Synthesis of five- and six-coordinate tris(pentafluoroethyl)fluorosilicates

Article abstract of DOI:10.1002/anie.201308059

The research area of perfluoroalkylsilanes is still in its infancy. Although there are already many examples of difluorotriorganylsilicates, the first example of a completely characterized trifluorotriorganylsilicate is presented, the dianion [Si(C₂F₅)₃F ₃]^2-. The strongly electron-withdrawing influence of the pentafluoroethyl groups appears to be a fundamental cause of the stability of this compound. This dianion is also the first structurally characterized example of a tris(pentafluoroethyl)silicon compound. The synthesis and complete characterization of [PPh₄]₂[Si(C₂F ₅)₃F₃] and [PPh₄][Si(C ₂F₅)₃F₂] along with the precursor [H(OEt₂)₂][Si(C₂F₅) ₃F₂] was achieved from SiCl₄ and LiC ₂F₅. Child's play: The chemistry of perfluoro-alkyl silanes is still at its infancy. The preparation and full characterization has now been achieved for [PPh₄]₂[Si(C₂F ₅)₃F₃] (see scheme and structure) and [PPh ₄][Si(C₂F₅)₃F₂] and their precursor [H(OEt₂)₂][Si(C₂F₅) ₃F₂] from SiCl₄ and LiC₂F ₅ as starting materials.

Full text of DOI:10.1002/anie.201308059

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Angewandte
Communications
DOI: 10.1002/anie.201308059
Perfluoroalkyl Silicon Compounds
Synthesis of Five- and Six-Coordinate
Tris(pentafluoroethyl)fluorosilicates**
Simon Steinhauer, Hans-Georg Stammler, Beate Neumann, Nikolai Ignatꢀev, and
Berthold Hoge*
Dedicated to Professor Werner Uhl on the occasion of his 60th birthday
Abstract: The research area of perfluoroalkylsilanes is still in
its infancy. Although there are already many examples of
difluorotriorganylsilicates, the first example of a completely
characterized trifluorotriorganylsilicate is presented, the di-
anion [Si(C2F5)3F3]2À. The strongly electron-withdrawing
influence of the pentafluoroethyl groups appears to be
a fundamental cause of the stability of this compound. This
dianion is also the first structurally characterized example of
a tris(pentafluoroethyl)silicon compound. The synthesis and
complete characterization of [PPh4]2[Si(C2F5)3F3] and [PPh4]
[Si(C2F5)3F2] along with the precursor [H(OEt2)2][Si-
(C2F5)3F2] was achieved from SiCl4 and LiC2F5.
Despite the knowledge of this fact for about 40 years, only
a few pentafluoroethyl silanes have been synthesized. More-
over, based on Sharpꢀs and Coyleꢀs observations, the existence
of silanes with more than two pentafluoroethyl substituents at
ambient temperature is very conceivable.
Herein we report our results on the combination of three
pentafluoroethyl substituents at a six-coordinate silicon
[5]
center. Reaction of SiCl4 with a tenfold excess of LiC2F5
T
he chemistry of (perfluoroalkyl)silanes has essentially been
restricted to trifluoromethyl derivatives. Owing to the high
and the subsequent cation exchange with [PPh4]Cl afforded
a mixture of [PPh4]2[Si(C2F5)3F3] and [PPh4][Si(C2F5)3F2].[6]
The course of this transformation involves the initial forma-
tion of (C2F5)3SiCl from SiCl4 and three equivalents of LiC2F5
(Scheme 1). The next two equivalents of LiC2F5 serve as
fluorophilicity of silicon, these compounds decompose by CF2
elimination and the formation of Si F bonds. Generally the
À
thermolability of (CF3)SiX3 derivatives increases with
increasing electronegativity of the substituent X.[1,2] With
regard to the pronounced group electronegativity of the CF3
unit, silanes with more than one CF3 substituent should be
highly sensitive. In keeping with this, (CF3)3SiCl and
(CF3)3SiNEt2 have not been isolated to date, and information
on their existence is limited to spectroscopic evidence.[2]
Despite claims otherwisein the literature,[3] Si(CF3)4 is still
elusive.
