Tautomeric properties, conformations and structure of N-(2- hydroxyphenyl)-4-amino-3-penten-2-on

Article abstract of DOI:10.1016/S0022-2860(98)00377-9

N-(2-hydroxyphenyl)-4-amino-3-penten-2-on (C₁₁H₁₃NO₂) has been studied by X-ray analysis. It crystallizes the orthorhombic space group P2₁2₁2₁ with a = 8.834(1), b = 10.508(2), c = 11.212(2

Full text of DOI:10.1016/S0022-2860(98)00377-9

Journal of Molecular Structure 470 (1998) 295±300
Tautomeric properties, conformations and structure of
N-(2-hydroxyphenyl)-4-amino-3-penten-2-on
*
M. Kabak, A. Elmali , Y. Elerman
Department of Engineering Physics, Faculty of Sciences, University of Ankara, 06100 BesËevler, Ankara, Turkey
Received 26 November 1997; revised 10 March 1998; accepted 10 March 1998
Abstract
N-(2-hydroxyphenyl)-4-amino-3-penten-2-on (C11H13NO2) has been studied by X-ray analysis. It crystallizes the ortho-
23
Ê
Ê 3
rhombic space group P212121 with a ˆ 8.834(1), b ˆ 10.508(2), c ˆ 11.212(2) A, V ˆ 1040.8(3) A , Z ˆ 4, Dc ˆ 1.22 g cm
and m(MoKa) ˆ 0.084 mm21. The structure was solved by direct methods and re®ned to R ˆ 0.038 for 1373 re¯ections
(I . 2s(I)). The title compound is photochromic and the molecule is not planar. Intramolecular hydrogen bonds occur between
Ê
Ê
the pairs of atoms N(1) and O(1) [2.631(2) A], and N(1) and O(2) [2.641(2) A], the H atom essentially being bonded to the N
¼
Ê
atom. There is also a strong intermolecular O±H O hydrogen bonding [2.647(2) A] between neighbouring molecules.
Tautomeric properties and conformations of the title compound were investigated by semi-empirical quantum mechanical
AM1 calculations and the results are compared with the X-ray results. q 1998 Elsevier Science B.V. All rights reserved.
Keywords: X-ray; Schiff base; AM1; Photochromic; Tautomerism
1. Introduction
Schiff base, including the non-aromatic aldehyde, 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 and their biologically active complexes
have been studied during the past decade [1]. Schiff
bases undergo photochromism and thermochromism
in the solid state by proton transfer [2]. Intramolecular
¼
¼
hydrogen bonds (either N±H O or N H±O) can
exist in aldimine compounds derived from the
aromatic aldehydes having a hydroxyl group in posi-
tion 2 to the aldehyde group [3]. The existence of the
enol (or predominantly enol) tautomer has been
established in all crystal structures of N-substituted
salicylaldimine listed so far in the Cambridge
Structure Database [3].
2. Experimental
The suitable crystals were obtained directly from
the synthesis of the compound. A solution of
0.003 mol of 2-aminophenol in 100 ml pure ethanol
was prepared and heated to boiling temperature.
Acetylacetone (0.003 mol) was dissolved in 50 ml of
hot ethanol. The mixture of the two solutions was then
re¯uxed for 3 h. Yellow needle crystals were formed
during the re¯uxions.
In this paper we investigated the structure of the
A crystal of dimensions 0.40 £ 0.30 £ 0.25 mm
* Corresponding author. Fax: 0090 312 2232395; e-mail:
elmali@science.ankara.edu.tr
was mounted on an Enraf±Nonius CAD-4
0022-2860/98/$ - see front matter q 1998 Elsevier Science B.V. All rights reserved.
PII S0022-2860(98)00377-9

