Synthesis and structure of ionic liquids containing the [Al(OC 6H4CN)4]- anion

Article abstract of DOI:10.1002/zaac.201300030

The synthesis, structure, and physical properties of ionic liquids (IL) bearing the novel [Al(O-C₆H₄-CN)₄]^- ion as counterion to the commonly used [NR₄]⁺, [PR ₄]⁺ and imidazolium ions are reported. Both the influence of the alkyl chain length as well as the functionalization with cyano groups is studied. These ILs are easily obtained by reaction of Ag[Al(O-C ₆H₄-CN)₄] with the corresponding ammonium, phosphonium, and imidazolium halides. The stability towards electrophilic cations was investigated. All prepared salts have a window for the liquid phase of ca. 200 °C and are thermally stable up to 450 °C. The solid-state structures reveal only weak cation-anion and anion-anion interactions in accord with the observed low melting points (glass transition points). Copyright

Full text of DOI:10.1002/zaac.201300030

ARTICLE
DOI: 10.1002/zaac.201300030
Synthesis and Structure of Ionic Liquids Containing the [Al(OC6H4CN)4]–
Anion
Henrik Lund,[a] Jörg Harloff,[a] Axel Schulz,*[a,b] and Alexander Villinger[a]
Keywords: Anions; Alkoxyaluminates; Coordination polymers; Ionic liquids; Structure
Abstract. The synthesis, structure, and physical properties of ionic
liquids (IL) bearing the novel [Al(O–C6H4–CN)4]– ion as counterion
to the commonly used [NR4]+, [PR4]+ and imidazolium ions are re-
ported. Both the influence of the alkyl chain length as well as the
functionalization with cyano groups is studied. These ILs are easily
ammonium, phosphonium, and imidazolium halides. The stability
towards electrophilic cations was investigated. All prepared salts have
a window for the liquid phase of ca. 200 °C and are thermally stable
up to 450 °C. The solid-state structures reveal only weak cation···anion
and anion···anion interactions in accord with the observed low melting
obtained by reaction of Ag[Al(O–C6H4–CN)4] with the corresponding points (glass transition points).
the basicity of these anion can be strongly reduced by addition
Introduction
of Lewis acids such as B(C6F5)3 yielding anions of the type
{E[CN·B(C6F5)]n}– (E = B, C, N; n = 4, 3, and 2).[16] A further
extension of this concept can be achieved by using spacer be-
tween the E–CN bond such as in [E(O–C6X4–CN)4]– (E = B,
Al; X = H, F). Salts bearing such anions are capable of forming
coordination polymers and Lewis-acid Lewis-base adduct
ions.[17] Moreover, anions of the type [E(O–C6X4–CN)4]– (E =
B, Al; X = H, F) should be suited for the design of new, nitrile
group containing ionic liquids. Many aluminum containing ILs
are known in the literature. For instance mixtures of aluminum
halide with organic halide salts form ILs of the first genera-
tion,[3a,18] or the combination of weakly coordinating cations
and anions such as (perfluorinated) tetraalkylaluminates[19] and
alkoxyaluminates.[20]
Ionic liquids (ILs), described for the first time by Walden in
1914,[1] can be referred to as an alternative to classic molecular
solvents. Due to the possibility of simple chemical modifica-
tion of the ionic components they are usually called designer
or green solvents.[2] Accordingly, in recent years a lot of re-
search was carried out to afford many applications for ILs like
usage in catalysis,[3] gas storage,[4] energetic materials,[5] syn-
thesis,[6] or for separation.[7] Introduction of functional organic
groups can lead to significant interactions (such as additional
van der Waals or hydrogen bonding) with the counterions and
thus can directly influence IL properties like viscosity, solubil-
ity behavior or melting point (Ͻ 100 °C).[3a,8] Especially, the
implementation of nitrile groups into the IL system is still of
high research interest,[9] since application in catalysis due to
In this study we report on the synthesis, structure, and prop-
erties of ammonium-, phosphonium-, and imidalzolium-based
ionic liquids bearing the [Al(O–C6H4–CN)4]– anion.
[10]
the coordination and immobilization in catalytic reactions
or as energetic materials are expected.[5,11] Cyano groups can
either be introduced in the cation or anion of the IL by formal
cyanization of the aryl or alkyl substituents/chains in the het-
erocyclic cation, e.g. imidazolium, pyridinium, or pyrazolium
ions. Another possibility to synthesize CN group containing
ILs is the utilization of small CN-based anions such as the
dicyanoamide- (DCA) ,[12] tricyanomethanide- (TCM)[13] or
tetracyanoborate-anions (TCB),[14] or similar CN group con-
taining anions.[15] Since CN groups are good donor ligands,
Discussion
1. Synthesis
Alkali and silver salts containing the [Al(O–C6H4–CN)4]–
ion are easily prepared by reaction of lithium aluminium hy-
dride (LAH) with four equivalents of the para-cyano-phenol
in good yields (Ͼ 80%) in 10–20 g scale experiments under
inert conditions.[17] With Li[Al(O–C6H4–CN)4] (1) as starting
material, the corresponding silver salt (2) is obtained by an
optimized metathesis reaction in thf with silver trifluoroacetate
(Agtfa) affording a colorless precipitate, which can easily be
purified by the addition of thf to remove impurities or starting
material by filtration. Finally, a colorless crystalline solid is
obtained in good yields (80% yield, Scheme 1).
