Amorphous aluminum bromide fluoride (ABF)

Article abstract of DOI:10.1002/anie.200460491

Different octahedral units form the basis of the structures of the very strong Lewis acids aluminum bromide fluoride (ABF) and aluminum chloride fluoride (ACF) (see picture). A structure model for these phases is established in which the octahedra are linked through μ-bridging fluorine atoms and μ₃-bridging X atoms (X = Cl, Br). The bridging of three octahedra by a heavy halogen atom explains the results of NMR and EXAFS analysis.

Full text of DOI:10.1002/anie.200460491

Angewandte
Chemie
1
3
Lewis Acids
have been observed by C NMR spectroscopy. ABF is
amorphous and shows no reflections in the XRD pattern.
During heating, it crystallizes suddenly at 4008C, forms
crystalline b-AlF and AlBr , and looses its very high Lewis
Amorphous Aluminum Bromide Fluoride
ABF)**
(
3
3
acidity.
The Lewis acidity of ABF is very high. It catalyzes the
isomerization reaction of 1,2-dibromohexafluoropropane to
Thoralf Krahl and Erhard Kemnitz*
2
,2-dibromohexafluoropropane at room temperature
Dedicated to Professor Gerd-Volker Röschenthaler
on the occasion of his 60th birthday
[
1d]
[
Eq. (2)], which requires an extremely strong Lewis acid.
CF
3
ꢀCFBrꢀCF
2
Br ! CF
3
ꢀCBr
2
ꢀCF
3
ð2Þ
Amorphous aluminum chloride fluoride (ACF; AlCl F , x =
x
3ꢀx
[
1]
We chose this particular reaction because it can be carried out
relatively easily. The acidity of certain Lewis acids has been
recently expressed quantitatively by calculation of the
fluoride ion affinity (FIA) at the MP2 level of theory. From
0
.05–0.3), which is of high interest for catalytic reactions,
[
2]
was discovered at DuPont in 1992. It exhibits an extraordi-
narily high Lewis acidity similar to the acidity of SbF , and in
5
some cases, has even higher acidity. ACF is prepared by mild
fluorination of AlCl with chlorofluorocarbons such as CFCl .
this point of view, molecular SbF is the strongest Lewis acid
5
3
3
[
5]
with the highest FIA. Molecular aluminum chloride and
molecular aluminum fluoride have just slightly lower acidities.
However, the fluorination is never carried out to completion;
the solid phase always contains some chlorine. The high Lewis
acidity is surprising since pure aluminum chloride and phases
of aluminum fluorides prepared in other ways are much
weaker Lewis acids. We recently reported the investigation of
However, the acid strength of solid acids such as AlF , ACF,
3
and ABF cannot be expressed in such terms, but a ranking of
the acid strengths can be done in terms of their reactivity.
Reaction (2) requires a highly acidic catalyst; it runs easily at
room temperature with the very strong Lewis acids SbF5,
ACF, and ABF but not with the strong acids AlCl and AlBr .
[
3]
ACF by several spectroscopic methods and presented our
initial findings on the structure of this amorphous compound.
However, the role of the residual chlorine in ACF is still
unclear, but it has been shown that the structures of ACF and
3
3
The IR spectrum of ABF (Figure 1) is typical for a
[
3]
AlCl differ with respect to the chlorine atoms.
network of corner-sharing AlF octahedra, which has the
3
6
Herein we report on the synthesis of amorphous alumi-
nominal composition of AlF . Two intensive bands at around
665 and 350 cm are attributed to the valence and deforma-
3
ꢀ1
num bromide fluoride ABF (AlBr F , with x = 0.13), which
x
3ꢀx
is very similar to ACF. EXAFS measurements on the
Br K edge of ABF enabled a more detailed study of the
structure than for ACF. Partially fluorinated samples of AlBr3
with the nominal composition AlBr F and AlBrF were also
2
2
investigated. From the analysis of the data measured by
1
9
F MAS NMR spectroscopy and Br K EXAFS, together with
[
3]
that gained during the investigation of ACF, a basic
structural model of the compounds ABF and ACF is
introduced for the first time.
