Pyrazolium- versus imidazolium-based ionic liquids: Structure, dynamics and physicochemical properties

Article abstract of DOI:10.1021/jp3107793

Ionic liquids (ILs) composed of two different pyrazolium cations with dicyanamide and bis(trifluoromethanesulfonyl)imide anions have been synthesized and characterized by NMR, Kamlet-Taft solvatochromic parameters, conductivity and rheological measurements, as well as ab initio calculations. Density functional calculations for the two pyrazolium cations, 1-butyl-2- methylpyrazolium [bmpz] and 1-butyl-2,3,5-trimethylpyrazolium [bm ₃pz], provide a full picture of their conformational states. Homo- and heteronuclear NOE show aggregation motives sensitive to steric hindrance and the anions' nature. Self-diffusion coefficients D for the anion and the cation have been measured by pulsed field gradient spin-echo NMR (PGSE-NMR). The ionic diffusivity is influenced by their chemical structure and steric hindrance, giving the order D_cation > D_anion for all of the examined compounds. The measured ion diffusion coefficients, viscosities, and ionic conductivity follow the Vogel-Fulcher-Tammann (VFT) equation for the temperature dependencies, and the best-fit parameters have been determined. Solvatochromic parameters indicate an increased ion association upon going from bis(trifluoromethanesulfonyl)imide to dicyanamide-based pyrazolium salts, as well as specific hydrogen bond donor capability of H atoms on the pyrazolium ring. All of these physical properties are compared to those of an analogous series of imidazolium-based ILs.

Full text of DOI:10.1021/jp3107793

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Article
Pyrazolium- Versus Imidazolium-Based Ionic Liquids:
Structure, Dynamics and Physico-Chemical Properties
Cinzia Chiappe, Angelo Sanzone, Daniele Mendola, Franca
Castiglione, Antonino Famulari, Guido Raos, and Andrea Mele
J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp3107793 • Publication Date (Web): 19 Dec 2012
Downloaded from http://pubs.acs.org on December 28, 2012
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Pyrazoliumꢀ versus Imidazoliumꢀbased Ionic
Liquids: Structure, Dynamics and Physicoꢀchemical
Properties
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Cinzia Chiappe,*1 Angelo Sanzone,1 Daniele Mendola,2 Franca Castiglione,2 Antonino
Famulari,2 Guido Raos,2 and Andrea Mele*2,3
1
Dipartimento di Chimica e Chimica Industriale ꢀ Università di Pisa, Via del Risorgimento, 35ꢀ
2
56126 Pisa, Italy. Eꢀmail: cinziac@farm.unipi.it Department of Chemistry, Materials and
Chemical Engineering “G. Natta”, Politecnico di Milano, Piazza L. Da Vinci 32, 20132 Milano,
Italy. 3 CNR – Istituto di Chimica per il Riconoscimento Molecolare, Via L. Mancinelli, 7, 20131
Milano; Eꢀmail:andrea.mele@polimi.it
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if
required according to the journal that you are submitting your paper to)
* Author to whom correspondence should be addressed. Eꢀmail: cinziac@farm.unipi.it. Ph.: +39ꢀ
050ꢀ2219669. Fax: +39ꢀ050ꢀ2219660. Eꢀmail: andrea.mele@polimi.it. Ph.: +39ꢀ02ꢀ23993006.
Fax: +39ꢀ02ꢀ23993180.
ABSTRACT. Ionic liquids (ILs) composed of two different pyrazolium cations with dicyanamide
and bis(trifluoromethanesulfonyl)imide anions have been synthesized and characterized by
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NMR, KamletꢀTaft solvatochromic parameters, conductivity and rheological measurements, as
well as ab initio calculations. Density functional calculations for the two pyrazolium cations, 1ꢀ
butylꢀ2ꢀmethylpyrazolium [bmpz] and 1ꢀbutylꢀ2,3,5ꢀtrimethylpyrazolium [bm3pz] provide a full
picture of their conformational states. Homoꢀ and heteronuclear NOE show aggregation motives
sensitive to steric hindrance and anions’ nature. Selfꢀdiffusion coefficients D for the anion and
the cation have been measured by pulsed field gradient spinꢀecho NMR (PGSEꢀNMR). The ionic
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diffusivity is influenced by their chemical structure and steric hindrance giving the order Dcation
>
Danion for all the examined compounds. The measured ion diffusion coefficients, viscosities and
ionic conductivity follow the VogelꢀFulcherꢀTammann (VFT) equation for the temperature
dependencies and the best fit parameters have been determined. Solvatochromic parameters
indicate an increased ion association on going from bis(trifluoromethanesulfonyl)imide to
dicyanamideꢀbased pyrazolium salts, as well as specific hydrogen bond donor capability of H
atoms on the pyrazolium ring. All these physical properties are compared to those of an
analogous series of imidazoliumꢀbased ILs.
Keywords: pyrazolium, ionic liquids, solvatochromic parameters, NOE, diffusion, DFT
calculations.
