Proton induced tautomeric switching in N-rich aromatics with tunable acid-base character

• 1,2,4triazolo[3,2-c][1,2,4]triazoles are organic compounds with remarkable acid-base properties.
• Neutral heterobicycles (HL) are amphoteric: they can form the base L− and the acid species H2L + and H3L++.
• With attractor groups at the heterobicycle, the compounds can be acids stronger than p-nitrophenol.
• The stable tautomer is 2H. By protonation, a switching to the 1H–3H tautomer is observed.

The acid-base properties of selected derivatives of the [1,2,4]triazolo[3,2-c][1,2,4]triazole fused aromatic system have been investigated by UV–vis spectroscopy. Neutral heterobicycles (HL) exhibit amphoteric behavior (they can deliver the N–H proton forming the conjugated base L− and can accept up to two protons, forming the species H2L+ and H3L++) and show an unprecedented tautomeric switching upon protonation, as revealed by single crystal X-ray analysis and confirmed by theoretical calculations. By varying the groups attached at the heterocycle, a remarkable shift of pKai values, up to 5–6 units, is observed. In particular, with strong electron attractor groups at position 7 (e.g. p-nitrophenyl or pentafluorophenyl) the neutral compounds are stronger acids than phenol or p-nitrophenol.

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