The inner-salt zwitterion, the dihydrochloride dihydrate and the dimethyl sulfoxide disolvate of 3,6-bis(pyridin-2-yl)pyrazine-2,5-dicarboxylic acid.

In the inner-salt zwitterion of 3,6-bis(pyridin-2-yl)pyrazine-2,5-dicarboxylic acid, (I), namely 5-carboxy-3-(pyridin-1-ium-2-yl)-6-(pyridin-2-yl)pyrazine-2-carboxylate, [C(16)H(10)N(4)O(4), (Ia)], the pyrazine ring has a twist-boat conformation. The opposing pyridine and pyridinium rings are almost perpendicular to one another, with a dihedral angle of 80.24 (18)°, and are inclined to the pyrazine mean plane by 36.83 (17) and 43.74 (17)°, respectively. The carboxy and carboxylate groups are inclined to the mean plane of the pyrazine ring by 43.60 (17) and 45.46 (17)°, respectively. In the crystal structure, the molecules are linked via N-H···O and O-H···O hydrogen bonds, leading to the formation of double-stranded chains propagating in the [010] direction. On treating (Ia) with aqueous 1 M HCl, the diprotonated dihydrate form 2,2'-(3,6-dicarboxypyrazine-2,5-diyl)bis(pyridin-1-ium) dichloride dihydrate [C(16)H(12)N(4)O(4)(2+)·2Cl(-)·2H(2)O, (Ib)] was obtained. The cation lies about an inversion centre. The pyridinium rings and carboxy groups are inclined to the planar pyrazine ring by 55.53 (9) and 19.8 (2)°, respectively. In the crystal structure, the molecules are involved in N-H···Cl, O-H···O(water) and O(water)-H···Cl hydrogen bonds, leading to the formation of chains propagating in the [010] direction. When (Ia) was recrystallized from dimethyl sulfoxide (DMSO), the DMSO disolvate 3,6-bis(pyridin-2-yl)pyrazine-2,5-dicarboxylic acid dimethyl sulfoxide disolvate [C(16)H(10)N(4)O(4)·2C(2)H(6)OS, (Ic)] of (I) was obtained. Here, the molecule of (I) lies about an inversion centre and the pyridine rings are inclined to the planar pyrazine ring by only 23.59 (12)°. However, the carboxy groups are inclined to the pyrazine ring by 69.0 (3)°. In the crystal structure, the carboxy groups are linked to the DMSO molecules by O-H···O hydrogen bonds. In all three crystal structures, the presence of nonclassical hydrogen bonds gives rise to the formation of three-dimensional supramolecular architectures.