On the structures of 5-(4-, 3- and 2-methoxyphenylazo)-3-cyano-1-ethyl-6- hydroxy-4-methyl-2-pyridone: An experimental and theoretical study

Article abstract of DOI:10.1016/j.dyepig.2014.01.007

In this work, a combined experimental and theoretical study on the structures of methoxy substituted 5-phenylazo-3-cyano-1-ethyl-6-hydroxy-4- methyl-2-pyridones has been reported. The dyes under the investigation have been thoroughly characterized. X-ray single-crystal analysis shows that 5-(4-methoxyphenylazo)-3-cyano-1-ethyl-6-hydroxy-4-methyl-2-pyridone crystallizes in the hydrazone form. Quantum chemical calculations of energies, geometrical structure and vibrational wavenumbers of the investigated dyes have been performed using density functional theory. The optimized geometrical parameters obtained by density functional theory calculations are in good conformity with the single-crystal data. The fundamental vibrational wavenumbers, as well as their intensities have been calculated and a good agreement between observed and scaled calculated wavenumbers has been achieved. Stability of the molecule arising from hyperconjugative interactions and charge delocalization has been analyzed using natural bond orbital analysis. Vibrational, nuclear magnetic resonance and natural bond orbital analysis confirm that the prepared dyes exist in the hydrazone tautomeric form in the solid state and dimethyl sulfoxide solution.

Full text of DOI:10.1016/j.dyepig.2014.01.007

Dyes and Pigments 104 (2014) 160e168
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
On the structures of 5-(4-, 3- and 2-methoxyphenylazo)-3-cyano-1-
ethyl-6-hydroxy-4-methyl-2-pyridone: An experimental and
theoretical study
a
a
b
b
ꢀ a
ꢁ
Jelena Mirkovic , Jelena Rogan , Dejan Poleti , Vesna Vitnik , Zeljko Vitnik ,
a,
ꢀ a
ꢁ
*
ꢁꢀ
Gordana Uscumlic , Dusan Mijin
a Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11120 Belgrade, Serbia
b
ꢁ
Department of Chemistry, IChTM e Institute of Chemistry, Technology and Metallurgy, University of Belgrade, Njegoseva 12, 11000 Belgrade, Serbia
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, a combined experimental and theoretical study on the structures of methoxy substituted 5-
phenylazo-3-cyano-1-ethyl-6-hydroxy-4-methyl-2-pyridones has been reported. The dyes under the
investigation have been thoroughly characterized. X-ray single-crystal analysis shows that 5-(4-
methoxyphenylazo)-3-cyano-1-ethyl-6-hydroxy-4-methyl-2-pyridone crystallizes in the hydrazone
form. Quantum chemical calculations of energies, geometrical structure and vibrational wavenumbers of
the investigated dyes have been performed using density functional theory. The optimized geometrical
parameters obtained by density functional theory calculations are in good conformity with the single-
crystal data. The fundamental vibrational wavenumbers, as well as their intensities have been calcu-
lated and a good agreement between observed and scaled calculated wavenumbers has been achieved.
Stability of the molecule arising from hyperconjugative interactions and charge delocalization has been
analyzed using natural bond orbital analysis. Vibrational, nuclear magnetic resonance and natural bond
orbital analysis confirm that the prepared dyes exist in the hydrazone tautomeric form in the solid state
and dimethyl sulfoxide solution.
Received 25 October 2013
Received in revised form
30 December 2013
Accepted 2 January 2014
Available online 14 January 2014
Keywords:
Pyridone
Azo dye
Azo-hydrazone tautomerism
X-ray crystallography
DFT calculation
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
and different approaches regarding this topic can be found in the
literature [6e9]. Arylazo pyridone dyes under investigation in this
Azo dyes represent a large group of colored organic compounds
and have been the subject of substantial interest due to their
physicochemical features and widespread commercial application.
Besides their traditional role in textile dyeing, cosmetics, leather
and food industries [1], azo dyes have been recently used in bio-
logical and medical studies [2], optical recording [3], dye-sensitized
solar cells [4] and high technology products and innovations [5].
Disperse dyes having pyridone as a coupling component have
gained a great significance in the dyestuff industry as a result of
their excellent coloration features, simplicity of preparation, good
light and wash fastness properties [1].
paper contain hydroxy group in the ortho position to the azo bridge
enabling intramolecular proton transfer and, thus, possibility for
existence of azo and hydrazone tautomers (Scheme 1). Numerous
investigations have been conducted regarding the tautomeric
structure of azo pyridone dyes in the solid state and solutions using
a variety of spectroscopic methods. The spectral data generally lead
to the conclusion that the tautomeric equilibrium of azo pyridone
dyes is in favor of the hydrazone tautomeric form in the solid state
and, depending on the dye structure and used solvent, the equi-
librium is predominantly in the favor of the hydrazone form in
different solvents [10e12].
Azo-hydrazone tautomerism occurs in azo dyes bearing groups
with labile proton conjugated with azo linkage. Because the tau-
tomers differ in physical and optical properties, their functional and
structural characterization has been of considerable controversy,
The molecular-level structure and supramolecular interactions
in the solid state of azo dyes can be investigated employing single-
crystal X-ray diffraction technique. This method is useful for
investigation of azo dye tautomerism, because it provides struc-
tural characterization of individual tautomers existing in the solid
state and, moreover, reveals molecular packing modes. The detailed
knowledge of the crystal structure of tautomers is crucial for un-
derstanding the difference in their physicochemical behavior. On
* Corresponding author. Tel.: þ381 11 3303671; fax: þ381 11 3370387.
E-mail address: kavur@tmf.bg.ac.rs (D. Mijin).
