Oligo(borolyl)benzenes-Synthesis and properties

Article abstract of DOI:10.1002/chem.201202345

Herein, we report the synthesis of boroles that are linked by a conjugated phenylene spacer. The characterization of these compounds was completed by NMR and UV/Vis spectroscopy, as well as X-ray crystal diffraction. Furthermore, the coordination behavior of these oligoboroles towards five electronically and sterically disparate pyridine derivatives was studied and revealed fundamental differences in the properties of the resulting adducts. The experimental results were supported by density functional theory (DFT) calculations that showed a charge-transfer effect upon formation of the pyridine-4-carbonitrile adduct. By chemical reduction of a tris(borolyl)-substituted benzene derivative, a hexaanion was isolated as a result of a two-electron reduction of each borolyl moiety. The interaction of the borolyl units through the aryl spacer, and the possible increase of the Lewis acidity due to the conjugation of the borolyl moieties, were investigated by base transfer reactions. Copyright

Full text of DOI:10.1002/chem.201202345

FULL PAPER
DOI: 10.1002/chem.201202345
OligoACTHNURGTNE(NUG borolyl)benzenes—Synthesis and Properties
Holger Braunschweig,*[a] Ching-Wen Chiu,[c] Alexander Damme,[a] Bernd Engels,[b]
Daniela Gamon,[a] Christian Hçrl,[a] Thomas Kupfer,[a] Ivo Krummenacher,[a]
Krzysztof Radacki,[a] and Christof Walter[b]
Abstract: Herein, we report the synthe-
sis of boroles that are linked by a con-
jugated phenylene spacer. The charac-
terization of these compounds was
completed by NMR and UV/Vis spec-
troscopy, as well as X-ray crystal dif-
fraction. Furthermore, the coordination
behavior of these oligoboroles towards
five electronically and sterically dispa-
rate pyridine derivatives was studied
and revealed fundamental differences
in the properties of the resulting ad-
ducts. The experimental results were
supported by density functional theory
By chemical reduction of
AHCTUNGTRENN(GUN borolyl)-substituted benzene deriva-
a
tris-
tive, a hexaanion was isolated as a
result of a two-electron reduction of
each borolyl moiety. The interaction of
the borolyl units through the aryl
spacer, and the possible increase of the
Lewis acidity due to the conjugation of
the borolyl moieties, were investigated
by base transfer reactions.
(DFT) calculations that showed
a
charge-transfer effect upon formation
of the pyridine-4-carbonitrile adduct.
Keywords: boron · heterocycles ·
Lewis acids
reduction
·
Lewis bases
·
Introduction
tadienyl cation. Their high level of reactivity, which is based
on the fact that boroles are highly antiaromatic, necessitates
steric protection.[6] This is commonly accomplished by the
use of aryl substituents on the BC4 core to prevent intramo-
lecular Diels–Alder reactions.[6,7] Although the synthesis of
a free borole was successfully accomplished in 1969,[6a] no
further studies were reported until 2008 when the first struc-
tural characterization of 1 was achieved.[8a] Since then, bor-
oles have increasingly moved into the focus of the chemical
communityꢀs attention, which is reflected by a multiplicity
of recent publications.[8] The high Lewis acidity of boroles
leads to the formation of stable Lewis acid–base adducts
with even weak bases such as ethers, nitriles, CO or sterical-
ly congested bases.[6b,8j,l] An intriguing example is the 2,6-lu-
tidine adduct of 1, which shows surprising behavior upon
UV irradiation (Scheme 1). A fast equilibrium between the
The incorporation of the element boron into conjugated or-
ganic frameworks has a significant impact on the optoelec-
tronic properties of the resulting materials.[1–3] This is mainly
due to the electron-deficient nature of tricoordinated boron,
a result of the vacant pz orbital on boron. The energetically
low-lying molecular orbitals enable efficient electron trans-
port through the organic p-system[1b,f,4] and promote their
application as ETL (electron transport layer) materials in
organic light-emitting diodes (OLEDs).[1a–c,4e,5] Within the
class of boron-containing p-conjugated systems, boroles are
of particular interest due to their similarity to the cyclopen-
[a] Prof. Dr. H. Braunschweig, A. Damme, Dr. D. Gamon, C. Hçrl,
Dr. T. Kupfer, Dr. I. Krummenacher, Dr. K. Radacki
Institut fꢁr Anorganische Chemie
Julius-Maximilians Universitꢂt Wꢁrzburg
Am Hubland, 97074 Wꢁrzburg (Germany)
Fax : (+49)931-31-84623
E-mail: h.braunschweig@uni-wuerzburg.de
[b] Prof. Dr. B. Engels, C. Walter
Institut fꢁr Physikalische und Theoretische Chemie
Bayerische Julius-Maximilians-Universitꢂt Wꢁrzburg
Am Hubland, 97074 Wꢁrzburg (Germany)
Scheme 1. Equilibrium between borole 1 and adduct 1c.
[c] Prof. C.-W. Chiu
Department of Chemistry
National Taiwan University
Roosevelt Rd. 1, 10617 Taipei (Taiwan)
free borole 1, the free base, and adduct 1c that was directly
connected to the color of the solution was found, revealing
thermochromic behavior. At room temperature, about 30%
of borole 1 remained uncoordinated, which resulted in a
blue-colored solution. Cooling shifted the equilibrium to the
side of the Lewis adduct. At À508C, only coordinated
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202345. It includes details on
electrochemical, computational and crystallographic procedures, and
UV/Vis spectra.
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borole was present and the solution appeared yellow. Irradi-
ation with UV light at low temperatures yielded an unantici-
pated rearrangement reaction of the borole adduct to a bor-
ataalkene 1c’, in which migration of the base from boron to
the adjacent carbon atom occurred.[8j]
Their strong Lewis acidity also enables boroles to activate
small molecules, such as H2.[8e] Oxidative addition reactions
of haloboroles to transition-metal complexes, as well as the
reduction of a carbene-stabilized haloborole leading to a nu-
cleophilic p-boryl anion, have received considerable atten-
tion.[8g] Other new reactivity patterns, such as the spontane-
ous conversion of haloboroles to spiro compounds and the
additional reduction-induced rearrangement to nido-carbor-
ane clusters, have been reported recently.[8h,k] The substitu-
ents on the butadiene backbone have been limited to
phenyl, pentafluorophenyl, tolyl, and, as recently reported,
thienyl groups.[6,8d,q] In addition to phenyl and halides, a
greater variation of substituents at the boron atom are toler-
ated, including ferrocenyl, cymantrenyl, amino, and various
aryl groups.[8b,m,o,p] We are interested in incorporating boroles
into p-conjugated systems to explore their electronic com-
munication behavior and the coordination chemistry of
these oligoboroles. To date, oligoboroles have been exclu-
sively prepared with organometallic fragments as spacers be-
tween individual borole units. Herein, we report the synthe-
sis and properties of the first examples that are linked
through an organic p-conjugated bridge.
Figure 2. Solutions of pentaphenylborole (1), 2, 3, and 4 in CH2Cl2.
Because of the poor solubility of compound 2 in non-
polar solvents, characterization by NMR spectroscopy was
performed at high temperatures. The 11B NMR spectrum in
[D8]toluene at 1008C displayed a broad resonance at d=
1
68 ppm. In the H NMR spectrum, the signals for the phenyl
groups of the tetraphenylbutadiene backbone overlap with
the signals from [D8]toluene. However, after subtraction of
the toluene protons, 40 phenyl protons that were associated
with 2 could be found as a broad multiplet at d=6.78–
7.05 ppm. Additional signals at d=7.43–7.45 and 7.65 ppm
are observed for three protons of the central benzene ring.
Furthermore, a spectrum recorded in CD2Cl2 at 358C con-
firmed these results; no overlap of the solvent and product
peaks was observed, and four multiplets at d=6.72–6.77,
7.00–7.01, 7.07–7.09, and 7.27–7.32 ppm were found for the
44 protons of 2. The low solubility of 2 in CD2Cl2 resulted in
a poorly resolved 11B NMR signal at around d=68 ppm.
The synthesis of 1,3,5-(BC4Ph4)3-C6H3 (3) was carried out in
the same manner as that described for 2; however, 3 was ob-
tained as a black solid and showed better solubility than 2.
