Identification of 1,3-dialkylimidazoIium salt supramolecular aggregates in solution

Article abstract of DOI:10.1021/jp0452709

The nature of the interactions between 1,3-dialkylimidazolium cations and noncoordinating anions such as tetrafluoroborate, hexafluorophosphate, and tetraphenylborate has been studied in the solid state by X-ray diffraction analysis and in solution by

Full text of DOI:10.1021/jp0452709

J. Phys. Chem. B 2005, 109, 4341-4349
4341
Identification of 1,3-Dialkylimidazolium Salt Supramolecular Aggregates in Solution
Crestina S. Consorti,† Paulo A. Z. Suarez,†,‡ Roberto F. de Souza,† Robert A. Burrow,§
David H. Farrar,| Alan J. Lough,| Watson Loh, Luis H. M. da Silva, and Jairton Dupont*,†
Laboratory of Molecular Catalysis, Institute of Chemistry, UFRGS, AV. Bento Gonc¸alVes,
9500 Porto Alegre 91501-970 RS, Brazil, Departamento de Qu´ımica, UFSM, Santa Maria, RS, Brazil,
Department of Chemistry, UniVersity of Toronto, Ontario, Canada M5S 3G3, and Instituto de Qu´ımica,
UNICAMP, CP l6154 Campinas 13083-970 SP, Brazil
ReceiVed: October 15, 2004; In Final Form: January 7, 2005
The nature of the interactions between 1,3-dialkylimidazolium cations and noncoordinating anions such as
tetrafluoroborate, hexafluorophosphate, and tetraphenylborate has been studied in the solid state by X-ray
1
diffraction analysis and in solution by H NMR spectroscopy, conductivity, and microcalorimetry. In the
solid state, these compounds show an extended network of hydrogen-bonded cations and anions in which
one cation is surrounded by at least three anions and one anion is surrounded by at least three imidazolium
cations. In the pure form, imidazolium salts are better described as polymeric supramolecules of the type
{[(DAI)3(X)]2+[(DAI)(X)3]2-}n (where DAI is the dialkylimidazolium cation and X is the anion) formed through
hydrogen bonds of the imidazolium cation with the anion. In solution, this supramolecular structural organization
is maintained to a great extent, at least in solvents of low dielectric constant, indicating that mixtures of
imidazolium ionic liquids with other molecules can be considered as nanostructured materials. This model is
very useful for the rationalization of the majority of the unusual behavior of the ionic liquids.
Introduction
as a three-dimensional network of anions and cations linked
together by weak interactions (mainly hydrogen bonds), as
Room-temperature ionic liquids (RTILs) emerged at the end
of the 1990s as a new and exiting field, opening new horizons
for many areas of chemistry, particularly in green chemistry.1
Imidazolium-based RTILs, such as 1-n-butyl-3-methylimida-
zolium tetrafluoroborate ([BMI]BF4) and the hexafluorophos-
phate analogue ([BMI]PF6),2 are liquids over a wide temperature
range, with negligible vapor pressure, relatively high conductiv-
ity, high thermal and relatively good chemical stability,3 and a
wide electrochemical window (up to 7 V).4 They have been
successfully used as media for organic chemistry,5 organome-
tallic catalysis,1 electrochemistry,6 and separation processes,7
etc. The physical-chemical properties of these ionic liquids
(ILs) are strongly influenced by the nature of the N-alkyl
imidazolium substituents and that of the counteranion.4,8
Much research effort has been devoted to the understanding
of the role of the different parameters that control the physical-
chemical properties of RTILs. Attempts to correlate the strength
of the hydrogen bonds between the cation and anion, the length
and/or symmetry of the N-alkyl imidazolium substituents, and
symmetry or size of the anion with the physical-chemical
properties (melting point, viscosity, conductivity, etc.) of these
materials have displayed only limited trends.8c Therefore,
knowledge of the structure of these ILs can play an important
role in their design and applications.8c
shown by X-ray crystallographic data.9 X-ray diffraction studies
of cocrystal (imidazolium salt and 1,4-phenylenediamine)
indicated that the general pattern of supramolecular organization
of the salt part remain the same in the crystal as in the pure
IL.10 It was also suggested by spectroscopic studies that molten
and crystalline pure 1-alkyl-3-methylimidazolium salts have a
similar structure in both solid and liquid states.11 In the past
decade, several authors observed similar weak interaction
networks for imidazolium cation salts. Using NMR and
conductivity studies, it was observed that in solution 1-ethyl-
3-methylimidazolium halides form strong hydrogen bonds in
polar solvents and exist in a quasi-molecular state in nonpolar
solvents.12 We have recently shown that small amounts of water
in fluorinated ILs, such as 1-butyl-3-methylimidazolium hexaflu-
orophosphate and tetrafluoroborate, have a dramatic effect on
the rate of diffusion of ionic and neutral species in these media.
