Hydrogen-bonding Behaviour of Benzene-1,2,4,5-tetracarboxylic Acid: Supramolecular Structures of Different Dimensionality in the 2:1 Adducts Formed with 4,4'-Bipyridyl and Hexamethylenetetramine

Co-crystallization of benzene-1,2,4,5-tetracarboxylic acid, C10H6O8, With 4,4'-bipyridyl, C10H8N2, or With hexamethylenetetramine, C6H12N4, from methanol solutions yields in each case a 2:1 salt, [(C10H9N2)(+)](2).[(C10H4O8)(2-)] (1) and [(C6H13N4)(+)](2).[(C10H4O8)(2-)] (2). In (1) the carboxylate anions lie across centres of inversion, but they contain no intramolecular O-H ... O hydrogen bonds: the cations and anions are linked by strong O-H ... N and N-H ... O hydrogen bonds into a chain-of-rings, and these chains are further linked into a three-dimensional framework structure by means of C-H ... O hydrogen bonds and aromatic pi ...pi stacking interactions. Compound (2) contains two independent three-molecule aggregates, comprising a central anion and two cations, linked to the anion by means of short N-H ... O hydrogen bonds. One of these aggregates is centrosymmetric, but the other is not, and the two types of anion both form two intramolecular O-H ... O hydrogen bonds. The two types of three-molecule aggregate, in which all the anions are virtually parallel, are linked by short C-H ... O hydrogen bonds into a molecular staircase.