Highly Electron-Deficient Dicyanomethylene-Functionalized Triarylboranes with Low-Lying LUMO and Strong Lewis Acidity

Article abstract of DOI:10.1021/acs.orglett.1c01983

A series of dicyanomethylene-functionalized triarylboranes is reported in this work, with low-lying LUMO energy levels at ca. -3.66 eV for FMesB-ACN. The single-crystal structures of the mono- and dianion of Mes*B-ACN were obtained via chemical reduction,

Full text of DOI:10.1021/acs.orglett.1c01983

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Letter
Highly Electron-Deficient Dicyanomethylene-Functionalized
Triarylboranes with Low-Lying LUMO and Strong Lewis Acidity
Guanming Liao, Xing Chen, Yali Qiao, Kanglei Liu, Nan Wang, Pangkuan Chen,* and Xiaodong Yin*
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ABSTRACT: A series of dicyanomethylene-functionalized triarylbor-
anes is reported in this work, with low-lying LUMO energy levels at
F
ca. −3.66 eV for MesB-ACN. The single-crystal structures of the
mono- and dianion of Mes*B-ACN were obtained via chemical
reduction, which revealed a conversion from a quinoidal to an
aromatic structure. The strong Lewis acidity of these compounds is
reflected in a fluoride-anion binding experiment. This work introduces
a facile strategy for modulating the electron deficiency of boron-
containing compounds.
anes to develop a series of acceptors with strongly suppressed
ricoordinated organoboranes are intrinsically electron-
Tdeficient and Lewis-acidic because of the vacant p orbital LUMOs and to further tune the electronic properties.
Herein we report the design and synthesis of a series of
dicyanomethylene-functionalized triarylboranes (Figure 1a).
Both 2,4,6-tri(t-butyl)phenyl (Mes*)7b,12 and FMes groups
were used for the stabilization of the tricoordinated boron
compound, and the dicyanomethylene was introduced in the
ortho position relative to the boron on the phenylene groups
to increase the electron deficiency. The resulting boron-doped
six-membered ring has four π-electrons, which possibly exhibit
antiaromatic character and thus enhance the Lewis acidity of
boron.13
of the boron atom. When incorporated into a π-conjugated
skeleton, the p−π* interactions between boron and the
jointing groups can tune the optical and electronic properties.1
On the basis of these characteristics, numerous boron-
containing organic π-conjugated materials have been con-
structed for various applications, such as anion sensing,2
organic electronics,3 catalysis,4 and biological imaging.5 In
particular, low-lying lowest unoccupied molecular orbital
(LUMO) energy levels of conjugated materials with the
introduction of electron-deficient boron are highly desirable in
n-type or ambipolar semiconductor material.6 Thus far, in the
development of boron-containing compounds with a low-lying
LUMO, a variety of structural modifications have been
explored, for example, introducing bulky protective groups
with electron-withdrawing properties (e.g., 2,4,6-tri-
(trifluoromethyl)phenyl (FMes)),7 embedding boron in
polycyclic aromatic hydrocarbons (PAHs),8 and placing the
triarylborane in conjugation with other strong electron-
acceptors.9 Because of the limited choices of the bulky
group, the first method usually does not have a very
pronounced effect, whereas the latter two usually suffer from
the challenging synthesis and poor solubility of the extended π-
conjugated skeletons. The cyano group is widely used as a
strong electron-withdrawing building block to improve the
electron-accepting properties of compounds. Among the
cyano-based acceptors, tetracyano-9,10-anthraquinonedime-
thane (TCAQ) has attracted increasing attention due to its
applications in electrical materials.10 In addition, TCAQ
achieves planarity and aromaticity during reduction, and the
charge is dispersed on the two dicyanomethylene groups and
the central anthracene.11 On the basis of the above, we tried to
combine the structural characteristic of TCAQ and triarylbor-
The syntheses of FMesB-ACN and Mes*B-ACN are
described in detail in the Supporting Information. TA was
prepared according to the procedure reported by the Piers’
group.14 As shown in Figure 1b, TA was treated with BCl3 in
CH2Cl2 to generate the chloroborane compounds (Cl-BA)
and then directly used for the next step without purification
beyond the removal of solvents and Me2SnCl2 byproduct.
F
Then, crude Cl-BA was treated with Mes-Li and Mes*-Li in
toluene to obtain FMesB-A and Mes*B-A, respectively.
