New ionic liquids from azepane and 3-methylpiperidine exhibiting wide electrochemical windows

Article abstract of DOI:10.1039/c0gc00534g

New ionic liquids based on azepanium and 3-methylpiperidinium cations have been synthesised; they exhibit moderate viscosities and remarkably wide electrochemical windows, thereby showing promise, inter alia, as electrolytes and battery materials, and as synthetic media.

Full text of DOI:10.1039/c0gc00534g

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COMMUNICATION
New ionic liquids from azepane and 3-methylpiperidine exhibiting wide
electrochemical windows†
Tayeb Belhocine,a Stewart A. Forsyth,b H. Q. Nimal Gunaratne,a Mark Nieuwenhuyzen,a Alberto V. Puga,a
Kenneth R. Seddon,a Geetha Srinivasana and Keith Whistonb
Received 9th September 2010, Accepted 5th November 2010
DOI: 10.1039/c0gc00534g
New ionic liquids based on azepanium and 3-
methylpiperidinium cations have been synthesised; they
exhibit moderate viscosities and remarkably wide electro-
chemical windows, thereby showing promise, inter alia, as
electrolytes and battery materials, and as synthetic media.
Teramoto et al. have reported the use of acyclic ammonium
or piperidinium ionic liquids in commercial lithium batteries
manufactured by Pionics Co., Ltd., with the safety advantage
of proven self-extinguishing properties on exposure to open
flames.8
Here, we report the syntheses and the physical and elec-
trochemical properties of two new families of ionic liq-
uids, based on 1-alkyl-1-methylazepanium‡ or 1-alkyl-1,3-
dimethylpiperidinium cations, which had been previously noted
in a very recent patent application from INVISTA.14 Such
compounds are derived from the parent alicyclic amines azepane
(azp)15 and 3-methylpiperidine (mbpip), respectively, which are
in turn obtained in industrial processes related to polyamide
production.16 As will be shown, these cyclic secondary amines
can serve as starting materials for the generation of room
temperature ionic liquids with the promise of wider elec-
trochemical windows and liquid temperature ranges. Their
physical properties are compared to those of previously re-
ported 1-alkyl-1-methylpyrrolidinium ([Rmpyrr]+) and 1-alkyl-
1-methylpiperidinium ([Rmpip]+) ionic liquids.
The initial development of ionic liquids was closely linked to
their application as electrolytes,1,2 given the set of favourable
properties for electrochemistry often exhibited by them, such as
inherent ionic conductivity, non-flammability, negligible volatil-
ity and wide potential windows.3 These desirable characteristics
show real promise to overcome major safety issues related to
energy devices, mostly lithium batteries, supercapacitors and
fuel cells.4–8
Over the past three decades, continuous improvements have
been made bringing to the scene water and air stable ionic liquids
showing conductivities on a par with those of conventional
organic solvent-supporting electrolyte systems.4,6,9 Furthermore,
the range of electrochemically robust cations and anions has
been extended in recent years, allowing the preparation of new
ionic liquids with enhanced applicability. In this regard, perfluo-
rinated anions (e.g. bis{(trifluoromethyl)sulfonyl}amide (bistri-
flamide; [NTf2]-),9,10 trifluorotris(pentafluoroethyl)phosphate10
and trifluoromethanesulfonate (triflate; [OTf]-)9 created a break-
through concerning anodic stability. Related investigations
showed that ionic liquids containing tetraalkylammonium
cations, either cyclic11 or acyclic,12 can surpass the perfor-
mance of imidazolium ones, further extending the cathodic
operational limits. In particular, 1,1-dialkylpyrrolidinium and
1,1-dialkylpiperidinium cations are the basis of ionic liquids
showing greater promise. Sakaebe and Matsumoto reported
the use of bistriflamide salts of these cations in lithium ion
batteries, concluding that 1,1-dialkylpiperidinium cations are
the best candidates in terms of Coulombic efficiency.13 Recently,
Results and discussion
The structures of the cations and anions of the newly
prepared cyclic ammonium ionic liquids are depicted in
Schemes 1 and 2. The 1-alkyl-1-methylazepanium and 1-alkyl-
1,3-dimethylpiperidinium cations are abbreviated as [Rmazp]+
and [Rmmbpip]+ respectively. The abbreviations [C4mazp]+ or
aThe QUILL Research Centre, School of Chemistry and Chemical
Engineering, Queen’s University Belfast, Stranmillis Road, Belfast, BT9
5AG, United Kingdom. E-mail: quill@qub.ac.uk
bINVISTA Intermediates, PO Box 401, Wilton, Redcar, TS10 4XY,
United Kingdom. E-mail: keith.whiston-1@invista.com
† CCDC reference number 792935. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0gc00534g
Scheme 1 Syntheses of ionic liquids derived from azepane.
