A HF Loaded Lewis-Acidic Aluminium Chlorofluoride for Hydrofluorination Reactions

Article abstract of DOI:10.1002/chem.202001627

The very strong Lewis acid aluminium chlorofluoride (ACF) was loaded with anhydrous HF. The interaction between the surface of the catalyst and HF was investigated using a variety of characterization methods, which revealed the formation of polyfluorides. Moreover, the reactivity of the HF-loaded ACF towards the hydrofluorination of alkynes was studied.

Full text of DOI:10.1002/chem.202001627

Accepted Article
Accepted Article
Title: A HF Loaded Lewis-acidic Aluminum Chlorofluoride for
Hydrofluorination Reactions
Authors: Thomas Braun, Maëva-Charlotte Kervarec, Erhard Kemnitz,
Gudrun Scholz, Svemir Rudić, Christian Jäger, Adam
Michalchuk, and Franziska Emmerling
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0
1/2020

Chemistry - A European Journal
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A HF Loaded Lewis-acidic Aluminum Chlorofluoride for
Hydrofluorination Reactions
[
a]
[a]
[a]
[c]
,[a]
Maëva-Charlotte Kervarec, Erhard Kemnitz, Gudrun Scholz, Svemir Rudić, Thomas Braun,*
[b]
[b]
[b]
Christian Jäger, Adam A. L. Michalchuk and Franziska Emmerling
[
a]
M.-C. Kervarec, Prof. Dr. E. Kemnitz, Priv.-Doz. Dr. G. Scholz, Prof. Dr. T. Braun
Department of Chemistry
Humboldt-Universität zu Berlin
Brook-Taylor-Straße 2, 12489 Berlin (Germany)
E-mail: thomas.braun@cms.hu-berlin.de
[
[
b]
c]
Prof. Dr. C. Jäger, Dr. A. A. L. Michalchuk, Priv.-Doz. Dr. F. Emmerling
BAM Federal Institute for Materials Research and Testing
Richard-Willstätter-Straße 12489 Berlin (Germany)
Dr. Svemir Rudic
ISIS Neutron and Muon Source, STFC,
Rutherford Appleton Laboratory, Chilton, Didcot (UK)
Supporting information for this article is given via a link at the end of the document.
Abstract: The very strong Lewis acid aluminum chlorofluoride (ACF)
a strong interaction of the adsorbed silane or germane at the
surface. MAS-NMR spectroscopy revealed considerable
was loaded with anhydrous HF. The interaction between the surface
a
of the catalyst and HF was investigated using
a
variety of
difference between the two materials. The data suggested an
interaction of the silanes at the surface, which might lead to a
silylium-like character of the silicon-containing species. For the
germanes, two distinct types of surface-bonded germane species
characterization methods, which revealed the formation of
polyfluorides. Moreover, the reactivity of the HF-loaded ACF towards
the hydrofluorination of alkynes was studied.
were detected.[17] An interaction of HF with β-AlF
007 using DFT calculations. The investigation reveals
was studied in
3
2
a
protonation of basic fluoride centers at the surface to yield a
polyfluoride-like structure, which can also interact with
undercoordinated Al sites.[ In another study, aluminum alkoxide
fluoride (AlF2.7(OR)0.3) was treated with gaseous HF at 100°C or
Introduction
31]
Fluorinated building blocks are of high interest, due to the often
drastic change in properties of fluorinated compounds, when
compared to their non-fluorinated analogs.[1,2] Correspondingly,
many studies focus on the incorporation of a fluorine atom into
organic molecules.[3–7] New pathways are being developed to
achieve the formation of C–F bonds, which is of high interest in
materials science, pharmaceuticals, and agrochemicals.[6–8]
Aluminum fluoride-based catalysts can be considered as the
strongest solid Lewis acids known.[9–11] Among them, aluminum
chlorofluoride (ACF, AlCl
of high interest, in part because of its large surface area and high
Lewis acidity, which is comparable to the one of SbF
.[12,13] Its
properties and reactivity have been intensely investigated,
showing that a variety of reactions can be mediated.[13–26] For
instance, H/D or Cl/F exchange reactions were reported at
ACF.[20,23,27–29] In the presence of a hydrogen source, such as
triethylsilane and triethylgermane, hydrodehalogenation, or
dehydrohalogenation of C–F bonds or C–Cl bonds were
achieved.[16,17,26,30] In the additional presence of benzene, Friedel-
2
3
00°C, resulting in the formation of an amorphous AlF phase with
2
-1
low surface area (100 m g ) which presumably contains surface-
immobilized HF.[32]
This paper covers studies on the properties of HF-loaded ACF.
