Colorimetric turn-on sensing of fluoride ions in H2O/CHCl 3 mixtures by pyridinium boranes

Article abstract of DOI:10.1039/b913030f

As part of our interest in chemistry of cationic boron compounds as anion receptors, we have synthesized 2-(4′-dimestylborylphenyl)-N-methyl- pyridinium triflate ([3]OTf) and 2-(5′-dimesitylborylthiophen-2′-yl) -N-methyl-pyridinium triflate ([4]OTf) by re

Full text of DOI:10.1039/b913030f

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PAPER
www.rsc.org/dalton | Dalton Transactions
Colorimetric turn-on sensing of fluoride ions in H2O/CHCl3 mixtures by
pyridinium boranes†
Casey R. Wade and Franc¸ois P. Gabba¨ı*
Received 1st July 2009, Accepted 29th July 2009
First published as an Advance Article on the web 3rd September 2009
DOI: 10.1039/b913030f
As part of our interest in chemistry of cationic boron compounds as anion receptors, we have
synthesized 2-(4¢-dimestylborylphenyl)-N-methyl-pyridinium triflate ([3]OTf) and
2-(5¢-dimesitylborylthiophen-2¢-yl)-N-methyl-pyridinium triflate ([4]OTf) by reaction of the
corresponding pyridyl derivatives with MeOTf. Both [3]OTf and [4]OTf extract fluoride ions
biphasically (H2O/CHCl3) to form the zwitterionic fluoroborates 3-F and 4-F, which have been
structurally characterized. Remarkably, fluoride binding induces a red-shift of the low energy
absorption band of these boranes (from 319 nm to 368 nm for 3/3-F and from 355 nm to 430 nm for
4/4-F) leading to the appearance of a yellow color. The origin of this turn-on colorimetric response is
attributed to an intramolecular charge transfer process involving the triorganofluoroborate moiety as
the donor and the pyridinium moiety as the acceptor.
Realizing this limitation, Wang recently reported a series
Introduction
of bifunctional molecules containing both a triarylborane and
The recognition of fluoride anions is attracting a great deal of
triarylamine moiety. In such systems, the emission of the triary-
interest because of the importance of this anion in dental health
lamine moiety is quenched by an intramolecular electron transfer
process.6 Because the emission of the triarylamine moiety can
be revived upon addition of fluoride to the boron center, such
derivatives behave as turn-on fluoride sensors.8 These derivatives
are neutral and can only be expected to be compatible with organic
solvents.
and its possible toxicity when administered in high doses. Because
of their inherent fluorophilicity, organoboranes have been recently
used as receptors for fluoride anions.1-9 Simple triarylboranes such
as tris(9-anthryl)borane3 or trimesitylborane5 complex fluoride in
non-polar organic solvents to form the corresponding fluorobo-
rate anions whose stability constants range from 105 to 106 M-1.
In the case of cationic boranes10,11 such as I (Scheme 1),12 fluoride
binding is assisted by favorable Coulombic attractions and occurs
in aqueous solutions where fluoride is highly hydrated.12-14
Arylboronic acids such as II4 have also been considered as
fluoride sensors (Scheme 2). In the case of II, formation of
the corresponding trifluoroborate species is accompanied by the
appearance of an intramolecular charge transfer band involving
the borate moiety as a donor and the electron deficient aryl
group as the acceptor.4 Appearance of this charge transfer band
results in an intense turn-on colorimetric response. Although this
strategy has proved quite general,7,11 it has not been applied to the
case of simple triorganoborane fluoride receptors. In this paper,
we describe cationic boranes with electron deficient pyridinium
moieties which serve for the turn-on colorimetric sensing of
fluoride ions in organic solvents or under biphasic conditions in
water/organic solvent mixtures.
Scheme 1
In most cases, the aromatic substituents of triarylboranes are
conjugated via the vacant boron p-orbital leading to the formation
of a chromophore whose absorption and emission are red-shifted
when compared to that of the individual aromatic substituents.3
Fluoride binding leads to population of the boron p-orbital,
interrupting conjugation and, as a result, providing a turn-off
response both in the absorption and emission spectra of the
borane. The turn-off rather than turn-on nature of the observed
response inherently limits the sensitivity of triarylborane-based
anion sensors.
