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.

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