Spectroscopic studies and keto-enol tautomeric effect of newer schiff bases of ortho-hydroxybenzaldehyde/ naphthaldehyde with 1,2-phenylenediamine and 4-aminophenyl ether

Article abstract of DOI:10.14233/ajchem.2013.13906

Hydroxyl Schiff bases derived from 1,2-phenylenediamine and 4-aminophenyl ether with o-hydroxy benzaldehyde or o-hydroxyl naphthaldehyde were used to investigate enol-imine and keto-amine tautomeric analysis. UV-visible spectra of the compounds have been

Full text of DOI:10.14233/ajchem.2013.13906

Asian Journal of Chemistry; Vol. 25, No. 8 (2013), 4215-4218
http://dx.doi.org/10.14233/ajchem.2013.13906
Spectroscopic Studies and Keto-Enol Tautomeric Effect of Newer Schiff Bases of ortho-Hydroxy-
benzaldehyde/naphthaldehyde with 1,2-Phenylenediamine and 4-Aminophenyl Ether
1,*
2
AMBREEN SHAH and ASIF A. SHAH
1
2
Dr. M. A.Kazi Institute of Chemistry, University of Sindh, Jamshoro, Pakistan
Mehran University Institute of Science and Technology Development, Jamshoro, Pakistan
*
Corresponding author: Fax: +92 22 2771373; Tel: +92 22 2772325; E-mail: anashba_shah@yahoo.com
Received: 14 April 2012; Accepted: 4 February 2013)
(
AJC-12930
Hydroxyl Schiff bases derived from 1,2-phenylenediamine and 4-aminophenyl ether with o-hydroxy benzaldehyde or o-hydroxyl
naphthaldehyde were used to investigate enol-imine and keto-amine tautomeric analysis. UV-visible spectra of the compounds have been
investigated tautomeric equilibrium (enol-imine O-H···N, keto- amine O···H-N forms) in different polar and non-polar solvents systems.
From FTIR spectra of these model compounds, were possible to assign the IR absorption for the C=O and C=N groups in both keto-
amine and enol imine forms, respectively. The keto-enol forms was observed in basic solution of polar and nonpolar solvents such as
ethanol, chloroform, DMF, benzene, cyclohexane, n-hexane and in acidic solutions of CHCl , benzene and n-hexane not in ethanol and
3
DMF. FTIR results were showed that all Schiff bases studied, favoured the enol-imine form over the keto form in a weakly polar solvent.
Key Words: Schiff base, Tautomerism, Keto-enamine, Enol-imine, Solvent effect.
6
-11
solid state and in solutions . UV-VIS spectroscopy of o-
INTRODUCTION
hydroxy Schiff bases were studied in both solid state and in
12,13
Schiff bases with an -OH group on the ortho position to
solutions
.
1
4,15
the imine group and their complexes derived from o- hydroxyl
benzaldehyde and o-hydroxyl naphthaldehyde, were exten-
sively studied and a number of them are being used as model
Herzfeld et al.
examined initially the role of acidity
and basicity of the solvent in spectra of some Schiff bases and
also observed their solvent effect in different solvents. In UV-
visible spectra of o-hydroxy Schiff bases, a new band is
observed at greater than 400 nm, in polar and non-polar
solvents. This result indicated that the absorption band at 400
nm belongs to the keto-amine from of the Schiff base. This
tatoumer is always observed when Schiff base is derived from
2-hydroxynaphthaldehyde and aromatic amines. In the o-
hydroxybenzaldehyde and aromatic amines, the keto amine is
not observed in polar and non-polar solvents, but it is observed
1
-4
for biological system . ortho-Hydroxyl Schiff base ligands
are of prime interest mainly due to the presence of (O-H···N
and O···H-N) type hydrogen bonding in tatomerism between
enol-imine and keto-enamine forms (Scheme-I).
5
Popovic et al. gave the first example of co-existance of the
keto-amine or enol-imine forms of diamines with Schiff bases
of naphthaldimine compound of N,N'-bis-(1-naphthaldimine)
o-phenylenediamine chloroform (1/1) at 200 K. In these
compounds, the short H- bonds between the -OH group on
the ortho position to the imino group and imine nitrogen were
played important role due to the stereochemistry of imine
compounds. Different spectroscopic techniques were used to
find the tatoumeric effect in o-hydroxy Schiff bases, both in
1
6,17
in acidic media . 2-Hydroxy naphthaldehyde was shown
two bands in the visible region located above 400 nm, which
are assigned to the keto form. In the solid state, 2-hydroxy
benzaldehyde, both forms are possible and O-H···N and O···H-N
18,19
20
intramolecular hydrogen bonding can occur .Antonov et al.
R
R
CH
N
N
HC
CH
N
H
N
H
HC
OH
OH
O
O
Scheme-I: Tautomerism in newer ortho-hdroxy Schiff bases

