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

Abstract

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.