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Table 1 Physical properties of [A]X (where X = [NTf2], [CF3CO2] or
[OTf]) and related piperidinium and pyrrolidinium salts
[A]+
Tg/◦Ca Tm/◦Ca h/cPb r/g cm-3b s/ms cm-1b
X- = [NTf2]-
[C4mazp]+
-69
-68
-79
-77c
-82c
-69
-70
315
358
160
182c
102c
315
392
136
76c
1.3661
1.3148
1.4156
1.3786c
1.4355c
1.3475
1.2982
1.3980
1.3931c
1.4539c
560
331
1117
1100c
2000c
550
[C6mazp]+
[MeOC2mazp]+
[C4mpip]+
[MeOC2mpip]+
[C4mmbpip]+
[C6mmbpip]+
318
[MeOC2mmbpip]+ -70
1192
2600c
3700c
Scheme 2 Syntheses of ionic liquids derived from 3-methylpiperidine.
[C4mpyrr]+
-89c
-91c
-19c
[MeOC2mpyrr]+
53c
[C4mmbpip]+ and [C6mazp]+ or [C6mmbpip]+ thus represent the
1-butyl and 1-hexyl cations, respectively. For the ether derived
side chain the abbreviation MeOC2 represents MeOCH2CH2.
The general synthetic approach for the preparation of these
ionic liquids is relatively straightforward (see Schemes 1 and
2). In both cases, the key step is the initial alkylation of
the azacycle.17 To minimise the formation of the dialkylated
products, azepane and 3-methylpiperidine were treated with
the desired alkylating agent (RBr, where R = butyl, hexyl,
MeOCH2CH2) in methanol at 0 ◦C in the presence of potas-
sium carbonate. In all cases, the corresponding cyclic tertiary
amines were obtained in 65–75% yield after distillation, and
characterised by 1H and 13C NMR spectroscopy and mass
spectrometry. The related ionic liquids [Rmazp][NTf2] and
[Rmmbpip][NTf2] were prepared in almost quantitative yield
by treating the corresponding cyclic tertiary amines with
iodomethane (this is illustrated with iodomethane, but other
alkylating agents can be used with ease), and subsequent
metathesis of the resultant iodide salts with Li[NTf2] in aqueous
solution (Schemes 1 and 2). These materials were obtained as
pale yellow liquids in all cases. The salts with trifluoroethanoate
or triflate anions were obtained by direct methylation of the
tertiary amines with methyl trifluoroethanoate or methyl triflate,
respectively, as low melting point solids and, in some cases, as
liquids (Table 1).
X- = [CF3CO2]-
[C4mazp]+
74
45
[C6mazp]+
[MeOC2mazp]+
[C4mmbpip]+
[C6mmbpip]+
-74
577
453
1.2197
1.1966
408
370
58
41
[MeOC2mmbpip]+ -61
X- = [OTf]-
[C4mazp]+
[C6mazp]+
55
98
[MeOC2mazp]+
[C4mmbpip]+
[C6mmbpip]+
-67
798
654
1.2929
1.2713
328
340
84
82
[MeOC2mmbpip]+ -60
a Glass transition temperatures (Tg) and melting points (Tm) were
re◦corded during the second heating cycles in the DSC traces recorded at
5 C min-1. b Density, viscosity and conductivity were measured at 25 ◦C
for neat ionic liquids. c From reference 20
In the case of [C4mazp][CF3CO2], suitable crystals for X-ray
diffraction were grown by slow evaporation from a saturated
solution in ethanol under a dry dinitrogen atmosphere.§ By
detailed examination of the crystal packing, it was observed that
each cation is coordinated by four anions. The closest cation–
anion contacts are established between carboxylate oxygen
atoms in the anions and hydrogen atoms in the a or b positions
from the nitrogen atom in the cation (see, for example, Fig. 1).
The mean O ◊ ◊ ◊ H distance, averaged over all O ◊ ◊ ◊ HC in the
Fig.
