Trishomoaromatic (B3N3Ph6) Dianion: Characterization and Two-Electron Reduction

Article abstract of DOI:10.1002/anie.201916651

Benzene, a common aromatic compound, can be converted into an unstable antiaromatic 8π-electron intermediate through two-electron reduction. However, as an isoelectronic equivalent of benzene, borazine (B₃N₃Ph₆), having weak aromaticity, undergoes a totally different two-electron reduction to afford (B₃N₃R₆)^2? homoaromatic compounds. Reported here is the synthesis of homoaromatic (B₃N₃Ph₆)^2? by the reduction of B₃N₃Ph₆ with either potassium or rubidium in the presence of 18-crown-6 ether. Theoretical investigations illustrate that two electrons delocalize over the three boron atoms in (B₃N₃Ph₆)^2?, which is formed by the geometric and orbital reorganization and exhibits (π,σ)-mixed homoaromaticity. Moreover, (B₃N₃Ph₆)^2? can act as a robust 2e reductant for unsaturated compounds, such as anthracene, chalcone, and tanshinones. This 2e reduction is of high efficiency and selectivity, proceeds under mild reaction conditions, and can regenerate neutral borazine.

Full text of DOI:10.1002/anie.201916651

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Accepted Article
Title: Trishomoaromatic (B3N3Ph6)-Dianion: Characterization and
Two-Electron Reduction
Authors: Wen-Xiong Zhang, Nan Li, Botao Wu, Chao Yu, Tianyu Li,
and Zhenfeng Xi
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201916651
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Trishomoaromatic (B3N3Ph6)-Dianion: Characterization and Two-
Electron Reduction
Nan Li,#[a] Botao Wu,#[a] Chao Yu,[a] Tianyu Li,[b] Wen-Xiong Zhang*[a] and Zhenfeng Xi*[a]
Dedicated to the 100th Birthday of Professor Youqi Tang
Abstract: Benzene, as a common aromatic compound, can be
converted to an unstable antiaromatic 8-electron intermediate
through two-electron reduction. However, as an isoelectronic
equivalent of benzene, borazine (B3N3R6) of weak aromaticity
undergoes the totally different two-electron reduction to afford
(B3N3R6)2- homoaromatic compounds. Here we report the synthesis of
homoaromatic (B3N3Ph6)-dianion by the reduction of B3N3Ph6 with
potassium or rubidium in the presence of 18-crown-6. Theoretical
investigation illustrates that two electrons delocalize over three boron
atoms in (B3N3Ph6)2-, which is formed by the geometric and orbital
reorganization and exhibits (,σ)-mixed homoaromaticity. Moreover,
(B3N3Ph6)-dianion can act as a robust 2e reductant for unsaturated
compounds, such as anthracene, chalcone and tanshinones. This 2e
reduction is of high efficiency, selectivity, mild conditions and re-
generation of neutral borazine.
18-Crown-6 to yield (B3N3Ph6)-dianion. Single-crystal X-ray
diffraction analysis reveals that it has a C3v-symmetric parallel
double-layer structure (Scheme 1B). Theoretical investigation
illustrates that two electrons delocalize over the three boron
atoms in (B3N3Ph6)2- and exhibit (,σ)-mixed homoaromaticity.
These results clearly show that borazine can accept two electrons
to its two degenerated LUMOs (2e’’) to give an unstable planar
intermediate which undergoes geometric and orbital
reorganization to give a parallel double-layer (B3N3Ph6)2- structure
(Scheme 1B). In addition, (B3N3Ph6)2- can act as a robust 2e
reductant for unsaturated compounds.
Aromaticity is an important and fascinating concept in organic
chemistry.[1,2] Benzene is a common aromatic compound with
planar D6h structure, which can be converted to dearomatized 1,4-
cyclohexadiene by a classic Birch reduction. Theoretically,
benzene could turn into an unstable antiaromatic intermediate
(6C, 8-electron)through two-electron reduction. This may lead to
rapid distortion of this intermediate based on pseudo-Jahn-Teller
effect, and form a C2-symmetric dearomatized dianion eventually
(Scheme 1A).[3] To maintain aromaticity of benzene, four-electron
reduction is required.[4] Borazine, as an isoelectronic equivalent of
benzene, has a planar D3h structure and equalized B-N bond
lengths (1.44 Å).[5,6] Despite bearing similar electronic structure,
the aromaticity of borazine is a long-controversial topic in history.