4
216 Shah et al.
Asian J. Chem.
RESULTS AND DISCUSSION
studied the tautomerism of 2-hydroxy naphthaldehyde Schiff
21
bases but Fernandez reported the structure and solvent effect
of ortho-hydroxyl Schiff bases ligands. The effect of substitutions
on the keto-enol equlirium mostly observed in crystalline state
and extra stabilization of keto form is derived from molecular
Newly synthesized Schiff bases from 1,2-phenylene-
diimine and 4-aminophenyl ether were shown in (Fig. 1). FT-
IR spectra of all compounds showed weak and broad absor-
-1
22
ption bands in the 3400-3200 cm region. This is interpreted
to be a sign of the presence of the enol forms schiff bases.
Other vibration bands with the wave numbers of 1625-1610
association, thus leading to its crystallization .
23
Recently, Zawadiak et al. investigated that the position
of substituted hydroxyl group of compound was shown clear
influence on UV absorption spectrum and keto enol equilibrium
-1
-1
-1
cm ν(C=N), 3080-3040 cm ν(CH, Ar-H), 1600-1580 cm
-1
24
ν(C=C) and 1293-1046 cm ν(C-O-C) were observed. From
the FTIR spectra of compounds, it was possible to assign the
IR absoption for C=O and C=N group in both keto and enol
form, it was also possible to assign other absorption which
exist constantly . This present work reports spectroscopic studies
of Schiff base formed by the reaction of 2-hydroxybenzal-
dehyde or 2-hydroxyl naphthaldehyde by the reaction with
different diamines; 1,2-phenylenediamine and 4-amino
phenyl ether, respectively.
8
were specific either to the keto or enol forms . Such as the
compound o-hydroxy naphthaldehyde crystallized as a mixture
of enol and keto forms with both amines. The enol form was
infrared from the presence of strong absorption in the region
EXPERIMENTAL
-1
of 3400-3200 cm , while the keto form shown by the strong
Melting points were recorded on a Gellenkamp capillary
melting apparatus (Gallenkemp, England), without correction.
Infrared spectra were recorded on a NicoletAvatar 330 FT- IR
-1
11
absorption in the 1700-1660 cm region .
The existence of intramolecular hydrogen bonding (O···H-
1
N) in solution, has been confirmed by H NMR spectroscopy.
(Thermo Nicolet Electron Corporation, USA) with attanulated
Mostly all compounds shown the enol-imine form dominates
total reflectance (ATR) accessory (Smart partner) within 4000-
-1
6
00 cm . Spectrophotometeric studies were carried out in
7
5
0
dimethyl formamide (DMF) and n-hexane on double beam
Hitachi 220 spectrophotometer [Hitachi (Pvt.) Tokyo, Japan],
with dual 1 cm silica cuvettees within 185-700 nm.All reagents
and chemicals were analytical grade, obtained from Aldrich
Chemical Company (USA).
(i)
6
40
Preparation of Schiff base: The Schiff bases was synthe-
25
sized by reported method, which was the following : The
Schiff base was synthesized by reacting (0.01 M) of diamine
compounds (1,2-phenylenediamine or 4-aminophenyl ether
dissolved in 10 mL of MeOH with (0.02 M) of o-hydroxy-
benzaldehyde/hydroxynaphthaldehyde in the presence of 0.5
gm of anhydrous sodium acetate. After one hour of reflux at
2
0
0
4000
3500
3500
3500
3000
3000
3000
2500
Wavenumber (cm
2000
–
1500
1500
1500
1000 600
1000 600
1000 600
1
1
1
)
)
)
70 ºC. The contents were cooled. The obtained precipitate were
100
filtered and recrystallized in ethanol. The Schiff bases were
isolated as a light yellow crystalline solid.
Bis-(O-hydroxylbenzaldehyde)1,2-phenylenediimine:
(ii)
90
Yield: 84.6 %; m.p.: 182.9 ºC. Analysis of (C28
H
20
N
2
2
O ):
60
Calcd.: C; 80.7 %, H; 4.8 %, N; 6.7 %. Found: C; 79.9 %, H;
-1
4
.69 %, N; 6.3 %. IR ν(cm ); 1610 (C=N), 1488 (C=C), 1277
(
C-O) and 1183 (C-N).
Bis-(o-hydroxylbenzaldehyde)4-aminophenylether:
30
0
Yield: 90 %; m.p.: 290.4 ºC. Analysis of (C20
H
16
2 2
N O ): Calcd.:
C; 75.9 %, H; 5.06 %, N; 8.8 %. Found: C; 73.8 %, H; 4.89 %,
4000
2500 2000
–
Wavenumber (cm
-1
N; 8.13 %. IR ν(cm ); 1608 (C=N), 1486 (C=C), 1276 (C-O)
and 1182 (C-N).
Bis-(o-hydroxylnaphthaldehyde)1,2-phenylenediimine:
75 (iii)
60
40
20
Yield: 73.9 %; m.p.: 176.6 ºC. Analysis of (C34
H
24
N
2
3
O ):
Calculate: C; 80.3 %, H; 4.72 %, N; 5.51 %. Found: C; 79.8
-1
%
, H; 4.89 %, N; 5.13 %. IR ν(cm ); 1610 (C=N), 1488 (C=C),
1
275 (C-O) and 1186 (C-N).
Bis-(o-hydroxylnaphthaldehyd)4-aminophenylether:
Yield: 89.7 %; mp: 177.2 ºC. Analysis of (C26
H
20
N
2
O ): Cal-
3
culate: C; 76.4 %, H; 4.9 %, N; 6.8 %. Found: C; 75.8 %, H;
-1
4
1
.89 %, N; 6.13 %. IR ν(cm ); 1611 (C=N), 1491 (C=C),
241 (C-O) and 1156 (C-N).
0
4000
2500 2000
–
Wavenumber (cm

