1,3-Bis{(E)-[4-(dimethylamino)benzylidene]amino}propan-2-ol: chain structure formation via an O—H⋯N hydrogen bond

The molecular and crystal structure of the title Schiff base derivative is reported. O—H⋯N hydrogen bonds link molecules into a supramolecular chain along a.

The asymmetric unit of the title compound, C 21 H 28 N 4 O, consists of two unique molecules linked by an O-HÁ Á ÁN hydrogen bond. The conformation of both C N bonds is E and the azomethine functional groups lie close to the plane of their associated benzene rings in each of the independent molecules. The dihedral angles between the two benzene rings are 83.14 (4) and 75.45 (4) . The plane of the one of the N(CH 3 ) 2 units is twisted away from the benzene ring by 18.8 (2) , indicating loss of conjugation between the lone electron pair and the benzene ring. In the crystal structure, O-HÁ Á ÁN hydrogen bonds together with C-HÁ Á ÁO hydrogen bonds link neighbouring supramolecular dimers into a three-dimensional network.

Chemical context
Schiff bases play important roles in the development of coordination chemistry related to catalysis, enzymatic reactions, and supramolecular architectures. Crystal structures of Schiff bases derived from substituted benzaldehydes and 1,3-diaminopropan-2-ol have been reported earlier Azam, Hussain et al., 2012;Rivera et al., 2016bRivera et al., , 2017Elmali, 2000). The title compound, (I), acts as an important raw material for the synthesis of Schiff base complexes. As an extension of our work on the synthesis and structural characterization of such Schiff base compounds, the crystal structure of the title compound is reported here.

Structural commentary
The title compound crystallizes with two unique molecules in the asymmetric unit. The conformers, labeled A and B, are shown in Fig. 1. Each molecule comprises a 1,3-diamino-2hydroxypropane bridge symmetrically substituted at the 1 and 3 positions by 4-(dimethylamino)phenyl]methylidene units. The conformational differences between the two molecules are extremely small, resulting in a superstructural motif. The two molecules are related by translation along the a-axis ISSN 2056-9890 direction. A structural overlay of the two independent molecules (r.m.s. deviation for fitting all non-H atoms = 0.097 Å ) is shown in Fig. 2. The disposition of the residues attached to the N2A and N2 positions can be described by the torsion angles N2A-C5A-C51A-C56A [À9.9 (11) in molecule A] and N2-C5-C51-C56 [À14.9 (11) in molecule B]. The two outer aromatic rings (C11-C16 and C51-C56) are inclined to one another by 83.14 (4) in molecule A and 75.45 (4) in molecule B.

Figure 3
Crystal packing of the title compound, indicating the O-HÁ Á ÁN hydrogen bonds (dashed lines), which result in chains along the a-axis direction.

Figure 1
The structure of the independent molecules A and B, showing the atomlabelling scheme. Displacement ellipsoids are drawn at the 50% probability level for non-H atoms.

Synthesis and crystallization
The title compound was prepared as described by (Rivera et al. 2016a). The crude product was recrystallized from diethyl ether solution by slow evaporation of the solvent, giving colorless crystals suitable for X-ray diffraction (m.p. 396.8-398 K; yield 40%).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The coordinates of the hydroxyl H atom were refined with U iso (H) = 1.5U eq (O). The remaining H atoms were positioned geometrically and allowed to ride on their parent atoms, with d(C-H) = 0.95 Å for aromatic and azomethine atoms, d(C-H) = 0.98 Å for methyl, d(C-H) = 0.99 Å for methylene, d(C-H) = 1.00 Å for tertiary CH. The U iso (H) values were constrained to 1.5U eq (C methyl ) or 1.2U eq (C) for the remaining H atoms. The structure shows signs of a superstructure. The two molecules are related by a translation of 1/2 along the a axis. However, if the structure is refined in a cell with the a axis halved, the displacement parameters of one NMe 2 group and some of the C atoms of the phenyl ring to which this group is attached are significantly enlarged (Fig. 4). Shifting one molecule by 1 2 in the a-axis direction, it becomes obvious how similar the two molecules are. Nevertheless, there are small differences in their overall conformation (Fig. 5). As a result of that, we opted to refine the structure using the larger unit cell with two molecules in the asymmetric unit.   (Sheldrick, 2015) and XP in SHELXTL-Plus (Sheldrick, 2008).

Figure 4
Perspective view of the molecule if the structure is refined in a cell with the a axis halved. SHELXL2014/7 (Sheldrick, 2015); molecular graphics: XP in SHELXTL-Plus (Sheldrick, 2008); software used to prepare material for publication: SHELXL2014/7 (Sheldrick, 2015).

1,3-Bis{(E)-[4-(dimethylamino)benzylidene]amino}propan-2-ol
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.