An estimate of the reduction potential of B(C6F 5)3 from electrochemical measurements on related mesityl boranes

Article abstract of DOI:10.1021/om050003q

MesB(C₆F₅)₂ (1) has been prepared from MesMgBr and FB(C₆F₅)₂·OEt₂, while Mes₂B(C₆F₅) (2) is readily available from CuC₆F₅ and Mes₂BBr. The reduction potential E^? of 1 vs Cp₂Fe^0/+ in THF is -1.72 V, while that of 2 is -2.10 V, and that of Mes₃B (3) is -2.73 V. ¹¹B and ¹H NMR show that neither 1 nor 2 binds THF significantly. These results have been used to estimate the reduction potential of B(C₆F₅)₃ in THF as -1.17 V vs Cp ₂Fe^0/+ or as -0.64 V vs SCE.

Full text of DOI:10.1021/om050003q

Organometallics 2006, 25, 1565-1568
1565
An Estimate of the Reduction Potential of B(C6F5)3 from
Electrochemical Measurements on Related Mesityl Boranes
Sarah A. Cummings,† Masanori Iimura,† C. Jeff Harlan,† Rebecca J. Kwaan,†
Isabelle Vu Trieu,† Jack R. Norton,*,† Brian M. Bridgewater,† Frieder Ja¨kle,*,‡
Anand Sundararaman,‡ and Mats Tilset§
Department of Chemistry, Columbia UniVersity, New York, New York 10027, Department of Chemistry,
Rutgers UniVersity-Newark, 73 Warren Street, Newark, New Jersey 07102, and Department of Chemistry,
UniVersity of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway
ReceiVed January 3, 2005
MesB(C6F5)2 (1) has been prepared from MesMgBr and FB(C6F5)2‚OEt2, while Mes2B(C6F5) (2) is
readily available from CuC6F5 and Mes2BBr. The reduction potential E° of 1 vs Cp2Fe0/+ in THF is
1
-1.72 V, while that of 2 is -2.10 V, and that of Mes3B (3) is -2.73 V. 11B and H NMR show that
neither 1 nor 2 binds THF significantly. These results have been used to estimate the reduction potential
of B(C6F5)3 in THF as -1.17 V vs Cp2Fe0/+ or as -0.64 V vs SCE.
obtained when traces of water form the highly acidic6 adduct
Introduction
H2OfB(C6F5)3.7
B(C6F5)3 has found wide application in the activation of
catalysts for olefin polymerization.1-3 It is known to be a
powerful acceptor for lone-pair donors,4 but there is relatively
little information on its ability to serve as a one-electron
acceptor. Some of us have reported the partial one-electron
oxidation of an azazirconacycle in the presence of B(C6F5)3 (eq
1).5
In neither case was the anion radical of B(C6F5)3 observed,
although some of us later prepared it by reducing B(C6F5)3 with
decamethylcobaltocene in THF (eq 3).8
Cp*2Co
•-
B(C6F5)3 THF, -50 °C8 B(C6F5)3
(3)
The reduction potential of B(C6F5)3 is therefore of interest,
but direct measurement has proven impossible. Little or no
signal is observed when we attempt a CV of B(C6F5)3,5,9
apparently because the radical anion becomes absorbed on
electrode surfaces.
Similar problems have been encountered in the electrochem-
istry of other triaryl boranes and have been solved by introducing
mesityl substituents; even a single mesityl generally provides
enough steric hindrance to preclude absorption of the radical
anion.10 We have therefore prepared the previously unreported
MesB(C6F5)2 (1) and Mes2B(C6F5) (2), examined the electro-
chemistry of 1, 2, and Mes3B (3), and used the results to estimate
the reduction potential of B(C6F5)3.
Green and co-workers have reported that B(C6F5)3 does serve
as a one-electron oxidant in eq 2 and that other products are
* To whom correspondence should be addressed. E-mail: jrn11@
columbia.edu; fjaekle@andromeda.rutgers.edu.
