Superacidic or not...? Synthesis, characterisation, and acidity of the room-temperature ionic liquid [C(CH3)3]+ [Al2Br7]-

Article abstract of DOI:10.1002/chem.201203260

The room-temperature ionic liquid (RT-IL) [C(CH₃) ₃]⁺ [Al₂Br₇]^- (m.p. 2 °C) was generated by bromide abstraction from tert-butyl bromide with the Lewis acid aluminum bromide in the absence of solvent. The crystal structure of the tert-butyl cation salt was determined by X-ray diffraction. NMR, IR, and Raman spectroscopy, as well as quantum-chemical and thermodynamic calculations, confirm the composition of this RT-IL. Thus, one may consider this RT-IL to be a readily accessible (and on a large scale) cationic Bronsted acid (protonated isobutene) with the potential for further reactivity. Based on the new absolute Bronsted acidity scale, we calculated an absolute pH _abs value of 171 for liquid bulk [C(CH₃)₃] ⁺ [Al₂Br₇]^-. This value is about as acidic as 100 % sulfuric acid (pH_abs=171) and, thus, on the edge of superacidity.

Full text of DOI:10.1002/chem.201203260

FULL PAPER
DOI: 10.1002/chem.201203260
Superacidic or Not…? Synthesis, Characterisation, and Acidity of the Room-
Temperature Ionic Liquid [CACHTNUGRTNEUNG
(CH3)3]+ [Al2Br7]ꢀ
Franziska Scholz, Daniel Himmel, Harald Scherer, and Ingo Krossing*[a]
Abstract: The room-temperature ionic
liquid (RT-IL) [C
(CH3)3]+ [Al2Br7]ꢀ
dynamic calculations, confirm the com-
position of this RT-IL. Thus, one may
consider this RT-IL to be a readily ac-
cessible (and on a large scale) cationic
Brønsted acid (protonated isobutene)
with the potential for further reactivity.
G
Based on the new absolute Brønsted
acidity scale, we calculated an absolute
pHabs value of 171 for liquid bulk [C-
(m.p. 28C) was generated by bromide
abstraction from tert-butyl bromide
with the Lewis acid aluminum bromide
in the absence of solvent. The crystal
structure of the tert-butyl cation salt
was determined by X-ray diffraction.
NMR, IR, and Raman spectroscopy, as
well as quantum-chemical and thermo-
AHCTUNGTRENNUNG
(CH3)3]+ [Al2Br7]ꢀ. This value is about
as acidic as 100% sulfuric acid (pHabs
=
171) and, thus, on the edge of supera-
cidity.
Keywords: carbocations
·
ionic
liquids · Lewis acids · superacidic
systems · X-ray diffraction
Introduction
bility of halometallate anions is usually coupled to the acidi-
ty of the underlying Lewis acid. As a quantitative scale of
acidity, fluoride-ion affinities (FIA), which hark back to the
pioneering work of Bartlett and co-workers,[11] are often
used. Applied to the given system, the FIA of monomeric
aluminum bromide (494 kJmolꢀ1) is a bit higher than that of
monomeric SbF5 (489 kJmolꢀ1).[12] Hence, monomeric gas-
eous aluminium bromide is a Lewis superacid, which is de-
fined as a molecular Lewis acid that is stronger than gaseous
monomeric SbF5,[12,13] and it is more suitable and easier-to-
handle for such transformations than SbF5 itself. In agree-
ment with this result, solutions of AlBr3 in CH2Br2/CH2Cl2
are known to stabilize tertiary alkyl cations at temperatures
of about ꢀ308C and the in situ solution-state stabilisation of
the tert-butyl cation in AlBr3 with liquid HBr as a solvent at
ꢀ258C has already been shown by NMR spectroscopy. No
isolation of the latter compound and related compounds has
hitherto been reported.[14–17] In the system AlBr3/Brꢀ, until
now, two counterions have been structurally characterised:
[AlBr4]ꢀ and [Al2Br7]ꢀ. The existence of [Al3Br10]ꢀ in
Alkyl cations have been the subject of many investiga-
tions[1–4] and they are key intermediates in many organic re-
actions. In magic acid, HFSO3/SbF5, Olah et al.[3,5] character-
ised tertiary alkyl cations at low temperatures by NMR
spectroscopy. However, attempts to confirm their structure
in the single-crystalline state were problematic. The first
structural characterisation of the tert-butyl cation was ach-
ieved in 1993 in the form of its [Sb2F11]ꢀ salt.[6] Buzek et al.[7]
confirmed these results by using ab initio calculations. Reed
ACTHNUTRGNEUNG
and co-workers[8–10] reported the isolation of the [C(CH3)3]+
ion and other simple tertiary alkyl cations at room tempera-
ture by using their substituted carborane anions as counter-
ions. Despite the very high stability of the latter salts, their
straightforward synthesis on a larger scale is restricted be-
cause the starting material, CH3[HCB11Me5X6] (X=Cl, Br),
is only available on a milligram scale. Herein, we were inter-
ested in whether a stable [CACHTNUGRTNEUNG
(CH3)3]+ salt was accessible with
a more straightforwardly accessible counterion.
