A novel azo-aldehyde and its Ni(II) chelate; Synthesis, characterization, crystal structure and computational studies of 2-hydroxy-5-{(E)-[4-(propan-2-yl) phenyl]diazenyl}benzaldehyde

Article abstract of DOI:10.1016/j.molstruc.2014.02.052

A novel azo-salicylaldeyde, 2-hydroxy-5-{(E)-[4-(propan-2-yl)phenyl] diazenyl} benzaldehyde and its Ni(II) chelate were obtained and characterized by analytical and spectral techniques. Molecular structure of the azo chromophore containing azo-aldehyde was determined by single crystal X-ray crystallography. X-ray data show that the compound crystallizes in the orthorhombic, Pbca space group with unit cell parameters a = 11.2706(9), b = 8.3993(7), c = 28.667(2) ?, V = 2713.7(4) ?³ and Z = 8. There is a strong phenol-aldehyde (OH)hydrogen bond forming a S(6) hydrogen bonding motif in the structure. There is also a weaker inter-molecular phenol-aldeyhde (OHO) hydrogen bonding resulting in a dimeric structure and generating a D22 (4) hydrogen bonding motif. Hydrogen bonded dimers are linked by p-p interactions within the structure. The azo-aldehyde ligand behaved as bidentate, coordinating through the nitrogen atom of the azomethine group and or oxygen atom of phenolic hydroxyl grop. Additionally, optimized structures of the three possible tautomers of the compound were obtained using B3LYP method with 6-311++G(d,p), 6-31G and 3-21G basis sets in the gas phase. B3LYP/6-311++G(d,p) level is found to be the best level for calculation. The electronic spectra of the compounds in the 200-800 nm range were obtained in three organic solvents.

Full text of DOI:10.1016/j.molstruc.2014.02.052

Journal of Molecular Structure 1065–1066 (2014) 191–198
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Journal of Molecular Structure
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A novel azo-aldehyde and its Ni(II) chelate; synthesis, characterization,
crystal structure and computational studies of 2-hydroxy-5-
{(E)-[4-(propan-2-yl)phenyl]diazenyl}benzaldehyde
Tug˘ba Eren a, Muhammet Kose a, Koray Sayin b, Vickie McKee c, Mukerrem Kurtoglu a,
⇑
a
_
Department of Chemistry, Kahramanmaraßs Sütçü Imam University, Kahramanmaraßs 46050, Turkey
b Department of Chemistry, Cumhuriyet University, Sivas 58140, Turkey
c Department of Chemistry, 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-salicylaldehyde and its
Ni(II) chelate were synthesized and
characterized by analytical and
spectroscopic studies.
ꢀ
ꢀ
The X-ray structure of the
azo-salicylaldehyde was determined.
The OHꢁꢁꢁO intra-intermolecular
hydrogen bondings and p–p interactions
stabilize the crystal structure.
ꢀ
Optimized structures of the three
possible tautomers were obtained using
B3LYP method.
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel azo-salicylaldeyde, 2-hydroxy-5-{(E)-[4-(propan-2-yl)phenyl]diazenyl} benzaldehyde and its
Ni(II) chelate were obtained and characterized by analytical and spectral techniques. Molecular structure
of the azo chromophore containing azo-aldehyde was determined by single crystal X-ray crystallography.
X-ray data show that the compound crystallizes in the orthorhombic, Pbca space group with unit cell
parameters a = 11.2706(9), b = 8.3993(7), c = 28.667(2) Å, V = 2713.7(4) Å3 and Z = 8. There is a strong phe-
nol-aldehyde (OHꢁ ꢁ ꢁO) hydrogen bond forming a S(6) hydrogen bonding motif in the structure. There is also
a weaker inter-molecular phenol-aldeyhde (OHꢁ ꢁ ꢁO) hydrogen bonding resulting in a dimeric structure and
Received 27 December 2013
Received in revised form 14 February 2014
Accepted 27 February 2014
Available online 6 March 2014
Keywords:
Azo-aldehyde
Ni(II) chelate
X-ray diffraction
Solvent effect
p–p Interactions
Quantum chemical studies
generating a D22(4) hydrogen bonding motif. Hydrogen bonded dimers are linked by
p–p interactions within
the structure. The azo-aldehyde ligand behaved as bidentate, coordinating through the nitrogen atom of the
azomethine group and or oxygen atom of phenolic hydroxyl group. Additionally, optimized structures of the
three possible tautomers of the compound were obtained using B3LYP method with 6-311++G(d,p), 6-31G and
3-21G basis sets in the gas phase. B3LYP/6-311++G(d,p) level is found to be the best level for calculation. The
electronic spectra of the compounds in the 200–800 nm range were obtained in three organic solvents.
