Pyrrolidinium imides: A new family of molten salts and conductive plastic crystal phases

Article abstract of DOI:10.1021/jp984145s

A new family of molten salts is reported, based on the N-alkyl, N-alkyl pyrrolidinium cation and the bis-(trifluoromethane sulfonyl)imide anion. Some of the members of the family are molten at room temperature, while the smaller and more symmetrical members have melting points around 100 °C. Of the room-temperature molten salt examples, the methyl butyl derivative exhibits the highest conductivity; at 2 × 10^-3 S/cm this is the highest molten salt conductivity observed to date at room temperature among the ammonium salts. This highly conductive behavior is rationalized in terms of the role of cation planarity. The salts also exhibit multiple crystalline phase behavior below their melting points and exhibit significant conductivity in at least their higher temperature crystal phase. For example, the methyl propyl derivative (mp = 12 °C) shows ion conductivity of 1 × 10^-6 S/cm at 0 °C in its higher temperature crystalline phase.

Full text of DOI:10.1021/jp984145s

4164
J. Phys. Chem. B 1999, 103, 4164-4170
Pyrrolidinium Imides: A New Family of Molten Salts and Conductive Plastic Crystal
Phases
,†
†
†
†
‡
D. R. MacFarlane,* P. Meakin, J. Sun, N. Amini, and M. Forsyth
Department of Chemistry and Department of Materials Engineering, Monash UniVersity,
Clayton, Victoria 3168, Australia
ReceiVed: October 23, 1998; In Final Form: February 2, 1999
A new family of molten salts is reported, based on the N-alkyl, N-alkyl pyrrolidinium cation and the bis-
(trifluoromethane sulfonyl)imide anion. Some of the members of the family are molten at room temperature,
while the smaller and more symmetrical members have melting points around 100 °C. Of the room-temperature
-
3
molten salt examples, the methyl butyl derivative exhibits the highest conductivity; at 2 × 10 S/cm this is
the highest molten salt conductivity observed to date at room temperature among the ammonium salts. This
highly conductive behavior is rationalized in terms of the role of cation planarity. The salts also exhibit
multiple crystalline phase behavior below their melting points and exhibit significant conductivity in at least
their higher temperature crystal phase. For example, the methyl propyl derivative (mp ) 12 °C) shows ion
-
6
conductivity of 1 × 10 S/cm at 0 °C in its higher temperature crystalline phase.
Introduction
factor was clearly important in that work, the presence of the
bis(trifluoromethane sulfonyl) imide ion (III) (otherwise often
referred to as simply the “imide” ion). This was shown to be
the origin of an at least 100 °C depression of the melting points
in the ammonium salt family based on this anion, compared
with the corresponding iodides. It was also, unexpectedly, the
progenitor of much higher fluidity and conductivity than
observed for related ions (for example the triflate ion) where
various salts of the same cation were able to be prepared. The
highest room-temperature conductivity observed in this family
Room temperature molten salts have been an area of interest
both for fundamental reasons, associated with the factors that
promote their low or nonexistent melting point and the intrinsic
ion conductivity displayed by these ionic liquids, and also for
application in a variety of electrochemical device fields. Molten
salts, sometimes also referred to as ionic liquids, generally have
long been of interest to Angell and co-workers because of their
range of behaviors, in particular with respect to glass formation.
1-4
Recently, Angell and co-workers
have investigated an
-4
of compounds was 8 × 10 S/cm, more than 1 order of
+
interesting series of Li molten salts for applications in lithium
battery technology. In other recent work,5 a family of room-
temperature molten salts based on the imidazolium ring (I) have
been developed with a view to applications such as photoelec-
trochemical solar cells and as media for “clean” industrial
processes. These imidazolium systems hold something of a
record for room-temperature conductivity (∼8 × 10 S/cm);
however, some doubts remain about the chemical and electro-
chemical stability of the 1-alkyl, 3-alkyl imidazolium family
because of possible reactions at the C(2) carbon. The 1-alkyl,
magnitude lower than closely related, in terms of molecular mass
-9
(slightly smaller), imidazolium salts. The difference between
the ammonium and imidazolium cations, of course, is stark. The
former cations have globular to rod-shaped geometries depend-
ing on the alkyl substituents involved, whereas the latter cations
are almost planar for the lower molecular weight members, e.g.,
the methylethyl imidazolium case.
-
3
In the case of the imide ion, the origin of some of its
interesting properties can be deduced in part from a recently
14
reported crystal structure. In this, it is clear that the charge
2
-alkyl, 3-alkyl imidazolium salts solve this problem, but are
delocalization expected in the imide ion extends only from the
central nitrogen to the neighboring sulfur atoms and not to any
great extent onto the four sulfonyl oxygen atoms. The result of
this partial delocalization is that the charge is substantially buried
within the molecule, shielded by the oxygens and terminal -CF3
groups from Coulombic interactions with neighboring cations.