Therefore, multinuclear NMR spectra of 3 were recorded at
ambient temperatures. The 11B NMR spectrum features one
broad signal at d=72 ppm. Like the chemical shift value ob-
served for 2, this is comparable to other boroles (e.g., penta-
phenylborole (1) d=65 ppm,[8a] 1-chloro-2,3,4,5-tetraphenyl-
borole d=66 ppm[8b]). The 60 phenyl protons are detected in
the region from d=6.55–7.14 ppm and a signal at d=
7.18 ppm was attributed to the three protons of the central
aryl ring. Finally, 1,4-(BC4Ph4)2-C6H4 (4) was obtained as a
green solid. It was found that oligoborole 4 was even less
soluble than 2. The 11B NMR spectrum of 4 features one
broad signal at d=71 ppm (see Table 1 for details).
Results and Discussion
Synthesis and characterization of oligoboroles: Boroles 2–4
were synthesized according to the known procedure for the
synthesis of pentaphenylborole (1) by reaction of the corre-
sponding dihaloboranes with 1,1-dimethyl-2,3,4,5-tetraphe-
nylstannole using the well-established tin–boron exchange
route.[6] Boroles 2–4 (Figure 1) are deeply colored in the
solid state and in solution and were obtained in reasonable
yields of 57–76%. 1,3-(BC4Ph4)2-C6H4 (2) precipitated as a
dark-green solid from a solution in toluene at À308C, which
was filtered and washed with hexane. In contrast to the
green color of the solid, 2 shows a blue color in solution
(Figure 2).
The UV/Vis spectra of 2–4 were measured and compared
with the parent molecule 1. Figure 3 shows the first absorp-
tion maxima of the four boroles 1–4. The absorption
maxima observed for 1–3 are found in similar regions
(Table 1). In comparison, a slightly red-shifted absorption
band was found for oligoborole 4, which is most likely due
to the para arrangement of the two borole units that allows
possible conjugation between them. This is in contrast to oli-
goboroles 2 and 3, in which the borole substituents are ori-
ented in a meta fashion. The first absorption maximum of 4
was observed at lmax =579 nm, which is red-shifted by about
14 nm with respect to 2 and 11 nm to 3. A more striking dif-
ference was found between the extinction coefficients of the
Figure 1. Synthesized bis- and trisboroles.
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OligoACHTUNGTRENNUNG(borolyl)benzenes—Synthesis and Properties
FULL PAPER
Figure 3. Section of the UV/Vis spectra of 1–4 recorded in CH2Cl2 at con-
[10]
centrations of 2ꢅ10À3 molLÀ1
.
Table 1. 11B NMR chemical shifts of free boroles 1–4 and borole Lewis
base adducts 2a–4e. Absorption maxima and extinction coefficients of
compounds 1–4 and 2a–4e.
Figure 4. Molecular structure of 1,3,5-(BC4Ph4)3-C6H3 (3) with thermal el-
lipsoids set at the 50% probability level. Hydrogen atoms and thermal el-
lipsoids of the aryl carbon atoms are omitted for clarity. Selected bond
lengths [ꢄ] and angles [8] are found in Table 2.
d
11B [ppm] lmax [nm] (e [LmolÀ1 cmÀ1])
1
2
3
4
65
68
562 (628)
565 (832)
351 (10604) 279 (18064) 261 (18148)
358[d]
72
568 (1395) 367 (18915) 267 (48545)
579 (785)
71[a]
387[d]
252[d]
the p-electron system of the central benzene ring is present
in 1. In 3, this interaction is reduced due to the presence of
three borole units connected to the aryl spacer. Therefore,
the relative amount of resonance stabilization gained from
the p-system is smaller for each borole unit. Furthermore,
the borole rings are bent out of the C6H3 plane, which re-
sults in a non-ideal planar environment around the boron
atom and can be specified by the angles between the center
of the borole ring, the boron atom, and the ipso-carbon
atom. The angles differ from the ideal value of 1808 (XB1-
B1-C5 163.78, XB2-B2-C10 169.38, and XB3-B3-C15 171.78).
These findings indicate a sterically and electronically con-
gested situation. In addition, all three borole groups are lo-
cated slightly below the plane of the C6H3 ring, hence the
angles between the centroid of the central benzene ring, the
ipso-carbon atom, and the boron atom are likewise smaller
than 1808 (XC6H3-C5-B1 169.38, XC6H3-C10-B2 168.48 and
2a 3.9
2b 4.0
2c 3.6[b]
2d 2.8
2e 4.9
350 (14444)
347 (13121) 255[d]
552 (782)
351 (16573)
359 (2218)
265 (46345)
513 (1543) 347 (16411)
341 (19880) 255 (69085)
342 (18150) 260[d]
540 (1208) 360 (24416)
349 (27990) 276 (116335)
3a 4.0
3b 4.3
3c 4.0, 70[b]
3d 3.3
3e 5.4
513 (2215) 334 (25555) 255 (74935)
[c]
4a
–
348 (9430)
345 (9602)
4b 3.8
[d]
4b’ 3.7, 67
–
[c]
4c
4d
4e
–
–
–
550[e]
450 (4040)
371 (19535)
[c]
[c]
345 (13859)
502 (1176) 340 (12392)
[a] 11B NMR spectrum recorded at 1008C. [b] 11B NMR spectrum record-
ed at À608C. [c] Not available due to poor solubility. [d] Not available.
[e] Absorption appears as a broad shoulder.
X
C6H3-C15-B3 168.88). The borole rings are nearly planar, a
common structural feature of boroles[6–9] that is reflected by
small torsion angles between À3.3(3) and +3.1(3)8. Similar-
ly to other boroles, such as 1-chloro-2,3,4,5-tetraphenylbor-
boroles 1–4. Increasing the number of borole substituents
on the benzene ring led to more intense absorptions, which
is ascribed to the chromophoric nature of boroles.
À
ole, the borole moieties in 3 feature alternating C C double
The molecular structure of 3 was studied by X-ray diffrac-
tion (Figure 4, Table 2). Dark-blue crystals were obtained
from a toluene/CH2Cl2 solution at À358C. The three borole
moieties were found to be in a propeller-like arrangement in
which the torsion angles between the planes are defined by
the borole rings and the central C6H3 ring. These angles
(39–428) are slightly larger than those of 1 (338). The bond
lengths between the boron atoms and the ipso-carbon atoms
(C5, C10, C15) are slightly longer (see Table 2) than those
observed in 1 (1.515(2) ꢄ). It is reasonable to suggest that
an interaction between the empty pz orbital at boron and
and single bonds.[8a,h] However, in contrast to 1, no intermo-
lecular contacts could be observed, which can also be attrib-
uted to steric effects.
Synthesis and characterization of Lewis adducts of oligobor-
oles: The coordination chemistry of the borole-substituted
benzenes was studied by the reaction of 2–4 with five differ-
ent pyridine derivatives (Scheme 2). These included 4-pico-
line (a), 3,5-lutidine (b), and the more sterically demanding
2,6-lutidine (c), which has been used for the formation of
frustrated Lewis pairs[9] and the adduct 1c mentioned
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H. Braunschweig et al.
Table 2. Selected bond lengths [ꢄ] and angles [8] of 3 and selected Lewis base adducts
of 2, 3, and 4.
the base. In the following discussion, we will first
focus on the less sterically demanding bases and
then turn to the 2,6-lutidine adducts of the boroles
3
2b
2d
3d
3e
4b
4c
À
B1 C1
À
B2 C6
1.575(4) 1.634(2) 1.639(2) 1.626(6) 1.631(4) 1.628(3) 1.642(5) 2–4. Similar to other borole adducts, such as the 4-
[a]
1.587(4) 1.622(2) 1.631(2) 1.632(6) 1.637(4)
1.585(4) 1.635(6) 1.621(4)
–
–
1.622(5)
–
picoline adduct of 1-chloro-2,3,4,5-tetraphenylboro-
le[8b] or the 3,5-lutidine adduct of 1-bromo-2,3,4,5-
tetraphenylborole,[8l] drastic color changes were ob-
served upon coordination of a base to the boron
atom of the borole ring (Figure 5).