† Columbia University.
‡ Rutgers University-Newark.
§ University of Oslo.
(1) For a review of the chemistry of B(C6F5)3 see: (a) Piers, W. E.;
Chivers, T. Chem. Soc. ReV. 1997, 26, 345-354. (b) Piers, W. E. AdV.
Organomet. Chem. 2005, 52, 1-76.
(2) For a review of such cocatalysts for olefin polymerization see: Chen,
E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391-1434.
(3) For a review of the activation of butadiene complexes for olefin
polymerization by B(C6F5)3 see: Erker, G. Chem. Commun. 2003, 1469-
1476. Recent examples can be found in: Strauch, J. W.; Faure´, J.-L.;
Bredeau, S.; Wang, C.; Kehr, G.; Fro¨hlich, R.; Luftmann, H.; Erker, G. J.
Am. Chem. Soc. 2004, 126, 2089-2104.
(4) For leading references see: (a) Bradley, D. C.; Harding, I. S.; Keefe,
A. D.; Motevalli, M.; Zheng, D. H. J. Chem. Soc., Dalton Trans. 1996,
3931-3936. (b) Do¨ring, S.; Erker, G.; Fro¨hlich, R.; Meyer, O.; Bergander,
K. Organometallics 1998, 17, 2183-2187. (c) Jacobsen, H.; Berke, H.;
Do¨ring, S.; Kehr, G.; Erker, G.; Fro¨hlich, R.; Meyer, O. Organometallics
1999, 18, 1724-1735. (d) Barrado, G.; Doerrer, L.; Green, M. L. H.; Leech,
M. A. J. Chem. Soc., Dalton Trans. 1999, 1061-1066. (e) Beckett, M. A.;
Brassington, D. S.; Coles, S. J.; Hursthouse, M. B. Inorg. Chem. Commun.
2000, 3, 530-533. (f) Blackwell, J. M.; Piers, W. E.; Parvez, M.; McDonald,
R. Organometallics 2002, 21, 1400-1407. (g) Fraenk, W.; Klapo¨tke, T.
M.; Krumm, B.; Mayer, P.; Piotrowski, H.; Vogt, M. Z. Anorg. Allg. Chem.
2002, 628, 745-750. (h) Erker, G. Dalton Trans. 2005, 11, 1883-1890.
(5) Harlan, C. J.; Hascall, T.; Fujita, E.; Norton, J. R. J. Am. Chem. Soc.
1999, 121, 7274-7275.
Results and Discussion
The relatively unhindered MesB(C6F5)2 (1) was prepared
straightforwardly (eq 4) from MesMgBr and FB(C6F5)2‚OEt2.
(6) Bergquist, C.; Bridgewater, B. M.; Harlan, C. J.; Norton, J. R.;
Friesner, R. A.; Parkin, G. J. Am. Chem. Soc. 2000, 122, 10581-10590.
(7) Beddows, C. J.; Burrows, A. D.; Connelly, N. G.; Green, M.; Lynam,
J. M.; Paget, T. J. Organometallics 2001, 20, 231-233.
(8) Kwaan, R. J.; Harlan, C. J.; Norton, J. R. Organometallics 2001, 20,
3818-3820.
(9) For B(C6F5)3 no current is observed between 1.2 and -2.8 V vs SCE
with a glassy carbon electrode in THF; ill-shaped curves were observed
with 0.1 N [Bu4N]ClO4 and a Pt electrode in THF and in CH2Cl2 (Fujita,
E., personal communication).
(10) Schulz, A.; Kaim, W. Chem. Ber. 1989, 122, 1863-1868.
10.1021/om050003q CCC: $33.50 © 2006 American Chemical Society
Publication on Web 03/03/2006

1566 Organometallics, Vol. 25, No. 7, 2006
Cummings et al.
