Products of the Reactions Between Pyridine and 1,2- or 1,3-phenylenediacetic Acid: Salts or Co-crystals?

Reaction of pyridine with 1,2- or 1,3-phenylenediacetic acids, denoted as 1,2-phdaH2 or 1,3-phdaH2, afforded two crystalline products, [PyH+ ][1,2-dphaH-] • 1,2-dphaH2 (1) and [PyH + ][1,3-dphaH-] (2) (PyH+ = C5H5NH+ , pyridinium cation). Compound 1 contains apart from protonated pyridine molecule also two 1,2-phenylenediacetic acid species, a semi-1,2-phenylenediacetate ion and a neutral acid molecule, one of each per formula unit. As such, it can be classified among co-crystals. The 1,2-phenylenediacetic acid species are assembled into double-chains via O-H•••O hydrogen bonds. Pyridinium cations are attached to these chains via N+ -H•••O-(carboxylate) interactions. The X-ray structure analysis of compound 2 revealed a hydrogen bond of moderate strength occurring between the pyridine nitrogen and oxygen atom of one COOH function, N•••H•••O = 2.539(2) Å. The position of hydrogen is almost half way between the two atoms. Compound 2 can neither be considered as a pure salt nor as a pure co-crystal. As in 1, the O-H•••O interactions link the 1,3-phenylenediacetic acid residues into chains.