Synthesis and Properties of Benzo [d] dithieno [b, f] borepins

Article abstract of DOI:10.1021/acs.organomet.7b00844

Borepin is a seven-membered unsaturated ring system containing a tricoordinate boron. Although many borepin compounds annulated with aromatic systems have been reported to date, only one [b,d,f]-annulated tetracyclic borepins has been synthesized. In the

Full text of DOI:10.1021/acs.organomet.7b00844

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis and Properties of Benzo[d]dithieno[b,f ]borepins
Yohei Adachi and Joji Ohshita*
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
S Supporting Information
*
ABSTRACT: Borepin is a seven-membered unsaturated ring
system containing a tricoordinate boron. Although many borepin
compounds annulated with aromatic systems have been reported
to date, only one [b,d,f ]-annulated tetracyclic borepins has been
synthesized. In the present study, we synthesized benzo[d]-
dithieno[b,f]borepins as a new class of building blocks for
functional materials motivated by the following structural
features: (1) the high chemical stability of dithieno[b,f ]borepin
species; (2) the highly conjugated tetracyclic system with a
borepin ring; and (3) electronic structures that are easily tuned
by the introduction of functional groups on the annulated
benzene ring. The prepared borepins were stable in air both in
solution and as solids, and the electronic states could be finely tuned by changing the substituents on the benzene ring.
Interestingly, DFT calculations revealed that the LUMO and LUMO+1 energy levels were strongly affected by the benzo[d]-
annulation, depending on the boron-bridged positions on the fused thiophene rings. NICS and HOMA methods proved that the
aromaticity of the borepin ring in the benzo[d]dithieno[b,f ]borepin system is relatively small. We also report an unexpected red
emission observed for the first time in borepin compounds for benzo[d]dithieno[b,f]borepin in the solid state.
INTRODUCTION
the properties of the systems, leading to desired function-
■
12−14
alities.
In this regard, borepin has been studied as a
Borepin, which is an unsaturated seven-membered ring system
with tricoordinate boron, has been investigated because it has
interesting aromaticity. Unlike other heteropins, such as
phosphepin and silepin, the borepin system in its neutral
state has an unoccupied p-orbital on boron that associates with
π and π* orbitals of the heptatriene moiety, leading to an
isoelectronic state with a tropylium cation. This fulfills the
bridging unit of π-electron systems. In 2009, the synthesis and
optical properties of benzo- and naptho-annulated borepins at
the [b] and [f ] CC bonds were reported by Piers et al. (1b−
1
5
d in Chart 1B). Annulation effectively extends the
conjugation while retaining the planar structure. The benzo-
fused compounds 1b and 1c emit in the blue spectrum with
good quantum yields (1b, 70%, and 1c, 39%, in CH Cl ),
2
2
̈
requirements for Huckel aromaticity of 6π electrons, resulting
suggesting potential applications as luminescent and sensor
materials. Tovar and co-workers prepared dibenzo[b,f ]borepins
protected with a phenyl (1e), tripyl (1f), or Mes* (1g) group at
in a planar structure. Much effort has been devoted to
1
theoretical and experimental approaches to examining the
2
−7
nature of borepin aromaticity. For example, Leusink and co-
workers clearly demonstrated the aromaticity of 3-phenyl-
16
the tricoordinate boron (Chart 1C). In contrast to the high
chemical stability of 1a, 1e is unstable in air and must be
handled in a glovebox. Borepin 1f is also unstable under
ambient conditions, despite the bulky tripyl group, and only 1g
with an extremely bulky Mes* protecting group is stable of
these three borepins. These results indicate that annulation
with benzene rings at the [b] and [f ] positions of the borepin
ring may not effectively stabilize the borepin system. In 2014,
Tovar’s group also reported the synthesis of thiophene-
1
benzo[d]borepin 1a (Chart 1A) by H NMR and absorption
2
spectroscopy in 1967. The aromaticity of borepin improves
the chemical stability. Tricoordinate boron species are usually
unstable and rarely able to be handled in air, unless the boron
center is kinetically stabilized by extremely bulky protecting
8
,9
groups, such as 2,4,6-tri-isopropylphenyl (tripyl), 2,4,6-tri-t-
10
butylphenyl (Mes*), and 2,4,6-tris(trifluoromethyl)phenyl
F
10
(
Mes). Despite compound 1a not having a sterically large
17
annulated borepins (1α and 1β in Chart 1D). Surprisingly,
although these compounds possess only a mesityl (2,4,6-
trimethylphenyl) group, which is less bulky than the tripyl and
Mes* groups, they are reasonably stable for several months
under ambient conditions, indicating the large stabilizing effect
of the thiophene annulation. In addition to these reports, many
group on the boron center, it was stable in air for several days in
the solid state, strongly implying that the aromaticity stabilized
the tricoordinate boron center.
2
Currently, π-electron systems bridged by nonconventional
heteroatoms, such as boron, silicon, germanium, and
phosphorus, have received much attention as a new strategy
to develop building units for π-conjugated functional
1
1
materials. Orbital interactions and/or electronic and struc-
tural perturbations by heteroatoms make it possible to control
Received: November 23, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00844
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Chart 1. Reported Structures of Borepins Fused with
Aromatic Systems
aromaticity. In addition, the detailed properties of 1h, including
the crystal structure and electrochemical properties have not
been reported. Therefore, the fundamental nature of [b,d,f ]-
annulated tetracyclic borepins has not been elucidated. In this
work, we introduced benzo-annulation at the [d] position of
dithieno[b,f ]borepin with a mesityl group on the boron as a
protecting group. We investigated the effects of benzo[d]-
annulation on the stability, electronic states, aromaticity, and
photophysical properties of the dithieno[b,f ]borepins. We
expected that introducing functional groups on the annulated
benzene ring at the borepin [d] position would be an effective
way to finely tune the electronic state of the borepin system. It
was also anticipated that extending the π-conjugated system at
the [d] position would enhance the π−π intermolecular
interaction in the condensed phase, which may result in
optoelectronic properties such as semiconducting behaviors in
the solid state. Benzo[d]dithieno[b,f ]borepins with a mesityl
group were prepared (Chart 1F), which showed high chemical
stability for several months in the solid state and for several
weeks in solution. The optical and electrochemical properties
were examined in detail and are discussed, along with computer
simulation results. The crystal structures were solved by single-
crystal X-ray diffraction analysis. We also report an unexpected
red emission in the crystal state.
RESULTS AND DISCUSSION
■
Synthesis. The synthetic route toward tetracyclic benzo-
[d]dithieno[b,f ]borepins is shown in Scheme 1 and included
dilithiation of the corresponding bis(bromothienyl)benzenes,
other borepin compounds annulated with aromatic systems,
2
,3,15,16,18a−d,19,21
5,6,17,18d,e,21
including benzene,
thiophene,
ben-
1
8f
7
15,18d,20
20
zothiophene, pyrrole, naphthalene,
and acenaphthylene have been reported. However, in most
acenaphthene,
followed by quenching with MesB(OMe) in ether in moderate
2
20
yields. The reaction solvent seems to be very important because
reports, the annulated borepins were [b,f ]-annulated tricyclic or
Lee et al. reported that a similar synthetic protocol in THF was
unsuccessful in the synthesis of the β-OMe compound.
21
[d]-annulated bicyclic systems. To our knowledge, no [b,d,f]-
annulated tetracyclic borepins have been reported, except for
Various groups (H, F, Me, and OMe) were introduced at the
4,5-positions on the annulated benzene ring. Trimethylsilyl
groups were introduced to the external α-positions of the
thiophene rings in the expectation that the chemical stability
21
borinic acid compound 1h (Chart 1E). In borepin 1h,
electron donation from the adjacent oxygen diminishes the
vacancy of the boron p-orbital, thereby suppressing the
Scheme 1. Synthetic Route toward Benzo[d]dithieno[b,f]borepins
B
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would be improved by kinetic stabilization of the borepin ring
and capping of the reactive thiophene α-positions. We also
attempted the synthesis of β-bridged benzo[d]dithieno[b,f]-
borepins possessing H or F atoms on the annulated benzene
ring. However, the attempted tetra-bromination of 1,2-bis(2-
thienyl)benzene with 4 equiv of N-bromosuccinimide (NBS)
provided a complex mixture, and the similar tetra-bromination
of 1,2-di(2-thienyl)-4,5-difluorobenzene gave the α-dibromi-
nated compound as the sole isolable product. The dilithiations
of 2-F and 2-OMe with lithium diisopropylamide (LDA) were
unsuccessful, providing complex mixtures, and the correspond-
ing trimethylsilyl-substituted bis(bromothienyl)benzene pre-
cursors could not be prepared. Therefore, unlike compounds α-
H, α-Me, β-Me, and β-OMe, trimethylsilyl groups were
introduced onto compounds α-F and α-OMe after forming
the borepin ring. Indeed, compound α-F was prepared via
dilithiation of α-F-H with tBuLi followed by silylation.