* Prof. Dr. A. Schulz
E-Mail: axel.schulz@uni-rostock.de
[a] Institut für Chemie
Universität Rostock
Albert-Einstein-Str. 3a
18059 Rostock, Germany
[b] Leibniz-Institut für Katalyse e.V.
an der Universität Rostock
Albert-Einstein-Str. 29a
18059 Rostock, Germany
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201200030 or from the au-
thor.
754
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2013, 639, (5), 754–764

Ionic Liquids Containing the [Al(OC6H4CN)4]– Anion
Scheme 1. Synthesis of 1 and 2.
The analogous ammonium-, phosphonium-, and imidalzol-
ium salts of [Al(O–C6H4–CN)4]– can be prepared in 70 to 90%
yields by reacting the corresponding halide salts with a slight
excess of 2 (1.1 equiv.) in acetonitrile (see Supporting Infor-
mation). After removal of the solvent, the raw product is dis-
solved in dichloromethane to remove the excess of 2 by fil-
tration. This procedure is repeated several times. Finally, the
purified product is washed three times with diethyl ether and
dried in high vacuum for 24 h (Scheme 2, Table 1).
To obtain nitrile rich ILs, bearing CN groups also in the
cation, two nitrile functionalized imidazolium salts (18, 19)
were prepared according to literature.[21] These cyano-substi-
tuted imidazolium halides can be used in analogous metathesis
reactions with Ag[Al(O–C6H4–CN)4] to form the correspond-
ing aluminate salts 20 and 21 as depicted in Scheme 3.
Scheme 3. Synthesis of nitrile group rich imidazolium halides.
solvents like CH2Cl2 consistent with the increasing amount of
nitrile groups.
The IR and Raman data of all considered salts show sharp
bands in the expected region 2200–2250 cm–1, which can be
assigned to the νCN stretching modes. As previously shown,
the coordination of electrophilic centers such as cations or
even Lewis acids e.g. B(C6F5)3 to a NC–R species causes a
significant band shift to higher wave numbers.[16,22] Hence,
both IR and Raman spectroscopy are particularly well suited
to study coordination effects. As pointed out in the X-ray sec-
tion (see below), only very weak interactions can be found
in the crystal structure of the aluminate salts bearing weakly
2. Properties
Alkali, ammonium, imidazolium, and silver salts of coordinating cations. The CN stretch vibration in these salts is
[Al(O–C6H4–CN)4]– are neither air nor moisture sensitive. The found in the range of 2213–2218 cm–1. A shift to slightly
aluminate salts 3–17, 18, and 20 are well soluble in aprotic, larger wave numbers (ca. 2225 cm–1) is found for the metal
polar solvents like thf, CH2Cl2, and acetonitrile but only ion containing salts.[17]
slightly soluble in non-polar solvents like benzene or hexane.
We also studied the stability of the [Al(O–C6H4–CN)4]– ion
Interestingly, the solubility is also limited for diethyl ether, a towards very strong Lewis acids such as [(C6H5)3C]+ (trityl).
rather polar solvent. Compounds 19 and 21 are less soluble in For instance, treatment of 2 with trityl bromide resulted in the
Scheme 2. Synthesized ammonium-, phosphonium and imidazolium salts of [Al(O–C6H4–CN)4]– (X = Cl, Br; R = alkyl or aryl substituent)
(exact assignment is given in Table 1).
Table 1. Synthesized ILs with the combination of R1/R2 for 3–9, 10–13, and 14–17, see Scheme 2.
R1
R2
R1
R2
R1
3
4
5
6
7
8
CH3
C2H5
C2H5
n-C3H7
n-C4H9
n-C8H17
CH3
CH3
C2H5
n-C3H7
n-C4H9
n-C8H17
9
C2H5
n-C4H9
C6H5
C6H5
C6H5
CH2–C6H5
n-C4H9
C2H5
C6H5
CH2–C6H5
14
15
16
17
C2H5
10
11
12
13
n-C4H9
n-C6H13
n-C8H17
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H. Lund, J. Harloff, A. Schulz, A. Villinger
ARTICLE
formation of 4-cyanophenyl trityl ether (22), which could be
All measurements were carried out in an inert atmosphere
isolated as a colorless precipitate. Presumably, trityl salt (DSC: N2; TGA: Ar). All results of the thermal analyses are
[(C6H5)3C][Al(O–C6H4–CN)4] was formed initially but de- summarized in Table 2.
composed to the corresponding trityl ether and the free Lewis
In case of salts bearing short alkyl chains the melting point
acid Al(O–C6H4–CN)3 as illustrated in Scheme 4. [(C6H5)3C]+ of the tetraalkylammonium salts is rather high with 100 °C or
seems to be the stronger Lewis acid resulting in the abstraction higher (cf. 3: 196.4, 4: 96.8, and 5: 135.1 °C). Therefore, these
of [O–C6H4–CN]– from the [Al(O–C6H4–CN)4]– ion yielding salts cannot be referred to as ionic liquids. For salts bearing
(C6H5)3C–O–C6H4–CN and Al(O–C6H4–CN)3. A similar longer alkyl chains (Cn, n = 3 and 4) no melting point can be
Lewis acid / Lewis base reaction is assumed for weakly coordi- discussed because all attempts to achieve a (crystalline) solid
nating anions of the type [Al(ORF)4]– in the presence of very failed. However, glass transition points of –7 and –15 °C were
electrophilic cations, where the decomposition is initiated found for 6 and tetrabutylammonium aluminate 7, respectively.
