Synthesis and anion affinity of a bidendate sulfonium fluorosilane lewis acid

Article abstract of DOI:10.1021/ol902641v

[Chemical equation presented] The cationic fluorosilane [1-Ant ₂FSi-2-Me₂S-(C₆H₄)]⁺ (2⁺) readily complexes fluoride Ions to afford the corresponding zwitterionic difluorosilicate complex 1-Ant₂F₂Si-2-Me ₂S-(C₆H₄) (2-F) with a binding constant in CHCl₃ of 7 (±1) × 10⁶ M^-1. Structural and computational results indicate that the high fluorophilicity of 2⁺ arises from both Coulombic and cooperative effects reflected by the formation of a Si-F-S bridge with a F→S distance of 2.741(3) A.

Full text of DOI:10.1021/ol902641v

ORGANIC
LETTERS
2010
Vol. 12, No. 3
600-602
Synthesis and Anion Affinity of a
Bidendate Sulfonium Fluorosilane Lewis
Acid
Youngmin Kim, Mieock Kim, and Franc¸ois P. Gabba¨ı*
Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843
francois@tamu.edu
Received December 7, 2009
ABSTRACT
The cationic fluorosilane [1-Ant2FSi-2-Me2S-(C6H4)]+ (2+) readily complexes fluoride ions to afford the corresponding zwitterionic difluorosilicate
complex 1-Ant2F2Si-2-Me2S-(C6H4) (2-F) with a binding constant in CHCl3 of 7 ((1) × 106 M-1. Structural and computational results indicate
that the high fluorophilicity of 2+ arises from both Coulombic and cooperative effects reflected by the formation of a Si-FfS bridge with a
FfS distance of 2.741(3) Å.
One of the main themes in the chemistry of polydentate
Lewis acids is the discovery of new molecular structures that
can support anion chelation. Modulating the structures and
varying the elements involved in anion binding provide an
effective way to control the affinity of such systems. To date,
a great deal of effort has been devoted to the chemistry of
In some of our recent exploratory studies, we have
boron-based polydentate Lewis acids1 and their use as sensors
demonstrated that the fluoride ion affinity of boranes could
for the potentially toxic fluoride anion.2 By contrast, and
be greatly increased by incorporation of proximal third row
onium functionality as in III.7 In addition to facilitating anion
despite the widespread use of tetracoordinate halosilanes as
Lewis acids in organic synthesis,3 much less is known about
binding via inductive and Coulombic effects, the phospho-
silicon-based polydentate Lewis acids.4–6 Some of the most
notable examples of such compounds include I,5 which
(4) Katz, H. E. J. Am. Chem. Soc. 1986, 108, 7640–7645. Jung, M. E.;
displays a high affinity for fluoride anions, and II,6 in which
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C.; Sakurai, H. J. Am. Chem. Soc. 1994, 116, 3595–6. Brondani, D.; Carre,
the silicon atom assists fluoride binding at the boron atom.
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10.1021/ol902641v  2010 American Chemical Society
Published on Web 01/07/2010

