Reinvestigation of the crystal structure of N-(4-chlorobenzylidene)-2-hydroxyaniline: a three-dimensional structure containing O—H⋯N, O—H⋯O and C—H⋯π(arene) hydrogen bonds

The molecules of the title compound are almost planar, with an intramolecular O—H⋯N hydrogen bond, and they are linked into a three-dimensional framework by a combination of one O—H⋯O hydrogen bond and two C—H⋯π(arene) hydrogen bonds.


Chemical context
Schiff bases exhibit a very wide range of biological activities (da Silva et al., 2011) and are also of interest because of the photochromic and thermochromic properties (Hadjoudis & Mavridis, 2004;Minkin et al., 2011). The molecular structure of N-(4-chlorobenzylidene)-2-hydroxyaniline (I) was reported in the space group P2 1 /n a number of years ago [CSD (Groom et al., 2016) refcode FAKDIE; Kamwaya & Khoo, 1985]. However, scrutiny of the reported structure reveals a number of unexpected features: the refinement was conducted in a non-standard monoclinic cell having < 90 ; the C-C distances in the aryl rings range between 1.336 and 1.427 Å ; no H atoms bonded to C atoms were included; and the C-O-H angle was reported as 88 , which seems very small, while the associated intramolecular HÁ Á ÁN distance was only 1.66 Å , which is very short, even for a strong O-HÁ Á ÁN hydrogen bond. Hence any conclusions drawn from the deposited atomic coordinates may be untrustworthy. The structures of several positional isomers of (I) have been reported recently (Kazak et al., 2004;Sundararaman et al., 2007;Saranya et al., 2015) and in view of these reports and of the widespread applications of Schiff bases, we have accordingly now collected a new data set for compound (I), whose structure we report here (Fig. 1).

Structural commentary
The molecular skeleton of compound (I) (Fig. 1) is very nearly planar: the r.m.s. deviation from the mean plane through all of the non-H atoms is only 0.043 Å , with a maximum displacement from this plane of 0.0900 (10) Å for atom Cl14. The ISSN 2056-9890 dihedral angle between the two aryl rings in the molecule of (I) is only 3.31 (9) . A fairly short intramolecular O-HÁ Á ÁN contact (Table 1) may be an influence on the molecular conformation. The C-C distances within the rings lie in the range 1.377 (3)-1.393 (3) Å for the hydroxylated ring, and 1.366 (3)-1.387 (3) Å for the chlorinated ring, much smaller than the range previously reported (Kamwaya & Khoo, 1985), while the C-O-H angle is 103.0 (18) . The inter-axial angle , as found here and as previously reported, 0 , are related by = (180 -0 ) and the atomic coordinates found here can be related to those reported previously, after inversion and a straightforward origin shift, by the transformation (x, y, Àz), suggesting that the previous determination may have in advertently used a left-handed axis set.

Supramolecular features
The supramolecular assembly is dominated by two C-HÁ Á Á(arene) hydrogen bonds (Table 1): that having atom C6 as the donor links molecules related by the 2 1 screw axis along (0.75, y, 0.75), and that having atom C15 as the donor links molecules related by the 2 1 screw axis along (0.25, y, 0.75), so forming two distinct types of chain parallel to [010]. In the first of these, the chlorinated ring provides both the donor and the acceptor, while in the second the hydroxylated ring provides both the donor and the acceptor (Fig. 2). The combination of these two chains links the molecules of (I) into sheets lying parallel to (001) (Fig. 2). Two sheets of this type, related to one another by inversion, pass through each unit cell, in the domains 0 < z < 0.5 and 0.5 < z < 1.0. Adjacent sheets are linked into a continuous three-dimensional framework by a combination of a short O-HÁ Á ÁO contact involving inversionrelated pairs of molecules (Fig. 3), and an aromaticstacking interaction. The aryl rings (C1-C6) and (C11-C16) in the molecules at (x, y, z) and (1 À x, 1 À y, 1 À z), respectively, which lie in adjacent sheets, make a dihedral angle of 3.31 (9) : the ring centroid separation is 3.773 (2) Å and the shortest perpendicular distance from the centroid of one ring to the plane of the other is 3.465 (2) Å , giving a ring centroid offset of ca 1.49 Å . In the earlier report (FAKDIE; Kamwaya & Table 1 Hydrogen-bond geometry (Å , ).

Figure 1
The molecular structure of compound (I), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level.

Figure 2
Part of the crystal structure of compound (I), showing the formation of a sheet lying parallel to (001) and built from C-HÁ Á Á(arene) hydrogen bonds. For the sake of clarity, H atoms bonded to C atoms but not involved in the motifs shown have been omitted. Khoo, 1985), the absence of any H atoms bonded to C atoms means that the C-HÁ Á Á(Arene) interactions were necessarily overlooked, and the apparent misplacement of the hydroxyl H atom noted above means that the intermolecular O-HÁ Á ÁO hydrogen bond was also overlooked.

Database survey
The structures of a number of Schiff bases which are isomeric with compound (I) have been reported in recent years (see Fig. 4). In each of compounds (II) (Kazak et al., 2004) and (III) (Sundararaman et al., 2007), the molecules are linked by O-HÁ Á ÁN hydrogen bonds to form chains of the C(7) and C(8) types, respectively, while in compound (IV) (Saranya et al., 2015) the sole O-HÁ Á ÁN interaction is intramolecular. The bromo derivative (V) (Jiao et al., 2006) is isomorphous with the chloro analogue (I), but these two compounds are not strictly isostructural in that the structure of (V) contains only one C-HÁ Á Á(arene) hydrogen bond, as compared with two such bonds in the structure of (I). On the other hand, compounds (III) and (VI) (Jothi et al., 2012) do appear to be isostructural. Finally, we note the isomeric nitrone (VII), which crystallizes in space group P1 with Z 0 = 2: each of the two types of molecule forms a C(4) chain built from C-HÁ Á ÁO hydrogen bonds (Vijayalakshmi et al., 2000).

Synthesis and crystallization
To a solution of 2-aminophenol (0.917 mmol) in ethanol (20 cm 3 ), an equimolar quantity of 4-chlorobenzaldehyde was added dropwise, with constant stirring, in the presence of a catalytic amount of glacial acetic acid. The mixture was then heated under reflux for 4 h. When the reaction was complete, as judged using thin layer chromatography, the reaction mixture was cooled to ambient temperature and the resulting solid product was collected by filtration and recrystallized from dimethyl sulfoxide, to give crystals suitable for singlecrystal X-ray diffraction; m.p. 358 K. Compound (I) and some closely related analogues.

Figure 3
Part of the crystal structure of compound (I), showing the O-HÁ Á ÁO interaction between an inversion-related pair of molecules. For the sake of clarity, the unit-cell outline and H atoms bonded to C atoms have been omitted. Atoms marked with an asterisk (*) are at the symmetry position (1 À x, 2 À y, 1 À z).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were located in difference maps. The H atoms bonded to C atoms were subsequently treated as riding atoms in geometrically idealized positions with C-H distances of 0.93 Å and with U iso (H) = 1.2U eq (C). For the H atom bonded to the O atom, the atomic coordinates were refined with U iso ( Data collection: APEX2 (Bruker, 2012); cell refinement: SAINT (Bruker, 2012); data reduction: SAINT (Bruker, 2012); program(s) used to solve structure: SHELXT2014 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015b) and PLATON (Spek, 2009).

N-(4-chlorobenzylidene)-2-hydroxyaniline
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.