Crystal structure of N-(1H-indol-2-ylmethylidene)-4-methoxyaniline

The molecule of the title compound contains an essentially planar indole ring system and a phenyl ring. In the crystal, the molecules are linked by a weak intermolecular C—H⋯O hydrogen bond and C—H⋯π interactions, forming a two-dimensional network structure.


Chemical context
Indole and its derivatives are useful starting compounds to derive pharmaceutical (Nalli et al., 2020) and biological materials (Arumugam et al., 2021). Indole can function as a hydrogen-bond donor because of the high acidity of the hydrogen atom at position 1. The introduction of a hydrogenbond acceptor to position 2 of the indole ring forms a five-toseven-membered intramolecular hydrogen-bonded ring (Nosenko, et al., 2008). In this work, a Schiff base including an indole ring, N-(indol-2-ylmethylidene)-4-methoxyaniline, was newly synthesized. Similar Schiff bases such as salicylideneamines often function as bidentate ligands (Wang et al., 2018). Whereas salicylideneamines form intramolecular hydrogen bonds between coordination site atoms, such intramolecular interactions are absent from the crystal structure of the title compound. We report herein on its molecular and crystal structure.

Structural commentary
The molecular structure of the title compound is shown in Fig. 1. The C N double bond adopts an E configuration. The indole moiety is almost planar with an r.m.s. deviation of 0.009 (1) Å . The bond lengths and angles in the title molecule are normal and agree with those in other indole imine compounds (IWIGUS; Suresh et al., 2016;KEVLON;Ho et al., 2006). The dihedral angle between the indole system and the benzene ring is 9.89 (5) . In the related compound IWIGUS, the dihedral angles between the indole system and the benzene ring disordered over two sets of sites are widened to 81.8 (3) and 85.2 (3) due to two isopropyl substituents in the benzene ring. There is no intramolecular hydrogen bond in the title compound, because the N2-H2Á Á ÁN3 angle is as small as 94.4 (10) ; however, the N2Á Á ÁN3 distance is 2.8633 (16) Å , and the N2-C4-C12-N3 torsion angle is 3.94 (19) . Although no intramolecular hydrogen bond is observed, a broad peak assigned for the N-H proton is seen in the 1 H NMR spectrum of the title compound in a CDCl 3 medium and this suggests that the compound forms an intramolecular hydrogen bond in solution (see Synthesis and crystallization).

Supramolecular features
The title compound contains an N-H group, which is a hydrogen-bond donor, and an imino group, which is a hydrogen-bond acceptor, but neither of them forms an intermolecular hydrogen bond in the crystal. Compounds containing a similar indol-2-ylmethylidene-aniline fragment with a cis-conformation of the C-C single bond between the N atoms often form dimers by intermolecular N-HÁ Á ÁN hydrogen bonds (see Database survey). However, in the crystal the molecules of the title compound are linked by a weak intermolecular C10-H10Á Á ÁO1 i hydrogen bond and C-HÁ Á Á interactions [C17-H17Á Á ÁCg1 i and C19-H19CÁ Á ÁCg2 i ; Cg1 is the centroid of the N2/C4-C6/C11 ring and Cg2 is the centroid of the C6-C11 ring; symmetry code: (i) 1 2 À x, 1 2 + y, 3 2 À z], forming columns along the b-axis direction (Fig. 2, Table 1). Besides this, the molecules belonging to different columns are joined by other C-HÁ Á Á interactions [C14-H14Á Á ÁCg1 ii and C15-H15Á Á ÁCg2 ii ; symmetry code: (ii) 3 2 À x, À 1 2 + y, 3 2 À z] (Fig. 3, Table 1). As a result, the intermolecular C-HÁ Á ÁO hydrogen bonds and C-HÁ Á Á interactions form a two-dimensional network structure (Fig. 4). One-dimensional column structure in the crystal of the title compound viewed along the a axis. The C-HÁ Á ÁO hydrogen bonds and the C-HÁ Á Á interactions are shown as dashed lines. H atoms not involved in these interactions are omitted for clarity. Table 1 Hydrogen-bond geometry (Å , ).

Figure 3
Part of the crystal structure of the title compound showing the formation of ribbons along the b-axis direction. The C-HÁ Á Á interactions are shown as dashed lines. H atoms not involved in these interactions are omitted for clarity.

Figure 1
The molecular structure of the title compound with atom labelling. Displacement ellipsoids are drawn at the 50% probability level. H atoms are represented by spheres of arbitrary radius.

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.42, update of May 2021; Groom et al., 2016) using ConQuest (Bruno et al., 2002) for indole derivatives gave 5272 hits, and for the (1H-indol-2-yl)methanimine skeleton gave 86 hits. Among these, the imino N atom bonded to an H atom gave one hit, to an N atom gave 24 hits, and to a C atom gave 61 hits. A search for the indol-2-ylmethylidene-aniline fragment gave 30 hits, and those containing a (1H-indol-2yl)methylidene-aniline fragment with a cis-conformation of the C-C single bond gave seven hits. These seven compounds  Greci & Sgarabotto, 1984). In the crystal of IWIGUS, which features a large dihedral angle between the indole and benzene rings, two neighbouring molecules are associated through pairs of N-HÁ Á ÁN intermolecular hydrogen bonds, forming a centrosymmetric dimer. The crystal structure of an indol-2-ylmethylidene-aniline compound without a hydrogen bond between the N-H and imino groups has not yet been reported. In an almost planar molecule without a bulky substituent such as the tile compound, the formation of a dimer by intermolecular N-HÁ Á ÁN hydrogen bonding is probably not appropriate for the crystal packing.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The H atom attached to N2 was located in a difference-Fourier map and freely refined. The Cbound H atoms were positioned geometrically and refined using a riding model: C-H = 0.93-0.96 Å with U iso (H) = 1.2U eq (C).

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 was performed using all reflections. The weighted R-factor (wR) and goodness of fit (S) are based on F 2 . R-factor (gt) are based on F. The threshold expression of F 2 > 2.0 sigma(F 2 ) is used only for calculating Rfactor (gt).