Electronically Stabilized Nonplanar Phenalenyl Radical and Its Planar Isomer

Stable
phenalenyl radicals have great potential as the basis for
new materials for applications in the field of molecular electronics.
In particular, electronically stabilized phenalenyl species that do
not require steric shielding are molecules of fundamental interest,
but are notoriously difficult to synthesize. Herein, the synthesis
and characterization of two phenalenyl-type cations is reported: planar
benzo­[i]­naphtho­[2,1,8-mna]­xanthenium
(8+) and helical benzo­[a]­naphtho­[8,1,2-jkl]­xanthenium (9+), which can be
reduced to the corresponding radicals. Radical 9 represents
the first stable, helical phenalenyl radical which does not rely on
bulky substituents to ensure its stability. Both cations are water-soluble,
and the radicals are stable for weeks at room temperature under air.
These compounds were characterized crystallographically, and also
by NMR, EPR, electrochemistry, and electronic spectra. The synthesis
of the previously reported compound benzo­[5,6]­naphthaceno­[1,12,11,10-jklmna]­xanthylium (5+), the largest
oxygen-containing polycyclic hydrocarbon, was undertaken for comparison
with 8+ and 9+, allowing
us to report its crystal structure here for the first time. The different
properties of these compounds and their radicals are explained by
considering their differing aromaticities using in-depth computational
methods.