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CHAPTER – II SCHIFF’S BASES AS LIGANDS
2.1 INTRODUCTION Synthesis of new metal complexes of different ligand systems is important
in chemistry, biology and medicine. In recent years, research in the field of
coordination chemistry of biologically important materials has gained
considerable momentum. This is because, metal complexes play vital roles in the
chemistry of living matter. Chlorophyll (a Magnesium complex) in photosynthesis,
Vitamin B12 (a Cobalt complex) in the formation of erythrocytes and haemo
proteins (Iron complex) in respiration are some of the early examples.
Metal complexes derived from heterocycles containing Nitrogen, Sulphur
and Oxygen as ligands have attracted increasing attention in recent years.
Chemical literature provides information on the complexation of transition metals
with ligands having donor atom such as Nitrogen, Sulphur and Oxygen. Many of
the salient features of these Nitrogen and Sulphur complexes have been brought
to light by various researchers in the field [Jungreis E. et al., 1969; Patai Sed.,
1970; Kennedy B.P. et al., 1972; Dutta R.L., 1975; Sibirskaia V.V., 1978; Becher
J. et al., 1982; Shah K.J. et al.,1985; Prisyazhnyuk A.I., 1986; Amina Rai R.A.
and Laxmi, 1986; Mukanti K. et al., 1986; Akbar Ali M. et al., 1986; Ahamad A.S.
et al., 1986; Syamal A. et al., 1986; Shoukry M.M. et al., 1987; Verma J.K., 1987;
Dutton J.C. et al., 1988; Revathy V. et al., 1990; Somasekharappa K.G., 1993].
The metal complexes of Schiff’s bases derived from salicyladehyde or derivatives
of salicyladehyde and the heterocyclic compounds containing Nitrogen, Sulphur
and Oxygen as ligand atoms are of interest, as simple structural models of more
complicated biological systems. Schiff’s bases, having multidentate coordination
sites are attractive analytical reagents, since many of them form complexes with
transition metal ions [Agett J. et al.,1981; Sharma R.C. et al., 1999; Sakyan I. et
al., 2004]. Developing highly functional chelating agents such as Schiff’s bases
has been a great concern of many analytical chemists. Many investigations have
been centered on the structure and bondings in Schiff’s bases, but a few have
27
been directly concerned with analytical applications. Schiff’s bases form
complexes with many transition metals to form either 1:1 (metal:ligand) or 1:2
(metal:ligand) complexes. The analytical methods based on complex formation
are used more frequently. Owing to the relatively simple preparation procedures
of Schiff’s bases, it is possible to obtain ligands of different design and
characteristics by selecting appropriate reactants.
In the subsequent pages of this Chapter, an attempt has been made to
outline a survey of relevant literature regarding the synthesis and analytical
applications of Schiff’s bases. The areas surveyed include formation of Schiff’s
bases, their biological activity, their metal complexes and analytical applications
of Schiff’s bases.
2.2 WHAT IS A SCHIFF’S BASE? A Condensation product of aromatic amines and aldehydes forming
azomethine is called as Schiff’s base. They have general formula RIRIIC=NRIII, where R is aryl or alkyl group. Schiff Hugo Josef in 1864 discovered Schiff’s
base.
2.2.1 Formation of Schiff’s Bases A Schiff’s base or azomethine, is nitrogen analog of an aldehyde or a
Ketone in which the C=O group is replaced by a C=N-R group. It is usually
formed by condensation of an aldehyde or a ketone with a primary amine
according to the following Scheme.
R NH2 + R C R CR
RN R + H2O
Primary amine Aldehyde or ketone Schiff's base
O
R=H
R may be alkyl or aryl group. Schiff’s bases, which contain aryl
substituents, are substantially more stable and more readily synthesized. While
28
those, which contain alkyl substituents are relatively unstable and readily
polymerizable, the aromatic aldehydes having effective configuration are more
stable [Abdul Rauf, 2005].
The formation of the Schiff’s bases from an aldehyde or a ketone is a
reversible reaction and generally takes place under acid or base catalysis.
+ R NH2 R C
NHR
OH
RC RR
O
Ketone or Aldehyde
Carbinolamine
C
NR
R + H2ORN-Substituted imine
(R=H)
The formation is generally driven to completion by separation of the
product or removal of water, or both. Many Schiff’s bases can be hydrolyzed
back to their aldehydes or ketones and amines by aqueous acid or base.
The mechanism of Schiff’s base formation is another variation on the
theme of nucleophilc addition to carbonyl group. In this case, the nucleophile is
the amine. In the first part of the mechanism, the amine reacts with the aldehyde
or ketone to give an unstable addition compound, called carbinolamine and this
subsequently loses water by acid or base catalyzed pathways. Since the
carbinolamine is an alcohol, it undergoes acid catalyzed dehydration.
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R2C
OH
N
H
R'H+
R2C N
OH2
HR'
C
R
RN R'H
+ H2O
C N
R
R R'+ H3O
(acid catalysed dehydration)
Typically the dehydration of the carbinolamine is the rate determing step
of Schiff’s base formation and this is why acids catalyze the reaction. Yet the acid
concentration cannot be too high, because amines are basic compounds. If the
amine is protonated and becomes non nucleophilic, equilibrium is pulled to the
left and carbinolamine cannot occur. Therefore many Schiff’s base synthesis are
best carried out at mildly acidic pH.
The dehydration of carbinolamine is also catalyzed by base. The reaction
is somewhat analogous to E2 elimination of alkyl halides, except that, it is not a
concerted reaction. It proceeds in two steps through an anionic intermediate. The
Schiff’s base formation is really a sequence of two types of reactions i.e. addition
followed by elimination.
