Hydrogen- and fluorine-bridged disilyl cations and their use in catalytic C-F activation

Article abstract of DOI:10.1021/ja061800y

The hydrogen-bridged disilyl cation 6 with an 1,8-naphthalenediyl backbone was synthesized and was characterized by NMR spectroscopy and X-ray crystallography, supported by quantum mechanical computations. The SiHSi linkage is symmetrical, corresponding to a single minimum potential, and the structural parameters are in agreement with the presence of a two electron-three center bond in 6. Treatment of disilyl cation 6 with alkyl fluorides yields the disilylfluoronium ion 10. The SiFSi group in the disilyl fluoronium ion 10 is symmetrical with an average SiF bond length of 175.9(8) and a bent angle β= 130°. Both cations catalyze the hydrodefluorination reaction of alkyl and benzyl fluorides to give alkanes.

Full text of DOI:10.1021/ja061800y

Published on Web 07/12/2006
Hydrogen- and Fluorine-Bridged Disilyl Cations and Their Use
in Catalytic C-F Activation
Robin Panisch, Michael Bolte, and Thomas Mu¨ller*
Contribution from the Institut fu¨r Reine und Angewandte Chemie der Carl Von Ossietzky
UniVersita¨t Oldenburg, Carl Von Ossietzky Str. 9-11 D-26129 Oldenburg, Federal Republic of
Germany, and Institut fu¨r Anorganische und Analytische Chemie der Goethe UniVersita¨t
Frankfurt, D-60439 Frankfurt/Main, Federal Republic of Germany
Received March 31, 2006; E-mail: thomas.mueller@uni-oldenburg.de
Abstract: The hydrogen-bridged disilyl cation 6 with an 1,8-naphthalenediyl backbone was synthesized
and was characterized by NMR spectroscopy and X-ray crystallography, supported by quantum mechanical
computations. The SiHSi linkage is symmetrical, corresponding to a single minimum potential, and the
structural parameters are in agreement with the presence of a two electron-three center bond in 6.
Treatment of disilyl cation 6 with alkyl fluorides yields the disilylfluoronium ion 10. The SiFSi group in the
disilyl fluoronium ion 10 is symmetrical with an average SiF bond length of 175.9(8) and a bent angle â )
130°. Both cations catalyze the hydrodefluorination reaction of alkyl and benzyl fluorides to give alkanes.
cations, 4.4 A similar bonding situation to that in 1 was found
in neutral silylated vinylboranes 5 which were described by
Wrackmeyer and co-workers.5 These authors also noticed an
enhanced reactivity of the Si-H linkage toward CC multiple
bonds in hydrosilylation reactions.5a,d
Introduction
Multiple-center bonding is a common feature in the chemistry
of electron-deficient compounds of the early s- and p-block
elements. For silicon this type of bonding is possible in low-
valent species such as silylium ions. Recently, disilyl cation 1
Although disilyl cation 1 is stable at room temperature, its
high reactivity prevented a thorough study of its chemistry. In
an attempt to increase the stability of the cation and to fix the
two silyl groups spatially in a suitable conformation for the
formation of the SiHSi linkage, we envisaged disilyl cation 6
with a naphthyl backbone as a target molecule. The close
relation of cation 6 to the isolobal hydrogen-bridged boranate
7, synthesized by Katz,6 is obvious and opens intriguing
possibilities7 for the follow-up chemistry based on disilyl cation
6. Here we describe the synthesis and structural characterization
with a symmetric SiHSi bridge was synthesized in our labora-
tories and was characterized by multinuclear NMR spectroscopy
supported by the results of quantum mechanical calculations.1
Consecutively, the group of Sekiguchi characterized silyl cation
2 by NMR spectroscopy2 and Lickiss and co-workers report in
a preliminary note selected details from the solid-state structure
of the tetrakis(pentafluorophenyl)borate ([B(C6F5)4]-, TPFPB)
salt of cation 3.3 Recently, Reed and colleagues were able to
crystallize carborane salts of simple hydrogen-bridged disilyl
(3) Lickiss, P. D.; Masangane, P. C. Sohal, W.; Veneziani, G. L. In
Organosilicon Chemistry V; Auner, N., Weis, J., Eds.; Wiley-VCH: New
York, 2003, p 45.
(4) Hoffmann, S. P.; Kato, T. Tham, F. S.; Reed, C. A. Chem. Commun. 2006,
767.
(5) (a) Wrackmeyer, B.; Tok, O. L.; Bubnov, Y. N. Angew. Chem., Int. Ed.
1999, 38, 124. (b) Wrackmeyer, B.; Tok, O. L. Magn. Reson. Chem. 2002,
40, 406. (c) Wrackmeyer, B.; Milius, W.; Tok, O. L. Chem. Eur. J. 2003,
9, 4732. (d) Wrackmeyer, B.; Tok, O. L.; Bubnov, Y. N. Appl. Organomet.
Chem. 2004, 18, 43. (e) Wrackmeyer, B.; Tok, O. L.; Khan, A.; Badshah,
A. Appl. Organomet. Chem. 2005, 19, 1249. (f) Wrackmeyer, B.; Tok, O.
L.; Khan, A.; Badshah, A. Appl. Organomet. Chem. 2006, 20, 99.
(6) (a) Katz, H. E. J. Am. Chem. Soc. 1985, 107, 1420. (b) Katz, H. E. J. Org.
Chem. 1985, 50, 5027.
(1) (a) Mu¨ller, T. Angew. Chem., Int. Ed. 2001, 40, 3033. (b) Mu¨ller, T. In
Organosilicon Chemistry V; Auner, N., Weis, J., Eds.; Wiley-VCH: New
York, 2003, p 34.