Surface and bulk characterization methods were used to
investigate the interaction of HF with the surface and the influence
on the bulk. The hydrofluorination of alkynes led to the formation
of hydrofluoroolefins (HFOs).
x
F3-x, x=0.05-0.3, patented by Dupont) is
5
Results and Discussion
Synthesis of HF-loaded-ACF
Freshly dried HF was condensed (8 wt. %) onto a defined amount
of ACF. A slight change in color was observed from yellowish to
brown-yellow. After that, a vacuum was applied for 15 minutes to
remove any excess of HF, which did not interact with the surface.
Crafts products can be generated when using Et
hydrogen source. However, when the reactions were performed
in C 12 without the addition of any Et SiH, 1-fluoropentane was
3
SiH as a
MAS NMR Spectroscopy
6
D
3
converted into traces of 2-fluoropentane and 3-fluoropentane,
which might be formed by dehydrofluorination and a subsequent
HF addition to the double bond of 2-pentene.[17]
The 27Al MAS NMR spectra shown in Figure 1 exhibit signals at δ
= -16 ppm for both ACF and the HF-loaded ACF samples. The
chemical shift is typical for ACF containing AlF
6
units and a few
The interaction of HSiEt
3 3
or HGeEt with the surface of ACF was
AlCl
x
F
6-x units.[14] However, the 27Al NMR signal of the HF-loaded
studied in some detail.[17,24] Pulse TA evidenced the presence of
1
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ACF appears more narrow, with a line-width decrease of about
250 Hz, which presumably indicates a slightly better-ordered
aluminum fluoride matrix as a consequence of HF loading.
Scheme 1. Schematic representation of possible interactions of HF with the
1
surface of ACF along with the corresponding H chemical shift values.
The 19F NMR MAS NMR spectra of HF-loaded ACF and ACF are
depicted in Figure 3. Both samples show a broad asymmetric
signal, which is caused by the fluorides in the bulk of ACF.[
Noticeably, the maximum of the peak is shifted from δ = -165 ppm
37][14]
(
ACF) to δ = -167 ppm (HF-loaded ACF), along with a diminished
line width of about 3 kHz, again indicating a slight reorganization
of the bulk structure in HF-loaded ACF, which is in accordance
with the 27Al MAS NMR spectra. Moreover, the signals are shifted
Figure 1. 27Al MAS NMR spectra of ACF (black) and HF-loaded ACF (red)
measured at 27.5 kHz rotation frequency.
towards the signals observed for -AlF
3
3
or -AlF , which appear
[
37]
at δ = - 172 ppm. All signals for the additional fluorine sites due
-
to [F(HF)
n
] anions can be expected in the same range and are
[
33,34]
covered by the broad network signal at δ = -167 ppm.
Note
1
Figure 2 shows the H MAS NMR spectrum for the ACF sample
that signals of solution 19F NMR spectra of polyfluoride anions
loaded with HF. Four different signals can be distinguished (see
also Figure 6 below). The signal for HF-loaded ACF at δ = 17 ppm
is typical for strongly bridged HF units and is in good agreement
-
[
F(HF)
3
] in p-toluidinium salts appear at δ = -147.2 and δ = -185.0
-
ppm, whereas the FHF anion shows a signal at δ = -155.4
ppm.[14,37,38] The sharp signal at δ = -135 ppm for the HF-loaded
ACF sample corresponds to some physisorbed HF and can be
-
[33,34]
with the values reported for FHF moieties.