Compound α-OMe was synthesized via lithiation with LDA,
because α-OMe-H was not able to be completely lithiated by
tBuLi at low temperature. All the prepared benzo[d]borepin
compounds (α-H, α-F, α-Me, α-OMe, β-Me, and β-OMe)
were identified by NMR (Figure S1−S55) and high-resolution
mass spectrometry. The compounds were stable under ambient
atmospheric conditions in the solid state for at least 3 months,
1
and no decomposition was observed in the H NMR spectra.
This high chemical stability is likely to be derived from the
dithieno[b,f ]- and benzo[d]-annulations.
To investigate the stability in solution, we monitored the H
1
NMR spectral changes in CDCl . The NMR sample sealed by a
3
Teflon tape with air was kept at room temperature under
ambient light. As shown in Figures 1 and S56−S71, the α-
bridged compounds are clearly more stable than the β-bridged
compounds. In the spectra of β-bridged compounds, the proton
signals from the mesityl groups (H in Figure 1) were gradually
c
decreased, and many unidentified peaks appeared after 1
month. From the integration ratios, approximately 70% of β-
Me and 60% of β-OMe were decomposed after 1 month. In
contrast, in the spectra of α-bridged compounds, the
1
Figure 1. Aromatic regions of the H NMR spectra of α-Me, α-OMe,
β-Me, and β-OMe; initial, after 2 weeks, and after 1 month in CDCl
.
3
The integration ratio is given relative to the initial signal. The residual
CHCl peak was used as the integration standard.
integrations of H protons of α-Me and α-OMe remained at
3
c
about 90%, and no dramatic changes were observed even after
Chart 2. Structural Representation of the Present Borepins
1
month under ambient conditions. These results indicated that
α-bridged borepins possessed quite high chemical stability,
making their application in the synthesis of functional materials
feasible. Compound α-F showed relatively low stability
compared with the other α-bridged borepins, and the stability
was lower than that of α-F-H (Figures S58−S60), although we
had expected that the bulky trimethylsilyl group may kinetically
stabilize the borepin ring.
Crystal Structures. Fortunately, we obtained crystals of the
benzo[d]dithieno[b,f ]borepins suitable for single-crystal X-ray
diffraction analysis from appropriate solvents, except for α-
OMe. Numbering the carbons in the present borepin structures
is shown in Chart 2. Figure 2 shows the crystal structures of α-
H, α-Me, and β-Me as typical examples. Those of the other
borepins are available in the Supporting Information (Figures
S72 and S73). Selected bond lengths and dihedral angles are
summarized in Table 1. The endocyclic dihedral angles ranged
from 10.3 to 20.6° for the α-bridged compounds, indicating a
lower degree of planarity of the borepin rings, relative to the β-
bridged compounds that had dihedral angles of 3.6−9.5°. The
benzene and thiophene rings were over and below the borepin
ring plane (Figure 2). The lower planarity of the α-borepins
compared with the β-borepins was likely due to steric repulsion
between the phenylene and thiophene C−H bonds. As shown
in Figures 2 and S73, β-Me and β-OMe have π-stacked packing
crystal structures with π−π distances of 3.621 Å for β-Me and
3.686 Å for β-OMe. As no clear π−π interaction was observed
in the crystal structure of 1β, the extended π-conjugation by
the benzo[d]-annulation seemed to enhance π−π interaction in
this system. In contrast, the α-bridged borepins (α-H, α-F, and
α-Me) did not exhibit π-stacking in the crystal structures. All
endocyclic B−C3 and B−C12 bonds of the synthesized
borepins were shorter than the exocyclic B−C15 (Mes)
bonds (Table 1), indicating delocalization of the π-electrons
17
C
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Figure 2. Crystal structures and packing diagrams of α-H (left), α-Me
(
middle), and β-Me (right) obtained at 123 K. Thermal ellipsoids are
at the 50% probability level. Hydrogen atoms are omitted for clarity.
Figure 3. Considerable competing factors affecting the photophysical
properties of the present borepins.
in the borepin rings. However, these endocyclic B−C bonds are
slightly longer than those of 1α (1.522(2) and 1.521(2) Å) and
difference essentially gives rise to smaller HOMO−LUMO gaps
for the β-bridged compounds. Indeed, 1β has a longer
wavelength at the absorption onset (390 nm) than that of 1α
17
1
β (1.533(2) and 1.538(2) Å). This difference suggests the
tetracyclic borepins have relatively little aromaticity compared
with 1α and 1β, likely because of the high aromaticity of the
annulated benzene rings, which suppresses the aromaticity of
the borepin rings. Similar electronic confinement has also been
17
(378 nm) in CHCl solution. Second, the planarity of the
3
compounds has an effect (Figure 3B). As we found in the
single-crystal structures of the borepins, the planarity changes
depending on the bridging positions; for example, β-Me
exhibited higher planarity than α-Me. Finally, benzo-[d]
annulation usually extends the conjugation to minimize the
HOMO−LUMO energy gap (Figure 3C). Benzo-[d] annulated
22
reported for phosphepin derivatives. Phosphepins annulated
with a nonaromatic ring at the [d] position showed more
extended conjugation than phosphepins annulated with an
aromatic ring at the same position. As 1β was reported to have
a highly planar structure, the relatively low planarity of β-Me
and β-OMe, which have no steric repulsion between the
phenylene and thiophene C−H bonds, can be explained by the
low aromaticity of the borepin ring derived from the annulation
of a highly aromatic benzene ring. Similar deviation of planarity
induced by benzo[d]-annulation was theoretically predicted in
23
borepin has similar Clar structure to triphenylene, suggesting
that the borepin ring in benzo-[d] annulated system has weak
24
aromaticity similar to the center ring in triphenylene (Figure
3C), which also exerts influence on the electronic states of the
whole systems. Similar effects of Clar structure on fused
18f
borepins have been recently reported.
Furthermore, the
1d
dibenzo[b,f ]borepin species.
donor−acceptor (D−A) interaction through the benzo[d]-
borepin core may affect the electronic states.
The absorption spectra of the borepins in THF are shown in
Figure 4. All spectra possessed multiple bands with shoulder
peaks in the region of 250−430 nm. For the α-bridged
compounds, the absorption edge of α-F was at almost the same
energy as that of α-H, whereas α-Me and α-OMe had red-
Optical Properties. Three important factors affect the
borepin electronic states (Figure 3). First, β-bridged borepins
have effective π-conjugation between the two thiophene rings
through the α,α’-ethenylene linkage, whereas α-bridged
borepins with a β,β’-ethenylene linkage have less effective
conjugation between the thiophene rings (Figure 3A). This
Table 1. Selected Bond Lengths and Dihedral Angels Obtained from X-ray Analysis
bond length (Å)
dihedral angle (deg)
B−C3, B−C12
C3−C4, C11−C12
C4−C5, C10−C11
C5−C10
B−C15
B−C3C4−C5, B−C12C11−C10
α-H
1.514(3), 1.531(2)
1.532(2), 1.524(2)
1.525(3), 1.530(3)
1.533(3), 1.543(2)
1.534(3), 1.532(3)
1.407(2), 1.410(2)
1.399(2), 1.397(2)
1.399(2), 1.399(2)
1.400(2), 1.400(3)
1.398(2), 1.400(2)
1.478(2), 1.475(3)
1.475(2), 1.478(2)
1.472(3), 1.471(3)
1.465(2), 1.460(3)
1.466(2), 1.466(3)
1.426(3)
1.420(2)
1.428(2)
1.419(3)
1.417(3)
1.591(2)
1.579(2)
1.586(3)
1.589(3)
1.592(2)
10.9, 13.4
10.3, 20.6
16.3, 15.4
9.5, 4.7
α-F
α-Me
β-Me
β-OMe
4.7, 3.6
D
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obtained from X-ray diffraction analysis. The fluorescence
quantum yields were low, at 2% for β-Me and too weak to
determine (<2%) for the other borepins (Table 2). Similar
Table 2. Optical and Electrochemical Properties of
Benzo[d]-Annulated Borepins
λabs
Egopt
λem
τ
Ered
Eox
f
onset
(nm)
max
c
a
b
d
e
sample
(eV)
(nm)
Φ (%) (ns)
(V)
(V)
α-H
381
379
390
415
399
407
3.25
3.27
3.18
2.99
3.11
3.05
383
383
392
432
400
409
g
g
g
g
2
g
0.25
−2.08
−2.03
−2.13
−2.14
−2.24
−2.26
0.90
h
α-F
0.21
0.30
0.22
1.01
α-Me
α-OMe
β-Me
β-OMe
0.94
0.73
0.98
0.79
Figure 4. Absorption spectra of benzo[d]-annulated borepins in THF.
