Spectral, structural and quantum chemical computational and dissociation constant studies of a novel azo-enamine tautomer

Article abstract of DOI:10.1016/j.molstruc.2014.06.033

We report here the synthesis of (6Z)-4-[(E)-(4-ethylphenyl)diazenyl]-6- {[(2-hydroxy-5-methylphenyl)amino]methylidene}cyclohexa-2,4-dien-1-one by the condensation reaction between 2-amino-4-methylphenol and 5-[(E)-(4-ethylphenyl) diazenyl]-2-hydroxybenzaldehyde in equimolar ratio in MeOH and characterized by elemental analyses, infrared, electronic, mass,¹H and ¹³C NMR spectroscopy. Molecular structure of the azo-enamine dye was also determined by single crystal X-ray diffraction technique. X-ray investigation of the dye showed that azo-enamine tautomer is favoured in the solid state. There is an intramolecular hydrogen bonding (N3-H···O1) in the molecule forming a S(6) graph set motif. Additionally, there is an intermolecular O2-H···O1 hydrogen bonding in the structure. The same intermolecular hydrogen bonding contacts are extended between the other symmetry-related molecules in their respective planes to form a 1D hydrogen bond chain. Self-isomerisation via intramolecular proton transfer was investigated by UV-Vis. spectra and theoretical calculations. Effects of polarity and temperature on UV-Vis. spectra were examined in detail. Moreover, acid dissociation properties of the polydentate compound was investigated at 25 ± 0.1 °C.

Full text of DOI:10.1016/j.molstruc.2014.06.033

Journal of Molecular Structure 1074 (2014) 449–456
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Spectral, structural and quantum chemical computational and
dissociation constant studies of a novel azo-enamine tautomer
Asuman Gözel a, Muhammet Kose a, Duran Karakaßs b, Hasan Atabey c, Vickie McKee d,
Mukerrem Kurtoglu a,
⇑
a Chemistry Department, Kahramanmaras Sutcu Imam University, 46050 Kahramanmaras, Turkey
b Chemistry Department, Cumhuriyet University 58140 Sivas, Turkey
c Chemistry Department, Gaziosmanpaßsa University, 60250 Tokat, Turkey
d Chemistry Department, Loughborough University, Leicestershire LE11 3TU, UK
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
A novel azo-enamine dye was
synthesized and characterized by
analytical and spectroscopic studies.
X-ray investigation of the dye showed
that azo-enamine tautomer is
favoured in the solid state.
Self-isomerisation via intramolecular
proton transfer was investigated by
UV–Vis. spectra and theoretical
calculations.
ꢀ
Acid dissociation constants for the
tautomer were determined
potentiometrically.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 May 2014
Received in revised form 9 June 2014
Accepted 9 June 2014
Available online 18 June 2014
We report here the synthesis of (6Z)-4-[(E)-(4-ethylphenyl)diazenyl]-6-{[(2-hydroxy-5-methylphenyl)
amino]methylidene}cyclohexa-2,4-dien-1-one by the condensation reaction between 2-amino-4-meth-
ylphenol and 5-[(E)-(4-ethylphenyl)diazenyl]-2-hydroxybenzaldehyde in equimolar ratio in MeOH and
characterized by elemental analyses, infrared, electronic, mass,1H and 13C NMR spectroscopy. Molecular
structure of the azo-enamine dye was also determined by single crystal X-ray diffraction technique. X-ray
investigation of the dye showed that azo-enamine tautomer is favoured in the solid state. There is an
intramolecular hydrogen bonding (N3–HꢁꢁꢁO1) in the molecule forming a S(6) graph set motif. Addition-
ally, there is an intermolecular O2–HꢁꢁꢁO1 hydrogen bonding in the structure. The same intermolecular
hydrogen bonding contacts are extended between the other symmetry-related molecules in their
respective planes to form a 1D hydrogen bond chain. Self-isomerisation via intramolecular proton
transfer was investigated by UV–Vis. spectra and theoretical calculations. Effects of polarity and
temperature on UV–Vis. spectra were examined in detail. Moreover, acid dissociation properties of the
polydentate compound was investigated at 25 0.1 °C.
