(E)-4-Bromo-2-[(phenyl­imino)­meth­yl]phenol: a new poly­morph and thermochromism

A novel poly­morph of (E)-4-bromo-2-[(phenyl­imino)­meth­yl]phenol is reported, with a dihedral angle between the planes of the two aromatic rings of 45.6 (1)°, significantly different to that of the previously published poly­morph. The structure contains an intra­molecular O—H⋯N hydrogen bond forming an S(6) ring.

Abstract

A new poly­morph of (E)-4-bromo-2-[(phenyl­imino)­meth­yl]phenol, C₁₃H₁₀BrNO, is reported, together with a low-temperature structure determination of the previously published poly­morph. Both poly­morphs were found to have an intra­molecular O—H⋯N hydrogen bond between the phenol OH group and the imine N atom, forming an S(6) ring. The crystals were observed to have different colours at room temperature, with the previously published poly­morph being more orange and the new poly­morph more yellow. The planarity of the mol­ecule in the two poly­morphs was found to be significantly different, with dihedral angles (Φ) between the two aromatic rings for the previously published ‘orange’ poly­morph of Φ = 1.8 (2)° at 120 K, while the new ‘yellow’ poly­morph had Φ = 45.6 (1)° at 150 K. It was also observed that both poly­morphs displayed some degree of thermochromism and upon cooling the ‘orange’ poly­morph became more yellow, while the ‘yellow’ poly­morph became paler upon cooling.

Introduction  

A wide range of N-salicylideneanilines, Schiff bases of salicyl­aldehyde derivatives with aniline derivatives, have been synthesized (Özek et al., 2007 ▸; Johmoto et al., 2012 ▸). The N-salicylideneaniline derivatives are inter­esting as they have generally been found to display thermochromism, with some also showing photochromism in the solid state (Cohen & Schmidt, 1962 ▸; Cohen et al., 1964 ▸; Fujiwara et al., 2004 ▸). The mechanism for the chromic colour change is believed to be due to a keto–enol tautomerism (Hadjoudis & Mavridis, 2004 ▸; Robert et al., 2009 ▸). The keto form is coloured, while the enol form is colourless and the switch can be induced either by changes in temperature or by irradiation. A link has been proposed between the thermochromic behaviour of a com­pound and the dihedral angle (Φ) between the two aromatic rings, with those having Φ < 25° being more likely to be strongly thermochromic (Hadjoudis & Mavridis, 2004 ▸; Robert et al., 2009 ▸). A larger interplanar angle allows increased orbital overlap and greater delocalization into the π-system, which reduces the basicity of the N atom and thus the thermochromism. The effect of substituents on the OH bond strength, nitro­gen-accepting ability and crystal packing have also been postulated as important in the chromic behaviour of the N-salicylideneanilines (Hadjoudis & Mavridis, 2004 ▸; Robert et al., 2009 ▸). It has also been observed that, in general, the N-salicylideneanilines that are more strongly coloured, typically red/orange, at room temperature, tend to be more strongly thermochromic than those that are paler, typically yellow, at room temperature (Ogawa et al., 2001 ▸; Fujiwara et al., 2009 ▸).

The structures of (E)-4-halogeno-2-[(phenyl­imino)­meth­yl]phenol have been reported for fluoro (Swetha et al., 2017 ▸), chloro (Bregman et al., 1964 ▸; Ogawa et al., 1998 ▸), bromo (Yan et al., 2014 ▸) and iodo (Swetha et al., 2019 ▸). Herein a new poly­morph of (E)-4-bromo-2-[(phenyl­imino)­meth­yl]phenol, denoted 1B, is reported together with a new low-temperature determination of the previously reported poly­morph, 1A (Yan et al., 2014 ▸). Both poly­morphs were found to be thermochromic to some extent.

