One-step access to luminescent pentaaryldiazaboroles via C-C double bond formation from imidoylstannanes

Article abstract of DOI:10.1021/ja305806r

A series of pentaaryl-substituted diazaboroles have been prepared for the first time by a novel strategy based on the C-C double bond formation from imidoylstannane reagents in the presence of dibromophenylboranes. The aryl substituents on the 4,5-position of the planar C₂N₂B core have substantial effects on their electronic structures. All the new diazaboroles are luminescent both in solution and in the solid state. DFT calculations indicate the 4,5-C-aryl substituents have significant contributions to the LUMOs.

Full text of DOI:10.1021/ja305806r

Communication
pubs.acs.org/JACS
One-Step Access to Luminescent Pentaaryldiazaboroles via C−C
Double Bond Formation from Imidoylstannanes
Dawei Tian, Jiang Jiang, Hongfan Hu, Jianying Zhang, and Chunming Cui*
State Key Laboratory of Elemento-Oganic Chemistry, Nankai University, Tianjin 300071, People’s Republic of China
S
* Supporting Information
Scheme 1. Normal Routes to Diazaboroles
ABSTRACT: A series of pentaaryl-substituted diaza-
boroles have been prepared for the first time by a novel
strategy based on the C−C double bond formation from
imidoylstannane reagents in the presence of dibromo-
phenylboranes. The aryl substituents on the 4,5-position of
the planar C2N2B core have substantial effects on their
metals and cyanide ions have been recently developed,8
electronic structures. All the new diazaboroles are
luminescent both in solution and in the solid state. DFT
calculations indicate the 4,5-C-aryl substituents have
significant contributions to the LUMOs.
tetraaryl-substituted α-diimines with bulky aryl substituents
that are required, in some cases, for the protection of a low-
coordinate central atom are still not accessible by these
methodologies. Inspired by the unique electronic properties of
peripheral aryl-substituted aromatic compounds and significant
electronic effects of the 4,5-C-substituents on N-heterocyclic
carbenes,9 we became interested in pentaaryldiazaboroles.
Surprisingly, such boracycles have not been reported so far.
We report here a facile one-step strategy for the synthesis of
the first pentaaryldiazaboroles based on a novel C−C double
bond formation reaction from imidoylstannane reagents
(Scheme 2) in the presence of a dibromophenylborane. The
eripheral aryl-substituted aromatic systems, such as
P
hexaarylbenzenes and pentaarylpyrroles, have potential
applications in molecular electronics and nanotechnology
because of their restricted intramolecular rotations of the
periphery aryl rings and unique photophysical properties.1 BN-
containing aromatic systems, where a CC bond is substituted
by an isoelectronic B−N unit in aromatic hydrocarbons, have
been one of the recent focuses in main group chemistry due to
their potential applications as optoelectronic materials.2
Therefore, many efforts have been devoted to the synthesis
of their derivatives.
