Zwitterionic Inorganic Benzene Valence Isomer with σ-Bonding between Two π-Orbitals

Article abstract of DOI:10.1021/jacs.8b08025

Despite the large number of plausible isomers of benzene (C₆H₆), only four valence isomers [(CH)₆] have been experimentally detected, all of which are nonionic and possess skeletal frameworks built from localized C-C bonds. Herein, we present the isolation of a diazatetraborabenzene analogue of a hypothetical zwitterionic valence isomer of benzene. Therein, two electrons are delocalized over the four boron atoms in the six-membered B₄N₂ ring, which is a result of the σ-bonding interaction between two odd-electron B-B π-orbitals. Simple treatment with a crown ether leads to the formation of a paramagnetic potassium-doped radical ion pair that exhibits a thermally populated triplet character.

Full text of DOI:10.1021/jacs.8b08025

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Communication
A Zwitterionic Inorganic Benzene Valence
Isomer with #-Bonding between Two #-Orbitals
Bochao Su, Kei Ota, Kai Xu, Hajime Hirao, and Rei Kinjo
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08025 • Publication Date (Web): 12 Sep 2018
Downloaded from http://pubs.acs.org on September 12, 2018
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A Zwitterionic Inorganic Benzene Valence Isomer with σ-
Bonding between Two π-Orbitals
Bochao Su,† Kei Ota,† Kai Xu,‡ Hajime Hirao*,‡ and Rei Kinjo*,†
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†Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Techno-
logical University, Singapore 637371, Singapore
‡Department of Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
Supporting Information Placeholder
ABSTRACT: Despite the large number of plausible isomers
of benzene (C6H6), only four valence isomers [(CH)6] have
been experimentally detected, all of which are nonionic and
possess skeletal frameworks built from localized C–C bonds.
Herein, we present the isolation of a diazatetraborabenzene
analogue of a hypothetical zwitterionic valence isomer of
benzene. Therein, two electrons are delocalized over the four
boron atoms in the six-membered B4N2 ring, which is a result
of the σ-bonding interaction between two odd-electron B–B
π-orbitals. Simple treatment with a crown ether leads to the
formation of a paramagnetic potassium-doped radical ion
pair that exhibits a thermally populated triplet character.
Since the first discovery of benzene by Faraday1 and the sub-
Figure 1. Benzene valence isomers. (A) Benzene (I) and its
known nonionic valence isomers II–V. (B) Zwitterionic va-
sequent identification of its cyclic structure by Kekulé,2 the
chemistry of benzene and its associated concept of aromatic-
lence isomer VI is formed from the ionic resonance form I’ of
ity have fascinated both synthetic and theoretical chemists
benzene. (C) Four-center two-electron bonding interaction
for over 190 years.3,4 Computational studies have identified
via σ-type overlap of two π-orbitals in I’.
more than 200 isomeric structures, all of which are at least
1
60 kcal∙mol‒ higher in energy than benzene.5,6 Four valence
isomers that consist of (CH)6 units have been experimentally
This is probably due to the polarity of the B–N units, which
characterized, i.e., cis-Dewar benzene (II), benzvalene (III),
in turn arises from the different number of valence electrons
of the boron and nitrogen atoms and their respective elec-
tronegativity (χB = 2.04, χN = 3.04). Given that B and N are
prismane (IV), and biscycloprop-2-enyl (V) (Figure 1A).7-10
Compounds I–V follow the octet rule as all the carbon atoms
are tetravalent in the absence of a formal charge localization.
isolobal to cationic and anionic CH fragments, respectively,
Interestingly, the calculated natural resonance theory (NRT)
the incorporation of these elements in the framework of
weight values for benzene suggest a slight admixture of ionic
benzene may furnish inorganic analogues of the proposed
canonical forms in the electronic structure.11 The ionic char-
zwitterionic valence isomers of benzene. Herein, we report
acter of benzene allows one to envision a number of zwitter-
the synthesis of diazatetraborinane 1 and its chemical reduc-
tion.22 The latter afforded the first valence isomer derivative
ionic valence isomers. For example, based on ionic resonance
form I', it may be postulated that zwitterionic valence isomer
of diazatetraborabenzene that is relevant to the envisioned
zwitterionic benzene valence isomer VI (Figure 1).
