Electron-donating tetrathienyl-substituted borole

Article abstract of DOI:10.1002/anie.201201156

Green borole debuts: A heteroaryl-substituted borole with peripheral thienyl groups has a significantly high-lying HOMO in addition to a low-lying LUMO, and thus unusual photophysical and electrochemical properties. These results highlight the highly electron-donating character of the borole ring. Copyright

Full text of DOI:10.1002/anie.201201156

Angewandte
Chemie
DOI: 10.1002/anie.201201156
Boron Heterocycles
Electron-Donating Tetrathienyl-Substituted Borole**
Takafumi Araki, Aiko Fukazawa, and Shigehiro Yamaguchi*
Borole is a boron-containing unsaturated five-membered ring
atom, because boron is a more electron-positive element than
carbon. In order to highlight this feature, we designed
a thienyl-substituted borole 3 (Scheme 1) with a pronounced
electron-donating character as a new example of borole-
with 4 p electrons, and is isoelectronic to a cyclopentadienyl
cation (C5H5 ).[1] The unusual electronic structures of borole-
+
based p-electron-containing materials have been predicted by
theoretical studies.[2,3] After the first synthesis of a borole
derivative, 1,2,3,4,5-pentaphenylborole (1a, Scheme 1), by
Eisch and coworkers,[4,5] only few reports on boroles have
appeared for almost four decades, and the crystal structure of
1a has only quite recently been determined by X-ray
crystallography.[6,7] Thereupon, the interest in this chemistry
has been dramatically revived.[6–16] Various intriguing borole
derivatives have been reported within the past several years,
including fused boroles,[8–11] ferrocenylborole,[6,12] halobor-
oles,[13] borole–metal complexes,[14] carbene-coordinated bor-
oles,[15] and the highly Lewis-acidic perfluorinated boroles.[8,16]
The most notable electronic feature of the borole skeleton
in comparison to other heteroles is its significantly low-lying
LUMO.[1,2] Therefore, borole-based p-conjugated compounds
have been widely recognized as electron-accepting systems.
The low-lying LUMOs of these compounds in combination
with their antiaromaticity allow them to undergo various
Scheme 1. Various pentaarylboroles. Fc=ferrocenyl.
based p-conjugated compounds. In a theoretical study, Ma
and co-workers suggested the unusual electronic structures of
borole/thiophene-alternating co-oligomers with a significant
contribution of a quinoid structure as well as a distinct
biradical character.[3] Herein, we report the synthesis of the
tetrathienyl-substituted borole 3 as the first example of
a heteroaryl-substituted borole. The study on the crystal
structure and fundamental properties of 3 demonstrated
several characteristic features of the borole as an electron-
donating building unit.
reactions, such as the formation of adducts with Lewis acids
[4]
and bases,[4c,13a,c,15] B C bond cleavage, Diels–Alder reac-
ꢀ
tions with alkynes,[4,16c] and reduction to a radical anion[12] or
dianion.[7,12,13b] Recently, the activation of H2 without any
transition metal has also been demonstrated by the use of
borole derivatives.[16b] The Lewis acidity of the perarylated
boroles 1 can be significantly enhanced by modification of the
peripheral aryl groups by introducing electron-withdrawing
substituents, as exemplified by perfluorophenylborole 2
described by Piers and co-workers.[16] The utilization of the
borole skeleton as an electron-accepting unit in the extended
p-conjugated system has been also demonstrated.[9b]
1-Mesityl-2,3,4,5-tetrakis(2-methylthienyl)borole
3 was
successfully obtained by a straightforward two-step synthesis
from bis(5-methyl-2-thienyl)acetylene 4 (Scheme 2). Alkyne
4 was first converted to 1,4-diiodobutadiene 5 via a zircona-
cyclopentadiene intermediate followed by treatment with I2
in the presence of CuCl, according to the method described by
Takahashi and co-workers.[17] Diiodide 5 was successively
treated with nBuLi and a 1.0m solution of MesBCl2 in toluene
to give a dark green solution. Purification by recrystallization
from hexane under an argon atmosphere afforded 3 in 27%
yield as an air- and moisture-sensitive green solid.[18]
In our studies, we noticed the other important feature of
the borole ring, its relatively high-lying HOMO compared
with other electron-accepting heteroles, such as silole and
phosphole.[2] This difference might be due to the inductive
effect that results from the s-donating character of the boron
The single crystals of the tetrathienylborole 3 were
obtained by slow evaporation of a solution of 3 in hexane,
and the structure was verified by X-ray crystallography
[*] T. Araki, Dr. A. Fukazawa, Prof. Dr. S. Yamaguchi
Department of Chemistry
Graduate School of Science, Nagoya University
Furo, Chikusa, Nagoya 464-8602 (Japan)
E-mail: yamaguchi@chem.nagoya-u.ac.jp
Homepage: http://orgreact.chem.nagoya-u.ac.jp/en/Home.html
Prof. Dr. S. Yamaguchi
CREST (Japan) Science and Technology Agency (JST)
Furo, Chikusa, Nagoya 464-8602 (Japan)
[**] This work was supported by a Grant-in-Aid (No. 19675001) from the
Ministry of Education, Culture, Sports, Science, and Technology of
Japan, and CREST (JST).