À
B3 C11
–
–
À
B1 C4
1.590(4) 1.629(2) 1.623(2) 1.626(6) 1.646(4) 1.627(3) 1.613(6)
[a]
À
B2 C9
1.578(4) 1.622(2) 1.624(2) 1.633(6) 1.621(4)
1.570(4) 1.626(6) 1.630(4)
–
–
1.644(5)
–
À
B3 C14
–
–
À
B1 C5
1.548(4) 1.621(2) 1.637(2) 1.625(6) 1.619(4) 1.619(3) 1.624(5)
[a]
À
B2 C10
1.545(4) 1.633(2) 1.627(2) 1.621(6) 1.606(4)
1.544(4) 1.622(6) 1.615(4)
–
–
1.615(5)
–
Figure 6 shows the UV/Vis spectra of the adducts
2a–e formed from borole 2, and Table 1 summariz-
es the observed absorption bands and extinction co-
efficients found for all adducts. For further spectral
details, see the Supporting Information. The solu-
tions of the adducts of 2–4 with 4-picoline (a), 3,5-
lutidine (b), and 4-dimethylaminopyridine (d)
appear yellow (Figure 5), which is reflected by UV/
Vis absorption bands at around lꢀ350 nm. This
was also observed in the aforementioned systems
and the previously reported 4-tert-butylpyridine and
4-dimethylaminopyridine adducts of 1-cymantrenyl-
2,3,4,5-tetraphenylborole.[8m]
The pyridine-4-carbonitrile adducts 2e, 3e, and
4e are deep red, and absorption maxima at around
lꢀ510 nm were found, in addition to the band at
around lꢀ350 nm. This difference is attributed to
the electron-withdrawing CN group at the base,
which leads to an additional charge-transfer band.
Theoretical investigations were performed on the
corresponding adducts of pentaphenylborole, 1a
and 1e (for details, see the Experimental Section).
The UV/Vis absorption spectra of these adducts
(1a: lmax =355 nm; 1e: lmax =510, 347 nm; see the
À
B3 C15
–
–
À
C1 C2
1.354(4) 1.358(2) 1.363(2) 1.369(5) 1.365(4) 1.361(3) 1.363(6)
[a]
À
C6 C7
1.362(4) 1.362(2) 1.361(2) 1.357(5) 1.354(4)
1.353(4) 1.355(5) 1.350(4)
–
–
1.347(6)
–
À
C11 C12
–
–
À
C2 C3
1.518(4) 1.498(2) 1.491(2) 1.489(5) 1.491(4) 1.500(3) 1.492(6)
[a]
À
C7 C8
1.525(4) 1.498(2) 1.497(2) 1.500(5) 1.493(4)
1.508(4) 1.499(5) 1.488(4)
–
–
1.485(5)
–
À
C12 C13
–
–
À
C3 C4
1.353(4) 1.356(2) 1.351(2) 1.353(6) 1.356(4) 1.353(3) 1.365(6)
1.339(4) 1.355(2) 1.497(2) 1.349(6) 1.365(4)
[a]
À
C8 C9
–
–
1.362(6)
–
À
C13 C14
1.360(4)
–
–
1.353(6) 1.363(4)
À
B1 N1
–
–
–
1.618(2) 1.594(2) 1.608(5) 1.656(4) 1.616(3) 1.664(6)
1.634(2) 1.605(2) 1.603(5) 1.614(4)
–
[a]
À
B2 N2
–
1.642(6)
–
99.4(3)
99.0(3)
–
À
B3 N3
–
1.597(5) 1.608(4)
–
C1-B1-C4
C6-B2-C9
104.9(2) 99.5(1)
104.7(2) 100.3(1) 99.9(1)
99.4(1)
99.4(3)
99.6(3)
99.3(3)
99.7(2)
99.9(2)
100.4(2)
99.9(2)
[a]
–
C11-B3-C14 104.9(2)
C1-B1-C5
C6-B2-C10
C11-B3-C15 128.6(2)
N1-B1-C1
N2-B2-C6
N3-B3-C11
C5-B1-XB1
C10-B2-XB2 169.3
C15-B3-XB3 171.7
–
–
–
125.9(2) 110.9(1) 109.5(1) 112.2(3) 117.8(2) 113.3(2) 103.1(3)
[a]
127.4(2) 110.6(1) 108.0(1) 110.4(3) 117.6(2)
114.6(3) 116.6(2)
–
–
118.4(3)
–
–
–
–
–
–
112.6(1) 116.8(1) 111.4(3) 108.0(2) 112.7(2) 114.6(3)
113.9(1) 110.6(1) 105.3(3) 103.0(2)
–
123.7
118.6
–
[a]
–
110.8(3)
–
120.6
119.8
–
–
110.2(3) 105.5(2)
–
163.7
119.9
117.3
–
119.9
125.6
121.5
136.3
129.1
125.3
126.0
[a]
–
–
[a] An inversion center is present.
above.[8j] Furthermore, electron-rich 4-dimethylaminopyri-
dine (d) and electron-poor pyridine-4-carbonitrile (e) were
used to vary both the steric and electronic characteristics of
Supporting Information) are comparable to the spectra ob-
tained for the corresponding adducts of 2, 3, and 4. The
computations revealed a LUMO that was significantly lower
Scheme 2. Synthesis of the borole adducts.
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OligoACHTUNGTRENNUNG(borolyl)benzenes—Synthesis and Properties
FULL PAPER
11B NMR resonance, which is in agreement with the pres-
ence of a tetracoordinate boron atom. Depending on the
pyridine base, small differences are observed in the
11B NMR chemical shifts of the adducts. For example, the
signals found for the adducts 2d (d=2.8 ppm) and 3d (d=
3.3 ppm) from 4-dimethylaminopyridine are shifted slightly
upfield compared to the signals for the adducts 2e (d=
4.9 ppm) and 3e (d=5.4 ppm) from pyridine-4-carbonitrile.
Adducts of boroles 2 and 3 with the same bases show only
small differences in their characteristics, such as the good
solubility in CH2Cl2. However, most adducts of 4 are nearly
insoluble in common organic solvents like CH2Cl2, toluene,
benzene, acetone, THF, or hexane. Figure 5 shows that only
suspensions of the adducts 4a, 4c, and 4e were obtained.
However, 4b was found to have a good level of solubility,
similar to that of 2a–3e, and multinuclear NMR spectra
could therefore be obtained. A 11B NMR signal at d=
3.8 ppm was detected that is comparable to the signals
found for 2b (d=4.0 ppm) and 3b (d=4.3 ppm). Single
crystals of 2b, 2d, 3d, 3e, and 4b, suitable for X-ray diffrac-
tion, were obtained by the diffusion of hexane into solutions
of the borole adducts in CH2Cl2. The molecular structures
display features typical of the boroles; for example, the pro-
peller-like arrangement of the phenyl groups and the planar-
ity of the borole ring (Figures 7–9).
À
The B C bond lengths in the adducts are longer than
Figure 5. Solutions/suspensions of the boroles 2–4 and their adducts 2a–
4e in CH2Cl2 at c=2ꢅ10À3 molLÀ1
those of the uncoordinated boroles 1 and 3, which is due to
a change in hybridization of the boron atoms from sp2 to
sp3. In the butadiene backbones of the five adducts, elonga-
.
À
tion of the C C single bonds was observed, as well as a de-
crease of the angle between the two endocyclic carbon
atoms next to the boron atom. For example, the angle C1-
B1-C4 in 3 is 104.9(2)8 but is reduced to 99.4(3)8 in the
adduct 3d and 99.7(2)8 in 3e. The most striking features are
À
the B N bond lengths, which depend both on the base and
À
the borole. Slightly shorter B N bonds were found for the
4-dimethylaminopyridine adduct of borole 2 (2d; 1.594(2),
1.605(2) ꢄ) compared to the corresponding 3,5-lutidine
adduct of 2 (2b; 1.618(2), 1.634(2) ꢄ) or 4 (4b; 1.616(3) ꢄ).
This can be attributed to the presence of the more electron-
rich and less sterically demanding base 4-dimethylaminopyr-
idine. The comparison of the adducts formed by borole 3
with two very different bases is even more interesting. All
À
B N bonds in adduct 3d are found to be very short
(1.608(3), 1.603(3), 1.597(3) ꢄ) due to the previously descri-
À
bed characteristic of the base. However, the B N bond
lengths in 3e show significant differences. Compared with
Figure 6. Section of the UV/Vis spectra of base-free borole 2 (blue; c=
3.0ꢅ10À4 molLÀ1) and the adducts 2a (yellow line; c=2.2ꢅ10À4 molLÀ1),
2b (yellow bars; c=2.2ꢅ10À4 molLÀ1), 2c (brown; c=2.4ꢅ10À4 molLÀ1),
2d (yellow dots; c=1.7ꢅ10À4 molLÀ1), and 2e (red; c=2.7ꢅ
10À4 molLÀ1) in CH2Cl2.
