N-heterocyclic carbene boranes as electron-donating and electron-accepting components of π-conjugated systems

Article abstract of DOI:10.1002/anie.201204050

Give and take: The introduction of NHC-borane moieties to thiophene-based π skeletons endows a zwitterionic character, which makes the π system electron-donating, while the NHC ring acts as an electron-accepting moiety. The NHC-borane-substituted thiophene underwent a clean photoisomerization with a drastic color change, however, the expanded bithiophene derivatives were inert to this photoreaction, showed low oxidation potentials, and formed a slipped π-stacked array in the crystal. Copyright

Full text of DOI:10.1002/anie.201204050

Angewandte
Chemie
DOI: 10.1002/anie.201204050
NHC–boranes
N-Heterocyclic Carbene Boranes as Electron-Donating and Electron-
Accepting Components of p-Conjugated Systems**
Kazuhiko Nagura, Shohei Saito, Roland Frçhlich, Frank Glorius,* and Shigehiro Yamaguchi*
N-Heterocyclic carbenes (NHCs)[1] react with trivalent bor-
anes to form Lewis base/acid complexes.[2] In the past several
years, this chemistry has attracted increasing attention.
Various useful NHC–boranes for organic synthesis have
been reported, such as hydrogen-radical donors,[3] hydride
sources,[4] substrates for cross-coupling reactions,[5] organo-
catalysts,[6] and frustrated Lewis pairs.[7] In addition, the
Figure 1. Electronic effects of the NHC–borane moiety.
removal of a substituent from the tetracoordinated boron in
NHC–boranes produces intriguing tricoordinated boron spe-
cies, such as borenium ions,[8] boryl radicals,[9] and boryl
anions.[10] However, studies of NHC–boranes from a materials
point of view have been limited to only a few examples. Piers
and co-workers reported NHC-coordinated boraacenes with
small HOMO–LUMO gaps, in which a saturated NHC
moiety plays a key role in the stabilization of the boraacene
skeletons, but does not fully participate in the p conjugation
because of the orthogonal conformation.[11] An unsaturated
NHC-bearing borabenzene with a coplanar geometry was
also reported by Herberich and co-workers, in which, how-
ever, the borabenzene and the NHC ring are not noticeably
conjugated to each other as a result of their different
electronic characters.[12]
We were interested in the potential of NHC–boranes as
components of p-conjugated skeletons. While many NHC
ligands with two aryl groups on the nitrogen atoms have been
synthesized, these aryl groups are introduced in order to
control the steric bulkiness of the ligands and suppress
dimerization, but do not significantly conjugate with the NHC
moiety because of their almost orthogonal conformations. In
contrast, the introduction of a boryl group to the ortho
positions of the N-aryl groups should fix the aryl–NHC
skeleton in a coplanar fashion, and thereby the aryl ring and
the NHC ring can fully conjugate to each other (Figure 1). We
envisioned in this structure that the zwitterionic character of
the NHC–borane moiety endows the aryl p skeleton not only
with a highly polar character, but also with a highly electron-
donating character, because the borate moiety is a strong
electron-donating group. At the same time, the NHC ring acts
as an electron-accepting moiety because of its cationic
character. In order to investigate the impacts of these
multifaceted electronic features, we chose thiophene and
bithiophene as representative aryl p skeletons. A series of
NHC–borane-substituted derivatives, 1–3, have been synthe-
sized and their reactivity and properties have been studied.