2.2.2 General method for the preparation of Schiff’s Bases The Schiff’s base can be prepared by refluxing equimolar quantities of any
primary amine and the aldehyde or ketone on a water bath for 2-5 hrs. Schiff’s
base thus obtained, is filtered and purified by recrystallisation from the
appropriate solvents (Yield 70-80%). A few examples are given below:
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Condensation reaction between salicyladehyde and aniline gives Schiff’s
base namely 2-(phenyl imino methyl) phenol [Vogel A.I., 1971].
H2N
OH
O
CH
OH
N+
Salicyladehyde Aniline 2-(phenyl imino methyl) Phenol
Condensation reaction between salicyladehyde and 2-aminobenzothiazole
yields 2-hydroxybenzylidine-2-aminobenzothiazole Schiff’s base. [Popov A.F.
et al., 1989]
OH
CH S
NN
2-Hydroxybenzylidine-2-aminobenzothiazole
Condensation reaction between 2-amino benzothiazole and derivatives of
benzaldehyde gives the Schiff's base [Amany M. Ibrahim, 1993].
S
NN CH
X X=H, p-N(CH3)2, p-OH, o-OH, p-Cl, m-Cl, p-Br, p-NO2
Condensation reaction of 2-hydroxyacetophenone with 1,2′ diamino cyclo-
hexane, 1,2-diphenyl ethylene diamine and 2,2′-diamino-1,1′ bi naphthalene
gives the Schiff’s bases of (I), (II) and (III) respectively [Wen Tao Gao et al.,
2002].
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C
OH
HN
HO
CNH
C
OH
HN
HO
CNH
HC
OH
N
HO
CHN
(I) (II) (III) Condensing 2-amino benzothiazole with 2-hydroxy-1-naphthaldehyde, 2-
hydroxy benzaldehyde, 4-methoxy benzaldehyde, 4-hydroxy benzaldehyde, and
benzaldehyde (a) and 2-amino-3-hydroxypyridine with 2-hydroxy-1-
naphthaldehyde and 2-hydroxy benzaldehyde (b) gives the Schiff’s bases [Raafat
M. Issa et al., 2008].
S N
N
CR
HN N C
H
R
OH
(a) (b)
(a) R=2-OH-1-naphthaldehyde, o-OH-Ph, p-OCH3-Ph and p-N(CH3)2-Ph.
(b) R=2-OH-1-naphthaldehyde and o-OH-Ph
2.2.3 General properties of the Schiff’s Bases
Pale yellow or orange needles, melting point ranges from 160-240°C,
above this, they undergo decomposition, nearly insoluble in water, but soluble in
aqueous alkaline solution, fairly soluble in benzene, dioxane and chloroform.
They are more soluble in hot ethanol or methanol. Hence these solvents are
generally used for recrystallization. IR spectra of the Schiff’s bases normally
shows C=N stretching frequency between 1562-1650 cm-1 and shift of this band
towards a lower frequency in all Schiff’s base metal complexes suggests that, the
coordination through nitrogen of the azomethine group might have taken place.
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2.3 SCHIFF’S BASES AS A LIGANDS Ligands which posses two or more donating groups may share more than
one pair of electrons with a single metal ion by coordinating around the central
metal ion. These ligands, generally known as multidentate ligands, are
specifically called bidentate, tridentate etc. Multidentate ligands, which coordinate
with metal ions to form complexes, are also called chelates. Instead of a linear
structure, they have a chelate ring structure.
A common case is where the chelating ligand has at least one acidic
group (-CO2H or -OH) or donor atom, as well as one or more basic atoms like
nitrogen. During the chelation, the acidic group loses a proton and becomes
anionic donor, thus resulting in charge neutralization. The basic nitrogen donates
a pair of electrons to the metal ion. A chelate ligand must possess two acidic or
two coordinating groups or one acidic and one coordinating group. Almost all
organic compounds contain –OH, -SH or –NH groups in some form. Molecules
containing Nitrogen, Oxygen, and Sulphur atoms frequently form coordinate
bonds with metal ions in forming chelate rings. These groups must be located in
the molecule in such positions that, the metal ion will be involved in the ring
formation of five or six atoms. An organic compound containing more than two
donor groups is capable of forming multiple rings with the metal ion resulting in
more stable chelate structure.
The Schiff’s base act as a bidentate monobasic donor for Cu(II), Co(II),
Ni(II), Mn(II) and Fe(II) and prominent sites of coordination are nitrogen of
azomethine group and oxygen of the hydroxyl group. Following are some of the
examples which show the complexing nature of Schiff’s bases.
2.3.1 Monodentate Schiff’s Bases as Ligands Ligands having one atom, which can be an electron donor often, function
as monodentate ligands. Schiff’s base ligand of bis [4-(4-Pyridyl methylene
amino) phenyl] ether (L1) and N,N′-bis (3-pyridyl methyl)-diphthalic diimide) (L2)
act as monodentate ligands. These ligands are used in the preparation of two
supramolecular coordination polymers. Ligand L1 forms an interestingly infinite
cross-linked double helical structure, where as (L2) formes the one-dimensional
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zig-zag chains, which are parallel with each other. Each ligand coordinates to two
Hg (II) ions and each Hg (II) ion is coordinated by two L2 to generate the
1D zig-zag chains, which are parallel with each other [Xiaokang Li et al., 2007].