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
reactions of salicylaldehyde and its derivatives with various
primary aromatic or aliphatic mono-, di-, tri- or polyamines result
The interest in studies of compounds derived from salicylalde-
hyde and its derivatives is increasing day by day [1]. Condensation
in compounds containing azomethine bond (AC@NA) known as
Schiff bases [2,3]. These compounds form coordination compounds
with several metal ions [4]. There are numerous papers published
on salicylaldehyde and its derivatives regarding their fluorescent
and use as colorimetric sensors [5,6], antitumor activity [7], and
DNA cleavage activity [8].
⇑
Corresponding author. Tel.: +90 344 280 1450; fax: +90 344 280 1352.
E-mail addresses: mkurtoglu@ksu.edu.tr, kurtoglu01@gmail.com (M. Kurtoglu).
http://dx.doi.org/10.1016/j.molstruc.2014.02.052
0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

192
T. Eren et al. / Journal of Molecular Structure 1065–1066 (2014) 191–198
Azo containing compounds are of interest due to their proper-
2.3. Crystallography
ties including the dyeing of textiles [9], optical switching, non-lin-
ear optical properties [10], and the preparation of photoactive
materials [11]. Synthesis of azo containing dyes dates back to the
second half of the 20th century, since then azo dyes have attracted
a great deal of attention and found many uses in textile industry as
well as material sciences [12]. Azo compounds are reported to be
involved in a number of biological systems such as inhibition of
DNA, RNA, and protein synthesis, carcinogenesis, and nitrogen
fixation [13,14]. In addition, azo compounds and their metal
chelates have shown a good anti-bacterial activity [15,16]. These
compounds are also used in analytical chemistry as indicators in
pH, redox, or complexometric titration [17,18].
There is a close relationship between the physico-chemical
properties of azo dyes and tautomerism [19]. Due to the affect of
tautomerism of azo dyes on different technical properties and dye-
ing abilities, theoretical and experimental studies of tautomers
have attracted much attention [20].
A single crystal of dimensions 0.46 ꢂ 0.45 ꢂ 0.15 mm3 was cho-
sen for the diffraction experiment. Data were collected at 150(2)K
on a Bruker ApexII 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 [22]. All the non-hydrogen
atoms were refined using anisotropic atomic displacement param-
eters and hydrogen atoms bonded to carbon atoms were inserted
at calculated positions using a riding model. Hydrogen atom
bonded to oxygen atom (O1) was located from difference maps
and refined with temperature factor riding on the carrier atom.
Details of the crystal data and refinement are given in Table 1.
Hydrogen bond parameters are listed in Table 2, bond lengths
and angles are given in Tables 1S and 2S.
2.4. Computational method
Because of the importance of azo-salicylaldehyde derivatives
and our continuing interest in synthesis of azo and azomethine
compounds, a novel azo-aldehyde compound, 2-hydroxy-5-{(E)-
[4-(propan-2-yl)phenyl]diazenyl}benzaldehyde, ipr-salH (Fig. 1)
and its Ni(II) complex were prepared and characterized by spectro-
scopic and analytic methods. Molecular structure of the title com-
pound was determined by single crystal X-ray diffraction studies.
The properties of the azo-aldehyde ipr-salH were determined com-
putationally by DFT method at the B3LYP/6-31G(d,p) level of the-
ory. Furthermore, the solvatochromic behavior of the prepared
azo compounds in various solvents was evaluated.
All calculations were made by using Gaussian 09 [23,24]. Den-
sity functional theory (DFT) methods [25,26], containing the Becke
three parameters hybrid exchange and the Lee–Yang–Parr correla-
tion hybrid functional (B3LYP) [27–29] were used in geometry
optimization. In the first step, tautomer (1) was fully optimized
by using B3LYP method with 6-311++G(d,p), 6-31G and 3-21G ba-
sis sets in gas phase. The best level was found to be B3LYP/6-
311++G(d,p). In the second step, tautomers (2) and (3) were fully
optimized by using the best level in gas phase. 6-311++G(d,p) is
a popular high angular momentum basis set which adds p func-
tions to hydrogen and d functions on heavy atoms. The transition
state (TS) method was used to search for the transition states of
all reaction paths [30,31]. The analyses of vibrational frequencies
indicated that optimized structures of transition states were at
stationary points corresponding to local minima with imaginary
frequencies which are equal to 1.