either by ligand (RFO–) or fluoride ion abstraction, finally lead- The tetraoctylammonium aluminate 8 with a melting point of
ing to [(RFO)3Al–F–Al(ORF)3]–.[23]
about 42 °C can be considered to be an ionic liquid. This trend
shows the known correlation between alkyl chain length of the
cation and melting point, which decreases with increasing
alkyl chain length.[24] A similar situation is found for symmet-
ric substituted ammonium salts bearing e.g. the tetracyanido-
borate anion, with decreasing melting points (cf. 230 °C for
the tetraethyl- and 80 °C for the tetrabutylammonium salt).[25]
In a series of [N{(CH2)n–1CH3}4]+ salts of the tetraphenylbor-
ate, [B(OC6H5)4]–, (n = 1, 2, 4, 6, 8, and 10) the melting point
diminishes with increasing chain lengths from 369 °C (n = 1)
to 102 °C (n = 10).[26] The melting points also decrease when
using asymmetric substituted cations as seen by exchange of
the tetraethylammonium- by triethylmethyl- and triethylbenz-
ylammnoium cation. The increase of the melting point by
40 °C when using tetraethyl- instead of triethylmethylammo-
nium is similar to the situation found for [B(CN)4]– salts with
the same ammonium cations, but here the difference of the
melting points is only 15 °C.[27] For compound 8 no crystalli-
zation point was observed during cooling the sample in the
Scheme 4. Decomposition of [Ph3C]+[Al(O-C6H4-CN)4]– in solution.
3. Thermal Analysis
Melting point (Tm) and glass transition (Tg) were determined
by DSC measurements. The sample was first cooled down to
–75 °C, held for a period of 5 min at this temperature followed
by heating above the melting point. Once the sample was com-
pletely liquid, the sample was cooled again down to –75 °C
and heated again above the melting point. This procedure was
repeated two times to ensure the results. The decomposition
temperature (TDec) was determined by TGA measurements.
The thermal stability is presented as the onset temperature of
the corresponding mass loss, obtained by integration of the TG
curve derivative (dTG). All data were revised by the use of the DSC experiment starting from molten 8. In the successive
Setsoft 2000 software package.
heating phase only a glass transition point was detected. The
Table 2. Thermal data of compounds 3-21 (Temperatures in °C).
Compound
Tg
Tm
TDec.
onset
peak
onset
peak
onset
peak
3
4
5
6
7
8
9
–
–
–
–
–
–
196.4
96.8
133.4
–
202.1
99.2
135.1
–
355.5
339.6
346.1
329.4
334.2
331.3
324.9
378.7
366.3
366.3
364.2
370.7
356.4
338.9
–6.7
–14.9
–38.2
–9.3
–5.9
–13.8
–36.9
–7.9
42.9
–
46.8
–
10
11
12
13
–26.7
5.8
15.4
–6.9
–25.7
8.5
16.9
–1.7
–
–
–
–
–
–
–
–
468.1
477.1
440.8
380.1
474.3
487.6
501.2
416.9
14
15
16
17
–18.3
–22.6
–24.0
–25.8
–19.4
–21.6
–23.0
–24.6
–
–
–
–
–
–
–
–
250.1
259.3
260.7
191.6
262.4
282.3
281.3
195.7
18
19
–32.0
–
–30.0
–
87.8
145.7
90.2
149.6
202.0
228.5
254.9
242.2
20
21
–24.4
–
–21.8
–
55.2
78.9
60.7
82.2
191.1
162.4
213.5
318.1
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Z. Anorg. Allg. Chem. 2013, 754–764

Ionic Liquids Containing the [Al(OC6H4CN)4]– Anion
fairly large difference between Tg and Tm has been reported
before for ionic liquids.[9,28]
By using asymmetric, partly alkyl-substituted phosphonium
cations highly viscous salts were obtained caused by less flexi-
bility of aryl substituent, but it was again impossible to isolate
crystalline solids with clearly defined melting points. Only
glass transition points were observed. The aryl/alkyl-substi-
tuted phosphonium salts (11–13) belong to the class of glass
forming ionic liquids due to their glass transitions between –7
to 16 °C. Interestingly, for [P(Ph)4][Al(O–C6H4–CN)4] (12)
with the highly symmetric [P(Ph)4]+ ion a rather high glass
transition point between 15–16 °C was observed, that is 10
or 20 °C higher as it is found for the asymmetric substituted
phosphonium salts 11 and 13. This compares to the melting
points of tetracyanoborate salts of [P(Ph)4]+ and [PEt(Ph)3]+,
that decrease by about 100 °C when asymmetric substituted
phosphonium cations are utilized.[29] The same trend is found
in the tetraphenylborate compounds of [P(Ph)4]+ (mp.: 310 °C)
and [PBz(Ph)3]+ (mp.: 230 °C).[30]
Figure 1. TG curves of symmetrical ammonium salts.
nium- (7) and tetrabutylphosphonium aluminate (10) is more
than 130 °C.
The imidazolate based aluminate containing ionic liquids of
this study are less stable compared to the ammonium salts.