N
O
CH
N
N CH
N
CH2N
C CO O
H2C CH2N
N
CH2
L1 L2
bis[4-(4-Pyridyl methylene amino) phenyl] ether (L1)
N,N'-bis(3-Pyridyl methyl)-diphthalic diimide) (L2)
A long, bis (monodentate), linking Schiff’s base ligand L (Py-CH=N-C6H4-
N=CH-Py) (N1 E, N4 E)-N1N4- (Pyridin-3-yl methylene) benzene-1, 4-diamine was
prepared from 1,4-phenylenediamine and 3-pyridine carboxaldehyde by the
Schiff’s base condensation. Ligand L has two terminal pyridyl groups capable of
coordinating to metals through their nitrogen atoms. The Schiff’s base was used
to prepare Zinc coordination polymers. X-ray crystallographic studies proved that
the Zinc metal chelate is coordinated by two Schiff’s base ligands (L) and four
aqua ligands. The geometry of the Zinc metal chelate can be described as a
distorted octahedron, in which the aqua ligands form an equatorial plane [Han Na
Kim et al., 2005].
N N N N
L= (N1 E, N4 E)-N1 N4- (Pyridin-3-yl methylene) benzene-1, 4-diamine.
Few examples for monodentate ligands are listed in Table 2.1
[José A. García-Vázquez et al., 1983; Elerman Y. et al., 1999; Tahereh Sedaghat
et al., 2008; 2009].
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Table 2.1: Few examples of monodentate Schiff’s bases
Monodentate Schiff’s bases
HO
H3CO
CN
C(CH3)3
C(CH3)3H
(E)-4-[(2,5-di-tert-butyl phenylimino) methyl]-2-methoxy phenol
C
H
N
R R=H, 2Me, 3Me, 4Me, 2,3 Me, 2, 4 Me (E)-N-benzylidenebenzenamine, (E)-N-benzylidene-2-methyl benzenamine, (E)-N-benzylidene-3-methyl benzenamine, (E)-N-benzylidene-4-methyl benzenamine, (E)-N-benzylidene-2, 4-dimethylbenzenamine
C
N
H3CSS
HN C
H
N
(E)-Methyl-2- (2-(pyridin-2-yl methyleneamino) cyclopent-1-enecarbodithioate
MeS
HS
N HOMe SnR2Cl2
n
N
n=1, R=Men=2, R=Bu,Ph
Methyl-2- [2-(acetyl acetoneimino) ethyl amino]-1-cyclopentene-1-dithiocarboxylate Sn (IV)
35
2.3.2 Bidentate Schiff’s Bases as Ligands Ligands having more than one atom, which can be an electron donor often
functions as bidentate ligands. The bis-bidentate ligands were prepared by
condensation reaction between salicylaldehyde and 4,4'-diaminodiphenyl ether.
Complexation of a new bis-bidentate schiff's base, 2-[{{4-[{3-{[(z)-1-(2-hydroxy
phenyl) methylidene] amino} phenyl) oxy] phenyl} imino) methyl]-1-benzenol
(APOPIB) with some mono, di and trivalent metal ions were investigated by the
theoretical calculations and conductance studies. Since APOPIB was able to
form a selective complex with Sm(III) ion (Kf =5.23± 0.24), it was applied
as a sensing material in a poly vinyl chloride (PVC) membrane sensor for
determination of Sm (III) ions [Mohammad Reza Ganjali et al., 2008].
O
N N
OH HO
2-[{{4-[{3-{[(Z)-1-(2-hydroxy phenyl) methylidene] amino} phenyl) oxy]phenyl} imino)
methyl] -1-benzenol (APOPIB)
A few examples for bidentate Schiff’s base ligands are given in Table 2.2
[Mahmoud A.S. Monsm et al., 1996; Viswanathamurthi P. et al., 2005; Siddappa
K. et al., 2008; Prashant Singh et al., 2009].
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Table 2.2: Few examples of bidentate Schiff’s bases
Bidentate Schiff’s bases
NC
C
N O
H
NH C
S
S CH2C6H5
C6H5
(E)-benzyl-2- [(Z)-2-hydroxyimino)-1,2-diphenylethylidene] hydrazine carbodithioate
H3CN
OH
R [R=Ph, 2-MeC6H4, 4-MeC6H4] (E)-2-[1-(o-tolyl imino) ethyl] phenol, (E)-2-[1-(phenyl imino) ethyl] phenol, (E)-2-[1-(p-tolyl imino) ethyl] phenol
O O
C
O
N
H
N C
HO
CH3
N′- (1-(2-hydroxy phenyl) ethylidene)-2-oxo-2H-chromene-3-carbohydrazide
COOHNS
O
OH 2-Hydroxy-4- {[2-oxo-2-(thiophen-2-yl) ethylidene]amino}benzoic acid [TEAB]
SO
N
HN
O
[N- [2-oxo-2- (thiophen-2-yl) ethylidene] pyridine-3-carboxamide] (TEPC)
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2.3.3 Multidentate Schiff’s Bases as Ligands Ligands having more than two atoms, which can be electron donors often,
function as multidentate ligands. They are specifically called as tridentate,
tetradentate, pentadentate, hexadentate, heptadentate ligands.
2.3.3.1 Tridentate Schiff’s Base Triaquotribenzo- [b,f,j][l,5,9]–triazacyclodecine Nickel (II) commonly called
(TRI) Ni(OH2)3 2+ is an example of tridentate Schiff’s base complex [Erno B. et al.,
1979].
N N
N
OH2
H2O OH2
Tridentate Schiff’s base Ni(II) complex
2.3.3.2 Tetradentate Schiff’s Base
A tetradentate Schiff’s base was prepared by the condensation reaction
between ethylenediamine and 2-hydroxy-4-methoxybenzophenone. This Schiff’s
base coordinates via the imine nitrogen and enolic oxygen atoms. Their Ni(II) and
Cu(II) complexes adopt a four coordinate square planar geometry, the Vo(IV)
complex is five coordinate square pyramidal and the heteroleptic complexes are
6-coordinate, octahedral geometry [Osowole A.A., 2008].