Up to now, it is widely accepted that borazine shows weak
aromaticity due to its limited -electron delocalization caused by
the electronegativity difference between boron and nitrogen
atoms.[5] This structural feature provides many unique properties
for borazine, which is remarkably different from benzene.
Although the reductions of arenes have been well studied, the
reductions of borazine and its derivatives still remain a mystery.
Herein, for the first time, we report the two-electron reduction of
borazine (B3N3Ph6) via potassium or rubidium in the presence of
Scheme 1. The 2e reductions of benzene and borazine.
[a]
Dr. N. Li, Mr. B. Wu, Mr. C. Yu, Prof. Dr. W.-X. Zhang, Prof. Dr. Z.
Xi.
Beijing National Laboratory for Molecular Sciences (BNLMS),
Key Laboratory of Bioorganic Chemistry and Molecular Engineering
of Ministry of Education, College of Chemistry, Peking University,
Beijing 100871 (China)
Fax: (+86)10-62751708
E-mail: wx_zhang@pku.edu.cn; zfxi@pku.edu.cn
Ms. T. Li, College of Chemistry, Beijing Normal University
# These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the
document.
[b]
Scheme 2. Synthesis of (B3N3Ph6)2- compounds 2a/2b and 3a/3b.
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As illustrated in Scheme 2, the phenyl-substituted borazine 1
was synthesized according to the literature procedure.[7]
Treatment of 1 with potassium in THF at RT for 6 h gave a dark
red solution. The expected ionic compound 2a was formed in 92%
NMR yield as tan solids after THF removal. When 18-Crown-6
(18C6) was added to the above solution of 2a, the corresponding
18C6-coordinated red crystals 3a was obtained in 60% yield. 3a
could also be synthesized by successively adding potassium and
18-Crown-6 to the solution of 1. Similarly, the rubidium componds
2b and 3b were obtained in 94% and 64% yields, respectively.
Borazines with alkyls on either N or B atoms could not be reduced
by alkali metals, which can be attributed to both electronic and
spatial effects (for experimental and DFT details, see SI). The
reduction reaction occurred with the similar phenomenon to
borazine B3N3Ph6 when borazine with p-ethylphenyl on N atoms
was used, giving red solids in 70% yield. Due to the low solubility
of this red solid in different solvents, we fail to get suitable crystals
for X-ray analysis. It can be seen from the above experiments that
the phenyl groups are essential for reduction reaction.
theory. The optimized geometry is consistent with the X-ray data.
Three 2p orbitals of B3 unit are taken as φa and φe (φex and φey)
1
and three 2p orbitals of N3 unit are taken as ϕa and ϕe (ϕex and
1
ϕe ). As shown in Figure 2, both ψ1e and ψ2e are composed of ϕe
y
and φe [ψ1e = ϕe+λφe, ψ2e = φe-λ’ϕe, (0 < λ, λ’ < 1)], and φa1 interacts
with ϕa1 to constitute ψ1a1 and ψ2 1. Although the orbital behavior
a
of C3v-symmetried (B3N3Ph6)2- is similar to borazine, its electron
configuration is different. Compared with D3h-symmetried
borazine, the energy level of ψ2 1 in (B3N3Ph6)2- is lower than ψ2
a
e
(in borazine: ψ2 and ψ2e ). This is caused by the directivity of
a
2’’
’’
orbitals’ interaction under unique geometric restraints, reflecting
the special conjugated effect between N and B atoms. To be
specific, ψ1
reveals negative conjugated interactions by
a
1
antiphase overlap of B, N-p orbitals, which can be illustrated by
the angle of antiphase p orbitals (61o measured) of Band Natoms.
Instead, the p-conjugated interaction between N and B atoms
turns to be positive in ψ2 due to the same phase interaction.