This reduced interaction appears, therefore, to be associated with
increased ion mobility and reduced lattice energy in the
crystalline state.
of lower conductivity.
In recent contributions from our laboratories,1
0-13
we have
reported a large family of low-symmetry quaternary ammonium
cation salts, many of which have subambient melting points;
some members of the family are, in fact, glass forming. This
work represented the first report of low molecular weight
(carbon atom number < 12) ammonium cation salts being
molten at room temperature. It was proposed that the depression
of the melting point compared with various related compounds
was due in part to disruption of the symmetry of the cation,
this serving to interfer with crystal packing and thereby
destabilize, relatively, the crystalline state. However, a second
*
To whom correspondence should be addressed. E-mail: d.macfarlane@
sci.monash.edu.au.
†
Department of Chemistry.
Department of Materials Engineering.
‡
In this contribution, we report the preparation and characteriza-
1
0.1021/jp984145s CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/26/1999

Pyrrolidinium Imides
J. Phys. Chem. B, Vol. 103, No. 20, 1999 4165
SCHEME 1
A ML 400 MacLab/4e with Echem v1.3.1 Software was used
for the electrochemical measurements. Glassy carbon was used
as the working and counter electrodes, and the quasireference
+
electrode was a silver wire (effectively forming a Ag/Ag
reference electrode in the melt). The scanning rate was 100 mV/
s. Before each measurement, the samples were bubbled with
dry, high-purity N2 for more than 20 min to remove dissolved
O2.
tion of another new family of molten salt-forming compounds,
which conceptually bridge the structural gap between the fully
three-dimensional ammonium ions and the close to planar two-
dimensional imidazolium ions. The new family of salts is based
on the pyrrolidinium cation (IV) where the two alkyl substituents
range from methyl up to butyl. In some cases, these salts are
shown to have melting points below room temperature. In some
cases, they also show interesting conductive behavior in their
crystalline state, in what we believe to be plastic crystal phases.
A Perkin-Elmer differential scanning calorimeter (DSC)
model 7 was used to measure the thermal properties at a
scanning rate of 10.0 °C/min and using aluminum sample pans.
Standard samples of oxalic acid dihydrate (mp 103 °C) and
indium (mp 156.60 °C) were used for the calibration of
temperature and power measurements in the DSC over the
temperature range from 30 to 170 °C. A standard sample of
cyclohexane (crystal-crystal transition temperature -87.06 °C
and mp 6.54 °C) was used to calibrate the DSC over the
temperature range from -150 to 40 °C. Before the DSC
measurement, all samples were dried under vacuum at room
temperature for at least 3 days.
Experimental Section
Pyrrolidinium bis(trifluoromethane sulfonyl) imide salts were
synthesized by reaction Scheme 1. The details of the synthesis
and characterization of the compounds are presented in the
appendix.
Results and Discussion
We begin by characterizing and discussing the properties of
these new substances as ionic liquids before proceeding to
investigate their conductive behavior in the solid state.
Molten Salt Properties. The properties of the salts prepared
Conductance measurements were carried out in a number of
locally designed multisample conductance cells, each designed
to optimally cover a particular range of conductivities; in this
-
9
-1
work, conductivities between 10 and 10 S/cm were en-
countered. Each consists of a block of aluminum into which
sample compartments were machined. The cell constant, b, of
the embedded stainless steel blocking electrodes in each
compartment was determined either by calibration before or after
each sample measurement with 0.01 M KCl solution at 25 °C
or by direct measurement of the empty cell capacitance, Co.
in this work are summarized in Table 1. An acronym, P , is
xx
used to distinguish between the salts by subscripting the number
of carbons in each of the alkyl substituents. The dimethyl (P )
11
and methyl ethyl derivatives (P ) have relatively high melting
1
2
points; however, with increasing alkyl chain length, the melting
point has dropped below room temperature at the methyl propyl
derivative. The melting points appear to fall steadily with
increasing alkyl chain length, as observed previously in the
-1
Cell constants were in the region of 1.0 cm for the low sample
-1
11
impedance cell and 0.0025 cm for the high sample impedance
cell. Conductivities were obtained by measurement of the
complex admittance of the cell between 20 Hz and 1 MHz using
a HP4284A impedance meter. The temperature was either
controlled at a set temperature or ramped at a steady 0.2 °C/
min under the control of a Shimaden digital temperature
controller. The type-T measurement thermocouple probe was
located in the aluminum block close to the sample compartment.