The liquid phase window also spans ca. 200 °C. This might be
explained by the increased reactivity of the imidazolate cation
compared to the ammonium cation. A possible decomposition
pathway is the reaction of the acidic proton attached to the C2
atom of the imidazole ion with one of the phenoxy substituents
of the anion at higher temperatures. This might be the reason
for the lower thermal stability, compared to the ammonium
salts. The introduction of a nitrile group in the cation leads to
a lower thermal stability compared to imidazolium aluminates
containing only alkyl chains as shown for compounds 20 and
21. The thermal stability of the corresponding halides (18 and
19) are similar to the reported values in the literature [decom-
position temperatures of 1-(2-cyanoethyl)-3-allyllimidazolium
chloride ([C8H12N3]Cl): 218 °C and 1-(2-cyanoethyl)-
3-cyanomethyl-imidazolium salts: [C8H9N4]Cl: 196 °C,
[C8H9N4][N(CN)2]: 176 °C].[11c,32]
The imidazolium ion containing salts 14–17 were obtained
as slightly viscous liquids with glass transition points between
–18 and –26 °C depending on the alkyl chain length. Again,
the glass transition decreases with increasing alkyl chain length
in the cation.
By introduction of cyano groups in the cation the melting
points raise to 60 °C (20) or even 82 °C (21), compared to
salts 14–18, but they still represent ionic liquids by definition.
The increase of the melting point can be attributed to stronger
electrostatic interaction of the polar CN groups of the cation
with the anion. Substitution of the anion in 18 and 19 by the
aluminate ion decreases the melting points by about 30 °C (18
versus 20) and 70 °C (19 versus 21). The influence of the
anion size on melting points has already been discussed in
literature.[31] Beside the size of the cation, the melting point is
influenced by e.g. kind and number of functional groups, abil-
ity to form hydrogen bonds and conformation flexibility.
The thermal stability was investigated by TGA measure-
ments in the temperature range from 25–600 °C in an argon
atmosphere. All compounds are stable up to 200 °C. The de-
composition of all compounds occurs in one endothermic step
with a mass loss of 50–85%. The TG curves for the com-
pounds with symmetrically substituted ammonium salts are
presented in Figure 1. The ammonium aluminates are stable up
to 380 °C depending on the chain length. Here thermal stability
decreases with increasing alkyl chain length in the cation.
This characteristics can be found in a series of tetra-n-alkyl
ammonium salts of [Al(O–C(CF3)3)4]– anions, with decreasing
decomposition points from [N(CH3)4]+ to [N(C4H9)4]+ in a
range from 400–250 °C.[20] With respect to the melting and
decomposition points, the liquid state is present between 200–
350 °C. By using asymmetric substituted ammonium cations,
e.g. compound 4 or 9, the thermal stability seems to be slightly
decreased.
4. Volume Based Thermodynamics
The thermal behavior, especially the melting point, is
closely related to the crystal structure and accordingly to the
overall ionic interaction.[33] An admissible quantity to cover
this interaction is the lattice potential energy UPOT. This pa-
rameter can easily be derived from the estimated molecular
volume VM as illustrated by Glasser and Jenkins with the con-
cept of volume based thermodynamics (VBT).[34] In absence
of experimental data (missing crystal data or density measure-
ments), the molecular volume can be derived from the volumes
of the elements.[35,36] Calculated VBT data from diffraction
experiments compared to values received from Hofmann val-
ues are summarized in Table 3.
The values obtained from diffraction experiments are in the
same magnitude as the values calculated from Hofmann’s vol-
ume of elements, by a maximum divergence of 3.5% in VM,
S°298, UPOT, and ΔHL for compounds 3–5, 18, and 19. The
potential lattice energy of the tetraalkylammonium compounds
Phosphonium aluminates (10–13) are found to be more
stable than the n-alkyl chain based ammonium salts. Here, the
difference of decomposition temperatures of tetrabutylammo- 3, 4, and 5 is slightly smaller than the values for analogue
Z. Anorg. Allg. Chem. 2013, 754–764
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H. Lund, J. Harloff, A. Schulz, A. Villinger
ARTICLE
Table 3. Thermodynamic data derived from VBT.
3
4
5
18
19
20
21
Hofmann’s approach
a)
VM /Å3
730.2
939.7
312.4
368.5
802.3
1029.6
304.4
360.5
826.4
1059.6
301.9
358.1
253.9
346.0
422.4
477.6
236.6
324.4
431.2
486.4
901.7
1153.5
294.7
351.0
805.9
1034.0
304.0
360.2
S°298 /J·K–1·mol–1
b)
c)
UPOT /kJ·mol–1
d)
ΔHL /kJ·mol–1
VBT
VM /Å3
732.5
942.6
312.2
368.3
829.2
1062.9
301.6
357.8
834.3
1069.4
301.1
357.3
263.1
357.4
418.0
473.3
244.5
334.3
427.1
482.3
–
–
–
–
–
–
–
–
e)
S°298 /J·K–1·mol–1
e)
UPOT /kJ·mol–1
e)
e)
ΔHL /kJ·mol–1
a) Values based on summation of Hofmann elements volumes[35]. b) So = 1246.5VM + 29.5. c) UPOT = 2I[(α / V1/3M) + β]. d) ΔHL = [232.8 /
298
V1/3M] + 110. e) Derived from VM estimated by diffraction experiment.
