Synthesis and spectroscopic properties of novel asymmetric Schiff bases

Article abstract of DOI:10.1016/j.saa.2010.05.027

Three novel diimine Schiff bases including two asymmetric imines (2-OH)R-CHN-C₆H₄-CHN-R'(2-OH) type [where R = R' = phenyl for H₂L¹; R = naphthyl, R' = phenyl for H ₂L² and R = R' = naphthyl for H₂L³] have been synthesized with a new two step method. For this purpose, the starting Schiff bases 4-nitrobenzylidene-2-hydroxyaniline (SB¹-NO ₂) and 4-nitrobenzylidene-2-hydroxy-3-naphthylamine (SB ²-NO₂) have been synthesized, previously. Nitro groups of them have been reduced into their amino derivatives (SB¹-NH ₂ and SB²-NH₂) with sodium dithionite as selective reductant and the other imino groups have been formed by adding salicylaldehyde or 2-hydroxy-1-naphthaldehyde to the same solutions. The structures of the diimine Schiff bases were confirmed by elemental analyses, ESI-MS, FT-IR, ¹H NMR and ¹³C NMR spectroscopy. The phenol-imine and keto-amine tautomerism of the Schiff bases were investigated by FT-IR, ¹H NMR, ¹³C NMR techniques and UV-vis spectra in different solvents (DMSO, methanol, chloroform, toluene and cyclohexane). The effects of acidic and basic media on the tautomeric equilibria were discussed.

Full text of DOI:10.1016/j.saa.2010.05.027

Spectrochimica Acta Part A 77 (2010) 304–311
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and spectroscopic properties of novel asymmetric Schiff bases
Özlem Güngör, Perihan Gürkan∗
Gazi University, Faculty of Arts and Science, Department of Chemistry, 06500 Teknikokullar, Ankara, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 April 2010
Received in revised form 13 May 2010
Accepted 15 May 2010
Three novel diimine Schiff bases including two asymmetric imines (2-OH)R-CH N-C6H4-CH N-R’(2-
OH) type [where R = R’ = phenyl for H2L1; R = naphthyl, R’ = phenyl for H2L2 and R = R’ = naphthyl for
H2L3] have been synthesized with a new two step method. For this purpose, the starting Schiff bases
4-nitrobenzylidene-2-hydroxyaniline (SB1-NO2) and 4-nitrobenzylidene-2-hydroxy-3-naphthylamine
(SB2-NO2) have been synthesized, previously. Nitro groups of them have been reduced into their amino
derivatives (SB1-NH2 and SB2-NH2) with sodium dithionite as selective reductant and the other imino
groups have been formed by adding salicylaldehyde or 2-hydroxy-1-naphthaldehyde to the same solu-
tions. The structures of the diimine Schiff bases were confirmed by elemental analyses, ESI-MS, FT-IR,
1H NMR and 13C NMR spectroscopy. The phenol-imine and keto-amine tautomerism of the Schiff bases
were investigated by FT-IR, 1H NMR, 13C NMR techniques and UV–vis spectra in different solvents (DMSO,
methanol, chloroform, toluene and cyclohexane). The effects of acidic and basic media on the tautomeric
equilibria were discussed.
Keywords:
Asymmetric diimine
Reduction
Tautomerism
Intramolecular hydrogen bonding
Solvent effect
Acid–base effect
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
step method [10–12]. At first, a Schiff base including a –NO2 group
was synthesized. Later, the nitro group was reduced into its amino
Schiff bases, products of the condensation reactions between
aldehydes and primary amines, are firstly described by H. Schiff
at the nineteenth century [1]. These classical type Schiff bases and
their complexes have been studied extensively [2,3].
derivative by using sodium dithionite as selective reductant and
the other imino group was formed by adding an aldehyde to the
same solution.
