Structure-spectroscopic relationship of co-crystals between a rare chromone structure type of barakol and some organic acids

Barakol, extracted from Cassia siamea Lamk. having an anxiolytic property, consists of a rare chromone core structure, therefore, in this work we investigate relation between the X-ray crystallography and physical properties such as spectroscopic data and the intermolecular interactions of the keto-enol forms based on the CSD search, especially e.g. hydrogen bonding, π-π interactions, ionic interactions and acid-base interactions and clarifies their ambiguous spectroscopic properties. The 1:1 molecular complexes of barakol and carboxylic acid (phthalic acid and 3-hydroxybenzoic acid) were prepared and the X-ray crystallographic studies revealed that the barakol-phthalate complex exists in an ion-pair complex. The formation of barakol-phthalate ion-pair complex is stabilized by the complementary of ion-ion interactions, π-π interactions and hydrogen bonding. The barakol-3-hydroxybenzoic acid complex is a π-π molecular complex. The co-crystallization of barakol-3- hydroxybenzoic acid complex is solely stabilized by π-π interactions. The spectroscopic studies including IR, ¹H NMR and ¹³C NMR are consistent with the results from the X-ray analysis. The ¹H NMR spectrum of the barakol and phthalic acid complex in a mixture of CDCl ₃-CD₃OD showed a dramatic spectral changes with downfield shifts of the ortho-protons of phthalic acid (Δδ_H(ppm) = 0.37), and the proton resonances of barakol (Δδ_H(ppm) = 0.31 for proton H3, 0.53 for proton H4, 0.57 for proton H6 and 0.33 for proton H9).