Borole-derived spirocyclic tetraorganoborate

Article abstract of DOI:10.1021/om301070q

Preparation of a novel conjugated tetraorganoborate is presented in a facile two-step procedure. Successful utilization as a halide abstraction reagent is demonstrated in a metathesis reaction with the platinum(II) boryl complex [(Cy₃P)₂Pt(Br)(BC₄Ph₄)], which was obtained by oxidative addition of 1-bromo-2,3,4,5-tetraphenylborole to a platinum(0) species. The novel compounds were investigated by means of spectroscopic and X-ray diffraction techniques.

Full text of DOI:10.1021/om301070q

Note
pubs.acs.org/Organometallics
Borole-Derived Spirocyclic Tetraorganoborate
Holger Braunschweig,* Christian Horl, Florian Hupp, Krzysztof Radacki, and Johannes Wahler
̈
Institut fur Anorganische Chemie, Julius-Maximilians-Universitat Wurzburg, Am Hubland, 97074 Wurzburg, Germany
̈
̈
̈
̈
S
* Supporting Information
ABSTRACT: Preparation of a novel conjugated tetraorganoborate is presented in a facile
two-step procedure. Successful utilization as a halide abstraction reagent is demonstrated in a
metathesis reaction with the platinum(II) boryl complex [(Cy3P)2Pt(Br)(BC4Ph4)], which
was obtained by oxidative addition of 1-bromo-2,3,4,5-tetraphenylborole to a platinum(0)
species. The novel compounds were investigated by means of spectroscopic and X-ray
diffraction techniques.
elimination reaction of [BF3·Et2O] with two equivalents of 5
yields lithium-1,2,3,4,6,7,8,9-octaphenyl-5-boraspiro[4.4]nona-
1,3,6,8-tetraenide (4) as an isolable compound (Scheme 1).
INTRODUCTION
■
Steric protection of the central structural motif is crucial for the
synthesis of boroles, a class of highly reactive compounds that
show a rich chemistry driven by inherent antiaromatic
destabilization.1 Recently, we isolated 1-bromo-2,3,4,5-tetra-
phenylborole (1)2 and 1-chloro-2,3,4,5-tetraphenylborole
(2),3,4 which serve as useful precursors for other borole
derivatives and borole-related chemistry (Chart 1).5 Examples
Scheme 1. Synthesis of 4
Chart 1. 1-Haloboroles 1−3, Perfluorinated Spiroborate 6,
and Spiro[4.4]nonatetraene (7)
The latter constitutes a tetraorganocycloborate where the
central boron atom is located in the spiro position connecting
two 1,2,3,4-tetraphenyl-1,3-butadiene moieties. The related
neutral group 14 derivatives with Si, Ge, Sn, and Pb in the
spiro position have been reported earlier.9 After optimization of
the reaction conditions, 4 was obtained in yields of 58% as a
bright yellow solid containing three equivalents of Et2O
for the versatile reactivity of the B−C bond in 2 include
substitution reactions,3,5 halide abstraction,3 oxidative addition
to platinum(0),6 and reduction processes.4,7 However, all
efforts to extend this series by 1-fluoro-2,3,4,5-tetraphenylbor-
ole (3) via the common synthetic methods (tin−boron
exchange reaction, salt elimination)8 met with no success so
far. In this contribution we present the synthesis of a
perphenylated spirocyclic tetraorganoborate (4) obtained
from the reaction of [BF3·Et2O] with 1,2,3,4-tetraphenyl-1,4-
dilithio-1,3-butadiene (5). The latter is readily accessible by
reductive dimerization of diphenylacetylene using lithium
metal.8 In addition, the facile application of 4 as a halide
abstraction reagent is demonstrated.
1
deduced from elemental analysis and H NMR spectroscopy.
