Methane activation by a borenium complex

Article abstract of DOI:10.1016/j.chempr.2021.03.010

The selective functionalization of methane under ambient conditions remains a formidable challenge for chemists. While most studies have focused on transition metal complexes, much less attention has been devoted to molecular complexes based on non-metal elements, despite them being more sustainable and less environmentally impactful. Here, we report that an N-heterocyclic carbene-stabilized borenium complex can activate methane under relatively mild conditions. The resulting methylborenium complex can readily transfer the methyl group to catecholborane (HBcat), which can lead to a synthetic cycle for the conversion of methane to MeBcat. Both experimental and theoretical mechanistic studies suggest that the C–H bonds of methane are activated via a σ-bond metathesis pathway. Such direct aliphatic C–H borylation can be extended to other alkanes, such as ethane and octane. The formed alkylborenium complexes can react with terminal alkynes via 1,2-alkylboration to install both the alkyl and boryl functionalities onto organic scaffolds.

Full text of DOI:10.1016/j.chempr.2021.03.010

ll
Article
Methane activation by a borenium complex
Yizhen Liu, Weishi Dong, Zhen
Hua Li, Huadong Wang
huadongwang@fudan.edu.cn
Highlights
Isolable-borenium-mediated
methane activation under
relatively mild conditions
Involvement of s-bond
metathesis in methane activation
supported by mechanism studies
A synthetic cycle for
methylcatecholborane synthesis
using methane and
catecholborane
Methane, the most abundant hydrocarbon in the world, is also the hardest to
transform, due to its particularly strong C–H bonds. Breaking these bonds
generally requires the involvement of reactive transition metal complexes. This
work describes a route to functionalize methane in the absence of metals by
employing a cationic boron species. This result could potentially lead to the
development of more sustainable and less environmentally impactful catalyst
systems for methane conversion.
Liu et al., Chem 7, 1–9
July 8, 2021 ª 2021 Elsevier Inc.
https://doi.org/10.1016/j.chempr.2021.03.010

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j.chempr.2021.03.010
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Article
Methane activation by a borenium complex
Yizhen Liu, Weishi Dong, Zhen Hua Li, and Huadong Wang1,2,*
1
1
1
SUMMARY
The bigger picture
The selective functionalization of methane under ambient condi-
tions remains a formidable challenge for chemists. While most
studies have focused on transition metal complexes, much less
attention has been devoted to molecular complexes based on
non-metal elements, despite them being more sustainable and
less environmentally impactful. Here, we report that an N-heterocy-
clic carbene-stabilized borenium complex can activate methane un-
der relatively mild conditions. The resulting methylborenium com-
plex can readily transfer the methyl group to catecholborane
Methane is the most abundant
hydrocarbon in the world. The
direct transformation of methane
into value-added chemicals under
ambient conditions is a highly
attractive but challenging
endeavor. While most studies
have focused on reactive
transition metal complexes, much
less attention has been devoted
to non-metal molecular
(
HBcat), which can lead to a synthetic cycle for the conversion of
methane to MeBcat. Both experimental and theoretical mechanistic
studies suggest that the C–H bonds of methane are activated via a s-
bond metathesis pathway. Such direct aliphatic C–H borylation can
be extended to other alkanes, such as ethane and octane. The
formed alkylborenium complexes can react with terminal alkynes
via 1,2-alkylboration to install both the alkyl and boryl functional-
ities onto organic scaffolds.
complexes, despite them being
more sustainable and less
environmentally impactful. In this
work, we show that a highly
electrophilic borenium complex
can activate methane under
relatively mild conditions. The
resulting methylborenium
INTRODUCTION
complex can readily transfer the
methyl group to catecholborane
As the main component of natural gas, methane is the most abundant hydrocarbon
in the world. While most globally produced methane (>90%) are used as an energy
source, the use of methane as a raw material for chemical production constitutes
(
HBcat), which can lead to a
synthetic cycle for the conversion
of methane to MeBcat.
1
only about 5% of methane consumption. Current industrial methane conversion
process relies on an indirect route, which initially converts methane into syngas,
Furthermore, such direct aliphatic
C–H borylation can be extended
to other alkanes, such as ethane
and octane.