2. Experimental
2.1. Reagents
All reagents and solvents were purchased from chemical com-
panies and used without further purification unless otherwise
noted. The UV–Vis spectra were taken in spectrophotometric grade
solvents.
2.5. Synthesis of 2-hydroxy-5-{(E)-[4-(propan-2-yl)phenyl]
diazenyl}benzaldehyde, pr-salH
i
The title compound, (ipr-salH), was synthesized following stan-
dard protocols [32], using p-isopropylaniline and salicylaldehyde
as starting materials: A mixture of 4-isopropylaniline (1.35 g,
10 mmol), water (50 mL) and concentrated hydrochloric acid
(2.5 mL, 30 mmol) was stirred until a clear solution was obtained.
2.2. Physical measurements
NMR spectra were performed using a Bruker Advance 400 MHz.
Spectrometer. Mass spectra were recorded on a Thermo Fisher
Exactive + Triversa Nanomate mass spectrometer. The infrared
spectra were obtained using a Perkin Elmer spectrum 100 FTIR
spectrophotometer. Carbon, hydrogen and nitrogen analyses were
performed with a model CE-440 elemental analyzer. Single crystals
suitable for X-ray diffraction study were obtained from CHCl3-
EtOH solution by slow evaporation at room temperature. Data col-
lection for X-ray crystallography was completed using a Bruker
APEX2 CCD diffractometer and data reduction was performed
using Bruker SAINT [21]. SHELXTL was used to solve and refine
the structures [22]. The UV–Vis spectra were measured with a
T80 + UV–Vis. Spectrometer PG Instruments Ltd.
Table 1
Crystallographic data for the ipr-salH compound.
Identification code
ipr-salH
C16H16N2O2
Empirical formula
Formula weight
Crystal size (mm3)
Crystal color
268.31
0.46 ꢂ 0.45 ꢂ 0.15
Orange
Crystal system
Space group
Orthorhombic
Pbca
Unit cell
a (Å)
b (Å)
11.2706(9)
8.3993(7)
28.667(2)
2713.7(4)
8
c (Å)
Volume (Å3)
Z
Abs. coeff. (mmꢃ1
Refl. collected
)
0.088
25992
Completeness to h = 28.02°
Ind. Refl. [Rint
R1, wR2 [I > 2
R1, wR2 (all data)
CCDC number
99.9%
]
r
3377 [0.0404]
0.0394, 0.1011
0.0550, 0.1093
947938
(I)]
Fig. 1. Synthesis of 2-hydroxy-5-{(E)-[4-(propan-2-yl)phenyl]diazenyl}benzalde-
hyde (ipr-salH). i: NaNO2/HCl, 273–278 K; ii: salicylaldehyde.

T. Eren et al. / Journal of Molecular Structure 1065–1066 (2014) 191–198
193
Table 2
state without decomposition. The ligand is soluble in a wide range
of common organic solvents such as methanol, acetonitrile, chloro-
form, dichloromethane, dimethyformamide and dimethylsulfoxide,
but is insoluble in water.
Hydrogen bonds for ipr-salH [Å and °].
DAHꢁ ꢁ ꢁA
d(DAH)
d(Hꢁ ꢁ ꢁA)
2.55
2.57
1.86
d(Dꢁ ꢁ ꢁA)
2.9819(13)
3.3056(16)
2.6537(13)
<(DHA)
O(1)AH(1)ꢁ ꢁ ꢁO(2)#1
C(16)AH(16)ꢁ ꢁ ꢁO(1)#2
O(1)AH(1)ꢁ ꢁ ꢁO(2)
0.88
0.93
0.88
111.2
136.8
149.9
The 1H and 13C NMR spectra of the azo-aldehyde ligand were re-
corded in CDCl3, and the spectral data are given in the experimen-
tal section. 1H and 13C NMR spectra are displayed in Figs. 1S and 2S
respectively. The 1H NMR spectrum displays a dublet at d 1.34 ppm
corresponding to protons of two identical methyl groups (CACH3),
a septet at d 2.99 ppm assigned to the proton of the isopropyl car-
bon atom (ACHA). The signal assigned to the carbonyl proton was
seen at d 10.28 ppm and one broad signal at d 11.33 ppm was as-
signed to phenolic protons (OH). All aromatic protons were seen
in the range of d 6.93–8.21 ppm (Fig. 1S).
