Solid-state esterification via ionic-to-covalent bond transformation in ionic molecular crystals consisting of disubstituted anthracene anion-cation combinations

Thermal solid-state reactions involving ionic-to-covalent bond transformation were achieved in the ionic molecular crystals of 1,8-bis[(pyridin-1-ium-1-yl)methyl]anthracene anthracene-1,8-dicarboxylate to produce the cyclic diester. The crystal structure given by X-ray diffraction analysis showed that the electrostatic interaction between the benzyl pyridinium cation and the carboxylate anion successfully works to locate these reaction sites close to each other and that there are channels for crystalline solvents and the liberated pyridine to escape. There by, the cyclic diester was selectively formed by the crystal-to-crystal reaction, which was proven by powder XRD profiles and optical microscopic and SEM images of the crystals before and after the reaction. This is the first thermal crystal-to-crystal condensation reaction. Another ionic molecular crystal of 1,8-bis[(pyridin-1-ium-1-yl)methyl]anthracene anthraquinone-1,8-dicarboxylate was also studied. There in, the corresponding cyclic diester was also obtained, but the crystals were transformed to amorphous solid by the reaction.