Keywords:
X-ray structure
Tautomer
Azo-enamine
Computational studies
Solvent effect
Dissociation constant
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
Hugo Schiff described the condensation between an aldehyde
and an amine to obtain the compounds, later known as Schiff base
[1]. These compounds represent important class of compounds in
⇑
Corresponding author. Tel.: +90 344 280 1450; fax: +90 344 280 1352.
E-mail addresses: mkurtoglu@ksu.edu.tr, kurtoglu01@gmail.com (M. Kurtoglu).
medical and pharmaceutical field [2] and it has many applications
http://dx.doi.org/10.1016/j.molstruc.2014.06.033
0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

450
A. Gözel et al. / Journal of Molecular Structure 1074 (2014) 449–456
in important biological field [3,4], further move it act as corrosion
inhibitors [5,6], catalysts [7,8]. Also, a large number of Schiff bases
have been investigated for their interesting and important proper-
ties, such as their ability to reversibly bind oxygen [9], photochro-
mic properties [10,11] and complexing ability towards some toxic
metals [12]. Schiff bases are generally bi- or tri dentate ligands
capable of forming very stable complexes with transition metal
ions. Recently, there has been a considerable interest in the chem-
istry of these compounds and their metal complexes due to analyt-
ical activities [13,14].
Compounds, such as o-hydroxy Schiff bases, exhibiting self-
isomerisation via intramolecular proton transfer are of consider-
able interest due to their potential usage in higher energy radiation
detectors, memory storage devices [15–17]. The presence of enol-
keto tautomerisms affects their photo-physical and photochemical
properties. o-Hydroxy Schiff bases may exist in two tautomeric
forms (enol and keto forms) in both solutions and the solid state
[18]. Self-isomerisation via proton transfer depends on several fac-
tors such as temperature, the substituent structure and solvent
polarity etc. [19]. o-Hydroxy Schiff bases containing second hydro-
xyl group in the aniline ring were reported to exhibit stronger tau-
tomerism due to this second hydroxyl group [20–25]. In recent
years, in addition to the experimental studies, quantum chemical
computational studies were used to investigate tautomerism and
intramolecular proton transfer process in ortho-hydroxy Schiff
base compounds [18,21]. Recently, we have reported an azo-o-
hydroxy Schiff base containing second hydroxyl group. X-ray
analysis on this compound revealed that the compound existed
in keto-amine form in the solid state [26].
In continuation of our work on azo Schiff bases [27,28], we
report herein the results of our studies on a novelazo-Schiff base
tautomer, (6Z)-4-[(E)-(4-ethylphenyl)diazenyl]-6-{[(2-hydroxy-5-
methylphenyl)amino]methylidene}cyclohexa-2,4-dien-1-one derived
from 5-[(E)-(4-ethylphenyl)diazenyl]-2-hydroxybenzaldehyde and
2-amino-4-methylphenol. Molecular structure of the tautomer
was determined by single crystal X-ray diffraction technique.
Self-isomerisation via intramolecular proton transfer for the
compound was investigated by UV–Vis. spectra and theoretical
calculations. Additionally, the dissociation constants of the azo-Schiff
base (ligand) were determined potentiometrically at 25 °C, in NaCl
(I = 0.1 mol dmꢂ3) in EtOH–H2O mixture (1/5, v/v).
ylphenyl)diazenyl]-2-hydroxybenzaldehyde (0.255 g, 1 mmol,
25 mL) was added to a methanolic solution of 2-amino-4-methyl-
phenol (0.125 g, 1 mmol, 20 mL) and refluxed for 2 h. After cooling
of the solution, the precipitate was separated, filtered, re-crystalli-
zation with MeOH, and dried over anhydrous calcium chloride
under vacuum.
Yield: 0.290 g (81%), color: red. M.p.: 213–214 °C. Analysis Calc.
for C22H21N3O2: C, 73.52; H, 5.89; N, 11.69%. Found: C, 73.26; H,
5.50; N, 11.59%. IR (KBr disc, cmꢂ1): 3435 (phenolic OAH),
ꢃ3100 (aromatic CAH stretching), 2920 (aliphatic CAH stretching),
1618 (C@O and CAN stretchings), 1518 and 1383 (symmetric and
asymmetric stretching vibrations of ethyl group), 1475 (N@N).