The intermediate Mes2BF is commercially available (Aldrich),
but steric hindrance made its reaction with nucleophiles difficult,
and the thermal instability of organometallic pentafluorophenyls
M-C6F5 (e.g., LiC6F5, BrMgC6F5) made it impossible to run
reaction 5 at elevated temperatures.
Mes2BF + M-C6F5 N Mes2B(C6F5) + M-F
(M ) Li, BrMg, Me3Si, Me3Sn)
(5)
As two of us had previously found CuC6F5 useful for
replacing Br with C6F5 on sterically congested boranes,12 we
tried the same reagent with Mes2BBr (eq 6). The reaction proved
straightforward and gave good yields of 2.
Mes2BBr + CuC6F5 f Mes2B(C6F5) + CuBr
(6)
An alternative boron starting material was Mes2BI, reported
by Power and co-workers as a byproduct in the preparation of
[MesPI]2 by reactions 7 and 8.13 Indeed, 2 was the major product
when Mes2BI was treated with AgC6F5 in DMF (eq 9).
Mes2BF + MesPH2 BuLi8 Mes2BPH(Mes) BuLi8
Mes2BP(Mes)Li (7)
Figure 1. Thermal ellipsoid representation of MesB(C6F5)2 (1).
See Table 1 for selected bond lengths and angles and Table 2 for
crystal data, data collection, and refinement parameters.
I2
9
Mes2BP(Mes)Li
8 Mes2BI + [Mes(I)P]2
(8)
(9)
Table 1. Selected Bond Lengths (Å) and Angles (deg) for 1
Mes2BI + AgC6F5 DMF8 Mes2B(C6F5) + AgI
Bond Lengths
B-C(11)
B-C(21)
B-C(31)
1.564(3)
1.590(3)
1.561(3)
Electrochemistry. While we know of no report of cyclic
voltammetry on compounds 1 and 2 prior to the present work,
there have been a number of reports (with different E° values)
of the reversible reduction of Mes3B (3) under various condi-
tions. In 1975 DuPont and Mills reported -3.0 V in THF at a
Pt electrode vs Ag/Ag+,14 which (if we take Ag/Ag+ as +0.49
vs SCE) suggests -2.5 V for 3 in THF vs SCE. The most recent
and presumably more reliable values suggest considerably less
negative reduction potentials for 3. In 1986 Okhlobystin and
co-workers reported -2.18 V in CH3CN vs SCE, and -2.13 V
in DMF vs SCE.15 In 1989, Schultz and Kaim reported -1.94
V in DMF vs SCE,10 and their value was used by Elschenbroich
and co-workers when working in glyme.16 In 1992, Okada and
co-workers reported -2.18 V in DMF vs SCE.17 The reduction
potential E° for 3 appears to vary little with solvent, as is
expected in view of the fact that 3 is an extremely weak Lewis
acid. Nevertheless, there is always an element of uncertainty
when electrode potential data of a given compound, recorded
with different solvents and supporting electrolytes and refer-
enced against different reference electrodes or internal refer-
ences, are to be compared. Therefore, we have investigated the
cyclic voltammograms of 1-3 under identical conditions.
The cyclic voltammograms of the boranes 1-3 were recorded
at 0 °C in THF containing 0.05 M [Bu4N][B(C6F5)4] as the
supporting electrolyte at Pt disk electrodes. The electrode
potentials are reported vs Cp2Fe0/+. After first recording the
Bond Angles
C(11)-B-C(21)
C(21)-B-C(31)
C(31)-B-C(11)
121.97(18)
117.94(17)
122.67(17)
The latter was, as reported by Bochmann and co-workers,11
readily accessible from BF3‚OEt2 and 2 equiv of C6F5MgBr.
FB(C6F5)2‚OEt2 + MesMgBr -78 to 20 °C8
Et2O
40%
MesB(C6F5)2 + MgBrF (4)
An X-ray structure of 1 shows the expected propeller shape
(Figure 1) with a trigonal planar environment at boron. Its
bulkier substituents cause the mesityl ring to make an angle of
72° with the BC3 plane, while the C6F5 rings make angles of
only 36° and 47°.
