Crystal structure of N,N′-dibenzylpyromellitic diimide

The title compound, C24H16N2O4 (systematic name: 2,6-dibenzylpyrrolo[3,4-f]isoindole-1,3,5,7(2H,6H)-tetraone), lies about a crystallographic inversion center at the center of the pyromellitic diimide moiety which is planar. In the crystal, intermolecular C—H⋯O hydrogen bonds and C—H⋯π interactions lead to the formation of a two-dimensional supramolecular network.


Chemical context
As a result of their potential applications in organic photovoltaics (Huang et al., 2014) and as molecular electronic devices  and energy storage devices (Song et al., 2010), several -conjugated, redox-active aromatic diimides including pyromellitic diimides, naphthalene diimides and perylene diimides have received considerable attention from materials chemists. Additionally, -conjugated aromatic diimides and their derivatives are used as rigid structural components in supramolecular assemblies for the exploitation of supramolecular interactions such as hydrogen-bonding and halogen-interactions (Hay & Custelcean, 2009;Lu et al., 2007;Gamez et al., 2007). Recently, our group reported a copper(I) coordination polymer with a pyromellitic diimide ligand, namely N,N 0 -bis [3-(methylthio)propyl]pyromellitic diimide, and revealed the presence of halogen-interactions between the chlorine atoms of a dichloromethane solvent molecule of crystallization and pyromellitic diimide rings (Park et al., 2011). In an extension of our studies of pyromellitic diimide derivatives, we have prepared the title compound by the reaction of pyromellitic dianhydride with 2phenyethylamine and we report its crystal structure here.

Structural commentary
The molecular structure of the title compound consists of a central pyromellitic diimide ring system with terminal benzyl groups on each of the inversion-related nitrogen atoms (Fig. 1). As the molecule is located about a crystallographic inversion centre, the asymmetric unit of the compound comprises one half-molecule. Short intramolecular C-HÁ Á ÁO contacts  (Table 1) enclose S(5) rings and may contribute to the planarity of the pyromellitic diimide ring system (r.m.s. deviation = 0.0145 Å ). The two terminal phenyl groups in the molecule are oriented away from each other, forming an elongated S-shaped conformation. The terminal phenyl ring is tilted by 72.97 (4) with respect to the mean plane of the central pyromellitic diimide moiety.

Supramolecular features
In the crystal, adjacent molecules are connected by weak C12-H12Á Á ÁO2 hydrogen bonds, A view of the molecular structure of the title compound, showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. H atoms are presented as small spheres of arbitrary radius and yellow dashed lines represent the intramolecular C-HÁ Á ÁO short contacts. [Symmetry code; (i) Àx + 2, Ày + 1, Àz.] Table 1 Hydrogen-bond geometry (Å , ).
Cg1 is the centroid of the C1-C6 ring.

Figure 2
Chains of the title compound formed through intermolecular C-HÁ Á ÁO hydrogen bonds (yellow dashed lines).
these into a chain propagating along [110]. Neighboring chains are linked through intermolecular C-HÁ Á Á interactions between a methylene H atom and the terminal phenyl ring, resulting in the formation of supramolecular layers extending parallel to the ab plane (black dashed lines in Fig. 3 and Table 1). These layers are separated from each other by 3.104 (3) Å . No intermolecularinteractions are found between the pyromellitic diimide moieties.

Synthesis and crystallization
The title compound was synthesized by the reaction of pyromellitic dianhydride with 2-phenylethylamine according to a literature procedure (Kang et al., 2015). X-ray quality single crystals were obtained by slow evaporation of a dichloromethane solution of the title compound.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were positioned geometrically with d(C-H) = 0.95 Å for Csp 2 -H and 0.99 Å for methylene, and were refined as riding with U iso (H) = 1.2U eq (C). program(s) used to solve structure: SHELXS97 (Sheldrick 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2010); software used to prepare material for publication: SHELXTL (Sheldrick, 2008). 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.