The protonation of CF3SO3H: Preparation and characterization of trifluoromethyldihydroxyoxosulfonium hexafluoridoantimonate, CF3SO3H2+SbF6 -

Article abstract of DOI:10.1002/ejic.201001339

Trifluoromethanesulfonic anhydride reacts with superacidic solutions AF/SbF₅ (A = H, D) to form their corresponding salts CF ₃SO₃A₂⁺SbF₆^-, which are protonated forms of trifluoromethanesulfonic acid and CF ₃SO₂F as by-product. The salts have been characterized by vibrational spectroscopy and single-crystal structural analysis. CF ₃SO₃H₂⁺SbF₆^- crystallizes in the triclinic space group Pβar {1}$ with two formula units in the unit cell: a = 709.29(5) pm, b = 763.46(5) pm, c = 882.15(6) pm; α = 71.884(6)°, β = 72.488(6)°, γ = 84.509(6)°; V = 432.97(5) A³. The cations are linked with two strong hydrogen bonds to SbF₆^- anions to form chains. The experimental data were also compared to quantum chemical calculations for the CF₃SO₃H₂(HF)₂⁺ cation.

Full text of DOI:10.1002/ejic.201001339

FULL PAPER
DOI: 10.1002/ejic.201001339
The Protonation of CF3SO3H: Preparation and Characterization of
Trifluoromethyldihydroxyoxosulfonium Hexafluoridoantimonate,
+
–
CF3SO3H2 SbF6
Theresa Soltner,[a] Nadine R. Goetz,[a] and Andreas Kornath*[a]
Dedicated to Prof. Dr. Peter Klüfers on the occasion of his 60th birthday
Keywords: Superacidic systems / Protonation / Structure elucidation / Sulfonium salts
Trifluoromethanesulfonic anhydride reacts with superacidic
solutions AF/SbF5 (A = H, D) to form their corresponding
salts CF3SO3A2+SbF6–, which are protonated forms of tri-
fluoromethanesulfonic acid and CF3SO2F as by-product. The
salts have been characterized by vibrational spectroscopy
in the unit cell: a = 709.29(5) pm, b = 763.46(5) pm, c =
882.15(6) pm; α = 71.884(6)°, β = 72.488(6)°, γ = 84.509(6)°; V
= 432.97(5) Å3. The cations are linked with two strong hydro-
–
gen bonds to SbF6 anions to form chains. The experimental
data were also compared to quantum chemical calculations
–
+
and single-crystal structural analysis. CF3SO3H2+SbF6 crys-
for the CF3SO3H2(HF)2 cation.
¯
tallizes in the triclinic space group P1 with two formula units
CF3)3 and SbF5.[4a] Due to the limited solubility of
B(OSO2CF3)3 in triflic acid, the highest acidity in the triflic
system is most likely reached with SbF5. At high SbF5 con-
centrations, the equilibrium described in Equation (2) com-
petes with the one described in Equation (1).[4a]
Introduction
Trifluoromethane sulfonic acid (triflic acid) was dis-
covered in 1954 and has been investigated intensively since
then.[1] The acid, its salts, and its organic derivatives (com-
monly known as triflates) are substances with a wide range
of applications in organic syntheses and polymerization re-
actions.[2] Recently it was shown by Schulz et al. that
CF3SO3(SiMe3)2+B(C6F5)4 can be utilized as a silylating
–
agent.[3] Triflic acid is one of the strongest monoprotic or-
ganic acids. Its acidity (H0 = –14.1) is comparable to per-
chloric acid (H0 ≈ –13), and it is classified as a superacid
by definition since its H0 value is lower than that of concen-
trated sulfuric acid (H0 = –12).[4] A study of the molecular
structure in the gaseous phase has been implemented and,
despite the low melting point (–40 °C) of triflic acid, a sin-
gle-crystal X-ray structure is known.[5] The self-ionization
equilibrium of triflic acid is described by Equation (1).
(2)
Indeed, Mootz et al. described a reaction of triflic acid
with SbF5 that forms an adduct, which has been identified
by a single-crystal structure.[6] The acidic cationic species of
the triflate superacidic system has not yet been identified.
Results and Discussion
Synthesis and Properties of CF3SO3H2+SbF6
–
The salt was prepared in three steps in quantitative yield
according to Equations (3), (4), and (5).
