Synthesis and application of strong Bronsted acids generated from the Lewis acid Al(ORF)3 and an alcohol

Article abstract of DOI:10.1021/om300776a

The strong neutral Bronsted acids [(R)OH→Al(OC(CF ₃)₃)₃] (R = -C(CF₃)₃ (1), -C₆F₅ (2), (-)-menthyl (3)) were synthesized by complexation of perfluoro tert-butyl alcohol, pentafluorophenol, and (-)-menthol with the Lewis superacid Al(OC(CF₃)₃)₃. The 1:1 composition of the compounds was proven by NMR (except for compound 2), IR, and partially Raman spectroscopy and X-ray crystallography. Of the structures, 2 crystallized with a coordinated toluene molecule, which might be seen as a frozen intermediate or prestep to form the classical Wheland complex of protonated toluene. This interaction was calculated to be exothermic by 32 (no dispersion) or 88 kJ mol^-1 (Grimmes D3 dispersion correction included ? RI-BP86/SV(P)). 1 proved suitable to protonate mesitylene and Et ₂O, giving the acidic cationic Bronsted acids [H(C ₆H₃(CH₃)₃)]⁺ (4b) and [H(OEt₂)₂]⁺ (5) with the respective weakly coordinating anion [Al(OC(CF₃)₃)₄]^-. In dichloromethane solution 4b decomposes at room temperature, leaving the room-temperature-stable salt [H(C₆H₃(CH₃) ₃)]⁺[((CF₃)₃CO)₃Al-F- Al(OC(CF₃)₃)₃]^- (4a; XRD). The acidities reached with 4 and 5 are discussed in terms of our recently introduced absolute Bronsted acidity scale. The absolute chemical potentials of a 0.001 M solution of protonated mesitylene and Et₂O amount to -944 and -1015 kJ mol^-1 or-in terms of absolute pH_abs values-to 165 and 178 and thus are at the threshold of superacidity of -975 kJ mol ^-1 or pH_abs of 171.

Full text of DOI:10.1021/om300776a

Article
pubs.acs.org/Organometallics
Synthesis and Application of Strong Brønsted Acids Generated from
the Lewis Acid Al(ORF)3 and an Alcohol
Anne Kraft,† Jennifer Beck,† Gunther Steinfeld,‡ Harald Scherer,† Daniel Himmel,† and Ingo Krossing*,†
†Institut fur Anorganische und Analytische Chemie, Freiburger Materialforschungszentrum (FMF) and Freiburg Institute for
̈
Advanced Studies (FRIAS), Universitat Freiburg, Albertstraße 19, 79104 Freiburg, Germany
̈
‡Zurcher Hochschule fur Angewandte Wissenschaften, Gruental, 8820 Wadenswil, Switzerland
̈
̈
̈
̈
S
* Supporting Information
ABSTRACT: The strong neutral Brønsted acids [(R)OH→Al(OC(CF3)3)3]
(R = −C(CF3)3 (1), −C6F5 (2), (−)-menthyl (3)) were synthesized by
complexation of perfluoro tert-butyl alcohol, pentafluorophenol, and
(−)-menthol with the Lewis superacid Al(OC(CF3)3)3. The 1:1 composition
of the compounds was proven by NMR (except for compound 2), IR, and
partially Raman spectroscopy and X-ray crystallography. Of the structures, 2
crystallized with a coordinated toluene molecule, which might be seen as a
frozen intermediate or prestep to form the classical Wheland complex of
protonated toluene. This interaction was calculated to be exothermic by 32
(no dispersion) or 88 kJ mol−1 (Grimmes D3 dispersion correction included
@ RI-BP86/SV(P)). 1 proved suitable to protonate mesitylene and Et2O,
giving the acidic cationic Brønsted acids [H(C6H3(CH3)3)]+ (4b) and
[H(OEt2)2]+ (5) with the respective weakly coordinating anion [Al(OC-
(CF3)3)4]−. In dichloromethane solution 4b decomposes at room temperature,
leaving the room-temperature-stable salt [H(C6H3(CH3)3)]+[((CF3)3CO)3Al−F−Al(OC(CF3)3)3]− (4a; XRD). The acidities
reached with 4 and 5 are discussed in terms of our recently introduced absolute Brønsted acidity scale. The absolute chemical
potentials of a 0.001 M solution of protonated mesitylene and Et2O amount to −944 and −1015 kJ mol−1 orin terms of
absolute pHabs valuesto 165 and 178 and thus are at the threshold of superacidity of −975 kJ mol−1 or pHabs of 171.
