Benzo[b]thiophene-fused boron and silicon ladder acenes

Article abstract of DOI:10.1021/om4004187

Synthetic routes to three new heteroacene compounds in which two benzo[b]thienyl units are bridged by dimethylsilyl and/or mesitylboryl units are described. In the Si/Si and Si/B compounds 1-syn and 3-syn, respectively, the sulfur atoms of the flanking benzo[b]thiophenes are disposed in meta positions on the central six-membered heterocycle, while in the B/B compound 4-anti, they are para to each other; the reasons for this lie in the synthetic method employed to prepare 4-anti. All three compounds were crystallographically characterized and their photophysical properties examined. A comprehensive examination of their photophysical properties shows that they are weakly absorbing and fluorescing materials, with the diborin derivative 4-anti exhibiting the most red-shifted absorption maxima and largest fluorescence quantum yield.

Full text of DOI:10.1021/om4004187

Article
pubs.acs.org/Organometallics
Benzo[b]thiophene-Fused Boron and Silicon Ladder Acenes
Lauren G. Mercier,†,§ Warren E. Piers,*,† Ross W. Harrington,‡ and William Clegg‡
†Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4
‡School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne, U.K. NE1 7RU
S
* Supporting Information
ABSTRACT: Synthetic routes to three new heteroacene
compounds in which two benzo[b]thienyl units are bridged
by dimethylsilyl and/or mesitylboryl units are described. In the
Si/Si and Si/B compounds 1-syn and 3-syn, respectively, the
sulfur atoms of the flanking benzo[b]thiophenes are disposed
in meta positions on the central six-membered heterocycle,
while in the B/B compound 4-anti, they are para to each
other; the reasons for this lie in the synthetic method
employed to prepare 4-anti. All three compounds were
crystallographically characterized and their photophysical
properties examined. A comprehensive examination of their
photophysical properties shows that they are weakly absorbing and fluorescing materials, with the diborin derivative 4-anti
exhibiting the most red-shifted absorption maxima and largest fluorescence quantum yield.
exocyclic Si−C bonds.29−31 While the sp3-hybridized Si center
INTRODUCTION
■
can lead to nonplanar conformations,13,32 in some instances
The use of highly conjugated organic molecules as semi-
conducting materials in various devices is growing, and as a
result there has been an increased interest in developing new
materials and new methods for their facile synthesis. While all-
fully planar structures are observed.33
Given our interest in boron- and silicon-modified hetero-
acenes, we targeted the benzothiophene-buttressed π systems
shown below in order to assess the effect of the differing B/Si
carbon acenes have played a significant role in organic
electronics,1,2 an important strategy for developing new
substitution patterns on the properties of the materials. Initial
plans focused on the syn isomers, where the thiophene sulfur
materials with modified electronic properties has involved the
atoms reside on the same side of the molecule. While the Si/Si
and Si/B combinations were readily accessed, the B/B
compound could be formed only as its anti isomer using the
methods developed. Herein we describe the synthesis,
structures, and properties of these novel heteroacenes.
incorporation of main-group elements into the structural
frameworks of these polycyclic aromatic hydrocarbon (PAH)
frameworks.1,3 Thus, the application of “ladder” type molecules
containing heteroatom elements has been a very active area of
research in main-group chemistry in the past decade.4
While researchers in this area have utilized the full palette of
main-group elements available, we have focused on hetero-
cycles containing boron5−12 and silicon13 in acenes and acene
analogues as a means of altering the properties of these
conjugated materials. Boron is one of the few (and from a
practical perspective, perhaps the only) choices for incorporat-
ing an electron-deficient element into a π-conjugated net-
work.14−16 A contribution from the empty p orbital associated
with the three-coordinate boron center to the LUMO surfaces
can lower the energy of this orbital,17−19 rendering materials
incorporating this element potential electron acceptors and
sensors for Lewis bases.20−23 Further, mixed heteroacenes in
which boron is partnered with a donor atom (N, P)24−28 can
exhibit strong intramolecular charge transfer absorption bands.