(1)
In this equilibrium, the CF3SO3– anion can be bound by
strong Lewis acids to form triflates, thereby increasing the
concentration of the acidic species. The highest acidities (H0
= –18.5) have been measured with Lewis acids B(OSO2-
(3)
(4)
(5)
[a] Department of Chemistry, Ludwig-Maximilian University,
Butenandtstrasse 5–13, 81377 Munich, Germany
Fax: +49-89218077487
E-mail: andreas.kornath@uni-muenchen.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201001339.
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Trifluoromethyldihydroxyoxosulfonium Hexafluoridoantimonate
In the first step, the superacid was formed to ensure the Table 2. Selected bond lengths [pm] and angles [°] for CF3SO3H,
–
+
CF3SO3H2+SbF6 , and the calculated CF3SO3H2(HF)2
.
–
highest possible concentration of H2F+SbF6 in an HF
CF3SO3HCF3SO3H2+ SbF6
obsd.[5b]
CF3SO3H2+CF3SO3H2(HF)2
–
+
solution, see Equation (3). In the second step, triflic an-
hydride, which is more volatile than CF3SO3H and there-
fore easier to condense quantitatively on a vacuum line, was
condensed onto the frozen superacid. During the warm-up
process of the reaction mixture, the superacid melted, and
then at –40 °C the HF reacted with triflic anhydride to form
CF3SO2F and CF3SO3H, with SbF5 catalyzing the cleavage.
CF3SO3H was then protonated according to Equation (5).
After removal of the excess amount of HF and CF3SO2F
obsd.
calcd.[a]
calcd.[b]
C(1)–S(1)
S(1)–O(1)
S(1)–O(2)
S(1)–O(3)
C(1)–F(1)
C(1)–F(2)
C(1)–F(3)
O(1)–H(1)
183.5(4)
153.4(3)
142.7(3)
141.4(4)
131.6(5)
131.3(4)
131.3(5)
99(9)
185.5(2)
150.5(2)
148.3(2)
140.5(2)
130.5(2)
130.7(2)
132.0(2)
84(4)
188.8
153.7
152.3
139.8
130.6
130.1
129.5
97.5
187.3
151.2
151.2
140.5
130.4
130.4
130.7
100.5
100.5
254.1
254.1
104.4
104.4
111.0
108.6
106.8
106.8
115.3
115.3
177.7
177.7
at –78 °C, only colorless crystals of CF3SO3H2+SbF6 ,
–
O(2)–H(2)
73(3)
97.5
O(1)···F(4a)
O(2)···F(9b)
O(1)–S(1)–C(1)
O(2)–S(1)–C(1)
O(3)–S(1)–C(1)
F(1)–C(1)–S(1)
F(2)–C(1)–S(1)
F(3)–C(1)–S(1)
S(1)–O(1)–H(1)
S(1)–O(2)–H(2)
O(1)–H(1)···F(4a)
O(2)–H(2)···F(9b)
252.5
242.6
which were stable up to –50 °C and suitable for single-crys-
tal X-ray structure analysis, remained [Equations (4) and
(5)]. Efforts to crystallize the cation as perfluoridoarsenate
or perfluoridogermanate salts using superacid systems HF/
AsF5 and HF/GeF4 were not successful. Clearly, the acidic
strength of these systems is not sufficient to protonate triflic
acid. The formation of an adduct of CF3SO3H with SbF5,
which was observed in the previous work of Mootz et al.,
was not found in the presence of HF.[6]
101.2(2)
105.2(2)
107.4(2)
109.2(3)
109.5(3)
110.2(3)
115(5)
102.26(9)
101.1(2)
111.8(2)
109.7(2)
107.5(2)
107.6(2)
117(3)
114(2)
173.09
162.84
103.8
105.3
113.2
106.7
106.2
107.1
114.6
117.3
[a] Calculated at the PBE1PBE/6-311G(3df,3pd) level of theory.
[b] Calculated at the PBE1PBE/6-311++G(3df,3pd) level of theory.
Symmetry operations: a: x, y, z + 1; b: x, y + 1, z.
–
Crystal Structure of CF3SO3H2+SbF6
Selected data of the X-ray data collection and refinement
are summarized in Table 1. CF3SO3H2+SbF6– crystallizes in
¯
the triclinic space group P1 (no. 2) with two formula units
in a unit cell. Bond lengths and selected angles are summa-
+
rized in Table 2. The structure of the CF3SO3H2 cation
and the hydrogen bonds to the nearest fluorine atoms of
the anions are shown in Figure 1. Figure 2 shows a section
of the crystal structure.
–
Figure 1. Fragment of the crystal structure of CF3SO3H2+SbF6
Table 1.
