Crystal structures of (E)-1-{3-[(5-fluoro-2-hydroxybenzylidene)amino]phenyl}ethanone and of a fourth polymorph of (E)-1-{3-[(2-hydroxy-3-methoxybenzylidene)amino]phenyl}ethanone

The molecules of the title compounds are effectively planar, apart from the methyl H atoms. In the crystals, C—H⋯O hydrogen bonds link the molecules into chains in one compound and into sheets in the other.


Chemical context
Schiff bases of general type RR 0 C NR 00 can exhibit very wide structural diversity and have found a wide range of applications (Jia & Li, 2015), ranging from anti-bacterial, anti-fungal and anti-tumour activity (Rani et al., 2015), via catalysis (Kumar et al., 2009), to use as organic photovoltaic materials (Jeevadason et al., 2014). The extensive patent literature on their medicinal applications has recently been reviewed (Hameed et al., 2017). With this great diversity of use in mind, we report herein on the molecular and supramolecular structures of two closely related Schiff bases,(E)-1-{3-[(5fluoro-2-hydroxybenzylidene)amino]phenyl}ethanone (I) and (E)-1-{3-[(2-hydroxy-3-methoxybenzylidene)amino]phenyl}ethanone (II). Compounds (I) and (II) were prepared by straightforward condensation reactions between 3-acetylaniline (3-aminoacetophenone) and the appropriately substituted salicylaldehydes. Their molecular constitutions differ only in the identity and location of a single substituent, 5-fluoro in (I) versus 3-methoxy in (II), but their crystallization behaviour is different. Compound (I) crystallizes in the monoclinic space group P2 1 /n with Z 0 = 1 (Fig. 1), while compound (II) crystallizes in the orthorhombic space group Pca2 1 with Z 0 = 2 (Figs. 2 and 3), and it will be convenient to refer to the molecules of (II) which contain the atoms N11 and N21 as molecules of types 1 and 2, respectively. Compound (II), in fact, represents the fourth polymorphic form of this compound to be identified. Three other forms, one in Pna2 1 with Z 0 = 2, and two others in P2 1 2 1 2 1 , each with Z 0 = 1, have recently been reported (Zbačnik et al., 2015). ISSN 2056-9890

Structural commentary
In each of compounds (I) (Fig. 1) and (II) (Figs. 2 and 3), the non-H atoms are almost coplanar. Thus in (I), the r.m.s. deviation of the non-H atoms from their mean plane is only 0.085 Å , with a maximum individual deviation from the plane of 0.196 (2) Å for the acetyl atom C18. Similarly, in compound (II), the r.m.s. deviations of the non-H atoms from the mean planes of the two molecules are 0.086 and 0.071 Å for molecules 1 and 2, respectively, with corresponding maximum deviations of 0.225 (5) and 0.211 (5) Å for atoms C118 and C218, respectively. In all of the molecules there is an intramolecular O-HÁ Á ÁN hydrogen bond (Tables 1 and 2); although this probably influences the orientation of the hydroxylated ring relative to the central spacer unit, it will not have any influence on the orientation of the acetylphenyl ring relative to the rest of the molecule. In the two molecules of (II), the deviation of the methoxy C atoms C128 and C228 from the planes of their adjacent aryl rings are 0.107 (9) and 0.049 (11) Å , respectively. Consistent with this, the pair of exocyclic C-C-O angles at each of the atoms C123 and C223 differ by ca 10 , as is generally observed in planar alkoxyarene derivatives (Seip & Seip, 1973;Ferguson et al., 1996). The dihedral angle between the mean planes of the two molecules in ( The structure of molecule 2 in compound (II), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level. Table 1 Hydrogen-bond geometry (Å , ) for (I). Symmetry code: (i) Àx þ 1 2 ; y À 1 2 ; Àz þ 3 2 . Table 2 Hydrogen-bond geometry (Å , ) for (II). Symmetry codes: (i) Àx þ 1; Ày; z þ 1 2 ; (ii) x; y À 1; z.

Figure 1
The molecular structure of compound (I), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level.

Figure 2
The structure of molecule 1 in compound (II), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level.

Supramolecular features
The supramolecular assembly in compound (I) is very simple, as shown in Fig. 4. In addition to the intramolecular hydrogen bond noted above, there is a single C-HÁ Á ÁO hydrogen bond (Table 1), which links molecules related by a 2 1 screw axis into C(8)chains running parallel to the [010] direction. Two chains of this type, related to one another by inversion, pass through each unit cell, but there are no direction-specific interactions between adjacent chains.
There are three C-HÁ Á ÁO hydrogen bonds in the structure of compound (II) ( Table 2): one of these links the two molecules within the selected asymmetric unit and the two others link these bimolecular aggregates into complex sheets, whose formation is readily analysed in terms of two one-dimensional sub-structures (Ferguson et al., 1998a,b;Gregson et al., 2000). The hydrogen bond having atom C227 as the donor links bimolecular aggregates related by translation to form a C 2 2 (16) chain running parallel to the [010] direction (Fig. 5), and that having atom C116 as the donor links aggregates related by a 2 1 screw axis into C 2 2 (17) chains running parallel to the [001] direction (Fig. 6). The combination of the orthogonal chains along [010] and [001] generates a sheet lying parallel to (100). Two sheets of this type, related to one another by the glide planes, pass through each unit cell but there are no directionspecific interactions between adjacent sheets.