spectroscopy. Addition of 5.0 equivalents of Cl-, Br- or I- as
tetrabutylammonium salts to a CDCl3 solution of [2]+ resulted
in the formation of the neutral silane 1 (δ(19F) -141.0 ppm)
indicating demethylation of the aryldimethylsulfonium moiety
of [2]+. By contrast, addition of TBAF to [2]+ resulted in the
formation of 2-F (Scheme 2). 2-F could be easily isolated from
Figure 1. “Onium-based strategy”: illustration of the forces and
bonding interactions involved in anion binding by a bidentate 3rd
row onium/Lewis acid.
Scheme 2.
Formation of 2-F and 19F NMR spectrum
nium ion of III becomes hypervalent and engages the anion
in a donor-acceptor interaction (Figure 1). Similar effects
have been observed in the chemistry of sulfonium boranes
as cyanide receptors.8 As part of our ongoing endeavor in
the chemistry of polydentate Lewis acids, we have now
decided to determine if this “onium-based strategy” (Figure
1) could be extended to silicon Lewis acids.
Triarylfluorosilanes bind fluoride anions to form the
corresponding triaryldifluorosilicates.9,10 Careful studies by
Tamao and Yamaguchi,10 who showed that Ant3SiF (Ant )
9-anthryl) can be used as a fluoride ion sensor in organic
solvents, led us to consider such fluorosilanes as a starting
point for our studies. To obtain a fluorosilane that could
easily be converted into a cationic derivative, we allowed
o-lithiothioanisole to react with di(9-anthryl)difluorosilane11
in THF at -78 °C (Scheme 1). The fluorosilane 1 was
converted into [2]OTf by reaction with MeOTf. Both 1 and
[2]OTf have been characterized by conventional spectro-
scopic methods. The 29Si NMR resonances of these new
compounds were detected at -2.4 ppm (1JSi-F ) 284.2 Hz)
the reaction of [2]OTf with tetrabutylammonium triphenylsi-
lyldifluoride in CH2Cl2. Some of its salient NMR spectroscopic
features in DMSO-d6 include: 1) two separate sulfur-bound
methyl signals detected at 2.40 and 2.90 ppm in the 1H NMR
2
spectrum; 2) two doublets at -49.5 and -65.1 with JF-F
)
62.4 Hz in the 19F NMR spectrum corresponding to the silyl-
bound fluorine atoms (Scheme 2); 3) a 29Si NMR resonance at
-92.7 ppm (dd, 1JSi-F ) 266.7 Hz, 1JSi-F ) 256.5 Hz) whose
chemical shift is comparable to that observed for [Ant2PhSiF2]-
(δ ) -97.2, JSi-F ) 262.9 Hz).10 Altogether, these spectro-
scopic features indicate the presence of two nonequivalent
fluorine atoms bound to the silicon center. As indicated by 19F
NMR, the chemical shift of one of these two fluorine nuclei is
significantly deshielded and exhibits a smaller 1JSi-F, possibly
suggesting the presence of an interaction with the neighboring
sulfonium center.12 The detection of two distinct sulfur-bound
methyl group resonances corroborates this possibility. To firmly
understand the coordination environment of the silicon and
sulfur atoms, the crystal structure of in 2-F was determined.
for 1 and -0.5 ppm (1JSi-F ) 282.0 Hz) for [2]+. In the 19
F
NMR spectra, the slilicon bound fluorine nucleus gives rise
to a resonance at -141.0 ppm for 1 and -138.0 ppm for
[2]+ whose chemical shift is close to that of Ant2PhSiF (δ
-143.7).10 Salt [2]OTf is stable in organic solvents but
decomposes in the presence of water.
Scheme 1. Synthesis of 1 and [2]OTf
With this new cationic silane in hand, we decided to study
the responses of [2]+ to different anions using 19F NMR
(8) Kim, Y.; Zhao, H.; Gabba¨ı, F. P. Angew. Chem., Int. Ed. 2009, 48,
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1987, 26, 760–5. Chuit, C.; Corriu, R. J. P.; Reye, C.; Young, J. C. Chem.
ReV. 1993, 93, 1371–1448. Pilcher, A. S.; Ammon, H. L.; Deshong, P.
J. Am. Chem. Soc. 1995, 117, 5166–5167. Yamaguchi, S.; Akiyama, S.;
Tamao, K. Organometallics 1999, 18, 2851–2854. Handy, C. J.; Lam, Y. F.;
Figure 2. (Left) Structure of 2-F (50% ellipsoid, H-atoms omitted
for clarity). Pertinent metrical parameters are provided in the text.
(Right) NBO contour plot showing the lp(F)fσ*(S-C) interaction
in 2-F.
DeShong, P. J. Org. Chem. 2000, 65, 3542–3543
.
Org. Lett., Vol. 12, No. 3, 2010
601