Scheme 2
Results and discussion
The reaction of 2-(4¢-lithiophenyl)pyridine and 2-(5¢-lithiothio-
phen-2¢-yl)pyridine with dimesitylboron fluoride (Mes2BF) af-
forded boranes 115 and 2 in moderate yields (Scheme 3). Sub-
sequent treatment of 1 and 2 with MeOTf in Et2O yielded the
Department of Chemistry, Texas A&M University, College Station, Texas
77843, USA. E-mail: francois@tamu.edu
† CCDC reference numbers 738717 and 738718. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b913030f
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Scheme 3 Synthesis of cationic boranes [3]OTf and [4]OTf.
Table 1 Crystal data, data collections, and structure refinements
new cationic boranes [3]OTf and [4]OTf which have been fully
characterized. These salts dissolve in polar solvents such as
acetone, acetonitrile and DMSO but are insoluble in hydrocarbon
solvents and diethyl ether. The H NMR spectra of [3]OTf and
Crystal data
3-F
4-F
1
Formula
C32.24H36.71BFN1.76O0.24 C28H31BFNS
[4]OTf in acetone-d6 feature all expected resonances for the
aromatic CH groups of the aryl, pyridinium, and mesityl groups.
The proton resonance of the N-methyl groups in [3]OTf and
[4]OTf appear at 4.43 ppm and 4.59 ppm, respectively. The broad
11B NMR signals at 76 ppm for [3]OTf and 68 ppm for [4]OTf
indicate that the boron atom of these derivatives adopts a trigonal
planar geometry in solution. Broad UV bands are observed at
319 nm and 355 nm for [3]OTf and [4]OTf, respectively, in CHCl3
solution and are assigned to the absorbance of the boron centered
chromophore.
Both [3]OTf and [4]OTf are quantitatively converted into the
zwitterionic fluoroborate adducts 3-F and 4-F upon addition of
TBAF (n-tetrabutylammonium fluoride) in CDCl3 (Scheme 4).
These zwitterions have been characterized by multinuclear NMR
spectroscopy and single crystal X-ray diffraction (Table 1). In both
cases, the 11B NMR signal is in the expected range for a four-
coordinate boron center (5.6 ppm for 3-F, 4.9 ppm for 4-F). The
19F NMR signals, which appear at -173 ppm for 3-F and -168 ppm
for 4-F, indicate the presence of fluoroborate moieties.2,3,5,14 The
structures of 3-F and 4-F (Fig. 1) confirm the presence of a fluorine
atom coordinated to the boron center via B(1)-F(1) lengths of
Mr
482.51
443.41
Crystal size/mm
Crystal system
Space group
0.24 ¥ 0.10 ¥ 0.05
Monoclinic
P2(1)/c
13.924(3)
12.520(3)
15.974(4)
101.333(3)
2730.4(11)
4
1.174
0.072
1034
110(2)
0.12 ¥ 0.11 ¥ 0.08
Tetragonal
I4(1)/a
22.51(3)
22.51(3)
˚
a/A
˚
b/A
˚
c/A
17.90(3)
b/◦
3
˚
V/A
9071(22)
16
1.299
0.168
3776
110(2)
w
-25 → +25
-25 → +23
-20 → +20
22185
3557 [0.0956]
3557
289
1.000
0.0839, 0.1290
Z
r
calc/g cm-3
m/mm-1
F(000)
T/K
Scan mode
hkl Range
w
-15 → +15
-14 → +14
-18 → +18
22904
Measd reflns
Unique reflns [Rint
]
4278 [0.0831]
4278
351
1.001
0.0986, 0.1433
Reflns used for refinement
Refined parameters
GooF
R1,a wR2b (all data)
-3
˚
rfin (max., min.)/e A
0.318, -0.272
0.404, -0.365
˚
ꢀ
ꢀ
ꢀ
ꢀ
1.482(3) and 1.460(5) A which are comparable to those found in
a R1 = ꢀFo| - |Fcꢀ/ |Fo|. b wR2 ([ w(Fo - Fc2)2]/[ w(Fo2)2])1/2
;
2
3,14
˚
triarylfluoroborate anions (1.47 A).
