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N-Salicylideneaniline (SA), C13H11NO, belongs to the large family of aromatic Schiff bases. It is of particular importance owing to its reversible photoreactivity. SA forms two photochromic polymorphs, both with two non-coplanar benzene rings. In addition, we have recently discovered a planar polymorph, named the [beta]-polymorph, which will be discussed in a subsequent paper. We report here the structure of the [alpha]2-polymorph in the orthorhombic crystal system. This compound exhibits a strong intra­molecular O-H...N hydrogen bond and the dihedral angle between the two rings varies with temperature.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S010827010500911X/sq1198sup1.cif
Contains datablocks global, I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S010827010500911X/sq1198Isup2.hkl
Contains datablock I

CCDC reference: 1254457

Comment top

Photochromism of aromatic molecules has been known and discussed for a long time (Senier & Shepheard, 1909; Senier et al., 1912). Schiff bases are important in diverse fields of chemistry and biochemistry owing to their biological activity (Lozier et al., 1975; Garnovskii et al., 1993) and can be classified according to their photochromic or thermochromic properties (Cohen et al., 1964; Hadjoudis et al., 1987). Some anils of salicylaldehyde have attracted the interest of chemists and physicists because of their reversible photoreactivity in the solid state. The design and synthesis of organic compounds with specific physical properties by methods of crystal engineering is under development.

Aromatic Schiff bases are a typical class of photochromic materials, involving both ESIPT (excited state intramolecular proton transfer) and cis–trans isomerization to form an orange–red coloured photoproduct from the colourless crystal. It is generally accepted that the stable form of the molecule of N-salicylideneaniline, SA, (I), in the ground state is a pale-yellow trans-enol isomer with an intramolecular hydrogen bond between the hydroxyl group and the N atom. Upon photoexcitation with ultraviolet (UV) light, this form undergoes an ultrafast proton transfer from the hydroxyl group to the N atom. Because of an electronic redistribution in the excited state, the two benzene rings rotate around the C7—N axis. A coloured species (dark orange), the keto form, is then produced in the excited singlet state. However, the details of the structural configuration of the keto form give rise to competition between forms in which the O atom and the imine H atom will be in cis or trans configurations with regard to the C7—N bond (Hadjoudis, 1995; Shen et al., 2000). The coloured species can be reversibly bleached, either by irradiation using visible light or upon heating the crystals. The photochromic reaction of SA has been extensively studied with various spectroscopic methods (Otsubo et al., 2002).

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In 1964, Cohen et al. observed polymorphism of anils and also determined the space group and lattice constants of (I), which are consistent with the corresponding values presented here. We report for the first time the complete structure of the α2-polymorph of SA in the ground state (Fig. 1). We selected a small crystal in order to perform some spectroscopic measurements as well.

Some of the distances and angles of (I) are very different from the values given by Destro et al. (1978) for the α1-polymorph, where some distances and angles were themselves judged to be out of the expected range. For example, the C6—O or C1—C7 distances of 1.320 (7) and 1.529 (5) Å for the α1-polymorph are very different from our values for the α2-polymorph, of 1.353 (3) and 1.450 (3) Å at room temperature and 1.352 (3) and 1.455 (3) Å at low temperature. Moreover, all the angles previously mentioned by Destro et al. (1978) as being out of the expected range exhibit satisfactory values in the present model.

The bond lengths and angles of (I) are in agreement with the International Tables for Crystallography (Allen et al., 1999). Some of these values will now be discussed in more detail by comparison with similar compounds (Bregman et al., 1964 Which?; Bürr & Hobson, 1969; Destro et al., 1978; Lindeman et al., 1982 Which?; Elmali & Elerman, 1997, 1998; Elmali et al., 1998; Burgess et al., 1999; Harada et al., 1999; Fukuda et al., 2003; Karadayi et al., 2003; Yeap et al., 2003).

Lindeman et al. (1982 Which?) suggested that the two compounds N-salicylidene-4-chloroaniline (planar) and N-salicylidene-4-bromoaniline (non-planar) differ only in their conformation, while the other geometric parameters are close to each other. On the other hand, Ogawa et al. (1998) reported that the lengths of each of those bonds which can change by tautomerism, specifically C1—C6, C1—C7, C6—O and C7—N, are significantly different between the OH and NH form.