On the other hand, the phenol-imine and keto-amine tau-
tomerism both in solution and solid state, which is a characteristic
feature of 2-hydroxy Schiff bases, are investigated using FT-IR [13],
UV–Vis [14], 1H NMR [15], 13C NMR [16], 15N NMR [17], 17O NMR
[18], mass [19] and X-ray crystallography techniques [20]. Many
factors, such as temperature, solvent polarity and chemical substi-
tution in the molecule influence the tautomeric equilibrium. With
the proton transfer from the hydroxyl oxygen to imino nitrogen
atom arise different conformations which are related to the pho-
tochromism and thermochromism [21] within the molecule.
In this work, synthesis and characterization of three novel asym-
metric diimine Schiff bases (H2L1, H2L2 and H2L3) is reported by
elemental analysis, FT-IR, 1H and 13C NMR spectroscopies and mass
spectra. We also studied their tautomeric properties both in solid
state and the solutions and discussed the differences of the tau-
tomeric behaviours of the two asymmetric imino groups in the
same molecule by FT-IR, NMR and UV–Vis spectroscopies.
Actually, simple condensation reaction between one mole of an
aromatic dialdehyde and two moles of primary amines or one mole
of an aromatic diamine and two moles of aldehydes gives symmet-
rical diimines, (∼N HC–Ar–CH N∼) and (∼CH N–Ar–N CH∼)
type, respectively. But, it cannot synthesize a stable compound
involving both –NH2 and –CHO groups in an aromatic ring since
a reaction formed between them. Therefore, Schiff bases including
two asymmetric imino groups attached to the same aromatic ring,
(∼N HC–Ar–N H∼) type, cannot be synthesized directly. Thus,
asymmetric diimines of these types were not investigated yet,
while many literatures concerning symmetrical diimines have been
published. Symmetrical diimines and their metal complexes have
been used as models for biological systems for their antimicrobial
and anticancer activities [4,5]. They have also been used in the cat-
alytic reactions [6] and oxidation reactions [7,8]. Additionally, they
have material properties and analytical applications such as ion
selective electrodes [9].
Recently, we reported synthesis and characterization of some
asymmetric diimines of (∼N HC–Ar–N CH∼) type with a new two
2. Experimental
2.1. Reagents and materials
∗
2-Aminophenol, 3-amino-2-naphthol, 4-nitrobenzaldehyde,
salicylaldehyde, 2-hydroxy-1-naphthaldehyde and sodium
Corresponding author. Tel.: +90 312 2021118.
E-mail address: pgurkan@gazi.edu.tr (P. Gürkan).
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.05.027

Ö. Güngör, P. Gürkan / Spectrochimica Acta Part A 77 (2010) 304–311
305
Fig. 1. Starting Schiff bases and related aldehydes of the asymmetric diimines.
Mattson 1000-FTIR spectrometer using KBr discs. The 1H NMR and
13C NMR spectra were recorded with a Bruker Avance DPX FT-NMR
spectrometer operating at 400 MHz (DMSO as solvent, TMS as inter-
nal standard). The mass spectra (LC-MS ESI) were recorded at 70 eV
on an Agilent 1100 MSD mass spectrometer. Ultraviolet–visible
spectra were recorded using Analytika Jena UV-200 spectropho-
tometer.
dithionite were purchased from Aldrich Chemical Company,
and used without further purification. The solvents used were of
spectroscopic grade.
2.2. Synthesis
2.2.1. Synthesis of the starting Schiff bases
The starting Schiff bases 4-nitrobenzylidene-2-hydroxyaniline
(SB1-NO2) and 4-nitrobenzylidene-2-hydroxy-3-naphthylamine
(SB2-NO2) were synthesized according to the published procedure
[22,23].
2.3.1. Absorption measurements of asymmetric diimine Schiff
bases
Ultraviolet-visible spectra were measured in DMSO, methanol,
chloroform, toluene and cyclohexane, over the wavelength range
270–600 nm at room temperature. Changes in the wavelength of
the maximum absorption (ꢀmax) were investigated in pure sol-
vents, acidic and basic media. 0.2 mL trifluoroacetic acid and 0.2 mL
triethylamine were added to the solutions of the diimines (5 mL)
in all polar and non-polar solvents to provide the acidic and basic
media, respectively. The molar extinction coefficients (ε) of the
bands in non-polar solvents for H2L2 and H2L3 cannot be calculated,
because of their low solubility.