Symmetry transformations used to generate equivalent atoms:
#1 ꢃx, ꢃy, ꢃz + 1; #2 x + 1/2, ꢃy + 1/2, ꢃz + 1.
This solution was cooled to 273–278 K and a solution of sodium
nitrite (0.96 g, 11.29 mmol) in water (3 mL) was added dropwise
while the temperature was maintained below 278 K. The resulting
mixture was stirred for 30 min in an ice bath. An o-hydroxybenzal-
dehyde (1.22 g, 10 mmol) solution (pH 9–10) was added gradually
to the solution of cooled 4-isopropylenzenediazonium chloride,
prepared as described above, and the resulting mixture was stirred
at 273–278 K for 60 min. The dark yellow product was filtered off,
washed with MeOH and dried in air. Crystals of 2-hydroxy-5-{(E)-
[4-(propan-2-yl)phenyl]diazenyl}benzaldehyde were obtained
from slow evaporation of an EtOH-CHCl3 mixture.
The 13C NMR spectrum of the azo-aldehyde exhibited the sig-
nals due to the presence of aromatic and aliphatic carbons. The
presence of carbonyl group is evident from the characteristic signal
at the farthest downfield ꢅ196.62 ppm corresponding to the car-
bon of ACHO group (C16) [33]. The peak at d 163.53 ppm was as-
signed to the aromatic carbon linked hydroxyl group (C13). The
i
aliphatic region of the 13C NMR spectrum of pr-salH showed two
peaks as seen in Fig. 2S. The medium intense peak CH3
(23.86 ppm, (C1, C3), was conveniently assigned to the methyl car-
bon of the isopropyl group. The peak ACHA (34.17 ppm, (C2) is due
to tertiary carbon according to the chemical shift. In the aromatic
Yield, 2.24 g, 83%, m.p. 368–369 K. Elemental analyses for
C
16H16N2O2 (F.W. 268.12 g/mol): Calcld. (%): C, 71.61; H, 6.01; N,
10.45. Found (%): C, 71.39; H, 5.69; N, 10.33. IR (KBr, cmꢃ1):
3435 (AOH stretching), 3050 (aromatic), 2958–2866 (CAH stretch-
ing of methyl), 1654 (C@O stretching), 1577 (AN@NA). 1H NMR
i
region of the 13C NMR spectrum of compound pr-salH, the peaks
at d 118.52–152.52 ppm were assigned to the aromatic carbons.
The peaks were then tentatively assigned by matching the ob-
served and calculated chemical shifts. The spectroscopic data ob-
tained in this work agree well with the previous work [33].
In the infrared spectrum of the azo-aldehyde (Fig. 3S), a broad
band at 3435 cmꢃ1 is attributed to intramolecular hydrogen
bonded phenolic AOH group. The spectrum of the compound
showed an intense carbonyl band (AC@O) at 1654 cmꢃ1, which
corresponds to a highly conjugated system [34]. The carbonyl
group produces a strong signal in the infrared spectrum, generally
(CDCl3 as solvent,
c in ppm): 1.34 (d, 6H, CH3), 2.99 (s (septet),
1H, CH), 6.93 (d, 1H, CH (aromatic)), 7.39 (d, 2H, CH (aromatic)),
7.94 (d, 2H, CH (aromatic)), 8.19 (d, 1H, CH (aromatic)), 8.21 (s,
1H, CH (aromatic)), 10.28 (s, 1H, HC@O), 11.33(s, 1H, OH). 13C
NMR: 23.86 (C1, C3), 34.17 (C2), 118.52–152.52 (aromatic C),
163.53 (C-OH), 196.62 (C@O). ESI mass (m/z): 269(40%) [M + H]+,
323(100%) [M + Na + CH3OH]+, 537(75%) [2 M + H]+, 553 (75%)
[2 M + ꢁOH]+.
2.6. Synthesis of [Ni(ipr-sal)2]ꢁ5H2O
To a hot solution (0.2 g, 0.75 mmol) of 2-hydroxy-5-{(E)-[4-
(propan-2-yl)phenyl]diazenyl}benzaldehyde in (35 mL) MeOH, a
hot solution of Ni(CH3COO)2ꢁ4H2O (0.09 g, 0.373 mmol, in 25 mL
MeOH) was added slowly. The resulting mixture was stirred and
refluxed on a hot plate for 3 h, and then concentrated to half its ini-
tial volume. After cooling for the mixtures overnight, the solid
product is precipitated and separated, washed with cold MeOH
and Et2O and dried in air.