Scheme 2. Synthesis of 3. Reagents and conditions: a) Cp2ZrCl2,
nBuLi, THF, ꢀ788C!RT, then I2, CuCl, ꢀ788C!RT; b) nBuLi, toluene,
RT, then MesBCl2, ꢀ788C!RT. Cp=cyclopentadienyl, Mes=2,4,6-
trimethylphenyl.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201156.
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(Figure 1). There is no noticeable intermolecular interaction
around the boron center in the packing structure. This
packing structure is different from that of pentaphenylborole
1a[6] (see the Supporting Information). The borole ring in 3
on the photophysical and electrochemical properties. To
assess these issues, DFT calculations for 3 in both the singlet
and triplet ground states were conducted at the B3LYP/6-
31G(d) level of theory. The structural parameters of the X-ray
crystal structure are in good agreement with those of the
optimized geometry in the singlet state, and not the triplet
state. In addition, the NMR signals of 3 at 258C did not show
any signal broadening (see Figure S5 in the Supporting
Information), thus suggesting that 3 exists in the singlet
ground state at this temperature. The nucleus-independent
chemical shift (NICS) calculations were also conducted at the
HF/6-31 + G(d,p)//B3LYP/6-31G(d) level to evaluate the
antiaromaticity of the borole ring.[19] The NICS(0) value of
the borole ring of 3 in the singlet state is + 12.7 ppm, which is
almost comparable with that calculated for pentaphenylbor-
ole 1a (+ 12.9 ppm). The incorporation of thienyl groups does
not affect the antiaromaticity of the borole ring, which is
consistent with the fact that the alternation of the bond
lengths in the borole ring in 3 is comparable to that in 1b.
Notably, this result is in contrast to the fact that the
thiophene-fused borole has an enhanced antiaromaticity,
which we have revealed recently.[11] On the other hand, the
calculated singlet–triplet energy difference (DES–T) was only
7.2 kcalmolꢀ1 for 3, which was significantly smaller than that
of pentaphenylborole 1a (15.4 kcalmolꢀ1).[7] This striking
effect on the DES–T is presumably attributable to the differ-
ence in the HOMO–LUMO gap. The lower aromaticity of
thiophene compared with that of benzene might give rise to
the contribution of quinoid structure to some extent.
Figure 1. Thermal ellipsoid plots of 3 (50% probability for thermal
ellipsoids). Hydrogen atoms are omitted for clarity.
adopts a planar geometry, in which the bond lengths within
ꢀ
the butadiene moiety show a large alternation: C1 C2:
ꢀ
ꢀ
1.363(6) ꢀ, C2 C3: 1.524(6) ꢀ, C3 C4: 1.369(6) ꢀ. The
alternation of the bond lengths within the butadiene moiety
of the borole ring is comparable to that observed for the
ꢀ
already reported 1-aryl-2,3,4,5-tetraphenylboroles 1b–e (C1
The tetrathienylborole 3 exhibits a bright green color in
toluene (Figure 2). This green color is noticeable, because all
the known pentaarylboroles 1 and 2 are dark blue. In the UV/
Vis/NIR absorption spectrum, 3 shows a weak absorption
band with the lmax of 735 nm (e = 1710). This maximum
wavelength is red-shifted by 175 nm and the molar absorption
coefficient is about five times larger than those of the
pentaphenylborole 1a (lmax = 560 nm, e = 360).[6] These dif-
ferences demonstrate the significant perturbation on the
electronic structure of borole by the peripheral thiophene
rings.
