Reversible σ-Bond Formation in Bowl-Shaped π-Radical Cations: The Effects of Curved and Planar Structures

The reversible formation of sigma-bonds between organic radicals has been widely investigated. However, reports on the formation of sigma-dimers from delocalized pi-radical cations are scarce. Herein, we report the reversible sigma-dimerization behavior of a bowl-shaped pi-radical cation generated from a nitrogen-embedded buckybowl, both in the crystalline state and in solution. The detailed structure of the sigma-dimer in the crystalline state was determined by a single-crystal X-ray diffraction analysis. The monomeric radical cation exists predominantly in solution at room temperature, while dimerization of the radical cations occurs through carbon-carbon sigma-bond formation upon reducing the temperature. H-1 NMR and optical spectroscopy measurements confirmed the formation of a adimer at low temperature. Comparative studies with a similar yet planar pi-conjugated system suggested that the curved structure of the bowl-shaped Jr-radical cation facilitates the sigma-dimerization at one of the internal sp2-hybridized carbon atoms. This trend was also observed for the nucleophilic addition reaction of methanol to the z-radical cations. The methoxylation reaction proceeded only for the curved r-radical cation. Theoretical calculations revealed that the large relief of structural strain at the a carbon atom during the dimerization or nudeophilic addition reactions accelerated the bond formation at the internal carbon atom of the curved radical cation.