2
the mixture of CO and H , under high temperature and high pressure and subse-
quently turns syngas into methanol and other hydrocarbons. Besides being energy
intensive, this indirect route is only economically feasible at a very large scale,
excluding much desired local, on-site applications. Compared with the indirect
route, direct functionalization of methane could be more advantageous from both
2
,3
economic and environmental points of view. Of particular interest is the develop-
ment of well-defined complexes capable of activating methane in homogeneous so-
lutions, which enable facile steric and electronic modification and are amenable to
4
experimental mechanism investigation. In order to overcome the low reactivity of
methane, molecular methane activation systems generally require the involvement
of reactive transition metal complexes. Recently, a groundbreaking report by Peri-
ana and co-workers showed that main group complexes could outperform transition
5
–8
9
metal complexes in methane activation.
They demonstrated that high-valent d
CCF can activate methane in the solution
CCF ) or
COOMe after reductive functionalization. While highly effi-
main group salts TI(O
2
ꢀ
CCF
3
)
3
and Pb(O
2
3 4
)
of CF
Pb(O
3
COOH at 180 C via electrophilic C–H activation, affording TI(O
2
3
2
CCF
3
)
2
and CF
3
cient and selective, this method has a number of limitations, such as the usage of
highly toxic metal complexes as well as harsh and corrosive conditions. The lack of
2 3 2 3 2
straightforward ways to oxidize TI(O CCF ) or Pb(O CCF ) to their high-valent
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counterparts could also hinder efforts to turn this stoichiometric reaction into a cat-
alytic one.
Besides electrophilic C–H activation, s-bond metathesis represents another attrac-
9
,10
tive pathway for the activation of the C–H bonds of methane.
primarily associated with rare earth complexes. Since the landmark discovery by
This reactivity is
1
1,12
Watson in 1983,
formula of Cp* MR (M = Sc, Y, Lu; R = alkyl or silyl) are known to react with methane in
a degenerate or non-degenerate way via s-bond metathesis.
a number of rare earth metallocene complexes with a general
2
1
1–16
However, the Cp*
ligand appears indispensable for such reactivity, thus significantly limiting the room
for further optimization of their electronic and structural properties. While similar
reactivity with methane has been extended to coordinatively unsaturated Ir(III) com-
1
7,18
plexes,
well-defined main group complexes capable of activating methane via
1
9
20,21
22–25
s-bond metathesis still remain elusive. Recently our group
and others
2
6
have reported that organoboranes or borenium complexes are capable of
activating H via s-bond metathesis. Particularly, we have shown that an N-hetero-
cyclic carbene (NHC) stabilized o-carboranyl-substituted hydroborenium complex
IMe B(H)Cb][B(C ] (IMe = 1,3,4,5-tetramethylimidazol-2-ylidene, Cb = o-car-
boran-1-yl, 1) can activate the C–C bonds of cyclopropanes via s-bond metathesis.
2
[
4
6
F
5
)
4
4
2
7
2
8
29
As the easily modified NHC ligand and o-carboranyl substituent of the borenium
complex enable judicious tuning of its electronic and steric properties, we ques-
tioned if such borenium complex could be applied in the activation of C–H bonds
of methane. The resulting borylation will extend the scope of catalyst-free aliphatic
3
0–35
C–H borylation reactions, which so far are limited to intramolecular processes,
and provide an alternative to the recently reported noble-metal catalyzed methane
3
6,37
borylation reactions.
Herein we describe the borylation of methane and other
alkanes with a borenium complex under relatively mild conditions. Our experimental
and theoretical mechanistic studies suggest that the C–H bond of methane is acti-
vated via a s-bond metathesis pathway.
RESULTS AND DISCUSSION
We started our investigation by examining the reactivity of the known borenium 1
6 5 6 4 2
with methane. Heating the solution of 1 in C D Br or o-C H F under 60 bar of
ꢀ
CH
4
to 100 C for 10 h only resulted in complete decomposition of 1. Reasoning
that the low thermal stability of 1 could be due to the lack of kinetic protection
around the borenium center, we replaced the o-carboran-1-yl substituent with
Me
bulkier 2-methyl-o-carboran-1-yl (Cb ) group. Additionally, 1,3-bis(2,3,4,5,6-pen-
F 38
tafluorobenzyl)imidazol-2-ylidene (IBn ) was employed as the NHC ligand. Be-
F
sides being bulkier compared with IMe
4
, IBn is a relatively electron-poor ligand,
which will likely increase the electrophilicity of the borenium center, thus promoting
2
1,27
F
Me
its activity in s-bond metathesis.