These favorable properties are tempered by the need to
stabilize the boron centers with sterically large protecting
groups, by generally nontrivial synthetic protocols. Integration
of silicon into conjugated frameworks, although electronically
saturated, also lowers the LUMO level through hyper-
conjugation of the π* system with the σ* orbitals of the
RESULTS AND DISCUSSION
■
The silicon−silicon fused molecule 1-syn was prepared in two
steps via the sequence shown in Scheme 1 starting from 3-
bromobenzo[b]thiophene. Lithiation with nBuLi followed by
addition of Me2SiCl2 efficiently installed the first dimethylsilyl
unit. Double deprotonation of this species required Schlosser’s
base34 in THF solutions at −20 °C to give clean conversion to
Special Issue: Applications of Electrophilic Main Group Organo-
metallic Molecules
Received: May 13, 2013
Published: July 3, 2013
© 2013 American Chemical Society
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trihalides might be feasible.20 The dibromo bis-
(benzothiophene)-substituted dimethylsilane precursor was
prepared by following the method of Baumgartner and co-
workers32 using selective lithium−halogen exchange of 2,3-
Scheme 1
dibromobenzo[b]thiophene in the 2-position and trapping in
1
situ with
/ equiv of Me2SiCl2. A second lithiation step
2
followed by trapping with Me2SnCl2 yielded the desired Si/Sn
compound 2-syn in very good yield; 2-syn was characterized by
multinuclear (1H, 13C, 29Si, 119Sn) NMR spectroscopy and mass
spectrometry. 2-syn could be converted to the Si/B compound
3-syn using a one-pot procedure in which treatment with BCl3
resulted in selective transmetalation with only the Me2Sn unit.
The diarylboron chloride was then protected by addition of a
mesityl lithium/toluene suspension to give 3-syn in 31% yield.
This compound is stable in air, but due to instability on silica
gel, crude products were purified by trituration in hexanes,
filtration to remove lithium chloride, and recrystallization of the
filtrate, manipulations that led to lowered isolated yields.
Characterization by multinuclear NMR spectroscopy revealed a
singlet at −16.6 ppm in the 29Si NMR spectrum and a broad
signal at 56 ppm in the 11B NMR spectrum. This latter signal is
characteristic of three-coordinate boron atoms with aryl
substituents35 and, more specifically, is near those measured
by Kawashima and co-workers for 1,4-heteroborins substituted
with pnictogens24,26 or chalcogens,25 as well as that measured
by Ashe and co-workers18 for diborapentacene. The resonance
is in the upfield end of the range for three-coordinate boron
atoms and is indicative of some conjugation between the boron
atom and the π framework of the molecule.
In an effort to induce boron−halogen exchange at both the
tin and silicon centers in 2-syn, it was treated with the more
Lewis acidic BBr3 reagent. Harsher conditions, including an
excess of BBr3, higher temperatures (50 °C), and a longer
reaction time (2 days), were required in order to drive the
reaction to one product.36 While both group 14 dimethyl units
were replaced, the product isolated upon treatment with MesLi
was the anti isomer 4-anti rather than the expected syn isomer.
DFT computations (B3LYP/6-31G**) predict that 4-anti is
only 0.1 kcal/mol more stable than the targeted 4-syn isomer,
suggesting that the preference for the 4-anti framework may be
kinetic in nature. Monitoring the reaction by NMR spectros-
copy showed that conversion of 2-syn to the Si/B−Br
intermediate upon treatment with BBr3 is facile at room
temperature (Scheme 3); this was indicated primarily by the
appearance of Me2SnBr2 in both the 1H and 119Sn NMR
spectra. Further addition of BBr3 showed no reaction at room
temperature, but upon heating, changes in the Me2Si region of
the 1H NMR spectrum and two new peaks at −17.6 and −19.4
ppm in the 29Si NMR spectrum suggested formation of the two
Si−Br products that result from bromine−methyl exchange at
silicon. This is all that can be gleaned from these experiments,
but we speculate that conversion of these intermediates to the
(unobserved) 2,3-dibromoborylbenzo[b]thiophene must occur
in order to account for the exclusive production of 4-anti in this
reaction. Bimolecular condensation of such ortho-disposed
dibromoboryl units to form diboraacene frameworks is
known.18,37−40 In the case of 2,3-dibromoborylbenzo[b]-
thiophene, the difference in Lewis acidity of the inequivalent
−BBr2 groups (the boryl group in the 2-position is expected to
be a weaker Lewis acid) favors the transition state leading to 4-
anti over that leading to 4-syn, and the former is isolated
exclusively. The equivalent boron nuclei in this compound
the dilithio species (monitored by quenching with D2O and
following by H NMR spectroscopy). Trapping this dilithio
1
reagent in situ with Me2SiCl2 resulted in the formation of 1-syn,
which was recrystallized from hot hexanes in an overall yield of
72% yield for the two steps. In addition to the expected
resonances in the 1H and 13C NMR spectra, 1-syn showed two
sharp peaks in the 29Si NMR spectrum at −15.6 and −19.4
ppm, similar to those measured by Baumgartner and co-
workers for 1,4-dihydro-1,4-silaphosphalins32 and indicative of
sp3-hybridized silicon atoms.