w = 1/[s (Fo2) + (ap)2 + bp]; p = (Fo + 2Fc2)/3 with a = 0.069 for 3-F
2
2
and 0.0415 for 4-F and b = 0 for 3-F and 21 for 4-F.
the boron center.3,5 However, in each case, this quenching is also
accompanied by the appearance of a new absorption band (at
368 nm for 3-F and 430 nm for 4-F) which is responsible for the
observed visible color change. The shape of the fluoride binding
isotherms suggests that the fluoride binding constants of these
cationic boranes are above the 107–108 M-1 range. Addition of
chloride, bromide, or iodide anions does not affect the UV-Vis
spectra of the cationic boranes, indicating that these anions do
not bind to the boranes. Although [3]OTf and [4]OTf are soluble
and undergo no noticeable decomposition in 9:1 H2O:MeOH
solvent mixtures, addition of fluoride to these solutions does not
result in the formation of the fluoroborates indicating that these
cationic boranes are not sufficiently Lewis acidic to overcome
the high hydration energy of the fluoride anion. Similarly, NMR
measurements in the same deuterated solvent mixture show no
indication of formation of the fluoroborate species.
Scheme 4 Synthesis of fluoroborates 3-F and 4-F.
Interestingly, conversion of [3]OTf and [4]OTf into 3-F and 4-F
is accompanied by the appearance of an intense yellow color in
solution (Fig. 2). These colorimetric changes indicate that both
[3]OTf and [4]OTf act as turn-on sensors for fluoride. In order
to better understand the origin of this turn-on response, the UV-
spectra of these compounds in CHCl3 solutions were monitored
upon incremental addition of TBAF (Fig. 3). In both cases,
addition of fluoride results in a quenching of the borane absorption
band indicating a break in conjugation due to fluoride binding to
Despite the inability of these compounds to complex fluoride
directly in aqueous media, we investigated their capacity to extract
9170 | Dalton Trans., 2009, 9169–9175
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Fig. 1 Structure of fluoroborates 3-F (left) and 4-F (right). Displacement ellipsoids are scaled to the 50% probability level.
fluoride from aqueous solutions into a less polar solvent
where binding might be more favorable. When CDCl3 solutions
of [3]OTf and [4]OTf (0.5 mL, 0.07 M) were layered with
solutions of TBAF in D2O (0.125 mL, 0.28 M, 1 eq.), the
organic layers quickly developed a yellow color indicative of
1
the fluoroborate species. Changes in both the H and 19F NMR
spectra were consistent with the formation of the corresponding
zwitterionic fluoride adducts 3-F and 4-F. Conversion to the
1
fluoroborate species was monitored by integration of H NMR
signals corresponding to the aromatic CH groups of the mesityl
substituents, which undergo an upfield shift (ca. 0.4 ppm) upon
1
fluoride binding. Based on integration of the H NMR spectra,
Fig. 2 Observed color change of solutions of [3]+ and [4]+ (0.017 M,
CHCl3) upon addition of TBAF.
[3]OTf was converted to 3-F in 50% yield while [4]OTf underwent
full conversion to 4-F. The greater fluoride affinity of [4]+ when
Fig. 3 Spectral changes in the UV-Vis absorption spectra of [3]OTf (top, 6.3 ¥ 10-5 M) monitored at 406 nm and [4]OTf (bottom, 5.6 ¥ 10-5 M) monitored
at 430 nm in CHCl3 upon incremental addition of a TBAF solution (3.4 ¥ 10-3 M in CHCl3).
