A two-coordinate boron cation featuring C-B + -C bonding

Article abstract of DOI:10.1038/nchem.1948

Two-coordinate boron cations (R 2 B +), referred to as borinium ions, are chemical species in which the boron bears only four valence electrons, and that are isoelectronic with hypothetical carbon dications (R 2 C 2+). Although lone-pair-donating substituents such as amino groups have enabled the isolation of several borinium ions, diarylated and dialkylated borinium derivatives remain entirely unexplored. Here, we present the synthesis, structure and reactivity of the dimesitylborinium ion, which displays unexpectedly high thermal stability. X-ray crystallography and 11 B NMR spectroscopy, supported by density functional theory calculations, reveal that the borinium ion adopts a linear two-coordinate structure in both the solid state and in solution. The boron centre is stabilized by p € bonding from the mesityl groups and is free from coordination by the counterion or solvent molecules. This diarylborinium ion possesses exceptional Lewis acidity, accepting a pair of electrons from CO 2 to cause an unusual deoxygenation reaction.

Full text of DOI:10.1038/nchem.1948

ARTICLES
PUBLISHED ONLINE: 11 MAY 2014 | DOI: 10.1038/NCHEM.1948
A two-coordinate boron cation featuring
C–B1–C bonding
Yoshiaki Shoji1 , Naoki Tanaka1, Koichiro Mikami2, Masanobu Uchiyama2,3 and Takanori Fukushima1
*
Two-coordinate boron cations (R2B1), referred to as borinium ions, are chemical species in which the boron bears only four
valence electrons, and that are isoelectronic with hypothetical carbon dications (R2C21). Although lone-pair-donating
substituents such as amino groups have enabled the isolation of several borinium ions, diarylated and dialkylated borinium
derivatives remain entirely unexplored. Here, we present the synthesis, structure and reactivity of the dimesitylborinium
ion, which displays unexpectedly high thermal stability. X-ray crystallography and 11B NMR spectroscopy, supported by
density functional theory calculations, reveal that the borinium ion adopts a linear two-coordinate structure in both the
solid state and in solution. The boron centre is stabilized by pp bonding from the mesityl groups and is free from
coordination by the counterion or solvent molecules. This diarylborinium ion possesses exceptional Lewis acidity, accepting
a pair of electrons from CO2 to cause an unusual deoxygenation reaction.
he octet rule is a fundamental principle of chemistry, describ- using dimethylamino and 2,2,6,6-tetramethylpiperidino groups
ing how main group atoms tend to combine so as to have eight as substituents (IV, Fig. 1b)24. Monoarylated and monoalkylated
T
electrons in their valence shell. It applies most rigorously to derivatives containing only one stabilizing amino group (V and
main group elements of the second period. Boron, the third VI, Fig. 1b) were reportedly hard to isolate because of their chemi-
element in the second period, accumulates only six valence electrons cal instability. Thus, a general consensus has developed that the
in its neutral three-coordinate state (borane), R3B (I, Fig. 1a) and it synthesis of a two-coordinate boron cation requires lone-pair-
is therefore favourable for it to accept an electron pair into its vacant donating substituents, such as amino groups, which can compen-
2p orbital to fulfil an octet. This strong Lewis acidity (as well as sate for the electron deficiency on boron through dative p-
boron’s low electronegativity) gives boron compounds a unique bonding from heteroatoms15,16,25,26. The chemistry of borinium
chemistry that plays a vital role in a wide range of fields, including ions that are devoid of such lone pair donation has
organic transformations1,2, supramolecular complexation3–5 and remained unexplored.
chemical bonding theory6–14
.
