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

•The azo-hydrazone tautomerism of three azopyrrole compounds have been studied.•One azopyrrole adopted azo form, whereas the other two crystallized in hydrazone form.•In polar solution, all the compounds mainly adopt azo form.•In apolar solution, however, two azopyrrole compounds mainly adopt hydrazone form.•Cooperative intramolecular hydrogen bonds help the transferring from azo to hydrazone.

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.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slide