Interaction Study of Co-Crystallization or Salt Formation Between 5-Hydroxyisophthalic Acid and 4,4′-Bipyridine using NMR and Powder X-Ray Diffraction

Abstract

When an organic acid and an organic base are used for the study of potential co-crystallization then on the basis of differences in their pKa salt, co-crystal or proton transfer is possible. A study of the Cambridge Structural Database (CSD) revealed that no cocrystal or salt formation has been observed between 5-hydroxyisophthalic acid (5-HIPA) and 4,4′-bipyridine (4,4-BIPY). This fact provided motivation for the study of interaction between these two leading to co-crystallizations or salt formation. On comparing differences in their pKa values these two lays in transition range where three possibilities as mentioned above may be present. In this study, it is emphasized on exploring interaction between 5-HIPA and 4,4-BIPY, using solution state NMR and powder X-ray diffraction (PXRD) and find feasibility of cocrystal formation, salt formation or proton transfer. After studying this system, evidence were found in support of proton transfer between 5-HIPA and 4,4-BIPY and molar ratio for maximum interaction between acid and base was also mapped and found to be 1:1.