salts of tetraphenylborate salts, but all in the range of 302– melting points of the aluminate based compounds 20 and 21
312 kJ·mol–1 (cf. UPOT values calculated from the published compared to 18 and 19.
crystal data of [N(CH2)n–1(CH3)4][B(C6H5)4] compounds in
kJ·mol–1: [N(CH3)4]+
[N(C3H7)4]+ = 357.2, and [N(C4H9)4]+ = 355.1).[37]
Furthermore, the influence of the use of the bulky aluminate
= = 369.0,
384.8, [N(C2H5)4]+
5. Structure Elucidation
Compounds 2–5, 18, 19, and 22 were analyzed by X-ray
anion instead of small anions like [N(CN)2]– or [B(CN)4]– is diffraction experiments. Compound 2 was recently published
illustrated by a decrease up to 200 kJ·mol–1 (cf. UPOT by our group and will not be discussed here (see Supporting
([N(CH3)4][N(CN)2])
=
501.3 kJ·mol–1
or
UPOT Information).[17] All attempts to obtain single crystals of alu-
minates containing compounds 6–17, 20, and 21 described
([N(C4H9)4][B(CN)4]) = 393.1 kJ·mol–1).[38]
In addition, the effect of the anion size can be illustrated by herein failed due to the formation of oil-like substances instead
the comparison of lattice energy values calculated from Hof- of solid or crystalline materials.
mann’s approach for compounds 18 versus 20 as well 19 ver-
Table 4 presents the X-ray crystallographic data. X-ray qual-
sus 21. In both cases, the values are decreased about 25–30%, ity crystals of all considered species were selected in Fomblin
as shown in Table 3 and this is in nice correlation with lower YR-1800 (Alfa Aesar) at ambient temperature. All samples
Table 4. Crystallographic details of 4, 5, 18, 19, and 22.
4
5
18
19
22
Chem. formula
C35H34AlN5O4
615.65
C36H36AlN5O4
629.68
colorless
orthorhombic
Pna21
11.691(1)
12.173(1)
23.449(1)
90
90
90
3337.1(3)
1.253
C9H12N3Br
242.13
colorless
monoclinic
P21/c
8.046(1)
7.828(1)
16.746(1)
90
93.942(1)
90
1052.2(1)
1.528
C8H9N4Br
241.10
colorless
monoclinic
P21/c
5.310(1)
9.674(1)
19.093(1)
90
94.253(2)
90
978.1(1)
1.637
C26H19NO
361.42
colorless
monoclinic
P21/c
22.636(3)
8.394(1)
21.292(3)
90
106.980(4)
90
3869.4(1)
1.241
Form. wght. /g·mol–1
Color
Crystal system
Space group
a /Å
b /Å
c /Å
α /°
β /°
colorless
triclinic
¯
P1
11.722(1)
11.850(1)
13.765(1)
82.815(2)
69.466(2)
67.822(2)
1658.1(1)
1.233
γ /°
V /Å3
ρcalc /g·cm–3
Z
2
0.11
4
0.11
4
3.87
4
4.16
8
0.08
μ /mm–1
λ /Å
T /K
0.71073
173
0.71073
173
0.71073
173
0.71073
173
0.71073
173
Measured reflections
Independent reflections
Reflections with I Ͼ 2σ(I)
Rint.
42734
10510
7170
44876
11553
9310
17276
3740
3156
13201
3513
2863
47001
12847
8430
0.043
0.041
0.021
0.035
0.048
F(000)
648
1328
488
480
1520
R1, R[F2 Ͼ 2σ(F2)]
wR2 (all data)
GooF
0.048
0.130
1.08
0.039
0.092
1.03
0.023
0.053
1.02
0.029
0.069
1.04
0.049
0.125
1.06
Parameters
CCDC
439
917050
420
917051
125
917052
118
917053
505
917054
758
www.zaac.wiley-vch.de
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2013, 754–764

Ionic Liquids Containing the [Al(OC6H4CN)4]– Anion
were cooled to –100(2) °C during the measurement. More de-
tails are found in the Supporting Information file.
[N(CH3)4][Al(O–C6H4–CN)4] (3)
Crystals of compound 3 were grown at room temperature
from a saturated acetonitrile solution by slow evaporation of
the solvent. Unfortunately the X-ray data, gained from the dif-
fraction experiment, were rather poor so that we abstain from
a detailed discussion. The connectivity of 3 is shown in Fig-
ure 2. The central aluminum atom of the anion sits in a dis-
torted tetrahedral environment formed by the phenoxy substit-
uents similar to the situation found for 2.[17] Only very weak
interionic interactions can be assumed since the distances be-
tween the ions are rather large.
Figure 3. ORTEP drawing of the asymmetric unit of 4. Thermal ellip-
soids with 50% probability at 173 K. Hydrogen atoms are omitted for
clarity. Disorder is not shown. Selected bond lengths /Å and angles
/°: Al–O1 1.75(1), Al–O2 1.721(1), Al–O3 1.737(1), Al–O4 1.733(1),
N1–C7 1.1479(2); O2–Al1–O4 114.34(5), O2–Al1–O3 108.1 (4), O4–
Al1–O3 108.21(4), O2–Al1–O1 108.63(4), O4–Al1–O1 107.20(4),
O3–Al1–O1 110.27(4).
by one formula unit, as shown in Figure 3. Only weak
cation···anion and anion···anion interactions are found. The
shortest cation–anion distance amounts to 3.085 Å (C–N1···H–
C29AЈ) indicating weak van der Waals interaction of the CN
group with the methyl group of the ammonium cation (cf.
