Ternary Assembly of Pyrazine 2,3-dicarboxylic acid with a Ditopic-Amine

Abstract

Ionic cocrystals have emerged as important class of supramolecular entities, having applications in electrolyte, fertilizers, and pharmaceuticals. There is a necessity to understand their self-assemblies to establish new formulations with energy consideration of different forms with same composition. Depending on the stoichiometry of the reactant 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 arial conditions (relative humidity 75-80 %), 1:1 salt and its ionic cocrystal with pyrazine 2,3-dicarboxylic acid were observed. The binary 1:1 salt had a composition H3anthraimida•pyzda•2H2O•CH3OH. It had interesting structure having hydrogen bonded cations and anions forming concave spaces to stablise clusters of hydrogen bonded solvent molecules that were held by hydrogen bonds. The ternary ionic cocrystal had a composition H3anthraimida•pyzda.•H2pyzda.H2O. In the self-assembly, the water molecule was holding the di-cation of Hanthraimmida, dianion of H2pyzda and a neutral H2pyzda. The water molecule held the other components by forming R12(6), R21(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 was attributed to the chain-like template formed between the anions and the neutral dicarboxylic acid molecules though charge-assisted hydrogen bonds, where the relatively larger-size cations were held. Structural studies and energy optimizations of the binary and the ternary assembly of the salt and the cocrystals were performed. The Hirshfeld analysis of both the forms suggested hydrophobic surfaces. The theoretical DFT-optimized energy has suggested that the observed form of the ionic cocrystal was not the lowest energy it had higher energy than a monoanionic cocrystal with identical composition. However, it had higher stability than the binary salt, which is attributed to its easy formation. The water molecules in the ionic cocrystal had contributed to hold the three components and the effect of it in stabilization other possible (differently proton transferred) forms of ionic cocrystals are presented. The assembly of the ionic cocrystal had the distinction to have one of the nitrogen atoms not participating in hydrogen bonds, thereby violated Etter-rule.