NHC–borane-substituted thiophenes 1a and 1b, which
bear a dimesitylboryl and a dibenzoborolyl group as boron
moiety, respectively, were successfully obtained by a two-step
synthesis from benzimidazolylthiophene 4 (Scheme 1). In the
[*] K. Nagura, Dr. S. Saito, 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
Dr. R. Frçhlich, Prof. Dr. F. Glorius
Organisch-Chemisches Institut
Westfꢀlische Wilhelms-Universitꢀt Mꢁnster
Corrensstrasse 40, 48149 Mꢁnster (Germany)
E-mail: glorius@uni-muenster.de
[**] This work was supported by a Grant-in-Aid (No. 19675001) from the
Ministry of Education, Culture, Sports, Science, and Technology
(Japan), CREST, JST, and the Deutsche Forschungsgemeinschaft
(IRTG Mꢁnster-Nagoya). K.N. thanks the JSPS Research Fellowship
for Young Scientists.
Scheme 1. Synthesis of 1–3. Reagents and conditions: a) MeI, CH2Cl2,
508C; b) nBuLi (2 equiv), diethyl ether, ꢀ788C!08C, then Mes2BF or
bromodibenzoborole; c) bromohexane, PhCN, 1508C; d) LDA
(4 equiv), THF, ꢀ788C!08C, then Mes2BF.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201204050.
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first step, 4 was converted to imidazolium salt 5 by alkylation
with an excess of MeI. The deprotonation and halogen/
lithium exchange reaction of 5 were simultaneously accom-
plished by treatment with two equivalents of nBuLi to
produce the corresponding NHC with a lithiated thienyl
group. The following reaction with Mes2BF or bromodiben-
zoborole afforded products 1a and 1b as stable colorless
compounds. In a similar manner, p-expanded bithiophenes 2
and 3, which have two NHC moieties at the 5,5’ and 4,4’
positions of the bithiophene skeleton, respectively, were also
synthesized from bis(benzimidazolyl)bithiophene precursors
6 and 7, in which the hexyl groups were introduced to gain
sufficient solubility, and four equivalents of LDAwere used to
simultaneously deprotonate the imidazolium and thiophene
rings. The yields of 2 and 3 were only moderate, partly
because of their instability in air (see below). The structures
of the NHC–boranes 1–3 were unequivocally identified by
NMR spectroscopy and mass spectrometry. The 11B NMR
spectra showed sharp and upfield-shifted signals around
ꢀ10 ppm, thus confirming the formation of tetracoordinated
borates by intramolecular coordination of NHC moieties.
To explore the fundamental characteristics of the NHC–
borane-substituted thiophene p skeletons, we first investi-
gated the properties and reactivities of 1a and 1b. The UV
absorption spectra in CH2Cl2 showed that both compounds
have weak-shoulder absorption bands at 327 and 312 nm,
respectively (Table 1). These derivatives show purple fluo-
compared to that in the excited state. The natural population
analysis (MP2/6-31G*//B3LYP/6-31G*)[14] demonstrated the
zwitterionic character of 1a in the ground state with the
positive charges on the carbene carbon atom (+ 0.319) and
the boron atom (+ 0.327) and the negative charges on the
carbon atom (C2) at the 2 position of thiophene (ꢀ0.419) and
the ipso carbon atoms (Cipso) of the mesityl groups (ꢀ0.232
and ꢀ0.242), respectively. Consequently, 1a has a large dipole
moment of 6.60 Debye.
Notably, 1a shows a high reactivity under photoirradia-
tion. Thus, upon irradiation of a solution of 1a in CH2Cl2 with
UV light under a nitrogen atmosphere, a photoreaction
smoothly took place with a drastic color change from colorless
to deep yellow (Figure 2). In the absorption spectra, an
intense new band appeared at 429 nm. The monitoring of the
1
reaction by H NMR spectroscopy demonstrated the clean
formation of a single product. Based on various kinds of NMR
1
measurements, including not only H and 11B NMR spectra,
but also COSY, HSQC, HMBC, and 2D NOESY (see the
Supporting Information), we determined that the product is
a thienyl-migrated 7-borabicyclo[4.1.0]hepta-2,4-diene 1a’
(Figure 2a). According to the TD-DFT calculation (B3LYP/
6-31G*), the intense and long-wavelength absorption band of
1a’ can be assigned to the ICT transition from the HOMO
that is localized on the borabicyclo[4.1.0]heptadiene skeleton
to the LUMO that is localized on the NHC moiety.
