Structural evidence of an intramolecular proton transfer leading to keto-amine tautomer in the crystals of Schiff bases derived from tyrosine and histidine esters

Three Schiff bases derived from of 2,4-dihydroxybenzaldehyde or 2,4-dihydroxyacetophenone and esters of tyrosine and histidine have been synthesized and the crystal and molecular structures determined by single crystal X-ray diffraction. The molecular structures of the three compounds are dominated by short intramolecular hydrogen bonds with distances N⋯O ranging from 2.536(2) to 2.588(2) Å and the hydrogen atom is bonded to the nitrogen. In the solid state, the structures are characterized by the keto-amine tautomer, whereas in the solution the phenol-imine form was detected by 1H NMR spectroscopy. Intermolecular interactions influencing crystal packing are discussed.