Tetra-branched tetra-cationic ionic liquids: Effects of spacer and tail structure on physical properties

Article abstract of DOI:10.1246/BCSJ.20200148

Electrolytes are key materials that support information society because they are included in most electronic devices, e.g., batteries, supercapacitors, fuel cells, and electrochromic devices. Ionic organic compounds, such as ionic liquids, have attracted increasing attention due to their designability and non-volatility. In this study, ten tetra-branched tetra-cationic ionic liquids, which consisted of a pentaerythritol-based core, alkylene or ethylenedioxy spacers, imidazolium cationic units, and short alkyl tails, were synthesized. The physical properties of the tetra-cations, including their glass transition and thermal decomposition temperatures, densities, viscosities, and ionic conductivities, were investigated. The tetra-cations were analyzed to determine the effects of the spacer and tail structure on these physical properties. The spacer unit located between the pentaerythrityl core and cationic unit was confirmed to be the key for improving ionic conductivity. A maximum ionic conductivity of 2.8 x 10^-4 S/cm (25 °C under anhydrous conditions) and a minimum viscosity of 1.6 Pa-sec (25 °C) were observed. While the physical values of the tetra-cationic ionic liquids are close to those of di-cationic ionic liquids, their structure-property relationship is similar to that of poly-cations rather than di-cations.

Full text of DOI:10.1246/BCSJ.20200148

Document type: Article
Tetra-Branched Tetra-Cationic Ionic Liquids: Effects of Spacer
and Tail Structure on Physical Properties
Taichi Ikeda
Research Center for Functional Materials, National Institute for Materials Science (NIMS),
1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
E-mail: IKEDA.Taichi@nims.go.jp
Received: May 11, 2020; Accepted: June 4, 2020; Web Released: June 11, 2020
Taichi Ikeda
Taichi Ikeda received his Ph.D. from Japan Advanced Institute of Science and Technology in 2002 under the
direction of Prof. Nobuhiko Yui. After working in the groups of Prof. Toshimi Shimizu (National Institute of
Advanced Industrial Science and Technology) and Prof. J. Fraser Stoddart (University of California, Los
Angeles), he joined in NIMS. His research interests include self-assembly, molecular machine, electrolyte
materials, and actuators.
Abstract
been revealed.17,18 Studies on polymer electrolytes also have a
long history, and many applications have been examined.19-22
Meanwhile, the intermediate molecular-weight region, between
that of mono-cationic ionic liquids and polyelectrolytes, has
not yet been well explored. Despite the limited number of exam-
ples, multi-cationic ionic liquids have been confirmed to exhibit
some superior properties compared to their mono-cationic
counterparts, such as high thermal stabilities,23-25 high capaci-
ties as supercapacitors,6,26,27 and higher performance as sta-
tionary phases for gas chromatography, etc.28,29 Among di-,30-32
tri-,33,34 and tetra-cationic ionic liquids,35 the latter are the most
unexplored because they are the most difficult to synthesize.
Although some groups have reported a facile synthesis of multi-
cationic ionic liquids utilizing oligosilsesquioxane frameworks,
no physical properties such as viscosity and ionic conductivity
have been reported so far.36,37
With the above background in mind, this study focused on
tetra-cationic ionic liquids. Generally, multi-cationic ionic
liquids are highly viscous and poorly ionically conductive
because of their high molecular masses; these physical proper-
ties are unfavorable for electronic materials. In order to im-
prove the ionic conductivity as much as possible, we focused
on a branched structure, since branched molecules, such as
dendrimers, have low viscosities.38 In our previous study, we
successfully prepared tetra-branched tetra-cationic ionic liquids
(Figure 1a); however, these tetra-cations exhibited low ionic
conductivities (the order of 10¹6 S cm¹1).39 Therefore, in this
study, improving molecular design for achieving high ionic
conductivity became the key challenge. Figure 1b shows the
molecular design concept of new tetra-cations investigated in
this study. Spacer unit (R1) was introduced between the penta-
erythrityl core and the cationic unit. It should be noted that the
1,2,3-triazolium cationic unit previously used was changed to
Herein, the synthesis of ten tetra-branched tetra-cationic
ionic liquids, which consist of a pentaerythritol-based core,
alkylene or ethylenedioxy spacers, imidazolium cationic units,
and short alkyl tails, is described. The physical properties of
the tetra-cations, including their glass transition and thermal
decomposition temperatures, densities, viscosities, and ionic
conductivities, were investigated. The tetra-cations were ana-
lyzed to determine the effects of the spacer and tail structure
on the above-mentioned physical properties. The spacer unit
located between the pentaerythrityl core and cationic unit was
confirmed to be the key for improving ionic conductivity. A
maximum ionic conductivity of 2.8 © 10¹4 S cm
¹1
(25 °C
under anhydrous conditions) and a minimum viscosity of 1.6
Pa s (25 °C) were observed. While the physical values of the
tetra-cationic ionic liquids are close to those of di-cationic ionic
liquids, their structure-property relationship is similar to that of
poly-cations rather than di-cations.
Keywords: Ionic liquids
j
Branched structure
j
Ion conductivity
1. Introduction
Electrolytes are key materials that support information
society because they are included in most electronic devices,
e.g. batteries,1-5 supercapacitors,6-8 fuel cells,9-11 and electro-
chromic devices,12-14 etc. Ionic organic compounds, such as
ionic liquids, have attracted increasing attention due to their
designability and non-volatility.15,16 Ionic organic compounds
cover a large molecular-weight range, from 102 Da (mono-
cationic ionic liquids) to 106 Da (polyelectrolytes). Mono-
cationic ionic liquids have been extensively studied over the
past two decades and the details of their physical properties have
1218 | Bull. Chem. Soc. Jpn. 2020, 93, 1218–1225 | doi:10.1246/bcsj.20200148
© 2020 The Chemical Society of Japan

