Polycyclic π-electron system with boron at its center

Article abstract of DOI:10.1021/ja3036042

We disclose a new planarized triarylborane in which the tri-coordinated boron atom is embedded in a fully fused polycyclic π-conjugated skeleton. The compound shows high stability toward oxygen, water, and silica gel, despite the absence of steric protection around the B atom. Reflecting the electron-donating character of the π-skeleton and the electron-accepting character of the B atom, this compound shows broad absorption bands that cover the entire visible region and a fluorescence in the visible/near-IR region. In addition, this compound shows dramatic property changes upon formation of a tetra-coordinated borate, such as thermochromic behavior in the presence of pyridine.

Full text of DOI:10.1021/ja3036042

Communication
pubs.acs.org/JACS
Polycyclic π-Electron System with Boron at Its Center
Shohei Saito,* Kyohei Matsuo, and Shigehiro Yamaguchi*
Department of Chemistry, Graduate School of Science, Nagoya University and JST-CREST, Furo, Chikusa, Nagoya 464-8602, Japan
S
* Supporting Information
ABSTRACT: We disclose a new planarized triarylborane
in which the tri-coordinated boron atom is embedded in a
fully fused polycyclic π-conjugated skeleton. The com-
pound shows high stability toward oxygen, water, and silica
gel, despite the absence of steric protection around the B
atom. Reflecting the electron-donating character of the π-
skeleton and the electron-accepting character of the B
atom, this compound shows broad absorption bands that
cover the entire visible region and a fluorescence in the
visible/near-IR region. In addition, this compound shows
dramatic property changes upon formation of a tetra-
coordinated borate, such as thermochromic behavior in the
presence of pyridine.
Figure 1. (a) Methylene-bridged planarized triarylborane 1 and (b)
fully π-conjugated planarized triarylborane 2.
To compensate for these drawbacks and produce a more
sophisticated B-containing π-system, we have now synthesized
a new planarized triarylborane 2, in which the three aryl groups
on the B atom are fully conjugated to one another (Figure 1b).
This compound can be regarded as a model of a B-doped
graphene fragment. We reveal that the high planarity indeed
allows this triarylborane skeleton to form a face-to-face π-
stacked structure. Moreover, this polycyclic π-skeleton shows
interesting photophysical properties due to the B atom, such as
long-wavelength absorption and emission as well as a
thermoresponsive color change in the presence of a Lewis base.
Our initial target molecule for a peripherally π-conjugated
triarylborane was fully planarized tris(benzo[b]thienyl)borane 2
(π = benzothiophene). We envisioned that this skeleton could
be constructed by three-fold intramolecular coupling from
tris(4-bromo-3-benzothienyl)borane 3 (Scheme 1a). The
precursor 3 was obtained in 64% yield from 4-bromo-3-
iodobenzothiophene. A crystal structure analysis confirmed that
3 has a propeller-like structure suitable for C3-symmetric three-
fold cyclization (Supporting Information (SI)). We first
attempted intramolecular Pd-catalyzed C−H arylation under
various conditions with bases such as K2CO3 and DBU.5
However, these reactions resulted in the undesired B−C bond
cleavage. Therefore, an intramolecular radical cyclization6 using
n-Bu3SnH with AIBN as initiator in toluene at 120 °C was next
conducted. Although this reaction did not produce the target
product, we were able to isolate a doubly cyclized compound 4
in 7% yield. This result suggested some important implications.
Thus, the B−C bonds of the triarylborane can remain intact
under the radical reaction conditions. The homocoupling of the
two C−Br bonds precedes the cross-coupling between the C−
Br and C−H bonds, resulting in the formation of the B-
containing seven-membered ring. Moreover, the doubly
cyclized product 4 is barely stable enough to isolate by column
chromatography on silica gel. These insights prompted us to
synthesize the next target 5, which has an anthracene skeleton
in place of one of the benzothiophene skeletons (Scheme 1b).