2.2.2. Synthesis of the asymmetric diimines: general procedure
Asymmetric diimines (H2L1, H2L2 and H2L3) were prepared with
a new two step method.
1 mmol of the starting Schiff base (0.2420 g for SB1-NO2 or
0.2920 g for SB2-NO2) was dissolved in ethanol: water 1:1 mixture
(60 mL) at 70 ◦C. 5 mmol (0.8707 g) of solid sodium dithionite was
slowly added to the solution in small portions during an hour and
stirred for 1 h at 50 ◦C for completion of reducing process. Conse-
quently, amino derivatives of the starting Schiff bases (SB1-NH2) or
(SB2-NH2) were formed in the solution. 1 mmol of appropriate alde-
hyde in 10 mL ethanol was added to this solution dropwise during
the period of an hour with stirring and was refluxed 5 h at 60 ◦C. The
resulting solution was evaporated at room temperature for approx-
imately 3 days, until a yellow-white precipitate was formed. The
crude product was treated with warm water and filtered twice.
If necessary, extracted with diethylether and recrystallized from
ethanol. Starting Schiff bases and appropriate aldehydes of the
diimines are shown in Fig. 1.
3. Results and discussion
The results of elemental analyses (C, H and N), the melting points
and the selected FT-IR data are presented in Table 1. The analytical
data for the new diimine Schiff bases are in good agreement with
the proposed molecular formulae.
As can be seen in Fig. 2, asymmetric diimines consist of two
different fragments bound to the central 1,4-phenylene ring: (1)
Salicyl/naphthaldimine fragment (left side) attached to the center
by nitrogen atom. It contains six-membered chelate ring formed
by intramolecular hydrogen bond. (2) Phenyl/naphthylazomethine
fragment (right side) linked by carbon atom to the center. It
contains five-membered chelate ring formed by (O-H. . .N) or
(O. . .H-N) type intramolecular hydrogen bond. Essentially, diimine
Schiff bases may exist in four possible tautomeric forms, as can be
seen in Fig. 2-II.
2.3. Instrumentation
Melting points were measured on a Gallenkamp apparatus using
a capilary tube. Carbon, nitrogen and hydrogen analyses were per-
formed on an Elementar Analysenssysteme GmbH vario MICRO
CHNS analyzer. Infrared absorption spectra were obtained from a

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Table 1
Analytical and FT-IR data of asymmetric diimine Schiff bases.
Comp. Formulae
Colour
Melt. point
(◦C)
Yield Elemental analysis found (calculated) (%)
(%)
FT-IR spectral data
C
H
N
ꢁOH
ꢁCH
ꢁC
C
ꢁCH
N
ꢁCO
(arom.)
(arom.)
H2L1
H2L2
H2L3
C
20H16N2O2
Orange
Orange
182–184
236–238
54
58
61
75.37 (75.95)
77.86 (78.69)
80.13 (80.77)
4.79 (5.06)
5.07 (4.92)
5.07 (4.81)
8.04 (8.86)
7.20 (7.65)
6.12 (6.73)
3553b 2759c
3567b 2753c
3600b 2000
3044
1632
1632
1638
1613-1593 1286
1620-1586 1313
1619-1549 1305
(316 g/mol)
C24H18N2O2
(366 g/mol)
3025
3037
C28H20N2O2
Dark orange 246–252
(416 g/mol)
b: broad, c: center.
Fig. 2. General structures (I) and possible tautomeric forms (II) of the asymmetric diimines.
Table 2
1H chemical shifts (ppm) of asymmetric diimine Schiff bases.a.
Comp.
Left side (six-membered ring)
Phenol-imine form
Right side (five-membered ring)
Phenol-imine form
Keto-amine form
CH–NH
CH
N
O–H. . .N
3J b
N–H. . .O
CH
N
O-H. . .N
Ar–H
H2L1
H2L2
H2L3
8.97
10.75
–
13.79
12.75
12.78
–
8.14
8.47
–
8.39
8.57
–
8.97
9.35
9.63
9.74
10.01
10.83
7.54–6.77
7.75–6.83
7.85–6.16
15.40
15.77
a
Relative to internal TMS in DMSO-d6 solution.
b
Coupling constant (Hz); 3J(NHCH)
.
