Fluoride ion complexation by a cationic borane in aqueous solution

Article abstract of DOI:10.1039/b616814k

The phosphonium borane [p-Mes₂B(C₆H ₄)PMePh₂]⁺ complexes fluoride in water containing 10% methanol with a binding constant of 1.0(±0.1) × 10³ M^-1 to afford the zwitterion p-Mes <

Full text of DOI:10.1039/b616814k

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Fluoride ion complexation by a cationic borane in aqueous solution{
Min Hyung Lee,a Tomohiro Agou,b Junji Kobayashi,b Takayuki Kawashima*b and Franc¸ois P. Gabba¨ı*a
Received (in Berkeley, CA, USA) 16th November 2006, Accepted 20th January 2007
First published as an Advance Article on the web 6th February 2007
DOI: 10.1039/b616814k
1-dimesitylboryl-4-diphenylphosphinobenzene13 and B-mesityl-
P-phenylphosphaborin,14 respectively (Scheme 1).15{ The identity
of these salts has been confirmed by multinuclear NMR and
elemental analysis. While the 1H NMR spectra show all the
expected resonance, the 11B NMR signal detected at 78.9 pm for
[1]I and 58.6 ppm for [2]I confirms the presence of a base-free
trigonal planar boron center. The 31P resonance detected at
21.9 ppm for [1]I and 3.1 ppm for [2]I is in agreement with the
presence of a phosphonium center. Both compounds feature a
broad absorption band whose lmax is red shifted when compared
to that of Mes3B (lmax = 347 nm (log e 3.96) for [1]I and 369 nm
(log e = 3.35) for [2]I).9
The phosphonium borane [p-Mes2B(C6H4)PMePh2]+ com-
plexes fluoride in water containing 10% methanol with a
binding constant of 1.0(¡0.1) 6 103 M21 to afford the
zwitterion p-Mes2FB(C6H4)PMePh2.
The design of fluoride sensors is an area of active investigations,
because of the possible toxicity of this anion which is present in
drinking water, toothpaste and osteoporosis drugs. The use of such
sensors can also be envisaged for tracking UF6 and phosphoro-
fluoridate nerve agents which release fluoride upon hydrolysis. A
large majority of the molecular sensors reported thus far are
unable to overcome the large hydration energy of fluoride
(504 kJ mol21) and cannot operate in water.1
The phosphonium borane salt [1]I is quantitatively converted
into zwitterionic 1-F when treated with TASF in THF.{ The 11B
NMR signal of the four coordinate boron center of 1-F appears at
9.8 ppm. The 19F NMR spectrum features a signal at 2175.5 ppm
which is comparable to the chemical shift observed in other
triarylfluoroborate complexes.5,8,9 Salt [2]I reacts with TBAF in
CHCl3 to afford 2-F as a mixture of two isomers which differ by
the up or down orientation of the fluorine with respect to the
phenyl group bound to phosphorus. Although the 11B (3.6 ppm)
and 19F NMR (2161.3 ppm) resonance of these two isomers
cannot be resolved, the 31P NMR spectrum features two peaks at
3.3 and 23.1 ppm. Unfortunately, conversion of [2]I into 2-F was
accompanied by formation of unidentified decomposition pro-
ducts even under anhydrous and anaerobic conditions. Further
decomposition is observed when the reaction is carried out in air or
in the presence of water. Because of the chemical instability of [2]+,
additional complexation experiments were carried out with [1]+.
Taking into account the strength of the B–F bond, oragnobor-
anes are receiving increasing attention as alternative fluoride
sensors.2 A majority of the recent research has been focused on
neutral monodentate derivatives such as tris(9-anthryl)borane
which capture fluoride in THF with association constants in the
105–106 M21 range.3–5 Although very selective for fluoride,
available data show that such receptors fail to capture this anion
in aqueous environments.6 Because of these difficulties, we and
others have explored the use of bidentate boranes such as I that
chelate fluoride.7–10 Such systems show an increased fluoride
affinity but have remained incompatible with aqueous solutions.
Stimulated by these challenges, we are now exploring the use of
cationic boranes whose fluoride affinity should be increased by
favorable Coulombic host–guest attractions. Our initial efforts in
this area have led us to consider the ammonium borane [II]+,11
which captures fluoride from water under biphasic conditions. We
have also shown that the phosphonium borane [III]+, binds
fluoride, chloride and bromide in THF.12 These recent results have
led us to question whether phosphonium boranes can be used for
the recognition of fluoride in water.
1
The H NMR spectrum of [1]+ in CDCl3 is not affected in the
presence of chloride and bromide indicating that the fluoride
complexation is selective. This selectivity probably arises from the
presence of four ortho-methyl substituents which lead to
We have decided to address this question by studying the
fluoride binding properties of the phosphonium borane iodide salts
[1]I and [2]I which could be easily obtained from the known
aDepartment of Chemistry, Texas A&M University, College Station,
Texas, 77843, USA. E-mail: francois@tamu.edu
bDepartment of Chemistry, Graduate School of Science, The University
of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan
{ Electronic supplementary information (ESI) available: Experimental and
spectroscopic details. See DOI: 10.1039/b616814k
Scheme 1 Reaction of phosphonium boranes with fluoride. Reagents
and conditions: (i) [Me3SiF2][S(NMe2)3], THF, 96%; (ii) TBAF, CHCl3,
25 uC.
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Chem. Commun., 2007, 1133–1135 | 1133