(2) Sekiguchi, A.; Muratami, A.; Fukaya, N.; Kabe, Y. Chem. Lett. 2004, 33,
530.
9
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J. AM. CHEM. SOC. 2006, 128, 9676-9682
10.1021/ja061800y CCC: $33.50 © 2006 American Chemical Society

Hydrogen- and Fluorine-Bridged Disilyl Cations
A R T I C L E S
Scheme 1. Synthesis of Disilyl Cation 6
Table 1. Selected NMR Parameter for Cations 6 and 10
of cation 6, the transformation of cation 6 to an unusual
disilylated fluoronium ion, and the use of both cations in a
catalytic hydrodefluorination reaction.8
|
1J(SiXSi)
Hz
|
,
3J(XSiCH3),
Hz
compd
X
δ
29 Si
δ
X
6a
H
H
H
F
54.4
62.5
54.4
77.2
77.2
45.7
42.3
3.34
4.06
3.33
1.8
6b
2.1c
Results and Discussion
6d
10a
10d
243.0
243.0
-144.0
-144.2
13.7
The precursor naphthyl silane 8 was obtained in satisfactory
9
F
yield from the reaction of 1,8-dilithionaphthalene 9 with
dimethylchlorosilane and characterized by standard techniques
(see Scheme 1 and, for details, the Experimental Section).
Hydridobridged disilyl cation 6 was prepared by reaction of 8
with trityl cation in benzene or toluene as described previously
for cation 1 (see Scheme 1).1 The counteranion in this reaction
was TPFPB. The salt 6·TPFPB was isolated as a white-brown
glassy amorphous solid, which was recrystallized from C6F6 to
obtain colorless crystals.
a In benzene-d6 at 303 K. b Computed at GIAO/B3LYP/6-311G(2d,p)//
MP2/6-31G(d,p). Chemical shifts δ reported relative to computed σ(29Si,
3
TMS) ) 329.14 and σ(1H, TMS) ) 32.06. c Mean value of three J(HH)
values: 4.7, 0.6, and 1.1 Hz. d In toluene-d8 at 303 K.
The 1,8-naphthalenediyl backbone confers to cation 6 high
stability, that is, while the cyclic cation 1 decomposes at
temperatures higher than room temperature, cation 6 is stable
for 3 days in toluene at 90 °C. This increased stability greatly
simplifies the manipulation of the compound and allows
studying its reactivity in detail.
Cation 6 is clearly identified by its characteristic 1H and 29Si
NMR resonances, which closely resemble those found for the
1
cation 1 and the related cation 2.1,2 The unusually shielded H
1
1
NMR signal for the SiH group in 6 (δ H ) 3.34, ∆δ H )
-1.85 compared to silane 8)10 indicates its hydrogen-bridged
structure. In addition, the deshielded 29Si NMR resonance at δ
1
29Si ) 54.4 splits in the H-coupled 29Si INEPT spectra into a
1
doublet of multiplets with a J(SiH) coupling constant of 46
1
Hz, which was also identified in the H NMR spectra. This
Figure 1. Ortep plot of the asymmetric unit of 6·TPFPB·C6F6. (Color code:
B, dark gray, F, medium gray; Si, light gray; C, white. Thermal ellipsoids
are drawn at the 50% probability level).
remarkably small 1J(SiH) coupling constant must be compared
1
to J(SiH) ) 193 Hz found for the precursor silane 8 and it is
characteristic for the SiHSi linkage in disilyl cations.1,2 A similar
reduction is found for the 3J(HSiCH3) coupling constant between
the methyl protons and the bridging hydrogen atom (3J(HSiCH3)
) 1.8 Hz in 6, compared to 3.3 Hz found for silane 8). The
NMR spectroscopic parameters are independent from the
solvent, as indicated by the nearly identical chemical shifts
detected in benzene-d6 and in toluene-d8 (see Table 1).11
Therefore, the interactions between solvent and cation are small
as long as arenes are used as solvents.11b
Colorless crystals suitable for X-ray analysis were obtained
by recrystallization of 6·TPFPB from C6F6. The solid-state
structure of the salt reveals an asymmetric unit that contains
the cation 6 and the TPFPB anion along with one molecule of
the solvent C6F6 (see Figure 1). The cation is well-separated
from the anion and from the C6F6 molecule. The nearest
approach between the cation 6 and C6F6 is 325.0(8) pm (between
F and C2), while the closest contact between 6 and the TPFPB
anion is 326.6(5) pm (between F and C1).14
(7) See, for example, the chemistry of the 1,8-substituted boranes and
gallanes: (a) Hoefelmeyer, J. D.; Schulte, M.; Tschinkl, M.; Gabba¨ı, F. P.
Coord. Chem. ReV. 2002, 235, 93. (b) Melaimi, M.; Gabba¨ı, F. P. AdV.
Organomet. Chem. 2005, 53, 61.
(8) Scott, V. J. C¸ elenligil-C¸ etin, R.; Ozerov, O. V. J. Am. Chem. Soc. 2005,
127, 2852.
(9) Neugebauer, W.; Clark, T.; Schleyer, P. v. R. Chem. Ber. 1983, 116, 3283.
(10) ∆δ is calculated as ∆δ ) δ(6) - δ(8).
The two silicon atoms in cation 6 are tetracoordinated. The
relatively small sum of bonding angles R involving silicon and
(11) (a) For R3Si+ solvent effects on δ 29Si up to 10 ppm are reported (R )
alkyl), e.g. R ) Et:
δ )
29Si (Et3SiC6H6)+ ) 92; δ 29Si (Et3SiC7H8)+
82.12,13 (b) Addition of solvents of higher nucleophilicity leads to reactions
between the cation and the solvent. For example, in the presence of pivaloyl
nitrile the corresponding iminium ion is formed as indicated by NMR
spectroscopy.