Crystal
data
and structure
refinement for
+
showing the CF3SO3H2 cation with interionic contacts (50%
–
CF3SO3H2+SbF6 .
probability ellipsoids for the non-hydrogen atoms). Symmetry op-
erations: a: x, y, z + 1; b: x, y + 1, z).
Empirical formula
Mr
T [K]
CH2F9O3SSb
386.836
100(2)
λ [pm]
71.073
Crystal system
Space group
a [pm]
b [pm]
c [pm]
α [°]
β [°]
triclinic
¯
P1 (no. 2)
709.29(5)
763.46(5)
882.15(6)
71.884(6)
72.488(6)
84.509(6)
432.97(5)
2
γ [°]
V [Å3]
Z
Final R indices [IϾ2σ(I)]
R indices (all data)
R1 = 0.0144; wR2 = 0.0342
R1 = 0.0164; wR2 = 0.0345
0.619/–0.526
Largest diff. peak/hole [eÅ–3]
+
As expected, the CF3SO3H2 cation contains one short
S=O double bond with a distance of 140.5(2) pm, which is
comparable to the S=O bonds of the CF3SO3H molecule.
The two S–O bonds with lengths of 148.3(2) and
150.5(2) pm are short for a formal single bond but, due to
–
Figure 2. Crystal structure of CF3SO3H2+SbF6 with interionic
the electron-withdrawing CF3 group, are in the expected re- contacts (50% probability ellipsoids for the non-hydrogen atoms).
Eur. J. Inorg. Chem. 2011, 3076–3081
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T. Soltner, N. R. Goetz, A. Kornath
FULL PAPER
gion [d(S–O) = 170 pm, d(S=O) = 146 pm]. The C–S dis-
tance of 185.5(2) pm is only slightly longer than that of un-
protonated triflic acid [183.2(3) pm]. Similarly, the CF3
group of the cation has typical C–F bonds and angles that
are almost unaffected by the protonation.
–
Each cation is connected to two neighboring SbF6
anions through strong, almost linear hydrogen bonds with
an O···F distance of 242.7(2) and 252.5(2) pm. This linkage
–
leads to (CF3SO3H2+SbF6 )n chains along the [011] axis.
–
The ideal octahedral structure of the SbF6 anion is dis-
torted to a quadratic bipyramid. The Sb–F distances in the
equatorial plane are almost identical [185.1(2) to
186.6(2) pm], whereas the trans-bridging Sb–F distances are
about 6 pm longer [190.6(2) and 192.7(2) pm].
–
Vibrational Spectra of CF3SO3A2+SbF6 (A = H, D)
–
The infrared and Raman spectra of CF3SO3H2+SbF6
and CF3SO3D2+SbF6– are shown in Figure 3. The observed
frequencies are listed in Table 3, together with the quantum-
chemical-calculated frequencies for CF3SO3A2(HF)2 (A =
+
H, D). The cation has C1 symmetry and therefore should
exhibit 24 fundamental vibrations. For the assignment of
the vibrational spectra, the spectra of triflic acid and the
+
theoretical calculations for CF3SO3A2(HF)2 were consid-
+
ered.[7] The 24 internal CF3SO3A2 vibrations have been
+
selected from the CF3SO3A2(HF)2 unit by inspection of
the Cartesian displacement coordinates of the vibrational
modes (A = H, D).
–
Figure 3. Vibrational spectra of CF3SO3H2+SbF6 : (a) Raman
spectrum, (b) IR spectrum; and of CF3SO3D2+SbF6 : (c) Raman
–
All spectra show frequencies for the CF3 group in regions
comparable to CF3SO3H. The ν(OH) band is detected at
3076 cm–1 and the ν(OD) band at 2162 cm–1. Both fre-
quencies agree well with the calculated values, and the
broad band shapes are typical for the OH stretching modes
that are involved in hydrogen bonds.[8]
spectrum, (d) IR spectrum.
tra have only been measured in the spectral region above
450 cm–1, and therefore only one infrared band at 691 cm–1
was detected.
