5-Methoxy-2-[(5-methoxy-1H-indol-1-yl)carbonyl]-1H-indole

The asymmetric unit of the title compound, C19H16N2O3, comprises three independent molecules (A, B and C). The inversion-related molecule of A is virtually superimposable upon the other two molecules. In each molecule, there is a twist in the link between the approximately syn carbonyl and amine groups [the N—C—C—O torsion angles range from 19.73 (19) to −21.2 (2)°]. Each molecule has a bent shape quantified in terms of the dihedral angle between the indole and indole fused-ring systems [range = 45.69 (5)–47.91 (5)°]. In the crystal, the A and B molecules form dimeric aggregates via ten-membered {⋯HNC2O}2 synthons, while the C molecules self-associate similarly but about a centre of inversion.

The asymmetric unit of the title compound, C 19 H 16 N 2 O 3 , comprises three independent molecules (A, B and C). The inversion-related molecule of A is virtually superimposable upon the other two molecules. In each molecule, there is a twist in the link between the approximately syn carbonyl and amine groups [the N-C-C-O torsion angles range from 19.73 (19) to À21.2 (2) ]. Each molecule has a bent shape quantified in terms of the dihedral angle between the indole and indole fused-ring systems [range = 45.69 (5)-47.91 (5) ]. In the crystal, the A and B molecules form dimeric aggregates via ten-membered {Á Á ÁHNC 2 O} 2 synthons, while the C molecules self-associate similarly but about a centre of inversion.
The financial support of the Deanship of Scientific Research and the Research Center of the College of Pharmacy, King Saud University is greatly appreciated. We also thank the Ministry of Higher Education (Malaysia) for funding structural studies through the High-Impact Research scheme (UM.C/HIR/MOHE/SC/12). that commercial melatonin preparations contain N-{2- [1-({3-[2-(acetylamino)ethyl]-5-methoxy-1H-indol-2-yl}methyl)-5-methoxy-1H-indol-3-yl]ethyl}acetamide (1) as a contaminant (Williamson et al., 1998). The title compound, namely (5-methoxy-1H-indol-1-yl)(5-methoxy-1H-indol-2-yl)methanone (I), can be elaborated to give compound 1, in four steps. The synthesis of compound 1 on a preparative scale is required for the development of an analytical method for the determination of MLT in the presence of this contaminant in commercial MLT preparations. Herein, the crystal and molecular structure of the title compound (I) is described in continuation of on-going studies of melatonin receptor ligands (Bedini et al., 2006;Attia et al., 2008;Attia et al., 2012).
Three crystallographically independent molecules comprises the asymmetric unit of the title compound (I), Fig. 1. In each molecule there is a twist in the link between the carbonyl and amine groups but, each of these is approximately syn with the N1-C9-C10-O2, N3-C28-C29-O5 and N5-C47-C48-O8 torsion angles being 19.73 (19), -20.34 (19) and -21.2 (2)°, respectively. Each molecule has a bent shape quantified in terms of the dihedral angle between the indole and indonyl fused ring systems. For the N1-containing molecule this angle is 45.69 (5)° which compares to 45.86 (5) and 47.91 (5)° in the other two molecules. If the inversion-related N1-containing molecule is overlapped with the N2and N3-containing molecules, it can be seen that all three molecules are virtually superimposable, as shown in Fig. 2. The major differences are apparent in the relative orientations of the terminal methoxy groups of the indonyl rings. In the N1and N-3 containing molecules, the methyl group is orientated in almost the opposite direction to that seen in the N2containing molecule. Further, in the N3-containing molecule, the methoxy group is slightly twisted out of the plane of the benzene ring to which it is connected. This is quantified in the values of the C16-C17-O3-C19, C35-C36-O6-C37 and C54-C55-O9-C57 torsion angles of -179.18 (14), 0.06 (2) and 168.68 (13)°, respectively.

Refinement
Carbon-bound H-atoms were placed in calculated positions [C-H = 0.95 to 0.98 Å, U iso (H) = 1.2U eq (C)] and were included in the refinement in the riding model approximation. The amino H-atoms were refined freely. The (5 11 17) reflection was omitted owing to poor agreement.

Figure 1
The molecular structure of the title compound (I), showing the atom-labelling and displacement ellipsoids drawn at the 50% probability level.  Overlay diagram of the N1-(red), N3-(green) and N5-(blue) containing molecules in (I) aligned so that the central amide residues are coincident.

Figure 3
A view of a dimeric aggregate in (I) sustained by N-H···O hydrogen bonds, shown as blue dashed lines. T min = 0.367, T max = 1.000 17372 measured reflections 9182 independent reflections 7534 reflections with I > 2σ(I)

5-Methoxy-2-[(5-methoxy-1H-indol-1-yl)carbonyl]-1H-indole
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.