(13) (a) Lambert, J. B.; Zhang, S.; Stern, C. L.; Huffman, J. C. Science 1993,
260, 1917. (b) Lambert, J. B.; Zhang, S.; Ciro, S. M. Organometallics 1994,
13, 2430.
(12) For a recent review on organometallic cations of group 14 elements
including silylium ions, see: Mu¨ller, T. AdV. Organomet. Chem. 2005,
53, 155.
(14) The distances between the silicon atoms and this fluorine atom are 351.6-
(5) and 389.9(6) pm, well beyond the sum of the van der Waals radii of
the elements Si and F (320 pm).
9
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A R T I C L E S
Panisch et al.
Figure 2. (a) Experimental molecular structure of disilyl cation 6. Thermal ellipsoids are drawn at the 50% probability level. (b) Computed structure of
disilyl cation 6 (at MP2/6-31G(d,p), molecular point group Cs; bond lengths are given in picometers).
its carbon neighbors for both silyl groups, ∑R, indicates,
however, considerable variations toward planarity (∑R ) 345.5°
and 346.7°, which must be compared to the value for ideal
tetrahedral environment ∑R ) 328.4° and for trigonal planar
coordination ∑R ) 360°). Such relative large ∑R values are
reported for complexes between silylium ions with solvent
molecules [e.g., ∑R ) 341.4(5)° and 342.6(5)° for [Et3-
SiC7H8]+]12,13,15, with anions [e.g., ∑R ) 345.0(10)° and 349.0-
(9)° for [Et3Si][Br6CB11H6]]12,15,16 and, quite recently, also for
the hydrogen-bridged disilyl cation 4 (∑R ) 350.1°).4 The
silicon atoms are slightly bent out of the plane defined by the
naphthyl ring [ϑ(Si1C1C9C10) ) 4° and (ϑ(Si2C8C9C10) ) 11°].
They form a six-membered ring in a distorted half-chair
conformation with the bridging hydrogen atom.
Although the position of the bridging hydrogen atom in 6
was detected in the differential Fourier analysis and the
coordinates of the hydrogen atom were refined, the exact
bonding parameters of the SiHSi linkage in cation 6 are subject
to appreciable errors. Within the error margins the SiHSi bond
is symmetrical with an SiHSi bond angle â ) 132(3)° (see
Figure 2). A slightly unsymmetrical structure, however, cannot
be excluded on the basis of these experimental data. In contrast,
the heavy atom distances in cation 6 are well-defined and can
be used to gauge the quality of geometries predicted by quantum
mechanical calculations.17 Ab initio calculations at the MP2/
6-31G(d,p) level of theory predict for cation 6 a geometry that
is very similar to the experimental structure (see Figure 2).18 In
particular, nearly all heavy atom distances predicted by the
calculations are within the error margins of the experimental
structure. In addition, the computations predict a symmetric
SiHSi bridge with a long Si-H distance of 160.7 pm. The close
accordance between theory and experiment is further substanti-
ated by the comparison of the computed δ 29Si NMR chemical
shift20 and the 1J(SiH) coupling constant22 for cation 6 with the
experimental data (see Table 1).1,12,23,24 A further analysis of
the computed spin-spin coupling constants22 for cation 6 reveals
that the unusually small 1J(SiH) coupling (calculated, 42.3 Hz;
experimental, 45.7 Hz) is a consequence of the strongly reduced
Fermi contact (Fc) term.25 This result suggests a very small
3s(Si)-orbital contribution for the Si-H bonds, in agreement
with a natural bond order (NBO) analysis at the HF/6-311G(d,p)
level of theory26 that indicates a contribution of the 3s(Si)
orbitals to the Si-H-Si bond of less than 10%. Therefore, the
accumulated experimental and theoretical data suggest for cation
6 a hydrogen-bridged structure with a symmetrical two electron-
three center SiHSi bond similar to that found previously for
(19) (a) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (b) Lee, C.; Yang, W.; Parr,
R. G. Phys. ReV. B 1988, 37, 785. (c) Becke, A. D. J. Chem. Phys. 1993,
98, 5648. (d) Johnson, B. G.; Gill, P. M. W.; Pople, J. A. J. Chem. Phys.
1993, 98, 5612.
(20) NMR parameters were calculated for the MP2/6-31G(d,p)-optimized
geometry using the gauge independent atomic orbital (GIAO) method21
and a 6-311G(2d,p) basis set.
(21) (a) Ditchfield, R. Mol. Phys. 1974, 27, 789. (b) Wolinski, K.; Hilton J. F.;
Pulay, P. J. Am. Chem. Soc. 1982, 104, 5667. (c) Cheeseman, J.; Trucks,
G. W.; Keith, T. A.; Frisch, M. J. Chem. Phys. 1996, 104, 5497.
(22) (a) Helgaker, T.; Watson, M.; Handy, N. C. J. Chem. Phys. 2000, 113,
9402. (b) Sychrovsky, V.; Ga¨fenstein, J.; Cremer, D. J. Chem. Phys. 2000,
113, 3530.
(23) 29Si NMR computations for silylium ions and related compounds: (a)
Maerker, C.; Schleyer, P. v. R. In The Chemistry of Organic Silicon
Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K.,
1998; Vol. 2, p 513. (b) Mu¨ller, T.; Zhao, Y.; Lambert, J. B. Organome-
tallics 1998, 17, 278. (c) Kraka, E.; Sosa, C. P.; Gra¨fenstein, J.; Cremer,
D. Chem. Phys. Lett. 1997, 9 (279).
(15) In the crystal structure of the salts [Et3SiC7H8]+·TPFPB and [Et3Si][Br6-
CB11H6] two independent cations were found.
