B–N–B Bond Embedded Phenalenyl and Its Anions

Despite
the great potential of heteroatom-containing polycyclic
aromatic hydrocarbons in organic optoelectronics, there are very limited
reports on heteroaromatics containing a B–N–B bond in
the π-scaffold. Herein, stable 1,9-dibora-9a-azaphenalenyl (DBAP) derivatives, named BNB-embedded phenalenyls,
are presented. The DBAP skeleton contains a three-center two-π-electron
B–N–B moiety with 12 π-electrons and can be regarded
as the isoelectronic structure of the phenalenyl cation. Chemical
reduction of the phenyl derivative of DBAP by potassium generated
the dianion containing 14 delocalized π-electrons, which can
be regarded as the isoelectronic structure of the phenalenyl anion.
The dianion is sandwiched and stabilized by two bulky [K([18]­crown-6)]+ counterions according to its X-ray structure. However, its
monoanion (an isoelectronic structure of the henalenyl radical) generated
by mixing equal moles of neutral compound and dianion gave an unusual
B–N–B-embedded benzo­[cd]­fluoranthene
dianion, which again was confirmed by X-ray crystallographic analysis.
The new dianion containing 20 π-electrons is highly aromatic
and is further stabilized by [K([18]­crown-6)]+ counterions.
An aromaticity driven rearrangement mechanism was proposed for this
unusual transformation. Based on X-ray structures and theoretical
calculations, the B–N–B moiety in the neutral and anionic
DBAP participates in the π-electron delocalization along the
whole DBAP skeleton like their phenalenyl cation/radical/anion counterparts,
but with more localized character. Therefore, our studies report the
first synthesis and characterization of a B–N–B-embedded
phenalenyl and its anionic species, which show unique electronic structure
and unusual reactivity different from that of their all-carbon phenalenyl
analogues.