The heteroatom bonds are those most affected by molecular self-isomerization (H-atom transfer). The C6—O bond is the most sensitive indicator of the type of tautomeric form. This is a single bond for the enol–imino tautomers, comparable with those in phenols [1.362 (15) Å], while it is shortened in the keto–amino tautomers. These values lie between those found in phenols and benzoquinones [1.222 (13) Å]. In contrast, the C7—N bond is lengthened to 1.339 (16) Å in the keto tautomer, while in the enol tautomer, it is 1.279 (8) Å. The C6—O and C7—N bond distances of the α2-polymorph are 1.352 (3) and 1.281 (3) Å, respectively, and correspond to the enol-isomer, in agreement with the expected ground-state conformation.

The wide spread of the C7N bond lengths has been the subject of many discussions, and several hypotheses have been invoked to explain the apparent shortening at room temperature (Bregman et al., 1964 Which?; Moustakali-Mavridis et al., 1978; Harada et al., 2004). A torsional vibration of the C—Ph and N—Ph bonds, resulting in a temperature dependence of the C7N bond length, has been postulated in analogy with benzylideneanilines, (E)-stilbenes, azobenzenes and 1,2-diphenylethanes (Ogawa et al., 1992, 1995; Harada et al., 1995, 1997, 2004; Harada & Ogawa, 2001). In the α2-polymorph, we also observe a decrease in the C7 N bond length, from 1.281 (3) to 1.267 (3) Å, with increasing temperature from 120 to 293 K, but librational shortening is improbable since the largest displacement component is along the bond. The other bond lengths (e.g. C1—C7 and N1—C8) change little with temperature, which prevents the use of Shomaker & Trueblood's rigid-body model (1968). The thermal displacements are approximately proportional to the temperature. A possible asphericity shift of the atomic positions due to the bonding electrons is then expected to be temperature independent and cannot explain the shortening of the bond.

Fig. 2 shows a difference electron-density map at 120 K, revealing the position of the H atom. The map clearly shows a peak that can be assigned to one H atom connected to atom O1. The H atom belonging to the oxygen site was found at an O1—H1A distance of 1.02 (4) Å and the C6—O1—H1A bond angle is 109 (2)°, as expected. A strong intramolecular hydrogen bond (O—H···N) occurs between atoms O1 and N1 [2.615 (3) Å], the H atom being bonded to atom O1. This distance is significantly shorter than the sum of the van der Waals radii for N and O (3.07 Å; Bondi, 1964) and is comparable with those observed for other similar compounds.

Since the same compound may occur in dimorphs, of which one is thermochromic and the others are photochromic, it seems that it is the crystal structure that determines this behaviour, rather than the molecule as such. From observations on some thermochromic and photochromic Schiff base compounds, it was proposed that molecules exhibiting thermochromism are planar, while those exhibiting photochromism are non-planar (Moustakali-Mavridis et al., 1978). Bregman et al. (1964 Which?) have suggested that photochromism of Schiff bases is related to the conformation and packing of molecules in the crystal state. Photochromic crystals are made up of non-planar molecules in which the aniline ring is significantly twisted out of the salicylidene moiety; thus, each molecule avoids tight packing forces. As the molecules pack loosely, there is sufficient room for the photo-induced isomerization of the molecules to occur in the crystal lattice.

The most striking feature in (I) is the twist of the aniline ring out of the C1—C7N1—C8 plane by 47.3° at 120 K, compared with 45.1° at room temperature. The twist of the benzylidene ring out of this plane is much smaller (7.1° at 120 K to 6.6° at room temperature). The angle between the two phenyl rings in the α2-polymorph is 54.1° at 120 K, compared with 51.4° at room temperature. In the α1 form, Destro et al. reported 49°. Only two other cases relating to this study were found. However, they concern non-planar compounds and no significant change could be detected: from 10 to 9.9° in 5-methoxysalicylaldimine (Popovic et al., 2002) and from 0.7 to 1.1° in 5-chlorosalicylideneaniline (mean value; Bregman et al., 1964 Which?). Therefore, the variation may not only be due to temperature variation but, more probably, the changes are more significant for a non-planar compound.

