Bis{4-[(2-hydroxy-5-methoxy-3-nitrobenzylidene)amino]phenyl} ether

The molecule of the title oxybis compound lies on a twofold rotational axis. The conformation of the title compound is discussed and compared to those of related structures. In the crystal, molecules of the title compound are assembled into layers parallel to the ab plane through C—H⋯O hydrogen bonds.


Chemical context
Bisthiosemicarbazones are formed by connecting separated thiosemicarbazone moieties through a pair of oxybisphenyl rings. These tetradentate ligands trap metals inside to form square-planar complexes (Alsop et al., 2005;Blower et al., 2003;Jasinski et al., 2003). The length of the C-C bond in the backbone affects the stability of the complexes. A higher number of C-C bonds obtained via alkylation or arylation allows metal ions to better fit inside the ligand cavity (Blower et al., 2003). These tetradentate ligands and transition-metal complexes exhibit promising anticancer and antibacterial activities (Lobana et al., 2009). In view of this and our research interest in the synthesis of oxybis Schiff base compounds, we herein report the crystal structure, supramolecular features and conformational comparison of the title compound.

Structural commentary
In the title compound ( Fig. 1), the asymmetric unit comprises one half of the oxybisbenzenyl molecule where the oxygen atom (O1) lies on a twofold rotation axis. The complete molecule is generated through the symmetry operation ISSN 2056-9890 Àx, y, 1 2 À z. The planes of the benzene rings bonded to the central oxygen atom form a dihedral angle of 66.0 (2) . The dihedral angle between the benzene and 4-methoxy-2-nitrophenol rings in the same half of the molecules is 4.9 (2) , indicating an almost coplanar arrangement of the benzene and phenol rings. The sp 2 -hybridized character of atoms N1 and C7 is confirmed by the N1-C7 [1.287 (6) Å ] bond length and C7-N1-C8 [121.9 (4) ] and N1-C7-C6 [121.7 (4) ] bond angles (Arafath et al., 2018). Each half of the molecule exhibits an imine E configuration with a C6-C7-N1-C8 torsion angle of 177.7 (4) . In the molecule, atom N1 of the imine moiety acts as a hydrogen-bond acceptor for the adjacent phenol group, forming an intramolecular O-HÁ Á ÁN hydrogen bond with an S(6) ring motif (Fig. 1, Table 1).

Database survey
In a search of the Cambridge Structure Database (CSD, version 5.40, last update August 2019; Groom et al., 2016), twelve structures containing the (1E,1 0 E)-N,N 0 -[oxybis(4,1-phenylene)]bis(1-phenylmethanimine) moiety with different substituents were found. The reference moiety is illustrated in Fig. 3. Details regarding different substituents (R 1 ) together with the dihedral and torsion angles for oxybisbenzenyl moiety in these structures are tabulated in Table 2. In analogy with the title molecule, the planes of the central benzene ring bonded to the central oxygen atom are always V-shaped with dihedral angle 1 in the range of 54.6-84.8 . The dihedral angle between the planes of central and terminal benzene rings exists in two conformations, viz. non-coplanar [dihedral 2 = 18.0-73.5 ] and nearly coplanar [dihedral 2 = 4.8-9.9 ]. In all of these structures, the imine C N double bond adopts an E configuration with torsion angles corresponding to C6-C7-N1-C8 in the range 172.9-180.0 .

Synthesis and crystallization
To a sample of 2-hydroxy-5-methoxy-3-nitrobenzaldehyde (0.98 g, 5.00 mmol) dissolved in 25.0 mL of methanol, 0.20 mL of glacial acetic acid were added, and the mixture was refluxed for 30 min. A solution of 4,4 0 -oxydianiline (0.50 g, 2.50 mmol) in 20.0 mL of methanol was added dropwise under stirring to the aldehyde solution. The resulting deep-red solution was refluxed for 4 h with stirring. The reaction scheme is shown in Fig. 4. The deep-red precipitate that formed was filtered off and washed with 5.0 mL of methanol and 5.0 mL of n-hexane.  Partial packing diagram for the title compound, showing intermolecular hydrogen bonds (cyan dotted lines). Hydrogen atoms not involved in hydrogen bonding are omitted for clarity. Symmetry codes: (i) À 1 2 + x, 1 2 + y, z; (ii) À1 + x, 1 + y, z. Table 1 Hydrogen-bond geometry (Å , ). Symmetry code: (i) x À 1 2 ; y þ 1 2 ; z.

Figure 1
The molecular structure of the title compound with displacement ellipsoids drawn at the 50% probability level. Intramolecular hydrogen bonds are shown as dashed lines. Atoms with the label suffix A are generated by the symmetry operation Àx, y, 1 2 À z.

Figure 3
Structural fragment for the CSD search.
The recovered product was dissolved in chloroform for recrystallization. Purple single crystals suitable for X-ray diffraction were obtained by slow evaporation of the solvent, m.p. 547-548 K, yield 96%. Analysis calculated for

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The phenolic hydrogen atom was located in a difference-Fourier map and refined freely. All other H atoms attached to C were positioned geometrically and refined using a riding model with C-H= 0.95-0.98 Å and U iso (H) = 1.2U eq (C) or 1.5U eq (C) for methyl H atoms. A rotating model was used for the methyl group. The crystal investigated was refined as a two-component pseudomerohedral twin resulting from a 180 rotation about the [001] reciprocal lattice direction, with a twin ratio of 0.977 (3):0.023 (3).

Funding information
The  Table 2 Selected dihedral and torsion angles ( ).
Dihedral 1 is the dihedral angle between the planes of the central benzene rings. Dihedral 2 is the dihedral angle between the planes of the central and terminal benzene rings.

Special details
Experimental. The following wavelength and cell were deduced by SADABS from the direction cosines etc. They are given here for emergency use only: CELL 0.71095 5.463 8.443 28.418 92.106 89.981 108.897 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. Refinement. Refined as a 2-component twin.