Synthesis and reactivity of a bis(dimesitylboryl)azaborine and its fluoride sensing ability

Article abstract of DOI:10.1039/b818505k

An azaborine bearing two dimesitylboryl groups on its periphery showed very strong light absorption and moderate photo-luminescence emission; the reaction of the title compound with fluoride ion resulted in multi-step fluoride ion complexation on the boro

Full text of DOI:10.1039/b818505k

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Synthesis and reactivity of a bis(dimesitylboryl)azaborine and its fluoride
sensing abilityw
Tomohiro Agou, Masaki Sekine, Junji Kobayashi and Takayuki Kawashima*
Received (in Cambridge, UK) 20th October 2008, Accepted 15th January 2009
First published as an Advance Article on the web 20th February 2009
DOI: 10.1039/b818505k
An azaborine bearing two dimesitylboryl groups on its periphery
showed very strong light absorption and moderate photo-
luminescence emission; the reaction of the title compound with
fluoride ion resulted in multi-step fluoride ion complexation
on the boron atoms of the dimesitylboryl groups, producing
Fig.
1 Azaborine and its periphery-functionalized analogue
mono- and bisfluoroborates.
(FG = functional group).
Triarylborane-based optical materials and anion sensors have
been extensively investigated. Because of its property as a
strong p-acceptor, as well as the switching of its optical
properties upon complex formation with Lewis bases,
triarylborane is one of the most powerful candidates for
organic functional materials.1
A synergic electronic interaction between several borane
units through p-orbitals is expected to achieve a further
decrease in the LUMO energy levels, resulting in effective
electron- and Lewis base-accepting materials that are
useful for n-type organic semiconductors and sensors.2,3 In
particular, the reaction of such oligoboryl p-conjugated
compounds with anions is interesting, because they can react
with anions in a multi-step fashion to give oligoanions, and the
absorption and emission colour can be changed by varing the
amount of guest anions.
Scheme 1
because it is expected to have dual functions. First, the Mes2B
group can improve the Lewis acidity of azaborine because of
its property as a p-acceptor that decreases the LUMO energy
level. Second, another donor–acceptor interaction between the
nitrogen atom and Mes2B groups is possible in addition to
that between the nitrogen atom and the central boron atom,
and the coordination of Lewis bases to the boron atoms can
switch the direction of donor–acceptor interactions, resulting
in multicolor sensing of Lewis bases.3c Here we report the
synthesis, structure, optical properties, and fluoride ion
complexation ability of a bis(dimesitylboryl)azaborine.
We have reported the syntheses, optical properties, and
reactivity of azaborines and their extended analogues
(Fig. 1).4 The introduction of various functional groups
around azaborines was achieved by taking advantage of
Bis(dimesitylboryl)azaborine 1 was synthesized from dibromo-
azaborine 2 as a pale yellow solid in moderate yield
(Scheme 1). 1 was stable towards air and moisture, because
of the effective steric protection of the three boron centres by
bulky groups, as shown by X-ray crystallographic analysis.6
The optical data of 1 and other azaborines are summarized
in Table 1. When electron-donating groups were introduced
dibromoazaborine as
a common intermediate. Donors
(amino groups) and electronically neutral functional groups
(carbon substituents) were successfully introduced. Azaborines
bearing a strong s-acceptor, such as ammonio or phosphonio
groups were also synthesized recently, and these cationic
azaborines showed enhanced Lewis acidity and water
solubility that are useful for anion sensing in aqueous media.5
Azaborines bearing strong p-acceptors, however, have not
previously been synthesized.
on the periphery of the azaborine core,
a substantial
bathochromic shift in lmax was observed (from 405 nm for 3
to 453 nm for 4), but there was little change in the extinction
coefficient (e).4c In contrast, the introduction of dimesitylboryl
groups resulted in a hypsochromic shift (from 405 nm for 3 to
377 nm for 1), and the value of e increased by a factor of
approximately 10 compared with other azaborines. To shed
light on such differences between the azaborine derivatives,
the electronic structures of azaborines were investigated by
theoretical calculations.
The introduction of a strong p-acceptor, such as the
dimesitylboryl (Mes2B) group should be worth investigating
Department of Chemistry, Graduate School of Science, The University
of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan.