The signal at δ =
1
1 ppm is as well compatible with the literature, suggesting the
1
associated with the narrow resonance at δ = 0 ppm in the H NMR
presence of [F(HF)
for solution spectra of tetrabutylammonium [F(HF)
n
]- moieties at the surface of ACF.[33] Signals
spectrum (Figure 2). Figure 4 displays the 19F MAS NMR spin
echo spectra for ACF and HF-loaded ACF. The signal that can be
assigned to terminal fluorine atoms bound at aluminum at δ = -
]- salts appear
3
-
at δ = 11.8 ppm and the FHF anion results in resonance at δ =
1
6.1 ppm.[33] The broad feature at δ = 6 ppm for the HF-loaded
208 ppm for ACF does not appear for the loaded sample,
ACF is different in line width and shape and includes at least two
signals, which could be assigned to larger polyfluoride clusters,
including the possibility of a second layer of HF.[33][34] We cannot
entirely exclude the presence of some adsorbed water molecules,
despite careful handling during the sample preparation, because
suggesting that those terminal fluorine atoms are not present
anymore.
the resonance at δ = 6 ppm is also typical for them (see also
below).[
35,36]
The narrow signal at δ = 0 ppm is typical for highly
mobile species, and might, therefore, be attributed to physisorbed
HF. Scheme 1 shows schematically four possible interactions of
HF with the surface of ACF, assigned to their corresponding
1
signals observed in the H spectra.
Figure 3. 19F MAS NMR spectra of ACF (black) and HF-loaded ACF (red)
measured at 27.5 kHz rotation frequency. Asteriks (*) represent spinning
sidebands.
Figure 2. 1H MAS NMR spectrum of HF-loaded ACF measured at 27.5 kHz
rotation frequency.
2
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Additionally, when the sample loaded with a lot of HF was put
under vacuum for 24 hours or heated, a drastic decrease in
intensity in the 19F MAS NMR of the bulk signal at δ = -167 ppm
was observed, indicating a removal of HF from the polyfluoride
network (see SI, Figure S2). The same trend was observed in the
1H MAS NMR (SI, Figure S1), where all the signals were reduced,
although the one at δ = 6 ppm seems to be less diminished.
1
A H-DQ MAS NMR experiment was performed for the sample
loaded with HF (Figure 7). Diagonal and off-diagonal peaks reveal
correlations of the signals at δ = 6 ppm and δ = 11 ppm. The signal
at δ = 17 ppm does not show any cross peak in the 1H DQ
experiment, which indicates the isolated character of the strongly
1
19
bridged HF with the surface of ACF. A H- F MAS NMR rotor-
synchronized correlation spectrum (Figure 8) shows cross-peaks
of all proton signals with the signal at δ = -167 ppm for the fluoride
bulk, which is superimposed by the 19F signals of the polyfluoride
Figure 4. 19F rotor synchronized spin-echo MAS NMR spectra of ACF (black)
and HF-loaded ACF (red) measured at 27.5 kHz rotation frequency.
anions [F(HF)
n
]-. No correlation was observed between the signal
at δ = -135 ppm in the 19F MAS NMR spectrum and the one at δ
In order to verify the presence of physisorbed HF, another batch
of HF-loaded ACF was synthesized in the same way as described
earlier, only using distinctly less HF. As a consequence, a
1
=
0 ppm in the H MAS NMR spectrum, probably due to the high
mobility of the physisorbed HF.
1
9
1
decrease of both F NMR (δ = -135 ppm) and H NMR (δ = 0
ppm) signal intensities was observed, supporting the assumption
that both signals correspond to physisorbed HF (Figure 5 and 6).
Moreover, partial removal of chemisorbed HF was observed as
well, clearly visible by the reduced intensity of the signal at δ = -
1
67 ppm in the 19F NMR spectrum as well as the signal at δ = 6
1
ppm in the H NMR spectrum. This indicates a diminished second
layer polyfluorides cluster when less HF is used during the loading
of ACF.
1
Figure 7. H DQ MAS NMR spectrum of HF-loaded ACF measured at 27.5 kHz
rotation frequency.