1. Introduction
Over the past ten years, room temperature ionic liquids (ILs) have aroused overwhelming
interest due to their unique physicoꢀchemical properties which make them useful in a broad range
of applications: synthesis, (bio)catalysis, separation processes, electrochemistry, materials
science, biotechnology and for the development of new electrical and electrochemical devices.1
Generally composed by a bulky organic cation and a polyatomic anion, ILs are characterized by
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an extreme flexibility and modularity: it is possible to tailor their properties for specific
applications through a suitable choice of the component ions. In recent years, new ILs have been
designed and synthesized to achieve important targets such as obtaining liquid salts characterized
by low viscosity, high conductivity, high electrochemical stability associated with low
flammability and low volatility. Despite of the wide selection of cations and anions giving ILs,
those based on 1,3ꢀdialkylimidazolium cations have dominated the field, probably due to their
wide liquid range, high conductivities and low viscosities which are necessary for a multitude of
applications.2 The remarkable number of studies carried out on these salts have shown that their
properties can be finely modulated by varying the alkyl groups, introducing specific groups on
the cation or changing the nature of the anion. Moreover, experimental measurements and
theoretical calculations have demonstrated that, depending on anion structure, imidazoliumꢀ
based ILs can give highly structured ion networks,3 in which polar and nonꢀpolar domains can be
formed as a function of the cation’s alkyl chain lengths.4 A high degree of structural and dynamic
heterogeneity has been evidenced both in the bulk and at the interfaces of these ILs: the highly
cohesive charged groups tend to segregate from the side chains if these are sufficiently long,
resulting in liquids with a heterogeneous structure at the nanometer scale, offering both ionic and
nonꢀpolar nanoꢀscale environments. Experiments carried out on other classes of ILs show that the
degree of selfꢀorganization characterizing these media depends however also on the cation and,
for example, it decreases on going from imidazolium to pyrrolidinium salts.5
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Pyrazoliumꢀbased ILs have been only marginally investigated6ꢀ7 although these salts,
structurally analogous to imidazolium basedꢀILs, are generally characterized by a relatively low
viscosity and a high conductivity, associated with relevant electrochemical properties. From the
structural viewpoint, the different position of the nitrogen atoms in the heteroaromatic ring
compared to the imidazolium series is expected to influence the charge distribution and
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consequently the nanostructuration (if any) of pyrazolium based ILs8, thus making these systems
appealing in our search for new modes of local ordering. This may prove to be useful for their
applications as reaction media or as electrolytes in electrochemical devices.
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Scheme 1. Pyrazolium basedꢀILs
We have synthesized four different alkylꢀsubstituted pyrazoliumꢀbased ILs: 1ꢀbutylꢀ2ꢀ
methylpyrazolium
bis(trifluoromethanesulfonyl)imide
([bmpz][Tf2N],
1),
1ꢀbutylꢀ2ꢀ
methylpyrazolium dicyanamide ([bmpz][N(CN)2], 2), 1ꢀbutylꢀ2,3,5ꢀtrimethylpyrazolium
bis(trifluoromethanesulfonyl)imide ([bm3pz][Tf2N], 3) and 1ꢀbutylꢀ2,3,5ꢀtrimethylpyrazolium
dicyanamide ([bm3pz][N(CN)2], 4). Structures and atom numbers are shown in Scheme 1. We
report the physicochemical characterization of these new systems and the comparison with the
properties of analogous imidazolium salts. The structural and physicochemical properties are
explored using modern twoꢀdimensional NMR techniques and quantum chemical ab initio
calculations. The main focus of the paper is the explanation of the ion transport properties,
linking the observed cation/anion structure and interactions to the measured ionic diffusivity,
viscosity and conductivity.
2. Experimental Section
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2.1 IL Synthesis. The pyrazolium salts1ꢀ4 were synthesized by methylation of pyrazole and 3,5ꢀ
dimethylpyrazole with dimethylcarbonate in the presence of sodium methoxide at the reflux
temperature, followed by alkylation of the resulting Nꢀmethyl derivatives with butyliodide, as
shown in Scheme 2. Anion exchange on 1ꢀbutylꢀ3ꢀmethylpyrazolium iodide and 1ꢀbutylꢀ2,3,5ꢀ
trimethylpyrazoliumiodide was carried out in water using LiTf2N or freshly prepared
Ag[N(CN)2], according to literature procedures. The synthesized salts were characterized by 1H
NMR, 13C NMR and ESIꢀMS.
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Scheme 2. Synthesis of the ILs.
2.2 NMR Measurements. All ionic liquids were dried for 5 h at 70 °C under mechanic pump
vacuum before NMR experiments. NMR spectra were carried on a Bruker Avance 500
spectrometer operating at 500 MHz proton frequency and equipped with a QNP four nuclei
switchable probe. Suitable amounts (0.5ꢀ1.0 g each) of dry 1, 2 and 3 were transferred in a 5 mm
NMR tube. A sealed capillary containing DMSOꢀd6 was used as internal chemical shift
reference. The NMR tubes were flameꢀsealed immediately after the transfer of ionic liquids. {1Hꢀ
19F}HOESY experiments9ꢀ10 were acquired using the inverse detected pulse sequence with 512
increments in the t1 dimension with 16 scans for each experiment, with a mixing time of 40 ms.
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Twoꢀdimensional correlation experiments by dipolar coupling in the rotating frame (ROESY)
were performed using a pulse sequence with 700 increments in the t1 dimension with 16 scans
for each experiment and a mixing time of 200 ms.
