A highly Lewis acidic triarylborane bearing peripheral o-carborane cages

Article abstract of DOI:10.1039/c1dt11064k

A triarylborane (2) bearing three o-carborane cages at peripheral positions on the aryl groups was prepared and its crystal structure was determined from X-ray diffraction study. Treatment of 2 with KF in the presence of 18-crown-6 led to the potassium salt, [2F]^-. A UV-vis titration experiment carried out in THF/H₂O (9/1 v/v) showed that 2 binds fluoride ions with a binding constant (K) of 4.8 × 10⁴ M^-1, which is an order-of-magnitude greater than K for the mono-carborane substituted triarylborane. The enhanced fluoride ion affinity of 2 indicates an apparent additive effect of multiple carborane substitutions on the Lewis acidity enhancement of the triarylborane. The highly Lewis acidic nature of 2 was further utilized in evaluating the fluoride ion affinity of tris(pentafluorophenyl)borane (B(C₆F₅)₃). A fluoride exchange reaction between [2F]^- and B(C₆F ₅)₃ resulted in 15 times higher fluorophilicity for B(C₆F₅)₃ than for 2. The lower Lewis acidity of 2 compared with B(C₆F₅)₃ was confirmed from its greater cathodic reduction potential. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1dt11064k

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PAPER
A highly Lewis acidic triarylborane bearing peripheral o-carborane cages†
Kang Mun Lee,a Jung Oh Huh,a Taewon Kim,b Youngkyu Do*a and Min Hyung Lee*b
Received 7th June 2011, Accepted 16th August 2011
DOI: 10.1039/c1dt11064k
A triarylborane (2) bearing three o-carborane cages at peripheral positions on the aryl groups was
prepared and its crystal structure was determined from X-ray diffraction study. Treatment of 2 with KF
in the presence of 18-crown-6 led to the potassium salt, [2F]-. A UV-vis titration experiment carried out
in THF/H2O (9/1 v/v) showed that 2 binds fluoride ions with a binding constant (K) of 4.8 ¥ 104 M-1,
which is an order-of-magnitude greater than K for the mono-carborane substituted triarylborane. The
enhanced fluoride ion affinity of 2 indicates an apparent additive effect of multiple carborane
substitutions on the Lewis acidity enhancement of the triarylborane. The highly Lewis acidic nature of
2 was further utilized in evaluating the fluoride ion affinity of tris(pentafluorophenyl)borane (B(C6F5)3).
A fluoride exchange reaction between [2F]- and B(C6F5)3 resulted in 15 times higher fluorophilicity for
B(C6F5)3 than for 2. The lower Lewis acidity of 2 compared with B(C6F5)3 was confirmed from its
greater cathodic reduction potential.
Introduction
Triarylboranes have attracted great interest for applications in
various chemical fields due to their high Lewis acidity. For ex-
ample, they have been utilized as effective cocatalysts in transition
metal-catalyzed olefin polymerizations.1–3 Recently, Stephan and
co-workers demonstrated that highly Lewis acidic triarylboranes
constitute a key component in frustrated Lewis pairs that promote
dihydrogen activation.4,5 It is also well established that triarylb-
oranes function as selective anion sensors for the detection of
harmful anions such as fluoride and cyanide.6–11 Thus, much effort
has been devoted to enhancing the Lewis acidity of triarylboranes
for such applications.
