a mixture of 2-OTf (21 mg) and KF (45 mg) in 0.5 mL of a
MeOH/H2O (3:2, v/v) solution results in the precipitation of
2-F (12 mg, 75% yield). These observations show that cationic
boranes such as 2þ canbeusedtosequesterF- ions under wet
conditions, making them compatible with inherently wet F-
salts (Scheme 1). The formation of such fluoroborate sulfo-
nium species is not unprecedented and has been recently
observed by our group in two prior instances.7
Scheme 1. Reactions Sequence Showing the Use of 2þ for the
Capture of Fluoride Ions and Their Triggered Releasea
The zwitterion 2-F can be easily recovered by filtration as
an air-stable and nonhygroscopic solid. The crystal struc-
ture of 2-F has been determined. It crystallizes in the
monoclinic P2(1)/c space group with two independent mole-
cules in the asymmetric unit (Figure 1). Both independent
a Edel corresponds to the energy provided by the lp(S)fp(B) and
lp(F)fσ*(S-C) interactions to the stability of 1 or 2-F, respectively.
mind, we have now developed a strategy which allows for (i)
the capture of F- under wet conditions and (ii) the triggered
release of F- and its transfer to organic substrates in dry
organic solutions. This work was partly inspired by the
contribution of DiMagno on “aromatic fluoride relay”, an
approach used to generate TBAF* starting from KF.4g
We have previously reported the synthesis and structure
of the sulfonium borane salt 2-OTf.5 Unlike other cationic
boranes, this derivative showed no affinity for F- in water.6
Interestingly, when this reaction is carried out in MeOH, 2-
OTf is readily converted into the corresponding fluoride
adduct 2-F. The 11B NMR resonance of 2-F at 9.0 ppm and
the 19F NMR signal at -161.1 ppm are consistent with the
presence of a typical triarylfluoroborate anion.6 Formation
of 2-F does not require the use of dry methanol. Instead, 2-F
also precipitates from concentrated MeOH/H2O solutions
containing large fractions of water. For example, sonicating
Figure 1. Left: Crystal structure of 2-F (only one independent
molecule is shown, ellipsoids are drawn at 50% probability levels,
hydrogen atoms omitted for clarity). Right: NBO contour plot
showing the lp(F)fσ*(S-C) donor-acceptor interaction.
molecules feature very similar structures. The boron-bound
fluorine atom is separated from the sulfur atom by an
average distance of 2.53 A, which is well within the sum of
van der Waals radii of the two elements (ca. 3.3 A).8 The
average F-S-CMe angle of 171.7° indicates that the
fluorine atom occupies an axial coordination site directly
opposite to one of the sulfur-bound methyl groups. These
geometrical parameters, which are reminiscent of those
observed in other fluoroborate sulfonium species,7 suggest
the presence of an interaction between the fluorine and
sulfur atom. To confirm this view, the structure of 2-F was
computationally optimized (DFT, functional: B3LYP;
mixed basis set: B, F: 6-31þg(d0); S: 6-31þg(d); C, H:
6-31 g) and subjected to a Natural Bond Orbital (NBO)
analysis. This analysis indicates that the short F-S separa-
tion present in 2-F corresponds to an lp(F)fσ*(S-C)
donor-acceptor interaction which contributes 7.0 kcal/mol
to the stability of the molecule (Figure 1). Altogether, these
results suggest that the ability of 2þ to complex F- in wet
methanol arises from favorable Coulombic effects which
are complemented by the formation of a B-FfS chelate
motif. Realizing that the absence of such interactions would
greatly increase the lability of the boron-bound fluoride anion,
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