Due to the steric constraints of the mesityl substituent, the
Lewis acidity of 1 is strongly reduced relative to that of
B(C6F5)3. The interaction between Et2O or THF and 1 is very
weak, although CH3CN binds noticeably to 1; the addition of
two drops of CH3CN to an NMR tube containing 1 in C6D6
shifts its ortho 19F signal upfield by 3.4 ppm, its para 19F signal
upfield by 12.3 ppm, and its meta 19F signal upfield by 4.3 ppm.
The chemical shift difference ∆δm,p between the meta and para
fluorine substituents has been reported to be very sensitive to
the electronic environment of fluorinated arylboranes.1b The
observed 8.0 ppm decrease in ∆δm,p confirms a considerable
degree of CH3CN binding to 1.
(12) Qin, Y.; Cheng, G.; Sundararaman, A.; Ja¨kle, F. J. Am. Chem. Soc.
2002, 124, 12672-12673.
(13) Pestana, D. C.; Power, P. P. Inorg. Chem. 1991, 30, 528-535.
(14) DuPont, T. J.; Mills, J. L. J. Am. Chem. Soc. 1975, 97, 6375-
6382.
(15) Urtaeva, Zh. Kh.; Bumber, A. A.; Okhlobystin, O. Yu. Dokl. Akad.
Nauk 1986, 286, 671-674. English translation: Dokl. Phys. Chem. 1986,
286, 85-88.
(16) Elschenbroich, C.; Ku¨hlkamp, P.; Behrendt, A.; Harms, K. Chem.
Ber. 1996, 129, 859-869 (correction, p 1407).
(17) Okada, K.; Sugawa, T.; Oda, M. J. Chem. Soc., Chem. Commun.
1992, 74-75.
The more hindered Mes2B(C6F5) (2) proved far more difficult
to prepare. An attempt at the comproportionation of Mes3B and
B(C6F5)3 gave negligible reaction even at elevated temperatures.
(11) Duchateau, R.; Lancaster, S. J.; Thornton-Pett, M.; Bochmann, M.
Organometallics 1997, 16, 4995-5005.

Estimate of the Reduction Potential of B(C6F5)
Organometallics, Vol. 25, No. 7, 2006 1567
V, for 2/2•- as -1.57 V, and for 3/3•- as -2.20 V, all reported
vs SCE in THF.
Correction of E° Values for the Coordination of THF? In
principle these E° values need correction for the coordination
of THF in equilibria like eq 11. The THF adducts (e.g., THF‚
1) have no low-lying vacant orbitals and cannot easily undergo
reduction. If we assume that these equilibria are rapidly
maintained on the time scale of the CV experiments, and know
their equilibrium constants Keq, we can determine the reduction
potentials E° for the free boranes from eq 12.20
Keq
MesB(C6F5)2
THF f BMes(C F )
6 5 2
+ THF y z
(11)
(12)
1
THF‚1
RT
nF
E ) E0 - ln(1 + Keq[THF])
However, Keq is small for both 1 and 2. The 11B chemical
shift of 1, δ 69.7 in toluene-d8, is little affected by THF: the
addition of 30 equiv (1.12 M) of THF moves it only to 68.9,
0.8 ppm upfield, whereas the addition of 20 equiv of THF (1.12
M) to the strong Lewis acid B(C6F5)3 moves its chemical shift
from δ 58.1 to δ 3.0, over 50 ppm upfield. Comparison of these
upfield 11B shifts suggests that less than 2% of 1 is complexed
in the presence of 30 equiv (1.12 M) of THF and that Keq is
<0.018, implying that the observed E is negligibly (<1 mV,
far less than the experimental uncertainty) different from the
E° for (free 1)/1-•.