1
t-C3-O-THP: Yield: 76%. H NMR (400 MHz, CD3CN):
¤ = 1.40-1.82 (m, 32H), 3.34 (s, 8H), 3.40-3.50 (m, 16H),
3.70-3.85 (m, 8H), 4.55 (t, J = 3.4 Hz, 4H); 13C NMR (100
MHz, CD3CN): ¤ = 21.4, 27.2, 31.8, 32.5, 47.2, 63.7, 65.9,
69.9, 71.2, 100.6.
1
t-C4-O-THP: Yield: 80%. H NMR (400 MHz, CD3CN):
¤ = 1.40-1.82 (m, 40H), 3.26-3.46 (m, 24H), 3.65 (m, 4H),
3.78 (m, 4H), 4.52 (t, J = 3.4 Hz, 4H); 13C NMR (100 MHz,
CD3CN): ¤ = 20.5, 26.3, 27.3, 27.4, 31.6, 46.3, 62.8, 67.9,
70.3, 71.9, 99.6.
Figure 1. Molecular design of tetra-cationic ionic liquids in
(a) previous study and (b) this study.
1
t-C5-O-THP: Yield: 72%. H NMR (400 MHz, CD3CN):
the imidazolium to avoid the risk of handling explosive azide
compounds. Here, the effects of the spacer and tail structure on
key physical properties (glass transition and thermal decom-
position temperatures, density, viscosity, and ionic conductiv-
ity) are reported. It was confirmed that the introduction of
spacer units could improve ionic conductivity by two orders of
magnitude.
¤ = 1.35-1.84 (m, 48H), 3.28-3.48 (m, 24H), 3.68 (m, 4H),
3.81 (m, 4H), 4.55 (t, J = 3.4 Hz, 4H); 13C NMR (100 MHz,
CD3CN): ¤ = 20.5, 23.9, 26.3, 30.3, 30.3, 31.6, 46.3, 62.7,
68.0, 70.2, 72.0, 99.5.
t-C6-THP:
Yield: 76%. 1H NMR (400 MHz, CD3CN):
¤ = 1.35 (m, 16H), 1.40-1.82 (m, 40H), 3.25-3.46 (m, 24H),
3.64 (m, 4H), 3.78 (m, 4H), 4.51 (t, J = 3.4 Hz, 4H); 13C NMR
(100 MHz, CD3CN): ¤ = 20.5, 26.4, 26.9, 30.4, 30.6, 31.6,
46.3, 62.7, 68.0, 70.3, 72.0, 99.6.
2. Experimental
Method. NMR spectra were recorded on a JEOL ECS-400
t-EG2-O-THP: Yield: 82%. 1H NMR (400 MHz, CD3CN):
¤ = 1.40-1.82 (m, 24H), 3.36 (s, 8H), 3.40-3.60 (m, 32H),
3.70-3.84 (m, 8H), 4.57 (t, J = 3.4 Hz, 4H); 13C NMR (100
MHz, CD3CN): ¤ = 20.4, 26.3, 31.5, 46.4, 62.7, 67.5, 70.5,
71.0, 71.2, 72.0, 99.7.
t-EG3-O-THP: Yield: 85%. 1H NMR (400 MHz, CD3CN):
¤ = 1.42-1.84 (m, 24H), 3.38 (s, 8H), 3.42-3.66 (m, 48H),
3.75-3.88 (m, 8H), 4.59 (t, J = 3.4 Hz, 4H); 13C NMR (100
MHz, CD3CN): ¤ = 20.4, 26.3, 31.5, 46.4, 62.8, 67.5, 70.5,
71.0, 71.2, 71.3, 71.3, 72.0, 99.8.
1
(400 MHz and 100 MHz for H and 13C nuclei, respectively)
with residual solvent as the internal standard. The density
was measured using 1 mL glass specific gravity pycnometer
(Thomas Scientific). The viscosity was measured using a
rolling-ball viscometer Lovis 2000 M/ME (Anton Paar).
Differential scanning calorimetry (DSC) was performed on a
Shimadzu DSC-60 Plus at a heating/cooling rate of 10