For the stabilisation of carbocations, the counterions need
to be rather stable, non-nucleophilic, and non-basic. The sta-
molten (KBr)yACHTUNGTERNN(UG AlBr3)1ꢀy, with y=0.25–0.33, has also been
suggested based on neutron-diffraction data.[18]
Herein, the properties of AlBr3 (in contrast to the non-ox-
idising magic acid) and the low nucleophilicity of the
[Al2Br7]ꢀ ion were found to be capable of stabilising the
tert-butyl cation at room temperature. The title compound,
[a] Dipl. Chem. F. Scholz, Dr. D. Himmel, Dr. H. Scherer,
Prof. Dr. I. Krossing
Institut fꢀr Anorganische und Analytische Chemie
Freiburger Materialforschungszentrum (FMF) and
Freiburg Institute for Advanced Studies (FRIAS)
Universitꢁt Freiburg
Albertstrasse 19, 79104 Freiburg (Germany)
E-mail: krossing@uni-freiburg.de
[CACTHNUTRGNEUNG
(CH3)3]+ [Al2Br7]ꢀ, exhibits a melting point of 28C and,
thus, can be classified as a room-temperature ionic liquid.[19]
We also evaluate the bulk acidity of this compound with re-
spect to the recently introduced absolute Brønsted acidity
scale.[20,21]
Supporting information for this article, including a detailed descrip-
tion of the quantum-chemical calculations, experimental procedures,
a full list of the calculated and experimental vibrational frequencies,
and crystallographic tables, is available on the WWW under http://
dx.doi.org/10.1002/chem.201203260.
Chem. Eur. J. 2013, 19, 109 – 116
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
109

Results and Discussion
HBr of about 0.3 bar remained unchanged for one year at
room temperature.
Synthesis of [C
G
+ A[Al2Br7]ꢀ: Our first attempts at the
CHTUNGTRENNUNG
preparation of [C
A
NMR spectroscopic characterisation of [C
[Al2Br7]ꢀ: With our optimised procedure in hand and under
ACHTUNGTRENNUNG
(CH3)3]+
sis of the tert-butyl cation from tBuBr and AlBr3 in solvent
was always accompanied by side reactions: In CH2Cl2,
halide exchange took place and the formation of AlCl3 was
observed; In CH2Br2, the reactants indeed dissolved as re-
ported.[14] However, in our hands, the existence of the tert-
butyl cation could not be verified by NMR spectroscopy.
Fluorinated aromatic solvents, such as ortho-difluoroben-
zene, immediately underwent alkylation. Finally, the reac-
tions between tert-butyl bromide and AlBr3 were carried out
without solvent ([Eq. (1)]).
ACHTUNGTRENNUNG
additional HBr pressure, as-prepared [CACTHNUTRGNEUNG
(CH3)3]+ [Al2Br7]ꢀ
was characterised by H, 13C, and 27Al NMR spectroscopy at
1
room temperature. The presence of the tert-butyl cation was
1
confirmed by characteristic shifts at H NMR: d=2.6 ppm,
13C NMR: d=325 (Cq) and 49 ppm (CH3), and 27Al NMR:
d=82, which were in accord with literature data.[1,2,15,22]
Some additional weak signals were also detected by NMR
spectroscopy, which suggested the formation of isobutene
oligomers (for the actual spectra, see the Supporting Infor-
mation).
+ A[Al2Br7]ꢀ and accompa-
Solid-state structure of [CACHTUNTRGENNUG(CH3)3] CHTUTGNRENNUGN
nying ab initio structure optimisations: Figure 2 shows the
asymmetric unit in the crystal structure of [C-
AHCTUNGTRENNUNG
(CH3)3]+ [Al2Br7]ꢀ (R1 =0.0491, wR2 =0.1111). The structural
details of the [Al2Br7]ꢀ ion are close to those found in K-
[Al2Br7][23] and NH [Al2Br7].[24] There are two independent
4A
Upon reaction for 15 min at 08C and subsequent warming
to room temperature, a highly viscous, air- and moisture-
sensitive liquid formed. Under an inert atmosphere, this sol-
ution turned out to be remarkably stable at room tempera-
ture (by NMR spectroscopy). At 28C, the oil reversibly crys-
tallised, thus characterising this compound as a RT-IL
(Figure 1).
G
CHTUNGTRENNUNG
cations in the asymmetric unit, one of which is intrinsically
disordered over two positions because its central C atom is
located on an inversion centre.
