Crystal structure and Hirshfeld surface analysis of 2,2′′′,6,6′′′-tetramethoxy-3,2′:5′,3′′:6′′,3′′′-quaterpyridine

The title 2,3′-bipyridine-based quaterpyridine derivative has a linear geometry. The pyridine rings are tilted slightly with respect to each other. In the crystal, π–π stacking and weak C—H⋯π interactions lead to formation of a two-dimensional layer structure.

In the title compound, C 24 H 22 N 4 O 4 , the four pyridine rings are tilted slightly with respect to each other. The dihedral angles between the inner and outer pyridine rings are 12.51 (8) and 9.67 (9) , while that between inner pyridine rings is 20.10 (7) . Within the molecule, intramolecular C-HÁ Á ÁO and C-HÁ Á ÁN contacts are observed. In the crystal, adjacent molecules are linked bystacking interactions between pyridine rings and weak C-HÁ Á Á interactions between a methyl H atom and the centroid of a pyridine ring, forming a twodimensional layer structure extending parallel to the ac plane. Hirshfeld surface analysis and two-dimensional fingerprint plots indicate that the most important contributions to the crystal packing are from HÁ Á ÁH (52.9%) and HÁ Á ÁC/CÁ Á ÁH (17.3%) contacts.

Chemical context
Polypyridines are considered to be strong and versatile chelating ligands for transition-metal ions (Adamski et al., 2014). This chelating nature provides complexes with diverse architectures possessing unique and useful photophysical properties (Zhong et al., 2013). Many structural studies of biand terpyridine-based metal complexes have been undertaken over the last decades (Kaes et al., 2000). When bi-or terpyridines are used as building blocks, sophisticated architectures such as helicates and cages can be obtained by self-assembly (Yeung et al., 2011;Glasson et al., 2008b). Although there are number of examples of bi-and terpyridine-based metal complexes with different geometries, structural reports of linear-type quaterpyridines are still scarce (Glasson et al., 2011b). Organic compounds bearing 2,3 0 -bipyridine have attracted much interest because of their unique properties such as proper coordination modes to late transition-metal ions and high triplet energy. As a result of these characteristics, they are widely used as ligands to develop blue phosphorescent materials (Zaen et al., 2019;Lee et al., 2018). However, no reports of a 2,3 0 -bipyridine-based quaterpyridine with a linear geometry have been published to date. Herein, we describe the molecular and crystal structures of the title compound, which can act as a potential multidentate ligand to various transition-metal ions. The molecular packing of the title compound was further examined with the aid of a Hirshfeld surface analysis.

Hirshfeld surface analysis
Hirshfeld surface analysis was performed using Crystal-Explorer (Turner et al., 2017) to quantify and visualize the various intermolecular close contacts in the molecular packing of the title compound. The Hirshfeld surface shown in Fig. 3 was calculated using a standard (high) surface resolution with the three-dimensional d norm surface mapped over a fixed colour scale of À0.1883 (red) to 1.2065 (blue) a.u.. In Fig. 3, except for three light-red spots, 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. The light-red spots on the surface indicate the  Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
The two-dimensional supramolecular network formed throughstacking interactions (black dashed lines) and intermolecular C-HÁ Á Á interactions (yellow dashed lines). For clarity, H atoms not involved in the intermolecular interactions have been omitted.

Figure 1
A view of the molecular structure of the title compound, showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. The intramolecular C-HÁ Á ÁO/N contacts are shown as yellow dashed lines.