Crystal structures and Hirshfeld surface analyses of 4,4′-{[1,3-phenylenebis(methylene)]bis(oxy)}bis(3-methoxybenzaldehyde) and 4,4′-{[(1,4-phenylenebis(methylene)]bis(oxy)}bis(3-methoxybenzaldehyde)

The title compounds, 4,4′-{[1,3-phenylenebis(methylene)]bis(oxy)}bis(3-methoxybenzaldehyde) (I) and 4,4′-{[(1,4-phenylenebis(methylene)]bis(oxy)}bis(3-methoxybenzaldehyde) (II), each crystallize with half a molecule in the asymmetric unit. The whole molecule of compound (I) is generated by twofold rotation symmetry, while the whole molecule of compound (II) is generated by inversion symmetry.

The title compounds, C 24 H 22 O 6 (I) and C 24 H 22 O 6 (II), each crystallize with half a molecule in the asymmetric unit. The whole molecule of compound (I) is generated by twofold rotation symmetry, the twofold axis bisecting the central benzene ring. The whole molecule of compound (II) is generated by inversion symmetry, the central benzene ring being located on an inversion center. In (I), the outer benzene rings are inclined to each other by 59.96 (10) and by 36.74 (9) to the central benzene ring. The corresponding dihedral angles in (II) are 0.0 and 89.87 (12) . In the crystal of (I), molecules are linked by C-HÁ Á ÁO hydrogen bonds and C-HÁ Á Á interactions, forming ribbons propagating along the [101] direction. In the crystal of (II), molecules are linked by C-HÁ Á ÁO hydrogen bonds, forming a supramolecular framework. The Hirshfeld surface analyses indicate that for both compounds the HÁ Á ÁH contacts are the most significant, followed by OÁ Á ÁH/HÁ Á ÁO and CÁ Á ÁH/HÁ Á ÁC contacts.
The molecular structure of compound (II) is shown in Fig. 2. The asymmetric unit consists of half a molecule, the other half being generated by inversion symmetry; the central benzene ring being situated about the inversion center. The outer benzene rings are parallel to each other and normal to the central benzene ring with a dihedral angle of 89.87 (12) . The methoxymethane and acetaldehyde groups adopt extended conformations, as can be seen from the torsion angles C5-C6-O2-C8 = 172.7 (2) Å and C7-C2-C1-O3 = À178.5 (3) . Here, atoms O2 and C8 deviate from the plane of the benzene ring by À0.025 (2) and À0.211 (4) Å , respectively, while atoms C1 and O1 deviate from the plane of the benzene ring by 0.023 (3) and 0.056 (2) Å , respectively.

Figure 1
The molecular structure of compound (I), with atom labelling (unlabelled atoms are related to labelled atoms by the symmetry operation Àx + 2, y, Àz + 1 In the crystal of (II), molecules are linked by C7-H7Á Á ÁO3 i and C12-H12Á Á ÁO2 ii hydrogen bonds (Table 2), forming a supramolecular framework, as shown in Fig. 5.
The Hirshfeld surfaces of compounds (I) and (II) mapped over d norm are given in Fig. 6a and 6b, respectively. Views of the intermolecular contacts in the crystals are shown in Figs. 7 and 8, for compounds (I) and (II), respectively. They are colour-mapped with the normalized contact distance, d norm , from red (distances shorter than the sum of the van der Waals radii) through white to blue (distances longer than the sum of the van der Waals radii). The blue region represents the positive electrostatic potential over the surface. The d norm The crystal packing of compound (I), viewed along the b axis. The C-HÁ Á ÁO hydrogen bonds (Table 1) are shown as dashed lines. For clarity, only the hydrogen atoms involved in hydrogen bonding have been included.

Figure 4
The crystal packing of compound (I), viewed along the b axis. The C-HÁ Á Á interactions (Table 1) are shown as dashed lines. For clarity, only the hydrogen atoms involved in these interactions have been included.

Figure 5
The crystal packing of compound (II), viewed along the b axis. the C-HÁ Á ÁO hydrogen bonds (Table 2) are shown as dashed lines. For clarity, only the hydrogen atoms involved in hydrogen bonding have been included.

Figure 6
The Hirshfeld surface mapped over d norm , for (a) compound (I) and (b) compound (II).

Figure 7
A view of the Hirshfeld surface mapped over d norm for compound (I), showing the various intermolecular contacts in the crystal.
surface was mapped over a colour scale in arbitrary units of À0.156 (red) to 1.705 (blue) for compound (I) and À0.207 (red) to 1.206 (blue) for compound (II), where the red spots indicate the intermolecular contacts involved in the hydrogen bonding.

Database survey
A search of the Cambridge Structure Database (CSD, Version 5.40, February 2019; Groom et al., 2016) for similar compounds gave one hit for 1,3-bis[(2-methoxyphenoxy)methyl]benzene (CSD refcode KACQEL; Bryan et al., 2003) but no hits for a 1,4-derivative. In KACQEL, the central benzene ring is inclined to the outer benzene rings by 67.60 (4) and 72.68 (6) , while the outer benzene rings are inclined to each other by 69.61 (6) . In compound (I), the central benzene ring is inclined to the outer benzene ring(s) by 36.74 (9) , while the outer benzene rings are inclined to each other by 59.96 (10) . In compound (II), the corresponding dihedral angles are 89.87 (2) and 0.0 , respectively.

Synthesis and crystallization
Compound (I): To vanillin (0.63 g, 4.1 mmol) dissolved in 20 ml DMF was added potassium carbonate (1.7 g, 12.5 mmol) and the mixture was stirred at room temperature followed by addition of 1,3-bis(bromomethyl)benzene (0.5 g, 1.9 mmol). The reaction was allowed to proceed for 12 h. Then the reaction mixture was partitioned between water and ethyl acetate. The ethyl acetate layer was collected and concen-      trated under reduced pressure. The crude product obtained was recrystallized by using ethyl acetate. Colourless block-like crystals were obtained on slow evaporation of the solvent (98%).
Compound (II): To vanillin (0.63 g, 4.1 mmol) dissolved in 20 ml DMF, was added potassium carbonate (1.7 g, 12.5 mmol) and the mixture was stirred at room temperature followed by addition of 1,4-bis(bromomethyl)benzene (0.5 g, 1.9 mmol). The reaction was allowed to proceed for 12 h. After the reaction mixture was partitioned between water and ethyl acetate, the ethyl acetate layer was collected and concentrated under reduced pressure. The crude product was recrystallized by using ethyl acetate. Colourless block-like crystals were obtained on slow evaporation of the solvent (98%).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. For both compounds, the hydrogen atoms were fixed geometrically and allowed to ride on their parent atoms: C-H = 0.93-0.97 Å with U iso (H) = 1.5U eq (Cmethyl) and 1.2U eq (N,C) for other H atoms.

Computing details
For both structures, data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2016/4 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and PLATON (Spek, 2009); software used to prepare material for publication: SHELXL2016/4 (Sheldrick, 2015) 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.

4,4′-{[(1,4-Phenylenebis(methylene)]bis(oxy)}bis(3-methoxybenzaldehyde) (II)
Crystal data  (4) 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.