An amphoteric switch to aromatic and antiaromatic states of a neutral air-stable 25π radical

Abstract

Ever since the discovery of the trityl radical, isolation of a stable and neutral organic radical has been a synthetic challenge. A (4n+1)π open-shell configuration is one such possible neutral radical but an unusual state between aromatic (4n+2)π and antiaromatic (4n)π electronic circuits. The synthesis and characterization of an air- and water-stable neutral 25π pentathiophene macrocyclic radical is now described. It undergoes reversible one-electron oxidation to a 24π antiaromatic cation and reduction to a 26π 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 4nπ system, while the anion adhered to aromatic nature. Computational studies revealed the delocalized nature of the unpaired electron as confirmed by EPR spectroscopy.

Keywords: electron transfer; macrocycles; polythiophenes; porphyrinoids; radicals.