Tautomeric properties, conformations and structure of N-(2-hydroxy-5- chlorophenyl) salicylaldimine

Article abstract of DOI:10.1016/S0022-2860(99)00074-5

The crystal structure of N-(2-hydroxy-5-chlorophenyl) salicylaldimine (C₁₃H₁₀NO₂Cl) was determined by X-ray analysis. It crystallizes orthorhombic space group P2₁2₁2₁ with a = 12.967(2) A,

Full text of DOI:10.1016/S0022-2860(99)00074-5

Journal of Molecular Structure 510 (1999) 207–214
www.elsevier.nl/locate/molstruc
Tautomeric properties, conformations and structure of
N-(2-hydroxy-5-chlorophenyl) salicylaldimine
a
a
a, b
A. Elmalia, M. Kabaka, b, , E. Kavlakoglu , Y. Elerman , T.N. Durlu
*
aDepartment of Engineering Physics, Faculty of Sciences, University of Ankara, 06100 Bes¸evler, Ankara, Turkey
bDepartment of Physics, Faculty of Art and Sciences, University of Kirikkale, Yahs¸ihan Campus, 71450 Kirikkale, Turkey
Received 21 October 1998; received in revised form 16 February 1999; accepted 16 February 1999
Abstract
The crystal structure of N-(2-hydroxy-5-chlorophenyl) salicylaldimine (C13H10NO2Cl) was determined by X-ray analysis. It
3
˚
˚
˚
˚
crystallizes orthorhombic space group P212121 with a ˆ 12.967(2) A, b ˆ 14.438(3) A, c ˆ 6.231(3) A, V ˆ 1166.5(6) A , Z ˆ
4, Dc ˆ 1.41 g cmϪ3 and m(MoKa) ˆ 0.315 mmϪ1. The title compound is thermochromic and the molecule is nearly planar.
Both tautomeric forms (keto and enol forms in 68(3) and 32(3)%, respectively) are present in the solid state. The molecules
…
˚
…
contain strong intramolecular hydrogen bonds, N1–H1 O1/O2 (2.515(1) and 2.581(2) A) for the keto form and O1–H01 N1
…
˚
for the enol one. There is also strong intermolecular O2–H O1 hydrogen bonding (2.599(2) A) between neighbouring
molecules. Minimum energy conformations AM1 were calculated as a function of the three torsion angles, u1(N1–C7–C6–
C5), u2(C8–N1–C7–C6) and u3(C9–C8–N1–C7), varied every 10Њ. Although the molecule is nearly planar, the AM1
optimized geometry of the title compound is not planar. The non-planar conformation of the title compound corresponding
to the optimized X-ray structure is the most stable conformation in all calculations. ᭧ 1999 Elsevier Science B.V. All rights
reserved.
Keywords: X-ray; Schiff base; AM1; Thermochromic; Tautomerism
1. Introduction
concluded that the significant difference lies in the
manner of molecular packing in the lattice and mole-
cules exhibiting thermochromy are planar while
photochromy are non-planar [4]. Namely, photo-
chromic salicylaldimines are packed rather loosely
in the crystal, in which non-planar molecules may
undergo some conformational changes, while thermo-
chromic salicylaldimines are packed tightly to form
one-dimensional columns. Recently, we reported the
structures and conformations of the non-planar N-
substituted salicylaldimines [5,6]. With the aim of
gaining a deep insight into the structural aspect
responsible for the observed phenomenon in the
solid state, conformational and crystallographic
analysis of the title compound (I) which is nearly
Schiff bases have been widely used as ligands in the
formation of transition metal complexes [1]. N-substi-
tuted salicylaldimines show photochromism and ther-
mochromism in the solid state. The chromism is
produced by intramolecular proton transfer from the
hydroxyl O to the imine N atom [2]. Proton transfer
causes a change in the p-electron configuration and
consequently in a molecular conformation [3]. On the
basis of structural studies on photochromic and ther-
mochromic salicylaldimine derivatives, it was
* Corresponding author. Fax: ϩ 90-312-223-2395.
E-mail address: kabak@science.ankara.edu.tr (M. Kabak)
0022-2860/99/$ - see front matter ᭧ 1999 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(99)00074-5