From the sum of the C-C+-C bond angles, 360.0(7)8, the
ordered carbocation was judged to be planar and its average
ꢀ
C C distance (1.45 ꢃ) is in good agreement with earlier
At 08C, the crystalline product can be stored for months
without decomposition. In a closed flask or an NMR tube,
slow partial decomposition occurs at room temperature, ac-
companied by the formation of (mainly) protonated isobu-
tene oligomers, until an equilibrium HBr pressure is
reached. This decomposition can be prevented, to a large
extent, by adding additional HBr pressure. Thus, a sealed
structures.[6,8] The solid-state packing of this AB salt is of a
NMR sample of [CACTHNUTRGNEUNG
(CH3)3]+ [Al2Br7]ꢀ with a pressure of
Figure 2. Molecular structure of [CACTHNUTRGNEUNG
(CH3)3]+ [Al2Br7]ꢀ. Thermal ellipsoids
are set at 50% probability (except for H atoms and the disordered
ꢀ
cation). Selected bond lengths [ꢃ] and angles [8]: Br11 Al1 2.222(2),
ꢀ
ꢀ
ꢀ
ꢀ
Br13 Al1 2.246(2), Br12 Al1 2.252(2), Br14 Al1 2.435(2), Br14 Al2
ꢀ
ꢀ
ꢀ
2.406(2), Br21 Al2 2.257(2), Br22 Al2 2.264(2), Br23 Al2 2.254(2); Al1-
Br14-Al2 108.03(7), Br11-Al1-Br12 114.26(9), Br11-Al1-Br13 112.89(9),
Br12-Al1-Br13 114.22(9), Br21-Al2-Br22 113.03(9), Br22-Al2-Br23
113.02(9), Br21-Al2-Br23 113.79(9), C11-C10-C13 120.2(7), C12-C10-C13
120.7(7), C11-C10-C12 119.1(7). Inset (top-left): Calculated bond lengths
Figure 1. Left: Liquid [C
G
[ꢃ] of [CACTHNGUTERNNUG
(CH3)3]+ with Cs symmetry (CCSD(T)/def2-TZVPP, MP2/def2-
Solid [C
A
G
QZVPP level).
110
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ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 109 – 116

Room-Temperature Ionic Liquid [CACHTUNGTRNEUNG
(CH3)3]+ [Al2Br7]ꢀ
FULL PAPER
distorted CsCl type (see the Supporting Information).
Anion–cation interactions were analysed by Hirshfeld sur-
face analysis (with Crystal Explorer).[25–28] For this analysis,
ꢀ
all of the C H distances were set by the program to 1.083 ꢃ
(derived from neutron-diffraction data). The Hirshfeld sur-
face of the disordered cation includes a volume of 107.7 ꢃ3,
whereas the non-disordered cation has a slightly smaller
Hirshfeld volume of 98.3 ꢃ3. Figure 3 shows the Hirshfeld
cation surface, surrounded by the eight nearest-neighbouring
[Al2Br7]ꢀ counterions.[29]
Figure 4. Experimental and calculated (scaling factor: 0.96, PBE0/def2-
TZVPP) IR and Raman spectra of [CACHTUNGTRNEUNG
(CH3)3]+ [Al2Br7]ꢀ.
in [CACHTNUGRTNEUNG
(CH3)3]+ [HCB11Me5Br6]ꢀ (2695 cmꢀ1)[8] and [C-
AHCTUNGTRENNUNG
(CH3)3]+ [Sb2F11]ꢀ (2830 cmꢀ1),[4] thus suggesting that the ba-
sicity of the [Al2Br7]ꢀ ion also lies in-between these two
anions.
Formation and phase-change thermodynamics: To investi-
gate the overall reaction thermodynamics that lead to solid
Figure 3. Hirshfeld surface of the [CACTHNUTRGNEUNG
(CH3)3]+ ion with surrounding coun-
terions. The red colour indicates contact distances that are shorter than
the sum of the van der Waals radii; the blue colour indicates contact dis-
tances that are longer than the sum of the van der Waals radii.
or liquid [CACTHNUTRGNEUNG
(CH3)3]+ [Al2Br7]ꢀ, we constructed a Born–
Fajans–Haber cycle (BFHC) by combining experimental
data, semi-empirical correlations, and highly correlated
quantum-chemical calculations up to the CCSD(T)-level
(for a detailed description, see Scheme 1 and the Supporting
Accompanying ab initio structure optimisations were per-
formed at the MP2 and CCSD(T) levels of theory with the
def2-TZVPP[30] basis sets, as well as with def2-QZVPP in
the case of MP2. At the MP2 level, the maximum deviation
between the triple-z- and quadruple-z-calculated bond
lengths was 0.002 ꢃ, which indicates almost-converged
structures with the smaller def2-TZVPP basis set. Both
methods yielded Cs structures that agreed to within 0.011 ꢃ,
with no imaginary frequencies for the MP2/def2-QZVPP
structure (no frequencies for CCSD(T)). At both levels, one
ꢀ
C H bond in every methyl group is clearly elongated by
Scheme 1. Born–Fajans–Haber cycle to assess the reaction enthalpy and
Gibbs reaction energy [kJmolꢀ1]. Values in italics are standard Gibbs en-
ergies; those in regular font are standard enthalpies at 298 K, 1 bar.