0143-7208/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2014.01.007

ꢀ
J. Mirkovic et al. / Dyes and Pigments 104 (2014) 160e168
161
4-methyl-2-pyridone following the modified procedure from the
literature [20]. N-ethyl cyanoacetamide was obtained from ethyl
cyanoacetate and ethylamine [21] and was further used in the re-
action with ethyl acetoacetate for preparation of 3-cyano-1-ethyl-
6-hydroxy-4-methyl-2-pyridone [22]. The azo colorants under the
study were characterized by melting point, FT-IR, UVeVis, 1H NMR,
13C NMR spectra, elemental analysis and ESI-MS spectra.
Scheme 1. Azo-hydrazone tautomerism in studied arylazo pyridone dyes, X ¼ 4-OMe
(1), 3-OMe (2) and 2-OMe (3).
2.2.1. 5-(4-Methoxyphenylazo)-3-cyano-1-ethyl-6-hydroxy-4-
methyl-2-pyridone (1)
the other hand, quantum chemical calculations are suitable for
investigation of geometry and the relative stabilities of distinct
tautomers, as well as the charge transfer through the molecule and
the possibility for the intramolecular hydrogen bonds. DFT calcu-
lations are often combined with X-ray crystallography because they
complement each other and provide a better insight into effects
controlling the predominance of one tautomer over the other [13e
15].
4-Anisidine (1.23 g, 10.0 mmol) was dissolved in concentrated
hydrochloric acid (2.5 mL) and cooled to ꢁ5 ꢀC. Sodium nitrite
(0.76 g, 11.0 mmol) was dissolved in cold water (4 mL) and added
dropwise in the solution of 4-anisidine. The mixture was stirred 1 h
and diazonium salt was obtained. 3-Cyano-1-ethyl-6-hydroxy-4-
methyl-2-pyridone (1.78 g, 10 mmol) was dissolved in the potas-
sium hydroxide (0.56 g, 10.0 mmol) water (4 mL) solution and
cooled to ꢁ5 ꢀC. Diazonium salt was added dropwise to a vigorously
stirred pyridone solution for 0.5 h and the mixture was additionally
stirred for 3 h. The temperature of the reaction mixture was
maintained at 0e5 ꢀC. The red solid was filtrated, washed with
water and dried. The crude product was recrystallized from chlo-
roform. Red crystalline solid; yield: 2.28 g (73%); m.p. 215.6e
Despite the commercial significance of pyridone based dyes,
only several papers concerning their crystal structures have been
published so far [16e19]. A general characteristic of these dyes is
crystallization in the hydrazone form. Hydrogen bonding and pep
stacking interactions are further responsible for the formation of
layered structures. Different dihedral angles between planes of
pyridone and phenyl rings, as well as different packing arrange-
ments are observed with a subtle change of functional groups in the
phenyl ring [17]. It is also found that alteration of N-alkyl chains in
pyridone ring results in dissimilar crystal packings [19].
In this study, three dyes with a methoxy group in different po-
sitions (o-, m- and p-) of phenyl moiety and the same 3-cyano-1-
ethyl-6-hydroxy-4-methyl-2-pyridone coupling component have
been synthesized (Scheme 1). Single-crystal structure of 5-(4-
methoxyphenylazo)-3-cyano-1-ethyl-6-hydroxy-4-methyl-2-
pyridone (1) has been reported. Quantum chemical calculations of
energies have been carried out in order to compare the energies of
the single-crystals and energy-minimized structures, as well as
energies of azo and hydrazone tautomers. In addition, the harmonic
vibrational frequencies, 1H and 13C chemical shifts have been
calculated in order to be compared with experimental data.
217.2 ꢀC; FT-IR (KBr, /cmꢁ1): 3433 (NH on hydrazone form), 2223
n
(CN), 1672, 1627 (C]O on heterocyclic); 1H NMR (200 MHz, DMSO-
d6): 1.13 (3H, t, J ¼ 7.0 Hz, CH3eCH2), 2.50 (3H, s, CH3), 3.80 (3H, s,
OCH3), 3.89 (2H, q, J ¼ 7.0 Hz, CH3eCH2), 7.06 (2H, d, J ¼ 9.0 Hz, Are
H), 7.71 (2H, d, J ¼ 9.0 Hz, AreH), 14.81 (1H, s, NH hydrazone); 13
C
NMR (50 MHz, DMSO-d6): 161.8 (Py), 161.2 (Py), 160.6 (Py), 158.9
(Ar), 134.8 (Ar), 122.9 (Py), 119.5 (Ar), 115.4 (Ar), 115.3 (CN), 99.6
(Py), 55.8 (OCH3), 34.5 (CH3CH2), 16.6 (CH3), 12.9 (CH3CH2); Anal.
Calcd for C16H16N4O3 (312.33): C, 61.53; H, 5.16; N, 17.94. Found: C,
61.39; H, 5.10; N, 17.82; ESI-MS (negative): m/z 311.2 [M-H]ꢁ, Uve
Vis (EtOH) (lmax/nm (log ε)): 460.0 (4.19), 273.5 (3.62).