Experimental  

Synthesis and crystallization  

(E)-4-Bromo-2-[(phenyl­imino)­meth­yl]phenol was synthesized by direct condensation of 5-bromo­salicyl­aldehyde and aniline in ethanol. The two materials (0.005 mol of each, 1.000 g of 5-bromo­salicyl­aldehyde and 0.466 g of aniline) were dissolved separately in ethanol (25 ml). The resultant solutions were combined and refluxed with stirring for 4 h. After removal of any precipitate, the solution was rotary evaporated until further precipitate formed, the solid filtered off, rinsed with ethanol and left to dry, giving a yield of 94% (1.304 g, 0.0047 mol). Yellow single crystals (of 1B) crashed out of the crude reaction mixture and orange single crystals (of 1A) were produced by recrystallization from ethanol.

Refinement  

Crystal data, data collection and structure refinement details are summarized in Table 1 ▸. All H atoms, apart from the OH hydrogen involved in the intra­molecular hydrogen bonding with the imine N atom, were positioned geometrically and refined using a riding model. The H atoms involved in the intra­molecular hydrogen bond were located in the Fourier difference map wherever feasible. In 1A, the O—H distance was restrained to 0.86 (1) Å.

Table 1. Experimental details.

For both structures: C₁₃H₁₀BrNO, M _r = 276.13, Z = 4. Experiments were carried out with Mo Kα radiation using an Oxford Diffraction Xcalibur (Sapphire3, Gemini ultra) diffractometer. An analytical absorption correction [CrysAlis PRO (Oxford Diffraction, 2010 ▸), based on expressions derived by Clark & Reid (1995 ▸)] was used. Refinement was with 2 restraints. H atoms were treated by a mixture of independent and constrained refinement.

	Polymorph 1A	Polymorph 1B
Crystal data
Crystal system, space group	Orthorhombic, P c a21	Monoclinic, C c
Temperature (K)	120	150
a, b, c (Å)	12.2768 (3), 4.4829 (1), 19.6694 (4)	25.8944 (13), 6.9439 (4), 6.1499 (4)
α, β, γ (°)	90, 90, 90	90, 91.381 (5), 90
V (Å3)	1082.52 (4)	1105.48 (11)
μ (mm−1)	3.77	3.69
Crystal size (mm)	0.46 × 0.20 × 0.05	0.58 × 0.49 × 0.22
Data collection
T min, T max	0.383, 0.847	0.190, 0.585
No. of measured, independent and observed [I > 2σ(I)] reflections	13133, 2215, 2133	7049, 2254, 2142
R int	0.043	0.051
(sin θ/λ)max (Å−1)	0.625	0.625
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.022, 0.053, 1.05	0.039, 0.100, 1.05
No. of reflections	2215	2254
No. of parameters	149	148
Δρmax, Δρmin (e Å−3)	0.39, −0.23	0.95, −0.34
Absolute structure	Flack x determined using 993 quotients [(I +) − (I −)]/[(I +) + (I −)] (Parsons et al., 2013 ▸)	Flack x determined using 1007 quotients [(I +) − (I −)]/[(I +) + (I −)] (Parsons et al., 2013 ▸)
Absolute structure parameter	−0.006 (8)	−0.010 (19)

Computer programs: CrysAlis PRO (Oxford Diffraction, 2010 ▸), SHELXS97 (Sheldrick, 2008 ▸), SHELXL2018 (Sheldrick, 2015 ▸) and OLEX (Dolomanov et al., 2009 ▸).

Results and discussion  

The structures of poly­morphs 1A and 1B are shown in Fig. 1 ▸. The structure of 1A at 120 K was consistent with the previously published structure at room temperature (Yan et al., 2014 ▸). The structure of 1A was obtained in the ortho­rhom­bic space group Pca2₁, while 1B was obtained in the monoclinic space group Cc. The com­pound consists of a hy­droxy-substituted phenyl ring linked via an imine group to a second unsubstituted phenyl group. In both poly­morphs, the structures were found to exist in the enol form, with C7=N1 bond lengths of 1.282 (4) Å for 1A and 1.284 (10) Å for 1B, indicating a double bond, and C1—O1 bond lengths of 1.350 (5) Å for 1A and 1.351 (9) Å for 1B, indicating a single bond. The structures showed quite different dihedral angles, with 1A having Φ = 1.8 (2)° at 120 K and 1B having Φ = 45.6 (1)° at 150 K. Upon cooling, the structures were both found to display some degree of thermochromism with 1A changing from orange at room temperature to yellow at 120 K and 1B, which was yellow at room temperature, becoming slightly paler at 150 K (Fig. 2 ▸). The differences in the thermo­chromic behaviour of the two poly­morphs are consistent with literature suggestions that a larger dihedral angle increases the overlap of the π-system reducing the nitro­gen basicity, disfavouring the keto form and thus also reducing the thermochromism of the com­pound.

