Pyrazinediium bis(3-carboxy-4-hydroxybenzenesulfonate) dihydrate

Pyrazine and 5-sulfosalicylic acid crystallize from a methanol solution containing water as the title salt, C4H6N2 2+·2C7H5O6S−·2H2O. The pyrazinediium cation sits on an inversion center. The component ions and water molecules are linked by intermolecular O—H⋯O, N—H⋯O and C—H⋯O hydrogen bonds into layers running parallel to the (10) plane.

Pyrazine and 5-sulfosalicylic acid crystallize from a methanol solution containing water as the title salt, C 4 H 6 N 2 2+ Á2C 7 H 5 O 6 S À Á2H 2 O. The pyrazinediium cation sits on an inversion center. The component ions and water molecules are linked by intermolecular O-HÁ Á ÁO, N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds into layers running parallel to the (102) plane.

Pyrazinediium bis(3-carboxy-4-hydroxybenzenesulfonate) dihydrate
Jia-Rong Wang, Ze-Hui Yang, Chun-Hai Liu and Ling-Lan Li S1. Comment 5-sulfosalicylic acid is strong organic acid (pKa = 2.85) which can release its sulfonic acid H to Lewis base N atoms, forming 1:1 molecular adducts in general (Smith et al., 2004 and2005). In order to gain more insight into these analogues, we have recently prepared an organic salt containing 5-H 2 SSA, whose crystal structure is reported here.
There are a half of a pyrazinediium cation, one each a 5-HSSAanion and a water molecule in the asymmetric unit.
Similarly to analogous organic adducts reported by others (Meng et al., 2008), the H atom is transferred from the sulfonic acid group to the pyrazine N atoms (Fig.1). Except for this, the other bond lengths and angles are usual.
By a combination of seven intermolecular hydrogen bonds (Table 1), these components are linked into a twodimensional layers (Fig.2) running parallel to the (102) plane in the crystal.

S2. Experimental
All the reagents and solvents were used as obtained without further purification. Equimolar amount of pyrazine (0.2 mmol, 16.0 mg) and 5-sulfosalicylic acid dihydrate (0.2 mmol, 50.8 g) were dissolved in 95% methanol (10 ml). The mixture was stirred for ten minutes at ambient temperature and then filtered. The resulting colorless solution was kept in air for two weeks. Plate shaped colorless crystals suitable for single-crystal X-ray diffraction analysis were grown by slow evaporation of the solvent.