Crystal structure and theoretical studies of the keto-enol isomerism of N,N′-bis(salicylidene)-o-phenylenediamine (salophen)

Article abstract of DOI:10.1016/j.saa.2012.09.002

The Schiff base N,N′-bis(salicylidene)-o-phenylenediamine (salophen) was prepared by the condensation of salicylaldehyde with o-phenylenediamine in ethanol solution. The compound was characterized by elemental analysis, infrared (IR), ¹H,

Full text of DOI:10.1016/j.saa.2012.09.002

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 99 (2012) 110–115
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
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Crystal structure and theoretical studies of the keto-enol isomerism of
N,N0-bis(salicylidene)-o-phenylenediamine (salophen)
Vinicius Z. Mota a, Gustavo S.G. de Carvalho b, Pedro P. Corbi b, Fernando R.G. Bergamini b,
André L.B. Formiga b, Renata Diniz a, Maria C.R. Freitas a, Adilson D. da Silva a, Alexandre Cuin a,
⇑
a Chemistry Department, Exact Sciences Institute, 36036-330 Juiz de Fora, MG, Brazil
b Institute of Chemistry, University of Campinas, UNICAMP, 13083-970 Campinas, SP, Brazil
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
"
Synthesis and structural
characterization of salophen are
presented.
"
"
"
The compound crystallizes in the
monoclinic system with Z = 4.
Tautomerism of salophen was
studied by calculations using DFT.
Experimental and simulated IR data
support the presence of salophen 1
in the solid sample.
a r t i c l e i n f o
a b s t r a c t
The Schiff base N,N0-bis(salicylidene)-o-phenylenediamine (salophen) was prepared by the condensation
of salicylaldehyde with o-phenylenediamine in ethanol solution. The compound was characterized by
elemental analysis, infrared (IR), 1H, 13C and 1HA15N HMBC nuclear magnetic resonance (NMR) spectro-
scopic measurements, and also by X-ray diffraction. The tautomerism of salophen was also studied by
calculations using density functional theory (DFT). Two of the three tautomers were shown to coexist.
A comparison of the DFT data of the three tautomers has shown that the most stable one is salophen
1, which is in accordance with experimental X-ray crystallographic data.
Article history:
Received 5 June 2012
Received in revised form 22 August 2012
Accepted 4 September 2012
Available online 21 September 2012
Keywords:
Salophen
Tautomerism
DFT
Ó 2012 Elsevier B.V. All rights reserved.
Salicylaldehyde
o-Phenylenediamine
Introduction
hyde. This class of compounds has received a special attention due
to its simple method of preparation and its versatility of applica-
The Schiff bases (RHC@NR) are a class of organic compounds
typically formed by condensation of a primary amine and an alde-
tions [1,2]. In special, tetradentate Schiff bases with an N2O2 donor
group are able to coordinate to different metal ions [3]. Metal com-
Abbreviations: Salophen, N,N’-bis(salicylidene)-o-phenylenediamine; NMR, nuclear magnetic resonance; HMBC, heteroatom multiple bond coherence technique; TMS,
tetramethylsilane; IR, infrared; UV–Vis, ultraviolet–visible; DMSO, dimethylsulfoxide; CH2Cl2, dichloromethane; DFT, density functional theory; B3LYP, Becke 3-Lee young
parr functional; PES, Potencial energy surface.
⇑
Corresponding author. Address: Departamento de Química, Instituto de Ciências Exatas, Universidade Federal de Juiz de Fora, 36.036-330 Juiz de Fora, MG, Brazil. Tel.: +55
32 3229 3310.
E-mail addresses: alexandre.cuin@ufjf.edu.br, alexandre_cuin@yahoo.com (A. Cuin).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.09.002

V.Z. Mota et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 99 (2012) 110–115
111
plexes with derivatives of salicylaldehyde and 1,2-ethlylenedi-
amine, which form the Schiff base known as ‘salen’, have been
extensively studied due to the catalytic action of such complexes
on oxygen insertion into organic substrates. Also, the bivalent me-
tal complexes with salen have been synthesized and considered as
models for investigation of metalloenzymes [4–6]. Other important
characteristics involving Schiff bases with the N2O2 group are re-
lated to the formation of intramolecular hydrogen bonds and the
coexistence of different isomeric forms [7]. Diimine Schiff bases
with OH groups in ortho positions to the amino groups are of inter-
est mainly due to the existence of OAHꢀꢀꢀN hydrogen bonds and
also due to the presence of tautomerism between keto-amine
and enol-imine forms. In these compounds, short hydrogen bonds
between the OH group in the ortho position and the imino group or
the imine nitrogen are observed, being attributed to stereochemi-
cal factors [8]. Investigations of the structural stability of Schiff
base compounds by experimental techniques and theoretical
methods have been made by quantum-chemical calculations,
which have been shown to be a suitable tool to study the tauto-
meric compounds.
clear multiple bond coherence technique (HMBC). Samples were
prepared in deuterated dimethylsulfoxide solutions (DMSO-d6).
The infrared spectrum was recorded on a Bomem FT-IR MB-102
spectrophotometer in the range 3500–400 cmꢁ1 with resolution
of 4.0 cmꢁ1. The sample was prepared as a KBr pellet. The UV–
Vis spectra were obtained by using an UV-1800 Shimadzu spectro-
photometer with DMSO as the solvent.
X-ray data collection and structure determination
Single-crystal X-ray data were collected at room temperature
on an Oxford GEMINI A-ultra diffractometer equipped with a
MoKa (k = 0.71073 Å) radiation source. Data collection, reduction
and cell refinement were performed using CrysAlis RED [10]. The
structure was solved and refined using SHELXL-97 [11]. Hydrogen
atoms were located from Fourier difference maps. Anisotropic
parameters were refined for all non-hydrogen atoms. The molecu-
lar structure (Fig. 3) was prepared by using ORTEP III [12].
Molecular modeling
Salophen or N,N’-bis(salicylidene)-o-phenylenediamine is a rep-
resentative molecule of Schiff bases with a N2O2 group, which is
able to present tautomerism. The possible tautomers of salophen
are described in Fig. 1. Despite the vast uses of salophen, its crys-
talline structure has not been described by X-ray diffractometry
or by calculations using density functional theory (DFT) until
now. This work describes crystallographic studies and theoretical
calculations of the different isomeric forms of salophen. A similar
work with salen was recently published [9]. Besides the crystallo-
graphic and theoretical studies, salophen was characterized by ele-
mental analysis, IR (infrared), UV–Vis (ultraviolet–visible), 1H, 13C
and 15N NMR spectroscopic measurements.
Geometry optimizations were carried out using the GAMESS
software [13] with convergence criterion of 10ꢁ5 a.u. in a conjugate
gradient algorithm. The 6-31G (d,p) basis set [14] was used for all
atoms. Density functional theory (DFT) calculations were per-
formed for three possible tautomers of salophen as described in
Fig. 1 by using the B3LYP [15] gradient-corrected hybrid functional
to solve the Kohn–Sham equations with a 10ꢁ5 convergence crite-
rion for the density change. The final geometries were confirmed as
minima of the potencial energy surface (PES) with calculation of
the hessians. Single point calculations were also performed using
the polarizable continuum method (PCM) to simulate water solva-
tion of tautomers for comparative purposes [16]. The harmonic
vibrational frequencies and intensities were calculated at the same
level of theory with the analytical evaluation of second derivatives
of the energy as a function of atomic coordinates, and the calcu-
lated intensities were used to generate the theoretical spectra. Fre-
quencies were scaled by a factor of 0.9614, as recommended by
Scott and Radom [17]. Simulated vibrational spectra were obtained
from the sum of the Lorentzian functions with 20 cmꢁ1 half-band-
widths using the software MOLDEN 4.7 [18].
Experimental
Physical techniques and materials
All reagents and solvents for synthesis and spectroscopic stud-
ies were purchased from Sigma–Aldrich laboratories and used as
received without further purification. The 1H and 13C NMR spectra
were recorded on a Bruker 250 MHz Avance II spectrometer oper-
ating at 250.1 MHz for 1H and 62.5 MHz for 13C. The 1H and 13C
NMR chemical shifts were referenced to an internal standard (tet-
ramethylsilane, TMS). The 15N NMR data were recorded on a Bru-
ker Avance III 400 MHz (9.395T) operating a 400.1 MHz. The 15N
NMR spectra were indirectly obtained by the [1HA15N] heteronu-
Synthesis of salophen
Salicylaldehyde (2.26 g, 18.5 mmol) was added to an ethanolic
solution containing 10 mL of 1,2-phenylenediamine (1.0 g,
Fig. 1. Possible tautomers of salophen: (a) salophen 1, (b) salophen 2, and (c) salophen 3.

