Aromatic versus Aliphatic Carboxyl Group as a Hydrogen Bond Donor in Salts and Cocrystals of an Asymmetric Diacid and Pyridine Derivatives

Abstract

A series of 15 salts and cocrystals of an asymmetric aromatic–aliphatic dicarboxylic diacid, N-(2-carboxyphenyl)glycine, and pyridine derivatives was synthesized in order to study whether there is a preference of pyridine binding to either of the carboxylic groups. In nine structures the pyridine was bonded to the aliphatic group, in three to both carboxylic groups, and in three to the aromatic group alone. The preference of pyridine derivatives to bond to the aliphatic group correlates with the more positive electrostatic potential on the aliphatic carboxyl hydrogen atom. The occurrence of proton transfer within the structures was found to follow the basicity of the pyridine derivative, four of the least basic pyridines forming cocrystals, two intermediate bases solids with proton disorder and the more basic pyridine derivatives salts. Proton transfer had a considerable effect on the crystal packing, cocrystals mostly comprising discrete molecular complexes, and salt chains of monoanions.