present Research Objective

The through literature review points to the necessity of structural characterisation of

schiff base and their metal complexes which can lead to valuable data for the design and

synthesis of newer material. Either varying the amine or the carbonyl subunit as well as

 

15

substituent gives different schiff base with different supramolecular bonding capabilities

that in turn enable great diversity of crystal packing. Present study aims at the synthesis

and spectral characterisation of some schiff bases and their transitional metal ion

complexes derived from 5-nitrosalicylaldehyde (nosal) and substituted aniline’s.

Specific features of this schiff base is that it shows an O-H----N or O----H-N type of

intra-molecular hydrogen bonding and related tautomerism between enol-imine and keto-

amine forms. This is due to the presence of an ortho hydroxyl group and the additional

non-bonding electron pair nitro group which acts as a hydrogen bond acceptor. Structural

characterisation data helps in suggesting the presence of a particular tautomer in the

crystal, which in turn depends upon the parent o-hydroxyl aryl aldehyde and the type of

N-substituents.

Position of the substituent on the N-substituent, their electron withdrawing or donating

ability and their stereo chemistry as well as hydrogen bond donor- acceptor properties are

also involved in stabilizing one or the other tautomer in the crystal. Since the structure of

the compound decides its function it is of utmost importance to deduce the structure of

the free Schiff base as well as the metal bonded schiff base with the help of various

spectral and solution studies.

 

16

Section II

1.6. Stability studies of metal complexes

An essential aspect to understanding the influence of metal ions on enzyme-catalyzed

reactions is the knowledge of how tight different metal ions bind to a wide variety of

substrates, products and effectors and that binding phenomena are altered by

experimental conditions(e.g., effect of pH, ionic strength, etc.). This necessitates the

experimental determination of stability constant for the metal ion- ligand complex. A

large number of organic compounds contain acidic and or basic groups, which govern

many of their chemical, physical and biological properties. For such compounds the

proportion of such species (neutral, molecule, anion, and cation) that are present at a

particular pH are determined by pKa values and can be calculated from the equation.

pKa = pH + log [Acid]/[Base]

In preparative chemistry, pKa values can be used to select conditions for the synthesis

specially by considering the effect of pH on reaction products and on the properties of

postulated intermediates not available for measurement. In analytical chemistry, pKa

values assist in the interpretation of pH titration where multiple acidic or basic sites are

present.

Werner34 rejected the usual concepts of valence and affinity or attraction of atoms and

established the coordination theory that accelerated the study of the nature of bonding in

coordination compounds. The efforts made by pioneers in the field such as Lewis35,

Kossel36, Sidgwick37 and others culminated in the proposals of different theories to

account for the nature of the chemical bond between the metal ion and the complexing

ligand molecule. Pauling38 put forth the valence bond theory based on hybridization of

orbital.

This theory, however, could not interpret the absorption spectra and the magnetic

properties of the complexes. However, these properties could be explained by the

molecular orbital theory as applied by Van Vleck39. Ilse and Hartman40 and Orgel41

applied the crystal field theory to explain the variation in the stabilities of the transition

metal complexes. Later both the molecular orbital theory and the crystal field theory, in

combination, gave rise to the ligand field theory. This theory proved remarkably

successful in interpreting the various properties of coordination compounds such as their

 

17

stabilities, magnetic measurements, spectral data, redox potentials, rates of reaction, and

steriochemical properties.

1.6.1. Significance of stability of complexes

In solutions, complexes result from the reversible association of one or more metal ions

and the ligand and hence the common laws of equilibria are applicable to them. The term

stability has been used to mean the amount of association that occurs in solutions

containing two or more species in equilibrium. Hence it follows that the greater the

association, the more stable is the complex. The term equilibrium constant, which is a

more general, now assumes a new name in the light of what has been stated above and is

referred to as the “stability constant”.

The measurement of stability constants is of primary importance in the proper

understanding of chelates. With the help of these values it is possible to calculate the

equilibrium concentration or activity of each of the species present in the solution under a

known set of experimental conditions. The values of stability constants facilitate the

formulation of conditions for complete or maximum formation of a given chelate. The

work of Bjerrum42 and Leden43 has evinced a great deal of interest in the investigation of

equilibria of metal chelates and ionic complexes in solution.