112
V.Z. Mota et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 99 (2012) 110–115
Fig. 2. Experimental NMR spectra: (A) HMBC 1HA15N spectrum and (B) 13C NMR spectrum.
Table 4
Assignments and experimental and calculated bands in the IR spectra (cmꢁ1).
Assignments
Experimental Salophen
1
Salophen
2
Salophen
3
mOH
mOH
mN@CAH
dCOH
3440
3088
3154
2905
1597
1558
1457
1270
1168
776
–
–
3007
–
–
–
–
1179
–
3205
3049
mCCaromatic 1614
2900
1600
1554
1475
1276
1156
747
+
mC@N
+
mCCaromatic + dCOH
1560
1481
1276
1149
760
dCOH
mCAO
mCCaromatic
dCOHasym
dHCH
–
734
726
mC@O
mCANAH
1536
2819 and
2866
1609 and
1489
1527
2982
–
–
dCANAH
1594 e
1491
Fig. 3. ORTEP representation of salophen.
respective band assignments, are presented in Table 4 while the
experimental and simulated spectra are shown in Fig. 5.
9.25 mmol) and the final solution was stirred for 2 h at room tem-
perature. A policrystalline compound was obtained and collected
by filtration, washed with cold ethanol and dried in a desiccator
under silica. Orange crystals suitable for X-ray studies were formed
after recrystallization in CH2Cl2. The yield was 84%, m.p. 154.4 °C.
Anal. Calcd (%) for C20H16O2N2: C, 75.9; H, 5.10; N, 8.86; found
(%): C, 75.8; H, 4.91; N, 8.84.
Experimental 13C, 1H and 15N NMR spectra of salophen are
shown in Fig. 2. In the 1H NMR spectrum of salophen the following
signals are observed (in ppm): 6.85–6.96 (4Haromatic, m), 7.30–7.59
(6Haromatic, m), 8.85 (2H, s, HC@N), 12.95 (2H, s, OH). The abbrevi-
ations ‘‘m’’ and ‘‘s’’ refers to multiplet and singlet, respectively. In
the 13C NMR spectrum three peaks at 164.4 ppm, 160.8 ppm and
142.6 ppm were observed, being assigned to C8 (C17), C2 (C24
)
and C10 (C15), respectively. In the 1HA15N HMBC NMR spectrum,
Results and discussion
the 15N (N9 or N16) signal is observed at 290.4 ppm. This signal is
coupled with the hydrogen atoms of OH, HC@N, and 4Haromatic
,
Spectroscopic studies
thus supporting the reported crystallographic structure.
The UV–Vis spectrum shows an absorption band with a maxi-
mum at 272 nm. This band is assigned to a transition between
The IR spectrum of salophen shows a strong absorption band
centered on 3440 cmꢁ1, being assigned to
m
OH and a weak absorp-
tion at 3049 cmꢁ1 due to
was assigned to combination modes of dCOH +
The dCOH and
CAO modes were observed at 1481 and 1276 cmꢁ1
m
NACAH. The absorption at 1614 cmꢁ1
the
p-orbital localized on the central bond of azomethine group.
Also, an absorption band centered at 334 nm was observed, being
assigned to an intramolecular charge transfer due to strong intra-
molecular hydrogen bonds of salophen.
m
C@N +
m
CCaromatic
.
m
,
respectively. The experimental and calculated IR data, as well the

