Journal of the Chemical Society. Perkin transactions II p. 1025 - 1030 (1983)
Update date:2022-09-26
Topics:
Desiraju, Gautam R.
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- sterches.X-Ray crystal structures of (1a and b) have been determined.The red crystals of (1a) are orthorombic, a = 10.026(7), b = 20.452(9), c = 10.111(9) Angstroem; 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) Angstroem, β = 91.82(3) deg; 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 hydroxazo 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 stucture 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).
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