This had resulted in the better understanding of the physical chemistry of solutions and

the development of different methods for examining the complex equilibria. It became

possible to get more accurate values of the stability constants of complexes, which

resulted in the development of quantitative theories to explain the interaction between the

metal ion, the ligand and the solvent. The work of Bjerrum was responsible for

rationalizing the solution chemistry of metal chelates, which were known to involve

successive equilibria.

1.7.1. Factors affecting stability of complexes

The important factors, which seem to influence the stability of complexes, are

1.7.1.1. Basicity of ligand The stability of a complex increases with the increase in the basicity of the ligand. The

transition metal ions and a few other metal ions tend to form covalent bonds with the

donor atoms. The ligands with higher basicity, as a rule, form stronger bonds with metal

 

18

ion, example, N and S donor groups give a stronger bond with metal ion than – O donor

group.

1.7.1.2. The size of the chelate ring Formation of five and six membered rings is most favored because such rings are free

from strain. According to Bayer’s strain theory, the most stable rings are six membered

rings which have no double bonds. A six membered ring with one double bond will be

somewhat more stable than a five membered ring with one double bond and considerably

stronger than a six membered ring with no double bond.

Even when polydentate ligands form chelates with a larger number of rings their stability

still depends more on the five or six membered rings present than on the larger rings.

Schwarzenbach and Ackermann44 found that the stability of the chelate decreases as the

size of the ring increases. An increase in the number of rings, however, results in greater

stability of the chelate. This is not due to any advantage of ring formation, but rather to

the increasing number of water molecules that are displaced from the metal coordination

sphere by one donor atom of polyfunctional reagent45.

1.7.1.3. Substitution effect Substitution of a group in the chelating agent may offset the stability of metal chelates in

one of the following two ways:

i) It may influence the basicity of the donor atom or may interfere with or enhance the

resonance of the chelating ring.

ii) The substituted group may, by purely steric effects46-48, prevent the ligand ion or

molecule from acquiring the orientation about the central metal ion49,50 most favorable to

chelation. Thus, any of the two factors operating in a chelate ring depends upon the

nature and relative position of the substituent.

1.7.1.4. Resonance in the chelate Calvin and Wilson51 showed that resonance might affect the formation and stability of a

chelate. They considered the formation of Cu2+ complexes of acetylacetone and some

other -diketones, of a group of substituted salicylaldehyde and 2-hydroxy-1-

naphthaldehyde and 3-hydroxy-2-naphthaldehyde. All gave the same chelated stable

rings and thus confirmed this view.

 

19

1.7.1.5. Steric factors

The formation of a complex compound may be hindered due to spatial interference

between different ligand molecules of the compound. Steric effects may also arise from

the necessity of bonding atoms of the ligand to group themselves around the central metal

atom in a specific way to give planar, tetrahedral or octahedral geometry. An implication

of steric factors or solvation factors on the stability of the complexes was pointed out by

Riley52 during his studies on the stability of copper complexes of substituted malonic

acids.

1.7.1.6. Effect of the metal ion

Factors governing the relative tendencies for various metals to combine with a given

donor are as follows,

i) The ionic forces which are related to both charge and radius of the metal ion.

ii) The relative tendencies of various metals to form homopolar bonds with electron

donors.

1.7.2. Literature Review for Stability Studies

In recent years many attempts have been made to enlist the metal cations on the basis of

their tendency to co-ordinate with one or two specific ligands. Pfeiffer et al53 claimed that

the order of the stability of salicylaldehyde ethylenediamine is Cu > Ni > Fe > Zn > Mg.

Mellor et al54 studied the stability of salicylaldehyde complexes in 50:50 v/v dioxane-

water medium. They found that chelates follow the order as Pd > Cu > Ni > Co > Zn >

Cd > Fe > Mn > Mg.

