Tetracoordinate Boron-Fused Double [5]Helicenes as Cathode Active Materials for Lithium Batteries

Article abstract of DOI:10.1021/acs.orglett.9b00337

Double [5]helicenes possessing two tetracoordinated boron atoms at the ring junctions were synthesized from glycine anhydride in three steps. The helicene with fluorine substituents on the boron atoms was employed as a cathode active material in a lithium battery to demonstrate moderate performance in the 1.8-3.5 V range with more than 63 mAh g^-1 discharge capacity for 20 cycles and high Coulombic efficiency of over 90%.

Full text of DOI:10.1021/acs.orglett.9b00337

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Tetracoordinate Boron-Fused Double [5]Helicenes as Cathode
Active Materials for Lithium Batteries
Susumu Oda, Takeshi Shimizu, Takazumi Katayama, Hirofumi Yoshikawa,* and Takuji Hatakeyama*
School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan
S
* Supporting Information
ABSTRACT: Double [5]helicenes possessing two tetracoor-
dinated boron atoms at the ring junctions were synthesized
from glycine anhydride in three steps. The helicene with
fluorine substituents on the boron atoms was employed as a
cathode active material in a lithium battery to demonstrate
moderate performance in the 1.8−3.5 V range with more than
63 mAh g−1 discharge capacity for 20 cycles and high
Coulombic efficiency of over 90%.
etracoordinate boron compounds have attracted signifi-
cant attention because of their excellent photophysical
of a lithium battery employing tetracoordinate boron com-
pounds as the electrode active material.10,11
The syntheses of 1a and 1b are summarized in Scheme 1.
Glycine anhydride was exposed to phosphorus pentachloride
T
properties,1 which have led to numerous applications in
optoelectronics, including organic light-emitting diodes
(OLEDs),2 organic field-effect transistors,3 and photorespon-
sive materials.4 To elicit diverse functionality, various types of
chelate ligands for the boron center, such as N,N-, N,C-, and
N,O-ligands, have been exploited. In particular, tetracoordinate
boron compounds employing pyridyl phenolate as N,O-chelate
ligands have proven to be promising light emission materials and
electron transport materials for OLEDs.5 In view of their
potential utility, further exploration of such novel functions is
highly desirable.
Scheme 1. Synthesis of Tetracoordinate Boron-Fused Double
Helicenes 1a and 1b
Helicenes are nonplanar screw-shaped molecules composed
of ortho-fused aromatic rings.6 Owing to their inherent chirality
and dynamic behavior involving self-assembly, they offer many
possible applications as asymmetric catalysts, molecular
machines, and in molecular recognition and organic electronics.
Recently, much effort has been devoted to accomplishing
multihelicity, which leads to dramatic changes in physical
properties as a result of increased distortion and multidimen-
sional intermolecular interactions.7,8 Despite the remarkable
progress in the study of helicenes with multihelicity, multiple
helicenes that possess tetracoordinated boron atoms have not
been reported to date.9 This is primarily because of a lack of
suitable synthetic methods.
Herein, we report a three-step synthesis of tetracoordinate
boron-fused double [5]helicenes, consisting of haloaromatiza-
tion, Suzuki−Miyaura coupling, and demethylative borylation.
The developed protocol is facile and scalable and provides access
to derivatives with different substituents on the boron atoms by
successive reaction with nucleophiles. Notably, a syn-racemic
isomer was selectively obtained by preferential crystallization.
Furthermore, the double helicene with fluorine substituents on
the boron atoms showed reversible reduction, a property that
can be exploited for preparing a cathode active material for
lithium batteries with moderate capacity and cycle stability. To
the best of our knowledge, this report presents the first example
and phosphoryl chloride to give tetrachloropyrazine in 63%
yield. The palladium-catalyzed Suzuki−Miyaura coupling
reaction of tetrachloropyrazine with 2-methoxyphenylboronic
acid proceeded at 100 °C to afford tetraarylpyrazine 2 in 78%
yield. In the presence of BBr3, demethylation took place
smoothly at 40 °C in toluene to form intermediate 3a, which was
then treated with phenylmagnesium bromide to give 1a in 35%
yield. Moreover, intermediate 3b was formed by using BI3,
followed by addition of AgF, to afford 1b in 45% yield. These
compounds showed substantial chemical stability against acidic
and basic conditions: no decomposition was observed on
Received: January 26, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00337
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Organic Letters
Letter
treatment with 0.5 N HCl or NaOH aqueous solutions.
However, gradual decomposition was observed during silica
gel column chromatography, presumably as a result of the
reaction between the B−O bonds and Si−OH groups on silica
gel.