296
M. Kabak et al. / Journal of Molecular Structure 470 (1998) 295±300
Table 1
Atomic coordinates ( £ 104) and equivalent isotropic displacement
diffractometer equipped with a graphite mono-
chromator. Cell constants were determined by least-
squares re®nement of diffractometer angles for 20
re¯ections collected in the range 3.158 , u , 7.258.
Three standard re¯ections were monitored every
120 min, but no considerable intensity variations
were recorded. A total of 1884 re¯ections were
recorded, with Miller indices hmin ˆ 2 9, hmax ˆ 9,
kmin ˆ 2 1, kmax ˆ 11, lmin ˆ 2 1, lmax ˆ 12. The
structure was solved by direct methods using
SHELXS86 [4]. The E-map computed from the
phase set with the best combined ®gure of merit
revealed the positions of all non-hydrogen atoms.
Full-matrix least-square re®nement of the fractional
coordinates of the non-hydrogen atoms with aniso-
tropic atomic displacement parameters was performed
SHELX97 [5]. Positions of H atoms (except hydroxyl
and amine H atoms) were generated from the assumed
geometries checked in Fourier maps and re®ned
isotropically. The hydroxyl and amine H atoms were
found in the difference Fourier maps calculated at the
end of the re®nement process as a small positive
electron density. Final R(F) and wR(F2) factors
were 0.038 and 0.109 for 134 parameters using
the I values of 1373 (I . 2s(I)) re¯ections. A
weighting scheme was used during the re®nement
as w ˆ 1/[\s2(Fo2) 1 (0.0807P)2 1 0.2220P],
where P ˆ [Fo2 1 2Fc2)/3. The highest and the
lowest peaks in the ®nal difference map were 0.21
Ê 2
parameters (A £ 103). Ueq is de®ned as one third of the trace of
orthogonalized Uij tensor
x
y
z
Ueq
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
C(9)
C(10)
C(11)
O(1)
O(2)
N(1)
6472(2)
4945(2)
4031(2)
2590(2)
1678(2)
513(2)
2298(2)
2877(2)
2555(1)
3440(1)
3264(1)
4312(2)
1783(1)
822(1)
77(1)
52(1)
54(1)
49(1)
76(1)
49(1)
50(1)
64(1)
72(1)
79(1)
65(1)
62(1)
63(1)
49(1)
1766(2)
1224(2)
779(2)
328(2)
424(2)
1118(1)
798(2)
271(2)
21470(2)
22279(2)
21708(2)
2304(2)
804(1)
356(2)
2187(2)
2871(2)
2589(2)
2068(1)
1846(1)
781(1)
842(2)
1766(2)
2231(2)
394(1)
4532(1)
1486(1)
2183(1)
1972(1)
Table 2
Ê
Bond distances (A) and angles (8) of X-ray, optimized X-ray, keto
and enol forms of molecule with esd's in parentheses
X-ray
X-opt.
Keto
Enol
1.491
C(1)±C(2)
C(2)±C(3)
1.504(3)
1.500
1.500
1.401(2)
1.370(2)
1.501(2)
1.330(2)
1.415(2)
1.400(2)
1.384(2)
1.371(3)
1.359(3)
1.377(3)
1.351(2)
1.256(2)
1.448
1.375
1.503
1.374
1.403
1.425
1.398
1.392
1.394
1.391
1.379
1.246
1.449
1.374
1.502
1.377
1.403
1.427
1.400
1.391
1.391
1.392
1.377
1.245
1.356
1.461
1.503
1.300
1.403
1.427
1.401
1.393
1.394
1.391
1.378
1.362
C(3)±C(4)
C(4)±C(5)
C(4)±N(1)
N(1)±C(6)
C(6)±C(7)
Ê 23
C(7)±C(8)
C(8)±C(9)
and 20.15 e A . Scattering factors were taken from
SHELX97.
C(9)±C(10)
Crystal data for (I), C11H13NO2, Mr
191.2 g mol21, orthorhombic P212121, a ˆ 8.834(1),
ˆ
C(10)±C(11)
C(7)±O(1)
C(2)±O(2)
Ê
Ê 3
b ˆ 10.508(2), c ˆ 11.212(2) A, V ˆ 1040.8(3) A ,
Z ˆ 4, Dc ˆ 1.22 gcm23, m(MoKa) ˆ 0.084 mm21
,
C(3)±C(2)±C(1)
C(4)±C(3)±C(2)
C(5)±C(4)±C(3)
N(1)±C(4)±C(3)
C(6)±N(1)±C(4)
C(7)±C(6)±N(1)
C(8)±C(7)±C(6)
C(9)±C(8)±C(7)
C(10)±C(9)±C(8)
119.9(2)
114.8
114.8
121.8
F(000) ˆ 408, T ˆ 292 K, R(F) ˆ 0.038 and wR(F2) ˆ
0.109 for 1373 observed re¯ections. Flack parameter
is found to be zero in the re®nement [6]. A list of
structure factors, H atom fractional atomic coordi-
nates and anisotropic atomic displacement parameters
for non-hydrogen atoms has been deposited with the
B.L.L.D. as Supplementary Publications No.
SUP26605 (10 pages).
124.9(2)
119.6(1)
120.8(1)
131.5(1)
116.2(1)
119.6(2)
119.9(2)
120.6(2)
126.6
116.1
122.6
125.8
120.6
121.2
119.8
120.1
120.3
121.4
117.2
122.1
121.7
117.1
120.5
126.7
116.9
122.3
125.7
120.1
120.5
120.1
120.1
120.5
120.9
117.9
121.9
122.0
117.5
120.6
128.1
113.4
120.0
124.6
119.5
121.3
120.2
119.3
121.0
121.4
116.8
123.5
121.1
117.6
110.9
C(11)±C(10)±C(9) 120.6(2)
C(10)±C(11)±C(6) 120.0(2)
Theoretical calculations were performed with the
semi-empirical quantum-mechanical program AM1
[7], which is part of the MOPAC package [8]. The
models of keto and enol forms of the title compound
were built and taken the values from Allen et al. [9].
C(11)±C(6)±C(7)
C(11)±C(6)±N(1)
O(1)±C(7)±C(8)
O(1)±C(7)±C(6)
O(2)±C(2)±C(1)
119.3(2)
124.5(2)
124.0(2)
116.6(1)
117.7(2)