Flat molecules have a characteristic packing arrangement, in stacks along the shortest axis, in which the molecules are inclined. Within each stack, the molecular planes pack with short intermolecular distances of the order of 3.5 Å, normal to the molecular planes. In contrast with flat molecules, rotation of the anil ring out of the plane in the α-polymorphs prevents any close parallel stacking and the structure is relatively open, with molecules arranged head-to-tail (Fig. 3). This packing was also observed for the 2-bromo-, 2-iodo- and 2-chloro-SA derivatives (Bregman et al., 1964 Which?; Bürr & Hobson, 1969; Elmali & Elerman, 1997). All of these have the same space group as the α2-polymorph and similar cells.

Experimental top

Crystals of (I) were obtained by recrystallization from methanol (m.p. ~322 K). The half-life of the spontaneous fading (dark reaction) of the red state is about 30 h at 298 K, a rate which is about 100 times slower than that of the α1 crystalline form. Cycling between the yellow and red states was performed 50000 times with no observed fatigue (Lo et al., 1971). Procedures for the recrystallization of the α1-polymorph from methanol and the α2-polymorph from petroleum ether have been reported by Cohen et al. (1964).

Refinement top

Since O, N, C, H do not possess a high enough enantiomorph-discriminating capacity with Mo radiation, 827 Friedel pairs were merged. The hydroxylic H atom was located in the difference electron-density (Fourier) map at the end of the refinement procedure and was refined isotropically. Carbon-bound H atoms were placed in calculated positions, with C—H distances of 0.93 Å, and then refined using a riding model, with Uiso(H) = 1.2Ueq(C). Please check added text.

Computing details top

Data collection: KM4 (Oxford Diffraction, 2001); cell refinement: KMRED (Oxford Diffraction, 2001); data reduction: XPREP (Siemens, 1998); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEPIII (Farrugia, 1997) and XP (Siemens, 1998); software used to prepare material for publication: SHELXL97.

