Crystal structures of 1,4-diazabicyclo[2.2.2]octan-1-ium 4-nitrobenzoate dihydrate and 1,4-diazabicyclo[2.2.2]octane-1,4-diium bis(4-nitrobenzoate): the influence of solvent upon the stoichiometry of the formed salt

Abstract

The 1:1 co-crystallization of 1,4-diazabicyclo[2.2.2]octane (DABCO) with 4-nitrobenzoic acid in ethanol–water (3/1) gave the salt dihydrate C6H13N2+·C7H4NO4−·2H2O, (1), whereas from methanol, the salt C6H14N22+·2C7H4NO4−, (2), was isolated. In (1), the cation and anion are linked by a strong N—H...O hydrogen bond, and the carboxylate anion is close to planar [dihedral angle between terminal residues = 6.83 (9)°]. In (2), a three-ion aggregate is assembled by two N—H...O hydrogen bonds, and the carboxylate anions are again close to planar [dihedral angles between terminal residues = 1.7 (3) and 5.9 (3)°]. Through the intervention of solvent water molecules, which self-assemble into helical supramolecular chains along thebaxis, the three-dimensional architecture in (1) is stabilized by water–DABCO O—H...N and water–carboxylate O—H...O hydrogen bonds, with additional stability afforded by C—H...O interactions. The global crystal structure comprises alternating layers of water molecules and ion pairs stacked along thecaxis. In the crystal of (2), the three-ion aggregates are assembled into a three-dimensional architecture by a large number of methylene–carboxylate/nitro C—H...O interactions as well as π–π contacts between inversion-related benzene rings [inter-centroid distances = 3.5644 (16) and 3.6527 (16) Å]. The cations and anions assemble into alternating layers along thecaxis.