VI (Figure 1B) could be formed via four-center two-electron12-
14
bonding interaction between two odd-electron π-orbitals
(Figure 1C).15-18 Thus far, however, such possible zwitterionic
A neutral diazatetraborinane derivative featuring a six-
membered B4N2 heterocycle (1) was obtained from the
deprotonation of 1,2-bis(anilide)-1,2-bis-
(dimethylamido)diborane with two equivalents of n-
butyllithium, followed by treatment with
bis(dimethylamido)diborondichloride (Figure 2). Tetra-
valence isomers have not yet been observed, neither experi-
mentally nor computationally.
Borazine (B3N3H6), also referred to as inorganic benzene, is
highly stable although it does not exhibit aromaticity on ac-
count of its relatively limited electronic delocalization.19-21
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Figure 2. Synthesis of diazatetraborinane 1 and its chemical reduction reactions. Reduction of 1 with potassium graphite (KC8),
followed by addition of one and two equivalents of 18-crown-6 ether (18C6) afforded 2 and 3, respectively. When the reaction
mixture is not immediately treated with crown ether after the reduction of 1, 4 is formed.
parallel to each other; i.e., they include a torsion angle of
27.2°. The B1−B4 (1.700(9) Å) and B2−B3 (1.673(9) Å) bond
distances are shorter than those in 1 (1.732(3) Å and 1.717(3)
hydrofuran (THF) was added to a solid mixture of 1 and five
equivalents of potassium graphite (KC8) at room tempera-
ture, which immediately afforded a dark purple solution.
Å, respectively) but slightly longer than the reported B–B
After filtration, a stoichiometric amount of 18-crown-6 ether
bond distances of diborane radical anions (1.636(7)−1.661(5)
Å).23-27 The distances between these boron atoms, i.e., B1−B2
(18C6) was added to the solution, which resulted in an im-
mediate color change from dark purple to dark blue. By slow
diffusion of hexane into the blue solution at ‒26 °C, single
crystals of dianionic product 2 were obtained in 68% yield.
(2.201 Å), B1−B3 (3.038 Å), B2−B4 (2.330 Å), and B3−B4 (2.176
Å), are longer than typical B−B single bonds (1.72 Å). Howev-
er, density functional theory (DFT) calculations performed
The 1H NMR spectrum of 2 shows only a broad singlet at 2.21
on 2 at the M06-2X/6-31G(d) level of theory indicated bond-
ppm, which corresponds to the protons of NMe2. In the 11B
NMR spectrum, a broad signal at 12.9 ppm was observed,
ing interactions between these four B atoms. The highest
occupied molecular orbital (HOMO) and the HOMO‒1 cor-
which is upfield-shifted relative to that of 1 (39.5 ppm), and
respond to σ-type bonding orbitals formed between two π(B–
thus indicative of an increase in the electron density on the
B) orbitals in the six-membered B4N2 rings (Figure 3B). Thus,
boron centers. A single-crystal X-ray diffraction analysis con-
the four B atoms in each B4N2 ring share two electrons,
firmed the ionic nature of 2, in which two potassium cations
which is consistent with the results of a natural population
are trapped by 18C6, whereas the C2-symmetric dianionic
analysis (NPA), suggesting that the B atoms in 2 are less posi-
fragment contains two six-membered B4N2 units that interact
tively charged (B1: 0.63; B2: 0.48; B3: 0.51; B4: 0.47) than
with one potassium ion each. Both potassium atoms are
those in 1 (B1: 0.71; B2: 0.79). A natural bond orbital (NBO)
bridged by a molecule of THF (Figure 3A). Each B4N2 ring
analysis yielded the following Wiberg bond index (WBI) val-
exhibits a distorted boat shape, whereby both N atoms are
ues: B1−B2 (0.17), B1−B3 (0.06) B1−B4 (1.02), B2−B3 (1.06),
bent toward the coordinating potassium atom (sum of the
B2−B4 (0.34), and B3−B4 (0.23), which confirm that the
bond angles around the N atoms: ΣN1 = 360.0°; ΣN2 =
bonding interaction between the B2 and B4 atoms is stronger
359.0°). The N atoms of the NMe2 groups at the B2 and B4
than that between the B1 and B3 atoms. Indeed, an atoms-in-
atoms display a pyramidal geometry (ΣN4 = 345.9°; ΣN6 =
molecules (AIM) analysis revealed the existence of a bond
345.0°), whereas those at the B1 and B3 atoms retain a planar
critical point between the B2 and B4 atoms (Figure 3C). In
geometry. The distance between the K1 and K2 atoms (3.660
the solid state, 2 is stable for several days at room tempera-
Å) is well within the sum of the van der Waals radii of two
ture, while it decomposes into an unidentified mixture after
potassium atoms (5.6 Å). The B1−B4 and B2−B3 bonds are not
6 hours in solution.