Figure 5. 19F MAS NMR spectra of HF-loaded ACF (red) and HF-loaded ACF
synthesized using less HF (blue) measured at 20 kHz rotation frequency.
Asterisks (*) represent spinning sidebands.
Figure 8. H-19F MAS NMR correlation spectrum for HF-loaded ACF measured
1
at 27.5 kHz rotation frequency.
With the aim to estimate a conceivable change of the chlorine
content in the sample loaded with HF, a 19F- Cl TRAPDOR NMR
experiment was carried out, and the spectra for ACF and the HF-
loaded ACF are depicted in Figure 9. The spectra are unique and
reveal the presence of chlorine atoms in the network of ACF and
35
1
Figure 6. H MAS NMR spectrum of HF-loaded ACF (red) and HF-loaded ACF
synthesized using less HF (blue) measured at 20 kHz rotation frequency.
3
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HF-loaded ACF. The experiments were performed under the
same experimental conditions (completely filled rotors, same
masses of the catalysts, the same number of accumulations), and
both samples showed signals with comparable peak areas, which
indicates that there is no distinct change in the chlorine content
after loading with HF.
Figure 10. FTIR spectra of ACF (black), HF-loaded ACF (red), HF-loaded ACF
200°C (green) and HF-loaded ACF 400°C (blue).
To explore further the vibrational spectra of HF-loaded ACF,
inelastic neutron scattering (INS) spectra were collected for both
the pure and HF-loaded ACF materials (Figure 11). Due to the
large incoherent scattering cross-section of hydrogen atoms as
compared with aluminum, chlorine, and fluorine atoms, the INS
signal of HF-loaded ACF is dominated (albeit not exclusively) by
H-containing vibrational modes. This is confirmed by noting the
absence of INS features in the pure ACF spectrum (Figure 11,
black). The INS spectrum of the HF-loaded material exhibits six
Figure 9. 19F-35Cl TRAPDOR MAS NMR spectra of ACF (black) and HF-loaded
ACF (red) measured at 27.5 kHz rotation frequency.
FTIR-ATR and INS
The FTIR spectrum shows four broad bands for the HF-loaded
-
1
-1
ACF sample (Figure 10) with maxima at 1665 cm , 1180 cm
,
-1
-1
-1
-1
050 cm-1, and 590 cm . All those bands are compatible with data
-1
vibrational bands: 658 cm , 1078 cm , 1192 cm , 1344 cm ,
1680 cm-1 and ca. 2292 cm . These vibrational frequencies are
consistent with those obtained by FTIR spectroscopy (Figure 10),
but are consistently blue shifted. This contrasts the established
1
-1
-
[39]
for [F(HF)
n
] anions in p-toluidinium hydrogen difluoride.
The
-1
band at 1665 cm is in the range of the stretching vibrations of
]- anions and the bands at 1180 cm-1 and 1050 cm-1 relate
[
F(HF)
n
…
]- anions.[39] Likewise, the band at
trend for HF upon cooling, in which the stronger H F hydrogen
to bending modes of the [F(HF)
n
90 cm-1 can be associated with a F-Al-F vibrational mode.[40]
bonding interactions at low temperatures are accompanied by red
shifted vibrational frequencies.[41] An alternative explanation for
5
Those values are also consistent with a DFT study on the
adsorption of HF at a β-AlF surface, which imparts different
this blue shift may reside in the chain length (n) or geometry of
3
-
-
[F(HF) ] clusters, both of which vary with temperature. Ab initio
frequency vibrational modes of FHF moieties, depending on the
binding mode at the respective under-coordinated Al sites.[31] With
a half monolayer coverage at a T1 termination, which relates to
under-coordinated Al moieties at a 100 surface, the stretching
n
calculations have suggested that (HF)n vibrational frequencies
vary semi-continuously with chain length.[42] Moreover, similar
calculations have demonstrated that cluster geometries influence
frequencies.[
43]
Dedicated
-
significantly
on
vibrational
frequency of adsorbed HF giving FHF moieties would then be
-
-
1
investigations into the thermal stability of [F(HF) ] clusters upon
1
654 cm , whereas at full monolayer coverage the HF stretch
n
-1 [31]
cooling on ACF surfaces are needed. Additional to the blue shifted
frequencies, the INS spectrum suggests the existence of an
additional vibrational band at 1344 cm-1 which is not visible in the
FTIR spectrum.[31] The vibrational band at 2292 cm-1 observed in
the INS spectrum is consistent with previously reported
combination bands observed by IR spectroscopy.[
frequency would appear at 1211 cm .
