(E)-N,N-Diethyl-4-{[(4-methoxyphenyl)imino]methyl}aniline: crystal structure, Hirshfeld surface analysis and energy framework

In the title compound, a benzylideneaniline Schiff base, the planes of the p-substituted aromatic rings subtend a dihedral angle of 46.01 (6)°.


Chemical context
Schiff bases are known for their distinctive azomethine group (-N CH-) and ease of synthesis, often by a simple condensation reaction.Brodowska & Łodyga-Chrus ´cin ´ska (2014, and references therein) have reviewed Schiff bases, covering their biological, antibacterial, antitfungal, biocidal, antimalarial and anticancer activities, together with their uses in technology, synthesis and chemical analysis.The -N CH-group plays an important role in forming stable metal complexes (Iqbal et al., 1995), and recently Boulechfar et al. (2023) have reviewed the history, synthesis and applications of Schiff bases and their metal complexes.
Herein, we describe the synthesis and crystal structure of the title benzylideneaniline Schiff base (E)-N,N-diethyl-4-{[(4methoxyphenyl)imino]methyl}aniline (I) and compare its structure and Hirshfeld surface to those of related compounds.

Structural commentary
The title compound crystallizes in the triclinic space group P1 with one molecule in the asymmetric unit (Fig. 1).The aromatic rings (A = C1-C6 and B = C8-C13) are inclined to each other by 46.01 (6) � , while the C4-N1-C7-C8 torsion angle is 176.9 (1) � .The configuration about the N1 C7 bond is E and its bond length is 1.2754 (15) A ˚.The major twist in the molecule occurs about the C4-N1 bond, as indicated by the C5-C4-N1-C7 torsion angle of À 41.89 (16) � .Atom C14 of the methoxy group lies almost in the plane of its attached ring [deviation = À 0.012 (1) A ˚].The N2/C15/C17 moiety is twisted by 12.85 (12) � from its attached ring and the C atom of the C16 methyl group is displaced from the C8-C13 ring by 1.329 (2) A ånd C18 is displaced in the opposite sense, by À 0.893 (2) A ˚, which we term a trans arrangement (see Database survey section).

Supramolecular features
In the crystal of I, the shortest contact involves a pair of very weak C-H� � �� interactions (Table 1).They link inversionrelated molecules to form dimers that stack along the a-axis direction (Fig. 2).
Compound II fits into the first cluster, whereas compounds I and III clearly fit into the second cluster.

to 1.315 A ˚for
Figure 1 A view of the molecular structure of I, with the atom labelling.The displacement ellipsoids are drawn at the 50% probability level.

Figure 2
A view along the b axis of the crystal packing of I.The C-H� � �� interactions are indicated by blue arrows (see Table 1).Only the H atoms involved in these interactions have been included.Another structural feature of compound I is the arrangement of the ethyl groups of the -N(C 2 H 5 ) 2 moiety.Here, they have a trans arrangement with one CH 3 group directed above the plane of the -CH 2 -N-CH 2 -unit and the other below (Fig. 1).A search of the CSD for benzylideneanilines with an N,N-diethylaniline group gave 12 hits.In nine of these struc-  A more extensive search for diethylaminobenzene derivatives gave over 300 hits for structures with the same search criteria as above.An analysis of the two CH 3 -CH 2 -N-CH 2 torsion angles is shown in a scatter plot (Fig. 3).It can be seen that the majority of compounds have either the cis (À /+ or +/À ) or the trans (+/+ or À /À ) arrangement.Some of the outliers indicate an intermediate state with one large torsion angle and the other quite small, for example, (2-diethylaminophenyl)diphenylmethanol (ERONDO; Al-Masri et al., 2004), whose structure is illustrated in Fig. 3. Finally, in one compound, viz.N,N,N 0 ,N 0 -tetraethyl-2,6-bis(phenylethynyl)thieno[2,3-f][1]benzothiophene-4,8-diamine (JOQZIA; Wen et al., 2015), a unique arrangement was observed with both ethyl groups having an extended conformation (see Fig. 3).

Hirshfeld surface analysis and two-dimensional fingerprint plots
The Hirshfeld surface (HS) analyses and the associated twodimensional fingerprint plots were performed with Crystal-Explorer17 (Spackman et al., 2021)    cant in the packing of the three compounds.The short contacts in the crystals of the three compounds are compared in Table S1 of the supporting information.It is not surprising that for II, with a total of seven C-H� � �� interactions in the crystal (Sundararaman et al., 2009), that there are a large number of C� � �H contacts.
The full two-dimensional fingerprint plots for I, II and III are given in Fig. 5.The contributions of the various interatomic contacts to the Hirshfeld surfaces for the three compounds are compared in Table 2.In all three compounds, the H� � �H contacts have a major contribution, i.e. 62.5% for I, 58.1% for the two independent molecule of II and 59.5% for III.The second most significant contributions are from the C� � �H/H� � �C contacts, 26.6, 29.4 and 29.8%, respectively, reflecting the presence of C-H� � �� interactions present in all three crystal structures.The other interatomic contacts, such as the N� � �H/H� � �N contacts, contribute from 5.1 to 6.3%, and the O� � �H/H� � �O contacts contribute from 4.6 to 6.0%.The C� � �C or O� � �O contacts contribute less than 1%.