Here, we report the isolation and full structural characterization
So, what would happen if borane were to lose one further chemi- of a diarylborinium ion (Mes2Bþ; Mes (mesityl) ¼ 2,4,6-trimethyl-
cal bond? In this context, two-coordinate cationic boron species phenyl, Fig. 1c). We further demonstrate the extraordinary reactivity
with only four valence electrons, referred to as ‘borinium ions’ of this borinium ion through a peculiar CO2 activation reaction. Key
(parent H2Bþ and II, Fig. 1a), are fascinating because they to the present achievement is the use of chemically inert and weakly
deviate significantly from the octet rule and may show unprece- nucleophilic counterions such as undecachloro carborane
dented reactivity arising from their extraordinary Lewis acidity. It ([HCB11Cl11]2) (refs 27, 28) and tetrakis(pentafluorophenyl)borate
is also interesting to note that the borinium ion is isoelectronic ([(C6F5)4B]2). Another part of the synthetic strategy is the use of
with the hypothetical two-coordinate carbon dication mesityl groups that offer steric bulk around the boron centre and
(III, Fig. 1a). In addition to extensive studies of three- and four- the possibility of pp electronic stabilization. Mesityl substituents
coordinate boron cations15–18, several efforts have been devoted to were used for the synthesis of the first silylium ion (Mes3Siþ) (ref. 29),
creating two-coordinate borinium ions15,16,19–26. For instance, where related steric and electronic constraints were necessary for
electron ionization of a variety of gas-phase boranes (R3B; R ¼ H, stabilization. We found that Mes2Bþ salts of [HCB11Cl11]2 (1a)
Me, F, Cl, and so on) under high-vacuum conditions was and [(C6F5)4B]2 (1b) (Fig. 1c) possess high thermodynamic
found to generate borinium ions (R2Bþ) that can be detected by stability in both the solid state and in solution and can therefore
mass spectrometry19,20
.
be readily obtained on a large synthetic scale.
The first attempt to synthesize a borinium ion in solution dates
back to 1958, where the reaction of chlorodiphenylborane
(Ph2BCl) with AlCl3 in nitrobenzene or methyl ethyl ketone was
reported to afford a mixture containing the diphenylborinium
ion (Ph2Bþ) (ref. 21). However, later re-examination revealed
that, rather than borinium ion formation, this reaction yields
three- and/or four-coordinate boron cations bonded to solvent
molecules22,23. In 1982, No¨th and co-workers demonstrated the
first successful isolation of a two-coordinate boron cation by
Results and discussion
For the synthesis of 1a, dimesitylfluoroborane (Mes2BF) was reacted
with 1 equiv. triethylsilylium carborane (Et3Si[HCB11Cl11]) (ref. 27)
in o-dichlorobenzene (ODCB) at 25 8C for 5 min. When hexane
vapour was allowed to diffuse into the resulting mixture, 1a
formed as a colourless crystalline material (86% yield). Although
compound 1a decomposes immediately upon exposure to air, it
exhibits surprisingly high thermal stability under an inert
1Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan, 2Graduate School of
Pharmaceutical Sciences, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, 3Advanced Elements Chemistry Research Team,
RIKEN Centre for Sustainable Resource Science (CSRS) and Elements Chemistry Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
e-mail: yshoji@res.titech.ac.jp
*
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1948
ARTICLES
c
a
R
++
C
+
Et3Si[HCB11Cl11
ODCB, 25 °C
]
B
R
R
B
R
+
B
R
R
R
R
R
+
++
–
B R
R B R
[HCB11Cl11]
R C R
1a
Mes2BF
I
II
III
[Et3Si(mesitylene)]+[(C6F5)4B]–
ODCB, 25 °C
+
B
+
b
+
[(C6F5)4B]–
N
B
N
N
B
R
1b
–
–
AlBr4
AlBr4
IV
V (R = Ph), VI (R = Me)
Figure 1 | Schematic representations of chemical compounds. a, Structures of a planar three-coordinate borane (I), a linear two-coordinate borinium ion (II)
and a hypothetical two-coordinate carbon dication (III). Their Lewis structures are shown in parentheses. Borinium ions II, consisting of sp-hybridized boron,
are characterized by the presence of two vacant 2p orbitals on a central boron atom. b, Typical examples of reported two-coordinate boron cations (IV–VI),
whose vacant 2p orbitals of boron are filled by dative p bonds with amino groups. c, Synthesis of Mes2Bþ salts of [HCB11Cl11]2 (1a) and [(C6F5)4B]2 (1b).