The conductance of the samples was determined from the first
real axis touchdown point in the Cole-Cole plot of the
impedance data. The complex electric modulus was calculated
from the complex admittance, A*, data via
ammonium salt family. Typical DSC traces are shown in
Figure 1. In the cases of P13 and P , it appears that the quenched
14
molten salt is glass forming, the glass transition temperatures,
Tg, measured on warming being -90 and -87 °C, respectively.
The thermal analysis traces also show classic evidence of
multiple crystallization and melting events in each case which
we attribute to the existence of several polymorphs of the
crystalline state in these compounds. Thus, in P , the exotherm
13
at -67 °C is associated with the crystallization of the super-
cooled liquid to what is thought to be a metastable phase because
of the subsequent further exotherm at -45 °C where it
transforms to the low-temperature stable phase, denoted phase
II. The progressive crystallization of first a metastable phase
prior to appearance of the lowest energy, maximally ordered
phase is well-known under these highly nonequilibrium condi-
tions. Phase II then transforms into phase I at -18 °C, which
subsequently melts at 12 °C. In the case of P12, the quenched
material shows a possible glass transition followed by sudden
crystallization and transformation around -100 °C; however,
as discussed further below, the glass transition is not that of
the quenched liquid state. Since the sample is crystalline at room
temperature at the beginning of the quench, the glass transition
must result from quenched-in disorder of the high-temperature
solid phase. P12 then shows a broad solid-solid transformation
to phase I which is complete by 14 °C (entropy of transition
M* ) iωC /A*
(1)
o
where M* is the complex electric modulus, ω the angular
frequency, Co the empty cell capacitance ()ꢀo/b), and ꢀï the
permittivity of free space. Before the conductivity measure-
ments, all samples were dried under vacuum for at least 3 days.
No evidence of electronic conduction was found; in particular,
the low-frequency behavior of the materials between the
blocking electrodes was strongly capacitive as expected for an
ion conductor/blocking electrode interface.
The densities of the liquid samples were determined by
weighing a measured volume of the material. The Ostwald
method was used in viscosity measurements of the liquid salts;
glycerol (viscosity 954 mPa s at 25 °C), cyclohexanol (57.5
mPa s), 1,2-propylene glycol (40.4 mPa s), diethylene glycol
-
1
-1
3.1 J K mol ) and a final melting at 86 °C. The thermogram
of P11 is much simpler than that of P12, showing only a weak
transition at 20 °C.
(
30.2 mPa s), ethanolamine (21.1 mPa s), ethylene glycol (16.1
Conductivities of P13 and P14 in the liquid state are shown as
a function of temperature in Figure 2, from which it can be
seen that the conductivity is slightly higher for the P14 derivative.
mPa s), and glycerol-water solution (60 wt % glycerol, 8.82
mPa s) were used as standards.

4166 J. Phys. Chem. B, Vol. 103, No. 20, 1999
MacFarlane et al.
TABLE 1: Properties of Pyrrolidinium Imide Saltsa
fluid
phase II f
phase I
(°C)
density/ viscosity
fluid
V
(cm
m
∆S
f
-1
-
3
3
T
g
∆S(IIfI) phase 1 (g cm
)
(mPas)
conductivity
(J K
-
1
((5%, (σ/10 S cm-1) mol-1) mol-1)
-4
acro-
nym
(°C)
((2 °C)
(J K
mp (°C) (20 °C)
structure
((1 °C)
mol-1) ((1 °C) ((0.02)
25 °C)
((5%, 25 °C) (20 °C) ((5%)
+
-
P
P
P
P
11
12
13
132
40
38
(
(
(
(
CH ) C H NN(SO CF )
3
2
4
8
2
-
3 2
+
-102 (pc)
-90
14
-18
-24
3.1
4.0
4.8
86
12
CH )(C H )C H NN(SO CF )
3
2
5
4
8
2
3 2
+
-
1.45
1.41
63
85
14
22
280
300
43
41
CH )(n-C H )C H NN(SO CF )
3
3
7
4
8
2
3 2
+
-
14
CH )(n-C H )C H NN(SO CF )
3 2
-87
-18
3
4
9
4
8
2
a
T
g
) glass transition temperature; V
m
) molar volume; ∆S(IIfI) ) entropy of phase II f phase I transition; ∆S ) entropy of fusion.
f
Figure 2. Conductivity versus temperature for methylpropyl pyrroli-
dinium imide (P13) and methylbutyl pyrrolidinium imide (P14) molten
salts. The lowest temperature data point for P13 corresponds to a sample
which has crystallized during the experiment (mp ) 12 °C). Unpub-
lished data for comparable ammonium salts is also shown.