Table 3
13C chemical shifts (ppm) of asymmetric diimine Schiff bases.a.
Comp.
Left side (six-membered ring)
Phenol-imine form
Right side (five-membered ring)
Phenol-imine form
Keto-amine form
CH–NH
CH
N
C–O
C
O
CH
N
C–O
Ar–C
H2L1
H2L2
H2L3
161.19
–
–
151.58
–
–
–
–
162.17
149.07
146.84
151.58
149.59
148.47
135.42–116.98
134.03–108.01
133.39–107.57
137.97
138.12
177.48
178.50
a
Relative to internal TMS in DMSO-d6 solution.

Ö. Güngör, P. Gürkan / Spectrochimica Acta Part A 77 (2010) 304–311
307
Fig. 3. 1H NMR spectra of H2L1 (a) and H2L2 (b).
3.1. IR spectra
tence of the NH bands are the other evidences for the phenol-imine
forms of the compounds in the solid state [21].
The main absorption bands are given in Table 1. In the solid state,
the existence of intramolecular hydrogen bonding and the pres-
ence of the tautomeric forms are investigated by FT-IR spectra. The
strong broad bands, within the range 2000–3500 cm−1, centered at
about 2750 cm−1, show the presence of (O-H. . .N) intramolecular
hydrogen bonding due to phenolic OH groups of the phenol-imine
forms [14]. The observations of the sharp and strong bands at
1613 cm−1 and 1593 cm−1 for H2L1; 1620 cm−1 and 1586 cm−1 for
H2L2; 1619 cm−1 and 1549 cm−1 for H2L3 can be assigned to the
stretching bands of different imino groups. The existence of the
phenolic C–O bands at between 1286 and 1313 cm−1 and nonexis-
3.2. 1H and 13C NMR spectra
1H and 13C NMR spectral data of the compounds are listed in
Tables 2 and 3. Tautomerism studies of ␣-hydroxynaphthaldimine
and salicylaldimine Schiff bases show that downfield shift of the
OH proton is clearly due to strong intramolecular (O-H. . .N) hydro-
gen bonding [24]. Phenol-imine tautomer occurs when forming
(O-H. . .N) hydrogen bond; contrary, keto-amine tautomer appears
in case of the formation of (N-H. . .O) type bond. In the 1H NMR and
13C NMR spectra of the diimines, aromatic protons and aromatic

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Ö. Güngör, P. Gürkan / Spectrochimica Acta Part A 77 (2010) 304–311
carbon atoms resonate at the range 6.16–7.85 ppm (multiplet) and
107.57–135.42 ppm respectively, as expected.
atoms of the keto-amine tautomer of the left side, respectively. The
phenolic (C–O) and imino (CH N) carbon atoms of the right side
are observed at ı 149.07 ppm and ı 149.59 ppm, which is similar to
the literature [16,25].
1H NMR and 13C NMR data for diimine H2L1 show that tau-
tomeric equilibrium favors the phenol-imine tautomer both on
the left and right sides of the molecule [Fig. 2-II (A)] in DMSO-
d6. In the 1H NMR spectrum of H2L1, as shown in Fig. 3(a), the
signals of the phenolic OH protons are observed at ı 13.79 ppm
1H NMR data of H2L3 show that keto-amine tautomer is domi-
nant in the six-membered chelate ring (left side) and phenol-imine
tautomer exists mostly in the five-membered chelate ring (right
side). As seen in Table 2, amino (NH. . .O) and enamine ( CH–NH)
protons related with the keto-amine tautomer in the left side are
occurred at ı 15.77 ppm as a singlet and at ı 8.47 ppm as a doublet
(1H, 3J NHCH = 8.57 Hz). (OH. . .N) proton related with phenol-imine
tautomer in the left side is observed at ı 12.78 ppm as a singlet. In
the right side, singlet peaks at ı 10.83 ppm (1H) and ı 9.63 ppm (1H)
are attributed to the phenolic (OH. . .N) and imino (CH N) protons
of the five-membered ring. In the proton de-coupled 13C NMR data
[Fig. 4(b)] of H2L3, the peaks at ı 178.50 ppm and ı 138.12 ppm
belong to (C O) and ( CH-–NH) carbon atoms of the keto-amine
tautomer of the left side, respectively. The phenolic (C–O) and imino
(CH N) carbon atoms of the right side (five-membered ring) are
observed at ı 148.47 ppm and ı 146.84 ppm, respectively.