Figure 2. Cyclic voltammogram of a solution of (a) 1 at a voltage
sweep rate of 5 V/s, (b) 2 at a voltage sweep rate of 10 V/s, (c) 3
at a voltage sweep rate of 1 V/s in THF/0.05 M [Bu4N][B(C6F5)4]
at T ) 0 °C at a Pt disk electrode (d ) 0.2-1.0 mm). The potential
scales are referenced to the ferrocene/ferrocenium couple.
1
The H NMR chemical shifts of 2 in toluene-d8 also show
voltammograms of the boranes without ferrocene present,
separate scans were recorded with ferrocene added for calibra-
tion purposes.
little change as THF is added, confirming that the equilibrium
constant for association of 2 with THF is also negligible and
that no correction is required to the E° measured for 2.
The equilibrium constant Keq for association of 3 (Mes3B)
with THF is surely even smaller than that for 2.
Estimate for E° of B(C6F5)3. By assuming a linear relation-
ship between the reduction potential of the substituted boranes
1, 2, and 3 and the number of mesityl substituents, we can
extrapolate an estimate for E° of B(C6F5)3. Our values of E°
for 1, 2, and 3 vs SCE lie on a reasonably straight line (Figure
3) and suggest that E° for free B(C6F5)3 in THF is about -0.64
V vs SCE. However, the accuracy of the extrapolation is limited
by the likelihood of variations in structure within this series of
compounds.
The cyclic voltammogram of 1 showed limited chemical
reversibility at scan rates slower than 0.5 V/s. The scans became
more reversible at faster scan rates. At 5 V/s (Figure 2a) the
reoxidation of the radical anion 1•- competed effectively with
its decomposition, resulting in a convincingly reversible vol-
tammogram. The reduction potential of 1 (eq 10, n ) 1), taken
as the midpoint between the reduction and oxidation peak
potentials, is -1.72 V vs Cp2Fe0/+. The position of the peaks
did vary somewhat from scan to scan (by up to 0.05 V) and
depended on the conditioning of the electrode.
MesnB(C6F5)3-n + e- h MesnB(C6F5)3-n
(10)
•-
Experimental Section
The cyclic voltammogram of 2 showed an ill-defined ir-
reversible reduction at 0.2 V/s, but a partially reversible
reduction (Figure 2b) above 10 V/s. The reduction potential for
2 (eq 10, n ) 2) is estimated at -2.10 V vs Cp2Fe0/+. Again,
the position of the peak potentials was highly dependent on the
electrode history (varying by up to 0.1 V), and the data presented
are taken from the most well-defined voltammograms. It is not
clear to us why the cyclic voltammetry behavior of 2 was less
chemically reversible, and also qualitatively less reproducible,
than that of 1 and 3.
General Comments. All reactions and manipulations were
carried out using standard vacuum-line, Schlenk, and glovebox
techniques, under an atmosphere of purified Ar/N2 unless otherwise
noted. All glassware was flamed out immediately prior to use or
dried overnight at 160 °C. All solvents were purified and dried
using standard procedures and were distilled immediately prior to
use.
AgC6F5 was prepared by a recently reported method.21 Mes2BI
was prepared from Mes2BP(Mes)Li, in 10% yield, by iodine
oxidation in ether as reported by Pestana and Power.13 The complex
The cyclic voltammogram of 3 exhibited a well-defined,
reversible wave centered at -2.73 V vs Cp2Fe0/+ (Figure 2c),
which was much more reproducible and independent of
electrode history than the voltammograms of 1 and 2.
To obtain E° values vs SCE in THF from the measured values
vs Cp2Fe0/+ in THF, an E° for Cp2Fe0/+ vs SCE in that solvent
is required. The value of E° (Cp2Fe0/+ vs SCE in THF) has
been given as +0.52 V with 0.2 M [Bu4N]PF6 at 20 °C18 and
as +0.547 with 0.1 M [Bu4N]PF6 at 20 °C.19 Using the average,
+0.53 V, our cyclic voltammetry data give E° for 1/1•- as -1.19
(18) Enemœrke, R. J.; Daasbjerg, K.; Skrydstrup, T. Chem. Commun.