Irving et al55 have correlated their data by plotting the stability constants against the

atomic numbers of the metals and observed the order of stability constant to be Mn < Fe

< Co < Ni < Cu < Zn . Martell et al56 indicated a general relationship between formation

constants of metal chelates and second ionization potentials of the metal ions. Irving et

al57 stated that the comparison of the stabilities of the complexes of different ligands is

most effective when the metals used are of similar type. Reversals in order may occur

when comparisons are made between complexes of dissimilar metals.

Feischer et al58 have discussed the stability of complexes formed by metal ion of closed

shell configurations such as lanthanides. Riley59 suggested that any factors that increases

the localization of negative charge in the coordinating ligands makes the electrons more

 

20

readily available and thus increases the ability of a base to coordinate. These ideas were

used to explain number of phenomena. Many attempts to establish a linear relation

between the basic strength of a ligand and the complex forming ability of the ligand are

on record.

The first attempt in this direction was made by Larsson60. However it is now an

established fact that when systems of sufficient structural similarity are compared, a

linear relationship between pK (complex) and pK (base) is obtained. Irving-Rossotti

technique61 has been applied to study formation constant of the reaction,

(CuA)2+ + L- → (CuAL)+

Where, A = dipyridyl and L = Glycine,α or β-alanine. Copper dipyridyl 1:1 is known to

form at lower pH and is stable at higher pH when the secondary ligand gets bound.

The values of the formation constant obtained in the mixed ligand systems are little less

than the first formation constant of Cu-acid simple systems. Irving-Rossotti technique62

has been applied to study the formation constant of the reaction (MA)- + L2- (MAL)3-

[where LH2 = catechol, pyrogallol and 2,3-dihydroxynaphthalene]. A similar equation

with L3- represents coordination of protocatechuic acid. Metal NTA 1:1 complexes were

formed at lower pH and are stable at higher pH.

The values of formation constants obtained for the mixed ligand system KMAL are found

to be less than the first formation constant in the binary system1MLK . This behavior may

be due to (i) the differences in electrostatic repulsions experienced by L2- to add to

M2+(aq.) (ML) or MNTA-, or NTA3- occupying more coordination positions around metal

ions than L2-.

The formation constants of binary copper complexes are higher than that of Ni-chelates,

Zn-chelates and Cd-chelates. However, in ternary system the magnitude of such

differences is small. The observation has been explained in terms of Jahn-Teller

distortion. Formation constants of the mixed ligand complexes (MAL), where M =

Cu(II), Ni(II), Zn(II) and Cd(II), A = dipyridyl or o-phenanthroline and L =

etylenediamine or propylenediamine, have been determined63 using an extension of

Irving-Rossotti titration technique.

It is found that MAMALK is nearly equal to M

MLK and this has been explained by considering

M → dipyridyl π- bonding. Irving-Rossotti titration technique64 has been applied to

 

21

mixed ligand formation constants of hetero chelates formed by the reaction MA + L →

MAL, where M = Ni2+ or Cu2+; A = dipyridyl (dipy), o-phenanthroline (o-phen),

iminodiacetic acid (IMDA) or nitrilotriacetic acid (NTA) and L=acetylacetone (acac) or

benzoylacetone(BA) or dibenzoylmethane (DBM). It is observed that dipyMLdipyMK (

)( is nearly

same as MMLK and M

MLK - MAMALK is more when A = IMDA or NTA. Thus, though β-

diketones are established π-bonding ligands, the formation constants of the ternary β-

diketones show same relationships to the corresponding binary complex as in cases of

ternary complexes containing σ-bonding secondary ligands.

Potentiometric titrations of the reaction mixtures containing metal-adenine (1:1)

complexes and secondary ligand 1,10-phenonthroline, 5-sulphosalicylic acid, cytosine,

adenine and 2,2`-bipyridyl in a 1:1:1 molar concentration indicate the formation of mixed

ligand 1:1:1 ternary complexes.