The two mesityl rings of Mes2Bþ are not coplanar to one another, as a consequence of steric effects and pp bonding with the boron atom. Mes ¼ 2,4,6-
trimethylphenyl; ODCB ¼ o-dichlorobenzene.
atmosphere, not showing decomposition until heated to 320 8C shift into account, coordination of the counter-anion or solvent
(Supplementary Fig. 3). Left standing under argon at ambient molecules to the cation centre seems unlikely. The 13C NMR
temperature, 1a showed no sign of decomposition after 1 year.
signals due to the aromatic carbons of 1a appear at 155.2 (Cortho),
The 11B NMR spectrum (128.0 MHz) of 1a at 25 8C in deuter- 158.9 (Cpara) and 119.2 ppm (Cmeta) (Supplementary Fig. 20). The
ated ODCB (ODCB-d4) shows a very broad signal from the former two signals are significantly deshielded compared to the
Mes2Bþ cation at d ¼ 93.3 ppm with a half-width (W1/2) of corresponding carbons of trimesitylborane (140.3 and 138.9 ppm
5,000 Hz, as well as three signals due to the carborane anion at for Cortho and Cpara, respectively, Supplementary Fig. 21), suggesting
d ¼ 22.4, 210.1 and 213.2 ppm (Supplementary Fig. 1). a certain degree of p donation from the Mes groups to the boron
Importantly, the 11B NMR signal of Mes2Bþ is significantly centre in 1a.
further downfield than those reported for other two-coordinate
X-ray structural analysis (see Supplementary Information) of 1a
boron cations (in the range 22.6–59.6 ppm)17,18, indicating excep- (Fig. 2a and Supplementary Fig. 3) reveals well-separated cations
tional electron deficiency at the boron centre. Taking this downfield and anions. The shortest distance between the boron atom of
a
b
C3
C2
C2
C3
C4
C1 B1
C11
B1
C12
C1
C10
C4
C13
C3* C2*
C15
C14
C6
C5
Figure 2 | Crystal structures of 1a and 1b (50% probability ellipsoids). a,b, Molecular structures of 1a (a) and 1b (b). Hydrogen atoms are not shown.
Boron, carbon, fluorine and chlorine atoms are in magenta, grey, yellow-green and green, respectively. For 1b, a quarter of the whole structure constitutes
the asymmetric unit; numbers with an asterisk are related by symmetry. Selected bond lengths (Å) and angles (deg) for 1a: C1–B1 ¼ 1.459(3), C1–C2 ¼
1.429(3), C1–C6 ¼ 1.422(3), C2–C3 ¼ 1.382(3), C3–C4 ¼ 1.398(3), C4–C5 ¼ 1.391(3), C5–C6 ¼ 1.384(3), C10–B1 ¼ 1.458(3), C10–C11 ¼ 1.428(3), C10–C15 ¼
1.431(3), C11–C12 ¼ 1.375(3), C12–C13 ¼ 1.395(3), C13–C14 ¼ 1.396(3), C14–C15 ¼ 1.374(3), C1–B1–C10 ¼ 172.1(2), dihedral angle (deg) between the mean
planes of the two aromatic rings ¼ 80.67(9). Selected bond lengths (Å) for 1b: C1–B1 ¼ 1.4568(18), C1–C2 ¼ 1.420(2), C2–C3 ¼ 1.378(3), C3–C4 ¼ 1.392(2).
In both 1a and 1b, the short Cipso–B bond and the quinoidal character in the mesityl groups of Mes2Bþ reflect the delocalization of positive charge through
pp bonding.
2
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Mes2Bþ and a chlorine atom of the carborane anion (4.562(2) Å) localized solely on the Mes groups. The quinoidal character of the
is much longer than the sum of the van der Waals radii (3.67 Å) boron–mesityl substituents, suggested by X-ray crystallography, is
(ref. 30). The geometry at boron in the cation is close to linear reflected in the HOMO–2 and HOMO–3 orbitals (Fig. 3), which
(/Cipso–B–Cipso ¼ 172.1(2)8), as expected for sp-hybridization. demonstrate p donation from the arene rings to the boron centre.
