Synthesis of B/Si bidentate Lewis acids, o-(fluorosilyl)(dimesitylboryl) benzenes, and their fluoride ion affinity

Article abstract of DOI:10.1021/ja710615r

o-(Fluorosilyl)(dimesitylboryl)benzenes have been prepared as colorless crystals by reacting fluorodimesitylborane with o-(fluorodimethylsilyl)phenyllithium and o-(fluorodiphenylsilyl)phenyllithium. The o-(fluorosilyl)(dimesitylboryl)benzenes serve as B/Si bidentate Lewis acid and efficiently capture fluoride ion from potassium fluoride in the presence of [2.2.2]cryptand or 18-crown-6 in toluene, giving the corresponding μ-fluoro bridged products. The structures were characterized by X-ray crystal structure analysis and multinuclear NMR spectroscopy. Fluoride ion affinities of the o-(fluorosilyl)(dimesitylboryl)benzenes were evaluated in comparison with non-silylated triarylborane. Copyright

Full text of DOI:10.1021/ja710615r

Published on Web 03/08/2008
Synthesis of B/Si Bidentate Lewis Acids,
o-(Fluorosilyl)(dimesitylboryl)benzenes, and Their Fluoride Ion Affinity
Atsushi Kawachi,* Atsushi Tani, Junpei Shimada, and Yohsuke Yamamoto
Department of Chemistry, Graduate School of Science, Hiroshima UniVersity, 1-3-1 Kagamiyama,
Higashi-Hiroshima 739-8526, Japan
Received November 26, 2007; E-mail: kawachi@sci.hiroshima-u.ac.jp
Homonuclear bidentate Lewis acids1 consisting of two identical
Scheme 1. Preparation and Fluoride Binding of 1 and 2
Lewis acidic sites have received much attention in the fields of
organic synthesis and molecular recognition because they can
efficiently accept Lewis bases compared to corresponding mono-
dentate Lewis acids. Since the Lewis acidity of tricoordinate boranes
and tetracoordinate silanes is well recognized and commonly used
in organic synthesis, it is natural that the chemistry of bidentate
Lewis acids having two boron centers2 or two silicon centers3 is
pursued with much interest. However, there are few examples of
bidentate Lewis acids having boron and silicon centers.4,5 Katz
prepared 1-dimethylboryl-8-fluorodimethylsilylnaphthalene (I) but
found that its fluoride complex II was less stable than its non-
silylated counterpart III.4a This may be due to the unsuitable
geometry of the 1,8-naphthylene backbone for the formation of the
Si(V)-F-B(IV) bridge.
distance (2.63 Å) of the minimal nonbonded approach between Si
and F,11 and those in 8 are comparable to the bridging Si-F bonds
in µ-fluoro-bis(silicates) (1.700(3)-2.369(3) Å)3a and in hexakis-
(fluorodimethylsilyl)benzene (av 2.39 Å).3b As the Si-F2 distance
shortens in the order of 7a > 7b > 8a > 8b, the sum of the three
C-Si-C angles increases in the order of 7a (349°) < 7b (352°) <
8a, 8b (354°), indicating the increase of the pentacoordination
character at the silicon atom. AIM analysis of 7a and 8b exhibited
In this context, we have focused our attention on the o-phenylene
backbone,2d,e,3a,c which affords a suitable scaffold to boron and
silicon to chelate fluoride ion. Here we report the facile preparation
of new B/Si bidentate Lewis acids, o-(fluorosilyl)(dimesitylboryl)-
benzenes6 1 and 2, and disclose their ability to bind fluoride ion
by forming a µ-fluoro bridge.
the bond path and the bond critical point between Si and F2 in
3
each compound:12 electron density F(r) ) 0.0212 e/a0 (7a) and
2
0.0346 e/a03 (8b); Laplacian value ∇ F(r) ) 0.0506 e/a05 (7a) and
5
0.0604 e/a0 (8b). As far as we know, this work is the first to
accomplish the solid-state characterization of the intramolecular Si-
(V)-F-B(IV) bridge.
Compounds 1 and 2 were readily prepared by reacting fluorodi-
mesitylborane with o-(fluorosilyl)phenyllithiums 37 and 4,8 respec-
tively, as shown in Scheme 1. In the presence of [2.2.2]cryptand
or 18-crown-6 1 and 2 efficiently capture fluoride ion from KF in
toluene, giving µ-fluoro bridged 7a (75%), 7b (83%), 8a (80%),
and 8b (85%) as colorless crystals after recrystallization from THF-
hexane.9
The binding modes of the fluoride ion in 7 and 8 were revealed
by X-ray crystallographic analysis. A summary of the bond lengths
and the bond angles of 7 and 8 is provided in the Supporting
Information (Table S-1), and the crystal structure of 8b is shown
in Figure 1 as a representative. Whereas 7a, 8a, and 8b exist as
solvent-separated ion pairs with K+ fully coordinated by the
cryptand or crown and THF molecules, 7b shows anion-cation
contact (Si-F1‚‚‚K+(18-crown-6)(thf) ) 2.808(3) Å). While
fluoride F2 is tightly bonded to the boron atom (B-F2 ) 1.491(2)
(7a); 1.492(4) (7b); 1.508(2) (8a); 1.514(3) (8b) Å)10 to form a
fluoroborate moiety, it simultaneously coordinates to silicon with
the following Si-F2 distances (2.5309(13) (7a); 2.4385(23) (7b);
2.2655(14) (8a); 2.2481(13) (8b) Å) and linear F1-Si-F2 ar-
rangement (175°-176°), forming a pseudopentacoordinate geom-
etry at the silicon atom. The Si-F2 distances are shorter than the
The structures of 7 and 8 in the THF solution are revealed by
multinuclear NMR analysis (Table S-1). Cryptand complex 7a and
crown ether complex 7b exhibit almost the same spectra, which
indicates that 7b also exists as a solvent-separated ion pair in THF.