À
À
À
B2 N2 and B3 N3, B1 N1 is about 4 pm longer. The elec-
tron-withdrawing properties of pyridine-4-carbonitrile are
expected to lead to weaker B N bonds, thereby easing the
À
sterically congested situation in 3e. However, in 3d, the
binding of the base is supposed to be much stronger and
therefore a more sterically strained situation is avoided by
rotation of the borole moiety (see Figure 9a). The base in
adduct 3e, which is coordinated to B1, is bound to the back
face of the molecule (Figure 9b), whereas the base in 3d
binds to the front face with respect to the central C6H3 ring.
in energy (À2.13 eV) in 1e than that of 1a (À0.86 eV),
whereas the HOMO of 1e (À6.29 eV) is only slightly lower
in energy than that of 1a (À6.09 eV). This imposes a smaller
HOMO–LUMO gap in 1e and a bathochromic shift in the
absorption maximum. The formation of all adducts 2a–4e
was also confirmed by a significant upfield shift of the
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H. Braunschweig et al.
This leads to different torsion angles (XB1-B1-C5-C16) of
the borole moiety in these two adducts (91.78 in 3d, 33.98 in
3e; Figure 9). Because of slight differences in the coordina-
tion of the other two bases, the torsion angles between the
remaining boroles differ, as indicated by the dihedral angle
XB2-B2-B3-XB3. In 3d, this angle is 33.98, compared with
16.88 in 3e (Figure 9).
The Lewis acid–base adducts of 2, 3 and 4 were prepared
in a similar manner to the corresponding 2,6-lutidine adduct
of 1. The adducts 2c, 3c, and 4c were red, and the UV/Vis
spectra show bands at around lmax =550 nm (Figure 6 and
Figure S5 in the Supporting Information). The borole ad-
ducts 2c and 4c show a comparable thermochromism to the
adduct with pentaphenylborole (1c). Cooling the solutions
of the adducts in CH2Cl2 to À508C led to a color change
from red to yellow, which is presumably due to a reduced
dissociation rate at these temperatures (Figure 10). In con-
trast to 1c, which was blue, it is reasonable to suggest that
for the red compounds 2c, 3c, and 4c, one base always stays
coordinated in the equilibrium between the oligoboroles
1
and 2,6-lutidine. The H NMR spectra of 2c and 4c show a
very broad resonance between d=2.0 and 3.0 ppm for the
methyl protons of the base. Cooling 2c leads to the splitting
and sharpening of the signals; at À308C, two signals at d=
1.32 and 3.31 ppm are found. This effect was also observed
for the methyl protons of 1c and is due to the hindered rota-
tion of the base at low temperatures.[8j] The poor solubility
of 4c in cold solutions is the reason why a low temperature
NMR spectrum could not be obtained.
Yellow single crystals of 4c, suitable for X-ray diffraction,
were be obtained by hexane diffusion into a solution of the
adduct in CH2Cl2. Again, the most distinct features of the
Figure 7. Molecular structure of adducts a) 2b and b) 2d with thermal el-
lipsoids set at the 50% probability level. Hydrogen atoms and thermal el-
lipsoids of the aryl carbon atoms and those of the Lewis bases are omit-
ted for clarity. Selected bond lengths [ꢄ] and angles [8] are found in
Table 2.
À
structure (see Figure 11) are the B N bonds, which are 2–
4 pm longer than those of 4b. This is indicated by steric con-
straints imposed by the two ortho-methyl groups of the ni-
trogen base. Surprisingly, upon cooling, no color change was
found in the case of adduct 3c. Furthermore, whereas the
solids 2c and 4c are yellow, the solid state of 3c is dark red.
Borole 3 was found to coordinate only to two equivalents of
2,6-lutidine, as verified by 11B NMR spectroscopy. One
signal corresponding to the tetracoordinate boron atom was
found at d=4.0 ppm, and another very broad signal was ob-
served at about d=70 ppm, which can be assigned to the un-
coordinated borole moiety. The 11B NMR spectrum remains
unchanged upon the addition of a third equivalent of base.
The 1H NMR spectrum at room temperature shows only
very broad signals due to the dissociation process, an effect
that was also previously found for the 2,6-lutidine adduct of
1. At À308C the signals appeared to be sharpened, giving
rise to two separated lines. The integration of the 63 aryl-
proton signals against the twelve methyl and six aryl protons
of the base support the indicated coordination of only two
molecules of 2,6-lutidine. Excess base remains uncoordinat-
ed, as verified by a methyl signal at d=2.46 ppm, whereas
the coordinated base shows two different methyl signals at
d=1.44 and 3.24 ppm. The solid-state structure of 3c con-
firmed the conformation of the adduct. Unfortunately, the
Figure 8. Molecular structure of adduct 4b with thermal ellipsoids set at
the 50% probability level. Hydrogen atoms and thermal ellipsoids of the
aryl carbon atoms and those of the Lewis bases are omitted for clarity.
Selected bond lengths [ꢄ] and angles [8] are found in Table 2.
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OligoACHTUNGTRENNUNG(borolyl)benzenes—Synthesis and Properties
FULL PAPER
Figure 9. Front and side views of the molecular structures of adducts a) 3d and b) 3e with thermal ellipsoids set at the 50% probability level. Hydrogen
atoms and thermal ellipsoids of the aryl carbon atoms and those of Lewis bases are omitted for clarity. Selected bond lengths [ꢄ] and angles [8] are
found in Table 2.
À
quality of the collected data does not allow a detailed dis-
cussion of the molecular structure. We assume that the ob-
served elongation of the B N bond, previously found for 4c
and the 2,6-lutidine adduct of 1, and the more sterically con-
gested borole 3 impede the addition of a third equivalent of
base.
Reduction of borole 3: The antiaromatic, five-membered
borole system is able to accept two electrons to form aro-
matic borole dianions that possess six p-electrons per borole
unit. As a result of a low-lying LUMO and a relatively low
reduction potential, boroles can be easily reduced by alkali
metals. To this end, we treated 3 in THF with either magne-
sium turnings or finely dispersed lithium metal to obtain the
hexaanions Mg3[3] and Li6[3], respectively (Scheme 3). The
transformation involved a characteristic color change of the
Figure 10. Solutions of the 2,6-lutidine adducts of boroles 2–4 in CH2Cl2
at room temperature (left) and À508C (right).
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H. Braunschweig et al.
Scheme 3. Synthesis of the hexaanions Mg3[3] and Li6[3].
obtained by the slow evaporation of a solution in THF and
proved to be stable enough for X-ray diffraction studies.
Unfortunately, the data obtained from these two experi-
ments are not suitable for a detailed discussion of structural
parameters. Nevertheless, the connectivity of Mg3[3] and
Li6[3] could be elucidated (Figure 12). The borole dianion
moieties are coordinated to either a magnesium or two lithi-
um cations, which are stabilized by solvent coordination. It
can be assumed that the application of a high vacuum re-
moves these solvent molecules and leads to the decomposi-
tion of the compounds. Moreover, several free THF mole-
cules are found in the unit cell of the hexaanions, making
the crystals even more susceptible to the evaporation of sol-
vent. However, the data do not allow a more in-depth dis-
cussion of the large torsion angles found for the borole dia-
nion moieties in Mg3[3] and Li6[3] with respect to the cen-
tral benzene ring, which stands in stark contrast to the tor-
sion angles in 3, which were found to be about 28 to 428.
One may suggest that this arises not only from steric conges-
tion present in the hexaanions but also from an interaction
of the empty pz orbital on the boron atoms with the p-
system of the central aryl ring in 3. This interaction is absent
after the reduction of the boroles and a larger torsion angle
may be expected. Another point supporting this argument is
the smaller torsion angle in 1 (338), which can be explained
by a stronger interaction of the empty boron orbital with
the p-electron density of the benzene ring.