commonly written HBF4 or HSbF6 acids only exist in the form
INTRODUCTION
■
15
H(H2O)n [BF4]− and H(HF)n [SbF6]−. So far the hypo-
thetical pure H2[B12F12] superacid has only been isolated as
[(H3O)2]2+[B12F12]2−·6H2O.17 In a broader sense, the almost
perfluorinated neutral radical [HCB11(CF3)nF11−n]• (n = 5, 6;
only stable at −60 °C) can be mentioned here, which was
synthesized by oxidation of the respective anion.18
+
+
Combining highly acidic systems with weakly coordinating
anions (WCAs) seems to be one of the promising strategies to
create highly reactive protonated cations.1,2 In recent years
superacids and their use to protonate arenes and fullerenes
were in the focus of experimental and theoretical work.1,3−8
While classical superacids such as HF/SbF5, HSO3F/SbF5
(Magic acid), and HBr/AlBr3 allow the synthesis of highly
reactiveoften oxidizedcations, their resulting anions are
often too nucleophilic, too basic, or too corrosive so that
decomposition reactions result.9,10 Therefore, a nonoxidizing
although extremely Brønsted acidic system with a conjugate
base as poorly nucleophilic as possible would be desirable.
Promising examples for this type of Brønsted acids are
Reed'sunfortunately still isolable only on a small scale
carborane acids H[CB11H6X6] (X = Cl, Br), which at a 1 equiv
level can protonate weak bases such as benzene11,12 and
fullerenes10 and were used for Et2Cl+ formation13 and
protonation-induced rearrangement reactions.14 Recent addi-
tions to these type of acids include H[RCB11F11] (R = H,
C2H5), C60(C2F5)5H, H[HCB11Cl11], and H2[B12X12] (X = Cl,
Br).4,15,16 To the best of our knowledge, no pure conjugate
acids of completely perfluorinated anions are currently known.
For example, due to acidic cleavage of the B−C bond,
H[B(C6F5)4] is not suitable for superacid chemistry and the
In analogy to the classical Lewis/Brønsted acid mixtures HF/
SbF5 and HSO3F/SbF5 we increased the Brønsted acidity of
perfluorinated or chiral alcohols by complexation with a
nonoxidizing Lewis superacid.5 Thus, stoichiometric reactions
of PhF→Al(ORF)3 or in situ prepared donor-free Al(ORF)3 (RF
= C(CF3)3) and the alcohols HO−R (R = −C(CF3)3 (1),
−C6F5 (2), (−)-menthyl (3)) form an alcohol−alane adduct,
which represents the protonated weakly coordinating anion
H[Al(OR)(ORF)3]. Neutral alcohol−water aluminum com-
plexes are only scarcely described in the literature, due to their
high tendency to hydrolyze with classical aluminum-based
Lewis acids such as trihaloalanes.19 In a wider perspective, also
the water and methanol complexes of E(C6F5)3 (E = B, Al) are
cited here.20 The properties of the employed alcohols clearly
influence the overall acidity of the prepared system: the pKa
Received: August 10, 2012
Published: October 3, 2012
© 2012 American Chemical Society
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values of HOC(CF3)3 and HOC6F5 are very similar (5.4/5.5 in
H2O, 20.55/20.11 in CH3CN, gas-phase acidities (GAs) 1357/
1343 kJ mol−1).21 Thus, both should qualify to form with
Al(ORF)3 the first completely perfluorinated, neutral, but still
isolable Brønsted acids 1 and 2. In support, we have calculated
the GAs of compounds 1−3 at the BP86/def-TZVP level as
1041, 1052, and 1169 kJ mol−1, respectively, ranking them
between the classical Brønsted acid H2SO4 and the very strong
neutral isolable carborane acid H[CB11H6Cl6] (Table 1).3
recommended to use 1 in the nonpolar solvent perfluorohex-
ane, in which the dissociation tendency is minimized. 3 is stable
in dichloromethane, while 2 is only scarcely soluble in common
solvents such as dichloromethane, SO2, and 1,2-F2C6H4.
Overall the calculated complexation enthalpies ΔrH°gas (eq 1,
(RI-)BP86/def-TZVP, 298 K) increase from HOC(CF3)3 over
HOC6F5 to menthol. This leveling is in agreement with the
enhanced stability and decreased acidity of the compounds in
solution as well as the calculated GAs. The NMR and IR
spectroscopic data as well as the X-ray crystal structure data
agree with 1:1 compositions of the compounds.