ΣrvdW(C–N) = 3.25 Å)[39] (Figure 4, left).
In addition, weak anion···anion interactions are observed
leading to the formation of a centrosymmetric dimer and a
staggered arrangement of two aluminate anions (Figure 4,
right). Two weak C–H···O interactions can be observed with a
short anion–anion distance of 3.315(2) Å (C3–H···O3Ј and
C3Ј–H···O3, cf. ΣrvdW(C–O) = 3.22 Å).[39]
The central aluminum atom of the [Al(O–C6H4–CN)4]
group in 4 is tetrahedrally distorted with O–Al–O angles vary-
ing between 107–114° indicating the flexibility of the
anion.[17] The Al–O bond length (average 1.736 Å) is in the
range of a typical polar covalent Al–O single bond (1.73–
1.74 Å in perfluorinated anions [Al(O–(CH(CF3)2)4]–).[40]
Similar values were reported for the corresponding lithium or
silver salts.[17,20,41] The average C–N distance of 1.146 Å (cf.
Figure 2. Ball-and-stick drawing of 3. Hydrogen atoms are omitted
for clarity. Disorder is not shown.
[N(CH2CH3)3(CH3)][Al(O–C6H4–CN)4] (4)
Compound 4 crystallizes in the triclinic space group P1 with Σrcov = 1.14 Å)[42] is in good agreement to those observed for
¯
two formula units per unit cell. The asymmetric unit is formed the analogous alkali salts.[17]
Figure 4. Left: Cation···anion interaction. Right: Anion···anion interaction in the solid state of [(CH2CH3)3(CH3)N][Al(O–C6H4–CN)4] (4).
Selected interionic distances /Å: N1–C29A’ 3.085(3), O3–C3’ and O3–C3’ 3.315(2). Symmetry codes left: C29A’ 2–x, 1–y, 1–z; right: C3’ and
O3’ 1–x, 1–y, 1–z.
Z. Anorg. Allg. Chem. 2013, 754–764
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759

H. Lund, J. Harloff, A. Schulz, A. Villinger
ARTICLE
Figure 5. Left: ORTEP drawing of the asymmetric units of 5, thermal ellipsoids with 50% probability at 173 K. Hydrogen atoms are omitted
for clarity. Disorder is not shown. Selected bond lengths /Å and angles /°: Al–O1 1.734(1), Al–O2 1.732(1), Al–O3 1.739(1), Al–O4 1.740(1),
N1–C25 1.141(2), O2–Al–O1 100.96(5), O2–Al–O3 112.12 (5), O1–Al–O3 116.66(4), O2–Al1–O4 113.54(5), O1–Al1–O4 106.26(5), O3–Al1–
O4 107.29(5). Right: Anion···anion interaction in 5. Interionic distances: O1–C11’ 3.367(2) Å. Symmetry code: C11’ –0.5+x, 0.5–y, z.
[N(CH2CH3)4][Al(O–C6H4–CN)4] (5)
Crystals of [N(CH2CH3)4][Al(O–C6H4–CN)4] (5) were
grown from a saturated acetonitrile solution. Compound 5
crystallizes in the orthorhombic space group Pna21 with four
units per cell. The asymmetric unit of 5 contains the ion pair,
which is shown in Figure 5 (left). In 5 the cation···anion dis-
tance is slightly larger than the sum of the van der Waals radii
with minimum distance of 3.368 Å between N1 and C31 (cf.
ΣrvdW(C–N) = 3.25 Å).[39] In contrast to the structure of 4 no
anion–anion dimer is found in the structure of 5. The anions
are connected by weak O···H–C interactions (Figure 5, right).
The distance O1–C11Ј is about 3,367(2) Å and slightly larger
than the sum of the van der Waals radii (cf. ΣrvdW(C–O) =
3.25 Å).[39]
Figure 6. Left: ORTEP drawing of the molecular structure of the ion
pair in the crystal of 18. Thermal ellipsoids with 50% probability at
173 K. Hydrogen atoms are omitted for clarity. Disorder is not shown.
Selected bond lengths /Å and angles /°: N1–C1 1.327(2), N2–C1
1.336(2), C9–N3 1.136(2); N1–C1–N2 108.2(1). Right: Unit cell of
18. View along the a axis. Hydrogen atoms are omitted for clarity.
Only very weak cation···anion and anion···anion interactions
can be discussed in the solid state. The structural parameters
of the anion in 5 are very similar to those discussed for com-
pound 4 and therefore will not discussed in detail.
This arrangement is formed by several weak cation···anion
and cation···cation interactions, which are mainly based on in-
terionic C···Br van der Waals contacts in a range between
3.485(1) (C1ЈЈЈ–Br1) and 3.674(1) Å (C8ЈЈЈЈ–Br1), as dis-
played in Figure 7 (cf. ΣrvdW(C–Br) = 3.53 Å). Due to the
length of the C1–Br1 distance no special C1–H···Br1 hydrogen
bond can be discussed. In addition a weak cation···cation C2–
N3ЈЈ contact of 3.275(2) Å can be found. Bond lengths and
angles of the imidazolium heterocycle (Figure 6) are in agree-
ment to reported values.[43] The C–N distance of the cyano
group (C9–N3 1.363(2) Å) is slightly shortened compared to
3-(1H-Imidazol-1-yl)propanenitrile (C–N bond length:
1.141(1) Å[21]) but lies in the range of a typical CN triple bond
(cf. Σrcov = 1.14 Å).[42]
1-(2-Cyanoethyl)-3-allyllimidazolium Bromide (18)
Colorless needle-like crystals of 18 were grown from a satu-
rated acetone solution after filtration of a boiling suspension.