Examination of the crystal structure of 2-F confirmed that
the silicon atom adopts a trigonal-bipyramidal geometry
(F(1)-Si(1)-F(2) ) 177.32(15)°, Σ(C-Si-C) ) 360°)
(Figure 2). Although close to those observed in the
[Ant3SiF2]- anion (1.710-1.716 Å),10 the silicon-fluorine
bond distances in 2-F (Si(1)-F(1) ) 1.706(3) Å; Si(1)-F(2)
) 1.732(3) Å) differ by almost 0.03 Å. Further inspection
of the structure indicates that the F(2) atom is separated from
S(1) by only 2.741(3) Å which is well within the sum of
van der Waals radii of two elements (ca. 3.27 Å). The
resulting F(2)-S(1)-C(8) angle of 166.5(2)° is also close
to linearity. Collectively, these metrical parameters suggest
the presence of a bonding interaction between the F(2) and
S(1) atoms. This view is confirmed by an NBO analysis
carried out at the DFT optimized geometry (Figure 2). This
analysis reveals a donor-acceptor interaction involving a
fluorine lone pair (lp(F)) as a donor and a carbon-sulfur
σ*-orbital (σ*(S-C)) as the acceptor (Figure 2). A deletion
calculation suggest that this interaction stabilizes the mol-
ecule by 5.60 kcal/mol. Presumably, existence of this
interaction is responsible for the lengthening of the Si(1)-F(2)
bond as well as for the somewhat deshielded 19F NMR
chemical shift measured for the F(2) nucleus.
Formation of 2-F can be followed by monitoring the
UV-vis absorption spectrum of [2]+ in chloroform upon
incremental addition of TBAF (Figure 3). Analysis of the
spectral changes indicate that formation of 2-F induces a blue
shift of the anthryl-based absorption bands. As previously
explained for other anthrylfluorosilane species,10 this re-
sponse can be assigned to a decrease in intramolecular
anthryl-anthryl π-stacking interactions induced by the
change in coordination geometry at the silicon center. Fitting
of this data to a 1:1 binding isotherm affords a fluoride
binding constant K ) 7 ((1) × 106 M-1. Under these
conditions, neutral silane 1 captures fluoride with an as-
sociation constant K ) 8((1) M-1. These experiments
demonstrate that the fluoride affinity of [2]+ exceeds that of
neutral silane by at least 5 orders of magnitude. The drastic
Figure 3
.
(Left) Absorbance change of a solution of [2]+ (5.00 ×
10-5 M) upon successive additions of fluoride in chloroform. (Right)
Binding isotherm measured at 403 nm and fitted with K ) 7 ×
106 M-1, ꢀ([2]+) ) 14 600 M-1 cm-1 and ꢀ(2-F) ) 2600 M-1 cm-1
.
enhancement observed in the fluoride ion affinity of [2]+ is
assigned to the presence of the sulfonium moiety which: (i)
provides a Coulombic and inductive drive for the formation
of the difluorosilicate compound and (ii) engages the fluoride
anion in a stabilizing lp(F)fσ*(S-C) interaction. Although
triarylfluorosilanes typically display a lower fluoride affinity
than triarylboranes,13 we note that the fluoride binding
constant of [2]+ in chloroform is almost equal to the value
of 6.5 ((0.5) × 106 M-1 measured for the boron-based
receptor [p-Mes2B(C6H4)PMePh2]+.14
The results reported in this communication indicate that
proximal third row onium ions can serve to enhance the
fluoride affinity of fluorosilanes via cooperative effects. These
results also demonstrate the viability of sulfonium ions as
Lewis acidic binding sites for fluoride binding.
Acknowledgment. This work was supported by NSF
(CHE-0646916), the Welch Foundation (A-1423), and the
U.S. Army Medical Research Institute of Chemical Defense.
Supporting Information Available: Experimental details.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL902641V
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