208
A. Elmali et al. / Journal of Molecular Structure 510 (1999) 207–214
Fig. 1. The (a) keto and (b) enol forms of the molecular structure of the title compound. Displacement ellipsoids are plotted at the 50%
probability level [14].
planar has been carried out and the results are
presented in this article.
lmin ˆ 0, lmax ˆ 8. The structure was solved by direct
methods using SHELXS86 [7]. The E-map computed
from the phased set with the best combined figure of
merit revealed the position of all non-hydrogen atoms
with anisotropic atomic displacement parameters was
performed SHELX97 [8]. Positions of H atoms
(except hydroxyl on the chlorophenyl side and
amine H atoms) were generated from the assumed
geometries checked in Fourier maps and refined
isotropically. The hydroxyl and amine H atoms were
found from the difference Fourier maps and calculated
at the end of the refinement process as a small positive
electron density. In order to consolidate the position
of the amine hydrogen atom we refined its occupation
factor and found the amine hydrogen atom 68% near
the N1 and 32% near the O1 (Fig. 1). Final R(F2) and
wR(F2) factor were 0.033 and 0.098 for 165
parameters using the I values of 1353 (I Ͼ 2s(I))
reflections. A weighting scheme was used during
the refinement as w ˆ 1=‰s2ꢀF02† ϩ ꢀ0:0517P†2 ϩ
0:2300PŠ where P ˆ ‰F02 ϩ 2Fc2Š=3. The highest and
the lowest peaks in the final difference map were 0.18
2. Experimental
The suitable crystals were obtained directly from
the synthesis of the compound. A solution of 2-amino-
4-chlorophenol (0.03 mol) in absolute ethanol was
prepared and an ethanolic solution of salicylaldehyde
(0.03 mol) was added slowly. When the reaction
mixture cooled, orange crystals were obtained.
A crystal of dimensions 1.00 × 0.35 × 0.35 mm was
mounted on a RIGAKU AFC7S diffractometer
equipped with a graphite monochromator. Cell
constants were determined by least squares refinement
of diffractometer angles for 25 reflections collected in
the range 1Њ Ͻ u Ͻ 20Њ. Three standard reflections
were monitored after every 150 reflections, but no
considerable intensity variations were recorded. A
total of 1776 reflections were recorded, with Miller
indices hmin ˆ Ϫ1, hmax ˆ 16, kmin ˆ 0, kmax ˆ 18,