5
(
at 20 °C), whereas an ammonium salt of comparable carbon
number (butyldimethyl propylammonium imide, N1134) has
-4
13
conductivity around 8 × 10 S/cm. The smaller imidazolium
cations, dimethyl imidazolium imide and methylethyl imid-
azolium imide, which are analogous to P11 and P12, respectively,
Figure 1. Differential scanning calorimeter thermograms of (a) P11
and P12 on warming at 10 °C after quenching from room temperature
and (b) P13 and P14 on warming at 10 °C after quenching from the
liquid state. Glass transition temperatures are obtained at the onset of
the transition.
-
3
have still higher conductivities, around 8 × 10 S/cm at 20
°
C. From this it would appear that the planarity of the
imidazolium ring is vital in supporting the high conductivity
exhibited by these compounds. The pyrrolidinium ring ap-
proaches this to an extent, but the presence of the substituents
above and below the plane of the ring clearly restricts the
mobility of this ring compared to its more planar imidazolium
relative. The pyrrolidinium ring itself is not planar, with the
nitrogen atom sitting above the plane of the four ring carbons;
however, in the methyl pyrrolidinium series, structural models,
Figure 3, show that the dihedral angle between the ring carbons,
the nitrogen, and the methyl group is ca. 163°, indicating a close
to planar arrangement of this subgroup.
This is unexpected given its higher viscosity and glass transition
temperature, evidence that the conductive and structural modes
of motion are not completely coupled in these compounds. Elias
and Elias have recently discussed15 the extent to which Walden’s
rule, i.e., the constancy of the product Λη where Λ is the molar
conductivity ()σVm), breaks down for the imidazolium chloride-
AlCl3 room-temperature molten salts. The deviations from
constancy were attributed to the formation of complex species
including ion pairs. Similar species are to be expected in the
present systems.
Cyclic voltammograms are shown in Figure 4 for P14 cycled
on a glassy carbon working electrode at 25 °C. Figure 3a shows
that the P14 is electrochemically stable at least to -3.0 V and
-
3
At room-temperature, P14 has a conductivity of 2.2 × 10
S/cm. By comparison, the analogous imidazolium salt (methyl-
butyl imidazolium imide) has conductivity 3.9 × 10- S/cm
3
+2.5 V vs Ag/Ag . The electrochemical window of stability is
+

Pyrrolidinium Imides
J. Phys. Chem. B, Vol. 103, No. 20, 1999 4167
Figure 5. Imaginary part of the electric modulus versus frequency,
plotted as isothermals, for methylpropyl pyrrolidinium imide.
Figure 3. Representation of the methylpropyl pyrrolidinium cation.
Structure was energy minimized within Discover 96 (Molecular
Simulation Incorporated, San Diego).
cation toward reduction. Extending the electrochemical excur-
sion, Figure 4b, introduces (i) oxidation processes at ∼+2.75
V, followed by a corresponding reduction event around -2.0
V in the reverse scan and (ii) reduction processes in the melt
just beyond -3.0 V, followed by a corresponding oxidation
process around +1.0 V. The large potential differences between
these melt breakdown processes and their corresponding reverse
reactions indicate the strongly irreversible nature of the reactions
involved.
Solid-State Behavior. As shown in Figure 2, there is an
unexpectedly high conductivity evident in the P13 sample after
it has crystallized during the experiment (the lowest temperature
point). Exploring this further, samples were quenched from
above their respective melting points in various cells designed
to explore different conductivity regimes and admittance spectra
gathered over a wide range of temperatures from below the glass
transition temperature to above Tm. An example of the isother-
mal, variable frequency behavior of M′′ is shown in Figure 5
for the solid forms of P13. In accordance with the observed dc
conductivity for the solid phases of these salts, the modulus
spectra exhibit characteristic conductivity loss peaks. At the peak
at each frequency and temperature, the following relationship
1
6
holds:
ωpeak ) σ /ꢀ ꢀ
o ∞
(2)
dc
where ꢀo and ꢀ∞ are the permittivities of free space and the
material respectively and σdc is the limiting low-frequency
conductivity. This relationship only holds exactly where the
relaxation process is governed by a simple exponential. Figure
5
shows that the isothermal conductivity loss peak is somewhat
broader than expected (1.14 decades of frequency) for a simple
exponential (or Debye) process. This broadening is typical of
many solid-state and viscous liquid ionic materials and is
understood to be associated with an intrinsically nonexponential
relaxation process for the motion of each ion and/or a distribu-
tion of relaxation times associated with different ionic sites and
environments. Nonetheless, eq 2 holds approximately under
these conditions.