Figure 11. Molecular structures of adducts a) 4c and b) 3c with thermal
ellipsoids set at the 50% probability level. Hydrogen atoms and thermal
ellipsoids of the aryl carbon atoms and those of the Lewis bases are omit-
ted for clarity. Selected bond lengths [ꢄ] and angles [8] are found in
Table 2.
solution from yellow to red. The initial yellow color of 3 in
THF arises from the formation of an adduct of 3 with the
solvent. The generation of the hexaanions was confirmed by
11B NMR spectroscopy. For both Mg3[3] and Li6[3], a signif-
icant upfield shift of the 11B NMR resonance to the range
expected for borole dianions was observed (Mg3[3] d=
26 ppm; Li6[3] d=25 ppm). For example, the potassium dia-
nion of 1 shows a 11B NMR signal at d=26 ppm.[6b,8c] The
hexaanions were found to be extremely sensitive to air and
moisture. Moreover, it was observed that the compounds de-
compose under prolonged exposure to vacuum, a phenom-
enon that has previously been observed for other oligobor-
ole polyanions.[8o] Single crystals of Mg3[3] and Li6[3] were
Base transfer and electrochemistry: By comparison, the
para-substituted borole groups in 4 are expected to allow in-
termolecular exchange interactions, whereas the meta orien-
tation of the borole units in 2 would make this less favora-
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OligoACHTUNGTRENNUNG(borolyl)benzenes—Synthesis and Properties
FULL PAPER
3,5-lutidine adduct of 1 (1d) in [D8]toluene was mixed with
an equimolar amount of 2 or 4. Subsequent heating of the
solutions at 608C for 18 h led to the formation of an equili-
brium between the free and coordinated boroles. Prolonged
heating of the mixture did not result in a change of the equi-
librium. In the reaction between 1d and 2, 73% of the base
1
was transferred from 1 to 2, as confirmed by H NMR spec-
troscopy. Moreover, two different species of the base ad-
ducts of 2, namely 2b’ and 2b (Scheme 4) were observed;
Scheme 4. Base-transfer reactions of a) 2 and b) 4.
only one borolyl moiety is coordinated to 3,5-lutidine in 2b’,
and 2b was only observed in small amounts (12%). In com-
parison, compound 4 only accommodates one equivalent of
base, giving rise to a single adduct (4b’) from the reaction
with 1d. In this sequence, 78% of the base is shifted from 1
to 4. These experiments show that the communication be-
tween the two borole groups is more efficient in 4 than in 2,
given that after coordination of one equivalent of 3,5-luti-
dine to 4 the other borolyl moiety has access to the electron
density from the base as well, which is mediated by the phe-
nylene spacer. In contrast, no significant electron delocaliza-
tion from the base to the free borole occurs in 2b’ and a
second equivalent of base can be added. The small concen-
tration of 2b is rather surprising if a statistical distribution
of 33% would be expected but, presumably, steric effects
explain this finding. In summary, 4 possesses no significantly
Figure 12. Molecular structures of a) Mg3[3] and b) Li6[3] with thermal
ellipsoids set at the 50% probability level. Hydrogen atoms and thermal
ellipsoids of the aryl carbon atoms and THF ligands are omitted for clari-
ty.
ble. Moreover, a higher Lewis acidity can be expected in 4
as a result of the mutual electron-withdrawing effect of the
two boroles. To get a better understanding of the Lewis
acidities of 2 and 4, these compounds were examined by
using base-transfer reactions. To this end, a solution of the
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H. Braunschweig et al.
1H, 13C: internal standard TMS, 11B: internal standard BF3·OEt2). NMR
spectra were obtained at 293 K if not identified otherwise. UV/Vis spec-
tra were measured on a JASCO V-660 UV/Vis spectrometer. Pentaphe-
nylborole (1),[6b] 1,3-(BBr2)2-C6H4,[11] 1,4-(BBr2)2-C6H4,[11] 1,3,5-(BBr2)3-
C6H3,[11] and 1,1-dimethyl-2,3,4,5-tetraphenylstannole[6b] were prepared
according to literature procedures. 4-Picoline, 3,5-lutidine, and 2,6-luti-
dine were dried over CaH2 prior to use. CCDC-888153 (2b), CCDC-
888154 (2d), CCDC-888155 (3), CCDC-888156 (3c), CCDC-888157 (3d),
CCDC-888158 (3e), CCDC-888159 (4b), CCDC-888160 (4c), CCDC-
888161 (Li6[3]) and CCDC-888162 (Mg3[3]) contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
higher Lewis acidity than 2. Thus the two borolyl moieties
in 4 are expected to adopt a bent configuration with respect
to each other so that no destabilizing conjugation takes
place. On the other hand, after coordination, the delocaliza-
tion of electron density through the organic linker should
stabilize the other borolyl group as well as the one coordi-
nated to the base.
To substantiate the results from the base-transfer reac-
tions, we attempted to study the electrochemical behavior of
the 1,3- and 1,4-bisACHTUNGTRENNUNG(borolyl)-substituted benzenes by cyclic
voltammetry. The measurements, however, were hampered
by their low solubility in THF and their high intrinsic reac-
tivity. As noted above, upon dissolving the boroles in THF,
the immediate formation of an adduct was observed, which
resulted in yellow solutions. Both isomers displayed a single,
irreversible redox event at À2.55 (2) and À2.75 V (4) against
Fc/Fc+, respectively (Figure S8 in the Supporting Informa-
tion). As well as a more negative reduction potential, bis-
Preparation of 1,3-bis(2,3,4,5-tetraphenylborole)benzene (2): A solution
of 1,1-dimethyl-2,3,4,5-tetraphenylstannole (1.00 g, 1.98 mmol) in toluene
(10 mL) was added to a solution of 1,3-(BBr2)2-C6H4 (0.41 g, 0.99 mmol)
in toluene (7 mL) over a period of 20 min, at which point the colorless
solution turned dark blue. The reaction mixture was stirred for 16 h at
ambient temperature. Filtration and evaporation gave a dark-green solid
which was washed with hexane (2ꢅ7 mL) and dried in vacuo to provide
2
in 73% yield (0.58 g, 0.72 mmol). 1H NMR (500.1 MHz, CD2Cl2,
313 K): d=6.72–6.77 (m, 15H; C6H5), 6.98–7.04 (m, 10H; C6H5), 7.07–
7.10 (m, 15H; C6H5), 7.27–7.32 ppm (m, 4H; C6H4); 11B NMR
(160.4 MHz, [D8]toluene, 373 K): d=68 ppm; 13C NMR (125.7 MHz,
CD2Cl2, 313 K): d=125.81, 126.04, 127.47, 127.52, 127.79 (CH), 128.52
(Cq), 129.63, 129.93 (CH), 136.79, 139.41 (Cq), 140.32, 144.66 ppm (CH);
elemental analysis calcd (%) for C62H44B2: C 91.86, H 5.47; found: C
91.72, H 5.37.
ACHTUNGTRENNUNG(borole) 4 showed a very broad wave, which could be indi-
cative of two coincident electron-reduction steps. However,
no clear separation of cathodic waves was observed under a
variety of conditions (solvent, electrolyte) that would indi-
cate some degree of electronic communication in the para-
substituted species 4.
Preparation of 1,3,5-tris(2,3,4,5-tetraphenylborole)benzene (3): The prep-
aration of 3 followed the procedure described above. Compound 3 was
obtained as a black solid in 57% yield (0.67 g, 0.56 mmol). 1H NMR
(500.1 MHz, CD2Cl2): d=6.55–6.59 (m, 9H; C6H5), 6.70–6.74 (m, 11H;
C6H5), 6.98–7.14 (m, 40H; C6H5), 7.18 ppm (s, 3H; C6H3); 11B NMR
(160.4 MHz, CD2Cl2): d=72 ppm; 13C NMR (125.7 MHz, CD2Cl2): d=
125.49, 125.72, 127.46, 127.60 (CH, C6H5), 128.42, 129.23 (CH, C6H3),
129.43, 129.82 (CH, C6H5), 136.61, 138.72 (Cq, i-C6H5), 144.76 (Cq, i-
Conclusion
We were able to synthesize and characterize several oligo-
boroles that consist of borole units connected through an
aryl spacer at either the meta- or para-positions. This leads
to differences in their Lewis acid–base behavior, which
arose from a more efficient communication of the borolyl
groups when they are configured in the para position, as
was confirmed by base-transfer reactions between pentaphe-
nylborole 1 and the oligoboroles 2 and 4. The formation of
Lewis base adducts was observed for all three oligoboroles
2, 3, and 4, revealing fundamental differences in their chem-
ical and photophysical properties. Moreover, the reduction
À
C6H3), 139.22, 162.77 ppm (Cq, B C=C); elemental analysis calcd (%) for
C90H63B3: C 91.85, H 5.40; found: C 91.64, H 5.31.
Preparation of 1,4-bis(2,3,4,5-tetraphenylborole)benzene (4): The prepa-
ration of 4 followed the procedure described above. 4 was isolated as a
dark-green solid in 76% yield (0.61 g, 0.75 mmol). 1H NMR (500.1 MHz,
CD2Cl2): d=6.76–6.78 (m, 7H; C6H5), 6.82–6.86 (m, 7H; C6H5), 6.99 (s,
4H; C6H4), 7.01–7.06 (m, 10H; C6H5), 7.10–7.19 (m, 12H; C6H5), 7.22–
7.26 ppm (m, 4H; C6H5); 11B NMR (160.4 MHz, CD2Cl2): d=71 ppm;
13C NMR (125.7 MHz, CD2Cl2): d=125.49, 125.84, 127.47, 127.49 (CH,
C6H5), 128.42, 129.23 (CH, C6H3), 129.57, 129.91 (CH, C6H5), 134.66 (Cq,
À
i-C6H5), 137.22 (Cq, i-C6H3), 137.85, 139.47 (Cq, i-C6H5), 163.33 ppm (B
C=C); elemental analysis calcd (%) for C62H44B2: C 91.86, H 5.47; found:
C 91.51, H 5.76.
of the trisACHTUNGTRENNUNG(borole) 3 to the hexaanions was accomplished by
reaction with lithium or magnesium. The incorporation of
borole units in extended p-conjugated structures is an excit-
ing avenue for further investigations because the element
boron imparts unique electronic and photophysical proper-
ties. These research endeavors are the subject of ongoing
studies in our laboratory.
