Crystal structure of diethyl 2-amino-5-{4-[bis(4-methylphenyl)amino]benzamido}thiophene-3,4-dicarboxylate

The title compound forms a head-to-head centrosymmetric dimer, involving a pair of intermolecular N—H⋯O hydrogen bonds. It also forms two intramolecular bonds between its amine and amide and the ester carbonyl groups.

In the title compound, C 31 H 31 N 3 O 5 S, the regioselective substitution of the thiophene is confirmed with the amine and the amide at the 2-and 5-positions, respectively. In the molecule, the thiophene ring is twisted by 12.82 (3) with respect to the aromatic ring of the benzamido group. Intramolecular N-HÁ Á ÁO hydrogen bonds are present involving the N atoms of the primary amine and the amide groups, forming S(6) ring motifs. In the crystal, centrosymmetrically related molecules are linked by pairs of N-HÁ Á ÁO hydrogen bonds involving the amide carbonyl O atoms and the primary amine N atoms to form dimers of R 2 2 (16) ring motif.

Chemical context
Azomethines are prepared by the condensation of amines with aldehydes. Using aromatic precursors, the reaction results in the preparation of conjugated azomethines having colors that are readily detectable in the visible spectrum (Dufresne et al., 2007). This is particularly the case with azomethines that are prepared from 2,5-diaminothiophene derivatives (Bolduc et al., 2013). These derivatives can be electrochemically oxidized (Yeh et al., 2016). The collective properties (reversible color change with applied potential) have proven ideal for use as electrochromic materials (Ma et al., 2016). While various azomethines have been studied for understating the impact of structure on the absorption and electrochemical properties (Liu et al., 2018), modifying the terminal amine has remained relatively underexplored. Such modification allows property tuning, including reversible oxidation. This is a key property for electrochromic use. Given the underexplored modification of 2-aminothiophenes, we investigated its conversion to a triphenylamide. The triphenylamide moiety was targeted because of its electrochemically reversible oxidation. Meanwhile, the amide functional group was chosen because of its robustness that could sustain electrochemical redox cycles. More importantly, it would be inert towards imination reactions for constructing conjugated azomethines having both various terminal groups and cores. Given the challenge of unequivocally identifying the configuration and absolute structural identification of aminothiophene derivatives with the concomitant limited number of reported triphenylamine amides, the X-ray crystal structure analysis of the title compound (I) was evaluated and it is reported on herein. ISSN 2056-9890

Structural commentary
In the molecule of I (Fig. 1), the mean plane through the 2,5diaminotihophene ring (r.m.s. deviation = 0.0116 Å ) is inclined to the C1-C6 benzene (ring A) by 12.82 (3) . The dihedral angles formed by the benzene rings A, B (C18-C23) and C (C25-C30) of the triphenylamide moiety are: A^B = 65.56 (3) , A^C = 55.22 (4) , B^C = 66.80 (4) . The O1-C7, N2-C7, N2-C8 and N3-C11 bond lengths are 1.2315 (13), 1.3644 (13), 1.3829 (13) and 1.3529 (14) Å , respectively. While the reactivity of the primary amine of I is less than that expected for typical arylamines owing to the electron-withdrawing esters, it nonetheless acts as a hydrogen donor. In fact, two N-HÁ Á ÁO intramolecular hydrogen bonds occur, one each between the ester carbonyl and its adjacent nitrogen, forming rings of S(6) graph-set motif ( Table 1). The intramolecular hydrogen bonds observed are consistent with those reported in other 2-amino-3-ester thiophenes (Dufresne & Skene, 2010a,b;Skene et al., 2006;Bourgeaux et al., 2006;Bolduc et al., 2010;Tshibaka et al., 2011;Furuyama et al., 2014). The crystal structure of I confirms the asymmetric substitution of thiophene by a primary amine and an amide. Of importance is that the thiophene substitution with the nitrogen atoms occurs at the 2,5positions, rather than the 3,4-positions. The primary amine at the 2-thiophene position is also confirmed. The 2,5-configuration is desired because extended degrees of conjugation result when the azomethines are formed in these positions with arylamines. The presence of ester functionalities at the 3,4-positions is also verified by the crystal structure.

Supramolecular features
In the crystal structure of I, centrosymmetrically related molecules are linked into head-to-head hydrogen-bonded dimers (Fig. 2) by pairs of N-HÁ Á ÁO hydrogen bonds (Table 1) involving the N3 amine atom and the O1 carbonyl atom of the amide group, forming rings of R 2 2 (16) graph-set motif. In this arrangement, the sulfur atoms of the two thiophenes are faceto-face and the two heteoratoms are separated by 3.5419 (4) Å . The crystal packing (Fig. 3) is further stabilized by van der Waals forces.

Figure 2
Supramolecular dimer of I showing the intermolecular hydrogen bonds as dotted lines.

Figure 1
The molecular structure of I with displacement ellipsoids drawn at the 50% probability level. H atoms are shown as spheres of arbitrary radius. Intramolecular hydrogen bonds are shown as dashed lines.
No differences between the N1-phenyl and C4-C7 bond distances were found. The three phenyl-N-phenyl dihedral angles of I are also consistent with the those of the reported structures.

Synthesis and crystallization
To a solution of 4-(di-p-tolylamino)benzoic acid (668 mg, 1.7 mmol, 1 eq) in anhydrous dichloromethane (15 mL) were added oxalyl chloride (0.21 mL, 2.3 mmol, 1.8 eq) and one drop of anhydrous DMF. The mixture was stirred for 16 h under nitrogen at room temperature. The solvent was removed under reduced pressure and the resulting 4-(di-ptolylamino)benzoyl chloride was dissolved in anhydrous THF (20 mL). The mixture was then added dropwise to a solution of diethyl 2,5-diaminothiophene-3,4-dicarboxylate (594 mg, 2.3 mmol, 1.1 eq) and Et 3 N (2.3 mmol, 0.32 mL, 1.1 eq) in anhydrous THF (5 mL Crystal packing of I approximately viewed along the a axis.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The amine H atoms were located in a difference-Fourier map and refined freely. All other H atoms were placed geometrically and refined with C-H = 0.95-0.99 Å , and with U iso (H) = 1.2U eq (C) or 1.5U eq (C) for methyl H atoms. A rotating model was used for the methyl groups. for Windows Taskbar v1.19 (Spek, 2009).; software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009) and publCIF (Westrip, 2010).

Special details
Experimental. X-ray crystallographic data for I were collected from a single crystal sample, which was mounted on a loop fiber. Data were collected using a Bruker Venture diffractometer equipped with a Photon 100 CMOS Detector, a Helios MX optics and a Kappa goniometer. The crystal-to-detector distance was 4.0 cm, and the data collection was carried out in 1024 x 1024 pixel mode. 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.