Figure 4. Cyclic voltammograms for P14 over various voltage ranges
(
a) cycling between and (b) extending the cycle to -3.5 V to +3.0 V
+
vs Ag/Ag at which point bulk oxidation and reduction processes in
+
the -3 V and +2.5 V vs Ag/Ag melt appear. The sign convertion
used in this plot has positive currents ) oxidation. Glassy carbon
working and counter electrodes. Scan rate, 100 mV/s; T ) 25 °C.
therefore in excess of 5.5 V. This window of electrochemical
stability is one of the widest yet observed in a room-temperature
molten salt. By comparison, the ammonium imide salts reported
in ref 11 showed an approximately 4.5-5.0 V electrochemical
window under similar experimental conditions. The pyrroli-
dinium salt is more stable by an additional 0.5 V toward
reduction; this presumably reflects additional stability of the
From the admittance data set, the DC conductivity was
obtained from the real axis touchdown in the impedance plane
and is presented in Figure 6. From this figure, it is clear that
the compounds all support a very substantial conductivity in
the crystalline state just below their respective melting points.
-8
For example, P12 shows conductivity around 10 S/cm at room
temperature, the conductivity rising steadily with activation

4168 J. Phys. Chem. B, Vol. 103, No. 20, 1999
MacFarlane et al.
Plastic crystal phases have been known and described since
23
the classic work of Timmermans. They have characteristically
-
1
-1
very low entropies of melting (e20 J K mol ) and below
the melting point often exhibit a series of solid-solid phase
transitions. The highest temperature crystalline phase (often
termed phase 1) is usually characterized by strong rotational
disorder of the whole molecular unit or some substantial
fragment thereof. Lying lower in energy than phase I are one
or more states of higher order that become stable at lower
temperature. The plastic crystal state is also understood to
contain many lattice vacancies as a result of the rotational
disorder, for which reason the material can exhibit plastic
material properties, substantial self-diffusion coefficients, and,
where appropriate, high ionic conductivity.
The behavior in the present pyrrolidinium family of com-
pounds appears to bear many features in common with the
observations of Ikeda and co-workers. All of the compounds
investigated here exhibit at least one conductive crystalline phase
and in several cases there is evidence of a series of conductive
phases with increasing temperature. The entropies of melting
of the present compounds, as listed in Table 1, are larger than
Figure 6. Log conductivity versus inverse temperature for pyrroli-
dinium imide salts in solid and liquid states. The DSC melting points
are arrowed.
energy 47 kJ mol-1 up to about 10-7 S/cm at 70 °C. At this
-
3
point, the conductivity begins to rise rapidly, reaching 7 × 10
-
1
-1
S/cm at 88 °C; the DSC thermogram shows a sharp melting
transition (onset 86 °C) in this region of temperature. There is
no significant step in conductivity in the region of temperatures
the maximum of 20 J K mol normally expected for plastic
-
1
crystals. The closest to this value is P12 with ∆Sf ) 38 J K
-
1
mol . By comparison, the entropies of melting of a series of
quaternary ammonium imide compounds reported by us in ref
13, which show only very low solid-state conductivity behavior
(14 °C) where the phase II f phase I transition is observed to
take place thermally. P13 shows a similarly sharp step in
conductivity around its final melting point from a solid-state
-
10
-1
-1
(σ < 5 × 10
S/cm), are in excess of 50 J K mol ; thus,
-
6
conductivity which reaches 10 S/cm at 0 °C, just before
melting begins. P14 shows almost identical behavior, the
transition being shifted down in temperature by approximately
it appears that the plastic crystal behavior in the present
compounds is associated with the pyrrolidinium cation. How-
ever, it is likely that these compounds have additional configu-
rational degrees of freedom associated with the imide ion which
become active on melting. The compounds studied by Ikeda et
al. contained either monatomic halide or other small anions,
and the compounds typically otherwise showing plastic crystal
behavior are usually relatively simple molecular crystals. Thus,
in the case of the present pyrrolidinium compounds where the
plastic crystal behavior is associated with only the cation and
the anion itself is capable of contributing significantly to ∆Sf,
it is likely that Timmerman’s condition no longer represents a
completely reliable indicator of plastic crystal behavior.
15 °C, consistent with the thermal analysis results. Interestingly,
although all of the salts reach very similar conductivities just
before the onset of melting, at any given temperature, for
example, -20 °C, P14 is 1 order of magnitude more conductive
in its crystal phase than is P13, which is similarly more
conductive than P12.
