The zwitterionic structure of 2-hydroxy-4-[(2-hydroxybenzylidene)amino]benzoic acid

Abstract

The title compound, C14H11NO4, exists in the solid phase in the zwitterionic form, 2-{[(4-carboxy-3-hydroxyphenyl)iminiumyl]methyl}phenolate, with the H atom from the phenol group on the 2-hydroxybenzylidene ring transferred to the imine N atom, resulting in a strong intramolecular N—H...O hydrogen bond between the iminium H atom and the phenolate O atom, forming a six-membered hydrogen-bonded ring. In addition, there is an intramolecular O—H...O hydrogen bond between the carboxylic acid group and the adjacent hydroxy group of the other ring, and an intermolecular C—H...O contact involving the phenol group and the C—H group adjacent to the imine bond, connecting the molecules into a two-dimensional network in the (10\overline{3}) plane. π–π stacking interactions result in a three-dimensional network. This study is important because it provides crystallographic evidence, supported by IR data, for the iminium zwitterionic form of Schiff bases.<!?tpb=12pt>