To this end, the introduction of electron-withdrawing groups
Chart 1
into triarylboranes has been widely exploited. This approach has
included the use of perfluorinated arenes,5,11–13 metal chelation,9,14
and cationic substituents,6,8,15–19 all of which were found to be
effective in increasing the Lewis acidity of the tri-coordinate
boron atom (Chart 1). While perfluorinated boranes are highly
Lewis acidic, they lack protecting groups around the boron
atom that may greatly limit their use under ambient atmosphere
conditions and in Lewis basic media. In contrast, the attachment
of electron-withdrawing substituents at the peripheral positions
of aryl moieties has been shown to increase Lewis acidity as
well as to retain high stability when the boron center has steric
protection from ortho-methyl groups. This approach has enabled
such boranes to be compatible with Lewis basic media even under
aqueous conditions adequate for anion sensing purposes.6,16–19
As a new approach to enhance the Lewis acidity of triarylb-
orane, our group recently demonstrated that the introduction
of closo-1,2-C2B10H12, so called o-carborane20 into triarylboranes
greatly increase their Lewis acidity (I in Chart 1).21 It was
revealed that both the inductive electron withdrawing effect of o-
carborane and the contribution of the carborane cage to LUMO
(lowest unoccupied molecular orbital) delocalization give rise to
substantial stabilization of the LUMO of boranes. In particular,
the increased Lewis acidity resulted in an increase in fluoride ion
affinity by two orders of magnitude compared with the fluorine-
substituted borane. This property of enhanced Lewis acidity by
the carborane cage led us to consider whether the Lewis acidity
of triarylborane could be further increased by the introduction of
multiple carboranyl groups. Since we note that the introduction
of multiple pentafluorophenyl12 or cationic aryl groups16 exhibits
aDepartment of Chemistry, KAIST, Daejeon, 305–701, Republic of Korea.
E-mail: ykdo@kaist.ac.kr; Fax: 82 42 350 2810; Tel: 82 42 350 2829
bDepartment of Chemistry and Energy Harvest-Storage Research Center,
University of Ulsan, Ulsan, 680–749, Republic of Korea. E-mail: lmh74@
ulsan.ac.kr; Fax: 82 52 259 2348; Tel: 82 52 259 2335
† CCDC reference numbers 837136 and 837137. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/c1dt11064k
11758 | Dalton Trans., 2011, 40, 11758–11764
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Scheme 1 The synthesis of triarylborane bearing three o-carborane cages (2) and its fluoride adduct ([2F]-).
apparent additive effects on Lewis acidity enhancement, it was
anticipated that multi-carborane substitution might also greatly
enhance the Lewis acidity of triarylboranes.
In this report, we synthesized a triarylborane bearing three o-
carborane cages at peripheral positions of the aryl groups (2)
and investigated its fluoride binding properties as a measure of
enhanced Lewis acidity. It is also shown for the first time that
the highly Lewis acidic nature of 2 could be used to evaluate the
fluoride ion affinity of tris(pentafluorophenyl)borane (B(C6F5)3).
The formation of 1 was characterized by multinuclear NMR
spectroscopy and elemental analysis. While 1H and 13C NMR
spectra show the expected resonances corresponding to the ArCB
(ArCB = 4-(2-Ph-o-C2B10-1-yl)-2,6-Me2-C6H2) moiety, 11B NMR
signals detected in the region of d -2 ~ -11 ppm in a 2 : 4 : 2 : 2 ratio
confirmed the presence of the closo-carborane cage. Furthermore
an X-ray diffraction study revealed the molecular structure of 1
(Fig. 1 and Table 1).† From the crystal structure, it can be seen
that both aryl ring planes are oriented roughly perpendicular to the
plane defined by C6–C1–C2–C11 (81.1◦ for C6H2Me2Br and 84.5◦
for C6H5) as has been similarly observed in other mono- and di-
arylcarborane compounds, indicating the existence of interactions
between the aryl p-systems and the tangential p-orbital on the cage
carbon atom.23,24
Results and discussion
Synthesis and characterization
Lithiation of 1 with n-BuLi followed by reaction with one
third equiv of BF3·OEt2 in ether afforded the triarylborane (2)
bearing o-carborane moieties at the peripheral positions of each
aryl group (45%). The identity of the white crystalline solid
The o-carborane substituted aryl starting compound, 1-bromo-4-
(2-phenyl-o-carboran-1-yl)-2,6-dimethylbenzene (1) was obtained
from the cage forming reaction between diarylalkyne, 1a22 and
decaborane (B10H14) in high yield (85%) (Scheme 1).