We did not envision that 1-syn would be a suitable starting
material for the preparation of Si/B and/or B/B compounds
through treatment with boron trihalides; therefore, we prepared
a related Si/Sn heterocycle (Scheme 2) as a compound for
which selective halogen exchange reactions with boron
Scheme 2
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Scheme 3
resonate at 59 ppm in the 11B NMR spectrum, near that
observed for 3-syn (vide supra).
The structures of the compounds 1-syn, 3-syn, and 4-anti
were investigated by X-ray diffraction, and the molecular
structures of the compounds are shown in Figures 1−3,
respectively, along with selected metrical parameters. Crystals
were grown by vapor diffusion of hexanes into saturated
dichloromethane/benzene solutions. 1-syn and 3-syn grew as
yellow crystals that could be analyzed on a conventional
diffractometer, while 4-anti was available only as a red
microcrystalline solid that required a synchrotron source for
analysis. Compound 1-syn crystallizes with a single molecule in
the asymmetric unit, while both 3-syn and 4-anti have two
molecules that are essentially identical. For 4-anti, both
molecules lie on a crystallographic inversion center, which is
not possible for a molecule with a syn configuration.
All three compounds are relatively planar structures with
angles between the least-squares planes defined by the five
atoms of the flanking thiophene groups measured at 3.30(12),
11.19(19)−12.00(19), and 0° for 1-syn, 3-syn, and 4-anti,
respectively. Most remarkably, the central disilin ring of 1-syn is
essentially planar even though it contains two sp3-hybridized
silicon atoms. As might be expected, however, within the
central rings of the three compounds, there is a great deal of
bond alternation. In particular, the internal C−C bonds are
clearly localized double bonds, the shortest being in the 1,4-
dihydro-1,4-disilin 1-syn (1.373(3) and 1.376(3) Å). In the 1,4-
Figure 1. Displacement ellipsoid (50%) diagram of 1-syn. Selected
bond distances (Å): Si(1)−C(1), 1.859(2); Si(1)−C(2), 1.852(2);
Si(2)−C(3), 1.865(2); Si(2)−C(4), 1.873(2); C(1)−C(4),1.373(3);
C(2)−C(3), 1.376(3); C(3)−C(16), 1.455(3); C(4)−C(5), 1.455(3);
C(5)−C(10), 1.408(3); S(1)−C(2), 1.744(2); S(1)−C(11),
1.729(3); S(2)−C(1), 1.742(2); S(2)−C(10), 1.731(2); C(11)−
C(16), 1.407(4). Selected bond angles (deg): C(1)−Si(1)−C(2),
105.11(9); C(3)−Si(2)−C(4), 108.45(10);. Selected torsion angles
(deg): Si(1)−C(2)−C(3)−Si(2), −5.5(3); Si(1)−C(1)−C(4)−Si(2),
1.9(3).
dihydro-1,4-diborin 4-anti these distances are somewhat longer
(1.388(2) Å), while the 1,4-dihydro-1,4-silaborin 3-syn exhibits
intermediate values (1.371(7)−1.380(8) Å). In the two
independent molecules in the unit cell for both 3-syn and 4-
anti, the sum of the angles about each boron atom is 360°, with
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carbon bond lengths similar to those of 3-syn, but the C(1)−
Si−C(2) angles are slightly larger (by ∼4°) in the former
compound. Packing interactions in the three compounds are
dominated by short S···H interactions41−43 between benzo-
thiophene sulfur atoms and aromatic C−H hydrogens on
adjacent molecules (Figures S1−S3, Supporting Information).