ꢀ
ꢀ
C2: 1.358–1.362 ꢀ, C2 C3: 1.518–1.529 ꢀ, C3 C4: 1.353–
1.362 ꢀ),[7] thus indicating the significant antiaromatic char-
acter of the borole ring in 3 (see below).
The other notable feature in the crystal structure of 3 is
the conformation of the peripheral thienyl groups. While the
dihedral angles between the borole ring and the aryl groups at
the 1, 3, and 4 positions are large (74.18, 71.28, and 43.48, for
the 1-mesityl, 3-thienyl, and 4-thienyl groups, respectively),
the dihedral angles between the borole ring and the thiophene
rings at the 2 and 5 positions are much smaller (10.78 and
38.28, respectively). These small dihedral angles at the 2 and
5 positions indicate the effective expansion of p conjugation
over the 2,5-dithienylborole framework. In line with this
ꢀ
point, the C C bonds between the borole and the aryl groups
at the 2 and 5 positions in 3 are shorter than those in 1b (3:
1.448(6) and 1.467(6) ꢀ, 1b: 1.486(2) ꢀ). This difference
might reflect the stronger interaction between the borole and
thiophenes in 3 compared with 1b. In addition, in 3 the sulfur
atoms of the thiophene rings at the 2 and 3 positions and the 4
and 5 positions, respectively, are facing each other, and are in
close proximity to each other, with interatomic distances of
3.432 ꢀ (2–3) and 3.495 ꢀ (4–5), respectively, which are
slightly smaller than the sum of the van der Waals radii of two
sulfur atoms (3.70 ꢀ).
Regarding the effect of the peripherally substituted
thienyl groups in 3 compared with the phenyl-substituted
analogues 1, we were interested 1) in the effect on the
aromaticity and the singlet–triplet energy difference, and 2) in
the electronic perturbation on the orbital levels and thereby
Figure 2. UV/Vis/NIR absorption spectrum of 3 in toluene. Inset:
photograph of 3 in toluene.
2
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To gain insights into the difference between 1 and 3, we
compared their electronic structures based on the DFT
calculations. Figure 3 shows the comparison of the HOMO
and LUMO energy levels for 1a and 3 together with those of
the tetrathienylheterole model compounds 6 and 7. While the
The unusual electronic structure of 3 can be experimen-
tally confirmed by cyclic voltammetry. We successfully
conducted the measurement by using inert nBu4N[B(C6F5)4]
as the supporting electrolyte (Figure 4).[22,23] Compound 3
shows a reversible redox wave that corresponds to one-
electron reduction at the low reduction poten-
tial of E1/2 = ꢀ1.57 V (vs Fc/Fc+). This potential
is more positive than that of ferrocenylborole
1e (E1/2 = ꢀ1.96 V) described by Braunschweig
and co-workers,[12] thus demonstrating the high
electron-accepting character of the tetrathienyl
derivative. Notably, 3 also shows a reversible
redox wave that corresponds to one-electron
oxidation at the significantly low oxidation
potential of E1/2 = ꢀ0.04 V (vs. Fc/Fc+). It is
remarkable that the p-conjugated skeleton
without any electron-donating groups, such as
an NR2 group, can have such a low oxidation
potential as 3.
In summary, we have succeeded in the
synthesis and characterization of the tetra-
thienyl-substituted borole 3, which is the first
example of
a heteroaryl-substituted borole
derivative. The combination of the electron-
deficient and antiaromatic borole ring with the
electron-donating aromatic thiophene rings
produced a new type of borole-based p-conju-
gated system with an unusual electronic struc-
ture. The incorporation of the electron-donating
thienyl groups on the borole skeleton accen-
tuates the electron-donating character of the
borole ring, and leads to the narrow HOMO–
LUMO gap and thus the characteristic absorp-
Figure 3. Pictorial representations of the Kohn–Sham HOMOs and LUMOs and their
energy levels for a series of pentaarylboroles, calculated at the B3LYP/6-31G(d) level of
theory.