The dihedral angle between the two mesityl groups is 80.67(9)8, a Based on second-order perturbation analysis, secondary orbital inter-
consequence of steric effects. The Cipso–B bond lengths (1.459(3) actions (p bonding) between boron and the Cipso–Cortho p bonds of
and 1.458(3) Å) are shorter than typical C–B single bond lengths the Mes groups certainly operate, providing Mes2Bþ with substantial
(1.59 Å) (ref. 31), reflecting the overall positive charge and some stabilization energy (43.28 kcal mol21) (Supplementary Fig. 9). The
degree of pp bonding. This is confirmed by the quinoidal character Wiberg bond index of the Cipso–B bond (1.065) also indicates
in the mesityl groups, where the mean length of the Cipso–Cortho partial p-bond character around the boron centre (Supplementary
bonds (1.428(3) Å) is longer than those of the other C–C bonds Fig. 10). The gauge-independent atomic orbital calculation on the
in the aromatic ring (1.379(3) and 1.395(3) Å).
counterion-free model of Mes2Bþ in vacuum (89.7 and 102.4 ppm
the and M06-2X/6-
For a better understanding of the structural features of Mes2Bþ we at
B3LYP/6-311þþG(3df,2pd)
performed density functional theory (DFT) calculations on a coun- 311þþG(3df,2pd) level of theory, respectively) virtually reproduces
terion-free model of Mes2Bþ (a singlet state in vacuum) at the the chemical shift of the boron centre of Mes2Bþ observed in
M06-2X/6-31 þ G(d) level of theory (see Supplementary ODCB-d4 (d ¼ 93.3 ppm). This result indicates that coordination of
Information). In the optimized structure (Fig. 3, Supplementary the counter-anion or solvent molecules to the boron centre of 1a
Fig. 7), all the C–B and C–C bond lengths agree well with those is insignificant.
observed for the crystal structure of 1a (Fig. 2a). This is consistent
We initially supposed that the isolation of such a Lewis-acidic
with well-separated cations and anions, as found experimentally. species as Mes2Bþ would require a counterion having the particular
The calculated dihedral angle between the two Mes groups (61.968) inertness of the [HCB11Cl11]2 carborane anion, as it does in
is smaller than that observed experimentally (80.67(9)8), most prob- silylium ion chemistry27,29. However, the more commonly available
ably due to the internal maximization of dispersion forces within the tetraarylborate anion [(C6F5)4B]2 is also applicable for the synthesis
isolated gas-phase cation relative to external dispersion forces arising of the borinium ion. In a manner similar to that for 1a, 1b was iso-
from cation/anion contacts in the crystal. The natural population lated (92% yield) as a stable crystalline material (decomposition
analysis charge distribution confirms that positive and negative temperature ¼ 192 8C, Supplementary Fig. 4) by the reaction of
charges are located predominately on the boron (þ1.277) and Cipso Mes2BF and [Et3Si(mesitylene)]þ[(C6F5)4B]2 (ref. 32) in ODCB
(20.657) atoms, respectively (Supplementary Fig. 8). As expected, at 25 8C (Fig. 1c). Single-crystal X-ray crystallography (Fig. 2b)
the lowest unoccupied molecular orbital (LUMO) and LUMOþ1 of as well as 1H, 11B and 13C NMR spectroscopy (Supplementary
Mes2Bþ (Fig. 3) involve the vacant 2p orbitals of boron, whilst the Figs 22, 2 and 23, respectively) showed that the structural features
highest occupied molecular orbital (HOMO) and HOMO–1 are of the Mes2Bþ ion in 1b are essentially identical to those observed
for 1a. Thus, the boron centre of Mes2Bþ is not subject to coordi-
nation by the anion. We would like to emphasize that the use of
such a readily available anion such as [(C6F5)4B]2 greatly enhances
the synthetic accessibility of the diarylborinium salt.
E (eV)
The Mes2Bþ ion has low-lying vacant orbitals (25.41 and
24.84 eV for LUMO and LUMOþ1, respectively, Fig. 3) on
the boron centre, which apparently does not accept significant
electron density from the counterions or chloroarene solvent mol-
ecules. Although Mes2Bþ is certainly stabilized by pp bonding
from the mesityl groups, its behaviour may differ from that of
conventional diaminoborinium ions15,16,24,25, which demonstrate
lone pair donation to the boron centre. The DFT calculations
of (Me2N)2Bþ as a model compound at the M06-2X/6-31þG(d)
level of theory revealed that the two molecular orbitals, consisting
of the vacant 2p orbitals on boron, lay at an energy level as high
as 23.73 eV (degenerated, Supplementary Figs 11 and 12).