While the bulky mesityl substituents on 3-syn and 4-anti tend
to disrupt face-to-face interactions in the packing of these
molecules, in 1-syn such features are observed between adjacent
molecules (separation distance of ∼3.54 Å), best observed
when the crystal packing is viewed down the c axis of the unit
cell (see Figure S1, Supporting Information).
Given the bond alternation in the central rings and the fact
that the central rings contain four π electrons, the
antiaromaticity of the central rings was investigated using
NICS(1)zz calculations using Gaussian 0344 (B3LYP/6-
31G**//B3LYP/6-31+G*). The diborin ring in 4-anti can be
considered as antiaromatic (+28.46), while the silaborin core of
3-syn (+11.02) and the disilin ring in 1-syn (+9.25) could
qualify as weakly antiaromatic (Table S1, Supporting
Information). As expected, the NICS(1)zz values become less
positive as silicon atoms are incorporated into the central ring;
the smallest positive value was calculated for 1-syn.
Photophysical data for the three new compounds are
summarized in Table 1, while further data are presented in
Figures S4−S6 and Table S2 in the Supporting Information.
Both the absorption and emission maxima for the three
substituted benzo[b]thiophenes shift to longer wavelengths as
more boron atoms are incorporated into the conjugated
framework. This is mainly a result of changes to the LUMO
energy surfaces. DFT calculations (B3LYP/6-31G**) show the
HOMO surfaces are of π character and contain a node through
the center of the molecule, while the LUMO orbitals are of π*
character with contributions from the heteroatoms (Figure 4).
Figure 2. Displacement ellipsoid (50%) diagram for one of the two
independent molecules of 3-syn. Selected bond distances (Å): C(1)−
C(4), 1.380(8); C(1)−Si(1), 1.867(6); C(2)−C(3), 1.371(7); C(2)−
Si(1), 1.856(5); C(3)−B(1), 1.559(8); C(4)−B(1), 1.559(8); C(1)−
S(1), 1.728(5); C(4)−C(5), 1.473(8); C(5)−C(10), 1.412(7); S(1)−
C(10), 1.718(7); C(2)−S(2), 1.726(5); C(3)−C(16), 1.476(7);
C(11)−C(16), 1.4114(8). Selected bond angles (deg): C(3)−B(1)−
C(4), 121.1(5); C(3)−B(1)−C(19), 119.8(4); C(4)−B(1)−C(19),
119.1(5); C(2)−Si(1)−C(1), 101.6(2). Selected torsion angles (deg):
Si(1)−C(2)−C(3)−B(1), 9.8(8); Si(1)−C(1)−C(4)-B(1), 2.7(8);
C(24)−C(19)−B(1)−C(3), 93.2(6); C(20)−C(19)−B(1)−C(3),
−83.0(7).
Figure 3. One of the two independent molecules of 4-anti. Selected
bond distances (Å): C(1)−C(2), 1.388(2), C(1)−B(1), 1.552(3);
C(2)−B(1), 1.556(2); C(2)−C(3), 1.441(2); C(3)−C(8), 1.415(2).
Selected bond angles (deg): C(1)−B(1)−C(2), 115.22(14); C(1)−
B(1)−C(9), 118.82(14); C(2)−B(1)−C(9), 125.93(16). Selected
torsion angle (deg): B(1)−C(1)−C(2)−B(1_01), 2.9(2).
Figure 4. Kohn−Sham orbital surfaces for the HOMOs and LUMOs
of 1-syn, 3-syn, and 4-anti prepared by Gaussian 03 at the B3LYP/6-
31G** level of theory. See Table S3 (Supporting Information) for the
calculated energies of these orbitals.
the mesityl units essentially perpendicular to the central six-
membered ring. For 3-syn, the endocyclic B−C bond lengths
are both 1.559(8) Å, slightly longer than those measured for 4-
anti (1.552(3), 1.556(2) Å). 1-syn exhibits internal silicon−
a
Table 1. Summary of Photophysical Properties of 1-syn, 3-syn, and 4-anti
b
c
d
λabs,max (nm) (log ε)
λonset (nm)
λem,max (nm)
Φ
τ (ns)
knr (107 ns−1
)
kr (107 ns−1
)
1-syn
3-syn
4-anti
307 (3.74); 296 (3.80); 266 (4.19)
315
409
484
327
0.01
0.01
0.15
3.7
0.45
48.2
26.8
220
0.27
2.22
0.31
379 (4.21); 311 (4.27); 270 (4.58); 234 (5.04)
427 (3.87); 356 (3.77); 293 (4.65); 281 (4.47)
425
666 (630)
1.76
a
b
c
Absorption and emission were measured in 2.25 × 10−5 M CH2Cl2 solution. Measured using a calibrated integrating sphere. Calculated from knr
=
d
(1 − ϕF)/τF. Calculated from kr = ϕF/τ.