OH
C NH3C
C
HO OCH3
N
Tetradentate Schiff’s base
38
Tetradentate Schiff’s base Ni(II) complex was synthesized from of
2-hydroxy 1-naphthaldehyde with aliphatic diamines. The mole ratio of Schiff’s
base ligands to the Ni(II) was 1:1 [Byeong-Goo Jeong et al., 1996].
O
HC N CH
O
N
Ni
(CH2)n
Tetradentate Ni(II) complex.
2.3.3.3 Pentadentate Schiff’s Base Pentadentate Schiff’s base ligand, (L) 3,3′-Thiodipropionic acid bis
(4-amino-5-ethylimino-2, 3-dimethyl-1-phenyl-3-pyrazoline) and its complexes
with Co(II), Ni(II) and Cu(II) were reported. [Sulekh Chandra et al., 2009]
N NN N
NMe
Me NH MeHN
N
Et Et
Me
C CO O
S
L 3,3′-Thiodipropionic acid bis (4-amino-5-ethylimino-2, 3-dimethyl-1-phenyl-3-
pyrazoline)
The Schiff’s base bis-[4-hydroxy coumarin-3-yl]-1N,5N-thiocarbo-
hydrazone, was prepared by the reaction of 4-hydroxy coumarin-3-carbaldehyde
with thiocabohydrazide in 2:1 molar ratio. The ligand and its binuclear complex
with Cu(II), Ni(II), Zn(II), Co(II), Mn(II), Fe(III) and Cr(III) ions were also reported
[Abou Melha K.S. et al., 2008].
O O
N
OH
N NN
O
H H
SO
OH
bis-[4-hydroxy coumarin-3-yl]-1N,5N-thiocarbohydrazone
39
2.3.3.4 Hexadentate Schiff’s Base Hexadentate Schiff’s base ligand (L) was prepared from the reaction of
two moles of 2-benzoyl pyridine and one mole of triethylene tetramine and its
Nickel complex was prepared and characterized by single crystal X-ray diffraction
studies. Each Nickel center is coordinated in a distorted octahedral fashion
consisting of three nitrogen atoms from the hexadentate ligand moiety, two
nitrogen atoms from two azido ligands and the last one from another terminal
azido ion [Soma Deoghoria A. et al., 2003].
N
C N NH
NH
N C
NL Hexadentate ligand
2.3.3.5 Heptadentate Schiff’s Base Heptadentate Schiff’s-base ligands tris[3-(salicylideneimino) propyl] amine
and tris [3-(4′-hydroxysalicylidene imino)-propyl] amine were by the condensation
reactions of tris(3-aminopropyl)amine with 3 equivalents of either salicylaldehyde
or the ring substituted salicylaldehyde, 4-hydroxy salicylaldehyde. Their nickel (II)
and copper(II) complexes were also synthesized [Hassan Keypour et al., 2002].
Heptadentate Schiff’s base metal complex
40
Schiff’s base ligands are capable of forming five or six membered chelate
rings resulting in a more stable chelate structure, which leads to the formation of
mixed ligand complexes. They often provide special sensitivity and selectivity for
analytical importance. Due to the great flexibility and diverse structural aspects of
Schiff’s bases, wide ranges of these compounds have been synthesized and
their complexation behaviors studied.
2.4 BIOLOGICAL ACTIVITY OF SCHIFF’S BASES Schiff’s bases and their metal complexes are extensively studied due to
synthetic flexibility, selectivity and sensitivity towards a variety of metal ions.
They are found useful in catalysis, in medicine as antibiotics and anti-
inflammatory agents and in the industry as anticorrosion agents [Mehta N.K. et
al., 1996; Sun B. et al., 2001; Britovsek G.J.P. et al., 2001; Boghaei D.M. et al.,
2002; Jin V.X. et al., 2005; Liu J. et al., 2006; Budakoti A. et al., 2006]. Schiff’s
base complexes are also noted for their significant antifungal and anti bacterial
activities. Nitro and halo derivative of Schiff’s bases are reported to have
antimicrobial and antitumor activities [Giordano R.S. et al., 1974].
A new anticarcinogenic agent of thiazole Schiff’s base like
N-Benzylidene thiazole-2-amine was prepared by condensation reaction between
2-amino thiozole and aromatic aldehydes. Reason for the biological activity of the
compound was discussed based on the presence of azomethine linkage and of
the thiazole moiety [Mazumdar A.K.D. et al., 1979].
N
SN C
H
N-Benzylidene thiazole-2-amine
Schiff’s bases derived from 4-aryl-2-aminothiazole and 2-amino
benzothiazole with salicyladehyde, 4-aryl-2-(2′-hydroxy aryl imino methyl)
thiazole methiodides, 4-aryl-2- (2′-hydroxy-α- substituted benzyl amino)-thiazoles
and 2-(2′-hydroxy phenyl or naphthyl)-3-(4′-aryl thiazol-2′-yl)-4-thiazolidones were
41
synthesized and biological screening studies of the Schiff’s bases and their metal
complexes reported [Dash B. et al., 1980].
Schiff’s bases derived from 4-substituted, 4,5-disubstituted-2-amino-
thiazoles, substituted 2-aminobenzothiazoles and p-hydroxy benzaldehydes were
synthesized. These compounds were characterized by elemental analysis, IR
and mass spectra and screened for their fungicidal activity [Dash B. et al., 1984].
Bidentate Schiff’s bases of 2-amino benzothiazole with 2-hydroxy-1-
naphthaldehyde [(E)-1-((benzo [d] thiazol-2-ylimino) methyl) naphthalene-2-ol]
were synthesized and their Chromium(III) complexes were screened for
microbiological activity against the fungi Aspergillus niger, Alternaria alternate
and bacterium Escerichia coli [Mishra V. and Saksena D.K., 1987].