M. Kabak et al. / Journal of Molecular Structure 470 (1998) 295±300
297
Fig. 1. The molecular structure of the title compound. Displacement ellipsoids are plotted at the 50% probability level.
The X-ray, keto and enol geometries were optimized
by using an AM1 method. To determine the confor-
mational energy pro®les, the optimized geometries
were kept ®xed and values of energies were calculated
as a function of the torsion angle u(C(4)±N(1)±C(6)±
C(11)) from 0 to 3608, varied every 108. The zero
values of the torsion angle u(C(4)±N(1)±C(6)±
C(11)) is 3.998 for optimized X-ray, 2 7.398 for
keto and 2 7.998 for enol structures. The results are
illustrated in Fig. 4.
3. Results and discussion
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 tautomer struc-
tures are listed in Table 2. The ORTEP [10] view of
the molecular structure of the title compound is given
in Fig. 1 and the crystal packing diagram in Fig. 2
[11].
Fig. 2. Crystal packing diagram for the title compound.

298
M. Kabak et al. / Journal of Molecular Structure 470 (1998) 295±300
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
Ê 23
¼
the H(1N) atom and revealing the presence of intramolecular N-H O hydrogen bonding. Contours are drawn from 2 0.17 to 0.34 e A at
Ê 23
intervals of 0.01 e A [16].
Thermochromic and photochromic properties of
the salicylideneanilines are a function of the 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 [12]. In agreement with the
above conclusions, the title compound is photo-
chromic [13] and the molecule is not planar; moieties
A(C(1)±(5), O(2)) and B(N(1), C(6)±(11), O(1))
map as a well designed small electron density
Ê 23
maximum of 0.21 e A (Fig. 3). The N(1)±H(1N),
¼
O(1) H(1N) and O(2) H(1N) distances are
¼
Ê
0.80(2), 2.31(2) and 2.00(2) A, respectively. The
¼
bond angles N(1)±H(1N) O(1) and N(1)±
¼
H(1N) O(2) are 105(1)8 and 138(2)8, respectively.
Ê
The C(2) ˆ O(2) bond [1.256(2) A] was determined
to be of bond order 1.5 from the values of the single
and double C±O bonds [9]. This along with the very
Ê
short C(3) ˆ C(4) bond [1.370(2) A] suggests the
[both planar with
a
maximum deviation of
presence of a signi®cant keto tautomer (Scheme 1).
Clearly, the keto tautomer is favoured over the enol
form.
Ê
20.020(2) A] are inclined at an angle of 32.8(1)8
re¯ecting mainly the twist about N(1)±C(6) [C(11)±
C(6)±N(1)±C(4) ˆ 237.1(3)8]. The distortion can be
seen in the crystal packing diagram (Fig. 2).
The crystal structure is stabilized by intermolecular
hydrogen bonds. The crystal structure determination
¼
indicates the existence of both N(1)±H(1N) O(1)
¼
and N(1)±H(1N) O(2) the bifurcated intramolecular
¼
Ê
hydrogen bonds as follows: N(1) O(1) 2.631(2) A
¼
Ê
and N(1) O(2) 2.641(2) A. These distances are
Ê
signi®cantly shorter than the sum, 3.07 A of the van
der Waals' radii for nitrogen and oxygen [14]. The
atom H(1N) was located from a difference Fourier
Scheme 1.