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and the cold bath was removed immediately. The solution turned light
yellow and eventually cloudy. The crude solution was filtered over
alumina and the filtrate concentrated in vacuo. The crude product was
used without further purification (211.8 mg, 97%) as an off-white
The LUMO surfaces of the two boron-containing molecules are
mainly focused on the borin rings and the flanking sulfur atoms.
Time-dependent density functional theory (TD-DFT) compu-
tations show that the HOMO−LUMO absorptions have low
oscillator strengths, and in the case of 4-anti it is symmetry
forbidden (Table S3, Supporting Information). Indeed, all
major transitions in these compounds have low oscillator
strengths (∼0.1−0.4), and thus they are weakly absorbing
materials. Electrochemical measurements show that quasi-
reversible one-electron reductions occur for each compound,
with the disilin being the most difficult to reduce (Figures S7
and S8 and Table S4, Supporting Information). Optical band
gaps obtained from the onset of absorption and the reduction
potential are in broad agreement with those computed using
DFT methods (Table S5, Supporting Information).
The two silicon-containing compounds, 1-syn and 3-syn,
exhibit fluorescence quantum yields of nearly 0 (0.01) and
relatively short fluorescence lifetimes (0.45 and 3.7 ns,
respectively) and have very large rates of nonradiative decay
(knr). This may be related to the conformational flexibility
imparted by the sp3 silicon centers, which renders these
molecules less rigid in comparison to the diborin 4-anti, which
exhibits a higher quantum yield of 0.15 and a much longer
fluorescence lifetime of 48.2 ns. Furthermore, the compound
displays a large Stokes shift of ∼230 nm (4.3 × 104 cm−1). The
reasons for this behavior are unclear at this point but do not
appear to be related to solvatochromism, since the absorption/
emission profile does not change significantly in solvents of
differing polarity23 (see Figure S4 and Table S2, Supporting
Information). One possibility is a lowering of the excited state’s
energy through orbital overlap between the boron p orbitals
and the π system of the molecular framework.23 The emission
profile underwent subtle changes at 173 K (Figure S6,
Supporting Information), but fluorescence lifetime measure-
ments at this temperature indicated the presence of two species
in solution with lifetimes of 38.5 ns (23.45%) and 54.7 ns
(76.55%). Perhaps the former results from a different
conformation at low temperature, but the results are not
conclusive enough to make firm conclusions.
1
solid. H NMR (400 MHz, CDCl3): δ 7.93 (dd, JHH = 7.8, 0.8 Hz,
2H), 7.78 (dd, JHH = 7.8 Hz, 0.8 Hz, 2H), 7.62 (s, 2H), 7.31 (ddd, JHH
= 7.4, 7.5, 1.4 Hz, 2H), 7.27 (ddd, JHH = 8.1, 7.5, 1.4 Hz, 2H), 0.78 (s,
6H) ppm. 13C{1H} NMR (100 MHz, CDCl3): δ 143.2, 141.7, 135.5,
134.5, 124.6, 124.0, 123.9, 122.6, −1.4 ppm. 29Si{1H} NMR (79 MHz,
CDCl3): δ −24.5 ppm. HRMS (EI): calcd for C18H16SiS2 [M+]
324.0463, found 324.0448.
Synthesis of 1,1,4,4-Tetramethylbis(benzo[b]thieno)[2,3-
b:3′,2′-e][1,4-dihydro-1,4]disilin (1-syn). A suspension of potas-
sium tert-butoxide (229 mg) and n-BuLi (1.6 M in hexanes, 1.28 mL)
was stirred for 5 min, and then a solution of bis(benzo[b]thiophene)-
dimethylsilane (166 mg, 0.51 mmol) in THF (5 mL) was added at
−20 °C. The orange-red suspension was stirred for 2 h at this
temperature, and neat Me2SiCl2 (0.07 mL) was added in one portion.