[
4a]
Solid AlBr is built up of discrete Al Br molecules,
3
2
6
[
4b]
whereas AlCl has a layered structure. The fluorination of
3
AlBr with CFCl is highly exothermic and can be performed
3
3
similar to that of ACF [Eq. (1)] to initially give CBrCl and
3
ABF.
AlBr
3
þ ð3ꢀxÞ CFCl
3
! AlBr
x
F
3ꢀx þ ð3ꢀxÞ CBrCl
3
ð1Þ
Figure 1. IR spectrum of ABF (AlBr0.13F2.87; CsI pellet).
The primary reaction product CBrCl is not stable in the
3
presence of a very strong Lewis acid such as ABF (see
Experimental Section). Thus, it undergoes dismutation and
slowly forms CCl , CBr Cl , CBr Cl, and CBr . These products
tion vibrations of the AlF octahedron, respectively. The large
6
4
2
2
3
4
width of the band indicates the high degree of amorphicity of
ABF. Interestingly, there is no significant difference between
[
3]
[*] T. Krahl, Prof. Dr. E. Kemnitz
the IR spectra of ABF and ACF.
Institut für Chemie
Humboldt-Universität zu Berlin
Brook-Taylor-Strasse 2, 12489 Berlin (Germany)
Fax: (+49)30-2093-7277
E-mail: erhard.kemnitz@chemie.hu-berlin.de
19
The F MAS NMR spectra of ABF, partially fluorinated
AlBr , and ABF exposed to air are given in Figure 2. All
3
spectra show the main signal between d = ꢀ160 and
ꢀ
170 ppm, which is typical for the m-F atoms of corner-
sharing AlF octahedra and is also observed in amorphous
[
**] This work was supported by the Deutsche Forschungsgemeinschaft
Ke 489/15-1). We thank Dr. J.-D. Grunwaldt and co-workers from
the ETH Zurich for performing the EXAFS measurements.
6
[
3,7]
and crystalline phases of AlF3.
The spectrum of ABF
(
shows an additional weak signal in the region from d = ꢀ200
Angew. Chem. Int. Ed. 2004, 43, 6653 –6656
DOI: 10.1002/anie.200460491
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6653

Communications
1
9
Figure 3. Measured (a) and simulated (b) F MAS NMR spectrum of
ABF. The fit (b) is the sum of the components (b1) to (b4). The
parameters of the lines are shown in Table 1. b1) Main signal, lines 5
and 6; b2) shoulder, lines 7 and 8; b3) spinning side bands, lines 2, 3,
1
9
Figure 2. F MAS NMR spectra of a) AlBr F, b) AlBrF , c) ABF
(
2
2
AlBr0.13F2.87), and d) ABF exposed to air for 18 h.
9, and 10; b4) organic residue, lines 1 and 4; c) difference between (a)
and (b). The same scale was used for all components.
to ꢀ210 ppm. According to the superposition model for the
F chemical shift, this signal is caused by terminal fluorine
atoms bound to only one aluminum atom at a distance of
1
9
[
6a]
19
[a]
around 1.7 Š (t-F).
The appearance of such types of
Table 1: Deconvolution of the F MAS NMR spectrum of ABF.
fluorine atoms has only been observed in ACF before and
No. Amplitude Position Width
Irel
[%]
Comment
[
7c]
recently also in alumina fluorinated with CHClF . Decon-
[a.u.]
[ppm]
[kHz]
2
1
9
volution of the F NMR spectrum of ABF with the program
1
2
3
4
5
6
7
8
9
0
1.04
8.72
3.34
3.72
51.30
45.81
6.75
1.38
5.33
4.56
ꢀ81.31 1.06
ꢀ80.18 8.60
ꢀ89.80 6.18
ꢀ122.64 0.35
ꢀ161.41 8.89
ꢀ169.84 6.56
ꢀ200.00 6.02
ꢀ210.00 1.19
ꢀ243.32 7.34
ꢀ252.38 5.15
–
–
–
organic
s.s.b.
s.s.b.