Conduction in Plastic Crystals. Much of this phase transition
and conductive behavior below the melting point is reminiscent
of the observations of Ikeda and co-workers1
7-21
in their
investigations of plastic crystal behavior in ammonium com-
pounds and also of Cooper and Angell22 in their study of plastic
crystal behavior in a methoxyethyl dimethyl ethylammonium
compound. These groups have described the properties of a
number of tetraalkylammonium and dialkylammonium com-
Angell and co-workers24-28 have shown that the disorder
present in a plastic crystal can be quenched into the structure
by rapid cooling in much the same way that liquid-state disorder
is quenched in a more normal glass-forming substance. The
quenched plastic crystal can then exhibit a glass transition which
is associated with the onset of the rotational motions responsible
for the plastic crystal behavior. This would appear to be the
phenomenon observed in the region of -100 °C in the
thermogram of P12, and for this reason the Tg noted in Table 1
has been indicated as pertaining to the plastic crystal phase.
-
-
-
-
pounds with anions such as the halides, Cl , Br , BF4 , I ,
-
and SCN . Via NMR relaxation measurements, Ikeda et al
showed that plastic crystal phases (otherwise known as rotator
phases or orientationally disordered phases) existed in these
compounds over quite wide ranges of temperature up to the
final melting point. The entropy of transition from the low-
temperature ordered state into the partially disordered state (in
some cases there existed a progression of several increasingly
disordered states) was a substantial fraction of the final entropy
of melting. The entropy of melting was, in most cases, in the
Ikeda and co-workers were able to show via NMR measure-
ments that the rotations responsible for the plastic crystal
behavior involved the cation in the case of their ammonium
salts. Therefore, we believe that the rotation involved in the
rotationally disordered state in the present compounds is most
likely to involve the cation executing a rotation about an axis
which bisects the pyrrolidinium ring, as shown in the molecular
representation in Figure 3. That there are several rotationally
disordered phases suggests either a progression through a state
of hindered rotation before full rotation is achieved in phase I
and/or an involvement of other rotational modes of this
molecule. Further details of these motions should be able to be
illucidated via multinuclear NMR relaxation experiments. Ikeda
and co-workers were able to definitively identify the charge
-
1
-1
vicinity of 2.5R (∼20 J K mol ), characteristic of plastic
crystals. The ionic conductivity of these compounds was found
to be significant in their plastic crystal phase, for example,
-
7
Et2NH2Br has σ ) 2.5 × 10 S/cm at 400 K in its most
disordered phase,21 which extends from 342 to 481 K. The
temperature dependence of conductivity was Arrhenius with
activation energy ) 58 kJ/mol. This activation energy was
comparable to that observed from cation NMR T1F relaxation
measurements. Trimethylethylammonium iodide exhibited two
1
9
disordered phases both of which were ionically conductive.

Pyrrolidinium Imides
J. Phys. Chem. B, Vol. 103, No. 20, 1999 4169
carrier responsible for the conduction in their plastic crystals
1-n-Butyl-1-methyl Pyrrolidinium Iodide (P14-I). The same
procedure was used as in P11-I. 1-Iodobutane (Aldrich) was
used instead of iodomethane, and the reacting mixture was
stirred at ∼70 °C (oil bath) instead of room temperature (yield
20
as the cation in some cases (e.g., in (CH3)4NSCN ). However,
19
in other cases (e.g., (CH3)3NCH2CH3I ) it appeared that it was
the anion which was the only species able to undertake rapid
translational motion in the two plastic crystal phases. It thus
appears that either ion may be responsible for the conduction
in the present materials; NMR self-diffusion measurements will
be undertaken to shed further light on this issue.
1
96%). H NMR (D2O, δ/ppm relative to TMS): 3.54-3.51 (m,
4H), 3.38-3.33 (m, 2H), 3.05 (s, 6H), 2.28-2.15 (m, 4H),
1.84-1.73 (m, 2H), 1.41 (sextuplet, 2H, J ) 7.4 Hz), 0.95 (t,
3H, J ) 7.4 Hz).
We return now to the observation that when P13 and P14 are
compared over a range of temperatures where both are in their
conductive plastic crystal phase, it appears that the larger
molecule P14 is the more conductive of the two by more than 1
order of magnitude. This indicates that the larger molecular
cation in its rotationally disordered arrangement creates a greater
degree of lattice expansion over the ordered phase and also
thereby a larger number of lattice vacancies.
1,1-Dimethyl Pyrrolidinium Bis(trifluoromethane Sulfonyl)
Imide Salt (P11). An amount of 1.60 g (0.0056 mol) lithium
bis(trifluoromethane sulfonyl) imide salt (3 M) was dissolved
in 2 g distilled water. In 2 g distilled water, 1.27 g (0.0056
mol) 1,1-dimethyl pyrrolidinium iodide was dissolved. The two
aqueous solutions were mixed together and then stirred at room
temperature for 3 h. The product (organic phase) was separated
from the aqueous phase by a separating funnel and was washed
with distilled water twice to remove any water-soluble impuri-
ties. The final product was dried under vacuum at room
temperature, and 1.79 g of the product was obtained (yield 84%).
