Fluoride ion chelation by a bidentate phosphonium/borane lewis acid

Article abstract of DOI:10.1021/ja804492y

The phosphonium borane [1-Mes₂B-2-MePh₂P-(C₆H₄)]+ ([2]+) has been synthesized as an iodide salt by alkylation of 1-Mes₂B-2-Ph₂P-(C₆H₄) with MeI. This novel cationic borane complexes fluoride to afford the corresponding zwitterionic fluoroborate complex 1-FMes₂B-2-MePh₂P-(C₆H₄) (2-F) with a binding constant in MeOH exceeding that of 1-Mes₂B-₄-MePh₂P-(C₆H₄) ([1]+) by at least 4 orders of magnitude. Structural and computational results indicate that the high fluorophilicity of [2]+ arises from both Coulombic and cooperative effects which lead to formation of a B-F→P interaction with a F→P distance of 2.666(2) A. These results, which are supported by NBO and AIM analyses, show that the latent phosphorus-centered Lewis acidity of the phosphonium moiety in [2]+ can be exploited to enhance fluoride binding via chelation. Copyright

Full text of DOI:10.1021/ja804492y

Published on Web 07/25/2008
Fluoride Ion Chelation By a Bidentate Phosphonium/Borane Lewis Acid
Todd W. Hudnall,† Young-Min Kim,† Magnus W. P. Bebbington,‡ Didier Bourissou,*,‡ and
Franc¸ois P. Gabba¨ı*,†
Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843, and Laboratoire He´te´rochimie
Fondamentale et Applique´e (UMR CNRS 5069), UniVersite´ Paul Sabatier, 118 route de Narbonne, F-31062
Toulouse Cedex 09, France
Received June 12, 2008; E-mail: francois@tamu.edu
Scheme 1a
Because of applications in dental care or in the treatment of
osteoporosis, the introduction of fluoride anions (F-) in drinking water,
toothpastes, and medications has become widespread. This recent trend
is raising concerns because high doses of F- have been associated
with dental and skeletal fluorosis, among other diseases.1 For these
reasons, discovering methods for the recognition of this anion has
become a topical research objective.2 One of the most successful
strategies adopted to date relies on the use of fluorophilic boron
compounds.3,4 Bidentate boranes5 such as I6 and II7 are among the
best receptors thus far identified. Their high fluorophilicity results from
cooperative effects involving the neighboring Lewis acidic centers
which both interact with the fluoride ion to form a chelate complex.
Such bifunctional boranes are however supplanted by cationic boranes
such as [1]+ whose enhanced fluorophilicity arises from favorable
Coulombic effects.8,9 In principle, boron based receptors that are
bidentate and cationic should capitalize on both Coulombic and
cooperative effects leading to a further enhancement of their F-
affinity.10 In this report, we provide a dramatic illustration of this
principle in the F- binding properties of [1-Mes2B-2-MePh2P-(C6H4)]+
(2+), the ortho isomer of 1+. We also demonstrate that the latent
phosphorus-centered Lewis acidity11 of the phosphonium moiety12,13
can be harnessed to enhance F- binding via chelation.
a Conditions: (a) MeI, CH2Cl2, 25 °C, 87%; (b) [S(NMe2)3][SiMe3F2],
CHCl3, 25 °C, 91%.
which substantially deviates from the ideal value of 120° and (ii) the
elongated C(1)-C(2) bond of 1.462(4) Å. Distortions are also evident
in the coordination geometry of the phosphorus atom with the sum of
the C(2)-P(1)-C(30), C(30)-P(1)-C(41), and C(41)-P(1)-C(2)
angles R equal to 335.2° rather than 328.4° which would be expected
if the phosphorus atom adopted a regular tetrahedral geometry.
With this new cationic borane in hand, we became eager to compare
its F- binding properties to those of its para isomer [1]+. Remarkably,
when equimolar amounts of [2]I and 1-F were mixed in CDCl3, 31
P
NMR spectroscopy indicated quantitative formation of [1]+ along with
a new species assigned to 2-F and characterized by a doublet at 28.3
ppm (eq 1). Encouraged by these results, we decided to compare the
F- binding constants (K) of [1]+ and [2]+. Since [2]I is not stable in
neutral water, the titrations were carried out in MeOH by monitoring
the absorbance of the boron-centered chromophore upon F- addition.
Fitting of the resulting isotherms to a 1:1 binding model, indicates
that the F- binding constant of [2]+ (K > 106 M-1) exceeds the
