Synthesis of tris- and tetrakis(pentafluoroethyl)silanes

Article abstract of DOI:10.1002/anie.201400291

The synthesis and complete characterization of functional, highly Lewis acidic tris(pentafluoroethyl)silanes as well as tetrakis(perfluoroalkyl)silanes Si(C₂F₅)₄ and Si(C₂F ₅)₃CF₃ by direct fluorination is described. The reaction of SiCl₄ with LiC₂F₅ invariably affords (pentafluoroethyl)fluorosilicates. To avoid silicate formation by fluoride transfer from LiC₂F₅ the Lewis acidity of the silane has to be decreased by electron-donating substituents, such as dialkylamino groups. The easily accessible Si(C₂F₅) ₃NEt₂ is a valuable precursor for a series of tris(pentafluoroethyl)silanes. Special eFfects: functional and highly Lewis acidic tris(pentafluoroethyl)silanes as well as the tetrakis(perfluoroalkyl) silanes Si(C₂F₅)₄ and Si(C₂F ₅)₃CF₃ are prepared. The two tetrakis species were formed in the direct fluorination of tris-(pentafluoroethyl)ethyl- and methylsilane. Si(C₂F₅)₄ was characterized by X-ray crystallography (see figure).

Full text of DOI:10.1002/anie.201400291

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DOI: 10.1002/anie.201400291
Direct fluorination Very Important Paper
Synthesis of Tris- and Tetrakis(pentafluoroethyl)silanes**
Simon Steinhauer, Julia Bader, Hans-Georg Stammler, Nikolai Ignatꢀev, and Berthold Hoge*
Dedicated to Professor G.-V. Rçschenthaler on the occasion of his 70th birthday
Abstract: The synthesis and complete characterization of
functional, highly Lewis acidic tris(pentafluoroethyl)silanes as
well as tetrakis(perfluoroalkyl)silanes Si(C2F5)4 and Si-
(C2F5)3CF3 by direct fluorination is described. The reaction
of SiCl4 with LiC2F5 invariably affords (pentafluoroethyl)-
fluorosilicates. To avoid silicate formation by fluoride transfer
from LiC2F5 the Lewis acidity of the silane has to be decreased
by electron-donating substituents, such as dialkylamino groups.
The easily accessible Si(C2F5)3NEt2 is a valuable precursor for
a series of tris(pentafluoroethyl)silanes.
oethyl)silanes will allow their straightforward synthesis and
isolation.
T
he thermal stability of trifluoromethylsilanes strongly
depends on the nature of the ancillary substituents. Their
decomposition is a result of the high fluorophilicity of silicon
Recently we succeeded in the preparation of the (per-
fluoroalkyl)fluorosilicates [Si(C2F5)3F2]À and [Si(C2F5)3F3]2À
from SiCl4 and LiC2F5 [Eq. (3)].[5]
À
and occurs through CF2 elimination with formation of Si F
bonds. The Ruppert–Prakash reagent, Si(CF3)Me3, is a fairly
stable and versatile reagent for the introduction of trifluor-
omethyl groups in organic and inorganic compounds.[1] The
decomposition temperatures of trifluoromethylsilanes
decrease with an increasing electron withdrawal by the
additional substituents. Owing to the pronounced group
electronegativity of the CF3 unit silanes with more than one
CF3 substituent are highly sensitive. Si(CF3)3NEt2, for exam-
ple, was generated and characterized at low temperatures, but
could not be isolated because of its thermolability.[2] Recently
the generation of Si(CF3)4 was reported by the reaction of
CF3H, KN(SiMe3)2(KHMDS), and SiCl4 at low tempera-
tures.[3] The complexity of the reaction mixture and the
instability of this compound prevented its unambiguous
characterization and isolation.
During the formation of these silicates, the transfer of
C2F5 groups as well as of fluoride ions occurs. The reaction is
initiated by the formation of Si(C2F5)3Cl from SiCl4 and three
equivalents of LiC2F5. Subsequently the strong Lewis acid
Si(C2F5)3Cl reacts with LiC2F5 to afford the silicates [Si-
(C2F5)3F2]À and [Si(C2F5)3F3]2À instead of the anticipated
Si(C2F5)4. The neutral pentafluoroethylsilanes are only inter-
mediates in this process and are not preparatively accessible
by the reaction of SiCl4 with various amounts of LiC2F5.
Clearly for the synthesis of neutral tris(pentafluoroethyl)si-
lanes from chlorosilanes and LiC2F5 it is crucial to prevent any
silicate formation. This can be achieved by reducing the Lewis
acidity of the silane by an electron donating alkyl-, aryl-, or
amino substituent.