For a sample of HF-loaded ACF, which was heated at 200°C for
4
hours, the signals appear (green spectrum in Figure 10) at the
same wavenumbers as for the sample, which did not undergo
thermal treatment, however, they are more narrow. This suggests
desorption of HF by heating the sample, and the observation is in
good agreement with the MAS NMR study, which also indicated
the presence of less HF after heating (see above). A spectrum of
another sample treated at 400°C was as well recorded, and it
44,45]
Noting the potential for contamination of ACF surfaces with water,
we additionally compare the INS spectra of ACF and HF-ACF with
that of ice 퐼 (see SI Figure S3). The clear absence of features in
ℎ
the INS spectrum strongly suggests no water is present in the
pure phase of ACF. Similarities between the INS spectrum of 퐼ℎ
showed only typical vibrational modes of the β-AlF
3
phase at 596
cm-1 and 650 cm , which correspond to the stretching and
-1
bending mode vibration of F-Al-F moieties, respectively.[37,40]
and HF-ACF are visible. Further analysis of relative scattering
intensities does not, however, suggest these similarities to result
Previous studies for ACF did not reveal any distinct bands, due to
its amorphous character.[13,14]
from the presence of ice 퐼 on the HF-ACF surface. The lack of
ℎ
water supports the aforementioned NMR results.
4
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XRD
In order to get a better understanding of the thermal behavior of
the samples, powder XRD data were recorded for ACF, HF-
loaded ACF, as well as for two HF-loaded ACF samples, which
were heated at 200°C and 400°C for thirty minutes. In the case of
HF-loaded ACF, the diffractogram reveals the presence of an
amorphous material, with a very slight crystalline character. This
result is consistent with the observation of the MAS NMR studies,
which indicate a slightly better-ordered aluminum fluoride (see
above). The diffractogram of the heated sample at 200°C does
not differ from the room temperature one, but the one heated at
4
00°C indicates the presence of a crystalline β–AlF
3
phase.[37,49]
Note that this behavior is slightly different from the one for pure
ACF. When the amorphous material was heated to 400°C, η-AlF
3
was obtained.[
the Ɵ-AlF
13,14]
When the temperature was increased to 500°C,
3
phase was formed, and finally, at 600°C, the β-AlF
3
Figure 11. INS spectrum of ACF (black) and HF-loaded ACF (red). In both
cases, the low frequency coherent inelastic scattering from the Al sample holder
has been subtracted.
phase was observed.[ This difference can be explained by the
fact that in the case of the loaded sample, HF is immediately
released and also might fluorinate part of the bulk, which will
cause the release of HCl, much faster than it might occur for ACF.
13]
Thermoanalysis
The thermal behavior of ACF was studied in the past.[13,14]
Differential thermal analysis (DTA) shows at approximately 400°C
a decomposition into gaseous AlCl
3 3
and crystalline phases of AlF
(
η-AlF , θ-AlF3, and ß-AlF
3
3
).[13,14] In contrast, the DTA of HF-
loaded ACF displays a continuous exothermic phase transition
between 50°C and 500°C. Indeed, during the heating time, HF is
released from the surface, as indicated by mass spectrometry,
and it might fluorinate the catalyst continuously[46–48]. The thermal
behavior resembles the one of aluminum alkoxide fluoride, which
was treated with diluted gaseous HF.[32] The material also shows
a broad desorption range for HF, with a maximum at 200°C. It is
also known that for surfaces covered with adsorbed species
compared to uncovered surfaces, the exothermic DTA curve has
a larger slope. Indeed, the DTA is extended to a wider
temperature range. The TG shows two mass losses around
200°C and 400°C, which correspond to a release of HF and a
Figure 13. Powder XRD pattern of ACF (black), HF-loaded ACF rt (red), HF-
slow fluorination and crystallization process. A total mass loss of
about 17% while heating from room temperature to 500°C was
observed, whereas a 10% mass loss is detected for unloaded
ACF under the same conditions. This indicates that the mass loss
related to the release of HF has a maximum of 8%.
