Lewis acid-base adducts of 1-mesityl- and 1-chloro-2,3,4,5- tetraphenylborole

Article abstract of DOI:10.1002/ejic.201201383

Electron-deficient borole compounds exhibit a pronounced Lewis acidity that is enhanced due to their antiaromatic character so that even weak donors datively coordinate to form Lewis acid-base adducts. This contribution presents the synthesis and structur

Full text of DOI:10.1002/ejic.201201383

FULL PAPER
DOI:10.1002/ejic.201201383
Lewis Acid–Base Adducts of 1-Mesityl- and 1-Chloro-
2,3,4,5-tetraphenylborole
Holger Braunschweig,*[a] Ching-Wen Chiu,[b] Daniela Gamon,[a]
Katrin Gruß,[a] Christian Hörl,[a] Thomas Kupfer,[a]
Krzysztof Radacki,[a] and Johannes Wahler[a]
Keywords: Lewis acids / Lewis bases / Boroles / Conjugation / Antiaromaticity
Electron-deficient borole compounds exhibit a pronounced
Lewis acidity that is enhanced due to their antiaromatic char-
acter so that even weak donors datively coordinate to form
Lewis acid–base adducts. This contribution presents the syn-
thesis and structural characterization of Lewis acid–base ad-
ducts formed by the reaction of 1-mesityl-2,3,4,5-tetraphen-
ylborole and 4-picoline as well as 1-chloro-2,3,4,5-tetraphen-
ylborole with various donors. The new compounds are char-
acterized by means of multinuclear NMR spectroscopy and
single-crystal X-ray diffraction techniques and compared to
related systems.
tiaromatic π conjugation induces a pronounced Lewis acid-
ity so that even weak Lewis bases such as CO,[19] ethers,[1,12]
nitriles,[1] as well as stronger donors like pyridine deriva-
tives,[4,11,12,16] N-heterocyclic carbenes (NHC),[7,12] or phos-
phanes[12] readily coordinate to the boron center. As a re-
sult, π conjugation within the five-membered ring is inter-
rupted, thus leading to enhanced stability of the Lewis
acid–base adducts. Such Lewis pairs are by no means unre-
active but provide the basis for further reactivity studies.
The pentaphenylborole–2,6-lutidine adduct [PhBC4Ph4(2,6-
Me2C5H3N)] (2), for example, shows remarkable thermo-
and photochromic behavior.[20] Furthermore, the chloro-
borole–SIMes adduct [ClBC4Ph4(SIMes)] (3) [SIMes =
N,NЈ-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene] serves
as a precursor to a highly unusual NHC-stabilized π-boryl
anion.[7] The latter exemplifies the interesting reduction
properties of boroles to produce aromatic dianions with six
π electrons.[5,24] The reduction sequence of free boroles in-
volves radical anionic intermediates with five π electrons,
as recently shown.[9,14] In this contribution, we report the
synthesis and characterization of a Lewis acid–base adduct
of 1-mesityl-2,3,4,5-tetraphenylborole with 4-picoline and,
in addition, a series of different Lewis adducts of 1-chloro-
2,3,4,5-tetraphenylborole.
Introduction
Interest in borole chemistry arises from the antiaromatic
character of the cyclic conjugated boracycle that hosts four
π electrons.[1] Starting in 1969,[2] when 1,2,3,4,5-pentaphen-
ylborole (1) was isolated as the first of its kind, and increas-
ing since its structural analysis in 2008,[3] a growing number
of publications emphasizes the significance of this area.[4–23]
Challenges to synthesis, structural and photophysical char-
acterization of boroles, as well as their rich and often unex-
pected reactivity can be ascribed to the inherent antiarom-
atic destabilization of boroles.[1–25] Typical reaction types
that cause disruption to the antiaromatic π system include
cycloaddition reactions[8,12,21,25] and the activation of small
molecules such as H2.[22] The essential strategy to isolate
a borole molecule is steric and electronic shielding of the
backbone in the C4B annulus. This has hitherto been
achieved by substitution with aryl or thienyl groups.[1,6,15]
The substituent at the boron atom is variable over a broad
range
from
halogen,[4,6,12]
amino,[4]
and
aryl
groups[2,5,6,14,15] to transition metals[10,23] and cyclopen-
tadienyl moieties in transition-metal complexes.[3,11,16,17]
Variation of the substituent at the boron center significantly
affects the HOMO–LUMO transition, thereby resulting in
different photophysical properties.[1] The vacant pz orbital
of the tricoordinate boron center in combination with an-
Results and Discussion
[a] Institut für Anorganische Chemie,
Julius-Maximilians-Universität Würzburg,
Am Hubland, 97074 Würzburg, Germany
Fax: +49-931-31-84623
The reduction chemistry of 1-mesityl-2,3,4,5-tetraphen-
ylborole (4) has recently been studied in detail by taking
advantage of a sterically shielded boron center.[14] That
steric bulk around the reactive centers in Lewis acid–base
pairs can lead to frustrated Lewis pairs (FLPs) has also been
shown by many examples.[26] The combination of 1 and 2,6-
E-mail: h.braunschweig@uni-wuerzburg.de
Homepage: http://www-anorganik.chemie.uni-wuerzburg.de/
Braunschweig/index.html
[b] Department of Chemistry, National Taiwan University,
Taipei, Taiwan 10617, Taiwan
Eur. J. Inorg. Chem. 2013, 1525–1530
1525
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org
FULL PAPER
lutidine results in an equilibrium between the Lewis acid–
base adduct (2) and the unreacted species at ambient tem-
perature.[20] This behavior is somewhat related to FLPs.
