Chiral crystallization of a zinc(II) complex

In this paper, we report on the chiral crystallization of achiral molecules.

The compound, {6,6 0 -dimethoxy-2,2 0 -[(4-azaheptane-1,7-diyl)bis(nitrilomethanylidyne)]diphenolato}zinc(II) methanol monosolvate, [Zn(C 22 H 27 N 3 O 4 )]Á-CH 3 OH, at 298 K crystallizes in the orthorhombic space group Pna2 1 . The Zn atom is coordinated by a pentadentate Schiff base ligand in a distorted trigonalbipyramidal N 3 O 2 geometry. The equatorial plane is formed by the two phenolic O and one amine N atom. The axial positions are occupied by two amine N atoms. The distorted bipyramidal geometry is also supported by the trigonality index (), which is found to be 0.85 for the molecule. In the crystal, methanol solvent molecule is connected to the complex molecule by an O-HÁ Á ÁO hydrogen bond and the complex molecules are connected by weak supramolecular interactions, so achiral molecules generate a chiral crystal. The Hirshfeld surface analysis suggests that HÁ Á ÁH contacts account for the largest percentage of all interactions.

Chemical context
Schiff bases and their coordination compounds play an important role in metal coordination chemistry owing to their thermal stability, relevant biological properties and high synthesis flexibility (Bartyzel, 2018;Siddiqui et al., 2006;Sacconi, 1966). These ligands are able to coordinate a wide variety of metal ions and to stabilize them in various oxidation states. The coordination geometry of the complexes depends upon the chemical structure of the chosen ligand, the coordination geometry preferred by the metal, the metal-to-ligand ratio, the reaction time and temperature (Thakurta et al., 2010;Fleck et al., 2013;Sanmartín et al., 2001;Khalaji et al., 2011). A number of zinc(II) complexes with different Schiff base ligands and their potential applications in sensing and as antibacterial and anticancer agents have been documented in the literature (Lui et al.,2019;Niu et al., 2015;Tang et al., 2013;AlDamen et al., 2016;Iksi et al., 2020). In addition, lanthanide Schiff base compounds are the subject of immense research interest because of their unique structures and their potential applications in advanced materials such as undoped semiconductors, magnetic, catalytic and florescent and non-linear optical materials (Li et al., 2016;Ishikawa et al., 2003;Long et al., 2018b).
In a previous work, we reported the crystal and molecular structure of a Cu II complex based on Schiff base ligand N1,N3bis(3-methoxysalicylicylidene)diethylenetriamine where two Schiff base ligands join two Cu II ions in a chelate-spacerchelate mode, in which the protonated aliphatic secondary amine moieties represents the spacer to form a double helix (Noor et al., 2018). In an another report, we redetermined the crystal structure of an organic-inorganic salt of an Mn II -Schiff base ligand complex Mn(C 18 H 18 N 2 O 4 )(H 2 O) 2 ]ClO 4 at 100 K. In contrast to crystal-structure determinations at room temperature (Akitsu et al., 2005, Bermejo et al., 2007, positional disorder of the ethylene bridge in the Schiff base and the perchlorate anion was not observed at 100 K (Noor et al. 2016). We now report the chiral crystallization on a zinc(II) complex.

Figure 1
The molecular structure of the title compound, showing the atomlabelling scheme.

Figure 2
A view of the O5-H5AÁ Á ÁO3 and C10-H10BÁ Á ÁO5 interactions (dashed lines). structure shows typical 'wings' (Fig. 4i). The percentage contribution to the Hirshfeld surface area by close contacts with H atoms inside the surface and H atoms outside is 57.4% ( Fig. 4ii), for O atoms inside the surface and H atoms outside it is 9.1% (Fig. 4iii), for H atoms inside the surface and O atoms outside it is 8.5% (Fig. 4iv), for C atoms inside the surface and H atoms outside it is 14.5% (Fig. 4v), and for H atoms inside the surface and C atoms outside it is 8.2% (Fig. 4vi). Hirshfeld surface analysis of the HÁ Á ÁO interaction clearly shows the close intermolecular contact near methanol, (d i is 1.1 Å and d e is 0.75 Å ).
The title compound was synthesized by the reaction of H 2 L (1 mmol, 0.399 mg) with Zn(OAc) 2 Á2H 2 O (1 mmol, 0.18 mg) in MeOH:H 2 O (v/v, 10:1) (50 cm 3 ) with a few drops of LiOH (1%). The reaction mixture was heated to 343 K for 1 h. The yellow solid obtained was filtered off and dried. (C N) 1618 cm À1 . Single crystals suitable for X-ray crystallography were obtained several days after dissolving the solid in 40 cm 3 of hot methanol.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2

Funding information
This work was partly supported by a Grant-in-Aid for Scientific Research (A) KAKENHI (20H00336).

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. Refinement. Reflections were merged by SHELXL according to the crystal class for the calculation of statistics and refinement. _reflns_Friedel_fraction is defined as the number of unique Friedel pairs measured divided by the number that would be possible theoretically, ignoring centric projections and systematic absences.