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Selfꢀdiffusion coefficients were measured by pulsed field gradient spinꢀecho NMR (PGSEꢀ
NMR) experiments. A pulsed gradient unit capable of producing magnetic field pulse gradients
in the zꢀdirection of 53 G cmꢀ1 was used. All the experiments were performed using the double
stimulated echo pulse sequence11 to suppress convection artifacts. The duration of the magnetic
field pulse gradients (δ) and the diffusion times (ꢀ) were optimized for each sample in order to
obtain complete dephasing of the signals with the maximum gradient strength. In each
experiment, a series of 32 spectra with 16K points were collected. For the investigated samples, δ
1
19
values were in the 2 ms and 6 ms ranges, respectively for H and F experiments, while the ꢁ
1
values were in the 0,4 s and 0,8 s ranges, respectively for H and 19F experiments. The pulse
gradients were incremented from 2 to 95% of the maximum gradient strength in a linear ramp.
Temperature was controlled with an air flow of 535 L hꢀ1 in order to avoid any temperature
fluctuations due to sample heating during the magnetic field pulse gradients. Variable
temperature experiments were performed changing the temperature from 300 K to 340 K in steps
of 5 K.
2.3 Conductivity, Viscosity and UV-VIS Measurements. Conductance measurements were
performed using a CON 510 bench meter supplied with conductivity/TDS electrode equipped
with a temperature sensor for automatic temperature compensation (cell constant of K = 1.0).
Viscosity was measured for all the dicyanamide samples using Brookfield DVꢀII + Pro
instrument. ESIꢀMS analyses were performed on a Finnigan LCQ Advantage (Thermo Finnigan,
San Jose, CA, USA) ion trap instrument equipped with an Excalibur software.
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Solvatochromic dyes like Reichardt’s betaine dye, N,Nꢀdiethylꢀ4ꢀnitroaniline and 4ꢀ
nitroaniline were dissolved in ionic liquids. These were taken in a quartz cell with light path
length of 1 mm on a Cary 2200 spectrophotometer (300ꢀ800 nm). Individual stock solutions of
Reichardt’s betaine dye, N,Nꢀdiethylꢀ4ꢀnitroaniline and 4ꢀnitroaniline were prepared in
dichloromethane. In order to prepare a given dye/ionic liquid solution, the appropriate amount of
the dye stock solution was micropipetted into a clean dry quartz cuvette. Residual
dichloromethane was accurately evaporated under gentle stream of argon gas. The CH2Cl2
removal was checked by NMR by monitoring the peak at 5.3 ppm. The ionic liquid was then
added to the cuvette. The cuvette was then capped and sealed and the sample was mixed for an
appropriate time before the experimental measurements.
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2.4 Computational Procedures. All calculations were performed with the GAMESSꢀUS
program.12 Stable conformations of the cations were obtained by density functional theory
(DFT)13. In particular, the B3LYP hybrid functional14 and the standard 6ꢀ311G** basis set15
were adopted. Three dihedral angles were defined in order to explore the conformational space
of the butyl chain: α1 (C(7)ꢀC(6)ꢀN(1)ꢀC(5)), α2 (C(8)ꢀC(7)ꢀC(6)ꢀN(1)) and α3 (C(9)ꢀC(8)ꢀC(7)ꢀ
C(6)), as shown in Figure 1. For all the dihedrals, the anti (α = 180°), gauche+ (α = +60°) and
gauche– (α = –60°) conformations were used as starting geometry and then fully minimized.
Accordingly, 27 conformations for cations 1 and 3 were generated. The conformers are
unambiguously identified by the triplet of α1, α2 and α3 values. The angles, energies and
populations of all conformations are given in Tables S1 and S2 of the Supplementary
Information (SI).
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Figure 1. Definition of the dihedral angles α1, α2 and α3 for the butyl chain of the examined
compounds. The pictures show their optimized minimumꢀenergy conformations.
Average interatomic distances, in which each conformation is weighted according to its
Boltzmann population (see below), were calculated in order to support the interpretation of NOE
data.16 Details are given in the SI.
3. Results and Discussion
3.1 DFT Calculations. The main purpose of the DFT calculations is to provide average
interatomic distances, suitable for the discrimination between interꢀ and intramolecular NOE
data.10 Nevertheless, the calculated structures deserve some comments considering that, at the
moment, no literature data are available on pyrazoleꢀbased systems, including single crystal Xꢀ
ray structures. At this stage of preliminary investigation on pyrazoleꢀbased ILs, it is interesting to
compare them with the related and extensively studied 1ꢀbutylꢀ3methylimidazoliumꢀbased
(bmim) analogues.