Table 1. Calculated Gas-Phase Acidities ((RI-)BP86/def-
Spectroscopic Characterization of the Acids. The IR
spectrum of 1 shows the O−H stretch at 3413 cm−1 as a
significantly (by 220 cm−1) red-shifted and broadened band in
comparison to that of free HOC(CF3)3, indicating the
formation of hydrogen bonds. This finding is in agreement
with the proton being localized at an oxygen atom and involved
in a hydrogen bond (to a (C−)F or (C−)O atom). The DFT-
optimized structure (see the Supporting Information) suggests
internal hydrogen bonding from a protonated oxygen atom to a
fluorine or oxygen atom of a OC(CF3)3 group. In the IR
spectra of 2 and 3 only weak O−H bands could be detected,
but all other bands are as expected and are in good agreement
with the calculations (see the Supporting Information).
TZVP) of 1−3 Compared to a Representative Selection of
a
Classical Brønsted Acids
Brønsted acid
GA (kJ mol−1
)
HBr
1341, [1333], 133222
1272, [1261], 126523,24
1233, 125523
1220, [1218], 120025
1209
H2SO4
HSO3F
HN(SO2CF3)2
(NC)2C(CNH)
menthol−Al(ORF)3 (3)
H[Al(OC6F5)(ORF)3] (2)
H[CB11H6Cl6]
H[Al(ORF)4] (1)
H[HCB11Cl11]
H[CB11F12]
1169
1052
1044, [1014]
1041
997, [965]
921, [891]
914
NMR experiments further suggest the stoichiometric
composition of the adducts 1 and 3. Unfortunately, 2 is
insoluble in all our commonly used solvents; therefore, no
solution NMR spectra could be recorded. In 3 the chiral
alcohol is coordinated to the Lewis acid Al(ORF)3 also in
dichloromethane solution. In the 27Al spectrum a broad signal
at 39 ppm (Δν1/2 = ∼600 Hz) was detected, which according
to our experience is only possible if the Lewis base is strongly
bound to Al(ORF)3. Apart from a very small signal of the free
alcohol, only one 19F signal in the typical chemical shift range of
H2B12F12
a
Values in brackets are taken from the literature;3 ORF = OC(CF3)3.
Values in italics are experimental data taken from the literature.
With the current work we present the easily available in large
scale Brønsted acid 1, which protonates weak bases such as
1,3,5-mesitylene and Et2O as their weakly coordinating anion
salts. Apart from [H−C6Me6]+[Al(ORF)4]−, which was
mentioned as a byproduct,26 no protonated arenes with the
[Al(ORF)4]− counteranion are currently known. By using the
chiral alcohol (−)-menthol we introduce chirality to the system,
which may find applications similar to those for classical chiral
anions and Brønsted acids:27 e.g., chiral solid catalysts based on
menthol were used for asymmetric Diels−Alder reactions.28
Further applications for 1−3 are in the focus of ongoing work.
Al(ORF)3 compounds at −75.6 ppm was detected.29
A
1
1
1
combination of H,13C HMBC, H,13C HSQC, and H,19F
1
HOESY experiments achieved the assignment of H and 13C
resonances. In agreement with the coordination to a very strong
1
Lewis acid, the H resonances of the hydroxyl and the OC−H
group are shifted downfield by 3 and 1.2 ppm, in comparison to
free (−)-menthol. Except for the signal of the proton of the
isopropyl group, all other proton resonances have higher
frequencies. Of the carbon resonances, the signal of the OC−H
group shows the most significant downfield shift and appears at
87.6 ppm versus 71.2 ppm for free (−)-menthol. All other
carbon resonances are shifted to high field, as expected when
the hydroxyl group bears an electron-withdrawing group (cf.
RESULTS AND DISCUSSION
Synthesis of the Neutral Acids. The simple route to
synthesize the neutral Brønsted acids 1−3 is shown in eq 1.
■
the Supporting Information).30 Finally, in the H,19F HOESY
1
NMR spectrum the cross peaks between the 19F signal and the
1H signals of the isopropyl and hydroxyl groups of (−)-menthol
show the small distance between menthol and Al(ORF)3. Taken
together, this proves that 3 does not dissociate in dichloro-
methane solution.