0.46
b
a
Absorption onset in 0.02 mM THF solution. Egopt (eV) = 1240/
shifted edges compared with those of α-H and α-F. Borepins α-
OMe and β-OMe, with electron-donating methoxy groups,
showed relatively red-shifted absorptions compared with those
of other borepins. The onset wavelengths of the absorptions
were shifted to longer wavelengths in the order of α-F ≤ α-H <
α-Me < β-Me < β-OMe < α-OMe. As the spectra varied
depending on the substituents on the benzene ring, it appears
that the substituents on the benzene ring can tune the
electronic structure of the whole borepin system. We predicted
that β-bridged borepins would show more red-shifted
absorptions compared with α-bridged compounds because of
abs
c
λ
(nm) In 0.02 mM THF solution, excited at 340 nm.
onset
d
e
Emission lifetime, excited at 318 nm. Reductive potential (vs Fc/
+
f
Fc ) obtained from the onset of cathodic wave. Oxidative potential
+
g
(vs Fc/Fc ) obtained from the onset of anodic wave. Too weak to
determine. Not detected.
h
barely emissive properties in solution have been reported for
17
1α (Φ = 5%) and 1β (Φ = 6%). The fluorescence lifetimes
were quite short, ranging from 0.21 to 1.01 ns. As compounds
α-OMe and β-OMe possessed electron-donating methoxy
groups with the electron-deficient borepin system, a D−A
interaction may be induced. We measured the fluorescence
spectra of α-OMe and β-OMe in toluene and DMF in addition
to THF (Figure S74). The spectra of α-OMe were shifted to
longer wavelengths as the solvent polarity increased, which was
likely derived from the intramolecular charge transfer (ICT)
character of the photoexited state, indicating an intramolecular
D−A interaction. In contrast, the spectra of β-OMe were
broadened in polar solvents; however, the fluorescence bands
were observed at similar wavelengths regardless of the solvent
polarity. This indicates that the electronic interaction between
the annulated benzene moiety and the dithieno[b,f]borepin
system is very different depending on the bridging position.
Even though the benzo[d]-annulated borepins have potentially
more extended conjugation than 1α and 1β from the benzo[d]-
annulation (Figure 3C), the red-shifts of absorption edge and
fluorescence wavelengths were slight, which was probably due
to competition between the lower planarity and higher π-
extension, which balance each other out. The trimethylsilyl
groups might also contribute to the slight red-shift. Indeed, the
absorption edges of α-F-H (372 nm) and α-OMe-H (406 nm)
in THF were slightly blue-shifted from those of the
corresponding silylated borepins α-F and α-OMe.
the electronic/structural features (Figure 3A,B). However, the
opt
estimated HOMO−LUMO energy gaps (E ) for the α-Me
g
and β-Me pair, as well as for α-OMe and β-OMe, were almost
the same. This indicated that the effect of the benzo-[d]
annulation on the energy levels strongly depended on the
bridging position of the thiophene rings. This is discussed in
detail later in the section on density functional theory (DFT)
calculations (vide infra).
The fluorescence spectra of the borepins in THF are shown
in Figure 5. Similar to the absorption spectra, the fluorescence
DFT Calculations and Evaluations of Aromaticity. To
further investigate the electronic structures of the tetracyclic
borepins, we carried out DFT calculations at the B3LYP/6-
31G(d) level of theory on the Gaussian 09 program. We also
performed calculations for the model compounds 1α-Si and
1β-Si without benzo[d]-annulation for comparison (Figure 6).
The calculated HOMO and LUMO energy levels and the
dihedral angles of the optimized structures are summarized in
Table 3. Similar to the crystal structures, the optimized
structures possessed nonplanar benzo[d]-annulated borepin
systems, whereas 1α-Si and 1β-Si were completely planar
(Table 3). α-Me and α-OMe had higher HOMO energy levels,
while α-F had a lower LUMO level, than those of α-H. These
energy shifts clearly reflected the electronic properties of the
substituents, that is, electron-donating methyl and methoxy
Figure 5. Fluorescence spectra of benzo[d]-annulated borepins in
THF.
spectra of α-H and α-F were similar, but the spectra for α-Me
and α-OMe showed bands in a lower energy region. The
maximum wavelengths of fluorescence were shifted to longer
wavelengths in the order of α-H = α-F < α-Me < β-Me < β-
OMe < α-OMe, which is almost consistent with the order of
the absorption edge. In the spectra of β-Me and β-OMe,
vibronic bands were clearly observed, likely because of the
higher structural rigidity of the β-bridged borepins compared
with the α-bridged compounds. This consideration is consistent
with the highly planar structure of the β-bridged borepins
E
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Figure 6. Frontier orbitals of the borepin core and dithieno[b,f ]borepins calculated from DFT at B3LYP/6-31G(d) level of theory. HOMO−1 is
shown for α-H and α-F because the HOMOs are localized on the mesityl group.
Table 3. Calculated Parameters of Dithieno[b,f]borepins
a
b
b
b
c
Egtheo (eV)d
dihedral angle (deg)
HOMO (eV)
LUMO (eV)
LUMO+1 (eV)
ΔE(L−L+1) (eV)
1
α-Si
0.0
15.9
15.3
15.4
15.1
0.0
−5.68
−5.86
−5.96
−5.69
−5.32
−5.53
−5.54
−5.28
−1.60
−1.81
−1.96
−1.74
−1.68
−1.81
−1.49
−1.40
−1.24
−0.94
−1.10
−0.86
−0.76
−1.09
−1.46
−1.39
0.36
0.87
0.87
0.87
0.92
0.72
0.04
0.01
4.08
4.05
4.00
3.95
3.63
3.72
4.05
3.88
e
α-H
e
α-F
α-Me
α-OMe
1
β-Si
β-Me
10.5
10.4
β-OMe
a
b
c
Between B−C3C4−C5 as shown in Table 1 and Chart 2. DFT calculated at B3LYP/6-31G(d) level. Energy gap between LUMO and LUMO
d
+
1. Energy gap between HOMO and LUMO. HOMO−1 is used for α-H and α-F because the HOMOs are localized on the mesityl group.
e
HOMO−1 because the HOMO is localized on the mesityl group.
substituents and electron-withdrawing fluorine atoms. The
bridged borepins and β-Me had π*(B ) symmetry, whereas
1
theo
calculated HOMO−LUMO energy gap (E ) of 1β-Si was
π*(A ) symmetry was observed in the LUMO of 1β-Si and β-
g
2
smaller than that of 1α-Si, which derived from the conjugated
thiophene system of 1β-Si (Figure 3A). In contrast, despite the
OMe. Meanwhile, the LUMO+1s of the borepins had opposite
symmetry (π*(A ) or π*(B )) to the LUMOs. This indicated
2
1
effective conjugation and higher planarity of the β-bridged
that the LUMO and LUMO+1 of the dithieno[b,f ]borepins
were composed of the π*(A ) or π*(B ) symmetry of the
theo
compounds, the Eg
values for α-Me and β-Me and for α-
2
1
OMe and β-OMe, were almost the same. These calculated
borepin core and that these energy levels were strongly
influenced by the bridged position (α- or β-) and benzo[d]-
annulation. As compared with the benzo[d]-annulated or
nonannulated compounds, the LUMO energy levels of the α-
bridged borepins were clearly lowered by benzo[d]-annulation,
whereas the LUMO+1 energy levels were raised. In contrast,
the LUMO energy levels of the β-bridged compounds were
elevated by benzo[d]-annulation, and LUMO+1 energy levels
results are consistent with the experimentally estimated energy
opt
gaps (Eg ), as discussed above. The calculated frontier orbitals
are shown in Figure 6. The HOMOs of the dithieno[b,f]-
borepins were similar regardless of the bridged position and
benzo[d]-annulation. In contrast, the LUMOs could be
classified into two orbital symmetries based on π*(A ) and
2
π*(B ) orbitals of the borepin core. The LUMO of the α-
1
F
DOI: 10.1021/acs.organomet.7b00844
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Article
were lowered. These LUMO and LUMO+1 energy shifts are
clearly shown by the energy differences between the LUMO
and LUMO+1 (ΔE(L−L+1)) in Table 3. For example, as a result
energy gaps to the β-bridged compounds. It was also noted that
the π*(B ) orbital clearly involved the vacant tricoordinate
1
boron p-orbital, while the π*(A ) did not. As the boron p-
2
of these energy shifts, β-Me and β-OMe had ΔE
values
orbital contributed less to the HOMO than the π*(B ) orbital,
(L−L+1)
1
of only 0.04 and 0.01, respectively. When we compared these
energy shifts with the orbital symmetry, we found that
benzo[d]-annulation effectively lowered the energy level of
the π*(B ) orbital played a role in the D−A interaction, and the
1
HOMO−π*(B ) transition seemed to be responsible for the
1
ICT state of the benzoborepins. This agreed with the fact that
the π*(B ) symmetry orbital, whereas annulation raised the
α-OMe with π*(B ) as the LUMO showed a solvatochromic
1
1
π*(A ) symmetry orbital energy level. To elucidate the effects
PL spectral feature, in contrast to β-OMe. Compound β-OMe
had nearly degenerate π*(B ) and π*(A ) orbitals and did not
2
of benzo[d]-annulation on the LUMO and LUMO+1 energy
levels, we performed DFT calculations on the simple
benzo[d]borepin compound without thiophene annulations
1
2
show solvatochromic behavior.
