Keto-enol tautomerism, conformations and structure of N-(2-hydroxy-5- methylphenyl), 2-hydroxybenzaldehydeimine

Article abstract of DOI:10.1016/S0022-2860(98)00604-8

N-(2-hydroxy-5-methylphenyl), 2-hydroxybenzaldehydeimine (C₁₄H₁₃NO₂) has been studied by X-ray analysis and AMI semi-empirical quantum mechanical method. It crystallizes in the monoclinic space group P2₁/c with

Full text of DOI:10.1016/S0022-2860(98)00604-8

Journal of Molecular Structure 477 (1999) 151–158
Keto–enol tautomerism, conformations and structure of
N-(2-hydroxy-5-methylphenyl), 2-hydroxybenzaldehydeimine
*
M. Kabak, A. Elmali, Y. Elerman
Department of engineering Physics, Faculty of Sciences, University of Ankara, 06100 Besevler, Ankara, Turkey
Received 1 July 1998; accepted 13 August 1998
Abstract
N-(2-hydroxy-5-methylphenyl), 2-hydroxybenzaldehydeimine (C14H13NO2) has been studied by X-ray analysis and AM1
semi-empirical quantum mechanical method. It crystallizes in the monoclinic space group P21/c with a ˆ 7.433(1), b ˆ
3
9.185(3), c ˆ 186(4) A, b ˆ 97.13(1)Њ, V ˆ 1164.3(5) A , Z ˆ 4, Dc ˆ 1.30 gcmϪ3 and m(MoKa) ˆ 0.087 mmϪ1. The structure
˚
˚
was solved by direct methods and refined to R ˆ 0.034 for 1407 reflections [I Ͼ 2s(I)]. The title compound is photochromic and
˚
the molecule is not planar. Intramolecular hydrogen bonds occur between the pairs of atoms N(1) and O(1) [2.582(1) A], and
˚
N(1) and O2) [2.655(1) A], the H atom essentially being bonded to the N atom. Adjacent molecules are linked via an
…
˚
intermolecular O–H O hydrogen bond [2.582(1) A] between neighbouring molecules. Tautomeric properties and conforma-
tions of the title compound were investigated by semi-empirical quantum mechanical AM1 calculations and the results are
compared with the X-ray results. ᭧ 1999 Elsevier Science B.V. All rights reserved.
Keywords: AM1; Photochromic; Schiff base; Tautomerism; X-ray
1. Introduction
tautomeric properties of the Schiff base including the
non-aromatic aldehyde and found out that the keto
tautomer is favoured over the enol form in the solid
state [4].
In this paper we investigated the structure of the N-
(2-hydroxy-5-methylphenyl), 2-hydroxybenzaldehy-
deimine, in order to reveal the presence of either the
enol or keto form (or the predominant presence of one
of them) in the crystalline state.
Schiff bases have been used extensively as ligands
in the field of coordination chemistry. Among these
bases, Schiff base compounds show photochromism
and thermochromism in the solid state by proton
transfer from the hydroxyl O atom to the imine N
atom [1]. Two types of intramolecular hydrogen
bonds (either N–H O or N H–O) can exist in
Schiff bases [2]. In the aldimine compounds made
from 2-hydroxy-1-naphthaldehyde both types of
hydrogen bonds were found [3]. The Schiff bases
…
…
2. Experimental
derived from salicylaldehyde always from the
…
The suitable crystals were obtained directly from
the synthesis of the compound. A solution of
0.002 mol of 2-hydroxy-5-methylaniline in 60 ml
pure ethanol was prepared and heated to boiling
temperature. 0.002 mol of salicylaldehyde was
N
H–O type of hydrogen bonding regardless of N-
substituent (alkyl or aryl) [3]. Recently we studied the
* Corresponding author. Fax: 90 312 223 2395;
e-mail: elerman@science.ankara.edu.tr
0022-2860/99/$ - see front matter ᭧ 1999 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(98)00604-8