A. Elmali et al. / Journal of Molecular Structure 510 (1999) 207–214
209
and Ϫ 0.21 e AϪ3. Scattering factors were taken from
SHELX97 [8].
˚
Table 1
Atomic coordinates ( × 104) and equivalent isotropic displacement
2
3
˚
parameters A × 10 ) (Ueq is defined as one third of the trace of the
orthogonalized Uij tensor)
Crystal data for N-(2-Hydroxy-5-chlorophenyl)-
salicylaldimine, C13H10NO2Cl, Mr ˆ 247.68 g molϪ1
,
˚
x
y
z
Ueq
orthorhombic P212121, a ˆ 12.967(2) A, b ˆ
3
˚
˚
˚
14.438(3) A, c ˆ 6.231(3) A, V ˆ 1166.5(6) A , Z ˆ
Cl(1)
O(1)
O(2)
N(1)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
C(9)
C(10)
C(11)
C(12)
C(13)
6749(1)
3608(1)
3097(1)
4834(1)
4394(1)
4283(2)
5119(2)
6119(2)
6266(1)
5422(1)
5595(1)
4862(1)
5742(1)
5653(1)
4707(1)
3834(1)
3896(1)
Ϫ1078(1)
1113(1)
Ϫ301(1)
389(1)
1438(1)
1924(1)
2273(1)
2167(1)
1694(1)
1313(1)
786(1)
Ϫ154(1)
Ϫ342(1)
Ϫ854(1)
Ϫ1181(1)
Ϫ1007(1)
Ϫ503(1)
Ϫ1915(1)
7157(2)
3284(2)
4518(2)
8210(3)
10158(3)
11219(3)
10418(3)
8534(3)
7418(3)
5530(3)
2652(3)
1478(3)
Ϫ376(3)
Ϫ1085(3)
109(3)
63(1)
48(1)
57(1)
36(1)
38(1)
47(1)
50(1)
50(1)
46(1)
36(1)
38(1)
34(1)
38(1)
39(1)
41(1)
41(1)
38(1)
4, Dc ˆ 1.41 g/cm3 and m(MoKa) ˆ 0.315 mmϪ1
,
F(000) ˆ 512, T ˆ 293(2) K, R(F2) ˆ 0.033 and
wR(F2) ˆ 0.098 for 1353 observed reflections. Flack
parameter is found to be Ϫ0.1(1) in the refinement [9].
A list of structural factors, H atom fractional atomic
coordinates and anisotropic atomic displacement
parameters for non-hydrogen atoms has been depos-
ited with the B.L.L.D. as supplementary publications
no. SUP 26619 (13 pp.).
Geometric optimization of keto form of I by the
AM1 semi-empirical quantum-mechanical method
[10], implemented with the MOPAC package [11]
running on a Pentium 133 PC, was calculated starting
from their crystallographic coordinates with hydrogen
1996(3)
Table 2
˚
Bond lengths (A) and angles (Њ) with e.s.d. in parantheses (including intra and intermolecular contacts)
Cl1–C10
O1–C1
O2–C13
N1–C7
N1–C8
C1–C2
C1–C6
C2–C3
C3–C4
C4–C5
C5–C6
C6–C7
C8–C9
C8–C13
C9–C10
C10–C11
C11–C12
C12–C13
1.744(2)
1.299(2)
1.343(2)
1.303(2)
1.403(2)
1.410(3)
1.432(2)
1.367(3)
1.398(3)
1.371(3)
1.409(2)
1.418(2)
1.383(2)
1.410(2)
1.376(3)
1.386(2)
1.378(2)
1.385(3)
C7–N1–C8
O1–C1–C2
O1–C1–C6
C2–C1–C6
C3–C2–C1
C2–C3–C4
C5–C4–C3
C4–C5–C6
C5–C6–C7
C5–C6–C1
C7–C6–C1
N1–C7–C6
C9–C8–N1
C9–C8–C13
N1–C8–C13
C10–C9–C8
C9–C10–C11
C9–C10–C11
D–H
0.89(3)
0.89(3)
1.01(7)
1.01(7)
0.80(4)
0.93
0.93
0.93
128.9(1)
122.3(2)
120.7(2)
117.1(2)
121.3(2)
121.5(2)
119.3(2)
120.6(2)
119.7(2)
120.2(2)
120.0(1)
121.1(2)
124.7(1)
120.6(2)
114.7(1)
118.7(2)
121.6(2)
119.5(1)
C11–C10–Cl1
C12–C11–C10
C11–C12–C13
O2–C13–C12
O2–C13–C8
118.8(1)
119.6(2)
120.4(2)
125.2(2)
115.8(2)
119.1(2)
C12–C13–C8
…
…
…
…
D–H A
D–H (symmetry code)
D
A
H
A
a
…
N1–H1 O1
N1–H1 O2
O1–H01 N1
O1–H01 O2
2.515(2)
2.581(2)
2.515(2)
3.231(2)
2.599(1)
3.418(2)
3.336(2)
3.791(2)
1.73(3)
2.21(3)
1.67(7)
2.52(7)
1.80(4)
2.82
145(3)
105(2)
139(7)
110(1)
173(2)
123(2)
110(2)
164(2)
a
…
a
…
…
a
…
O2–H02 O1 ( Ϫ x ϩ 1/2, Ϫ y, ϩ z Ϫ 1/2)
…
…
C12–H12 O1 ( Ϫ x ϩ 1/2, Ϫ y, ϩ z Ϫ 1/2)
C12–H12 O2 ( Ϫ x ϩ 1/2, Ϫ y, ϩ z Ϫ 1/2)
2.90
2.89
…
C7–H7 Cl1( Ϫ x ϩ 1/2, Ϫ y, ϩ z ϩ 1/2)
a The population parameters of the H1 and H01 are 68(3) and 32(3)%.

210
A. Elmali et al. / Journal of Molecular Structure 510 (1999) 207–214
Fig. 2. Crystal packing along the c axis. For clarity purposes only the keto form has been retained [15].
˚
˚
atoms placed at a distance of 0.93 A from their target
total energy were calculated as a function of the three
torsion angles u1(N1–C7–C6–C5), u2(C8–N1–C7–
C6) and u3(C9–C8–N1–C7) from 0 to 360Њ, varied
every 10Њ. The molecular energies were calculated as
a function of each u, keeping the other u values
constant. The optimized torsion angles of u1(N1–
C7–C6–C5), u2(C8–N1–C7–C6) and u3(C9–C8–
N1–C7) are Ϫ 179.5, Ϫ 179.1 and Ϫ 26.7Њ, respec-
tively. We discussed both, the enol and the keto forms
˚
carbon, 0.80 A from O2 and 0.89 A from N1 atom.
Geometry optimization of the crystal structure of I
implemented were carried out using the Fletcher–
Powell–Davidson algorithm [12,13] implemented in
the package and the PRECISE option to improve the
convergence criteria. To determine the conforma-
tional energy profiles, the optimized geometry of
keto form I was kept fixed, and values of the AM1