E-mail: takayuki@chem.s.u-tokyo.ac.jp; Fax: +81-3-5800-6899;
Tel: +81-3-5800-6899
w Electronic supplementary information (ESI) available: Synthesis,
characterization and X-ray crystallographic analysis of 1, Cartesian
coordinates and molecular orbital diagrams for 10 and 30, 11B NMR
spectra of the reaction mixture of 1 with (n-Bu)4NF, the UV-Vis
titration curve, and 11B NMR and UV-Vis spectra of the reaction
mixture of 1 and NaCN. CCDC 697739. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/b818505k
Theoretical calculations were performed on model
azaborines using the Gaussian 03 program package at
B3LYP/6-31G(d) level of theory.7 In the case of azaborine
30, the LUMO is attributed to the p* orbital of azaborine,
mainly constructed from the vacant 2p orbital of the boron
ꢀc
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Table 1 Optical properties of azaborines in cyclohexane at 298 K
lmax/nm (log e)
lem/nm (F)
Stokes shift/cmꢁ1
1
377 (4.96)
405 (4.02)
453 (3.80)
402 (0.21)
421 (0.48)
501 (0.51)
1.65 ꢂ 103
0.94 ꢂ 103
2.12 ꢂ 103
3a
4b
Scheme 2
1 was treated with an excess amount of (n-Bu)4NF in THF,
and the reaction mixture was analyzed by negative-mode FAB
mass spectroscopy to show the formation of monofluoro-
borate 5a or 5b as well as bisfluoroborate 6a or 6b
(Scheme 2). This reaction was also monitored by 11B NMR
spectroscopy. First, a broad signal around dB 72 corres-
ponding to the Mes2B groups was diminished rapidly and
completely disappeared after the addition of more than 2
equivalents of fluoride ion, accompanied by the appearance
of a new signal at dB 5 due to the formation of fluoroborates.
On the other hand, another signal around dB 58, which is the
resonance of the boron nucleus in the azaborine core,
remained throughout the reaction. Judging from these data,
fluoride ion is thought to attack the Mes2B groups selectively
to form fluoroborate 5a and bisfluoroborate 6a. The forma-
tion of 6a can be explained by considering electrostatic
repulsion between the two fluoroborate moieties. 6b has two
closely located anionic centres and should be more unstable
than 6a. However, the main reason of the formation of 5a
rather than 5b is more obscure. The LUMO of 1 is distributed
over the two Mes2B groups (vide supra), and thus fluoride ion
may attack these groups kinetically, but we do not have
information about the thermodynamic stability balance
between 5a and 5b. Precise theoretical investigation on the
stability of these fluoroborates is now under way.
a
b
Ref. 4a. Ref. 4c.
atom in the azaborine core. In contrast, the LUMO of 10 is
distributed over the H2B groups and benzene rings, and the
atomic orbitals of the boron or nitrogen atom in the azaborine
core are not involved. In contrast, the HOMO and LUMO+1
of 10 are p and p* orbitals of azaborine, respectively, and thus
the photo-excitation of 10 is attributable to the intramolecular
charge transfer (ICT) from the nitrogen atom to the H2B
groups, which is quite different from the excited states of other
azaborines that are similar to the p–p* excited state of
anthracene.
The calculations also indicated that the energy levels of
HOMO, LUMO and LUMO+1 of 10 decreased compared
with the energy levels of the corresponding orbitals of 30
because of the electronic effect of the H2B groups (Fig. 2).