It was thus proposed that “wet” ILs may not be regarded as
homogeneous solvents but have to be considered as “nano-
structured” materials, through hydrogen-bond interactions that
can possess in some cases polar and nonpolar regions.13 In this
respect, it is also known that imidazolium ILs possess dual
behavior when used as a stationary phase for gas chromatog-
raphy. They act as a low-polarity stationary phase to nonpolar
compounds, while molecules with proton donor groups are
strongly retained.14 We have already shown by X-ray studies
of 1-n-butyl-3-methylimidazolium tetraphenylborate that the
hydrogen atoms of the imidazolium ring form three dimension
networks through weak C-H-π bonds and that this type of
supramolecular structure is maintained to a some extent even
in solution, at least in solvents with low dielectric constants,
such as chloroform.15 In this work, we report the X-ray
diffraction, 1H NMR, conductivity, and microcalorimetry results
More than a decade ago it was proposed that in the solid
state 1-ethyl-3-methylimidazolium chloride should be considered
* To whom correspondence should be addressed. E-mail:
dupont@iq.ufrgs.br. Fax: +55-5133167304.
† UFRGS.
‡ Present address: Department of Chemistry, UNB, Bras´ılia, DF, Brazil.
§ UFSM.
| University of Toronto.
UNICAMP.
10.1021/jp0452709 CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/17/2005

4342 J. Phys. Chem. B, Vol. 109, No. 10, 2005
Consorti et al.
CHART 1: 1-n-Butyl-3-methylimidazolium-Based ILs
SCHEME 1: Synthesis of the Symmetric Imidazolium
ILs
of a series of 1,3-N-disubstituted imidazolium salts, showing
that these systems are better described as polymeric supramol-
ecules with weak interactions in pure form, whereas mixtures
of imidazolium salts with other molecules may be regarded as
nanostructured materials.
Results and Discussion
The Synthesis of 1,3-Dissubstituted Imidazolium-Based
ILs. The nonsymmetric 1-n-butyl-3-methylimidazolium salts
(Chart 1) have been prepared by simple anion metathesis as
described previously.16
The symmetric 1,3-di-R-(+)-methylbenzyl imidazolium cat-
ions associated with tetrafluoroborate, hexafluorophosphate, and
tetraphenylborate anions have been prepared in good yields
(>90%) by the reaction of 2 equiv of R-(+)-1-phenetylamine
with glyoxal and formaldehyde followed by treatment with
hydrochloride acid and anion metathesis (Scheme 1).17
Solid-State Structures. The molecular structures of [DMBI]-
BF4, [DMBI]PF6, and [DMBI]BPh4 have been investigated by
single-crystal X-ray diffraction crystallographic analysis. Dia-
grams of the structures are shown in Figure 1 for [DMBI]BF4
and [DMBI]PF6. Crystallographic data and details of the
structure determination are presented in Table 3. Tables of
atomic coordinates, hydrogen coordinates, complete list of bond
distances and angles, and anisotropic thermal parameters are
supplied as Supporting Information.
Figure 1 shows the expected structural features in [DMBI]-
BF4 and [DMBI]PF6 for the imidazolium cation and the BF4
and PF6 anions that are slightly distorted with tetrahedral and
octahedral geometries, respectively. Each imidazolium ring is
surrounded by three anions, and in turn, each anion is surrounded
by three imidazolium cations through hydrogen bonds of the
H-imidazolium ring with the F atom of the anions. Note that
there are also significant hydrogen bonds between the hydrogens
of the R-methylbenzyl (MB) N-imidazolium substituents and F
of the anions. Therefore, when we take into account all of the
relevant hydrogen bonds involving the hydrogens of both
imidazolium ring and those of the MB N-imidazolium substit-
uents, each cation is surrounded by six anions (BF4 or PF6).
The crystallographic structures of the two salts are quite similar;
both resemble a structure previously reported for [BMI]BPh4.15
In both compounds, the molecules pack in stacks of cations
and anions with an extended network of cations and anions
connected by hydrogen bonds (Figure 2).
Figure 1. View of the close contacts between the cation and the BF4
(A) and PF6 (B) anions (only relevant parts shown) for [DMBI]BF4
and [DMBI]PF6 (the sum of van der Waals radii for the H and F atoms
is 2.67 Å).18
relatively important hydrogen bonds between the methyl group
of the benzylic moiety and the fluorine atoms. The most
important interactions are 2.448(3) Å (H27A‚‚‚F3) in [DMBI]-
BF4 and 2.408(4) Å (H26‚‚‚F4) in [DMBI]PF6.
Although there are also significant interactions between the
hydrogen of the N-alkyl hydrogen substituents and the hydrogen
of the phenyl ring with the anions, this structural pattern of one
cation hydrogen bonded with at least three anions and one anion
hydrogen bonded with at least three cations is a general trend
for the imidazolium salts9,15,17,19-24 as pointed out in a recent
survey.25
This structural pattern is also observed in the case of [DMBI]-
BPh4 in which the interaction between the cation and anion
involves weak C-H‚‚‚π hydrogen bonds (see Figure 3 and
Table 2). The crystal of [DMBI]BPh4 consists of two indepen-
dent molecules (parts A and B in Figure 3) found in the unit
cell. These structures are quite similar to each other, and the
conformational feature of the cation that differentiates A and B
is the torsion angle of the phenyl moiety around C41-C40-
C30-C31 [structure A of Figure 3, 150.29(29)°] or C51-C50-
C60-C61 [structure B of Figure 3, 161.68(30)°] of one of the
MB side chains.