M. Kabak et al. / Journal of Molecular Structure 470 (1998) 295±300
299
Fig. 4. AM1 calculated conformation energies for X-ray, keto and enol structures vs the u(C(4)±N(1)±C(6)±C(11)) torsion angle.
There is also a strong intermolecular hydrogen
bond between O(1) and O(2)ii [2.647(2) A]
Ê
[symmetry code(ii): x 2 1/2, 2 y 1 1/2, 2 z] atoms
of neighbouring molecules (Fig. 2). The O(1)±H(1O)
methyl group. Burgi and Dunitz carried out an
extensive theoretical and experimental study on the
non-planar conformation of the N-benzylideneaniline
and related compounds [15]. 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 the increasing non-planarity; and (b)
the p-electron systems itself divisible into two
components, including, on the one hand, delocaliza-
tion 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 essentially 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).
Ê
distance is 0.90(2) A.
The optimized bond distances and angles of X-ray
structure and keto tautomer are in good agreement,
but enol form shows some differences (Table 2).
The conformations of the optimized X-ray geometry
are in good agreement with the keto tautomer. The
X-ray structure and keto tautomer are more stable than
the enol tautomer as seen in Fig. 4. The non-planar
conformations corresponding to zero values of
u(C(4)±N(1)±C(6)±C(11)) are the most stable
conformation for X-ray, keto and enol structures.
The energies corresponding to this non-planar
conformations are 247.95 kcal/mol for X-ray,
248.24 kcal/mol for keto and 2 40.66 kcal/mol for
enol structures. The energy pro®le as a function of
u(C(4)±N(1)±C(6)±C(11)) shows one maximum at
34.08 for X-ray, 37.48 for keto and 28.08 for enol
structures. This energy barrier arises from the steric
interactions between the hydroxyl substituent and the
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

300
M. Kabak et al. / Journal of Molecular Structure 470 (1998) 295±300
[5] G.M. Sheldrick, SHELX97, Program for the Re®nement of
È
Crystal Structures, Univ. of Gottingen, Germany, 1997.
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 rotated phenyl ring
might also contribute to the conformational energy
of the title compound.
[6] H.D. Flack, Acta Cryst. A39 (1983) 876±881.
[7] M.J.S. Dewar, E.G. Zeobisch, E.F. Healy, J.J.P. Stewart,
AM1, a new general purpose quantum mechanical molecular
model, J. Am. Chem. Soc. 107 (1985) 3902±3909.
[8] J.P. Stewart, MOPAC Version 6.0, Quantum Chemistry
Program Exchange 581, Indiana Univ., USA, 1989.
[9] F.H. Allen, O. Kennard, D.G. Watson, L. Brammer, A.G.
Orpen, J. Chem. Soc. Perkin Trans. 11 (1987) S1±S67.
[10] C.K. Johnson, ORTEPII, Report ORNL-5138. Oak Ridge
National Laboratory, Tennessee, USA, 1976.
[11] A.L. Spek, PLATON. Program for the Automated Analysis of
Molecular Geometry. Version of February 1996. University of
Utrecht, The Netherlands.
[12] I.M. Mavridis, E. Hadjoudis, A. Mavridis, Acta Cryst. B34
(1978) 3709±3715.
References
[13] M. Kabak, J. Org. Chem., submitted.
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[14] A. Bondi, J. Phys. Chem. 68 (1964) 441±451.
[15] H.B. Burgi, J.D. Dunitz, Helv. Chim. Acta 54 (1971) 1255±
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Tetrahedron 43 (1987) 1345±1360.
 Â
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