While 1 needs to be synthesized from freshly prepared donor-
free Al(ORF)3, 2 and 3 can be generated by substitution of the
PhF ligand in PhF→Al(ORF)3 with HOC6F5 or menthol. After
the compounds are crystallized at −20 or −40 °C, they can be
isolated as colorless powders in yields between 44 (2) and 88%
(1). 1 is best stored at −40 °C, whereas 2 and 3 can be handled
at room temperature. 1 is highly soluble in PhF, 1,2-F2C6H,
CH2Cl2, and SO2 but tends to dissociate in solution (see
below). In donor solvents such as SO2, Et2O, and MeCN
adduct formation is almost impossible to suppress, as will be
discussed later. In THF polymerization occurs. Thus, it is
19F NMR spectra of 1 in SO2 and 1,2-F2C6H4 show two
signals in the characteristic range for the chemically equivalent
CF3 groups of perfluoro tert-butoxy moieties (Table 2). The
ratio of the integrals of these two signals is 1:3. Whereas in SO2
solution the line width of the two signals is small, they are
significantly broadened in 1,2-F2C6H4. When the 1,2-F2C6H4
solution of 1 was cooled to 0 and −35 °C, the signals became
considerably sharper (see the Supporting Information).
Obviously, there is a chemical exchange between the two
different perfluoro tert-butoxy groups in the 1,2-F2C6H4
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solution is not suitable for discussion (see the Supporting
Information). However, the solution of the single-crystal data is
also in agreement with the assignment of 1 as H[Al(OC-
(CF3)3)4].
Table 2. Experimental NMR Data (ppm) of 1, 3, and 4a
(ORF = OC(CF3)3)
b
compound
solvent
SO2
1H
19F
27Al
assignment
1
−75.4
−74.3
−74.4
−75.5
35.3 SO2→Al(ORF)3
HORF
The aluminum atoms in 2·tol and 3 have distorted-
F
tetrahedral coordination, and the sums of the RO−Al−ORF
4.96
4.96
reference SO2
free HORF
(ORF = OC(CF3)3) bond angles are 347° in 2·tol and 341° in
3.
1
1,2-F2C6H4
1,2-F2C6H4→
Al(ORF)3
The average Al−O distance (∼169.4 pm, OC(CF3)3) in both
structures is consistent with the values for Al(ORF)3 complexes
already described in the literature.29,32 In tetracoordinate Al
compounds using C6F5O and menthoxy as anionic ligands,
dAl−O is significantly shorter than in the dative Al−O bonds to
the coordinated alcohols (188.5 pm in 2·tol vs 178.7 pm in
AlMe2(OC6F5)[N(C2H4)3CH]33/184.2 pm in Al2(OC6F5)6,34
average 183.0 pm in 3 vs 177.4 pm in [Li(thf)2(μ-O(−)-
Ment)2Al(H)2]35 (Ment = menthyl) but similar to that in a
bridging situation (184.1 pm in [Me2Al(μ-O(−)-Ment)]236).
Due to the high Lewis acidity of Al(ORF)3, the C−O distances
of the coordinated alcohols are strongly elongated in
comparison to those in the free alcohols (140.6 vs 136.0
pm37 in 2·tol, 151.1 vs 142.8 pm38 in 3). In 3 the position of
one of the hydrogen atoms of the two independent moieties
could not be found on the difference Fourier map and needed
to be restrained. Nevertheless, their position at the oxygen
atom of menthol was already proven by NMR spectroscopy and
is in agreement with 3 being the weakest acid of all three
compounds (cf. also the high aqueous pKa value of menthol of
19).39
4.26
4.40
−74.6
−75.6
HORF
a
3
CD2Cl2
39
Ment−OH→
Al(ORF)3
reference CD2Cl2
4b SO2
1.39
2.77
2.92
4.57
7.67
free menthol
−75.7
36.3 [H(C6H3(CH3)3)]
[Al(ORF)4]
a
Only the H−O signal is given. For all other 1H chemical shifts see the
b
Supporting Information. Spectra measured in SO2 were calibrated
using a reference sample of HOC(CF3)3 in SO2.