S
NN
OH
(E)-1-((benzo [d] thiazol-2-ylimino) methyl) naphthalene-2-ol
Mixed ligand transition metal complexes of Schiff’s bases were
synthesized and anti bacterial, antifungal activities were evaluated including
toxicological studies [Saidul Islam M. et al., 2001]. Co (II) and Ni (II) complexes
with a Schiff’s base, N-(2-furanyl methylene)-2-amino thiadiazole were prepared
and characterized. These Schiff’s bases and their complexes were screened for
antibacterial activity against bacterial stains, e.g. Escherichia coli,
Staphylococcus aureus and Pseudomonas aeruginosa [Zahid H. Chohan et al.,
2002].
N
O
N N
S
N- (2-furanyl methylene)-2-aminothiadiazole
Benzoisothiazole, benzothiazole and thiazole Schiff’s bases were
synthesized and tested for invitro studies with the aim of identifying novel lead
42
compounds which are active against human and cattle infectious diseases or
against drug resistant cancers for which no definite cure or efficacious vaccine is
available at present. In particular, these complexes were evaluated for invitro,
activity against representatives of different virus species, such as Yellow fever
virus and bovine viral diarrhoea virus and it is also tested for gram positive and
negative bacteria [Vicini Paola et al., 2003].
Schiff’s base derived from 4-aminobenzoic acid was used for antibacterial
activity [Jigna Parekh et al., 2005]. A Series of Schiff’s bases were synthesized
by the condensation reaction of 2-amino nicotinic acid with salicyaldehyde,
5-bromo salicyladehyde, 5-nitrosalicyladehyde and 5-methoxy salicyladehyde,
condensation reaction of 2-amino-1,3,4-thiadiazole with the furfuraldehyde,
thiophene-2- carboxaldehyde and pyrrole-2-carboxaldehyde. Condensation
reaction of 5-amino-1,3,4-thiadiazole-2-thiole with furfuraldehyde, thiophene-2-
carboxaldehyde, 4-bromothiophene-2-carboxaldehyde, pyrrole-2-carboxaldehyde,
salicyladehyde and pyridine-2-carboxaldehyde and their antibacterial activities
were reported [Abdul Rauf, 2005].
Co(II), Ni(II), Cu(II) and Zn(II) Vanillidene alanine complexes were
synthesized, characterized, their geometry and the ferromagnetic nature, X-ray
powder diffraction and biological activity studies were reported[Selwin Joseyphus
R. et al., 2006].
4-Ethyl-6-{(E)-1-{(3-nitrophenyl) imino] ethyl} benzene-1, 3-diol and their
metal complexes were prepared and their antibacterial activitity studied on
bacteria Pseudomonas vulgaris [Nair R. et al., 2006].
Ni(II) Schiff’s base complex, derived from salicyladehyde and o-amino
benzoic acids were tested for their antibacterial activities against several human
pathogenic bacteria [Morad F.M. et al., 2007].
COOH
N C
HHO
2-(2-hydroxy benzylidene amino) benzoic acid
43
Coordination complexes of Cu(II), Co(II), Ni(II), Mn(II) and Fe(II) with
Schiff’s bases derived from 3-(4-Chlorophenoxy methyl)-4-amino-5-mercapto-
1,2,4-triazole and salicyladehyde were synthesized and characterized. The
antibacterial activities of these Schiff’s bases and their metal complexes were
screened by cup plate method [Vidyavathi Reddy et al., 2008].
Schiff’s base of dapsone and 2-azetidinones of 4,4′-diaminodiphenyl
sulphone were synthesized. 4,4′ –diamino diphenylsulphone was condensed with
various aromatic or heterocyclic aldehydes in ethanol in the presence of
concentrated sulphuric acid as a catalyst to yield the Schiff’s base. These Schiff’s
bases on treatment with chloroacetylchloride in the presence of triethylamine,
gave substituted 2-azetidinone, which was evaluated for the invitro activity
against several microbes [Wadher S.J. Puranik et al., 2009].
H2N NH2 + R CO
2 H
Reflux
N NS CH RHCR
Chloroacetyl chlorideTriethylamineSchiff's base (1)
N NSHC R
O X
HCR
X O Azetidinone(II)
DapsoneAldehyde
SO
O
O
O
O
O
X= Cl, F, -OMe, -NO2
Synthesis of Schiff’s base and azetidinone, R=aromatic group
Schiff’s bases were synthesized by the reaction of naphtha [1,2-d] thiazol-
2-amine with various substituted aromatic aldehydes. 2-(2′-Hydroxy) benzylidene
aminonaphtha thiazole was converted to its Co(II), Ni(II) and Cu(II) metal
44
complex on treatment with metal salts in ethanol. All the complexes were
evaluated for their antibacterial activities by paper disc diffusion method with
Gram-positive Staphylococcus aureus, Staphylococcus epidermidis and Gram-
negative Escherichia coli and Pseudomonas aeruginosa bacteria. The minimum
inhibitory concentrations of all the Schiff’s bases and metal complexes were
determined by agar streak dilution method. All the compounds moderately
inhibited the growth of Gram-positive and Gram-negative bacteria. Schiff’s base
2-(2′-hydroxy) benzylidene aminonaphtho thiazole showed maximum inhibitory
activity and among metal complexes, Cu(II) metal complex was found to be most
potent. The results also indicated that, the metal complexes are better
antibacterial agents as compared to the Schiff’s bases [Faizul Azam et al., 2007].
2.5 SCHIFF’S BASE METAL COMPLEXES Schiff’s base metal complexes are very important in coordination
chemistry due to their facial synthesis, involvement in catalytic processes and
discovery that the proteins and enzymes require two or more metal ions for their
activity. Schiff’s bases have remarkable property of forming complexes. Several
reports are available only for the synthesis, characterization of Schiff’s bases and
their metal complexes.
