Intermolecular proton transfers in the solid state. Conversion of the hydroxyazo into the quinone hydrazone tautomer of 2-amino-3-hydroxy-6-phenylazopyridine. X-ray crystal structures of the two forms

Abstract

The thermal transformation of the low (1a) to the high temperature (1b) form of 2-amino-3-hydroxy-6-phenylazopyridine has been studied by differential scanning calorimetry and variable temperature i.r. spectroscopy. Form (1a) is converted into (1b) through two intermediate phases and the i.r. spectra show changes in the >C=C< and -N=N- stretches. X-Ray crystal structures of (1a and b) have been determined. The red crystals of (1a) are orthorhombic, a= 10.026(7), b= 20.452(9), c= 10.111(9)Å; the space group is Pbca and the structure has been refined to R 0.080 for 443 non-zero reflections. The blue crystals of (1b) are monoclinic, a= 22.666(6), b= 3.886(4), c= 11.373(7)Å,β = 91.82(3)°; the space group is P2₁/c and the structure has been refined to R 0.048 for 607 non-zero reflections. The bond lengths in (1a and b) are in accord with a hydroxyazo and a quinone hydrazone structure, respectively. The hydrogen bonding in each form can only be explained if (1a) is the azo and (1b) the hydrazone tautomer. In the crystal structure of (1a), the hydroxy-group is close to an azo-group of an adjacent molecule and the solid state tautomerization can be accounted for by an intermolecular co-operative shift of protons across the various hydrogen bonds in structure (1a).