152
M. Kabak et al. / Journal of Molecular Structure 477 (1999) 151–158
Fig. 1. The molecular structure of the title compound. Displacement ellipsoids are plotted at the 50% probability level [11].
˚
dissolved in 30 ml of hot ethanol. The mixture of the
two solutions was then refluxed for 4 h. Yellow pris-
matic crystals were formed during the refluxions.
A crystal of dimensions 0.35 × 0.30 × 40 mm was
mounted on an Enraf–Nonius CAD-4-diffractometer
equipped with a graphite monochromator. Cell
constants were determined by least-squares refine-
ment of diffractometer angles for 20 reflections
collected in the range 2.65Њ Ͻ u Ͻ 9.18Њ. Three stan-
dard reflections were monitored every 120 min, but no
considerable intensity variations were recorded. A
total of 1610 reflections were recorded, with Miller
and Ϫ0.14 eAϪ3. Scattering factors were taken from
Shelx97 [6].
Crystal data for (I), C14H13NO2, Mr
ˆ
227.3 gmolϪ1, monoclinic P21/c, a ˆ 7.433(1), b ˆ
˚
9.185(3), c ˆ 17.186(4) A, b ˆ 97.13(1)Њ, V ˆ
3
1.30 gcmϪ3 and
˚
1164.3(5) A ,
Z
ˆ
4, Dc
ˆ
m(MoKa) ˆ 0.087 mmϪ1, F(000) ˆ 480, T ˆ
292 K, R(F) ˆ 0.034 and wR(F2) ˆ 0.095 for 1407
observed reflections. Flack parameter is found to be
zero in the refinement [7]. A list of structure factors, H
atom fractional atomic coordinates and anisotropic
atomic displacement parameters for non-hydrogen
atoms has been deposited with the B.L.L.D. as
Supplementary Publications No. SUP… (13 pages).
Theoretical calculations were performed with the
semi-empirical quantum-mechanical program AM1
[8], which is part of the Mopac package [9]. The
models of keto and enol forms of the title compound
were built and taken the values from Allen et al.
[10]. The X-ray, keto and enol geometries were opti-
mized by using AM1 method. To determine the
conformational energy profiles, the optimized
geometries were kept fixed and values of energies
were calculated as a functions of the torsion angles
indices hmin ˆ Ϫ8, hmax ˆ 2, kmin ˆ 0, kmax ˆ 10, lmin
ˆ
Ϫ18 and lmax ˆ 18. The structure was solved by direct
methods using Shelxs86 [5]. The E-map computed
from the phase set with the best combined figure of
merit revealed the positions of all non-hydrogen
atoms. Full-matrix least-square refinement of the frac-
tional coordinates of the non-hydrogen atoms with
anisotropic atomic displacement parameters was
performed using Shelx97 [6]. Positions of H atoms
(except hydroxyl 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 in the difference Fourier
maps calculated at the end of the refinement process
as a small positive electron density. Final R(F) and
wR(F2) factors were 0.034 and 0.095 respectively, for
162 parameters using the I values of 1407 [I Ͼ 2s(I)]
reflections. A weighting scheme was used during the
refinement as w ˆ 1/[\s2(Fo2) ϩ (0.0555P)2 ϩ
0.2427P], where P ˆ (Fo2 ϩ 2Fc2)/3. The highest and
the lowest peaks in the final difference map were 0.13
u1[C(5)-C(6)-C(7)-N(1)],
u2[C(6)-C(7)-N(1)-C(7)]
and u3[C(7)-N(1)-C(8)-C(9)] from 0Њ to 360Њ, varied
every 10Њ. The molecular energies were calculated
as a function of each torsional angle (u), keeping the
other torsional angles constant. The zero values of
the torsion angles u1[C(5)-C(6)-C(7)-N-N(1)],
u1[C(6)-C(7)-N(1)-C(7)] and u3[C(7)-N(1)-C(8)-
C(9)] are Ϫ179Њ, Ϫ179Њ and Ϫ28Њ for optimized
X-ray and keto structures and Ϫ173Њ, Ϫ179Њ and

M. Kabak et al. / Journal of Molecular Structure 477 (1999) 151–158
153
Fig. 2. Crystal packing diagram for the title compound [12].
Fig. 3. The difference Fourier map for the least-squares plane through the chelate atoms O(2), C(2), C(3), C(4) and N(1) showing the position of
Ϫ3
…
˚
the H(1N) atom and revealing the presence of intramolecular N–H O hydrogen bonding. Contours are drawn from Ϫ0.15 to 0.45 eA at
intervals of 0.01 eA [13].
Ϫ3
˚