LUMO of 3 is only 0.04 eV lower than that of 1a, 3 has a much
higher-lying HOMO than 1a. This difference in the HOMO
level, which amounts to 0.54 eV, is obviously the origin of the
significantly red-shifted lmax of 3. We confirmed by the TD-
DFT calculation that the absorption band at 735 nm can
indeed be assigned to the HOMO–LUMO transition. This
difference in the HOMO level reflects the highly electron-
donating character of the thiophene rings as well as the higher
coplanarity of the 2,5-diarylborole moiety in 3.
More importantly, the tetrathienylborole 3 can be recog-
nized as a highly electron-donating p-conjugated system that
also possesses a highly electron-accepting character. Even
compared with the phosphole oxide congener 7, which
=
contains the highly electron-withdrawing P O group, borole
Figure 4. Cyclic voltammogram of 3 in CH2Cl2 (1.0 mm), measured
with nBu4N[B(C6F5)4] (0.1m) as a supporting electrolyte at a scan rate
3 has a much lower-lying LUMO by 0.53 eV. Moreover, the
HOMO level of 3 is also slightly higher than that of a silole
derivative 6, which is a congener with an electron-positive
silicon atom. As a consequence, the lmax of 3 is significantly
longer than those of the known analogous silole and phosp-
hole oxide derivatives 6’ (1,1-dimethyl-2,5-dithienylsilole;
lmax 415 nm)[20] and 7’ (1-phenyl-2,3,4,5-tetrathienylphosphole
oxide; lmax 434 nm).[21] These comparisons clearly demon-
strate the uniqueness of the borole skeleton as the building
unit for p-conjugated systems.
of 100 mVsꢀ1
.
tion and electrochemical properties, which are significantly
different from those of the known phenyl-substituted boroles.
This result has shed light on the hidden nature of boroles,
which are electron-accepting as well as electron-donating
heteroles. These findings should provide an important guide-
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Chiu, A. Damme, K. Ferkinghoff, K. Kraft, K. Radacki, J.
Wahler, Organometallics 2011, 30, 3210.
[14] H. Braunschweig, C.-W. Chiu, K. Radacki, P. Brenner, Chem.
Commun. 2010, 46, 916.
[15] a) H. Braunschweig, C.-W. Chiu, K. Radacki, T. Kupfer, Angew.
Chem. 2010, 122, 2085; Angew. Chem. Int. Ed. 2010, 49, 2041;
b) H. Braunschweig, C.-W. Chiu, T. Kupfer, K. Radacki, Inorg.
Chem. 2011, 50, 4247.
[16] a) C. Fan, W. E. Piers, M. Parvez, Angew. Chem. 2009, 121, 2999;
Angew. Chem. Int. Ed. 2009, 48, 2955; b) C. Fan, L. G. Mercier,
W. E. Piers, H. M. Tuononen, M. Parvez, J. Am. Chem. Soc. 2010,
132, 9604; c) C. Fan, W. E. Piers, M. Parvez, R. McDonald,
Organometallics 2010, 29, 5132.
[17] a) C. Xi, S. Huo, T. H. Afifi, R. Hara, T. Takahashi, Tetrahedron
Lett. 1997, 38, 4099; b) A. J. Ashe, J. W. Kampt, S. Pilotek, R.
Rousseau, Organometallics 1994, 13, 4067; c) U. Bankwitz, H.
Sohn, D. R. Powell, R. West, J. Organomet. Chem. 1995, 499,
C7 – C9; d) W. P. Freeman, T. D. Tilley, O. M. Liable-Sands,
A. L. Rheingold, J. Am. Chem. Soc. 1996, 118, 10457.
[18] The reaction of di(2-thienyl)acetylene without methyl groups
resulted in formation of a complex mixture, thus demonstrating
the importance of the methyl group for the isolation of the
tetrathienylborole.
line for the design of more fascinating boron-based p
materials.
Received: February 11, 2012
Published online: && &&, &&&&
Keywords: antiaromaticity · borole · boron · conjugation ·
.
thiophene
[1] H. Braunschweig, T. Kupfer, Chem. Commun. 2011, 47, 10903.
