Crystal structure of diethyl 3,3′-[(4-nitrophenyl)methylene]bis(1H-indole-2-carboxylate)

In the title compound, the two indole ring systems are approximately perpendicular to one another, making a dihedral angle of 89.7 (5)°. In the crystal, pairs of N—H⋯O hydrogen bonds link the molecules into the inversion dimers, which are further linked into supramolecular chains.


Chemical context
Bis(indolyl)methane derivatives are abundantly present in various terrestrial and marine natural resources (Porter et al., 1977;Sundberg, 1996). They are important antibiotics in the field of pharmaceuticals with diverse activities, such as anticancer, antileishmanial and antihyperlipidemic (Chang et al., 1999;Ge et al., 1999). On the other hand, bis(indoly)methane derivatives can also be used as a precursor for MRI necrosis avid contrast agents (Ni, 2008). In recent years, we have reported the synthesis and crystal structures of some similar bis(indoly)methane compounds (Sun et al., 2012(Sun et al., , 2015Li et al., 2014;Lu et al., 2014). Now we report herein on the crystal structure of the title bis(indoly)methane compound.

Structural commentary
The molecular structure of the title compound is shown in Fig. 1. The overall conformation of the molecule is affected by ISSN 2056-9890 intramolecular C10-H10AÁ Á ÁCg3 and C21-H21AÁ Á ÁCg1 interactions (Table 1). The two indole ring systems are nearly perpendicular to one another [dihedral angle = 89.7 (5) ] while the benzene ring (C2-C7) is twisted to the N2/C8-C15 and N3/C19-C26 indole ring systems by dihedral angles of 52.6 (4) and 88.2 (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 the N2/C8-C15 and N3/C19-C26 indole ring systems are 12.5 (4) and 4.9 (5) , respectively.

Supramolecular features
In the crystal, pairs of N2-H2AÁ Á ÁO3 i and N3-H3AÁ Á ÁO5 ii hydrogen bonds link the molecules into the inversion dimers, which are further shown as supramolecular chains propagating along the b-axis direction (Table 1 and Fig. 2). In the crystal, weak C-HÁ Á ÁO hydrogen bonds and C-HÁ Á Á interactions are also observed, linking the chains to form a three-dimensional supramolecular structure.

Figure 2
A packing diagram of the title compound. The N-HÁ Á ÁO Hydrogen bonds are shown as dashed lines.

Synthesis and crystallization
Ethyl indole-2-carboxylate (1.88 g, 10 mmol) was dissolved in 20 ml ethanol; commercially available 4-nitrobenzaldehyde (0.76 g, 5 mmol) was added and the mixture was heated to reflux temperature. Concentrated HCl (0.5 ml) was added and the reaction was left for 1 h. After cooling, the yellow product was filtered off and washed thoroughly with ethanol. The reaction was monitored with TLC (AcOEt:hexane = 1:3). Single crystals of the title compound suitable for X-ray analysis were obtained by slow evaporation of a methanol solution (yield 93%).

Diethyl 3,3′-[(4-nitrophenyl)methylene]bis(1H-indole-2-carboxylate)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.39 e Å −3 Δρ min = −0.28 e Å −3 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 > 2sigma(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.