The 1:1 co-crystal of 2-bromonaphthalene-1,4-dione and 1,8-dihydroxyanthracene-9,10-dione: crystal structure and Hirshfeld surface analysis

The 1:1 co-crystal comprising two fused-ring molecules features significant hydrogen bonding between the 1,8-dihydroxyanthraquinone coformers with the main links between the resulting dimeric aggregates and the bromonaphthoquinone coformer being of the type C—H⋯O.


Chemical context
The formation of co-crystals is one of the major activities of crystal engineering endeavours and is motivated by various considerations. The concept of non-covalent derivatization of active pharmaceutical ingredients (API's) by this technology, in the hope of producing new formulations with improved bioavailability, etc. is a prominent motivation for investigation (Duggirala et al., 2016;Bolla & Nangia, 2016). Over and above this are applications ranging from enhancing non-linear optical materials, crystallization of materials that normally do not crystallize, optical resolution, etc. (Aakerö y, 2015). The above notwithstanding, the title co-crystal, (I), was isolated serendipiously during attempts to react 2-bromonaphthoquinone with 1,8-dihydroxyanthraquinone. Subsequently, it was shown that an equimolar ethyl acetate (or ethanol) solution of 2-bromonaphthoquinone and 1,8-dihydroxyanthraquinone could be co-crystallized to give the same product. Herein, the crystal and molecular structures of (I) are described along with a detailed analysis of the supramolecular association by means of an analysis of the Hirshfeld surfaces. ISSN 2056-9890

Structural commentary
The molecular structures of the constituents of (I) are shown in Fig. 1, the asymmetric unit comprising one molecule each of 2-bromonaphthoquinone, Fig. 1a, and 1,8-dihydroxyanthraquinone, Fig. 1b. The six carbon atoms comprising the cyclohexa-2,5-diene-1,4-dione ring of the naphthoquinone molecule are not strictly planar with the r.m.s. deviation being 0.030 Å ; the maximum deviations are 0.025 (1) and À0.031 (2) Å for the C4a and C4 atoms, respectively. The appended Br1, O1 and O4 atoms lie, respectively, 0.077 (1), 0.078 (1) and À0.117 (1) Å out of the plane with the Br1 atom lying to one side of the ring and the carbonyl-O atoms to the other. Overall, the r.m.s. deviation for the best plane defined by the 13 non-H atoms comprising the naphthoquinone molecule is 0.060 Å , with the maximum deviations being 0.093 (1) Å for atom Br1 and À0.099 (1) Å for the O4 atom, again with these atoms lying to opposite sides of the plane. With respect to the anthraquinone molecule, the r.m.s. deviation for the 18 non-H atoms is 0.022 Å with the maximum deviations being 0.039 (2) Å for C(13) and 0.026 (1) Å for the C19 and C23 atoms. As seen from Fig. 1b

Figure 1
The molecular structures of (a) 2-bromonaphthoquinone and (b) 1,8dihydroxyanthraquinone, i.e. the coformers comprising the asymmetric unit of (I), showing the atom-labelling scheme and displacement ellipsoids at the 70% probability level.