1
A view of an ion pair in the crystal structure of
˚
structure, is 2.51(8) A, considerably shorter than the sum of the
[C4mazp][CF3CO2]. Dotted red lines indicate hydrogen bonding. Dis-
tances (A) are shown in black. White = hydrogen; grey = carbon; blue =
18
˚
van der Waals radii (2.70 A). This indicates the presence of
˚
significant hydrogen bonding in this structure, a phenomenon
which has only recently achieved universal recognition.19 This
is as anticipated, as the hydrogen bond is between a cationic
CH moiety and an anionic oxygen functionality. To the best of
our knowledge, this is the first reported example of a crystal
structure for any salt containing a 1,1-dialkylazepanium cation.
The physicochemical properties of the newly synthesised
ionic liquids, compared to their piperidinium and pyrrolidinium
analogues, are summarised in Table 1. All the new ionic
nitrogen; red = oxygen and green = fluorine.
liquids containing 1-alkyl-1-methylazepanium or 1-alkyl-1,3-
dimethylpiperidinium are either liquid at room temperature or
have a melting point below 100 ◦C. All ionic liquids with the
[NTf2]- an◦ion are liquids with glass transition temperatures
below -68 C. As for the salts with [CF3CO2]- or [OTf]- anions,
it can be seen in Table 1 that the replacement of the 1-alkyl
group in the cation with a chain containing an ether linkage
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results in a noticeable reduction of the melting temperatures.
For example, [C4mazp][CF3CO2] exhibits a melting point above
◦
room temperature (Tm = 74 C), while the etherated analogue
[MeOC2mazp][CF3CO2] is liquid at room temperature (Tg
=
-74 ◦C), despite the fact the chain length is four in both cases. In
other words, substituting a methylene group by an oxygen atom
changes the liquid range by 148 ◦C. A similar effect has been
previously observed for piperidinium and pyrrolidinium ionic
liquids.20
The densities for the new azepaniu◦m and 3-methyl-
piperidinium ionic liquids, measured at 25 C, lie in the range
1.30 to 1.42 g cm-3 for the bistriflamide salts and the individual
values are listed in Table 1. These data are compared to those of
previously reported bistriflamide analogues of piperidinium and
pyrrolidinium in Fig. 2.20 In every case where analogous ionic
liquids containing either butyl or methoxyethyl as side chain on
the cation are compared, higher densities are observed for the
latter, as in the case of [C4mazp][NTf2] and [MeOC2mazp][NTf2]
(1.3661 and 1.4156 g cm-3, respectively). This effect is repre-
sented by solid blue lines in Fig. 2. The effect of the different sizes
of the rings in each cation also has a noticeable effect upon the
densities, as represented by the green dashed lines in Fig. 2; the
smaller the ring, the higher the density. Finally, comparing the
two isomers 1-methylazepanium and 1,3-dimethylpiperidinium
reveals a decrease in density, as represented by the red dashed
line in Fig. 2.
Fig. 3 Viscosities (25 ◦C) for ionic liquids with [NTf2]- anion derived
from 3-methylpiperidine (triangles), azepane (diamonds), piperidine
(squares) and pyrrolidine (circles).
> [Rmpip]+ > [Rmpyrr]+. These results show that an expansion
in the cation core size from 5-membered through to 7-membered
rings leads to a progressive increase in viscosity. A similar
effect is observed with the introduction of asymmetry via a
methyl substituent in the case of the piperidinium based ionic
liquids. For example, the viscosity is increased from 182 cP for
[C4mpip][NTf2] to 315 cP for [C4mmbpip][NTf2].
The different structural features in the cations also have a
noticeable effect on the conductivities of the ionic liquids based
on the [NTf2]- anion (Table 1 and Fig. 4). The introduction of an
ether linkage to the side chain of the cation leads to an increase
in ionic conductivity and a corresponding decrease in viscosity.
In addition, a decrease in the ring size of the cation induces an
increase in conductivity in the order [Rmazp]+ ª [Rmmbpip]+ <
[Rmpip]+ < [Rmpyrr]+, which reflects the decreasing viscosities.
For example, the conductivity at 25 ◦C is reduced from 1100
mS cm-1 for [C4mpip][NTf2]20 to 560 for [C4mazp][NTf2].
◦
Fig. 2 Densities (25 C) for ionic liquids with the [NTf2]- anion,
and the relationship between them, for the cations derived from 3-
methylpiperidine (triangles), azepane (diamonds), piperidine (squares)
and pyrrolidine (circles).