Although the cation is surrounded by three BPh4 anions, only
one hydrogen of the imidazolium ring has a significant
interaction with the anion, while the other hydrogen bonds are
resulting from the close contact of the hydrogens of the MB
side chains. This is at first glance quite surprising because it is
expected that the most important hydrogen bond should involve
the most acidic NCHC imidazolium ring as observed in the
Table 1 reports the distribution of intermolecular C-H‚‚‚F
angles and H‚‚‚F distances for [DMBI]PF6 and [DMBI]BF4.
The BF4 anion leads to slightly longer C-H‚‚‚F bonds with
respect to the PF6 anion, while in the PF6 anion, the C-H donors
approach more linearly toward the fluorine.
The nearest interionic interaction was H2-F3 [2.327(3) Å]
for [DMBI]BF4 and H2-F4 [2.204(1) Å] for [DMBI]PF6, as
observed for other 1,3-dialkylimidazolium salts.19 There are also

1,3-Dialkylimidazolium Salt Supramolecular Aggregates
J. Phys. Chem. B, Vol. 109, No. 10, 2005 4343
TABLE 1: Nearest Hydrogen Interactions in [DMBI]PF6
and [DMBI]BF4
d(C-H) d(H‚‚‚F) d(C‚‚‚F) angle (CHF)
C-H‚‚‚F
(Å)
(Å)
(Å)
(deg)
a
[DMBI]PF6
C2-H2‚‚‚F4
0.95
2.20
2.58
2.54
2.52
2.56
2.59
2.64
2.59
2.56
2.60
2.52
2.64
2.63
2.41
3.083(3)
3.462(3)
3.171(3)
3.387(3)
3.252(3)
3.468(3)
3.530(3)
3.460(3)
3.533(3)
3.285(3)
3.423(3)
3.529(3)
3.512(3)
3.334(3)
153.5
154.7
124.3
151.0
130.1
153.2
155.8
152.8
170.9
127.1
153.6
155.3
154.4
164.6
C4-H4‚‚‚F2 (a)
C4-H4‚‚‚F4 (a)
C5-H5‚‚‚F1 (b)
C5-H5‚‚‚F5 (b)
C12-H12‚‚‚F1 (b)
C16-H16‚‚‚F5
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.98
0.98
0.98
0.95
0.95
0.95
C16-H16‚‚‚F6
C17-H17B‚‚‚F1 (b)
C17-H17C‚‚‚F1 (c)
C17-H17C‚‚‚F3 (c)
C22-H22‚‚‚F3 (a)
C23-H23‚‚‚F2 (d)
C26-H26‚‚‚F4
b
[DMBI]BF4
C2-H2‚‚‚F3
0.95
2.33
2.58
2.65
2.51
2.53
2.53
2.45
2.55
2.59
2.57
2.45
3.145(4)
3.126(3)
3.320(4)
3.279(4)
3.201(4)
3.387(4)
3.319(4)
3.352(3)
3.474(4)
3.398(3)
3.306(4)
143.8
116.8
128.0
137.6
127.9
149.5
151.4
137.4
155.9
146.3
145.9
C4-H4‚‚‚F1 (a)
C5-H5‚‚‚F4 (b)
C12-H12‚‚‚F2 (b)
C14-H14‚‚‚F1 (c)
C15-H15‚‚‚F1 (d)
C16-H16‚‚‚F4
0.95
0.95
0.95
0.95
0.95
0.95
1.00
0.95
0.95
0.98
C20-H20‚‚‚F1 (a)
C24-H24‚‚‚F2 (e)
C26-H26‚‚‚F1 (a)
C27-H27A‚‚‚F3
a Symmetry transformations used to generate equivalent atoms: (a)
1
1
1
1
x + /2, -y + /2, -z + 1; (b) -x + /2, -y + 1, z - /2; (c) -x, y +
1/2, -z + /2; (d) -x + /2, -y, z - /2. b Symmetry transformations
3
1
1
1
1
used to generate equivalent atoms: (a) -x + /2, -y, z - /2; (b) -x
1
1
1
1
+ 1, y + /2, -z + /2; (c) x + 1, y, z; (d) x + /2, -y - /2, -z + 1;
1
1
(e) -x + /2, -y - 1, z - /2.
Figure 2. View of the crystal structure of (A) [DMBI]BF4 along the
crystallographic y axis and (B) [DMBI]PF6 along the crystallographic
z axis. Hydrogen atoms have been omitted for clarity; BF4 and PF6
solid state can be represented as {[(DAI)3(X)]2+[(DAI)(X)3]2-}n,
where DAI is the dialkylimidazolium cation and X is the anion.
Solution Structures. Previous 1H NMR studies showed that
[BMI]BPh4 behaves as a contact ion pair in CDCl3 and that
these ion pairs form floating aggregates through hydrogen
bonds.15 Figure 5 shows the chemical-shift dependence of the
-
-
anions are shown as tetrahedra and octahedra, respectively.
structure of the analogous [BMI]BPh4 salt.15 However, the steric
congestion imposed by MB side chains restricts the effective
approximation of the phenyl rings of the anions to the NCHN
hydrogen to form a hydrogen bond.