oron-containing π-conjugated materials have been exten-
B
sively studied because of their intriguing electronic
features, including strong electron-accepting character and
strong Lewis acidity, which make promising themselves for use
as emissive organic solid materials, two-photon absorbing
materials, electron-transporting materials, and chemosensors
for anions.1 In designing new B-containing π-systems, one of
the central issues to overcome is the intrinsic instability against
moisture and oxygen. Two approaches have been generally
adopted. One is the introduction of bulky substituents to
sterically protect the Lewis acidic B atom.2 However, bulky
substituents prevent intermolecular interactions in the solid
state, which is disadvantageous for gaining high carrier
transporting properties. The other approach is the replacement
of B−C bonds with B−N or B−O bonds, which has yielded a
number of fascinating π-systems with reasonable stabilities.3
However, interaction between the lone-pair electrons of the
heteroatom and the vacant p orbital of the B atom diminishes
the inherent virtue of the boron compounds, i.e., the electron-
accepting character and the Lewis acidity.
We recently disclosed the structural constraint as an
alternative strategy for stabilizing organoboron compounds.
We synthesized a triphenylborane 1 (π = benzene in Figure
1a), rigidly fixed in a planar fashion with three methylene
tethers, and demonstrated that this compound showed a high
stability toward water, oxygen, amines, and silica gel.4 In
addition, the completely planarized skeleton effectively spreads
the π-conjugation among the three phenyl groups through the
B atom, leading to some intriguing properties. However, as the
sp3 carbon bridges do not expand the π-electron delocalization,
1 showed only an absorption band in the UV region. Moreover,
the dimethyl groups on the bridging methylene moieties in 1
still prevented intermolecular contact in the crystalline state.
Received: April 21, 2012
Published: May 17, 2012
© 2012 American Chemical Society
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coordinated B atom is embedded in the center of the 10-ring-
fused π-skeleton in the form of a borepin2d,e-like seven-
membered ring (Figure 2a). The high planarity is confirmed by
Scheme 1. Synthesis of Planarized Triarylboranes
Figure 2. (a) Crystal structure of 5 (50% probability for thermal
ellipsoids) and (b) π-stacked dimer in the crystal packing.
the small mean plane deviation of 0.15 Å as well as the small
dihedral angles between the anthracene and benzothiophene
planes (1.5° and 4.0°, SI). The central B atom is confined to
the planar π-skeleton with the sum of the three C−B−C angles
of 360.0°. The B−Cthienyl (1.508(2) and 1.513(2) Å) and B−
C
anthryl (1.539(2) Å) bond lengths are much shorter than those
of triphenylborane (1.57−1.59 Å)10 and trimesitylborane
(1.57−1.58 Å),11 and comparable to those of the methylene-
bridged triphenylborane 1 (1.52 Å).4 Importantly, in the crystal
packing, 5 forms π-stacked dimers with a mean plane distance
of 3.53 Å (Figure 2b). This is an unprecedented structure for a
triarylborane-based π-system. The bulky Mes groups are
arranged almost perpendicular to the π-system, and octanes
are located above and below the π-stacked dimer.
The planarized borane 5 showed interesting photophysical
properties (see Figure 3a). Its toluene solution exhibited a
purple color and showed broad absorption bands that covered
the entire visible region of 400−730 nm, with maximum
wavelengths (λabs) of 527, 563, 620, and 668 nm. These
We envisioned that the incorporation of the anthracene unit
would allow us to employ the oxidative cyclization for
constructing a fully ring-fused π-skeleton in the final step.7
For the synthesis of 5, anthrylbis(bromobenzothienyl)-
boranes 7 were prepared as the key precursor from 4-bromo-
3-iodobenzothiophene and anthryldimethoxyboranes 6. Radi-
cal-promoted intramolecular homocoupling from 7 was then
conducted using (TMS)3SiH with 1,1′-azobis(cyclohexane-
carbonitrile) (ABCN) as an initiator to give the cyclized
products 8a and 8b in 33% and 71% yields, respectively. Crystal
structural analysis of 8a revealed the perpendicular arrangement
of the anthryl group to the di(benzothienyl)borane plane (SI).