The stability of the keto-amine form in the ␣-
hydroxynaphthylidene derivatives is expected to be larger
than salicylidene derivatives, since proton transfer from hydroxyl
oxygen to the imino nitrogen does not affect D2h symmetry of
the naphthalene ring. On the other hand, for the salicylidene
derivatives, the same process results in the loss of the aromaticity
of the benzene ring [24]. Although naphthalene rings are present
both in the left and right side of H2L3, keto-amine tautomer
occurs only in the six-membered chelate ring at the left side. In
conclusion, we may consider that the size of the ring (five- or
six-membered), formed with hydrogen bonding between hydroxyl
oxygen and imino nitrogen atom, is more important than keeping
the aromaticity.
(1H) and
ı 9.74 ppm (1H) as singlets. OH signals are also
defined with D2O exchange. The signal at ı 13.79 ppm can be
attributed to the phenolic proton of the left side of the molecule,
because of the increasing probability of (O-H. . .N) intramolec-
ular hydrogen bonding in six-membered chelate ring. Lower
ppm value of the other phenolic proton indicates decreasing
intramolecular hydrogen bonding effect in the five-membered
chelate ring. Imino protons are observed at 8.97 ppm as one
signal, it may be because of overlapping. Whereas in the pro-
ton de-coupled 13C NMR spectra, the peaks at ı 161.19 ppm
and ı 162.17 ppm are ascribed to two imino carbon atoms
(Table 3).
1H NMR [Fig. 3(b)] and 13C NMR [Fig. 4(a)] data of H2L2 in DMSO-
d6 show that an equilibrium between two tautomers occurs in the
left side (keto-amine tautomer is redundant), but almost all the
phenol-imine tautomer formed in the right side of the molecule
[Fig. 2-II (A and D)]. In the left side of H2L2: amino (NH. . .O) and
enamine ( CH–NH) protons of keto-amine tautomer are observed
at ı 15.40 ppm as a singlet and at ı 8.14 ppm as a doublet peak (1H,
3JNHCH = 8.39 Hz). (OH. . .N) and (CH N) protons of phenol-imine
tautomer are observed as small singlet peaks at ı 12.75 ppm and
at ı 10.75 ppm, respectively. In the right side of H2L2: phenolic
(OH. . .N) and imine (CH N) protons of phenol-imine form occur
at ı 10.01 ppm (1H) and at ı 9.35 ppm (1H) as singlet peaks. In the
proton decoupled 13C NMR spectrum of H2L2 [Fig. 4(a)], the peaks at
ı 177.48 and ı 137.97 ppm belong to ( CH–NH) and (C O) carbon
Fig. 4. 13C NMR spectra of H2L2 (a) and H2L3 (b).

Ö. Güngör, P. Gürkan / Spectrochimica Acta Part A 77 (2010) 304–311
309
3.3. UV–vis spectra
UV–vis spectroscopy is known to be a very sensitive method
for studying tautomeric equilibrium in Schiff bases involving a
hydroxyl group in ortho position to the azomethine group. Presence
of a new band above 400 nm indicates the existence of keto-amine
tautomer of the Schiff bases, while phenol-imine tautomer shows
absorption below 400 nm [20].