0.02 ꢃ compared to the other two, owing to hyperconjuga-
tion with the empty p orbital on the central C atom. This
result is in agreement with the IR spectra, in which the
ꢀ
main C H stretch of the cation appears red-shifted at
ꢀ1
ꢀ
2787 cm , when compared to the C H stretches of free iso-
Information). Lattice Gibbs energies/enthalpies were calcu-
lated according to a recently published approach that was
optimised for ionic liquids.[34,35] The condensed-phase ther-
modynamics that were established with this cycle were in
very good agreement with the experimental observations:
The reaction of liquid tert-butyl bromide with solid AlBr3
was calculated to be slightly exothermic and exergonic for
the formation of solid—as well as liquid—[C-
butene (2893–3086 cmꢀ1).[31] Similar calculations at the
ccsd(t)/cc-pVXZ (X=T, Q) level were published very re-
cently, with almost identical results.[32]
Vibrational spectroscopy: The IR and Raman measurements
show the expected bands and are in good agreement with
the calculated (PBE0/def2-TZVPP; Figure 4) and experi-
mental spectra of [C
A
(CH3)3]+ [Al2Br7]ꢀ. In both cases, the main driving force for
ACHTUNGTRENNUNG
C H stretch of the cation appears at 2787 cmꢀ1. Interesting-
ly, this stretching frequency is in-between the values found
the reaction is the gain in lattice/solvation energy in the con-
densed solid/liquid phase from the gaseous ions. The gas-
ꢀ
Chem. Eur. J. 2013, 19, 109 – 116
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
111

I. Krossing et al.
Table 1. IR and Raman bands of liquid [C
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
[C(CH3)3]
+A[Al2Br7]ꢀ
G
E
K
E
[C
(CH3)3]
[Sb2F11]ꢀ
Assignment
Raman
IR
Raman[24]
IR[24]
Raman[23]
IR[23]
Calculated
Experimental
Calculated
3056 (0.56)
Experimental
3018 (0.05)
3060 (0.16)
3054 (0.16)
2978 (0.10)
2853 (0.20)
2837 (0.38)
2995 (0.02)
na CH3
2950 (0.04)
2809 (0.12)
2978 (0.57)
2834 (0.70)
2953 (0.07)
2787 (0.37)
na CH3
ns CH3
2850
2830
1455
1498 (0.03)
1447 (0.61)
1533 (0.04)
1458 (0.14)
dbend CH3
dbend CH3
dbend CH3
1444 (0.13)
1488 (0.01)
1430 (0.10)
1366 (0.15)
1347 (0.15)
1324 (0.13)
1307 (0.12)
1275 (0.23)
1250 (0.25)
1059 (0.11)
978 (0.10)
1463 (0.01)
1396 (0.01)
1366 (0.01)
1295 (0.04)
1281 (0.04)
1241 (0.01)
1220 (0.01)
1089 (0.01)
992 (0.01)
1450
1295
dbend CH3
dbend CH3
dbend CH3
dbend CH3
dbend CH3
dbend CH3
1368 (0.57)
1303 (0.65)
1271 (0.67)
1251 (0.61)
1062 (0.58)
1283 (0.40)
1290
dbend CH3/na CC
drock CH3/ns CC
drock CH3/na CC
drock CH3/na CC
drock CH3/na CC
drock CH3/na CC
ns CC
1065 (0.15)
968 (0.09)
1070
962
951ACTHNUTRGNE(UNG 0.57)
951 (0.09)
942 (0.09)
825 (0.16)
950 (0.01)
815 (0.02)
765 (0.01)
732 (0.01)
821 (0.55)
716 (0.17)
682 (0.10)
717 (0.55)
684 (0.56)
430 (1.0)
ns CCC/dbend CH3
drock CH3
696 (0.05)
440 (1.0)
667
ꢀ
442 (vw)
425 (vw)
442 (vs)
427 (s)
na,t Al Br
ꢀ
428 (0.15)
411 (0.17)
401 (0.17)
332 (0.16)
420 (0.03)
341 (0.04)
na,t Al Br
ꢀ
410 (0.81)
ds C C/dbend CH3
ꢀ
398 (s)
350 (m)
na,t Al Br
ꢀ
332 (0.57)
350 (vw)
na,b, ns,t, ds Al Br
347
306
287 (0.87)
da CC
ꢀ
284 (0.10)
207 (0.24)
186 (1.0)
156 (0.29)
112 (0.18)
90 (0.43)
48 (0.28)
283 (0.03)
280 (0.86)
286 (vw)
201 (vs)
286 (m)
208 (m)
198 (m)
na,b, ns,t, ds Al Br
ꢀ
ns,t, na,b, ds Al Br
ꢀ
202 (1.0)
202 (0.56)
156 (0.55)
111 (0.56)
ns,t, na,b, ds Al Br
ꢀ
125 (m)
114 (m)
99 (m)
ns,t, ds Al Br
ꢀ
ds Al Br
da Al Br
ds Al Br
ꢀ
99 (0.37)
78 (0.27)
ꢀ
[a] Frequencies are given in cmꢀ1, intensities in parentheses are normalised to 1.0; italic font indicates the frequencies of the [Al2Br7]ꢀ ion, regular font
indicates the frequencies of the [CACTHNUTRGNEUNG
(CH3)3]+ ion. n: stretching mode, d: deformation mode, rock: rocking mode, bend: bending mode, a: antisymmetric
mode, s: symmetric mode, b: bridged mode, t: terminal mode.