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compared to [3]+ can be assigned to the electron withdrawing
properties9 and possibly the lower steric requirement of the five-
membered thienyl group. By layering a solution of [3]OTf with
a total of four equivalents of TBAF solution (0.500 mL) the
conversion to 3-F increased to 74%. We performed the same
experiments by layering with a NaF/D2O (0.125 mL, 0.28 M,
1 eq.) solution. Under these conditions only 5% conversion to
3-F and 30% conversion to 4-F was observed thus indicating that
the n-tetrabutylammonium cation acts a phase transfer agent for
fluoride anions. The percent conversion to 3-F remained low when
layered with a total of four equivalents of NaF/D2O (0.500 mL)
while 4-F was formed in a 42% yield (Fig. 4). In the experiments
performed with NaF, a slight upfield shift of the pyridinium CH
signals is observed for the remaining [4]OTf. Similar changes were
observed in the spectra of [3]OTf upon layering with NaF/D2O
and are attributed to the changing ionic strength of the organic
layer.
The structures of [3]+, [4]+, 3-F and 4-F have been optimized
using DFT methods (B3LYP, 6-31+g(d¢) for all atoms) and
subjected to single point energy calculations using the Polarizable
Continuum Model16 (PCM) and chloroform as a solvent. Inspec-
tion of the frontier orbitals shows that the lowest unoccupied
molecular orbital (LUMO) of each molecule is localized on the
pyridinium portion of the molecule while the highest occupied
molecular orbital (HOMO) is localized exclusively on the mesityl
rings (Fig. 5). Although common TD-DFT methods cannot
Fig. 4 1H NMR spectra of [4]OTf in CDCl3 upon layering with a D2O
solution of TBAF (top) or NaF (bottom).
be used to study such chromophores,17 it appears reasonable
to assume that the lowest energy absorption band of these
compounds is dominated by a HOMO-LUMO transition. Because
of the localization of the frontiers orbital, these transitions can be
regarded as intramolecular charge transfer transitions involving
the boron moiety as the donor and the pyridinium moiety as the
Fig. 5 Rendering of the HOMOs and LUMOs of [3]+ and [3]F (top) and [4]+ and [4]F (bottom) along with relative orbital and HOMO-LUMO gap
energies. Isovalues are set at 0.03.
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acceptor. In agreement with this assumption, we note that the
HOMO-LUMO energy gap undergoes a noticeable decrease on
going from the cationic borane [3]+ and [4]+ to the corresponding
fluoroborate species 3-F and 4-F, respectively. Presumably, this
fluoride induced decrease of the HOMO-LUMO gap of the
compounds is responsible for the experimentally observed red-
shift, thus allowing to rationalize the turn-on response given by
[3]+ and [4]+ in the presence of fluoride.
1.88 mmol) at -78 ◦C. The resulting reaction mixture was stirred
at -78 ◦C for 1 h and mixed with a solution of Mes2BF (0.529 g,
1.97 mmol) in THF (10 mL). The reaction was stirred overnight
at room temperature before removing the solvent in vacuo. The
solids were extracted with CH2Cl2 (20 mL) and filtered through
Celite to remove LiF. The filtrate was evaporated to dryness
and the residue recrystallized from Et2O/pentane to afford 2-(4¢-
1
dimestylborylphenyl)pyridine (1) (0.400 g, 53% yield). H NMR
(399.9 MHz, CDCl3): d 2.04 (s, 12H, Mes-CH3), 2.33 (s, 6H,
Mes-CH3), 6.85 (s, 4H, Mes-CH), 7.25-7.28 (m, 1H, Pyr-CH),
Experimental
3
7.64 (d, JH-H = 8.42 Hz, 2H, Ph-CH), 7.75-7.81 (m, 2H,
General considerations
Pyr-CH), 7.98 (d, 3JH-H = 7.98 Hz, 2H, Ph-CH), 8.72 (d, 3JH-H
=
4.58 Hz, 1H, Pyr-CH). 11B NMR (128.2 MHz, CDCl3): d 75.10.
Without further purification, 1 (0.100 g, 0.248 mmol) was treated
with MeOTf (0.100 mL, 0.420 mmol) in 10 mL Et2O at room
temperature. A white solid precipitated upon standing overnight.