Yield, 0.65 g, 73%, m.p. 537–538 K. Elemental analyses for
C
32H40N4NiO9 (F.W. 683.37 g/mol): Calcld. (%): C, 56.24; H, 5.90;
N, 8.20. Found (%): C, 55.57; H, 5.985; N, 8.48. IR (KBr, cmꢃ1):
3559 (AOH stretching), 3050 (CAH aromatic), 2961–2866 (CAH
aliphatic), 1626 (C@O stretching), 1464 (AN@NA), 501 (Ni-O),
464 (Ni-O).
i
Fig. 2. The proposed structure of Ni(II) chelate of pr-salH azo-aldehyde.
3. Result and discussion
3.1. Synthesis and spectroscopic studies
The azo-aldehyde (ipr-salH) was synthesized from the solution of
4-isopropylaniline diazonium salt and salicylaldehyde using the
classical reaction for the preparation of azo compounds (Fig. 1).
i
The Ni(II) complex ½Nið pr-salÞ2ꢄ ꢁ 5H2O was prepared by the reaction
of two equivalents of ipr-salH and one equivalent of Ni(CH3COO)2ꢁ
4H2O in MeOH (Fig. 2). The compounds were characterized using
microanalyses and spectroscopic techniques. The microanalytical
results are in good agreement with formulation. Both the ligand
and its Ni(II) complex are stable at room temperature in the solid
Fig. 3. Crystal structure of ipr-salH with atom labeling (thermal ellipsoid 50%
probability), hydrogen bonding is shown as dashed lines.

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T. Eren et al. / Journal of Molecular Structure 1065–1066 (2014) 191–198
Fig. 4. Hydrogen bonded dimers, hydrogen bonds are shown as dash lines.
Fig. 5. Hydrogen bonding network in crystal packing, hydrogen bonding is shown as dashed lines.
around 1715 or 1695 cmꢃ1. However, a conjugated carbonyl will
produce a signal at a lower wavenumber as a result of electron
delocalization via resonance effects. Aldehydes generally exhibit
one or two signals (CAH stretching) between 2700 and
2850 cmꢃ1. Bands at 2958–2866 cmꢃ1 are assigned to asymmetric
and symmetric stretching vibrations of isopropyl group, respec-
tively. The azo-aldehyde compound, ipr-salH shows a band at
1481 cmꢃ1 could be assigned to the AN@NA azo chromophore
that the oxygen atom of the carbonyl group is coordinated to the
metal(II) ion. The bonding of the metal ion to the ligand through
phenolic and carbonyl oxygen atoms is further supported by the
presence of new bands in the 501 and 464 cmꢃ1 due to the
t(MAOphenoxy) and t(NiAOcarbonyl) vibrations, respectively.
The ESI mass spectrum of the title ligand showed signals at m/z
269 (40%), 323(100%) and 537(75%) assigned to [M + H]+,
[M + Na + CH3OH]+ and [2 M + H]+, respectively.
group. The azo-aldehyde showed a band at 1382 cmꢃ1 for
m(C@C)
of aromatic rings. The band at about 1261 cmꢃ1 is attributed to
3.2. X-ray structure of the ligand (ipr-salH)
i
t
(CAO)(phenolic) stretching in the compound pr-salH [35].
A perspective view of the ipr-salH azo-aldehyde is shown in Fig. 3.
The compound crystallizes in the non-centric orthorhombic Pbca
In the infrared spectrum of the Ni(II) complex (Fig. 4S), C@O
stretching shifted to lower wavenumbers (1626 cmꢃ1) denoting

T. Eren et al. / Journal of Molecular Structure 1065–1066 (2014) 191–198
195
Fig. 6.
pꢁ ꢁ ꢁp Interactions within the structure.
lazo-salicylaldehydes published. Additionally, each hydroxyl group
can also make hydrogen bonds with the aldehyde oxygen of the
neighboring molecule resulting in either chain or dimeric struc-
ture. Four different hydrogen bonding motifs, labeled type 1–4,
appear in this series (Fig. 7 and Table 3).
Table 3
Crystallographically characterized azo-salicylaldehydes.
