Crystal structure and Hirshfeld surface analysis of 1,2-bis(2′,6′-diisopropoxy-[2,3′-bipyridin]-6-yl)benzene

The title molecule adopts a helical structure, in which two 2,3′-bipyridyl units are twisted up and down relative to the plane of the central benzene ring. Weak intermolecular C—H⋯π interactions lead to formation of a two-dimensional supramolecular network. Hirshfeld surface analysis indicates that the molecular packing in the title compound is mainly dominated by intermolecular H⋯H and H⋯C/C⋯H interactions.


Chemical context
Phosphorescent transition metal complexes based on platinum metal cations have attracted enormous current interest owing to their applications as electroluminescent devices, e.g. as phosphorescent organic light-emitting diodes (PhOLEDs) or light-emitting electrochemical cells (LEECs) (Cebriá n & Mauro, 2018). In particular, platinum complexes bearing tetradentate ligands are of great interest as blue phosphorescent materials because of their pure blue emission and high efficiency (Fleetham et al., 2014). It is well known that the origin of emission in platinum complexes results mainly from an intra-ligand charge transfer (ILCT) mixed with a metal-toligand charge-transfer transition (MLCT) (Yersin et al., 2011). In order to achieve blue phosphorescent materials, the design of ligands with a large triplet energy needs to be taken into account as the first step.
Our interest has been focused on the development of a suitable tetradentate ligand based on 2,3 0 -bipyridine with a large triplet energy (Lee et al., 2017). Moreover, the crystal structures of 2,3 0 -bipyridine-based tetradentate ligands have aroused our curiosity, because the knowledge of the coordination mode(s) to a metal ion are of paramount importance in understanding its chemical and physical properties. Herein, we describe the molecular and crystal structures of the title compound that can act as a tetradentate ligand to various ISSN 2056-9890 transition metal ions. In addition, the molecular packing of the title compound was examined with the aid of a Hirshfeld surface analysis.
The two 2,3 0 -bipyridyl units are attached at the 1,2-positions of the central benzene in an up-and down-fashion with the C10-C11-C16-C17 torsion angle being À10.8 (2) , which is believed to reduce the steric hindrance between the two 2,3 0bipyridyl units. In combination with this torsion angle, the consecutive connections of five aromatic rings in the title molecule lead to a helical structure. The central benzene unit occupies ortho-positions relative to the N atoms (N2 and N3) of the two inner pyridine rings, while the outer pyridine rings containing N1 and N4 are substituted relative to the inner pyridine rings at the meta-positions. An intramolecular C-HÁ Á Á interaction between aromatic H3 and the centroid of the N3/C17-C21 ring as well as C-HÁ Á ÁN/O hydrogen bonds (Table 1, shown as yellow and black dashed lines in Fig. 1, respectively) assists in the stabilization of the helical structure.

Supramolecular features
In the crystal structure, the title molecules are interlinked by further C-HÁ Á Á interactions (Table 1, yellow dashed lines in Fig. 2) between (methyl)H32AÁ Á ÁCg1 i and between (methyl)-H37CÁ Á ÁCg2 ii [Cg1 and Cg2 are the centroids of the N3/C17-C21 and C11-C16 rings, respectively; symmetry codes refer to Table 1], forming a two-dimensional supramolecular network parallel to the ac plane, in which molecules with right-and lefthanded helical structures are alternately arranged. These layers are stacked in an ABAB fashion along the b-axis direction whereby no significant intermolecular interactions between the layers are observed.  Table 1 Hydrogen-bond geometry (Å , ).

Figure 1
The molecular structure of the title compound with the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level; H atoms not involved in intramolecular interactions were omitted for clarity. The minor part of the disordered isopropyl group is drawn by twocoloured dashed lines. Black and yellow dashed lines represent intramolecular C-HÁ Á ÁN/O hydrogen bonds and C-HÁ Á Á interactions.