F
Oxidation of MesB-A and Mes*B-A with CrO3 in refluxing
acetic acid produced carbonyl-functionalized triarylboranes
FMesB-AQ and Mes*B-AQ, which were further transformed
to dicyanomethylene-functionalized compounds under Knoe-
F
venagel conditions to yield MesB-ACN and Mes*B-ACN,
respectively. Dicyanomethylene-functionalized triarylboranes
Received: June 14, 2021
Published: July 12, 2021
https://doi.org/10.1021/acs.orglett.1c01983
© 2021 American Chemical Society
Org. Lett. 2021, 23, 5836−5841
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Figure 2. (a) UV−vis spectra (1 × 10−5 M in THF) and (b) cyclic
voltammetry data of carbonyl- and dicyanomethylene-functionalized
compounds in THF for Mes*B-AQ and CH2Cl2 for the rest.
Figure 1. (a) Molecular design and (b) synthesis routes of
dicyanomethylene-functionalized triarylboranes. aThe yield is esti-
F
mated starting from TA. (c) ORTEP plots of MesB-AQ, Mes*B-
AQ, FMesB-ACN, and Mes*B-ACN with thermal ellipsoids drawn at
observed (see Table 1). The electrochemical properties of
these compounds were examined by cyclic voltammetry in
50% probability.
Table 1. Photophysical and Electrochemical Data of
Carbonyl- and Dicyanomethylene-Functionalized
Compounds
exhibited sufficient stability and could be purified by column
chromatography on silica gel in an ambient atmosphere
without noticeable decomposition. These molecular structures
were fully characterized by nuclear magnetic resonance
(NMR) spectroscopy, high-resolution mass spectrometry,
and X-ray crystallography. Single crystals of FMesB-A,
Mes*B-A, FMesB-AQ, Mes*B-AQ, FMesB-ACN, and
Mes*B-ACN were obtained by the slow evaporation of
hexane/dichloromethane solutions. The structures of these
compounds are shown in Figure 1c and Figure S1. Selected
bond lengths and angles are listed in Table S4. It is noteworthy
that the dihedral angles between the two fused benzene rings
a
b
c
compound
λonset (nm) Ered (V)
LUMO (eV)
LUMO (eV)
FMesB-AQ
Mes*B-AQ
FMesB-ACN
Mes*B-ACN
368
366
395
392
−1.46
−1.73
−1.14
−1.28
−3.34
−3.07
−3.66
−3.52
−3.24
−2.84
−3.65
−3.38
b
a
Half-wave potentials of first reductive waves versus Fc+/0
.
ELUMO
=
−(4.8 + Ered)/eV.7b,16 DFT-calculated results by Gaussian 09 at the
c
B3PW91/6-311+G* level of theory.
F
of carbonyl-bridged triarylboranes are 6.1(1)° for MesB-AQ
CH2Cl2 and THF (for Mes*B-AQ), respectively (Figure 2b).
In the carbonyl-bridged triarylboranes, the first reversible
reduction waves were observed at E1/2 = −1.46 (FMesB-AQ)
and −1.73 V (Mes*B-AQ, vs Fc+/0), which further positively
and 8.94 (6)° for Mes*B-AQ, which indicates that the
boraanthracene skeleton adopts an almost planar geometry.
However, the dihedral angles between the two fused benzene
rings of dicyanomethylene-functionalized compounds are
F
shifted to −1.14 V vs Fc+/0 for MesB-ACN and −1.28 V vs
F
21.23(9)° for MesB-ACN and 19.62(8)° for Mes*B-ACN,
Fc+/0 for Mes*B-ACN, respectively. This observation clearly
indicates that the dicyanomethylene group significantly
contributes to lowering the LUMO levels of these compounds
revealing a butterfly-like structure due to the steric hindrance
between the dicyanomethylene moieties and the boraanthra-
cene skeleton.
