Chemistry Reference
In-Depth Information
R
R
1)
n
BuLi
THF
2) RX
Cr(CO)
3
Cr(CO)
3
Cr(CO)
3
33
34a
:R=D
34b
:R=CH
3
6
-Cr(CO)
3
-complexes of biphenylene [
54
]
Scheme 10 Synthesis of unsymmetric, substituted
Z
6
-
Z
6
)[
54
]or
metal and ligand, the metal atom migrates between either benzene (
Z
4
cases of
IRHR in [N]phenylenes, DFT calculations show that the migration occurs through
mechanism in which the metal atom “walks” the periphery of the PAH.
In the case of the former, the mono-chromium tricarbonyl complex of
biphenylene 33 can be selectively lithiated on the complexed benzene ring and
subsequently substituted with an electrophile to give two non-degenerate ligand
sites on biphenylenes 34a/b (Scheme
10
)[
54
]. The equilibrium of IRHR of these
species was studied by NMR at 130
C. Whereas 34a equilibrates to a ratio of 1:1 of
the two ligand sites after about 20 h, decomposition of the complex competes with
IRHR in 34b and after 56 h equilibrium had still not been reached. The activation
for IRHR in 34a was determined to be 32.2 kcal mol
1
, which expectedly lies
between the analogous values in naphthalene (28.5 kcal mol
1
) and biphenyl
(35.1 kcal mol
1
).
More recently it has been shown that cyclobutadiene-metal complexes of the
longer linear [N]phenylenes can undergo photo-thermal reversible IRHR [
55
,
56
].
Systems such as these have potential for a variety of applications including solar
energy storage and switches. The cyclopentadienyl-cobalt {CpCo} complexes
of substituted linear [3]-, [4]-, and [5]phenylenes can be isolated directly from
the [2
4
-
Z
4
)[
55
,
56
] ligands. In both the
Z
6
-
Z
6
and
Z
4
-
Z
cyclobutadiene (
Z
2] cycloaddition of the appropriate precursor with BTMSA,
under modified conditions employing stoichiometric [CpCo(CO)
2
] and THF as a
co-solvent. As indicated by deshielding in the NMR and calculated NICS values,
complexation of {CpCo} has an aromatizing effect on the entire PAH; however this
is most strong on the metal-bound cyclobutadiene ring.
When irradiated with light [N]phenylene complexes 35-37a undergo IRHR to
give the higher energy haptomers 35-37b (Scheme
11
) reaching a photostationary
state as indicated in Table
4
. The calculated heats of activation for the thermal
reversal of the unsubstituted [N]phenylenes are all very close in value, around
27 kcal mol
1
. However, the exothermicity increases going down the series from
[3]- to [5]phenylene. This trend can be attributed to the decreasing HOMO-LUMO
gap and increasing antiaromaticity along the series, which result
þ
2
þ
in stronger
{CpCo} bonds.
The
-stacking interaction of aromatic materials in the solid state is a key
variable to consider in achieving high charge mobilities for electronic applications.
The [2.2]paracyclophane motif, with an inter-ring distance shorter than the 3.4
˚
necessary for effective
p
-stacking, has been a useful tool for investigating the
effects of stacking on optoelectronic properties [
57
]. In 2005 Leung and coworkers
p