Next, we carried out nucleus-independent chemical shift
(NICS) calculations to evaluate the aromaticity of the borepin
ring in the tetracyclic borepin systems. The NICS calculations
were performed at B3LYP/6-31+G(d,p) level of theory. First,
we computed NICS based on the optimized structures from the
(Figure 7). As predicted, in the benzo[d]borepin system, the
theo
theo
DFT calculations (NICS(±1) and NICS(0) in Tables 4
theo
and S1, respectively). The NICS(1) values of 1α-Si and 1β-
Si were negative enough to presume that the borepin ring
theo
possess certain aromaticity. However, the NICS(±1) values
of the tetracyclic borepins were obviously more positive than
those of 1α-Si and 1β-Si. We then performed NICS
calculations based on the crystal structures obtained from the
exp
exp
X-ray diffraction studies (NICS(±1) and NICS(0) in
exp
Tables 4 and S1, respectively). The calculated NICS(±1)
values were very similar to the corresponding NICS(±1)
theo
.
The NICS(±1) values depended on the side of the borepin
plane the ghost atom was placed on (NICS(+1) and
NICS(−1)) (Figure 8). This is due to the nonplanarity of
Figure 7. Energy diagrams of 3H-benzo[d]borepin calculated from the
DFT at B3LYP/6-31G(d) level of theory.
π*(B ) symmetry orbital (LUMO) was stabilized, and the
1
π*(A ) symmetry orbital (LUMO+1) was destabilized from the
2
original borepin core π* orbitals, which was consistent with the
results of calculations for the present dithieno[b,f ]borepin
systems. As 1α-Si and 1β-Si possess π*(B ) and π*(A )-based
Figure 8. Schematic representation of NICS(±1).
1
2
LUMOs, respectively, the benzo[d]-annulation provided
effective stabilization of the π*(B )-based LUMO for the α-
the tetracyclic borepin rings. However, the differences were
1
bridged borepins, whereas annulation destabilized the π*(A )-
small and nearly negligible. All the NICS values were only
2
theo
based LUMO for the β-bridged borepins. As a result of
competition between the effects of the bridged position, the
planarity, and the orbital symmetry of the LUMO and LUMO
slightly negative, at most −3.80 ppm for NICS(−1)
of β-
OMe, indicating their weak aromaticity. To confirm this lack of
aromaticity, we estimated the harmonic oscillator model of
aromaticity (HOMA) values using the bond lengths in the
+1, the α-bridged borepins showed similar HOMO−LUMO
Table 4. Calculated Parameters of Dithieno[b,f]borepins
NICS(+1)theo (δ, ppm)a
,
c
NICS(−1) (δ, ppm) ,
theo
ad
NICS(+1)exp (δ, ppm) ,
bc
NICS(−1) (δ, ppm) ,
exp
bd
HOMAtheo
a
HOMAexp
b
f
1
α-Si
−4.89
−1.69
−2.19
−2.41
−2.65
−4.51
−2.62
−2.91
e
0.59
0.45
0.45
0.46
0.48
0.54
0.44
0.46
(0.63)
α-H
−2.72
−3.23
−3.30
−3.59
e
−2.40
−1.66
−2.11
−2.53
−2.28
−2.66
0.50
0.48
0.50
α-F
α-Me
α-OMe
f
1β-Si
(0.57)
β-Me
−3.38
−3.80
−2.34
−3.17
−3.37
−3.15
0.47
0.49
β-OMe
a
b
c
Based on optimized structures from DFT calculations with respect to the borepin ring. Based on crystal structures. Ghost atom located at 1 Å
d
from the borepin ring center at the same direction of the mesityl group on boron. DFT calculated at B3LYP/6-31+G(d,p) level. Ghost atom
located at 1 Å from the borepin ring center at the opposite direction of the mesityl group on boron. DFT calculated at B3LYP/6-31+G(d,p) level.
e
f
17
The same with NICS(+1) because of the planar structure. Estimated based on the X-ray data of 1α and 1β that possess no trimethylsilyl groups.
G
DOI: 10.1021/acs.organomet.7b00844
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Organometallics
Article
borepin ring. The “optimal” bond length (R ) in a fully
opt
aromatic system can be estimated from eq 1 using the single
(
R ) and double (R ) bond lengths. The concept of the HOMA
s d
method is based on the difference between Ropt and the
corresponding bond lengths in real compounds or calculated
structures (R ), as shown in eqs 1−3:
i
Ropt = (R + ωR )/(1 + ω)
s
d
(1)
(2)
2
2 −1
α = 2[(Ropt − R ) + (R − R ) ]
s
opt
d
1
N
2
HOMA = 1 −
∑
α(Ropt − Ri)
(3)
i
where ω is taken as 2, N is the number of bonds in the cyclic
2
5
system, and α is a constant for each bond type. In general,
Figure 9. Cathodic CVs of benzo[d]-annulated borepins in DMF with
HOMA values over 0.5 indicate aromatic, values between 0 and
.5 indicate nonaromatic systems, and negative values indicate
0
.1 M tetrabutylammonium perchlorate as supporting electrolyte at a
−
1
0
scan rate of 100 mV s .
antiaromatic systems. First, we calculated the HOMA values of
the parent rings. On the basis of the optimized structures at
B3LYP/6-31G(d) level of theory, the HOMA values of 1H-
borepin, thiophene, and benzene were obtained as 0.65, 0.88,
and 0.99, respectively, indicating the relatively weak aromatic
character of borepin. Then, we estimated HOMA values using
order of α-F > α-H > α-Me > α-OMe > β-Me > β-OMe,
reflecting the electron-donating or electron-withdrawing
properties of the substituents. Anodic CVs were also
investigated. As shown in Figure S75, irreversible oxidative
waves were detected for most samples, but not for α-F. The
oxidative onset potentials Eox were lowered with the
introduction of electron-donating methoxy groups, again
implying efficient control of the electronic state by the nature
of the substituents.
Emission Properties in the Crystal State. Finally, we
investigated the emissive properties of tetracyclic borepins in
the solid state. The recrystallized samples were barely emissive
(Φ < 2%), similar to the situation in solution, except for α-H
(Φ = 18%) and β-OMe (Φ = 6%), and the emission color was
blue except for α-H (Figures 10 and 11). To our surprise, the
the optimized structures of the borepin rings in the present tri/
theo
tetracyclic systems (HOMA
in Table 4). These values of
benzo[d]-annulated borepins were nearly 0.5 and clearly
smaller than those of 1α-Si and 1β-Si as well as the parent
borepin. We also estimated HOMA values from the crystal
exp
structures (HOMA ). Those of 1α and 1β were also listed in
parentheses in Table 4 for comparison. Tendency of the
exp
changes of HOMA
values of tri/tetracyclic borepins,
theo
including 1α and 1β was consistent with that of HOMA
.
This clearly suggests the reliability of those data, regardless of
what structures were used, i.e., theoretical or experimental. The
exp
HOMA values of benzo[d]-annulated borepins were nearly
0
.5, again indicating their nonaromaticity. The HOMA values
of the fused thiphene and benzene rings in the tri/tetracyclic
systems were highly aromatic and are close to those of the
parent rings (Table S2), consistent with the expectation based
on the Clar structures (Figure 3C). On the basis of the NICS
and HOMA estimations, it is concluded that the tetracyclic
borepins is nearly “nonaromatic”. Taking the short endocyclic
B−C bond lengths in the borepin ring and the observed high
chemical stability of the compounds into account, the borepin
ring in the present system is likely to have aromaticity to a
certain extent, although the aromaticity of the tetracyclic
borepins is less than that of 1α-Si/1α and 1β-Si/1β based on
the NICS and HOMA values as mentioned above.
Figure 10. Fluorescence spectra of borepins in this study as solids.
Electrochemical Properties. We carried out cyclic
voltammetry measurements (CVs) to evaluate the redox
properties of the tetracyclic borepins in DMF containing
tetrabutylammonium perchlorate (0.1 M) under nitrogen. All
borepins showed pseudo- or irreversible reductive waves in
(excitation: 370 nm).
cathodic CVs, and their onset potentials (E ) ranged from
red
+
−
2.03 to −2.26 V vs Fc/Fc (Figure 9, Table 2). The E
red
values of the α-bridged borepins were higher than those of the
β-bridged borepins, indicating their greater electron-accepting
properties. This contrasted with the relationship between 1α
and 1β, where the half-wave reduction potentials E1
Ag/Ag ) of 1α and 1β were −2.42 and −2.26 V, respectively,
but was consistent with the order of the calculated energy levels
(vs
/2 red
+
17
Figure 11. Photographs of borepins in this study under ambient (top)
(vide supra). The Ered values shifted to lower potentials in the
and UV (bottom) light.