Vol. 25, No. 8 (2013)
00 (iv)
Spectroscopic Studies and Keto-Enol Tautomeric Effect of Newer Schiff Bases 4217
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
1
90
60
30
0
4
000
3500
3000
2500
Wavenumber (cm
2000
–
1500
1000 600
1
)
0.4
Fig. 1. IR spectra of new Schiff bases: (i) Bis-(o-hydroxylbenzaldehyde)1,2-
phenylenediimine; (ii) Bis-(o-hydroxylnaphthaldehyde)1,2-
phenylenediimine; (iii) Bis-(o-hydroxylbenzaldehyde)4-
aminophenylether; (iv) Bis-(o-hydroxylnaphthaldehyde)4-
aminophenylether
0
.2
0
180 200
300
400
500
600
Wavelength (nm)
in CDCl
3
. The signals at proton of OH and NH groups were
Fig. 3. UV-visible spectra of compound bis-(o-hydroxynaphthaldehyde)
,2-phenylenediimine in n-hexane
1
involved in the intra molecular hydrogen bonding. According
to NMR spectra, the proton exists in the spectra of all four
compounds indicated the presence of enol forms except the
Schiff base o-hydroxynaphthaldehyde due to the mixture of
keto-enol forms .
Electronic absorption spectral properties: The UV-
visible spectra of the compounds were studied in polar DMF
and non-polar to n-hexane solvents . The spectra are presented
in Figs. 2-5 and summarized in Table-1.All compounds studied
show one absorption band in the range of less that 400 nm but
the new band (greater than 400 nm) belongs to the keto form of
2
.0
.8
.6
.4
.2
.0
.8
.6
1
4
1
1
1
1
0
0
13
15,16
the o-hydroxy Schiff bases in polar and non-polar solvents
.
The band was observed in o-hydroxy naphthaldehyde 1,2-
phenylendiimine at greater than 400 nm in both polar DMF
and non-polar n-hexane solutions. The enol-imine tautomer
is dominant only in the polar solutions of DMF, in contrast to
the keto-enamine tautomer. Solvent effects can be explained
in term of the capability of solvents to form hydrogen bonding
both as proton donors and as proton acceptors, thereby permi-
tting proton transfers that result in formation of the keto form.
0.4
0.2
0
1
80 200
300
400
500
600
Wavelength (nm)
Fig. 4. UV-visible spectra of compound bis-(o-hydroxybenzaldehyde)4-
amino phenylether in n-hexane
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
1
80 200
300
400
500
600
180 200
300
400
500
600
Wavelength (nm)
Wavelength (nm)
Fig. 5. UV-visible spectra of compound bis-(o-hydroxynaphthaldehyde)4-
amino phenylether in n-hexane
Fig. 2. UV-visible spectra of compound bis-(o-hydroxybenzaldehyde)1,2-
phenylenediimine in n-hexane

4
218 Shah et al.
Asian J. Chem.
TABLE-1
ELECTRONIC SPECTRAL DATA OF
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66560
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56880
53720
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78400
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4
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