The decrease of HOMO level (0.85 eV) is bigger than that in
the LUMO level (0.65 eV), resulting in the increase in
HOMO–LUMO energy gap (+0.20 eV), which is correlated
with the blue-shift of the absorption maximum of
1
(vide supra). Due to the decreased LUMO level, 1 is expected
to have enhanced Lewis acidity and exhibit strong complexa-
tion ability against Lewis bases. In addition, as 1 has three
boron atoms that are capable of acting as Lewis acid centres,
stepwise complexation of fluoride ion up to three centres can
be possible, resulting in multi-step fluoride ion sensing. The
strong light-absorption and emission properties of 1 are also
useful for optical sensing, and thus the detection of a Lewis
base by 1 was investigated.z
The UV-Vis titration of 1 with (n-Bu)4NF was carried out in
THF at 298 K, and the absorption spectra were gradually
changed with two sets of isosbestic points, indicating the
stepwise formation of 5 and 6 (Fig. 3). The complex formation
constants K1 and K2 were determined by using a non-linear
least square fitting method.8 Although the value of K1 is
too high to be determined exactly, it was estimated to be
4108 Mꢁ1. This value is quite high for a triarylborane without
any additional effect to enhance Lewis acidity, such as chela-
tion or electrostatic effects (cf. Mes3B: K = 3.3 ꢂ 105 Mꢁ1 in
THF9). The value of K2 (= 7(1) ꢂ 105 Mꢁ1) was much lower
than that of K1, indicating that the formation of the borate ion
in 5 may assist charge transfer from the nitrogen to the
remaining boryl group, resulting in decreasing Lewis acidity
of the boryl group.
The complex formation could also be monitored by fluor-
escence spectroscopy (Fig. 4). When 2 equivalents of fluoride
ion was added, the fluorescence maximum around 400 nm
disappeared, and a new broad emission band developed,
indicating the quantitative formation of fluoroborate 5.
Further addition of fluoride ion resulted in the blue-shift of
the emission band on accordance with the conversion of 5 to 6.
Because of much smaller value of K2 than that of K1 as well as
Fig. 2 Calculated energy diagram of model azaborines 10 and 30.
ꢀc
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calculations indicated that the LUMO of the bis(dimesitylboryl)-
azaborine is mainly constructed from vacant 2p orbitals on
Mes2B groups unlike other azaborines. Therefore, its light-
absorption originates from an intramolecular charge transfer
from the nitrogen atom to the Mes2B groups. Theoretical
calculations also indicated the enhanced Lewis acidity of the
bis(dimesitylboryl)azaborine, as revealed by the complexation
titration with fluoride ion monitored by UV-Vis and
fluorescence spectroscopy.
Notes and references
z Cyanide ion complexation was also investigated by the reaction of 1
and NaCN in THF-d8 or THF, and the formation of the corres-
ponding cyanide complex was confirmed by 11B NMR and UV-Vis
spectroscopy. See ESIw for details.
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Fig. 3 UV-Vis spectral change of 1 upon the addition of (n-Bu)4NF
in THF at 298 K ([1] = 1.0 ꢂ 10ꢁ5 M).
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6 Crystallographic data for 1: C64H76B3N, M = 891.69, monoclinic,
space group P21/n, a = 14.1943(11), b = 23.5686(13), c =
16.7739(11) A, b = 108.4178(10)1, V = 5324.1(6) A3, T = 120 K,
Z = 4, m(Mo-Ka) = 0.062 mmꢁ1, 40 560 reflections measured,
11 920 unique (Rint = 0.0424) which were used in all calculations.
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respectively. GOF = 1.081.
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Fig. 4 Fluorescence spectra of 1 upon the addition of (n-Bu)4NF in
THF at 298 K ([1] = 1.0 ꢂ 10ꢁ6 M).
low concentration of 1, a large excess amount of fluoride ion
was necessary for this step. Because of its good optical proper-
ties (intense light-absorption and emission) as well as strong
complexation ability, the detection of fluoride ion under a sub-
micromolar condition can be accomplished easily. These
results revealed that the introduction of Mes2B groups into
an azaborine framework is a powerful strategy to enhance
both the Lewis acidity and the detection ability.
In conclusion, a new azaborine bearing two Mes2B groups
as a strong p-acceptor has been synthesized from the corres-
ponding dibromoazaborine. The bis(dimesitylboryl)azaborine
exhibited quite strong light-absorption and moderate photo-
luminescence in the near UV to violet region. Theoretical
8 The analysis was carried out by using a program developed by
Prof. Yasuhisa Kuroda. Prof. Yasuhisa Kuroda, Department
of Biomolecular Engineering, Kyoto Institute of Technology,
Sakyo-ku, Kyoto 606-8585, Japan. E-Mail: ykuroda@kit.ac.jp;
Tel and Fax: +81-75-724-7830.
¨
9 S. Sole and F. P. Gabbaı, Chem. Commun., 2004, 1284.
´
ꢀc
This journal is The Royal Society of Chemistry 2009
1896 | Chem. Commun., 2009, 1894–1896