Cooperative intramolecular hydrogen bonding induced azo-hydrazone tautomerism of azopyrrole: Crystallographic and spectroscopic studies

Abstract

The azo-hydrazone tautomerism of three azopyrrole compounds, which were synthesized by the reactions of 2-acylbenzenediazonium salt with pyrrole and meso-diethyl-2,2′-dipyrromethane, have been studied. In the solid state, 2-(2-acylphenyl)diazopyrrole adopted an azo tautomeric form, whereas 5,5′-bis(2-acylphenyl)diazo-dipyrromethane and 5-(2-acylphenyl)diazo-dipyrromethane crystallized in the hydrazone form. In polar solution, all of the compounds mainly adopt an azo form. In an apolar solution, however, 5,5′-bis(2-acylphenyl)diazo-dipyrromethane and 5-(2-acylphenyl)diazo-dipyrromethane mainly adopt hydrazone form. It is cooperative intramolecular hydrogen bonds that influence the azopyrrole tautomerisation from the azo to the hydrazone form.