Crystal structure of N,N′-dibenzyl-3,3′-dimethoxybenzidine

The title compound comprises a central o-dianisidine unit and terminal benzyl groups. In the molecule, the O atom are involved in intramolecular hydrogen bonding. In the crystal, adjacent molecules are linked by C—H⋯O interactions, forming a one-dimensional ladder along the a axis.


Chemical context
Benzidine derivatives have received increasing attention in recent years beacuse of their applications in a wide variety of domains, for instance as building blocks in the construction of functionalized organic/organometallic materials and as sensor materials (Hmadeh et al., 2008;Satapathi, 2015;Nagaraja et al., 2017). The chemical and physical properties of benzidinebased compounds have enabled their use in cell biology as staining reagents (Liu et al., 2004). Benzidine derivatives are also relevant examples of simple redox systems, which could find applications as OLEDs (Zhang et al., 2004) or electroactive organic polymeric compounds (D'Eramo et al., 1994). Recently, we have reported copper(I) coordination polymers based on pyromellitic diimide derivatives, and shown that photoluminescence emission peaks are shifted depending on the solvent (Kang et al., 2015). In an extension of previous research, we have synthesized a benzidine derivative as a diamine intermediate, in which a benzidine moiety was used instead of a pyromellitic diimide spacer unit, and report its crystal structure here. ISSN 2056-9890

Structural commentary
The molecular structure of the title compound consists of a central dimethoxybenzidine unit and two terminal benzyl groups (Fig. 1). The molecule lies about a crystallographic inversion centre at the midpoint of the C4-C4(Àx, Ày, Àz + 1) bond, thus the asymmetric unit contains one half-molecule. The dihedral angle between the terminal phenyl and phenylene rings of a benzidine unit is 48.68 (6) . Disorder was modelled for the methylene C atom of the benzyl group over two sets of sites with an occupancy ratio of 0.779 (18):0.221 (18). The biphenyl moiety is strictly planar [dihedral angle between rings = 0 ; maximum deviation of 0.015 (2) Å for atom C3]. There is no pronounced anisotropy in the aryl anisotropic displacement parameters, indicating that there is no disorder or dynamic twisting process to accommodate the steric crowding of the ortho H atoms of the biphenyl moiety (El-Shafei et al., 2003). The molecular conformation is in part influenced by the formation of weak intramolecular N1-H1Á Á ÁO1 hydrogen bonds that enclose S(5) rings ( Fig. 1, Table 1).

Supramolecular features
In the crystal, neighbouring molecules are linked by C10-H10Á Á ÁO1 hydrogen bonds (Table 1; yellow dashed lines in Fig. 2) that generate R 2 2 (24) rings. These contacts stack adjacent molecules, forming a one-dimensional ladder-like structure (Fig. 2). Neighbouring stacks of molecules in these ladders are not connected but lie parallel to the (012) plane (Fig. 3).

Theoretical calculations
DFT calculations have been performed to support the experimental values on the basis of the diffraction study using the GAUSSIAN09 software package (Frisch et al., 2009). Full geometry optimizations were performed using B3LYP levels of theory with a 6-311G* basis set. The bond lengths of the optimized parameter are in excellent agreement with the experimental crystallographic data (Table 2). Interestingly, however, while the central biphenyl conformation from the crystal structure is found to be planar, that from the DFT calculations shows an angle of 37.67 between the two aromatic rings, Fig. 4. Furthermore, the dihedral angle between the terminal phenyl and phenylene rings of the title compound is 48.68 (6) from the crystallographic data but 76.69 from the DFT calculation. Similarly, as a result of the twisted conformation found in the DFT calculations, the lengths of the intramolecular N-HÁ Á ÁO hydrogen bonds from the X-ray and DFT calculation data are also slightly different, at 2.33 and 2.21 Å , respectively.

Synthesis and crystallization
A mixture of o-dianisidine (4.88 g, 20 mmol), benzaldehyde (4.71 g, 40 mmol) and acetic acid (2.47 g, 40 mmol) in 30 mL of toluene and 7 mL of ethanol was heated at refluxed for 6 h. The central biphenyl conformation from the crystallographic data is planar (a), while that from the DFT calculations is twisted (b).

N,N′-dibenzyl-3,3′-dimethoxy-1,1′-biphenyl-4,4′-diamine
Crystal data 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.