Table 1 Crystallographic data and parameters for 1 and 2
Compound
1
2·CH2Cl2
Formula
C16H23B10Br
403.37
Monoclinic
P21/n
12.9548(7)
11.2634(6)
14.4557(7)
90.00
100.072(3)
90.00
2076.80(19)
4
1.290
1.977
816
C49H71B31Cl2
1066.07
Monoclinic
P21/c
19.882(2)
14.743 (2)
23.618(3)
90.00
111.808(6)
90.00
6427.3(12)
4
Formula weight
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Z
rc/g cm-3
1.102
0.135
2216
296(2)
m/mm-1
F(000)
T/K
296(2)
f, v
scan mode
hkl range
measd reflns
unique reflns [Rint
f, v
-14 → +14, -12 → +12, -16 → +16
-18 → +20, -15 → +15, -24 → +24
18512
36715
]
3293 [0.0286]
3293
286
0.0325
0.1002
1.001
0.356, -0.333
7878 [0.0514]
7878
865
0.0794
0.2606
1.042
0.899, -0.565
reflns used for refinement
refined parameters
a
R1 (I > 2s(I))
b
wR2 all data
GOF on F2
-3
˚
rfin (max/min)/e A
ꢀ
ꢀ
ꢀ
ꢀ
1/2
a R1 = ꢀFo| - |Fcꢀ/ |Fo|. b wR2 = {[ w(Fo2 - Fc2)2]/[ w(Fo2)2]}
.
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carbon atoms (Ccage) are oriented almost perpendicular to the
CAr–Ccage–Ccage–CPh plane (72.7◦/88.6◦, 82.4◦/88.0◦, 87.3◦/88.4◦
for each ArCB moiety; Ar = 2,6-Me2-Ph). Although the angles
for the Ar ring planes deviate to a certain extent from an
ideal 90◦, probably due to steric congestion around the B(1)
boron center, the deviation for the Ph rings is very small (<2◦)
and is even smaller than that observed for 1 (5.5◦). It was
previously shown from molecular orbital calculations for the
mono-carborane substituted I that the higher-lying unoccupied
orbital (LUMO+1) is dominated by the appended Ph group and
the cage carbon atoms.21 Thus, the perpendicular orientation of the
Ph rings could reflect an appreciable p-interaction between the Ph
group and the cage carbon atom, although the cage carbon atoms
contribute to LUMO delocalization at the same time. Similar to
other triarylboranes possessing ortho-methyl groups, 2 is air and
moisture stable.
Fig. 1 The crystal structure of 1 (40% ellipsoid). H atoms are omitted for
clarity. Selected bond lengths (A): C(1)–C(2) 1.738(3), C(1)–C(6) 1.504(3),
C(2)–C(11) 1.503(3).
˚
2 was fully characterized by multinuclear NMR spectroscopy,
1
elemental analysis, and X-ray diffraction study. The H NMR
Compound 2 was further converted into its fluoride adduct
[2F]- by reaction with KF in the presence of 18-crown-6 in toluene
(Scheme 1). Although a crystal structure of this adduct could
not be obtained, the formation of [2F]- was fully confirmed by
multinuclear NMR spectroscopy. Unlike the neutral compound 2,
the 1H NMR spectrum of [2F]- shows two distinct methyl proton
resonances as well as two aromatic CH proton resonances for the
ArCB moieties, which indicates steric congestion around the boron
center. The 11B NMR spectrum features two broad signals in the
region of d -5 ~ -13 ppm indicative of the o-carborane cage and
also shows a weak peak at around d +1.3 ppm attributable to a
tetra-coordinate boron atom. The 19F NMR signal detected at d
-173 ppm is consistent with the formation of a four-coordinate
triarylfluoroborate, thus indicating the binding of fluoride to the
trigonal boron atom of 2.
