N,N′-Bis(pyridin-2-yl)benzene-1,4-diamine–quinoxaline (2/1)

The asymmetric unit of the title compound, 2C16H14N4·C8H6N2, consits of one molecule of N,N′-bis(pyridin-2-yl)benzene-1,4-diamine (PDAB) and one half-molecule of quinoxaline (QX) that is located around an inversion centre and disordered over two overlapping positions. The PDAB molecule adopts a non-planar conformation with an E configuration at the two partially double exo C N bonds of the 2-pyridylamine units. In the crystal, these self-complementary units are N—H⋯N hydrogen bonded via a cyclic R 2 2(8) motif, creating tapes of PDAB molecules extending along [010]. Inversion-related tapes are arranged into pairs through π–π stacking interactions between the benzene rings [centroid–centroid distance = 3.818 (1) Å] and the two symmetry-independent pyridine groups [centroid–centroid distance = 3.760 (1) Å]. The QX molecules are enclosed in a cavity formed between six PDAB tapes.

The asymmetric unit of the title compound, 2C 16 H 14 N 4 Á-C 8 H 6 N 2 , consits of one molecule of N,N 0 -bis(pyridin-2-yl)benzene-1,4-diamine (PDAB) and one half-molecule of quinoxaline (QX) that is located around an inversion centre and disordered over two overlapping positions. The PDAB molecule adopts a non-planar conformation with an E configuration at the two partially double exo C . . . N bonds of the 2-pyridylamine units. In the crystal, these self-complementary units are N-HÁ Á ÁN hydrogen bonded via a cyclic R 2 2 (8) motif, creating tapes of PDAB molecules extending along [010]. Inversion-related tapes are arranged into pairs throughstacking interactions between the benzene rings [centroid-centroid distance = 3.818 (1) Å ] and the two symmetry-independent pyridine groups [centroid-centroid distance = 3.760 (1) Å ]. The QX molecules are enclosed in a cavity formed between six PDAB tapes.

B. Wicher and M. Gdaniec
Comment N,N'-Di(pyridin-2-yl)benzene-1,4-diamine (PDAB) is a very versatile supramolecular reagent. It has been shown that it can cocrystallize with the aromatic base, phenazine, forming cocrystals with the 1:4 molar ratio (Gdaniec et al., 2005). In this cocrystal the PDAB molecule is centrosymmetric and adopts a nearly planar conformation and a Z,Z form, i.e. the configuration at the partially double exo C≐ N bonds of its two 2-pyridylamine units is Z. The PDAB molecules are hydrogen bonded to phenazine molecules but, most importantly, their π-faces are directed to the edges of the phenazine molecules arranged via π-π stacking interactions into quartets. To check whether a similar packing motif will be observed for a compound containing the pyrazine fragment but a reduced π-system compared to phenazine, an attempt was made to cocrystallize PDAB with quinoxaline (QX). Cocrystallization was successful when PDAB was dissolved in molten QX (m.p. 301 K) and the solution was slowly evaporated at 331 K yielding the title molecular complex with 2:1 PDAB/QX ratio ( Fig.   1). In contrast with the PDAB/phenazine cocrystal, in the title complex the PDAB molecule is nonplanar and adopts an E,E form that promotes formation of a cyclic R 2 2 (8) motif via N-H···N hydrogen bond between the self-complementary 2-pyridylamine units (Table 1). These cyclic motifs assemble PDAB molecules into tapes extending along [010]. The tapes related by inversion center are arranged into pairs through π-π stacking interactions between the benzene rings [centroidcentroid distance 3.818 (1) Å] and the two symmetry independent pyridine groups [centroid-centroid distance 3.760 (1) Å] ( Fig. 2). Similar tape motifs have been observed in two of the three PDAB polymorphs (Bensemann et al., 2002;Wicher & Gdaniec, 2011), however these polymorphic structures were not stabilized by π-π stacking interactions between the tapes.
The QX molecule, that is not hydrogen bonded to PDAB, is enclosed in a centrosymmetric cavity formed between six PDAB tapes (Fig. 3). This leads to a disorder of the non-centrosymmetric QX molecule which in the cavity is located, with equal occupancies, in two alternative overlapping positions. Thus QX molecule in this crystal structure simulates the shape of a naphthalene molecule.
As there are no specific interactions between QX and PDAB molecules the driving force for the complex formation with PDAB is different in the two cocrystals with the aromatic heterobases containing the pyrazine ring.
Experimental N,N'-Di(pyridin-2-yl)benzene-1,4-diamine (0.07 g, 0.27 mmol) was dissolved in an excess of the melted quinoxaline. The solution was heated at 331 K and after a few days colourless crystal suitable for X-ray analysis were obtained.