measurable range and is at least 4 orders of magnitude higher than
that measured for [1]+ (K ) 400 ((50) M-1). In an effort to identify
the origin of this difference, we isolated 2-F from the reaction of [2]I
with [S(NMe2)3][Me3SiF2] (TASF) and proceeded with its character-
ization. The 11B NMR signal of the four coordinate boron center of
2-F appears at 5.7 ppm. The 19F NMR spectrum features a broad signal
at -158 ppm which is comparable to the chemical shift observed in
other triarylfluoroborate complexes.8 The 31P NMR signal at 28.3 ppm
appears as a doublet because of coupling to the fluorine nucleus (JP-F
) 24.3 Hz). Although the JP-F coupling constant is much smaller than
The iodide salt of [2]+ could be easily obtained by reaction of the
corresponding neutral phosphinoborane 1-Mes2B-2-Ph2P-(C6H4)14 with
methyl iodide in dichloromethane (Scheme 1). This salt has been
characterized by multinuclear NMR spectroscopy as well as by single
crystal X-ray diffraction. The 1H NMR of [2]I displays broad multiple
signals for the protons on the mesityl substituents, indicative of a
congested structure about the boron center. The phosphorus bound
methyl group gives rise to a characteristic doublet (2JH-P ) 13.5 Hz)
at 2.86 ppm and the 31P NMR resonance appears at 23.9 ppm. In
CHCl3, [2]I features a broad absorption band at 347 nm corresponding
to the triarylborane chromophore. Observation of this band indicates
that the boron center remains coordinatively unsaturated as in the case
of the para isomer [1]I.8 In the crystal, the boron center of [2]+ is
trigonal planar (Σ(C-B-C) ) 359.7°) and separated from the phosphorus
atom P(1) by only 3.494(3) Å. This short separation indicates that the
unsaturated boron center is sterically encumbered. This conclusion is
in agreement with (i) the large B(1)-C(1)-C(2) angle (130.1(3)°)
1
reported for JP-F values,15 the down-field shift of the 31P NMR
resonance in 2-F suggests that the fluorine atom may actually be
chelated by the borane and phosphonium moieties of [2]+.
This conclusion was confirmed by determination of the X-ray crystal
structure of 2-F (Figure 1). Indeed, the boron bound fluorine atom
F(1) (B(1)-F(1) ) 1.482(3) Å) is located only 2.666(2) Å away from
the P(1) atom, which is well within the sum of the van der Waals
radii of the two elements (ca. 3.45 Å).16 Another conspicuous feature
† Texas A&M University.
‡ Universite´ Paul Sabatier.
9
10890 J. AM. CHEM. SOC. 2008, 130, 10890–10891
10.1021/ja804492y CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
obtained from these calculations afforded ∆H ) -7.6 kcal/mol
for the reaction shown in eq 1. The exothermicity computed for
this reaction is in line with experimental findings and indicates that
the fluoride ion affinity (FIA) of [2]+ significantly exceeds that of
[1]+. Furthermore, [1]+ and [2]+ have virtually identical LUMO
energies (-2.57 eV for [1]+ and -2.56 eV for [2]+) thus suggesting
that inductive effects in these cationic boranes19 do not play a major
role in the increased FIA of [2]+.
In summary, the results reported herein show that the F- affinity
of cationic boranes can be drastically enhanced via cooperative
effects arising, in the present case, from the enforced proximity of
a Lewis acidic phosphonium and borane moiety.
Figure 1. Crystal structure of [2]+ (left) and 2-F (right) (50% ellipsoid,
mesityl groups represented by thin lines, H-atoms omitted). Pertinent
parameters are provided in the text.
Acknowledgment. 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 as well
as by the Centre National de la Recherche Scientifique, the
Universite´ Paul Sabatier, and the Agence Nationale de la Recherche
(program BILI).
Supporting Information Available: Experimental and computa-
tional details. Crystallographic data for [2]I and 2F in CIF format. This
material is available free of charge via the Internet at http://pubs.acs.org.
Figure 2. AIM and NBO analyses of the B-FfP interaction. (Top) AIM
electron density map with relevant bond paths and bond critical points;
(bottom) NBO contour plot showing the lp(F)fσ*(P-C) interaction.
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