The DFT calculation on cations 1 and 3 show wellꢀdefined conformational properties of their
butyl chain. The populations of the different conformers may be calculated according to Eq. (1):
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α1
,
α2
,
α
3 ) / kT
α1
e−E(
) = ∑α ∑α ∑α
P(α1
,
α2
,
α3
(1)
e−E(
,
α2 ,α3 ) / kT
1
2
3
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,α2 ,α3 )
where k is Boltzmann’s constant, T is the absolute temperature (=298 K) and E(α1 is
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total energy of a specific conformation. From the Boltzmann probabilities, the distribution for a
single torsion angle (α1, for example) were calculated as:
P(α1 ) =
P(α1
3
,
α2
,
α3
)
(2)
∑α ∑α
2
These distributions are displayed in the forms of histograms in Figure 2. For both pyrazoleꢀbased
cations 1 and 3, α1 angle is centred around ±90°, arbitrarily labelled as up (u) or down (d). The
corresponding conformers account for about the 80% of the population in 1, 100% in 3, where
the inꢀplane arrangement of the side chain is forbidden by steric the hindrance of the methyl
groups on either sides of N(1). These findings confirm that also in the pyrazolium cations the
butyl chain preferentially assumes the outꢀofꢀplane conformation, similarly to what already found
for the bmim derivatives, both by molecular dynamics (MD) simulations and NMR
spectroscopy17 and by solid state structural studies.18ꢀ22
Three major families of conformations in cations 1 and 3 can be identified on the basis of the
distributions of α2 and α3: antiꢀanti (tt), gaucheꢀanti (gt) and antiꢀgauche (tg). In grouping the
conformers together, all the possible combinations of gauche+ and gauche– have been
considered and summed up. For both pyrazole cations 1 and 3, the families of conformations
mentioned above account for about 82% of the overall population. The tt conformers are the
most abundant class of rotamers for 1 (38.1%), whilst the tg conformers account for the 36.1% of
the population of 3. It is interesting to note that the calculations predict that also the gauche±
conformations for α3 are appreciably populated. The same was found for the butyl chain of bmim
by MD and experimentally confirmed by NMR.17 The conformational flexibility of the chain is
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drastically reduced in the crystal state, where the α3 dihedral of the butyl chain of a set of bmim
derivatives was found to populate the t conformation only.18ꢀ22 Examples of significantly
populated conformations for 1 and 3 are showed in Figure S4 and S5 of SI, respectively.
At this preliminary stage of investigation on pyrazoleꢀbased ILs, DFT calculations indicate that
the conformational properties of the butyl chain of cation of compounds 1 and 3 are similar to
those of the homologous chain in the reference imidazolium cation. A good match is observed,
despite of the fact that the comparison of DFT calculations on gas phase cations (this work) with
MD data on bulk ionic liquid17 is naturally biased by the different starting systems, the presence
of the counterion, intermolecular interactions (in the case of [bmim][BF4]), and computational
approaches.
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Figure 2. Histograms showing the distribution of the dihedral angles α1, α2 and α3 calculated at
DFT level for cation 1 (bmpz, top) and cation 3 (bm3pz, bottom).
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3.2 Internuclear Contacts via Nuclear Overhauser Enhancements. The main purpose of the
NMR investigation is the assessment of possible aggregation motives in the bulk liquid, due to
the presence of local order or nanostructuration.4a This can be done exploiting the intermolecular
Nuclear Overhauser Effect (NOE). According to the methodological approach proposed by
Oysteryoung and coworkers,10 rotating frame NOE correlation experiments (ROESY) were
carried out to observe specific cationꢀcation organization. This approach has been successfully
applied several times in the study of the imidazolium series of ILs.23ꢀ27 The separation of the
intraꢀ and intermolecular dipolar contacts can be done a posteriori by applying the distance
threshold of 4 Å for vanishing ROEs. The average distances between the H atoms of interest in
cations 1 and 3 were calculated at DFT level (see the SI) and used to assign intraꢀ and
intermolecular ROEs, e.g. detectable ROEs between protons more that 4 Å apart within the same
cation should be considered as intermolecular contacts.
The intermolecular ROEs observed in the pure liquids 1, 3 and 4 are summarized in Tables 1, 2
and 3, respectively.
Table 1. ROESY (1Hꢀ1H) intermolecular interactions for compound 1. Legend: w = weak, m =
medium, s = strong, * = no intermolecular interaction. Headꢀtoꢀhead and headꢀtoꢀtail motives are
indicated in the table with grey and striped cells, respectively.
H3 H4 H5 H6 H7 H8 H9 H10
H3
-
*
*
w
*
*
w
*
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H4
H5
H6
H7
H8
H9
*
-
*
-
*
*
-
*
*
*
-
*
w
*
*
-
w
w
w
*
m/s
s
*
*
*
*
*
w
w
*
*
*
w
w
*
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*
*
w
*
*
*
*
*
*
*
w
s
ꢀ
w
*
w
H10 *
m/s s
s
-
Table 2. ROESY (1Hꢀ1H) intermolecular interactions for 3; w = weak, m = medium, s = strong, *
= no intermolecular interaction. Headꢀtoꢀhead and headꢀtoꢀtail motives are indicated in the table
with grey and striped cells, respectively.
H11 H4 H12 H6 H7 H8 H9 H10
H11 -
*
-
*
*
ꢀ
*
w
*
-
*
*
*
*
-
*
*
w
*
*
-
m/w *
H4
*
w
*
*
*
*
ꢀ
w
H12 *
*
w
*
*
*
H6
H7
H8
*
*
*
*
*
w
*
*
*
*
*
*
*
*
*
*
*
m
w
-
H9 m/w w
H10 *
*
w
m w
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Table 3. ROESY (1Hꢀ1H) intermolecular interactions for 4; w = weak, m = medium, s = strong, *
= no intermolecular interaction. Headꢀtoꢀhead and headꢀtoꢀtail motives are indicated in the table
with grey and striped cells, respectively.