ESI–MS (m/z (rel. intensity) assignment): 360 (18%) [M + H]+, 382
(100%) [M + Na]+. NMR: 1H (d-DMSO as solvent, d in ppm), 15.07
(s, 1H, NH), 10.41 (s, 1H, phenolic OH), 9.25 (s, 1H,@CHANA),
6.80–8.32 (10H, aromatic), 2.64 (q (quartet), 2H, PhACH2AC),
2.26 (s, 3H, PhACH3), 1.24 (t, 3H, PhACACH3). 13C NMR (d-DMSO
as solvent,
d
in ppm):166.93 (ACHANH), 114.17–159.87
(C, aromatic), 15.35, 20.21 and 28.01 (C, ethyl (PhACH2ACH3)
and methyl (Ph-CH3) groups.
X-ray crystallography
A single crystal of dimensions 0.33 ꢄ 0.25 ꢄ 0.08 mm3 was cho-
sen for the diffraction experiment. Data were collected at 150(2) K°
on a BrukerApexII CCD diffractometer using Mo K
a radiation
(k = 0.71073 Å). The structure was solved by direct methods and
refined on F2 using all the reflections [30].
All the non-hydrogen atoms were refined using anisotropic
atomic displacement parameters and hydrogen atoms bonded to
carbon atoms were inserted at calculated positions using a riding
model. Hydrogen atoms bonded to oxygen and nitrogen atoms
were located from difference maps and refined with temperature
factors riding on the carrier atom. Details of the crystal data and
refinement are given Table 1. Hydrogen bond parameters are given
in Table 2 and bond lengths and angles are given in Table S1.
Computational method
All calculations about tautomers were made with GaussView
5.0.8 and Gaussian 09 IA32W Rev. A.02 programmes [31,32]. In
the first step, all tautomers were fully optimized by using B3LYP/
3-21G in gas phase. B3LYP method is hybrid density functional
theory method and it was used in all calculations with 3-21G basis
set [33–37]. In the second step, the most stable and reactive
tautomer was determined by using total energy, Gibbs free energy,
some molecular orbital energies and contour diagrams.
Experimental
General
All starting materials were obtained from Aldrich and Fluka, and
were used without further purification.
Potentiometric studies
The 13C and 1H NMR spectral measurements were performed on
a BrukerAvance 400 (400 MHz). Mass spectrum was recorded on a
Thermo Fisher Exactive + Triversa Nanomate mass spectrometer.
The electronic spectra in the 200–900 nm range were obtained on
a Shimadzu UV-1800 UV–Vis spectrophotometer. IR spectrum
was recorded on a Perkin Elmer Paragon 1000 PC using KBr pellet.
CHN analysis was performed using a CE-440 Elemental analyser.
Melting point was uncorrected and is in degree Celsius. Data collec-
tion for X-ray crystallography was completed using a Bruker APEX2
CCD diffractometer and data reduction was performed using Bruker
SAINT. SHELXTL was used to solve and refine the structures [29].
EtOH, NaCl, CuCl2, NiCl2 and CoCl2 were purchased from Merck,
potassium hydrogen phthalate (KHP) and borax (Na2B4O7) from
Fluka, 0.1 M NaOH and 0.1 M HCl as standard from Aldrich. All
reagents were of analytical quality and were used without further
purification. For the solutions, CO2-free double-distilled deionized
water was obtained with an aquaMAX™-Ultra water purification
system (Young Lin Inst.). Its resistivity was 18.2 M
X cm pH-metric
titrations were performed using the Molspin pH meter™ with a
Orion 8102BNUWP ROSS ultra-combination pH electrode. The tem-
perature in the double-wall glass titration vessel was constantly
controlled using a thermostat (DIGITERM 100, SELECTA) and kept
at 25.0 0.1 °C. The cell solution was stirred during the titration
at constant. 0.05 m (mol/kg) potassium hydrogen phthalate
(KHP) and 0.01 m (mol/kg) borax (Na2B4O7) were prepared for
calibration of electrode systems. The electrode was calibrated
according to instructions of the Molspin Manuel [38]. The pH
Preparation of (6Z)-4-[(E)-(4-ethylphenyl)diazenyl]-6-{[(2-hydroxy-
5-methylphenyl)amino]methylidene}cyclohexa-2,4-dien-1-one
The azo Schiff base tautomer (Scheme) was prepared according
to the following procedure: A methanolic solution of 5-[(E)-(4-eth-

A. Gözel et al. / Journal of Molecular Structure 1074 (2014) 449–456
451
Table 1
Spectroscopy
Crystallographic data for the compound.