One of the fundamental properties required for evaluating the
applicability of ionic liquids as electrolytes is a low viscosity,
since the electrolytes in many electrochemical devices are
required to operate in ambient temperature ranges. It is now
well established that the viscosity of ionic liquids amongst other
physicochemical properties can be dramatically affected by a
slight modification of their cations. Table 1 shows the dynamic
viscosites for the different ionic liquids at 25 ◦C. For ionic liquids
with the [NTf2]- anion, introduction of an ether bond in the side
chain of the cation reduces the viscosity in all cases, as can be
seen in Fig. 3. For example, the viscosity is reduced from 315 cP
for [C4mazp][NTf2] to 160 cP for [MeOC2mazp][NTf2] by simply
replacing the butyl group in the cation with MeOCH2CH2. The
different cation cores are also compared in Fig. 3, showing
that viscosities decrease in the order [Rmazp]+ ª [Rmmbpip]+
◦
Fig. 4 Conductivities (25 C) for ionic liquids with [NTf2]- anion
derived from 3-methylpiperidine (triangles), azepane (diamonds), piperi-
dine (squares) and pyrrolidine (circles).
The utility of a liquid for electrochemical applications is
frequently reflected in the width of the electrochemical window,
combined with low viscosity, and hence high diffusivity. For
conventional solvents, this window is less than 3 V,4,21,22 whereas
for the ionic liquids presented here, values in the range 5.0 to
6.5 V were obtained. The lowest value in this range (see Fig. 5 and
Table 2) is over 0.5 V larger than for the analogous
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Table 2 Electrochemical windows (DE) specifying cathodic (Ec) and
anodic limits (Ea) of [A]X (where X = [NTf2], [CF3CO2] or [OTf]) and
related pyrrolidinium salts
Finally, some of the newly synthesised salts were also used as
solvents for chemical reactions such as the Heck coupling, which
is known to proceed well in other ionic liquids.24 These new
ionic liquids compare well with those already published in the
literature. In a further example, the O-ethanoylation of glucose
with ethanoic anhydride in [C4mmbpip][N(CN)2] gave a yield of
87%, essentially identical to that reported in the literature.25
[A]+
DE/Va
Ec/Va
Ea/Va
X- = [NTf2]-
[C4mazp]+
6.50
6.25
5.50
6.25
6.00
6.00
5.50
5.50
-3.25
-3.25
-2.75
-3.00
-3.00
-3.00
-2.75
-3.00
3.25
3.00
2.75
3.25
3.00
3.00
2.75
2.50
[C6mazp]+
[MeOC2mazp]+
[C4mmbpip]+
[C6mmbpip]+
[MeOC2mmbpip]+
[C4mpyrr]+
Conclusions
We have synthesised two new families of ionic liquids
from the readily available starting materials, azepane and 3-
methylpiperidine, by reaction with a range of standard alkylating
reagents. Salts with the bistriflamide anion are liquid in all cases,
whereas salts with the trifluoroethanoate or triflate anions are
low melting solids when the cation core is substituted with alkyl
chains, but liquids when the core is substituted with alkoxyalkyl
chains. Examining these two series revealed systematic changes
in density, viscosity and conductivity, which correlated strongly
with alicyclic ring size, chain length and the degree of oxygen
substitution in the chain, thus allowing them to be optimised
by design to produce ionic liquids for specific electrochemical
and/or solvent applications. The current evidence suggests that
the azepanium-based ionic liquids are superior for battery
applications to the more conventionally used pyrrolidinium
systems.
[MeOC2mpyrr]+
X- = [CF3CO2]-
[MeOC2mazp]+
5.00
5.25
-3.00
-3.00
2.00
2.25
[MeOC2mmbpip]+
X- = [OTf]-
[MeOC2mazp]+
5.50b
5.50b
-2.75b
-3.00b
2.75b
2.50b
[MeOC2mmbpip]+
a Recorded by cyclic voltammetry vs. Ag/Ag+ using glassy carbon
working of 3 mm diameter and Pt coil counter electrodes. b Recorded
for 0.1 M solutions in ethanenitrile because of the high viscosity of the
neat liquids.
Notes and references
‡ Alternative names for azepane include hexamethyleneimine (HMI),
hexahydro-1H-azepine, perhydroazepine or azacycloheptane.15
§ Crystal data for [C4mazp][CF3CO2]: C13H24F3NO2, T = 293(2) K,
˚
orthorombic, Pbna, a = 12.2840(2), b = 13.2290(2), c = 18.3910(3) A, V =
3
-3
˚
2988.63(8) A , Z = 8, Dc = 1.259 mg m , reflections collected/unique =
31969/2619 (Rint = 0.0964), m = 0.108 mm-1, S = 1.062, R1 = 0.0789 (I >
2s), wR2 = 0.2428 (all data). CCDC reference number 792935.
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(blue), and (b) [C4mim][NTf2] (red) on glassy carbon electrodes vs.
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