2.2.2. 5-(3-Methoxyphenylazo)-3-cyano-1-ethyl-6-hydroxy-4-
methyl-2-pyridone (2) and 5-(2-methoxyphenylazo)-3-cyano-1-
ethyl-6-hydroxy-4-methyl-2-pyridone (3)
The synthesis of the compounds 2 and 3 were the same as for
compound 1 using corresponding anisidine. Compound 2: Dark
orange powder; yield: 2.09 g (67%); m.p. 211.0e211.5 ꢀC; FT-IR (KBr,
2. Experimental part
n
/cmꢁ1): 3434 (NH on hydrazone form), 2223 (CN), 1676, 1628 (C]
O on heterocyclic); 1H NMR (200 MHz, DMSO-d6): 1.14 (3H, t,
J ¼ 7.1 Hz, CH3eCH2), 2.50 (3H, s, CH3), 3.81 (3H, s, OCH3), 3.88 (2H,
q, J ¼ 7.5 Hz, CH3eCH2), 6.87 (1H, d, J ¼ 8.4 Hz, AreH), 7.27e7.44
(3H, m, AreH), 14.54 (1H, s, NH hydrazone); 13C NMR (50 MHz,
DMSO-d6): 161.0 (Py), 160.6 (Py), 160.1 (Py), 159.4 (Ar), 142.7 (Ar),
131.0 (Py), 123.3 (Ar), 115.3 (CN), 113.0 (Ar), 109.9 (Ar), 103.2 (Ar),
101.1 (Py), 55.6 (OCH3), 34.6 (CH3CH2), 16.7 (CH3), 12.9 (CH3CH2);
Anal. Calcd for C16H16N4O3 (312.33): C, 61.53; H, 5.16; N, 17.94.
Found: C, 61.64; H, 5.08; N, 17.86; ESI-MS (negative): m/z 311.3 [M-
H]ꢁ, UveVis (EtOH) (lmax/nm (log ε)): 436.0 (4.37), 276.0 (3.85).
Compound 3: Orange-red powder; yield: 1.81 g (58%); m.p.
2.1. Materials and measurement
All reagents were purchased from either Aldrich, Fluka and
Merck and were used without any further purification. The melting
points were determined in capillary tubes on an automated melting
point system Stuart SMP30. The IR spectra were acquired using a
Bomem MB-Series 100 Fourier Transformation-infrared (FT-IR)
spectrophotometer in the form of KBr pellets. The 13C and 1H
spectra were taken on a Varian Gemini 2000 (50 Hz and 200 Hz
respectively) in deuterated dimethyl sulfoxide (DMSO-d6) with
tetramethylsilane (TMS) as an internal standard. All spectral mea-
surements were carried out at room temperature (25 ꢀC). Electro-
spray ionization mass spectra (ESI-MS) were recorded on an LCQ
Advantage ion trap in acetonitrile. Elemental analysis was per-
formed using a Vario EL III elemental analyzer. The ultraviolete
visible (UVeVis) absorption spectra were recorded on a Schimadzu
1700 spectrophotometer in the region 200e700 nm.
271.1e273.3 ꢀC; FT-IR (KBr, /cmꢁ1): 3441 (NH on hydrazone form),
n
2223 (CN), 1676, 1624 (C]O on heterocyclic); 1H NMR (200 MHz,
DMSO-d6): 1.13 (3H, t, J ¼ 7.0 Hz, CH3eCH2), 2.51 (3H, s, CH3), 3.89
(2H, q, J ¼ 6.7 Hz, CH3eCH2), 3.99 (3H, s, OCH3), 7.12 (1H, t,
J ¼ 6.7 Hz, AreH), 7.26 (1H, t, J ¼ 6.7 Hz, AreH), 7.47 (1H, d,
J ¼ 7.8 Hz, AreH), 7.85 (1H, d, J ¼ 7.8 Hz, AreH), 14.95 (1H, s, NH
hydrazone); 13C NMR (50 MHz, DMSO-d6): 161.4 (Py), 160.9 (Py),
160.0 (Py), 149.0 (Ar), 142.3 (Ar), 131.0 (Ar), 128.2 (Py), 122.1 (Ar),
121.5 (Ar), 115.7 (CN), 112.6 (Ar), 101.0 (Py), 56.6 (OCH3), 34.5
(CH3CH2), 16.5 (CH3), 12.9 (CH3CH2); Anal. Calcd for C16H16N4O3
(312.33): C, 61.53; H, 5.16; N,17.94. Found: C, 61.42; H, 5.19; N,17.85;
2.2. Synthesis
All investigated arylazo pyridone dyes were synthesized from
the corresponding diazonium salt and 3-cyano-1-ethyl-6-hydroxy-

ꢀ
162
J. Mirkovic et al. / Dyes and Pigments 104 (2014) 160e168
ESI-MS (negative): m/z 311.3 [M-H]ꢁ, UveVis (EtOH) (lmax/nm
(log ε)): 454.5 (4.53), 277.5 (3.85).
Table 2
Selected bond distances and angles for compound 1.
ꢂ
Bond distances/A
C5eN1
C8eN2
C8eC9
C8eC12
1.411(3)
1.331(3)
1.429(4)
1.461(4)
1.354(4)
1.470(4)
C11eO2
C11eN4
C12eO3
C12eN4
N1eN2
1.212(3)
1.396(3)
1.230(3)
1.380(3)
1.294(3)
2.3. X-ray crystallography
Single crystals of compound 1 suitable for X-ray diffraction
measurement were grown from chloroform by slow evaporation in
a refrigerator for three weeks. Room temperature (20 ꢀC) single-
crystal X-ray diffraction data were collected on an Oxford Gemini
C9eC10
C10eC11
Bond angles/ꢀ
C5eN1eN2
120.0(2)
C8eN2eN1
121.2(2)
S diffractometer equipped with a sapphire 3 CCD detector, using
ꢂ
monochromatized Cu K
a
radiation (
l
¼ 1.5418 A). A multi-scan
correction for absorption was applied. The structure was solved
by direct methods (SIR97) [23] and refined on F2 by full-matrix
least-squares using the programs SHELXL97 [24] and WinGX [25].