The 1H and 13C NMR spectra were recorded in d6-DMSO and
data are given in the experimental part. The 1H NMR spectrum of
the title ligand displays a tripletat d 1.24 ppm and a quartet at d
2.69 ppm corresponding to ethyl group protons (PhACH2ACH3)
(Fig. 1). A singlet at d 2.26 ppm was assigned to the protons of
methyl group. Two broad signals at d 10.25 and 15.07 ppm could
be assigned to the protons of enamine (CANHAC) and phenol
(PhAOH), respectively. A singlet at d 9.23 ppm was assigned to
the proton of (ANACH@) [41]. All the other aromatic protons were
seen in the range of 6.80–8.32 ppm.
Empirical formula
C22H21N3O2
Formula weight
Crystal size (mm3)
Crystal color
Crystal system
Space group
Unit cell a (Å)
b (Å)
359.42
0.57 ꢄ 0.19 ꢄ 0.08
Red
Monoclinic
P21/n
9.0973 (12))
8.1678 (11)
24.883 (3)
90
91.238 (2)°
90
1848.5 (4)
4
0.084
18,483
99.8 %
4599 [0.0560]
0.0500, 0.1114
0.0985, 0.1310
994,909
c (Å)
a
(°)
b (°)
(°)
c
In the 13C NMR spectrum of the compound aliphatic carbon
shifts were observed at 15.35, 20.21 and 28.01 ppm assigned to
ethyl and methy groups. The signal at d 166.93 ppm could be
assigned to the carbon (ACHANH). All other aromatic carbon shifts
were observed in the range of d 114.17–159.87 ppm (Fig. S1). The
NMR data showed that azo-enamine tautomer is maintained in
DMSO and agreed with values reported in literature [26].
The ESI mass spectrum of the synthesized azo-azomethine tau-
tomer was performed in MeOH and is shown in Fig. S2. Two signals
at m/z 382 (100%) and 360 (18%) were assigned to molecular ion
signals [M + Na]+ and [M + H]+, respectively.
Volume (Å3)
Z
Abs. coeff. (mmꢂ1
Refl. collected
)
Completeness to h = 28.02°
Ind. Refl. [Rint
R1, wR2 [I > 2
]
r
(I)]
R1, wR2 (all data)
CCDC number
Table 2
Hydrogen-bond geometry (Å, °).
The IR spectrum of the dye shows several bands in 4000–
400 cmꢂ1 region. The bands of the dye are also listed in experimen-
tal section. The functional groups of the dye have been identified
from the infrared spectrum. The band at 1618 cmꢂ1 was assigned
DAHꢁꢁꢁA
DAH
HꢁꢁꢁA
1.64 (2)
DꢁꢁꢁA
DAHꢁꢁꢁA
O2AH2ꢁꢁꢁO1i
0.94 (2)
0.987 (19)
2.5831 (17)
2.5625 (19)
177 (2)
141.1 (16)
N3AH3ꢁꢁꢁO1
1.718 (19)
to C12@O1 and C15AN3 stretchings (Fig. S3). The azo
v(N@N)
stretching band of the compound is observed in the spectrum at
1475 cmꢂ1. A broad peak at 3435 cmꢂ1 was assigned to phenolic
(OAH) stretching. The peaks observed at 1518 and 1383 cmꢂ1 are
attributed to symmetric and asymmetric stretching vibrations of
ethyl group. The aromatic and aliphatic CAH stretchings in the
compound were observed at ꢃ3100 cmꢂ1 (aromatic CAH
stretching) and 2920 cmꢂ1 (aliphatic CAH stretching).
Symmetry code: (i) ꢂx + 3/2, y + 1/2, ꢂz + 1/2.
electrode was calibrated with buffer solution of pH 4.005 (KHP)
and pH 9.180 (Na2B4O7) at 25.0 ( 0.1) °C [39]. Also, the potentio-
metric cell was calibrated in the H2O–EtOH mixture solvent before
each experiment. An automatic burette was connected to Molspin
pH-mV-meter. A solution of the compound in EtOH–H2O mixture
(1/5, v/v) (10ꢂ3 M) and NaOH (0.025 M) were prepared. NaCl (Rie-
del-de Haën) stock solution (1.0 M) was prepared. All potentiomet-
ric pH measurements were made on solutions in a 100 mL double-
walled glass vessel. The cell was equipped with magnetic stirrer.