Scheme 2. New Strategy for the Synthesis of the Peripheral
Aryl-Substituted Diazaboroles 1−7
1,3,2-Diazaboroles, isoelectronic with pyrroles, are important
aromatic boracycles that have been studied for decades since
the first synthesis in 1973.3 The primary concerns on the
molecules have been centered on their aromaticity and
electronic properties in comparison to pyrroles,4 and synthesis
of N-heterocyclic boryl anions.5 However, the study of
diazaboroles was to a great extent limited to 1,3,2-
benzodiazaboroles (Chart 1, A)6 and 1,3-N- and B-substituted
Chart 1
overall reaction leads to the one-step coupling of the two
imines and the borane, with the formation of a C−C double
bond and the five-membered ring closure. To the best our
knowledge, this type of coupling reaction has not been applied
for the synthesis of α-diimine-derived main group and
transition metal complexes. This protocol allows direct access
to a series of peripheral aryl-substituted diazaboroles and does
not need α-diimine ligands. Interestingly, the resulting
pentaaryldiazaboroles are luminescent both in solution and in
derivatives (B); the 4,5-C-aryl derivatives (C) are extremely
rare, probably due to the lack of synthetic approaches.7 The
widely employed methods to synthesize diazaboroles developed
by Weber and Schmid require suitable α-diimines, which must
be reduced by highly active metals in the processes (Scheme
1).4a,b Although several synthetic routes to α-diimine ligands
based on aldimine coupling reactions mediated by lanthanide
Received: June 21, 2012
Published: August 30, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Communication
the solid state, in contrast to well-documented benzodiaza-
boroles.6
δ6.46 ppm, which falls in the range for four-coordinate boron
species.10 However, the donor−acceptor complex is not stable
in solution and slowly eliminates Me3SnBr at ambient
temperature. Furthermore, it was found that reaction of the
aminodibromoborane (iPr)2NBBr2 with an ArN
CAr′(SnMe3) directly led to the elimination of Me3SnBr with
the formation of the corresponding boryl-substituted imine
ArNC(Ph)[BBrN(iPr2)],11 while the C−C coupling product
has not been observed, indicating that relatively strongly Lewis
acidic boranes such as PhBBr2 are required for the coupling.
The most prominent and interesting features of these
diazaboroles can be seen in their UV−vis and emission spectra
(Figure 2). The diazaboroles 1−7 are fluorescent both in
The desired imidoylstannane reagents were obtained in
excellent yields by reactions of different aryl-substituted
imidazoyl chlorides with Me3SnLi. Subsequent reaction of the
imidoylstannane reagents with PhBBr2 or MesBBr2 (Mes =
2,4,6-Me3C6H2) in refluxing n-hexane directly yielded the
expected pentaaryldiazaboroles 1−6 in modest yield (Scheme
2). The phenyl-linked bis-diazaborole 7 can be obtained with
1,4-(BBr2)2C6H4 by the same strategy.
These new compounds were isolated as pale yellow or yellow
1
crystals and have been fully characterized by H, 11B, and 13C
NMR and EI mass spectroscopy and elemental analysis. The
molecular structures of 2 and 3 have been determined by
single-crystal X-ray analysis and are shown in Figure 1. The
Figure 2. Luminescent spectra of 1−8 in THF (left) and in the solid
state (right).
solution and in the solid state. For comparison, the absorption
and luminescence properties of the diazaborole
(CHNAr)2BC6H5 (8, Ar = 2,6-iPr2C6H3),5b without aryl
substituents on the 4,5-position, have also been investigated.
The absorption and luminescence maxima measured in THF
are listed in Table 1. The absorption and fluorescence
Figure 1. ORTEP drawings of 2 (left) and 3 (right) with 30% ellipsoid
probability. Selected bond length and angles (deg): for 2, B1−N2
1.436(2), B1−N1 1.438(2), N1−C1 1.4224(18), N2−C2 1.4198(18);
for 3, B1−N2 1.4358(19), B1−N1 1.4375(19), N1−C1 1.4118(16),
N2−C2 1.4062(17).
Table 1. Absorption and Emission Data for Compounds 1−8
in THF and in the Solid State (in Parentheses)
planar central C2N2B cores of 2 and 3 are surrounded by the
propeller-like twisted aryl rings with torsion angles of 29.8°(B),
80.7°(N), 57.4°(C), 66.1°(C), and 72.2°(N) in 2 and 32.1°(B),
76.2°(N), 49.6°(C), 43.9°(C), and 80.2°(N) in 3, respectively.