The S=O stretching vibration is observed at nearly con-
stant wavenumbers around 1400 cm–1, which is comparable
+
to that observed for the H3SO4 and H2SO3F+ cations.[8,9]
Theoretical Calculations
This is expected due to the similarity of the cations and the
absence of interionic interactions. The two S–O stretching
The calculation of the gas-phase structure of the free
+
vibrations at 1073 and 956 cm–1 are better described as CF3SO3A2 cation (A = H, D) was performed with the
modes between the sulfur atom and the hydroxy group, PBE1PBE method using the 6-311G(3df,3pd) basis set.
which itself participates in the S–O–H···F bonds found in Subsequently, vibrational frequencies in the harmonic
the crystal structure. The C–S stretching vibration occurs approximation as well as IR and Raman intensities were
at a notably low wavenumber of 267 cm–1. This is a lower calculated. The calculated geometry of the free cation
value than that observed for the triflic acid (312 cm–1). shows a fair agreement with the one experimentally found
Clearly, the protonation of the triflic acid leads to a weaker in the crystal structure (Table 2), but large differences occur
C–S bond, which is in accord with the crystallographic re- between the calculated and experimental frequencies. The
sults.[5a,7b]
O–H stretching vibrations (νs = 3589 and νas = 3578 cm–1)
For the SbF6– anion with ideal octahedral symmetry, two in particular were overestimated by up to 500 cm–1 com-
vibrations in the infrared and three vibrations in the Raman pared to the experimental data (νs = 3076 cm–1). This is
spectra with mutual exclusions are expected. As found in due to the strong hydrogen bonds confirmed by the crystal
–
the crystal structure, the SbF6 is at least distorted to D4h structure, since the formation of hydrogen bonds usually
symmetry for which five vibrations in the infrared and five leads to a decrease of the corresponding O–A stretching
vibrations in the Raman spectrum with mutual exclusions frequency. To validate this, we added two HF molecules to
are expected. This lowered symmetry suffices to explain the the free CF3SO3A2+ cation, and this CF3SO3A2(HF)2+ unit
number of observed vibrations. However, the infrared spec- was calculated by the PBE1PBE method using a 6-
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Eur. J. Inorg. Chem. 2011, 3076–3081

Trifluoromethyldihydroxyoxosulfonium Hexafluoridoantimonate
–
Table 3. Experimental vibrational frequencies [cm–1] of CF3SO3A2+SbF6 and calculated vibrational frequencies [cm–1] of
+
CF3SO3A2(HF)2 (A = H, D).
[a] Calculated at the PBE1PBE/6-311++G(3df,3pd) level of theory. Frequencies are scaled with an empirical factor 0.98. IR intensity in
kmmol–1. Raman activity is relative to a scale on which the most intense band is 100.
311++G(3df,3pd) basis set. The calculated geometry of the solution of HF/SbF5, which formed CF3SO3H in situ and
–
CF3SO3A2+ cation in the CF3SO3A2(HF)2+ unit is compar- was instantly protonated. The resulting CF3SO3H2+SbF6
able to that found in the crystal structure. The addition of salt was characterized by vibrational spectroscopy and a
the two HF molecules to the free cation leads only to small single-crystal structure analysis. In the solid state, strong
changes of the geometric parameters of the cation, but sig- hydrogen bonds (O–H···F) between the cation and the
nificantly influences of the vibrational modes observed. In fluorine atoms of the anions were observed. Therefore,
+
particular, the O–H stretching modes of the cation (νs = theoretical calculations of the free CF3SO3A2 cation did
3059 and νas = 3006 cm–1) were redshifted, due to the for- not describe the experimental vibrational spectra suffi-
mation of S–O···H···F–H hydrogen bonds into a region that ciently. To consider the influences of the hydrogen bonds,
+
agrees fairly well with the experimental data. Overall, we an CF3SO3H2(HF)2 unit was calculated, which contains
find a satisfying agreement between the experimentally ob- two hydrogen bonds (O–H···F) between the hydroxy groups
served frequencies and those calculated for the of the cation and the fluorine atom of the HF molecules.
+
+
CF3SO3A2(HF)2 cation, although the CF3SO3A2(HF)2
This simplified model of the solid state led to a sufficient
agreement between calculated and experimentally obtained
vibrational spectra and geometric parameters.
unit represents a very simplified model of the solid state.
–
The formation of CF3SO3H2+SbF6 can be seen as a re-
Conclusion
sult of a competitive reaction between CF3SO3H/SbF5 and
+
The CF3SO3H2 cation was prepared and identified for HF/SbF5, and it therefore indicates that CF3SO3H/SbF5 is
the first time. The synthesis was successfully executed a weaker conjugated Brønsted–Lewis superacid than the
through the reaction of (CF3SO2)2O with a superacidic HF/SbF5 system.
Eur. J. Inorg. Chem. 2011, 3076–3081
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T. Soltner, N. R. Goetz, A. Kornath
FULL PAPER
–
–
Synthesis of CF3SO3D2+SbF6 : CF3SO3D2+SbF6 was prepared in
Experimental Section
–
a procedure analogous to CF3SO3H2+SbF6 , using DF instead of
HF.