a
1
Compared with ψ1 1, more bonding orbital characteristics are
a
found in ψ2 and it shows a stabilizing effect, thus reducing the
a
1
energy level of ψ2 1. This special interactions between B, N-p
a
orbitals result in ψ2 possessing two electrons as the HOMO of
a
1
(B3N3Ph6)2-, which is much different from trishomoaromatic
dianions of 1,3,5-triboracyclohexanes that only p orbitals of B
atoms participate in the bonding.[9]
Figure 1. Molecular structures of 3a and 3b. Hydrogen atoms, solvent
molecules, and the counterions (M+)2(18-Crown-6)2(THF)2 (M = K or Rb) are
omitted for clarity. Selected bond lengths [Å] and angles [o] for 3a: B1-N1
1.508(7), B1-N3 1.513(7), B2-N1 1.493(7), B2-N2 1.520(7), B3-N2 1.520(7), B3-
N3 1.485(8), B1-B2 1.983(7), B1-B3 1.971(7), B2-B3 1.997(7), N1-B1-N3
116.4(5), B1-N3-B3 82.2(4); for 3b: B1-N1 1.52(1), B1-N3 1.53(1), B2-N1
1.53(1), B2-N2 1.54(1), B3-N2 1.54(1), B3-N3 1.54(1), B1-B2 2.00(1), B1-B3
2.00(1), B2-B3 1.99(1), N1-B1-N3 114.6(9), B1-N3-B3 81.1(6).
Single-crystal X-ray diffraction analyses reveal that 3a and 3b
have similar molecular configurations, thus only metric
parameters of 3a is discussed here (Figure 1). In the dianionic
fragment of 3a, three boron atoms and three nitrogen atoms
belong to two different planes which are parallel to each other,
while the neutral B3N3Ph6 1 exhibits a planar [B3N3] core. The B-
N bonds in 3a [1.485(8)-1.520(7) Å] are slightly longer than those
in 1 [1.433(4)-1.447(3) Å].[7] The distances between two boron
atoms (B1-B2 1.983(7) Å, B1-B3 1.971(7) Å, B2-B3 1.997(7) Å) in
the [B3]-plane are significantly longer than typical B−B single
bonds (1.72 Å). This comparison shows there is no bond behavior
of B−B single bond in 3a. The angles of N1-B1-N3 and B1-N3-B3
in 3a are 116.4(5)o and 82.2(4)o, which are different from the
neutral one (118.6(2)o, 121.1(2)o for each).
Figure 2. (a) Schematic π-FMO diagrams of theoretical C3v-(B3N3Ph6)2- system.
Mulliken symbols are used for showing molecular orbitals; (b) The orbital
diagrams of C3v-(B3N3Ph6)2- system. The isovalue of ψ2a1, ψ1e and ψ1a1 is 0.05,
while the isovalue of ψ2e is 0.03 for clarity. Orange and blue dots represent B
and N atoms, respectively. The phenyls on B, N atoms are omitted for clarity.
To gain further insight into the electronic structures, DFT
calculations and analyses for 3a were conducted at the B3LYP-
D3BJ/def2-TZVP level using Gaussian 16 and Multiwfn.[8] The
orbital interaction was discussed by using the perturbation MO
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In two-electron occupied ψ2
,
apart from conjugated
Absorption, excitation and emission spectra of 3a and 3b were
also investigated (for details, see SI). Compared with 1 (250-300
nm absorption range),[7] 3a and 3b undergo bathochromic shifts
to give absorption range at 325-400 nm, 330-475 nm respectively.
As for the steady-state excitation spectrum, 3a and 3b display
almost the same excitation maxima at 354 nm. In the PL spectrum,
3a and 3b show fluorescence at 473 nm and 489 nm individually.
The steady-state excitation data correspond well with the
simulated results calculated under (TD)PBE0/6-311G** level of
theory.[16] The calculation results show that (B3N3Ph6)2- has the
characteristic of valence electronic transition (HOMO→LUMO),
and its S1 is close to doublet degeneracy.
Taking 2a as example, the applications of (B3N3Ph6)2-
homoaromatic compounds as a 2e reductant were depicted in
Table 1. Gratifyingly, the homogeneous reaction between
anthracene 4a and 2a/diphenylamine proceeded efficiently at
room temperature, giving the desired dihydro compound 4b in
91% yield.[17] As a contrast, under the heterogeneous condition of
K/diphenylamine, the reaction rate decreased obviously. For
chalcone 5a with α, β-unsaturated carbonyl moiety, the reduction
took place smoothly, affording 1,3-diphenyl-1-propanone 5b in
90% yield. Natural products like tanshinone I 6a and tanshinone
IIA 7a were treated with 2a to lead to the formation of the
corresponding phenolic compounds 6b (93% yield) and 7b (95%
yield). It is worth mentioning that the neutral B3N3Ph6 could be
readily recovered by flash column chromatography after the
above reduction reaction. In all, 2a as a robust 2ereductant shows
excellent efficiency in reducing unsaturated compounds.