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1
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B
C
D
Figure 3. Crystallographic characterization and computational results on the molecules prepared in this work. (A) Solid-state
structures of 1 (top left), the dianionic part of 2 (top right), 3 (bottom left), and 4 (bottom right) (hydrogen atoms, solvent mole-
cules, and the counterion in 2 are omitted for clarity). All ellipsoids are shown at 50% probability. (B) Plots of the HOMO (left)
and HOMO‒1 (right) of 2. (C) The topology of the Laplacian distribution of the charge density in the six-membered B4N2 ring of
2 with bond critical points (blue dots). (D) Plot of the SOMO of 3-D.
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To prevent the potassium atoms from interacting with the
a colorless solution, indicative of thermal isomerization
(Figure S19−S21). After work-up, i.e., treatment with two
equivalents of 12C4, single crystals of 4 were obtained in 71%
yield, and an X-ray diffraction study revealed the dianionic
tetraborane structure of 4. Compound 4 represents a rare
example of long-chain homocatenated tetraboranes.31 It
seems quite likely that 4 was formed via a ring-opening reac-
tion coupled with an electron-precise B–B bond formation32,33
from an in-situ-generated dianionic intermediate (Figure
S22). This notion is consistent with the results of the NBO
and AIM analyses of 2, which show bonding interactions
among the four B atoms, especially between the B2 and B4
atoms.
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six-membered B4N2 rings, two equivalents of 18C6 were add-
ed immediately after the reduction reaction. The reduction
of 1 with KC8 (5 equiv.) in THF, followed by treatment with
18C6 (2 equiv.), afforded a dark blue solution, which is NMR
silent even at low temperature (−60 °C). Slow diffusion of
hexane into this THF solution afforded dark needle-shaped
crystals of 3 in 63% yield. We found that 3 could also be pre-
pared by adding 18C6 to a THF solution of 2. An X-ray dif-
fraction analysis at 103 K revealed the involvement of an ani-
onic B4N2 unit and two potassium atoms that are coordinat-
ed by 18C6 and two molecules of THF in the unit cell (Figure
3A). The shortest distance between the B4N2 anion and the
[K(18C6)(THF)2]+ counterions is greater than 3.0 Å, indicat-
ing weak interactions. The boat-shaped geometry of the B4N2
ring of 3, as well as its structural parameters, is similar to
those of 2, suggesting that the four B atoms in 3 share elec-
trons, as seen in 2. The paramagnetic nature of 3 was con-
firmed by means of electron paramagnetic resonance (EPR)
spectroscopy and SQUID magnetic measurements. The EPR
spectrum of 3 shows a broad unresolved resonance both in
THF solution (g = 2.00476; −90 ~ 20 °C) and in the solid state
(g = 2.00484; −150 ~ 20 °C) (Figure S13). The observed tem-
perature dependence of both the EPR spectrum of the THF
solution and the magnetic susceptibility for the crystalline
sample of 3 fits well with the Bleaney–Bowers equation,28
from which a singlet ground state was estimated with the
single-triplet gaps (2J) of −419.15 cm−1 (−1.2 kcal∙mol−1) by the
EPR measurements (Figure S13) and −647.08 cm−1 (−3.7
kcal∙mol−1) by the SQUID magnetic measurements (Figure
S14), respectively. This observation agrees reasonably well
with the DFT results, which show that the singlet ground
state 3-S is by 7.8 kcal∙mol−1 more stable than the triplet
state. Significantly, the most stable triplet state consists of a
doublet state of 3 (3-D) and a neutral potassium atom
[K(18C6)(THF)2] rather than a triplet state of 3 (3-T). Such
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In conclusion, we have demonstrated that a diazatetrabor-