Hirshfeld surface analysis
In order to quantify the various intermolecular interactions in the molecular packing of the title compound, a Hirshfeld surface analysis was carried out using CrystalExplorer (Turner et al., 2017). In Fig. 3, which shows the Hirshfeld surface mapped over the normalized contact distance (d norm ), the light-red spot on the surface indicates contact points with atoms participating in intermolecular C-HÁ Á Á interactions, corresponding to the H32A and pyridine-C20 atoms (Table 2). Except for this light-red spot, the overall surface mapped over d norm is covered by white and blue colours, indicating that the distances between the contact atoms in intermolecular contacts are nearly the same as the sum of their van der Waals radii or longer. Therefore, there are no effective intermolecular interactions apart from the C-HÁ Á Á interactions in the molecular packing. These features are confirmed in the two-dimensional fingerprint plots, Fig. 4a-e, delineated into overall, HÁ Á ÁH, HÁ Á ÁC/CÁ Á ÁH, HÁ Á ÁO/OÁ Á ÁH and HÁ Á ÁN/ NÁ Á ÁH contacts, respectively. Their relative contributions of interatomic contacts to the Hirshfeld surface are summarized in Table 3. As shown in Fig. 4b and Table 3, the most widely scattered points in the fingerprint plot are related to HÁ Á ÁH contacts, which make a 65.2% contribution to the Hirshfeld surface. The sharp peak at d e = d i = 1.0 Å in the fingerprint plot delineated into HÁ Á ÁH contacts ( Fig. 4b)   A view of the Hirshfeld surface of the title compound mapped over d norm , showing HÁ Á ÁC contacts of intermolecular C-HÁ Á Á interactions using a fixed colour scale of À0.1511 (red) to 1.6184 (blue) a.u.

Table 2
Summary of selected short interatomic contacts (Å ) in the title compound.

Figure 2
Layer formed through intermolecular C-HÁ Á Á interactions (yellow dashed lines). The disordered isopropoxyl group and H atoms not involved in intermolecular interactions are not shown for clarity. Colour codes: grey = carbon, blue = nitrogen, red = oxygen and white = hydrogen.
shortest interatomic HÁ Á ÁH contact between symmetryrelated isopropoxy-H34C atoms (Table 2), whereas two pairs of the flanking broad peaks, symmetrically disposed with respect to the diagonal, at d e + d i $ 2.1 and 2.2 Å , result from interatomic HÁ Á ÁH contacts between the isopropoxy-H34B and -H31F atoms and between the benzene-H18 and isopropoxy-H31E atoms, respectively ( Table 2). The central green strip in Fig. 4b, centered at d e + d i = 2.8 Å along the diagonal, indicates the presence of a large number of loose HÁ Á ÁH contacts in the molecular packing. The second largest contribution (22.7%) to the Hirshfeld surface of the title compound is due to interatomic HÁ Á ÁC/CÁ Á ÁH contacts ( Fig. 4c and Table 3), drawn on the fingerprint plot as a pair with a symmetrical wing-like shape on the left and right side with respect to the diagonal. The peaks at d e + d i $ 2.7 Å in the fingerprint plot delineated into HÁ Á ÁC/CÁ Á ÁH contacts ( Fig. 4c) reflect the presence of short C-HÁ Á Á interactions between the isopropoxy-H32A and pyridine-C20 atoms (Table 2). In the fingerprint plot delineated into HÁ Á ÁO/OÁ Á ÁH contacts (Fig. 4d), the 6.5% contribution to the Hirshfeld surface (Table 3) originates from C-HÁ Á ÁO hydrogen bonding. A pair of broad peaks at d e + d i $ 2.6 Å in Fig. 4d corresponds to hydrogen bonding between the pyridine-H25 and O4 atoms (Table 2). Although NÁ Á ÁH/HÁ Á ÁN contacts with a contribution of 4.3% to the Hirshfeld surface ( Fig. 4e and Table 3) were observed, their interatomic distances are longer than the sum of their van der Waals radii and therefore they do not specifically contribute to the molecular packing. Finally, the small contributions from the remaining interatomic contacts (Table 3), i.e. CÁ Á ÁC (0.9%), NÁ Á ÁC/CÁ Á ÁN (0.4%) and OÁ Á ÁC/CÁ Á ÁO (0.1%), have a negligible effect on the molecular packing.
In summary, the Hirshfeld surface analysis and twodimensional fingerprint plot reveal that the molecular packing in the title compound is dominated by intermolecular van der Waals interactions between neighbouring H atoms as well as by C-HÁ Á Á interactions.

Synthesis and crystallization
All experiments were performed under a dry N 2 atmosphere using standard Schlenk techniques. All solvents were freshly distilled over appropriate drying reagents prior to use. All starting materials were commercially purchased and used without further purification. The 1 H NMR spectrum was recorded on a Bruker Advance 400 MHz spectrometer. The two starting materials, 6-bromo-2 0 ,6 0 -diifluoro-2,3 0 -bipyridine and 1,2-bis(2 0 ,6 0 -difluoro-2,3 0 -bipyridine)benzene were synthesized according to a slight modification of the previous synthetic methodology reported by our group Oh et al., 2013). Details regarding the synthetic procedures and reagents are presented in Fig. 5.

1,2-Bis(2′,6′-diisopropoxy-[2,3′-bipyridin]-6-yl)benzene
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.