The cold bath was removed immediately and the reaction mixture
stirred at room temperature for 16 h. The solvent was removed in
vacuo, and the solid crude mixture was triturated in hexanes. Solids
were removed by filtration, and the filtrate was concentrated to give 1-
syn as a yellow solid (80%). X-ray-quality crystals were obtained by
1
recrystallization from hot hexanes. H NMR (400 MHz, CDCl3): δ
8.08 (ddd, JHH = 8.0, 1.3, 0.80 Hz, 2H), 8.04 (ddd, JHH = 7.8, 1.3. 0.70
Hz, 2H), 7.45 (ddd, JHH = 7.9, 7.0, 1.3 Hz, 2H), 7.40 (ddd, JHH = 7.8,
29
29
7.0, 1.3 Hz, 2H), 0.78 (s, J
= 6.8 Hz, 6H), 0.64 (s, J SiH = 6.8 Hz,
SiH
6H) ppm. 13C{1H} NMR (100 MHz, CDCl3): δ 146.8, 144.9, 144.5,
142.9, 124.5, 124.4, 124.1, 122.6, 1.69, 0.11 ppm. 29Si{1H} NMR (79
MHz, CDCl3): δ −15.6, −19.4 ppm. HRMS (EI): calcd for
C20H20Si2S2 [M+] 380.0545, found 380.0528.
Synthesis of 1,1,4,4-Tetramethylbis(benzo[b]thieno)[2,3-
b:3′,2′-e][1,4-dihydro-1,4]silastannin (2-syn). This compound
was obtained on the basis of a procedure reported by Baumgartner
and co-workers.32 n-BuLi (1.6 M in hexanes, 0.69 mL) was added to a
solution of bis(3-bromobenzo[b]thiophene)dimethylsilicon32 (0.265
g, 0.55 mmol) in diethyl ether (90 mL) at −78 °C. After 55 min,
dimethyltin dichloride (134 mg, 0.61 mmol) in THF (0.5 mL) was
added dropwise and the solution was immediately warmed to room
temperature. After several hours it turned cloudy white and was stirred
at room temperature for a total of 16 h. The crude reaction mixture
was filtered through Al2O3 and the solvent removed in vacuo.
Evaporation of ether yielded a white foam that gave an off-white solid,
2-syn (0.207 g, 80%). 1H NMR (400 MHz, CDCl3): δ 8.04 (dd, JHH
7.0, 1.4 Hz, 2H), 7.87 (dd, JHH = 7.6, 1.4 Hz, 2H), 7.43 (ddd, JHH
=
=
CONCLUSIONS
■
7.4, 7.4, 1.0 Hz, 2H), 7.39 (ddd, JHH = 7.4, 7.4, 1.4 Hz, 2H), 0.71 (s,
119SnH
117SnH
In summary, we describe methodology to obtain a family of
benzothiophene flanked heteroacenes that contain disilin,
diborin, and silaborin six-membered-ring cores. The silicon-
containing compounds are prepared as their syn isomers, in
which the sulfur atoms of the benzothiophene wings are syn to
each other across the molecular core. In the case of the diborin
structure, the synthetic methodology employed leads to kinetic
favoring of the anti isomer. Each compound has been fully
characterized both structurally and in solution. The compounds
are generally poor chromophores, but the diborin species with
its electron-deficient, antiaromatic core offers distinctive
photophysical features in comparison to the silicon-containing
compounds.
6H), 0.68 (s, J = 58.5 Hz, J
= 56.0 Hz, 6H) ppm. 13C{1H}
NMR (100 MHz, CDCl3): δ 146.3, 146.0, 143.9, 143.7, 124.7 (1J119SnC
1
1
119SnC
119
= 11.1 Hz), 124.6, 124.3 ( J SnC = 19.5 Hz), 122.6, 2.2, −7.45 ( J
1
= 360.3 Hz) ppm. 119Sn{1H} NMR (149 MHz,
117SnC
= 375.4 Hz, J
CDCl3): δ −109.9 ppm. 29Si{1H} NMR (79 MHz, CDCl3): δ −14.7
ppm. HRMS (EI): calcd for C20H20SiS2Sn [M+] 471.9798, found
471.9795.