The title compound (ipr-salH) exhibits the dimeric structure
(type 2). All of the 5-arylazo-salicyclaldehyde derivatives pub-
lished show simple intramolecular hydrogen bonding. 4-Ethyl
compound (3) shows the type 1 however the closely related 4-bro-
mo (4) and 4-chloro (5) compounds adopt the chain structure (type
3). 4-Methyl compound (2) was initially published by two separate
groups in monoclinic forms exhibiting dimeric structure (type 2),
however the same compound was later reported as an orthorhom-
bic form showing the chain structure (type 3). Both monoclinic and
orthorhombic forms of 4-methyl compound (2) exhibit similar
Compound no. R1
R2 R3
R4
Structure type References
1
2
H
H
H
H
H
H
H
Cl
H
H
H
F
H
H
H
H
H
H
H
Br
Cl
H
i-pr
Me
Et
Br
Cl
H
H
H
H
H
Type 2
Type 2,3
Type 1
Type 3
Type 3
Type 2
Type 1
Type 1
Type 1
Type 1
Type 1
This work
[33,37,38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
3
H
4
H
5
H
6
H
p p stacking (phenyl–phenyl) interactions. Each polymorph was
ꢃ
7
8
9
10
11
12
OMe
OMe
OMe
OMe
OMe
OMe
re-crystalised in different solvents (monoclinic form in DMSO,
orthorhombic form in MeOH). The reason why the same compound
shows two distinct crystal systems may be due to a small energy
gap between these two forms. Related 2-chloro analogue (6) shows
the dimeric structure (type 2).
[46]
[47]
H
H
OMe
H
H
n-butyl Type 4
[48]
Most of the 5-arylazo substituted 3-metoxysalicyclaldeyhde
derivatives (7–11) adopt the type 1 structure which only has an
intramolecular hydrogen bonding (OHꢁ ꢁ ꢁO). There is only one com-
pound which shows no intramolecular hydrogen bond (OHꢁ ꢁ ꢁO)
but involves in intermolecular hydrogen bonding with neighboring
molecules producing hydrogen bonded chain. This is rather unu-
sual phenomenon for salicyclaldehyde derivatives. The compound
(12) shows no intramolecular hydrogen bond (OHꢁ ꢁ ꢁO) but in-
volves in intermolecular hydrogen bonding (OHꢁ ꢁ ꢁO) with adjacent
molecules forming hydrogen bonded chain (type 4).
space group with unit cell parameters a = 11.2706(9), b = 8.3993(7),
c = 28.667(2) Å, V = 2713.7(4) Å3 and Z = 8. All the bond lengths and
angles are within the normal ranges. The aldehyde (C15@O2) and
azo (N1@N2) distances are 1.2257(15) and 1.2574(14) Å
respectively, which are in the range of C@O and N@N double bond
character. Aromatic rings (C4AC9) and (C10AC15) adopt the trans
configuration with regard to the azo double bond (AN@NA) with
the torsion angle C(7)AN(1)AN(2)AC(10) of 179.62(9)°. In the struc-
ture, dihedral angles between two aromatic rings (C4AC9 and
C10AC15) is 5.82(5)°.
3.3. Optimized structures of the azo aldehyde
There is a strong intra-molecular hydrogen bonding between
phenolic O1AH1 with aldehyde oxygen (O1AO2 = 2.6535(14) Å)
forming a S(6) hydrogen bonding motif. There is also a weaker in-
ter-molecular O1AH1ꢁ ꢁ ꢁO2⁄ [ꢃx, ꢃy, ꢃz + 1] hydrogen bonding
joining two molecules together generating a D22(4) hydrogen bond-
ing motif (Fig. 4). There are also CH(aldehydic)ꢁ ꢁ ꢁO interactions link-
ing the molecules into hydrogen bonding network (Fig. 5 and
Table 2) [36].
Fig. 8 shows the optimized structures of three possible tautom-
ers (1), (2) and (3). Calculated bond lengths and angles of tautom-
ers (1–3) were given in Tables 1S and 2S, respectively.
Compared with experimental data and calculated bond lengths/
angles of tautomer (1), it can be seen that B3LYP/6-311++G(d,p) le-
vel is the best level. As can be seen from Table 1S, the differences
average between experimental and calculated bond lengths of tau-
tomer (1) is 0.008 Å. According to the Table 2S, the differences
average between experimental and calculated bond angles of tau-
tomer (1) is 0.6°. These results show that there is a good agreement
with experimental and optimized structure of tautomer (1). The
structures of tautomers (2) and (3) were optimized with B3LYP/
6-311++G(d,p).