Schiff’s base derived from the self-condensation of o-aminobenzaldehyde
was used for the preparation of Nickel (II) complex [Larry T. Taylor et al., 1969].
N
C
CH
N C
HN
H
N
CH
Ni
Nickel complex of Schiff's base derived from self-condensation of
o-aminobenzaldehyde
45
Dash et al synthesized and studied the structural features of Co(II), Ni(II),
Cu(II) and Zn(II) complexes of Schiff’s bases derived from 4-aryl-2-aminothiazoles
and salicyladehyde [Dash B. et al., 1980].
C
HC
R
SC
N
N
MO
C
H
M=Co(II), Ni(II), Cu(II), Zn(II), R=H, Cl, CH3, OCH3, C2H5
Schiff's base metal complexes derived from 4-aryl-2-aminothiazoles and salicyladehyde
Schiff’s bases synthesized by condensation of salicyladehyde and
2-amino-4-phenyl-5-arylazothiazoles and their metal complexes with Pd(II),
Rh(III), Ru(III), Fe(III) were characterized and their structural studies were
reported [Sharma C.L. et al., 1986].
Ligand 5-bromo-2-hydroxy benzylidine-2-amino benzothiazole and its
complexes with Co(II), Cu(II) and Ni(II) were synthesized and characterized. It
was suggested that, two ligands with water molecules coordinate to each metal
atom by hydroxyl oxygen and imino nitrogen to form high spin distorted
octahedral complexes with Co(II), Ni(II), and Cu(II) [Saydam S. et al., 2001].
S
N
N
M O
N
N
SO
OH2
H2O
Br
Br
M=Co(II), Ni(II), Cu(II) Metal complexes of 5-bromo-2-hydroxy benzylidine-2-amino benzothiazole
46
Cu(II), Ni(II), Mn(II), Zn(II) Schiff’s base complexes derived from
o-phenylendiamine and acetoaetanilide were synthesized and characterized,
magnetic susceptibility data and conductance values were reported [Raman N.
et al., 2001].
N
N
N Me
O
H
N Me
O
H
M
H
H
Ph
Ph
M=Cu(II), Ni(II), Mn(II), Zn(II)
Schiff’s base metal complexes derived from o-phenylendiamine and acetoaetanilide
Cu(II), Co(II) and Mn(II) complexes of Schiff’s base ligand derived from
2,2′-bis(p-methoxyphenylamine) and salicyladehyde were synthesized and
spectral properties and electrochemical behaviors investigated [Xishi Tai et al.,
2003].
N N
OOM
M=Cu(II), Co(II), Mn(II)
Metal complexes of Schiff’s base ligand derived from 2-2′-bis(p-methoxy
phenylamine) and salicyladehyde
Cu(II), Mn(II), Ni(II) and Zn(II) metal complexes with novel
heterocyclic Schiff’s bases derived from 5-Phenyl azo-salicyladehyde and
o-amino benzoic acid were synthesized, characterized. These Schiff’s bases
behave as neutral tri dentate ligands forming chelates with (1:1) metal ligand
stoichiometry [Refut M.S. et al., 2006].
47
2.6 SCHIFF’S BASES AS ANALYTICAL REAGENTS Schiff’s bases chelate with many transition metals either in weakly acidic
medium or in weakly alkaline medium to form either 1:1 (metal: ligand) or 1:2
(metal: ligand) complexes. Hence Schiff’s bases derived from aromatic amines
and aromatic aldehydes have wide variety of applications in many fields. Schiff’s
bases are attractive as analytical reagents because, they enable simple and
inexpensive determination of various organic and inorganic substances. In
general, there are two principal ways of their analytical applications: first
determination of organic compounds bearing an amino or an active carbonyl
group by the formation of coloured (Chromophore containing), fluorescent or
insoluble Schiff’s bases and secondly, the determination of various metal ions as
well as amino and carbonyl compounds, using complex formation reactions. The
analytical methods based on complex formation are used more frequently. Owing
to the relatively simple preparation procedures of Schiff’s bases, it is possible to
obtain ligands of different design and characteristics by selecting appropriate
reactants.
Glyoxal bis(2-hydroxyanil) was used as a sensitive reagent for the
determination of calcium and also used as metal indicator in the chelatometric
titration of Calcium. The reagent behaves as quadridentate ligand, forming a 1:1
chelate [Lapin L.N. et al., 1958; Gavrilova L.G. et al., 1970; Honjo et al., 1978]. O
N
O
N
CHCH
M
Glyoxal bis (2-hydroxyanil)
Schiff’s base reagents derived from pyridinaldehyde, pyridine -2-aldehyde-
2′-pyridyl hydrazone and pyridine-2-aldehyde-2′-quinolylhydrazone were used as
extraction photometric reagents for the determination of Pd(II) and for Cu(II) [Bell
C.F. et al., 1965]. The only disadvantage of this reagent in comparison with
cuprion reagent was the lack of selectivity [Cameron A.J. et al., 1968; Sims G.G.
et al.,1969; Zatka V. et al., 1971].
48
Schiff’s bases of o-hydroxy aromatic aldehyde (salicyladehyde) with
aliphatic and aromatic mono and diamines (1,2-benzene diamine, aniline,
2-aminophenol and their derivatives) which were used in the determination of
Cu(II), Be(II), Mg(II), Ca(II), Al(III), Ga(III), Sc(III), Mn(II), Fe(III), U(VI) [Jungreis
E. et al., 1969; Morishige K., 1978; Cheng K.L. et al., 1982; Ren K., 1993].
These Schiff’s bases are used as volumetric, gravimetric, fluorimetric,
spectrophotometric reagents.