All non-hydrogen atoms were refined anisotropically. Positions of H
atoms bonded to C atoms were calculated at ideal positions and
refined by the riding model with Uiso ¼ 1.5Ueq(C) for methyl, and
Uiso ¼ 1.2Ueq(C) for remaining H atoms. H2 atom connected to N1
activation energy [30], the B3LYP method was not able to locate 2-
hydroxy-6-pyridone isomers which exist as vary shallow minima
on the potential the energy surface. To resolve that problem DFT
M06 and M06-2X calculations with 6-311þþG(d,p) basis set are
carried out to compare the energy differences between single-point
energy of the single-crystal structure of 1 and energies of the fully
optimized structures for both pyridine-2,6-dione and 2-hydroxy-6-
pyridone isomers for all investigated compounds.
The harmonic frequencies were calculated by B3LYP method
using 6-311þþG(d,p) basis set and then scaled by 0.9668 [31]. The
assignments of the calculated wavenumbers were aided by the
animation option of Gauss View 5.0 graphical interface from
Gaussian programs, which gave a visual presentation of the shape
of the vibrational modes [32].
The nuclear magnetic resonance (NMR) chemical shifts calcu-
lations were performed using Gauge-Invariant Atomic Orbital
(GIAO) method [33] at B3LYP/6-311þþG(d,p) level with SMD model
[34] for simulation of solvent (DMSO). The 1H and 13C isotropic
chemical shifts were referenced to the corresponding values for
TMS, which were calculated at the same level of theory.
The natural bonding orbitals (NBO) calculations [35] were per-
formed using NBO 3.1 program as implemented in the Gaussian 09
[27] package at the B3LYP/6-311G(d,p) level in order to understand
various second order interactions between the filled and vacant
orbitals of the investigated molecule, which is a measure of the
intramolecular delocalization and hyperconjugation.
was found in DF map and freely refined with isotropic displacement
parameter. The figures were produced with Mercury [26]. Crystal
data and refinement results are listed in Table 1, while selected
bond distances and angles are given in Table 2.
2.4. Computational studies
All the density function theory (DFT) calculations were per-
formed using Gaussian 09 Revision D.01 program package [27] on
the B3LYP [28], M06 and M06-2X [29] level of theory with the
default convergence criteria and without any constraint on the
geometry.
In the present work all the possible isomers of 1 are generated
and fully optimized at the DFT B3LYP/6-311þþG(d,p) level. 24
stable isomers of 1 are obtained. Two most stable isomers, pyri-
dine-2,6-dione and 2-hydroxy-6-pyridone are used for further ge-
ometry and energy studies. The harmonic vibrational frequencies
were derived from the numerical values of these second derivatives
and used to obtain the Gibbs energy contributions at 298.15 K and
standard pressure. Structural parameters (bond lengths) were
extracted directly from the data files following the geometry opti-
mizations. Because of well known lack to underestimate the
3. Results and discussion
3.1. Synthesis
Table 1
Crystal data and structure refinement for compound 1.
Azo dyes 1e3 having the same pyridone skeleton are prepared
via classical coupling reactions between 3-cyano-1-ethyl-6-
hydroxy-4-methyl-2-pyridone and diazonium salts of the corre-
sponding anisidine. Coupling reactions are performed under the
alkaline conditions and pure azo pyridone dyes are obtained after
recrystallization in a good yield. Dye 1 has been reported earlier in
order to study azo-hydrazone tautomerism [36], but without
analytical and experimental data. Dyes 2 and 3 have not been
registered in the literature so far. The FT-IR spectra of the com-
pounds clearly show the existence of the hydrazone form in the
solid state. This is concluded on the basis of the two intense
carbonyl bands in each spectrum appearing within the region
1627e1676 cmꢁ1. The vibrations in the region 3431e3440 cmꢁ1 are
assigned to the imino group of hydrazone unit. The aromatic CeH
Empirical formula
M/g molꢁ1
C16H16N4O3
312.33
Crystal system
Space group
Orthorhombic
P212121
7.5800(8)
10.1990(8)
19.753(2)
90
90
90
ꢂ
a/A
ꢂ
b/A
ꢂ
c/A
ꢀ
ꢀ
a
/
b
/
ꢀ
g
/
3
ꢂ
V/A
1527.1(2)
Z
4
rc/g cmꢁ3
1.359
0.800
m
/mmꢁ1
F(000)
q
656
4.48e70.06
range for data collection/ꢀ
stretching vibrations are observed in the region 2919e3071 cmꢁ1
,
Range of h, k, l
ꢁ8 ꢂ h ꢂ 9, ꢁ12 ꢂ k ꢂ 9, ꢁ20 ꢂ l ꢂ 23
Reflections collected
Independent reflections (Rint
Data/restraints/parameters
S
3777
while the aliphatic CeH stretching vibrations are found between
2850 and 2980 cmꢁ1. The band at 2223 cmꢁ1 in FT-IR is assigned to
C^N stretching vibrations. The data obtained from NMR spectra
also strongly suggest the existence of the hydrazone tautomer of all
three dyes in DMSO-d6 solution. The 1H NMR shifts assigned to the
NeH proton of the hydrazone form are within the range 14.54e
14.95 ppm. This is in accordance with chemical shifts of the imino
)
2511(0.0216)
2511/0/212
1.044
R indices [I > 2
R indices (all data)
s
(I)]
R1 ¼ 0.0464, wR2 ¼ 0.1217
R1 ¼ 0.0558, wR2 ¼ 0.1309
0.0(4)
0.246, ꢁ0.196
Flack parameter
ꢂꢁ3
Drmax
,
Drmin/e A

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J. Mirkovic et al. / Dyes and Pigments 104 (2014) 160e168
163
group in DMSO-d6 for analogous pyridone azo dyes reported by
Peng et al. ( 14.30e16.09) [37] and Alimmari et al. ( 14.27e14.93)
d
d
[38]. Comparing the 1H NMR spectra of the compounds 1e3, one
can notice that the chemical shift of NH group for o-substituted dye
is slightly larger than for m- and p-substituted compounds. This is
the reflection of intramolecular hydrogen bond formed between
the hydrazone hydrogen atom and the oxygen atom of methoxy
group in ortho position to the hydrazone unit. In the present study,
signals for two carbons of carbonyl groups, expected in hydrazone
form, are observed at 160e160.6 and 160.6e161.2 ppm, respec-
tively, in 13C NMR spectra for the molecules 1e3. Peng et al. [37]
have reported that chemical shifts of the phenyl carbon atoms
connected to the azo group in azo and hydrazone form of the
arylazo pyridone dyes differ for w14 ppm. It is, also, reported that
these shifts for hydrazone form are the range 134.1e140.7, which
match with shifts of the same carbon atoms of 1e3, proving the
presence of hydrazone form. The signals at 122.9e131.0 ppm are
assigned to carbons of hydrazone C]N group and agree with
literature data.