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H11 H4 H12 H6 H7 H8 H9 H10
H11 -
*
-
*
*
ꢀ
*
w
*
-
*
*
*
*
-
*
*
*
*
*
-
w
w
*
*
m
*
*
*
*
w
-
H4
*
H12 *
*
w
*
*
w
H6
H7
H8
H9
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
ꢀ
w
*
*
H10 *
m *
w
The cations can be depicted as a polar head (the charged pyrazolium ring: C(3), C(4), C(5),
C(6), C(10), C(11)) and an apolar tail (the nꢀbutyl chain). Two distinct aggregation motives can
be identified within the ROE matrices: headꢀtoꢀtail (hꢀt) and headꢀtoꢀhead (hꢀh). The
corresponding contacts are highlighted in the tables in different ways (grey for hꢀh interaction
and striped for hꢀt), while those contacts which are likely to originate from both intraꢀ and interꢀ
molecular interactions are not considered.
Both hꢀh and hꢀt arrangements are present in all compounds. It is not straightforward to assign
a relative weight to these arrangements on the basis of ROE data.24 On the other hand, no
experimental evidence for significant tailꢀtoꢀtail (tꢀt) or “micelle” aggregations can be found. The
same type of local structuration has been found for 1ꢀbutylꢀ3ꢀmethylimidazolium Tf2N and 1ꢀ
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butylꢀ2,3ꢀdimethylimidazolium ILs. The comparison of the results of Table 1 with those of
Tables 2 and 3 suggests that the contribution of the hꢀh motif decreases on going from
pyrazolium 1 to dimethylpyrazolium 3 and 4.
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Further information on the aggregation motives and cationꢀanion interactions in the pure
liquids can be obtained by heteronuclear (1Hꢀ19F)ꢀNOE experiments (HOESY) of samples 1 and
3, where the cations contain H atoms whilst the anions contain F atoms. The (1Hꢀ19F)ꢀHOESY
experiments on 1 and 3, presented below in Figures 3 and 4, show different interactions between
the fluorinated anion and the cations.
Figure 3. (1Hꢀ19F)ꢀHOESY spectra of 1. Top trace: 1HꢀNMR spectrum with atom numbering for
assignment. Vertical trace: 19FꢀNMR spectrum.
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Figure 4. (1Hꢀ19F)ꢀHOESY spectra of 3. Top trace: 1HꢀNMR spectrum with atom numbering for
assignment. Vertical trace: 19FꢀNMR spectrum.
The small but appreciable selectivity observed in HOESY of 1 suggests a correlated orientation
of the anion and cation. The Tf2N anion is close in space to the C(5)ꢀNꢀNꢀC(3) frame
(intermolecular distance of dipoleꢀcoupled nuclei < 4 Å, see Figure 1 for atom numbering) but
does not show any appreciable interaction with HꢀC(4). This finding suggests a strikingly
different behavior of pyrazoleꢀbased ILs compared to the homologous imidazolium derivatives,
in which the H atom between the nitrogens displays some unique properties and specific
interactions (acidity, Hꢀbond donation, etc.). However, the NOEs of CF3 groups with H atoms
bonded to C(7), C(8) and C(9) of the butyl chain indicate a significant interaction of the anion
with the apolar tail of the cation, which was not observed either in the case of [bmim][BF4]28 or
in the case of NꢀbutylꢀNꢀmethyl pyrrolidinium Tf2N–.29
The HOESY spectrum of 3 indicates that the anion interacts in a nonꢀselective way with the
cation, thus suggesting an even minor degree of structuration with respect to 1, probably due to
the increased steric hindrance brought about by the methyl substitution.
3.3 Diffusion Coefficients, Density, Viscosity and Conductivity. Selfꢀdiffusion coefficients
(D), density (ρ), viscosity (η) and conductivity (σ) were measured for all pyrazolium based ILs
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after accurate drying, typically in the ranges T=293ꢀ353 K (for σ and η) and T=300ꢀ340 K (for
D). Values measured at room temperature are given in Table 4, together with those reported for
1ꢀethylꢀ2ꢀmethylpyrazolium dicyanamides ([empz][N(CN)2]) and three analogous imidazolium
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salts:8
1ꢀethylꢀ3ꢀmethylimidazolium
dicyanamides
([emim][N(CN)2]),
1ꢀbutylꢀ3ꢀ
methylimidazolium dicyanamide ([bmim][N(CN)2]) and 1ꢀbutylꢀ3ꢀmethylimidazolium
bis(trifluoromethanesulfonyl)imide ([bmim][Tf2N]). Table 4 contains also the KamletꢀTaft
parameters π* (polarizability), α (hydrogen bond donor ability) and β (hydrogen bond acceptor
ability), determined using three solvatochromic probes.