More interestingly, 2 shows a migration of the Lewis base
from the boron atom to the adjacent carbon atom (com-
pound 5, Scheme 1) upon irradiation with UV light at
–50 °C, which is thermally reversible. With this background
in mind, we targeted the combination of 4 and 4-picoline,
which should result in a similar spatial configuration
around the boron center in the product 6, and therefore a
similar reactivity is anticipated. In addition, 4-picoline (pKb
= 8.0)[27] is less basic than 2,6-lutidine (pKb = 7.3)[27] as
deduced from experimental and theoretically predicted pKb
values in aqueous solution, which should promote FLP be-
havior over Lewis adduct formation.
Figure 1. Molecular structure of 6 in the solid state with hydrogen
atoms and thermal ellipsoids of peripheral atoms omitted for clar-
ity. Thermal ellipsoids are set at 50% probability.
and 11B NMR spectroscopy as well as the characteristic
dark green color of the solution that is indicative of free
4.[14] A similar base-transfer experiment has been used to
demonstrate the enhanced Lewis acidity in bis(borolyl)fer-
rocene [Fe(CpBC4Ph4)2] (Cp = η5-C5H4) relative to the
monofunctionalized derivative [(CpH)Fe(CpBC4Ph4)].[16]
Thus, we have to state that 4 and 4-picoline form no FLP
(but a classical adduct) despite the fact that they constitute
the weaker Lewis acid and the weaker Lewis base relative
to 1 and 2,6-lutidine.
The second part of this work concerns the synthesis of
Lewis adducts formed by 1-chloro-2,3,4,5-tetraphenyl-
borole and a series of Lewis bases (Scheme 2). Coordina-
tion of ethers to haloboranes often involves cleavage of a
C–O bond owing to the highly reactive nature of the Lewis
acid. Contrary to our expectations, we were able to isolate
the moderately stable Lewis acid–base adduct
[BrBC4Ph4(thf)] (8) by reaction of 1-bromo-2,3,4,5-tetra-
phenylborole (9) with thf, which undergoes thf cleavage in
solution only at slightly elevated temperatures.[12] The reac-
tion of 1-chloro-2,3,4,5-tetraphenylborole (10) with thf
gives the analogous compound [ClBC4Ph4(thf)] (11), which
was obtained in a yield of 53% as a yellow solid. The 11B
NMR resonance was detected at δ = 12.4 ppm, which is
similar to that of 7 (δ = 10.3 ppm). Under an inert gas, 11
shows no signs of decomposition or ether cleavage reactions
in the solid state and in solution. The solid-state structure
of 11 (Figure 2) obtained by single-crystal X-ray diffraction
shows bond parameters very similar to those of 8. The B1–
Scheme 1. Reactivity of 1 towards 2,6-lutidine and of 4 towards 4-
picoline as well as Lewis base transfer from 6 to 1.
Surprisingly, the reaction of 4 with 4-picoline entails an
immediate color change from dark green to yellow upon
addition of the Lewis base to a solution of 4 at ambient
temperature. This indicates the irreversible formation of the
Lewis acid–base adduct [MesBC4Ph4(4-MeC5H4N)] (6)
rather than an FLP. Thus, the 11B NMR spectroscopic reso-
nance of 6 was observed at δ = 4.5 ppm in the typical range
of four-coordinate boron. The solid-state structure was con-
firmed by single-crystal X-ray diffraction (Figure 1). The
B1–N1 distance of 6 [1.635(2) Å] is considerably shorter
than that of 2 [1.657(3) Å],[20] thus indicating a stronger
dative interaction between donor and acceptor in complex
6 than in 2. The B1–C5Ј separations of 6 [1.639(2) Å] and
2 [1.635(3) Å] are very similar.