LUMO+1 (–4.84)
LUMO (–5.41)
The Mes2Bþ ion, featuring low-lying vacant orbitals, possesses
exceptional Lewis acidity and readily engages in a peculiar deoxy-
genation reaction of CO2. When an ODCB solution of 1a at 25 8C
was exposed to CO2 gas at 1 bar pressure, colourless crystals
immediately began to form. X-ray crystallography revealed that
the resulting material is a salt of the MesC;Oþ aroyl cation with
[HCB11Cl11]2 as counterion (2a, Supplementary Fig. 6). As with
other reported examples of aroyl cations33, crystalline 2a showed a
strong C–O stretching vibration at 2,276 cm21 in its infrared
spectrum. Similarly, 1b reacted with CO2 to afford the MesC;Oþ
salt of [(C6F5)4B]2 (2b). The isolated yields of 2a (37%) and 2b
(33%) are comparable with one another. The reaction of Mes2Bþ
and CO2 should also produce oxoborane MesB¼O (ref. 34)
and its aggregates (boroxin and dioxadiboretane); however, these
could not be isolated because of the occurrence of side reactions
with unreacted Mes2Bþ, resulting in a complex mixture. We
considered that the observed arylation–deoxygenation is triggered
by the coordination of an oxygen atom of CO2 to the boron
centre of Mes2Bþ, leading to polarization of CO2. Most probably,
HOMO (–11.66)
HOMO–1 (–11.70)
HOMO–3 (–12.29)
HOMO–2 (–11.81)
Figure 3 | DFT calculation of Mes2B1 at the M06-2X/6-311G(d) level.
Localized Kohn–Sham orbitals of Mes2Bþ in its optimized geometry are
shown. Energy levels (eV) are given in parentheses. The low-lying LUMO
and LUMOþ1 of Mes2Bþ involve the vacant 2p orbitals of boron. The
HOMO–2 and HOMO–3 reflect the quinoidal character of Mes2Bþ arising
from pp bonding between the mesityl groups and boron.
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1948
ARTICLES
ΔE (kcal mol–1
)
B
+
O
C
O
10.0
+
TS1
(+8.6)
B
+
O
C
B
O
CP1
(+4.4)
+
Mes2B
+
O
C
O
0.0
RT
(0.0)
TS2
(–1.1)
B
O
+
B
Oxoborane
+
O
C
O
CP2
+
(–5.7)
C
O
CP3
(–9.0)
+
MesC
O
–10.0
PD
(–10.6)
+
B
O
C
O
+
B
O
C
O
Figure 4 | Energy diagram of the arylation–deoxygenation of CO2 with Mes2B1 to form MesC;O1 at the PCM(o-dichlorobenzene)-M06-2X/6-
3111G(d,p)//M06-2X/6-311G(d) level. Energy changes at the M06-2X/6-311þG(d,p) level are shown in kcal mol21. RT, CP, TS and PD indicate reactant,
complex (intermediate), transition state and product, respectively. At the initial stage, the oxygen atom of CO2 approaches the boron centre of Mes2Bþ to
give CP1. C–C bond formation then occurs to form CP2 through TS1. The Cipso–B bond dissociation in CP2 through TS2 occurs to form CP3, which finally
disproportionates into MesC;Oþ and oxoborane MesB¼O.
the resulting positively charged carbon atom in CO2 then forms than RT. The Cipso–B bond dissociation in CP2 through TS2 occurs
a covalent bond with the negatively charged Cipso atom of with a small energy barrier (4.6 kcal mol21) to afford CP3, which
Mes2Bþ. This scenario was rationalized by DFT calculations using exothermically (1.6 kcal mol21) disproportionates into MesC;Oþ
artificial force-induced reaction (AFIR) analysis based on an and oxoborane MesB¼O. This arylation–deoxygenation of CO2
M06-2X method.
with a Lewis acid is unprecedented and provides a sharp contrast to
As shown in Fig. 4, the oxygen atom of CO2 approaches the common reactions between CO2 and nucleophiles.
boron centre of Mes2Bþ to form CP1 with a small endothermicity
In summary, we have synthesized and isolated a two-coordinate
(4.4 kcal mol21). The Cipso–Bþ–Cipso angle (179.98 in RT) is boron cation with C–Bþ–C bonding. With its relatively easy
deformed by 37.38 and the C¼O bond length (1.16 Å in RT) is synthetic accessibility and unique oxophilicity, the diarylborinium
elongated by ꢀ3%. C–C bond formation in the four-membered ion provides a platform for molecular Lewis-acid chemistry
ring CP2 via TS1 requires
a
small activation energy and may offer a powerful tool for exploring new chemical
(4.2 kcal mol21). CP2 lies in an energy state 5.7 kcal mol21 lower transformations15,16,35–38
.
4
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1948
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Methods
References
The handling of air- and/or moisture-sensitive compounds was performed either using
standard Schlenk-line techniques or in a glove box under argon. Anhydrous hexane was
dried by passage through an activated alumina column and a Q-5 column (Nikko
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Mes2BF (ref. 39), Et3Si[HCB11Cl11] (ref. 27) and [Et3Si(mesitylene)]þ[(C6F5)4B]2
(ref. 32) were prepared according to reported procedures.
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]
Et3Si[HCB11Cl11] (59.0 mg, 9.26 × 1022 mmol) was added Mes2BF (24.8 mg,
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2.43 (s, CH3, 12H), 2.17 (s, CH3, 6H). 11B NMR (128 MHz, ODCB-d4): d (ppm)
93.3 (W1/2 ¼ 5,000 Hz), 22.4, 210.1, 213.2. 13C NMR (100 MHz, ODCB-d4):
d (ppm) 158.9 (Cpara), 155.2 (Cortho), 119.2 (Cmeta), 47.3 (CH), 23.2 (two signals,
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Synthesis of Mes2B1[(C6F5)4B]– (1b). Compound 1b was obtained as colourless
crystals (92% yield) from Mes2BF and [Et3Si(mesitylene)]þ[(C6F5)4B]2 in a
manner similar to that for 1a. d.p. (in a sealed tube under argon): 192 8C. FT-IR
(ATR): n (cm21) 2,958, 2,929, 2,867, 1,644, 1,606, 1,557, 1,514, 1,458, 1,409, 1,383,
1,372, 1,288, 1,271, 1,160, 1,084, 1,034, 975, 862, 770, 754, 683, 662. 1H NMR
(400 MHz, ODCB-d4): d (ppm) 6.74 (s, Ar-H, 4H), 2.43 (s, CH3, 12H), 2.17
(s, CH3, 6H). 11B NMR (128 MHz, ODCB-d4): d (ppm) 93.5 (W1/2 ¼ 4,400 Hz),
216.5. 13C NMR (100 MHz, ODCB-d4): d (ppm) 159.0 (Cpara), 155.2 (Cortho),
1
1
148.5 (Cborate; dm, JC–F ¼ 241 Hz), 138.3 (Cborate; dm, JC–F ¼ 245 Hz), 136.4
1
(Cborate; dm, JC–F ¼ 245 Hz), 124.5 (Cborate; br), 119.2 (Cmeta), 22.9 (CH3), 22.7
(CH3), one peak of the aromatic carbon, ipso position to the boron, was not
observed. 19F NMR (376 MHz, ODCB-d4): d (ppm) 2132.8, 2163.4, 2167.2.
1H, 13C, 11B, 19F NMR spectra of 1b are shown in Supplementary Figs 22, 23, 2
and 24, respectively.
Synthesis of MesC;O1[HCB11Cl11 2 (2a). An ODCB solution (3.0 ml) of 1a
]
(200 mg, 0.259 mmol) was degassed by freeze–pump–thaw cycles (three times) and
then exposed to CO2 gas (1.0 l) at 25 8C under 1 bar pressure, whereupon
colourless crystals formed. After hexane vapour was allowed to diffuse into the
reaction mixture, the resulting crystals were filtered off, washed with hexane
(5.0 ml) and dried under reduced pressure to give 2a as colourless crystals
(64.4 mg, 9.63 × 1022 mmol) in 37% yield. d.p. (in a sealed tube under Ar):
344 8C. FT-IR (ATR): n (cm21) 3,031, 2,964, 2,832, 2,859, 2,175, 1,598, 1,539,
1,504, 1,463, 1,379, 1,345, 1,327, 1,300, 1,227, 1,213, 1,120, 1,010, 955, 903, 863,
740, 715, 671. 1H NMR (400 MHz, ODCB-d4): d (ppm) 6.82 (s, Ar-H, 2H), 3.02
(br, CH, 1H), 2.32 (s, CH3, 6H), 2.20 (s, CH3, 3H). 11B NMR (128 MHz, ODCB-
d4): d (ppm) 22.4, 210.0, 213.1. 13C NMR (100 MHz, ODCB-d4): d (ppm) 165.0
(C;O), 160.7 (Cpara), 155.2 (Cortho), 132.2 (Cmeta), 84.7 (Cipso), 47.6 (CH), 24.3
(CH3), 21.5 (CH3). 1H, 13C, 11B NMR spectra of 2a are shown in Supplementary
Figs 25, 26 and 27, respectively.
Synthesis of MesC;O1[(C6F5)4B]– (2b). Compound 2b was obtained as colourless
crystals (33% yield) from 1b in a manner similar to that for 2a. d.p. (in a sealed tube
under Ar): 197 8C. FT-IR (ATR): n (cm21) 2,958, 2,926, 2,876, 2,852, 2,190, 1,644,
1,597, 1,514, 1,459, 1,411, 1,384, 1,367, 1,301, 1,276, 1,217, 1,146, 1,085, 1,032, 975,
925, 906, 863, 773, 756, 727, 700, 684, 661. 1H NMR (400 MHz, ODCB-d4): d (ppm)
6.83 (s, Ar-H, 2H), 2.32 (s, 6H), 2.21 (s, 3H). 11B NMR (128 MHz, ODCB-d4):
d (ppm) 216.6. 13C NMR (100 MHz, ODCB-d4): d (ppm) 164.9 (C ; O), 160.3
1
(Cpara), 154.9 (Cortho), 148.5 (Cborate; dm, JC–F ¼ 242 Hz), 138.3 (Cborate; dm,
1JC–F ¼ 242 Hz), 136.5 (Cborate; dm, 1JC–F ¼ 245 Hz), 124.2 (Cborate; br), 84.1 (Cipso),
23.7 (CH3), 20.7 (CH3). 19F NMR (376 MHz, ODCB-d4): d (ppm) 2132.7, 2162.9,
2166.8. 1H, 13C, 11B, 19F NMR spectra of 2b are shown in Supplementary Figs 28,
29, 30 and 31, respectively.
30. Mantina, M., Chamberlin, A. C., Valero, R., Cramer, C. J. & Truhlar, D. G.
Consistent van der Waals radii for the whole main group. J. Phys. Chem. A
113, 5806–5812 (2009).
Crystallographic data deposition. Crystal data of 1a, 1b and 2a are available from
the Cambridge Crystallographic Data Centre under reference numbers CCDC-
978055, 978056 and 978057 (www.ccdc.cam.ac.uk/data_request/cif).
31. Olmstead, M. M., Power, P. P., Weese, K. J. & Doedens, R. J. Isolation and X-ray
crystal structure of the boron methylidenide ion [Mes2BCH2]2 (Mes¼2,4,6-
Me3C6H2): a boron–carbon double bonded alkene analog. J. Am. Chem. Soc.
109, 2541–2542 (1987).
Received 22 January 2014; accepted 7 April 2014;
published online 11 May 2014
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1948
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Acknowledgements
This work was supported by KAKENHI (22750046). The authors thank C.A. Reed for his
instruction regarding carboranes synthesis and valuable discussions. RIKEN Integrated
Cluster of Clusters (RICC) provided the computer resources for the DFT calculations.
Author contributions
Y.S. and T.F. conceived and designed the work. Y.S. and N.T. performed the experiments.
Y.S., N.T. and T.F. analysed the experimental data. K.M. and M.U. performed the DFT
calculations and analysed the computational data. Y.S., M.U. and T.F. co-wrote the paper.
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Additional information
Supplementary information and chemical compound information are available in the
online version of the paper. Reprints and permissions information is available online at
www.nature.com/reprints. Correspondence and requests for materials should be
addressed to Y.S.
triarylborirene ring system by tricoordinate boron and facile ring opening with Competing financial interests
tetracoordinate boron. J. Am. Chem. Soc. 112, 1847–1853 (1990).
The authors declare no competing financial interests.
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