1
The H NMR spectrum of 1a shows eight peaks in the
aromatic region, except for those of the phenyl substituents on
the boron atoms. Given that four peaks should be observed from
a single isomer, this result indicates that 1a exists as a mixture of
two isomers in 1:1 ratio (Figure 1). Based on the stereo-
Figure 3. ORTEP drawing of syn_rac_1a obtained by X-ray
crystallographic analysis. Thermal ellipsoids are shown at 50%
probability. Hydrogen atoms are omitted for clarity.
small nucleus-independent chemical shift (NICS(0)) value of
−1.9 (Figure S6).
Notably, recrystallization of 1a from benzene gave only a
single crystal of syn_rac_1a. Although 1H NMR analysis
indicated the existence of anti_rac_1a, its single crystal was
not obtained despite multiple attempts using various solvents
(benzene, dichloromethane, toluene, diethyl ether, dioxane, and
carbon tetrachloride). This suggests that 1a preferentially
crystallizes as syn_rac_1a under rapid equilibrium between
syn- and anti-racemic isomers in the solution state. The single
crystal of syn_rac_1a was then subjected to 1H NMR
spectroscopy in CDCl3 at low temperature (−40 °C), which
revealed a mixture of the two isomers in a 1:1 ratio. To confirm
the stereochemistry of 1a in the solid state, powder X-ray
diffraction (XRD) analysis of the solid obtained after simple
evaporation was performed (Figure S2). The powder XRD
pattern of 1a is in good agreement with that simulated from the
single-crystal data of syn_rac_1a. Based on these results, we
conclude that 1a is in rapid equilibrium between the syn- and
anti-racemic isomers in the solution state but forms the syn-
racemic isomer in the solid state. We assume that the rapid
isomerization proceeds through the cleavage of the B−N or B−
O bond. Similar results were obtained with 1b by means of 1H
NMR analysis, DFT calculations, and X-ray crystallographic
analysis (Figures S1, S4, and S18).
Figure 1. 1H NMR spectrum of 1a in CDCl3 at 25 °C.
chemistry of the substituents on boron (syn/anti) and the
helicenes (P/M), these double helicenes afforded four possible
diastereomers (Figure 2): syn_racemic (PP/MM), syn_meso
Figure 2. Relative electron energies (ΔE) of 1a diastereomers
calculated at the B3LYP/6-31G(d) level of theory.
The photophysical properties of 1a and 1b in CH2Cl2 are
summarized in Figure 4. The absorption maxima of 1a and 1b
were 574 and 553 nm, respectively. Time-dependent DFT
calculations suggest that the absorption bands of 1a and 1b
correspond to the S0−S1 transitions of the syn-racemic (PP/
MM) and anti-racemic (PP/MM) isomers because their
excitation energies are almost identical (syn_rac_1a, 2.14 eV;
anti_rac_1a, 2.17 eV; syn_rac_1b, 2.26 eV; anti_rac_1b, 2.29
eV; Tables S1 and S2). They showed red emission bands at 653
and 623 nm (Φf = 0.20, 0.29), respectively.
(PM), anti_racemic (PP/MM), and anti_meso (PM). Density
functional theory calculations revealed that syn_rac_1a and
anti_rac_1a were the most and second-most stable diaster-
eomers, respectively. The meso diastereomers were relatively
unstable (6.27 and 2.77 kcal/mol, respectively), which is
commonly observed in highly fused double helicenes.9c−e
Owing to the small energy difference (0.05 kcal/mol) between
syn_rac_1a and anti_rac_1a, they were identified as the two
observed isomers.
The helical structure of syn_rac_1a was determined by X-ray
crystallography (Figure 3). The four coordinated boron atoms
of syn_rac_1a adopt a tetrahedral geometry. The B−C, B−O,
and B−N bond lengths are 1.618, 1.451−1.467, and 1.605−
1.611 Å, respectively, which are comparable to those of reported
phenol−pyridyl boron complexes.5c Although the C3−C3′ and
C3−N1 bond lengths are similar to those of the reported
pyrazine ring, the dihedral angles between the planes of the
terminal rings (A−A′, 40.4°; B−B′, 51.9°) indicate low
aromaticity of the central pyrazine ring because of strong
distortion. This observation is consistent with the relatively
The electrochemical properties of 1a and 1b were studied by
cyclic voltammetry (Figure S5). One reversible reduction wave
was observed for 1a (E1/2red = −1.06 V), whereas two reversible
red
reduction waves were observed for 1b (E1/2 = −0.97 and
−1.64 V). The second reduction wave of 1a was not observed
within a potential window of CH2Cl2 (ca. −2.0 V). The two-
electron reduction of 1b is not attributed to the two
diastereomers because the LUMO energy levels of syn_rac_1b
(−3.00 eV) and anti_rac_1b (−2.93 eV) are very close and
cannot be distinguished by cyclic voltammetry. Encouraged by
the reduction property and low LUMO energy level of 1b, we
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DOI: 10.1021/acs.orglett.9b00337
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Organic Letters
Letter
S9). The initial discharge capacity (180 mAh g−1) was much
higher than the theoretical capacity (106 mAh g−1), although the
subsequent discharge capacities were close to the theoretical
capacity, and a long plateau was not observed after the second
discharge process. Moreover, the Coulombic efficiency at the
first cycle exceeded 180%, which indicates that the initial process
is irreversible due to electrolyte decomposition and solid-
electrolyte interface formation and is peculiar to ionic liquids.13
Therefore, to evaluate the battery performance of 1b, the
discharge curves were obtained after the second discharge
process. The second discharge capacity of 1b was 93 mAh g−1,
which corresponds to 88% of the theoretical capacity, and the
capacity retention after the 20th discharge process was 67%.