solution. This was proven by a 19F,19F EXSY experiment (see
the Supporting Information). Most likely the mechanism for
this chemical exchange is a replacement of an alcohol molecule
bound to Al(ORF)3 by a solvent molecule. This leads to an
equilibrium between the free alcohol HOC(CF3)3 and the
ligands of 1,2-F2C6H4→Al(ORF)3 through the formation of
H[Al(ORF)4]. In SO2 this chemical exchange is much slower
if present at allbecause in SO2→Al(ORF)3 the donor is more
strongly bound by about 39 kJ mol−1 than in 1,2-F2C6H4→
Al(ORF)3 (ΔrH°(dissociation) = +49 (SO2) vs +10 kJ mol−1
(1,2-F2C6H4), (RI-)BP86/def-TZVP, see also ref 29). This
suggestion was confirmed by the following observations. The
proton chemical shift of the OH group of HOC(CF3)3 in the
SO2 solution of 1 is identical with the chemical shift of the pure
alcohol in the same solvent. Furthermore, and in contrast to the
1,2-F2C6H4 solution, a broad aluminum signal similar to that for
the known SO2→Al(ORF)3 was detected in the SO2 solution.29
Crystal Structures of the Acids. The molecular structures
of 2·tol (tol = toluene) and 3 were determined by X-ray
diffraction studies (Figures 1 and 2).31 We also collected
multiple crystal structure data of independent batches of 1, but
due to the strong disorder of the compound, the structure
The hydrogen atom on the oxygen atom in 2·tol was located,
and the position was refined isotropically in the final model. A
hydrogen bond between the O−H donor and the π electron
cloud of the toluene ring was observed: cf. the related
B(C6F5)3·H2O·C6H6.41 The oxygen atom is positioned over
three carbon atoms of the toluene ring with an O···toluene
centroid distance of 318 pm. This distance lies between those
42
of [(C2B10H10Hg)3·H2O]2·C6H6 (299(1) pm) and B-
41
(C6F5)3·H2O·C6H6 (323.1(3) pm). The short contact
distance is nicely visualized with the Hirshfeld surface of
toluene (Figure 2b). Furthermore, the influence of the proton
on the electrostatic potential of toluene is significant (see the
Supporting Information), although the structural data of
toluene remain almost unchanged. This type of NH−/OH−π
intermolecular interaction was in the focus of several gas-phase
experimental and theoretical studies.43 They presume an
increase of the hydrogen bond strength with toluene in
comparison to benzene due to the stronger π-electron density
caused by the additional electron-donating methyl group.41
Thus, the calculated interaction energy (ΔrH°gas(298 K))
between 2 and toluene is significant at 32 kJ mol−1 but also
includes π stacking, as indicated by the frontier orbitals as well
as from the mentioned electrostatic interaction (see the
Supporting Information).44 Using Grimme's D3-dispersion
correction (2010), the calculated interaction energy increases
to 88 kJ mol−1 (with D2-dispersion correction 86, with D3-
dispersion correction 88 kJ mol−1, (RI-)BP86/SV(P)).45
Overall the constitution of the interaction of 2 with toluene
might be seen as a frozen intermediate or prestep to form a
classical Wheland complex, in this case protonated toluene.
Successful isolation of a protonated arene as an intermediate of
electrophilic aromatic substitution was only realized when using
the strongest acid 1 (vide infra).
Figure 1. Molecular structure of 3. Displacement ellipsoids are drawn
at the 50% probability level. The asymmetric unit contains two
independent moieties, only one of which is shown. Selected distances
in pm: Al1−O4 = 183.5(5), CMent−O4 = 151.4(1.0).
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Figure 2. (a) Section of the molecular structure of 2·tol. Displacement ellipsoids are drawn at the 50% probability level. Selected distances in pm:
Al−O4 = 188.49(12), O4−C13 = 140.56(19), av Al−O = 169.4, O4−H = 0.80(2), O4−Ctol = 335, 307, and 318 (C19, C20, C21); distances within
toluene C19−C20 = 138.9(3), C20−C21 = 138.9(2), C21−C22 = 138.5(3), C22−C23 = 138.4(3), C23−C24 = 139.8(2), C24−C25 = 150.6(3),
C19−C24 = 139.0(2). The distance between the toluene plane and the oxygen atom of HOC6F5 is 302 pm, and the angle between the planes is 13°.
(b) Hirshfeld surface40 of the coordinated toluene molecule, showing the strong OH−π intermolecular interaction.