F
to ca. −3.66 eV for MesB-ACN. These results demonstrate
The photophysical properties of carbonyl- and dicyano-
methylene-functionalized compounds were recorded in
tetrahydrofuran (THF) (Figure 2a). A slight red shift in the
absorption of the dicyanomethylene-functionalized derivatives
compared with the carbonyl-bridged compounds can be
the strong electron-accepting ability of triarylboranes bearing a
dicyanomethylene group among the majority of the reported
triarylboranes.15 Density functional theory (DFT) calculations
F
F
of MesB-AQ, Mes*B-AQ, MesB-ACN, and Mes*B-ACN
were conducted using the Gaussian 09 D.01 soft package at the
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B3PW91/6-311+G*//B3LYP/6-31G** level of theory, re-
spectively. The LUMO energy levels exhibited good agreement
with the values estimated from CV data, as shown in Table 1
and Figure S12. Time-dependent (TD)-DFT calculations at
the PBE0/6-311+G* level of theory were also conducted to
get an in-depth understanding of the vertical transitions of the
dicyanomethylene-functionalized compounds. (See Figures
S13 and S14 and Table S7 for details.) In addition, all of
these compounds are nearly nonemissive in THF solution, and
the photoluminescence spectra are shown in Figure S9.
Encouraged by the excellent reversibility of the electro-
chemical reduction, spectro-electrochemistry was utilized to
monitor the reduction processes of dicyanomethylene-
functionalized compounds. Figure S16b shows the different
absorption spectra of Mes*B-ACN at sufficiently negative
potentials to confirm the reduction reaction. The one-electron
reduction of Mes*B-ACN to its radical anion results in new
peaks growing around 682 nm. Further reduction is
accompanied by spectral changes, with the emergence of a
new absorption maxima at 551 nm, which is indicative of the
generation of the dianionic species Mes*B-ACN2−. Reverting
the applied voltage to 0 V resulted in the recovery of the
absorption spectrum of the neutral species. The chemical
reductions of Mes*B-ACN were carried out using CoCp*2
(Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl) and KC8 for
single- and double-electron reduction, respectively (see Figure
S16a). Upon the addition of 1.2 equiv of CoCp*2 to a solution
of Mes*B-ACN in THF, the color of the solution changed
from yellow to red, displaying broad absorption bands around
693 nm, which is consistent with the electrochemically reduced
radical anion Mes*B-ACN−•. A solution of dianion Mes*B-
ACN2− was obtained by adding 2.4 equiv of KC8, resulting in
the appearance of the absorption bands at 518 nm. TD-DFT
calculations of reduced products were conducted at the
UPBE0/6-311+G* level of theory based on the optimized
structures, as shown in Figure S16c and Table S9. The broad
absorption peak of Mes*B-ACN−• at ca. 700 nm can be
assigned to a transition of highest occupied spin orbital
(HOSO) → lowest unoccupied spin orbital (LUSO) of both α
and β spin orbitals. For Mes*B-ACN2−, the absorption at ca.
530 nm can be attributed to a transition of HOMO−1 →
LUMO+3. Electron paramagnetic resonance (EPR) data of
Mes*B-ACN−• were recorded in THF solution at room
temperature, and the spectrum is shown in Figure S8, revealing
a broad signal without a hyperfine structure. The spin density
of Mes*B-ACN−• was calculated at the UB3LYP/6-31G**
level of theory, which shows that the radical spin is mainly
distributed over the dicyanomethylene part with a relatively
smaller distribution on the boraanthracene skeleton. Therefore,
the lack of a hyperfine structure in the EPR could be due to the
conjugation over the boraanthracene skeleton. In addition,
attempts to reduce FMesB-ACN by treatment with an excess of
CoCp*2 or KC8 in THF failed due to the poor stability of the
reduction product.
Figure 3. ORTEP plots of (a) [Mes*B-ACN]−• [CoCp*2]+ and (b)
[Mes*B-ACN]2− K22+, with thermal ellipsoids drawn at 50%
probability.
much smaller than those of Mes*B-ACN (19.62(8)°),
suggesting that the fused benzene rings of the two reduced
products are nearly coplanar. It is noteworthy that the bond
lengths of the C−C bond adjacent to the dicyanomethylene
group are 1.442(8) Å for the Mes*B-ACN−• and 1.497(5) Å
for the Mes*B-ACN2−, which are much longer than that in the
neutral compound (1.365(2) Å), indicating the formation of
single-bond character in dianionic species. As a result, the
conformation changed from a butterfly to a coplanar structure
during the reduction process. The elongated lengths of the C−
C double bond in the reduced species result in an increment of
torsion angles for the twisted dicyanomethylene group to
boraanthracene skeleton from 44.1(8)° for Mes*B-ACN−• to
68.3(6)° for Mes*B-ACN2−, respectively. In addition, the
bond lengths of C−N in cyano group exhibit pronounced
elongation from 1.147(2) and 1.143(2) Å of neutral molecule
to 1.160(8) and 1.168(7) Å for Mes*B-ACN−• and 1.158(6)
and 1.156(6) Å for Mes*B-ACN2−, which is similar to the
reduction of TCAQ.10a,11 Proton and carbon NMR were
recorded for dianionic species in THF-d8, which exhibit a
relative upfield shift in comparison with that of neutral
compounds (see Figures S10 and S11).