benezene analogue 2 of a hypothetical zwitterionic valence
isomer of benzene can be synthesized by simple reduction of
diazatetraborinane 1 with KC8 followed by the addition of 2
eq 18-crown-6 ether. X-ray analysis and DFT calculation dis-
close that two electrons are delocalized over the four boron
atoms in the six-membered B4N2 ring with exhibiting a σ-
bonding interaction between two B-B π-orbitals, which is
further supported by the generation of the long-chain homo-
catenated tetraborane 4. Treatment of 1 with twice amount
of 18-crown-6 ether for 2 leads to the formation of a para-
magnetic potassium-doped radical ion pair 3 that exhibits a
thermally populated triplet character. These results in their
entirety show that this method represents a useful strategy
for the construction of benzene valence isomers that are not
accessible via conventional hydrocarbon systems, indicating
that this method may potentially lead to a variety of mole-
cules with unique bonding.
ASSOCIATED CONTENT
Supporting Information
+
‒
⦁
alkali metal-doped radical ion pairs of the type M2 A⦁ (M =
alkali metal; A = organic molecule) are considered to be pro-
spective molecular materials with large nonlinear optical
(NLO) responses.29 A comparison of the calculated and ex-
perimental geometric parameters for 3 suggests that, even at
103 K, 3 should be best described as a thermally populated
triplet state.30 Indeed, a triplet was still detected experimen-
tally even at 103 K by the SQUID measurements. These re-
sults indicate the significant role of K+. While the localization
of two negative charges in the (N2B4)2- core must be essen-
tially disfavored, such electronic state is allowed if there is a
potassium cation in close proximity, because a K+ cation sta-
bilizes the negative charges and thus the closed-shell singlet
state 2. Without K+, the electrons will get delocalized in a
wider rage to gain greater stabilization, which may lead to
Synthesis, NMR spectra, and crystallographic data (CIF) of 1-
4; EPR spectroscopy; SQUID magnetic measurements; UV-
Vis spectroscopy; Cyclic voltammograms; Computational
details. This material is available free of charge via the Inter-
net at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*rkinjo@ntu.edu.sg
*hhirao@cityu.edu.hk
Notes
B.S. and K.O. contributed equally to the study. The authors
declare no competing financial interests.
the triplet state 3.
The singly occupied molecular orbital
(SOMO) of 3-D corresponds to the σ-type bonding orbital
formed between two π(B–B) orbitals (Figure 3D), confirming
that the four B atoms in the B4N2 ring share one electron. An
NBO analysis yielded WBI values between those boron atoms
of B1−B2 (0.11), B1−B3 (0.03), B2−B4 (0.18), and B3−B4 (0.11).
The spin density is mainly localized on the two B atoms (B1:
0.09; B2: 0.31; B3: 0.08; B4: 0.31). The synthesis and isolation
of 3-D via independent control reactions have not been suc-
cessful thus far.
ACKNOWLEDGMENT
This work was financially supported by Nanyang Technologi-
cal University (NTU), Singapore, and the Singapore Ministry
of Education (MOE2015-T2-032) (R.K.). H.H. acknowledges
financial support from City University of Hong Kong
(7200534 and 9610369). We thank Dr. Y. Li (NTU) for assis-
tance in X-ray analyses.
When crown ether was not added immediately after the
reduction of 1, the dark purple solution gradually turned into
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26
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28
29
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32
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44
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50
51
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53
54
55
56
57
58
59
60
6
ACS Paragon Plus Environment