The solid was filtered, washed with Et2O (3 ¥ 5 mL) and pentane
(3 ¥ 5 mL), and dried under vacuum to afford [3]OTf (0.115 g,
2-(4¢-Bromophenyl)pyridine was synthesized by a published
procedure.18 NaF and dimesitylboron fluoride were purchased
from Aldrich. n-Bu4NF·3H2O (TBAF) and 2-(2-thienyl)pyridine
were purchased from Alfa Aesar and used as received. Solvents
were dried by passing through an alumina column (n-hexane,
CH2Cl2) or refluxing under N2 over Na/K (Et2O and THF).
Air-sensitive compounds were handled under a N2 atmosphere
using standard Schlenk and glovebox techniques. UV-vis spectra
were recorded on an Ocean Optics USB4000 spectrometer with an
Ocean Optics ISS light source. Elemental analyses were performed
at Atlantic Microlab (Norcross, GA). NMR spectra were recorded
on Varian Unity Inova 400 FT NMR (399.59 MHz for 1H,
375.99 MHz for 19F, 128.19 MHz for 11B, 100.45 MHz for 13C)
spectrometer at ambient temperature unless otherwise stated.
Chemical shifts d are given in ppm, and are referenced against
external Me4Si (1H, 13C) and BF3·Et2O (11B, 19F).
1
82% yield). H NMR (399.9 MHz, acetone-d6): d 2.03 (s, 12H,
Mes-CH3), 2.30 (s, 6H, Mes-CH3), 4.43 (s, 3H, N-CH3), 6.89 (s,
3
4H, Mes-CH), 7.71 (d, JH-H = 7.88 Hz, 2H, Ph-CH), 7.82 (d,
3JH-H = 7.88 Hz, 2H, Ph-CH), 8.25-8.27 (m, 2H, Pyr-CH), 8.78 (t,
3JH-H = 8.06 Hz, 1H, Pyr-CH), 9.24 (d, 3JH-H = 6.05 Hz, 1H, Pyr-
CH). 13C NMR (100.5 MHz, CDCl3): d 21.22 (Mes-p-CH3), 23.47
(Mes-o-CH3), 47.48 (pyr-CH3), 118.92, 122.11, 127.25, 128.21,
128.43, 129.50, 133.55, 136.45, 139.50, 140.72, 141.05, 145.26,
147.63, 149.42, 155.67. 11B NMR (128.2 MHz, acetone-d6): d
76.3. 19F NMR (375.97 MHz, acetone-d6): d -77.8 (OTf). Anal.
Calcd for C31H33BNSF3O3: C, 65.61; H, 5.86. Found: C, 65.71;
H, 5.92.
Crystallography
Single crystals of 3-F could be obtained by slow evaporation of a
solution of [3]OTf and TBAF in CH3CN/acetone. Single crystals
of 4-F could be obtained by cooling a concentrated acetone-d6
solution of [4]OTf and TBAF. The crystallographic measurements
were performed using a Bruker-AXS APEX-II CCD area detector
diffractometer, with a graphite-monochromated Mo Ka radiation
Synthesis of 2-(5¢-dimesitylboryl-thiophen-2¢-yl)-
N-methyl-pyridinium triflate ([4]OTf)
n-BuLi (1.30 mL, 2.5 M in hexanes, 3.26 mmol) was added to a
THF (10 mL) solution of 2-(2-thienyl)pyridine (0.500 g, 3.1 mmol)
at -78 ◦C. The resulting reaction mixture was stirred at -78 ◦C for
1 h and mixed with a solution of Mes2BF (0.873 g, 3.26 mmol)
in THF (10 mL). The reaction was stirred overnight at room
temperature before removing the solvent in vacuo. The crude
product was extracted with CH2Cl2 (20 mL) and filtered through
Celite to remove LiF. The filtrate was evaporated to dryness and
the residue was recrystallized from Et2O/pentane to afford the
crude product 2-(2-pyridyl)-5-dimesitylboryl-thiophene (0.70 g,
55% yield). 1H NMR (399.9 MHz, CDCl3): d 2.16 (s, 12H, Mes-
CH3), 2.32 (s, 6H, Mes-CH3), 6.84 (s, 4H, Mes-CH), 7.16-7.21 (m,
˚
(l = 0.71069 A). A specimen of suitable size and quality was
selected and mounted onto a nylon loop. The structure was solved
by direct methods, which successfully located most of the non-
hydrogen atoms. Subsequent refinement on F2 using the allowed
location of the remaining non-hydrogen atoms. The structure
solution and refinement were carried out using the SHELXTL/PC
package (version 6.10).19
Theoretical calculations
Density functional theory (DFT) calculations (full geometry
optimization) were carried out with Gaussian03 using the gradient-
corrected Becke exchange functional (B3LYP) and the Lee-Yang-
Parr correlation functional. A 6-31+g(d¢) basis set was used for
all atoms. Frequency calculations were also carried out on the
optimized geometry. In the case of 4-F, an imaginary frequency
corresponding to low energy rotation of a para-methyl group of a
mesityl ligand was observed and ignored.
