An experimental screen for quinoline/fumaric acid salts and co-crystals

Abstract

An experimental screen has been carried out for salts and co-crystals of quinoline (C₉H₇N) and fumaric acid (C₄H₄O₄), including solution-based co-crystallisation from a variety of solvents, solvent-assisted and solvent-free co-grinding, and direct co-crystallisation of the starting materials. Characterised products include the previously published 1 : 1 salt, C₉H₈N⁺·C₄H₃O₄⁻, and a new 2 : 1 quinoline/fumaric acid co-crystal, (C₉H₇N)₂·C₄H₄O₄. Attempts to influence the crystallisation outcome by addition of 6-methylquinoline yielded a second co-crystal, also with an inherent 2 : 1 quinoline/fumaric acid ratio, as a solid solution containing ca. 75% 6-methylquinoline and 25% quinoline. The corresponding co-crystal with pure 6-methylquinoline, (C₁₀H₉N)₂·C₄H₄O₄, was prepared, but the analogous structure with pure quinoline could not be obtained. Energy minimisation using dispersion-corrected density functional theory (DFT-D) calculations suggests that the hypothetical polymorph of the 2 : 1 quinoline/fumaric acid co-crystal derived from the structure of the 2 : 1 6-methylquinoline/fumaric acid co-crystal is ca. 8 kJ mol⁻¹ less stable than the structure that is actually observed.