spectrum exhibits one methyl and one aromatic CH proton
resonance indicating that the three ArCB moieties are in the
same chemical environment in solution. Similarly, the 13C NMR
spectrum also shows resonances corresponding to a single type of
ArCB, suggesting that all three ArCB moieties are freely rotating in
solution. Although the 11B NMR signal attributable to the trigonal
boron atom was not observed despite a prolonged acquisition time,
the two broad 11B NMR signals detected in the region of d -2 ~
-10 ppm confirm the presence of o-carboranyl boron atoms. The
crystal structure of 2 belonging to the monoclinic space group
P21/c (Fig. 2 and Table 1) was unequivocally determined by the
X-ray diffraction method.†
Fluoride ion binding properties
To investigate the fluoride ion binding properties of 2, UV-vis
titrations were carried out. The experiment was conducted in a
THF/H2O (9/1 v/v) medium to compare the binding ability of 2
with that of the previously reported mono-carborane substituted
I.21 Compound 2 features a low-energy absorption band at 324 nm
(log e = 4.30) assignable to the dominant p(Mes)–pp(B) transition
in the borane moiety (Fig. 3).10,13,18,25
Upon addition of incremental amounts of fluoride, the intensity
of the absorption band gradually decreases as a result of fluoride
binding to the tri-coordinated boron atom of 2. An estimation
from the 1 : 1 binding isotherm gives a binding constant (K) of
4.8 ¥ 104 M-1 in THF/H2O (9/1 v/v). Comparison of this K value
with that of I (K = 5.0 ¥ 103 M-1) under the same conditions reveals
that 2 has a 10-fold increase in fluorophilicity. Considering that
the binding constant of Mes2B(C6H5) (K = 5.0 ¥ 106 M-1 in THF)
which lacks the two ortho-methyl groups is 10 times larger than
that of Mes3B (K = 3.3 ¥ 105 M-1 in THF),21 this result implies that
the introduction of three o-carborane cages into the triarylborane
intrinsically enhances the fluorophilicity of the boron atom by
more than one order-of-magnitude when compared to the mono-
substituted triarylborane. The high fluoride ion affinity of 2 also
reflects the increased Lewis acidity of the boron atom, and thus it
can be suggested that the multiple carborane substitution has an
apparent additive effect on the Lewis acidity enhancement of the
triarylborane.
Fig. 2 The crystal structure of 2 (40% ellipsoid). H atoms and one CH2Cl2
molecule are omitted for clarity. Selected bond lengths (A) and angles (deg):
B(1)–C(3) 1.573(6), B(1)–C(19) 1.585(6), B(1)–C(35) 1.588(6), C(1)–C(2)
1.736(5), C(17)–C(18) 1.751(5), C(33)–C(34) 1.735(6); C(3)–B(1)–C(19)
120.3(4), C(3)–B(1)–C(35) 119.6(4), C(19)–B(1)–C(35) 120.1(4).
˚
The B(1) boron atom adopts a trigonal planar geometry as
judged from summation of the three C–B–C angles (R(C–B–C)
=
360◦). The three o-carborane cages are attached at the para-
positions of the aryl groups, and the six ortho-methyl groups
surround the central tri-coordinate boron atom. As similarly
noted in the structure of 1, the two aryl rings on the carboranyl
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for all of the species in equilibrium. The fluorine signal at d -
188 ppm was newly observed and it could be assigned to the B–
F fluorine of [FB(C6F5)3]-,27 indicating that B(C6F5)3 abstracts a
fluoride ion from [2F]-.
Most interestingly, even though the intensity of the fluorine
signal at d -169 ppm, ascribable to [2F]-, was largely reduced, it
was greater than the expected intensity that would have remained
after complete abstraction by B(C6F5)3. This indicates that 2
competes with B(C6F5)3 for fluoride ion binding.
From the integral ratio of the two fluorine signals and the
equations below (eqns (1) and (2)), the relative fluorophilicity (K)
of the two compounds was calculated to be 15.0, which indicates
that B(C6F5)3 (KB(C 5 3 ) is 15 times more fluorophilic than 2 (K2).
F
)
6
[2F]− + B(C6F )3 ⎯K⎯→2 +[FB(C6F )3]−
(1)
5
5
KB(C
F
[2][FB(C6F )3]
)
3
6
5
5
Fig. 3 Spectral changes in the UV-vis absorption of 2 in THF/H2O (9 : 1
v/v) (2.45 ¥ 10-5 M) upon addition of Bu4NF (0 – 2.34 ¥ 10-4 M). The inset
shows the absorbance at 324 nm as a function of [F-]. The line corresponds
to the binding isotherm calculated with K = 4.8 ¥ 104 M-1.