phase reaction is energetically highly disfavoured, owing to
charge separation. The calculated enthalpy of fusion of [C-
ACHTUNGTRENNUNG
(CH3)3]+ [Al2Br7]ꢀ (+17 kJmolꢀ1) is typical for ILs (15–
25 kJmolꢀ1),[34] whilst the slightly negative Gibbs energy of
fusion (ꢀ8 kJmolꢀ1) corresponds to a melting point that is
somewhat below room temperature.
In agreement with Equation (2), we observed that a
sample of [C
(CH3)3]+ [Al2Br7]ꢀ in a sealed NMR tube with
AHCTUNGTRENNUNG
0.3 bar HBr pressure remained unchanged for one year at
room temperature. Separate experiments (see the Support-
ing Information) showed that the reaction of AlBr3 with
0.5 equivalents of isobutene under an atmosphere of HBr
(1.6 equiv) gave a mixture of hydrocarbons and higher-mass
carbocations. No terminal-alkene moieties were observed by
NMR spectroscopy. This result implies that the formation of
Investigation of the (super-?)acidity and absolute acidity of
[CACHTUNGTRENNUNG(CH3)3] CHTUNGTRENNUNG
+ A[Al2Br7]ꢀ: Farcasiu et al.[36] and Kramer[37] have
previously suggested that the mixture of HBr and AlBr3 is
one of the strongest superacids known, although this state-
ment has been disputed.[33] The acidic properties of HBr/
2AlBr3 are apparently strong enough to shift the equilibri-
um in Equation (2) almost completely to the right-hand
side.
[C
companied by polymerisation and yields a mixture of com-
(CH3)3]+ [Al2Br7]ꢀ from isobutene and HBr/AlBr3 is ac-
ACHTUNGTRENNUNG
112
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Chem. Eur. J. 2013, 19, 109 – 116

Room-Temperature Ionic Liquid [CACHTUNGTRNEUNG
(CH3)3]+ [Al2Br7]ꢀ
FULL PAPER
pounds that appear to be ki-
netically stable under these
conditions. In the absence of
extra HBr pressure, the forma-
tion of higher-mass carbo-
A
from
pure
[C-
ACHTUNGTRENNUNG
This formation could largely
be prevented through the law
of mass action by the applica-
tion of slight HBr pressure
(see above).
Scheme 2. Born–Fajans–Haber cycle to assess the Gibbs energy of decomposition at 298 K, 1 bar. a) Experi-
mental value;[40] b) value calculated by using a ccsd(t)/MP2 compound method (for details, see the Supporting
Information); c) calculated MP2/(Al: aug-cc-pV
ACHTUNGTREN(NUGN T+d)Z, Br: aug-cc-pVTZ-PP) reaction energy with thermal/
entropic corrections at the BP86/def-TZVP level; d) value calculated by using the COSMO-RS model.[38,39]
Based on these above-sum-
marised observations, we will now analyse the reactions that
are responsible for the Brønsted acidity of [C-
1 m solution in the IL as the standard state (denoted as
(solv)). By using these conventions, the law of mass action
for this autoprotolysis is given by Equation (6).
ACHTUNGTRENNUNG
(CH3)3]+ [Al2Br7]ꢀ in bulk medium. In contrast to molecular
liquids, this “autoprotolysis reaction” of the pure [C-
ACHTUNGTRENNUNG
(CH3)3]+ [Al2Br7]ꢀ liquid is somewhat more complicated.
2
KAP ¼ aðHBrðsolvÞÞ ꢁ aðAl3Brꢀ10ðsolvÞÞ
Our main assumptions are: 1) That the tert-butyl cation acts
as a Brønsted acid ([Eq. (3)]).
