Formation of R44(8)$R_4^4(8)$ ring in chloride salts of 8-hydroxyquinolinium derivatives: synthesis, structural, and theoretical studies

Abstract

Abstract The molecular structures of chloride salts of 2- and 5-substituted derivatives of 8-hydroxyquinoline, Me–H2Q+·Cl– ([C10H10NO]Cl) and Cl–H2Q+·Cl– ([C9H7ClNO]+·Cl–), were determined by single crystal X-ray diffraction methods; the latter is a new polymorph. In the crystal structures of these salts, several intra- and inter-molecular interactions result in a step-shaped centrosymmetric R 4 4 ( 8 ) $R_4^4(8)$ ring. Unlike most quinolinium salts, there was no solvent present in these structures. Protonation of the quinoline N atom had an effect on the N···O bite distances and C–N–C angle and greater conjugation of the benzene ring with a hydroxyl group was also observed. π–π Interactions between each pair of quinolinium rings were observed in Me–H2Q+·Cl– but not in Cl–H2Q+·Cl–. In addition, the quantum chemical calculations were performed on the new structures as well as similar compounds for comparison. The optimized structures were compared with the experimental observations for the effect of protonation and of hydrogen bonding interactions.