No intermolecular π−π interactions have been observed in the
solid state. This structural feature resembles those observed in
their isoelectronic pentaphenylpyrroles.1j
λ
λ
λ
Stokes shifts
abs a
ex b
em c
d
compd
(nm)
(nm)
(nm)
(cm−1
)
ΦF
1
406
(407)
415
(367)
471
(513)
3400
(5080)
0.82
(0.04)
e
2
3
4
5
6
7
8
344
317
484
8410
0.16
(0.40)
(452)
(432)
(528)
(3180)
346
(380)
363
(366)
471
(491)
7670
(5950)
0.61
(0.15)
The mechanism of the C−C double bond formation has not
been fully established. However, it is quite possible that the first
step involves the formation of a donor−acceptor complex
between an imidoylstannane and PhBBr2. This complex, upon
heating, eliminates Me3SnBr to generate a carbene intermedi-
ate, Ar(PhBrB)N-C(Ar′), which subsequently dimerizes to
form the CC bond with the elimination of PhBBr2 and
formation of the five-membered ring system. The isolation of
the donor−acceptor complex E (Scheme 3) at low temperature
and its subsequent transformation to 4 by heating strongly
support this mechanism. The intermediate E has been
388
(393)
403
(364)
472
(469)
4950
(4120)
0.59
(0.03)
398
(407)
416
(427)
481
(499)
4340
(4530)
0.06
(0.03)
411
(409)
422
(367)
480
(494)
3500
(4210)
0.90
(0.04)
439
(440)
432
(362)
495
(566)
2580
(5060)
0.90
(0.01)
280
(355)
323
(333)
b
406
(407)
11080
(3600)
0.76
(0.22)
a
c
Absorption maximum. Wavelengths for excitation. Emission
maximum. Absolute quantum yield determined by a calibrated
integrating sphere system within 3% errors. Data in parentheses
d
1
characterized by H, 11B, and 13C NMR spectroscopy. The
e
1H NMR spectrum clearly shows a singlet at δ 0.45 ppm with
tin satellites (2JSn−H = 56 Hz), indicating the presence of Me3Sn
group. The 11B NMR spectrum of E shows the resonance at
refer to the solid-state data (the solid samples were obtained by
grinding crystals).
Scheme 3. Synthesis of the Intermediate E
properties were also investigated in n-hexane and dichloro-
methane (Table S4). The results indicated that the absorption
and fluorescence properties of these compounds were only
slightly affected by solvent polarity. Notably, it can be seen
from Table 1 and Figure 2 that introduction of aryl groups to
the 4,5-positions of the C2N2B core results in the apparent red
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shifts of the absorption and emission maxima compared to
those of the 4,5-dihydro derivative 8. The absorption maxima
for these compounds range from 344 to 439 nm, while the
emission maxima shift from violet to blue, and even to green in
comparison to that of 8. The absorption maxima of 1, 4, 5, and
6 are red-shifted relative to the other compounds since the
small C6H5 groups on the 4,5-position in these compounds
would result in their relatively small torsion angles and thus the
enhanced π-conjunction with the central core. The fluorescence
maxima of 1−7 are within the narrow range from 471 to 495
nm, indicating that the substituents of the aryl rings on the
central core have a limited effect on their emission maxima.
However, the quantum yields (ΦF) of these compounds are
apparently affected by the steric hindrance of the aryl group on
the boron and nitrogen atoms. The highest ΦF values (0.90)
are observed for 6 and 7. The boron atom is efficiently
protected by the Mes group in 6, while the extended π system
in 7 may electronically stabilize the boron center. For the four
4,5-C6H5-substituted derivatives 1, 4, 5, and 6, the quantum
yields decrease [6 (0.90) > 1 (0.71) > 4 (0.59) > 5 (0.06)] with
the decrease of the steric hindrance around the boron atom.
The very low ΦF observed for 5 is most likely due to the poor
protection for the three-coordinate boron center from the
attack by solvent molecules.12 Compounds 1, 3, and 8 have
similar steric effects around the boron atoms, and relatively high
quantum yields (0.82, 0.61, and 0.76) have been observed for
these compounds, indicating the necessity of bulky groups
around the boron atom. The relatively low ΦF (0.16) of 2 may
result from the fact that the ideal planar geometry for the
excited state can hardly be approached by the big naphthyl
groups.
