Crystal structure and DFT study of (E)-N-[2-(1H-indol-3-yl)ethyl]-1-(anthracen-9-yl)methanimine

In the title compound, the indole ring system makes a dihedral angle of 63.56 (8)° with the plane of the anthracene ring. The conformation about the C=N bond of the –CH2–CH2–N=CH– bridge linking the two units is E. In the crystal, the indole H atom is involved in an intermolecular N—H⋯π interaction with the benzene ring of the indole group, leading to the formation of chains along [010].


Chemical context
Tryptamine is a biogenic serotonin-related indoamine and is the decarboxylation product of the amino acid tryptophan. 2-(1H-Indol-3-yl)ethanamine is an alkaloid found in plants and fungi and is a possible intermediate in the biosynthetic pathway to the plant hormone indole-3-acetic acid (Takahashi, 1986). It is also found in trace amounts in the mammalian brain, possibly acting as a neuromodulator or neurotransmitter (Jones, 1982). There are seven known families of serotonin receptors, which are tryptamine derivatives. All of them are neurotransmitters. Hallucinogens all have a high affinity for certain serotonin receptor subtypes and the relative hallucinogenic potencies of various drugs can be gauged by their affinities for these receptors (Glennon et al., 1984;Nichols & Sanders-Bush, 2001;Johnson et al., 1987;Krebs-Thomson et al., 1998). The structures of many hallucinogens are similar to serotonin and have a tryptamine core. Indole analogues, especially of tryptamine derivatives, have been found to be polyamine site antagonists at the N-methyl-daspartate receptor (NMDAR; Worthen et al., 2001). Indole and its derivatives are secondary metabolites that are present in most plants (such as unripe bananas, broccoli and clove), almost all flower oils (e.g. jasmine and orange blossoms) and coal tar (Waseem & Mark 2005;Lee et al., 2003). In the pharmaceutical field, it has been discovered that it has antimicrobial and anti-inflammatory properties (Mohammad & Moutaery, 2005). The present work is part of an ongoing structural study of Schiff bases and their utilization in the synthesis of new organic and polynuclear coordination compounds, and their application in fluorescence sensors (Faizi & Sen, 2014;Faizi et al., 2016). We report herein the crystal structure of (E)-N-[2-(1H-indol-3-yl)ethyl]-1-(anthracen-9-yl)methanimine, (I), and its DFT computational calculation. Calculations by density functional theory (DFT) on (I), carried out at the B3LYP/6-311 G(d,p) level, are compared with the experimentally determined molecular structure in the solid state.

Supramolecular features
In the crystal, the indole H atom forms an intermolecular N-HÁ Á Á interaction, linking molecules to form chains along the b-axis direction ( Fig. 2 and Table 1). There are also C-HÁ Á Á interactions present, involving the central ring and terminal rings of the anthracene unit, linking the chains to form layers parallel to the bc plane ( Fig. 2 and Table 1). The molecular structure of compound (I), with the atom labelling. Displacement ellipsoids are drawn at the 50% probability level.

Figure 2
A view along the a axis of the crystal packing of compound (I), showing the layer-like structure. Weak N-HÁ Á Á and C-HÁ Á Á interactions are shown as blue dashed lines (see Table 1). Table 1 Hydrogen-bond geometry (Å , ).

DFT study
Calculations by density functional theory DFT-B3LYP, with basis set 6-311 G(d,p), of bond lengths and angles were performed. These values are compared with the experimental values for the title system (see Table 2). From these results we can conclude that basis set 6-311 G(d,p) is better suited in its approach to the experimental data. The LUMO and HOMO orbital energy parameters are considerably answerable for the charge transfer, chemical reactivity and kinetic/thermodynamic stability of (I). The DFT study of (I) revealed that the HOMO and LUMO are localized in the plane extending from the whole anthracene ring to the indole ring, and electron distribution of the HOMO-1, HOMO, LUMO and LUMO+1 energy levels are shown in Fig. 3. The molecular orbital of HOMO contains both and character, whereas HOMO-1 is dominated by -orbital density. The LUMO is mainly composed of density, while LUMO+1 has both and electronic density. The HOMO-LUMO gap for (I) was found to be 0.12325 a.u. and the frontier molecular orbital energies, E HOMO and E LUMO , were found to be À0.196412 and À0.07087 a.u., respectively.

Synthesis and crystallization
80 mg (0.435 mmol) of 2-(1H-indol-3-yl)ethanamine (tryptamine) were dissolved in 10 ml of absolute ethanol. To this solution, 89 mg (0.434 mmol) of anthracene-9-carbaldehyde in 5 ml of absolute ethanol were added dropwise under stirring. The mixture was stirred for 10 min, two drops of glacial acetic acid were added and the mixture was refluxed for a further 2 h.
The resulting yellow precipitate was recovered by filtration, washed several times with small portions of ice-cold ethanol and then with diethyl ether to give 140 mg (87%) of compound (I). Dark-yellow block-like crystals suitable for X-ray analysis were obtained within 3 d by slow evaporation of a solution in methanol.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The N-H H atom was located from a difference-Fourier map and constrained to ride on the parent atom: N-H = 0.86 Å and U iso (H) = 1.2U eq (N). All Cbound H atoms were positioned geometrically and refined using a riding model, with C-H = 0.93-0.97 Å and U iso (H) = 1.2U eq (C).
The DFT quantum-chemical calculations were performed at the B3LYP/6-311 G(d,p) level (Becke, 1993;Lee et al., 2003) as implemented in GAUSSIAN09 (Frisch et al., 2009). DFT structure optimization of (I) was performed starting from the X-ray geometry. Electron distribution of the HOMO-1, HOMO, LUMO and LUMO+1 energy levels for compound (I). Table 2 Comparison of selected geometric data for (I) (Å , ) from X-ray and calculated (DFT) data.

(E)-N-[2-(1H-Indol-3-yl)ethyl]-1-(anthracen-9-yl)methanimine
Crystal data C 25 H 20 N 2 M r = 348.43 Orthorhombic, P2 1 2 1 2 1 a = 6.0044 (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.