V.Z. Mota et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 99 (2012) 110–115
113
Crystal structure
The compound belongs to the monoclinic system, and the a, b
and c axes are: 5.9636(2) Å, 16.5213(4) Å and 16.3116(4) Å, respec-
tively, and the b angle is 91.568(2)°. The structure has a symmetry
element of screw 21 perpendicular to plane c. The crystallographic
data are presented in Table 1.
The ORTEP representation of the compound is represented in
Fig. 3, which is the asymmetric unit, so there is no atom in any spe-
cial position. As can be seen in Fig. 3 there are no solvent molecules
in this structure, and there are four molecules in the unit cell of the
compound (Z = 4). There are
p–p stacking interactions, where the
distance centroid–centroid is 3.636(8)°A. This is the only intermo-
lecular interaction since intermolecular hydrogen bonds were not
observed. However, there are intramolecular hydrogen bonds
formed by the atoms N16 and O24, and also O1 and N9, respectively,
with distances of 2.613(1) Å and 2.564(1) Å, see Table 2. As shown
in Fig. 3 free salophen is not a planar molecule as it appeared to be
in some complexes described in the literature [19]. The angle be-
tween ring 1 (C2AC7) and ring 2 (C10AC15), is 3.20(8)°, and the tor-
sion angle between ring 2 (C10AC15) and ring 3 (C18AC23) is
56.88(3)°. This twisting may be caused by steric effects.
Molecular modeling
The geometries of three possible salophen tautomers were ob-
tained by theoretical calculations using density functional theory
(DFT). The tautomers were labeled as salophen 1, salophen 2 and
salophen 3, being the structure of salophen 1 equal to the structure
observed after X-ray refinement. The optimized structures for the
three tautomers are presented as supplementary material #1. As
observed, salophen 1 is the most stable structure based on DFT.
Salophen 2 and salophen 3 are shown to be 3.5 kcal molꢁ1 and
10.3 kcal molꢁ1 higher in energy than salophen 1, respectively.
Inclusion of solvation using PCM decreases the energy differences,
salophen 2 and salophen 3 were shown to be +1.39 kcal molꢁ1 and
+6.10 kcalꢀmolꢁ1 higher in energy, respectively. The same effect
was described before by Flores-Leonar et al. [20]. In both cases,
the energy difference between salophen 1 and salophen 2 is lower
Fig. 4. Experimental bond distances (Å) and angles (°) for N2O2 group.
Fig. 5. Experimental and calculated spectra of salophen.