Fig. 2. ORTEP-like representation of compound 1 with the atomic-numbering scheme.
Displacement ellipsoids are drawn at the 50% probability level and the H atoms are
shown as small spheres of arbitrary radii. The intramolecular hydrogen bond is shown
as a dashed line.
the C1 atom from the methoxy group. The dihedral angle between
benzene (C2eC7) and the pyridone (N4, C8eC12) ring is only 3.3ꢀ.
The investigated dye 1 crystallizes in the hydrazone form. This
can be deduced from the position of the hydrogen atom on N1
UVeVis absorption spectra of the dyes 1e3 in ethanol are given
in Fig. 1. Strong absorption bands at 460.0, 436.0 and 454.5 nm for
1e3, respectively, are ascribed to
idone ring and phenyl ring in hydrazone form. Weak bands
appearing at 273.5e277.5 nm correspond to ne * transitions be-
pep* transitions between pyr-
ꢂ
rather than O3 and from the bond lengths of C12eO3 (1.230 (3) A)
p
ꢂ
and C8eN2 (1.331 (3) A) exhibiting a double bond character.
tween the phenyl ring and middle hydrazone unit [19]. Methoxy
group exerts positive mesomeric effect in para and ortho positions
and causes a batochromic shift of the longest wavelength when
compared with meta substituted dye where only positive inductive
effect is present. Ortho substituted dye show a small hypsochromic
shift with respect to para substituted dye due to the steric effect
preventing pyridone and phenyl rings lying in the same plane and,
thus, inhibiting effective delocalization.
Another indicator supporting the hydrazone tautomer of 1 is a
ꢂ
noticeable difference between N1eC5 (1.411 (3) A) and N2eC8
ꢂ
(1.331 (3) (A)) bond lengths expected for this tautomer. The N1eN2
ꢂ
bond amounts 1.294 (3) A, which is shorter than NeN single bond
but not short enough to be regarded as a double bond, indicating
extended delocalization of electron density from electron donating
methoxy group to pyridone ring through the hydrazone unit. This
may be one of the reasons for planarity of the molecule.
Comparing the averaged bond lengths of the compound 1 with
the bond distances of the compounds having the same pyridone
backbone described in literature (4e6, Scheme 2) the significant
difference is noticed for the C5eN1eN2eC8 fragment [17,19]. Due
to the presence of nitro group in ortho position in the phenyl moiety
of these compounds, shortening of N1eN2 bond and elongations of
the C5eN1 and N2eC8 bonds are observed for the compound 1. A
possible explanation may be the withdrawing nature of the nitro
group having the opposite effect to the charge transfer present in
the investigated compound 1, as well as the presence of an addi-
tional intramolecular hydrogen bond formed between nitro group
and NH group. The steric hindrance of nitro group in C.I. Disperse
Yellow 119 (4) is responsible for the noticeable different dihedral
angle between pyridone and phenyl rings (16.9ꢀ) with respect to
the investigated compound 1, as expected.
3.2. Crystal structure of azo dye 1
Compound 1 crystallizes in orthorhombic space group P212121,
with 4 molecules per unit cell. The molecular structure of 1 with the
atomic-numbering scheme is shown in Fig. 2. The main structural
feature of 1 is its planarity, with the expected exception of terminal
CH3 group from the ethyl radical. The maximal deviation from
ꢂ
least-squares plane through skeletal C, N and O atoms is 0.207 A for
According to the observed hydrazone form, an intramolecular
and short NeH/O hydrogen bond of the S(6) type, with N$$$O
ꢂ
distance of 2.602(3) A is noticed (Table 3). This hydrogen bond
completes pseudo six-membered ring whose presence may be
another reason for the planarity of the molecule. Some authors
suggest that this intramolecular hydrogen bond contributes to
stabilization of the hydrazone form in the solid state [17,18].
In the solid state, molecules are held together by a complex
system of hydrogen bonds,
pep and van der Waals interactions
making first a 2D layered structure and then a 3D framework. On
one hand, there is one series of corrupted, herringbone-like layers
approximately parallel to the bc-plane (Fig. 3(a)). Inside the indi-
vidual segments of this layer all molecules are oriented in the same
way and connected by CeH/N/O hydrogen bonding interactions
(Fig. 3(b)). The following short contacts: C1eH1A/O3, C3eH3/O2
and C6eH6/N3 (Table 3) are found in these layers. On the other
hand, when the packing is projected onto bc-plane another set of
Fig. 1. UVeVis spectra of the investigated compounds 1e3.

