Crystal structure of 2,3′-bipyridine-2′,6′-dicarbonitrile

The asymmetric unit of the title disubstituted 2,3′-bipyridine, contains four independent molecules (namely, A, B, C and D). The conformations of the molecules differ, as seen from the dihedral angles between the two pyridine rings in each molecule. They vary from 5.51 (9)° for molecule B to 25.25 (8)° for molecule A.

The title compound, C 12 H 6 N 4 , crystallizes with four independent molecules (A, B, C and D) in the asymmetric unit. The dihedral angles between the two pyridine rings in each molecule are 25.25 (8) in A, 5.51 (9) in B, 11.11 (9) in C and 16.24 (8) in D. In the crystal, molecules A and B are linked by C-HÁ Á ÁN hydrogen bonds to form layers extending parallel to the ab plane, while molecules C and D are linked by C-HÁ Á ÁN hydrogen bonds forming -C-D-C-D-chains propagating along the b-axis direction. The layers and the chains are stacked alternately along the c axis through offsetand C NÁ Á Á [N-topyridine-centroid distance = 3.882 (2) Å ] interactions, resulting in the formation of a supramolecular framework.

Chemical context
Bipyridine ligands with the C^N chelating mode to transition metal ions, such as 2,3 0 -bipyridine, are considered to be strong candidates for the synthesis of blue phosphorescent heavy transition metal complexes because of their larger triplet energy (T 1 ) compared with phenylpyridine-based C^N chelating ligands (Reddy & Bejoymohandas, 2016). In particular, the triplet energy of fluorine-functionalized 2,3 0 -bipyridine (T 1 : 2.82 eV) is larger than that of alkoxyfunctionalized analogue, 2 0 ,6 0 -dimethoxy-2,3 0 -bipyridine (T 1 : 2.70 eV) (Lee et al., 2017;Kim et al., 2018). Therefore, the introduction of electron-withdrawing groups into the C-coordinating pyridine group is highly desirable in order to develop blue phosphorescent metal complexes. To design a suitable ligand possessing a large triplet energy is still a main issue in the organic light-emitting diodes (OLEDs) research area because developing blue phosphorescent materials remains a problem that has not been solved so far. Although there are a number of advantages in 2,3 0 -bipyridine ligands, incorporating the substituents into the ligand framework is difficult owing to the low selectivity and reactivity of the pyridine ring (Oh et al., 2013). In addition, structural examples of bipyridine-bearing electron-withdrawing groups are very scarce.
Herein, for potential applications for the development of blue phosphorescent materials, we describe the synthesis and ISSN 2056-9890 crystal structure of the title compound, 2,3 0 -bipyridine-2 0 ,6 0dicarbonitrile.

Structural commentary
As shown in Fig. 1, the asymmetric unit of the title compound contains four crystallographically independent molecules (A, B, C and D). The dihedral angles between the two pyridine rings in each molecule are 25.25 (8) in A, 5.51 (9) in B, 11.11 (9) in C and 16.24 (8) in D. In order to investigate the conformational similarity between the four molecules, the r.m.s. overlay fits of the 16 non-H atoms of each molecule were calculated using the AutoMolFit routine in PLATON (Spek, 2009). As shown in Fig. 2, and as expected in view of the values of the dihedral angles, the largest overlay fit of 0.197 Å is observed for molecules A and B, while the smallest r.m.s. overlay fit of 0.060 Å is observed for molecules C and D.

Figure 1
The molecular structure of the four independent molecules (A, B, C and D) of the title compound, with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.39, last update May 2018; Groom et al., 2016) for 2 0 ,6 0 -disubstituted 2,3 0 -bipyridines, gave a number of hits. The majority of them involve iridium or platinum complexes of the difluoro and dimethoxy analogues of the title compound. As explained in the Chemical context, such compounds, particularly blue iridium complexes of 2 0 ,6 0 -difluoro-2,3 0 -bipyridine, have been synthesized to study their phosphorescence (e.g. Lee et al., 2009) and electroluminescence (e.g. Xu et al., 2015) efficiency. As there are no reports of the crystal structures of either 2 0 ,6 0 -difluoro-2,3 0 -bipyridine nor 2 0 ,6 0 -dimethoxy-2,3 0bipyridine, it is not possible to compare their conformations with those of the four independent molecules of the title compound.

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 Avance 300 MHz spectrometer. The fluorinated bipyridine, 2 0 ,6 0 -difluoro-2,3 0 -bipyridine, was synthesized according to previous reports (Lee et al., 2009). (a) View along the c axis of the layer formed by C-HÁ Á ÁN hydrogen bonds between molecules A and B; (b) view along the c axis of the chains formed by C-HÁ Á ÁN hydrogen bonds between molecules C and D [symmetry codes: (i) Àx + 1, y + 1 2 , Àz + 1 2 ; (ii) Àx + 2, y À 1 2 , Àz + 1 2 ; (iii) x, y + 1, z; colour codes: grey = carbon, blue = nitrogen and white = hydrogen].

Figure 4
The supramolecular framework formed via intermolecularstacking (black dashed lines) and C NÁ Á Á (yellow dashed lines) interactions involving the four independent molecules (colour codes: gray = molecule A, red = molecule B, blue = molecule C and green = molecule D). All H atoms have been omitted for clarity.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were positioned geometrically and refined using a riding model: C-H = 0.95 Å with U iso (H) = 1.2U eq (C). program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2010) and PLATON (Spek, 2009); software used to prepare material for publication: SHELXTL (Sheldrick, 2008) and publCIF (Westrip, 2010). 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.