First Applications of 1 to Protonate Ether and
Mesitylene. Synthesis. The protonated 1,3,5-mesitylene salt
[H(C6H3(CH3)3)]+[Al(ORF)4]− (4b)an isolated Wheland
complexwas synthesized by mixing 1 (prepared in situ) with
mesitylene in perfluorohexane at −20 °C (eq 2). The yellow
reaction mixture is highly acidic and reactive; thus, in situ
preparation is recommended. 4b is highly soluble in SO2 but
needs to be handled at −20 °C. In a similar reaction carried out
in dichloromethane single crystals of the well-ordered salt
[H(C6H3(CH3)3)]+[(FRO)3Al−F−Al(ORF)3]− (4a) were iso-
Figure 3. Molecular structure of 4a. Displacement ellipsoids are drawn
at the 50% probability level. The disorder of one of the C(CF3)3
lated. From earlier work it is known that the counterion
groups is removed for clarity. Selected distances in pm: Al1−F1 =
[(FRO)3Al−F−Al(ORF)3]− is a frequently observed decom-
176.4(3), Al2−F1 = 177.0(3), av Al−O = 169.7, C71−C72 =
position product of [Al(ORF)4]− that results from the action of
144.3(9), C72−C73 = 144.9(9), C73−C74 = 136.1(8), C74−C75 =
the very strong Lewis acid Al(ORF)3 on the [Al(ORF)4]−
139.7(1.0), C75−C76 = 139.4(1.0), C76−C71 = 135.6(9). Al1−F1−
counterion.46
Al2 = 176.0(2)°. The hydrogen atoms at C72 were located and the
1
positions were refined isotropically in the final model.
The H NMR spectrum of 4b in SO2 shows the typical
signals of protonated mesitylene (Table 2).12 Since an excess of
mesitylene and free alcohol was used in this reaction, the
resonances of these compounds were also detected. However,
also smaller signals of several side products were observed (for
a discussion see the Supporting Information).47 In the 19F and
27Al spectra the characteristic signals for [Al(ORF)4]− were
detected.48,49
It is noteworthy that just a weak interaction between the
anion and the cation was observed. The shortest distance
between the CH2 group of the cation and a fluorine atom of the
anion is about 299 pm; thus, the distance is at the sum of the
van der Waals radii considering weak hydrogen bonds.
However, the structural data of the cation slightly differ from
those reported with the carborane anions, and the cyclo-
hexadienyl character is more pronounced in 4a.11 Comparison
of experimental and calculated data is best for the cation of 4a
(Table 3). This reveals that the pseudo-gas-phase conditions are
strongly introduced with the μ-F bridged anion, indicating the
aluminates as very weakly coordinating.2
Analysis of the Absolute Acidities. Furthermore, we
evaluated the strengths of the Brønsted acidity of solutions of
the cationic acids protonated mesitylene and diethyl ether in
the respective solvents SO2 and Et2O. In a Born−Haber−
Fajans cycle (see the Supporting Information) using the
experimental gas-phase basicity of mesitylene and calculated
Gibbs solvation energies (COSMO and rCCC), we calculated
1 can also be used to protonate Et2O to [H(OEt2)2]+[Al-
(ORF)4]− (5) just by mixing pure 1 with Et2O, giving the
known signals in the NMR spectra.49 However, due to the high
tendency of 1 to dissociate in solution, the formation of Et2O→
Al(ORF)3 was also observed with 24 mol % next to 76 mol % 5
(see the Supporting Information).34
Crystal Structure. Structural data of 4a are typical for
protonated mesitylene as a classical σ complex (the H−C−H
angle at C72 is around 97°) and the μ-F bridged anion, already
described in the literature (Figure 3).11,12,46 Therefore, a
detailed structural discussion is not given here (see the
Supporting Information).
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1−3 protonated weakly coordinating anions, which are
promising starting materials to stabilize new protonated species.
As a first test, 1 was used to protonate mesitylene (4a,b) and
Et2O (5), underlining the potential of these acids. However,
choosing the ideal combination of nonbasic solvent and weak
but non-σ-basic base is necessary to reach the high possible
Brønsted acidity and to suppress the stable σ-adduct formation.
Here Et2O already presents problems, as shown by the roughly
24% Et2O→Al(ORF)3 byproduct observed during the proto-
nation of ether. Further applications of 1−3 are in the focus of
current experiments. An evaluation of the absolute acidities of
solutions of 4 and 5 revealed that the weakly coordinating
anion [Al(ORF)4]− tolerates remarkable acidities up to a level
similar to that of neutral liquid H2SO4 with a pHabs of 171.
Table 3. Comparison of the Experimental and Calculated
Bond Lengths (pm) of Mesitylenium Cations
pbe0/def2-
TZVPP
[H(C6H3(CH3)3)]
[CB11H6Br6]11
bond
4a
C71−C72 144.3(9)
C71−C76 135.6(9)
C72−C73 144.9(9)
C73−C74 136.1(8)
C74−C75 139.7(1.0)
C75−C76 139.4(1.0)
147.7
136.2
147.7
136.2
141.0
141.0
141.4(1.2)
138.7(1.1)
142.5(1.2)
138.4(1.1)
137.0(1.0)
138.0(1.0)
the pKa value of protonated mesitylene in SO2 to be 13.