3
1H, Pyr-CH), 7.46 (d, JH-H = 3.85 Hz, 1H, Thioph-CH), 7.69-
7.72 (m, 2H, Pyr-CH), 7.74 (d, 3JH-H = 3.58 Hz, 1H, Thioph-CH),
3
8.58 (d, 1H, Pyr-CH, JH-H = 4.67 Hz). 11B NMR (128.2 MHz,
acetone-d6): d 67.35. Without further purification, 2 (0.100 g,
0.240 mmol) was treated with MeOTf (0.10 mL, 0.420 mmol) in
10 mL Et2O at room temperature. A white solid precipitated upon
standing overnight. The solid was filtered, washed with Et2O (3 ¥
5 mL) and pentane (3 ¥ 5 mL), and dried under vacuum to afford
1
[4]OTf (0.125 g, 89% yield). H NMR (399.9 MHz, acetone-d6):
Synthesis of 2-(4¢-dimesitylborylphenyl)-N-methyl-pyridinium
triflate ([3]OTf)
d 2.14 (s, 12H, Mes-CH3), 2.29 (s, 6H, Mes-CH3), 4.59 (s, 3H,
3
N-CH3), 6.89 (s, 4H, Mes-CH), 7.60 (d, JH-H = 3.85 Hz, 1H,
3
n-BuLi (0.790 mL, 2.5 M in hexanes, 1.97 mmol) was added to
a THF (10 mL) solution of 2-(4¢-bromophenyl)pyridine (0.440 g,
Thiophene-CH), 8.00 (d, JH-H = 3.85 Hz, 1H, Thiophene-CH),
3
3
8.24 (t, JH-H = 6.41 Hz, 1H, Pyr-CH), 8.37 (d, JH-H = 8.06 Hz,
Dalton Trans., 2009, 9169–9175 | 9173
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1H, Pyr-CH), 8.72 (t, 1H, Pyr-CH, 3JH-H = 7.88 Hz), 9.24 (d, 1H,
Pyr-CH, 3JH-H = 6.23 Hz). 13C NMR (100.5 MHz, acetone-d6): d
21.20 (Mes-p-CH3), 23.61 (Mes-o-CH3), 48.64 (pyr-CH3), 128.08,
129.24, 131.62, 135.86, 140.31, 140.79, 141.33, 141.54, 143.02,
146.43, 148.56, 150.28, 156.43. 11B NMR (128.2 MHz, acetone-
d6): d 68.4. 19F NMR (375.97 MHz, acetone-d6): d -77.6 (OTf).
Anal. Calcd for C29H31BNS2F3O3: C, 60.73; H, 5.45. Found: C,
60.89; H, 5.53.
a colorimetric turn-on response upon fluoride binding. This turn-
on response originates from an intramolecular charge transfer
band involving the fluoroborate moiety as a donor and the
pyridinium ring as the acceptor.
