Controlled shift in the tautomeric equilibrium of 4-((phenylimino)methyl)naphthalen-1-ol

4-((Phenylimino)methyl)naphthalen-1-ol and 4-((phenylimino)methyl)-2-(piperidin-1-ylmethyl)naphthalen-1-ol have been synthesized and their tautomeric properties were investigated using molecular spectroscopy (UV–Vis absorption/emission and NMR), X-ray crystallographic analysis and quantum-chemical calculations. The results show that with the implementation of a flexible piperidine ring a controlled shift in the tautomeric equilibrium will be achieved upon protonation/deprotonation in acetonitrile. The addition of a metal salt also shifts the tautomeric equilibrium, but at very high concentrations, which is caused not by a complex formation, but a special effect of the salt addition.

► 4-((Phenylimino)methyl)-2-(piperidin-1-ylmethyl)naphthalen-1-ol was studied.
► The controlled shift of the tautomeric equilibrium through structural modifications was achieved.
► The inclusion of the piperidine ring leads to shift of the equilibrium to the enol tautomer.
► Upon protonation the equilibrium shifts to the keto tautomer in a controlled and reversible manner.
► The salt addition leads to shift in the tautomeric equilibrium, independly on the kind of salt used.