31kJmolꢀ1
ð6Þ
¼ expðꢀ
Þ ¼ 3:7 ꢁ 10ꢀ6
R ꢁ 298:15K
ꢀ(solv))=2a
With aACHTUNGTENR(UNNG Al3Br10 ACHUTGTNRNEUNG AHCTNUREGTG(NNUN HBrCAHTNUGTREN(GUNN solv)), one obtains Equa-
tion (7).
pffiffiffiffiffiffiffiffiffiffiffiffiffi
3
aðHBrðsolvÞÞ ¼
K
AP=4 ¼ 9:8 ꢁ 10ꢀ3 and
2) That the formed isobutene undergoes cationic polymer-
isation ([Eq. (4)]).
ð7Þ
aðAl3Brꢀ10ðsolvÞ ¼ 1:95 ꢁ 10ꢀ2
In terms of our recently introduced absolute Brønsted
acidity scale,[20,21] the absolute chemical potential of the
proton, mabs(H+), was defined as the lowering of its chemical
potential compared to its standard state in the gas phase.
mabs(H+) corresponds to the pressure of a (hypothetical)
ideal gas of protons and to an “absolute pH value” pHabs
(note that RTln10=5.71 kJmolꢀ1 at 298.15 K), according to
Equation (8) and (9).
On the way to the neutral polyisobutene, the observed
higher-mass carbocations that result from the addition of
isobutene to the tert-butyl cation are the natural intermedi-
ates.
pðHþ; idealÞ
3) That the solvated proton decomposes the [Al2Br7]ꢀ
anion, with the formation of HBr and [Al3Br10]ꢀ ([Eq. (5)]).
mabsðHþÞ¼RT lnð
Þ ¼ ꢀ5:71kJmolꢀ1 ꢁ pHabs
1bar
ð8Þ
ð9Þ
with
pðHþ; idealÞ
pHabs ¼ ꢀ logð
Þ
1bar
Therefore, the overall decomposition reaction should be
the sum of Equations (3)–(5). We constructed a BFHC
(Scheme 2) that analysed the decomposition of [C-
In a solvent (S) at room temperature, the chemical poten-
tial of the proton follows Equation (10).
ACHTUNGTRENNUNG
(CH3)3]+ [Al2Br7]ꢀ. By combining the available experimental
mabsðHþÞ ¼ maꢂbsðHþ; SÞ þ RT ln aðHþ; SÞ
¼ DsolvGꢂðHþ; SÞ ꢀ 5:71kJmolꢀ1 ꢁ pH with mꢂabsðHþ; SÞ ð10Þ
¼ DsolvGꢂðHþ; SÞ
values, high-level quantum-chemical calculations, and
COSMO-RS[38,39] to calculate the Gibbs solvation energies,
the underlying energetics could be assessed.
As a standard state for the IL-forming [C
(CH3)3]+ and
ACHTUNGTRENNUNG
[Al2Br7]ꢀ ions, we used the pure liquid [C(CH3)3]+ [Al2Br7]ꢀ
G
To calculate the absolute chemical potential of the proton
IL in the COSMO-RS calculations (denoted as (IL) in
Scheme 2) so that their activities were set to 1 and could be
removed from the decomposition equilibrium. For solid
in the [CACTHNGUTERNNUG
(CH3)3]+ [Al2Br7]ꢀ IL, we constructed a BFHC
(Scheme 3) to establish the standard Gibbs solvation energy
of the proton in the pure IL. As a basis for this assessment,
we suggest that the solvation of the proton in the IL only
proceeds by interactions with the only base that is present,
poly
ACHTUNGTRENNUNGisobutene, the activity could also be set to 1. For the de-
scription of the [Al3Br10]ꢀ ion and HBr, we used an ideal
Chem. Eur. J. 2013, 19, 109 – 116
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113

I. Krossing et al.
stable when the H0 value is smaller than ꢀ17. However,
Olah at al. stated that “when solutions of alcohols in fluoro-
sulfonic acid or fluorosulfonic acid/sulfur dioxide were pre-
pared, tertiary carbonium ions could be generally observed,
but peak broading (due to exchange) and side products are
observed.”[42]
Based on this observation, we can estimate the acidity of
such a solution, that is, its pH value, absolute acidity, and H0
value, to evaluate the boundary values of acidity under
which the tBu+ cation is stable.
Neat HSO3F has a H0 value of ꢀ15.1. Considering an au-
toprotolysis constant of 10ꢀ7, this value corresponds to a pH
value of 3.5. In other words, an indicator base system Ind/
IndH+ in HSO3F that is half protonated at a H0 value of
Scheme 3. Born–Fajans–Haber cycle to assess the standard Gibbs energy
of solvation in [CACHTUNGTRENNUNG
(CH3)3]+ [Al2Br7]ꢀ at 298 K, 1 bar: a) Experimental
value;[40] b) value calculated by using a ccsd(t)/MP2 compound method
(for details, see the Supporting Information); c) calculated MP2/(Al: aug-
cc-pVACHTUNGTRENNUNG(T+d)Z, Br: aug-cc-pVTZ-PP) reaction energy with thermal/en-
tropic corrections at the BP86/def-TZVP level; d) value calculated by
using the COSMO-RS model.[38,39] For full details, see the Supporting In-
formation.