Unexpectedly, the different behavior of 2 and 6 correlates
neither with the basicity of 2,6-lutidine (pKb = 7.3) and 4-
picoline (pKb = 8.0) nor with the Lewis acidity of the un-
complexed boroles 1 and 4 (see below).[27] It was shown
earlier that the reaction of 1 and 4-picoline gives the Lewis
acid–base adduct [PhBC4Ph4(4-MeC5H4N)] (7) without dif-
ficulty.[20] As expected, mixtures of 4 and 2,6-lutidine show
the unaltered presence of both compounds in solution ac-
cording to 1H and 11B NMR spectroscopy. Likewise, no
dative interaction is observed between 4 and weak donors
such as diethyl ether or thf, whereas 1 is readily complexed
by the latter, which indicates a stronger relative Lewis acid-
ity of 1 than of 4.[1] This assumption was unequivocally
confirmed by a Lewis base transfer experiment. Addition
of 1 equiv. of 1 to a solution of 6 in CD2Cl2 results in a
complete transfer of 4-picoline to 1 as deduced from H variety of Lewis bases.
Scheme 2. Synthesis of Lewis adducts of different boroles with a
1
Eur. J. Inorg. Chem. 2013, 1525–1530
1526
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org
FULL PAPER
O1 atom separations [11: 1.573(4) Å; 8: 1.572(2) Å] are reduction is presumed to play a role in the course of the
equal within the limits of measurement. As previously fast reaction.
shown, the aromatic system K(thf)2[ClBC4Ph4] (12) is ob-
However, when a stronger σ donor such as an N-hetero-
tained by reduction of 10 with potassium graphite in thf as cyclic carbene (NHC) is coordinated to 10, as in
a solvent. Although the isolated yields of 12 are low, it is [ClBC4Ph4(SIMes)] (3), cleavage of the B–Cl bond and for-
worth noting that the boron–halogen bond remains intact, mation of an NHC-stabilized π-boryl anion can be achieved
which is uncommon for haloborane species under such re- under the same reducing conditions.[7] Given the impor-
ducing conditions. Accordingly, formation of 11 prior to tance of 1-haloborole–carbene adducts for subsequent
transformations, we sought to broaden the scope of such
species and prepared an exemplary series of carbene ad-
ducts. Although the synthesis of 3 has been reported ear-
lier,[7a] the solid-state structure is hitherto unknown and is
reported here.
Reaction of 10 with N,NЈ-bis(2,4,6-trimethylphenylimid-
azol-2-ylidene (IMes) or N-(2,6-diisopropylphenyl)-2,2,4,4-
tetramethylpyrrolidine-5-ylidene (Caac) proceeds readily as
indicated by a rapid color change from deep purple to yel-
low when the donor is added to solutions of 10. The Lewis
pairs [ClBC4Ph4(IMes)] (13) and [ClBC4Ph4(Caac)] (14)
show typical 11B NMR spectroscopic chemical shifts at
lower frequencies (13: δ = –3.0 ppm, 14: δ = –1.9 ppm) rela-
tive to 10 (δ = 66.4 ppm). The 11B resonances reflect those
of the analogous bromo derivatives [BrBC4Ph4(SIMes)] (15:
δ = –6.2 ppm) and [BrBC4Ph4(Caac)] (16: δ = –4.8 ppm).
Analysis of the solid-state structures of 3, 13, and 14 was
carried out by means of single-crystal X-ray diffraction.
The structural data (Table 1) are unobtrusive and compare
well with the structures of 15 and 16. The longest B1–
Ccarbene distance was found in 3 [1.649(2) Å], whereas the
shortest value was found in 14 [1.634(3) Å]; this follows the
trend previously observed between 15 [1.655(3) Å] and 16
[1.628(2) Å]. This bonding situation reflects the stronger σ-
Table 1. 11B NMR spectroscopic chemical shifts [ppm], bond
lengths [Å], and angles [°] of 3, 6, 11, 13, and 14.