NICS_zz values were calculated at the B3LYP/6-31G**
level of theory for the optimized structures, as shown in Figure
S15.17 In the neutral molecule, the NICS(0)_zz value of the
boron-doped six-member ring is 27.44 indicating pronounced
antiaromaticity, whereas the NICS(1)_zz values are 9.34 and
6.41, respectively. For Mes*B-ACN−•, the NICS(0)_zz value
is ca. 9.76, and the NICS(1)_zz values are −8.07 and −8.15,
signifying a decrease in the antiaromatic character. For Mes*B-
ACN2−, the NICS(0)_zz value is ca. −11.43, and the NICS(1)
_zz values are −28.27 and −26.30, revealing a dramatic switch
to aromatic character, which is consistent with the reduction of
TCAQ.10a,11
F
The Lewis acidity of MesB-ACN was first tested by the
Gutmann−Beckett method18 to obtain the acceptor number
(AN) at ca. 29 in CDCl3 (Figure S2 and Table S5), which is
higher than that of FMes containing borafluorene (AN =
14.1).19 In addition, the fluoride anion (F−) binding
experiments were conducted via both UV−vis and NMR
spectroscopy. The UV−vis absorption spectra showed that the
absorption around 268 nm decreased, and a new band
X-ray crystallographic analysis confirmed the generation of
the corresponding monoanion (Mes*B-ACN−•) and dianionic
species (Mes*B-ACN2−), as shown in Figure 3. Some selected
bond lengths of the reduced products are given in Table S4. In
the crystal structures, the B−CMes* bond length of Mes*B-
ACN−• (1.559(9) Å) is shorter than that in the neutral state
(1.583(2) Å) or the dianionic species (1.597(6) Å). The
dihedral angles between the two fused benzene rings of
Mes*B-ACN−• (2.1(8)°) and Mes*B-ACN2− (4.2(2)°) are
F
appeared at 318 nm, adding F− to MesB-ACN in THF (see
Figure 4a). The fitting of the titration data indicated a very
strong binding process with a binding constant toward F− to be
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Letter
In this work, we reported the synthesis of a series of
dicyanomethylene-functionalized triarylboranes. The introduc-
tion of the dicyanomethylene group dramatically enhances the
electron-deficient character, as evidenced by low-lying LUMO
energy levels. The single-crystal structures exhibit the typical
quinoidal character with a butterfly-like structure. The
spectroscopy and structural character during the reduction of
the dicyanomethylene-functionalized compounds were studied.
The mono- and dianion species of Mes*B-ACN were obtained
via reduction with CoCp2* and KC8, respectively, which
exhibited a molecular configuration transformation from a
butterfly-like to a planar skeleton with an aromatization of the
boron-doped six-membered ring reflected via DFT calcu-
lations. The fluoride anion titration confirmed the pronounced
Lewis acidity of these compounds, with the binding constant of
FMesB-ACN to fluoride anion in THF of >1 × 107 M−1. This
work presents a series of highly electron-deficient triarylborane
compounds with a novel skeleton and therefore could improve
the structural and functional diversity of boron-containing
molecules.
ASSOCIATED CONTENT
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sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01983.
Figure 4. UV−vis absorption spectral changes upon the addition of
TBAF·3H2O to a THF solution of (a) FMesB-ACN and (b) Mes*B-
ACN (c = 1.0 × 10−5 M). Inset: Illustration of F-bonded FMesB-ACN
and Mes*B-ACN from DFT calculations at the B3LYP/6-31+G(d,p)
level of theory.
