1,4-Dihydrobenzo[g]quinoxaline-2,3-dione

The title compound, C12H8N2O2, was prepared by the reaction of the diethyl ester of naphthalenebis(oxamate) with tert-BuNH2. The molecule is nearly planar, with an r.m.s. deviation of 0.017 Å from the plane through all 16 non-H atoms. In the crystal, a three-dimensional network is formed, composed of layers of molecules along the b- and c-axis directions, due to the formation of intermolecular N—H⋯O hydrogen bonds, as well as of chains along the a-axis direction due to parallel displaced sandwich-type π–π interactions with average distances between the interacting molecules in the range 3.35–3.40 Å.

The title compound, C 12 H 8 N 2 O 2 , was prepared by the reaction of the diethyl ester of naphthalenebis(oxamate) with tert-BuNH 2 . The molecule is nearly planar, with an r.m.s. deviation of 0.017 Å from the plane through all 16 non-H atoms. In the crystal, a three-dimensional network is formed, composed of layers of molecules along the b-and c-axis directions, due to the formation of intermolecular N-HÁ Á ÁO hydrogen bonds, as well as of chains along the a-axis direction due to parallel displaced sandwich-typeinteractions with average distances between the interacting molecules in the range 3.35-3.40 Å .
Data collection: CrysAlis CCD (Oxford Diffraction, 2006); cell refinement: CrysAlis CCD; data reduction: CrysAlis RED (Oxford Diffraction, 2006); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: WinGX (Farrugia, 2012) and publCIF (Westrip, 2010 Weak interactions such as intermolecular π interactions or the formation of hydrogen bonds are essential for molecular recognition and self organization in biological systems and supramolecular chemistry (Burrow et al., 1996;Chowdhry et al., 1996;Dai et al., 1997). Oxtoby et al. (2005) synthesized derivatives of dihydroquinoxalinedione containing a hydrophilic oxalamide-based "terminus". This oxalamide serves to increase the water solubility of the organic molecules and allows H 2 O molecules to be hydrogen bonded to the organic molecules as well as to each other. In this perspective, 1,4 dihydrobenzo[g]quinoxaline-2,3-dione hydrate (1×3H 2 O) has been crystallized by slowly cooling a boiling solution of the powder in DMF/water to room temperature (Oxtoby et al., 2005). Infinite arrays containing two different alternating head-to-tail π interactions parallel to the crystallographic a axis were observed in the solid state of 1×3H 2 O, with the π stacks being orthogonal to chains of H 2 O molecules and held together by R 2 2 (8) hydrogen-bonding interactions. Bis(oxamate) molecules (type I molecules, cf. Figure 4) have been widely used in order to produce a series of complexes with various magnetic interactions (Pardo et al., 2008). In previous work, our interest was devoted to a study of the impact of π interactions in the formation of thin films using trimetallic bis(oxamato) type complexes (Eya'ane Meva, 2009;Bräuer et al., 2006) as well as the investigation of electronic effects on magnetic J couplings (Rüffer et al., 2007). With the aim of using type I-NR 2 molecules as ligands for the formation of transition metal complexes Rüffer et al., 2012), we became interested in their synthesis. Generally, type I molecules are reacted with an excess of a primary amine (Ruiz et al., 1997) resulting in the formation of type I-NR 2 molecules. On the other hand, quinoxaline derivatives are generally synthesized by refluxing diamines and oxalic acid in HCl (Oxtoby et al., 2005). We report here, that the reaction of the diethyl ester of naphtalene-bis(oxamate) (Rüffer et al., 2007), Fig. 4, with an excess of tert-BuNH 2 in MeOH does not give the corresponding type I-NR 2 molecule, but instead forms 1,4-dihydrobenzo[g]quinoxaline-2,3-dione (1). A similar reaction has been already described by Munoz et al. (1998), who treated ophenylenebis(oxamate) with [Me 4 N]OH and Fe(ClO 4 ) 3 to obtain an analogous derivatized product in the form of its Fe(III) complex.
In the crystal structure of 1 the formation of a three-dimensional network is observed. Intermolecular hydrogen bonds between individual molecules of 1, cf. Table 1, form two-dimensional layers. A representative view of one selected twodimensional layer is illustrated in Figure 2, showing the two-dimensional layers extending along the crystallographic band c-axes. Within the two-dimensional layers the formation of dimers of 1 with R 2 2 (8) type hydrogen bond interactions is also observed, as reported for 1×3H 2 O (Oxtoby et al., 2005), cf. N2-H2N···O2 ii in Table 1 and Figure 2. However, due to the N1-H1N···O1 i , hydrogen bond cf. Additionally, individual molecules of 1 interact with each other by means of π interactions. They are approximately arranged in a parallel-displaced sandwich type configuration and thus form one-dimensional layers along the crystallographic a-axis, cf. Figure 3. Within such a one-dimensional chain, a head-to-tail arrangement is observed, as reported previously for 1×3H 2 O (Oxtoby et al., 2005). Moreover, by analogy with 1×3H 2 O, the π stacks of 1 are arranged orthogonal to the two-dimensional layers formed by intermolecular hydrogen bonds. The combinations of both supramolecular arrangements finally give rise to a three-dimensional network structure.

Experimental
Diethyl N,N′-naphthalene-bis(oxamate) was synthesized according to the literature (Rüffer et al., 2007). To a solution of diethyl naphthalene-bis(oxamate) (1.5 g, 9.49 mmol) in MeOH (50 ml) three equivalents of a solution of tert-BuNH 2 (2.05 g, 28.47 mmol) in MeOH (25 ml) were added. The solution was refluxed for 30 minutes, cooled to room temperature and concentrated to 30 ml. Diethyl ether (100 ml) was added and the resulting brown precipitate was filtered and dried on air. Yellow crystals of 1 were obtained by solvent diffusion of a dilute MeOH solution of 1 against Et 2 O at room temperature. Yield: 0.7 g, 80%.

Refinement
C-bonded H atoms were placed in calculated positions and constrained to ride on their parent atoms, with a C-H distance of 0.93 Å and U iso (H) = 1.2U eq (C). The N-bonded H atoms were located on a difference Fourier map and refined freely. The high R factor, low ratio of observed to unique reflections and relatively high su values indicate that the crystals were not of good quality and were very weakly diffracting.

Figure 4
Synthesis of 1. Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.