The crystal structures of two novel polymorphs of bis(oxonium) ethane-1,2-disulfonate

Two novel crystal forms of bis(oxonium) ethane-1,2-disulfonate, 2H3O−·C2H4O6S2 2−, are reported. Polymorph II has monoclinic (P21/n) symmetry, while the symmetry of form III is triclinic (P ). Both structures display extensive networks of O—H⋯O hydrogen bonds.


Chemical context
Sulfonic acids are commonly used in salt formation in the pharmaceutical industry, especially for poorly or non soluble in water drugs (Neau & Loka, 2018). Salts of ethane-1,2-disulfonic acid account for 0.38% of all the FDA-approved commercially marketed salts (Steele & Talbir, 2016) and therefore its toxicology, dosage (Saal & Becker, 2013) and various physico-chemical properties are widely studied (Black et al., 2007;Elder et al., 2010). In our laboratory, ethane-1,2disulfonic acid is commonly used in the salt screening for increasing solubility as well as improving the crystallinity of various researched active pharmaceutical ingredients (APIs).

Structural commentary
The sulfonate anion in all polymorphs, including the previously determined form (Mootz & Wunderlich, 1970, refcode HOEDSO;Sartori et al., 1994, refcode HOEDSO01) has a nearly identical geometry. In all cases, the center of the C-C bond is located on an inversion center, and the C-S and C-O distances in all cases are within 3. The sulfonate group adopts the geometry of an open umbrella with the C-S-O bond angles of 106.51 (6), 105.82 (6), 107.23 (6) for Form II (Fig. 1) and 106.16 (11), 106.21 (10), 107.20 (12) for Form III (Fig. 2). The values of all O-S-O angles are above 110 [112.91 (7), 111.48 (7), 112.37 (7) for Form II and 111.31 (11),113.45 (11), 112.00 (12) for Form III]. In this way, the molecular symmetry of the sulfonate group becomes slightly distorted C 3V . In all crystals, the oxonium cations have a pyramidal geometry with slightly elongated O-H distances for one H atom. This is most likely an effect of the fast exchange of a proton (H atom) between the sulfonate group and the water molecules.
The biggest differences between forms are observed in the density of the crystal, as well as in the packing coefficient (Kitajgorodskij, 1973). The lowest values of both parameters are attributed to Form III (1.60 g cm À3 and 0.67, respectively), which suggests that this polymorph is the least stable. Form II presented here has a slightly better packing index than previously reported for Form I (Mootz & Wunderlich, 1970;Sartori et al., 1994) 0.75 versus 0.73. On the other hand, the density is lower: 1.78 versus 1.82 g cm À3 , respectively.

Figure 2
The molecular structure of anion cation pair of Form III, with the atomlabelling scheme. Displacement ellipsoids are drawn at the 50% probability level and hydrogen bonds are shown in torquoise. Unlabelled atoms are related to labelled ones by the symmetry operator (Àx + 1, Ày + 1, Àz + 1).

Figure 1
The molecular structure of an anion-cation pair of Form II, with the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level and hydrogen bonds are shown in torquoise. Unlabelled atoms are related to labelled ones by the symmetry operator (Àx + 1, Ày + 1, Àz + 1).

Figure 3
The crystal packing of Form II, viewed along the a axis. The ethane-1,2disulfonate dianions are coloured in green, while oxonium cations are red and hydrogen bonds are shown in torquoise.
reported form, where the hydrogen-bond network is built from alternate anion-cations layers, in Form II such layers could not be distinguished. The supramolecular behaviour of Form III is significantly different. In this case (Table 2 and Fig. 4), the anion-cation hydrogen-bond network forms separate layers parallel to the ab plane built from sulfonate anions surrounded by oxonium cations with no interactions between the planes.

Database survey
As mentioned above, the crystal structure of a different polymorphic form of oxonium ethane-1,2-disulfonate has been previously reported (Mootz, & Wunderlich, 1970, refcode HOEDSO;Sartori et al., 1994, refcode HOEDSO01). Apart from these structures, there are 12 hits for ethane-1,2-disulfonate salts in the Cambridge Structural Database (CSD, Version 5.40; ConQuest 2.02; Groom et al., 2016), one of which is disordered. The geometry of the sulfonate group in all of the anions is nearly the same, with slightly distorted C 3v molecular symmetry for the open-umbrella geometry. The average values of the C-S-O and O-S-O bond angles are very close to those reported in this paper: 105.9AE0.8 and 112.8AE0.9 , respectively.

Synthesis and crystallization
Both crystals were obtained from an aqueous solution during unsuccessful salt formation with an unnamed free base (API) in water. Firstly, columnar crystals of Form III that appeared to be unstable were grown from the thick oil and within time transformed into prismatic crystals of Form II.

Refinement
Crystal data, data collection and structure refinement details are summarized in

Figure 4
The crystal packing of Form III, viewed along the a axis. For both structures, data collection: COLLECT (Hooft, 1998); cell refinement: HKL SCALEPACK (Otwinowski & Minor, 1997); data reduction: HKL DENZO and SCALEPACK (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015b); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: enCIFer (Allen et al., 2004).  (2) 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.

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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )