Two-dimensional hydrogen-bonded supramolecular networks in the compounds of benzene-1,2,4,5-tetracarboxylic acid (pyromellitic acid) with 2,2'-biimidazole and 4,4'-dimethyl-2,2'-bipyridine.

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, C6H8N4(2+),C10H4O8(2-), (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 group P1. The asymmetric unit of (I) is composed of two independent ion pairs. Both the 2,2'-biimidazole-3,3'-diium dication and the PMA(2-) 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 linked via carboxyl-carboxyl O-H···O hydrogen bonding, resulting in a two-dimensional supramolecular sheet with the R6(5)(34) motif extending in the (1-21) 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 an R2(2)(8)R4(4)(30) motif. The chains are connected by water-carboxyl O-H···O hydrogen bonds to form a two-dimensional network parallel to the (011) plane. The 4-methyl-2-(4-methylpyridin-2-yl)pyridinium cations lie between the two-dimensional supramolecular layers linked via N-H···O hydrogen-bonding interactions.