ꢀ
164
J. Mirkovic et al. / Dyes and Pigments 104 (2014) 160e168
Scheme 2. Molecular structures of the compounds having the same pyridone backbone described in the literature.
highly corrupted layers oriented perpendicular to the c-axis, with
partially overlapped aromatic rings is visible (Fig. 4(a)). Among
these layers possible hydrogen bonding contacts involve two out of
three methyl groups: C1eH1B/O2, C1eH1C/O2 and C13e
H13C/N1 (Table 3). Practically all short contacts around C1H3
methyl group are concentrated in one hemisphere explaining why,
as described above, C1 atom deviates from the mean-plane of the
skeleton atoms. Within these layers the molecules are stacked
energy gap between the single-crystal and energy-minimized
structures in the same hydrazone form (157.20 and
158.05 kcal molꢁ1 for 1), which may be the reflection of the
energy compensation for the formation of hydrogen bonding and
pep stacking between molecules. These results are according to
the previously reported results for compounds 4 and 5 (Scheme
2) [17]. The results also indicate that the pyridine-2,6-dione
isomer is more thermally stable than the 2-hydroxy-6-pyridone
together by parallel-displaced
pep
interactions, with details
isomer for 1 with the energy gaps of 8.68 and 6.21 kcal molꢁ1
,
shown in Fig. 4(b). The dihedral angle between mean-planes of the
which may be regarded as theoretical support for the crystalli-
zation in the hydrazone form for 1. Results for compounds 2 and
3 show an additional increase of the energy gap (8.27 and 7.11
and 15.84 and 7.61 kcal molꢁ1) between pyridine-2,6-dione and
2-hydroxy-6-pyridone isomers. The largest energy gap observed
and difference in the energy of two conformers with different
orientation of o-methoxyphenyl group around PheN bond of 6.61
and 5.84 kcal molꢁ1 (correspondingly to the used DFT models) for
ortho substituted dye may be attributed to an additional stabili-
zation of the hydrazone form through formation of the intra-
molecular hydrogen bond between methoxy and imino group,
which is not present in m- and p-substituted dyes.
All shown results are consistent with the aforementioned ten-
dency between the energy-minimized pyridine-2,6-dione and 2-
hydroxy-6-pyridone isomers. In summary, both, the geometry
and energy studies, demonstrate the existence of the hydrazone
form for this family of azo dyes in the solid state [16e18].
benzene and pyridone ring from adjacent molecules is 10.6ꢀ. As a
ꢂ
consequence, C/C distances vary from 3.532(4) A for C10/C4 to
ꢂ
4.117(4) A for C12/C7 contacts. These values indicate moderate to
weak pep interactions.
3.3. Optimized geometry
In the present work all the possible isomers of 1 are generated
and fully optimized at DFT-B3LYP/6-311þþG(d,p). 24 stable iso-
mers (the four tautomers with corresponding number of rotational
conformers, Scheme 3) of 1 are obtained. The Gibbs energy and the
statistical Boltzmann distribution weighted values at 298.15 K and
standard pressure of all isomers are presented in Table S1. The
optimized geometries of all isomers are confirmed to be located at
the local true minima on the potential energy surface, as they do
not contain imaginary frequencies.
The results show that two most stable geometries belong to
pyridine-2,6-dione (I-a) and 2-hydroxy-6-pyridone (II-b) isomer
(Fig. 5). The obtained results are consistent with the previous report
concerning 5-arylazo-6-hydroxy-4-phenyl-3-cyano-2-pyridones
[38]. The Gibbs energies and statistical Boltzmann distribution
weights show that in the solid state only pyridine-2,6-dione (I-a)
isomer is present, whereas hydroxy-6-pyridone (II-b) could be
present in no more than 0.0002%, and all other isomers are un-
traceable. In accordance to that, in the further calculations only
these two isomers for all investigated compounds are used for the
geometry and energy studies.
The optimized geometries for the geometry studies of 1e3 have
been obtained using the B3LYP, M06 and M06-2X methods with 6-
311Gþþ(d,p) basis set (Table S2). All methods agree that the most
stable geometry belongs to pyridine-2,6-dione (I-a) isomer. The
most stable geometries of pyridine-2,6-dione isomers of com-
pounds 1e3 are shown in Figs. S1eS3 and calculated bond lengths
are listed in Table S3 along with the experimental data for 1. The
optimized bond lengths for compounds 1e3 are correlated with
data determined by X-ray for compound 1. The results show a very
good agreement between experimental and calculated bond
lengths of all heavy atoms for all used DFT methods (R2 > 0.985).
The relations for the bond lengths between calculated and exper-
imental values and the corresponding R2 and RMSD values are
presented in Table 4.
3.4. Vibrational and NMR analysis
The harmonic vibrational frequencies for the azo dye 1 have
been calculated by using DFT method at 6-311þþG(d,p) level. In
order to compare the theoretical results using the experimental
values, a scaling factor of 0.9668 is applied to all of the calculated
frequencies. Experimental and scaled theoretical spectra for the
qualitative comparison are shown in Fig. S4. Some of the char-
acteristic frequencies for the title compounds are given in
Table S4. The broad band at 3433 cmꢁ1 in FT-IR spectrum
assigned to NeH stretching vibration shows deviation of
273 cmꢁ1 with scaled frequency value at 3160 cmꢁ1 by DFT
method. This deviation may be due to the presence of
Table 3
Geometry of possible hydrogen bonds for compound 1.