Since the tautomeric equilibria strongly depend on the solvent
polarity and nature of media, UV–vis spectra of the asymmetric
diimines are measured in polar (DMSO, methanol and chloro-
form) and non-polar (toluene and cyclohexane) solvents and also
in acidic and basic media. Fig. 5 shows absorption spectra of the
diimines in DMSO and toluene. Concentration of the solutions are
between 10−4 and 10−5 M. Trifluoroacetic acid (CF3COOH) and
triethylamine ((C2H5)3N) are added to achieve acidic and basic
media, respectively. The absorption maxima (ꢀmax), molar extinc-
tion coefficients (ε) and calculated percentages of the keto-amine
tautomers are given in Table 4. Fig. 6 shows UV spectra of H2L2
in different solvents (a), in acidic (b) and basic media (c), respec-
tively.
3.3.1. Structure and solvent effect on the tautomeric equilibria
H2L1 does not show any absorption maxima above 400 nm in
all of the pure solvents studied, except in MeOH. This indicates
that H2L1 exists in completely phenol-imine form in solutions
(Fig. 5). The ꢀmax of the bands resulting from the phenol-imine
form exhibits a small bathochromic shift from polar solvent
DMSO (356 nm) to the non-polar solvent cyclohexane (362 nm).
The existence of small amount of (17%) keto-amine tautomer
in methanol may be explained as the forming of intermolecu-
lar hydrogen bonding with methanol oxygen and imino nitrogen
atoms.
Fig. 5. Absorption spectra of diimines in DMSO (a) and toluene (b).
Table 4
Absorption maxima (ꢀmax) and percentage of keto-amine tautomer for the dimine Schiff bases in various solvents, acidic and basic solutions.
Solvent
Tautomer
H2L1
H2L2
H2L3
ꢀmax., nm (Є× 104, M−1 cm−1
)
Pure
solvent
Acidic
media
Basic
media
Pure
solvent
Acidic
media
Basic
media
Pure
solvent
Acidic
media
Basic
media
DMSO
Phenol-imine
Keto-amine
356(1.4)
–
330(0.9)
413(0.4)
358(1.8)
327(1.6)
327(1.5)
327(1.6)
303(1.3)
331(1.0)
461(2.2)
485(2.2)
64
303(1.0)
331(0.9)
460(1.6)
483(1.5)
61
303(1.7)
332(1.4)
461(3.2)
485(3.1)
65
–
453(2.6)
476(2.6)
62
452(2.5)
475(2.3)
62
453(2.6)
476(2.6)
61
a
%
0
30
0
Methanol
Chloroform
Toluene
Phenol-imine
Keto-amine
350(0.8)
318(0.7)
362(0.7)
324(1.3)
348(1.1)
450(2.8)
471(2.7)
69
370(1.1)
439(2.2)
66
325(1.1)
298(1.1)
329(1.1)
458(2.5)
480(2.3)
70
331(0.8)
364(0.6)
447(0.7)
480(sh)b
47
335(1.2)
443(0.2)
400(0.9)
442(0.5)
453(2.2)
469(2.2)
67
462(2.3)
476(2.3)
65
%
17
58
40
Phenol-imine
359(1.9)
326(1.9)
359(1.8)
329(1.2)
399(1.3)
452(1.2)
469(1.1)
51
309(0.7)
378(1.1)
452(2.7)
473(2.1)
70
322(1.3)
456(2.1)
62
303(0.9)
332(1.1)
461(1.4)
481(1.2)
54
382(1.1)
303(1.0)
331(1.1)
461(1.4)
482(1.2)
55
Keto-amine
–
415(2.9)
–
463(2.6)
489(1.7)
71
%
0
60
0
Phenol-imine
358(1.5)
330(1.3)
359(1.5
326(1.3)
383(1.4)
380(2.8)
320(1.4)
365(1.0)
454(1.2)
475(1.1)
45
328
382
349
382
461
488
66
329
373
462
482
48
Keto-amine
–
412(2.0)
–
451(4.5)
480(0.3)
62
478(0.2)
13
478
22
%
0
59
0
Cyclohexane
Phenol-imine
362(1.7)
324(1.3)
355(2.3)
319
382
453
474
31
319
373
452
482
60
320
381
453
482
47
320
382
459
480
28
320
373
462
494
60
320
382
Keto-amine
%
–
0
411(1.6)
56
–
0
473
26
a
As keto-amine percent.