A. Elmali et al. / Journal of Molecular Structure 510 (1999) 207–214
211
Scheme 1.
of the single molecule in order to clarify the position
of the proton and predominant form of the molecule. It
is well known that the AM1 calculations underesti-
mate the hydrogen bond interactions and in this way
when the planarity of the molecule is due to these kind
of bonds optimized molecular structure are not planar.
We found the heat of formation energy for both the
enol and the keto optimized geometry. Due to the
underestimation of the hydrogen bond interactions
of the AM1 method, the heat of formation of the
enol form [Ϫ19.6 kcal molϪ1] is less than the keto
form [Ϫ15.6 kcal molϪ1] of compound I. In all calcu-
lations it was found that the most stable conformation
is the optimized non-planar X-ray structure.
and the molecules were close packed along the c axis;
and the molecule is thermochromic. The dihedral
angle between A(O1,C1–C7) and B(N1,O2,Cl1,C8–
C13) [both A and B are planar with a maximum devia-
˚
tion of 0.039(1) A of C7 atom] is 4.87(6)Њ. The torsion
angles N1–C7–C6–C5[ Ϫ 176.8(2)Њ], C8–N1–C7–
C6[Ϫ179.8(2)Њ], and C7–N1–C8–C9 [Ϫ2.4(3)Њ] also
indicate that the molecule is almost planar. N-(2-
Hydroxy-5-chlorophenyl) salicylaldimine contains
strong bifurcated intramolecular hydrogen bonds
…
˚
˚
between N1–H1 O1 [2.515(2) A] and N1–
…
H1 O2[2.581(2) A]. These distances are signifi-
cantly shorter than the sum of the van der Waals
radii of O and N [3.07 A] [16]. In the present study,
˚
…
the intramolecular N1 O1 hydrogen bond length is
comparable with that observed for 1-[N-(4-methyl-2-
pyridyl)aminomethylidene]-2(1H)-naphthalenone
3. Results and discussion
˚
[2.532(3) A] [17] 2-salicylideneamino-4,5,6,7-tetra-
˚
hydrobenzo[b]thiophene-3-carbonitrile [2.626(2) A]
Fractional atomic coordinates and equivalent
isotropical thermal parameters for non-hydrogen
atoms are given in Table 1. Bond distances and
bond angles for X-ray are listed in Table 2. The
ORTEP view of the molecular structure of I is given
in Fig. 1 [14] and the crystal packing diagram in Fig. 2
[15].
Thermochromic and photochromic properties of
the salicylideneanilines are a function of their crystal
and molecular structure [2]. From some thermo-
chromic and photochromic Schiff base compounds,
it was proposed that molecules exhibiting thermo-
chromy are planar, while those exhibiting photo-
chromy are non-planar and planarity of the molecule
makes it possible for the proton to the transfer through
the hydrogen bond in the ground state with small
energy requirement [3,4]. In agreement with the
above conclusions, I is planar (the most deviated
[18] and for N-(5-bromosalicylidene)-2-aminopyri-
˚
dine [2.581(6) A] [19]. The amine and hydroxyl H
atoms were located from a difference Fourier map at
the end of the refinement process as small positive
electron densities. To clarify the atomic position of
the intermediate H atom between N1 and O1 atom we
freed the occupation factor during the refinement
process and found the H1 is convergent 68(3)% to
N1 and the H01 32(3)% to O1 atom, respectively.
The related geometrical parameters of intramolecular
hydrogen bonds and angles are given in Table 2. The
˚
O1–C1 bond length [1.299(2) A] is not included in
either the single or double bond character. By virtue
of the strong intermolecular hydrogen bond between
˚
O1 and O2 (Ϫx ϩ 1/2,Ϫy,z Ϫ 1/2) atoms [2.599(1) A]
the C1–O1 bond length is seemingly single bond in
character. Of course this bond is a bit smaller than the
˚
˚
single bond values in enols [1.333(7) A] [20], but on
atom from the molecular plane is O1[Ϫ0.185(1) A])

212
A. Elmali et al. / Journal of Molecular Structure 510 (1999) 207–214
Fig. 3. AM1 calculated conformation energy for X-ray structure of the u1(N1–C7–C6–C5) torsion angle.
Fig. 4. AM1 calculated conformation energy for X-ray structure of the u2(C8–N1–C7–C6) torsion angle.

