A 1:2 co-crystal of 2,2′-dithiodibenzoic acid and benzoic acid: crystal structure, Hirshfeld surface analysis and computational study

The asymmetric unit of the title 1:2 co-crystal has a half molecule of twofold symmetric dithiodibenzoic acid and a full molecule of benzoic acid. These are connected into three-molecule aggregates via hydroxy-O—H⋯O(hydroxy) hydrogen bonds.


Chemical context
Molecular recognition represents an essential aspect in the crystal engineering of co-crystals as it dictates how supramolecular aggregates are formed, whether through shape, size or functional complementarity, to give a distinct connectivity and pattern (Meng et al., 2008). To date, various supramolecular frameworks comprising homo-synthons, occurring between the same functional groups, as well as heterosynthons, occurring between disparate functional groups, have been described. Molecules with carboxylic acid functionality remain at the forefront of co-crystal technology based on hydrogen-bonded synthons (Duggirala et al., 2015). Despite expectations to the contrary, the carboxylic acidÁ Á Ácarboxylic acid homo-synthon, i.e. association through the formation of an eight-membered {Á Á ÁHOC O} 2 synthon, only forms in about one-third of structures where they potentially can occur (Allen et al., 1999). The remaining structures of carboxylic acids are dominated by hetero-synthons involving carboxylic acid with other functional groups, such as a pyridyl residue (Shattock et al., 2008). This relatively low probability is due to competing supramolecular interactions that hinder the ISSN 2056-9890 formation of the homosynthon (Steiner, 2001). A related issue concerns the formation of co-crystals involving different carboxylic acids (Seaton, 2011). Here, different crystalline outcomes may be envisaged and in terms of co-crystals, cocrystals involving the same molecules associating via a symmetric carboxylic acid homosynthon might be isolated, or a co-crystal comprising different molecules, via a nonsymmetric homo-synthon might be formed. In this context, in a recent study, the characterization of the 2:1 co-crystal between 2,2 0 -dithiodibenzoic acid (DTBA) and 3-chlorobenzoic acid showed the formation of a homo-synthon between two DTBA molecules with each of the terminal carboxylic acid residues of the two-molecule aggregate engaged in non-symmetric homo-synthons with two 3-chlorobenzoic acid molecules, giving rise to a hydrogenbonded four-molecule aggregation pattern (Tan & Tiekink, 2019). In continuation of these studies, herein, the crystal and molecular structures of the title 1:2 co-crystal of DTBA and benzoic acid (BA) are described as well as an analysis of the calculated Hirshfeld surface and the calculation of some specific interaction energies through a computational chemistry approach.

Structural commentary
The title co-crystal (I) was the result of crystallization of a powder resulting from the solvent-assisted (methanol) grinding of a 1:1 mixture of 2-thiobenzoic acid and benzoic acid. X-ray crystallography showed the asymmetric unit of the resultant crystals to comprise half a molecule of 2,2 0 -dithiodibenzoic acid (DTBA), as this is disposed about a crystallographic twofold axis of symmetry, Fig. 1(a), and a molecule of benzoic acid (BA) in a general position, Fig. 1(b). Such oxidation of the original 2-thiobenzoic acid to DTBA is well known in co-crystallization studies (Broker & Tiekink, 2007;Gorobet et al., 2018). In terms of stoichiometry, the formation of the title 1:2 co-crystal is consistent with the 1:1 stoichiometry of the original grinding experiment.
The twofold-symmetric DTBA molecule is twisted about the disulfide bond with the C3-S1-S1 i -C3 i torsion angle being À83.19 (8) ; symmetry operation (i): 1 À x, y, 1 2 À z. This almost orthogonal disposition is also seen in the dihedral angle between the benzene rings of 71.19 (4) . The presence of a carboxylic acid group is readily confirmed by the disparity in the C1-O1, O2 bond lengths, i.e. 1.317 (2) and 1.229 (2) Å , respectively. This group is practically co-planar with the benzene ring to which it is bonded, as seen in the dihedral angle of 4.82 (12) . This co-planar arrangement allows for a significant intramolecular S O interaction, i.e. S1Á Á ÁO2 = 2.6712 (12) Å , as the carbonyl-O2 atom is orientated towards a disulfide-S1 atom (Nakanishi et al., 2007).
The presence of a carboxylic acid group in the molecule of BA is confirmed by the C8-O3, O4 bond lengths of 1.318 (2) and 1.233 (2) Å , respectively. As for the DTBA molecule, the carboxylic acid group is close to co-planar with the benzene ring to which it is bound, forming a dihedral angle of 3.65 (15) .

Supramolecular features
The geometric parameters characterizing the interatomic contacts, as identified in PLATON (Spek, 2009), in the crystal of (I) as are given in Table 1. The molecular packing of the crystal structure is mainly governed by hydrogen bonds formed between the carboxylic groups of DTBA and BA, whereby each terminus of the former connects via hydroxy-O-HÁ Á ÁO(hydroxy) hydrogen bonds, leading to a nonsymmetric, eight-membered {Á Á ÁHOC=O} 2 homo-synthon as shown in the two views of Fig. 2 The molecular structures of (a) 2,2 0 -dithiodibenzoic acid and (b) benzoic acid in (I), showing the atom-labelling scheme and displacement ellipsoids at the 70% probability level. The molecule in (a) is disposed about a twofold axis of symmetry with unlabelled atoms related by the symmetry operation: 1 À x, y, 1 2 À z. Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (ii) x À 1 2 ; y À 1 2 ; z; (iii) x þ 1 2 ; y þ 1 2 ; z.
interactions, to form non-symmetric, ten-membered {OÁ Á ÁHCCC} 2 homo-synthons leading to supramolecular layers in the ab plane, Fig. 2(b). Owing to the nearly rightangle relationship between the rings in the DTBA molecule, and the co-planarity between the carboxylic acid groups and the respective rings they are connected to, the layers also have a similar topology. Adjacent layers inter-digitate with other layers, on both sides, i.e. approximately orthogonally, as highlighted in Fig. 2(c). As illustrated in Fig. 2(d), the connections between layers are of two types and includestacking interactions between DTBA and BA rings with the inter-centroid (C2-C7)Á Á Á(C9-C14) iv separation being 3.8093 (10) Å , an angle of inclination of 8.36 (8) and an offset of 1.40 Å for symmetry operation (iv): 1 À x, 1 À y, 1 À z.

