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

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