Atmospheric CO2 was excluded from the titration cell with a purg-
ing steam of purified N2. The system was maintained at an ionic
strength of 0.1 M by NaCl as a supporting electrolyte. In experi-
ment, a solution containing about 0.01 mmol of ligand was placed
in the cell. The required amount of NaCl (1.0 M) and HCl (0.1 M)
were added.
X-ray structure
Molecular structure of the azo-enamine tautomer is shown in
Fig. 2. The compound crystallizes in monoclinic crystal system,
P21/n space group with unit cell parameters a = 9.0973(12),
b = 8.1678(11), c = 24.883(3) Å, b = 91.238(2)°, V = 1848.5(4) Å3
and Z = 4. All bond lengths and angles are within the normal
ranges. All bond lengths and angles in the phenyl rings have nor-
mal Csp2–Csp2 values and are in the expected ranges (Table S1).
X-ray investigation of the compound showed that azo-enamine
tautomer is favoured in the solid state. The C12AO1 bond length
of 1.290(2) Å indicates a double bond character, whereas the
C15AN3 bond length of 1.307(2) Å is longer than a double bond
(C@N) character and the C11AC15 bond length of 1.405(2) Å is
Finally, doubly distilled deionised water was added to the cell to
a total volume of 50 mL and titration was started. The pH data
points were collected after each addition of 0.03 cm3 of the stand-
ardised NaOH solution. The HYPERQUAD computer program was
used for the calculation of the acid dissociation constants [40].
longer than
a double bond (C@C) character. Aromatic rings
(C9AC14) and (C15AC20) adopt the trans configuration with
regard to the azo double bond (AN2@N3A) with a distance of
1.2656(19) Å the torsion angle C6AN1AN2AC9 of 179.69(14) Å
which is in good agreement with published values [27]. In order
to confirm the existence of azo-enamine tautomer in the solid
state, the harmonic oscillator model of aromaticity (HOMA)
indexes for rings are calculated using the following equation
[18,42].
Results and discussion
Synthesis and solubility
The azo-Schiff base ligand was prepared by the condensation
between one equivalent of 2-amino-4-methylphenol with one
equivalent of 5-[(E)-(4-ethylphenyl)diazenyl]-2-hydroxybenzalde-
hyde in MeOH as shown in Scheme 1. The compound is stable at
room temperature and insoluble in water and n-hexane and it is
soluble in common organic solvents such as DMSO, DMF, EtOH,
MeOH, CH3CN, CH3Cl and CH2Cl2. Elemental analysis data are given
in the experimental section and are in well agreement with the
theoretical values. The data are in good agreement with single
crystal X-ray determination.
"
#
n
X
a
n
HOMA ¼ 1 ꢂ
ðRi ꢂ RoptÞ2
i¼1
n is the number of bonds in ring,
a is the constant equal to 257.7
and Ropt is equal to 1.388 Å for CAC bonds. For the purely aromatic
compounds, HOMA index is equal to 1 but, for non-aromatic

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A. Gözel et al. / Journal of Molecular Structure 1074 (2014) 449–456
O
O
NH2
N
N
i
H
ii
O
OH
NH
N
N
N
N
N
OH
OH
azo-enamine form
OH
azo-imine form
O
N
N
NH
hydrozone-imine form
Scheme 1. Synthesis of (6Z)-4-[(E)-(4-ethylphenyl)diazenyl]-6-{[(2-hydroxy-5-methylphenyl)amino]methylidene}cyclohexa-2,4-dien-1-one dye and its possible tautomers.
(i) HCl/NaNO2, salicylaldehyde, 0–5 °C and (ii) 2-amino-4-methylphenol, MeOH, reflux.
Fig. 1. 1H NMR spectrum of azo-enamine compound.

A. Gözel et al. / Journal of Molecular Structure 1074 (2014) 449–456
453
Optimized structures
Optimized structures of tautomers (1–3) were represented in
Fig. S4. Experimental and calculated bond lengths and angles of
tautomer (2) were given in Table S1.