Conclusions
A new family of salts based on the pyrrolidinium family of
cations and the bis(trifluoromethane sulfonyl) imide ion have
been described. The methyl propyl (P13) and methyl butyl (P14)
derivatives have subambient melting points and are thus stable
molten salts at room temperature. P14 has the highest room
temperature conductivity, and by analogy with trends seen in
related families of compounds, it is possible that slightly higher
conductivities may yet be observed somewhere in the vicinity
of the P23 or P15 members of the family. All of the compounds
described exhibit polymorphism in the crystalline state possibly
including plastic crystal phases which are able to support
substantial conductivity in its crystalline lattice via hopping
between lattice vacancies. Given the large size and mass of the
mobile ions involved, the conductivity exhibited by the plastic
1
H NMR (DMSO-d6, δ/ppm relative to TMS): 3.46-3.43 (m,
4
H), 3.08 (s, 6H), 2.12-2.07 (m, 4H).
In all of the following, the same procedure as above for P11
was used, substituting the appropriate pyrrolidinium iodide
starting material.
1
-Ethyl-1-methyl Pyrrolidinium Bis(trifluoromethane Sulfo-
1
nyl) Imide Salt (P12). Yield 94%. H NMR (DMSO-d6, δ/ppm
relative to TMS): 3.48-3.38 (m, 6H), 2.96 (s, 6H), 2.11-2.07
(
m, 4H), 1.29-1.24 (m, 3H).
1-Methyl-1-n-propyl Pyrrolidinium Bis(trifluoromethane Sul-
fonyl) Imide Salt (P13). Yield 86%. Anal. Calcd for C10H18-
N2F6S2O4: C, 29.41; H, 4.44; N, 6.86; F, 27.92; S, 15.70; O,
1
5.67. Found: C, 29.47; H, 4.69; N, 6.93; F, 27.98; S, 15.85;
1
-6
O, 15.08. H NMR (DMSO-d6, δ/ppm relative to TMS): 3.51-
crystal material, of the order of 10 S/cm, at 0 °C is impressive.
Future work will attempt to further characterize the rotational
and diffusional motions in the solid phases by NMR and also
to investigate the conductive properties of dopant ions in these
structures.
3
4
.39 (m, 4H), 3.28-3.24 (m, 2H), 2.99 (s, 3H), 2.10-2.08 (m,
H), 1.78-1.68 (m, 2H), 0.92 (t, 3H, J ) 7.4 Hz).
1
-n-Butyl-1-methyl Pyrrolidinium Bis(trifluoromethane Sul-
1
fonyl) Imide Salt (P14). Yield 88%. H NMR (DMSO-d6, δ/ppm
relative to TMS): 3.52-3.39 (m, 4H), 3.33-3.27 (m, 2H), 2.98
(
2
s, 6H), 2.15-2.04 (m, 4H), 1.74-1.64 (m, 2H), 1.32 (sextuplet,
Appendix
H, J ) 7.4 Hz), 0.94 (t, 3H, J ) 7.4 Hz).
Synthetic and Characterization Details. 1,1-Dimethyl Pyr-
rolidinium Iodide (P11-I). An amount of 6.30 g (0.069 mol)
References and Notes
1
-methyl pyrrolidine (Aldrich, all chemicals were used as
(
1) Xu, K.; Zhang, S.; Angell, C. A. J. Electrochem. Soc. 1996, 143
received except as otherwise specified) was mixed with 15 g
acetonitrile (Fluka); 12.08 g (0.085 mol) iodomethane (Aldrich)
was added dropwise into the pyrrolidine solution, and N2
bubbling was used. The mixture was stirred at room temperature
overnight. The solvent was removed by distillation, and the solid
product was washed with petroleum spirit X4 three times. The
final product was dried under vacuum at room temperature for
(
11), 3548-3554.
(
2) Liu, C.; Angell, C. A. Solid State Ionics 1996, 86-88, 467-473.
(3) Xu, K.; Day, N. D.; Angell, C. A. J. Electrochem. Soc. 1996, 143
(
9), L209-L211.
(
4) Angell, C. A.; Xu, K.; Liu, C. Arizona State University; U.S. Patent
5
48,670 A.
5) Bonhote, P.; Dias, A. P.; Papageorgiou, N.; Kalyanasundaram, K.;
Gratzel, M. Inorg. Chem. 1996, 35, 1168.
6) (a) Wilkes, J. S.; Levisky, J. A.; Wilson, R. A.; Hussey, Ch. L.
(
(
more than 48 h, and 14.9 g of the product was obtained (yield
Inorg. Chem. 1982, 21, 1263. (b) Wilkes, J. S.; Zaworotko, M. J. J. Chem.
