Crystal structures of 1,4-diazabicyclo[2.2.2]octan1-ium 4-nitrobenzoate dihydrate and 1,4-diazabicyclo[2.2.2] octane-1,4-diium bis(4-nitro-benzoate): the influence of solvent upon the stoichiometry of the formed salt

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+center dot C7H4NO4-center dot 2H(2)O, (1), whereas from methanol, the salt C6H14N22+center dot 2C(7)H(4)NO(4)(-), (2), was isolated. In (1), the cation and anion are linked by a strong N-H center dot center dot center dot O hydrogen bond, and the carboxylate anion is close to planar [dihedral angle between terminal residues = 6.83 (9)degrees]. In (2), a three-ion aggregate is assembled by two N-H center dot center dot center dot O hydrogen bonds, and the carboxylate anions are again close to planar [dihedral angles between terminal residues = 1.7 (3) and 5.9 (3)degrees]. Through the intervention of solvent water molecules, which self-assemble into helical supramolecular chains along the b axis, the three-dimensional architecture in (1) is stabilized by water-DABCO O-H center dot center dot center dot N and water-carboxylate O-H center dot center dot center dot O hydrogen bonds, with additional stability afforded by C-H center dot center dot center dot O interactions. The global crystal structure comprises alternating layers of water molecules and ion pairs stacked along the c axis. 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 center dot center dot center dot O interactions as well as pi-pi contacts between inversion-related benzene rings [inter-centroid distances = 3.5644 (16) and 3.6527 (16) angstrom]. The cations and anions assemble into alternating layers along the c axis.