Hydrogen bonded dimers are linked by
p–p interactions within
the structure. There are two sets of stacking; first, aldehyde-
p–p
phenyl group (C11AC16) is stacked with the same section of neigh-
bor molecule (symmetry operation: ꢃx, 1 ꢃ y, 1 ꢃ z), C13AC15⁄ is
separated by 3.355 Å (Fig. 6); second, C8AC9 edge of one of
molecule is stacked with C14AC16 edge of the adjacent molecule
(symmetry operation: x, 1 + y, z).
It is informative to compare the structure of the title compound
with related 5-arylazo-salicylaldehyde derivatives reported in lit-
erature (Table 3). We have examined the structures of the
5-arylazo-salicylaldehydes located in CCDC. Strong intramolecular
hydrogen bond (OHꢁ ꢁ ꢁO) was observed for almost all of the 5-ary-
3.4. The stability of tautomers
Some quantum chemical parameters such as total energy
(ETotal) and Gibbs free energy (G°), were calculated to predict the
stability of tautomers in gas phase and given in Table 4.

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T. Eren et al. / Journal of Molecular Structure 1065–1066 (2014) 191–198
Fig. 7. Hydrogen bonding motifs reported for azo-salicyclaldeyhdes.
Fig. 8. Optimized structures of tautomers (1–3) with atom labeling.
ETotal ¼ EThermal þ ERotational þ EVibrational þ EElectronic
These parameters, which are ETotal and G°, were used to predict the
stability. According to these parameters, ranking of tautomer
stability should be as follow,
where ETotal, EThermal, ERotational, EVibrational and EElectronic are total en-
ergy, thermal energy, rotational energy, vibrational energy and elec-
tronic energy of tautomers, respectively. Total energy and Gibbs
free energy include the zero-point energy and thermal corrections.
ð1Þ > ð2Þ ꢆ ð3Þ ðaccording to the ETotal
ð1Þ > ð3Þ ꢆ ð2Þ ðaccording to the GꢇÞ
Þ

T. Eren et al. / Journal of Molecular Structure 1065–1066 (2014) 191–198
197
LUMO:
HOMO
(1)
(2)
(3)
Tautomer
Tautomer
Tautomer
Fig. 9. Contour diagrams of frontier molecular orbitals for tautomers (1–3).
Fig. 10. The relative energy profile of tautomers (1–3), TS1 and TS2.
The tautomer with the lowest energy has the most stability. Accord-
ing to these rankings, the most stable tautomer is tautomer (1). The
stabilities of tautomers (2) and (3) 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. 9.
Table 4
Quantum chemical parameters of tautomers (1–3).
Tautomer (1)
Tautomer (2)
Tautomer (3)
ETotal (kJ molꢃ1
G° (kJ molꢃ1
)
ꢃ2308329.6
ꢃ2308502.7
ꢃ2308212.0
ꢃ2308387.8
ꢃ2308211.8
ꢃ2308388.4
)
HOMO is localized on (N@N) bond while LUMO is mainly
delocalized on all atoms of molecules for tautomers (1) and (2).
It can be said that the reactive regions of tautomers (1) and (2)
are on N atoms. For tautomer (3), HOMO and LUMO are mainly
delocalized on all atoms of molecule.
and conversion between tautomers 1 and 2 is more than conver-
sion between tautomers 2 and 3.
3.6. UV.-Vis. absorption spectra
3.5. Transition states
The UV.-Vis. absorption spectra of the azo aldehyde (ipr-salH)
and its Ni(II) chelate were investigated 200–800 nm range in
different solvents (2 ꢂ 10ꢃ5 M CHCl3, MeOH and DMF). The absorp-
tion spectra of the compounds in different solvents are shown in
Fig. 5S and the corresponding absorption wavelength maxima,
absorbance and solvent properties (dielectric constant and dipole
moment) are given in Table 5. The azo aldeyhde (ipr-salH) gave a
maximum absorbance band (k1max) at 339 nm (CHCl3), 349 nm
(MeOH) and 408 nm (DMF). These absorption bands were assigned
The transformations between tautomers (1–3) were investi-
gated by using transition state method. The transition states (TS)
were calculated to explain the transformations. Imaginary frequen-
cies (IF) indicate that the calculated structures are transition states.
The total energies of tautomers (1–3), TS1 and TS2 were calculated.
