Hydrogen bonding properties and intermediate structure of N-(2-carboxyphenyl)salicylidenimine

The hydrogen bonding properties, the nature of the tautomeric structure and dimerization of N-(2-carboxyphenyl)salicylidenimine 1 has been studied. The crystal and molecular structure of 1 has been determined by single-crystal X-ray diffraction analysis. This compound forms a dimer in the solid state, which is held together by two strong intermolecular O–H · · · O bridges [O(1) · · · O(2a) = 2.455(1) Å]. This dimeric structure is further stabilized by two intramolecular N–H · · · O hydrogen bonds [N(1) · · · O(1) = 2.654(1) and N(1) · · · O(2) = 2.663(1) Å]. In this way, an eight-membered pseudocycle is created. However, in methanol or acetonitrile solution, no dimerization was observed according to 1H-NMR, IR and UV measurements. It was also found that this compound exists as an intermediate form between a phenol-imine and an o-quinoid structure. In addition, a derivative soluble in chloroform, 2-[2-hydroxy-5-(2-butyl)benzylideneamino]benzoic acid 3, was examined for comparison.