A. Elmali et al. / Journal of Molecular Structure 510 (1999) 207–214
213
Fig. 5. AM1 calculated conformation energy for X-ray structure of the u3(C9–C8–N1–C7) torsion angle.
the contrary, O1–C1 bond length is greater than the
double bonds in aldehydes and ketones [1.210(8) A]
compound I, semi-empirical calculations using the
AM1 molecular orbital method were carried out.
The AM1 optimized geometry of the crystal structure
is non-planar, while the X-ray structure is nearly
planar. The angle between two planar Schiff base
moieties A(O1, C1–C7) and B(N1, O2, Cl1, C8–
C13) is 24.7Њ in the AM1 optimized geometry. The
energy profile as a function of u1(N1–C7–C6–C5)
shows a maxima at 0Њ. The largest energy barrier at
˚
[20]. The short C2yC3 bond length value
˚
[1.367(3) A] is characteristic for the quinoidal effects
for the structures [20,21]. In the present work the O1–
C1 bond length is smaller than the mean value of the
previous similar works N-(2-hydroxyphenyl)salicyl-
˚
aldimine [1.398(1) and 1.387(2) A] [22] and o-(sali-
˚
cylideneaminium)phenol chloride [1.355(2) A] [23].
˚
Previous designations and the very short C2yC3
0Њ is due to the non-bonded H1–H7 [1.05 A] interac-
tions between Schiff base moieties. The smallest
˚
bond [1.367(4) A] suggest the presence of a signifi-
cant keto tautomer (Scheme 1).
energy barrier at 70Њ arises from the steric interactions
between H1–H7 [1.88 A]. The energy profile as a
˚
There is also a strong intermolecular hydrogen
bond between O1 and O2 (Ϫx Ϫ 1/2, Ϫy, ϩ z Ϫ 1/2)
function of u2(C8–N1–C7–C6) has an energy barrier
near 20Њ because of steric interactions between H9 and
˚
[2.599(2) A] atoms of neighbouring molecules. The
˚
crystal is built up of chains along the c axis that they
do not bear any interactions between them (Fig. 2). The
secondary structure is formed by molecules linked via
H1 [2.13 A]. The conformational energy as a function
of u3(C9–C8–N1–C7) shows very small differences.
The non-planar conformation of I corresponding to
the optimized X-ray structure is the most stable
conformation (see Figs. 3–5). Burgi and Dunitz
carried out an extensive theoretical and experimental
study on the non-planar conformation of N-benzylide-
neaniline and related compounds [26]. Their explana-
tion for the non-planarity of N-benzylideneaniline
involves a competition between two principal factors:
(1) the interaction of the ortho hydrogen on the aniline
ring and the hydrogen on the bridge carbon, which is
…
O2–H O1 (Ϫx ϩ 1/2, Ϫy,z ϩ 1/2) hydrogen bonds
…
…
reinforces by weak C–H O contacts C12 O1/O2
(Ϫx ϩ 1/2, Ϫy,z Ϫ 1/2) [24,25]. C–H atoms have a
tendency to form short intermolecular contacts to
…
oxygen atoms. C–H O contacts occur frequently in
the crystal structures; they play a significant role in
determining the packing arrangements of some organic
crystal structures.
In order to define the conformational flexibility of

214
A. Elmali et al. / Journal of Molecular Structure 510 (1999) 207–214
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geometries show that the dihedral angle between
A(O1, C1–C7) and B(N1,O2,Cl1,C8–C13) planes
are 24.7 and 21.4Њ in keto and enol forms, respec-
tively. Because the AM1 method does not appreciate
the intra- and intermolecular hydrogen bond interac-
tions, the planarity of the single molecule decreases in
the AM1 calculations. Therefore the enol form has the
lowest heat of formation than the keto form of the
isolated moelcule. The theoretical results strongly indi-
catethatthemoststableconformationisprimarilydeter-
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¨
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