Table 4. Physicoꢀchemical properties of pyrazoliumꢀ and imidazoliumꢀbased ILs at T=298 K.
b
b
Ionic Liquid
Dcation
Danion
(m2/s)
ρ (g/mL)σ (mS/cm) η (cP)
α
β
π∗
(m2/s)
[empz][N(CN)2]a
[bmpz][Tf2N]
1.16c
1.40
1.07
1.38
1.05
1.43
1.08 c
1.06 c
17
24.6
0.428 Nd 1.11
3.20
8.50
2.60
3.20
4.60
28.0
11.0
67.1 3.2*10ꢀ11 2.6*10ꢀ11 0.69 0.205 1.04
[bmpz][N(CN)2]
[bm3pz][Tf2N]
[bm3pz][N(CN)2]
[bmim][Tf2N]
30.2
0.46 0.554 1.13
0.234 1.02
0.13 0.586 1.13
76.9 2.4*10ꢀ11 2.0*10ꢀ11
77.0 1.9*10ꢀ11
d
40.0 4.5*10ꢀ11 3.6*10ꢀ11 0.64 0.248 0.97
[emim][N(CN)2]a
[bmim][N(CN)2]a
16.1
0.53 nd 1.07
0.46 0.708 1.12
29.3
a
c
From ref 8. bThe D values are measured at T=305 K.
From ref 8. Determined at 293 K.
dNot determined due to the immediate disappearing of the colour after addition of the Reichardt’s
dye.
The selfꢀdiffusion coefficients D of the cations and the fluorineꢀcontaining anions were
measured independently by PFGSEꢀNMR experiments in the 19F and H frequency domains,
1
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respectively. The observed echo intensity I is related to the experimental parameters by the
5
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StejskalꢀTanner equation:30
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
δ
3






I = I0 exp −
(
γ
g
δ
)
2 D ꢀ−


(3)


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
where I0 is the echo intensity without field gradient, γ is the gyromagnetic ratio of the observed
nucleus, g is the magnetic field gradient strength, δ is the duration of the field gradient and ꢁ is
the interval between the two gradient pulses.
The measured diffusion coefficient of the cation and anion (for ILs 1 and 3) follow the general
trend Dcation > Danion. This relationship points out that the cations’ hydrodynamic radius is smaller
than that of the anions, as previously reported for other classes of ILs.29,31 The presence of two
methyl groups in the cation (for ILs 3 and 4) leads to a reduction of the diffusion coefficient.
Similarly, the presence of the dicyanamide counterion slows down the diffusion of the bm3pz
cation, in comparison with Tf2N.
All ILs display an approximate Arrheniusꢀtype dependence of the diffusion coefficients on
temperature:
D = D0e−E / RT
D
(4)
where ED is and activation energy for diffusion and R is the gas constant. The results are given in
Table 5.
Table 5. Results of the Arrhenius fits of the ion diffusion coefficients.
ED(cation)
D0
R2
ED(anion)
D0
Ionic liquid
R2
(kJ molꢀ1) (m2sꢀ1)
(kJ molꢀ1)
[bmpz][Tf2N]
32.4
1.2*10ꢀ5 0.997 33.1
1.3*10ꢀ5 0.997
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[bm3pz][Tf2N]
35.5
41.7
3.0*10ꢀ5 0.993 38.8
2.7*10ꢀ4 0.997
9.1*10ꢀ5 0.996
[bm3pz][N(CN)2]
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For the present salts, in analogy with imidazolium based ILs, density and viscosity increase
changing the anion from [N(CN)2]ꢀ to [Tf2N]ꢀ and their temperature dependencies approximately
follow the Arrhenius equation (5) over the examined temperature range (20ꢀ80 °C),
η
η
=
η
0eE / RT
(5)
where Eη is the activation energy for viscous flow. However, the Arrhenius plots (ln
η
vs. 1/T) of
all investigated ILs show a slight upward curvature. Better fits of the viscosities were obtained
using the VogelꢀFulcherꢀTammann (VFT) law :
0
η
= AeB /(T −T )
(6)
where
Α, B and T0 are fitting parameters. Unlike the Arrhenius law, this allows a divergence of
the viscosity at a finite T, consistently with a possible transition of these systems to supercooled
and then to a glassy state (the fitted T0 cannot be identified with a true glass transition
temperature, though). The best parameters and correlation coefficients are reported in Table 6,
together with the Arrhenius parameters. As generally found with other ILs, the VFT equation is
able to predict more rigorously the viscosity behavior with temperature but, due to the limited
accessible range of temperatures, does not allow to obtain clearꢀcut trends for the VFT
parameters.
Table 6. Results of the Arrhenius and VFT fits of the viscosities.
Ionic liquid
R2
B
T0
R2
Eη
η0
Α
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(kJ molꢀ1) (cP)
(cP) (K)
(K)
5
6
7
8
[bmpz][Tf2N]
28.3
29.9
24.6
32.3
6.7ꢂ10ꢀ4 0.990 0.91 356
3.7ꢂ10ꢀ4 0.992 0.40 523
1.5ꢂ10ꢀ3 0.994 0.29 513
1.3ꢂ10ꢀ4 0.990 0.76 224
215 0.998
198 0.999
186 0.999
224 0.997
[bm3pz][Tf2N]
[bmpz][N(CN)2]
[bm3pz][N(CN)2]
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The anion nature exerts a drastic effect on conductivity, as observed for viscosity. The
conductivity of the present pyrazolium salts increases going from ILs having as counteranion
[Tf2N]ꢀ to [N(CN)2]ꢀ. As expected on the basis of the temperature dependence of the viscosities,
also the conductivities follow approximately the Arrhenius equation (7):
σ
σ
=
σ
0e−E / RT
(7)
where Eσ is the activation energy for conductivity. Also these data have been fitted using the
VFT equation:
0
σ
= Ae−B/ R(T −T )
.
