Tautomerism of 4-hydroxyterpyridine in the solid, solution and gas phases: an X-ray, FT-IR and NMR study

The keto–enol equilibrium between 2,6-bis(2′-pyridyl)-4-pyridone 1b and 2,6-bis(2′-pyridyl)-4-hydroxypyridine 1a was evaluated using infrared spectroscopy, variable temperature 1H and 13C NMR spectroscopy and X-ray crystallography. These studies show that the less polar hydroxy tautomer 1a is the predominant species in the gas phase. The solution-state studies show the more polar keto form 1b to be predominant but not exclusive, and the ratio of tautomers depends on the polarity and hydrogen-bonding ability of the solvent as well as temperature. In the solid-state both species are present in a 1∶1 ratio and form a dimeric structure held together by a strong CO· · ·H–O hydrogen bond between the tautomers.