Spectroscopic studies of 2,6-dihydroxypyridine and some of its derivatives. Tautomerism in solution, and very strong hydrogen bonding in solid 2,6-dihydroxypyridine and its N-methylated derivative

Abstract

Ultraviolet, infrared, and proton magnetic resonance spectra were measured for 2,6-dihydroxypyridine, 6-hydroxy-1-methylpyrid-2-one, α,α,γ-trichloro- and α,α,γ-tribromo-glutaconimide, with special reference to the hydroxypyridone-pyridinediol-glutaconimide equilibrium. The ratio [pyridol]/[pyridone] is much smaller for 2,6- dihydroxy-than for 6-methoxy-2-hydroxy-pyridine, especially in hydroxylic solvents. The hydroxypyridone form is the main or predominant tautomer for 2,6-dihydroxypyridine in water, dimethyl sulphoxide, ethanol, and 5% ethanol-95% cyclohexane, and for the 1- methylated derivative in ethanol. The glutaconimide form is the main or predominant tautomer for 2,6-dihydroxy-pyridine in dioxan, and for the 1-methylated derivative in water, dioxan, and chloroform. In the solid state 2,6-dihydroxypyridine and the 1-methylated derivative show a very broad intense infrared OH band with a plateau extending from c. 1300 cm^-1 to, respectively, 800 and 450 cm^-1, and with some transmission ?windows?; there is very strong hydrogen bonding, various aspects of which will be discussed. The solids probably consist of 6-hydroxypyrid- 2-one molecules linked by very short unsymmetrical hydrogen bonds; though, for the unmethylated compound, an alternative ?macromolecular? structure, consisting of units in which the heavy-atom skeletons have C_2v symmetry, and are linked together by symmetrical hydrogen bonds, cannot be ruled out entirely on the present evidence.