6
11
13
3
14
12.4
δ(11B)
4.5
–3.0
–3.3
–1.9
B1–C1
B1–C4
1.645(2) 1.600(4) 1.626(3) 1.628(2) 1.647(2)
1.629(2) 1.613(4) 1.630(3) 1.631(2) 1.643(2)
B1–Cl1
B1–C5Ј
–
1.866(3) 1.909(2) 1.903(2) 1.898(2)
1.639(2)
–
–
–
–
C1–C2
C2–C3
1.358(2) 1.347(4) 1.355(3) 1.354(2) 1.355(3)
1.501(2) 1.515(4) 1.494(3) 1.491(2) 1.489(2)
1.353(2) 1.341(4) 1.343(3) 1.350(2) 1.362(2)
1.635(2) 1.573(4) 1.637(3) 1.649(2) 1.634(3)
C3–C4
B1–E[a]
C1–B1–C4
B1–C1–C2
C1–C2–C3
C2–C3–C4
C3–C4–B1
C1–B1–Cl1
C4–B1–Cl1
C1–B1–C5
C4–B1–C5
99.7(2)
102.3(2) 100.5(2) 100.8(2) 99.2(1)
108.2(2) 107.4(2) 107.7(2) 107.6(2) 108.2(2)
111.7(2) 111.6(2) 111.7(2) 111.8(2) 112.2(2)
111.7(2) 111.5(2) 112.3(2) 112.5(2) 111.7(2)
108.9(2) 107.3(2) 107.7(2) 107.2(2) 108.4(2)
–
–
111.6(2) 108.0(2) 107.9(1) 107.9(1)
115.1(2) 106.4(2) 106.6(1) 108.4(1)
121.6(2)
111.2(2)
–
–
–
–
–
–
–
–
C1–B1–E[a] 102.6(1) 110.7(2) 116.4(2) 116.8(2) 109.2(2)
C4–B1–E[a] 108.9(2) 112.5(2) 115.4(2) 113.4(2) 118.0(2)
Cl1–B1–E[a]
–
104.8(2) 109.3(2) 110.5(1) 113.0(1)
C5Ј–B1–E[a] 111.7(2)
–
–
–
–
Figure 2. Molecular structures of 11, 13, 3, and 13 in the solid state
with hydrogen atoms and thermal ellipsoids of peripheral atoms
omitted for clarity. Thermal ellipsoids are set at 50% probability.
[a] E defines the respective donor atom of the Lewis base (6: N1;
11: O1; 3, 13, 14: C5).
Eur. J. Inorg. Chem. 2013, 1525–1530
1527
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org
FULL PAPER
Synthesis of 7: In a J. Young NMR spectroscopy tube, 4-picoline
(6.3 mg, 6.75 μmol) was added to a solution of 1 (30.0 mg,
6.75 μmol) in CH2Cl2 (0.6 mL), thus resulting in an immediate
color change of the solution from dark blue to yellow. After slow
evaporation of the solvent, the solid was washed with hexane and
dried under vacuum to yield 7 (30.9 mg, 5.75 μmol, 85%) as a yel-
donor strength of the Caac than the used NHC do-
nors.[28,29]
Conclusion
1
low solid. H NMR (500 MHz, CD2Cl2): δ = 2.45 (s, 3 H, 4-CH3
We have analyzed a series of Lewis acid–base adducts of
boroles with different electron-pair donors. We showed that
the relative Lewis acidity of [PhBC4Ph4] (1) is significantly
increased over that of [MesBC4Ph4] (4). This was performed
by a Lewis base transfer experiment from complex
[MesBC4Ph4(4-MeC5H4N)] (6) to free borole 1, which
proved to be a spontaneous and quantitative reaction. In
addition, the haloborole [ClBC4Ph4] (10) forms an unusual
thermally stable Lewis adduct with thf and various carb-
enes, which were analyzed by means of single-crystal X-ray
diffraction. The latter might serve as useful precursors for
novel π-boryl anion derivatives potentially formed by two-
electron reduction.
Pic), 6.65–6.67 (m, 4 H, C6H5), 6.84–6.90 (m, 6 H, C6H5), 6.95–
7.03 (m, 10 H, C6H5), 7.15–7.22 (m, 3 H, C6H5), 7.28–7.29 (m, 2
H, CH Pic), 7.40–7.42 (m, 2 H, C6H5), 8.54–8.55 (m, 2 H, CH
Pic) ppm. 11B NMR (160 MHz, CD2Cl2): δ = 3.5 ppm. 13C NMR
(126 MHz, CD2Cl2): δ = 21.71 (CH3), 124.55, 125.67, 126.69,
127.46, 127.52, 127.63, 129.19, 130.67, 133.71, 145.83 (CH), 140.58,
143.35, 150.44, 154.34, 158.49 (Cq) ppm. UV/Vis (CH2Cl2): λmax
(ε) = 355 (8340 Lmol–1 cm–1) nm. C40H32BN (537.51): calcd. C
89.38, H 6.00, N 2.61; found C 89.93, H 6.40, N 2.87.
Lewis Base Transfer from 6 to 1: In a J. Young NMR spectroscopy
tube, 1 (16.5 mg, 37.1 μmol) was added in one portion to a solution
of 6 (21.5 mg, 37.1 μmol) in CD2Cl2 (0.6 mL). The initially yellow
solution turned dark blue upon addition of 1 and within 5 min
1
changed to dark green. H and 11B NMR spectra measured after
50 min show the characteristic sets of signals of 7 and 4 indicative
of a quantitative transfer of 4-picoline from 6 to 1.
Experimental Section
Synthesis of 11: A solution of BCl3 (4.5 mL, 2.08 m, 1.10 g,
9.36 mmol) in hexane was added dropwise to a cooled (–45 °C)
solution of Me2SnC4Ph4 (0.96 g, 1.90 mmol) in CH2Cl2 (6 mL)
within 5 min. A dark purple solution was formed immediately. The
reaction mixture was stirred at –20 °C for 3.5 h and at 0 °C for 1 h.