Sharp and Coyle reported on the different thermal
stability of Si(C2F5)F3 and Si(CF3)F3 [Eq. (1), (2)].[4] The
pentafluoroethyl derivative seems to be considerably more
stable. Based on this result the question is intriguing as to
whether the thermal stability of tris- and tetrakis(pentafluor-
[*] S. Steinhauer, Dr. J. Bader, Dr. H.-G. Stammler, Prof. Dr. B. Hoge
Fakultꢀt fꢁr Chemie, Universitꢀt Bielefeld
Treatment of alkyl-, aryl-, and aminotrichlorosilanes with
three equivalents of LiC2F5 furnishes the corresponding
tris(pentafluoroethyl)silanes as colorless liquids which are
stable at room temperature [Eq. (4)].[6] While Si(CF3)3NEt2 is
described as thermally highly sensitive compound, Si-
(C2F5)3NEt2 does not decompose when heated to 1808C for
2 h.
Universitꢀtsstrasse 25, 33615 Bielefeld (Germany)
E-mail: b.hoge@uni-bielefeld.de
Dr. N. Ignat‘ev
Merck KGaA, PM-ABE
Frankfurter Strasse 250, 64293 Darmstadt (Germany)
[**] The Merck KGaA (Darmstadt (Germany)) is acknowledged for
financial support and the Solvay GmbH (Hannover (Germany)) for
providing chemicals. We want to thank Prof. Dr. L. Weber for helpful
discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201400291.
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Reaction of Si(C2F5)3NEt2 with gaseous HCl or HBr leads
to the corresponding chloro- and bromosilane in high a yield
[Eq. (5)].[4] Owing to the pronounced Lewis acidity of Si-
(C2F5)3F, the combination of Si(C2F5)3NEt2 with anhydrous
HF gave rise to the formation of the salt [NEt2H2][Si-
(C2F5)3F2] [Eq. (6)]. In contrast to this reaction, treatment of
neat Si(C2F5)3Cl or Si(C2F5)3Br with SbF3 yields Si(C2F5)3F
[Eq. (7)] as a colorless liquid which solidifies at À1108C. A
twinned crystal was obtained by in situ crystallization of the
liquid sealed in a glass capillary by manually generating
a suitable seed crystal slightly below the melting point first,
followed by slowly cooling to À1738C. The fluorosilane,
Si(C2F5)3F, crystallizes in the monoclinic space group P21/
n (Figure 1).
Table 1: Fluoride ion affinity (FIA) of some Lewis acids (B3LYP/6-
311+ +G(2d)).[10]
Lewis acid
FIA [kJmolÀ1 [a]
]
Si(C2F5)3F
AsF5
P(C2F5)3F2
PF5
420.0
416.5
389.3
357.1
299.3
SiF4
[a] FIA=ÀDrGV.
According to these calculations, the Lewis acidity of Si-
(C2F5)3F is comparable with that of AsF5.
In keeping with the investigations of Sharp and Coyle
heating of gaseous Si(C2F5)3F for 1 h at 1808C led to the
disintegration of most C2F5 groups by elimination of CF(CF3).
The resulting reaction mixture contains Si(C2F5)F3, SiF4, cis-
and trans-perfluorobut-2-ene, as well as some other fluoro-
carbons [Eq. (9)].
Thermolysis of Si(C2F5)3F at 508C for 60 h proceeds
differently. cis- and trans-perfluorobut-2-ene can only be
detected as traces in the reaction mixture. Instead Si(C2F5)2F2
and Si(C2F5)4 are formed by ligand exchange [Eq. (10)]. This
Figure 1. Molecular structure of Si(C2F5)3F (thermal ellipsoids are set
at 50% probability). Selected bond lengths [pm] and angles [8]: Si–F1
156.7(1), Si–C1 192.9(1), Si–C3 193.0(2), Si–C5 192.4(2), C1–F2
135.9(2), C1–F3 136.0(2), C2–F4 132.7(2), C2–F5 132.5(2), C2–F6
132.3(2); F1–Si–C1 108.4(1), F1–Si–C3 108.8(1), F1–Si–C5 108.5(1),
C1–Si–C3 109.9(1), C1–Si–C5 110.2(1), C5–Si–C3 111.0(1); F1–Si–C3–
C4 19.9(3), F1–Si–C1–C2 29.1(3), F1–Si–C5–C6 32.8(3).
surprising result is in accord with a high thermal stability of
Si(C2F5)4. Because Si(C2F5)4 is not accessible by the reaction
of Si(C2F5)3Cl and LiC2F5, a different synthetic strategy to
access this compound was designed. The reaction of SiMe4
and Si2Me6 with elemental fluorine has been investigated by
Lagow and co-workers.[11] The direct fluorination of methyl-
silanes gives SiF4, fluorocarbons, and only partially fluori-
nated methylsilanes as pyrophoric and explosive compounds.