The compound crystallizes in the monoclinic space group
P21/c with four formula units per unit cell. The position of the
allyl group in 18 was found to be disordered and split into two
parts. In the unit cell the imidazolate rings are arranged in
stacked layers in direction of the a axis, as shown in Figure 6.
The imidazolate heterocycles are found to be twisted by 180°
from layer to layer.
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Ionic Liquids Containing the [Al(OC6H4CN)4]– Anion
Figure 7. Interionic interactions in the unit cell of 18. Interionic dis-
tances /Å: C1–Br1 3.619(1), C3Ј–Br1 3.654(1), C2–N3ЈЈ 3.275(2),
C1ЈЈЈ–Br1 3.485(1), C8ЈЈЈЈ–Br1 3.674(1). Symmetry codes: C3Ј –1+x,
y, z; N3ЈЈ x, 1+y, z; C1ЈЈЈ 1–x, 1–y, 1–z; C8ЈЈЈЈ 1–x, 2–y, 1–z.
Figure 9. Interionic interactions in the unit cell of 19. Interionic dis-
tances /Å: C1–Br1 3.651(2), C7Ј–Br1 3.432(2), C1ЈЈЈBr1 3.579(2);
C4ЈЈЈBr1 3.644(2), N4C3ЈЈ 3.090(2), C4ЈN3ЈЈ 3.134(2). Symmetry
codes: C4Ј, C7Ј 0.5–x, 0.5+y, 0.5–z; C3ЈЈ, N3ЈЈ –0.5–x, 0.5+y, 0.5–z;
C1ЈЈЈ, C4ЈЈЈ –1+x, y, z.
1-(2-Cyanoethyl)-3-cyanomethyl-imidazolium Bromide (19)
intermolecular interactions are observed. The C–O–C angle
amounts to 121.55(8)° and lies in the expected range for or-
ganic ethers.[45] The C–N distance of 1.146(2) Å compares
well with the distances found for 3, 4, and 5 (vide supra).
Single crystals of 19 were obtained by slow cooling of a
boiling acetonitrile solution. Like 18, also 19 crystallizes in the
monoclinic space group P21/c with four ion pairs in the unit
cell (Figure 8).
The smallest cation···anion distance in the unit cell is found
between the methylene carbon of the cyanomethyl-group C7Ј
and Br1 with 3.432(2) Å. Additional weak van der Waals C–
Br interactions are found ranging from 3.579(2) to 3.651(2) Å
[see Figure 9, cf. ΣrvdW(C–Br) = 3.53 Å]. Cation···cation inter-
actions are found between nitrile groups towards the partially
positive charged carbons in the neighborhood to imidazolate-
chain nitrogen atoms (N4–C3ЈЈ 3.090(2) Å, C4Ј–N3ЈЈ
3.134(2) Å, cf. ΣrvdW(C–N) = 3.25 Å). All interionic interac-
tions are depicted in Figure 9. The structural parameters of the
imidazolium heterocycle and CN groups are similar to those
found for 18.
Conclusions
This work focuses on the synthesis of new aluminum con-
taining ionic liquids by the reaction of Ag[Al(O–C6H4–CN)4]
with different alkyl chain bearing ammonium, phosphonium,
and imidazolium halides. In addition, the influence of CN
groups attached to imidazolate cations and the stability towards
electrophilic cations was investigated. All compounds are eas-
ily prepared by salt metathesis reaction in good yields. Ther-
mal analyses of the aluminate salts point out that the combina-
tion of cations bearing short alkyl chained with [Al(O–C6H4–
CN)4]– ions lead to the formation of compounds with melting
points of about 100 °C. Ionic liquids with lower melting or
glass transition points can be prepared if cations with longer
Triphenylmethyl-4-cyanophenyl Ether (22)[44]
Colorless crystals of 22 were obtained from acetonitrile ac- alkyl chains are utilized. All prepared salts have a window for
cording to Scheme 4. The ether crystallizes with eight formula the liquid phase of ca. 200 °C and are thermally stable up to
units per unit cell in the monoclinic space group P21/c. The 450 °C.
asymmetric unit is formed by two independent molecules.
The solid-state structures reveal only weak cation···anion
Since the structural parameters are very similar for both inde- and anion···anion interactions in accord with the low melting
pendent molecules only one molecule of triphenylmethyl-4- points (glass transition points). Introduction of cyano groups
cyanophenyl ether is depicted in Figure 10. No significant into cations results in higher melting points (glass transition
Figure 8. Left: ORTEP drawing of the ion pairs in the crystal of 19. Thermal ellipsoids with 50% probability at 173 K. Hydrogen atoms are
omitted for clarity. Selected bond lengths /Å and angles /°: N1–C1 1.334(2), N2–C1 1.327(2), N3–C6 1.142(2), N4–C8 1.139(2), N2–C1–N1
107.93(1). Right: Unit cell of 19. View along the a axis. Hydrogen atoms are omitted for clarity.