DeH/A
d(DeH)/ d(H/A)/ d(D/A)/ :(DHA)/ꢀ Symmetry code
ꢂ
ꢂ
ꢂ
A
A
A
N1eH2/O3
C1eH1A/O3
C1eH1B/O2
0.97(4) 1.82(4) 2.602(3) 136(3)
0.96
0.96
2.656
2.763
3.461(4) 142
3.402(4) 125
ex þ 1/2, ey, z þ 1/2
ex þ 1/2, ey þ 1,
z þ 1/2
C1eH1C/O2
C3eH3/O2
C6eH6/N3
0.96
0.93
0.93
2.888
2.493
2.543
2.588
3.817(4) 163
3.420(3) 174
3.386(4) 151
3.530(4) 167
ex, y þ 1/2, ez þ 3/2
ex þ 1/2, ey, z þ 1/2
x, y þ 1,þz
The results for the Gibbs energies determined with M06 and
M06-2X methods summarized in Table 5, demonstrate a large
C13eH13C/N1 0.96
ex, y e 1/2, ez þ 3/2

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J. Mirkovic et al. / Dyes and Pigments 104 (2014) 160e168
165
Fig. 3. Crystal packing of compound 1: (a) space filling model viewed along b-axis; (b) projection showing intralayer CeH/N/O hydrogen bonding interactions as thin lines.
ꢂ
intramolecular hydrogen N1eH2/O3 bonding (1.767 A). While
the C]O and C]N stretching vibrations were seen at 1672,
1627 cmꢁ1 and 1398 cmꢁ1, respectively, the same bands were
calculated as 1676, 1612 cmꢁ1 and 1386 cmꢁ1, respectively, i.e.
there is a very good agreement between experimental and
calculated data. These stretching modes are also consistent with
those reported in the literature [39].
A powerful way to predict and interpret the structure of azo-
hydrazone tautomers is to combine computer simulation with
NMR methods. The experimental and theoretical values for 1H and
13C NMR of the 1e3 are given in Tables S5 and S6.
3.5. Natural bond orbital (NBO) analysis
NBO analysis has been performed on 1 in order to elucidate
intramolecular charge transfer (ICT), rehybridization and delo-
calization of electron density within the molecule. The impor-
tant interactions between occupied Lewis type (bonding or lone
pair) NBO orbitals and unoccupied (antibonding or Rydberg)
non Lewis NBO orbitals of investigated dye 1 are given in
Table S7.
The natural bond orbital analysis provides an efficient method
for studying intra and intermolecular bonding, and it also provides
a convenient basis for investigating charge transfer in molecular
systems. The larger the E(2) value, the more intensive is the inter-
action between electron donors and electron acceptors.
The intramolecular interactions for dye 1 are formed by the
orbital overlap between bonding NeN, CeC, CeN, CeH and C]N,
C]O antibonding orbitals which results in an ICT causing stabili-
zation of the system.
The 1H NMR spectra of dyes 1e3 measured in DMSO-d6 at
room temperature show a characteristic singlet for the CH3 group
at the 4-position at 2.5 ppm, with calculated chemical shift values
at 2.57e2.61. Two signals, triplet and quartet, at 1.13e1.14 and
3.88e3.99 ppm mark the ethyl hydrogens (Table S5). The chemical
shift values for ethyl hydrogen atoms (with respect to TMS) are
1.26e1.27 and 4.17e4.26 ppm in DMSO solution. The experimental
peaks of aromatic hydrogen atoms are found at 7.06e8.38 ppm
and correlate well with calculated chemical shifts. The hydrogen
atom of eNH group appears at chemical shift of 14.54e14.95 ppm,
with theoretical peaks at 14.56e14.76 ppm. These results suggest
that dyes 1e3 exist in the hydrazone tautomeric form in DMSO
solution.
The intramolecular hyperconjugative interaction of
s(N1e
N2) distributes to
s*(N1eC5), N2eC8, C8eC9 and C4eC5,
leading to stabilization of 0.9e2 kcal molꢁ1, as shown in
Table S7.
The
and C6eC7) lead to stabilization of 6.8e32.7 kcal molꢁ1, as shown
in Table S7. The same kind of interaction is calculated among (N2e
C8) with *(N1eC5, N2eC8, C9eC10 and C12eO3) with stabiliza-
tion of 6.7e18 kcal molꢁ1. This
* interaction shows that there
p / p* interactions of p(N1eC5) with p*(N2eC8, C3eC4
In the 13C NMR spectra for the dyes 1e3, the two carbon atoms
(C11 and C12) of the carbonyl groups resonate at 160e160.6 and
160.6e161.2 ppm, respectively, and show good agreement with
computed values at 168.66e169.07 and 168.97e169.76 ppm. The
carbon atom of C]N group has peak at 122.9e130.25 ppm with
respect to TMS in experimental 13C NMR spectra for dyes 1e3,
which matches well with the theoretically calculated chemical shift
at 129.32e131 ppm. These signals characteristic for carbonyl
groups and C]N group again suggest that dyes 1e3 exist in
hydrazone tautomeric form. The calculated chemical shifts in 13C
NMR spectra show excellent agreement with the experimental data
(Table S6).
p
p
p
/ p
is charge transfer through the molecule. It can be said that there is
an electronedonor interaction of OCH3 group to the molecule, LP(2)
O2 / LP(1)C2, as well as electroneacceptor interaction of CN group
through
19.61, respectively.
The interaction energy, related to the resonance in the molecule,
is electron withdrawing to the ring through *(N4eC11, C10eC11
p(C9eC10) / p*(N3eC14), with values of E(2) 57.1 and
s
and N4eC12, C8eC12) bond from the lone pairs LP(2)O2 and LP(2)
O3 which leads to moderate stabilization energy of 13e

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J. Mirkovic et al. / Dyes and Pigments 104 (2014) 160e168
166
Fig. 4. (a) Projection of compound 1 onto the bc-plane showing intra- and interlayer CeH/N/O hydrogen bonding interactions as thin lines. (b) Details of
pep stacking
interactions (Centroids, Cg, are shown as white spheres of arbitrary radii).