(16) Xie, Z.; Bau, R.; Benesi, A.; Reed, C. A. Organometallics 1995, 14, 3933.
(17) All computations were done with Gaussian 03, Revision B.03 (Frisch, M.
J. et al. Gaussian, Inc., Pittsburgh, PA, 2003).
(18) Cation 6 was initially optimized using Cs symmetry at the hybrid density
functional B3LYP level of theory19 in connection with the 6-31G(d,p) basis.
A subsequent frequency calculation was used to verify the nature of the
stationary point as a minimum. This structure was used as a starting point
for the subsequent MP2/6-31G(d,p) optimization. The geometry (see
Supporting Information) obtained at the B3LYP/6-31G(d,p) level is very
close to that obtained at the MP2 level; i.e., d(SiSi) ) 301.2 pm, d(SiH) )
162.1 pm and R(SiHSi) ) 136.5°. A second minimum structure of 6 at
B3LYP/6-31G(d,p) has C2 symmetry. 6(C2) is not only very close in energy
to 6, but it is also geometrically nearly identically to 6 (see Supporting
Information for further details).
(24) For a monograph on the methodology, see: Calculation of NMR and EPR
Parameter; Kaupp, M., Malkin, V. G., Bu¨hl, M., Eds; Wiley-VCH:
Weinheim, 2004.
(25) The FC contribution to the 1J(SiH) for 6 is calculated to be 40.2 Hz. This
results in an overall 1J(SiH) ) 42.3 Hz. These values must be compared
with an FC contribution of 177.4 Hz predicted for silane 8 and an overall
1J(SiH) coupling of 1J(SiH) ) 177.0 Hz. (B3LYP/6-311G(2d,p)//MP2/6-
31G(d,p)).
(26) (a) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.;
Weinhold, F. NBO 4.0; Theoretical Chemistry Institute, University of
Wisconsin: Madison, WI, 1996. (b) Reed, A. E.; Curtiss, L. A.; Weinhold,
F. Chem. ReV. 1988, 88, 899.
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Hydrogen- and Fluorine-Bridged Disilyl Cations
A R T I C L E S
Scheme 2. Reaction of Cation 6 with Trityl Cation To Give Fluoronium Ion 10
Scheme 3. Reaction of Cation 6 with Alkyl and Benzyl Fluorides
To Give Fluoronium Ion 10
is not reactive toward cation 6 and no conversion is observed,
even after prolonged heating in toluene.
Therefore, there is a qualitative discrimination in the reactivity
of cation 6 toward compounds with different kinds of C-F
bonds, with benzylic C-F bonds being most reactive, interme-
diate reactivity versus C(sp3)-F groups, and no reactivity versus
C(aromatic)-F functionalities.
The 29Si NMR chemical shift of δ 29Si ) 77.2, which is
detected for cation 10, indicates considerable charge accumula-
tion at the silicon atoms. This chemical shift and the doublet
splitting [1J(SiF) ) 243 Hz] are characteristic for the bissilylated
fluoronium ions, and similar values are reported for fluoronium
ion 12.2 The corresponding 19F NMR [δ 19F ) -144.0; 1J(FSi)
cation 1.1,27 The geometric parameters of the SiHSi linkage in
6 resemble that reported for the cations 3 and 4. That is, the
experimental SiH bond length in all three cations is very similar
[165.5 pm (3), 163 pm (6) and 161 pm (4, R ) Me)]28 and it
slightly decreases with increasing SiHSi bent angle â [â ) 97.6°
(3), 132(3)° (6), 160.1° (4, R ) Me)].3,4 Obviously the SiHSi
bent angle â is a function of the ring size, steadily increasing
from the four-membered cyclic cation 3 to the six-membered 6
and the acyclic cations 4. In agreement, computations at the
MP2/6-311G(d,p) level for cation 4 (Me), valid strictly only
for the gas phase, predict a linear SiHSi linkage [r(SiH) ) 160.8
pm, â ) 180°].29
Hydride abstraction from cation 6 by trityl cation is an
extremely slow process and it results in the exclusive formation
of fluoronium ion 10 after 9 h at 75 °C in benzene. A possible
intermediate in this reaction is the disilyl dication 11, which
instantaneously reacts with the weakly coordinating anion
TPFPB with formation of 10.30 The only detectable byproducts
in this reaction are B(C6F5)3 and triphenylmethane, Ph3CH
(Scheme 2). After washing with pentane, the salt 10·TPFPB was
crystallized from toluene.
) 243 Hz] and 13C NMR data of oniumion 10 provide further
evidence for its constitution (see Table 1 and Experimental
Section). The 19F NMR data might be compared to that of the
neutral compound 13 (δ 19F ) -113.8), which shows at 240 K
a degenerate fluorine exchange that is fast on the NMR time
scale31, and to that of the unsymmetrical fluorine bridged silicate
anion 14 (δ 19F ) -70).32,33 It is noteworthy that the fluorine
atom in cation 10 is the most shielded along this series and
that the 1J(FSi) coupling constant is only slightly reduced
compared to that of PhSiMe2F [1J(FSi) ) 278 Hz]34 in contrast
to that found for the dynamic molecule 13 [1J(FSi) ) 127 Hz].31
As already found for cation 6, no solvent effects on the NMR
chemical shifts of cation 10 were detected when the solvent
was varied from benzene-d6 to toluene-d8 (see Table 1). This
indicates also in this case negligible cation/solvent interactions
at room temperature in solution.