The total energy profile of them was represented in Fig. 10.
As can be seen from Fig. 10, tautomer (1) has the lowest energy.
IF of TS1 and TS2 was found as ꢃ632.39 and ꢃ161.09 cmꢃ1, respec-
tively. The conversion from tautomer 1 and 2, proton transfers
from O1 to O2. For TS2, the structure rotates around C14AC16.
There is a two proton transfer steps. The imaginary frequency of
TS1 is higher than TS2. It indicates that TS1 is more near to reality
to
p ?
p⁄ transitions of the aromatic rings. As the polarity of the
solvent increases, maximum absorption wavelength shifted to
longer values (batochromic effect). In CHCl3 and MeOH, there is
also absorption as shoulder (k2max) at 419 and 421 nm, respec-
tively, was assigned to n ? p⁄ transition. In DMF which has higher

198
T. Eren et al. / Journal of Molecular Structure 1065–1066 (2014) 191–198
Table 5
The UV–Vis. data of the azo-aldehyde and its Ni(II) chelate in various solvents (kmax, nm/cmꢃ1, absorbances,
e
values).
Solvent
CHCl3
Dielectric constant
4.8
Dipole moment
1.04
Compound
k1max (nm/cmꢃ1) (abs.,
e
ꢂ 103)
k2max (nm/cmꢃ1) (abs.,
e
ꢂ 103)
ipr-salH
339/29,499 (0.241, 12.05)
349/23,202 (0.283, 14.15)
350/28,571 (0.271, 13.55)
359/27,855 (0.291, 14.55)
408/24,510 (0.217, 10.85)
408/24,510 (0.276, 13.8)
431/23,202 (0.047, 2.35)
419/23,866 (0.098, 4.9)
417/23,981 (0.092, 4.6)
421/23,753 (0.104, 5.2)
452/22,124 (0.238, 11.9)
[Ni(ipr-sal)2]ꢁ5H2O
MeOH
DMF
33
38
1.70
3.82
ipr-salH
[Ni(ipr-sal)2]ꢁ5H2O
ipr-salH
[Ni(ipr-sal)2]ꢁ5H2O
446/22,422 (0.288, 14.40)
kmax: Maximum absorption wavelength; abs.: maximum absorption intensity;
e
: molar absorptivity coefficient (L molꢃ1 cmꢃ1); sample concentration was 2.0 ꢂ 10ꢃ5 mol Lꢃ1
.
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In the UV–Vis. absorption spectra of the complex [Ni(ipr-sal)2]-
ꢁ5H2O, maximum absorption wavelengths shifted to longer values
(batochromic effect) compared to the azo-aldehyde ligand indicat-
ing the complex formation. Absorption intensities also shifted to
higher values compared to the ligand (hyperchromic effect). In all
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4. Conclusions
In conclusion,
p ?
p⁄ (k1max) and n ? p⁄ (k2max) transitions.
a
novel azo-salicyclaldeyhde compound,
2-hydroxy-5-{(E)-[4-(propan-2-yl)phenyl]diazenyl}benzaldehyde,
ipr-salH and its Ni(II) chelate were prepared and characterized by
analytic and spectroscopic tecniques. X-ray structure of the ligand
indicated that there is a strong intramolecular hydrogen bonding
(OHꢁ ꢁ ꢁO) in the structure. There is also inter-molecular phenol-
aldeyhde (OHꢁ ꢁ ꢁO) hydrogen bondings resulting a dimeric struc-
ture and stabilizing the structure. Optimized structures of the three
i
possible tautomers of the pr-salH compound were obtained using
B3LYP method with 6-311++G(d,p), 6-31G and 3-21G basis sets in
the gas phase and B3LYP/6-311++G(d,p) level is found to be the
best level for calculation. In the UV–Vis. absorption spectra of
the complex [Ni(ipr-sal)2].5H2O, maximum absorption wavelengths
shifted to longer values (batochromic effect) compared to the
azo-aldehyde ligand indicating the complex formation.
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Acknowledgements
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The authors thank to Research Found of Kahramanmaras Sutcu
Imam University for financial support (Project No.: 2011/8-5 YLS).
The authors are grateful to the Department of Chemistry, Lough-
borough University for providing X-ray laboratory for data
collection.
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CCDC 947938 (ipr-salH) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, by e-mailing da-
ta_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crys-
tallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK.
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article can be found, in the online version, at http://dx.doi.org/
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