General procedure and characterization data for adducts 1a, 1e, and 2a–
4e:
Pentaphenylborole-1-(4-methylpyridine)
(1a):
4-Picoline
(6.6 mL,
67.5 mmol) was added to a solution of 1 (30.0 mg, 67.5 mmol) in CD2Cl2,
at which point the color of the solution changed from dark blue to
yellow. The solvent was removed and the residue washed with hexane
(2ꢅ2 mL) and dried in vacuo to provide 1a as a yellow solid in 85%
yield (30.9 mg, 57.5 mmol). 1H NMR (500.1 MHz, CD2Cl2): d=2.45 (s,
3H; NC5H4-4-CH3), 6.65–6.67 (m, 4H; C6H5), 6.84–6.90 (m, 6H; C6H5),
6.95–7.03 (m, 10H; C6H5), 7.15–7.22 (m, 3H; C6H5), 7.29 (d, J=6.0 Hz,
2H; NC5H4-4-Me), 7.40–7.42 (m, 2H; C6H5), 8.55 ppm (d, J=6.3 Hz, 2H;
NC5H4-4-Me); 11B NMR (160.4 MHz, CD2Cl2): d=3.5 ppm; 13C NMR
(125.7 MHz, CD2Cl2,): d=21.71 (CH3, NC5H4-4-CH3), 124.55, 125.67,
126.69, 127.46, 127.52, 127.63, 129.19, 130.67, 133.71 (CH, C6H5, NC5H4-4-
Me), 140.58, 143.35 (Cq, C6H5, NC5H4-4-Me), 145.83 (CH, NC5H4-4-Me),
Experimental Section
General conditions: All manipulations were performed in a glove box
(MBraun) or under an atmosphere of argon by using standard Schlenk
techniques. Solvents were dried according to standard procedures or by
using an MBraun solvent purification system (SPS) and were stored
under an argon atmosphere over molecular sieves. CD2Cl2, [D8]THF, and
[D8]toluene were degassed by using three freeze–pump–thaw cycles and
À
150.44, 154.34 (Cq, C6H5), 158.49 ppm (Cq, B C=C); elemental analysis
calcd (%) for C40H32BN (537.51): C 89.38, H 6.00, N 2.61; found: C 89.93,
H 6.40, N 2.87.
stored over molecular sieves. NMR spectra were recorded on
a
Bruker 500 Avance spectrometer (1H: 500.1, 11B: 160.4, 13C: 125.7 MHz;
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OligoACHTUNGTRENNUNG(borolyl)benzenes—Synthesis and Properties
FULL PAPER
Pentaphenylborole-1-(pyridine-4-carbonitrile) (1e): 1H NMR (500.1 MHz,
CD2Cl2): d=6.48–6.50 (m, 2H; NC5H4-4-CN), 6.64–6.66 (m, 4H; C6H5),
6.88–7.03 (m, 16H; C6H5), 7.16–7.25 (m, 3H; C6H5), 7.38–7.40 (m, 2H;
C6H5), 7.68–7.69 (m, 2H; NC5H4-4-CN), 8.86–8.87 ppm (m, 2H; NC5H4-
4-CN); 11B NMR (160.4 MHz, CD2Cl2): d=4.7 ppm; 13C NMR
(125.7 MHz, CD2Cl2): d =115.15 (Cq, NC4H4-C-CN), 125.02 (Cq, NC4H4-
C-CN), 125.05, 125.98, 126.19, 127.57, 127.80, 127.95, 128.06, 130.55,
133.39 (CH, C6H5, NC4H4-C-CN), 139.99, 142.49 (Cq, C6H5), 147.19 (CH,
(CH, NC5H4-4-CN), 129.15 (CH, C6H5), 130.60 (CH, C6H4), 130.64 (CH,
C6H5), 137.72 (CH, C6H4), 140.01, 142.91 (Cq), 147.02 (CH, NC5H4-4-
À
CN), 150.93 (Cq), 158.72 ppm (Cq, B C=C); elemental analysis calcd (%)
for C74H52B2N4: C 87.23, H 5.14, N 5.50; found: C 86.90, H 5.31, N 5.03.
1,3,5-Tris[2,3,4,5-tetraphenylborole-1-(4-methylpyridine)]benzene
(3a):
1H NMR (500.1 MHz, CD2Cl2): d=2.21–2.27 (m, 9H; NC5H4-4-(CH3)),
6.73–6.92 (m, 66H; C6H5, NC5H4-4-Me), 7.32 (s, 3H; C6H3), 8.34–
8.37 ppm (m, 4H; NC5H4-4-Me); 11B NMR (160.4 MHz, CD2Cl2): d=
4.0 ppm; 13C NMR (125.7 MHz, CD2Cl2): d=21.52 (CH3, NC5H4-4-
(CH3)), 124.08, 125.13, 125.90, 127.10, 127.31, 128.69, 129.44, 131.00 (CH,
À
NC4H4-C-CN), 151.20 (Cq, C6H5), 158.35 ppm (Cq, B C=C); elemental
analysis calcd (%) for C40H29BN2 (548.50): C 87.59, H 5.33, N 5.11;
found: C 87.14, H 5.35, N 5.42.
À
C6H5), 134.95 (Cq, B C=C), 140.90, 144.35 (CH, C6H3), 145.79, 149.60
1,3-Bis[2,3,4,5-tetraphenylborole-1-(4-methylpyridine)]benzene
(2a):
À
(CH, NC5H4-4-Me), 152.83 ppm (Cq, B C=C); elemental analysis calcd
1H NMR (500.1 MHz, CD2Cl2): d=2.38 (s, 6H; NC5H4-4-(CH3)), 6.73–
6.75 (m, 8H; C6H5), 6.84–6.85 (m, 12H; C6H5), 6.90–6.91 (m, 16H;
C6H5), 6.93–6.96 (m, 4H; C6H5), 7.08–7.09 (m, 3H; C6H4), 7.12–7.13 (m,
4H; NC5H4-4-Me), 7.78–7.80 (m, 1H; C6H4), 8.47–8.48 ppm (m, 4H;
NC5H4-4-Me); 11B NMR (160.4 MHz, CD2Cl2): d=3.9 ppm; 13C NMR
(125.7 MHz, CD2Cl2): d=21.62 (CH3, NC5H4-4-(CH3)), 124.31, 125.00
(CH, C6H5), 125.40 (CH, NC5H4-4-Me), 126.33 (CH, C6H4), 126.66,
127.30, 127.45 (CH, C6H5), 129.39 (CH, C6H4), 130.39 (CH, C6H5), 130.84
(CH, C6H4), 137.96, 140.80 (Cq), 143.78 (CH, NC5H4-4-Me), 145.84,
(%) for C108H84B3N3: C 89.07, H 5.81, N 2.89; found: C 89.71, H 5.95, N
3.02.
1,3,5-Tris[2,3,4,5-tetraphenylborole-1-(3,5-dimethylpyridine)]benzene (3b):
1H NMR (500.1 MHz, CD2Cl2): d=1.70 (s, 18H; NC5H4-3,5-(CH3)2),
6.70–6.76 (m, 24H; C6H5), 6.81–6.91 (m, 27H; C6H5), 7.01–7.03 (m, 12H;
C6H5), 7.30 (s, 3H; C6H3), 8.00–8.02 ppm (m, 6H; NC5H4-3,5-(Me)2);
11B NMR (160.4 MHz, CD2Cl2): d=4.3 ppm; 13C NMR (125.7 MHz,
CD2Cl2): d=18.18 (CH3, NC5H4-3,5-(CH3)2), 124.24, 125.08, 126.87,
127.66, 129.18, 130.98 (CH, C6H5), 134.26, 134.73, 140.88 (Cq), 141.29,
À
150.08, 153.26 (Cq), 158.80 ppm (Cq, B C=C); elemental analysis calcd
À
144.23 (CH, NC5H4-3,5-(Me)2), 145.15, 149.25 (Cq), 152.83 ppm (Cq, B
C=C); elemental analysis calcd (%) for C111H90B3N3: C 88.98, H 6.05, N
(%) for C74H58B2N2: C 89.16, H 5.86, N 2.81; found: C 88.47, H 6.28, N
2.68.