Colorless crystals from the salt 10·TPFPB were obtained from
toluene solution. One molecule of the solvent toluene is found
along with the cation 10 and the TPFPB anion in the asymmetric
unit (see Figure 3a). While the cation and solvent molecule are
well-separated [smallest C(toluene)-Si distance r(Cmeta/Si) )
460.1(1.2) pm; smallest C(toluene)-F(10) distance r(Cpara/F)
) 550.3(1.0) pm], several relatively close contacts between the
anion and cation can be detected (see Figure 3b). However, all
contacts are beyond the sum of the van der Waals radii,
The same cation 10 is obtained under much milder conditions,
when fluorodecane is used in the reaction with cation 6. In this
way, cation 10 is obtained nearly quantitatively from cation 6
along with decane after 3 h of stirring at 60 °C (Scheme 3).
With benzyl trifluoride the reaction proceeds even at room
temperature very fast and the only product detected besides the
salt 10·TPFPB is toluene. Interestingly, hexafluorobenzene, C6F6,
(27) IR spectroscopy could be used to further characterize cation 6 and would
give additional insight on the bonding. Computations predict an intensive
band corresponding to νas(Si-H-Si) ) 1966 cm-1 (at B3LYP/6-31G(d,p),
unscaled harmonic frequency). All attempts however to record an IR
spectrum of 6‚TPFPB, either dissolved in benzene-d6 or without solvent,
led to inconclusive results, most probably due to decomposition of 6 in
the IR cell.
(28) The average SiH bond length for each cation is reported.
(29) Optimized structure at D3d symmetry (see Supporting Information). Similar
theoretical results are reported at the density functional level in ref 4.
(30) Preliminary evidence for the intermediate formation of disilyl dication 11
is provided by the reaction kinetics as obtained from 29Si NMR spectros-
copy. The disappearance of the NMR signal of the cation 6 follows a first-
order rate law, while the appearance of the NMR signals of cation 10 is
independent from the concentrations of cation 6 and 10 (zero-order rate
law). This is in agreement with the existence of a short-living intermediate
11.
(31) Ebata, K.; Inada, T.; Kabuto, C.; Sakurai, H. J. Am. Chem. Soc. 1994,
116, 3595.
(32) Tamao, K.; Hayashi, T.; Ito, Y.; Shiro, M. J. Am. Chem. Soc. 1990, 112,
2422.
(33) Tamao, K.; Hayashi, T.; Ito, Y.; Shiro, M. Organometallics 1992, 11, 2099.
(34) Marsmann, H. In NMRsBasic Principles and Progress; Diehl, P., Fluck,
E., Gu¨nther, H., Kosfeld, R., Seelig, J., Eds.; Springer: New York, 1981;
Vol. 17, p 65.
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A R T I C L E S
Panisch et al.
Figure 3. (a) Ortep plot of the asymmetric unit of 10·TPFPB·C7H8. (Color code: B, dark gray; F, medium gray; Si, light gray; C, white. Thermal ellipsoids
are drawn at the 50% probability level). (b) Closest contacts between the cation 10 and the TPFPB anion in the solid-state structure of 10·TPFPB [Si1‚‚‚F44
) 334.1(1.3) pm; Si2‚‚‚F44 ) 336.8(1.1) pm; F1‚‚‚F44 ) 293.8(8) pm].
Scheme 4. Reaction of Fluoronium Ion 10 with Triethylsilane To
Give Cation 6
Scheme 5. Suggested Catalytic Hydrodefluorination Reaction
Figure 4. Ortep plot of cation 10 (bond lengths are given in picometers;
color code: F, medium gray; Si, light gray; C, white. Thermal ellipsoids
are drawn at the 50% probability level).
suggesting the absence of covalent interactions between anion
and cation.11,35
The molecular structure of cation 10 reveals for the six-
membered silicon-containing heterocycle a twisted half-chair
conformation with tetrahedral coordinated silicon atoms (see
that found in the neutral compound 13 (168.4 pm) and the
Figure 4). Just as in the case of the hydrogen-bridged cation 6,
shorter SiFbr bond in the unsymmetrical fluorine-bridged silicate
both silicon atoms show, however, strong variations toward a
anion 14 (189.8 pm).
Interestingly, the fluoronium ion 10 is converted to the
trigonal planar coordination, as measured by ∑R ) 345.9° and
347.7°. The most prominent feature of the cation 10 is a
hydrogen-bridged cation 6 in 1 h by reaction with excess of
symmetric bent Si-F-Si linkage (bent angle â ) 129.9°) with
triethylsilane in toluene at 50 °C (Scheme 4). The byproduct
SiF bond lengths that are clearly elongated compared to Si-F
formed in this reaction is triethylsilyl fluoride. This result opens
bonds in organosubstituted silyl fluorides (175.5 and 176.3 pm
the possibility for the catalytic cycle, shown in Scheme 5, by
in cation 10 compared to 159.4 pm as the average Si-F bond
which fluoroalkanes, RF, are converted to hydrocarbons, RH,
length for tetracoordinated silicon fluorides,36 see also Figure
by treatment with excess of Et3SiH in the presence of catalytic
4). The SiF bond lengths in cation 10 are intermediate between
amounts of either cation 6 or 10. A quite similar catalytic C-F
activation was recently described by Ozerov and co-workers,
which utilized as catalyst a cationic silicon compound, generated
from triethylsilane and the precatalyst trityl TPFPB.8
(35) Sum of van der Waals radii: FF ) 270 pm.
(36) Kaftory, M.; Kapon, M.; Botoshansky M. In The Chemistry of Organosilicon
Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: New York, 1998;
Vol 2, p 181.