1
imidazolium hydrogens observed in the H NMR spectra of
The three-dimensional arrangement of the imidazolium
cations is formed through chains of the imidazolium rings with
weak π‚‚‚π stacking interactions. This molecular arrangement
generates channels in which the tetraphenylborate anions are
accommodated as chains (Figure 4). A similar arrangement was
observed in the structure of the analogous [DMBI]BF4 and
[DMBI]PF6 salts (Figure 2).
The conformation adopted by the two MB moieties in
[DMBI]BPh4 is rather different from those found in compounds
of the analogous series [DMBI]BF4 and [DMBI]PF6. The
general trend observed in these series was that an increase in
anion size reflects a change in conformation of the two MB
groups to reach a more efficient packing, from stacked phenyl
groups in [DMBI]BF4 to anti-position phenyl groups in [DMBI]-
BPh4. The torsion angles of phenyl groups are 8.67(24)° for
[DMBI]BF4 (C21-C20-C10-C11), 28.18(24)° for [DMBI]-
PF6 (C21-C20-C10-C11), and 161.68(30)° (C51-C50-
C60-C61) and 150.29(29)° (C41-C40-C30-C31) for the two
independent structures found in [DMBI]BPh4.
[BMI]BPh4 and [DMBI]BPh4 in CDCl3 with the concentration.
As can be seen in Figure 5, when we change the N-alkyl
groups in [BMI]BPh4 for the relatively bulky MB group in
[DMBI]BF4, the shielding effect in the imidazolium anions
decreases dramatically. This is basically due to the steric
hindrance of the MB group, which avoids close contact between
the anion and cation, restricting the C-H‚‚‚π interaction. It is
noteworthy, that no significant C-H‚‚‚π interaction involving
the H2 imidazolium hydrogen was detected in the crystals by
X-ray diffraction analysis (Figure 3). However, this C-H‚‚‚π
interaction is apparently present in solution and occurs prefer-
entially with the H2 hydrogen in concentrations up to 46 mmol
L-1. For higher concentration, H4 and H5 appear downfield to
the H2. This result corroborates our previous observations
obtained for [BMI]BPh4, for which contact ionic pair formation
occurs via H2 in low concentrations and via H4 and H5 for
higher concentrations.15
Figure 6 shows the H2 chemical-shift dependence with the
concentration for [BMI]BPh4 and [DMBI]BPh4 in CD3CN
solutions. The 1H NMR spectra of [BMI]BPh4 and [DMBI]BPh4
in CD3CN show signals shifted to higher frequencies than those
obtained in CDCl3 (for example, 90 mmol L-1 CD3CN solutions
of [BMI]BPh4 and [DMBI]BPh4 shows H2 signals at 8.0 and
8.8 ppm, and in CDCl3 solutions, these peaks appear at 4.4 and
The monomeric unit, constituted by at least one cation
surrounded by three anions and vice versa, is apparently
independent of the nature of imidazolium substituents or on the
type of anion (halogen, BF4, PF6, BPh4, etc.). Therefore, the
general structural organization of the imidazolium salts in the

4344 J. Phys. Chem. B, Vol. 109, No. 10, 2005
Consorti et al.
Figure 3. View of the close contacts between the imidazolium cation and the BPh4 anions (only the relevant parts shown) with the hydrogen bonds
with the phenyl centroid indicated (bond distances and angles summarized in Table 2) of the two molecules (A, left, and B, right) founded in the
unit cell.
TABLE 2: Nearest C-H‚‚‚π Ring Interactions in Solid
in CHCl3 solution, the conductance decreases continuously with
the dilution, indicating the existence of equilibrium between a
contact ion pair and charged aggregates. At higher salt
concentrations, the equivalent conductance in CHCl3 begins to
fall as a consequence of higher aggregate formation. Measure-
ments in mixtures of the solvents CHCl3 and CH3CN, with
slightly higher dielectric constants (2.25 acetonitrile wt %),
a
State for [DMBI]BPh4
d(C-H) d(H‚‚‚π) d(C‚‚‚π) angle (CH-π)
C-H‚‚‚π
(Å)
(Å)
(Å)
(deg)
[DMBI]BPh4 (A)
C40-H40‚‚‚π1
1.00
2.62
2.84
2.94
2.97
3.589(4)
3.749(10)
3.521(4)
3.816(4)
163.8
160.8
119.2
149.1
C36-H36‚‚‚π2
0.95
0.98
0.95
C37-H37B‚‚‚π3 (a)
C14-H14‚‚‚π4 (b)
however, show a minimum conductivity point at 3 mmol L-1
.
This behavior has been described for tetraalkylammonium salts
for which a contact ion pair and solvent-separated ion pair
equilibria are predominant at low concentrations and equilibria
between charged triple ions and ion pairs at higher concentra-
tions.28
[DMBI]BPh4 (B)
C50-H50‚‚‚π1 (c)
C67-H67C‚‚‚π3 (d)
C24-H24‚‚‚π4 (e)
C66-H66‚‚‚π2 (f)
1.00
2.71
2.79
2.83
2.88
3.657(4)
3.469(4)
3.713(4)
3.762(10)
157.6
126.9
155.7
155.9
0.98
0.95
0.95
Scheme 2 shows a simplified model in which different species
can coexist in equilibrium at different concentrations. Such
multiple-ion formation can explain the conductivity behavior
observed with ILs.
a The centroids are of the following atoms with symmetry operations:
(a) 1 - x, -0.5 + y, 2 - z; (b) 2 - x, -0.5 + y, 2 - z; (c) x, -1 +
y, z; (d) 1 - x, -0.5 + y, 1 - z; (e) 2 - x, -0.5 + y, 1 - z; (f) x, -1
+ y, z.