Hirshfeld surface analysis
The Hirshfeld surface analysis of title 1:1 co-crystal, (I), was performed as per recent publications on co-crystals  and provides more detailed information on the supramolecular association formed by the individual coformers and overall packing in the crystal. The Hishfeld surfaces are mapped over d norm , Figs. 3 and 4, the calculated electrostatic potential, Figs. 5 and 6, and shape-index, Figs. 7 and 8. The donors and acceptors of intermolecular hydroxy-O-HÁ Á ÁO(carbonyl) hydrogen-bonds between anthraquinone molecules are viewed as bright-red spots labelled with '1' and '2' on the Hirshfeld surfaces mapped over d norm in Fig. 3a. On the Hirshfeld surface mapped over the calculated electrostatic potential, the respective donors and acceptors appear as the blue (positive potential) and red regions (negative potential) in Fig. 5a. The presence of faint-red spots near carbon atoms C11, C19, Fig. 3a, and near the atoms C15 and C20, Fig. 3b, also indicate the links between molecules through short interatomic CÁ Á ÁC contacts, Table 2. These short contacts are also illustrated by white dashed lines in Fig. 6a. Links between the coformers involving their carbonyl-C4 O4 and C20 O20 groups through short interatomic CÁ Á ÁO/OÁ Á ÁC contacts, Table 2, are viewed as a pair of bright-and faint-red spots near these atoms in Fig. 3b and 4b. This is also illustrated by the black dashed lines on the Hirshfeld surface mapped over electrostatic potential in Fig. 6b. The donors and acceptors of intermolecular C-HÁ Á ÁO(carbonyl) interactions can be viewed as bright-red spots having labels '3'-'5' in Figs. 3 and 4, and as blue and red regions, respectively, in Fig. 5. The comparatively weak anthraquinone-C15-HÁ Á ÁO4 hydrogen bond is represented with faint-red spots near these atoms in Two views of the Hirshfeld surface for the anthraquinone molecule in (I) mapped over d norm over the range À0.120 to 1.190 au.   Table 2 Summary of short inter-atomic contacts (Å ) in (I).
2.27 -x, 2 À y, Àz ecules within shape-index-mapped Hirshfeld surfaces highlighting intermolecular O-HÁ Á ÁO, C-HÁ Á ÁO,stacking and C-OÁ Á Á interactions influential on the packing are illustrated in Figs. 7 and 8. The two-dimensional fingerprint plots for the individual naphthoquinone and anthraquinone molecules, and for the overall co-crystal are illustrated in Fig. 9a. The plots delineated into HÁ Á ÁH, OÁ Á ÁH/HÁ Á ÁO, CÁ Á ÁH/HÁ Á ÁC, CÁ Á ÁC and CÁ Á ÁO/OÁ Á ÁC contacts (McKinnon et al., 2007) are shown in Fig. 9b-f, respectively; the relative contributions from various contacts to the Hirshfeld surfaces are quantitatively summarized in Table 3. The different immediate environments of intermolecular interactions around the naphthoquinone and anthraquinone coformers result in different shapes and a distinct distribution of points in the respective delineated fingerprint plots: there is a clear distinction between these and those for the overall co-crystal.
The fingerprint plots delineated into HÁ Á ÁH contacts arise from relatively low percentage contributions to their respective Hirshfeld surfaces, Table 3, as a result of their relatively their low contents in the molecules and the involvement of Two views of the Hirshfeld surface for the naphthoquinone molecule in (I) mapped over d norm over the range À0.125 to 1.157 au.