Synthesis of 1,1-Dimethyl-4-mesitylbis(benzo[b]thieno)[2,3-
b:3′,2′-e][1,4-dihydro-1,4]silaborin (3-syn). BCl3 gas (714 mg, 6.1
mmol) was condensed into a solution of 2-syn (225 mg, 0.48 mmol)
in toluene (45 mL) at −78 °C. The reaction mixture turned a light
brown-yellow and was gradually warmed to room temperature then
stirred for 60 min. At this point, solvent was removed in vacuo and the
crude mixture was brought into an inert-atmosphere glovebox and
redissolved in a minimal amount of toluene (2 mL). A suspension of
mesityllithium (239 mg, 1.94 mmol) in toluene (2 mL) was then
added dropwise and the reaction mixture stirred for 16 h at room
temperature. Toluene was removed in vacuo and the crude mixture
filtered over Fluorisil and washed with dichloromethane. After removal
of CH2Cl2, the crude solid was triturated in hexanes and filtered to
EXPERIMENTAL SECTION
General experimental procedures are described in the Supporting
Information.
■
Synthesis of Bis(benzo[b]thiophene)dimethylsilane. n-BuLi
(1.6 M in hexanes, 0.84 mL) was added to a solution of 3-
bromobenzo[b]thiophene (285 mg, 1.34 mmol) in diethyl ether (80
mL) at −78 °C, and the solution turned yellow. After 30 min, neat
dimethylsilicon dichloride (0.08 mL, 0.67 mmol) was added dropwise
1
yield 3-syn as a yellow solid (66.5 mg, 31%). H NMR (400 MHz,
CDCl3): δ 7.96 (d, JHH = 8.0 Hz, 2H), 7.29 (ddd, JHH = 7.4, 7.4, 1.3
Hz, 2H), 7.17 (d, JHH = 8.3 Hz, 2H), 7.11 (ddd, JHH = 7.6, 7.6, 1.3 Hz,
2H), 6.99 (s, 2H), 2.50 (s, 3H), 2.03 (s, 6H), 0.70 (s, 6H) ppm.
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13C{1H} NMR (100 MHz, CDCl3): δ 158.6, 145.9, 144.3, 137.2,
ACKNOWLEDGMENTS
■
137.0, 127.6, 125.3, 124.8, 124.2, 121.9, 21.8, 21.5, −0.79 (two peaks
for carbon atoms bonded to the boron atom were not observed due to
quadrupolar relaxation) ppm.11B{1H} NMR (128 MHz, CDCl3): δ
56.9 ppm. 29Si{1H} NMR (79 MHz, CDCl3): δ −16.6 ppm. HRMS
(EI): calcd for C27H25BSiS2 [M+] 452.1260, found 452.1282.
W.E.P. acknowledges the NSERC of Canada for a Discovery
Grant and the Canada Council of the Arts for a Killam
Research Fellowship (2012-14). L.G.M. thanks Alberta
Innovates Technology Futures for a Studentship. W.C. thanks
the UK Engineering and Physical Sciences Research Council
for equipment funding and Diamond Light Source for access to
synchrotron facilities.
Synthesis of l,4-Dimesitylbis(benzo[b]thieno)[2,3-b:2′,3′-
e][1,4-dihydro-1,4]diborin (4-anti). BBr3 gas (∼3 mL, ∼10
equiv) was condensed into a thick-walled glass vessel containing
solid 2-syn (149 mg, 0.32 mmol) at −78 °C and gradually warmed to
room temperature. The reaction mixture was stirred at 50 °C for 2
days. Excess BBr3 was removed in vacuo, and the crude mixture was
brought into a glovebox and redissolved in a minimal amount of
toluene (2 mL). A suspension of mesityllithium (81 mg, 0.64 mmol)
in toluene (2 mL) was then added dropwise and the reaction mixture
stirred for 16 h at room temperature. Toluene was removed in vacuo
and the crude mixture filtered through an Acros disk, washing with
dichloromethane. After removal of CH2Cl2, 4-anti was obtained as a
red solid (75 mg, 45%). X-ray -quality crystals were grown by slow
evaporation of hexanes into a concentrated CH2Cl2 solution of 4-anti
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