An Amphoteric Switch to Aromatic and Antiaromatic States of a Neutral Air-Stable 25π Radical

Ever since the discovery of the trityl radical, isolation of a stable and neutral organic radical has been a synthetic challenge. A (4n+1)pi open-shell configuration is one such possible neutral radical but an unusual state between aromatic (4n+2)pi and antiaromatic (4n)pi electronic circuits. The synthesis and characterization of an air-and water-stable neutral 25 pi pentathiophene macrocyclic radical is now described. It undergoes reversible one-electron oxidation to a 24 pi antiaromatic cation and reduction to a 26 pi aromatic anion, thus confirming its amphoteric behavior. Structural determination by single-crystal X-ray diffraction studies revealed a planar configuration for the neutral radical, antiaromatic cation, and aromatic anion. In the solution state, the cation shows the highest upfield chemical shift ever observed for a 4n pi system, while the anion adhered to aromatic nature. Computational studies revealed the delocalized nature of the unpaired electron as confirmed by EPR spectroscopy.