A similar reaction has been reported between perfluorinated
2,2′-dilithio-1,1′-biphenyl and BCl3 to form the analogue
spirocyclic borafluorene derivative 6 (Chart 1), which is
considered as an activator for zirconocene-catalyzed ethylene
polymerization.10 The 11B NMR resonance of 4 was found at δ
= −1.4 ppm thus, in the range of sp3-hybridized boron in
tetrahedral geometry. However, the resonance is significantly
shifted to higher frequency by about 10 to 20 ppm compared to
organocycloborate derivatives comprising a saturated organic
backbone11 or the acyclic tetravinylborate (Li[B(CHCH2)4],
δ = −16.1 ppm).12 This is presumably a consequence of spiro-
RESULTS AND DISCUSSION
■
Reaction of excess [BF3·Et2O] with 5 at low temperature (−78
°C) gives a complex mixture of products. However, salt
Received: November 8, 2012
Published: November 28, 2012
© 2012 American Chemical Society
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Note
conjugation effects, which are discussed for the neutral
isoelectronic spiro[4.4]nonatetraene (7).13
Scheme 2. Synthesis of the Platinum-Substituted Boroles 8
and 10[B(C4Ph4)2]
Single crystals suitable for X-ray diffraction were obtained by
diffusion of diethyl ether into a solution of 4 in thf. The solid-
state analysis reveals a separated ion pair with the lithium cation
being coordinated by four thf molecules (Figure 1). The
Figure 1. Molecular structure of 4(thf)4 in the solid state with
additional solvent molecule (thf) and hydrogen atoms omitted for
clarity. Thermal ellipsoids are set at 50% probability. Selected bond
lengths [Å] and angles [deg]: B1−C1 1.627(2), B1−C4 1.637(2),
B1−C5 1.635(2), B1−C8 1.623(2), C1−C2 1.368(2), C2−C3
1.481(2), C3−C4 1.365(2), C5−C6 1.365(2), C6−C7 1.480(2),
C7−C8 1.363(2); C1−B1−C4 100.1(2), C1−B1−C5 112.2(2), C1−
B1−C8 115.3(2), C4−B1−C5 114.3(2), C4−B1−C8 116.6(2), C5−
B1−C8 99.1(2), B1−C1−C2 107.9(2), C1−C2−C3 112.0(2), C2−
C3−C4 111.8(2), C3−C4−B1 108.0(2).
anionic moiety shows an idealized D2d symmetry, and the fused
five-membered rings are essentially planar with an interplanar
torsion angle of 89.7(1)°. The B−C bond distances (1.623(2)
to 1.637(2) Å) as well as the C−C single (1.480(2) to 1.481(2)
Å) and CC double bonds (1.363(2) to 1.368(2) Å) of the
spirocycle are comparable to those found in Lewis acid−base
adducts of boroles with various Lewis bases.2a,3,7,14
Hence, the structure resembles that of a conjugated
butadiene moiety similar to E,E-1,2,3,4-tetraphenyl-1,3-buta-
diene (C−C: 1.483(2) Å; CC: 1.353(2) Å).15 The anionic
boron center is well-shielded, and in principle, charge
delocalization should be possible over the entire conjugated
system.
As a suitable target to probe a potential applicability of 4 in
organometallic synthesis, we chose the platinum(II) boryl
complex (8) obtained by oxidative addition of 1 to [Pt(PCy3)2]
(Cy = cyclohexyl). As reported earlier, halide abstraction from
the chloro derivative [(Cy3P)2Pt(Cl)(BC4Ph4)] (9) affords the
cationic T-shaped complex 10[B(ArF)4] (ArF = C6H3-3,5-
(CF3)2) when Na[B(ArF)4] is used in a metathesis reaction.6,16
The oxidative addition of the B−Br bond of 1 to [Pt(PCy3)2]
proceeds readily at ambient temperature, as indicated by a rapid
color change from deep purple to dark red (8: λmax = 487 nm)
upon addition of [Pt(PCy3)2] to a solution of 1 (Scheme 2).
After extraction of the crude product, 8 was obtained as red
crystals suitable for X-ray diffraction. The 11B NMR resonance
was found at δ = 93.4 ppm as a broad signal in the expected
range for a borole system in η1-coordination to platinum(II).6
The 31P{1H} NMR resonance at δ = 34.6 ppm (1JPt−P = 2942
Hz) is in agreement with a planar trans-Pt(II) boryl complex.17
Structural analysis substantiates the observed spectroscopic
findings, and the structural parameters are very similar to those
found for 9 (Figure 2).6
Figure 2. Molecular structure of 8 in the solid state with hydrogen
atoms omitted for clarity. Thermal ellipsoids are set at 50% probability.
Selected bond lengths [Å] and angles [deg]: Pt1−P1 2.351(1), Pt1−
P2 2.356(1), Pt1−Br 2.640(1), Pt1−B1 1.996(3), B1−C1 1.624(4),
B1−C4 1.620(4), C1−C2 1.361(4), C2−C3 1.525(4), C3−C4
1.350(4), B1−Pt1−Br1 174.7(1), P1−Pt1−P2 172.2(1).
solubility of both compounds (Scheme 2). The product
10[B(C4Ph4)2] precipitated from solution along with LiBr.