Figure 1.

Illustration of the structures of (a) 1A and (b) 1B at 120 (2) and 150 (2) K, respectively, with the atomic numbering schemes depicted. Anisotropic displacement parameters are shown at the 50% probability level.

Figure 2.

Illustration of the colour change observed upon cooling (a) 1A and (b) 1B.

An intra­molecular O1—H1⋯N1 hydrogen bond, involving the phenol OH group and imine N atom, was identified in the structures of both poly­morphs and creates an S(6) ring. The hydrogen-bonding parameters were almost identical in the two structures, with a donor–acceptor distance of ∼2.59 Å and a hydrogen-bond angle of ∼150° (Tables 2 ▸ and 3 ▸.). The packing of the two poly­morphs was unsurprisingly significantly different given the large difference in the dihedral angles. In poly­morph 1A, the mol­ecules are essentially planar and orientated diagonally such that the plane of the mol­ecule is perpendicular to the bc plane and, as a result of the 2₁ screw axis, the diagonal slant of alternate mol­ecules along the a-axis direction essentially align in opposite directions (Fig. 3 ▸ a). It was also noted that there were short π-type contacts between the C=N group and the phenol ring in the 01 direction, with a centroid-to-centroid (C=N) distance of 3.326 (1) Å. These can be seen on the Hirshfeld surface of 1A as red dots (Fig. 4 ▸ a). In poly­morph 1B, although the mol­ecules themselves are twisted, the mol­ecules are orientated relative to each other such that they create planes parallel to the ac plane direction (see Fig. 3 ▸ b).

Table 2. Hydrogen-bond geometry (Å, °) for poly­morph 1A .

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O1—H1⋯N1	0.86 (1)	1.82 (3)	2.593 (4)	150 (5)

Table 3. Hydrogen-bond geometry (Å, °) for poly­morph 1B .

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O1—H1⋯N1	0.86 (11)	1.82 (11)	2.590 (10)	148 (10)

Figure 3.

(a) Illustration of the packing for 1A, looking down the a axis; mol­ecules in blue are in-plane behind those in element colours. (b) View of polymorph 1B, looking down the c axis.

Figure 4.

The Hirshfeld surface plot (top) and fingerprint plot (bottom) for (a) 1A and (b) 1B.

Examining the Hirshfeld fingerprint plots (Turner et al., 2017 ▸) for the two structures highlights the differences in the two structures, not least in the shapes of the two plots (Fig. 4 ▸). For 1A, the O⋯H and Br⋯H contacts are quite obvious, while in 1B the H⋯H and C⋯H contacts are significantly more pronounced, slightly masking the O⋯H and Br⋯H contacts. These differences are very apparent on the Hirshfeld surface for both com­pounds with a greater number of red spots on the surface of 1A that are more noticeable than for 1B, showing that 1A has more short contacts.

The two poly­morphs of (E)-4-bromo-2-[(phenyl­imino)­meth­yl]phenol reported herein are particularly inter­esting as part of a study into N-salicylideneanilines because they show significantly different mol­ecular conformations and colours at room temperature. In line with the literature, the extent of the thermochromism was found to be linked to the dihedral angle, with 1A [Φ = 1.8 (2)°] showing a greater colour change upon cooling than observed for 1B [Φ = 45.6 (1)°].

Supplementary Material

CCDC references: 2025079, 2025078