(2)
K =
=
[2F][B(C6F )3]
K2
5
To further elucidate this result, we compared the reduction
potentials of 2 and B(C6F5)3. From the cyclic voltammogram
shown in Fig. 5, it can be seen that 2 undergoes two sequential
Comparison of fluorophilicity of 2 with B(C6F5)3
B(C6F5)3 has been regarded as one of the strongest Lewis acidic
compounds known.3,26 Despite its facile conversion into the well-
characterized [FB(C6F5)3]-,27,28 evaluation of the fluorophilicity
of B(C6F5)3 using common fluoride sources has not yet been
reported, probably due to its high air and moisture sensitivity. As
the Lewis acid 2 was shown to be highly Lewis acidic, we decided to
determine the fluorophilicity of B(C6F5)3 by the fluoride exchange
reaction with readily accessible [2F]-. This experiment may also
serve as an estimation of the Lewis acidity of 2 in comparison to
the family of highly Lewis acidic fluorinated triarylboranes.2,3,29
Because of the limited choice of reaction solvent for B(C6F5)3,
which excludes any use of coordinating solvents, the fluoride
exchange reaction was carried out in toluene-d8, a solvent that
all of the reaction species were soluble in. After dissolving the
prescribed amounts of [2F]- and B(C6F5)3 (initial concentration
ratio = 1 : 0.890), the reaction mixture was monitored by 19F NMR
spectroscopy. As shown in Fig. 4, fluorine signals could be assigned
Fig. 5 Cyclic voltammograms of 2 (1 mM) in DMSO (3 cycles, scan
rate = 50 mV s-1).
Fig. 4 A 19F NMR spectrum for the reaction between [2F]- and B(C6F5)3 in toluene-d8 (initial concentration ratio = 1 : 0.890).
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reductions, although the second reduction is observed as a weak
shoulder at around -1.6 V. Three consecutive measurements
reveal that the reduction processes are chemically reversible but
electrochemically quasi-reversible.
EA1110 (FISONS Instruments) by the Environmental Analysis
Laboratory at KAIST. UV-vis spectra were recorded on a Jasco
V-530 spectrophotometer. Cyclic voltammetry experiments were
performed using an AUTOLAB/PGSTAT12 system.
Since it was previously shown that the LUMO of mono-
substituted I mainly resides on the central borane moiety with
a contribution from the carborane cage,21 the first reduction
observed for 2 could be ascribed to the reduction at the boron
atom of the borane moiety. The second reduction, whose potential
value is similar to the reduction range observed for 1-Ar-2-Ph-o-
carboranes,23,30 could be assigned to reduction at the carborane
cage of 2. In particular, the first reduction appears at an E1/2 of
-1.35 V indicating that 2 is highly Lewis acidic, in accordance with
its high fluoride affinity. Although it is known that the reduction
potential of B(C6F5)3 cannot be determined experimentally due to
Synthesis of 1
To a toluene solution (50 mL) of decaborane (B10H14, 0.29 g,
2.4 mmol) and 1a (0.57 g, 2.0 mmol) was added an excess of
Et2S (5 equiv) at room temperature. After heating to reflux, the
reaction mixture was stirred for 3 days. The solvent was removed
under vacuum and the resulting solid was purified by column
chromatography on alumina using toluene as eluent, affording 1 as
a white solid (0.69 g, 85%). Recrystallization from EtOAc/MeOH
gave single crystals suitable for X-ray structure determination. 1H
NMR (CDCl3): d 2.24 (s, 6H, ArCB-CH3), 7.07 (s, 2H, ArCB-CH),
7.16 (t, J = 8.0 Hz, 2H, Ph-CH), 7.25 (t, J = 8.0 Hz, 1H, Ph-CH),
7.43 (d, J = 8.0 Hz, 2H, Ph-CH). 13C NMR (CDCl3): d 23.80
(ArCB-CH3), 84.33 (CB-C), 85.18 (CB-C), 128.33, 129.12, 129.93,
130.02, 130.26, 130.59, 130.63, 138.32. 11B NMR (CDCl3): d -2.5
(br s, 2B), -9.2 (br s, 4B), -10.4 (br s, 2B), -11.4 (br s, 2B). Anal.