a
1
interactions between B and N atoms, there is also a multicenter
interaction among three B atoms. Localized Orbital Locator (LOL)
function map shows electrons delocalized among three B atoms
(Figure 3a). An atoms-in-molecules (AIM) analysis presents the
existence of a bond critical point (BCP) at the B3 center (Figure
3b). Meanwhile the dotted lines around the BCP represent the
electron delocalized area. These results demonstrate a 3c-2e
interaction in the B3 center obviously. The multicentered bond
order is 0.571, which further proves the 3c-2e bond. The
NICS(0)ZZ and NICS(1)ZZ of the B3 unit are -11.55 and -8.78 ppm,
which indicate homoaromatic characters. The angle of 25.5o
between the axes of B p-orbitals and the center of the B3 plane
reflects the (,σ)-mixed homoaromaticity, which shows
asymmetric NICS values with respect to the plane of the centers
of cyclic delocalization (Figure 3c). It could also be verified in
NICS(Z)ZZ curve (Figure 3d).[10] The NICS minimum is only
located at 42 pm above the plane of the centers. The
homoaromaticity of B3 unit could be further verified by the induced
current visualization (Figure 3e). It shows that the diamagnetic
current appears on the B3 plane to resist the external magnetic
field. Thus, (,σ)-mixed homoaromaticity in 3a is significantly
different from these reported Hückel

B3 systems,[11]
monohomoaromatic B3 systems,[12] (+σ)-double aromatic B3
systems,[13] σ-aromatic B3 systems [14] and others.[15]
Table 1 The reduction chemistry of compound 2aa
a Reaction conditions: substrates (0.07 mmol), reductant 2a (0.077 mmol), THF
(5 mL), 5-60 mins, RT. The values in parentheses refer to the yields of isolated
product; b Diphenylamine (0.154 mmol) as an additive; c MeOTf (0.14 mmol) is
used to trap diphenol compounds.
In summary, the two-electron reduction of borazine B3N3Ph6 via
potassium or rubidium in the presence of 18-Crown-6 is achieved
for the first time to yield (B3N3Ph6)-dianion. The (B3N3Ph6)-dianion
exhibits the C3v-symmetric parallel double-layer structure and the
(,σ)-mixed homoaromaticity. Furthermore, (B3N3Ph6)-dianion
can act as a robust 2ereductant for unsaturated compounds, such
as anthracene, chalcone and tanshinones under mild conditions.
The unique chemical bonding modes of trishomoaromatic
(B3N3Ph6)-dianion disclosed in this work would undoubtedly
Figure 3. Theoretical analysis of (B3N3Ph6)2- shown as (a) LOL contour map on
the Z = 1.0 Bohr over the B3 plane; (b) The topology of the 2elec on the Z = 1.0
Bohr over the B3 plane; (c) The angle between the axes of B p-orbitals and the
center of the B3 plane is 25.5o; (d) The distribution of NICS(Z)ZZ values; (e) The
induced current sectional view of the Z = 1 Bohr over the B3 unit.
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Acknowledgements
This work was supported by the National Natural Science Foundation of
China (Nos. 21725201, 21890721, 21690061). We thank Prof. Shang-Da
Jiang and Dr. Zifeng Zhao for the help of experimental measurements. We
thank Prof. Shengfa Ye, Prof. Jun Zhu, Dr. Junnian Wei, Dr. Rui Feng and
Mr. Zhe Huang for the theoretical discussions. Theoretical calculations of
this work was supported by High-performance Computing Platform of
Peking University
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10.1002/anie.201916651
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Nan Li,# Botao Wu,# Chao Yu, Tianyu Li,
Wen-Xiong Zhang,* Zhenfeng Xi*
Page No. – Page No.
Trishomoaromatic (B3N3Ph6)-Dianion:
Characterization and Two-Electron
Reduction
Trishomoaromatic (B3N3Ph6)-Dianion: The two-electron reduction of borazine
(B3N3Ph6) is achieved for the first time to yield the stable (B3N3Ph6)-dianion which
exhibits the C3v-symmetric parallel double-layer structure and the (,σ)-mixed
homoaromaticity. The (B3N3Ph6)-dianion can act as a robust 2e reductant for
anthracene, chalcone and tanshinones with high efficiency, selectivity, mild
conditions and re-generation of neutral borazine.
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