[
6b]
DMFIT using Gaussian line shapes and two lines for each
of the signals shows that the m-F:t-F ratio of intensities of the
fluorine signals is 92.3:7.7 (Figure 3 and Table 1). The small
and narrow signals at chemical shifts of about d = ꢀ80 and
–
organic
48.8
43.5
6.4
1.3
–
bridging fluorine, 92.3%
ꢀ
120 ppm can be assigned to the CF and CF groups of the
3 2
organic residue. The signals of the terminal fluorine atoms
disappear when the sample is exposed to atmospheric
moisture (Figure 2d), and the structure of ABF changes
irreversibly.
terminal fluorine, 7.7%
s.s.b.
s.s.b.
1
–
[a] The parameters of the Gaussian line fit are given. Only the lines 5 to 8
The fingerprint region of the X-ray absorption near edge
spectra (XANES) of the Br K edge of AlBr , AlBr F, AlBrF ,
were considered for relative intensity. (I ) (s.s.b.=spinning side band).
rel
3
2
2
and ABF are shown in Figure 4. The spectra of ABF and
AlBr differ from each other, whereas the spectra of AlBr F
3
2
and AlBr are almost the same. The spectrum of AlBrF is a
3
2
superposition of the spectra of AlBr and ABF—a linear
ABF. Thus, the measurements confirm that AlBrF2 is a
3
combination of both data sets delivers the best fit result at
mixture of the two phases, AlBr and ABF, and that the
3
2
9% AlBr and 71% ABF. From these results, it is evident
remaining bromine in ABF does not derive from unreacted
AlBr3.
3
that the bromine atoms in ABF and AlBr differ from each
3
other—the only partially fluorinated compound AlBrF2
contains two different types of bromine atoms: one similar
The FT EXAFS spectra of the Br K edge of AlBr and
ABF (Figure 5) clearly show that the mean distance between
3
to the bromine in AlBr and another similar to the bromine in
bromine and the surrounding atoms in AlBr is smaller than
3
3
6
654
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Angewandte
Chemie
Figure 4. Br K-edge XANES spectra of a) AlBr , b) AlBr F, c) AlBrF ,
3
2
2
and d) ABF (AlBr0.13F2.87). The dashed lines indicate characteristic fea-
tures. E =photon energy.
P
Figure 5. Fourier-transformed EXAFS spectra of the Br K-edge of a)
that found in ABF, that is the AlꢀBr bonds lengthen during
the fluorination. The long AlꢀBr distances are surprising, but
can be explained with the model established in the following.
The results gained from catalysis studies as well as IR and
AlBr0.13F2.87 (ABF) and b) AlBr . The transformed spectra are not phase-
3
corrected. The original EXAFS function is shown in the insets.
1
9
F NMR measurements, and thermoanalysis show that the
solid phases ABF and ACF are very similar to each other.
Nevertheless, there are some spectroscopic methods, which
could only be used with one of the phases (EXAFS for ABF,
[
3]
ESR for ACF ). The combination of the results of work on
[
3]
ACF and the work on ABF presented here allows us to
propose a simple geometric model for the structure of such
compounds. It is assumed that ACF and ABF are built up of
the same basic units; the structure is explained with a model
based on linked polyhedra.
Scheme 1. Structural octahedral units proposed in ABF and ACF
(X=Cl, Br). Al atoms in the middle of the octahedra are not shown.
The sinuous lines indicate the bond to the next aluminum atom.
The following assumptions are made:
1
2
)
)
The aluminum atoms are octahedrally coordinated.