After filtration and extraction, the product was recrystallized
from CH2Cl2/hexane at −30 °C to give a reddish-brown solid
of 10[B(C4Ph4)2] in 85% yield. According to 1H NMR
spectroscopy and elemental analysis the product includes 1.5
equivalents of CH2Cl2. The 11B NMR resonance of the borole
unit was detected at δ = 63.9 ppm as an extremely broad signal,
whereas the 11B NMR signal of the anion (δ = −1.7 ppm)
remains almost unaltered compared to 4. The 31P{1H} NMR
resonance detected at δ = 53.5 ppm (1JPt−P = 2848 Hz) is
consistent with formation of the cationic T-shaped 14-electron
complex.6,16 The lowest energy electronic absorption was
observed at λmax = 521 nm to be almost identical to that found
for 10[B(ArF)4] (λmax = 524 nm).6 Hence, substitution of the
counteranion has no striking effect on the absorption properties
of the cationic moiety. This can be judged as an indication of
similar coordinating properties between the novel anion and
[B(ArF)4]− in the present case. The absorption bands of the
anion are also very similar in 4 (λmax = 356 nm) and
10[B(C4Ph4)2] (λmax = 358 nm).
Slow diffusion of hexane into a solution of 10[B(C4Ph4)2] in
CH2Cl2 yielded single crystals suitable for X-ray diffraction
(Figure 3). As expected from the spectroscopic data, the ion
pair is well-separated in the solid state. The cationic T-shaped
fragment shows a nearly planar coordination of platinum as well
The metathesis reaction of 4 with 8 was conveniently
accomplished in benzene solution supported by the good
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Note
EXPERIMENTAL SECTION
■
General Considerations. All syntheses were carried out under an
argon atmosphere with standard Schlenk and glovebox techniques. 1-
Bromo-2,3,4,5-tetraphenylborole (1),2a 1,4-dilithio-1,2,3,4-tetraphenyl-
buta-1,3-diene containing 0.5 equivalent of diethyl ether (5),8 and
[Pt(PCy3)2]18 were prepared according to published procedures.
[BF3·Et2O] was freshly distilled prior to use. Hexane, benzene,
tetrahydrofuran (thf), and diethyl ether were dried by distillation over
sodium, and dichloromethane (dcm) was dried by distillation over
P2O5 under argon and stored over molecular sieves. C6D6 and CD2Cl2
were degassed by three freeze−pump−thaw cycles and stored over
molecular sieves. NMR spectra were recorded on a Bruker Avance 500
NMR spectrometer (500 MHz for 1H, 160 MHz for 11B, 202 MHz for
31P{1H}, 194 MHz for 7Li, 126 MHz for 13C{1H}) at 296 K. Chemical
shifts (δ) are given in ppm and are referenced against external Me4Si
(1H, 13C), [BF3·Et2O] (11B), 85% H3PO4 (31P), and 1 M LiCl (7Li).
Synthesis of [Li(Et2O)3][B(C4Ph4)2] (4). A solution of [BF3·Et2O]
(257 mg, 1.81 mmol) in benzene (3 mL) was added dropwise to a
solution of 5 (1.00 g, 2.45 mmol) in benzene (15 mL) within 5 min at
RT. The mixture was stirred for 22 h, resulting in formation of a
colorless precipitate and a brown solution. After filtration, all volatiles
were removed under vacuum. Et2O (3 mL) was added to give a dark
brown oil that started to crystallize within minutes. The yellow solid
(682 mg, 716 mmol, 58%) was filtered, washed with Et2O (3 × 2 mL),
and dried under vacuum. Single crystals of [Li(thf)4][B(C4Ph4)2]
(4(thf)4) suitable for X-ray diffraction were obtained by diffusion of
Et2O into a solution of 4 in thf. 4 shows a good solubility in benzene
Figure 3. Molecular structure of 10[B(C4Ph4)2] in the solid state with
additional solvent molecules (CH2Cl2) and hydrogen atoms omitted
for clarity. Thermal ellipsoids are set at 50% probability. Selected bond
lengths [Å] and angles [deg]: B1−C1 1.628(5), B1−C4 1.621(5),
B1−C5 1.629(5), B1−C8 1.628(5), C1−C2 1.365(4), C2−C3
1.474(4), C3−C4 1.355(4), C5−C6 1.369(5), C6−C7 1.478(4),
C7−C8 1.355(4), Pt1−B2 1.953(3), Pt1−P1 2.328(1), Pt1−P2
2.330(1), B2−C9 1.584(4), B2−C12 1.578(5), C9−C10 1.352(4),
C10−C11 1.518(4), C11−C12 1.353(4); C1−B1−C4 99.4(3), C1−
B1−C5 123.5(3), C1−B1−C8 108.4(3), C4−B1−C5 112.1(3), C4−
B1−C8 114.5(3), C5−B1−C8 99.6(3), P1−Pt1−P2 166.4(1), B2−
Pt1−P1 95.1(1), B2−Pt1−P2 98.5(1).
