Tautomerism in Schiff bases derived from 3-hydroxysalicylaldehyde. Combined X-ray diffraction, solution and solid state NMR study

The crystal structure of six Schiff bases derived from 3-hydroxysalicylaldehyde has been determined by single crystal X-ray diffraction. In the solid state, these compounds form dimers linked by two strong intermolecular O–H⋯O bridges resulting in ten-membered pseudocycles. From the observed geometrical parameters together with previous results obtained for N-(3-hydroxysalicylidene)isopropylamine, it can be concluded that these compounds undergo a fast proton exchange between two forms, phenol–imine and keto-amine. The tautomeric equilibrium mixture is greatly influenced by the physical state of the examined compounds. This is clearly seen when comparing results obtained from liquid and solid-state 13C NMR. For derivatives derived from aliphatic amines the keto-amine tautomer is always predominant; for compounds obtained from substituted anilines the tautomeric concentration is variable depending on the nature of the substituent. An excellent correlation between C–O bond lengths and 13C chemical shifts is observed.