Supramolecular isomerism and polymorphism in dianion salts of pyromellitic acid

The X-ray crystal structures of several salts of pyromellitic acid, H₄PMA, have been determined. The structures of the pyridinium, [C₆H₅NH]₂[H₂PMA], 1, 3-methylpyridinium, [3-CH₃C₆H₄NH]₂[H₂PMA], 2a, [3-CH₃C₆H₄NH]₂-[H ₂PMA]·[3-CH₃C₆H₄N], 2b, 4-methylpyridinium, [(4-CH₃C₆H₄N)₂H][4-CH ₃C₆H₄NH][H₂PMA], 3, 2-ethylpyridinium, [2-CH₃CH₂C₆H₄N]₂[H ₂PMA], 4, 2,4,6-trimethylpyridinium, [2,4,6-(CH₃)₃C₆H₄NH] ₂[H₂PMA], 5a and 5b, and quinolinium, [C₉H₇NH]₂[H₂PMA], 6, salts were characterized and analyzed in terms of their networking and crystal packing. The structures of 1-6 reveal the existence of OD, 1D, 2D, or 3D O-H⋯O^- hydrogen bonded networks that are sustained and controlled by self-assembly of H₂PMA^2- anions. Five structures, 1, 2a, 3, 4, and 6 form OD complexes, whereas 1D, 2D, and 3D networks occur in 5a, 2b, and 5b, respectively. The relatively high occurrence of the OD species might be attributable to a greater likelihood that more efficient crystal packing should generally occur in OD complexes when compared to higher dimensional networks, in which open spaces are more likely. The formation of such a variety of networks by a single tecton, supramolecular isomerism, resembles that which has been observed in coordination polymer networks in which 4,4'-bipyridine or related ligands are used as spacer ligands.