Electronic and structural effects of stepwise borylation and quaternization on borirene aromaticity

Article abstract of DOI:10.1021/ja3110126

Room-temperature photolysis of the aminoboryl complex [(OC) ₅Cr-B-N(SiMe₃)₂] in the presence of a series of mono- or bis(boryl) alkynes bis{bis(dimethylamino)boryl}ethyne, 1-phenyl-2-bis(dimethylamino)borylethyne, and 1-trimethylsilyl-2- bis(dimethylamino)borylethyne led to the isolation of hitherto unknown borylborirenes in resonable yields, that is, [(RC-CR′)(μ-BN(SiMe ₃)₂)] (7, R = B(NMe₂)₂, R′ = Ph; 8, R = R′ = B(NMe₂)₂; 9, R = B(NMe ₂)₂, R′ = SiMe₃). The borirenes were isolated and spectroscopically characterized by multinuclear NMR, IR, and UV/vis spectroscopy, crystallography, and elemental analysis. Reactivity studies of the borirenes demonstrated their behavior toward different Lewis bases. The isolated adducts and the parent borirenes were compared to 1,3-mesityl-2- phenylborirene and its adducts. To gain insight into the electronic structure and to evaluate the influence of the exocyclic groups, Density Functional Theory (DFT) calculations were carried out.

Full text of DOI:10.1021/ja3110126

Article
pubs.acs.org/JACS
Electronic and Structural Effects of Stepwise Borylation and
Quaternization on Borirene Aromaticity
Holger Braunschweig,*,† Alexander Damme,† Rian D. Dewhurst,† Sundargopal Ghosh,‡ Thomas Kramer,†
Bernd Pfaffinger,† Krzysztof Radacki,† and Alfredo Vargas†
†Institut fur Anorganische Chemie, Julius-Maximilians-Universitat Wurzburg, Am Hubland, 97074 Wurzburg, Germany
̈
̈
̈
̈
‡Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India
S
* Supporting Information
ABSTRACT: Room-temperature photolysis of the amino-
boryl complex [(OC)5CrBN(SiMe3)2] in the presence of
a series of mono- or bis(boryl) alkynes bis{bis-
(dimethylamino)boryl}ethyne, 1-phenyl-2-bis-
(dimethylamino)borylethyne, and 1-trimethylsilyl-2-bis-
(dimethylamino)borylethyne led to the isolation of hitherto
unknown borylborirenes in resonable yields, that is, [(RC
CR′)(μ-BN(SiMe3)2)] (7, R = B(NMe2)2, R′ = Ph; 8, R = R′ = B(NMe2)2; 9, R = B(NMe2)2, R′ = SiMe3). The borirenes were
isolated and spectroscopically characterized by multinuclear NMR, IR, and UV/vis spectroscopy, crystallography, and elemental
analysis. Reactivity studies of the borirenes demonstrated their behavior toward different Lewis bases. The isolated adducts and
the parent borirenes were compared to 1,3-mesityl-2-phenylborirene and its adducts. To gain insight into the electronic structure
and to evaluate the influence of the exocyclic groups, Density Functional Theory (DFT) calculations were carried out.