All volatiles were removed under vacuum at 0 °C, and the residue
was dissolved in CH2Cl2 (4 mL) at –20 °C. After the addition of
thf (0.3 mL), an immediate color change from purple to orange
was observed. The mixture was stirred at ambient temperature for
10 min before the solvents and the Me2SnCl2 byproduct were re-
moved by sublimation (35 °C, 1ϫ10–3 mbar). The orange-colored
residue was dissolved in thf (3 mL), and hexane (2.5 mL) was added
after filtration to obtain a yellow solid that was washed with hexane
(3ϫ 2 mL) and dried under vacuum. [ClBC4Ph4(thf)] (11) (0.48 g,
1.01 mmol, 53%) was obtained as a pale yellow solid. Single crys-
tals suitable for X-ray diffraction were obtained by diffusion of
General Considerations: All syntheses were carried out under argon
with standard Schlenk and glovebox techniques. 1-Mesityl-2,3,4,5-
tetraphenylborole,[14] 1-chloro-2,3,4,5-tetraphenylborole,[4] 1,1-di-
methyl-2,3,4,5-tetraphenylstannole, 1,2,3,4,5-tetraphenylborole,[1]
IMes, SIMes,[30] and Caac[28] were prepared according to published
procedures. 4-Picoline was dried with CaH2 and distilled under ar-
gon. Hexane, benzene, diethyl ether, and tetrahydrofuran (thf) were
dried by distillation from Na/K alloy under argon and stored over
molecular sieves. Likewise, dichloromethane (CH2Cl2) was dried by
distillation from P2O5. CDCl3 was dried with CaH2, distilled under
argon, and stored over molecular sieves. CD2Cl2 was degassed with
three freeze–pump–thaw cycles and stored over molecular sieves.
Elemental analyses were obtained with an Elementar Vario
MICRO cube instrument. NMR spectra were recorded with a
Bruker Avance 500 NMR spectrometer (500 MHz for 1H, 160 MHz
for 11B, 126 MHz for 13C{1H}). Chemical shifts are given in ppm
and are referenced against external Me4Si (1H, 13C) and [BF3·Et2O]
(11B).
1
hexane into a solution of 11 in thf. H NMR (500 MHz, CD2Cl2):
δ = 1.89–1.94 (m, 4 H, CH2), 4.30–4.32 (m, 4 H, CH2), 6.85–6.87
(m, 4 H, C6H5), 7.02–7.08 (m, 8 H, C6H5), 7.12–7.15 (m, 4 H,
C6H5), 7.20–7.22 (m, 4 H, C6H5) ppm. 11B NMR (160 MHz,
CD2Cl2): δ = 12.4 ppm. 13C{1H} NMR (126 MHz, CD2Cl2): δ =
25.33, 74.74, (CH2), 125.64, 126.38, 127.71, 128.04, 129.40, 130.23
(CH), 138.89, 141.23, 144.17 (br.), 151.71 (Cq) ppm. C32H28BClO
(474.84): calcd. C 80.94, H 5.94; found C 81.09, H 6.02.
Synthesis of 6: A solution of 4-picoline (22.0 mg, 236 μmol) in hex-
ane (2 mL) was added dropwise to a cooled (–40 °C) suspension of
4 (131 mg, 232 μmol) in hexane (2 mL), thus resulting in the forma-
tion of a yellow suspension. The mixture was warmed to ambient
temperature and stirred for 1 h. The solid was filtered, washed with
hexane (2ϫ 1 mL), and recrystallized from thf/hexane (1:1) at
–30 °C. [MesBC4Ph4(4-pic)] (6) (73.0 mg, 126 μmol, 54%) was ob-
tained as a pale yellow solid. Single crystals suitable for X-ray dif-
Synthesis of 13: Benzene (20 mL) was added to a mixture of 10
(400 mg, 0.99 mmol) and IMes (302 mg, 0.99 mmol) at ambient
temperature. After stirring for 20 min, the solvent was removed un-
fraction were obtained by diffusion of hexane into a solution of 6
1
in thf. H NMR (500 MHz, CDCl3): δ = 1.05 (s, 3 H, o-CH3 Mes), der vacuum to give a brown solid, which was washed with hexane
2.30 (s, 3 H, p-CH3 Mes), 2.31 (s, 3 H, 4-CH3 Pic), 2.81 (s, 3 H, o- (20 mL) and extracted with diethyl ether (100 mL). The diethyl
CH3 Mes), 6.58 (br. s, 1 H, m-CH Mes), 6.72–6.73 (m, 4 H, C6H5),
6.82–6.97 (m, 17 H, C6H5 and m-CH Mes), 7.00–7.02 (m, 2 H, CH
Pic), 8.53–8.54 (m, 2 H, CH Pic) ppm. 11B NMR (160 MHz,
CDCl3): δ = 4.5 ppm. 13C{1H} NMR (126 MHz, CDCl3): δ =
21.02, 21.42, 23.76, 24.38 (CH3), 124.23, 125.25, 125.89, 127.04,
127.09, 129.07, 129.28, 129.65, 130.17, 143.56 (CH), 134.19, 140.01,
141.36 (br.), 142.73, 143.66, 143.86, 149.60, 152.52, 158.57 (br., Cq)
ppm. UV/Vis (CH2Cl2): λ (ε) = 327 (br. sh, 8000 Lmol–1 cm–1) nm.