As shoulder.
b

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Ö. Güngör, P. Gürkan / Spectrochimica Acta Part A 77 (2010) 304–311
As can be seen the UV–vis spectra of H2L2 and H2L3 in Fig. 5,
3.3.2. Acid and base effect on the tautomeric equilibria in various
solvents
H2L1 exists completely as phenol-imine form in pure solvents
and in basic media studied, except in MeOH, but keto-amine
tautomer appears in all of the acidic solutions. The keto-amine
tautomer is dominant in acidic solutions of methanol, chloroform,
toluene and cyclohexane with the ratio of 58%, 60%, 59% and 56%,
respectively. It can be explained that attachment of an acidic proton
to the imino nitrogen atom in acidic media, enables the formation
the new bands, that are observed at >400 nm in all polar and non-
polar solvents, indicate keto-amine tautomer. Most of the peaks
belonging to both of the tautomers are observed as double humped,
different from that of the H2L1. It is most probably resulting from
the naphthylidene rings in H2L2 and H2L3. Keto-amine tautomer
is dominant (between 54 and 70%) in DMSO, methanol and chlo-
roform, whereas phenol-imine form is abundant (between 68 and
87%) in non-polar solvents.
Fig. 6. Absorption spectra of H2L2 in various solvents (a), in acidic (b), in basic (c) solutions.

Ö. Güngör, P. Gürkan / Spectrochimica Acta Part A 77 (2010) 304–311
311
of the keto-amine tautomer. New keto-amine bands are observed
between 400 and 415 nm, while the bands of phenol-imine forms
show a blue shift between ꢂꢀmax = 26–38 nm. The only basic media
forming the keto-amine tautomer is found as MeOH for H2L1. It may
be resulting from the loss of the phenolic proton by the influence
of the base.
solutions. Whereas, the new bands were observed at >400 nm in
all solvents indicated keto-amine tautomer for H2L2 and H2L3.
Acknowledgement
The authors are grateful to Research Foundation of Gazi Univer-
sity for supporting this study with the project F.E.F.05/2008-07.
For H2L2 and H2L3, the keto-amine tautomer ratio increases both
in the acidic and basic chloroform, toluene and cyclohexane solu-
tions with respect to the pure solvent media, while it is almost the
same in the acidic and basic DMSO solutions.
References
[1] H. Schiff, J. Ann. Chem. 150 (2) (1869) 193–200.
˙
[2] H. Ünver, M. Yıldız, A. Kiraz, N.O. Iskeleli, A. Erdönmez, B. Dülger, T.N. Durlu, J.
3.4. Mass spectra
Chem. Crystallogr. 36 (3) (2006) 229–237.
[3] K.V. Sashidhara, J.N. Rosaiah, G. Bhatia, J.K. Saxena, Eur. J. Med. Chem. 43 (2008)
2592–2596.
[4] L. Shi, H.M. Ge, S.H. Tan, H.Q. Li, Y.C. Song, H.L. Zhu, R.X. Tan, Eur. J. Med. Chem.
42 (2007) 558–564.
[5] S.M. Abdallah, G.G. Mohamed, M.A. Zayed, M.S. Abou El-Ela, Spectrochim. Acta
Part A 73 (2009) 833–840.
[6] D.M. Boghaei, S. Mohebi, Tetrahedron 58 (2002) 5357–5366.
[7] M.L.P. Santos, A. Faljoni-Alario, A.S. Mangrich, A.M.C. Ferreira, J. Inorg. Biochem.
71 (1–2) (1998) 71–78.
[8] S. Arunachalam, N.P. Priya, C. Jayabalakrishnan, V. Chinnusamy, Spectrochim.
Acta Part A 74 (2009) 591–596.
[9] C. Hazneci, K. Ertekin, B. Yenigul, E. Cetinkaya, Dyes Pigments 62 (2004) 35–41.
[10] D. Nartop, P. Gürkan, N. Sarı, S. C¸ ete, J. Coord. Chem. 61 (21) (2008) 3516–3524.