Hirshfeld surface analysis and computational study
To gain better understanding of the nature of the intermolecular interactions identified in (I), the co-crystal and its individual components were subjected to a Hirshfeld surface analysis through the mapping of the normalized contact distance (d norm ) as well as calculation of the interaction energies using CrystalExplorer (Turner et al., 2017) and in accord with a recent study (Tan & Tiekink, 2018). Briefly, the d norm maps were obtained through the calculation of the internal (d i ) and external (d e ) distances to the nearest nucleus (Spackman & Jayatilaka, 2009), while the interaction energies were calculated using a dispersion-corrected CE-B3LYP/ 6-31G(d,p) quantum level of theory, as available in Crystal-Explorer (Turner et al., 2017). The total intermolecular energy is the sum of energies of four main components, comprising electrostatic, polarization, dispersion and exchange-repulsion with scale factors of 1.057, 0.740, 0.871 and 0.618, respectively (Mackenzie et al., 2017).
The d norm mapping of the three-molecule aggregate is shown in Fig. 3. In general, the prominent hydrogen-bond interactions are readily identified from the intense red spots on the Hirshfeld surface which are dominated by the strong hydroxy-O-HÁ Á ÁO(carbonyl) hydrogen bonds. The calculation of the relevant interaction energies shows that it is the strongest among all of the specified contacts present in the crystal with the calculated (total) energy of À71.7 kJ mol À1 , Table 2. By contrast, the diminutive red spots observed around the atoms involved in the benzene-C-HÁ Á ÁO(hydroxy) contacts, Table 1, are indicative of weak interactions, and this is confirmed through the calculated interaction energy of merely À7.1 kJ mol À1 . The shortinteraction involving the DTBA and BA benzene rings, mentioned in Supramolecular features, has an interaction energy of À21.7 kJ mol À1 , i.e. more stable than the C-HÁ Á ÁO interactions. The energy calculation reveals that such an interaction is mainly dispersive in nature, cf Table 2, with the electrostatic character of the corresponding benzene rings being complementary, as demonstrated from the electrostatic surface mapped onto the Hirshfeld surfaces of the individual components of (I), Fig. 4(a) and (b), and the molecular dimer sustained bycontacts in Fig. 4(c).
A quantitative analysis of the Hirshfeld surfaces was performed through the generation of two  Table 2 Interaction energies (kJ mol À1 ) for selected close contacts.

Figure 3
The Hirshfeld surface mapped with d norm for the DTBA molecule in (I) over the range À0.753 to 1.252 a.u., shown interacting with nearneighbour BA molecules connected through hydrogen bonds (green dashed lines).

Figure 6
Energy framework of (I) as viewed down along the a-axis direction, showing the (a) electrostatic potential force, (b) dispersion force and (c) total energy diagrams. The cylindrical radii are proportional to the relative strength of the corresponding energies and they were adjusted to the same scale factor of 50 with a cut-off value of 5 kJ mol À1 within 4 Â 4 Â 4 unit cells.
(external) contacts that contribute about 2.1% to the overall Hirshfeld surface.
In order to study the overall topology of the energy distributions in the crystal of (I), the energy framework was generated for a cluster of 4 Â 4 Â 4 unit cells using the same quantum level of theory as mentioned for the interaction energy model. As shown in Fig. 6(a)-(c), the crystal is significantly governed by electrostatic force owing to the strong O-HÁ Á ÁO interactions that result in an alternate V-shape energy topology across the b-axis direction. A relatively less significant, but essential dispersion contribution is also observed and arises from theinteractions spanning all benzene rings. Overall, it can be concluded that these interacting forces directed the assembly of the molecules in (I).  (9) found in the structure of a 1:1 co-crystal of DTBA with trans-1,2-bis(4-pyridyl)ethene (Broker & Tiekink, 2007) and the widest angle of 100.98 (17) was observed in in a co-crystal salt, i.e. [NH 4 ][DTBA_H]DBTA (Murugavel et al., 2001).

Synthesis and crystallization
All chemicals were of reagent grade and used as received without purification. 2-Thiobenzoic acid (Merck; 0.154 g, 0.001 mol) was mixed with benzoic acid (R&M; 0.122 g, 0.001 mol) and ground for 15 minutes in the presence of a few drops of methanol. The procedure was repeated three times.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. The carbon-bound H atoms were placed in calculated positions (C-H = 0.93 Å ) and were included in the refinement in the riding-model approximation, with U iso (H) set to 1.2U eq (C). The oxygen-bound H atoms were located from difference-Fourier maps and refined without constraint.

Funding information
The support of Sunway University for studies in co-crystals,   (Rigaku OD, 2018); cell refinement: CrysAlis PRO (Rigaku OD, 2018); data reduction: CrysAlis PRO (Rigaku OD, 2018); program(s) used to solve structure: SHELXT (Sheldrick, 2015b); program(s) used to refine structure: SHELXL (Sheldrick, 2015a); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012), OLEX2 (Dolomanov et al., 2009) and Mercury (Macrae et al., 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.