28 kcal molꢁ1 (Table S7). These interactions confirm the existence
of charge transfer in the 1. The most important interaction energies
4. Conclusion
of LP(1)N4 /
p
*(C11eO2) and LP(1)N4 /
p
*(C12eO3) are 47.37
Three methoxy substituted 5-phenylazo-3-cyano-1-ethyl-6-
hydroxy-4-methyl-2-pyridones have been synthesized and thor-
oughly characterized. According to the spectral data, dyes exist in
the hydrazone form in the solid state and in DMSO-d6. X-ray single-
crystal analysis of 5-(4-methoxyphenylazo)-3-cyano-1-ethyl-6-
hydroxy-4-methyl-2-pyridone revealed that this dye crystallizes
in hydrazone form. The main structural feature of the molecule is
planarity. In the solid state, molecules are held together by a
and 65.38 kcal molꢁ1, respectively.
The intramolecular NeH/O hydrogen bond is formed by the
orbital overlap between the lone pairs of electrons LP(O) and
s
*(NeH) which results in ICT causing stabilization of the H-
bonded systems. Therefore, hydrogen bonding interaction leads
to an increase in electron density (ED) of the NeH antibonding
orbital. The increase of the population of C]O antibonding
orbital weakens the C]O bond. NBO analysis of azo dye 1
makes clear evidence of the formation of a strong H-bond in-
complex system of hydrogen bonds,
pep and van der Waals in-
teractions creating parallel zigzag pseudo-chains along the b-axis.
DFT calculations of the compounds 1e3 show that the most stable
geometry belongs to the hydrazone form. The results also indicate
that the hydrazone form is more thermally stable than the azo
form. A large energy gap between the single-crystal and energy-
minimized structures in the same hydrazone form is observed for
the dye 1 (157.20 and 158.05 kcal molꢁ1 determined with M06 and
M06-2X methods, respectively) which is due to supramolecular
teractions between the LP(O) and
The stabilization energies E(2) associated with hyper-
conjugative interaction LP(1)O3 *(N1eH2), and LP(2)
O3 /
*(N1eH2) are obtained as 3.89 and 12.5 kcal molꢁ1
s*(NeH) antibonding orbital.
/
s
s
,
respectively (Table S7), which quantify the extend of intra-
molecular hydrogen bonding, causing additional stabilization of
the hydrazone tautomer.

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J. Mirkovic et al. / Dyes and Pigments 104 (2014) 160e168
167
Scheme 3. Possible tautomers of the investigated compound 1.
Fig. 5. The most stable geometries of 1 (hydrazone (I-a) and azo (II-b) tautomer).
Table 4
Correlation equations between calculated and experimental bond lengths.
Compound
B3LYP method
M06 method
M06-2X method
1
dcal. ¼ 0.9896dexp. þ 0.0202
(R2 ¼ 0.9883, RMSD ¼ 0.0098)
dcal. ¼ 0.9910dexp. þ 0.0188
(R2 ¼ 0.9892, RMSD ¼ 0.0094)
dcal. ¼ 0.9913dexp. þ 0.0186
(R2 ¼ 0.9878, RMSD ¼ 0.0101)
dcal. ¼ 0.9789dexp. þ 0.0283
(R2 ¼ 0.9869, RMSD ¼ 0.0103)
dcal. ¼ 0.9825dexp. þ 0.0234
(R2 ¼ 0.9865, RMSD ¼ 0.0105)
dcal. ¼ 0.9817dexp. þ 0.0246
(R2 ¼ 0.9860, RMSD ¼ 0.0107)
dcal. ¼ 1.0100dexp. ꢁ 0.0121
(R2 ¼ 0.9883, RMSD ¼ 0.0100)
dcal. ¼ 1.0143dexp. ꢁ 0.0179
(R2 ¼ 0.9878, RMSD ¼ 0.0103)
dcal. ¼ 1.0127dexp. ꢁ 0.0156
(R2 ¼ 0.9875, RMSD ¼ 0.0104)
2
3
Table 5
Calculated Gibbs energy (
DG in a.u.) and dipole moment (p in D) for the investigated compounds 1e3 on M06 and M06-2X (in brackets) models.
Structure
Compound
1
2
3
D
G
p
DG
p
DG
p
The single-crystal structure
in the hydrazone form
The energy-minimized structure
in the pyridine-2,6-dione form
The energy-minimized structure
in the 2-hydroxy-6-pyridone form
ꢁ1063.0712 (ꢁ1063.3555)
ꢁ1063.3217 (ꢁ1063.6073)
ꢁ1063.3079 (ꢁ1063.5974)
9.40
(9.09)
9.31
(10.05)
9.32
(10.50)
e
e
e
e
ꢁ1063.3225 (ꢁ1063.6088)
ꢁ1063.3093 (ꢁ1063.5974)
8.38
(6.97)
8.43
ꢁ1063.3231 (ꢁ1063.6085)
ꢁ1063.2984 (ꢁ1063.5964)
10.25
(10.08)
9.40
(7.27)
(10.54)

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J. Mirkovic et al. / Dyes and Pigments 104 (2014) 160e168
168
crystal structures and spectroscopic properties. Dyes Pigments 2011;92:916e
interactions in the solid state. The harmonic vibrational frequencies
and NMR calculations provide chemical shifts that are consistent
with experimental data. The NBO calculations confirm the exis-
tence of charge transfer in the molecule 1 and intramolecular
hydrogen bonding causing additional stabilization of the hydrazone
tautomer. It can be concluded that both experimental and theo-
retical data support the existence of the hydrazone tautomer of the
dyes 1e3 in the solid state and DMSO solution.
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Acknowledgment
This work has been financially supported by Ministry of Edu-
cation, Science and Technological Development, Republic of Serbia,
under Grants no. 172035, 172013, III45007 and HP-SEE European
project.
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Appendix B. Supplementary data
CCDC reference number 966964 for 1 at 291 K contains sup-
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