Crystal structure of N,N′-bis(2,4-difluorobenzoyloxy)benzene-1,2:4,5-tetracarboximide

The title compound crystallizes with half a molecule in the asymmetric unit and shows Car—H⋯F interactions in the crystal packing.


Chemical context
Heterocycles are key compounds in synthetic chemistry. In addition to their applications in drugs, bioactive and tautomeric compounds (D'Errico et al., 2012;Piccialli et al., 2007;, aromatic heterocycles play an important role in modern materials chemistry, because they are used as building blocks of active molecules in many emerging fields of advanced materials, for example, conducting polymers (Heeger, 2010), organic field-effect transistors (Miao, 2014), organic solar cells (Nielsen et al., 2015), liquid crystals (Centore et al., 1996) and nonlinear optically active compounds (Carella et al., 2007;Centore et al., 1999).
Aromatic diimides, in particular, are a class of heterocyclic compounds well known for their outstanding properties as n-type organic semiconductors. Very high electron mobilities have been measured for perylenediimides (Schmidt et al., 2007) and naphthalenediimides (Yan et al., 2009). The research on n-type organic semiconductors has also shown that electron mobilities and device performances can be improved by extensive replacement of H atoms by fluorine (Facchetti et al., 2003).

Structural commentary
Molecules of the title compound in the crystal lie on crystallographic inversion centres and have C i point-group symmetry. ISSN 2056-9890 Thus, the asymmetric unit is formed by half a molecule, as shown in Fig. 1. The difluorophenyl ring is disordered over two orientations that differ by a rotation of 180 around the C6-C7 bond. The atomic positions for the two orientations of the difluorophenyl ring are completely superimposed for all atoms, except for the ortho-F atom, for which split positions were observed. The final refined occupancy factors of the two components of disorder are 0.947 (4) and 0.053 (4). Molecules adopt a nonplanar conformation, mainly because of a torsion around the O3-N1 bond. In particular, the pentaatomic ring (atoms C2/C3/C4/N1/C5) is planar within 0.0164 (14) Å , while the phenyl ring (C7/C8a/C9/C10/C11/C12a) is planar within 0.002 (2) Å . The dihedral angle between the mean planes of the two rings is 86.14 (8) .
The C6-O3 bond length [1.402 (3) Å ] is significantly longer than the mean value for esters of aromatic acids (1.337 Å ; Allen et al., 1987). This suggests a reduced contribution of the minor resonance form of the ester group, in which one of the lone pairs of the alkoxy oxygen forms a double bond with the carbonyl C atom that breaks its double bond with the other O atom, thereby giving it a negative charge. This, in turn, can be due to the presence of the electronegative N atom bonded to O3.

Supramolecular features
There are weak hydrogen-bond donors (C ar -H) and strong hydrogen-bond acceptors (carbonyl O atoms) in the title compound. Moreover, F atoms are present as well, whose low hydrogen-bonding-acceptor capability, if any, has been the subject of a long debate in the literature (Dunitz & Taylor, 1997;Dunitz, 2004). Actually, it is now established that the C-HÁ Á ÁF interaction is generally weak and does not play a significant structural role in crystal packing when stronger and more polarizable acceptors than the C-F group are present. On the other hand, when the carbon acidity is suitably enhanced, and in the absence of competing acceptors, the (weak) hydrogen-bonding nature of the C-HÁ Á ÁF interaction is revealed (Thalladi et al., 1998).
The most acidic C ar -H group in the title compound is C9-H, because it has two ortho C-F-group neighbours. It is involved in weak hydrogen-bonding interactions with fluorine, as shown in Fig. 2 and Table 1. In particular, C9-H acts as bifurcated donor to the F1A and F2 atoms. In the first case, R 2 2 (8) ring motifs are formed across inversion centres. In the second case, chain patterns running parallel to a À b + c/2 are formed. These patterns are quite similar to the supramolecular synthons II and IV reported in the Scheme 2 of Thalladi et al. (1998). It is quite remarkable that significant C-HÁ Á ÁF Partial crystal packing of the title compound, showing the C-HÁ Á ÁF and C-HÁ Á ÁO interactions as dashed lines. Only the most populated orientation of the disordered difluorophenyl ring is shown. Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx þ 1; y À 1 2 ; Àz þ 1 2 ; (ii) Àx; Ày; Àz; (iii) Àx; y À 1 2 ; Àz À 1 2 ; (iv) x; Ày þ 3 2 ; z À 1 2 .

Figure 1
The molecular structure of the title compound, with displacement ellipsoids drawn at the 30% probability level. Only the most populated orientation of the disordered difluorophenyl ring is shown. [Symmetry code: (i) Àx + 1, Ày, Àz + 1.] ZALKUV. The hits found are crystal structures determined at temperatures in the range 90-298 K. In the 47 hits, the N1-O3 distance (DIST1) ranges between 1.375 and 1.408 Å , with an average value of 1.388 (6) Å . On the other hand, the distance O3-C6 (DIST2) is between 1.350 and 1.423 Å , with an average value of 1.393 (15) Å . The histogram of the distribution of DIST1 over the 47 hits found is shown in Fig. 3. The values of DIST1 and DIST2 found in the title compound [N1-O3 = 1.381 (2) Å and O3-C6 = 1.402 (3) Å ] are fully consistent with the average values determined from the 47 hits.

Synthesis and crystallization
N,N 0 -Dihydroxybenzene-1,2:4,5-tetracarboximide (Centore & Carella, 2013) (1.000 g, 4.030 mmol) was suspended in 20 ml of dry pyridine and the system was kept under stirring at room temperature. 2,4-Difluorobenzoylchloride (1.991 g, 11.28 mmol) was added dropwise and the previous suspension turned into a dark solution. The solution was warmed and boiled gently for 45 min. Absolute ethanol (2 ml) and, after 2 min, distilled water (0.2 ml) were then added and the system cooled slowly to room temperature and filtered. The white crystals were washed on the filter with absolute ethanol. From the recovered material it was possible to isolate several single crystals suitable for X-ray analysis. The yield was 55% (m.p. 604 K).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The H atoms were generated stereochemically and were refined by the riding model. For all H atoms, U iso (H) = 1.2U eq (carrier). The difluorophenyl ring is disordered over two orientations, which differ by a rotation of 180 around the phenyl to carbonyl bond. Split positions were only observed for the ortho-F atom. The two split positions were refined by applying SADI restraints on bond lengths. The final refined occupancy factors of the two components of disorder are 0.947 (4) and 0.053 (4).   Data collection: COLLECT (Nonius, 1999); cell refinement: DIRAX/LSQ (Duisenberg et al., 2000); data reduction: EVALCCD (Duisenberg et al., 2003); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL2016 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and Mercury (Macrae et al., 2008); software used to prepare material for publication: WinGX (Farrugia, 2012).

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. The difluorophenyl ring is disordered over two orientations. The two split positions were refined by applying SADI restraints on bond lengths.