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-hydroxysalicylalde hyde 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 geo metrical parameters together with previous results obtained for N-(3-hydrox ysalicylidene)isopropylamine, it can be concluded that these compounds unde rgo a fast proton exchange between two forms, phenol-imine and keto-amine. The tautomeric equilibrium mixture is greatly influenced by the physical st ate of the examined compounds. This is clearly seen when comparing results obtained from liquid and solid-state C-13 NMR. For derivatives derived from aliphatic amines the keto-amine tautomer is always predominant; for compou nds obtained from substituted anilines the tautomeric concentration is vari able depending on the nature of the substituent. An excellent correlation b etween C-O bond lengths and C-13 chemical shifts is observed.