ꢀ15.1 would have a pK
(IndH+) value of 3.5. This result can
aACHTUNGTRENNUNG
be used to derive a relationship between the pH and H0
values in HSO3F. Thus, we obtain Equation (14) and, with
the assumption that the activity coefficient of the neutral
base is 1 and is unaffected by changes in concentration, we
obtain Equation (15).
that is, the [Al2Br7]ꢀ ion in the IL.
To place this value in context, the calculated standard
Gibbs solvation energy of the proton in the [C-
A
standard values for fluorosulfonic acid (ꢀ924 kJmolꢀ1) and
sulfuric acid (ꢀ966 kJmolꢀ1).[21] The solvation constant of
the proton is calculated by Equation (11) or, in logarithmic
form, by Equation (12).
½Indꢃ
H0 ¼ ꢀ15:1 þ log
ð14Þ
ð15Þ
½IndHþꢃ
aðIndÞ
½Indꢃ
pH ¼ 3:5 þ log
¼ 3:5 þ log
ꢀ log fꢄ
aðIndHÞ
½IndHþꢃ
2
aðHBrðsolvÞÞ ꢁ aðAl3Brꢀ10ðsolvÞÞ
KsolvðHþ; ILÞ ¼
pðHþðgÞÞ ꢁ 1barꢀ1
ð11Þ
The ion activity coefficient (fꢄ) can be calculated from
the ion strength by using Debye–Hꢀckel theory. From both
equations, we obtain a relationship between pH and H0
(Equation (16).
945kJmolꢀ1
¼ expð
Þ ¼ 3:1 ꢁ 10165
R ꢁ 298:15K
DsolvGꢂðHþ; ILÞ
¼ log aðHBrðsolvÞÞ
RT ln 10
165:5 ¼ log Ksolv
¼
H0 ¼ ꢀ18:6 þ pH þ log fꢄ
ð16Þ
ð12Þ
þ2 log aðAl3Brꢀ10ðsolvÞÞ þ pHabs
No concentrations of added alcohol were reported by
Olah et al.; thus, we used a concentration of 0.1 molLꢀ1
tBuOH as a basis for our model calculations. Considering
the fact that two equivalents of HSO3F are consumed per al-
cohol molecule (the formed water molecule is also protonat-
ed), we assumed an ion strength of 0.2 molLꢀ1. With a rela-
tive dielectric constant of 120,[43] the Debye–Hꢀckel limiting
Using the calculated equilibrium activities of the solvated
HBr and [Al3Br10]ꢀ particles, the absolute acidity in the
liquid [CACHTUNGTRENNUNG
(CH3)3]+ [Al2Br7]ꢀ IL at room temperature was cal-
culated according to Equation (13).
pHabs ¼ 165:5 ꢀ logð9:8 ꢁ 10ꢀ3Þ ꢀ 2 ꢁ logð1:95 ꢁ 10ꢀ2
¼ 170:9 or mabsðHþÞ ¼ ꢀ976kJmolꢀ1
Þ
ð13Þ
law gave a fꢄ value of 0.76. From the autoprotolysis equi
ACHTUNGTRENNUNGlib-
AHCTUNGRTENrNGUN ium (pKap =7.0), we obtained the pH value according to
Equation (17).
A practically identical absolute acidity of pure sulfuric
acid, with pHabs =170.8 or pHabs(H+)=ꢀ975 kJmolꢀ1, was
recently established. Assuming an error bar of 15–
20 kJmolꢀ1 for both calculations, we conclude that the ther-
½SO3Fꢀꢃ
pH ¼ 7 þ log aðSO3FꢀÞ ¼ 7 þ log
¼ 7 þ logð0:2Þ þ logð0:76Þ ¼ 6:2
þ log fꢄ
1molLꢀ1
ð17Þ
modynamic acidity of liquid [CACHTUNGTRNEUNG
(CH3)3]+ [Al2Br7]ꢀ is about
the same as that of neutral anhydrous sulfuric acid.
Inserting this value into Equation (16) gives a H0 value
according to Equation (18).
Bracketing the stability of the tBu+ cation in molecular
acids: By using a ladder of indicator bases, Hammett and
Deyrup[41] introduced the Hammett acidity function (H0) for
the quantification of acid strength in non-aqueous media.
The tBu cation is known to be unstable in neat sulfuric acid,
which has an experimentally determined H0 value of ꢀ12.0.
Reed and co-workers reported[9] that the tBu cation is only
H0 ¼ ꢀ18:6 þ 6:2 þ logð0:76Þ ¼ ꢀ12:6
ð18Þ
Recently, we calculated the absolute chemical standard
potential (=Gibbs standard solvation energy) of the proton
in HSO3F (i.e., at pH 0) to be ꢀ924 kJmolꢀ1. The absolute
114
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ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 109 – 116

Room-Temperature Ionic Liquid [CACHTUNGTRNEUNG
(CH3)3]+ [Al2Br7]ꢀ
FULL PAPER
AHCTUNGTRENNUNG
acidity and the absolute pH value of HSO3F with pH 6.2 are
calculated according to Equations (19) and (20).
pound reversibly crystallised in a refrigerator at 28C.