1999, 343-344.
(19) Ruiz, J.; Astruc, D. C. R. Acad. Sci. 1998, 21-27.
(20) For examples of the use of such relationships see: (a) Gagne´, R.
R.; Allison, J. L.; Ingle, D. M. Inorg. Chem. 1979, 18, 2767-2774. (b)
Fujita, E.; Creutz, C.; Sutin, N.; Szalda, D. J. J. Am. Chem. Soc. 1991,
113, 343-353. The theory is summarized in: Bard, A. J.; Faulkner, L. R.
Electrochemical Methods: Fundamentals and Applications; Wiley: New
York, 2001; pp 36-37.
(21) Tyrra, W.; Wickleder, M. S. Z. Anorg. Allg. Chem. 2002, 628,
1841-1847.

1568 Organometallics, Vol. 25, No. 7, 2006
Cummings et al.
Table 2. Summary of Data Collection, Solution, and
Refinement Parameters for 1
empirical formula
fw
C21H11BF10
464.11
T (K)
233(2)
wavelength (Å)
cryst syst/space group
a (Å)
b (Å)
c (Å)
0.71073
monoclinic, Cc
13.877(10)
20.404(10)
7.706(5)
R (deg)
90
â (deg)
116.153(14)
90
1959(2)
γ (deg)
volume (Å3)
Z
4
D
calc (Mg/m3)
1.574
1.646
absorp coeff (mm-1
)
cryst size (mm3)
0.70 × 0.40 × 0.40
1.92 to 28.04
-18 e h e 17,
-9 e k e 23,
-10 e l e 10
4733/3780 [Rint )0.0680]
SADABS
3780/2/294
1.058
R1 ) 0.0393
wR2 ) 0.1089
R1 ) 0.0423
wR2 ) 0.1117
θ range for data collection (deg)
index ranges
no. reflns collected/unique
absorp corr
no. of data/restraints/params
goodness-of-fit on F2
final R indices [I > 2σ(I)]
Figure 3. Reduction potential (vs SCE) vs number of mesityl
substituents on boranes 1, 2, and 3. The reduction potential for
B(C6F5)3 can be extrapolated to -0.64 V vs SCE.
R indices (all data)
[Cu(C6F5)]4(toluene)2 was prepared according to literature proce-
dures22 and dried at 60 °C under high vacuum for 24 h to remove
the toluene. Mes2BBr was prepared from [CuMes]5(toluene) and
BBr3.23
ature for 16 h. It was then cooled to room temperature, the insoluble
precipitate was removed by filtration, and the volatile components
were removed under vacuum. The spectroscopic yield of 2 is 90%
X-ray diffraction data were collected on a Bruker P4 diffracto-
meter equipped with a SMART CCD detector, and crystal data,
data collection, and refinement parameters are summarized in Table
2. The structures were solved using direct methods and standard
difference map techniques and were refined by full-matrix least-
squares procedures on F2 with SHELXTL (Version 5.03).24
MesB(C6F5)2 (1). Two equivalents (40 mmol) of C6F5MgBr in
Et2O (40 mL of an 1.0 M solution) was added to BF3 etherate (2.5
g, 18 mmol) in 40 mL of ether and stirred for 3 h at 0 °C, giving
an orange-brown solution of crude FB(C6F5)2‚OEt2. One equivalent
(20 mmol) of MesMgBr (20 mL of a commercial 1.0 M ether
solution) was then added at 0 °C, and the combined solution allowed
to warm to room temperature while stirring overnight. Removing
all volatiles left an orange-brown oil. Extracting with 3:1 toluene-
hexane (with sonication), allowing the suspension to settle, remov-
ing and filtering the top layer, and removing the solvent under
vacuum gave an orange oil; upon standing a colorless solid
separated from a brown oil. Recrystallization of the solid from ether
1
according to H NMR analysis; sublimation at 75 °C under high
1
vacuum gave 0.67 g (66%) of pure product. H NMR (500 MHz,
CDCl3): δ 6.81 (4H), 2.30 (6H), 2.08 (12H). 19F NMR (470.4 MHz,
CDCl3): δ -131.9, -151.9, -162.8. 13C NMR (125.7 MHz,
CDCl3): δ 146.2 (CF), 142.7 (CF), 137.5 (CF), 141.2, 140.9, 140.6,
129.0, 120.0, 22.9, 21.5. 11B NMR (160.4 MHz, CDCl3): δ 72.6.
