Preparation and characterization of new room temperature ionic liquids

Article abstract of DOI:10.1002/1521-3765(20020816)8:16<3671::AID-CHEM3671>3.0.CO;2-9

A new series [C_nO_mmim][X] of imidazolium cation-based room temperature ionic liquids (RTILs), with ether and alcohol functional groups on the alkyl side-chain has been prepared. Some physical properties of these RTILs were measured, namely solubility in common solvents, viscosity and density. The solubility of LiCl, HgCl₂ and LaCl₃ in room temperature ionic liquids was also determined. The features of the solid-liquid phase transition were analysed, namely the glass transition temperature and the heat capacity jump associated with the transition from the non-equilibrium glass to the metastable supercooled liquid. These properties were compared with those reported for the 1-n-alkyl-3-methylimidazolium [C_nmim][X] series. While the density and solid-liquid phase transition properties are similar for both series, the new RTILs present a considerably lower viscosity and an increased ability to dissolve HgCl₂ and LaCl₃ (up to 16 times higher).

Full text of DOI:10.1002/1521-3765(20020816)8:16<3671::AID-CHEM3671>3.0.CO;2-9

FULL PAPER
Preparation and Characterization of New Room Temperature Ionic Liquids
[b]
LuÌs C. Branco,[a] Joaƒo N. Rosa,[a] Joaquim J. Moura Ramos, and Carlos A. M. Afonso*[a]
Abstract: A new series [CnOmmim][X]
of imidazolium cation-based room tem-
perature ionic liquids (RTILs), with
ether and alcohol functional groups on
the alkyl side-chain has been prepared.
Some physical properties of these RTILs
were measured, namely solubility in
common solvents, viscosity and density.
The solubility of LiCl, HgCl2 and LaCl3
in room temperature ionic liquids was
also determined. The features of the
solid liquid phase transition were ana-
lysed, namely the glass transition tem-
perature and the heat capacity jump
associated with the transition from the
non-equilibrium glass to the metastable
supercooled liquid. These properties
were compared with those reported for
the 1-n-alkyl-3-methylimidazolium
[Cnmim][X] series. While the density
and solid liquid phase transition prop-
erties are similar for both series, the new
RTILs present a considerably lower
viscosity and an increased ability to
dissolve HgCl2 and LaCl3 (up to 16 times
higher).
Keywords: alkylation
¥
green
chemistry ¥ ionic liquids ¥ phase
transitions ¥ solvent effects
traditional organic solvents (OS)in aqueous-OS biphasic
systems, including selective extraction of metal ions[7, 8] and for
OS-scCO2 extraction.[9, 10] Applications have also been found
in different areas, including gas chromatography[11] as sta-
tionary phases, in electrochemistry[12] as solvents and electro-
lytes, and in pervaporation.[13]
Future applications of RTILs to other important processes
may require the synthesis of new ionic liquids with suitable
properties. For instance, improved solubility of more specific
solutes including very polar substrates or catalysts may be
called for. This can be achieved by including different
substituent groups R with potential complexation properties
in the basic ionic liquid structure.
Herein, we report the preparation of a new series of ionic
liquids [CnOmmim][X], based on the imidazolium cation that
contains additional functional groups, such as ether and
alcohol, on the alkyl group R. This new series of RTILs was
prepared, characterised, and some physical properties were
studied.
Introduction
Although known since the beginning of the 21st century, only
recently have room temperature ionic liquids (RTILs)at-
tracted significant and growing interest, especially those
based upon the 1-n-alkyl-3-methylimidazolium cation.[1, 2]
They have intrinsically useful properties, such as thermal
stability, high ionic conductivity, negligible vapour pressure
and a large electrochemical window. Depending on the anion
and alkyl group of the imidazolium cation, the RTILs can
solubilise carbonyl compounds, alcohols, alkyl halides, super-
critical CO2 (scCO2), and also transition metal complexes.
Furthermore, they can have low miscibility with dialkyl ethers,
alkanes, water and can be insoluble in scCO2.[1, 3, 4] The RTILs
can be used as an alternative recyclable and environmentally
benign reaction media for chemical processes. Examples of
their usefulness in biocatalysis[5] and catalysis have been
reported, for example hydroformylation, hydrogenation,
olefin oligomerisation, Heck reactions, alkylation, Friedel
Crafts reactions, Diels Alder reactions and Baylis Hillman
reactions.[6] Moreover, RTILs have successfully replaced
Results and Discussion
[a] Prof. C. A. M. Afonso, L. C. Branco, J. N. Rosa
Departamento de QuÌmica
The structures of the ionic liquids [CnOmmim][X] prepared
and studied in the present work are shown in Table 1. A
previously reported methodology[1, 14] was used for the
preparation of these new ionic liquids. Alkylation of methyl-
imidazole with an alkyl halide is followed by halogen (ClÀ or
BrÀ)exchange with slight excess (1.1 equiv)KPF 6, NaBF4 or
NaCF3CO2 (NaTFA)in order to reduce the amount of
remaining halogen content. The volatile components are then
Centro de QuÌmica Fina e Biotecnologia
√
Faculdade de Ciencias e Tecnologia
Universidade Nova de Lisboa, 2829-516 Caparica (Portugal)
Fax : (‡351)21-294-8550
E-mail: cma@dq.fct.unl.pt
[b] Prof. J. J. Moura Ramos
Centro de QuÌmica-Fisica Molecular, Complexo-I
¬
Instituto Superior Tecnico
Av. Rovisco Pais, 1049-001 Lisboa (Portugal)
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C. A. M. Alfonso et al.
FULL PAPER
Table 1. Structure of room temperature ionic liquids of the series
[CnOmmim][X].
Table 2. Solubility of new RTILs in some common solvents.