1H NMR (104.27 MHz, 298 K): d=2.6 ppm (br s, 9H; CH3); 13C NMR
(100.6 MHz, 298 K): d=325 ppm (br s, 1C; Cq), 49 (d, 9C; CH3);
27Al NMR (104.27 MHz, 298 K): d=82 ppm; IR (diamond ATR): n˜ =440
(100), 696 (5), 968 (9), 1065 (15), 1283 (40), 1458 (14), 1533 (4), 2787
(37), 2953 (7), 3018 cmꢀ1 (5%); FT-Raman: n˜ =78 (27), 99 (37), 202
(100), 341 (4), 420 (3), 732 (1), 765 (1), 815 (2), 950 (1), 992 (1), 1089 (1),
1220 (1), 1241 (1), 1281 (4), 1295 (4), 1366 (1), 1396 (1), 1463 (1), 1488
(1), 2809 (12), 2950 (4), 2995 cmꢀ1 (2%).
mabsðHþÞ ¼ DsolvGꢂðHþÞ ꢀ ðRT ln 10Þ ꢁ pH
ð19Þ
¼ ꢀ924kJmolꢀ1 ꢀ 5:71kJmolꢀ1 ꢁ 6:2 ¼ ꢀ959kJmolꢀ1
mabsðHþÞ 959kJmolꢀ1
ð20Þ
pHabs ¼ ꢀ
¼
¼ 168:0
5:71kJmolꢀ1
RT ln 10
Crystal data for [C
clinic; space group C2c; a=27.727(2), b=13.114(1), c=13.854(1); a=
908, b=95.974(2)8, g=908; V=5010.0(7) ꢃ3; Z=12; 1calcd =2.667 gcmꢀ3
(000)=3648; l=0.71073 ꢃ; T=153(2) K; absorption coefficient=
16.889 mmꢀ1
absorption correction: multi-scan; min =0.2852; Tmax
+ A[Al2Br7]ꢀ: C4H9Al2Br7; Mw =670.39; mono-
ACHTUGTNRENG(NU CH3)3] CHTUTGNRENNUGN
This result allows us to bracket the stability of the dis-
;
+
solved C
(CH3)3 ion between H0 values of ꢀ12.0 (in pure
FACHTUNGTRENNUNG
;
T
=
H2SO4) and ꢀ12.6 (in HSO3F solution, see above). Within
the error bars of our calculations, the absolute acidities of a
0.1m solution of the tBu cation prepared from tBuOH in
HSO3F (pHabs =168) and that of our RT-IL [C-
0.7456. Data for the structure were collected on a Bruker SMART
APEX2 CCD area detector diffractometer with MoKa radiation (l=
0.71073 ꢃ). A single crystal was coated with perfluoroether oil at ꢀ408C
and mounted onto a 0.2 mm Micromount. The structure was solved by
using direct methods with SHELXTL[44] and OLEX2[45] and refined by
least squares on weighted F2 values for all reflections. The final refine-
ments converged at R1 =0.0491 and wR2 =0.1111 for all reflections (I>
2s(I)). The hydrogen atoms were included in the refinement at calculated
positions by using a riding model. All attempts to locate the positions of
the hydrogen atoms in the difference Fourier maps were futile.
ACHTUNGTRENNUNG
(CH3)3]+ [Al2Br7]ꢀ (pHabs =171) are very similar.
Conclusion
CCDC-821960 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.
In summary, we have shown that this simplest tertiary carbo-
cation salt can be easily synthesised and handled at room
temperature. Therefore, it can be used as a readily available
source of the highly Brønsted acidic tert-butyl cation. Thus,
we assume that this acidic IL, which is readily available in a
straightforward manner and in large quantities, represents
an interesting starting material or reaction medium for fur-
ther reactions and investigations.
Acknowledgements
This work was supported by the University of Freiburg, the FRIAS, the
DFG, and the ERC. We also thank the Fonds der Chemischen Industrie
for their support.
On the basis of our recently established absolute Brønsted
acidity scale,[20,21] the medium acidity of bulk [C-
ACHTUNGTRENNUNG
(CH3)3]+ [Al2Br7]ꢀ and a 0.1m solution of the tBu cation that
was prepared from tBuOH in HSO3F were almost identical.
This investigation allowed us to make the first quantitative
comparison between the acidities of an acidic molecular
medium and an acidic ionicliquid medium. With this high
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Received: September 12, 2012
Published online: November 23, 2012
116
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Chem. Eur. J. 2013, 19, 109 – 116