An Air- and Moisture-Stable Fulvalene Radical Cation in a Reversible Three-State Redox Interconversion

Abstract

Pentafulvalene has attracted considerable interest owing to its unique cross-conjugated electronic structure, but its intrinsic instability has limited detailed studies of its redox chemistry, particularly with respect to accessing and characterizing its radical form. Herein, we report an imidazolium-substituted zwitterionic fulvalene (IZFv) that undergoes reversible interconversion among three isolable redox states, including a fulvalene radical cation. Single-crystal X-ray diffraction, spectroscopy, and complementary NICS, HOMA, and AICD analyses reveal systematic oxidation-state-dependent changes in the structure and local aromaticity of the fulvalene framework. Notably, the dication (IZFv2+) shows the closest bond-metric and aromaticity correspondence to parent pentafulvalene (Fv) while remaining isolable and amenable to handling under ambient conditions. The radical cation (IZFv•+) is stable toward air and moisture (t1/2 = 15.2 days), exhibits a distonic electronic structure in which the unpaired spin resides predominantly on the fulvalene core while the positive charge is accommodated primarily by the imidazolium-derived carbon centers, and shows red photoluminescence at 650 nm. To the best of our knowledge, this work provides the first isolation of an oxidatively generated radical cation based on a sulfur-free, unannulated pentafulvalene scaffold and establishes an experimentally accessible platform for examining redox-dependent structural and electronic evolution within the fulvalene π-framework.