The target [IBn B(H)Cb ][B(C
Cb (3) with [Ph C][B(C ] in the solution of o-
6 4 2
C H F /hexane with a yield of 78% (Scheme 1). Borenium 2 was characterized by
6 5 4
F ) ] (2) was
F
Me
prepared by treating IBn B(H)
2
3
6 5 4
F )
1
multinuclear NMR analysis and HR-MS. In the H NMR spectrum of 2, a broad singlet
corresponding to the hydrogen substituent on the borenium center was observed at
1
1
4
.38 ppm. The B NMR spectrum of 2 consists of a broad singlet at 66.2 ppm, which
is assigned to the tri-coordinated borenium center. This signal is substantially down-
1Department of Chemistry and Shanghai Key
3
9
Laboratory of Molecular Catalysis and Innovative
Materials, Fudan University, Songhu Road 2005,
Shanghai 200438, China
field shifted relative to the corresponding resonance in 1 (54.2 ppm), indicating
increased positive charge on the borenium center of 2 compared with 1. The identity
of 2 was further confirmed using derivatization. Treatment of 2 with 1 equiv of pyri-
dine gave pyridine coordinated boronium complex 2-py, which was characterized by
X-ray diffraction (Figure S69). Borenium 2 shows higher thermal stability compared
2Lead contact
*Correspondence: huadongwang@fudan.edu.cn
https://doi.org/10.1016/j.chempr.2021.03.010
2
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C6F5
N
N
H
B
C
C6F5
H
C
O
O
[Ph3C][B(C6F5)4]
B
CH3 + "[Bcat][B(C6F5)4]"
3
O
2
Ph3CH
C6F5
(i)
(iii)
HB
O
C6F5
N
N
N
N
H
B
(ii)
CH3
B
C6F5
C6F5
C
C
C
C
CH4
H-H
B(C6F5)4-
B(C6F5)4-
:
B-H
2
4
Scheme 1. Synthetic cycle for the conversion of methane into MeBcat initiated by borenium 2 and
À
the molecular structure of methylborenium 4 (for clarity, the [B(C6F5)4] anion and hydrogen
atoms were removed)
ꢀ
(
(
(
i) o-C
6
H
4
F
2
/hexane, 25 C, 78% yield.
ꢀ
ii) CH
4
(60 bar), o-C
6
H
4
F
2
, 110 C, 61% yield.
ꢀ
iii) C
6
D
5
Br, 110 C, >95% NMR yield.
ꢀ
6 4 2
with 1. Upon 10 h of heating in a o-C H F solution at 100 C, about 60% of 2 was
found decomposed, in contrast to complete decomposition of 1 under identical
conditions.
With the new borenium in hand, we set out to examine the reactivity of 2 with
ꢀ
methane. Heating the solution of 2 in o-C
6
H
4
F
2
under 60 bar of CH
4
to 100 C re-
Me
F
sulted in the formation of the C–H borylation product [IBn B(Me)Cb ][B(C
6 5 4
F ) ]
(
4), which can be isolated in 29% yield after recrystallization. Increasing the reaction
ꢀ
temperature to 110 C resulted in a higher isolation yield of 61% (Scheme 1). Replac-
ing isolated 2 with in situ generated 2 (from equimolar of 3 and [Ph C][B(C ]) pro-
3
6 5 4
F )
vided a lower isolation yield (28%). Further increase of the reaction temperature to
ꢀ
1
30 C has little impact on the isolation yield (60%). Decreasing the pressure of
methane to 30 bar led to a diminished isolation yield (29%). Analysis of the overhead
gas by GC revealed the presence of H gas. The o-C solvent appears critical for
the activation of methane, as employing C Br or o-C Cl as solvent led to the
2
6 4 2
H F
6
H
5
6
H
4
2
formation of a trace amount of 4 (NMR yield < 10%). The molecular structure of bore-
nium 4 was determined using a combination of multinuclear NMR analysis, HR-MS,
1
and single-crystal X-ray analysis. In the H NMR spectrum of 4, a singlet at 1.99 ppm
was observed, which was assigned to the methyl substituent on the borenium center.