The metal ions studied are Cu(II), Ni(II), Zn(II), Co(II), Mg(II) and Ca(II) at 45 ± 0.1oC

and μ = 0.1 M (KNO3). The stability constants of these metal ternary complexes and 1:2

metal-adenine complexes are being reported65. Potentiometric studies66 have been carried

out to determine the multiple equilibria involved in copper(II)-L-arginine (A)-L-histidine,

histamine or imidazole (B) mixed systems at 37oC and I = 0.15mol dm-3 (NaClO4). The

protonation constants for L-argenine and its binary stability constants with copper(II)

have been obtained under identical conditions.

Data treatment indicates the presence of CuAH, CuA2H and CuA2H2 binary complexes in

the copper(II)-L-argenine binary system and the mixed species of the stoichiometry

CuABH, CuABH2, CuAB, CuAB2H and CuAB2 in the mixed ligand systems. The results

suggest an increased stability for the mixed complexes as compared to the statistical case.

The nature of coordination sites of the various complex species detected is discussed in

terms of their stability constant data. Ternary complex systems metal(II)-salicylic acid

derivatives (as primary ligands) – acidic and basic amino acids (as secondary ligands)

have been investigated by potentiometric technique67.

Formation constant values of the various binary and ternary complexes liable to exist in

such systems have been determined at 25oC and μ = 0.2mol dm-3 (KCl). The order of

stability of the binary and ternary complexes in terms of nature of the metal ion salicylic

acid derivative and amino acid as well as the stability of the ternary complex is compared

 

22

to binary amino acid complex.

Equilibrium study on the mixed ligand formation of M2+ ions ( M = Co, Ni, Cu and Zn)

with 6-aminopenicillanic acid (apaH+-) and nucleic bases adenine, guanine, thymine,

uracil (BH) and cytosine (B) in aqueous solution, ionic strength, I = 0.1M (NaNO3) and

at 37oC has provided evidence of formation of variety of complexes of the types, M(apa),

M(B), M(apa)(B), M(apa)(B)(OH) and M(apa)(B)(OH)2 . Stability of these complexes

has been characterized68 by log MBapaM ))(( and log KM values. Stability of M(apa), M(B)

and M(apa)(B) complexes has been found to follow the order : CuII > NiII > CoII > ZnII

with regard to the metal ions, and the order : guanine > adenine > uracil > thymine >

cytosine with regard to the nucleic bases.

Acidity of the coordinated water molecules in M(apa)(B) complexes has been found to

increase with the stability constant of such complexes. Cytosine is found to form

complexes of least stability with all the four metal ions. Complexation reactions of Cu+2,

Ni+2, Co+2 and Cd+2 with 2,2'-bipyridyl (Dpy) as primary ligand and salicylidene aniline

(SA), salicylidene p-anisidine (SPA) and salicylidene p-nitroaniline (SPNA) as secondary

ligands were studied by modified Irving-Rossotti69 method. Measurements were made on

a pH meter. Solutions were prepared in water-ethanol medium at 25oC, measurements

were made in different solutions having ionic strengths µ = 0.05M; 0.1M and 0.2M

NaNO3 stability constants at 0.0 ionic strength was determined by extrapolation, values of

free energy ∆F have been calculated by using Gibbs-Helmholtz equation: -∆F = RTln k.

There is a slow decrease in stability constants with increasing ionic strengths. The order

of stability constants was Cu(II) > Ni(II) > Co(II) > Cd(II).

Stability constants for the complexes having N,O type of multidentate Schiff base

ligands(bis(salicylaldehyde)ethylendiamine(SED),bis(salicylaldehyde)propylendiamine(S

PD),bis(salicylaldehyde)diethylenetriamine(SDT),bis(salicylaldehyde)triethylenetetraami

ne (STT), and bis(salicylaldehyde)tetraethylenepentaamine(STP)) with Co(II) and Zn(II)

were determination by a potentiometric method70 in a 70% dioxane-30% water mixture

and ethanol, respectively stability constants for the complexes increased in the order of

SPD<SED<SDT<STT<STP with the increasing number of donor atoms employed.