Table 1: Photophysical Properties of 1–3.
Compd
Solvent
Absorption[a]
labs [nm] loge
Fluorescence
lem [nm]
Stokes shift
Dn [cmꢀ1
]
1a
C6H12
C6H6
349
342
327
327
312
425
408
2.88
3.03
3.15
3.18
3.94
4.08
3.79
429
440
441
443
440
477
448
5340
6510
7910
8000
8820
2570
2190
CH2Cl2
DMSO
CH2Cl2
CH2Cl2
CH2Cl2
1b
2
3
[a] The maximum wavelength of the longest absorption band and its
molar absorption coefficient were determined by a PeakFit program.
Figure 2. a) Photoreaction of 1a with pictures of the solution before
and after irradiation by UV light (365 nm), and b) absorption spectral
change of a solution of 1a in CH2Cl2 (0.03 mm) upon light irradiation.
rescences with maxima (lem) at 441 nm for 1a and 440 nm for
1b. Thus, the large Stokes shifts of 7900–8800 cmꢀ1 are one of
the features of this skeleton, and are indicative of their
significant structural change in the excited state. According to
the molecular orbital calculations at the B3LYP/6-31G*
level,[13] the HOMO and LUMO of 1a are mainly localized
on the dimesitylthienylborane moiety and the NHC moiety,
respectively (see the Supporting Information). The TD-DFT
calculation at the same level of theory suggested that the
longest absorption band is assignable to the intramolecular
charge transfer (ICT) transition from HOMO to LUMO.
However, despite the ICT character, no red-shift was
observed in the fluorescence when the solvents were changed
from nonpolar to polar. Instead, a blue shift was observed in
the absorption spectra of 1a from 349 nm in cyclohexane to
327 nm in DMSO. This negative solvatochromism demon-
strates the larger dipole moment of 1a in the ground state
Recently, Braunschweig and co-workers reported the
formation of NHC-coordinated borabicyclo[4.1.0]hepta-2,4-
diene derivatives by reduction of BHCl2·IMe (IMe = 1,3-
dimethylimidazol-2-ylidene) with sodium naphthalenide in
THF.[15] In this reaction, the borabicyclo[4.1.0]heptadiene
skeleton is likely produced by the [2+1] cycloaddition of an
in situ generated borylene with naphthalene. On the other
hand, Wang and co-workers have recently pioneered the
photoisomerization of 2-(2-borylaryl)-substituted N-hetero-
aryl compounds, which also affords the borabicyclo-
[4.1.0]hepta-2,4-diene skeleton.[16] Its structure has been
unambiguously determined by X-ray crystallography.[16e]
The photoreaction we now observed can be regarded as
2
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a carbene-analogous reaction of the photoisomerization
described by Wang.[17] Different from the photoisomerization
of the N-heteroaryl-coordinated substrate, the NHC-coordi-
nated product 1a’ emits fluorescence. The fluorescence
maximum was shifted from 441 nm for 1a to 459 nm for 1a’
in CH2Cl2. In addition, unlike the reaction of the N-hetero-
aryl-coordinated substrate, the NHC-coordinated product 1a’
does not revert to starting material 1a under thermal
conditions. We confirmed that a chlorobenzene solution of
1a’ remained intact, even after heating at 1208C for two
hours. The high thermal stability of 1aꢀ results from the more
persistent coordination of the NHC to the borabicyclo-
[4.1.0]heptadiene skeleton.