According to Sharp and Coyle, the thermal stabilities of
(CF3)SiF3 and (C2F5)SiF3 differ considerably, which may be
rationalized by the increased tendency to extrude CF2 in
comparison to CF(CF3) [Eqs. (1),(2)].[4]
Scheme 1. The multistep reaction of SiCl4 with an excess of LiC2F5.
fluoride sources, which convert the Lewis acidic (C2F5)3SiCl
into the tris(pentafluoroethyl)difluorosilicate ion. Addition of
a further fluoride ion by release from the sixth equivalent of
LiC2F5 eventually furnished the six-coordinate tris(penta-
fluoroethyl)trifluorosilicate dianion.
The preparation of salts containing exclusively the five-
coordinate anion [Si(C2F5)3F2]À has been achieved by treating
the initially obtained reaction mixture of SiCl4 and LiC2F5 in
diethyl ether with dry HCl gas [Eq. (3)]. The compound
[H(OEt2)2][Si(C2F5)3F2] was isolated after filtration of the
resulting mixture and removal of all volatile components in
[*] S. Steinhauer, Dr. H.-G. Stammler, B. Neumann, Prof. Dr. B. Hoge
Fakultꢀt fꢁr Chemie, Universitꢀt Bielefeld
Universitꢀtsstrasse 25, 33615 Bielefeld (Germany)
E-mail: b.hoge@uni-bielefeld.de
N. Ignat’ev
Merck KGaA
Frankfurter Strasse 250, 64293 Darmstadt (Germany)
[**] Merck KGaA (Darmstadt, Germany) is acknowledged for financial
support and Solvay GmbH (Hannover, Germany) for providing
chemicals. We wish to thank Prof. Dr. L. Weber for helpful
discussions.
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vacuo. The product is a colorless liquid with a vapor pressure
of less than 1 mbar at 208C.
The composition of the product [H(OEt2)2][Si(C2F5)3F2]
can be rationalized from the integration of the 1H NMR
spectrum of the neat liquid (Figure 1). The data of the cation,
[H(OEt2)2]+, are in agreement with literature data.[7] The
resonance in the 29Si NMR spectrum is observed at À111 ppm
Figure 2. Molecular structure of [Si(C2F5)3F3]2À in [PPh4]2[Si-
1
which is split into a triplet of septets owing to J(Si,F) and
(C2F5)3F3]·CH2Cl2 (ellipsoids are set at 50% probability). Selected bond
lengths [pm] and angles [8]: Si1–F1 169.2(3), Si1–F2 168.6(3), Si1–F3
168.5(3), Si1–C1 202.8(5), Si1–C3 205.0(5), Si1–C5 202.2(5); F1-Si1-F3
92.0(1), F2-Si1-F3 90.0(1), F1-Si1-C3 91.6(2), F1-Si1-C5 87.4(2), C3-Si1-
C5 93.7(2).
2J(Si,F) couplings of 318 Hz and 37 Hz, respectively.
F bond lengths are about 169 pm and do not vary significantly.
À
The Si C3 bond of the pentafluoroethyl group trans to the
fluorine atom is slightly elongated (205 pm) with respect to
À
the Si C distances of the trans oriented pentafluoroethyl
À
À
groups (Si C5 202 pm and Si C1 203 pm). All of the
intermolecular contacts exceed the sum of the corresponding
van der Waals radii, and therefore do not influence the
molecular structure of the anion.
The first-order 29Si NMR resonance of the [Si(C2F5)3F3]2À
ion exhibits the expected doublet of triplet splitting with two
different J(Si,F) couplings of 315 Hz and 202 Hz. A further
septet splitting is caused by a J(Si,F) coupling of 32 Hz. A
possible coupling to the CF3 groups, the J(Si,F) coupling,
1
2
3
could not be observed. The chemical shift of À180.8 ppm lies
in the expected range for six-coordinate silicon species.