C43H38BN (579.59): calcd. C 89.11, H 6.61, N 2.42; found C 88.71,
H 6.79, N 2.39.
ether solution was dried under vacuum to give [ClBC4Ph4(IMes)]
(13) as a yellow solid (351 mg, 0.50 mmol, 51%). Single crystals
suitable for X-ray diffraction were obtained by diffusion of hexane
1
into a solution of 13 in CH2Cl2. H NMR (500 MHz, CD2Cl2): δ
= 1.78 (s, 12 H, o-CH3 Mes), 2.42 (s, 6 H, p-CH3 Mes), 6.56–
6.60 (m, 4 H, CH Mes), 6.89–6.95 (m, 14 H, C6H5), 6.96 (s, 2 H,
CHimidazoline), 7.11–7.14 (m, 6 H, C6H5) ppm. 11B NMR (160 MHz,
CD2Cl2): δ = –3.0 ppm. 13C{1H} NMR (126 MHz, CD2Cl2): δ =
18.66, 21.23 (CH3), 124.77, 124.87, 125.30, 126.97, 127.22, 129.31,
130.50, 130.57 (CH), 134.68, 136.12, 139.82, 140.96, 142.50, 151.10
Eur. J. Inorg. Chem. 2013, 1525–1530
1528
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org
FULL PAPER
(Cq) ppm. C49H44BClN2 (707.16): calcd. C 83.22, H 6.27, N 3.96;
found C 82.81, H 6.11, N 3.24.
11.9493(6) Å, b = 14.6325(9) Å, c = 22.0119(13) Å; α = 90°, β =
91.340(3)°, γ = 90°; V = 3847.7(4) Å3; Z = 4; ρcalcd. = 1.221gcm–3;
μ = 0.137 mm–1; F(000) = 1496; T = 173(2) K; R1 = 0.0694, wR2
= 0.0993; 8218 independent reflections (2θ Յ 53.64°) and 484 pa-
rameters.
Synthesis of 14: Benzene (10 mL) was added to a mixture of 10
(200 mg, 0.50 mmol) and N-(2,6-diisopropylphenyl)-2,2,4,4-tetra-
methylpyrrolidine-5-ylidene (141 mg, 0.50 mmol) at ambient tem-
perature. After stirring for 20 min, the solvent was removed under
vacuum to give a brown solid, which was washed with hexane
(10 mL) and extracted with diethyl ether (50 mL). The diethyl ether
solution was dried under vacuum to give [ClBC4Ph4(Caac)] (14) as
a yellow solid (161 mg, 0.23 mmol, 46%). Single crystals suitable
for X-ray diffraction were obtained by diffusion of hexane into a
Crystal Data for 14: C48H51BClN; Mr = 688.16; yellow block,
0.316ϫ0.213ϫ0.178 mm; monoclinic, space group P21/n; a =
9.712(9) Å, b = 18.521(16) Å, c = 21.201(18) Å; α = 90°, β =
96.53(3)°, γ = 90°; V = 3789(6) Å3; Z = 4; ρcalcd. = 1.206 gcm–3; μ
= 0.136 mm–1; F(000) = 1472; T = 273(2) K; R1 = 0.0410, wR2 =
0.1066; 7504 independent reflections (2θ Յ 52.3°) and 468 param-
eters.
1
solution of 14 in CH2Cl2. H NMR (500 MHz, CD2Cl2): δ = 0.64
3
3
(d, J = 6.45 Hz, 6 H, CH3 dipp), 1.04 (d, J = 6.50 Hz, 6 H, CH3
dipp), 1.25 (s, 6 H, CH3), 1.89, (s, 6 H, CH3), 2.28 (s, 2 H, CH2),
2.38–2.46 (m, 2 H, CH iPr), 6.63–6.65 (m, 4 H, C6H5), 6.84–6.89
(m, 6 H, C6H5), 6.99–7.08 (m, 6 H, C6H5 and CH dipp), 7.10–7.15
(m, 6 H, C6H5), 7.29–7.32 (m, 1 H, CH dipp) ppm. 11B NMR
(160 MHz, CD2Cl2): δ = –1.9 ppm. 13C{1H} NMR (126 MHz,
CD2Cl2): δ = 23.83, 26.84, 29.87, 30.22 (CH3), 51.13 (CH2), 29.51,
124.90, 125.00, 125.28, 126.92, 127.12, 129.11, 130.40, 130.83 (CH),
55.32, 80.53, 134.43, 140.13, 144.37, 146.00, 152.25 (Cq) ppm.