Z. Anorg. Allg. Chem. 2013, 754–764
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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761

H. Lund, J. Harloff, A. Schulz, A. Villinger
ARTICLE
Figure 10. Right: ORTEP drawing of the molecular structure of 22 (only one independent molecule is shown). Thermal ellipsoids with 50%
probability at 173K. Hydrogen atoms are omitted for clarity. Selected bond lengths /Å and angles /°: N1–C7 1.146(2), O1–C1 1.364 (1),
O1–C8 1.462(1); C1–O1–C8 121.54(8). Left: Unit cell of 22, view along the b axis.
was calculated based on a two point calibration (melting points of In
and Zn) using the Mettler-Toledo STARe Software.
points) as shown for imidazolium salts bearing terminal CN
groups.
TGA Measurement was done with a Setaram LapSys 1600 TGA-
DSC in an argon atmosphere (heating-rate 5 K·min–1).
Experimental Section
General Informations: All manipulations were carried out in an argon
atmosphere using standard Schlenk or drybox techniques.
For TGA measurements the sample was weight into an alumina cruci-
ble (about 15 to 25 mg) in an inert atmosphere in a glove box. After
placement on the TGA rod, the furnace was evacuated for 5 min and
flushed with argon afterwards to ensure an inert atmosphere. All data
were corrected by baseline substraction. No temperature correction
was applied. The thermal stability is presented as the onset temperature
of the corresponding mass loss, obtained by integration of the TG
curve derivative (dTG). All data were revised by the use of the Setsoft
2000 software package.
Tetrahydrofuran (thf) and diethyl ether were dried with Na/benzophe-
none. Acetonitrile was dried with CaH2. Dichloromethane was dried
with P4O10, distilled, and refluxed over CaH2 and distilled again. All
solvents were freshly distilled prior use.
X-ray Structure Determination: X-ray quality crystals were selected
in Fomblin YR-1800 perfluoroether (Alfa Aesar) at ambient tempera-
tures. The samples were cooled to 173(2) K during measurement. The
data were collected with a Bruker Apex Kappa-II CCD diffractometer
using graphite monochromated Mo-Kα radiation (λ = 0.71073 Å). The
structures were solved by direct methods (SHELXS-97) and refined
by full-matrix least-squares procedures (SHELXL-97). Semi-empirical
absorption corrections were applied (SADABS). All non hydrogen
atoms were refined anisotropically, hydrogen atoms were included in
the refinement at calculated positions using a riding model.
Li[Al(OC6H4CN)4], Ag[Al(OC6H4CN)4], 1-(2-cyanoethyl)-3-allyllim-
idazolium bromide, and 1-(2-cyanoethyl)-3-cyanomethyl-imidazolium
bromide were synthesized by modified literature procedures.[17,21]
General Procedure for the Synthesis of Ammonium, Phosphonium
and Imidazolium Salts of the Tetrakis(4-cyanophenoxy)aluminate:
The desired ammonium, phosphonium, or imidazolium halide
(1 mmol) was dissolved in acetonitrile (10 mL) and added dropwise to
a stirred solution of 2 (1.1 mmol) in acetonitrile (40 mL). The mixture
was allowed to stir for 60 min in the dark, followed by removal of the
solvent and extraction with dichloromethane (20 mL). The dichloro-
methane solution was filtered (F4), the solvent was removed and the
crude product was dissolved in dichloromethane and filtered again.
The solvent was removed and the product was washed three times with
ethyl ether (5 mL), followed by drying in vacuo for 1 d. The com-
pounds were isolated in 70–90% yield. NMR, IR, and Raman data can
be found in the Supporting Information.
1
NMR: H and 13C spectra were obtained with Bruker AVANCE 250
(250 MHz), AVANCE 300 (300 MHz) spectrometer and were refer-
enced externally. CD3CN was dried with CaH2 and distilled.
IR: Nicolet 380 FT-IR with a Smart Orbit ATR device was used.
Raman: Bruker VERTEX 70 FT-IR with RAM II FT-Raman module,
equipped with a Nd:YAG laser (1064 nm), or Kaiser Optical Systems
RXN1-785 nm was used.
CHN Analyses: Analysator Flash EA 1112 from Thermo Quest or
C/H/N/S-Mikronalysator TruSpec-932 from Leco were used.
Crystallographic data (excluding structure factors) for the structures in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK.
Copies of the data can be obtained free of charge on quoting the
depository numbers CCDC-917050 (4), -917051 (5), -917052 (18),
-917053 (19), and -917054 (22) (Fax: +44-1223-336-033; E-Mail:
deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk)
DSC: DSC 823e from Mettler-Toledo (heating-rate 5 K·min–1) was
used.
A sample of approximately 1 to 2 mg was placed in a aluminium cruci-
ble in case of solid samples. For liquid and high viscous samples a 15
to 20 mg sample was used. The closed crucible was placed in the
furnace. The closed furnace was flushed with nitrogen and the sample Supporting Information (see footnote on the first page of this article):
was measured using a heating rate of 5 °C per minute. The heat flow NMR, IR, and Raman spectroscopic data of the title compounds.
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Ionic Liquids Containing the [Al(OC6H4CN)4]– Anion
[19] T. D. Westmoreland, N. Ahmad, M. C. Day, J. Organomet. Chem.
1970, 25, 329–335.
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