Figure 5
Views of the Hirshfeld surfaces for the (a) anthraquinone and (b) naphthoquinone molecules in (I) mapped over the electrostatic potential in the range AE0.059 au. The red and blue regions represent negative and positive electrostatic potentials, respectively.  Table 2, is evident in the respective plot as a single peak at d e + d i $ 2.2 Å . The donors and acceptors of the naphthoquinone-H3 and anthraquinone-O20(carbonyl) atoms are viewed as a thin, long spike at d e + d i $ 2.2 Å in each of the fingerprint plots of OÁ Á ÁH/HÁ Á ÁO contacts, Fig. 9c; the spikes for the donor and acceptor interactions are viewed separately in the plots for the naphthoquinone and anthraquinone coformers, respectively. The O-HÁ Á ÁO interactions instrumental in linking anthraquinone molecules are evident in the respective OÁ Á ÁH/HÁ Á ÁO delineated plot, Fig. 9c, and is characterized by a pair of short spikes at d e + d i $ 2.3 Å where in the acceptor spike is merged within the plot of the aforementioned C3-HÁ Á ÁO ii interaction. The other intermolecular C-HÁ Á ÁO contacts involving anthraquinone-H13 and -H17, and naphthoquinone-O1 and -O4(carbonyl) atoms are viewed as a pair of short spikes at d e + d i $ 2.4 Å in the donor and acceptor regions of their respective plots in Fig. 9c. The points corresponding to anthraquinone-C15-H15Á Á ÁO4(carbonyl) interactions and other short interatomic OÁ Á ÁH contacts, Table 2, are merged within the plots.
A pair of short peaks at d e + d i < 2.9 Å , i.e. less than sum of their van der Waals radii, in the fingerprint plot delineated into CÁ Á ÁH/HÁ Á ÁC contacts for anthraquinone, Fig. 9d, are indicative of short interatomic CÁ Á ÁH contacts, Views of Hirshfeld surfaces for the molecules in (I) mapped over the electrostatic potential highlighting (a) short interatomic CÁ Á ÁC contacts as with white dashed lines in the stacking of anthraquinone molecules in the range AE0.059 au and (b) short interatomic CÁ Á ÁO/OÁ Á ÁC contacts as black dashed lines between approximately co-planar anthraquinone and naphthoquinone molecules in the range AE0.060 au. (C1-C4,C4a,C8a) and (C4a,C5-C8,C8a) rings and is highlighted as the parabolic distribution of points in Fig. 9e, having high density at around d e = d i $ 1.8 Å . The parabolic distribution of points with the peak at d e = d i $ 1.6 Å in the plot for the anthraquinone coformer, Fig. 9e, indicates links between these molecules through short interatomic CÁ Á ÁC contacts along the b axis. The presence of CÁ Á ÁC contacts in (I) results in an overall 9.3% contribution to the Hirshfeld surface.
Although the naphthoquinone-bromide substituent makes a notable contribution to the Hirshfeld surface, Table 3, it does not form inter-atomic contacts with other atoms less than sum of the respective van der Waals radii. Therefore, it exerts no significant influence on the packing. The small contribution from OÁ Á ÁO contacts also has a negligible effect on the packing.

Database survey
The coformers comprising (I) are relatively unexplored in the crystallographic literature (Groom et al., 2016). For example, the structure of 2-bromonaphthoquinone has only been reported on one previous occasion, namely in its pure form (Gaultier & Hauw, 1965). This structure presents the same features as the molecule in (I) with the r.m.s deviation of the 13 fitted atoms being 0.059 Å , cf. 0.060 Å in (I). More attention has been directed towards 1,8-dihydroxyanthraquinone. The structure of the pure molecule was originally reported in 1965 (Prakash, 1965) and a recent study focussed upon the several polymorphic forms of this compound (Rohl et al., 2008). In all known forms of 1,8-dihydroxyanthraquinone, an essentially planar molecule is observed along with the two intramolecular hydroxy-O-HÁ Á ÁO(carbonyl) hydrogenbonds persisting as in (I). A co-crystal of 1,8-dihydroxyanthraquinone is also known, i.e. a 3:1 co-crystal with acetic acid (Cheuk et al., 2015). This structure is particularly notable in that there are six independent 1,8-dihydroxyanthraquinone molecules in the asymmetric unit, each with the same conformation as in the parent compound and in (I), along with two independent acetic acid molecules.

Synthesis and crystallization
Compound (I) was isolated during attempts to chemically bond 2-bromonaphthoquinone and 1,8-dihydroxyanthraquinone under basic conditions. Upon work up of the reaction mixture, the crude material was obtained after evaporation of all the volatiles. This was filtered through a short column of silica gel eluting with CH 2 Cl 2 /hexane (1:1 v/v) and a single, yellow fraction was collected. After evaporation of the solvent under reduced pressure, a yellow solid was obtained. This was recrystallized from ethyl acetate solution to give small orangered crystals with yields of 78-85% based upon the quantity of 1,8-dihydroxyanthraquinone initially used. Notably, the  Table 4 Summary of C OÁ Á Á contacts (Å , ) in (I).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. Carbon-bound H atoms were placed in calculated positions (C-H = 0.95 Å ) and were included in the refinement in the riding-model approximation, with U iso (H) set to 1.2U eq (C). The O-bound H atoms were located from a difference map but refined with O-H = 0.84AE0.01 Å and U iso (H) = 1.5U eq (O).  SHELXS (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010). 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.