We have previously reported that (3-boryl-2-thienyl)-2-
thiazole-based p-conjugated system 11 has a characteristic
electron-accepting feature, in which the intramolecular coor-
dination of the thiazole to the boron moiety effectively
decreases the LUMO level.[18] In sharp contrast, notably, we
now found that the intramolecular coordination of the NHC
to the boron moiety has a totally opposite electronic effect
(Figure 4). The calculations showed that the NHC–borane-
substituted bithiophene 2’, which is a model for 2, has a much
higher-lying HOMO (by 0.32 eV) compared with that of
a non-borylated congener 12, while the LUMO level is not
significantly influenced by intramolecular coordination. Thus,
the NHC–borane moiety is a powerful skeleton to make the
p skeleton electron-donating in nature.
To gain insights into the mechanism of this reaction, we
conducted the structural optimization in
the excited state at the B3LYP/def2-
SV(P) level (see the Supporting Infor-
mation). In the local-minimum structure
1a* in the S1 excited state, one of the
mesityl groups is located closer to the
2 position of the thiophene moiety com-
pared to the initial ground-state structure
(Figure 3). The distance between the C2
atom of the thiophene and the Cipso atom
of the mesityl group becomes shorter,
from 2.588 ꢀ in 1a to 2.408 ꢀ in 1a*,
ꢀ
accompanied by a narrowing of the C2
ꢀ
B Cipso angle from 101.988 in 1a to 92.798
in 1a*. This structural change results
from the ICT transition from the HOMO
that is largely localized on the mesityl
moiety to the LUMO that is localized on
the NHC ring, thus reducing the electron
density of the Cipso atom. From this
excited-state structure, the electron-rich
thienyl C2 atom should migrate to the
electron-poor Cipso atom of the mesityl
group, followed by the formation of the
Figure 4. Electronic effects of the introduction of boron moieties to a thienylthiazole skeleton
and a NHC–borane-substituted thiophene skeleton, calculated at the B3LYP/6-31G* level.
borabicyclo[4.1.0]heptadiene skeleton,
while maintaining the NHC coordination
to the boron atom. Thus, an important
requisite for this photoreaction is that the HOMO is localized
on the electron-rich mesityl moiety in the ground state.
Moreover, it is interesting to note that the dibenzoborole
derivative 1b did not undergo the photoisomerization. This
result suggests that the rigidity of the dibenzoborole moiety
can suppress photoisomerization.
Bithiophene derivatives 2 and 3 showed interesting
electrochemical properties, which reflected the electronic
impact of the NHC–borane moiety. Cyclic voltammograms
were measured in CH2Cl2 with Bu4NPF6 as a supporting
electrolyte (Figure 5). While 3 showed an irreversible oxida-
tion wave with the peak potential Epc of + 0.27 V (vs. Fc/Fc+),
2 exhibited a reversible oxidation wave with the half potential
E1/2 of + 0.28 V. Their low oxidation potentials are consistent
with their high HOMO levels (2: ꢀ4.78 eV; 3: ꢀ4.74 eV),
which were obtained by DFT calculations. These compounds
are prone to oxidation in air as a result of these low oxidation
potentials. The reversible oxidation wave for 2 demonstrates
the importance of the substitution pattern for the stabilization
of the produced radical cation.
In the absorption spectra in CH2Cl2, the longest-wave-
length absorption bands of 2 and 3 are red-shifted and
stronger compared with those of the monothiophene deriv-
atives 1. The longest lmax values determined by a PeakFit
Figure 3. Optimized structures of 1a a) in the ground state, and b) in
the S1 excited state, calculated at the B3LYP/def2-SV(P) level.
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rable to those of the known NHC–borane derivatives
( ꢁ 1.65 ꢀ).[20] The intramolecular coordination of the car-
bene to the boron atom fixes the thiophene-NHC moiety in
a planar fashion. The dihedral angle between the thiophene
and NHC rings is 10.308. In addition, the bithiophene moiety
also has a completely planar conformation with the s-trans
geometry (dihedral angle: 0.008). As a consequence of this
high planarity, 2 forms a slipped face-to-face p-stacking array.
