Crystal structure of dimethyl 3,3′-[(3-nitrophenyl)methylene]bis(1H-indole-2-carboxylate) ethanol monosolvate

In the title compound, the planes of the two indole ring systems are approximately perpendicular to each other, with a dihedral angle of 89.3 (5)°.

In the title compound, C 27 H 21 N 3 O 6 ÁC 2 H 5 OH, the indole ring systems are approximately perpendicular to each other, with a dihedral angle of 89.3 (5) ; the plane of the benzene ring is oriented with respect to the indole ring systems at 49.9 (5) and 73.4 (3) . In the crystal, molecules are linked by N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds and weak C-HÁ Á Á interactions into a threedimensional supramolecular architecture. A void of 33.0 (7) Å 3 is observed in the crystal structure. The solvent ethanol molecule acts as a donor, forming an O-HÁ Á ÁO hydrogen bond, reinforcing the framework structure.

Chemical context
Indole derivatives are found abundantly in a variety of natural plants and exhibit various physiological properties (Poter et al., 1977;Sundberg, 1996). Among them, bis-indolymethane derivatives are found to be potentially bioactive compounds (Chang et al., 1999;Ge et al., 1999). In recent years, the synthesis and application of bis-indolymethane derivatives have been widely studied. The title compound is one of the bis-indolymethane derivatives as a precursor for MRI contrast agents (Ni, 2008). We report herein the synthesis and crystal structure of the title compound.

Structural commentary
The molecular structure of the title compound is shown in Fig. 1. The two indole ring systems are nearly perpendicular to each other [dihedral angle = 89.3 (5) ] while the benzene ring (C1-C6) is twisted to the N1/C8-C15 and N2/C18-C25 indole ring systems with dihedral angles of 49.9 (5) and 73.4 (3) , respectively. The carboxyl groups are approximately coplanar with the attached indole ring systems, the dihedral angles between the carboxyl groups and the mean plane of attached indole ring system are 10.0 (3) and 4.0 (4) . The nitro group is also nearly coplanar with the attached benzene ring, the dihedral angle being 7.7 (7) . A void of 33.0 (7) Å 3 is observed ISSN 1600-5368 in the crystal structure. The solvent ethanol molecule acts as a donor, forming an O-HÁ Á ÁO hydrogen bond, reinforcing the framework structure.

Supramolecular features
In the crystal, the organic molecules and ethanol solvent molecules are linked by classic N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds and weak C-HÁ Á Á interactions involved the benzene rings, forming the three-dimensional supramolecular architecture (Table 1).

Synthesis and crystallization
Methyl indole-2-carboxylate (17.5 g, 100 mmol) was dissolved in 200 ml methanol; commercially available 3-nitrobenzaldehyde (7.6 g, 50 mmol) was added and the mixture was heated to reflux temperature. Concentrated HCl (3.7 ml) was added and the reaction was left for 1 h. After cooling the white product was filtered off and washed thoroughly with methanol. The reaction can be followed by thin-layer chromatography (CHCl 3 -hexane = 1:1 v/v). The yield was 90%. Crystals of the title compound suitable for X-ray analysis were obtained by slow evaporation of an ethanol solution.   Table 1 Hydrogen-bond geometry (Å , ).

Figure 1
The molecular structure of the title molecule. showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level. program(s) used to refine structure: SHELXTL (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008). 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. 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.  (4)