3
loaded ACF 400°C (blue) and β-AlF (purple).
Surface area determination and pore size analysis
To get information on the morphology properties of the HF-loaded
ACF sample, nitrogen sorption studies were performed without
any initial degassing process by heating or applying vacuum
(
Figure 14). According to the isotherm curve, they revealed a low
intensity of adsorbed N
, reaching only 30.5 cm3g-1 when
compared to 99 cm3g-1 of adsorbed N
for ACF. Noticeably, the
2
2
loaded sample adsorb and desorb much less than ACF; thus,
ACF has a very fast adsorption till 0.1 in relative pressure, with a
3
-1
high adsorption value (70 cm g ) and then reaches the maximum
3
-1
value (99 cm g ) slowly. Both ACF and the loaded ACF exhibit a
type I isotherm, which indicates a microporosity according to the
BET model. The surface area calculated by the BET model
showed a smaller value for the loaded sample. This is probably
because the pores are filled by HF, leaving low capacity for the N
to adsorb. This diminished surface area was as well observed in
2
Figure 12. TA-MS curves for HF-loaded ACF; m19 corresponds to HF.
5
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the case of HF-loaded aluminum alkoxide, however, the sample
was less affected, and a value of 100 m2g-1 was obtained.[32] The
re-determined value for the pure ACF is in accordance with
previous studies determined by the BET method (Table 1).[13,19]
These observations are confirmed by studies on the pore size
distribution using non-local Density Functional Theory (NLDFT),
where two distinct kinds of pores for ACF were identified (Figure
Deuterated acetonitrile and pyridine-adsorption studied by
DRIFTS
Diffuse Reflectance Infrared Fourier Transform Spectroscopy
(DRIFTS) studies on the adsorption of deuterated acetonitrile
were performed on HF-loaded ACF and ACF itself (see SI, Figure
S4). A band of low intensity with a shift of about 67 cm-1 to higher
energy was detected for the CN vibration for the HF-loaded ACF
sample (see SI, Table S1). This large shift indicates that free
Lewis acidic sites are still present on the HF-loaded ACF, but in a
very small amount. They are as strong as found for ACF since an
absorption band with the same energy was found for the surface-
bound deuterated acetonitrile.[19] However, the intensity of the
bands is much lower for the HF-loaded ACF sample than for ACF,
which is in good agreement with the assumption that most of the
sites are blocked by HF. Additionally, pyridine was as well
adsorbed on the surface of HF-loaded ACF. Two bands with low-
intensity at 1540 cm-1 and 1490 cm-1 indicate the presence of
Brönsted-acidic sites, but there is no indication for Lewis-acidity
15). The studies reveal only one kind of pores for the loaded
sample with HF with a diameter range between 14-17 Å. For ACF,
micropores of two different sizes were distinguished: one with a
diameter of about 12 Å, and another type with a diameter of 20 Å.