As can be seen from Table S1, compared with experimental
datum and calculated bond lengths/angles of tautomer (2), it can
be seen that B3LYP/3-21G level is the appropriate level. The
differences average between experimental and calculated bond
lengths of tautomer (2) is 0.012 Å and the differences average
between experimental and calculated bond angles is 2.4°. These
results show that there is a good agreement with experimental
and optimized structure of tautomer (2).
Fig. 2. Molecular structure of the azo-enamine tautomer with atom numbering
thermal ellipsoid 50% probability, intra-molecular hydrogen bonding is shown as
dash lines.
The stability and reactivity of tautomers
Total energy (E), Gibbs free energy (G°) were calculated to
predict the stability of tautomers at B3LYP/3-21G level in gas
phase and given in Table S2. Total energy (ETotal) comprises
electronic energy (Eelec), zero-point energy (ZPE), vibrational
compounds it is equal to 0. The HOMA indexes in the range of
0.900–0.990 or 0.500–0.800 show that the rings are aromatic or
the non-aromatic, respectively [18,42]. HOMA indexes for the rings
of C3AC8, C9AC14 and C16AC21 were found to be 0.9993, 0.5959
and 0.9996, respectively. These results revealed that the rings
C3AC8 and C16AC21 show aromatic character however C9AC14
ring deviates from aromaticity.
energy (Evib), rotational energy (Erot), translational energy (Etransl
)
and represented in Eq. (1) and Gibbs free energy (G) includes
entalphy (H), entropy (S) and it represented in Eq. (2).
There is an intramolecular hydrogen bonding (N3AHꢁꢁꢁO1) in
the molecule forming a S(6) graph set motif. Additionally, there
is an intermolecular O2AHꢁꢁꢁO1 hydrogen bonding in the structure
(symmetry code: ꢂx + 3/2, y + 1/2, ꢂz + 1/2). The same intermolec-
ular hydrogen bonding contacts are extended between the other
symmetry-related molecules in their respective planes to form a
1D hydrogen bond chain (Fig. 3).
In the structure, the azo benzene ring slightly twisted with
respect to the central benzene ring. The mean planes of C3AC8
and C16AC21 are at 3.13 (10)° and 4.15 (10)° to the central
(C9AC14) ring, respectively. This is possibly a consequence of
the intermolecular interactions in the lattice. The molecules
show two sets of interactions with neighbouring dimeric units.
First, C3AC12 section of the molecules is stacked with the
same section of an adjacent molecule under symmetry operation
of 1ꢂx, 1ꢂy, ꢂz (C3/C8AC9/C14 centroid–centroid distance
is 3.599 Å). Second, the central benzene ring (C9AC14) is
stacked with 4-methylphenol unit of a neighbouring compound
ETotal ¼ Eelec þ ZPE þ Evib þ Erot þ Etransl
G ¼ H ꢂ TS
ð1Þ
ð2Þ
ETotal and G can be used to explain the stability. The tautomer
with the lowest energy has the most stability. The stability ranking
of tautomers should be:
ð2Þ > ð1Þ > ð3Þðaccording to theETotal
ð2Þ > ð1Þ > ð3Þðaccording to theGÞ
Þ
Taken into account E and G, the most stable tautomer is tauto-
mer (2). But the stabilities of tautomers (2) and (1) are close to
each other.
Contour diagram of HOMO and LUMO can be used to predict the
reactive region of tautomers. Contour diagrams of frontier molecu-
lar orbitals of tautomers were represented in Fig. 5.
HOMO of tautomers (1) and (2) is mainly delocalized on all
atoms of molecules while LUMO is localized on oxygen, nitrogen
and middle benzene ring. If mentioned tautomers are taken into
account as tridentate ligands, electrons are given from HOMO,
HOMOꢂ1 and HOMOꢂ2 of tautomers. The energies of LUMO,
HOMO, HOMOꢂ1 and HOMOꢂ2 were given in Table S3.
The tendency of donating electron increases with increasing
of orbital energies. The total energies of HOMO, HOMOꢂ1 and
HOMOꢂ2 are important to determine the most reactive tautomer.
under symmetry operation of x, ꢂ1 + y,
centroid–centroid distance is 3.531 Å) (Fig. 4). Hydrogen bonded
chains are linked by interactions and crystal packing of the
compound is determined by intermolecular hydrogen bonding
and interactions. The packing plot of the compound is
shown in Fig. 4.
z (C9/C14–C16/C21
p–p
p–p
Fig. 3. 1D hydrogen bonded chain within the structure.