The Si–F distance of 156.7(1) pm is in the expected range
(d(Si–F) 158.5(1) pm in Si(C6F5)3F,[7] d(Si–F) 156(1) pm in
SiF4[8]). The F-Si-C angles are slightly more acute than the C-
À
Si-C angles. The C F bond lengths for the CF2 groups exceed
À
those of the CF3 group (CF2: dø(C-F) 135.9(2) pm, CF3: dø(C-
F) 132.5(6) pm). The different dihedral angles F1-Si-C-C
reflect a distortion from local C3 symmetry (19.9(1)8, 29.1(1)8,
and 32.8(1)8).
This result is not necessarily due to the instability of Si C
bonds towards elemental fluorine as the expected products
Si(CF3)4 and Si(CF3)3F themselves are thermally sensitive
compounds which could spontaneously decompose. There-
fore we investigated the direct fluorination of Si(C2F5)3C2H5
[Eq. (11)].
To demonstrate the high Lewis acidity of Si(C2F5)3F it was
treated with [P(C2F5)3F3]À salts, whereby the phosphorane,
P(C2F5)3F2, and ionic liquids with the [Si(C2F5)3F2]À ion
[Eq. (8)] are cleanly generated.[4] In Table 1 some calculated
fluoride ion affinities (FIA)[9] are given for comparison.
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The gas phase reaction of Si(C2F5)3C2H5 with five
equivalents of elemental fluorine produces a mixture of
Si(C2F5)4, Si(C2F5)3F, Si(C2F5)2F2, SiF4, HF, and C2F6. By
isothermal distillation Si(C2F5)4 can be isolated in a 30%
yield.
The 29Si NMR spectrum of Si(C2F5)4 shows a nonet at d =
À35.2 with the expected 2J(Si,F) coupling of 39 Hz (Figure 2).
Figure 3. Molecular structure of Si(C2F5)4 in the crystal (thermal
ellipsoids are set at 50% probability). Selected bond lengths [pm] and
angles [8]: Si–C1 194.2(1), Si–C3 194.5(1), Si–C5 194.7(1), Si–C7
194.4(1), C1–F1 136.8(2), C1–F2 136.3(1), C2–F3 132.7(2), C2–F4
133.1(2), C2–F5 133.6(2); C1–Si–C3 112.1(1), C1–Si–C5 113.4(1), C1–
Si–C7 103.0(1), C3–Si–C5 102.4(1), C7–Si–C3 113.5(1), C7–Si–C5
112.8(1).
Isothermal distillation of the reaction mixture yields
a colorless liquid containing Si(C2F5)3CF3 (40%), Si(C2F5)3F
(54%), and Si(C2F5)2F2 (6%). After 14 h at room temper-
ature 90% of the initial amount of Si(C2F5)3CF3 had decom-
posed to Si(C2F5)3F and C2F4 [Eq. (13)].
Figure 2. Top : 29Si NMR spectrum of neat Si(C2F5)4 with [D6]acetone as
external lock. Bottom: 13C satellites of the central part of the nonet
1
2
*
( J(Si,C)=82 Hz ( )), J(Si,C)=8 Hz ( )).
*
In conclusion, silicon carbon bonds of alkylsilanes are
clearly inert to a direct fluorination in the gas phase. Thus,
Si(C2F5)4 and Si(C2F5)3CF3 are accessible by direct fluorina-
tion of Si(C2F5)3C2H5 and Si(C2F5)CH3, respectively. Both
compounds represent first examples of tetrakis(perfluoroal-
kyl)silanes. Whereas Si(C2F5)4 decomposes only at elevated
temperatures, Si(C2F5)3CF3 already liberates difluorocarbene
at room temperature. Pentafluoroethylsilicon compounds are
thermally significantly more stable than their corresponding
trifluoromethyl counterparts. Owing to its thermal sensitivity,
Si(CF3)3NEt2 could not be isolated to date. The pentafluor-
oethyl derivative, Si(C2F5)3NEt2, does not decompose on
heating to 1808C and is accessible on a large scale by the
reaction of SiCl3NEt2 and LiC2F5. Based on the versatile
reactivity of the silicon–nitrogen bond, this compound is
a potential starting material for the synthesis of a series of
tris(pentafluoroethyl)silanes.