According to our recently reported acidity scale,7,8 we
calculated the absolute chemical potentials of a 0.001 M
solution of protonated mesitylene and Et2O as −944 and
−1015 kJ mol−1 orin terms of absolute pHabs valuesto 165
and 178 (using the rCCC model with the anchor points ΔsolvG°
= −898 kJ mol−1 for H+ in SO2 and −998 kJ mol−1 for H+ in
Et2O).7 Since we established the border of superacidity (neutral
sulfuric acid) at an absolute chemical potential of −975 kJ
mol−1 or an absolute pHabs value of 171, we conclude that the
weakly coordinating anion [Al(ORF)4]− tolerates remarkable
acidities up to a superacidic environment, as for protonated
mesitylene in SO2 solution. It appears that the counterion
[(FRO)3Al−F−Al(ORF)3]− is slightly more tolerant against
protons and the salt [H(C6H3(CH3)3)]+[(FRO)3Al−F−Al-
(ORF)3]− (4a) is therefore stable at room temperature. This
is in agreement with earlier experiences with this anion.
EXPERIMENTAL SECTION
■
General Considerations. All reactions were performed under an
argon atmosphere using standard vacuum and Schlenk techniques or a
glovebox. PhF→Al(OC(CF3)3)3 was synthesized according to ref 32.
AlEt3 (>93% purity) and HOC6F5 were used as provided. PhF,
perfluorohexane, SO2, Et2O, toluene, 1,2-F2C6H4, and 1,3,5-mesitylene
were dried over CaH2. HOC(CF3)3 was dried over P4O10. Solvents
were distilled prior to use or directly added to the reaction mixture
using condensation techniques. All reactions were performed in special
double J. Young flasks sealed with grease-free Teflon or glass valves or
special J. Young NMR tubes to exclude air and moisture. NMR spectra
were recordedif not otherwise mentionedat room temperature in
CD2Cl2, SO2, or 1,2-F2C6H4 on a Bruker Biospin Avance II+ 400 MHz
WB or a Bruker DPX 200 spectrometer with the software Topspin 2.1.
19F NMR spectra in 1,2-F2C6H4, which were recorded without
deuterium lock, were calibrated to the 19F solvent signal at −139 ppm.
Spectra measured in SO2 were calibrated using a reference sample of
HOC(CF3)3 in SO2.
Strengths of the Do→Al(ORF)3 Adduct Formation as a
Stability Governing Factor. Summarizing all hitherto
available experimental and calculated data of L→Al(ORF)3 (L
= menthol, Et2O, C6F5OH, HOC(CF3)3, SO2, PhF, 1,2-
F2C6H4), it is obvious that in the presence of σ donors such as
Et2O and SO2 or in this case menthol the reasonably polar, but
nonbasic and accordingly more weakly bound ligands PhF, 1,2-
F2C6H4, and HOC(CF3)3 are easily substituted (Figure 4).
For single-crystal measurements, the temperature-sensitive crystals
were mounted in perfluoroether oil on MicroMounts at −20 to −30
°C using a low-temperature mounting device. X-ray crystallographic
data were collected on a Bruker Quazar APEX2 CCD area detector
diffractometer or a Rigaku R-Axis SPIDER diffractometer with Mo Kα
X-ray (0.710 73 Å) sources. Structures were solved by direct methods
in the SHELXS/XL51 program or with OLEX2.52 Disorder in the
anions (OC(CF3)3 groups) was restricted with SADI and SAME
instructions. The hydrogen atoms, which were not found on the
difference Fourier map, were added with HFIX instructions. ORTEP
was used for the graphics of the crystal structures.53 Crystal structure
data of 2·tol: a = 10.7030(2) Å, b = 10.8399(2) Å, c = 15.5105(3) Å, α
= 72.0560(10)°, β = 80.9760(10)°, γ = 77.2220(10)°, P1, Z = 2, R1 =
̅
Figure 4. Ordering of the donor strength of several solvents and σ
donors in the shown equilibrium according to their calculated
complexation enthalpies (Do = σ donor, L = ligand at the (RI-
)BP86/def-TZVP level50).
0.0375, wR2 = 0.0842. Crystal structure data of 3: a = 11.4149(3) Å, b
= 17.9673(6) Å, c = 15.8175(4) Å, α = 90°, β = 96.312(2)°, γ = 90°,
P21, Z = 4, R1 = 0.0685, wR2 = 0.1680. Crystal structure data of 4a: a
= 10.447(2) Å, b = 33.489(7) Å, c = 15.380(3) Å, α = 90°, β =
105.82(3)°, γ = 90°, P21/c, Z = 4, R1 = 0.0701, wR2 = 0.1820. CCDC
862693 (2·tol), 865147 (3), and 865081 (4a) contain 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.