454
A. Gözel et al. / Journal of Molecular Structure 1074 (2014) 449–456
Fig. 4. Packing diagram of the azo-enamine dye viewing down a axis showing
p–p
stacking interactions between hydrogen bonded molecules.
According to Table 2, total energies of tautomers (2) and (3) are
close to each other, yet tautomer (3) is very unstable. Based on this
comparison, the most reactive tautomer was found as tautomer
(2). Electron density surface of tautomer (2) was given in Fig. 6.
The highest electron density surface is on the oxygen atom num-
bered with O1.
Electronic absorption spectra
Electronic spectroscopy is very useful tool for studying tautom-
erism in Schiff bases involving ahydroxyl group in ortho position to
the azomethine group. o-Hydroxy Schiff bases may exist in two
tautomeric forms (enol and keto forms) in solid state and solvent
media [18].
To examine the behavior of the title compound in solution, elec-
tronic spectra in four organic solvents with different polarity
(CHCl3, MeOH, DMSO and DMF) were performed in the wavelength
range 200–800 nm at room temperature. UV–Vis. spectra of the
title compound are shown in Fig. 7.
Fig. 6. Electron density surface of tautomer (2).
azomethine groups). Absorption intensity of the
increased with increasing the solvent polarity (hyperchromic
effect). The
pꢅ transitions in DMF and DMSO shifted to higher
p ?
pꢅ transitions
In all four solvents, maximum absorptions observed in the
range of 320–410 nm were assigned to the
of the aromatic rings and active functional groups (azo and
p ?
pꢅ transitions
p
?
Fig. 5. Contour diagrams of frontier molecular orbitals for tautomers (1–3).

A. Gözel et al. / Journal of Molecular Structure 1074 (2014) 449–456
455
Non-linear optic properties
Non-linear optical properties (NLO) is current research field
because of its importance in providing the key functions of
frequency shifting, optical modulation, optical switching, optical
logic and optical memory. The non-linear optical properties of
molecules increase with conjugation of
p electrons on molecule
or adding donor/acceptor groups to molecule. Quantum chemical
calculations can be used to describe the relationship between
the electronic structures of molecules and their non-linear
optical properties. Non-linear optical properties of tautomer (2)
which has delocalized
level.
p electrons was studied by B3LYP/3-21G
The total static dipole moment
(l), the average linear
Fig. 7. UV–Vis. spectra of the compound (10ꢂ5 M) in various solvent.
polarizability ( ) and first hyperpolarizability (b) can be calculated
a
by using the Eqs. (1), (2), (1)–(3), respectively and given in Table S4
[43]:
wavelengths (batochromic effect) with higher absorption intensi-
ties than in CHCl3. Additionally, new absorption bands at 465,
465 and 470 nm were observed in MeOH, DMF and DMSO, respec-
tively. The intensity of these new bands increased with decreasing
the solvent polarity (hyperchromic effect). The earlier computa-
tional and experimental studies on o-hydroxy Schiff bases indicate
that presence of a new band above 400 nm is indicative of the exis-
tence of keto-enamine tautomer in solution [21]. No new band due
to keto-amine tautomer was observed in CHCl3.
The electronic absorption spectra of the azo-azomethine com-
pound at different volume ratios of the applied pair solvents
DMF/H2O and DMSO/H2O were also measured. The absorption
curves of the dye in DMF/H2O and DMSO/H2O mixtures are shown
in Fig. S5. In DMF/H2O, the absorption bands shifted to lower wave-
lengths (hypsochromic effect) but with higher absorption intensi-
ties (hyperchromic effect). No particular differences were
observed in UV–Vis absorption spectra of the compound in DMF/
H2O (1:1 and 1:2 v/v ratios) solutions. However, in DMSO/H2O,
as amount of water increased, the absorption bands shifted to
lower wavelengths (hypsochromic effect) with lower absorption
intensities (hyporchromic effect) (Fig. S5).