11B chemical shifts (δ ) 6 (7); 7 (8)) fall in the region typical for
tetracoordinate borates. In 19F NMR spectra the terminal fluorine
(F1) signal resonates in the low field region (δ ) -147 (7); -145
2
(8)) and is accompanied by JF-F coupling (9 Hz (7); 18 Hz (8)),
whereas the bridging fluorine (F2) exhibits a broad signal in the
high field region (δ ) -152 (7); -148 (8)). The coordination of
F2 to silicon is also reflected in the 29Si NMR spectra: (i) 29Si
signals (δ ) 6.5 (7); -32.6 (8)) were shifted upfield by 14 ppm in
7 and by 29 ppm in 8 relative to those in 1 and 2, respectively; (ii)
Si-F1 coupling constants in 7 and 8 (260-266 Hz) were reduced
relative to those of 1 (278 Hz) and 2 (284 Hz), suggesting axial
elongation of the Si-F1 bonds in the pentacoordinate geometry;
and (iii) Si-F2 coupling constants increase from 7 Hz in 7 to 17
Hz in 8 with reduced Si-F2 distances. These trends are consistent
with the general tendency that aryl substitution increases the Lewis
acidity of the silicon to stabilize pentacoordinate silicates.9
Exchange of the two fluorines in 8 was observed by variable-
temperature 19F NMR analysis. The two fluorine signals in both
9
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J. AM. CHEM. SOC. 2008, 130, 4222-4223
10.1021/ja710615r CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
In conclusion, o-(fluorosilyl)borylbenzenes 1 and 2 have high
fluoride ion affinity compared to the triarylborane 9, which is in
clear contrast to 1-boryl-8-silylnaphthalene I.4a Changing the
substituents on the boron and silicon centers may lead to diversity
of the B/Si bidentate Lewis acids for selective binding of a particular
Lewis base, such as detection of a specified halide ion.
Acknowledgment. This work was supported by Grants-in-Aid
for Scientific Research, Nos. 17550038 and 18037052, the latter
of which corresponds to Priority Area “Advanced Molecular
Transformations of Carbon Resources” from the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
Figure 1. Crystal structure of 8b (30% thermal ellipsoids). K+(18-crown-
6)(thf)2 and hydrogen atoms are omitted for clarity.
Supporting Information Available: Summary of structural data;
experimental details; tables of crystallographic data for 7a, 7b, 8a, and
8b (PDF/CIF); and computational works. This material is available free
of charge via the Internet at http://pubs.acs.org.
Scheme 2. Competition Reactions of Fluoride Ion Affinity
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8a and 8b underwent coalescence at 361 K in DMSO-d6 (k361
)
4.3 × 103 s-1, ∆Gq ) 15 kcal/mol). In contrast, 7 exhibited no
361
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Fluoride ion affinities of 1 and 2 were investigated by competition
experiments with non-silylated triarylborane13 9 and its fluoroborate
10, as shown in Scheme 2. Mixing equimolar amounts of 1 and 10
in THF at room-temperature gave 7b and 9 (10/7b ) 5:95; K298
)
3.6 × 102, ∆G298 ) -3.5 kcal/mol).14,15 Phenyl derivative 2 also
captured fluoride ion from 10 to give 8b and 9 (10/8b ) 22:78;
K298 ) 13, ∆G298 ) -1.5 kcal/mol).14,15 Thus, the presence of
o-fluorosilyl groups tended to increase fluoride ion affinity of the
triarylboranes.16 These results also suggested that fluoride ion
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mol).14
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F]-): their structures were optimized at the B3LYP/6-31G(d) level,
and their energies were obtained at the MP2/6-31+G(d,p) level.17
∆G calculated for {1 + [2-F]- f [1-F]- + 2} (-1.68 kcal/mol) is
consistent with ∆G estimated from the equilibrium ratio of 7b/8b
(-1.8 kcal/mol). We noticed that the Si-F2 distance in [2-F]-
(2.405 Å) is longer than that in the crystal structures of 8a and 8b,
whereas the Si-F2 distance in [1-F]- (2.533 Å) is equal to that in
the crystal structure of 7a. It is plausible that elongation of the
Si-F2 bond in [2-F]- is due to steric repulsion between phenyl
groups and mesityl groups, and this is the case of 8 in THF.17 Total
stability of 8 may be determined by balance between attractive
interaction between Si and F2 and steric repulsion between the aryl
groups, which destabilizes 8 in THF and lowers fluoride ion affinity
of 2 relative to 1.
to be between 1.39 and 1.48 Å (CSD version 5.28).
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(14) Molar ratios were determined by measuring the integral ratios in the 19F
NMR spectra at 25 °C, see the Supporting Information.
(15) Reaction of 7b and 9 afforded 1 and 10 in the ratio of 10/7b ) 2:98.
Reaction of 8b and 9 afforded 2 and 10 in the ratio of 10/8b ) 21:79.
(16) Fluoride binding constants of triarylboranes were reported to be g105
M-1
: (a) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc.
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Chem. 2006, 45, 9137. See also ref 2g.
(17) The 29Si shifts predicted from the optimized structures with the GIAO
method at the RHF/6-311+G(2d,p) level were identical to the 29Si shifts
in THF solution: δ 6.39 for [1-F]-; δ -30.28 for [2-F]-.
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