(8)
The best parameters and associated correlation coefficients are reported in Table 6, together with
the Arrhenius parameters. Notice the fairly good agreement between the T0’s extracted from
viscosity and conductivity measurements.
Table 7. Results of the Arrhenius and VFT fits of the conductivities.
B
T0
Eσ
σ0
Α
Ionic liquid
R2
R2
kJ molꢀ1 mS cmꢀ1
mS cmꢀ1
29.4
39.5
K
K
[bmpz][Tf2N]
[bm3pz][Tf2N]
13.0
15.0
0.66ꢂ103 0.990
1.2ꢂ103 0.992
209 202
282 193
0.998
0.999
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[bmpz][N(CN)2]
12.1
1.1ꢂ103 0.996
15ꢂ103
0.986
96.0
79.0
298 174
271 213
0.999
0.997
[bm3pz][N(CN)2] 20.8
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Viscosity and diffusion are different manifestations of the fluid dynamics probed at molecular
and atomistic level respectively. For pure ionic liquids29 the activation energies needed for these
processes is quite similar, while IL mixtures32 revealed different values due presumably to nanoꢀ
scale heterogeneity of these mixtures. For the two IL compounds [bmpz][Tf2N] and
[bm3pz][Tf2N], the values of the activation energies for the viscosity Eη are quite similar to those
for the diffusive motion ED. The variation of the cation structure does not produce a large change
in these physicoꢀchemical properties. On the other hand, [bm3pz][N(CN)2] has a value of ED
significantly higher than Eη. The comparison with [bm3pz][Tf2N] confirms that the dicyanamide
anion increases the energy barrier for the diffusion. For all the studied compounds the
conductivity process needs lower activation energies compared with viscosity and diffusion.
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Figure 5. Walden plot at 25 °C: 1 [bm3pz][(CN)2N], 2 [bm3pz][Tf2N], 3 [bmpz][Tf2N], 4
[bmim][Tf2N],
5
[bmpz][(CN)2N] and],
6
[bmim][(CN)2N], 7 [empz][(CN)2N], 8
[emim][(CN)2N].
The comparison of the values of viscosity and conductivity for [bmpz][Tf2N] and
[bm3pz][Tf2N] shows that the introduction of two methyl groups on the cation core has only a
moderate effect on both these properties, whereas a more significant effect is played by the same
groups in the dicyanamideꢀbased ILs. The low effect of the methyl groups in the case of
bis(trifluoromethanesulfonyl)imides might have been anticipated, as it has been recently shown7
that
also
the
alkyl
chain
length
in
1ꢀalkylꢀ2,3,5ꢀtrimetheylpyrazolium
bis(trifluoromethanesulfonyl)imides not significantly affect viscosity and conductivity.
Considering that in the case of imidazolium salts the same structural features (alkyl chain length
and presence of methyl group(s) on cation core) strongly affect both properties, the lower
sensitivity of the pyrazolium cationꢀbased ILs to ionic sizes might be attributed to the different
anionꢀcation interaction ability and therefore to a different attitude of imidazolium and
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pyrazolium cations to give organized threeꢀdimensional structures (ion pairs and larger
aggregates).
7
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9
The unique properties of ILs arising from their nature of charged species able to aggregate are
often expressed in terms of ionicity and defined on the basis of the Walden plot (Figure 5).
According to the empirical Walden rule, the ionic mobility (represented by the equivalent
conductivity Λ) is related to the fluidity (1/η) of the medium through which the ions move. The
data for a 0.01 M aqueous KCl solution are taken as reference. All the data from Table 4 are
plotted in Figure 5, which shows that the pyrazoliumꢀbased salts lie below the soꢀcalled Walden
product line, roughly within the same region of the analogous imidazolium salts. The distance
between the ideal line and the experimental data increase on going from [Tf2N]ꢀ to [N(CN)2]ꢀ
based ILs and, in the case of [Tf2N]ꢀ, from pyrazolium to imidazolium. The departure of
[bmpy][Tf2N] and [bm3py][Tf2N] from the ideal Walden line is lower than that of [bmim][Tf2N].
Since these deviations are generally considered indicative of the existence of a significant
fraction of ion pairs and/or aggregation, the plot suggests a dependence of these phenomena on
anion nature (both for pyrazolium and imidazolium based ILs aggregation increases on going
from bis(trifluoromethylsulfonylimides) to dicyanamides) and cation nature (in the case of
bis(trifluoromethylsulfonylimides)ꢀbased ILs, pyrazolium salts are nearest to the ideal line
whereas a more complex behaviour characterizes dicyanamideꢀbased salts).