C48H51BClN (688.20): calcd. C 83.77, H 7.47, N 2.04; found C
84.26, H 7.49, N 2.08.
Acknowledgments
We are grateful to the German Science Foundation (DFG) for fin-
ancial support.
[1] J. J. Eisch, J. E. Galle, S. Kozima, J. Am. Chem. Soc. 1986, 108,
379–385.
Crystal Structure Data: The crystal data of 3, 11, 13, and 14 were
collected with a Bruker X8-APEX II diffractometer with a CCD
area detector and multilayer-mirror-monochromated Mo-Kα radia-
tion. The crystal data of 6 were collected with a Bruker APEX
diffractometer with CCD area detector and graphite-monochro-
mated Mo-Kα radiation. The structures were solved by direct meth-
ods, refined with the SHELX software package[31] and expanded
by using Fourier techniques. All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were assigned to idealized geomet-
ric positions and included in the structure factor calculations.
CCDC-908226 (3), -908227 (6), -908228 (11), -908229 (13), and
-908230 (14) 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.
[2] J. J. Eisch, N. K. Hota, S. Kozima, J. Am. Chem. Soc. 1969, 91,
4575–4577.
[3] H. Braunschweig, I. Fernández, G. Frenking, T. Kupfer, An-
gew. Chem. 2008, 120, 1977–1980; Angew. Chem. Int. Ed. 2008,
47, 1951–1954.
[4] H. Braunschweig, T. Kupfer, Chem. Commun. 2008, 4487–
4489.
[5] C.-W. So, D. Watanabe, A. Wakamiya, S. Yamaguchi, Organo-
metallics 2008, 27, 3496–3501.
[6] C. Fan, W. E. Piers, M. Parvez, Angew. Chem. 2009, 121, 2999–
3002; Angew. Chem. Int. Ed. 2009, 48, 2955–2958.
[7] a) H. Braunschweig, C.-W. Chiu, K. Radacki, T. Kupfer, An-
gew. Chem. 2010, 122, 2085–2088; Angew. Chem. Int. Ed. 2010,
49, 2041–2044; b) H. Braunschweig, C.-W. Chiu, T. Kupfer, K.
Radacki, Inorg. Chem. 2011, 50, 4247–4249.
[8] H. Braunschweig, C.-W. Chiu, J. Wahler, K. Radacki, T. Kup-
fer, Chem. Eur. J. 2010, 16, 12229–12233.
Crystal Data for 3: C49H46BClN2; Mr = 709.14; yellow block, [9] H. Braunschweig, F. Breher, C.-W. Chiu, D. Gamon, D. Nied,
K. Radacki, Angew. Chem. 2010, 122, 9159–9162; Angew.
Chem. Int. Ed. 2010, 49, 8975–8978.
[10] H. Braunschweig, C.-W. Chiu, K. Radacki, P. Brenner, Chem.
Commun. 2010, 46, 916–918.
[11] H. Braunschweig, A. Damme, D. Gamon, T. Kupfer, K. Rad-
acki, Inorg. Chem. 2011, 50, 4250–4252.
[12] H. Braunschweig, C.-W. Chiu, A. Damme, K. Ferkinghoff, K.
Kraft, K. Radacki, J. Wahler, Organometallics 2011, 30, 3210–
3216.
0.20ϫ0.20ϫ0.10 mm; monoclinic, space group P21/n;
a =
11.972(2) Å, b = 14.729(3) Å, c = 22.019(5) Å; α = 90°, β =
91.166(10)°, γ = 90°; V = 3882.2(14) Å3; Z = 4, ρcalcd.
=
1.213 gcm–3; μ = 0.136 mm–1; F(000) = 1504; T = 100(2) K; R1 =
0.0476, wR2 = 0.1078; 7701 independent reflections (2θ Յ 52.28°)
and 484 parameters.
Crystal Data for 6: C43H38BN; Mr = 579.55; yellow needles,
¯
0.19ϫ0.07ϫ0.06 mm; hexagonal, space group R3;
a
=
[13] H. Braunschweig, T. Kupfer, Chem. Commun. 2011, 47, 10903–
41.2393(15) Å, b = 41.2393(15) Å, c = 11.6474(5) Å; α = 90°, β =
90°, γ = 120°; V = 17154.7(11) Å3; Z = 18; ρcalcd. = 1.010 gcm–3; μ
= 0.057 mm–1; F(000) = 5544; T = 100(2) K; R1 = 0.0708, wR2 =
0.1095; 7828 independent reflections (2θ Յ 52.84°) and 410 param-
eters.
