Hydrogen bonded binary molecular adducts derived from exobidentate N-donor ligand with dicarboxylic acids: Acid⋯imidazole hydrogen-bonding interactions in neutral and ionic heterosynthons

Four new binary molecular compounds between a flexible exobidentate N-heterocycle and a series of dicarboxylic acids have been synthesized. The N-donor 1,4-bis(imidazol-1-ylmethyl)benzene (bix) was reacted with flexible and rigid dicarboxylic acids viz., cyclohexane-1,4-dicarboxylic acid (H2chdc), naphthalene-1,4-dicarboxylic acid (H2npdc) and 1H-pyrazole-3,5-dicarboxylic acid (H2pzdc), generating four binary molecular complexes. X-ray crystallographic investigation of the molecular adducts revealed the primary intermolecular interactions carboxylic acid⋯amine (via O–H⋯N) as well as carboxylate⋯protonated amine (via N–H+⋯O−) within the binary compounds, generating layered and two-dimensional sheet type H-bonded networks involving secondary weak interactions (C–H⋯O) including the solvent of crystallization. Depending on the differences in pKa values of the selected base/acid (ΔpKa), diverse H-bonded supramolecular assemblies could be premeditated. This study demonstrates the H-bonding interactions between imidazole/imidazolium cation and carboxylic acid/carboxylate anion in providing sufficient driving force for the directed assembly of binary molecular complexes. In the two-component solid form of hetero synthons involving bix and dicarboxylic acid, only H2chdc exist as cocrystal with bix, while all the other three compounds crystallized exclusively as salt, in agreement with the ΔpKa values predicted for the formation of salts/cocrystals from the base and acid used in the synthesis of supramolecular solids.