When we attempted the oxidative cyclization of 8a using FeCl3,
only a black complex mixture was obtained. This is probably
due to the low selectivity of the intramolecular cyclization over
the intermolecular reactions.8 The poor solubility of the highly
planar product may also be a problem. To solve these
problems, we decided to introduce bulky mesityloxy (-OMes)
groups to the 4,5-positions of the anthracene unit, which should
suppress the intermolecular reaction as well as the aggregation
of the product and, at the same time, increase the reactivity at
the 1,8-positions, resulting in selective intramolecular cycliza-
tion.9 In fact, the reaction of 8b (R = OMes) with FeCl3
successfully produced 5 in 63% yield as a deep purple solid.
Notably, product 5 as well as all the precursors 6−8 are stable
enough to handle in the air and isolate by silica gel column
chromatography without any special precautions. Thus, a
glovebox system is unnecessary for the entire synthetic process.
The high stability of 5 despite the absence of steric protection is
attributable to the effect of the structural constraint. The rigid
and cyclic π-skeleton around the B atom retards a
decomposition process that proceeds through the reaction
with Lewis bases, because a tetra-coordinated intermediate is
destabilized and/or a C−B bond cleavage from the
intermediate is prevented by the chelating effect.4 The high
thermal stability of 5 is also demonstrated by thermogravi-
metric analysis: the decomposition temperature for a 5% weight
loss was 424 °C.
Figure 3. (a) UV/vis absorption (black line) and vis/near-IR
fluorescence (red line) spectra of 5 in toluene along with the
oscillator strengths (blue bars) obtained by the TD-DFT (B3LYP/6-
31G*) calculation. (b) Kohn−Sham molecular orbitals of 5 calculated
at the B3LYP/6-31G* level.
Single crystals of 5 suitable for X-ray diffraction analysis were
obtained by vapor diffusion of octane into a toluene solution of
the compound. The crystal structure confirmed that the tri-
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absorption maxima are significantly longer than the longest λabs
of 1 (320(sh) nm), demonstrating the impact of the peripheral
expansion of π-conjugation. Moreover, this compound showed
fluorescence in the vis/near-IR region with maximum wave-
length (λem) of 729 nm. Since most triarylboranes fluoresce
only in the visible region,12 this near-IR fluorescence is a unique
property for the present B π-system, although the quantum
yield is low, 0.016. These absorption and emission bands
showed negligible or only subtle solvent effects (SI).
To elucidate the photophysical properties, a DFT (B3LYP/
6-31G*) calculation was conducted for 5 (Figure 3b).13 This
compound has both a rather high-lying HOMO and a low-lying
LUMO. The HOMO is mainly delocalized over the “oxy-
phenylbenzothiophene dimer” moiety, while the HOMO−1 is
delocalized over the entire π-skeleton with a contribution from
the p orbital of the B atom. On the other hand, the LUMO is
mostly localized on the borylanthracene moiety with a
contribution from the p−π* interaction.1 The LUMO level of
5 (−2.56 eV) is much lower than that of 1 (−1.56 eV). The
TD-DFT calculation at the same level of theory suggested that
the broad visible absorption bands consist of two transitions
accompanied by vibronic structures, which are assignable to the
HOMO→LUMO and the HOMO−1→LUMO transitions.
Their calculated transition energies are 1.86 eV (666 nm) and
2.38 eV (522 nm), respectively. The smaller oscillator strength
for the longer-wavelength absorption band was also reproduced
by the TD-DFT calculation (SI). This calculation demonstrated
that the key factor for realizing absorption and emission at
significantly long wavelengths is to incorporate the electron-
accepting tri-coordinated B atom into the highly electron-
donating polycyclic π-skeleton.
To experimentally gain insights into the electronic structure,
cyclic voltammetry was conducted in THF using n-Bu4NPF6 as
the supporting electrolyte (SI). Notably, 5 showed reversible
redox waves both for oxidation and reduction, demonstrating
that the generated radical cation and radical anion are stable
under the measurement conditions. The first half-reduction and
oxidation potentials (E1/2) were −1.37 and 0.60 V (vs Fc/Fc+),
respectively.
The most important structural feature of 5 is that the tri-
coordinated B atom is surrounded by a rigid and planar π-
skeleton. We are interested in whether this B compound still
maintains Lewis acidity and forms a complex with a Lewis base.
