Crystal structure of diethyl 3,3′-[(2,4-dichlorophenyl)methylidene]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 80.9 (5)°. In the crystal, pairs of N—H⋯O hydrogen bonds link the molecules into inversion dimers and these are further linked by N—H⋯O and hydrogen bonds and short Cl—Cl contacts into supramolecular chains.

In the title compound, C 29 H 24 Cl 2 N 2 O 4 , the mean planes of the two indole ring systems (r.m.s. deviations = 0.1249 and 0.0075 Å ) are approximately perpendicular to one another, with a dihedral angle of 80.9 (5) between them. The benzene ring is inclined to the mean planes of the two indole ring systems by 76.1 (3) and 78.3 (4) . Weak intramolecular C-HÁ Á Á interactions affect the molecular conformation. In the crystal, pairs of N-HÁ Á ÁO hydrogen bonds link the molecules into inversion dimers which are further linked into supramolecular chains by N-HÁ Á ÁO hydrogen bonds and short Cl-Cl contacts.

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, displaying anticancer, antileishmanial and antihyperlipidemic properties (Chang et al., 1999;Ge et al., 1999). Furthermore, bis(indolyl)methane derivatives can also be used as precursors 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). We report here the molecular and crystal structure of the title bis(indoly)methane derivative.

Structural commentary
The molecular structure of the title compound is shown in Fig. 1. The overall conformation of the molecule is affected by intramolecular C4-H4AÁ Á ÁCg5 and C15-H15AÁ Á ÁCg1 ISSN 2056-9890 interactions (Cg1 and Cg5 are the centroids of the N1,C8,C3,C2,C9 and C24-C29 rings, respectively), Fig. 1, Table 1. The two indole ring systems are nearly perpendicular to one another, subtending a dihedral angle of 80.9 (5) while the C24-C29 benzene ring is inclined to the N1/C2-C9 and N2/ C13-C20 indole ring systems by dihedral angles of 76.1 (3) and 78.3 (4) , respectively. The carboxyl groups lie close to the planes of the indole ring systems to which they are bound, with dihedral angles between the carboxyl groups and the mean planes of the N1/C2-C9 and N2/C13-C20 indole ring systems of 8.3 (5) and 5.6 (3) , respectively.

Synthesis and crystallization
Ethyl indole-2-carboxylate (1.88 g, 10 mmol) was dissolved in 20 ml ethanol; commercially available 2,4-dichlorobenzaldehyde (0.88 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 white A packing diagram of the title compound. Hydrogen bonds (Table 1) and ClÁ Á ÁCl contacts are shown as dashed lines. 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. Intramolecular C-HÁ Á Á(ring) contacts (Table 1) are shown as dotted black lines with ring centroids displayed as coloured spheres.
product was filtered off and washed thoroughly with ethanol. The reaction was monitored with TLC (AcOEt:hexane = 1:3). Colourless block-like crystals of the title compound suitable for X-ray analysis were obtained in 92% yield by slow evaporation of an ethanol solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were positioned geometrically with N-H = 0.86 Å and C-H = 0.93-0.98 Å , and constrained to ride on their parent atoms with U iso (H) = xU eq (C,N), where x = 1.5 for methyl H atoms and 1.2 for all others.  program(s) used to refine structure: SHELXTL (Sheldrick, 2008); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Diethyl 3,3′-[(2,4-dichlorophenyl)methylidene]bis(1H-indole-2-carboxylate)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.69 e Å −3 Δρ min = −1.13 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.