With this multiple-ion model it is possible to understand the
minimum observed in the equivalent conductivity versus
concentration curve, which corresponds to a region where the
two equilibria coexist with the same strength. This behavior is
not limited to BMI cations. It is also observed for [DMBI]BPh4,
and it is expected to be a general property of imidazolium salts
as shown in Figure 8. The equivalent conductivity versus
concentration data obtained for [BMI]PF6, [BMI]BF4, [DMBI]-
PF6, and [BMI]BF4 in CHCl3/CH3CN 2.8 wt % solutions show
identical trends.
6.2 ppm). As can be seen in Figure 6, the range of variation of
these chemical shifts in CD3CN with the concentration (0.5 ppm
for [BMI]BPh4 and 0.2 ppm for [DMBI]BPh4) is not as
important as those observed in CDCl3 (up to 1.0 and 0.5 ppm,
respectively). These results indicate that, in contrast to CDCl3
solutions, [BMI]BPh4 and [DMBI]BPh4 dissolved in CD3CN
behave most probably as solvent separated ion pairs. Notewor-
thy, evidence for hydrogen bonds between the imidazolium cat-
ion and the tetrafluoroborate anion has also been obtained by
NMR investigations with [EMI]BF4 (1-ethyl-3-methylimidazo-
lium tetrafluoroborate) IL26 and for [BMI]BF4 in acetonitrile.27
The formation of IL aggregates was also investigated by
microcalorimetry. At low concentrations (the infinite dilution
limit), the solution enthalpies of these compounds may be
expressed as the sum of different contributions (eq 1)
1
The association process shown by H NMR can be also
conveniently studied by conductometry. The conductivity of the
salts solutions, in different compositions was measured at 25
°C using different mixtures of CHCl3 and CH3CN, to control
the dielectric constant of the medium. The behavior of [BMI]-
BPh4 solutions is shown in Figure 7, where the equivalent
conductance versus IL concentration in different solvents is
plotted.
It can be seen from Figure 7 that the addition of CH3CN to
a CHCl3 solution of [BMI]BPh4 increases the conductance
dramatically. For CH3CN solutions, the equivalent conductance
of the IL solution increases at lower concentrations, as expected
for the ionization of the imidazolium salt with the dilution, but
∆Hsol ) ∆Hion-ion + ∆Hsolv-solv + ∆Hion-solv
(1)
where the first two terms represent, respectively, the enthalpy
change for separation of the ions and of the solvent molecules
(both positive) and the last one reflects the enthalpy of the ion-
solvent interaction, expected to be negative. A positive enthalpy
of solution would infer that the sum of the two processes is
more energetic than the contribution from ion solvation, and a
negative value represents the opposite situation.
As the imidazolium salt concentration is increased, the
formation of aggregates may occur, as previously discussed

1,3-Dialkylimidazolium Salt Supramolecular Aggregates
J. Phys. Chem. B, Vol. 109, No. 10, 2005 4345
Figure 4. (A) Packing diagram of [DMBI]BPh4 showing alternating columns parallel to the crystallographic z axis of the DMBI cations and BPh4
anions. (B) Side view of the columns.
TABLE 3: Summary of the Crystal Data and Structure Refinement for [DMBI]BF4 and [DMBI]PF6
[DMBI]BF4
[DMBI]PF6
[DMBI]BPh4
C43H41BN2
empirical formula
formula weight
temperature
crystal system
space group
C19H21BF4N2
C19H21F6N2P
364.19
150(2) K
orthorhombic
P212121
422.35
100(2) K
orthorhombic
P212121
596.59
100(2) K
monoclinic
P21
unit cell dimensions
a ) 10.7207(3) Å,
b ) 10.8545(2) Å,
c ) 15.4841(4) Å
a ) 10.2149(5) Å,
b ) 12.5025(4) Å,
c ) 15.0057(6) Å
a ) 8.484(5) Å,
b ) 19.347(5) Å,
c ) 19.586(5) Å,
â ) 90.506(5)°
3215(2) Å3
4
volume
Z
density (calculated)
absorption coefficient
F(000)
1801.85(8) Å3
4
1916.40(14) Å3
4
1.343 Mg/m3
1.464 Mg/m3
1.233 Mg/m3
0.070 mm-1
1272
0.107 mm-1
0.207 mm-1
760
872
crystal size
θ range for data collection
index ranges
0.30 × 0.30 × 0.25 mm3
2.63-27.49°
-13 < h < 13,
-14 < k < 14,
-20 < l < 20
22 610
0.20 × 0.10 × 0.05 mm3
4.21-27.49°
-13 < h < 13,
-16 < k < 16,
-19 < l < 19
4379
2.96-26.35°
0 < h < 10,
0 < k < 23,
-23 < l < 23
6087
reflections collected
independent reflections
completeness
absorption correction
maximum and minimum transmission
data/restraints/parameters
goodness-of-fit on F2
2366 [R(int) ) 0.024]
4379 [R(int) ) 0.0000]
6087
89.9%
multiscan (Denzo-SMN)
99.7%
99.5%
multiscan (Denzo-SMN)
0.974 and 0.968
2345/0/238
multiscan (Denzo-SMN)
0.990 and 0.960
4379/0/257
6087/1/834
1.032
1.042
0.883
final R indices (I > 2σ(I))
R1 ) 0.0464
wR2 ) 0.1166
R1 ) 0.0530, wR2 ) 0.1210
0.2(13)
R1 ) 0.0434
wR2 ) 0.0736
R1 ) 0.0897, wR2 ) 0.0818
0.16(11)
R1 ) 0.0461
wR2 ) 0.1036
R1 ) 0.0692, wR2 ) 0.1112
-3(3)
R indices (all data)
absolute structure parameter
extinction coefficient
0.010(2)
0.0049(7)
0.0125(12)
largest difference peak and hole
0.57 and -0.47 e Å-3
0.210 and -0.192 e Å-3
0.208 and -0.207 e Å-3
based on conductivity and NMR measurements, which will
contribute to an increase or decrease of the observed enthalpy
of solution. Again, whether ion-pair formation leads to an
increase or decrease in the system enthalpy would depend on
the balance between ion-solvent interactions (which are being
disrupted) and ion-ion interactions (which are being formed).