To study this issue, we treated 5 with several Lewis bases
(Scheme 2).14 Upon the addition of n-Bu4NF or n-Bu4NCN to
at 1.6 ppm in CDCl3 by the addition of an excess amount of n-
Bu4NF. The binding constant (K) of 5 with a fluoride ion in
THF was determined by UV/vis titration to be 1.3 × 105 M−1,
which is slightly lower than those of the methylene-bridged
triphenylborane 1 (7.0 × 105 M−1)4 and trimesitylborane (3.3
× 105 M−1).14a This comparison demonstrates that 5 keeps its
Lewis acidity, but the degree is diminished by the structural
restriction. Interestingly, 5 has a much lower-lying LUMO than
1 (vide supra). Nevertheless, the Lewis acidity of 5 is weaker
than that of 1, demonstrating the impact of the more rigid
structure of 5.
When 5 was treated with a weaker Lewis base, pyridine, an
interesting thermochromism was observed: the color of a
pyridine/THF (1:3) solution of 5 changed dramatically from
purple at a higher temperature to yellow at a lower temperature
(see Figure 4a). We quantitatively analyzed this phenomenon
Figure 4. (a) Thermochromism of a 1:3 pyridine/THF solution of 5.
(b) Absorption spectral change of 5 (4.66 × 10−5 M) in a 1:15
pyridine/THF mixed solvent.
by UV/vis titration as well as variable-temperature UV/vis
absorption spectra. The binding constant of 5 with pyridine in
THF had the very low value of K = 0.35 M−1 at 23 °C. As the
temperature decreased from 60 to −80 °C, the absorption
bands at 563 and 668 nm decreased in intensity, and the
intensity of the bands at 485 and 461 nm increased, with
isosbestic points at 508 and 397 nm, as shown in Figure 4b.
Based on these results, the parameters for the equilibrium
between 5 and the 5·pyridine complex were estimated to be
ΔH = −21.3 kJ mol−1 and ΔS = −76.2 J K−1 mol−1. As a result,
the Gibbs energy change (ΔG) for this equilibrium becomes
negative at low temperature but positive at high temperature,
and thereby this system shows thermochromism. The weak
Lewis acidity of 5 is responsible for this thermoresponsive
behavior.
Finally, the structure of the tetra-coordinated 5·F− was
confirmed by X-ray crystallography as a K+([2.2.2]cryptand)
salt (Figure 5):15 the polycyclic π-skeleton still has high
planarity, and only the central B atom deviates from the π-
plane. The dihedral angles between the anthracene and
Scheme 2. Interconversion between 5 and the
Corresponding Borate by Reaction with a Lewis Base
a THF solution of 5, the color of the solution dramatically
changed from purple to yellow. The UV/vis spectra showed
that as the amount of the base increased, the absorption bands
of 5 around 670 and 560 nm disappeared and a new band at
478 nm appeared, which is assignable to a tetra-coordinated B
species (SI). In line with this change, the 11B NMR spectrum of
5 was converted from a broad peak at 39.5 ppm to a sharp one
Figure 5. X-ray crystal structure of 5·F−[K(cryptand)]+ (50%
probability for thermal ellipsoids). (a) Top view, in which
[K(cryptand)]+ was omitted for clarity, and (b) side view.
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(2) (a) Yamaguchi, S.; Shirasaka, T.; Akiyama, S.; Tamao, K. J. Am.
Chem. Soc. 2002, 124, 8816. (b) Yuan, Z.; Entwistle, C. D.; Collings, J.
C.; Albesa-Jove, D.; Batsanov, A. S.; Howard, J. A. K.; Taylor, N. J.;
́
Kaiser, H. M.; Kaufmann, D. E.; Poon, S.-Y.; Wong, W.-Y.; Jardin, C.;
Fathallah, S.; Boucekkine, A.; Halet, J.-F.; Marder, T. B. Chem. Eur. J.
2006, 12, 2758. (c) Wakamiya, A.; Mishima, K.; Ekawa, K.;
Yamaguchi, S. Chem. Commun. 2008, 579. (d) Mercier, L. G.; Piers,
W. E.; Parvez, M. Angew. Chem., Int. Ed. 2009, 48, 6108. (e) Caruso,
A., Jr.; Siegler, M. A.; Tovar, J. D. Angew. Chem., Int. Ed. 2010, 49,
4213.