Caution! Avoid contact with any of these compounds and note that
HF burns skin and causes irreparable damage. Safety precautions
should be taken when using and handling these materials.
Theoretical Calculations: Theoretical calculations were carried out
on the free CF3SO3H2+ cation and the CF3SO3H2(HF)2+ unit using
the Gaussian 03 program.[12] The highest level of theory employed
for each system was the PBE1PBE density functional approach
Chemicals: Trifluoromethanesulfonic anhydride (99%, ABCR) was
used without further purification. SbF5 (ABCR) was distilled three
times through a Vigreux column under a flow of dry nitrogen at
atmospheric pressure, then purified by trap-to-trap distillation un-
der vacuum. HF (Linde) was first trap-to-trap-distilled under vac-
uum and then dried with fluorine for two weeks in a stainless-steel
pressure cylinder. DF was prepared from dried CaF2 and D2SO4,
distilled under vacuum, and then dried with fluorine for two weeks
in a stainless-steel pressure cylinder. D2SO4 was obtained by a reac-
tion of D2O with SO3, which was trap-to-trap-condensed from
oleum (65% SO3, Merck).
+
+
with a 6-311G(3df,3pd) basis set for CF3SO3H2 and CF3SO3D2
and a 6-311++G(3df,3pd) basis set for CF3SO3H2(HF)2 and
,
+
+ [13]
CF3SO3D2(HF)2
.
Structural optimizations were performed
using the GDIIS algorithm with tight convergence criteria.[14] Opti-
mized geometries and vibrational frequencies were calculated in
each case. The calculation of the CF3SO3H2(HF)2 unit was con-
+
sidered to simulate the hydrogen bonds found in the crystal struc-
–
ture of CF3SO3H2+SbF6 . The resulting theoretical vibrational
modes represent the experimental spectra more closely than that of
the free cation.[9]
Supporting Information (see footnote on the first page of this arti-
cle): Computational details.
Equipment and Instrumentation: All synthetic work and sample
handling was performed by employing standard Schlenk tech-
niques using a stainless-steel vacuum line. Superacid reactions were
carried out in FEP ampules, which were sealed at one end and
closed with a stainless-steel valve. All reaction vessels as well as the
stainless-steel line were dried with fluorine prior to use. Infrared
spectra of dry powders were recorded at –100 °C with a Bruker
Vertex 80V FTIR spectrometer (3500–450 cm–1). The infrared spec-
tra were obtained using a single-crystal CsBr plate coated with the
neat sample in a cooled cell. Raman spectra of the solids in a glass
cell cooled with liquid nitrogen were recorded with a Bruker
MultiRAM FT-Raman spectrometer with Nd:YAG laser excitation
(λ = 1064 nm, 3500–250 cm–1).
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Crystal Structure Determination: X-ray diffraction studies were car-
ried out with an Oxford Xcalibur3 diffractometer with a Spellman
generator (voltage 50 kV, current 40 mA) and a Kappa CCD area
detector (Mo-Kα, λ = 0.71073 Å, graphite monochromator) at
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ABSPACK multiscan method.[11] All atoms, including protons,
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freely. All non-hydrogen atoms were refined anisotropically.
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[11] SCALE3 ABSPACK, An Oxford Diffraction Program, 1.0.4
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[12] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
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Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K.
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Hratchian, J. B. Cross, C. Adamo, J. Jaramillo, R. Gomperts,
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A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu,
–
Synthesis of CF3SO3H2+SbF6 : In
a typical reaction, SbF5
(1.00 mmol, 220 mg) was condensed into an FEP reactor at
–196 °C, followed by the addition of a large excess amount of anhy-
drous HF (3 mL, SbF5/HF = 1:300). The mixture was warmed to
–40 °C to form the superacid. The vessel was then cooled to
–196 °C, and (CF3SO2)2O (1.00 mmol, 282 mg) was condensed
onto the frozen superacid. The reaction mixture was warmed up to
–60 °C for 5 min, then cooled to –78 °C. The excess amounts of
HF and CF3SO2F were removed in a dynamic vacuum at –78 °C.
The resulting colorless crystals of CF3SO3H2+SbF6– were obtained
in quantitative yield (387 mg, 1.00 mmol). The solid is stable below
–50 °C and hydrolyzes rapidly upon exposure to moisture. The ob-
tained crystals were suitable for single-crystal X-ray diffraction.
–
CCDC-782908 (for CF3SO3H2+SbF6 ) 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.
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 3076–3081

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Published Online: June 6, 2011
Eur. J. Inorg. Chem. 2011, 3076–3081
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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