Energy frameworks
A comparison of the energy frameworks calculated for I, showing the electrostatic potential forces (E ele ), the dispersion forces (E dis ) and the total energy diagrams (E tot ), are shown in Fig. 6.Those for compounds II and III are given, respectively, in Figs.S3 and S4 of the supporting information.The energies were obtained by using wave functions at the HF/3-2IG level of theory.The cylindrical radii are proportional to the relative strength of the corresponding energies (Spackman et al., 2021;Tan et al., 2019).They have been adjusted to the same scale factor of 90 with a cut-off value of 6 kJ mol À 1 within a radius of 3.8 A ˚of a central reference molecule.
For all three compounds, the major contribution to the intermolecular interactions is from dispersion forces (E dis ), reflecting the absence of C-H� � �O or C-H� � �N hydrogen bonds in the crystals.The colour-coded interaction mappings within a radius of 3.8 A ˚of a central reference molecule and the various contributions to the total energy (E tot ) for compounds I, II and III are given in Figs.S5, S6 and S7, respectively, of the supporting information.

Synthesis and crystallization
Compound I was synthesized by condensing p-diethylaminobenzaldehyde and p-methoxyaniline (1:1) dissolved in methanol.The reaction mixture was heated under reflux for 6 h at �363 K and then cooled to room temperature.The precipitated product was dissolved in methanol.Yellow prismatic single crystals of I were obtained by slow evaporation of the solvent at room temperature over a period of ca 15 d.
A Shimadzu IR Affinity-1 Fourier transform infrared (FT-IR) spectrometer was used to record the FT-IR spectrum of I using the KBr pellet technique in the range 400-4000 cm À 1 (Fig. S8 of the supporting information).The absorption band at 1603 cm À 1 confirms the formation of the C N groups.The aromatic ring C C stretching vibrations are observed in the range 1468-1585 cm À 1 .The aromatic C-H in-plane bending modes are observed in the region 1005-1292 cm À 1 , whereas the out-of-plane bending modes are observed in the range 762-973 cm À 1 .
The 1 H and 13 C nuclear magnetic resonance (NMR) spectra of compound I (Fig. S9 of the supporting information) were recorded using a Bruker Advance Neo 400 MHz NMR spectrometer.Deuterated chloroform (CDCl 3 -d) was employed as the solvent, with tetramethylsilane (TMS) serving as the internal standard.In the 1 H NMR spectrum of I, the singlet peak at 8.30 ppm is attributed to the azomethine (-N CH-) proton, while signals observed at 7.73, 7.18, 7.16 and 6.89 ppm are attributed to the aromatic protons.Additionally, there are sharp singlet peaks at 3.80 ppm, corresponding to the methoxy protons (O-CH 3 ).The protons of the diethylamino group were detected at 1.19 ppm as a triplet (CH 3 ) and at 3.41 ppm as a quartet (CH 2 ).In the 13 C NMR spectrum of I, the resonance at 158.70 ppm signifies the presence of the azomethine (-N CH-) unit, 55.51 ppm is associated with the CH 3 -O group, 44.51 ppm is related to the methylene C atoms of the (CH 3 CH 2 ) 2 -N group and 12.62 ppm corresponds to the methyl C atoms of the (CH 3 CH 2 ) 2 -N group.
An SDT Q600 V20.9 Build 20 TA instrument were used to measure the thermogravimetric analysis (TGA) and the differential thermal analysis (DTA) in the temperature range 303-723 K (Fig. S10  heating rate of 20 K min À 1 .A small peak observed at �377 K (Fig. S10) in the DTA curve corresponds to the melting point of the material.The material is stable up to 483 K, after which it starts to decompose.

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.

Figure 4 The
Figure 4The Hirshfeld surfaces of compounds (a) I, (b) II and (c) III, mapped over d norm in the colour ranges of 0.00 to 1.41, À 0.08 to 1.26 and À 0.02 to 1.22 a.u., respectively.
following the protocol of Tan et al. (2019).The Hirshfeld surfaces for compounds I, II and III are compared in Fig. 4. The absence of promient red spots indicate that short contacts are not particularly signifi-research communications 204 Subashini et al. � C 18 H 22 N 2 O Acta Cryst.(2024).E80, 201-206

Figure 6
Figure 6The energy frameworks calculated for I, viewed along the b-axis direction, showing the electrostatic potential forces (E ele ), the dispersion forces (E dis ) and the total energy diagrams (E tot ).

Table 2
Relative percentage contributions of close contacts to the Hirshfeld surfaces of compounds I, II and III.IIa and IIb refer to the two independent molecules of compound II.

Table 3
Experimental details.