Moreover, previously discussed data have suggested the pos-
sibility of further ion association, with the formation of triple
ions and higher aggregates, because of hydrogen bonding. These
further interactions would add another term to the observed
enthalpy of solution (eq 1), which again may lead to an increase
or decrease in enthalpy, depending on the balance between
interactions that are disrupted and the ones that are formed.
This picture may be better understood by analyzing the data
shown in Figures 10 and 11. These data represent the changes
of solution enthalpies of four imidazolium salts in chloroform.

4346 J. Phys. Chem. B, Vol. 109, No. 10, 2005
Consorti et al.
Figure 7. Equivalent conductance of [BMI]BPh4 solutions in (2)
CHCl3, (9) CHCl3/CH3CN (2.25 wt % CH3CN), (O) CHCl3/CH3CN
(4.45 wt % CH3CN), and (4) CH3CN at various IL concentrations.
Figure 5. Variation of the 1H NMR chemical shifts of the imidazolium
cations with concentration in CDCl3. (2) H4 and H5 and (9) H2 in
[DMBI]BPh4 and (3) H4, (0) H5, and (O) H2 in [BMI]BPh4.
Figure 8. Conductivity of (9) [BMI]PF6, (2) [BMI]BF4, (4) [DMBI]-
PF6, and (0) [DMBI]BF4 in CHCl3/CH3CN (2.8 wt % in CH3CN).
direction, leading to lower enthalpy values for the first two
imidazolium salts and to an increase in enthalpy for the latter.
Again, these trends are consistent with different contributions
from solvation and formation of charged triple ions. As the
concentration is further increased, another deviation is observed
in these curves, reversing the enthalpy variation and suggesting
the appearance of another process in solution. According to the
previous discussion, we propose that this is the formation of
higher aggregates. For [DMBI]PF6 and [DMBI]BF4, the forma-
tion of aggregates is detected at ca. 25 mmol L-1, while it is
observed at a larger concentration, ca. 110 mmol L-1, for [BMI]-
BF4.
1
Figure 6. Variation of H NMR chemical shifts of the imidazolium
cation ([DMBI]BPh4 and [BMI]BPh4) with the concentration in CD3-
CN. (9) H2 [BMI]BPh4 and (2) H2 [DMBI]BPh4.
Apart from the measurements with [BMI]PF6, for which,
because of its low solubility, only a few data points could be
obtained, all of the other three curves show a constant enthalpy
of solution at low concentrations. These values are positive for
salts [DMBI]PF6 and [DMBI]BF4, which possess chemically
related cations, indicating that the sum of the enthalpies because
of the two first processes of eq 1 is greater than the absolute
value for the enthalpy change because of ion solvation by
chloroform. For [BMI]PF6 and [BMI]BF4, the solution enthal-
pies are negative, indicating the opposite situation and a more
energetic ion solvation. As the concentration increases, these
enthalpy values start to change. This occurs at ca. 7, 11, and
60 mmol L-1 respectively for [DMBI]BF4, [DMBI]PF6, and
[BMI]BF4. Interestingly, these changes occur in an opposite
The data presented in Figure 11, for the solution of [BMI]-
BPh4 and [DMBI]BPh4 in acetonitrile and chloroform, allow
the assessment of solvent contributions to these solution
processes. The solution enthalpies measured in acetonitrile are
more positive, as a consequence of higher cohesive energy for
this more polar solvent, which increases the enthalpy change
because of the separation of solvent molecules (as represented

1,3-Dialkylimidazolium Salt Supramolecular Aggregates
J. Phys. Chem. B, Vol. 109, No. 10, 2005 4347
Figure 9. ∆Hsol changes of (4) [DMBI]PF6 and (0) [DMBI]BF4 in
chloroform with the imidazolium salt concentration.
