Ternary assembly of pyrazine 2,3-dicarboxylic acid with a ditopic amine†

Abstract

Ionic cocrystals have emerged as an important class of supramolecular entities, with applications in electrolytes, fertilizers, and pharmaceuticals. It is necessary to understand their self-assembly to establish new formulations based on energy considerations for different forms with the same composition. Depending on the stoichiometry of the reactants used in the reaction of the ditopic amine 9-N-(3-imidazolylpropylamino)methyl anthracene (Hanthraimmida) with pyrazine 2,3-dicarboxylic acid (H₂pyzda) in methanol under ambient conditions (relative humidity, 75–80%), a salt and its ionic cocrystal with pyrazine 2,3-dicarboxylic acid were observed in a 1 : 1 ratio. The binary 1 : 1 salt had a composition of H₃anthraimmida·pyzda·2H₂O·CH₃OH. It exhibited an interesting structure with hydrogen-bonded cations and anions forming concave spaces to stabilise clusters of solvent molecules that were held by hydrogen bonds. The ternary ionic cocrystal had a composition of H₃anthraimida·pyzda·H₂pyzda·H₂O. In the self-assembly, the water molecule was holding the di-cation of Hanthraimmida, the dianion of H₂pyzda and a neutral H₂pyzda. The water molecule held the other components by forming R²₁(6), R¹₂(4) and R²₁(5) synthons with extensive C–H⋯X (X = O or N) bonds. The self-recognition of the H₂pyzda acid in this ionic cocrystal could be attributed to the chain-like template formed between the anions and neutral dicarboxylic acid molecules through charge-assisted hydrogen bonds; this template held the relatively large-sized cations. Structural studies and energy optimization of the binary and ternary assemblies of the salt and its cocrystals were performed. Hirshfeld analysis of both forms suggested the presence of hydrophobic surfaces. The theoretical DFT-optimized energy suggested that the observed form of the ionic cocrystal did not have the lowest energy but had higher energy than a monoanionic cocrystal with identical composition. However, it exhibited higher stability than the binary salt, which can be attributed to its easy formation. The water molecules in the ionic cocrystal contributed to holding the three components, and their effects on the stabilization of other possible (differently proton transferred) forms of ionic cocrystals are presented. In the assembly of the ionic cocrystal, one of the nitrogen atoms distinctly did not participate in hydrogen bonding, thereby violating the Etter rule.