Entry
Ionic liquid
H2O[gLÀ1
]
Et2O
EtOAc
EtOH[a]
X
N
N
1
2
3
4
5
6
7
8
9
[C2OHmim][PF6]
[C2OHmim][BF4]
[C2OHmim][TFA]
[C3Omim][PF6]
[C3Omim][BF4]
[C5O2mim][PF6]
[C5O2mim][BF4]
[C4mim][PF6]
[C4mim][BF4]
[C8mim][PF6]
[C8mim][BF4]
[C10mim][BF4]
misc
misc
misc
misc
imisc
imisc
imisc
imisc
imisc
imisc
imisc
imisc
imisc
imisc
imisc
imisc
pm
pm
pm
pm
misc
misc
misc
pm
pm
pm
R
Ionic liquid
Cation
R
Anion
XÀ
misc
pm
[C2OHmim][Cl]
[C2OHmim][PF6]
[C2OHmim][BF4]
[C2OHmim][TFA]
[C3Omim][Cl]
[C3Omim][PF6]
[C3Omim][BF4]
[C4OHmim][Br]
[C5O2mim][Cl]
[C5O2mim][PF6]
[C5O2mim][BF4]
(CH2)2OH
(CH2)2OH
(CH2)2OH
(CH2)2OH
(CH2)2OMe
(CH2)2OMe
(CH2)2OMe
(CH2)4OH
ClÀ
0.38[b]
0.43[b]
0.57[b]
misc
imisc
imisc
imisc
imisc
imisc
imisc
imisc
À
PF6 À
pm
BF4
misc
misc
misc
misc
misc
À
CF3CO2
ClÀ
10
11
12
0.22[b]
0.30[b]
0.59[b]
À
PF6 À
BF4
BrÀ
ClÀ
(CH2)2O(CH2)2OMe
(CH2)2O(CH2)2OMe
(CH2)2O(CH2)2OMe
[a] Ethanol 96% (v/v). [b] Determined by UV spectroscopy; misc:
miscible; imisc: immiscible; pm: partially miscible.
À
PF6 À
BF4
removed by stirring the RTIL at 70 908C for 24 48 h under
vacuum. Stirring is essential, otherwise removal of the volatile
components is incomplete as a result of slow diffusion of the
volatile molecules in the viscous RTIL. The remaining water
[C4mim][PF6] (entry 6 vs entry 8). Among all RTILs studied,
[C8mim][PF6] presents the lowest water solubility (entry 10).
In ethyl acetate, the RTILs of entries 1 5 are partially
miscible while the others (entries 6 12)are immiscible. On
À
content in the RTIL is higher in case of BF4 anion [2.3 to
the other hand, in 96% ethanol (v/v)the RTILs of entries 4 7
4.7 mg (H2O)mLÀ1 (RTIL)] than for PF6 [0.9 to 2.0 mg
are partially miscible while all the others (entries 1 3 and 8
12)are miscible. Additionally, all RTILs studied are immis-
cible in diethyl ether, and miscible in acetone and in
dichloromethane. They also show low solubility in hexane
and petroleum ether.
À
(H2O)mLÀ1 (RTIL)] which is in agreement with the observed
contents for the [Cnmim][X] series.[3c] The samples used for
microanalysis and calorimetric experiments were further
purified by filtration through silica and left under high
vacuum (6 Â 10À6 mbar)at room temperature in order to
remove traces of halogen and volatile components, respec-
tively.
In general, the introduction of hydroxyl or ether functional
groups in the alkyl chain considerably modifies the solubility
À
behaviour, while modification of the anion (PF6À, BF4
TFAÀ)does not seem to have any significant influence.
,
All new RTILs were obtained in medium-to-high yields
(75 95%). They are stable in air, water and in other common
organic solvents, such as ethers, hexane, dichloromethane,
ethyl acetate and ethanol. No decomposition was observed in
the range of temperatures tested (up to 908C).
Viscosity: Table 3 gives viscosity data for some RTILs at
different temperatures (10, 20 and 308C). From this data it can
be concluded that both RTIL series (the new [CnOmmim][X]
and the [Cnmim][X])display qualitatively similar viscosity
behaviour.
Some physical properties of this new series of RTILs were
studied, namely their solubility in some common organic
solvents and in water, solubility of inorganic salts in the RTIL,
viscosity and density. The analysis of the solid liquid phase
transition allowed the determination of the glass transition
temperature, Tg, and of the heat capacity jump, DCp
associated with the glass transition. Some of these properties
were compared with those previously reported for the RTIL
series [Cnmim][X] (n ˆ 4, 6, 8 and 10)containing different
Table 3. Viscosity (cP)data for several RTILs at different temperatures.
Entry
Ionic liquid
cP(308C)
cP(208C)
cP(108C)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
[C2OHmim][PF6]
[C2OHmim][BF4]
[C3Omim][PF6]
[C3Omim][BF4]
[C5O2mim][Cl]
[C5O2mim][PF6]
[C5O2mim][BF4]
[C4mim][PF6]
[C4mim][BF4]
[C8mim][Cl]
[C8mim][PF6]
[C8mim][BF4]
[C10mim][BF4]
[C2mim][BF4]
[C6mim][Cl]
82.7
148.8
279.5
70.9 (258C)90.9
148.1
138.0
283.7
212.3
189.2
172.8[a]
65.2[c]
337.0 (258C)
425.2[e]
82.1
223.1
43.0[g]
716.0 (258C)[g]
363.0[g]
177.0[g]
157.6
283.6
262.8
613.4
425.8
377.0
308.3[b]
104.9[d]
607.5
374.3
1515.8
1034.9
860.4
615.0
185.9
anions (XÀ ˆ ClÀ, PF6 and BF4 ).
