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-phdaH(2) or 1,3-phdaH(2), afforded two crystalline products, [PyH+][1,2-dphaH(-)]center dot 1,2-dphaH(2) (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 center dot center dot center dot O hydrogen bonds. Pyridinium cations are attached to these chains via N+-H center dot center dot center dot 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 center dot center dot center dot H center dot center dot center dot O = 2.539(2) angstrom. 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 center dot center dot center dot O interactions link the 1,3-phenylenediacetic acid residues into chains.