Schiff’s base of salicylidene-o-aminophenol and Schiff’s base
2-hydroxyaniline-N-salicylidene were used to study the fluorescence properties of
some metal complexes of Zinc, Tin, Scandium, Aluminium, Galium [Kiyotoshi
Morisige, 1978, 1980]
HC OHN OH
2-Hydroxyaniline-N-salicylidene
Aromatic 2-hydroxy aldehyde was condensed with a number of aliphatic
and aromatic amines to form a series of bi, tri and tetra dentate Schiff’s bases
like bis-salicylidene-o-phenylenediamine, salicylidene ethylimine, N,N′-bis
(salicylidene)-2,3-diamino benzofuran and salicylidene anthranilic acid. These
were used as highly sensitive extraction photometric reagents for Cu(II)
(extraction with chloroform, methyl isobutyl ketone and toluene) [Agett J. et al.,
1981].
3-Hydroxy picolinaldehyde Azine was used as photometric reagent for
Ag(I), Cu(II), Hg(II), Pt(IV), Co(II), Ni(II), Zn(II), Pd (II), Cd(II), Mn (II) and Fe(II, III)
[Cheng K.L., 1982].
N
OH N
OH
NN
3-Hydroxypicolinaldehyde Azine
49
N,N′-bis (3-carboxysalicylidene) trimethylenediamine was used for the
determination of trace amounts of Mercury by anode stripping voltammetry
method and the method was used for the determination of mercury in natural
water samples [Kuai-Zhi Liu et al., 1990].
Heteroaromatic Schiff’s bases, 2-(3-pyridyl methyl iminomethyl) phenol,
2-(2-pyridyl iminomethyl) phenol, 2-(2-amino-3-pyridyl iminomethyl) phenol,
N,N′-bis (salicylidene)-2,6-pyridine diamine and 2-(2-amino-4-methoxymethyl-6-
methyl-3-pyridyl methyl imino methyl) phenol were used as reagents for the
spectrophotometric and spectrofluorimetric determination of Copper by extraction
method. The spectrophotometric determination of Cu(I) after extraction was very
sensitive and selective with regard to Cd(II) and Pb(II). Highest sensitivity was
achieved with compound 2-(2-amino-4-methoxy methyl-6-methyl-3-pyridyl methyl
imino methyl) phenol [Zvjezdana Cimerman et al., 1997]. OH
N
N
OH
NN
NH2 2-(3-pyridyl methyl imino methyl) phenol, 2-(2-amino-3-pyridyl imino methyl) phenol
OH
N
N
N
NN
OH
OH
2-(2-pyridyl imino methyl) phenol, N, N′-bis (salicylidene)-2,3-pyridinediamine
NN N
OH
OH NH3C
CH2OCH3
NH2
N
OH
N,N′-bis(salicylidene)-2,6-pyridinediamine, 2-(2-amino-4-methoxy methyl-6-methyl-3-
pyridyl methyl imino methyl) phenol.
50
Schiff’s base of N-furoylphenyl hydroxyl amine forms complexes with
Co(II), Cu(II), Zn(II) and Fe(II). The reagent is used for the gravimetric
determination of Co(II), Cu(II), Zn(II) and Fe(II) through the precipitation of their
complexes. [Moustafam M. et al., 1997]
The Schiff’s base 2-(2-pyridyl methylene amino) phenol (PMAP) was
investigated as a spectrophotometric reagent for the determination of Iron in
caustic soda, cotton yarn and fabric, woolen fabric and industrial water [Grabaric
Z. et al., 1999].
OH
N
N 2-(2-pyridyl methylen amino) phenol
Nickel was determined by flow injection spectrophotometry at 370 nm after
extraction of Nickel(II)bis(acetylacetone)ethylenediaminate chelate into
chloroform using phosphate buffer (pH 7). Calibration graph was linear up to
25µg mL-1 Nickel. The system was applied to the determination of Nickel in
Nickel-Copper alloys and in synthetic electroplating solutions [Chimpalee N.
et al., 2000].
Salicylhydrazidone-2′-Hydroxy acetophenone was used as the spectro-
photometric reagent for the determination of Al(III) at pH 4.6 and was also used
for the determination of microgram quantities of sulfanilamide [Raluca Mocanu
et al., 2000].
Schiff’s base metal complexes of N,N′-cis-1,2-cyclohexylene bis
(salicylideneaminato) Cobalt(II), [Co(II)(c-Salcn)] and N′-(±)-trans-1,2-cyclo-
hexylenebis(salicylideneaminato)Cobalt(II), [Co(II)(t-Salcn)] were synthesized
and characterized by elemental analysis, Melting points, IR, electronic, and 1H
and 13C NMR spectra. Their thermo gravimetric studies were also discussed
[Felicio R.C. et al., 2001].
51
Salen-type Schiff’s base is obtained by condensing ethyl-o-hydroxy-
benzene with ethylene diamine and 1-ethyl-salicylidene bis ethylene diamine.
This reagent was used as spectrophotometric reagent for the determination of
Mn(II) to a concentration range of 10-70 µg/mL. This method was successfully
applied for the determination of to Mn(II) present in pharmaceutical products
[Tantaru Gladiola et al., 2002].
Schiff’s bases synthesized by the condensation reaction between
5-[3-(1,2,4– triazolyl-azo]-2,4-dihydroxy benzaldehyde with 1,3-diaminopropane
and 1,6-diaminohexane were used for the spectrophotometric determination of
Cobalt (II) [Khedr Abdalla M. Gaber Mohamed et al., 2005].
Sensitive chromogenic reagent N,N′-bis(3-methylsalicylidene)-ortho
phenylene diamine was used in the spectrophotometric determination of Nickel.
At pH 8 the ligand reacts with Nickel to form 1:1 complex. The method was
successfully applied for the determination of trace amounts of Nickel in some
natural food samples [Ali Reza Fakhuri et al., 2005].