1
and thf as well as a poor solubility in Et2O. H NMR (500 MHz,
C6D6): δ 6.87−6.92 (m, 8H, C6H5), 6.96−6.99 (m, 8H, C6H5), 7.04−
7.11 (m, 16H, C6H5), 7.51−7.52 (m, 8H, C6H5). 11B NMR (160 MHz,
C6D6): δ −1.4. 13C{1H} NMR (126 MHz, C6D6): δ 125.67, 126.05,
127.91, 128.35, 129.37, 130.56 (CH), 140.53, 143.42, 152.49, 156.99
(br) (C). 7Li NMR (194 MHz, C6D6): δ −1.8. λmax (ε) = 356 nm (34
490 L mol−1 cm−1; measured in Et2O). Anal. Calcd (%) for
C68H70BLiO3: C 85.70; H 7.40. Found: C 85.41; H 7.09.
Synthesis of [(Cy3P)2Pt(Br)(BC4Ph4)] (8). A solution of [Pt-
(PCy3)2] (30.0 mg, 39.7 μmol) in benzene (0.4 mL) was added
dropwise to a stirred solution of 1 (20.0 mg, 44.7 μmol) in benzene
(0.3 mL) at RT, resulting in an immediate color change from deep
purple to dark red. After 10 min the solvent was removed under
vacuum, and the residue was extracted with hexane (4 × 0.3 mL).
Within 5 h red crystals (suitable for X-ray diffraction) of 8 (33.2 mg,
24.9 μmol, 63%) containing 1.5 equivalents of hexane were obtained
from the solution. The product was washed with hexane (0.3 mL) and
as a planar borole ring with bond lengths and angles similar to
those found in 10[B(ArF)4].6 The anionic moiety of 10[B-
(C4Ph4)2] displays significant distortion of the central
spirocyclic framework in comparison to 4. This is indicated
by a decreased interplanar torsion angle of 82.7(2)° between
the fused C4B rings, resulting in C−B−C angles ranging from
108.4(3)° to 123.5(3)° in 10[B(C4Ph4)2] compared with
112.2(2)° to 116.6(2)° in 4. The bond distances show no
unusual deviation. Considering the spectroscopic findings in
solution, this structural change is most likely caused by crystal-
packing effects.
1
dried under vacuum. H NMR (500 MHz, C6D6): δ 1.19−1.38 (m,
18H, Cy), 1.64−1.77 (m, 31H, Cy), 2.18 (br s, 11H, Cy), 2.79 (br s,
6H, Cy), 6.91−7.00 (m, 12H, C6H5), 7.12−7.15 (m, 4H, C6H5), 7.87−
7.89 (m, 4H, C6H5). 11B NMR (160 MHz, C6D6): δ 93.4 (br).
13C{1H} NMR (126 MHz, C6D6): δ 26.85, 28.14 (m), 31.13 (m)
(CH2), 37.86 (m), 125.80, 126.83, 127.45, 127.90, 130.00, 132.37
(CH), 138.36, 141.45, 142.77, 154.86 (C). 31P{1H} NMR (202 MHz,
C6D6): δ 34.6 (s, 1JP−Pt = 2942 Hz). λmax (ε) = 487 nm (1407 L mol−1
cm−1; measured in CH2Cl2). Anal. Calcd (%) for C73H107BBrP2Pt: C
65.81; H 8.09. Found: C 65.90; H 8.09.