154
M. Kabak et al. / Journal of Molecular Structure 477 (1999) 151–158
Table 1
Ϫ35Њ for enol structure, respectively. The results are
illustrated in Figs. 4–6.
Atomic coordinates (× 104) and equivalent isotropic displacement
parameters (A × 103). Ueq is defined as one third of the trace of
2
˚
orthogonalized Uij tensor
Atom
x
y
z
Ueq
3. Results and discussion
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)
C(14)
N(1)
O(1)
O(2)
8599(1)
9158(2)
8893(2)
8070(2)
7511(1)
7745(1)
7244(1)
7110(1)
5833(2)
5475(2)
6433(2)
7712(2)
8079(1)
4121(2)
7503(1)
8859(1)
9334(1)
Ϫ1275(1)
Ϫ2655(1)
Ϫ3049(1)
Ϫ2117(1)
Ϫ788(1)
Ϫ330(1)
1093(1)
3081(1)
3943(1)
5344(1)
5864(1)
5028(1)
3626(1)
6280(2)
1643(1)
Ϫ861(1)
2744(1)
9076(1)
9382(1)
1012(1)
10611(1)
10342(1)
9578(1)
9341(1)
8410(1)
8711(1)
8449(1)
7867(1)
7563(1)
7831(1)
8799(1)
8666(1)
8381(1)
7571(1)
48(1)
57(1)
59(1)
55(1)
50(1)
44(1)
46(1)
45(1)
53(1)
58(1)
59(1)
55(1)
46(1)
90(1)
46(1)
61(1)
59(1)
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, keto and enol structures are
listed in Table 2. Ortep [11] view of the molecular
structure of the title compound is given in Fig. 1 and
the crystal packing diagram in Fig. 2 [13].
Thermochromic and photochromic properties of
the salicylideneanilinies are a function of the crystal
and molecular structure. Ab-initio calculations on
benzylideneaniline and related molecules have
shown that rotations about the Ph-N bond of up to
45Њ from planar conformation are stabilizing, while
rotations about the Ph-C bond are destabilizing,
and the most stable free-molecule conformation is
Table 2
˚
Bond distances (A) and angles (Њ) of X-ray, otimized X-ray, keto and enol forms of molecule with esd’s in parentheses [symmetry code (ii):
2Ϫx, 1/2Ϫy, 1.5Ϫz]
X-ray
Opt. X-ray
Keto
Enol
X-ray
Opt. X-ray
Keto
Enol
C(1)-C(2)
C(1)-C(6)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
C(6)-C(7)
C(7)-N(1)
C(8)-C(9)
C(8)-C(13)
C(8)-N(1)
C(9)-C(10)
C(10)-C(11)
C(10)-C(14)
C(11)-C(12)
C(12)-C(13)
C(13)-O(2)
1.414(2)
1.427(2)
1.359(2)
1.395(2)
1.352(2)
1.411(2)
1.405(2)
1.301(1)
1.384(2)
1.391(2)
1.412(1)
1.379(2)
1.383(2)
1.505(2)
1.375(2)
1.384(2)
1.353(1)
0.99(2)
1.467
1.464
1.350
1.438
1.355
1.442
1.386
1.354
1.407
1.426
1.405
1.398
1.398
1.481
1.390
1.397
1.377
1.01
1.466
1.464
1.352
1.436
1.355
1.442
1.386
1.354
1.408
1.426
1.407
1.398
1.396
1.483
1.391
1.398
1.380
1.01
1.417
1.409
1.382
1.402
1.384
1.410
1.465
1.292
1.410
1.425
1.407
1.396
1.398
1.484
1.390
1.403
1.376
—
O(1)-C(1)-C(2)
O(1)-C(1)-C(6)
C(2)-C(1)-C(6)
C(3)-C(2)-C(1)
C(2)-C(3)-C(4)
C(5)-C(4)-C(3)
C(4)-C(5)-C(6)
C(7)-C(6)-C(5)
C(7)-C(6)-C(1)
C(5)-C(6)-C(1)
N(1)-C(7)-C(6)
C(9)-C(8)-C(13)
C(9)-C(8)-N(1)
C(13)-C(8)-N(1)
C(10)-C(9)-C(8)
C(9)-C(10)-C(11)
C(9)-C(10)-C(14)
C(11)-C(10)-C(14)
C(12)-C(11)-C(10)
C(11)-C(12)-C(13)
O(2)-C(13)-C(12)
O(2)-C(13)-C(8)
C(12)-C(13)-C(8)
C(7)-N(1)-C(8)
122.5(1)
120.6(1)
116.9(1)
121.1(1)
121.8(1)
119.1(1)
121.3(1)
119.3(1)
120.8(1)
119.8(1)
123.8(1)
120.1(1)
122.9(1)
117.0(1)
121.7(1)
117.6(1)
121.0(1)
121.4(1)
121.7(1)
120.6(1)
124.2(1)
117.4(1)
118.4(1)
126.4(1)
120.6
122.9
116.6
121.9
120.9
120.0
122.1
118.0
123.4
118.6
126.0
117.7
122.7
119.9
122.2
118.9
119.6
120.5
120.2
120.4
122.3
117.3
120.7
123.8
120.4
122.9
116.6
121.3
120.9
120.8
121.2
117.9
123.4
118.6
126.3
117.7
122.4
119.9
120.8
120.3
119.6
120.5
120.2
120.0
121.9
117.3
120.7
124.0
113.9
125.8
120.2
120.0
120.1
120.2
121.1
116.8
124.9
116.8
123.2
117.4
123.4
119.4
121.4
120.1
119.9
120.6
120.3
120.1
120.8
118.1
120.6
122.6
…
N(1) H(1N)
…
O(1) H(1N)
1.59(2)
2.582(1)
0.99(2)
2.582(1)
2.655(1)
—
2.10
—
0.97
2.79
2.76
—
1.251
2.10
—
0.97
2.79
2.76
—
1.251
—
—
0.97
2.88
2.77
0.97
1.365
O(1)-O(2)ii
…
O(2) H(1O)
…
O(1) N(1)
O(2)-N(1)
O(1)-H(1N)
O(1)-C(1)
1.291(1)

M. Kabak et al. / Journal of Molecular Structure 477 (1999) 151–158
155
Fig. 4. AM1 calculated conformation energies for X-ray, keto and enol structures vs the u1[C(5)–C(6)–C(7)–N(1)] torsion angle.
Fig. 5. AM1 calculated conformation energies for X-ray, keto and enol structures vs the u2[C(6)–C(7)–N(1)–C(8)] torsion angle.

156
M. Kabak et al. / Journal of Molecular Structure 477 (1999) 151–158

M. Kabak et al. / Journal of Molecular Structure 477 (1999) 151–158
157
Scheme 1. Enol and keto tautomerism of the title compound.
non-planar [1]. From some thermochromic and photo-
chromic Schiff base compounds, it was proposed that
molecules exhibiting thermochromy are planar, while
those exhibiting photochromy are non-planar [14]. In
agreement with the above conclusions, the title
compound is photochromic (M. Kabak, unpublished
results) and the molecule is not planar; moieties
A[C(1)-C(7), O(1)] and B[N(1), C(8)-C(14), O(2)]
neighbouring molecules (Fig. 5). The O(1)-H(10)
distance is 0.99(2) A.
˚
In order to define the conformational flexibility of
the title molecule, semi-empirical calculations using
the AM1 molecular orbital method were carried out.
The molecular energy can be divided into bonded and
non-bonded contributions. The bonded energy is
considered to be independent of torsional angles
changes. The non-bonded energy (EN) is then further
separated into torsional (ET), steric (ES) and electro-
static (EES) contributions.
[both planar with
a
maximum deviation of
˚
Ϫ0.031(1) A] are inclined at an angle of 24.3(1)Њ.
The distortion can be seen in the crystal packing
diagram (Fig. 2).
EN ˆ ET ϩ ES ϩ EES
ꢀ1†
The crystal structure is stabilized by inermole-
cular hydrogen bonds. The crystal structure determi-
nation indicates the existence of both N(1)-
where torsional energy (ET) is that part of the torsional
energy which does not arrive from the steric or
electro-static term. The optimized bond distances
and angles of X-ray structure and keto tautomer are
in good agreement. Conformation profiles of the X-
ray structure, keto–enol tautomers are given in Figs.
4–6. The conformations of the optimized X-ray
geometry are in good agreement with the keto
tautomer. The non-planar conformations corre-
sponding to zero values of u1[C(5)-C(6)-C(7)-N(1)],
u2[C(6)-C(7)-N(1)-C(7)] and u3[C(7)-N(1)-C(8)-
C(9)] are the most stable conformation for X-ray,
keto and enol structures. The energies corresponding
…
…
H(1N) O(1) and N(1)-H(1N) O(2) the bifurcated
intramolecular hydrogen bonds as follows:
…
˚
…
˚
N(1) O(1) 2.582 A and N(1) O(2) 2.655 A.
These distances are significantly shorter than the
˚
sum, 3.07 A, of the van der Waals radii for nitrogen
and oxygen [15]. The atom H(1N) was located from
a difference Fourier map as a well defined small
Ϫ3
˚
electron density maximum of 0.13 eA (Fig. 6).
…
The N(1)-H(1N), O(1) H(1N) distances are
0.98(2), 1.75(2) and 2.39(2) A, respectively. The