Three different types of octahedra are present (see
To ensure charge balance, the ratio between 2 and 3 must
be 1:3. The overall sum formula can be formulated as [Al(m-
Scheme 1):
F)
5
(t-F)]·3[Al(m -X)
1
(m-F) =
5
]·n[Al(m-F) =2], where n is a var-
6
=
2
3
=
3
2
0
2
2
a) [Al(m-F) =2
6
] octahedra (1), in which all fluorine atoms link
iable parameter. This formula can be simplified to Al4+nX(t-
1
two octahedra,
F)(m-F)10+3n. Substitution of n as xꢀ4 yields AlX (t-F) (m-
x
x
1
=
2
ꢀ
b) [Al(m-F)=2
5
(t-F)]
octahedra (2), in which five fluorine
F) , or AlX F . For the ABF investigated in this
3ꢀ2x x 3ꢀx
atoms link two octahedra and one fluorine atom is
publication n equals 3.69.
1
9
terminal,
This model can explain the F NMR and Br K EXAFS
spectroscopic data. The following conclusions can be drawn:
1) The formal charge of the octahedra of type 3 is higher than
1
=
6
+
2
c) [Al(m -X)
1
(m-F) =2
5
]
octahedral (3), in which one atom X
Cl or Br) links three octahedra and five fluorine atoms
link two octahedra.
3
=
3
(
that of the normal octahedra 1 found in AlF . Thus, the
3
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6655

Communications
electron deficiency of the central Al atom of 3 is higher
obtained. The sample had a composition of AlBr0.13
.8% carbon.
AlBr F and AlBrF : The reaction was carried out similarly.
F2.87 and contained
0
than that of the Al atoms in AlF . Thus, ABF and ACFare
3
2
2
more Lewis acidic than AlF3.
Perfluorooctane was added until the solid AlBr was totally immersed
3
2
) From n ꢁ 0 follows x ꢂ 0.25. AlX0.25F2.75 is the limiting
composition for a compound called “ABF” or “ACF”.
This agrees quite well with the value 0.3 both given by
before the reaction. After mixture had been frozen, the desired
amount of CFCl3 (for AlBr F 1 equiv, for AlBrF 2 equiv) was
2
2
condensed on the solid and then the workup proceeded as described
for ABF.
[
2]
19
duPont and determined by F NMR spectroscopy for
ACF.
[
3]
3
) The ABF structure investigated contains 7.7% terminal
Received: April 28, 2004
fluorine atoms with chemical shifts between d = ꢀ200 and
1
9
ꢀ
210 ppm in the F NMR spectrum (Table 1). The
Keywords: aluminum · EXAFS spectroscopy · Lewis acids ·
NMR spectroscopy · structure elucidation
.
amount of t-F and Br should be the same. Its formula
can then be formulated as AlBr0.13(t-F)0.22(m-F)2.65. The
amount of t-F is a little higher than that of bromine.
[
1] a) G. C. Krespan, V. A. Petrov, Chem. Rev. 1996, 96, 3269 – 3301;
b) C. G. Krespan, D. A. Dixon, J. Fluorine Chem. 1996, 77, 117 –
4
) The mean distance between a m -bridging halogen atom X
3
and aluminum should be comparatively high. The EXAFS
spectra (Figure 5) show clearly that the mean distance of
bromine to its nearest neighboring atoms is higher in ABF
than in crystalline AlBr —the m -bridging Br atoms have
126; c) V. A. Petrov, C. G. Krespan, B. E. Smart, J. Fluorine
Chem. 1996, 77, 139 – 142; d) V. A. Petrov, C. G. Krespan, B. E.
Smart, J. Fluorine Chem. 1998, 89, 125 – 130; e) V. A. Petrov, C. G.
Krespan, J. Fluorine Chem. 2000, 102, 199 – 204.
3
2
an AlꢀBr distance of 2.38 Š and the terminal Br atoms
[
[
[
2] A. C. Sievert, G. C. Krespan, F. J. Weigert (DuPont Co.), US-A
[
4a]
5.157.171, 1992.
have a AlꢀBr distance of 2.19 to 2.20 Š.