Single crystals were obtained by in situ crystallization slightly
below the melting point of À828C. Si(C2F5)4 crystallizes in the
monoclinic space group P21/c (Figure 3)[12] with two nearly
congruent molecules per asymmetric unit, so only one will be
discussed in the following. The coordination of the silicon
atom strongly deviates from tetrahedral symmetry with two
smaller C-Si-C angles of about 1028 (C3-Si-C5 and C7-Si-C1)
and of about 1138 for the remaining ones. As for Si(C2F5)3F
the C–F distances of the CF2 groups are slightly elongated in
comparison with those of the CF3 groups (CF2, dø(C–F)
136.4(3) pm; CF3, dø(C–F) 133.0(3) pm).
To our knowledge this is the first synthesis of a tetrakis(-
perfluoroalkyl)silane by direct fluorination. Larger quantities
of Si(C2F5)4 could also be obtained by thermolysis of liquid
Si(C2F5)3F at 708C for 2 weeks and subsequent isothermal
distillation of the crude product.
Received: January 10, 2014
Revised: February 6, 2014
Published online: April 1, 2014
The remarkable difference in the thermal stability of
trifluoromethyl- and (pentafluoroethyl)silanes can be ration-
alized by the different tendency to eliminate the correspond-
ing carbene, CF2 or CF(CF3), respectively. To further study
this different behavior we also investigated the reaction of
Si(C2F5)3CH3 with elemental fluorine [Eq. (12)].
Keywords: direct fluorination · fluorine · Lewis acids · silanes
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[12] Data for X-ray structure determination of Si(C2F5)3F were
[1] G. K. S. Prakash, A. K. Yudin, Chem. Rev. 1997, 97, 757 – 786.
[2] a) H. Beckers, PhD Thesis, Bergische Universitꢀt Wuppertal—
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Anorg. Allg. Chem. 1988, 563, 38 – 47.
[3] G. K. S. Prakash, P. V. Jog, P. T. D. Batamack, G. A. Olah,
Science 2012, 338, 1324 – 1327.
[4] K. G. Sharp, T. D. Coyle, Inorg. Chem. 1972, 11, 1259 – 1264.
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Steinhauer, N. Ignatyev, M. Schulte (BASF SE), DE
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[6] B. Hoge, S. Steinhauer, N. Ignatyev, M. Schulte (Merck Patent
GmbH), DE 102012006896 A1, 2012.
[7] A. D. Dilman, V. V. Levin, A. A. Korlyukov, P. A. Belyakov,
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[8] M. Atoji, W. N. Lipscomb, Acta Crystallogr. 1954, 7, 597.
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2004. Complete citation is given in supporting informations.
[11] a) R. E. Aikman, R. J. Lagow, Inorg. Chem. 1982, 21, 524 – 526;
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collected on a Bruker Nonius KappaCCD diffractometer, for
Si(C2F5)4 on an Agilent SuperNova diffractometer with EOS
detector, at 100(2) K using MoKa radiation (l = 71.073 pm). The
structures were solved by direct methods and refined by full-
matrix least-squares cycles (program SHELX-97: G. M. Shel-
drick, Acta. Cryst. Sect. A 2008, 64, 112 – 122). All atoms were
refined anisotropically. Data for Si(C2F5)3F: colorless crystal,
Mr = 404.15 gmolÀ1, monoclinic space group P21/n, a = 858.3(1),
b = 1382.7(1), c = 1061.7(1) pm, b = 112.64(1)8, V= 1162.9(2) ꢂ
106 pm3, Z = 4, 1calcd = 2.308 gcmÀ3, F(000) = 776; 11821 reflec-
tions up to q = 25 collected, thereof 9621 with I > 2s(I), 2048
independent reflections, 209 parameters. R-values: R1 = 0.0462
for refl. with I > 2s(I), wR2 = 0.124 for all data. Data for
Si(C2F5)4: colorless crystal, Mr = 504.17 gmolÀ1
, monoclinic
space group P21/c, a = 1220.99(3), b = 1232.30(3), c =
2004.00(5) pm, b = 91.144(2)8, V= 3014.7(1) ꢂ 106 pm3, Z = 8,
1calcd = 2.222 gcmÀ3, F(000) = 1936; 50510 reflections up to q =
30 collected, 8822 independent reflections, thereof 6722with I >
2s(I), 523 parameters. R-values: R1 = 0.0316 for refl. with I >
2s(I), wR2 = 0.076 for all data. CCDC 973139 (Si(C2F5)3F) and
973140 (Si(C2F5)4) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif..
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