À
À
Solubility of RTILs in some solvents: In Table 2 we present
the solubilities of the new RTILs [CnOmmim][X] together
with those of with the RTIL series [Cnmim][X]. It can be
observed that the new RTILs [C2OHmim][PF6], [C2OH-
857.4[f]
135.0
416.6
66.5[g]
1922.4
294.3
846.3
mim][BF4],
[C2OHmim][TFA],
[C3Omim][PF6]
and
[C3Omim][BF4] (entries 1 5)are miscible with water, as is
the case for the RTIL [C4mim][BF4] (entry 9). For the others,
RTILs (entries 6 8 and 10 12)the solubility in water was
determined by UV spectroscopy and it was found that the
[C6mim][PF6]
[C6mim][BF4]
680.0[g]
314.0[g]
À
RTILs with PF6 anion have lower solubility in water when
À
[a] 204.0;[15] [b] 371.0;[15] [c] 91.4;[15] [d] 154.0;[15] [e] 452.0;[15] [f] 866.0;[15]
[g] literature values.[15, 16]
compared to those with the BF4 anion. The RTIL
[C5O2mim][PF6] has lower solubility in water than the RTIL
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Ionic Liquids
3671 3677
The viscosity of ionic liquids is essentially influenced by
their hydrogen bonding ability and by the strength of their
van der Waals interactions, being strongly dependent of the
À
anion type. We observe that all the RTILs containing the BF4
À
anion have lower viscosity than the RTILs with PF6 anion,
which is in agreement with reported data.[3c] This difference is
even more noticeable at the lower temperature (108C). For
À
example, in the [Cnmim][PF6] series, RTILs with PF6 have
viscosities that are more than twice the value of the viscosity
À
of RTILs with BF4 (entries 8, 11, 16 vs 9, 12, 17). This
À
À
significant difference of viscosities between PF6 and BF4
anions is less pronounced for the [CnOmmim][X] series
(entries 1, 3, 6 vs 2, 4, 7).
The structure of the cation also influences the viscosity of
the ionic liquid. In our studies we concluded that the viscosity
of the RTIL [C2OHmim][PF6] is less than half as large as the
RTILs [C3Omim][PF6] and [C4mim][PF6] (entry 1 vs entries 3
and 8). The RTIL [C2OHmim][BF4] presents an intermediary
viscosity between the RTILs [C2mim][BF4] and [C4mim][BF4]
(entry 14 < entry 2 < entry 9). The viscosity of the RTIL
[C5O2mim][Cl] is lower than the viscosity of the RTILs
[C6mim][Cl] and [C8mim][Cl] (entry 5 vs entries 15 and 10).
In Figure 1 we present the viscosity of some RTILs plotted
as a function of the temperature, which allows a better
comparison between the RTILs.
From this study, we can conclude that ionic liquids display a
very large range of viscosities, depending on the cation/anion
combination. On the other hand, the temperature coefficient
of the viscosity at room temperature (that expresses the
activation energy for the viscous flow)is high when compared
to ™normal∫ solvents.
From the data displayed in Figure 1, it can be concluded
that with regard to the viscosity, the best RTILs are
[C2OHmim][PF6], [C2OHmim][BF4], [C3Omim][BF4] and
[C4mim][BF4], since they have a lower viscosity, which is an
important issue in some applications.
Figure 1. Viscosity of a series of RTILs as a function of the reciprocal of
the temperature
Table 4. Densities at 258C of several RTILs.
Density: In Table 4 we present density data at 258C for the
prepared RTILs. In general, the new RTIL [CnOmmim][X]
series have higher densities than the [Cnmim][X] series. The
Entry
Ionic liquid
Density[gmLÀ1
]
1
2
3
4
5
6
7
8
[C2OHmim][PF6]
[C2OHmim][BF4]
[C3Omim][PF6]
[C3Omim][BF4]
[C5O2mim][Cl]
[C5O2mim][PF6]
[C5O2mim][BF4]
[C4mim][Cl]
1.48
1.33
1.40
1.26
1.14
1.32
1.22
1.08[a]
1.31[b]
1.26[c]
1.03[a]
1.29[a]
1.00[a]
1.19[d]
1.08
À
RTILs containing the PF6 anion have higher densities than
those with BF4À or ClÀ anions. As would be expected, a larger
alkyl chain on the [Cnmim] series results in a decrease of
density (butyl > hexyl > octyl > decyl)(entries 8, 9 and 10 vs
entries 13, 14 and 15).
Among the new RTIL [CnOmmim][X] series, the densities
decrease in the order [C2OHmim] > [C3Omim] > [C5O2mim]
(entries 1 and 2 vs entries 3 and 4 vs entries 6 and 7). The
RTILs [C3Omim][X] and [C5O2mim][PF6] present a very
similar density to those from the [Cnmim][X] series with
comparable chain size, respectively [C4mim][X] and
[C6mim][PF6].
9
[C4mim][PF6]
[C4mim][BF4]
[C6mim][Cl]
[C6mim][PF6]
[C8mim][Cl]
[C8mim][PF6]
[C8mim][BF4]
[C10mim][BF4]
10
11
12
13
14
15
16
1.04
[a] Reported value;[3c] [b] 1.36;[3c] [c] 1.12;[3c] [d] 1.22.[3c]
Melting point and glass transition temperature: The solid
liquid phase transition of these RTILs has been studied by
differential scanning calorimetry (DSC, Table 5). One of the
features arising from this study is that most of these molten
salts are very good glass-formers (they present a weak
tendency to crystallise). They can be cooled from the
equilibrium liquid state down to low temperatures without
crystallising, entering the metastable supercooled liquid state,
and suffering a glass transition leading to an out-of-equili-
brium glassy state. Consequently, these RTILs, that are easily
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Indeed, values of DCPm of 23 JKÀ1 molÀ1 and 53 JKÀ1 molÀ1
are reported respectively for glycerol (Tg ˆ 185 K)and
toluene (Tg ˆ 113 K),[17] while for polymeric systems we have
DCPm ˆ 10 JKÀ1 molÀ1 for polyethylene (Tg ˆ 140 K)and
DCPm ˆ 36 JKÀ1 molÀ1 for poly(vinyl acetate)( Tg ˆ 302 K).[18]
The high DCPm at Tg found for ionic liquids arises from the fact
the each molecular unit is in fact composed by two sub-units
(the cation and the anion)that have, in the supercooled liquid
state, an appreciable relative mobility.
Table 5. Thermal data of several RTILs obtained by DSC analysis.
