Chemistry Reference
In-Depth Information
S
S
a
S
S
S
S
S
S
18
SS
SS
17
19
b
3.13
3.19
3.55
0
3.55
X
Z
X
3.22
3.21
3.21
3.55
3.55
3.19
3.13
Y
Z
0
Y
Fig. 5 (a) Structures of phenalenyl radicals with disulfide bridges; (b) single-crystal structures of
17 [
18
-
20
]. Reprinted with permission from [
19
]. Copyright 2007 American Chemical Society
-dimer (3.13-3.22
ยด
)is
even shorter than that of 10, indicating a strong intermolecular interaction. Other
contacts in the lattice, such as S-S interactions between different
p
overlap of SOMOs. The intermolecular distance in this
-dimers, were
also observed (Fig.
5b
), but the dimers remain isolated with electrons trapped
within dimer pairs, which was further demonstrated by the conductivity
measurements revealing that this material was an insulator [
19
]. It is worth noting
that, in spite of the absence of bulky substituents, no
p
-dimerization was formed in
the solid state, showing that extension of spin delocalization indeed provided an
alternative way to stabilize neutral radicals. The attempt to prepare a more spin
delocalized system, a tetrathiophenalenyl 18, was performed in 2008; however, the
reduction of the corresponding cation led to formation of a closed-shell dimer with
an S-S
s
bond, and only a weak ESR signal can be observed in solution [
20
]. The
phenalenyl with three disulfide bridges and a threefold symmetrical molecular
skeleton, a hexathiophenalenyl 19, has not been made so far, but was expected to
show an extensively spin delocalization nature and intriguing magnetic and elec-
tronic properties.
Modifications within the phenalenyl moiety instead of around the phenalenyl
unit were also investigated to stabilize the open-shell system; for example,
incorporation of heteroatoms such as the nitrogen atom is an effective way to
stabilize phenalenyl radical. However, the electronic structure and the spin density
can be altered dramatically due to the higher electron negativity of the nitrogen
atom and the existence of lone pair electrons; thus it is questionable whether they
can be regarded as phenalenyl radicals anymore. On the basis of the position in
which nitrogen is introduced, the nitrogen-containing phenalenyl derivatives can be
divided into two types:
s
a
type and
b
type (Fig.
6
). For
a
type compounds, the spin