The results are summarized in Table 1. This might indicate a
complete filling of the smaller pores by HF, with an additional
partial filling of the bigger pores, which are present on ACF, hence
being reduced in size in the loaded sample. X-ray data on
2
polyfluoride anions in KHF revealed an F-H-F distance of about
2
.277 ± 0.006 Å.[
45,50]
Data for larger anions were also reported
-
such as for H
Å.[51,52]
4
F
5
, which reach an F-H-F distance of 2.453 ± 0.002
(
see SI, Figure S5). [53–55] Again, in the case of HF-loaded
Table 1. Surface area and pore size distribution of ACF and HF-loaded ACF
aluminum alkoxide, when photoacoustic IR of adsorbed pyridine
on the surface was performed, the band for the Lewis acid sites
was hardly detected.[32] However, in this case, the Lewis acidity
could be recovered by desorbing the HF at 350°C in the presence
of dichlorodifluoromethane (R12).[32]
Surface
area by
Pore
Pore size
Å]
Material
volume
[
2
3
BET [m /g]
[cm /g]
0
.022 &
.05
0.01
ACF
215
44
12 & 20
14-17
0
Reactivity
ACF--HF
Various reports demonstrate catalytic hydrofluorination reactions
of alkynes, using transition metal complexes and different HF
sources.[56–62] HF sources including adducts such as DMPU/HF,
3 x
Et N·3HF, and Pyridine·(HF) have been used for fluorination, as
for instance, demonstrated by Olah in the hydrofluorination of
alkynes using pyridinium poly(hydrogenfluoride).[
59,62–65]
At 0°C,
more than 70% yield of the corresponding Markownikow alkyl
fluorides as products were obtained. Recently, many research
groups focus on gold-based catalysts in combination with HF-
sources to develop hydrofluorination processes of alkynes.[
Indeed, the groups of Nolan, Toste, Miller, and Sadighi, used
56,66,67]
3
Et HF·3HF together with various catalysts and succeeded in the
formation of monofluorinated alkenes.[57,66,68,69] Hammond
reported on a gold-catalyzed hydrofluorination of alkynes to yield
olefins with DMPU/HF as well as with Pyridin·HF.[59,70] Moreover,
the synthesis of β-fluorovinyl sulfones was studied by the group
of Fustero, using copper-based catalysts together with
2
Figure 14. N sorption isotherm of ACF (black) and HF-loaded ACF (red).
Et
reported by O’Hagan for hydrofluorinations of alkynyl sulfides
using Et N·HF as HF source and TiF or BF .Et O as Lewis
acids.[71] Thibaudeau demonstrated the applicability of the
superacid HF-SbF in the hydrofluorination of ynamides at low
3
N/HF.[60] Conversions in the presence of Lewis acids were
3
4
3
2
5
temperatures, but it turned out that using pure anhydrous HF gave
better yields.[72] Finally, a stable ion exchange resin loaded with
anhydrous HF reagent was developed by Hammond, allowing for
a hydrofluorination of alkenes and functionalized alkynes and
ring-opening of aziridines.[73,74]
The ACF-loaded HF was tested in the hydrofluorination reactions
of alkynes at 70°C, and the reactions were monitored over seven
days (Scheme 2). As shown in Scheme 3, in all cases, the
fluorination occurred to give the Markownikow product. A second
hydrofluorination was observed in certain cases, and fluorination
takes place at the already fluorinated carbon. However, in
Figure 15. Pore size distribution of the two materials determined by NLDFT.
6
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comparison to other catalytic systems, such as the Au complex
catalyzed DMPU/HF fluorination by Hammond et al., the
conversions are low. They also reported for the same olefin that
on using only fluorinating reagents, such as Pyridine/HF,
The loaded HF can further be transferred to alkynes by
hydrofluorination under mild conditions to yield fluorinated
alkenes, which are currently of interest as fluorinated building
blocks.[76–78]
+
-
4
DMPU/HF, or even Bu N OTf /HF, no conversion was
observed.[59,75]
Experimental Section
General procedures
ACF was synthesized according to the literature.[13] HF (gift from Solvay
Fluor GmbH) was dried using potassium hexafluoronickelate prior to the
loading onto the surface of ACF.[79] In a flow of an argon stream, HF was
condensed onto the drying agent, forming a deep red solution. The dry HF
was then condensed onto 3 g of ACF for 30 minutes, and the excess of
HF was then removed under vacuum. Before and after the loading with HF,
the material was weighted and an increase of about 8% in weight was
observed for the loaded sample.
Scheme 2. synthesis (up) and reactivity (bottom) of HF-loaded-ACF.
A second batch of HF-loaded ACF was synthesized, in the same way, as
described, but a smaller amount of pre-dried HF was condensed onto the
surface of ACF to obtain an HF moistened ACF.
CAUTION: Appropriate safety precautions must be taken when using HF,
which is a highly toxic and irritant compound. Severe burns can be caused
if HF comes in contact with the skin.