bond angles N(1)-H(1N) O(1) and N(1)-
H(1N) O(2) are 139(1)Њ and 95(2)Њ, respectively.
˚
…
…
to
this
non-planar
conformations
are
Ϫ16.9 kcal molϪ1 for X-ray, Ϫ16.9 kcal molϪ1 for
˚
The C(1)yO(1) bond [1.291(1) A] was determined
keto and Ϫ20.0 kcal molϪ1 for enol structures.
to be of bond order 1.5 from the values of the single
and double C–O bonds [10]. This along with the
The energy profile of the u1 shows one maxima near
360Њ for the X-ray, keto and enol structures. This
energy barrier arises from the steric interactions of
H(N1) with H(C7). The energy profile as a function
u2 shows one maxima near 337Њ for X-ray and keto
structures near 350Њ for enol structures due to steric
˚
short C(3)–C(4) bond [1.405(2) A] suggests the
presence of a significant keto tautomer (Scheme 1).
There is also a strong intermolecular hydrogen
bond between O(1) and O(2)ii [2.582(1) A];
˚
[symmetry code (ii): 2Ϫx, 1/2Ϫy, 1.5Ϫz] atoms of

158
M. Kabak et al. / Journal of Molecular Structure 477 (1999) 151–158
interactions between H(N1) and H(C9) for X-ray and
keto structures and H(O1) and H(C9) for enol struc-
tures. The conformational energy as a function of u3
shows very small differences and the energy barrier is
less than 15 kcal molϪ1. Burgi and Dunitz carried out
an extensive theoretical and experimental study on the
non-planar conformation of the N-benzylideneaniline
and related compounds [16]. Their explanation for the
non-planarity of N-benzylideneaniline involves a
competition between two principal factors: (a) the
interaction of the ortho hydrogen on the aniline ring
and the hydrogen on the ‘bridge’ carbon, which is
repulsive in the planar conformation but is reduced
with increasing non-planarity; and (b) the p-electron
system, itself divisible into two components,
including, on the one hand, delocalization between
the –CHyN-double bond and the aniline phenyl
ring, (which is maximized for a planar conformation)
and, on the other hand, delocalization of the nitrogen
lone pair electrons into the aniline ring which is essen-
tially zero for the planar conformation but increases
with increasing non-planarity (where the lone pair
density on the nitrogen may interact with the p-
system of the ring).
In summary, semi-empirical AM1 calculations
show a good agreement between X-ray and keto struc-
tures. The AM1 optimized geometries of X-ray, keto
and enol structures of the title compound corre-
sponding to non-planar conformation is the most
stable conformation. The results strongly indicate
that the minimum energy conformation is primarily
determined by non-bonded hydrogen–hydrogen
repulsions. Although non-bonded repulsions are
largely responsible for the conformational differences
of the title compound, interaction between the N-lone
pair and the p-electrons of the rotted phenyl ring
might also contribute to the conformational energy
of the title compound.
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