Experimental procedures, X-ray analysis, UV−vis spec-
tra, electrochemical data, computational studies, and
characterization data like NMR and high-resolution
mass spectrometry (HRMS) spectra (PDF)
Accession Codes
CCDC 2010875, 2013629−2013630, and 2068081−2068085
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.ca-
m.ac.uk/data_request/cif, or by emailing data_request@ccdc.
cam.ac.uk, or by contacting The Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
+44 1223 336033.
much higher than 1 × 107 M−1 (see Figure S5).20 The 11B
NMR signal shifted to ca. 2.6 ppm in THF-d8, indicating the
formation of tetracoordinated borate, whereas 1H NMR
spectrum also exhibited a clean and completely conversion
(see Figure S3). It is noteworthy that treatment with water
cannot convert the F− complex of the NMR sample back to the
tricoordinated boron, revealing the strong F− affinity. The
electrospray ionization (ESI) mass spectrometry was also able
AUTHOR INFORMATION
■
F
to detect the ion peaks of MesB-ACN-F− (Figure S4). The
Corresponding Authors
Mes*-substituted boranes seldom show adequate anion
binding owing to the steric hindrance of the Mes* group.9a
Interestingly, we observed that Mes*B-ACN was able to form
coordination complexes with the fluoride anion via both UV−
vis spectra and 11B NMR (Figure 4b, Figure S6). On the basis
of this spectral change, the binding constant of Mes*B-ACN
toward F− was determined to be K = 7.20 × 105 M−1 (log K =
5.86; see Figure S7),20 which is remarkable because the
binding constant of other Mes*-functionalized boranes toward
F− usually cannot be determined. In the 11B NMR spectrum, a
new signal appeared at 3.3 ppm upon the addition of ca. 2.0
equiv of fluoride ions in THF-d8; however, 1H NMR
spectroscopy showed complex signals for the fluoride ion-
binding products, which indicates evidence of side reactions
(Figure S6). The fluoride ion affinities (FIAs) were computa-
Xiaodong Yin − Key Laboratory of Cluster Science, Ministry of
Education of China, Beijing Key Laboratory of
Photoelectronic/Electrophotonic Conversion Materials, School
of Chemistry and Chemical Engineering, Beijing Institute of
Technology, Beijing 102488, P. R. China; orcid.org/
0000-0001-7644-6744; Email: yinxd18@bit.edu.cn
Pangkuan Chen − Key Laboratory of Cluster Science, Ministry
of Education of China, Beijing Key Laboratory of
Photoelectronic/Electrophotonic Conversion Materials, School
of Chemistry and Chemical Engineering, Beijing Institute of
Technology, Beijing 102488, P. R. China; orcid.org/
0000-0002-8940-7418; Email: pangkuan@bit.edu.cn
Authors
F
tionally evaluated for MesB-ACN and Mes*B-ACN at M06-
Guanming Liao − Key Laboratory of Cluster Science, Ministry
of Education of China, Beijing Key Laboratory of
Photoelectronic/Electrophotonic Conversion Materials, School
of Chemistry and Chemical Engineering, Beijing Institute of
Technology, Beijing 102488, P. R. China; School of
Chemistry and Chemical Engineering, Jiangxi Science and
2X/6-311+G(d,p)//B3LYP/6-31+G(d,p) to ca. 386.8 (92.44
kcal/mol) and 292.5 kJ/mol (69.9 kcal/mol), respectively.
F
(See Table S11 for details.) The FIA of MesB-ACN exhibits
pronounced affinity for the fluoride anion, which is in line with
the highest values previously reported in triarylboranes.19,21
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Xing Chen − Key Laboratory of Cluster Science, Ministry of
Education of China, Beijing Key Laboratory of
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of Chemistry and Chemical Engineering, Beijing Institute of
Technology, Beijing 102488, P. R. China
Yali Qiao − Beijing National Laboratory for Molecular
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Education Centre for Excellence in Molecular Sciences,
Institute of Chemistry, Chinese Academy of Sciences
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Kanglei Liu − Key Laboratory of Cluster Science, Ministry of
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Nan Wang − Key Laboratory of Cluster Science, Ministry of
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Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was funded by the National Natural Science
Foundation of China (21772012) and the Beijing Institute of
Technology Research Fund Program for Young Scholar. We
acknowledge the Analysis and Testing Center of the Beijing
Institute of Technology for the NMR, MS, and X-ray single-
crystal diffraction characterization. We acknowledge the
Analysis Center of the Institute of Chemistry, Chinese
Academy of Sciences (ICCAS) for the X-ray single-crystal
diffractions. G.L. also thanks the Project of the Science Funds
of Jiangxi Education Office (GJJ180629) and the Project of
Jiangxi Science and Technology Normal University
(2016XJZD009) for financial support
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