Figures top
[Figure 1] Fig. 1. The molecular structure of (I), showing the atom-numbering scheme and the intramolecular hydrogen bonding (dashed line). Displacement ellipsoids are plotted at the 50% probability level.
[Figure 2] Fig. 2. Difference electron-density map for (I) at 120 K. The map clearly reveals a peak assigned to one H atom connected to O1. Contour interval 0.03 e Å−3.
[Figure 3] Fig. 3. The packing arrangement in the plane (0kl) of the α2-polymorph of salicylideneaniline.
2-(phenyliminomethyl)phenol top
Crystal data top
C13H11NODx = 1.280 Mg m3
Mr = 197.23Melting point: 49 K
Orthorhombic, P212121Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2ac 2abCell parameters from 7113 reflections
a = 6.0750 (11) Åθ = 3.6–26.4°
b = 11.6306 (15) ŵ = 0.08 mm1
c = 14.484 (2) ÅT = 120 K
V = 1023.4 (3) Å3Prism, yellow
Z = 40.18 × 0.14 × 0.06 mm
F(000) = 416
Data collection top
Oxford Model CCD area-detector
diffractometer
1004 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tubeRint = 0.074
Graphite monochromatorθmax = 26.4°, θmin = 3.6°
oscillation scansh = 77
5769 measured reflectionsk = 1414
1237 independent reflectionsl = 1718
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.041Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.101H atoms treated by a mixture of independent and constrained refinement
S = 1.03 w = 1/[σ2(Fo2) + (0.0566P)2]
where P = (Fo2 + 2Fc2)/3
1237 reflections(Δ/σ)max < 0.001
140 parametersΔρmax = 0.18 e Å3
0 restraintsΔρmin = 0.19 e Å3
Crystal data top
C13H11NOV = 1023.4 (3) Å3
Mr = 197.23Z = 4
Orthorhombic, P212121Mo Kα radiation
a = 6.0750 (11) ŵ = 0.08 mm1
b = 11.6306 (15) ÅT = 120 K
c = 14.484 (2) Å0.18 × 0.14 × 0.06 mm
Data collection top
Oxford Model CCD area-detector
diffractometer
1004 reflections with I > 2σ(I)
5769 measured reflectionsRint = 0.074
1237 independent reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0410 restraints
wR(F2) = 0.101H atoms treated by a mixture of independent and constrained refinement
S = 1.03Δρmax = 0.18 e Å3
1237 reflectionsΔρmin = 0.19 e Å3
140 parameters
Special details top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. The value of the hydrogen bond is normalized in PLATON.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger. The maximal resolution has been set to 0.8 Å during the refinements.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
C10.3867 (4)0.37561 (19)0.51910 (16)0.0189 (5)
C20.4975 (4)0.3728 (2)0.43423 (16)0.0233 (5)
H2A0.63920.34270.43160.028*
C30.4008 (5)0.41346 (19)0.35480 (17)0.0263 (6)
H3A0.47480.40930.29870.032*
C40.1914 (5)0.4608 (2)0.35951 (18)0.0267 (6)
H4A0.12670.48970.30610.032*
C50.0774 (4)0.4660 (2)0.44174 (16)0.0263 (6)
H5A0.06240.49850.44350.032*
C60.1713 (4)0.4226 (2)0.52202 (16)0.0206 (6)
C70.4952 (4)0.33256 (19)0.60175 (15)0.0201 (5)
H7A0.64150.30940.59820.024*
C80.5122 (4)0.2868 (2)0.75854 (15)0.0212 (5)
C90.4144 (5)0.2065 (2)0.81543 (17)0.0281 (6)
H9A0.27450.17890.80170.034*
C100.5240 (5)0.1672 (2)0.89276 (18)0.0335 (7)
H10A0.45900.11150.92990.040*
C110.7299 (5)0.2098 (2)0.91549 (18)0.0347 (7)
H11A0.80280.18300.96770.042*
C120.8263 (5)0.2925 (2)0.86000 (17)0.0298 (6)
H12A0.96320.32260.87550.036*
C130.7183 (4)0.3308 (2)0.78106 (16)0.0252 (6)
H13A0.78390.38570.74340.030*
N10.3943 (3)0.32578 (17)0.67917 (13)0.0219 (5)
O10.0561 (3)0.42616 (15)0.60183 (13)0.0303 (5)
H1A0.147 (7)0.389 (3)0.653 (2)0.070 (12)*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
C10.0172 (12)0.0149 (11)0.0246 (12)0.0030 (10)0.0011 (11)0.0020 (9)
C20.0183 (12)0.0185 (12)0.0330 (14)0.0012 (11)0.0003 (12)0.0027 (10)