Figure 11. ∆Hsol changes of (9) [BMI]BPh4 and (4) [DMBI]BPh4 in
acetonitrile and (2) [BMI]BPh4 and (0) [DMBI]BPh4 in chloroform.
the detection of supramolecular floating aggregates by NMR,
conductometry, and microcalorimetry. Of note is that these
structures (possessing mono-, di-, and tricharged species) can
be maintained even in the gas phase as observed by fast atomic
bombardment (FAB)29 and electrospray ionization (ESI)30 mass
spectrometry experiments. Therefore, imidazolium ILs mixed
with other molecules may be regarded as nanostructured
materials. This model, involving supramolecular aggregates, can
account for most of the unique physical-chemical properties
such as an increase in their electrical conductivity in the presence
of nonpolar molecules (carbon dioxide),31 increasing the dif-
fusion coefficient of both polar and nonpolar compounds by
the addition of controlled amounts of water in imidazolium
salts,13 dual behavior when used as a stationary phase for gas
chromatography,14 “polarity”,32 proton transfer “support” prop-
erties when used as electrolytes for fuel cells,33 miscibility with
both polar and nonpolar molecules,34 steric and electronic
protecting properties for nanoparticles,35 stabilization of en-
zymes,36 support properties for liquid membranes,37 the forma-
tion of inclusion compounds with aromatic molecules,38 etc.
Figure 10. ∆Hsol changes of (2) [BMI]PF6 and (9) [BMI]BF4 in
chloroform with the imidazolium salt concentration.
in eq 1). The data for solution in chloroform conform to the
pattern observed in Figure 9, whereas those obtained in
acetonitrile do not show any significant change in solution
enthalpies, within the investigated concentration range, sug-
gesting that because of its greater ability to solvate ions (as
measured, for instance, by its dielectric constant), no ion pairing
is observed.
In summary, 1,3-dialkylimidazolium salts form supramol-
ecules in solid state through hydrogen bonds. The monomeric
unit is composed of at least three cations associated with one
anion and one cation with at least three anions. These hydrogen-
bonded structures are maintained to a great extent even in
solution in solvents of low dielectric constant, evidenced through
Experimental Section
General Considerations. All preparative procedures were
carried out under dry argon using standard Schlenk tube
techniques. The solvents were distilled from appropriate drying
agents under argon. The NaBPh4, NaBF4, and NaPF6 salts were
used as purchased from Aldrich Chemical Co. 1-n-butyl-3-
methylimidazolium chloride,16 1-n-butyl-3-methylimidazolium
tetrafluoroborate,16 1-n-butyl-3-methylimidazolium hexafluo-
rophosphate,16 and 1-n-butyl-3-methylimidazolium tetraphe-
nylborate15 were prepared as described earlier. Infrared (Nujol
mulls or KBr pellets) spectra were recorded using a Bomem
B-102 spectrometer. The 1H and 13C NMR spectra were
SCHEME 2: Multiple-Ion Equilibria at Different Ionic Concentrations

4348 J. Phys. Chem. B, Vol. 109, No. 10, 2005
Consorti et al.
recorded on a Varian Inova 300 instrument. Conductivity
measurements were carried out in a Schlenk tube using a
commercial platinum electrode calibrated at 298 K by using 1
mol L-1 aqueous KCl solution and performed with a CG 859
Schott Gerate Wheatstone bridge.39 Calorimetric experiments
were carried out in a 1200 PL-DSC, under nitrogen flux, with
a heating rate varying from 1 to 10 °C. Enthalpies of solution,
∆Hsol, for the imidazolium salts in different solvents were
measured using the Thermometric 2225 Solution Calorimeter
(Thermometric AB, Sweden), operating at 25 °C, according to
the instructions of the manufacturer. This unit was inserted into
the water bath of another calorimeter (Thermometric 2277), with
temperature stability of (0.0001 °C. Glass ampules were loaded
with 3-4 mg of RTIL and sealed by a flame, keeping the
ampules under controlled cooling. These ampules were then
broken into a 100 mL vial, containing pure solvent or RTIL
solution. The measured enthalpy changes were corrected for
an almost negligible endothermic effect because of the breaking
of ampules, which was discounted based on measurements using
empty ampules. Each value reported represents an average of,
at least, two measurements. Observed deviations are smaller
than 1%. Measuring the solution enthalpies of KCl in water
produced values within 0.8% of those in the literature,40
validating the whole procedure.
(C); 136.6 (C2); 129.3 (Co); 129.2 (Cm); 126.8 (Cp); 121.0
(C4 and C5); 60.0 (NCH); 20.5 (CH3).
1-3-Di(R-(+)-methylbenzyl)imidazolium Hexafluorophos-
phate ([DMBI]PF6). Potassium hexafluorophosphate (1.16 g,
6.2 mmol) and 1-3-di(R-(+)-methylbenzyl)imidazolium chloride
(1.9 g, 6.1 mmol) were diluted in 5 mL of acetone. After stirring
at room temperature for 24 h, the sodium chloride was filtered
off and the solution was concentrated giving a pale yellow solid.
The residue was crystallized from dichloromethane/hexane.