Thus, it is recommended to use 1 in perfluorohexane, since
otherwise stable σ-adduct formation is difficult to suppress, as
already discussed with the NMR experiments. To take full
advantage of the high acidity of 1, the choice of the right
solvent (1,2-F2C6H4 or, even better, perfluorohexane) and a
weak but non-σ-basic base such as mesitylene is the best
combination for successful protonations.
For reflective IR measurements a Nicolet Magna-IR 760 Fourier-
transform spectrometer with a Diamond cell was used (OMNIC
software, usually 32 scans). The data were ATR corrected using the
default values (Diamond ATR correction: angle of reflection 45°,
number of reflections 1, index of refraction 1.50). The samples were
prepared in a glovebox, squeezed in the Diamond cell, and measured
immediately under dry air. Raman spectra were recorded on a Bruker
RAM II Fourier-transform Raman module for the VERTEX 70
spectrometer (Nd:YAG laser, excitation wavelength 1064 nm, software
package OPUS). The samples were prepared in a glovebox and
enclosed in flame-sealed glass tubes to exclude air and moisture.
Details on the quantum chemical calculations are included with the
Supporting Information.
CONCLUSION
■
The high stability of the corresponding anion is the key to high
protic acidity of neutral Brønsted acids. It can be realized with a
large anion sizeto delocalize the negative chargeand
shielding induced by inert substituents such as halides.1
Starting from the Lewis superacid Al(ORF)3 and bulky alcohols
such as HOC(CF3)3, HOC6F5, and menthol, we created with
7489
dx.doi.org/10.1021/om300776a | Organometallics 2012, 31, 7485−7491

Organometallics
Article
Complex 1. A mixture of AlEt3 (0.075 g, 0.66 mmol) in
perfluorohexane (3−5 mL) was cooled to 253 K in a special J.
Young flask sealed with greaseless glass valves. HOC(CF3)3 (0.5 mL,
3.4 mmol, 5.2 equiv) was added with strong stirring over 30 min. The
reaction mixture was warmed to 273 K for 5 min. Afterward
H[Al(OC(CF3)3)4] was crystallized at 233 K. After the solvent was
removed at 253 K, the compound could be isolated as a colorless
powder (0.56 g, 88%). 1H NMR (400.17 MHz, SO2, 298 K): δ 4.96 (s,
1H, free HOC(CF3)3). 19F NMR (376.53 MHz, SO2, 298 K): δ −74.3
(s, 9F, HOC(CF3)3), −75.4 (s, 27F, 3 × C(CF3)3), in a 1:3 ratio. 27Al
NMR (104.27 MHz, SO2, 298 K): δ 35.3 (bs, SO2→Al(OC-
ASSOCIATED CONTENT
■
S
* Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(CF3)3)3).54 IR (Diamond ATR, corrected): ν
̃
464 (w), 539 (w),
■
We thank the Fonds der Chemischen Industrie, the Albert-
581 (w), 728 (m), 890 (w), 938 (w), 979 (s), 1104 (m), 1149 (w),
1222 (m), 1257 (vs), 1301 (m), 1359 (w), 3413 (w, O−H).
Ludwigs-Universitat Freiburg, the ERC with the UniChem
̈
Project, and the DFG for support.
Complex 2. In one neck of a special double-necked J. Young flask
sealed with greaseless glass valves PhF→Al(OC(CF3)3)3 (0.135 g, 0.16
mmol) was dissolved in 1,2-F2C6H4 (2 mL) at 253 K; in the other
neck C6F5OH (0.030 g, 0.16 mmol, 1 equiv) was dissolved in toluene
(2 mL). The 1,2-F2C6H4 phase was slowly covered with the toluene
phase. The title compound crystallized at 253 K between the two
layers and could be isolated as a colorless powder. The yield was
determined in a reaction made in 1,2-F2C6H4 (0.066 g, 44%). 2 is
insoluble in SO2, 1,2-F2C6H4, dichloromethane, and toluene; there-
fore, no NMR spectra could be recorded. IR (Diamond ATR,
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Data for 4b are as follows. H NMR (400.17 MHz, SO2, 233 K): δ
(e) Boere,
́
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■
In the version of this paper published on Oct 3, 2012, incorrect
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