H
DOI: 10.1021/acs.organomet.7b00844
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Article
emission spectrum of α-H in the crystal state contained broad
bands in the 550−700 nm region, giving a reddish emission. To
our knowledge, such a phenomenon has not been previously
observed for borepin derivatives. To elucidate the reason for
this unusual red emission, we prepared an amorphous sample
of α-H by heating it in air on a hot plate at 200 °C, followed by
quickly cooling the melted sample to room temperature. The
amorphous form was confirmed by powder XRD analysis
spectrometers. BF
3
·Et
2
O and CF
3
COOH were used as the external
standards for 11B and F NMR measurements, respectively. High-
resolution mass spectra were obtained on a Thermo Fisher Scientific
LTQ Orbitrap XL spectrometer at N-BARD, Hiroshima University.
X-ray Diffraction Analysis. Single crystal X-ray diffraction data of
α-F and α-OMe were collected at 123 K on a Bruker AXS SMART
APEX II ULTRA diffractometer at Natural Science Center for Basic
Research and Development (N-BARD), Hiroshima University, using
Mo Kα radiation monochromated with a multilayered confocal mirror.
The structures were solved by Intrinsic Phasing on the SHELXT-
2014/4 program and expanded using Fourier techniques. Non-
hydrogen atoms were refined anisotropically, whereas hydrogen atoms
were included but not refined (SHELXL-2014/6). All other
calculations were performed using the APEXII crystallographic
software package of Bruker AXS. The data of α-H, α-Me, and β-
Me, in contrast, were collected at 123 K on a Rigaku R-AXIS RAPID
diffractometer using graphite-monochromated Mo Kα radiation. The
structures were solved by Direct Method (SIR92) and expanded using
Fourier techniques. Non-hydrogen atoms were refined anisotropically,
whereas hydrogen atoms were refined using the riding model by the
full-matrix least-squares method. Graphical crystal structures were
generated using Mercury 3.9 (Cambridge Crystallographic Data
Centre). All other calculations were performed using the CRYSTAL-
STRUCTURE crystallographic software package of the Rigaku
Corporation. Powder XRD patterns were obtained on a Rigaku
Ultima IV multipurpose X-ray diffraction system.
19
(
Figure S76). As shown in Figure 10, the red emission at
approximately 600 nm was absent in the spectrum of the
amorphous sample of α-H. This indicated that crystal packing
of α-H is required for the red emission. As described above, α-
H has no π−π intermolecular interactions in the crystal
structure (Figure 2). Therefore, this red emission is not likely
to be due to excimer formation. Despite our attempts, we have
not yet obtained sufficient experimental data to discuss this
phenomenon in detail. Further investigations to determine the
reason for this unexpected red emission are in progress and will
be reported elsewhere.
CONCLUSIONS
■
Herein, we have reported the synthesis and properties of new
benzo[d]dithieno[b,f ]borepins. Benzo[d]-annulation slightly
increased deviations from planarity in the systems because of
the high aromaticity of the benzene ring. As a consequence, the
aromaticity of the borepin ring in the present systems was
relatively lower than that of 1α and 1β, based on the results of
our experimental and theoretical measurements. Despite the
low aromaticity of the borepin ring, these compounds were
quite stable under ambient conditions, both in solution and in
solid form, especially the α-bridged borepins. Furthermore, the
electrochemistry of the whole structure was influenced by
substituents on the fused benzene ring, as indicated by optical
and electrochemical measurements. This influence suggested
that the introduction of functional groups on the benzene ring
is an effective way to control the electronic structures of
benzo[d]dithieno[b,f ]borepin systems. Moreover, the β-
bridged β-Me and β-OMe compounds showed π-stacking in
the crystal structures, derived from the π-extension. This result
indicated that benzo[d]-annulation can affect not only the
electronic structure but also the packing form in the crystal
structure. Finally, we reported the first example of the chromic
red emission of a borepin derivative, as observed for α-H in the
crystal state. On the basis of the high chemical stability, tunable
electronic states, and interesting emissive properties, benzo-
Photophysical Measurements. UV−vis absorption spectra were
measured with a Shimadzu UV-3600 plus spectrometer. Photo-
luminescence (PL) spectra were measured with a HORIBA
FluoroMax-4 spectrophotometer. The absolute PL quantum yields
were determined by using a HORIBA FluoroMax-4 spectrophotom-
eter attached to an integration sphere. Fluorescence decay measure-
ments were performed on a HORIBA DeltaFlex modular fluorescence
lifetime system, using a Nano LED pulsed diode excitation source.
Electrochemical Measurements. CVs were measured with a
HOKUTO DENKO electrochemical measurement system HZ-7000
in a solution of 0.1 M tetrabutylammonium perchlorate (TBAP) in
DMF at analyte concentrations of 2.0 mM using a three-electrode
system with a Pt plate counter electrode, a Pt wire working electrode,
+
and an Ag/Ag reference electrode.
Computational Methods. DFT calculations were performed on a
32
Gaussian 09 program at B3LYP/6-31G(d) level of theory. NICS
nucleus-independent chemical shift) calculations were carried out
(
using GIAO method at B3LYP/6-31+G(d,p) level of theory. To
estimate HOMA values, we employed HOMHED indexes for C−C
2
5b
and C−S bonds, whereas an experimental HOMA index was used
2
5a
for C−B bond.
1,2-Bis(3-thienyl)-4,5-difluorobenzene (1-F). A mixture of 17.9
g (47.9 mmol) of 3-tributylstannylthiophene, 4.34 g (16.0 mmol) of
1
,2-dibromo-4,5-difluorobenzene, 559 mg (0.484 mmol) of Pd-
(PPh ) , and 40 mL of toluene was heated to reflux for 2 days. The
[d]dithieno[b,f]borepins are highly promising as a new class of
3 4
mixture was cooled to room temperature and poured into a KF
aqueous solution with stirring. After 3 h, the resulting suspension was
filtered with Celite and washed with toluene. The organic phase was
separated and washed twice with water and then once with brine. After
drying the organic phase over anhydrous magnesium sulfate, the
solvent was evaporated and the residue was purified by silica gel
chromatography with hexane as the eluent to give 4.33 g (15.5 mmol,
97% yield) of 1-F as a white solid. H NMR (400 MHz, CDCl
7.24 (2H, t, JH−F = 9.6 Hz, Bz), 7.19 (2H, dd, J = 3.0 and 5.1 Hz, Th),
building blocks for functional materials.
EXPERIMENTAL SECTION
■
General. All reactions were carried out under dry argon. Toluene,
THF, and diethyl ether were purchased from Kanto Chemical Co.,
Ltd. and were distilled from calcium hydride and stored over activated
molecular sieves under argon until use, whereas chloroform was
distilled from calcium hydride immediately before use. All other
chemicals were purchased from Wako Pure Chemical Industries, Ltd.
1
) δ:
3
7.04 (2H, dd, J = 1.3 and 3.0 Hz, Th), 6.74 (2H, dd, J = 1.3 and 5.1
2
6
13
and TCI Co., Ltd. Starting materials 2-tributylstannylthiophene, 3-
Hz, Th). C NMR (126 MHz, CDCl
3
) δ: 149.2 (dd, JC−F = 15 and
2
7
28
tributylstannylthiophene, 1,2-dibromo-4,5-dimethylbenzene, 1-
251 Hz), 140.0, 132.0 (t, JC−F = 5 Hz), 128.5, 125.2, 123.4, 118.7 (q,
JC−F = 6 Hz). HRMS (APCI) Calcd for C H F S : M : 278.00300.
Found: 278.00299.
2
9
30
30
+
H, 2-OMe, and 3′-OMe were prepared according to the
14
8 2 2
literature. Mesityldimethoxyborane (MesB(OMe) ) was prepared by a
2
method similar to the synthesis of dimethoxytripylborane, using
4,5-Bis(3-thienyl)-o-xylene (1-Me). 1-Me was prepared from
18.4 g (49.3 mmol) of 3-tributylstannylthiophene, 4.24 g (16.1 mmol)
of 1,2-dibromo-4,5-dimethylbenzene, 557 mg (0.482 mmol) of
Pd(PPh ) , and 40 mL of toluene as a white solid (2.66 g, 9.84
31
mesityl bromide instead of tripyl bromide. Abbreviations Bz, Th, and
Mes were used for the following NMR assignments stand for fused
benzene and thiophene ring, and mesityl group, respectively. NMR
spectra were recorded on Varian System 500 and 400MR
3
4
1
mmol, 61% yield) in a manner similar to that above. H NMR (400
I
DOI: 10.1021/acs.organomet.7b00844
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Organometallics
Article
MHz, CDCl ) δ: 7.24 (s, 2H, Bz), 7.16 (2H, dd, J = 3.0 and 4.9 Hz,
with hexane as the eluent to give 1.74 g (3.20 mmol, 72% yield) of 3-H
3
1
Th), 7.03 (2H, dd, J = 1.3 and 3.0 Hz, Th), 6.78 (2H, dd, J = 1.3 and
as a colorless viscous oil. H NMR (400 MHz, CDCl ) δ: 7.51−7.42
3
.9 Hz, Th), 2.32 (6H, s, CH3). 13C NMR (126 MHz, CDCl ) δ:
(4H, m, Bz), 6.58 (2H, s, Th), 0.19 (18H, s, Si-CH ). C NMR (126
13
4
3
3
1
42.0, 135.9, 132.8, 131.4, 129.0, 124.5, 122.4, 19.4. HRMS (APCI)
MHz, CDCl ) δ: 142.1, 140.3, 136.4, 134.8, 130.4, 127.7, 115.2, −0.3.