1
13
The H- C HSQC spectrum revealed this signal is correlated to a broad singlet at
1
3
1
2
0.5 ppm of the C{ H} NMR spectrum, which is close to those observed in other
F
F
methylboranes, such as MeBAr
2
(Ar = 2,4,6-tris(trifluoromethyl)phenyl), 19.9
4
0
11
ppm). The B NMR spectrum of 4 reveals a broad signal at 82.7 ppm, in agree-
ment with the existence of a tri-coordinated borenium center. In the solid structure
of 4, the borenium center remains tri-coordinated as evidenced by the sum of
ꢀ
:
C–B–C angles of the borenium center of 360.0 . The corresponding B–C bond
4
1
˚
lengths are in the range of 1.534(3)–1.609(3) A, typical for B–C single bonds.
1
3
When C enriched methane was applied in the borylation reaction, as anticipated,
1
13
1
the H NMR of the resulting 4- C exhibited a doublet ( JC–H = 120.5 Hz) at
1
3
13
1
1
.99 ppm, assigned to the boron linked CH
3
moiety. In the C{ H} NMR spectrum
1
3
of 4- C, a broad singlet at 20.5 ppm with enhanced intensity was observed, and this
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1
13
signal appears as a quartet with JC–H of 120.5 Hz in the C NMR spectrum. The
F
Me + 13
appearance of a major peak corresponding to [IBn B(Me)Cb
]
-
C in HR-MS
1
3
13
spectrum of 4- C further confirms that the formation of 4- C is the result of
1
3
13
CH
4
activation. After 4 was treated with 50 bar of C enriched methane in o-
ꢀ
13
C
6
H
4
F
2
at 110 C for 24 h, we found no detectable amount of 4- C in the reaction
1
mixture via H NMR analysis. The facile activation of methane with 2 as well as the
lack of reactivity of 4 are in contrast to the well-studied rare earth metallocene sys-
tems, in which Cp*
2 2 3
MH complexes show no reactivity with methane and Cp* MCH
1
3
11,12,14
(
M = Sc, Y, Lu) complexes readily undergo isotopic exchange with CH .
4
To understand the mechanism of this methane C–H borylation reaction, we
measured the initial rates of the reaction under CH and CD independently (Table
4
4
S4). The resulting primary KIE value of 3.3(2) indicates that the activation of the C–H
bond of methane is involved in the rate determining step. A similar KIE value (4.0)
2 2 3
was also observed in Cp* ScCH CMe -mediated methane activation reported by
1
4
Tilley et al. Furthermore, measuring the initial rates of the reaction with variant 2
concentrations (0.01 to 0.08 M) and methane pressures (51 to 79 bar) allows us to
determine the rate law of this reaction, which shows first-order dependence on
both 2 and the pressure of methane (Figures S10 and S11). Eyring analysis over a
ꢀ ꢀ ꢀ
3
0 C range (100 C to 130 C) was also conducted, which revealed the enthalpy
s
À1
s
and entropy barriers (DH = 9.0(2) kcal mol , DS = -63.1(4) e.u.) for this reaction
Figure S9), consistent with a highly ordered transition state. Collectively, these
(
experimental data are all in agreement with a s-bond metathesis mechanism in
which the C–H and B-H bonds are cleaved in a single step.
To further elucidate the mechanism of the methane activation process, we carried
À
out density functional theory (DFT MN15) calculations (the [B(C
6
F
5
)
4
]
anion was
4
2
omitted). This methane C–H borylation reaction is almost thermodynamically
o
À1
neutral with DG = +3.8 kcal mol at 298 K. The C–H activation process takes place
via a four-centered transition state (TS) without prior coordination of methane with 2
À1
(
Figure 1). The calculated free energy barrier is 28.2 kcal mol , in excellent agree-
À1
ment with the experimental value of 27.8(4) kcal mol (298 K). In TS, the C1-H1 bond
˚
is nearly broken with a bond length of 1.455 A, whereas the C1-B1 bond is almost
˚
formed at a distance of 1.734 A, suggesting TS as a late transition state in regard
to the formation of 4. Both H1 and H2 atoms are bonded to the B1 atom with a
˚
˚
B1-H1 distance of 1.267 A and a B1–H2 distance of 1.273 A. The distance between
˚
˚
H1 and H2 is 1.059 A, substantially longer than the one for free H (0.74 A). Accord-
2
ingly, the Wiberg bond indices (WBI) revealed fractional C1–H1 (0.211), B1–
C1(0.768), B1–H1 (0.399), B1–H2 (0.671), and H1–H2 (0.192) interactions. Overall,
4
3
the bonding picture of TS agrees with a s-bond metathesis. Notably, Goldfuss
and co-workers reported a similar transition state in their theoretical investigation
4
4
3 4
of the reaction between BH and CH .