Calcd for C16H23B10Br: C, 47.64; H, 5.75. Found: C, 47.82; H,
5.47.
the instability of its radical anion, an estimated reduction potential
12
of B(C6F5)3 was reported to be ca. -1.17 V vs. Fc0/+
.
Therefore,
comparison of the reduction potential of 2 with that of B(C6F5)3
is consistent with the lower Lewis acidity of 2 demonstrated in the
fluoride ion exchange experiment.
Conclusions
We prepared and characterized a highly Lewis acidic triarylborane
(2) that bears three o-carborane cages at the peripheral positions
of the aryl groups. It was demonstrated by fluoride ion binding
experiment that the introduction of multiple carborane moieties
into the triarylborane has an apparent additive effect on the Lewis
acidity enhancement of the triarylborane. The highly Lewis acidic
nature of 2 was successfully utilized in evaluating the fluoride ion
affinity of B(C6F5)3, and the result was corroborated by comparing
the reduction potentials of 2 and B(C6F5)3. Although less Lewis
acidic than B(C6F5)3, compound 2 constitutes a novel type of
highly Lewis acidic triarylborane that also possesses high chemical
stability.
Synthesis of 2
A hexane solution of n-BuLi (2.5 M, 0.55 mL, 1.37 mmol) was
added to a solution of 1 (0.50 g, 1.24 mmol) in Et2O (20 mL)
at -78 ◦C. After stirring for 30 min, the reaction mixture was
allowed to warm to 0 ◦C and was stirred for another 20 min. The
mixture was then recooled to -78 ◦C, and BF3·OEt2 (0.05 mL,
0.4 mmol) was added via syringe. After stirring for 1 h, the reaction
mixture was allowed to warm to room temperature and then
stirred overnight. After quenching with saturated aqueous NH4Cl
(30 mL), the organic layer was separated, and the aqueous layer
was extracted with Et2O (20 mL). The combined organic layers
were dried over MgSO4 and concentrated under reduced pressure.
The yellow residue was dissolved in a THF/MeOH mixed solvent.
Cooling of the solution afforded 2 as a white crystalline solid
(0.18 g, 45%). Single crystals suitable for X-ray diffraction study
Experimental section
General considerations
1
were grown through cooling a CH2Cl2 solution of 2. H NMR
All operations were performed under an inert nitrogen atmosphere
using standard Schlenk and glove box techniques. Anhydrous
grade solvents (Aldrich) were dried by passing through an
activated alumina column and stored over activated molecular
(CDCl3): d 1.44 (s, 18H, ArCB-CH3), 6.83 (s, 6H, ArCB-CH), 7.11
(t, J = 8.0 Hz, 6H, Ph-CH), 7.24 (t, J = 8.0 Hz, 3H, Ph-CH), 7.38
(d, J = 8.0 Hz, 6H, Ph-CH). 13C NMR (CDCl3): d 22.61 (ArCB
-
CH3), 84.65 (CB-C), 85.03 (CB-C), 128.05, 129.89, 129.93, 130.65,
130.68, 131.92, 140.18, 146.89 (B-CArCB). 11B NMR (CDCl3): d
-2.6 (br s, 6B, CB-B), -10.5 (br s, 24B, CB-B). Anal. Calcd for
C48H69B31: C, 58.76; H, 7.09. Found: C, 58.48; H, 7.48.