In conclusion, the chemistry of perfluoroalkyl silanes is
still at its infancy. Although several examples of triorganyl
difluorosilicates are known,[11] the dianion [Si(C2F5)3F3]2À is
the first example of a fully characterized triorganyl trifluoro-
silicate. The strong electron-withdrawing effect of the penta-
fluoroethyl groups seems to be essential to stabilize this
unusual compound. More generally, this compound is the first
Figure 1. 1H NMR spectrum (top) and 29Si NMR spectrum (bottom) of
neat [H(OEt2)2][Si(C2F5)3F2] (RT, [D6]acetone as external lock).
structurally characterized example of
a tris(perfluoro-
Adding [H(OEt2)2][Si(C2F5)3F2] to an aqueous KF solu-
tion leads to the selective formation of the six-coordinate
silicate, which can be transformed into its tetraphenylphos-
phonium salt upon addition of [PPh4]Cl. Using this procedure
allows the isolation of [PPh4]2[Si(C2F5)3F3] [Eq. (4)]. The
alkyl)silicon compound.[12]
We succeeded in the preparation and full characterization
of [PPh4]2[Si(C2F5)3F3] and [PPh4][Si(C2F5)3F2] and their
precursor [H(OEt2)2][Si(C2F5)3F2] from SiCl4 and LiC2F5 as
starting materials, and a mechanism for the multistep
generation of these silicates was presented. Future studies
in our laboratory are focused on neutral tris(pentafluoro-
ethyl)silanes and the most challenging silane Si(C2F5)4.
Experimental Section
All of the chemicals were obtained from commercial sources and used
without further purification. Standard high-vacuum techniques were
employed throughout all preparative procedures. Nonvolatile com-
product crystallizes with one molecule CH2Cl2 in the triclinic
[8]
¯
space group P1. The molecular structure of the dianion
[Si(C2F5)3F3]2À is depicted in Figure 2. The substituents of
pounds were handled in
a dry N2 atmosphere using Schlenk
[Si(C2F5)3F3]2À show a meridional constitution, as it is found
techniques. The NMR spectra were recorded on a Bruker Model
for the isoelectronic phosphate ion, [P(C2F5)3F3]À.[9,10] The Si
Avance III 300 spectrometer (29Si 59.6 MHz, 19F 282.4 MHz, 1H
À
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Angewandte
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300.1 MHz) with positive shifts being downfield from the external
standards (TMS (29Si, 1H), CCl3F (19F)). ESI mass spectra were
recorded using an Esquire 3000 ion-trap mass spectrometer (Bruker
Daltonik GmbH, Bremen, Germany) equipped with a standard ESI/
APCI source. C, H, and N analyses were carried out with a HEKAtech
Euro EA 3000 apparatus.
Synthesis of [PPh4][Si(C2F5)3F2]: HC2F5 (24 g, 200 mmol) is
condensed at À858C onto a degassed solution of 1.6m n-butyllithium
in hexane (100 mL, 160 mmol) in diethyl ether (500 mL). After
stirring for 20 min a sample of SiCl4 (3.4 g, 20 mmol) was added. The
reaction mixture was allowed to slowly warm to room temperature
within 150 min. At RT the reaction mixture was stirred under an HCl
atmosphere. The precipitate was filtrated off, and the solvent was
removed at reduced pressure. The remaining liquid (10.5 g) with an
estimated composition [H(OEt2)2][Si(C2F5)3F2] was dissolved in
CH2Cl2 and a solution of [PPh4]Cl (6.6 g, 17.6 mmol) in CH2Cl2 was
added. After evaporating the solvent at reduced pressure, [PPh4]
[Si(C2F5)3F2] (13.4 g, 17.6 mmol, 88% with respect to SiCl4) was
obtained as a colorless solid that decomposes above 908C.
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[7] P. W. Siu, K. Hazin, D. P. Gates, Chem. Eur. J. 2013, 19, 9005 –
9014, and references therein.
[8] Data collection for X-ray structure determination of [PPh4]2[Si-
(C2F5)3F3]·CH2Cl2 was performed on a Bruker Nonius Kap-
paCCD diffractometer at 100(2) K using graphite-monochro-
mated MoKa radiation (l = 71.073 pm). The structure was
solved by direct methods and refined by full-matrix least-squares
cycles (program SHELX-97: G. M. Sheldrick, Acta. Crystallogr.