10914.
[14] H. Braunschweig, V. Dyakonov, J. O. C. Jiminez-Halla, K.
Kraft, I. Krummenacher, K. Radacki, A. Sperlich, J. Wahler,
Angew. Chem. 2012, 124, 3031–3034; Angew. Chem. Int. Ed.
2012, 51, 2977–2980.
[15] T. Araki, A. Fukazawa, S. Yamaguchi, Angew. Chem. 2012,
124, 5580–5583; Angew. Chem. Int. Ed. 2012, 51, 5484–5487.
[16] H. Braunschweig, C.-W. Chiu, D. Gamon, M. Kaupp, I.
Krummenacher, T. Kupfer, R. Müller, K. Radacki, Chem. Eur.
J. 2012, 18, 11732–11746.
[17] H. Braunschweig, A. Damme, D. Gamon, H. Kelch, I. Krum-
menacher, T. Kupfer, K. Radacki, Chem. Eur. J. 2012, 18, 8430–
8436.
Crystal Data for 11: C32H28BClO; Mr = 474.80; yellow block,
0.14ϫ0.16ϫ0.33 mm; orthorhombic, space group Pca2(1); a =
10.9923(14) Å, b = 11.8853(15) Å, c = 19.727(3) Å; α = 90°, β =
90°, γ = 90°; V = 2577.3(6) Å3; Z = 4; ρcalcd. = 1.224 gcm–3; μ =
0.171 mm–1; F(000) = 1000; T = 173(2) K; R1 = 0.0717, wR2
=
0.1485; 6486 independent reflections (2θ Յ 56.92°) and 316 param-
eters.
[18] H. Braunschweig, C.-W. Chiu, A. Damme, B. Engels, D. Ga-
mon, C. Hörl, T. Kupfer, I. Krummenacher, K. Radacki, C.
Walter, Chem. Eur. J. 2012, 18, 14292–14304.
Crystal Data for 13: C49H44BClN2; Mr = 707.12; colorless block,
0.20ϫ0.19ϫ0.18 mm; monoclinic, space group P21/n;
a =
Eur. J. Inorg. Chem. 2013, 1525–1530
1529
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org
FULL PAPER
[19] A. Fukazawa, J. Dutton, C. Fan, L. G. Mercier, A. Y. Hough-
ton, Q. Wu, W. E. Piers, M. Parvez, Chem. Sci. 2012, 3, 1814–
1818.
[20] K. Ansorg, H. Braunschweig, C.-W. Chiu, B. Engels, D. Ga-
mon, M. Hügel, T. Kupfer, K. Radacki, Angew. Chem. 2011,
123, 2885–2888; Angew. Chem. Int. Ed. 2011, 50, 2833–2836.
[21] C. Fan, W. E. Piers, M. Parvez, R. McDonald, Organometallics
2010, 29, 5132–5139.
[22] C. Fan, L. G. Mercier, W. E. Piers, M. Parvez, J. Am. Chem.
Soc. 2010, 132, 9604–9606.
[23] H. Braunschweig, C. Hörl, F. Hupp, K. Radacki, J. Wahler,
Organometallics 2012, 31, 8463–8466.
[26]
[27]
[28]
S. W. Stephan, G. Erker, Angew. Chem. 2010, 122, 50–81; An-
gew. Chem. Int. Ed. 2010, 49, 46–76.
G. I. Hawe, I. Alkorta, P. L. A. Popelier, J. Chem. Inf. Model.
2010, 50, 87–96.
V. Lavallo, Y. Canac, C. Präsang, B. Donnadieu, G. Bertrand,
Angew. Chem. 2005, 117, 5851–5855; Angew. Chem. Int. Ed.
2005, 44, 5705–5709.
[29] T. Dröge, F. Glorius, Angew. Chem. 2010, 122, 7094–7107; An-
gew. Chem. Int. Ed. 2010, 49, 6940–6952.
[30] A. J. Arduengo III, R. Krafczyk, R. Schmutzler, H. A. Craig,
J. R. Goerlich, W. J. Marshall, M. Unverzagt, Tetrahedron
1999, 55, 14523–14534.
[31] G. Sheldrick, Acta Crystallogr., Sect. A 2008, 64, 112–122.
Received: November 16, 2012
[24] G. E. Herberich, B. Buller, B. Hessner, W. Oschmann, J. Or-
ganomet. Chem. 1980, 195, 253–259.
[25] J. J. Eisch, J. E. Galle, B. Shafii, A. L. Rheingold, Organometal-
lics 1990, 9, 2342–2349.
Published Online: January 29, 2013
Eur. J. Inorg. Chem. 2013, 1525–1530
1530
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim