Bis(6-nitro-1,10-phenanthrolin-1-ium) 2,5-di­carb­oxy­terephthalate

In the structure of the title 2:1 proton-transfer compound, 2C(12)H(8)N(3)O(2) (+)·C(10)H(4)O(8) (2−), the 6-nitro-1,10-phenanthroline mol­ecules act as proton sponges, accepting protons from pyromellitic acid. The –NO(2) 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 –NO(2) 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.