Acknowledgements
This work was supported by the National Science Foundation
(CHE-0646916), the Welch Foundation (A-1423), the Petroleum
Research Funds (Grant 44832-AC4) and the US Army Medical
Research Institute of Chemical Defense.
Generation of 3-F and 4-F
3-F and 4-F were prepared and characterized by multinuclear
NMR spectroscopy in situ by addition of TBAF to solutions
of [3]OTf and [4]OTf in acetone-d6. Data for 3-F: 1H NMR
(399.9 MHz, acetone-d6): d 1.94 (s, 12H, Mes-CH3), 2.08 (s, 6H,
Mes-CH3), 4.34 (s, 3H, N-CH3), 6.45 (s, 4H, Mes-CH), 7.10-
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3
7.30 (bm, 3H), 8.03-8.07 (m, 3H), 8.59 (t, JH-H = 8.06 Hz,
3
1H, Pyr-CH), 9.00 (d, JH-H = 6.05 Hz 1H, Pyr-CH). 13C NMR
(100.5 MHz, dmso-d6): d 20.56 (Mes-o-CH3), 24.90 (Mes-o-CH3),
46.96 (pyr-CH3), 125.34, 125.99, 126.35, 128.03, 129.72, 130.60,
133.51, 140.60, 144.81, 146.30, 154.12, 157.08, 169.42. 11B NMR
(128.2 MHz, acetone-d6): d 5.64. 19F NMR (375.97, MHz, acetone-
1
d6): d -176.0. Data for 4-F: H NMR (399.9 MHz, acetone-d6):
d 2.03 (s, 12H, Mes-CH3), 2.09 (s, 6H, Mes-CH3), 4.51 (s, 3H,
3
N-CH3), 6.47 (s, 4H, Mes-CH), 6.90 (d, JH-H = 3.48 Hz, 1H,
3
4
Thioph-CH), 7.66 (dd, JH-H = 3.66 Hz, JH-F = 2.56 Hz, 1H,
3
Thioph-CH), 7.74 (t, JH-H = 6.31 Hz, 1H, Pyr-CH), 8.18 (d,
3
3JH-H = 8.24 Hz, 1H, Pyr-CH), 8.36 (t, JH-H = 7.78 Hz, 1H,
3
Pyr-CH,), 8.81 (d, JH-H = 6.23 Hz 1H, Pyr-CH). 13C NMR
(100.5 MHz, acetone-d6): d 20.92 (Mes-p-CH3), 24.90 (Mes-
o-CH3), 48.67 (pyr-CH3), 120.68, 123.58, 123.88, 129.13, 129.39,
130.15, 131.44, 132.22, 135.62, 141.60, 144.28, 146.72, 152.38. 11
B
NMR (128.2 MHz, acetone-d6): d 4.93. 19F NMR (375.97, MHz,
acetone-d6): d -167.5.
Titration of [3]OTf and [4]OTf with fluoride in CHCl3
Solutions of [3]OTf (3.0 mL, 6.3 ¥ 10-5 M, CHCl3) and [4]OTf
(3.0 mL, 5.6 ¥ 10-5 M, CHCl3) were titrated with incremental
(5 mL) amounts of fluoride anions by addition of a 3.4 ¥ 10-3
M
solution of TBAF in CHCl3.
Biphasic fluoride capture
1H NMR spectra of solutions of [3]OTf and [4]OTf in CDCl3
(0.500 mL, 0.07 M) were collected after layering and shaking
with 0.125 mL (1 eq. F-) and/or 0.500 mL (4 eq. F-) of D2O
solutions containing TBAF or NaF (0.28 M). Conversion to the
fluoroborate species 3-F and 4-F was monitored by integration,
specifically the signals corresponding to the mesityl aromatic
CH groups which are shifted significantly upfield upon fluoride
binding.
Conclusion
In conclusion, we report the synthesis and structures of two
new cationic boranes. These new cationic boranes selectively bind
fluoride ions in organic solvents. These compounds are sufficiently
fluorophilic to capture fluoride ions under biphasic conditions in
H2O/CHCl3. In addition to being selective, these new boranes give
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