[a]
[b]
[c]
Ionic liquid
TG
[8C]
DCP
DCPm
Tf[d]
[8C]
[JgÀ1 KÀ1
]
[JmolÀ1 KÀ1
]
[C2OHmim][PF6]
[C2OHmim][Cl]
[C2OHmim][BF4]
[C2OHmim][TFA]
[C3Omim][PF6]
[C3Omim][Cl]
[C3Omim][BF4]
[C4OHmim][Br]
[C5O2mim][PF6]
[C5O2mim][Cl]
[C5O2mim][BF4]
[C4mim][Cl]
[C4mim][PF6]
[C4mim][BF4]
[C6mim][Cl]
[C6mim][PF6]
[C6mim][BF4]
[C8mim][Cl]
À 72
0.46
0.72
0.56
0.74
125
117
120
178
À 111
À 84
À 89
26
À 60
À 88
À 83
À 69
À 85
À 86
0.52
0.56
0.63
0.46
0.79
0.65
92
128
148
151
174
177
No apparent correlation can be found between the heat
capacity jump, DCPm, at the glass transition, and the glass
transition temperature or the molar mass of the ionic liquids.
41[e]
10[e]
À 81[e]
À 80[e]
À 97[e]
À 75[e]
À 78[e]
À 82.4[e]
À 87[e]
À 82[e]
À 80.5[e]
À 24.7[e]
Solubility of inorganic salts in RTILs: Recently, ionic liquids
have often been discussed as promising solvents for green
chemistry and clean synthesis.
À 61[e]
The use of RTILs as a new recyclable reaction media for a
considerable range of organic transformations, including
transition metal catalysis has already been demonstrated.
Nonetheless, the use of RTILs has been mainly restricted to
the [Cnmim][X] series. Meanwhile, in our experience, the
extension of this medium to other organic transformations
suffers from the drawback of the considerably low solubility
of some substrates and catalysts on the RTIL [Cnmim][X]
series. To circumvent this limitation, we performed a com-
parative study of solubility of Li, Hg and La chlorides between
the [Cnmim][X] and [CnOmmim][X] series as a prospective
study. The three salts tested were chosen in order to allow us
to predict the potential solubility of each RTIL of different
types of a diverse range of transition metals and inorganic
species that could eventually be used in different applications.
Solubility data for LiCl, HgCl2 and LaCl3 in several RTILs
is presented in Table 6.
[C8mim][PF6]
[C8mim][BF4]
[C10mim][BF4]
[a] Glass transition temperature (TG)determined as the onset temperature
in the heating mode (108CminÀ1); [b] heat capacity jump (DCP); [c] molar
heat capacity jump (DCPm); [d] melting temperature (Tf); [e] reported
values.[3a,c]
manipulated in the supercooled liquid state, show a strong
DSC signature of a glass transition (on cooling, as well as on
heating). Among the ionic liquids studied, only
[C3Omim][PF6] readily crystallises on cooling. As a conse-
quence, no glass transition signal was observed in this case.
For a heating rate of 108CminÀ1, the onset of the melting peak
appeared at 158C, and the maximum of this peak occurred at
268C. The molar melting enthalpy was found to be DHm ˆ
14.3 kJmolÀ1.
Table 6. Observed solubility constants (Ks)of LiCl, HgCl and LaCl3 in
2
The DSC thermogram recorded on heating of some of the
studied molten salts, namely of [C3Omim][Cl] and
[C5O2mim][PF6], show three different signals: the heat flow
jump arising from the glass transition, a broad exothermic
signal caused by cold recrystallisation (crystallisation on
heating), and the melting (endothermic) peak. In most cases,
the melting peak of these ionic liquids appears above room
temperature. This means that, when manipulated as liquids
near room temperature, they are, in fact, in the metastable
supercooled liquid state, rather than in the equilibrium liquid
state.
Another feature of the room temperature ionic liquids
studied in the present work is that they show a strong jump in
the molar heat capacity associated with the glass transition. In
fact, the sigmoidal change in the heat flux, that is the DSC
signature of the glass transition, arises from a change in the
heat capacity, DCP, when the sample is heated from the glassy
state to the metastable supercooled liquid (or cooled from the
supercooled liquid to the glassy state). The jump in the molar
heat capacity, DCPm, is higher than 90 JKÀ1 molÀ1 for all the
RTILs studied in the present work. These high values of DCPm
indicate that many degrees of freedom are released on heating
above Tg. A very high value of the heat capacity jump at the
glass transition appears as a specific feature of ionic liquids.
several RTILs.
Entry Ionic liquid
Ks[a] (LiCl)[b] Ks[a] (HgCl2)[c] Ks[a] (LaCl3)[c]
1
2
3
4
5
6
7
8
[C2OHmim][PF6] 144.47
44.64
84.73
50.13
220.86
295.34
147.48
174.17
4.06
32.47
54.01
37.61
180.27
379.23
97.22
292.46
6.58
[C2OHmim][BF4]
[C3Omim][PF6]
[C3Omim][BF4]
[C5O2mim][Cl]
[C5O2mim][PF6]
[C5O2mim][BF4]
[C4mim][PF6]
18.46
12.44
14.43
9.98
35.52
21.36
12.08
15.54
35.32
56.02
12.64
9
[C4mim][BF4]
[C8mim][PF6]
[C8mim][BF4]
[C10mim][BF4]
41.41
32.98
35.92
2.12
10.92
8.49
53.25
47.12
10
11
12
[a] Observed Ks (10À6 g of saltgÀ1 of RTIL); [b] determined by flame
photometry; [c] determined by inductively coupled plasma spectroscopy
(ICP).
We found that LiCl has, in general, a low solubility in the
RTILs studied, except for [C2OHmim][PF6] (entry 1). The
solubility of LiCl in the RTIL with [C3Omim] cation structure
is similar to the solubility in the RTIL with the [C4mim] cation
structure (entries 3 and 4 vs entries 8 and 9). The same is
observed between the RTILs [C5O2mim][PF6] and
[C8mim][PF6] (entry 6 vs entry 10).
3674
¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
0947-6539/02/0816-3674 $ 20.00+.50/0
Chem. Eur. J. 2002, 8, No. 16

Ionic Liquids
3671 3677
Our results show that the solubility of LiCl in RTIL
[C5O2mim][BF4] is less than half of that in the RTIL
[C8mim][BF4], which is unexpected on account of the
presence of the ether functionality (entry 7 vs entry 11). In
general, we did not detect a considerable difference in
solubility for LiCl between the new RTIL [CnOmmim][X]
and the [Cnmim][X] series. Among the RTILs, [C2OH-
mim][PF6] is a notable exception which appears to be the
appropriate RTIL to solubilise LiCl and comparable alkaline
metals.
With regard to the solubility of HgCl2, we verified that the
new RTILs [CnOmmim][X] allow a higher dissolution than the
[Cnmim][X] series: both [C2OHmim][PF6] and [C3Omim]-
[PF6] solubilise an amount of HgCl2 eleven or more times
higher than [C4mim][PF6] (entries 1 and 3 vs entry 8). The
RTIL [C3Omim][BF4] was found to solubilise HgCl2 three and
five times better than the RTILs [C2OHmim][BF4] and
[C4mim][BF4], respectively (entry 4 vs entries 2 and 9). In
the same line, RTILs containing the [C5O2mim] cation
solubilise HgCl2 five to six times better than those having
the [C8mim] cation (entries 5 and 6 vs entries 10 and 11).
From the above observations, we can conclude that the new
RTILs (in particular [C3Omim][BF4], [C5O2mim][BF4] and
[C5O2mim][PF6])are more efficient at solubilising HgCl 2 and
possibly other transition metals.
Similar to HgCl2, LaCl3 is more soluble in the [CnOm-
mim][X] series than in the [Cnmim][X] series: both [C2OH-
mim][PF6] and [C3Omim][PF6] solubilise LaCl3 five or more
times better than [C4mim][PF6] (entries 1 and 3 vs entry 8).
LaCl3 has a solubility in [C3Omim][BF4] which is three to
sixteen times higher than in [C2OHmim][BF4] and
[C4mim][BF4], respectively (entry 4 vs entries 2 and 9). The
RTILs containing the [C5O2mim] cation solubilise LaCl3 five
to eleven times better than those containing the [C8mim]
cation (entries 6 and 7 vs entries 10 and 11). From the above
results obtained for LaCl3, we expect that the most appro-
priate RTILs to solubilise lanthanides are [C5O2mim][BF4]
and [C3Omim][BF4].
changes on the solubility of inorganic salts, arising from the
possibility of new interactions occurring. Taking advantage of
the general properties of RTILs and the presence of these
extra potential complexing groups, new specific applications
of the RTILs [CnOmmim][X] series presented here are
expected to emerge.
Experimental Section
General remarks: All glassware was oven-dried and cooled in a desiccator
(P2O5 desiccant)prior to use. Commercially supplied reagents were used as
supplied.
All aqueous solutions were prepared with distilled water.
The room temperature ionic liquids (RTIL)1- n-butyl-3-methylimidazo-
lium hexafluorophosphate [C4mim][PF6], 1-n-butyl-3-methylimidazolium
tetrafluoroborate [C4mim][BF4], 1-n-octyl-3-methylimidazolium hexa-
fluorophosphate [C8mim][PF6], 1-n-octyl-3-methylimidazolium tetrafluor-
oborate [C8mim][BF4], 1-n-decyl-3-methylimidazolium hexafluorophos-
phate [C10mim][PF6], 1-n-decyl-3-methylimidazolium tetrafluoroborate
[C10mim][BF4] were prepared according to reported procedures.[1c, 14]
1H and 13C NMR spectra were recorded on a Bruker AMX400 spectrom-
eter. Chemical shifts are reported downfield in ppm from a tetramethylsi-
lane reference. [D6]toluene was used as an internal reference in the case of
neat samples. IR spectra were recorded on a Mattson Instruments model
Satellite FTIR as thinly dispersed films. The samples for elemental analysis
¬
¬
¬
were performed by Laboratorio de Analises at Instituto Superior Tecnico
(Lisbon). The differential scanning calorimetry (DSC) experiments were
performed with a 2920MDSC system from TA Instruments Inc. The water
content of each RTIL was determined by a volumetric Aquastar Karl
Fischer titration. Each RTIL sample (2 mL scale)used for elemental
analysis and DSC experiments was further purified by flash column
chromatography (eluent: dichloromethane)followed by removal of the
volatile components under high vacuum (6 Â 10À6 mbar)at room temper-
ature for 24 h.
Viscosity: The viscosity of each RTIL was measured with a Brookfield,
Model RVTDV-II viscosimeter. For each analysis, an 8-mL sample was
used and the measurements were performed in duplicate. The temperature
of the samples was maintained to Æ0.18C by means of an external
temperature controller.
Density: The density of each RTIL was determined with a picnometer. For
each analysis a 1-mL sample was used and the mass of that volume of liquid
was determined. The measurements were repeated three times and the
average value is reported. All measurements were taken at room temper-
ature (25 Æ 18C).
Solid liquid phase transition analysis: The calorimetric measurements
were performed with a 2920MDSC system from TA Instruments Inc. Dry
high-purity He gas with a flow rate of 30 cm3 minÀ1 was purged through the
sample. Cooling was accomplished with the liquid nitrogen cooling
accessory (LNCA)which provides automatic and continuous programmed
sample cooling down to À1508C.
In general, the solubility behaviour of HgCl2 and LaCl3 is
similar for the RTILs [CnOmmim][X] and [Cnmim][X]. The
À
RTILs containing BF4 are better solvents than the corre-
À
sponding RTILs with the PF6 anion. The cation structure of
the RTIL also influences the solubility of HgCl2 and LaCl3,
which increases with chain size: [C2OHmim] < [C3Omim] <
[C5O2mim] and [C4mim] < [C8mim]. In contrast, LiCl pres-
ents very different solubility behaviour in the RTILs studied.
The solubility profile presented here for LiCl, HgCl2 and
LaCl3 should be valid for alkaline, transition metals and
lanthanides salts, respectively.
The baseline was calibrated by scanning the temperature domain of the
experiments with an empty pan. The temperature calibration was
performed taking the onset of the endothermic melting peak of several
calibration standards: n-decane (Tm ˆ 243.75 K), n-octadecane (Tm
ˆ
301.77 K), hexatriacontane (Tm ˆ 347.30 K), indium (Tm ˆ 430.61 K)and
tin (Tm ˆ 506.03 K). The organic standards were high-purity Fluka prod-
ucts, while the metal standards were supplied by TA Instruments Inc. The
enthalpy was calibrated with indium (melting enthalpy DHm ˆ 28.71 JgÀ1).
Solubility of inorganic salts in RTILs: The solubility of inorganic salts in
each RTIL was measured by Inductively Coupled Plasma spectroscopy
(ICP)for Hg and La, and by flame photometry (Corning flame photometer
410)for Li.
Conclusion
The new [CnOmmim][X] RTILs described here show a set of
properties (solubility in common solvents, viscosity, solid
liquid phase transition, density and solubility of inorganic
salts)that could extend the range of applications this new
media has to offer. We have shown that the presence of
hydroxyl or ether functional groups promotes remarkable
For preparation of the samples we used 1.00 g of each ionic liquid followed
by the addition of excess inorganic salt (HgCl2, LiCl and LaCl3). This
mixture was stirred at room temperature for 48 h and then filtered, weighed
(range: 150 380 mg)and diluted with distilled water (100 mL). By means
of this procedure we prepared the initial solutions that, in a number of
cases, were diluted to the calibration range of the method used.
Chem. Eur. J. 2002, 8, No. 16
¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
0947-6539/02/0816-3675 $ 20.00+.50/0
3675

C. A. M. Alfonso et al.
FULL PAPER
Preparation of ionic liquids
[C3Omim][Cl]: The same procedure was followed as that described above
for [C2OHmim][Cl], except for the use of 2-chloroethylmethyl ether
(87 mL, 0.95 mol)instead of chloroethanol. The product [C 3Omim][Cl]
(89.00 g, 80%)was obtained as a slightly yellow oil that solidified on
cooling. 1H NMR (400 MHz, D2O, 25 8C): d ˆ 3.28 (s, 3H), 3.73 (t,
J(H,H) ˆ 12.0 Hz, 2H), 3.80 (s, 3H), 4.29 (t, J(H,H) ˆ 12.0 Hz, 2H), 7.34
(s, 1H), 7.40 (s, 1H), 8.64 (s, 1H); 13C NMR (100 MHz, D2O, 25 8C): d ˆ
35.53, 48.73, 58.09, 69.72, 122.49, 123.55, 136.40; IR (film): nÄmax ˆ 653.9,
760.2, 833.5, 968.3, 1013.8, 1083.2, 1171.0, 1285.1, 1453.1, 1572.9, 1572.9,
[C2OHmim][Cl]: 1-Chloroethanol (64 mL, 0.95 mol)and 1- methylimida-
zole (50 mL, 0.63 mol)were added to a round-bottomed flask fitted with a
reflux condenser for 24 h at 808C with stirring until two phases formed. The
top phase, containing unreacted starting material, was decanted and diethyl
ether (50 mL)was added with thorough mixing. The diethyl ether was
decanted followed by addition of fresh solvent and this step was repeated
twice. The bottom phase was washed with diethyl ether (4 Â 25 mL), heated
at 808C and stirred under vacuum (0.5 mmHg)for 2 d. The product was
obtained as a slightly yellow liquid which solidified on cooling (84.00 g,
82%). 1H NMR (400 MHz, D2O, 25 8C): d ˆ 3.80 (s, 3H), 3.84 (d, J(H,H) ˆ
4.6 Hz, 2H)4.22 (t, J(H,H) ˆ 8.8 Hz, 2H), 7.36 (s, 1H), 7.41 (s, 1H), 8.66 (s,
1H); 13C NMR (100 MHz, D2O, 25 8C): d ˆ 35.73, 51.52, 59.79, 122.43,
123.59, 136.37; IR (film): nÄmax ˆ 648.5, 751.1, 840.8, 945.1, 1068.9, 1167.9,
1339.9, 1452.3, 1574.4, 1643.1, 2888.3, 2961.0, 3160.8, 3392.2 cmÀ1; elemental
analysis calcd (%)for C 6H12N2OCl (163.63): C 44.04, H 7.30, N 17.12;
found: C 44.08, H 6.92, N 17.10.
1640.4, 2834.3, 3433.7 cmÀ1; elemental analysis calcd (%)for C H14N2OCl
7
(177.65): C 47.33, H 7.90, N 15.75; found: C 47.29, H 7.65, N 15.55.
[C3Omim][PF6]: The same procedure was followed as that described above
for [C2OHmim][PF6]: [C3Omim][Cl] (25.00 g, 0.14 mol)and KPF salt
6
(29.00 g, 0.16 mol)were used, and the product [C 3Omim][PF6] (37.00 g,
93%)was obtained as a colourless liquid; water content of 1.74 mg
(H2O)mLÀ1 (RTIL); 1H NMR (400 MHz, neat, 258C): d ˆ 3.27 (s, 3H),
3.73 (brs, 2H), 3.88 (s, 3H), 4.30 (brs, 2H), 7.35 (s, 1H), 7.42 (s, 1H), 8.42 (s,
1H); 13C NMR (100 MHz, neat, 258C): d ˆ 36.89, 50.61, 59.19, 70.99,
124.27, 124.73, 138.12; IR (film): nÄmax ˆ 652.8, 749.6, 836.9, 968.7, 1013.8,
1084.6, 1169.8, 1226.5, 1340.5, 1453.5, 1574.0, 2839.9, 2902.6, 2943.0, 3125.7,
3170.4, 3595.1, 3667.8 cmÀ1; elemental analysis calcd (%)for C 7H14N2OPF6
(287.17): C 29.38, H 4.58, N 9.79; found: C 29.30, H 4.60, N 9.80.
[C2OHmim][PF6]: [C2OHmim][Cl] (25.00 g, 0.15 mol)was transferred to a
plastic Erlenmeyer flask (250 mL). Acetone (150 mL) was added followed
by KPF6 (31.00 g, 0.17 mol). This mixture was stirred at room temperature
for 24 h. The resulting waxy solid precipitate was collected by filtration and
washed with acetone (2 Â 100 mL). The organic layer was collected, dried
(MgSO4), filtered and the solvent removed in vacuum to give the product
[C2OHmim][PF6] (37.00 g, 91%)as a colourless liquid; water content of
[C3Omim][BF4]: The same procedure was followed as that described above
for [C2OHmim][BF4]: [C3Omim][Cl] (25.00 g, 0.14 mol)and NaBF salt
4
0.85 mg (H2O)mLÀ1 (RTIL). H NMR (400 MHz, neat, 258C): d ˆ 3.88 (s,
1
(17.00 g, 0.16 mol)were used, and the product [C 3Omim][BF4] (31.00 g,
96%)was obtained as a colourless liquid; water content of 2.55 mg
(H2O)mLÀ1 (RTIL). 1H NMR (400 MHz, neat, 258C): d ˆ 3.27 (s, 3H), 3.72
(brs, 2H), 3.78 (s, 3H), 4.28 (brs, 2H), 7.33 (s, 1H), 7.39 (s, 1H), 8.50 (s,
1H); 13C NMR (100 MHz, neat, 258C): d ˆ 38.06, 51.27, 60.63, 72.25,
125.03, 126.09, 139.12; IR (film): nÄmax ˆ 653.5, 756.7, 835.2, 1075.3,1171.3,
1286.5, 1340.1, 1454.3, 1574.4, 2838.7, 2902.6, 2943.1, 3123.2, 3163.1, 3563.8,
3H), 3.91 (t, J(H,H) ˆ 9.8 Hz, 2H), 4.29 (t, J(H,H) ˆ 9.2 Hz, 2H), 4.74 (s,
3H), 7.42 (s, 1H), 7.48 (s, 1H), 8.69 (s, 1H); 13C NMR (100 MHz, neat,
258C): d ˆ 36.10, 52.00, 60.20, 122.90, 124.10, 136.80; IR (film): nÄmax ˆ 651.8,
703.1, 750.2, 839.5, 943.0, 1068.4, 1170.2, 1247.3, 1360.3, 1452.3, 1579.2,
1705.0, 2896.2, 2967.5, 3172.3, 3412.6, 3591.4 cmÀ1; elemental analysis calcd
(%)for C 6H12N2OPF6 (273.14): C 26.48, H 4.07, N 10.29; found: C 26.53, H
4.08, N 10.20.
3635.4 cmÀ1
;
elemental analysis calcd (%)for
C
7H14N2OBF4 ¥
[C2OHmim][BF4]: [C2OHmim][Cl] (25.00 g, 0.15 mol)was transferred to a
plastic Erlenmeyer flask (250 mL). Acetone (150 mL) was added followed
by NaBF4 (19.00 g, 0.17 mol). This mixture was stirred at room temperature
for 24 h. The resulting waxy solid precipitate was collected by filtration and
washed with acetone (2 Â 100 mL). The combined layer was collected,
dried (MgSO4), filtered and the solvent removed in vacuum to give the pro-
duct [C2OHmim][BF4] (30.00 g, 93%)as a colourless liquid; water content
0.5H2O(238.01): C 35.29, H 6.30, N 11.76; found: C 35.67, H 6.12, N 11.81.
[C5O2mim][Cl]: The same procedure was followed as that described above
for [C2OHmim][Cl], except for the use of 1-chloro-2-(2-methoxyethoxy)-
ethane (98 mL, 0.95 mol)instead of 1-chloroethanol. The product
[C5O2mim][Cl] (104.00 g, 75%)was obtained as a slightly yellow liquid;
water content of 4.31 mg (H2O)mLÀ1 (RTIL). 1H NMR (400 MHz, neat,
258C): d ˆ 3.30 (s, 3H), 3.53 (brs, 2H), 3.65 (brs, 2H), 3.90 (brs, 2H), 3.99
(s, 3H), 4.44 (brs, 2H), 7.58 (s, 1H), 7.66 (s, 1H), 8.92 (s, 1H); 13C NMR
(100 MHz, neat, 258C): d ˆ 36.31, 50.06, 58.57, 68.94, 71.43, 72.00, 123.52,
123.9, 137.71; IR (film): nÄmax ˆ 654.9, 763.1, 846.8, 920.3, 1025.4, 1104.6,
1171.6, 1246.9, 1297.5, 1355.1, 1453.5, 1572.3, 1665.2, 1713.5, 2878.8, 2980.7,
3067.2, 3144.2, 3381.4 cmÀ1; elemental analysis calcd (%)for C 9H18N2O2Cl ¥
0.2H2O (225.31): C 47.93, H 8.08, N 12.43; found: C 47.90, H 8.13, N 12.80.
1
of 4.68 mg (H2O)mLÀ1 (RTIL). H NMR (400 MHz, neat, 258C): d ˆ 3.75
(m, 2H), 4.16 (t, J(H,H) ˆ 10.0 Hz, 2H), 4.61 (s, 3H), 7.30 (s, 1H), 7.35 (s,
1H), 8.58 (s, 1H); 13C NMR (100 MHz, neat, 258C): d ˆ 35.10, 51.50, 59.70,
122.40, 123.60, 136.30; IR (film): nÄmax ˆ 652.2, 702.7, 755.2, 846.2, 1059.1,
1169.6, 1252.2, 1287.3, 1340.3, 1452.9, 1575.4, 1705.0, 2894.3, 2966.2, 3123.4,
3164.8, 3546.5 cmÀ1; elemental analysis calcd (%)for C 6H12N2OBF4 ¥
0.5H20 (223.99): C 32.17, H 5.60, N 12.51; found: C 32.31, H 5.36, N 12.53.
[C5O2mim][PF6]: The same procedure was followed as that described
[C2OHmim][TFA]: [C2OHmim][Cl] (3.00 g, 0.013 mol)was transferred to
above for [C2OHmim][PF6]: [C5O2mim][Cl] (25.00 g, 0.11 mol)and KPF
6
a
plastic Erlenmeyer flask (1000 mL). Acetone (20 mL) was added
salt (23.00 g, 0.13 mol)were used, and the product was obtained
[C5O2mim][PF6] (34.00 g, 94%)as a colourless liquid; water content of
followed by NaCF3CO2 (2.81 g, 0.021 mol). This mixture was stirred at
room temperature for 24 h. The resulting waxy solid precipitate was
collected by filtration and washed with acetone (2 Â 25 mL). The organic
layer was collected, refiltered and the solvent removed in vacuum to give
1
2.02 mg (H2O)mLÀ1 (RTIL). H NMR (400 MHz, neat, 258C): d ˆ 3.48 (s,
3H), 3.71 (brs, 2H), 3.82 (brs, 2H), 4.05 (brs, 2H), 4.10 (s, 3H), 4.51 (brs,
2H), 7.59 (s, 1H), 7.68 (s, 1H), 8.73 (s, 1H); 13C NMR (100 MHz, neat,
258C): d ˆ 35.26, 49.05, 57.45, 67.88, 69.47, 70.88, 122.46, 122.90, 136.45; IR
(film): nÄmax ˆ 653.2, 750.9, 842.7, 1025.0, 1104.2, 1170.4, 1198.7, 1247.9,
1299.2, 1354.2, 1435.7, 1575.2, 1620.3, 2884.6, 2929.7, 3122.8, 3167.9, 3413.0,
3645.4 cmÀ1; elemental analysis calcd (%)for C 9H18N2O2PF6 ¥ H20 (349.22):
C 30.95, H 5.73, N 8.02; found: C 30.72, H 5.35, N 8.05.
the product [C2OHmim][TFA] (2.81 g, 94%)as
a colourless liquid.
1H NMR (400 MHz, neat, 258C): d ˆ 3.73 (m, 2H), 4.15 (brs, 2H), 4.59
(s, 3H), 7.35 (s, 1H), 7.43 (s, 1H), 8.86 (s, 1H); 13C NMR (100 MHz, neat,
258C): d ˆ 36.06, 48.36, 60.06, 119.13, 123.12, 135.16; IR (film): nÄmax ˆ 721.4,
833.1, 945.3, 1075.9, 1135.3, 1341.1, 1426.7, 1574.2, 1689.7, 2885.9, 2964.3,
3286.3 cmÀ1
; elemental analysis calcd (%)for C 8H12N2O3F3 ¥ 0.5H2O
(250.19): C 38.37, H 5.18, N 11.11; found: C 38.22, H 5.20, N 11.17.
[C5O2mim][BF4]: The same procedure was followed as that described
above for [C2OHmim][BF4]: [C5O2mim][Cl] (25.00 g, 0.11 mol)and NaBF 4
salt (14.00 g, 0.13 mol)were used, and the product was obtained
[C5O2mim][BF4] (29.00 g, 97%)as a colourless liquid; water content of
[C4OHmim][Br]: The same procedure was followed as that described
above for [C2OHmim][Cl], except for the use of bromobutanol (133 mL,
0.95 mol)instead of chloroethanol. The product [C 4OHmim][Br] (114.00 g,
79%)was obtained as a colourless liquid. 1H NMR (400 MHz, neat, 258C):
d ˆ 0.68 (brs, 2H), 0.94 (m, 2H), 2.31 (m, 2H), 2.68 (brs, 2H), 3.23 (s, 1H),
4.60 (s, 3H), 6.87 (s, 1H), 6.94 (s, 1H), 8.46 (s, 1H); 13C NMR (100 MHz,
neat, 258C): d ˆ 26.23, 28.44, 36.07, 44.81, 60.05, 119.13, 123.15, 135.17; IR
(film): nÄmax ˆ 625.0, 762.1, 831.6, 903.3, 1086.3, 1171.6, 1283.6, 1331.0, 1380.4,
1450.0, 1550.3, 1581.7, 1639.2, 2616.6, 2837.8, 2946.1, 3399.5 cmÀ1; elemental
analysis calcd (%)for C 8H16N2OBr (235.90): C 40.70, H 6.78, N 11.87;
found: C 40.65, H 6.90, N 11.41.
1
2.30 mg (H2O)mLÀ1 (RTIL). H NMR (400 MHz, neat, 258C): d ˆ 3.31 (s,
3H), 3.53 (brs, 2H), 3.65 (brs, 2H), 3.89 (brs, 2H), 4.01 (s, 3H), 4.45 (brs,
2H), 7.58 (s, 1H), 7.67 (s, 1H), 8.93 (s, 1H); 13C NMR (100 MHz, neat,
258C): d ˆ 35.20, 49.95, 57.46, 67.83, 69.40, 70.90, 122.41, 122.84, 136.61; IR
(film): nÄmax ˆ 653.6, 757.1, 845.8, 923.9, 1063.2, 1171.7, 1248.3, 1286.5, 1354.0,
1454.1, 1574.6, 1631.1, 1719.0, 2885.4, 2926.5, 3120.5, 3161.2, 3394.3,
3609.7 cmÀ1; elemental analysis calcd (%)for C 9H18N2O2BF4 (273.06): C
39.59, H 6.60, N 10.26; found: C 39.40, H 6.46, N 10.31.
3676
¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
0947-6539/02/0816-3676 $ 20.00+.50/0
Chem. Eur. J. 2002, 8, No. 16

Ionic Liquids
3671 3677
Branco, C. A. M. Afonso, Tetrahedron 2001, 57, 4405 4410; r)V.
Acknowledgements
¡
Calo, A. Nacci, L. Lopez, A. Napola, Tetrahedron Lett. 2001, 42,
4701 4703; s)J. Peng, Y. Deng, Tetrahedron Lett. 2001, 42, 5917
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u)D. W. Morrison, D. C. Forbes, J. H. Davis, Tetrahedron Lett. 2001,
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√
We would like to thank FundaÁaƒo para a Ciencia e Tecnologia (project
PRAXIS/C/QUI/10069/98 and Ph.D. grant for J.M.N.R., ref. PRAXIS
XXI/BD/18286/98)and the European Union (project QLK3-1999-01213)
for financial support.
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Received: January 14, 2002 [F3794]
Chem. Eur. J. 2002, 8, No. 16
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