2.80; found: C 88.32, H 6.38, N 2.74.
1,3-Bis[2,3,4,5-tetraphenylborole-1-(3,5-dimethylpyridine)]benzene (2b):
1H NMR (500.1 MHz, CD2Cl2): d=2.07 (s, 12H; NC5H3-3,5-(CH3)2),
6.76–6.79 (m, 8H; C6H5), 6.85–6.94 (m, 32H; C6H5), 7.03–7.05 (m, 2H;
C6H4), 7.09–7.12 (m, 1H; C6H4), 7.49–7.52 (m, 2H; NC5H3-3,5-(Me)2),
7.76–7.78 (m, 1H; C6H4), 8.19–8.21 ppm (m, 4H; NC5H3-3,5-(Me)2);
11B NMR (160.4 MHz, CD2Cl2): d=4.0 ppm; 13C NMR (125.7 MHz,
CD2Cl2): d=18.49 (CH3, NC5H3-3,5-(CH3)2), 124.34, 125.33 (CH, C6H5),
126.77 (CH, C6H4), 127.17, 127.53, 129.22 (CH, C6H5), 129.91 (CH,
C6H4), 130.85 (CH, C6H5), 135.31 (Cq), 137.42 (CH, C6H4), 140.73 (Cq),
141.92, 143.96 (CH, NC5H3-3,5-(Me)2), 144.29, 149.72 (Cq), 158.55 ppm
1,3-Bis[2,3,4,5-tetraphenylborole-1-(2,6-dimethylpyridine)]-5-(2,3,4,5-tetra-
phenylborole)benzene (3c): 1H NMR (500.1 MHz, CD2Cl2, 243 K): d=
1.42 (s, 6H; NC5H3-2,6-(CH3)2), 3.25 (s, 6H; NC5H3-2,6-(CH3)2), 6.15–
6.17 (m, 2H; C6H3), 6.24–6.26 (m, 4H; C6H5), 6.45–6.46 (m, 4H; C6H5),
6.51–6.53 (m, 2H; NC5H3-2,6-(Me)2), 6.60–6.62 (m, 4H; C6H5), 6.83–7.05
(m, 48H; C6H5), 7.29–7.30 (m, 2H; NC5H3-2,6-(Me)2), 7.53–7.56 (m, 2H;
NC5H3-2,6-(Me)2), 8.42 ppm (s, 1H; C6H3); 11B NMR (160.4 MHz,
CD2Cl2, 213 K): d=4.0 (Bq), 70 ppm (Btr); elemental analysis calcd (%)
for C104H81B3N2: C 89.79, H 5.87, N 2.01; found: C 89.38, H 6.21, N 1.97.
1,3,5-Tris[2,3,4,5-tetraphenylborole-1-(4-dimethylaminopyridine)]benzene
À
(Cq, B C=C); elemental analysis calcd (%) for C76H62B2N2: C 89.06, H
(3d): 1H NMR (500.1 MHz, CD2Cl2): d=2.79 (s, 18H; NC5H4-4-(N-
6.10, N 2.73; found: C 88.12, H 6.41, N 2.76.
AHCTNUERTGGUN(NN CH3)2)), 5.99–6.01 (m, 6H; NC5H4-4-(N(Me)2), 6.74–6.77 (m, 12H;
1,3-Bis[2,3,4,5-tetraphenylborole-1-(2,6-dimethylpyridine)]benzene (2c):
1H NMR (500.1 MHz, CD2Cl2, 213 K): d=1.32 (s, 6H; NC5H3-2,6-
(CH3)2), 3.31 (s, 6H; NC5H3-2,6-(CH3)2), 5.82–5.83 (m, 3H; C6H5), 6.42–
6.44 (m, 4H; C6H5), 6.75–7.06 (m, 42H; C6H5, C6H4, NC5H3-2,6-(Me)2),
7.39–7.41 (m, 2H; NC5H3-2,6-(Me)2), 8.20 ppm (s, 1H; C6H4); 11B NMR
(160.4 MHz, CD2Cl2, 213 K): d=3.6 ppm; 13C NMR (125.7 MHz, CD2Cl2,
213 K): d=23.40, 26.67 (CH3, NC5H3-2,6-(CH3)2), 119.90, 123.42, 123.53,
124.75, 124.85, 125.49, 126.36, 126.50, 126.72, 127.10, 127.71, 129.48 (CH),
129.82 (Cq), 130.58, 138.83 (CH), 139.80, 139.83, 140.18, 144.23, 145.30,
148.42, 149.64, 157.36, 159.32, 160.18, 162.69 ppm (Cq); elemental analysis
calcd (%) for C76H62B2N2: C 89.06, H 6.10, N 2.73; found: C 88.71, H
6.18, N 2.64.
C6H5), 6.81–6.87 (m, 36H; C6H5), 6.97–6.99 (m, 12H; C6H5), 7.44 (s, 3H;
C6H3), 8.06–8.07 ppm (m, 6H; NC5H4-4-(N(Me)2)); 11B NMR
(160.4 MHz, CD2Cl2): d=3.3 ppm; 13C NMR (125.7 MHz, CD2Cl2): d=
39.35 (CH3, NC5H4-4-(NACHTNUGTREN(NUG CH3)2)), 106.12 (CH, NC5H4-4-(N(Me)2)),
123.68, 124.85, 126.95, 127.22, 129.58, 131.29 (CH, C6H5), 134.84 (CH,
C6H3), 141.55, 145.12 (Cq), 145.86 (CH, NC5H4-4-(N(Me)2)), 148.92,
À
155.20 (Cq), 160.25 ppm (Cq, B C=C); elemental analysis calcd (%) for
C
111H93B3N6: C 86.38, H 6.07, N 5.45; found: C 86.24, H 6.26, N 5.52.
1,3,5-Tris[2,3,4,5-tetraphenylborole-1-(pyridine-4-carbonitrile)]benzene
1
(3e): H NMR (500.1 MHz, CD2Cl2): d=6.76–6.78 (m, 13H; C6H5), 6.83–
6.94 (m, 47H; C6H5), 7.28 (s, 3H; C6H3), 7.32–7.33 (m, 6H; NC5H4-4-
CN), 8.63–8.64 ppm (m, 6H; NC5H4-4-CN); 11B NMR (160.4 MHz,
CD2Cl2): d=5.4 ppm; 13C NMR (125.7 MHz, CD2Cl2): d=115.24 (Cq,
NC5H4-4-CN), 124.14 (Cq, NC5H4-4-CN), 124.74, 125.70, 127.37, 127.61
(CH, C6H5), 127.85 (CH, NC5H4-4-CN), 129.26, 130.68 (CH, C6H5),
134.89 (CH, C6H3), 139.98, 143.30 (Cq), 146.90 (CH, NC5H4-4-CN), 150.61
1,3-Bis[2,3,4,5-tetraphenylborole-1-(4-dimethylaminopyridine)]benzene
(2d): 1H NMR (500.1 MHz, CD2Cl2): d=2.94 (s, 12H,; NC5H4-4-(N-
ACHTUNGTRENNUNG(CH3)2)), 6.28–6.29 (m, 4H; NC5H4-4-(NCAHTUNGTRNE(NUGN CH3)2)), 6.78–6.80 (m, 8H;
C6H5), 6.83–6.87 (m, 12H; C6H5), 6.90–6.95 (m, 20H; C6H5), 7.06–7.14
(m, 3H, C6H4), 7.81–7.83 (m, 1H; C6H4), 8.12–8.14 ppm (m, 4H; NC5H4-
4-(N(Me)2)); 11B NMR (160.4 MHz, CD2Cl2): d=2.8 ppm; 13C NMR
À
(Cq), 159.21 ppm (Cq, B C=C); elemental analysis calcd (%) for
C
108H75B3N6: C 87.10, H 5.08, N 5.64; found: C 86.99, H 5.25, N 5.57.
(125.7 MHz, CD2Cl2): d=39.57 (CH3, NC5H4-4-(N
ACHTUGNTRENUN(NG CH3)2)), 106.42 (CH,
1,4-Bis[2,3,4,5-tetraphenylborole-1-(4-methylpyridine)]benzene (4a): Ele-
mental analysis calcd (%) for C74H58B2N2: C 89.16, H 5.86, N 2.81;
found: C 88.22, H 5.94, N 2.66.