3
+
+
Calcd for C H S : [M + H] : 271.06097. Found: 271.06137.
HRMS (APCI) Calcd for C H Br S Si : [M + H] : 542.92975.
16
15
2
20 25
2
2
2
4
,5-Bis(2-thienyl)-o-xylene (1′-Me). 1′-Me was prepared from
Found: 542.93011.
1
8.5 g (49.6 mmol) of 2-tributylstannylthiophene and 6.27 g (23.8
mmol) of 1,2-dibromo-4,5-dimethylbenzene, 825 mg (0.714 mmol) of
Pd(PPh ) , and 60 mL of toluene as a colorless oil (3.99 g, 14.8 mmol,
2% yield) in a manner similar to that above. H NMR (400 MHz,
CDCl ) δ: 7.29 (2H, s, Bz), 7.24 (2H, dd, J = 0.9 and 4.0 Hz, Th), 6.94
4,5-Bis(2-bromo-5-trimethylsilyl-3-thienyl)-o-xylene (3-Me).
3-Me was prepared from 1.16 g (2.71 mmol) of 2-Me, 4.90 mL (5.54
mmol) of 1.13 M LDA in THF/hexane, 0.86 mL (6.81 mmol) of
trimethylsilyl chloride, and 30 mL of ether as a white solid (1.03 g,
3
4
1
6
1
1.79 mmol, 66% yield) in a manner similar to that above. H NMR
3
(
2H, dd, J = 2.8 and 4.0 Hz, Th), 6.85 (2H, dd, J = 0.9 and 2.8 Hz,
(400 MHz, CDCl ) δ: 7.26 (2H, s, Bz), 6.57 (2H, s, Th), 2.34 (6H, s,
3
13
13
Th), 2.31 (6H, s, CH3). C NMR (126 MHz, CDCl ) δ: 142.7, 136.4,
1
C H S : [M + H] : 271.06097. Found: 271.06158.
C−CH ), 0.19 (18H, Si-CH ). C NMR (126 MHz, CDCl ) δ: 142.1,
3
3
3
3
32.2, 131.1, 126.8, 126.7, 125.4, 19.3. HRMS (APCI) Calcd for
140.0, 136.6, 136.2, 132.1, 131.4, 114.9, 19.6, −0.3. HRMS (APCI)
+
+
Calcd for C H Br S Si : M : 569.95322. Found: 569.95313.
1
6
15
2
22 28
2
2
2
1
,2-Bis(2-bromo-3-thienyl)benzene (2-H). To a solution of 3.18
4,5-Bis(3-bromo-5-trimethylsilyl-2-thienyl)-o-xylene (3′-Me).
To a solution of 565 mg (0.965 mmol) of 2′-Me in 15 mL of ether was
added 1.20 mL (1.97 mmol) of 1.64 M nBuLi in hexane at −78 °C
over a period of 5 min, and the mixture was stirred at this temperature
for 20 min. To this was added 0.31 mL (2.45 mmol) of trimethylsilyl
chloride at this temperature and the mixture was stirred at room
temperature overnight. The resulting mixture was hydrolyzed with
water. The organic layer was separated, and the aqueous layer was
extracted with 50 mL of ethyl acetate. The combined organic layers
were washed twice with water and then once with brine. After drying
over anhydrous magnesium sulfate, the solvent was evaporated, and
the residue was purified by silica gel chromatography with hexane as
g (13.2 mmol) of 1-H in 260 mL of CHCl was added 4.70 g (26.4
3
mmol) of NBS in several portions at 0 °C, and the reaction mixture
was stirred overnight at room temperature. The reaction mixture was
washed twice with water and then once with brine. After drying the
organic phase over anhydrous magnesium sulfate, the solvent was
evaporated to give the crude product, which was then purified by silica
gel chromatography with hexane as the eluent to give 4.11 g (10.3
mmol, 78% yield) of 2-H as a light yellow solid. H NMR (400 MHz,
CDCl ) δ: 7.47 (4H, m, Bz), 7.09 (2H, d, J = 5.6 Hz, Th), 6.55 (2H, d,
J = 5.6 Hz, Th). C NMR (126 MHz, CDCl ) δ: 140.9, 134.7, 130.7,
29.6, 127.9, 125.0, 110.7. HRMS (APCI) Calcd for C H Br S : M :
1
3
13
3
+
1
3
14 8 2 2
97.84287. Found: 397.84280.
,2-Bis(2-bromo-3-thienyl)-4,5-difluorobenzene (2-F). 2-F
the eluent to give 328 mg (0.572 mmol, 59% yield) of 3′-Me as a
1
1
white solid. H NMR (400 MHz, CDCl ) δ: 7.31 (2H, s, Bz), 6.99
3
1
3
was prepared from 831 mg (2.99 mmol) of 1-F, 1.19 g (6.66
mmol) of NBS, and 30 mL of CHCl in a manner similar to that
above. The crude product was purified by silica gel chromatography
with hexane as the eluent to give 2-F as a white viscous oil (845 mg,
.94 mmol, 65% yield). H NMR (400 MHz, CDCl ) δ: 7.29 (2H, t,
JH−F = 9.5 Hz, Bz), 7.10 (2H, d, J = 5.7 Hz, Th), 6.51 (2H, d, J = 5.7
Hz, Th). C NMR (126 MHz, CDCl ) δ: 149.5 (dd, J
52 Hz), 138.9, 131.5 (t, JC−F = 5 Hz), 129.2, 125.5, 119.6 (q, JC−F = 6
(2H, s, Th), 2.34 (6H, s, C−CH ), 0.24 (18H, s, Si-CH ). C NMR
(126 MHz, CDCl ) δ: 142.0, 141.1, 137.1, 136.1, 132.4, 130.4, 111.5,
19.6, −0.4. HRMS (APCI) Calcd for C H Br S Si : M : 569.95322.
Found: 569.95319.
3
3
3
3
+
22
28
2
2
2
1
1
Compound α-H. To a solution of 265 mg (0.486 mmol) of 3-H in
5 mL of ether was added 0.61 mL (1.0 mmol) of 1.64 mol/L nBuLi in
hexane at −78 °C over a period of 5 min, and the mixture was stirred
at this temperature for 20 min. To this was added 145 mg (0.755
3
13
= 15 and
3
C−F
2
+
Hz), 111.4. HRMS (APCI) Calcd for C H Br F S : M : 433.82402.
mmol) of MesB(OMe) at this temperature, and the mixture was
14
6
2
2
2
2
Found: 433.82404.
,5-Bis(2-bromo-3-thienyl)-o-xylene (2-Me). 2-Me was pre-
pared from 813 mg (3.01 mmol) of 1-Me, 1.06 g (6.00 mmol) of NBS,
stirred at room temperature overnight. The resulting mixture was
hydrolyzed with water, and 30 mL of ethyl acetate was added. The
organic layer was separated, and the aqueous layer was extracted with
30 mL of ethyl acetate. The combined organic layers were washed
twice with water and then once with brine. After drying over
anhydrous magnesium sulfate, the solvent was evaporated. The crude
product was purified by preparative GPC with toluene as the eluent to
give 163 mg (0.316 mmol, 65% yield) of α-H as a white solid. This
solid was recrystallized by slow evaporation from hexane for XRD
4
and 30 mL of CHCl in a manner similar to that above. The crude
3
product was purified by silica gel chromatography with dichloro-
methane as the eluent to give 2-Me as a light yellow solid (1.16 g, 2.71
1
mmol, 90% yield). H NMR (400 MHz, CDCl ) δ: 7.25 (2H, s, Bz),
3
7
.08 (2H, d, J = 5.6 Hz, Th), 6.54 (2H, d, J = 5.6 Hz, Th), 2.35 (6H, s,
13
CH3). C NMR (126 MHz, CDCl ) δ: 140.9, 136.5, 132.0, 131.8,
1
3
+
1
29.8, 124.8, 110.4, 19.6. HRMS (APCI) Calcd for C H Br S : M :
analysis and emission measurements in the solid state. H NMR (400
16
12
2 2
4
25.87417. Found: 425.87448.
MHz, CDCl ) δ: 8.38−8.34 (2H, m, Bz), 8.17 (2H, s, Th), 7.59−7.54
3
4
,5-Bis(3,5-dibromo-2-thienyl)-o-xylene (2′-Me). 2′-Me was
(2H, m, Bz), 6.94 (2H, s, Mes), 2.40 (3H, s, p-CH , Mes), 2.11 (6H, s,
3
13
prepared from 1.09 g (4.02 mmol) of 1′-Me, 2.93 g (16.5 mmol) of
o-CH , Mes), 0.36 (18H, s, Si−CH ). C NMR (126 MHz, CDCl3)
3
3
NBS, 40 mL of CHCl , and 40 mL of acetic acid in a manner similar to
δ: 151.6, 151.3, 149.1 (br, B−C), 141.4 (br, B−C), 138.6, 137.8 (Th
ring CH), 137.2, 133.2, 131.4 (Bz ring CH), 127.2 (Bz ring CH),
127.1 (Mes ring CH), 23.0 (o-CH , Mes), 21.5 (p-CH , Mes), − 0.1
3
that above. The crude product was purified by preparative GPC with
toluene as the eluent to give 2′-Me as a colorless viscous oil (879 mg,
3
3
1
11
1
.50 mmol, 37% yield). H NMR (400 MHz, CDCl ) δ: 7.26 (2H, s,
(Si−CH ). B NMR (160 MHz, CDCl ) δ: 51.1 (w = 1900 Hz).