(3) (a) Bosdet, M. J. D.; Sorensen, T. S.; Piers, W. E.; Parvez, M.
Angew. Chem., Int. Ed. 2007, 46, 4940. (b) Brothers, P. J. Chem.
Commun. 2008, 2090. (c) Marwitz, A. J. V.; Matus, M. H.; Zakharov,
L. N.; Dixon, D. A.; Liu, S.-Y. Angew. Chem., Int. Ed. 2009, 48, 973.
(d) Tsurumaki, E.; Hayashi, S.; Tham, F. S.; Reed, C. A.; Osuka, A. J.
Am. Chem. Soc. 2011, 133, 11956. (e) Hatakeyama, T.; Hashimoto, S.;
Seki, S.; Nakamura, M. J. Am. Chem. Soc. 2011, 133, 18614.
(4) Zhou, Z.; Wakamiya, A.; Kushida, T.; Yamaguchi, S. J. Am. Chem.
Soc. 2012, 134, 4529.
(5) (a) Reisch, H. A.; Bratcher, M. S.; Scott, L. T. Org. Lett. 2000, 2,
1427. (b) Alberico, D.; Scott, M. E.; Lautens, M. Chem. Rev. 2007, 107,
174. (c) Pascual, S.; Mendoza, P.; Echavarren, A. M. Org. Biomol.
Chem. 2007, 5, 2727.
benzothiophene units are 3.0° and 13.4°, and that between the
two benzothiophene units is 16.3°. The B−C bond lengths
(1.579(9), 1.584(8), and 1.596(9) Å) are longer than those of 5
but still much shorter than those of standard triarylfluor-
oborates (1.62−1.68 Å).16 The sum of the three C−B−C
angles of 337.1° in 5·F− corresponds to the small tetrahedral
character of 73%,17 indicating that the structural constraint
significantly restrains the deformation to the tetrahedral
structure. This fact should be relevant to the weak Lewis
acidity.
In summary, we have succeeded in the synthesis of a new
planarized triarylborane, in which tri-coordinated B is
embedded in the electron-donating polycyclic π-skeleton. The
compound shows remarkable chemical and thermal stabilities
and can be handled without special care. Different from the
previous methylene-bridged planar triphenylborane, the highly
planar skeleton indeed forms a face-to-face π-stacked structure.
In light of this structural feature as well as its interesting
electronic features with the narrow HOMO−LUMO gap, this
expanded π-conjugated borane should have significant potential
for optoelectronic applications. The interconversion between
the tri-coordinated borane and the tetra-coordinated borate is
the other fascinating feature of this boron-centered π-system,
which results in dramatic property changes, such as
thermochromic behavior in the presence of pyridine. Further
studies on the application of this boron π-system as well as the
synthesis of more sophisticated planar boron-centered π-
skeletons are currently underway in our laboratory.
(6) (a) Harrowven, D. C.; Guy, I. L.; Nanson, L. Angew. Chem., Int.
́
Ed. 2006, 45, 2242. (b) Rajca, A.; Miyasaka, M.; Xiao, S.; Boratynski,
P. J.; Pink, M.; Rajca, S. J. Org. Chem. 2009, 74, 9105. (c) Castillo, R.
R.; Burgos, C.; Vaqueromgrt, J. J.; Alvarez-Builla, J. Eur. J. Org. Chem.
2011, 619.
(7) (a) Watson, M.; Fechtenkotter, A.; Mullen, K. Chem. Rev. 2001,
̈
̈
101, 1267. (b) Wu, J.; Pisula, W.; Mullen, K. Chem. Rev. 2007, 107,
718.
̈
(8) King, B. T.; Kroulík, J.; Robertson, C. R.; Rempala, P.; Hilton, C.
L.; Korinek, J. D.; Gortari, L. M. J. Org. Chem. 2007, 72, 2279.
(9) (a) Davis, N. K. S.; Thompson, A. L.; Anderson, H. L. Org. Lett.
2010, 12, 2124. (b) Davis, N. K. S.; Thompson, A. L.; Anderson, H. L.