NC5H4-4-(N(CH3)2)), 123.98, 125.19 (CH, C6H5), 126.38 (CH, C6H4),
127.21, 127.36, 129.36 (CH, C6H5), 130.19 (CH, C6H4), 131.01 (CH,
C6H5), 137.90 (CH, C6H4), 141.33, 144.45 (Cq), 145.77 (CH, NC5H4-4-
ACHTUNGTRENNUNG
À
(N(Me)2)), 149.39, 155.53 (Cq), 159.53 ppm (Cq, B C=C); elemental anal-
ysis calcd (%) for C76H64B2N4: C 86.53, H 6.12, N 5.31; found: C 85.89, H
6.28, N 5.35.
1,4-Bis[2,3,4,5-tetraphenylborole-1-(3,5-dimethylpyridine)]benzene (4b):
1H NMR (500.1 MHz, CD2Cl2): d=2.25 (s, 12H; NC5H3-3,5-(CH3)2),
6.71–6.73 (m, 8H; C6H5), 6.88–7.03 (m, 32H; C6H5), 7.23 (s, 3H; C6H4),
7.58–7.60 (m, 2H; NC5H3-3,5-(Me)2), 8.31–8.35 ppm (m, 4H; NC5H3-3,5-
(Me)2); 11B NMR (160.4 MHz, CD2Cl2): d=3.8 ppm; 13C NMR
(125.7 MHz, CD2Cl2): d=18.62 (CH3, NC5H4-3,5-(Me)2), 124.38, 125.47,
127.30, 127.48, 129.24, 130.72 (CH, C6H5), 132.89 (CH, C6H4), 135.64,
140.71 (Cq), 142.24, 143.96 (CH, NC5H4-3,5-(Me)2), 144.25, 149.76 (Cq),
1,3-Bis[2,3,4,5-tetraphenylborole-1-(pyridine-4-carbonitrile)]benzene (2e):
1H NMR (500.1 MHz, CD2Cl2): d=6.69–6.71 (m, 8H; C6H5), 6.85–6.99
(m, 32H; C6H5), 7.14–7.18 (m, 3H; C6H4), 7.53–7.54 (m, 4H; NC5H4-4-
CN), 7.61–7.63 (m, 1H; C6H4), 8.73–8.74 ppm (m, 4H; NC5H4-4-CN);
11B NMR (160.4 MHz, CD2Cl2): d=4.9 ppm; 13C NMR (125.7 MHz,
CD2Cl2): d=115.21 (Cq, NC5H4-4-CN), 124.55 (Cq, NC5H4-4-CN), 124.88,
125.81 (CH, C6H5), 127.39 (CH, C6H4), 127.47, 127.73 (CH, C6H5), 128.17
À
159.00 ppm (Cq, B C=C); elemental analysis calcd (%) for C76H62B2N2:
C 89.06, H 6.10, N 2.73; found: C 88.63, H 6.18, N 2.76.
Chem. Eur. J. 2012, 00, 0 – 0
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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H. Braunschweig et al.
1,4-Bis[2,3,4,5-tetraphenylborole-1-(2,6-dimethylpyridine)]benzene (4c):
Elemental analysis calcd (%) for C76H62B2N2: C 89.06, H 6.10, N 2.73;
found: C 88.42, H 6.13, N 2.65.
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(4d): Elemental analysis calcd (%) for C76H64B2N4: C 86.53, H 6.12, N
5.31; found: C 85.75, H 6.13, N 5.91.
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1,4-Bis[2,3,4,5-tetraphenylborole-1-(pyridine-4-carbonitrile)]benzene (4e):
Elemental analysis calcd (%) for C74H52B2N4: C 87.23, H 5.14, N 5.50;
found: C 87.57, H 5.27, N 5.87.
Trismagnesium[1,3,5-tris(2,3,4,5-tetraphenylborole)benzene]
(Mg3[3]):
THF (2 mL) was added to a mixture of solid 3 (50.0 mg, 42.5 mmol) and
magnesium turnings (6.00 mg, 0.26 mmol). The resulting suspension was
stirred for 16 h at ambient temperature. Subsequent removal of the
excess magnesium by filtration and layering the filtrate with hexane
(40 mL) yielded an orange precipitate, which was decanted and redis-
solved in THF. After slow evaporation of the solvent, Mg3[3] was ob-
tained as yellow crystals. The yield could not be determined due to insta-
bility under reduced pressure. 1H NMR (500.1 MHz, [D8]THF): d=6.35–
6.38 (m, 6H; C6H5), 6.69–6.70 (m, 8H; C6H5), 6.78–6.86 (m, 36H; C6H5),
7.04–7.06 (m, 10H; C6H5), 7.20 ppm (s, 3H; C6H3); 11B NMR
(160.4 MHz, [D8]THF): d=26 ppm; 13C NMR (125.7 MHz, [D8]THF):
À
d=112.83 (Cq, B C=C), 117.97, 121.87 (CH, C6H5), 123.53 (Cq), 126.70,
126.86, 130.62, 133.15 (CH, C6H5), 135.42 (CH, C6H3), 146.05 (Cq), 148.20
(Cq), 148.64 ppm (Cq).
Hexakislithium[1,3,5-tris(2,3,4,5-tetraphenylborole)benzene]
(Li6[3]):
THF (2 mL) was added to a mixture of solid 3 (100 mg, 85 mmol) and
lithium powder (3.5 mg, 0.50 mmol). The resulting suspension was stirred
for 16 h at ambient temperature. Subsequent removal of the excess lithi-
um by filtration and layering with hexane (40 mL) yielded a dark-red
precipitate, which was decanted and redissolved in THF. After slow evap-
oration of the solvent, Li6[3] was obtained as yellow crystals. The yield
could not be determined due to instability under reduced pressure.
1H NMR (500.1 MHz, [D8]THF): d=6.47–6.50 (m, 6H; C6H5), 6.73–6.79
(m, 20H, C6H5), 6.83–6.87 (m, 24H; C6H5), 6.99–7.01 (m, 10H; C6H5),
7.02–7.06 ppm (m, 3H; C6H3); 11B NMR (160.4 MHz, [D8]THF): d=
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25 ppm; 13C NMR (125.7 MHz, [D8]THF): d=115.66 (Cq, B C=C),
À
117.62 (Cq), 119.71, 122.97 (CH, C6H5), 126.48, 127.12, 131.83, 133.30
(CH, C6H5), 138.07 (CH, C6H3), 143.80 (Cq), 146.60 (Cq), 147.29 ppm
(Cq).
Acknowledgements
The authors are grateful for financial support from the DFG. This study
was performed within the framework of the Graduate College GRK
1221.
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OligoACHTUNGTRENNUNG(borolyl)benzenes—Synthesis and Properties
FULL PAPER
[10] Due to solubility problems that result from an increasing number of
borolyl substituents on the benzene ring, the effective concentra-
tions are somewhat lower than specified. Hence, the extinction coef-
ficients listed in Table 1 are not in full agreement with the spectral
data.
[11] M. C. Haberecht, J. B. Heilmann, A. Hagniri, M. Bolte, J. W. Bats,
H. W. Lerner, M. C. Holthausen, M. Wagner, Z. Anorg. Allg. Chem.
2004, 630, 904–913.
Received: July 2, 2012
Published online: && &&, 0000
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H. Braunschweig et al.
A bunch of boroles: Two and three
borolyl units were attached to a ben-
zene ring (see figure). The adduct
chemistry of these oligoboroles was
found to be strongly dependent on the
electronic and steric properties of the
pyridine base that was employed. The
degree of Lewis acidity of the oligo-
boroles was examined by base-transfer
reactions.
Boroles
H. Braunschweig,* C.-W. Chiu,
A. Damme, B. Engels, D. Gamon,
C. Hçrl, T. Kupfer, I. Krummenacher,
K. Radacki, C. Walter ....... &&&& — &&&&
Oligo
ACHTUNGTRENNUNG(borolyl)benzenes—Synthesis
and Properties
OligoACTHNUGRTENUNG(borolyl)benzenes
Up to three borolyl groups have been installed on a
benzene ring, yielding bis- and trisboroles. The adduct
formation of these oligoboroles with a variety of pyridine
bases is described. In addition, the interplay of the highly
Lewis acidic moieties in the meta- and para-substituted
bisboroles is addressed by base-transfer reactions. For
more details, see Full Paper by H. Braunschweig et al. on
p. ff.
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ÝÝ
These are not the final page numbers!