1).4f A highly unusual “partially-cleaved” borirene was observed
INTRODUCTION
■
upon reaction of 1,3-mesityl-2-phenylborirene with [Pt(PEt3)3]
Borylated molecular and polymeric species have become of
great importance due to their broad application in organic and
inorganic synthesis and materials science.1 In particular, the
design of π-conjugated organic compounds displaying three-
coordinate boron centers constitutes a highly topical area of
research, because functional materials based thereon are
promising materials for use in light emitting devices, sensors,
and electronic circuits.1d,e,g,j−l Syntheses of these important
materials rely heavily on classical methods established in
organoboron chemistry such as hydroboration,1a silicon−boron
exchange,1g and metal-mediated C−H bond borylation.1m,n
Transition metal borylene complexes of the type [LnMB
N(SiMe3)2] (M = Cr, Mo, W) serve as facile synthetic sources
of [:BN(SiMe3)2] both under thermal and photochemical
conditions.2 Borylene transfer is also an excellent tool for the
functionalization of carbon double3 and triple bonds,4 thus
providing new synthetic approaches for π-conjugated boron-
containing molecules far beyond the conventional method-
ologies. The borylene-based functionalization of alkynes is not
only applicable to a wide range of substrates, including terminal
and internal alkynes, diynes, and transition metal-bound alkynyl
moieties, but also proceeds with remarkably high yields. This
functionalization of carbon−carbon triple bonds upon
photolysis provides simple access to borirenes,4 the boron-
containing analogue of the cylopropenylium cation.5 Although
several reports from our laboratories4 and those of others6 are
available on borirenes, in general little is known about their
properties and reactivity. Known reactivity patterns are scarce
and commonly result in cleavage of the aromatic ring induced
by protic substrates,4f,6e hydroboranes,7 or UV light (Scheme
yielding a B−C σ-complex.8
If borirenes and the borylene transfer reaction are to play any
part in the construction of useful boron-containing materials,
methods of functionalization and modulation of the π system
that do not involve rupture of the ring, will need to be
developed. For these reasons, we explored the synthesis of
borirenes with both one and two boryl substituents, which are
well-known to be excellent σ-donor groups (as well as π-
acceptors, although only when coplanar), as well as potential
points of further chemistry via cross-coupling protocols. Filling
the empty p orbital of boron by the addition of a Lewis base is a
well-known reaction, but in boron-containing heterocycles,
such adduct formation disrupts any π-conjugation involving the
boron atom, and thus also “switching off” any aromaticity or
antiaromaticity in the ring. This phenomenon has recently
proven useful for modulating the photophysical properties of
boroles.9 Thus, herein we have attempted to quaternize
borirene boron atoms using Lewis bases, a possibility hinted
at but only briefly explored by Eisch in an early report.6a
In this Article, we present the photochemical borylene
transfer from borylene complex 1 to a set of alkynes substituted
by different functional groups, for example, B(NMe2)2, Ph, and
SiMe3. The results of the reaction of borylene complexes with
substituted alkynes led to the isolation of the new boryl-
functionalized borirenes 7−9. We then investigated the
reactivity of aminoborirenes (7−9) in a comparative study
Received: November 8, 2012
Published: January 10, 2013
© 2013 American Chemical Society
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a
Scheme 1. Two Syntheses of Borirenes and Their Key
Reactivity Patterns
Scheme 2. Synthesis of the Borirenes 5−9
a
A = SiMe3.
the presence of the requisite alkynes for 12 h at room
temperature, which resulted in the formation of the amino-
borirenes 6−9.
All reactions were monitored by multinuclear NMR
spectroscopy, which revealed gradual consumption of the
starting materials and quantitative formation of the borirenes,
as indicated by the presence of new resonances in the 11B NMR
spectrum at δ = 30−23 ppm. Compounds 7−9 were purified by
distillation under reduced pressure. The compounds were
isolated as air- and moisture-sensitive light yellow solids (7 and
8) or a light yellow oil (9) in yields between 56% and 86%. The
borirenes 7 and 9 show considerably decreased stability
indicated by the change of their color to black-yellow/black-
red after elongated storage under argon in a glovebox or a
Schlenk tube at ambient temperature. Nervertheless, 7 and 8
can be stored at low temperatures without decomposition.
Borirene 8 was found to be stable under the same conditions as
the triaryl borirene 5.
with 1,3-dimesityl-2-phenylborirene (5). Once these borirenes
were available in sufficient quantities, theoretical calculations
combined with spectroscopic data were employed to elucidate
the electronic structure of these species, particularly with
respect to the extent of π-delocalization within the BCC ring
and the exocyclic groups. The quaternization and dequaterniza-
tion of a number of borirenes was accomplished with a range of
Lewis bases, and a number of the adducts were structurally
characterized.
Table 1. Relevant Bond Lengths (Å) and Angles (deg) for
6−8
RESULTS AND DISCUSSION
■
Synthesis of Borirene Precursors. [(Me2N)2BCCPh]
(2) was prepared according to a literature procedure.10
Treatment of [LiCCSiMe3] in hexane with 1 equiv of
ClB(NMe2)2 yielded 3. Compound 4 was obtained by reaction
of [BrMgCCMgBr] with 2 equiv of ClB(NMe2)2. After
workup, 3 was isolated as a colorless liquid, showing
spectroscopic characteristics similar to those reported for an
analogous compound,11 and 4 as a colorless solid (Figure 1).
Single crystals of 4 suitable for X-ray analysis were obtained by
storing a concentrated pentane solution at −30 °C for 3 weeks.