114
V.Z. Mota et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 99 (2012) 110–115
Table 1
the two structures observed for the aniline crystal) [22]. These data
are additional evidence that the salophen 1 tautomer is the most
probable one to exist in the solid state.
Crystallographic data of salophen.
Empirical formula
C20H16N2O2
The theoretical IR spectra for the three tautomeric forms of
salophen were obtained by calculation of the hessians, showing
no imaginary frequencies. The calculated absorption profiles are
not conclusive in specifying the presence of only salophen 1 in
the solid state, since they are very similar. The simulated spectra
for salophen 1, salophen 2 and salophen 3 are presented in supple-
mentary material #2, while the experimental and calculated spec-
tra, for salophen 1 are presented in Fig. 5.
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
316.35
Monoclinic
P21/c
5.9636(2)
16.5213(4)
16.3116(4)
90
91.568(2)
90
1606.52(8)
4
1.308
0.71073
3982
2715
218
a
(°)
b (°)
c
(°)
V (Å3)
Since the single crystal X-ray characterization and the DFT
structures show that salophen 1 is the preferred tautomer, we have
compared theoretical and experimental IR spectra for salophen 1.
Z
d (calc) (g/cm3)
k (MoK
a)(Å)
Measured reflections
Independent reflections
Parameters
Rint
R
The simulated vibrational spectrum for salophen
absorption bands at 3154 cmꢁ1 3088 cmꢁ1 2905 cmꢁ1 and
2923 cmꢁ1, which corresponds to vOH and
CH stretching modes
for carbons C17 and C8 (Fig. 1a). The combination band between
the C@N stretching, the dCOH bending and CCaromatic stretching
vibrations of the phenylic rings is observed at 1597 cmꢁ1 [23].
The band which corresponds to a combination between CC ring
stretchinꢁg1 and dCOH bending appears as shoulder at
1558 cm . Moreover another combination band is observed at
1457 cmꢁ1 which corresponds to dCOH bending and
CCaromatic
stretching of the benzylic ring. The CAO stretching, the CC
1 presents
,
,
m
0.0243
0.0365
0.09
m
m
wR
S
1.041
m
a
Table 2
m
Intramolecular H-bond and angle.
m
m
DAHꢀꢀꢀA
O1AH1AN9
DAH (Å)
HꢀꢀꢀA (Å)
1.670
1.780
DꢀꢀꢀA (Å)
2.564
2.613
DAHꢀꢀꢀA (°)
150.0
147.0
stretching for the rings, the asymmetric dCOH bending and the
symmetric dCH bending of the benzylic ring are observed at
1270 cmꢁ1, 1168 cmꢁ1, 776 cmꢁ1 and 734 cmꢁ1, respectively.
The simulated spectra for salophen 2 and salophen 3 present
0.980
0.940
O
24AH24AN16
m
C@O bands at 1536 cmꢁ1 and 1527 cmꢁ1, respectively. It is also
than the thermal energy kT (with k the Boltzman constant and T
the ambient temperature in Kelvin). Based only on the energetic
difference, it is possible to suggest that salophen 2 can coexist with
salophen 1. Nevertheless, the greater energy difference between
salophen 1 and salophen 3 excludes the possibility of the existence
of salophen 3. The main calculated and experimental bond dis-
tances and angles for the salophen tautomers are shown in Table 3.
The experimental bond distances and angles for the N2O2 group are
described in Fig. 4a and b, respectively. As observed, the bond dis-
tances and angles for the optimized structure of salophen 1 are in
good agreement with the experimental X-ray crystallographic
data. The simulated C@N, CAN and CAOH bond distances for salo-
phen 1 (Fig. 1a) match those observed experimentally in the X-ray
crystallographic studies. Moreover, the experimental and theoreti-
cal values for the CAO bond length are near to that reported in the
literature for a crystal of phenol (1.372 Å) [21]. The theoretical and
experimental values for the CAN bond length are also in good
agreement with that reported for aniline (1.398 Å and 1.386 Å for
possible to identify in the simulated spectrum of salophen 2 vibra-
tion modes due to
NH at 2819 cmꢁ1 and 2866 cmꢁ1, while
dCANAH vibration bands are seen at 1609 cmꢁ1 and 1489 cmꢁ1
m
.
In the spectrum of salophen 3 the mCANAH stretching mode ap-
pears at 2982 cmꢁ1, while the absorption bands at 1593 and
1491 cmꢁ1 were assigned to dCANAH symmetrical and asymmetri-
cal modes. The IR stretching modes for C@O and NAH groups were
not observed in the calculated spectrum of salophen 1. Since the
mC@O and mNAH stretching modes have not been noted in the
experimental IR spectrum, the tautomer obtained in the solid state
was confirmed as salophen 1.
Conclusion
N,N0-bis(salicylidene)-o-phenylenediamine has been synthe-
sized and structurally characterized by single crystal X-ray diffrac-
tometry and DFT studies. IR, UV–Vis, 1H, 13C NMR and 15N NMR
Table 3
Experimental and calculated bond distances and angles of salophen.
Theoretical values
Experimental values
Bond
Distance (Å)
Salophen 1
Bond
Distance (Å)
Salophen 1
Salophen 2
Salophen 3
C@N
CAN
CAOH
CNAH
C@O
1.28
1.42
1.35
–
1.29
1.40
1.34
1.04
1.26
–
1.40
–
1.05
1.26
C8@N9/C17@N16
C10AN9/C15AN16
C2AO1/C23AO24
–
–
1.278(2)/1.278(2)
1.410(2)/1.420(2)
1.351(2)/1.349(2)
–
–
–
Angle (°)
121.23
121.20
121.42
121.90
123.47
119.41
Angle (°)
121.3(2)
121.2(1)
121.5(1)
121.9 (1)
123.4 (1)
119.4 (1)
O1AC2AC7
122.00
122.43
123.20
123.29
126.24
120.56
122.33
122.00
122.97
122.18
120.61
124.40
O1AC2AC7
O
24AC23AC18
C7AC8AN9
18AC17AN16
C8AN9AC10
17AN16AC15
O
24AC23AC18
C7AC8AN9
18AC17AN16
C8AN9AC10
17AN16AC15
C
C
C
C

V.Z. Mota et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 99 (2012) 110–115
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compound, mainly the HC@N imine group. Single crystal X-ray dif-
fraction revealed that salophen crystallizes in a monoclinic system
with a, b and c cell parameters 5.9636(2) Å, 16.5213(4) Å and
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Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.09.002.
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