3
3
3
1/2
13
+
Bz), 6.92 (2H, s, Th), 2.34 (6H, s, CH3). C NMR (126 MHz,
HRMS (APCI) Calcd for C H BS Si : M : 514.18063. Found:
29 35 2 2
CDCl ) δ: 138.3, 138.0, 132.9, 132.3, 129.3, 112.7, 109.7, 19.6. HRMS
514.18103.
3
+
(
APCI) Calcd for C H Br S : M : 581.69519. Found: 581.69531.
Compound α-F-H. Compound α-F-H was prepared from 369 mg
16
10
4 2
1
,2-Bis(2-bromo-5-trimethylsilyl-3-thienyl)benzene (3-H).
(0.847 mmol) of 2-F, 1.05 mL (1.72 mmol) of 1.64 M nBuLi in
To a solution of 1.79 g (4.47 mmol) of 2-H in 45 mL of ether was
added 8.1 mL (9.2 mmol) of 1.13 M LDA in THF/hexane at −78 °C
over a period of 5 min and the mixture was stirred at this temperature
for 10 min then at 0 °C for 50 min. To this was added 1.23 mL (9.74
mmol) of trimethylsilyl chloride at −78 °C, and the mixture was
stirred at room temperature overnight. The resulting mixture was
hydrolyzed with water. The organic layer was separated and the
aqueous layer was extracted with 100 mL of ethyl acetate. The
combined organic layers were washed twice with water and once with
brine. After drying over anhydrous magnesium sulfate, the solvent was
evaporated, and the residue was purified by silica gel chromatography
hexane, 203 mg (1.06 mmol) of MesB(OMe) , and 5 mL of ether in a
2
manner similar to that above. The crude product was purified by silica
gel chromatography with a mixed solvent of hexane/dichloromethane
= 2/1 as the eluent to give α-F-H as a white solid (199 mg, 0.498
mmol, 58% yield). This solid was recrystallized by slow evaporation
from acetonitrile containing a small amount of dichloromethane to
give single crystals for XRD analysis and emission measurements in the
1
solid state. H NMR (400 MHz, CDCl ) δ: 8.14 (2H, t, J
= 10.7
3
H−F
Hz, Bz), 7.99 (2H, d, J = 5.1 Hz, Th), 7.96 (2H, d, J = 5.1 Hz, Th),
6.92 (2H, s, Mes), 2.39 (3H, s, p-CH , Mes), 2.08 (6H, s, o-CH ,
3
3
Mes). 13C NMR (126 MHz, CDCl ) δ: 148.8 (dd, J
= 15 and 252
3
C−F
J
DOI: 10.1021/acs.organomet.7b00844
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Hz), 148.8, 144.6 (br, B−C), 140.5 (br, B−C), 138.7, 137.7, 135.9 (Th
by vapor diffusion with acetonitrile as the precipitant to give single
ring CH), 131.1 (Th ring CH), 130.5 (t, J = 4 Hz), 127.2 (Mes ring
crystals for XRD analysis and emission measurements in the solid
C−F
1
CH), 119.6 (q, J
= 6 Hz, Bz ring CH), 22.8 (o-CH , Mes), 21.4 (p-
state. H NMR (400 MHz, CDCl ) δ: 7.85 (2H, s, Bz), 7.35 (2H, s,
C−F
3
3
11
19
CH , Mes). B NMR (160 MHz, CDCl ) δ: 50.9 (w = 900 Hz).
F
Th), 6.90 (2H, s, Mes), 4.10 (6H, s, OCH ), 2.39 (3H, s, p-CH ,
3
3
1/2
3
3
1
3
NMR (470 MHz, CDCl ) δ: −138.3 (m). HRMS (APCI) Calcd for
Mes), 1.98 (6H, s, o-CH , Mes), 0.31 (18H, s, Si-CH ). C NMR
3
3
3
+
C H BF S : M : 406.08273. Found: 406.08289.
(126 MHz, CDCl ) δ: 161.8, 149.0, 145.7 (br, B−C), 144.9 (Th ring
2
3
17
2
2
3
Compound α-Me. Compound α-Me was prepared from 238 mg
0.416 mmol) of 3-Me, 0.52 mL (0.85 mmol) of 1.64 M nBuLi in
CH), 144.0 (br, B−C), 137.9, 137.4, 135.9, 126.9 (Mes ring CH),
(
125.0, 112.9 (Bz ring CH), 56.0 (OCH ), 22.7 (o-CH , Mes), 21.3 (p-
3
3
11
hexane, 104 mg (0.541 mmol) of MesB(OMe) , and 5 mL of ether in
CH , Mes), −0.1 (Si-CH ). B NMR (160 MHz, CDCl ) δ: 54.4
2
3
3
3
+
a manner similar to that above. The crude product was purified by
preparative GPC with toluene as the eluent to give α-Me as a white
solid (115 mg, 0.212 mmol, 51% yield). This solid was recrystallized
by slow evaporation from acetonitrile containing a small amount of
(w = 1700 Hz). HRMS (APCI) Calcd for C H BO S Si : M :
1/2 31 39 2 2 2
574.20176. Found: 574.20239.
Compound α-F. To a solution of 57.2 mg (0.141 mmol) of α-F-H
in 5 mL of ether was added 0.19 mL (0.31 mmol) of 1.61 M tBuLi in
pentane at −78 °C over a period of 5 min and the mixture was stirred
at this temperature for 30 min. To this was added 0.055 mL (0.44
mmol) of trimethylsilyl chloride at this temperature, and the mixture
was stirred at room temperature overnight. The resulting mixture was
hydrolyzed with water, and 30 mL of ethyl acetate was added. The
organic layer was separated, and the aqueous layer was extracted with
50 mL of ethyl acetate. The combined organic layers were washed
twice with water and then once with brine. After drying over
anhydrous magnesium sulfate, the solvent was evaporated, and the
residue was purified by preparative GPC with toluene as the eluent to
give 40.6 mg (0.0737 mmol, 52% yield) of α-F as a white solid. This
solid was recrystallized by slow evaporation from acetonitrile to give
dichloromethane to give single crystals for XRD analysis and emission
1
measurements in the solid state. H NMR (400 MHz, CDCl ) δ: 8.15
3
(
2
2H, s, Th), 8.10 (2H, s, Bz), 6.92 (2H, s, Mes), 2.48 (6H, s, CH , Bz),
.39 (3H, s, p-CH , Mes), 2.10 (6H, s, o-CH , Mes), 0.36 (18H, s, Si-
3 3
3
CH ). NOESY NMR was taken to assign the two signals at 8.15 and
3
1
13
8
.10 ppm in the H NMR spectrum (Figure S39). C NMR (126
MHz, CDCl ) δ: 151.4, 151.3, 148.6 (br, B−C), 141.6 (br, B−C),
3
1
1
38.6, 137.6 (Th ring CH), 137.1, 136.0, 132.2 (Bz ring CH), 131.1,
27.1 (Mes ring CH), 22.9 (o-CH , Mes), 21.5 (p-CH , Mes), 19.8
3
3
11
(
CH , Bz), −0.1 (Si-CH ). B NMR (160 MHz, CDCl ) δ: 50.3 (w
3
3
3
1/2
+
=
1900 Hz). HRMS (APCI) Calcd for C H BS Si : M : 542.21193.
31 39 2 2
Found: 542.21210.