To examine the possibility of extending this stoichiometric methane borylation reac-
tion to a catalytic one, we studied the reactivity of 4 with catecholborane (HBcat), a
6 5
common borylation reagent. When equimolar of 4 and HBcat was heated in C D Br
ꢀ
to 110 C for 18 h, neutral borane 3 and MeBcat were observed in the reaction
1
mixture along with around 60% of unreacted 4 as indicated by H NMR spectros-
copy. When the amount of HBcat was increased to two equivalents, 4 was
ꢀ
completely consumed after 6 h of heating at 110 C, and 3 and MeBcat were quan-
4À
titatively generated (Scheme 1). Decomposition of the B(C
6
F
5
)
anion (about 70%
, (C )Bcat
and some unidentified compounds among the decomposition products. Methyl
1
9
according to F NMR spectroscopy) was also observed with B(C
6
F
5
)
3
6 5
F
4
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2 4
Figure 1. Geometry of TS (hydrogen atoms except H and those in CH are omitted for clarity)
exchange likely takes place between 4 and HBcat, affording MeBcat and 2, and the
resulting 2 reacts further with HBcat to yield 3 and [Bcat][B(C ], the latter of which
is known to undergo decomposition to form B(C and (C
with this hypothesis, heating of equimolar of 2 and HBcat in C
titatively affords 3, with both B(C and (C
6 5 4
F )
4
5
6
F
5
)
3
6
F
5
)Bcat. Consistent
ꢀ
6
D
5
Br to 110 C quan-
6
F
5
)
3
6 5
F )Bcat being present in the reaction
mixture. Without regeneration of 2, it is unlikely 2 can catalyze methane borylation
with HBcat. Indeed, after the solution of HBcat in the presence of 20 mol% of 2 in
ꢀ
o-C
6
H
4
F
2
was heated at 110 C under 60 bar of CH
4
for 24 h, no formation of MeBcat
F
was observed. 3 was the only observable species containing IBn moiety in the reac-
tion mixture. Nonetheless, the successful transfer of the methyl group from 4 to
HBcat revealed a synthetic cycle for the conversion of methane to MeBcat, which
could serve as a proof of concept for the development of borenium catalyzed
methane borylation reaction.
After achieving borylation of methane with borenium 2, we set out to explore
whether such a borylation reaction can be extended to other alkanes (Table 1). Treat-
ꢀ
ment of 2 with ethane (30 bar) at 110 C resulted in the formation of ethane activation
F
Me
product [IBn B(Et)Cb ][B(C
6
F
5
)
4
] (5) in 46% yield after recrystallization. Decreasing
ꢀ
the reaction temperature to 90 C improved the isolation yield to 56%, which can be
ꢀ
attributed to the thermal instability of 5 at a higher temperature (t1/2 z 10 h at 110 C
in o-C
6
H
4
F
2
). No ethylene glycol was detected by gas chromatography-mass spec-
was applied,
trometry (GC-MS) if conventional oxidative workup with NaOH/H
2
O
2
indicating the absence of bisborylation products. Besides gaseous alkanes, we
also examined the reactivity of 2 against n-octane. Heating 2 with octane (300 equiv)
ꢀ
in o-C
6
H
4
F
2
at 110 C led to the formation of octyl-substituted borenium 6 in 18%
NMR yield along with a substantial amount of decomposition products. The lower
yield compared with methane or ethane could be due to the decreased polarity of
the n-octane/o-C
6 4 2
H F solvent, which enhances the interaction between the ion
pairs, resulting in acceleration of decomposition. GC-MS analysis of the mixture
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Table 1. Borylation of alkanes with borenium 2
ꢀ
Substrates
CH CH (30 bar)
Temperature ( C)
Products
Yield
a
b
3
3
90
56%
a
c
n-octane (300 equiv)
110
18%
a
b
toluene (350 equiv)
110
44%
d
(
p: m = 3:2)
a
2
6 4
(0.16 mmol) in o-C H F (3 mL).
b
Isolation yield.
NMR yield.
c
d
1
Determined by H NMR analysis.