˚
sieves (5 A). Spectrophotometric grade THF (Aldrich) was
used for absorption measurements. Commercial reagents were
used as obtained without any further purification from Aldrich
(BF3·OEt2, n-BuLi (2.5 M solution in n-hexanes), 18-crown-6,
diethyl sulfide, KF, tetra-n-butylammonium fluoride (TBAF)) and
Katchem (Decaborane, B10H14). B(C6F5)3 (Strem) was used after
recrystallization from hexane. 1a was synthesized in a manner
analogous to reported procedures.22 Deuterated solvents from
Cambridge Isotope Laboratories were used. NMR spectra were
recorded on a Bruker Avance 400 spectrometer (400.13 MHz for
1H, 100.62 MHz for 13C), a Bruker AM 300 spectrometer (96.29
MHz for 11B) and a Bruker DRX300 spectrometer (282.38 MHz
for 19F) at ambient temperature. Chemical shifts are given in ppm,
and are referenced against external Me4Si (1H, 13C), BF3·OEt2
(11B), and CFCl3 (19F). Elemental analyses were performed on an
Synthesis of [K·(18-crown-6)][2F]
Toluene (2.0 mL) was added to a mixture of the solids 2 (30 mg,
0.031 mmol), 18-crown-6 (16 mg, 0.061 mmol), and KF (18 mg,
0.031 mmol), and the reaction mixture was stirred at room
temperature for 2 h. After removal of excess KF by filtration,
the filtrate was dried in vacuo. The resulting white solid was
recrystallized from THF/n-hexane at -20 ◦C, affording [K·(18-
1
crown-6)][2F] (28 mg, 69%) as colorless microcrystals. H NMR
(THF-d8): d 1.11 (s, 9H, ArCB-CH3), 1.73 (s, 9H, ArCB-CH3), 3.60
(s, 24H, crown), 6.48 (s, 3H, ArCB-CH), 6.70 (s, 3H, ArCB-CH),
11762 | Dalton Trans., 2011, 40, 11758–11764
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7.13 (t, J = 8.0 Hz, 6H, Ph-CH), 7.24 (t, J = 8.0 Hz, 3H, Ph-
CH), 7.40 (d, J = 8.0 Hz, 6H, Ph-CH). 13C NMR (acetone-d6):
d 20.93 (ArCB-CH3), 25.37 (ArCB-CH3), 70.86 (crown), 86.63 (CB-
C), 89.47 (CB-C), 125.39, 129.00, 129.28, 129.61, 130.08, 130.81,
131.44, 131.63, 141.83, 145.14 (B–CMes). 11B NMR (THF-d8): d
+1.3 (br s, B-F), -5.0 (br s, 6B, CB-B), -12.8 (br s, 24B, CB-B).
19F NMR (THF-d8): d -173. Anal. Calcd for C60H93B31FKO6: C,
55.28; H, 7.19. Found: C, 55.26; H, 7.35.
Chen and T. J. Marks, Chem. Rev., 2000, 100, 1391–1434 and references
therein.
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X-ray structure determination
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A specimen of suitable size and quality was coated with Paratone
oil and mounted onto a glass capillary. The crystallographic
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˚
Mo-Ka radiation (l = 0.71073 A). The structure was solved by
direct methods and all nonhydrogen atoms were subjected to
anisotropic refinement by full-matrix least-squares on F2 using
the SHELXTL/PC package. Hydrogen atoms were placed at their
geometrically calculated positions and were refined riding on the
corresponding carbon atoms with isotropic thermal parameters.
The detailed crystallographic data are given in Table 1.
UV-vis titration experiments
A solution of 2 (3.0 mL, 2.45 ¥ 10-5 M, THF/H2O 9/1 v/v)
was titrated with incremental amounts of fluoride anion by the
addition of TBAF solution (1.58 ¥ 10-2 M in THF). The absorption
was monitored at lmax = 324 nm. In fitting the experimental points,
a correction was applied to account for the absorption of [2F]- at
324 nm. The experimental data obtained were fitted to a 1 : 1
binding isotherm (Fig. 3).
Cyclic voltammetry
Cyclic voltammetry measurements were carried out in DMSO
with a three-electrode cell configuration consisting of platinum
working and counter electrodes and a Ag/AgNO3 (0.01 M
in CH3CN) reference electrode at room temperature. Tetra-n-
butylammonium hexafluorophosphate (0.1 M) was used as the
supporting electrolyte. The reduction potentials were recorded at
a scan rate of 50 mV s-1 and are reported with reference to the
ferrocene/ferrocenium (Fc/Fc+) redox couple.
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Acknowledgements
Financial supports from the National Research Foundation of
Korea (No. 2010-0008264 for Y. Do and No. 2010-0007796 for
M.H. Lee) and the Priority Research Centers Program of the NRF
(No. 2009-0093818 for M.H. Lee) funded by the Korea Ministry of
Education, Science and Technology are gratefully acknowledged.
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