Synthesis and spectroscopic properties of Schiff bases derived from 3-hydroxy-4-pyridinecarboxaldehyde

A series of six new Schiff bases has been prepared by reacting aniline and 4-R-substituted anilines (R=CH3, OCH3, Br, Cl, NO2) with 3-hydroxy-4-pyridinecarboxaldehyde. The 1H, 13C, 15N and 17O NMR data of these compounds are used to discuss the tautomerism. 15N NMR and 17O NMR chemical shifts established the tautomer existing in solution as the hydroxy/imino. 13C CPMAS NMR confirms that the same tautomer is found in the solid state. The stabilities of the tautomeric forms have been approached using density functional calculations (B3LYP/6-31G**) in the gas phase. In all cases the neutral hydroxy/imino with E configuration is more stable than the oxo/enamino form (by ∼22 kJ mol−1) and significantly more stable than the betaine (by ∼75 kJ mol−1).