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Hydrogen- and Fluorine-Bridged Disilyl Cations
A R T I C L E S
Table 2. Catalytic Hydrodefluorination Reactions
triethylsilane were distilled freshly from calcium hydride. Fluorodecane,
hexafluorobenzene, and trifluorotoluene were distilled from P4O10. All
reactions were carried out under inert conditions. Trityl TPFPB was
prepared according to modified literature procedures.39,40 IR spectra
were recorded on a Shimadzu FTIR-8300 spectrometer. Elemental
analyses were carried out on a Foss-Heraeus CHN-O-RAPID. NMR
spectra were recorded on a Bruker DPX-250 spectrometer at 303 K.
1H and 13C NMR spectra were calibrated using residual solvent signals
δ 1H(C6D5H) ) 7.20, δ 13C(C6D6) ) 128.0, δ 1H(C6D5CD2H) ) 2.03,
entry
RF
RH
T,
°
C
mol %
TONa
1
2
3
C10H21F
C10H21F
C6H5CF3
C10H22
C10H22
C6H5CH3
25
25
2.2 (6)
45
35
19
2.9 (10)
-15 to 25
4.7 (6)b
a Turnover numbers. b Addition of 6‚TPFPB at -15 °C.
Preliminary test reactions were performed with fluorodecane
and benzyl trifluoride as substrates, and the results are sum-
marized in Table 2. In a typical reaction equimolar amounts of
fluorodecane and triethylsilane were dissolved in toluene and a
small quantity of the salt 6·TPFPB was added at room
temperature. After 30 min at room temperature, the alkyl fluoride
was consumed and the reaction was completed. Fluoronium
cation 10 shows comparable reactivity in this hydrodefluorina-
tion reaction. With benzyl trifluoride the reaction is highly
exothermic and therefore the catalyst was added at -15 °C and
then the temperature was allowed to rise to room temperature.
Interestingly, when this reaction was monitored by NMR
spectroscopy, no other fluorine-containing compound but benzyl
trifluoride and triethylfluorosilane were observed.
δ
13C(C6D5CD3) ) 22.4. 19F NMR spectra were calibrated using external
19F(C6F6) ) -162.9 vs CFCl3. 29Si NMR spectra were calibrated
δ
using external Me2SiHCl (δ 29Si ) 11.1 vs TMS). 29Si NMR spectra
were recorded using the INEPT pulse sequence.
Compound 8.9 n-Butyllithium (12.5 mmol, 2.5 M, 5 mL) was added
slowly to a stirred solution of 10 mmol (2.07 g) of 1-bromonaphthalene
in 30 mL of dry diethyl ether at -40 °C, and the mixture was stirred
for a further 30 min. Stirring was stopped and the supernatant clear
solution was decanted off via a flexible Teflon tube. The precipitate
was washed three times with 50 mL portions of dry hexane. Then 14
mmol of n-butyllithium (2.5 M, 5.2 mL) in hexanes and 14 mmol (1.6
g) of TMEDA were added to the resulting white precipitate. The
reaction mixture was heated to reflux for 3 h. The resulting brown
suspension was cooled to -30 °C and was diluted with 30 mL of diethyl
ether. Then 25 mmol (2.5 g) of dimethylchlorosilane was added. The
dark brown suspension was allowed to warm to room temperature and
heated to reflux for 1 h. Excess dimethylchlorosilane was removed at
reduced pressure. After usual aqueous workup, the crude product was
purified by column chromatography on silica using hexane as eluent
(Rf ) 0.64) to yield 2 g (8.2 mmol) of 2 as a colorless oil (82% yield).
1H NMR (250.131 MHz, C6D6): δ 0.42 (d, 3JHH ) 3.6 Hz, 12H, SiMe2),
Conclusions
The synthesis and structural characterization of bissilylated
onium salts 6·TPFPB and 10·TPFPB are reported. In the absence
of nucleophiles, these cations are persistent, even in boiling
toluene. Both cations, hydronium ion 6 and fluoronium ion 10,
are characterized by a symmetric bent SiXSi linkage (X ) H,
F). Although structurally closely related, the bonding in both
cations is different. That is, while in 6 and related cations2-4
electrondeficient multicenter bonding prevails,1 Lewis acid base
interactions seem to dominate in fluoronium ion 10. Noteworthy,
in contrast to related boron and carbon systems e.g. 7 and 15,
16,5,37,38 in which unsymmetrical E-X‚‚‚E groups (E ) B, C;
3
1
5.29 (sept, JHH ) 3.6 Hz, JSi-H ) 193 Hz, 2H, Si-H), 7.27-7.33
(m, 2H), 7.69-7.76 (m, 4H). 13C{1H} NMR (62.902 MHz, C6D6): δ
-0.7 (CH3), 124.7 (C-3/6), 131.5 (C-2/7), 134.3 (C-10), 136.0 (C-4/
5), 138.3 (C-1/8), 142.0 (C-9). 29Si NMR (49.696 MHz, C6D6): δ
1
2
3
-19.63 (d, JSi-H ) 193 Hz, JSi-H ) 6.7 Hz, JSi-H ) 2.3 Hz).
Elemental analysis calcd/found (C14H20Si2): C 68.78, H 8.25/C 68.58,
H 8.37. IR: νSi-H ) 2166 cm-1 (film).
6·TPFPB. Method a: From 8 by Reaction with Trityl TPFPB.
A solution of 0.5 mmol (460 mg) of trityl TPFPB in benzene was slowly
added to a stirred solution of 0.5 mmol (122 mg) of 1,8-bis-
(dimethylsilyl)naphthalene in 2 mL of benzene at room temperature.
The reaction was monitored by the disappearance of the orange color
of the trityl cation. Stirring was stopped and the two phases were
allowed to separate. The lower phase was transferred into an NMR
tube and investigated by NMR spectroscopy. The brown salt was
purified by washing with pentane to obtain 415 mg (0.45 mmol, 90%
yield) 6·TPFPB as a glassy solid.
