| Summary: | In the structure of the title 2:1 proton-transfer compound, 2C12H8N3O2+·C10H4O82−, the 6-nitro-1,10-phenanthroline molecules act as proton sponges, accepting protons from pyromellitic acid. The –NO2 group of one of the 6-nitro-1,10-phenanthrolin-1-ium cations is disordered and was refined with a site-occupancy ratio of 0.624 (15):0.376 (15). Two –COOH(–COO−) groups of the 2,5-dicarboxyterephthalate dianion are disordered and were refined with site-occupancy ratios of 0.769 (4):0.231 (4) and 0.766 (5):0.234 (5). The –NO2 group of the second cation is also disordered about a pseudo-twofold rotation axis and was refined with a site-occupancy ratio of 0.903 (3):0.097 (3). There is an intramolecular O—H...O hydrogen bond in the anion. The phenanthroline rings of the two cations are inclined to one another by 31.3 (1)°. In the anions, considering the major components only, the carboxylic acid groups (–COOH) are inclined to the benzene ring by 17.3 (2) and 22.3 (3)°. The carboxylate groups (–COO−) are twisted by 9.3 (2) and 13.6 (6)° with respect to the benzene ring. In the crystal, adjacent 2,5-dicarboxyterephthalate anions are linked via O—H...O hydrogen bonds, forming chains propagating along [010]. The cations are attached to the chain of anions by N—H...O hydrogen bonds.
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