MAS NMR spectroscopy
The MAS NMR spectra were recorded on a Bruker AVANCE 600
0
spectrometer (B = 14.1 T) at room temperature using a 2.5 mm magic
angle sample spinning (MAS) probe and the MAS frequency was 27.5 kHz.
The spectra shown in Figure 5, 6 and in the SI were recorded on a Bruker
AVANCE 400 spectrometer (B
0
= 9.4 T) at room temperature, using the
same 2.5 mm rotor. The rotation frequency used was 20 kHz. The
chemical shifts are given with respect to (i) the CFCl
3
standard for 19F, (ii)
for 27Al.
3
1
TMS for H, and (iii) and aqueous solution of AlCl
Data analysis was performed with the software TopSpin 2.1.
Scheme 3: Reactivity of various alkynes in the presence of HF-loaded-ACF.
It should be mentioned that in a comparable case, the reactivity
of HF-loaded aluminum alkoxide was found to be significantly
reduced compared to High Surface-AlF (HS-AlF
3 3
).[32] Indeed,
FTIR/INS
All FT-IR spectra were acquired with a Bruker Vertex 70 spectrometer
equipped with an ATR unit (diamond).
when this material was tested in the isomerization reaction of 1,2-
dibromohexafluoropropane and in the dismutation reaction of
dichlorodifluoromethane, no conversions at all were observed for
both reactions, even when the temperature was increased up to
Inelastic neutron scattering spectra were collected on the indirect
geometry instrument TOSCA[80,81] at the ISIS Neutron and Muon Facility.
Samples (0.928 g HF-loaded ACF) were loaded under argon atmosphere
into Al sachets and sealed into Al sample holders before loading onto the
instrument. The samples were cooled to < 20 K prior to data collection in
order to minimize Debye-Waller dampening. Data were collected in the
forward and backwards scattering geometries and summed.
2
50°C.[32] Those reactions can be usually performed at room
temperature, by HS-AlF
3
or ACF, reaching 99% conversion.[9]
Conclusion
Thermoanalysis
The thermal behavior of the solids was studied by conventional thermal
analysis (TA) in an argon atmosphere. A NETZSCH Thermo analyzer
STA409C Skimmer, being additionally equipped with a conventional high-
temperature SiC oven, was used to record the thermoanalytical curves. An
online-coupled BALZERS QMG 421 enabled MS-analysis of evolved
gases. A DTA‐TG sample carrier system with platinum crucibles (beaker,
HF was successfully immobilized at the surface of the ACF, by
forming polyfluoride structures. The Lewis and Brønsted acidity
are strongly reduced, presumably due to the interactions with
surface Lewis sites as well as with surface fluorides. Thus, loading
HF at the surface of ACF does not result in the generation of a
5
superacid similar to systems such as SbF /HF. Comparable
0.8 mL) and Pt/PtRh10 thermocouples were used. Measurements were
observations were reported by Kemnitz et al. for a compound with
HF immobilized at aluminum alkoxide fluoride.[32] MAS NMR and
X-Ray data of HF-loaded ACF indicate a slight reorganization of
the bulk. Heating leads to fluorination and crystallization
processes, which might even already be initiated at room
temperature. This would also result in a reduced Lewis-acidity.
performed under an argon atmosphere by applying a heating rate of 10
Kꢀmin–1
.
XRD
X‐ray powder diffraction measurements were performed on an STOE Stadi
MP diffractometer equipped with a Dectris Mythen 1K linear silicon strip
7
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Chemistry - A European Journal
10.1002/chem.202001627
FULL PAPER
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Entry for the Table of Contents
A variety of characterization methods such as MAS NMR spectroscopy, XRD, FTIR, INS, and surface analysis methods were used to
investigate the interaction of HF with the surface of the nanoscopic Lewis acidic aluminum chlorofluoride (ACF). The studies reveal
the formation of polyfluorides at the surface of ACF, as well as a slight reorganization of the bulk and a decrease in the acidity. The
material was tested in the hydrofluorination of alkynes and showed good results in hydrofluorination.
1
0
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