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
l
a
¼
¼
lx2
þ
ly2
þ
lz2
ð1Þ
ð2Þ
ð3Þ
1
ð
axx
þ
ayy
þ
azz
Þ
3
h
i
1=2
À
Á
À
Á
À
Á
2
2
2
b ¼ bxxx þbxyy þbxzz þ byyy þbxxy þbyzz þ bzzz þbxzz þbyzz
The calculated polarizability and hyperpolarizability for
tautomer (2) are 48.7674 Å3 and 6.00 ꢄ 10ꢂ26 cm5/esu. These
values are greater those of urea
(a
= 3.8312 Å3 and
b = 0.37289 ꢄ 10ꢂ30 cm5/esu), respectively. Therefore, tautomer
(2) can be used as a non-linear optical material.
Acid dissociation constants
Acid dissociation constants were potentiometrically calculated
from a series of several independent measurements. The ligand
species, which occurred in solution under experimental conditions,
are LH53+, LH42+, LH+3, LH2, LHꢂ and L2ꢂ. If LH35+ denotes the fully
protonated form of the ligand, its acid dissociation equilibriums
are as follows:
LH35þ þ H2O ꢀ LH2þ þ H3Oþ Ka1 ¼ ½LH24þꢆ½H3Oþꢆ=
½
LH35þꢆ
ð4Þ
ð5Þ
ð6Þ
ð7Þ
ð8Þ
The UV–Vis. spectra of the compound in DMF and DMSO
solvents were performed at 20–80 °C range (Fig. S6). In DMF,
considerable increase both the absorption wavelengths and
intensities were observed when temperature increased from
20 to 40 °C. However, no particular differences were observed
after 40 °C. Interestingly, in DMSO, as the temperature
4
LH24þ þ H2O ꢀ LHþ þ H3Oþ Ka2 ¼ ½LHþꢆ½H3Oþꢆ=
½
LH24þꢆ
3
3
LHþ3 þ H2O ꢀ LH2 þ H3Oþ Ka3 ¼ ½LH2ꢆ½H3Oþꢆ=
½
LHþ3 ꢆ
LH2
LHꢂꢆ
LH2 þ H2O ꢀ LHꢂ þ H3Oþ Ka4 ¼ ½LHꢂꢆ½H3Oþꢆ=
½
ꢆ
LHꢂ þ H2O ꢀ L2ꢂ þ H3Oþ Ka5 ¼ ½L2ꢂꢆ½H3Oþꢆ=
½
increased, the intensity of the band due to the
p ?
pꢅ
transitions shifted lower values but intensity of the band due
to azo-enamine tautomer increased. Again, no particular change
was observed after 40 °C.
Hence, titration curves and distribution diagrams illustrated in
Fig. 8. Five dissociation constants were determined for the ligand
as 3.45 (pKa1), 4.78 (pKa2), 6.75 (pKa3), 9.27(pKa4) and 9.96 (pKa5
)
(b)
(a)
11
100
80
60
40
20
0
LH
2
L
LH
10
9
3
LH
5
LH
4
8
LH
7
6
5
4
3
0
1
2
3
4
5
6
4
6
8
10
mL NaOH
pH
Fig. 8. (a) Titration curves of the ligand; (b) Species distribution curves for ligand (25.0 0.1 °C, I: 0.1 mol dmꢂ3 by NaCl).

456
A. Gözel et al. / Journal of Molecular Structure 1074 (2014) 449–456
Table 3
www.ccdc.cam.ac.uk/data_request/cif, by e-mailing data_re-
quest@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallo-
graphic Data Centre 12 Union Road Cambridge CB2 1EZ, UK Fax:
+44(0)1223-336033. Supplementary data associated with this arti-
cle can be found, in the online version, at http://dx.doi.org/
10.1016/j.molstruc.2014.06.033.
Acid dissociation constants of the compound in EtOH–H2O mix. (1/5, v/v)
(25.0 0.1 °C, I = 0.1 mol dmꢂ3 by NaCl, 0.05 mmol HCl).
Species
Log ba
pKa Values
LH4
LH3
LH2
LH
28.80 0.05
25.22 0.03
18.92 0.03
9.69 0.03
3.58 0.02
6.30 0.01
9,23 0.03
9.69 0.03
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This work is supported by the Scientific Research Project Fund
of Kahramanmaras Sutcu Imam University under the Project Num-
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_
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Appendix A. Supplementary material
CCDC 994909 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge via