Halogen Bond as Controlling the Crystal Structure of 4‑Amino-3,5-Dihalogenobenzoic Acid and Its Effect on the Positional Ordering/Disordering of Acid Protons

Crystal structures and thermal behaviors of 4-amino-3,5-dihalogenobenzoic acid (AXBA: X = Cl and I) were studied by single-crystal X-ray diffractometry and differential scanning calorimetry. It was found that all the crystal structures of AClBA and AIBA as well as ABrBA at room temperature belong to the same space group with two crystallographically distinguishable dimer units. The dimer was located in a highly symmetric circumstance; for example, its CO and C–O bonds were indistinguishable due to the presence of 2-fold rotational and mirror symmetry elements so that two hydrogen bonds should be equivalent within each dimer. Such symmetric dimer structures are very rare among carboxylic acid crystals. Stabilization of such a symmetric dimer structure is attributed probably to notably strong and angular-dependent interdimer interactions: halogen bonds and NH···O hydrogen bonds. The strength of the halogen bond, among different halogens, was in the order of I > Br > Cl from the interhalogen-atomic length as compared with its ordinary van der Waals’ diameter. AIBA revealed no phase transition even down to 110 K, while AClBA revealed it at around 140 K. The difference in the thermal behaviors perhaps originates from the different strengths of halogen bonds in the AIBA and AClBA crystals.