N
NHO
H3C
OHH3C N, N′-bis (3-methyl salicylidene)-ortho phenylene diamine
Schiff’s bases derived from 2-thiophene carbaldehyde with 2-amino-
thiazole were used for the spectrophotometric determination of Ni(II). The
method was applied for the determination of Ni(II) in various synthetic and natural
samples [Maria Pleniceance et al., 2005].
2-Ethanolimino-2-pentylidino-4-one Schiff’s base derived from mono-
ethanolamine and acetyl acetone was used as spectrophotometric reagent for
the determination of Fe(III), Cu(II), and UO2(II). Fe(III) complex was detected at
λmax 440 nm, (pH 3.5), Cu(II) complex at λmax 340 nm (pH 6) and UO2 complex at
52
λmax 370 nm (pH 4). Beer Lambert’s law obeyed in a concentration range of
0.5 to 3.0 x10-4 [Adel S. Orabi et al., 2005].
N,N′ bis-salicylidene-o-phenylene di imine was used as analytical reagent
to determine trace concentration of metal ions like Fe(II), V(II) and Co(II) by
spectrophotometric method [Naser Eltaher Eltayeb et al., 2005].
Two new chemically modified silica gel covalently bonded with 4,4′-
diaminodiphenyl ether (DDE), 4,4′-diaminodiphenyl sulfone salicyladehyde
Schiff’s bases were synthesized and were characterized by FTIR and BET
surface area measurement techniques. These synthesized chelating materials
were tested for the pre concentration of metal ions like Zn(II), Mn(II), Cr(III) in
batch and column techniques with variation in parameter in competitive and non
competitive conditions. The method was found very useful in recovery of the
metal ions from dilute aqueous solutions [Dey R.K. et al., 2006].
Schiff’s bases prepared by the condensation of aromatic mono and
diamines derivatives and were used as fluorometric analytical reagents
[Mohamed N. Ibrahim et al., 2007].
Extractive spectrophotometric determination of Ca(II), Mg(II), Cr(III),
Fe(II), Zn(II), Cd(II), Ni(II), Mn(II), and Co(II) were done using some chiral Schiff’s
bases Benzaldehydene-(S)-2-amino-3-phenyl propanol (I), o-hydroxy
benzaldehydene-(S)-2-amino-3-phenyl-propanol (II), Benzaldehydene-(S)-2-
amino-3-methylbutanol (III) as complexing agents [Giray Topal et al., 2007].
CH N
OH
H
CH N
OH
H
OH
H3CCH3CH N
OH
H
OH
(I) (II) (III)
Schiff’s bases derived from aromatic aldehydes namely Vanillin and
p-dimethyl amino benzaldehyde were used for the spectrophotometric
determination of metronidazole in tablets [Siddappa K. et al., 2008].
53
2.7 CONCLUSIONS The Schiff’s bases are among the most widely used ligands due to their
facile synthesis, remarkable versatility and good solubility in common solvents.
Thus, they have played an important role in the development of coordination
chemistry, as they readily form stable complexes with most of the metals. The
research field dealing with Schiff’s base metal complexes is very broad due to their
potential interest for a number of interdisciplinary areas including bioinorganic
chemistry, catalysis and magneto chemistry. In the area of bioinorganic chemistry,
the interest in the Schiff’s base complexes lies with the synthetic models for the
metal containing sites in metalloproteins and enzymes [Ouyang X.M. et al., 2002;
Jayabalakrishnan C. et al., 2002; Sharghi H., 2003; Cozzi P.G., 2004; Shalin
Kumar et al., 2009]. Therefore several reports are available only in synthesis and
characterization of Schiff’s bases.
Various Schiff’s bases of aromatic amines and aromatic aldehydes have a
wide variety of applications in many fields like biology, inorganic and analytical
chemistry. They are used in optical and electrochemical sensors as well as in
various chromatographic methods, to enable detection with selectivity and
sensitivity [Lawrence J.F. et al., 1976; Valcarcel M. et al., 1994; Spichiger-Keller
U., 1998].
Among the organic reagents, actually used Schiff’s bases show excellent
characteristics, structural similarities with natural biological substances with
relatively simple preparation procedures and synthetic flexibility accompanied
with suitable structural properties. [Jungreis E. et al., 1969; Patai Sed, 1970]
Schiff’s bases derived from thiazoles and benzothiazoles have been
widely used as antifungal, antibacterial and anticancer agents. This versatile
bioactivity is due to presence of multifunctional groups [Ivanovskii V.I., 1978;
Venugopala K.N. et al., 2003; Vashi K. et al., 2004; Tellez F. et al., 2004; Chohan
Z.H. et al., 2004].
Benzothiazoles are used for the production of dyes with photosensitizing
properties [Kiyotoshi Morisige, 1980]. A number of researchers have carried out
the research with o-Vanillin and 2-aminobenzothiazoles metal complexes [Giusti
54
A. et al., 1982; Sokolowska Gajda J. et al., 1992; Velcheva E.A. et al., 2004;
Tellez F. et al., 2006; Yin H.D. et al., 2006]. But Schiff’s base of 2-amino
benzothiazole and its derivatives with o-Vanillin and the metal complexes along
with the use of these Schiff’s bases as analytical reagent was not done. This
stimulated our research interest in the present investigation.
Synthesis and characterization of Schiff’s bases derived from 2-amino
benzothiazole, 2-amino-4-methylbenzothiazole, 2-amino-6-chlorol benzothiazole
and 2-amino-6-bromo benzothiazole with o-Vanillin and their Copper complexes
are done in the present work. Also new spectrophotometric methods for the
determination of Copper ions from these Schiff’s bases were designed. This
method was successfully applied for the estimation of trace amounts of Copper
ions from the industrial effluents.
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