C30.0350 (15)0.0182 (12)0.0255 (13)0.0047 (12)0.0048 (13)0.0018 (10)
C40.0363 (15)0.0175 (12)0.0264 (13)0.0043 (12)0.0112 (13)0.0021 (11)
C50.0217 (13)0.0198 (12)0.0373 (15)0.0027 (11)0.0081 (13)0.0006 (11)
C60.0180 (12)0.0179 (12)0.0259 (13)0.0008 (10)0.0012 (11)0.0012 (10)
C70.0179 (12)0.0166 (11)0.0258 (12)0.0017 (10)0.0009 (12)0.0035 (10)
C80.0238 (12)0.0192 (11)0.0206 (12)0.0081 (11)0.0005 (11)0.0040 (9)
C90.0305 (14)0.0248 (13)0.0290 (13)0.0021 (12)0.0020 (13)0.0037 (11)
C100.0483 (18)0.0253 (13)0.0269 (13)0.0046 (13)0.0035 (14)0.0024 (11)
C110.0459 (18)0.0318 (15)0.0264 (13)0.0116 (14)0.0069 (14)0.0003 (12)
C120.0283 (14)0.0324 (14)0.0287 (13)0.0067 (12)0.0037 (13)0.0049 (12)
C130.0263 (14)0.0253 (14)0.0241 (13)0.0041 (12)0.0037 (11)0.0017 (11)
N10.0198 (10)0.0217 (10)0.0243 (10)0.0024 (9)0.0013 (10)0.0023 (9)
O10.0230 (10)0.0343 (10)0.0337 (10)0.0063 (8)0.0057 (9)0.0021 (8)
Geometric parameters (Å, º) top
C1—C21.402 (3)C8—C91.379 (3)
C1—C61.419 (3)C8—C131.391 (4)
C1—C71.455 (3)C8—N11.429 (3)
C2—C31.376 (3)C9—C101.381 (3)
C2—H2A0.9300C9—H9A0.9300
C3—C41.388 (4)C10—C111.385 (4)
C3—H3A0.9300C10—H10A0.9300
C4—C51.379 (4)C11—C121.384 (4)
C4—H4A0.9300C11—H11A0.9300
C5—C61.390 (3)C12—C131.391 (3)
C5—H5A0.9300C12—H12A0.9300
C6—O11.352 (3)C13—H13A0.9300
C7—N11.281 (3)O1—H1A1.02 (4)
C7—H7A0.9300
C2—C1—C6118.5 (2)C9—C8—C13119.8 (2)
C2—C1—C7119.7 (2)C9—C8—N1118.6 (2)
C6—C1—C7121.7 (2)C13—C8—N1121.5 (2)
C3—C2—C1121.4 (2)C8—C9—C10120.1 (3)
C3—C2—H2A119.3C8—C9—H9A120.0
C1—C2—H2A119.3C10—C9—H9A120.0
C2—C3—C4119.1 (2)C9—C10—C11120.6 (3)
C2—C3—H3A120.4C9—C10—H10A119.7
C4—C3—H3A120.4C11—C10—H10A119.7
C5—C4—C3121.3 (2)C12—C11—C10119.5 (3)
C5—C4—H4A119.3C12—C11—H11A120.2
C3—C4—H4A119.3C10—C11—H11A120.2
C4—C5—C6120.0 (2)C11—C12—C13120.0 (3)
C4—C5—H5A120.0C11—C12—H12A120.0
C6—C5—H5A120.0C13—C12—H12A120.0
O1—C6—C5119.5 (2)C12—C13—C8119.9 (3)
O1—C6—C1121.0 (2)C12—C13—H13A120.0
C5—C6—C1119.6 (2)C8—C13—H13A120.0
N1—C7—C1121.6 (2)C7—N1—C8118.94 (19)
N1—C7—H7A119.2C6—O1—H1A109 (2)
C1—C7—H7A119.2
C6—C1—C2—C30.5 (3)C6—C1—C7—N16.3 (3)
C7—C1—C2—C3179.5 (2)C13—C8—C9—C102.2 (3)
C1—C2—C3—C41.6 (3)N1—C8—C9—C10179.9 (2)
C2—C3—C4—C51.1 (3)C8—C9—C10—C111.8 (4)
C3—C4—C5—C60.4 (4)C9—C10—C11—C120.1 (4)
C4—C5—C6—O1178.7 (2)C10—C11—C12—C131.2 (4)
C4—C5—C6—C11.5 (3)C11—C12—C13—C80.8 (4)
C2—C1—C6—O1179.2 (2)C9—C8—C13—C120.9 (3)
C7—C1—C6—O11.9 (3)N1—C8—C13—C12178.6 (2)
C2—C1—C6—C51.0 (3)C1—C7—N1—C8177.7 (2)
C7—C1—C6—C5178.0 (2)C9—C8—N1—C7134.6 (2)
C2—C1—C7—N1174.8 (2)C13—C8—N1—C747.7 (3)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O1—H1A···N11.02 (3)1.72 (4)2.615 (3)145 (3)

Experimental details

Crystal data
Chemical formulaC13H11NO
Mr197.23
Crystal system, space groupOrthorhombic, P212121
Temperature (K)120
a, b, c (Å)6.0750 (11), 11.6306 (15), 14.484 (2)
V3)1023.4 (3)
Z4
Radiation typeMo Kα
µ (mm1)0.08
Crystal size (mm)0.18 × 0.14 × 0.06
Data collection
DiffractometerOxford Model CCD area-detector
diffractometer
Absorption correction
No. of measured, independent and
observed [I > 2σ(I)] reflections
5769, 1237, 1004
Rint0.074
(sin θ/λ)max1)0.625
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.041, 0.101, 1.03
No. of reflections1237
No. of parameters140
H-atom treatmentH atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å3)0.18, 0.19

Computer programs: KM4 (Oxford Diffraction, 2001), KMRED (Oxford Diffraction, 2001), XPREP (Siemens, 1998), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), ORTEPIII (Farrugia, 1997) and XP (Siemens, 1998), SHELXL97.

Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O1—H1A···N11.02 (3)1.72 (4)2.615 (3)145 (3)
 

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