Soc. Chem. Commun. 1992, 965.
1
9
5%). H NMR (D2O, δ/ppm relative to TMS): 3.57-3.52 (m,
H), 3.17 (s, 6H), 2.26-2.24 (m, 4H).
4
(7) Matsunaga, M.; Inoue, Y.; Morimitsu, M.; Hosokawa, K. Proc.
Electrochem. Soc. 1993, 93-99 (Molten Salt Chemistry and Technology),
1
-Ethyl-1-methyl Pyrrolidinium Iodide (P12-I). The same
507.
procedure was used as in P11-I. 1-Iodoethane (Aldrich) was
(
8) Seddon, K. R. J. Chem. Technol. Biotechnol. 1997, 68, 351-356.
1
used instead of iodomethane (yield 65%). H NMR (D2O, δ/ppm
(9) Fuller, J.; Carlin, R. T.; De Long, H. C.; Haworth, D. J. Chem.
relative to TMS): 3.54-3.41 (m, 6H), 3.05 (s, 6H), 2.23-2.22
Soc., Chem. Commun. 1994, 299.
(10) Sun, J.; Forsyth, M.; MacFarlane, D. R. Ionics 1997, 3, 356.
(
m, 4H), 1.42-1.36 (m, 3H).
(
11) Sun, J.; Forsyth, M.; MacFarlane, D. R. J. Phys. Chem. 1998, 102,
858.
12) Sun, J.; Forsyth M.; MacFarlane, D. R. Molten Salt Forum 1998,
5-6, 585-588.
13) MacFarlane, D. R.; Sun, J.; Forsyth, M.; Meakin, P.; Amini, N. In
preparation.
1
-Methyl-1-n-propyl Pyrrolidinium Iodide (P13-I). The same
8
procedure was used as in P11-I. 1-Iodopropane (Aldrich) was
used instead of iodomethane, and the reacting mixture was
(
(
stirred at ∼60 °C (oil bath) instead of room temperature (yield
1
8
4
1
6%). H NMR (D2O, δ/ppm relative to TMS): 3.54-3.52 (m,
H), 3.34-3.28 (m, 2H), 3.05 (s, 6H), 2.30-2.14 (m, 4H),
.89-1.76 (m, 2H), 0.98 (t, 3H, J ) 7.4 Hz).
(14) Golding, J.; MacFarlane, D. R.; Spiccia, L.; Forsyth, M.; Skelton,
B. W.; White, A. H. Chem. Commun. 1998, 15, 1593-1594.
(15) Elias, A. M.; Elias, M. E. Molten Salt Forum 1998, 5-6, 617.

4170 J. Phys. Chem. B, Vol. 103, No. 20, 1999
MacFarlane et al.
(
16) Angell, C. A. Annu. ReV. Phys. Chem. 1992, 43, 693.
(22) Cooper, E. I.; Angell, C. A. Solid State Ionics 1986, 18/19, 570.
(23) Timmermans, J. J. Phys. Chem. Solids, 1961, 18, 1.
(17) Hattori, M.; Fukada, S.; Nakamura, D.; Ikeda, R. J. Chem. Soc.,
Faraday Trans. 1990, 86, 3777.
18) Iwai, S.; Hattori, M.; Nakamura, D.; Ikeda, R. J. Chem. Soc.,
Faraday Trans. 1993, 89, 827.
19) Ishida, H.; Furukawa, Y.; Kashino, S.; Sato, S.; Ikeda, R. Ber.
Bunsen-Ges. Phys. Chem. 1996, 100, 433-439.
20) Tanabe, T.; Nakamura, D.; Ikeda, R. J. Chem. Soc., Faraday Trans.
991, 87 (7), 987-990.
21) Shimizu, T.; Tanaka, S.; Onoda-Yamamuro, N.; Ishimaru, S.; Ikeda,
R. J. Chem. Soc., Faraday Trans. 1997, 93 (2), 321-326.
(24) Atake, T.; Angell, C. A. J. Phys. Chem. 1979, 83 (25), 3218-
(
3223.
(25) Angell, C. A.; Busse, L. E.; Cooper, E. I.; Kadiyala, R. K.; Dworkin,
A.; Ghelfenstein, M.; Szwarc, H.; Vassal, A. J. Chem. Phys. 1985, 82, 267-
274.
(26) Angell, C. A. J. Non-Cryst. Solids 1991, 131-133, 13-31.
(27) Angell, C. A. Pure Appl. Chem. 1991, 63 (10), 1387-1392.
(28) Fan, J.; Cooper, E. I.; Angell, C. A. J. Phys. Chem. 1994, 98, 9345-
9349.
(
(
1
(