Sect. A 2008, 64, 112 – 122). All non-hydrogen atoms were
refined anisotropically, hydrogen atoms isotropically. Data for
Elemental analysis(%) calcd for C30H20F17PSi: C 47.2, H 2.6%;
found: C 47.2%, H 2.7%. 29Si NMR (Et2O, RT): d = À109.3 ppm (t,
1
sept, J(Si,F) = 315 Hz, 2J(Si,F) = 37 Hz); 19F NMR (Et2O, RT): d =
À83.6 (t, 9F, CF3, 4J(F,F) = 7.5 Hz), À113.0 (m, 2F, SiF2, 1J(Si,F) =
315 Hz), À126.9 ppm (t, 6F, CF2, 3J(F,F) = 7.5 Hz); ESI-MS [m/z]:
negative: 422.7 (32, [Si(C2F5)3F2]À), 322.7 (100, [Si(C2F5)2F3]À), 222.7
(9, [Si(C2F5)F4]À), positive: 339.0 (100, [PPh4]+); IR (ATR):
n˜ [cmÀ1] = 3091 vw, 3066 vw, 1588 w, 1485 w, 1439 m, 1319 m, 1183
vs, 1131 vs, 1107 vs, 1037 s, 996 s, 970 s, 923 w, 882 w, 840 vw, 815 m, 757
w, 745 w, 721 s, 689 s, 617 w, 592 w, 557 w, 524 vs, 507 vs, 453 w, 426 m,
401 w.
[PPh4]2[Si(C2F5)3F3]·CH2Cl2:
colorless
crystal,
Mr =
1205.83 gmolÀ1, triclinic space group P1, a = 1032.51(3), b =
1303.55(3), c = 2144.09(7) pm, a = 99.30(1), b = 102.39(1), g =
¯
107.77(1)8, V= 2602.6(1) ꢃ 106 pm3, Z = 2, 1calcd = 1.539 gcmÀ3
,
F(000) = 1224; 42294 reflections up to q = 27 collected, 11585
independent reflections, thereof 8480 with I > 2s(I), 703 param-
eters. R values: R1 = 0.0809 for reflections with I > 2s(I), wR2 =
0.24 for all data. CCDC 941548 contains 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.
Synthesis of [PPh4]2[Si(C2F5)3F3]: HC2F5 (5.4 g, 45 mmol) was
condensed at À858C onto a degassed solution of 1.6m n-butyllithium
in hexane (25 mL, 40 mmol) in diethyl ether (500 mL). After stirring
for 20 min a sample of SiCl4 (0.63 g, 3.8 mmol) was added. The
reaction mixture was allowed to warm up to room temperature within
150 min. At RT the reaction mixture was stirred under an HCl
atmosphere. The precipitate was filtered off, and the solvent was
removed at reduced pressure. The remaining liquid was extracted
with an aqueous KF solution (0.54 g in 50 mL H2O). To the aqueous
phase, a solution of [PPh4]Cl (3.1 g (8.3 mmol) in 20 mL H2O) was
added. The white precipitate was separated by centrifugation, washed
with water, and dried under reduced pressure. [PPh4]2[Si(C2F5)3F3]
(2.1 g, 1.9 mmol, 50% with respect to SiCl4) was obtained as
a colorless solid that decomposes above 1108C.
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29Si NMR (CH3CN, RT): d = À180.8 ppm (t, d, sept, 1J(Si,F) =
315 Hz, 1J(Si,F) = 202 Hz, 2J(Si,F) = 32 Hz); 19F NMR (CH3CN, RT):
d = À80.5 (m, 3F, CF3), À81.5 (m, 6F, CF3), À109.4 (m, 1F, SiF,
1J(Si,F) = 202 Hz), À122.3 (m, 2F, CF2), À123.3 (m, 4F, CF2),
À133.6 ppm (m, 2F, SiF2, 1J(Si,F) = 315 Hz); ESI-MS [m/z]: negative:
422.7 (1, [Si(C2F5)3F2]À), 322.7 (100, [Si(C2F5)2F3]À), 222.7 (19, [Si-
(C2F5)F4]À) 118.9 (3, C2F5À), positive: 339.0 (100, [PPh4]+); IR (ATR):
n˜ [cmÀ1] = 3086 vw, 3059 vw, 1586 w, 1484 w, 1439 m, 1315 m, 1302 m,
1188 vs, 1159 vs, 1108 vs, 1097 vs, 995 s, 935 s, 751 w, 722 s, 689 s, 588 w,
562 w, 549 w, 524 vs, 465 w, 424 w, 401 m.
Received: September 13, 2013
Revised: October 7, 2013
Published online: November 29, 2013
Keywords: perfluoroalkyl groups · silicates · silicon ·
.
weakly coordinating anions
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[1] H. Beckers, PhD Thesis, Bergische Universitꢁt Wuppertal—
Gesamthochschule, 1987.
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