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indicates that CHCl3 provides a more competitive medium for
fluoride binding.
Remarkably, [1]+ is able to capture fluoride from water under
biphasic conditions as indicated by NMR spectroscopy. For
example, shaking a biphasic mixture consisting of 0.1 mL of a
TBAF in solution in D2O (1.3 6 1021 M) and 1 mL of a solution
of [1]+ in CDCl3 (1.3 6 1022 M) results in a 76% conversion of
[1]+ into 1-F after a few minutes. Such experiments can be carried
out in air and without decomposition of [1]+ or 1-F. The behavior
of [1]+ parallels that of [II]+ which also capture fluoride under
these conditions. In order to provide additional comparative
data, we have carried out the same experiments using I10 and
1-(dimesitylboryl)-8-(109-bora-99-thiaanthryl)naphthalene9
and
found that these chelating boranes fail to capture fluoride under
these biphasic conditions.
Fig. 1 Crystal structure of 1-F (50% ellipsoids, H-atoms omitted for
Encouraged by these results, we decided to attempt the use of
[1]+ in water. Although salt [1]I is poorly soluble in water, 5 6
1025 M solution can be readily prepared in H2O–MeOH (9 : 1 v/v)
mixtures. Under these conditions, the UV spectrum of [1]+, which
features a broad band at 321 nm, remains unchanged after several
hours thus pointing to the water stability of the phosphonium
borane (Fig. 2). The emission spectrum of [1]+ shows an intense
green fluorescence (lmax = 495 nm) which can be easily observed
with the naked eye (Fig. 2). This emission is significantly red
shifted when compared to that observed in CHCl3 (lmax = 450 nm).
This red shift can be correlated to the high polarity of the aqueous
medium and suggest that the excited state bears some intramo-
lecular charge transfer character.16 Remarkably, addition of
fluoride quenches both the green fluorescence and low energy
absorption band of [1]+ thus indicating that fluoride complexation
also occurs in H2O–MeOH (9 : 1 v/v) (Fig. 2). The stability
constant of 1-F under these conditions is equal to 1.0(¡0.1) 6
103 M21. The use of a phosphate buffer solution (pH 7, 5 6
1022 M) does not affect the magnitude of this binding constant
confirming that the fluoride and not hydroxide binds to boron.
This experiment also indicates that [H2PO4]2 does not bind to [1]+.
While addition of KF (80 eq.) to a 5 6 1025 M of [1]I in H2O–
MeOH (9 : 1 v/v) results in a 78% absorbance decrease at 321 nm,
addition of KCl, KBr, NaNO3 or [nBu4N]I (80 eq.) only results in
an absorbance decrease of less than 1.5% at the same wavelength.
These experiments show that the boron center of [1]+ has no
appreciable affinity for chloride, bromide, iodide and nitrate and is
highly selective for fluoride. When evaluated under the same
conditions (H2O–MeOH (9 : 1 v/v)), neutral I and cationic [II]+ do
clarity, C-atoms not labelled).
substantial steric crowding around the boron center thus
preventing approach of larger anions. The presence of two phenyl
substituents appended to the phosphonium center may also help to
disperse the positive charge thereby lessening its effect on the Lewis
acidity of the boron center. Compound 1-F crystallizes as a CHCl3
¯
solvate in the P1 space group with no unusually short
intermolecular contacts.§ The structure of 1-F confirms that the
fluorine atom is coordinated to the boron center (Fig. 1). The B–F
˚
bond length of 1.476(4) A is comparable to those found in
5
triarylfluoroborate anions (1.47 A), thus indicating the presence of
˚
a usual polar covalent B–F linkage. Accordingly, the sum of the
coordination angles at boron (g(C–B–C) = 339.4u) indicates
substantial pyramidalization.
Addition of fluoride to a CHCl3 solution of [1]+ results in
quenching of the absorption band of the borane at 347 nm.
This phenomenon4 reflects conversion of the phosphonium borane
into the corresponding phosphonium fluoroborate 1-F whose
stability constant is equal to 6.5(¡0.5) 6 106 M21. Under these
conditions, Mes3B does not form any detectable quantities of
[Mes3BF]2 while the neutral bidentate Lewis acid I captures
fluoride with an association constant of 2.1(¡0.2) 6 104 M21
.
These experiments attest to the fluoride affinity displayed by