J. Am. Chem. Soc. 2011, 133, 30. (c) Zeng, L.; Jiao, C.; Huang, X.;
Huang, K.-W.; Chin, W.-S.; Wu, J. Org. Lett. 2011, 13, 6026.
(10) Zettler, F.; Hausen, H. D.; Hess, H. J. Organomet. Chem. 1974,
72, 157.
(11) (a) Blount, J. F.; Finocchiaro, P.; Gust, D.; Mislow, K. J. Am.
Chem. Soc. 1973, 95, 7019. (b) Olmstead, M. M.; Power, P. P. J. Am.
Chem. Soc. 1986, 108, 4235.
(12) (a) Agou, T.; Kobayashi, J.; Kawashima, T. Chem. Eur. J. 2007,
13, 8051. (b) Wakamiya, A.; Mori, K.; Yamaguchi, S. Angew. Chem., Int.
Ed. 2007, 46, 4273. (c) Kano, N.; Furuta, A.; Kanbe, T.; Yoshino, J.;
Shibata, Y.; Kawashima, T.; Mizorogi, N.; Nagase, S. Eur. J. Inorg.
Chem. 2012, 1584.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures; X-ray crystal structures (PDF, CIF)
of 5 (CCDC-873940), 5·F−[K(cryptand)]+ (CCDC-873941),
3, and 8a; theoretical calculations; cyclic voltammogram;
titration experiments; and complete ref 13. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
s_saito@chem.nagoya-u.ac.jp; yamaguchi@chem.nagoya-u.ac.jp
Notes
The authors declare no competing financial interest.
(13) The DFT calculations were performed by Gaussian 09 program
(SI).
ACKNOWLEDGMENTS
(14) (a) Sole,
́
S.; Gabbaï, F. P. Chem. Commun. 2004, 1284.
■
(b) Sundararaman, A.; Venkatasubbaiah, K.; Victor, M.; Zakharov, L.
This work was supported by a Grant-in-Aid (No. 19675001)
from the Ministry of Education, Culture, Sports, Science, and
Technology, Japan and CREST, JST. S.S. is grateful to the
Japan Society for the Promotion of Science for a Grant-in-Aid
for Research Activity Start-up. The authors thank Prof. A.
Osuka, Dr. N. Aratani, and Dr. T. Tanaka (Kyoto University)
for measurement of variable-temperature UV/vis absorption
spectra.
N.; Rheingold, A. L.; Jakle, F. J. Am. Chem. Soc. 2006, 128, 16554.
̈
(c) Pakkirisamy, T.; Venkatasubbaiah, K.; Kassel, W. S.; Rheingold, A.
L.; Jakle, F. Organometallics 2008, 27, 3056.
̈
(15) In the structure of 5·F−[K(cryptand)]+, two crystallographically
independent molecules were observed, one of which showed a
disorder in the core π-skeleton. Thus, the structural features of 5·F−
are discussed for the other, non-disordered molecule.
(16) All the crystal structures of triarylborane fluoride complexes
registered in the CCDC database were examined for reference.
(17) Toyota, S.; Oki, M. Bull. Soc. Chem. Jpn. 1992, 65, 1832.
̅
REFERENCES
■
(1) Reviews: (a) Entwistle, C. D.; Marder, T. B. Angew. Chem., Int.
Ed. 2002, 41, 2927. (b) Entwistle, C. D.; Marder, T. B. Chem. Mater.
2004, 16, 4574. (c) Jakle, F. Coord. Chem. Rev. 2006, 250, 1107.
̈
(d) Yamaguchi, S.; Wakamiya, A. Pure Appl. Chem. 2006, 78, 1413.
(e) Bosdet, M. J. D.; Piers, W. E. Can. J. Chem. 2008, 86, 8. (f) Wade,
C. R.; Broomsgrove, A. E. J.; Aldridge, S.; Gabbai, F. P. Chem. Rev.
̈
2010, 110, 3958. (g) Jakle, F. Chem. Rev. 2010, 110, 3985. (h) Hudson,
Z. M.; Wang, S. Dalton Trans. 2011, 40, 7805.
̈
9133
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