Anal. Calcd for C24H22BF5: C, 69.25; H, 5.33. Found: C, 69.30;
H, 5.15.
Formation of 2 was also observed (19F NMR) when a J-Young
NMR tube was charged with Mes2BI (ca. 0.020 g, 0.054 mmol),
excess AgC6F5 (0.055 g, 0.200 mmol), and DMF-d7, degassed by
freezing under vacuum, wrapped in aluminum foil, and kept at 30
°C for 2 h. The reaction appeared to be complete after 9 h.
Cyclic voltammetric measurements were performed with a
three-electrode CV cell, using an EG&G-PAR Model 273 poten-
tiostat/galvanostat driven by an external HP 33120 function
generator. The signals were fed to a National Instruments DAQ
interface card for on-line processing on a personal computer using
in-house-designed National Instruments LabView software. The
working electrodes were Pt-disk electrodes (d ) 0.2-1.0 mm
depending on voltage scan rates), the counter electrode was a Pt
wire, and the Ag-wire reference electrode assembly was filled with
acetonitrile/0.01 M AgNO3/0.1 M [Bu4N]BF4. The reference
electrode was calibrated against Cp2Fe in the THF/[Bu4N][B(C6F5)]4
electrolyte.
1
and hexane at -30 °C gave colorless 1 in 40% yield. H NMR
(300 MHz, C6D6): δ 6.68 (2H), 2.11(3H), 1.99 (6H). 19F NMR
(282 MHz, C6D6): δ -128.3, -143.9, -159.8. 13C NMR (125.7
MHz, CDCl3): δ 146.2 (CF), 142.7 (CF), 137.5 (CF), 141.2, 140.9,
140.6, 129.0, 120.0, 22.9, 21.5. 11B NMR (32.1 MHz, toluene-d8):
δ 69.7. Anal. Calcd for C21H11BF10: C, 54.35; H, 2.39. Found: C,
54.17; H, 1.83.
Mes2B(C6F5) (2) was prepared by cooling a solution of CuC6F5
(0.60 g, 2.60 mmol) in toluene (20 mL) to -37 °C and adding it
to a solution of Mes2BBr (0.81 g, 2.45 mmol) in toluene (25 mL)
at the same temperature. Upon heating the mixture to 85 °C, a white
precipitate gradually formed; the mixture was kept at this temper-
Acknowledgment. This material is based upon work sup-
ported by the National Science Foundation grants CHE-0211310
and CHE-0451385. We thank Prof. W. E. Geiger, Dr. E. Fujita,
Prof. B. T. Donovan-Merkert, and Prof. F. Gabba¨ı for helpful
discussions and Prof. G. Parkin for assistance with the X-ray
structure determinations.
(22) (a) Cairncross, A.; Sheppard, W. A.; Wonchoba, E. Org. Synth. 1980,
59, 122. (b) Sundararaman, A.; Lalancette, R.; Zakharov, L. N.; Rheingold,
A. L.; Ja¨kle, F. Organometallics 2003, 22, 3526-3532.
Supporting Information Available: Complete details of the
crystallographic study (CIF and PDF) for complex 1. This material
is available free of charge via the Internet at http://pubs.acs.org.
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OM050003Q