Tautomerism in N-(2-hydroxy-1-naphthylidene)amino acids and the search for an answer to the difficult question about where the proton belongs

•Tridentate O,N,O′-Schiff bases with amino acid moieties were synthesized and characterized.•Order–disorder phase transition occurs for N-(2-hydroxy-1-naphthylidene)-l-valine.•All three Schiff bases exist uniformly in keto-amine tautomeric structure in solid state.•NMR and UV–Vis spectroscopy show that the keto-amine form is slightly more stable in solution.

N-(2-hydroxy-1-naphthylidene)-l-valine 1, N-(2-hydroxy-1-naphthylidene)-l-phenylalanine 2, and N-(2-hydroxy-1-naphthylidene)-l-threonine 3 were prepared and characterized with spectroscopic methods, elemental analyses, and values of optical rotation. Compound 1 undergoes a solid state order-disorder phase transition at 231 K. The X-ray structures of the high and low temperature phase of 1 have been determined. Single crystal X-ray structures of 2 and 3 have been determined as well. The tautomerism of N-(2-hydroxy-1-naphthylidene)amino acid derivatives is discussed controversial in the literature. A bond lengths statistical analysis shows that all three compounds exist uniformly in the keto-amine form in the solid state. Quantum chemical calculations, NMR, and UV–Vis spectroscopy were used to obtain further insight into the existence of phenol-imine and keto-amine structures in this class of compounds.

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