[2] U. Salzner, J. B. Lagowski, P. G. Pickup, R. A. Poirier, Synth.
Met. 1998, 96, 177.
[3] H. Cao, J. Ma, G. Zhang, Y. Jiang, Macromolecules 2005, 38,
1123.
[4] a) J. J. Eisch, N. K. Hota, S. Kozima, J. Am. Chem. Soc. 1969, 91,
4575; b) J. J. Eisch, J. E. Galle, J. Organomet. Chem. 1977, 127,
C9; c) J. J. Eisch, J. E. Galle, S. Kozima, J. Am. Chem. Soc. 1986,
108, 379; d) J. J. Eisch, J. E. Galle, B. Shafii, A. L. Rheingold,
Organometallics 1990, 9, 2342.
[5] P. J. Fagan, W. A. Nugent, J. C. Calabrese, J. Am. Chem. Soc.
1994, 116, 1880.
[6] H. Braunschweig, I. Fernꢁndez, G. Frenking, T. Kupfer, Angew.
Chem. 2008, 120, 1977; Angew. Chem. Int. Ed. 2008, 47, 1951.
[7] C.-W. So, D. Watanabe, A. Wakamiya, S. Yamaguchi, Organo-
metallics 2008, 27, 3496.
[19] a) P. v. R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao, N. J. R.
van Eikema Hommes, J. Am. Chem. Soc. 1996, 118, 6317; b) H.
Fallah-Bagher-Shaidaei, C. S. Wannere, C. Corminboeuf, R.
Puchta, P. v. R. Schleyer, Org. Lett. 2006, 8, 863.
[8] P. A. Chase, W. E. Piers, B. O. Patrick, J. Am. Chem. Soc. 2000,
122, 12911.
[20] M. Katkevics, S. Yamaguchi, A. Toshimitsu, K. Tamao, Organo-
metallics 1998, 17, 5796.
[9] a) S. Yamaguchi, T. Shirasaka, S. Akiyama, K. Tamao, J. Am.
Chem. Soc. 2002, 124, 8816; b) A. Wakamiya, K. Mishima, K.
Ekawa, S. Yamaguchi, Chem. Commun. 2008, 579.
[10] S. Kim, K. Song, S. O. Kang, J. Ko, Chem. Commun. 2004, 68.
[11] A. Iida, S. Yamaguchi, J. Am. Chem. Soc. 2011, 133, 6952.
[12] H. Braunschweig, F. Breher, C.-W. Chiu, D. Gamon, D. Nied, K.
Radacki, Angew. Chem. 2010, 122, 9159; Angew. Chem. Int. Ed.
2010, 49, 8975.
[21] C. Hay, C. Fischmeister, M. Hissler, L. Toupet, R. Rꢂau, Angew.
Chem. 2000, 112, 1882; Angew. Chem. Int. Ed. 2000, 39, 1812.
[22] All attempts for the cyclic voltammetry (CV) measurement with
nBu4NPF6 as electrolyte were unsuccessful because of the
decomposition of 3 under the applied conditions. The use of
nBu4NB(C6F5)4 (see Ref. [23]) with its less nucleophilic anion as
electrolyte was essential.
[23] R. J. LeSuer, W. E. Geiger, Angew. Chem. 2000, 112, 254;
Angew. Chem. Int. Ed. 2000, 39, 248.
[13] a) H. Braunschweig, T. Kupfer, Chem. Commun. 2008, 4487;
b) H. Braunschweig, C.-W. Chiu, J. Wahler, K. Radacki, T.
Kupfer, Chem. Eur. J. 2010, 16, 12229; c) H. Braunschweig, C.-W.
4
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Angew. Chem. Int. Ed. 2012, 51, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
Boron Heterocycles
Green borole debuts: A heteroaryl-sub-
stituted borole with peripheral thienyl
groups has a significantly high-lying
HOMO in addition to a low-lying LUMO,
and thus unusual photophysical and
electrochemical properties. These results
highlight the highly electron-donating
character of the borole ring.
T. Araki, A. Fukazawa,
S. Yamaguchi*
&&&& — &&&&
Electron-Donating Tetrathienyl-
Substituted Borole
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5
These are not the final page numbers!