Hassan, Fatma S. M.71 studied stability constants of the binary and ternary Ni(II), Pd(II),

and Pt(II) complexes containing salicylaldehyde-3-amino-1,2,4 triazole Schiff base (L1)

 

23

and a second ligand, cysteine (L2) in a 1:1:1 molar ratio were determined pH-metrically

at 25, 30, 35, and 40oC in 50% (vol./vol.) aqueous ethanol at an ionic strength I = 0.1M

(NaCl). stability constants of the mono and mixed ligand complexes of Ni(II), Pd(II),

and Pt(II) have been evaluated.

The difference in stability constants ∆logKM = log KM (Schiffbase)M (Schiffbase)(Cys)

- log KMM (Cys) is found to be positive showing a cooperative behavior of the ligands.

Thermodynamic parameters ∆Ho and ∆So were calculated for systems controlled by

variation in temperatures. Sadikoglu, M72and coworker Serin, S., prepared 3 Schiff base

ligands from a mixture of 2-hydroxy-1-naphtaldehyde with 2-methyl-5-chloroaniline,

2,5-dichloroaniline and 3-chloroaniline, respectively (A1, A2 and A3) and their Ni(II)

complexes. These complexes were characterized by elemental analysis, IR, electronic

spectra and magnetic susceptibility techniques.

The stoichiometry and the stability constants of the complexes were determined by

applying the conventional spectrophotometry of continuous variation method (Job's

method). The stabilities of Ni(II) complexes of the naphtaldimine Schiff bases containing

the electron donor or acceptor groups on the 2- and 5-position of Phenyl ring were

studied as connected with the role of the molecular structure. It was observed that the

Ni(II) complex of A1 Schiff base containing an electron donor -CH3 group on the 2-

position of Phenyl ring was more stable than the other complexes. Binary and ternary

complexes of the type M-L and M-X-L [M = Cu(II), Ni(II), Mn(II), and Fe(III)]; X = 5-

chlorosalicylidene-2-chlorobenzylamine and L = 5-chlorosalicylidene-4-

aminobenzenesulphonamide have been examined pH-metrically at 27oC and μ = 0.1M

(KCl) in 75:25 % (vol/vol) dioxane-water mixture73.

The logarithms of the values of formation constants for M-L and M-X-L systems [M =

Cu(II), Ni(II), Mn(II), and Fe(III)] are calculated as 7.64 and 5.96; 8.05 and 4.71; 7.86

and 4.60; 7.95 and 4.66; 10.07 and 8.88 respectively. Binary and ternary complexes of

the type M-L and M-X-L {M = Cu(II), Zn(II), Mn(II), Ni(II), and Co(II), X = [1-phenyl-

3-methyl-5-hydroypyrazol-4-yl)-2’,3’-dimethylaniline, Y = 2-hydroxy-1-naphthadehyde}

have been examined pH-metrically at 29oC and μ = 0.1 M (KCl) in 75:25 % (vol/vol)

dioxane-water mixture74.

The values of formation constants for M-L and M-X-L systems [M = Cu(II), Zn(II),

 

24

Mn(II), Ni(II), and Co(II)] are calculated. Binary and ternary complexes of the type M-Y

and M-X-Y[M = Co(II), Ni(II), Cu(II) and Zn(II); X = N-(2-hydroxy-1-naphthylidene)-

2,6-diisopropylaniline and Y = N-(2-hydroxybenzylidene)-2,3-dimethylaniline] were

studied pH-metrically75 at 27±0.5oC and μ = 0.1 M in 75: 25% (vol/vol)1,4-dioxne-water

medium. The value of log are slightly lower than log and higher than log ,

which is due to the fact that the tendency of the secondary ligand (Y) to get bound with

aquated metal ion [M(aq)]2+ is more than to combine with the metal ion already bound

with primary ligand (X).

The relative stability (∆ log KT) values of the ternary complexes with corresponding

binary complexes for all the metal(II) ions in the present study is negative indicating that

ternary 1:1:1 (M-X-Y) complexes are less stable than binary 1:1 (M-Y) complexes. In the

ternary system studied, the order of stability constants of mixed ligand complexes with

respect to the metal ions was found to be Cu(II) > NI(II) > Co(II) > Zn(II), which is same

as in the corresponding binary (M-Y) systems. This is in accordance with the Irving-

Williams series of stability constant.

 

25

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