Database survey
The structures of Schiff bases derived from hydroxyaryl aldehydes have recently been the subject of a general survey, in which a number of structural errors, often involving misplaced H atoms, were pointed out (Blagus et al., 2010). Part of the crystal structure of compound (I), showing the formation of a hydrogen-bonded C(8) chain running parallel to the [010] direction. For the sake of clarity, the H atoms not involved in the motif shown have been omitted. Hydrogen bonds are drawn as dashed lines and the atoms marked with an asterisk (*) or a hash (#) are at the symmetry positions ( 1 2 À x, À 1 2 + y, 3 2 À z) and ( 1 2 À x, 1 2 + y, 3 2 À z), respectively.

Figure 5
Part of the crystal structure of compound (II), showing the formation of a hydrogen-bonded C 2 2 (16) chain running parallel to the [010] direction. For the sake of clarity, the H atoms not involved in the motif shown have been omitted, and the hydrogen bonds are drawn as dashed lines.

Figure 6
Part of the crystal structure of compound (II), showing the formation of a hydrogen-bonded C 2 2 (17) chain running parallel to the [001] direction. For the sake of clarity, the H atoms not involved in the motif shown have been omitted, and the hydrogen bonds are drawn as dashed lines.
Compound (III) is isomorphous with compound (I): as in (I), the structure of (III) contains an intramolecular O-HÁ Á ÁN hydrogen bond and the non-H atoms are effectively coplanar. The structure of (III) also contains an intermolecular C-HÁ Á ÁO hydrogen bond, although this is nowhere mentioned in the original report (De et al., 2009); this interaction forms C(8) chains along [010], exactly the same as those in the structure of (I), so that (I) and (III) are, in fact, isostructural despite their different patterns of substitution.
Three other polymorphic forms of compound (II) have recently been reported and are described as forms I, II and II,I respectively (Zbačnik et al., 2015). Form I is orthorhombic in space group Pna2 1 with Z 0 = 2, and forms II and III both crystallize in space group P2 1 2 1 2 1 with Z 0 = 1, so that the Pca2 1 form reported here can be regarded as form IV. All three forms, I-III, can be crystallized from ethanol solutions under different conditions and a crucial factor in determining which polymorph is obtained appears to be the filtration process used prior to crystallization. By contrast, the form described here was crystallized from a solution in dichloromethane. In all of the molecules in forms I-III, there is an intramolecular O-HÁ Á ÁN hydrogen bond and, in every case, the non-H atoms are effectively co-planar as found here for (I) and (II). The supramolecular assembly differs in all three polymorphs I-III: form II contains no intermolecular hydrogen bonds; in form III two C-HÁ Á ÁO hydrogen bonds generate a C(8)C(10)[R 1 2 (6)] chain of rings; and in form I, three C-HÁ Á ÁO hydrogen bonds generate sheets in which the component sub-structures both involve molecules related by an n-glide plane, in contrast to the sheets found for form IV reported here.

Synthesis and crystallization
For the synthesis of compounds (I) and (II), 3-acetyl aniline (0.740 mmol) and a catalytic quantity of acetic acid were added to solution of the appropriate aldehyde, 5-fluorosalicylaldehyde for (I) or 3-methoxysalicylaldehyde for (II) (0.740 mmol) in ethanol (20 cm 3 ), and these mixtures were then heated under reflux for 5 h. The mixtures were then cooled to ambient temperature and the solvent was removed under reduced pressure. The solid residues were then washed with cold ethanol and dried under reduced pressure. Crystals suitable for single crystal X-ray diffraction were grown by slow evaporation, at ambient temperature and in the presence of air, of solutions in dimethylsulfoxide for (I) and in dichloro-

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. For compound (II), one bad outlier reflection (8,1,3) was omitted from the data set before the final refinements. All H atoms were located in difference-Fourier maps. The C-bound H atoms were subsequently treated as riding atoms in geometrically idealized positions: C-H 0.93-0.96 Å with U iso (H) = 1.5U eq (C-methyl) and 1.2U eq (C) for other C-bound H atoms. The methyl groups were permitted to rotate but not to tilt. For the H atoms bonded to O atoms, the atomic coordinates were refined with U iso (H) = 1.5U eq (O), giving the O-H distances shown in Tables 1 and 2. The correct orientation of the structure of (II) relative to the polar axis direction was established by means of the Flack x parameter (Flack, 1983), x = À0.04 (16)  For both structures, data collection: APEX3 (Bruker, 2016); cell refinement: SAINT (Bruker, 2016); data reduction: SAINT (Bruker, 2016); 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). 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.

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.