Compound α-OMe-H. Compound α-OMe-H was prepared from
30 mg (0.501 mmol) of 2-OMe, 0.63 mL (1.0 mmol) of 1.64 M
single crystals for XRD analysis and emission measurements in the
1
2
solid state. H NMR (400 MHz, CDCl ) δ: 8.13 (2H, t, J
= 10.7
3
H−F
nBuLi in hexane, 123 mg (0.640 mmol) of MesB(OMe) , and 10 mL
Hz, Bz), 8.02 (2H, s, Th), 6.94 (2H, s, Mes), 2.40 (3H, s, p-CH ,
2
3
1
3
of a 1:1 mixed solvent of ether and THF in a manner similar to that
above. The crude product was purified by silica gel chromatography
with a mixed solvent of hexane/dichloromethane = 1/1 as the eluent
Mes), 2.09 (6H, s, o-CH , Mes), 0.36 (18H, s, Si-CH ). C NMR
3
3
(126 MHz, CDCl ) δ: 152.6, 149.3, 148.7 (dd, J
= 15 and 252 Hz),
3
C−F
138.6, 137.5 (Th ring CH), 137.4, 130.5 (t, JC−F = 4 Hz), 127.2 (Mes
to give α-OMe-H as a white solid (91.9 mg, 0.214 mmol, 43% yield).
ring CH), 119.5 (q, JC−F = 6 Hz, Bz ring CH), 22.9 (o-CH
21.5 (p-CH
detected. 11B NMR (160 MHz, CDCl
NMR (470 MHz, CDCl
3
, Mes),
1
H NMR (400 MHz, CDCl ) δ: 8.08 (2H, d, J = 5.1 Hz, Th), 7.94
3
, Mes), −0.2 (Si-CH ). Two signals for B−C were not
3
3
19
(
2H, d, J = 5.1 Hz, Th), 7.80 (2H, s, Bz), 6.93 (2H, s, Mes), 4.09 (6H,
3
) δ: 50.8 (w1/2 = 1300 Hz). F
13
s, OCH ), 2.39 (3H, s, p-CH , Mes), 2.09 (6H, o-CH , Mes).
C
3
) δ: −138.9 (m). HRMS (APCI) Calcd for
: M : 550.16178. Found: 550.16180.
Compound α-OMe. To a solution of 67.2 mg (0.156 mmol) of α-
3
3
3
+
NMR (126 MHz, CDCl ) δ: 150.5, 148.3, 138.8, 137.4, 135.3 (Th ring
C H33BF S Si
29 2 2 2
3
CH), 130.8 (Th ring CH), 127.5, 127.1 (Mes ring CH), 113.2 (Bz ring
CH), 55.9 (OCH ), 22.8 (o-CH , Mes), 21.4 (p-CH , Mes). Two
OMe-H in 10 mL of a 1:1 mixed solvent of ether and THF was added
0.35 mL (0.40 mmol) of 1.13 M LDA in THF/hexane at −78 °C over
a period of 5 min, and the mixture was stirred at 0 °C for 30 min. To
this was added 0.060 mL (0.47 mmol) of trimethylsilyl chloride at this
temperature, and the mixture was stirred at room temperature
overnight. The resulting mixture was hydrolyzed with water, and 30
mL of ethyl acetate was added. The organic layer was separated, and
the aqueous layer was extracted with 50 mL of ethyl acetate. The
combined organic layers were washed twice with water and then once
with brine. After drying over anhydrous magnesium sulfate, the solvent
was evaporated, and the residue was purified by preparative GPC with
3
3
3
signals for B−C were not detected, probably due to their low
intensities. 11B NMR (160 MHz, CDCl ) δ: 49.7 (w = 1100 Hz).
3
1/2
+
HRMS (APCI) Calcd for C H BO S : M : 430.12270. Found:
25
23
2 2
4
30.12262.
Compound β-Me. Compound β-Me was prepared from 141 mg
(
0.247 mmol) of 3′-Me, 0.45 mL (0.74 mmol) of 1.64 M nBuLi in
hexane, 85.7 mg (0.446 mmol) of MesB(OMe) , and 5 mL of ether in
2
a manner similar to that above. The crude product was purified by
preparative GPC with toluene as the eluent, followed by silica gel
chromatography with a mixed solvent of hexane/dichloromethane =
1
4
/1 as the eluent to give β-Me as a white solid (60.7 mg, 0.112 mmol,
5% yield). This solid was recrystallized by slow evaporation from
toluene as the eluent to give 25.5 mg (0.044 mmol, 28% yield) of α-
1
OMe as a white solid. H NMR (400 MHz, CDCl
) δ: 8.09 (2H, s,
), 2.40
, Mes), 0.36 (18H, s, Si-CH ).
) δ: 151.7, 151.0, 148.1, 138.7, 137.24
(Th ring CH), 137.15, 127.5, 127.1 (Mes ring CH), 113.4 (Bz ring
CH), 56.0 (OCH ), 22.9 (o-CH , Mes), 21.5 (p-CH , Mes), −0.1 (Si-
). B NMR (160 MHz, CDCl ) δ: 50.5 (w1/2 = 1500 Hz). HRMS
3
acetonitrile containing a small amount of dichloromethane to give
Th), 7.78 (2H, s, Bz), 6.94 (2H, s, Mes), 4.09 (6H, s, OCH
(3H, s, p-CH , Mes), 2.11 (6H, o-CH
C NMR (126 MHz, CDCl
3
single crystals for XRD analysis and emission measurements in the
3
3
3
1
13
solid state. H NMR (400 MHz, CDCl ) δ: 8.22 (2H, s, Bz), 7.34 (2H,
3
3
s, Th), 6.89 (2H, s, Mes), 2.44 (6H, s, CH , Bz), 2.39 (3H, s, p-CH ,
3
3
13
Mes), 1.98 (6H, o-CH , Mes), 0.30 (18H, s, Si-CH ). C NMR (126
3
3
3
3
3
11
MHz, CDCl ) δ: 162.0, 146.0 (br, B−C), 144.5 (Th ring CH), 138.1,
CH
3
3
3
+
1
37.32, 137.25, 135.9, 132.5 (Bz ring CH), 128.6, 126.9 (Mes ring
(APCI) Calcd for C31
574.20233.
H39BO S Si : M : 574.20176. Found:
2 2 2
CH), 22.7 (o-CH , Mes), 21.3 (p-CH , Mes), 19.7 (CH , Bz), 0.0 (Si-
3
3
3
CH ). One signal for B−C was not detected, probably due to the low
3
11
intensity. B NMR (160 MHz, CDCl ) δ: 55.1 (w = 1600 Hz).
3
1/2
ASSOCIATED CONTENT
■
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00844.
+
HRMS (APCI) Calcd for C H BS Si : M : 542.21193. Found:
31
39
2
2
5
42.21222.
Compound β-OMe. Compound β-OMe was prepared from 334
S
mg (0.553 mmol) of 3′-OMe, 0.70 mL (1.1 mmol) of 1.64 M nBuLi in
hexane, 145 mg (0.755 mmol) of MesB(OMe) , and 5 mL of ether in
2
a manner similar to that above. The crude product was purified by
preparative GPC with toluene as the eluent, followed by silica gel
chromatography with a mixed solvent of hexane/dichloromethane =
Experimental details, NMR spectra, fluorescence spectra,
anodic CVs, and theoretical calculations (PDF)
X-ray crystallographic data (XYZ)
1
/1 as the eluent to give β-OMe as a white solid (110 mg, 0.192
mmol, 35% yield). This solid was recrystallized from dichloromethane
K
DOI: 10.1021/acs.organomet.7b00844
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Accession Codes
2003, 244, 1−44. (d) Baumgartner, T.; Rea
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12) (a) Shuto, A.; Kushida, T.; Fukushima, T.; Kaji, H.; Yamaguchi,
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u, R. Chem. Rev. 2006, 106,
4
(
CCDC 1587388−1587392 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
6
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925. (c) Yamaguchi, S.; Wakamiya, A. Pure Appl. Chem. 2006, 78,
413−1424. (d) Jak
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Y.; Jak
(
̈
AUTHOR INFORMATION
■
*
H. Chem. Phys. Lett. 2001, 339, 161−169. (b) Murata, H.; Kafafi, Z.
H.; Uchida, M. Appl. Phys. Lett. 2002, 80, 189−191.
Corresponding Author
Tel.: +81-82-424-7743. Fax: +81-82-424-5494. E-mail: jo@
hiroshima-u.ac.jp.
(14) Parke, S. M.; Boone, M. P.; Rivard, E. Chem. Commun. 2016, 52,
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485−9505.
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(16) Caruso, A., Jr.; Siegler, M. A.; Tovar, J. D. Angew. Chem., Int. Ed.
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17) Levine, D. R.; Siegler, M. A.; Tovar, J. D. J. Am. Chem. Soc. 2014,
36, 7132−7139.
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c) Levine, D. R.; Caruso, A., Jr.; Siegler, M. A.; Tovar, J. D. Chem.
ORCID
Joji Ohshita: 0000-0002-5401-514X
Notes
The authors declare no competing financial interest.
2
(
1
(
ACKNOWLEDGMENTS
■
(
This work was supported by JSPS KAKENHI Grant NOS.
JP17H06890 AND JP17H03105. We thank Prof. Kazuyoshi
Tanaka (Kyoto University) for his kind support on NICS
calculations.
Commun. 2012, 48, 6256−6258. (d) Messersmith, R. E.; Siegler, M.
A.; Tovar, J. D. J. Org. Chem. 2016, 81, 5595−5605. (e) Levine, D. R.;
Messersmith, R. E.; Siegler, M. A.; Tovar, J. D. Can. J. Chem. 2017, 95,
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H.; Tovar, J. D. J. Org. Chem. 2017, 82, 13440−13448.
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DOI: 10.1021/acs.organomet.7b00844
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
revision A.02; Gaussian, Inc.: Wallingford, CT, 2009.
M
DOI: 10.1021/acs.organomet.7b00844
Organometallics XXXX, XXX, XXX−XXX