Moreover, high Coulombic efficiency of more than 90% was
maintained between the second and 20th processes. Notably,
tetraarylpyrazine 2 did not show the redox property and the
battery performance (Figures S11 and S12).
In summary, we synthesized tetracoordinate boron-fused
double [5]helicenes through demethylative borylation and
successive treatment with nucleophiles such as Grignard
reagents and silver fluoride. Stereoselective formation of syn-
racemic isomer through preferential crystallization was con-
firmed by X-ray crystallography. These double helicenes showed
strong red emissions and reversible reduction waves. In
particular, the double helicene with fluorine substituents
exhibited two reversible reductions and a low LUMO energy
level. The lithium battery fabricated using this helicene as the
cathode active material showed moderate performance,
capacity, and cycle stability. The key to success is the use of
an ionic liquid as the electrolyte to suppress the leaching of the
electrode active material. These results not only extend the
feasibility of organic electrode materials for lithium batteries but
also encourage the further development of organoboron-based
materials.
Figure 4. Normalized absorption (red) and fluorescence (blue) spectra
of (a) 1a and (b) 1b (0.02 mM in CH2Cl2) with photophysical data.
Abbreviations: ε, absorption coefficient at absorption maximum; Φf,
absolute PL quantum yield; τs, fluorescence lifetime measured at an
emission maximum; kr, radiative rate constant; knr, nonradiative rate
constant.
investigated its performance as an electrode active material for
lithium batteries.
After careful screening of the electrolytes, lithium bis-
(trifluoromethanesulfonyl)imide (LiTFSI) in ionic liquid, 1,2-
dimethyl-3-propylimidazolium bis(trifluoromethanesulfonyl)-
imide (DMPImTFSI),12 was found to afford a stable cycle
performance owing to its low solubility, which suppressed the
dissolution of the cathode active material into the electrolyte
solution (Figure 5). The discharge curve showed a long plateau
at 2.2 V in the first cycle but exhibited a gentle slope between 3.0
and 3.2 V after the second cycle. This result correlates well with
the reduction potentials of the corresponding battery of 1b, as
determined by cyclic voltammetry (E1/2red = 2.1, 3.1 V) (Figure
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.9b00337.
Synthesis, analytical data, NMR spectra, DFT studies, CV
measurements, X-ray crystallography, fabrication of
lithium battery (PDF)
Accession Codes
CCDC 1890482 and 1890488 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.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.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: yoshikawah@kwansei.ac.jp.
*E-mail: hatake@kwansei.ac.jp.
ORCID
Susumu Oda: 0000-0003-1088-1932
Takuji Hatakeyama: 0000-0002-7483-9525
Notes
Figure 5. (a) Discharge−charge curves of 1b during 20 cycles. (b)
Cycle performance of 1b. LiTFSI/DMPImTFSI was used as an
electrolyte.
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.orglett.9b00337
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Organic Letters
Letter
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ACKNOWLEDGMENTS
■
This study was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas “π-System Figuration”
(JP17H05164), Grant-in-Aid for Scientific Research
(JP18H02051), Challenging Research (Exploratory,
JP17K19164), and Grant-in-Aid for Early-Career Scientists
(18K14228) from the Japan Society for the Promotion of
Science (JSPS), Iketani Science and Technology Foundation,
Mitsubishi Foundation, Sumitomo Foundation, and Foundation
of Kinoshita Memorial Enterprise. Preliminary measurements of
the X-ray crystal structure analysis were performed at BL40XU
in the SPring-8 with the approval of JASRI (2016A1052,
2016B1059, 2017A1132, 2017B1073, 2018A1114) and with the
help of Dr. Nobuhiro Yasuda (JASRI) and Mr. Soichiro
Nakatsuka (Kwansei Gakuin University). Powder X-ray
diffraction analysis was performed with the help of Prof. Daisuke
Tanaka and Mr. Yoshinobu Kamakura (Kwansei Gakuin
University). We are grateful to Mr. Yasuhiro Kondo (JNC
Petrochemical Corp.) for providing experimental support.
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