B3LYP/6-31G(d,p) level.16 The calculated bond parameters are
well correlated to those determined by single-crystal X-ray
analysis (see Tables S2 and S3). The HOMO and LUMO of 3
are shown in Figure 3, while those of 1 and 2 are shown in
Figure 3. Calculated frontier orbitals for 3.
Figures S2 and S4. The HOMOs of 1−3 are located in the core
ring with noticeable contributions from the B-aryl and 4,5-C-
aryl rings, while the LUMOs mainly correspond to the two 4,5-
aryl rings, indicating the significant contribution of the 4,5-aryl
rings to the LUMOs of 1−3. In addition, the LUMO is
delocalized over the 4,5-aryl rings and central C2N2 segment.
These orbital interactions may decrease the LUMO level,
resulting in the red-shifted absorption and emission of these
compounds. The calculated LUMO levels disclosed the order
of 1 (−0.64) > 2 (−1.11) > 3 (−1.36 eV) (Table S5),
indicating that the LUMOs are significantly stabilized by
electron-withdrawing groups on the 4,5-phenyl rings.
Compounds 1−8 are also luminescent in the solid state. Red-
shifts of the emission maxima and relatively large Stokes shifts
in the solid state compared to their THF solutions (Figure 2)
are generally observed, except for 8. The large red-shift of the
absorption maximum found for 2 in the solid state in
comparison to that in solution (Table 1) may be attributed
to the increased π−π interactions between the two naphthyl
rings due to the relatively large torsion angles between the
naphthyl rings and the C2N2B ring observed in the solid state.
Notably, the quantum yields in the solid state appear to be
related to the 4,5-substituents of the central ring. Low quantum
yields have been observed for 1 (0.04), 4 (0.03), 5 (0.03), and
6 (0.04), featuring the same C6H5 groups in the 4,5-positions.
In contrast, 2, featuring 4,5-naphthyl groups, has the highest ΦF
(0.40) in the solid state, presumably due to the fact that
rotation of the naphthyl rings is restricted in the solid state.
Furthermore, the weak intermolecular CH···π interactions
between the naphthyl rings (Figure S7) observed in the
structure of 2 may restrict the rotations of the naphthyl rings,
leading to the enhanced emission in the solid state compared to
that in solution (aggregation-induced emission enhance-
ment).13 A relatively high ΦF (0.15) is also observed for 3, in
which the strong electron-withdrawing CF3 group may result in
a small degree of the π bonding character between the 4,5-
carbon atoms and the neighboring carbon atoms of the phenyl
rings, thus suppressing the molecular motion in the solid state.
It is noted that the luminescence properties for diazaborole
derivatives in the solid state have only been reported very
recently.14 However, a fair number of boron fluorophores have
been reported to be luminescent in the solid state.15
In summary, the first series of pentaaryldiazaboroles have
been prepared by a novel one-step strategy based on the C−C
double bond formation from imidoylstannane reagents in the
presence of a dibromophenylborane. The preliminary photo-
physical studies indicated that these diazaboroles are
luminescent both in solution and in the solid state and are
distinct from other diazaboroles in their electronic structures.
DFT calculations disclosed significant contributions of the 4,5-
C-aryl rings to the LUMOs. In addition, the present studies
allowed facile access to peripheral aryl-substituted aromatic
diazaboroles in one step. This strategy is promising for the one-
pot synthesis of other main group N-heterocyclic systems.
Further studies on the ring systems are currently in progress.
ASSOCIATED CONTENT
* Supporting Information
■
S
Synthesis and characterization of the new compounds in this
paper, DFT calculations for 3, and CIF files for 2 and 3. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
cmcui@nankai.edu.cn
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the 973 program (Grant No.
2012CB821600) and the National Natural Science Foundation
of China for the support of the work.
To have an insight on the effects of the 4,5-substituents,
preliminary DFT calculations were performed on 1−3 at the
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Journal of the American Chemical Society
Communication
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