X ) H, F) corresponding to double minimum potentials were
found, the disilyl cations 6 and 10 prefer a symmetric structure
equivalent to a single minimum potential. A full account on
the bonding in bridged disilyl cations and the differences in
the carbon systems will be given elsewhere.
The disilyl cations 6 and 10 can be easily interconverted by
reaction with alkylfluorides or silanes; thus, there is potential
for a catalytic dehydrofluorination process of environmentally
important fluoroalkanes to give hydrocarbons. This is shown
here for some selected examples, and the scope and limitations
of these processes are currently being tested in our laboratories.
Method b: From 10·TPFPB by Reaction with Triethylsilane.
Triethylsilane (1.4 mmol, 167 mg) was added at room temperature to
a stirred solution of 0.5 mmol of 10·TPFPB in benzene. After 1 h at 50
°C the reaction mixture was transferred to an NMR tube and
investigated by NMR spectroscopy. After evaporation of the solvent
and washing with small portions of pentane, 304 mg (0.33 mmol, 66%
yield) of 6·TPFPB was obtained as brown glassy powder. Colorless
crystals could be obtained by crystallization of the crude product from
1
hexafluorobenzene at 7 °C. H NMR (250.131 MHz, C6D6): δ 0.39
(d, 3JH-H ) 1.8 Hz, 12H, SiMe2), 3.34 (sept, 3JH-H ) 1.8 Hz, 1JSi-H
)
45.7 Hz, 1H, µ-H), 7.18-7.23 (m, 2H, H2/7), 7.32-7.38 (m, 2H, H3/
6), 7.74-7.82 (m, 2H, H4/5). 13C NMR (62.902 MHz, C6D6): δ -2.8
(SiMe2), 126.4 (C1/8), 126.6 (C3/6), 133.2 (C10), 133.8 (C2/7), 134.3
(C4/5), 137.0 (d, 1JC-F ) 244.1 Hz, CF, [B(C6F5)]4), 137.6 (C9), 138.9
Experimental Section
General. Diethyl ether and hexane were distilled from sodium/
benzophenone. Benzene, benzene-d6, toluene, toluene-d8, and pentane
were distilled from sodium. TMEDA, dimethylchlorosilane, and
1
1
(d, JC-F ) 246.3 Hz, CF, [B(C6F5)]4), 149.1 (d, JC-F ) 240.4 Hz,
CF, [B(C6F5)]4). 29Si NMR (49.696 MHz, C6D6): δ 54.4 (dm, JSi-H
1
(37) Wang, H.; Webster, C. E.; Pe´rez, L. M.; Hall, M. B.; Gabba¨ı, F. P. J. Am.
Chem. Soc. 2004, 126, 8189.
(38) Kawai, H.; Takeda, T.; Fujiwara, K.; Suzuki, T. J. Am. Chem. Soc. 2005,
127, 12172.
(39) Bauch, C. Ph.D. Thesis, Goethe Universita¨t Frankfurt/Main, 2003.
(40) (a) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 245. (b) Chien,
J. C. W.; Tsai, W.-M.; Rausch, M. D. J. Am. Chem. Soc. 1991, 113, 8570.
9
J. AM. CHEM. SOC. VOL. 128, NO. 30, 2006 9681

A R T I C L E S
Panisch et al.
2
) 45.7 Hz, JSi-H ) 7.2 Hz). 19F NMR (235.334 MHz, C6D5CD3): δ
-175.2 (s, 1JSi-F ) 288.7 Hz). 29Si NMR (49.696 MHz, C6D6): δ 31.6
-166.7 (t, 3JFF ) 18 Hz, 8F), -162.7 (t, 3JFF ) 20.5 Hz, 4F), -132.0
(m, 8F).
(d, JSi-F ) 288.7 Hz). Decane: 13C NMR (62.902 MHz, C6D6): δ
1
13.9 (CH3), 22.7, 33.3, 29.7, 30.2 (4×CH2). GC/MS (EI) m/e (%): 142
(5), 113 (3), 99 (4), 98 (4), 85 (15), 84 (6), 71 (30), 70 (13), 57 (88),
55 (27), 43 (100), 41 (82), 29 (50), 27 (30). Toluene: 1H NMR (250.131
MHz, C6D6): δ 2.16.
X-ray Crystallographic Analysis. A single crystal of 6·TPFPB
suitable for X-ray crystallographic analysis was grown by recrystalli-
zation from hexafluorobenzene, whereas that of 10·TPFPB was grown
by recrystallization from toluene. X-ray crystal structure analyses were
performed on a STOE IPDS [Mo KR radiation, λ ) 71.073 pm, graphite
monochromator at 173(2) K].
10 TPFPB. Method a: From the Reaction of 6·TPFPB with
Trifluorotoluene. A solution of 0.5 mmol of 6·TPFPB in benzene was
cooled to 8 °C. After slowly adding 0.17 mmol (25 mg) of neat
trifluorotoluene, the cooling was removed. The reaction mixture was
stirred at room temperature for 30 min. After the stirring was stopped,
a two-phase reaction mixture was obtained. The lower phase was
transferred into an NMR tube and investigated by NMR spectroscopy.
The dark red TPFPB salt of cation 10 was purified by washing with
pentane to yield 460 mg (0.49 mmol, 98% yield) of 10·TPFPB.
Method b: From the Reaction of 6·TPFPB with Fluorodecane.
As described in method a, but upon adding 0.5 mmol (62 mg) of neat
fluorodecane at room temperature, the reaction mixture was warmed
to 60 °C for 3 h to yield 432 mg (0.46 mmol, 92% yield) of 10·TPFPB.
