Crystal structure of 2,3-bis(4-methylphenyl)benzo[g]quinoxaline

The synthesis and crystal structure of 2,3-di-p-tolylbenzo[g]quinoxaline, a potential ligand for OLED IrIII complexes, are reported.


Chemical context
Quinoxalines are well-known nitrogen-containing heterocyclic compounds, and substituted quinoxalines are important ligands with transition metals (Achelle et al., 2013;Floris et al., 2017;Tariq et al., 2018). They act as chelating agents bearing ring complexes bounded by a benzene ring and a pyrazine ring. We have reported, for example, deep-red emissive iridium(III) complexes containing 2,3-diphenylquinoxaline (dpqH), in which red emissions contributed to the conjugated structure of the dpq ligand (Song et al., 2015). The use of long conjugated compounds as metal coordination ligands could be an approach to develop novel emitters toward red-shift emission up to near-infrared (NIR) wavelengths due to intersystem crossing (Ahn et al., 2009). Recently, 2,3-diphenylbenzoquinoxaline (dpbqH), a more -extended ligand than dpqH, has been introduced, and its iridium(III) complex showed bathochromic shifted emission at 763 nm (Kim et al., 2018). The aromatic rings in dpqH formed dimeric aggregates byinteractions, and these dimers interact via van der Waals interactions in the solid state (Cantalupo et al., 2006;Kim et al., 2018). In this work, we have synthesized 2,3-di-ptolylbenzo[g]quinoxaline (dmpbqH) from the reaction of 4,4dimethylbenzil with 2,3-diaminonaphthalene, and investigated its single crystal structure.

Database survey
A search of the Cambridge Structural Database (CSD; Groom et al., 2016) via the WebCSD interface in February 2018 returned several entries for crystal structures related to 2,3-disubstituted benzoquinoxalines. In 2,3-diphenylbenzoquinoxaline, the two phenyl rings form dihedral angles of 43.42 (3) and 46.89 (3) with the benzoquinoxaline plane, a little larger than those of the title compound. The packing in the crystals is described as having a herringbone motif (REKDIV, Cantalupo et al., 2006;REKDIV01, Chan & Chang, 2016). There are three entries for metal complexes with this ligand. In the crystal lattice of a bis-cyclomanganese complex, the molecules are -stacked in a parallel head-to-tail pattern with a mean inter-planar distance between the benzoquinoxaline planes of 3.5 Å (DECTAH; Djukic et al., 2005). In addition we also found two octahedral Ir III complexes (VEHCAN and VEHCER; Chen et al., 2006).
There are three entries for crystal structures related to 2,3bis(2-pyridyl)benzoquinoxaline. In the distorted octahedral Co III complex (JUHVIR; Escuer et al., 1991), the Co III atom is situated in the benzoquinoxaline plane, coordinated by one pyridyl N atom and one quinoxaline N atom. In the octahedral Re V complex (HAYSAB; Bandoli et al., 1994), the Re V atom is chelated by two pyridyl N atoms of the bis(2-pyrid- Table 1 Hydrogen-bond geometry (Å , ).

Figure 3
Crystal packing of the title compound, showing molecules linked by intermolecular C-HÁ Á Á bonds (dashed lines).

Figure 1
Molecular structure of the title compound, showing the atom-numbering scheme and 30% probability ellipsoids for non-H atoms.
yl)benzoquinoxaline ligand. Finally, in the square-planar Pt II complex (AYAMIW; Cusumano et al., 2004), the benzoquinoxaline moiety lies almost perpendicular to the square plane giving the molecule an unusual L-shaped geometry.

Synthesis and crystallization
Chemicals were obtained commercially in reagent grade and used as received. Solvents were dried using standard procedures as described in the literature. 1 H NMR spectra were recorded with a 300 MHz Varian Mercury model in CDCl 3 . 4,4-Dimethylbenzil (3 mmol), 2,3-diaminonaphthalene (4.4 mmol), and iodine (0.37 mmol) were dissolved slowly in acetonitrile (10 ml), and stirred for 10 minutes at room temperature. The reaction mixture was poured into water, extracted with ether and dried over anhydrous MgSO 4 . After volatiles had been removed under reduced pressure, the product was purified by silica gel chromatography using an eluent of hexane/ethyl acetate (20:1). Pale-yellow single crystals of the title compound were obtained from dichloro-methane/hexane (1:1)

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The C-bound H atoms were positioned geometrically and refined using a riding model, with d(C-H) = 0.93-0.96 Å , and with U iso (H) = 1.2U eq (C) for aromatic-H and 1.5U eq (C) for methyl-H atoms, respectively.  Data collection: SMART (Bruker, 2012); cell refinement: SAINT (Bruker, 2012); data reduction: SAINT (Bruker, 2012); program(s) used to solve structure: SHELXS2013 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: WinGX (Farrugia, 2012). 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.