[1]+ which exceeds that of both monodentate and bidentate
neutral boranes by several orders of magnitude. We note in
passing that the fluoride binding constants of Mes3B and I in
CHCl3 is much lower than those measured in THF (K = 3.3(¡0.4)
6 105 for Mes3B and K . 108 M21 for I).9,10 This difference
Fig. 2 Spectral changes in UV-vis absorption (left) and emission (middle) of a solution of [1]I in H2O–MeOH (9 : 1 v/v) (5 6 1025 M) upon addition of
KF (3 6 1021 M in H2O). The picture on the right shows the emission of the solution before and after fluoride addition under a hand-held UV lamp.
1134 | Chem. Commun., 2007, 1133–1135
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not bind fluoride. Finally, addition of an aqueous solution of Al3+
to a solution containing 1-F leads to complete regeneration of [1]+
within a few minutes. This result shows that fluoride binding by
[1]+ is readily reversible.
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In summary, we report an unprecedented boron-based fluoride
receptor ([1]+) which operates in largely aqueous solutions. This
receptor is selective for fluoride and gives rise to a fluorescent
ratiometric response in the presence of the analyte. The ability of
this cationic receptor to overcome the high hydration enthalpy of
fluoride can be correlated to favorable Coulombic effects which
enhance the Lewis acidity of the boron center. We are currently
investigating the use of such receptors for the binding of cyanide
which has been shown to interact with triarylboranes.3e
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6 For water compatible aluminum and tin-based fluoride sensors
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We gratefully acknowledge the following agencies for funding:
the Korea Research Foundation Grant (KRF-2005-214-C00211),
the U. S. Army Medical Research Institute of Chemical Defense,
the Welch Foundation (Grant A-1423), the PRF (Grant 44832-
AC4), the 21st Century Center of Excellence Program for
Frontiers in Fundamental Chemistry, the Scientific Research (S)
from the Ministry of Education, Culture, Sports, Science and
Technology of Japan and the Japan Society of the Promotion of
Science for Young Scientists (research fellowship).
Notes and references
{ [1]I: MeI (0.15 mL) was added to a solution of 1-dimesitylboryl-4-
diphenylphosphinobenzene (400 mg, 0.78 mmol) in Et2O (20 mL) at room
temperature. The mixture was stirred overnight and the solid formed was
collected by filtration. After washing with Et2O (10 mL), drying in vacuo
afforded [1]I as a yellow solid (350 mg, 69% yield) (Found: C, 67.54; H,
6.13. C37H39BIP requires C, 68.12; H, 6.03%).
1-F: [1]I (100 mg, 0.15 mmol) and TASF (42 mg, 0.15 mmol) were mixed
in THF (20 mL) at room temperature. After stirring for 30 min, the
mixture was filtered. The filtrate was exposed to a vacuum to afford a white
solid. This solid was washed with Et2O and dried under vacuum yielding
1-F (78 mg, 96% crude yield). Colorless single crystals of 1-F?CHCl3 could
be obtained in a 50–60% yield (not optimized) by partial evaporation of a
CHCl3 solution of 1-F (Found: C, 68.56; H, 6.07. C38H40BCl3FP requires
C, 68.75; H, 6.07%.).
12 T. Agou, J. Kobayashi and T. Kawashima, Inorg. Chem., 2006, 45,
9137–9144.
13 Z. Yuan, N. J. Taylor, Y. Sun, T. B. Marder, I. D. Williams and
L.-T. Cheng, J. Organomet. Chem., 1993, 449, 27–37.
14 T. Agou, J. Kobayashi and T. Kawashima, Org. Lett., 2005, 7,
4373–4376.
15 Related fluorinated phosphonium boranes have been recently reported:
G. C. Welch, R. R. S. Juan, J. D. Masuda and D. W. Stephan, Science,
2006, 314, 1124–1126.
¯
§ Crystal data for 1-F: C38H40BCl3FP, Mr = 663.83, triclinic, P1, a =
˚
12.2069(11), b = 12.6236(12), c = 13.1483(12) A, a = 71.916(2), b =
3
˚
76.183(2), c = 62.797(2)u, V = 1701.7(3) A , Z = 2, T = 110(2) K, m(Mo-
Ka) = 0.348 mm21, 10398 reflections collected, 7517 unique (Rint = 0.0697),
R1 = 0.0660 [I . 2s(I)], wR2 = 0.1630 (all data). CCDC 627715. For
crystallographic data in CIF or other electronic format see DOI: 10.1039/
b616814k.
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Chem. Commun., 2007, 1133–1135 | 1135