Method c: From the Reaction of 6‚TPFPB and Trityl TPFPB.
Trityl TPFPB (0.3 mmol, 275 mg) in 2 mL of benzene was added to
a stirred solution of 0.3 mmol of preformed 6·TPFPB in 1 mL of
benzene, and the mixture was heated to 75 °C for 9 h. Stirring was
stopped and the two phases were allowed to separate. The lower phase
was transferred into an NMR tube and investigated by NMR spectros-
copy. Colorless crystals from 10·TPFPB could be obtained by crystal-
lization of the crude product from toluene at -35 °C to yield 220 mg
(0.23 mmol, 78% yield) of 10·TPFPB. 1H NMR (250.131 MHz,
Crystal data for (6‚TPFPB·C6F6): C44H19BF26Si2, MW ) 1108.58,
crystal dimension 0.17 × 0.17 × 0.09 mm3, monoclinic, space group
P21/c, a ) 1498.08(8) pm, b ) 1071.69(4) pm, c ) 2741.62(17) pm,
â ) 93.974(5)°, V ) 4.3910(4) nm3, Z ) 4, Dcalc ) 1.590 Mg m-3
.
F(000) ) 2200, 2Θmax ) 52.12°, 60 772 reflections measured, of which
8603 were unique [R(int) ) 0.0951].The final R factor was 0.0753
with wR2 ) 0.1143 for 8603 observed reflections with I > 2σ(I). (RW
) 0.1416 all data, 60 772 reflections), GOF ) 1.060. The Cambridge
database file is CCDC 600351.
Crystal data for (10‚TPFPB·C7H8): C45H26BF21Si2, MW ) 1032.65,
crystal dimension 0.33 × 0.30 × 0.26 mm3, triclinic, space group P-
1, a ) 1035.20(10) pm, b ) 1386.04(13) pm, c ) 1564.53(15) pm, R
) 85.862(8)°, â ) 85.879(8)°, γ ) 74.786(7)°, V ) 2.1572(4) nm3, Z
) 2, Dcalc ) 1.590 Mg m-3. F(000) ) 1036, 2Θmax ) 50.8°, 26 358
reflections measured, of which 7842 were unique [R(int) ) 0.0540].
The final R factor was 0.0334 with wR2 ) 0.0812 for 7842 observed
reflections with I > 2σ(I). (RW ) 0.0493 all data, 26 358 reflections),
GOF ) 0.914. The Cambridge database file is CCDC 600350.
3
C6D6): δ 0.36 (d, JH-F ) 13.7 Hz, 12H), 7.12-7.17 (m, 2H), 7.30-
7.37 (m, 2H), 7.75-7.79 (m, 2H). 13C NMR (62.902 MHz, C6D6): δ
2
2
-1.0 (d, JC-F ) 15.4 Hz, CH3), 125.1 (d, JC-F ) 11.2 Hz, C-1/8),
126.3 (d, JC-F ) 2.8 Hz, C-3/6), 133.5 (C-10), 134.3 (d, JC-F ) 8.2
4
3
3
Hz, C-2/7), 135.1 (C-4/ 5), 136.4 (d, JC-F ) 6.0 Hz, C-9), 137.0 (d,
1JC-F ) 244.1 Hz, CF, [B(C6F5)]4), 138.9 (d, JC-F ) 246.3 Hz, CF,
1
Acknowledgment. This paper is dedicated to Prof. H.-U.
Siehl on the occasion of his 60th birthday. This work was
supported by a scholarship of the Fonds der chemischen
Industrie to R.P.
[B(C6F5)]4), 149.1 (d, JC-F ) 240.4 Hz, CF, [B(C6F5)]4). 29Si NMR
1
(49.696 MHz, C6D6): δ 77.2 (d, 1JSi-F ) 243 Hz). 19F NMR (235.334
3
3
MHz, C6D6): δ -166.7 (t, JFF ) 18 Hz, 8F), -162.7 (t, JFF ) 20.5
Hz, 4F), -144.0 (s, JSi-F ) 243 Hz, Si-F, 1F), -132.0 (m, 8F).
1
Typical Procedure for the Catalytic Hydrodefluorination of
Fluorohydrocarbons. A mixture of 4.5 mmol (720 mg) of fluoro-
decane, 4.5 mmol (520 mg) of triethylsilane, 0.6 mL of toluene, and
hexafluorobenzene as internal standard was stirred at room temperature.
6·TPFPB (0.1 mmol) in 1 mL of benzene was added via a flexible
Teflon tube. The progress of the reaction was followed by NMR
spectroscopy. After 1 h at room temperature fluorodecane and tri-
ethylsilane were completely consumed, and only triethylsilyl fluoride
was formed, as indicated by 19F and 29Si NMR. Decane was identified
by 13C NMR and GC/MS. In the case of benzyl trifluoride as starting
material, the reaction was done in benzene and the product toluene
Supporting Information Available: Complete description of
the computations including optimized geometries of cations 4
(Me) and 6 at different levels of theory and full citation of ref
17; correlation between computed and experimental 13C NMR
chemical shifts for cation 6; 1H, 13C, 29Si, 19F NMR spectra of
6·TPFPB and 10·TPFPB; NMR spectra for the catalytic hydro-
defluorination of trifluorotoluene; and CIF files for 6·TPFPB
and 10·TPFPB. This material is available free of charge via the
Internet at http://pubs.acs.org.
1
was detected by H NMR. Et3SiF: 19F NMR (235.334 MHz, C6D6) δ
JA061800Y
9
9682 J. AM. CHEM. SOC. VOL. 128, NO. 30, 2006