[11] Ö. Güngör, M.Sc. Thesis, Institute of Science and Technology, Gazi University;
2008.
LC–mass spectra of the compounds give the molecular ions at
the desired position m/z: 316 [M]+, 356 [M−10H]+ and 417 [M+H]+,
for the H2L1, H2L2 and H2L3. Similar fragmentation pathways are
followed by the molecular ion of the Schiff bases. In the spec-
tra of H2L1, H2L2 and H2L3, the highest intensity peaks occurred
214 (100%), m/z 264 (100%) and m/z 314 (100%), corresponding
to m/z [M−102], may be ascribed to the elimination of the same
fragment.
4. Conclusion
[12] K. Sürücüog˘lu, Ph.D. Thesis, Institute of Science and Technology, Gazi Univer-
Three
novel
asymmetric
diimine
Schiff
bases
sity, 2008.
(∼N HC–Ar–N CH∼) type were synthesized by using a new
two step method and they were characterized by elemental
analyses, ESI-MS, FT-IR, 1H NMR and 13C NMR spectroscopy. The
phenol-imine and keto-amine tautomerism of the Schiff bases
were investigated by FT-IR, 1H NMR, 13C NMR and UV–vis spec-
troscopies. According to the FT-IR spectra, both the left and right
sides of the diimines were in phenol-imine form in solid state.
1H NMR and 13C NMR data in DMSO-d6 for diimine H2L1 showed
that tautomeric equilibrium favored the phenol-imine tautomer
in both sides of the molecule. However, an equilibrium between
two tautomers occurred (keto-amine tautomer was dominant) in
the left side, but phenol-imine tautomer formed in the right side
of the H2L2 and H2L3. UV–vis spectra of the asymmetric diimines
were measured in polar and non-polar solvents and also in acidic
and basic media. H2L1 did not show any absorption maxima above
400 nm in all of the pure solvents studied, except in MeOH. This
indicated that H2L1 existed in completely phenol-imine form in
[13] S.R. Salman, N.A.I. Saleh, Spectrosc. Lett. 30 (7) (1997) 1289–1300.
[14] H. Nazır, M. Yıldız, H. Yılmaz, M.N. Tahir, D. Ülkü, J. Mol. Struct. 524 (2000)
241–250.
[15] K. Ambroziak, Z. Rozwadowski, T. Dziembowska, B. Bieg, J. Mol. Struct. 615 (1-3)
(2002) 109–120.
[16] Z. Rozwadowski, K. Ambroziak, T. Dziembowska, M. Koftica, J. Mol. Struct. 643
(1-3) (2002) 93–100.
[17] N.S. Golubev, S.N. Smirnov, P.M. Tolstoy, S. Sharif, M.D. Toney, G.S. Denisov,
H.H. Limbach, J. Mol. Struct. 844–845 (2007) 319–327.
[18] J.C. Zhuo, Magn. Reson. Chem. 37 (4) (1999) 259–268.
[19] S.R. Salman, F.S. Kamounah, Spectr. Lett. 35 (2002) 327–335.
[20] H. Ünver, M. Yıldız, D.M. Zengin, S. Özbey, E. Kendi, J. Chem. Crystallogr. 31
(2001) 211–216.
[21] A. Ohshima, A. Momotake, T. Arai, J. Photochem. Photobiol. A: Chem. 162 (2004)
473–479.
[22] F.G. Singleton, C.B. Pollard, J. Am. Chem. Soc. 62 (1940) 288–2289.
[23] S.A. Mahgoub, M.Z.A. Badr, A.A.A. Abd El-Hafez, Egyptian J. Chem. 34 (2) (1991)
165–170.
[24] Z. Popovic, V. Roje, G. Pavlovic, D.M. Calogovic, G. Giester, J. Mol. Struct. 597
(2001) 39–47.
[25] Z. Rozwadowski, K. Ambroziak, M. Szypa, E. Jagodzinska, S. Spychaj, W. Schilf,
B. Kamienski, J. Mol. Struct. 734 (1–3) (2005) 137–142.