Crystal structure and Hirshfeld surface analysis of 3,4-dihydro-2H-anthra[1,2-b][1,4]dioxepine-8,13-dione

The dihedral angle between the mean plane of the anthraquinone ring system and the dioxepine ring in the title compound is 16.29 (8)°. The packing is consolidated by C—H⋯O, π–π and C=O⋯π interactions.


Chemical context
Anthraquinone derivatives, which are extracted from the seeds of the Rubiaceae family of shrubs, include alizarin (1,2dihydroxyanthraquinone; C 14 H 8 O 4 ) and other polycyclic aromatic hydrocarbons. The colour of anthraquinone-based compounds can be modified by the type and position of the substituents attached to the anthraquinone nucleus (Nakagawa et al. 2017;Cheuk et al., 2015;Tonin et al., 2017). Besides their application as pigments or dyes in textile, photographic, cosmetic and other industries (Wang et al., 2011), anthraquinone derivatives have been used for centuries for medical applications, for example, as laxatives (Oshio et al., 1985), antioxidants (Yen et al., 2000), antimicrobial (Xiang et al., 2008;Yadav et al., 2010) and anitiviral (Alves et al., 2004) agents. Their redox properties and cytotoxicity have been investigated recently (Okumura et al., 2019). Anthraquinone derivatives exhibit various applications in supramolecular and electro-analytical chemistry (Czupryniak et al., 2012). ISSN 2056-9890 As part of our studies in this area, the synthesis and structure of the title compound, (I), are described along with a detailed analysis of its supramolecular associations through an analysis of the Hirshfeld surfaces.

Structural commentary
Compound (I) crystallizes in space group P2 1 /n with one molecule in the asymmetric unit: it consists of three fused sixmembered rings and one seven-membered ring as shown in Fig. 1. The fused-ring system is close to planar with an r.m.s. deviation for all non-hydrogen atoms of 0.039 Å (the dihedral angle between the aromatic rings of the anthraquinone unit and the central ring range from 1.5 to 1.9 ). The dioxepine ring is inclined to the mean plane of the anthraquinone ring system by 16.29 (8) .

Figure 3
Partial crystal packing for (I) showing the C-HÁ Á ÁO hydrogen bonds and the offset -(purple) and C OÁ Á Á (green) interactions between inversion-related molecules.
0.050 a.u. within the Hartree-Fock level of theory. Molecular sites evidenced in red correspond to positive potential energy and in blue to negative potential energy (Spackman et al., 2008). As illustrated in Fig. 5, the overall fingerprint plot for (I) and those delineated into HÁ Á ÁH, HÁ Á ÁO/OÁ Á ÁH, CÁ Á ÁH/HÁ Á ÁC and CÁ Á ÁC show characteristic pseudo-symmetric wings in the d e and d i diagonal axes. The most important interaction is HÁ Á ÁH, contributing 43% to the overall crystal packing, which is reflected in Fig. 5b as widely scattered points of high density due to the large hydrogen content of the molecule, with small split tips at d e ' d i ' 1.2 Å . The contribution from the OÁ Á ÁH/ HÁ Á ÁO contacts (27%) [note that the OÁ Á ÁH interactions make a larger contribution (14.6%) than the HÁ Á ÁO interactions (12.4%)], corresponding to C-HÁ Á ÁO interactions, is represented by a pair of sharp spikes characteristic of a strong hydrogen-bond interaction, d e + d i ' 2.35 Å (Fig. 5c). The significant contribution from CÁ Á ÁH/HÁ Á ÁC contacts (13.8%) to the Hirshfeld surface of (I) reflect the short CÁ Á ÁH/HÁ Á ÁC contacts, and the distribution of points has characteristic wings, Fig. 5d, with d e + d i '2.55 Å . The distribution of points in the d e = d i ' 1.75 Å range in the fingerprint plot delineated into CÁ Á ÁC contacts indicates the existence of weakstacking interactions between the central anthracene ring and the C6-C11 and C1-C4/C13-C14 rings ( Fig. 4b and 5e). Aromaticinteractions are indicated by adjacent red and blue triangles in the shape-index map (Fig. S1b)and also by the flat region around these rings in the Hirshfeld surfaces mapped over curvedness in Fig. S1c.
The contribution of 3.2% from CÁ Á ÁO/OÁ Á ÁC contacts is due to the presence of short interatomic C OÁ Á Á contacts, and is apparent as the pair of parabolic tips at d e + d i ' 3.2 Å in Fig. 5f.

Synthesis and crystallization
Under argon, alizarin (0.50 g, 2.0 mmol) was treated with 1,3-dibromo-propane (0.42 g, 2.0 mmol) in dimethylformamide (30 ml) in the presence of anhydrous potassium carbonate (1.0 g, 7.2 mmol) with stirring and heated to 393 K for 24 h. The reaction mixture was evaporated to dryness under vacuum and the resulting crude product was acidified with 12 N hydrochloric acid, extracted with chloroform (3 Â 30 ml) and then chromatographed on a silica gel column with dichloromethane/petroleum ether (1/1) as eluent, which yielded 200 mg (35%) of 1,2-propylenedioxyanthraquinone as a yellow compound (Fig. 6). Colourless needles were obtained by slow evaporation of a dichloromethane/petroleum ether (1:1) solution. Synthesis pathway leading to the formation of the title compound.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were placed in calculated positions and refined in the riding model: C-H = 0.95-0.99 Å with U iso (H) = 1.2U eq (C). The reflection (011), affected by the beam-stop, was removed during refinement.

3,4-Dihydro-2H-anthra[1,2-b][1,4]dioxepine-8,13-dione
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.