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Hydrodynamic theory relates the conductivity
Einstein equation:
σ and diffusion through the approximate Nernstꢀ
nq2
σNE
=
(
D+ + D−
)
(9)
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where n is the number density of ions, q=±e is the ion charge, k is Boltzmann’s constant, T is the
absolute temperature and D+ and Dꢀ are the cation and anion diffusion coefficients, respectively.
The conductivities calculated from equation (9) are typically greater than the experimentally
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measured values (
σ
exp) and this can be attributed to the neglect of ionic correlation effects. A
expT vs. (D++Dꢀ) is reported in Figure 6 for compounds 1 and 3. The plots demonstrate
that σexp<σNE, as expected, but their values are still of the same order of magnitude, indicating a
graph of
σ
significant but not overwhelming degree of association.
Figure 6. Graph of σT versus (D++Dꢀ) for [bmpz][Tf2N] a) and [bm3pz][Tf2N] b). The red lines
are linear fits to the data.
The different propensity of the ILs depending on cation and anion structure to give ion
association affects also the solvatochromic parameters.33 As previously observed for
imidazoliumꢀbased ionic liquids, the comparison of the KamletꢀTaft parameters found for the
bis(trifluoromethanesulfonyl)imide and dicyanamideꢀbased pyrazolium salts shows that
[N(CN)2]ꢀbased ILs are characterized by a slightly higher dipolarity/polarizability (π*), by a
significantly higher hydrogen bond basicity (β), a parameter which is determined by the anion
nature, and by a lower hydrogen bond acidity (α). The latter property, which is primarily
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determined by the cation nature, is normally modulated by the anion basicity: a strong interaction
between anion and cation, in other words ion association, reduces the cation’s propensity to
interact with Reichardt’s dye. The solvatochromic parameters of pyrazolium and imidazolium
salts show similar variations on going from (trifluoromethanesulfonyl)imides to dicyanamides
suggesting an increased ion association in both systems. In addition, the 1,2ꢀdialkylpyrazoliumꢀ
based ILs have a hydrogen bond donor ability comparable to the 1,3ꢀdialkylimidazolium salts.
The α values characterizing [empz][N(CN)2], [bmpz][N(CN)2] and [bmpz][Tf2N] are similar to
those reported for analogous imidazolium salts, despite of the absence of the acidic proton at
C(2). In pyrazolium salts the solventꢀsolute interaction with the dyes probably involves the other
protons close to the nitrogen atoms. Only the introduction of two other methyl groups at C(3) and
C(5) on the pyrazolium ring determines indeed a drastic reduction of the hydrogen bond donor
ability. The interaction of the dye, as well as of the counteranion, with the protons H(3) and H(5)
is eliminated and the interaction with H(7), H(8) and H(9) is reduced, probably by steric effects.
Conclusions
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We have presented the synthesis and physicoꢀchemical characterization of a new series of
pyrazoliumꢀbased ILs. An environmental friendly procedure, based on the use of
methylcarbonate as methylating agent, has been utilize for the first time to synthesize
methylpyrazolium based ILs. NMR NOE experiments show specific headꢀtoꢀtail and headꢀtoꢀ
head aggregation motives, which are quite sensitive to steric effects produced by alkyl
substitution of the cation. The nature of the anion also seems to influence the degree of
nanostructuration. The heteronuclear HOESY experiments indicate the formation of loose ion
pairs, easily disrupted by steric effects, thus confirming the tunability of the ion ordering in these
systems.
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Walden plot and KamletꢀTaft solvatochromic parameters suggest an increased ion association
on going from bis(trifluoromethanesulfonyl)imide to dicyanamideꢀbased pyrazolium salts.
Furthermore, solvatochromic measurements show that 1,2ꢀdialkylpyrazoliumꢀbased ILs have a
hydrogen bond donor ability comparable to the 1,3ꢀdialkylimidazolium salts, probably involving
protons at C(3) and C(5) close to the nitrogen atoms. Only the introduction of two other methyl
groups at these positions on the pyrazolium ring determines indeed a drastic reduction of the
hydrogen bond donor ability.
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The ion diffusion measured by NMR methods is significantly influenced not only by the cation
size and steric hindrance (Dcation1 > Dcation3), but also by the shape and size of the anion (Dcation3
>
Dcation4). The temperature dependencies of the viscosities, conductivities and diffusion show a
similar Arrheniusꢀtype behaviour characterized by close values of the activation energies for
shear flow and diffusion processes. The VFT equation has also been used to fit the experimental
parameters.
Chemical modulation of the steric hindrance of cation and the choice of the counterꢀion open
the possibility of an easy and extensive tunability of pyrazoleꢀbased ILs, as demonstrated by the
Walden plot analysis, in order to design and tailor their physicoꢀchemical properties for specific
applications.
Supporting Information
NMR spectra of titled compounds, dihedral angles, interatomic distances, conformational
energies of pyrazolium cations calculated at DFT level. This material is available free of charge
via the Internet at http://pubs.acs.org.
Acknowledgments. The authors acknowledge the support of Regione Lombardia and CILEA
Consortium through a LISA Initiative (Laboratory for Interdisciplinary Advanced Simulation)
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2011 grant (http://lisa.cilea.it). FC, GR, AF and AM wish to thank the EU for funding (FP7 –
GREENLION Project – Contract nº 285268).
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