25
Yield: 92% (2.37 g, 5.61 mmol). Mp ) 108 °C. [R]D ) +
13.9° (C ) 0.2.22 g/100 mL; CH2Cl2). IV(KBr): 3155, 3109,
and 3064 cm-1 (νC-H aromatic); 2995 and 2950 cm-1 (νC-H
aliphatic); 841 cm-1 (νP-F); 556 cm-1 (δF-P-F). 1H NMR
(CDCl3): 8.82 (s, 1H, H2); 7.38 (m, 5H, Ho, Hm, and Hp);
7.20 (m, 2H, H4 and H5); 5.67 (q, 1H, NCH, 3JHH ) 7.0 Hz);
3
1.94 (d, 3H, CH3, JHH ) 7.0 Hz). 13C NMR (CDCl3): 137.4
(C); 133.2 (C2); 129.4 (Co and Cm); 126.8 (Cp); 121.1 (C4
and C5); 60.2 (NCH); 20.5 (CH3).
1-3-Di(R-(+)-methylbenzyl)imidazolium Tetraphenylbo-
rate ([DMBI]BPh4). The same procedure was used as for
[DMBI]PF6. [DMBI]BPh4 was obtained starting from [DMBI]-
Cl (1.07 g, 3.26 mmol) and NaBPh4 (1.8 g, 5.2 mmol).Yield:
93% (2.94 g, 4.93 mmol). Mp ) 176 °C. [R]D25 ) + 16.6° (C
) 1.75 g/100 mL; CH2Cl2). IV(KBr): 3153, 3132, 3117, 3080,
3055, 3035, and 3013 cm-1 (νC-H aromatic); 2998 and 2983
Single-crystal determinations were done on a Nonius κ-CCD
single-crystal diffractometer with ψ and ω scans with κ offsets
using Mo KR radiation (0.710 73 Å). Data reduction with Lorenz
and polarization corrections was made using Denzo/SMN.41
SIR9742 or SHELXS43 were used for data solution, and
SHELXL44 was used for data refinement.
1
cm-1 (νC-H aliphatic). H NMR (CDCl3): 7.48 (m, 16H, Co
and Cm, BPh4); 7.42 (m, 5H, Ho, Hm, and Hp, Ph); 6.93 (m,
4H, Cp, BPh4); 6.26 (s, 1H, H2); 6.15 (m, 2H, H4 and H5);
3
3
4.70 (q, 1H, NCH, JHH ) 7.0 Hz); 1.45 (d, 3H, CH3, JHH
)
7.0 Hz). 13C NMR (CDCl3): 163.7 (C, BPh4); 137.3 (C, Ph);
136.0 (Co, BPh4); 132.8 (C2); 129.3 (Co, Ph); 129.2 (Cm, Ph);
126.1 (Cp, Ph); 125.8 (Cm, BPh4); 122.0 (Cp, BPh4); 120.2 (C4
and C5); 59.2 (NCH); 21.0 (CH3).
1-3-Di(R-(+)-methylbenzyl)imidazolium Chloride ([DM-
BI]Cl). To a stirred mixture of R-(+)-methylbenzylamine (8.3
g, 69.4 mmol) and toluene (20 mL) at 0 °C, formaldehyde (3.95
mL, 34.7 mmol) was added over the course of 10 min. After
the addition, the mixture was warmed to room temperature,
stirred for 30 min, and lowered again to 0 °C. HCl (2.6 mL,
34.7 mmol) and subsequently glyoxal (2.85 mL, 34.72 mmol)
were added to the reaction mixture. After a reflux for 24 h, the
solvent was removed in vacuo and 30 mL of dichloromethame
was added. After filtration, the solvent was removed in vacuo.
The residue was crystallized from acetone at 0 °C. Yield: 70%
(7.6 g, 24.3 mmol). Mp ) 199 °C. [R]D25 ) + 29.8° (C ) 1.91
g/100 mL; CH2Cl2). IV (KBr): 3138, 3097, and 3057 cm-1
(νC-H aromatic); 2961, 2924, 2867, and 2801 cm-1 (νC-H
Acknowledgment. Thanks are due to CNPq, FAPERGS, and
FAPESP for partial financial support and to NSERC (Canada)
for financial support for the diffractometer. We also thank CNPq
for a fellowship to C. S. C.
Supporting Information Available: Tables of full crystal
data, atomic coordinates, calculated hydrogen coordinates,
anisotropic thermal parameters, a complete list of bond lengths
and angles, and CIF files. This material is available free of
charge via the Internet at http://pubs.acs.org.
1
aliphatic). H NMR (CDCl3): 10.4 (s, 1H, H2); 8.11 (s, 2H,
References and Notes
H4 and H5); 7.54 (m, 2H, Ho); 7.37 (m, 3H, Hm and Hp); 5.96
3
3
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25
Yield: 90% (2.06 g, 5.67 mmol). Mp ) 110 °C. [R]D ) +
17.1° (C ) 1.46 g/100 mL; CH2Cl2). IV(KBr): 3176, 3156,
3116, 3098, and 3067 cm-1 (νC-H aromatic); 2988, 2980, and
2935 cm-1 (νC-H aliphatic); 1074 cm-1 (νB-F). 1H NMR
(CDCl3): 9.18 (s, 1H, H2); 7.37 (m, 5H, Ho, Hm, and Hp);
7.28 (m, 2H, H4 and H5); 5.73 (q, 1H, NCH, 3JHH ) 6.9 Hz);
3
1.94 (d, 3H, CH3, JHH ) 7.0 Hz). 13C NMR (CDCl3): 137.6

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