2 2
after NaOH/H O workup revealed the presence of n-octanol and no other isomers
were detected, implying that the primary C–H bonds is substantially favored over the
secondary ones in this borylation reaction, a phenomenon also observed in transi-
4
6
tion-metal-catalyzed borylation reactions. In the case of toluene, however, the
borylation took place at the aryl ring, resulting in the formation of a mixture of
4
7
para- and meta-tolylborenium complexes (7) in a 3:2 para:meta ratio. In agreement
with this observation, theoretical calculations indicate that the free energy barrier for
À1
the activation of the benzylic C–H bond (30.0 kcal mol ) is higher than those for the
À1
À1
activation of aryl C–H bonds (27.5 kcal mol for para-C–H; 28.0 kcal mol for meta-
C–H) (Figure S71).
One attractive feature of this alkane borylation reaction is that the formed alkylbore-
nium complexes can be directly employed in alkyne functionalization, introducing
both the alkyl and boryl moieties onto organic scaffolds. Treatment of 4 with tri(iso-
6 4 2
propyl)silylacetylene in o-C H F at room temperature results in the quantitative for-
mation of syn-1,2-methylboration product 8 (Scheme 2). After recrystallization, 8 can
be isolated in 55% yield and its identity was confirmed by multinuclear NMR analysis,
HR-MS, and single-crystal X-ray analysis. Similar reactivity was also observed for 5,
which affords syn-1,2-ethylboration product 9 in 66% isolation yield. While the
4
8
49–51
25,52
works from Ingleson, Shoji,
and others
have shown that arylborenium,
6
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C6F5
C6F5
R
N
N
Si(iPr)3
N
N
R
B
C
B
C
C6F5
(iPr)3Si
H
C6F5
H
2
5 oC, o-C6H4F2
C
C
B(C6F5)4-
B(C6F5)4-
4
5
: R = Me
: R = Et
8: R = Me
9: R = Et
Scheme 2. Synthesis of borenium 8 and 9 via 1,2-alkylboration and the molecular structure of
À
methylborenium 8 (for clarity, [B(C
alkenyl moiety were removed)
6
F
5
)
4
]
anion and the hydrogen atoms except the one on the
arylborinium, or arylboranes can react with alkynes to afford 1,2-arylboration prod-
ucts, uncatalyzed 1,2-alkylboration of alkynes, to the best of our knowledge, has not
been reported previously.
Conclusions
To conclude, we found that a well-defined NHC carbene-stabilized hydroborenium
complex can react with methane or ethane under relatively mild conditions to afford
the corresponding methyl- or ethylborenium complexes. Mechanistic studies indi-
cate that the C–H bond of methane is activated via a s-bond metathesis pathway.
The utility of the methylborenium complex as a methyl source was evidenced by
the methyl exchange reaction with HBcat as well as selective 1,2-alkylboration of
alkynes. This work not only offers a route to functionalize alkanes in the absence of
metals but also underscores the potential of main group elements mediated
s-bond metathesis, which could rival d- or f-elements in enthalpically strong bond
activation.
SUPPLEMENTAL INFORMATION
Supplemental information can be found online at https://doi.org/10.1016/j.chempr.
2
021.03.010.
EXPERIMENTAL PROCEDURES
Resource availability
Lead contact
Further information and requests for resources should be directed to and will be ful-
filled by the lead contact, Huadong Wang (huadongwang@fudan.edu.cn).
Materials availability
Unique and stable reagents generated in this study will be made available on
request, but we might require a payment and/or a completed materials transfer
agreement if there is potential for commercial application.
Data and code availability
The crystal structure data of complexes 3, 2-py, 4, and 8 have been deposited in
the Cambridge structural database under reference numbers 2005260, 2008556,
2
005185, and 2008557.
ACKNOWLEDGMENTS
Financial support from the National Natural Science Foundation of China (21573044,
2
1672039, 21871051, 21873019, and 22071027) and Shanghai Science and Tech-
nology Committee (19DZ2270100) is gratefully acknowledged. We also thank the
High Performance Computer Center of Fudan University for the allocation of
Chem 7, 1–9, July 8, 2021
7

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j.chempr.2021.03.010
a
borenium complex, Chem (2021), https://doi.org/10.1016/
ll
Article
computing time. We thank Dr. Yue-Jian Lin for help on the crystallography study and
Li-Ping Wang for assistance on NMR measurement.
AUTHOR CONTRIBUTIONS
H.W. conceived the research and designed the project. Y.L. and W.D. performed the
experimental work. Z.H.L. undertook the DFT calculations. H.W. wrote the manu-
script with input from all authors. All authors analyzed the results and commented
on the manuscript.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: July 23, 2020
Revised: September 9, 2020
Accepted: March 10, 2021
Published: April 9, 2021
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