Two products from the proton-transfer reactions of benzene-1,2,4,5-tetracarboxylic acid (pyromellitic acid, PMA) with 2,2′-biimidazole and 4,4′-dimethyl-2,2′-bipyridine, namely 2,2′-biimidazole-3,3′-diium 2,5-dicarboxybenzene-1,4-dicarboxylate, C6H8N42+,C10H4O82−, (I), and 4-methyl-2-(4-methylpyridin-2-yl)pyridinium 2,4,5-tricarboxybenzoate monohydrate, C12H13N2+·C10H5O8−·H2O, (II), have been prepared and their structures determined. Both compounds crystallize in the space groupP\overline{1}. The asymmetric unit of (I) is composed of two independent ion pairs. Both the 2,2′-biimidazole-3,3′-diium dication and the PMA2−anion are located on special positions (inversion centres). The protonated 2,2′-biimidazole-3,3′-diium ring H atoms are involved in hydrogen bonding with carboxylate O atoms to form one-dimensional hydrogen-bonded chain structures. Adjacent chains are further linkedviacarboxyl–carboxyl O—H...O hydrogen bonding, resulting in a two-dimensional supramolecular sheet with theR65(34) motif extending in the (1\overline{2}1) plane. In (II), classical O—H...O hydrogen-bond-linked anion–anion units are extended into a one-dimensional chain running parallel to the [100] direction, giving anR22(8)R44(30) motif. The chains are connected by water–carboxyl O—H...O hydrogen bonds to form a two-dimensional network parallel to the (01\overline{1}) plane. The 4-methyl-2-(4-methylpyridin-2-yl)pyridinium cations lie between the two-dimensional supramolecular layers linkedviaN—H...O hydrogen-bonding interactions.
Diosgenin hemihydrate