3] T. Krahl, R. Stösser, E. Kemnitz, G. Scholz, M. Feist, G. Silly, J.-Y.
BuzarØ, Inorg. Chem. 2003, 42, 6474 – 6483.
4] a) S. I. Troyanov, Zh. Neorg. Khim. 1994, 39, 552 – 555; b) S. I.
Troyanov, Zh. Neorg. Khim. 1992, 37, 266 – 272.
5
) AlꢀBr bonds are very sensitive to moisture. During
exposure to air, not only are the acidic centers on the
surface blocked, but the relevant structural elements
explained above are destroyed. The resulting compound
[5] a) K. O. Christe, D. A. Dixon, D. McLemore, W. W. Wilson, J. A.
Sheehy, J. A. Boatz, J. Fluorine Chem. 2000, 101, 151 – 153;
b) H. D. B. Jenkins, H. K. Roobottom, J. Passmore, Inorg. Chem.
can be formulated approximately as Al(OH) F . In the
y
3ꢀy
1
9
F NMR spectrum no signals for terminal fluorine are
seen, and the signals are shifted to slightly lower field
Figure 2D). The latter is also observed on comparing the
2
003, 42, 2886 – 2893.
6] a) B. Bureau, G. Silly, J.-Y. BuzarØ, J. Emery, Chem. Phys. 1999,
49, 89 – 104; b) D. Massiot, F. Fayon, M. Kapron, I. King, S. Le
[
(
2
[
3]
spectra of AlF and AlF ·3H O.
3
3
2
CalvØ, B. Alonso, J.-O. Durand, B. Bujoli, Z. Gan, Magn. Reson.
Chem. 2002, 40, 70 – 76.
[
7] a) P. J. Chupas, M. F. Circoalo, J. C. Hanson, C. P. Grey, J. Am.
Chem. Soc. 2001, 123, 1694 – 1702; b) J. L. Delattre, P. J. Chupas,
C. P. Grey, A. M. Stacy, J. Am. Chem. Soc. 2001, 123, 5364 – 5365;
c) P. J. Chupas, C. P. Grey, J. Catal. 2004, 224, 69 – 79.
Experimental Section
Solid-state NMR spectra were measured with a Bruker AVANCE
00 MHz spectrometer equipped with a 2.5-mm Bruker MAS probe
head at a rotation frequency of 30 kHz and a resonance frequency of
4
1
9
3
76 MHz for the F nucleus.
Measurements of X-ray absorption spectra were carried out at
HASYLAB on the beamline X1 (DESY, Hamburg, Germany) in
transmission mode. The edge energy of the Br K-shell (13474 eV) was
calibrated with gold foil (Au L -edge energy 13733 eV). Samples
2
were mixed with hexagonal boron nitride and pressed into pellets in a
dry box.
All preparations were carried out under standard Schlenk
conditions. Solvents were dried by condensing over molecular
sieves (4 Š) before use. AlBr3 (Reakhim, p.a.), CFCl3 (Fluka,
9
9.5%), and perfluorooctane (ABCR, 95%) were used for the
syntheses. The composition of the samples was checked by Br, C, and
F analysis.
Synthesis of ABF: The reaction between AlBr3 and CFCl3 is
strongly exothermic and should not be carried out at room temper-
ature. AlBr (11.0 g, 41.4 mmol) was placed with a magnetic stirrer in
3
a 250-mL round-bottomed flask equipped with a dry ice condenser.
The flask was evacuated and cooled with liquid nitrogen. Six
equivalents of CFCl (248 mmol) were condensed on the solid. The
3
flask was warmed up to around 200 K (dry ice/2-propanol) and stirred
at this temperature for 1 h. The start of the reaction was indicated by
the yellow color of the solid. The flask was warmed to room
temperature and refluxed for one more hour. The liquid was then
evaporated in vacuum, and a fine orange-yellow powder was
6
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Angew. Chem. Int. Ed. 2004, 43, 6653 –6656