Importantly, in this array, the electron-rich thiophene rings
are overlapped with the electron-deficient benzimidazolidene
moieties of the adjacent molecules. The interfacial distance of
these two rings is about 3.51 ꢀ. Because this overlapping is
also observed in the structure of the monothiophene deriv-
ative 1a (see the Supporting Information), the electrostatic
interaction between the thiophene and benzimidazolidene
moieties may be one of the dominant factors that determine
the packing structure.
In summary, we disclosed the multifaceted electronic
impacts of the NHC–borane moiety attached to the terminal
positions of the thiophene p skeletons. In the monothiophene
derivative, photoisomerization smoothly proceeds to give
a borabicyclo[4.1.0]-hepta-2,4-diene skeleton. In contrast, the
bithiophene derivatives do not undergo the photoreaction
and demonstrate interesting electronic properties. The most
notable impact of the NHC–borane moiety is its polar and
electron-donating character, so that this moiety should be
a powerful tool to produce a highly electron-donating p-
conjugated skeleton. At the same time, the benzimidazoli-
dene moiety in the terminal NHC–borane unit serves as an
electron-deficient moiety, which is beneficial to forming
a face-to-face p-stacking array in the solid state through the
electrostatic interaction. Making the best use of these
characteristics of the NHC–borane moiety would lead to the
production of a more fascinating optoelectronic material.
Further studies along this line are in progress in our
laboratory.
Figure 5. Cyclic voltammograms of a) 2 and b) 3 in CH2Cl2 with
Bu4NPF6 at the scan rate of 100 mVsꢀ1
.
program are 425 nm and 408 nm, respectively. The fluores-
cence spectra show that 2 and 3 emit weak sky-blue (lem
477 nm, FF 0.05) and blue (lem 448 nm, FF 0.02) fluorescen-
ces. The crucial difference from the monothiophene deriva-
tive 1 is that the bithiophene derivatives are inert to photo-
isomerization. This behavior is due to the delocalization of
their HOMOs, not on the mesityl moieties, but over the
bithiophene skeleton. As
a result, photoisomerization
observed for 1a no longer proceeds for 2 and 3 (see above).
According to the calculations, there is also a notable differ-
ence between 2 and 3. Namely, while the LUMO of 2 is
delocalized over the entire NHC–bithiophene framework,
that of 3 is mostly localized on the NHC ring (for the pictorial
presentation of the LUMO of 3, see the Supporting Informa-
tion). The NHC ring serves as the electron-accepting p ske-
leton, although the LUMO level itself is not particularly low.
We finally succeeded in the crystal structure analysis of
the p-expanded bithiophene derivative 2 (Figure 6).[19] The
boron atoms have a slightly distorted tetrahedral geometry
Received: May 24, 2012
Published online: && &&, &&&&
Keywords: boron · intramolecular coordination · Lewis pairs ·
.
N-heterocyclic carbenes · p-conjugated systems
ꢀ ꢀ
with the sum of the C B C bond angles of 337.78. The length
ꢀ
of the NHC C B coordination bond (1.657(3) ꢀ) is compa-
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Chem. 2010, 122, 2524; Angew. Chem. Int. Ed. 2010, 49, 2474;
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
NHC–boranes
Give and take: The introduction of NHC–
borane moieties to thiophene-based
p skeletons endows a zwitterionic char-
acter, which makes the p system electron-
donating, while the NHC ring acts as an
electron-accepting moiety. The NHC–
borane-substituted thiophene underwent
a clean photoisomerization with a drastic
color change, however, the expanded
bithiophene derivatives were inert to this
photoreaction, showed low oxidation
potentials, and formed a slipped p-
stacked array in the crystal.
K. Nagura, S. Saito, R. Frçhlich,
F. Glorius,*
S. Yamaguchi*
&&&& — &&&&
N-Heterocyclic Carbene Boranes as
Electron-Donating and Electron-
Accepting Components of p-Conjugated
Systems
6
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!