CONCLUSION
■
In conclusion, we have reported a two-step synthesis protocol
and concomitant characterization of the novel borate salt 4
consisting of a bulky perphenylated spirocyclic tetraorganobo-
rate. In particular, it was demonstrated that 4 can be effectively
employed as a halide abstraction reagent. According to this,
reaction with the platinum boryl complex 8 results in formation
of the cationic T-shaped derivative 10[B(C4Ph4)2] in high
yield. The structural and spectroscopic parallels of 10[B-
(C4Ph4)2] and 10[B(ArF)4] give a first indication of possible
applications. The facile access and good solubility of 4 as well as
the tendency of its derivatives to form single crystals of good
quality are considered to be advantageous in a variety of
reactions. Further research will include investigations of the
scope of 4 in related reactions and catalysis
Synthesis of [(Cy3P)2Pt(BC4Ph4)][B(C4Ph4)2] (10[B(C4Ph4)2]). A
solution of 4 (15.1 mg, 15.8 μmol) in benzene (0.5 mL) was added
dropwise to a stirred solution of 8 (20.0 mg, 15.5 μmol) in benzene
(0.5 mL) at RT, resulting in an immediate color change from dark red
to reddish-brown. After 1 h the mixture was cooled to −30 °C, and
after warming to RT a reddish solid precipitated. The solid was
filtered, washed with benzene (3 × 0.3 mL), and dried under vacuum.
After extraction with dcm (0.5 mL), hexane (0.5 mL) was diffused into
the solution at −30 °C to yield 10[B(C4Ph4)2] (26.0 mg, 13.2 μmol,
85%) as a reddish-brown solid containing 1.5 equivalents of dcm.
Single crystals suitable for X-ray diffraction were obtained by the same
1
method. H NMR (500 MHz, CD2Cl2): δ 1.19−1.42 (m, 30H, Cy),
1.79−1.87 (m, 18H, Cy), 2.00−2.02 (m, 12H, Cy), 2.19−2.24 (m, 6H,
Cy), 6.66−6.68 (m, 4H, C6H5), 6.74−7.78 (m, 4H, C6H5), 6.87−6.96
(m, 20H, C6H5), 6.99−7.07 (m, 20H, C6H5), 7.15−7.25 (m, 12H,
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Note
C6H5). 11B NMR (160 MHz, CD2Cl2): δ −1.7, 63.9 (br). 13C{1H}
2011, 50, 2833−2836. (b) Braunschweig, H.; Damme, A.; Gamon, D.;
Kupfer, T.; Radacki, K. Inorg. Chem. 2011, 50, 4250−4252.
(c) Fukazawa, A.; Dutton, J.; Fan, C.; Mercier, L. G.; Houghton, A.
Y.; Wu, Q.; Piers, W. E.; Parvez, M. Chem. Sci. 2012, 3, 1814−1818.
4
NMR (126 MHz, CD2Cl2): δ 26.13, 27.42 (vt, N = |2JC−P + JC−P| =
3
11.6 Hz), 30.97 (CH2), 35.27 (vt, N = |1JC−P + JC−P| = 27.0 Hz),
122.73, 124.36, 126.64, 127.22, 127.26, 128.12, 128.26, 128.55, 129.39,
129.52, 130.19, 130.94 (CH), 135.56, 136.37 (br), 138.87, 144.14,
145.64, 149.32, 158.18, 161.15 (br) (C). 31P{1H} NMR (202 MHz,
́
(15) Lopez-Serrano, J.; Duckett, S. B.; Dunne, J. P.; Godard, C.;
Whitwood, A. C. Dalton Trans. 2008, 26, 4270−4281.
1
CD2Cl2): δ 53.5 (s, JP−Pt = 2848 Hz). λmax (ε) = 521 nm (831 L
(16) (a) Braunschweig, H.; Radacki, K.; Rais, D.; Scheschkewitz, D.
Angew. Chem., Int. Ed. 2005, 44, 5651−5654. (b) Braunschweig, H.;
Radacki, K.; Uttinger, K. Chem.Eur. J. 2008, 14, 7858−7866.
mol−1 cm−1), 358 nm (27 620 L mol−1 cm−1; measured in CH2Cl2).
Anal. Calcd (%) for C121.5H129B2Cl3P2Pt: C 73.91, H 6.59. Found: C
73.88, H 6.68.
(17) Braunschweig, H.; Brenner, P.; Muller, A.; Radacki, K.; Rais, D.;
̈
Uttinger, K. Chem.Eur. J. 2007, 13, 7171−7176.
ASSOCIATED CONTENT
* Supporting Information
(18) Yoshida, T.; Otsuka, S. Inorg. Synth. 1979, 19, 101−107.
■
S
Experimental details including X-ray crystallographic data (CIF
files) and UV/vis spectra of 4, 8, and 10[B(C4Ph4)2]. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Fax: (+49) 931-31-84623. E-mail: h.braunschweig@uni-
wuerzburg.de.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful to the German Science Foundation (DFG) for
financial support.
■
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