Synthesis of the Borirenes. Compound 58 was synthe-
sized according to the literature. The borirenes 6,4e 7, 8, and 9
were prepared by photochemical borylene transfer. As shown in
Scheme 2, a pale yellow hexane solution of 1 was irradiated in
compound
6
7
8
space group
C1−C2
B1−C1
B1−C2
C1−C3/B3
P1
P21/c
C2/c
̅
1.357(2)
1.487(2)
1.490(2)
1.469(2)
1.366(2)
1.478(2)
1.498(2)
1.462(2)
12.4(2)
1.382(2)
1.478(3)
1.476(2)
1.564(2)
68.8(1)
∠BCC plane−C3Ph/B3NN 44.9(1)
plane
C2−B2/C4
1.462(2)
1.558(2)
1.560(2)
77.7(1)
∠BCC plane−C4Ph/B2NN 37.7(1)
72.0(2)−78.9(2)
plane
B1−N1
∠C1−B1−C2
1.419(2)
54.2(1)
1.423(2)
54.7(1)
1.430(2)
55.8(1)
Differential thermal analysis showed melting points below
100 °C for the borirenes 5−8 and in all cases decomposition
over 200 °C. The data indicate that borirenes 5−8 are
thermally very stable. The 11B NMR signals of the borirenes are
in the expected region. The borirenes 8 (δ = 31.6 ppm) and 9
(δ = 31.9 ppm) show only one resonance indicating
isochronism for the ring boron atom and the exocyclic boron
atom, whereas 7 shows two signals (δ = 31.3 ppm, 27.9 ppm).
The signals of 7−9 are downfield shifted by 5 ppm in
comparison to bis(borirenes).4e Surprisingly, the signal of the
Figure 1. Molecular structure of 4. Ellipsoids drawn at the 50%
probability level. Hydrogen atoms are omitted for clarity. Relevant
bond lengths (Å): C1−C2 1.211(1), B1−C1 1.556(1).
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triarylborirene 5 (δ = 32.8 ppm)8 has nearly the same shift,
suggesting that the group at the ring boron atom has a
negligible influence on the chemical shift in the 11B NMR
spectrum. The basic structural features of 7−9 are very similar
to those reported for borirenes with other substituents. The 1H
and 13C NMR spectra of 7−9 showed only one signal for the
N(SiMe2)2 moiety, indicating free rotation around the B−N
bond at room temperature in solution. In addition, the 13C
NMR spectra of these compounds show the presence of ring
carbon resonances (7, δ = 175.95, 166.23; 8, δ = 186.46; 9, δ =
182.25) observed as broad singlets. The values are downfield-
shifted as compared to those reported in the literature.4a,12
Other NMR spectroscopic data are consistent with proposed
structures.
6 and 1.382(2) Å in 8 is typical for the change of hybridization
of the two carbon atoms from sp to sp2. Comparison of the
C1−C2 bonds of 6−8 reveals that the more B(NMe2)2 groups
are bound to the BCC ring, the longer is the C1−C2 bond. The
B1−C1 and B1−C2 bonds show lengths between 1.476(2) and
1.498(2) Å, again comparable with those of triarylborirenes
(1.474(4) Å, 1.459(4) Å),8 bis(silyl)aminoborirenes4a
(1.485(3) Å), bis(borirenes)4e (e.g., 1.473(3) Å, 1.498(2) Å),
metal borirene complexes4b,d,f (e.g., 1.501(2) Å, 1.474(2) Å),
and the one known ferroborirene4g (1.49(4) Å). Comparison of
the ring B−C bonds of 6, 7, and 8 reveals a shortening upon
augmentation of the number of B(NMe2)2 groups, a trend
closely related to that for the C1−C2 bonds. Only in 7 does the
B1−C2 bond show a slight elongation as compared to that of 6.
Presumably due to steric hindrance, the exocyclic phenyl
groups of 6 are not in plane with the BCC ring (44.9(2)°,
37.7(2)°), a fact that is also found for all three aryl groups in 5.
In contrast, the phenyl group in 7 is nearly coplanar with the
BCC ring with the distortion being only 12.4(2)°. This has also
been observed in a platinum borirene complex4b,f and indicates
conjugation with the BCC ring. The distance between the BCC
ring and the exocyclic boryl groups in 7 and 8 is typical for a
diaminoboryl group conjugated with an alkene17 and is close to
the sum of covalent radii of boron and carbon (1.57 Å).18
These distances in 8 are longer than in 7 and the B1C1 distance
(1.556(1) Å) in the parent alkyne 4. The disordered exocyclic
boryl group (B2NN plane) of 7 is distorted between 72(2)°
and 78.9(2)° from the BCC ring, thus minimizing steric
congestion. For 8 the same effect is found; the distortions are
68.8(1)° (B2NN plane) and 77.7(1)° (B3NN plane). The B1−
N1 distance in 6, 7, and 8 is also influenced by the number of
B(NMe2)2 groups, thus showing a greater elongation the more
B(NMe2)2 groups are attached to the BCC ring.
The infrared spectra of 7−9 showed a medium absorption in
the range of 1685−1614 cm−1, characteristic for the BCC ring
breathing of the borirenes (Table 2).12 The values measured in
Table 2. Experimental and Calculated IR Data of the Ring
Breathing Mode of Representative Borirenes (cm−1)
5
6
7
8
9
exp
1708
1759
1685
1732
1663
1711
1641
1696
1617
1671
calcd
the solid state are about 50 cm−1 lower than the values
calculated for the molecules in the gas phase. To establish a
trend, the frequencies of bis(trimethylsilyl)- and 1-trimethylsil-
yl-2-phenyl-aminoborirene (1693 cm−1, 1633 cm−1) were
calculated. Comparison of the values indicates that the bond
strength in the BCC ring seems to be influenced by the
exocyclic groups over the following series: aryl > boryl > silyl,
which can probably be explained by the extent of electron
donation by the functional groups.
Single crystals of 6 and 8 suitable for X-ray analysis were
obtained by storing a pentane solution at −30 °C for 1 week,
and of 7 by slow evaporation of a saturated pentane solution in
the freezer of a glovebox at −35 °C (Figure 2). The C1−C2
bonds of 6−8 are minimally longer than those in free ethylene
(1.339 Å)13 and cis-stilbene (1.339(5) Å).14 They are similar to
those of triaryl cyclopropenylium cations7 (∼1.37 Å),
triarylborirenes8 (1.370(4) Å), bis(silyl)aminoborirenes4a
(1.376(4) Å), bis(borirenes)4e (e.g., 1.362(3) Å, 1.367(2) Å),
metal borirene complexes4b,d,f (e.g., 1.367(4) Å, 1.363(2) Å),
the one known ferroborirene4g (1.371(3) Å), and a cis-1,2-
bis(boryl)alkene15 (1.354(9) Å). The elongation from 1.19(2)
Å in diphenylacetylene16 and 1.211(1) Å in 4 to 1.357(2) Å in
These structural findings for 6, 7, and 8 have been
interpreted as indicative of extensive delocalization of the two
π-electrons over a three-centered bonding molecular orbital
comprised of the pz atomic orbitals of boron and carbon.19 The
number of B(NMe2)2 groups in 6, 7, and 8 influences the bond
lengths in a clear trend, which indicates stronger delocalization
in the BCC ring the more B(NMe2)2 groups are attached to it.
As the boryl groups are quasi-orthogonal to the BCC ring, no
π-conjugation between them and the BCC ring can be
postulated. To better understand these facts, we performed
DFT calculations, which are discussed below.
To gain insight into the electronic structure of the borirenes,
UV/vis spectroscopic measurements were performed in hexane.
In the case of 5, peaks at 260 and 248 nm (19 548, 19 536 L
Figure 2. Molecular structures of 6−8. Ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Relevant bond lengths
(Å) and angles (deg) are shown in Table 1.
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mol−1 cm−1), which are characteristic for the aryl groups, and
another at 208 nm (65 297 L mol−1 cm−1) were observed. Also,
the spectra of 7 showed a characteristic absorption (248.5 nm;
18 164 L mol−1 cm−1) for the phenyl group. In the spectra of 8
a shoulder at 225 nm was identified, whereas that of 9 shows no
characteristic absorptions.
Reactivity of the Borirenes. A huge variety of reports
about partial or total quaternization of boron centers exist in
the literature. These reactions are useful for, among other
things, the sensing of anions.1l This reaction is restricted to
electron deficient boranes, thus quaternization of aminoboranes
is much more rare, as the p orbital of boron is significantly filled
by the π-basic amino group. To gather more information about
the reactivity of borirenes and to probe the degree of
aromaticity, we investigated the reactivity of 5 and 7,8 toward
Lewis bases as shown in Scheme 3.
Figure 3. Temperature-dependent formation of 15 as noted by 11B
NMR.
of pyridine to a hexane solution of 5. The base can be removed
from adducts 10−14 by addition of BPf3 (Pf = C6F5,
pentafluorophenyl), yielding the parent borirene and corre-
sponding base adduct of BPf3 (Scheme 5). This reaction
suggests that borirenes are weaker Lewis acids than BPf3.
Scheme 3. Reactivity of Aminoborirenes toward Lewis
a
Bases
Scheme 5. Dequaternization of Borirenes
a
A = SiMe3.
The aminoborienes 7−9 did not react with DMAP (4-
(dimethylamino)pyridine), pyridine, PCy3, or PMe3 at ambient
temperature or at 80 °C, whereas NMR spectroscopy showed
immediate reaction with IMe (1,3-dimethyl-2,3-dihydro-1H-
imidazole) at ambient temperature. The base adducts 10−12
were isolated after workup as colorless solids in resonable yield.
In the case of 7−9, no quaternization of the exocyclic boryl
groups was observed even upon addition of an excess of IMe.
The triaryl borirene 5 does not react with phosphorus-
centered Lewis bases, but it forms stable adducts with IMe (13)
and DMAP (14), whereas pyridine does not form an adduct at
ambient temperature. Upon cooling to −40 °C, adduct
formation (15) was observed (Scheme 4, Figure 3). Never-
theless, 15 can be precipitated as a pale yellow solid by addition
The melting and decomposition points of 10−15 determined
by differential thermal analysis indicate a reduced thermal
stability of the borirenes quaternized with IMe in comparison
to the parent borirenes, whereas the DMAP (14) and pyridine
(15) adducts of 5 show no decomposition <320 °C.
Scheme 4. Reactivity of 5 toward Lewis Bases
The 11B{1H} NMR spectra showed in all cases new
resonances at δ ≈ −20 ppm, which is high-field shifted by
about 50 ppm in comparison to the parent borirenes, thus
indicating the formation of a quaternized boron center. The
exocyclic boryl groups of 10 (δ = 33.9 ppm), 11 (δ = 33.8
ppm), and 13 (δ = 34.8 ppm) are only slightly high-field shifted
with respect to the parent borirenes. The signals for 14 and 15
are found at around δ = 0 ppm and obscured by the signal of
the borosilicate glass tube. The presumed shifts are consistent
with the lower Lewis basicity of pyridine bases as compared to
NHCs (N-heterocyclic carbene) and literature 11B NMR data
of other pyridine adducts of simple alkyl- and arylboranes,
which are close to 0 ppm.20 The carbon atoms of the BCC ring
were detected at δ = 165.94 ppm (11) and δ = 178.84, 165.94
ppm (12). For the carbene carbon atom coordinated to the ring
boron atom, resonances were found for 11 and 12 at δ = 172.05
1
ppm. In both cases, the signal is a quartet with a JBC ≈ 71 Hz
coupling. Other NMR spectroscopic data are consistent with
proposed structures.
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IR spectra of 10−15 showed typical absorptions between
1693 and 1573 cm−1 in the solid state for CC double bonds
(Table 3). The experimental values fit to those predicted via
group, the bonds are somewhat longer than the sum of the
covalent radii (ΣBC(sp2) = 1.57 Å). The distance between the
ring boron and the base is minimally influenced by the base or
the functional groups at the borirene. The average distance of
1.62 Å fits to those observed in the pyridine base adducts of
boroles.20c These structural findings for 10−14 are interpreted
as indicative for annihilation of delocalization of the two π-
electrons over the three-centered bonding molecular orbital
comprised of the pz atomic orbitals of boron and carbon.19
The compounds 10−14 were also investigated by UV/vis
spectroscopy. Compound 10 shows a maximum at 250 nm (13
425 L mol−1 cm−1) typical for a phenyl group and nearly
identical to the absorption in the parent borirene 7 (248.5 nm;
18 164 L mol−1 cm−1). In the spectrum of 11, a shoulder at 224
nm was identified, again similar to the parent borirene 8, which
shows one at 225 nm. The spectrum of 12 shows no
characteristic absorption. The same fact is found in the parent
borirene 9. This can be explained by the lack of chromophores
that absorb in the appropriate range. The absorption spectra of
13 and 14 are quite similar. That of 13 shows a maximum at
245 nm (13 011 L mol−1 cm−1) and a shoulder at 285 nm. That
of 14 shows a striking maximum at 258 nm (34 345 L mol−1
cm−1) and a shoulder at 281 nm.
Computational Studies. To analyze the bonding
situations in the synthesized borirenes and the adducts in
more detail, we performed DFT calculations at the B3LYP/6-
311G* and OLYP/TZP levels. The so-called “fragment
approach” as implemented in the Amsterdam Density Func-
tional (ADF) program was employed for the bonding analysis.
The major aim of the analysis was to quantify the relative
ease with which charge flux is contained in the simplest
borirene center, the BCC ring. We thus systematically
compared the effects of various side-chains on the energy of
bonding for some strategically selected bonds. Although the
dynamics of polarity in molecular systems have been tradition-
ally related to charge delocalization on extended π-systems,
boron possesses the unusual property of acting as multi-
symmetry charge flux “adapter”; that is, it is able to
Table 3. Experimental and Calculated IR Data of the CC
Stretch in Quaternized Borirenes (cm−1)
10
11
12
13
14
15
exp
1650
1705
1615
1661
1573
1686
1693
1763
1687
1733
1680
calcd
computational methods well. The values for the CC stretch
decrease over the series aryl > boryl > silyl, thus resembling the
trend observed for the nonquaternized borienes. The influence
of the base seems not to be as great as that of the exocyclic
groups. Even so, the values for 13−15 show that a stronger
base yields higher frequencies for the CC stretch, thus
indicating reduced delocalization in the ring.
The C1−C2 bonds of the quaternized borirenes 10−14 are
very similar, regardless of the functional groups or base on the
BCC ring (Figure 4, Table 4). They are shorter than in the
parent borirenes, indicating a lesser degree of delocalization
over the ring and the electron density being more centered
between C1−C2. The intracyclic B−C bonds have lengths near
the sum of the covalent radii of boron and carbon (1.57 Å);20c
however, they are elongated with regard to the parent
borirenes. An aryl or amino group at the ring boron atom
has no effect on the ring B−C bonds, whereas the ring B−C
bonds adjacent to a boryl or mesityl group are slightly longer
than those next to a phenyl or silyl group. The exocyclic boryl
and mesityl groups are distorted from the plane of the BCC
ring, whereas the phenyl groups are in plane with the BCC ring
except for that in 13. The orientation of the exocyclic groups in
10, 11, and 13 does not differ from those of the parent
borirenes, whereas in 14 the aforementioned coplanarization of
the phenyl group takes place. If the exocyclic group on the
boron is an amino group, the distance matches the sum of
covalent radii of boron and nitrogen (1.55 Å);18 if it is a mesityl
Figure 4. Molecular structures of 10−14. Ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Relevant bond
lengths (Å) and angles (deg) are mentioned in Table 4.
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Table 4. Relevant Bond Lengths (Å) and Angles (deg) of 10−14
compound
10
11
12
13
14
space group
P21/c
P21/c
P1
̅
P1
̅
P1
̅
C1−C2
B1−C1
B1−C2
1.330(2)
1.561(2)
1.628(2)
1.469(2)
18.3(1)
1.553(2)
63.4(1)
1.560(2)
1.626(2)
49.24(8)
1.339(2)
1.590(2)
1.584(2)
1.556(2)
66.3(1)
1.556(2)
67.7(1)
1.569(2)
1.629(2)
49.7(1)
1.341(2)
1.581(3)
1.608(2)
1.843(2)
1.315(2)
1.589(2)
1.619(2)
1.458(2)
36.4(1)
1.327(2)
1.563(2)
1.619(2)
1.460(2)
3.2(1)
C1−C3/B3/Si1
∠BCC plane−C3Ph/B3NN plane
C2−B2/C4
∠BCC plane−C4Mes/B2NN plane
B1−N1/C6
1.556(2)
77.00(1)
1.554(3)
1.612(3)
48.89(9)
1.460(2)
56.2(1)
1.463(2)
68.9(1)
1.609(3)
1.621(2)
49.9(1)
1.614(2)
1.612(2)
48.39(7)
B1−C5/N2
C1−B−C2
Table 5. Bonding Energy Decomposition Participation (kcal mol−1) and N/C−B Bond Distance (Å) for Some Selected
Borirene Systems
bidirectionally convey charge flow between two different
elements simultaneously through both σ and π symmetry
bonds.21 In the model molecule (H3Si)2NBC2(SiH3)2, for
instance, the N−B bond is composed of σ and π bonds, the
latter to a smaller extent. In the BCC core, the π covalent
contribution to bonding energy indicates cyclic delocalization,
suggesting some aromatic character, which is also supported by
computed NICS(0) and NICS(1) values. The lone pair (LP)
donation of N and of the π-(CC) bond to the empty pz
orbital on the ring boron was also quantified by Natural
Bonding Analysis (NBO), revealing that the extent of the
donation process can be modulated through modification of the
substituents on the CC linkage.4e
replaced by a phenyl group, in line with a longer N−B bond.
This reflects the reduced LP(N)→pz(Bborirene) donation that
can be ascribed to the increased pz(B)←π(CC) donation,
which in turn is due to the fact that the π(CC)→B(NMe2)2
charge flux is suppressed. Indeed, the B(NMe2)2 unit cannot
accept charge from π(CC) because either the pz(Bexocyclic
)
orbital is perpendicular to the π(CC) or if an overlap can
occur, the LP(N) on the other side is already donating to the
pz(Bexocyclic) orbital (see Figure 5). Moreover, it is interesting to
Table 5 shows the different values of bonding energy
contribution to the N/C−B and the B−C2 bonds. In all cases,
the orbital interaction is predominant with respect to the
electrostatic contribution; thus electron delocalization in the
BCC core does take place. Orbital interaction is much larger in
the R−B bond when R is an amino group, due to the LP(N)→
pz(Bborirene) donation, and is commensurate with a shorter N−B
bond. This orbital interaction is modified upon change of the
substituents bound to CC. When the CC unit is bound to
two B(NMe2)2 units, the Eorb(N−Bborirene) is smaller as
compared to the case when one of the B(NMe2)2 groups is
Figure 5. The proposed donations and suppressed donations in
isomers of 8 in which the boryl [BN2] plane is perpendicular to (left)
or coplanar with (right) the borirene π system.
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Figure 6. The LUMO (lowest unoccupied molecular orbital) (top) to HOMO−2 (bottom) levels in 5, 8, and 7.
note that in the N−B−C−Ph chain the orbitals of each atom
are prone to extensive π-electron delocalization, in agreement
with the shorter NB−CPh bond as compared to the NB−
CB(NMe2)2 bond, amounting to 1.481 and 1.499 Å,
respectively.
simultaneously accommodate and donate charge density by
way of both resonance and induction.
There is a tendency for the B−C2 bonds to have a larger Eorb
when the boron is sp2 hybridized, except for the case, although
not by much, when both carbons in the CC unit are bound
to B(NMe2) side chains. Furthermore, the CC bond is
longer when bound to sp2 boron, reflecting the existence of
π(CC)→pz(B) donation relative to the quaternary boron
case, in which NBO analysis accordingly indicates no
considerable π(CC)→pz(B) interaction. The lack of LP-
(N)→pz(B) donation in the case of 8 is also supported by the
fact that the B−C2 orbital interaction energy value for either sp2
or sp3 hybridized boron is of the same order of magnitude.
In a borirene unit, charge flux does not depend solely on π
delocalization, and thus aromaticity plays a minor role due to
boron’s multisymmetry charge flux bridging capabilities; that is,
the boron atom can convey charge flow even through induction
and not only through resonance. The two forms of electron
donation to boron, LP→pz and π→pz, are complementary and
highly coupled, which in turn depend on the borirene side-
chains; this dependency is due to geometrical and electronic
reasons.
Inspection of molecular orbitals indicates that in the borirene
core, there are substantial B−C2 σ-like bonds generated from
the boron-centered p orbital parallel to the borirene plane. In 7,
the borirene clearly exhibits a strong additional stabilization
from the CC π system (see its HOMO−1 in Figure 6),
which is additionally antiphase to the LP(N); thus the π(C
C)→pz(B) donation is not countered by the LP(N)→pz(B)
stabilization. Second-order perturbation analysis in NBO basis
indicates an energy of 100 kcal/mol corresponding to the
π(CC)→pz(B) donation. However, for 8 (Tables 5−7), the
π(CC)→pz(B) stabilization amounts to 104 kcal/mol; it is
thus likely that inductive effects are responsible for the
observed higher ΔEorb(B−C2) value in 7. Moreover, for the
all-aryl borirene 5, the π(CC)→pz(B) donation is also of the
same value (105 kcal/mol), which, when compared to 8,
suggests that the inductive effect in the borirene core is
independent of any lone pair donation to the boron (e.g.,
LP(N)→pz(B), p(aryl)→pz(B)). One thus sees here the
capacity of boron to behave as what could be called a three-
way multisymmetry charge flux bridge; that is, boron’s unique
feature of having an unoccupied pz orbital allows it to
To determine the effect of substitution of the borirene ring
on the π and σ delocalization in the borirene ring, nucleus-
independent chemical shift (NICS) values were calculated for
the sequentially borylated borirenes 6−8 (Table 7) and the
adduct 11 at the ring centroid (NICS(0))22 and 1.0 Å above
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Journal of the American Chemical Society
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Table 6. Bond Lengths (Å), Natural Charge (e), and Energy Contribution (kcal/mol) in a Range of Borirenes and Base-
Stabilized Borirenes
Table 7. Nucleus-Independent Chemical Shift (NICS)
Values for 6−8 and 11
CONCLUSIONS
■
This Article describes the synthesis of borirenes by photo-
chemical borylene transfer to a series of different functionalized
alkynes in high yield. The series of borirenes synthesized
allowed us to systematically compare the properties of
borirenes depending on the functional groups in 1, 2, and 3
positions. All borirenes were found to possess remarkable
thermal stability. We were also able to identify the ring-
breathing band in the IR spectrum, giving us an additional tool
to identify borirenes and to evaluate the bonding situation in
the ring. The X-ray structures of the borirenes 6−8 showed
clearly that the replacement of the phenyl groups by the
B(NMe2)2 group yields more conjugation in the BCC ring. To
evaluate the reactivity of borirenes, we reacted amino- and
arylborirenes with different Lewis bases. The reaction yielded
borirenes with a quaternized ring boron atom. The formation of
the adduct was obviously influenced by the strength of the base
and the extent of electronic saturation of the ring boron atom.
The base can be removed by addition of BPf3, thus making the
quaternization process a reversible method for switching the
borirene aromaticity on and off. The joint computational−
experimental study reveals that in a borirene unit charge flux
does not depend solely on π-delocalization, and thus
aromaticity plays a minor role thanks to boron’s ability to
convey charge flux through induction with the same ease as
through resonance.
compound
6
7
8
11
NICS(0)
NICSzz(0)
NICS(1)
NICSzz(1)
−19.48
−14.58
−10.47
−13.41
−20.15
−19.82
−11.67
−16.04
−19.68
−25.23
−13.10
−19.83
−21.92
−30.20
−9.48
−17.02
the ring (NICS(1)).23,24 The NICSZZ(1)25 value, which
represents the tensor z-components of the value rather than
the isotropic NICS value, is considered to give the best measure
of aromaticity in π systems. Our results show that aromaticity
increases regularly with increasing replacement of phenyl by
boryl substituents (6→8), which is in line with our above
finding (Figure 5) that the perpendicular and relatively
electronically saturated diaminoboryl groups do not signifi-
cantly communicate with the borirene π system. Somewhat
counterintuitively, this indicates that the phenyl groups are
effectively stronger π-acids than the B(NMe2)2 groups in this
case. Unsurprisingly, the “aromaticity” is lowest for the carbene
adduct 11. The NICSZZ(0)25 values, which give a measure of
the extent of delocalization in the σ-system, also increase as
strongly σ-donating boryl groups are successively added (6→
8). The σ delocalization of 11, which has suffered a loss of
delocalization in its π system, is even higher than in 6−8.
1910
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Journal of the American Chemical Society
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ASSOCIATED CONTENT
■
S
* Supporting Information
D.; Horl, C.; Kupfer, T.; Krummenacher, I.; Radacki, K.; Walter, C.
̈
Experimental details, UV/vis spectra of 7−14, crystallographic
data of complexes 4, 6−8, and 10−14 in cif format, and details
about DFT calculations. This material is available free of charge
via the Internet at http://pubs.acs.org.
Chem.-Eur. J. 2012, 18, 14292−14304.
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AUTHOR INFORMATION
■
Corresponding Author
h.braunschweig@uni-wuerzburg
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the European Research Council
(ERC Advanced Grant to H.B.). We thank Mr. C. Horl for
helpful discussions.
̈
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