¹1
°C min under N2 flow. Thermogravimetric analysis (TGA)
was performed with a Shimadzu DTG-60 at a heating rate of
¹1
10 °C min under N2 flow. Ionic conductivity was measured
General Procedure of t-R1-OH Synthesis. t-R1-O-THP
(7.7 mmol) and pyridinium p-toluenesulfonate (PPTS, 3.2
mmol) were dissolved in MeOH (300 mL). The reaction mix-
ture was stirred for 16 h at 70 °C under N2 atmosphere. The
solvent was removed by evaporation. The residue was directly
subjected to column chromatography (SiO2, CH2Cl2/Ace-
tone = 1/1; Acetone/MeOH = 9/1).
using two-terminal impedance spectroscopy on a Solartron SI
1260 with a 1296 Dielectric Interface. The sample was placed
on a disk-type blocking electrode (diameter: 5 mm) of a
Solartron 12962A dielectric sample holder. The spacing
between two electrodes was set to 300 ¯m. The frequency was
swept from 1 MHz to 1 Hz by applying a sinusoidal voltage of
10 mV. The sample holder was placed inside a programmable
thermostat chamber ESPEC SH-221 filled with dry N2 gas. The
sample was completely dried at 100 °C for 1 h before starting
the measurement. The impedance data were collected from 100
to 10 °C at a cooling rate of 20 °C h¹1 (0.33 °C min¹1). The data
were processed using ZViewμ Ver. 3.3c software.
1
t-C2-OH: Yield: 83%. H NMR (400 MHz, CD3CN): ¤ =
3.06 (br, 4H), 3.38-3.48 (m, 16H), 3.56 (br, 8H); 13C NMR
(100 MHz, CD3CN): ¤ = 46.3, 61.8 70.7, 73.7.
t-C3-OH:
Yield: 88%. 1H NMR (400 MHz, CD3CN):
¤ = 1.72 (m, 8H), 3.03 (t, J = 5.4 Hz, 4H), 3.34 (s, 8H), 3.48 (t,
J = 6.2 Hz, 8H), 3.59 (q, J = 6.0 Hz, 8H); 13C NMR (100 MHz,
CD3CN): ¤ = 34.2, 46.9, 61.3, 70.6, 71.8.
General Procedure of t-R1-O-THP Synthesis.
Tetra-
hydropyranyl (THP) derivative of OH-R1-O-THP (65 mmol)
was dissolved in DMF (50 mL). NaH (60 wt% dispersion in oil,
3.0 g) was added portion-wise. The reaction mixture was stirred
for 30 min at room temperature. After adding pentaerythrityl
tetrabromide (13 mmol), the reaction mixture was stirred for
24 h at 100 °C under N2 atmosphere. The solvent was removed
by evaporation. The residue was dissolved in diethyl ether (150
mL) and washed with water (50 mL) using a separation funnel.
The organic layer was recovered, dried with MgSO4, filtrated
and concentrated with an evaporator. The crude product was
purified by column chromatography (SiO2, Hex/AcOEt = 1/1).
t-C4-OH:
Yield: 74%. 1H NMR (400 MHz, CD3CN):
¤ = 1.46-1.58 (m, 16H), 2.79 (t, J = 5.2 Hz, 4H), 3.30 (s,
8H), 3.37 (t, J = 6.0 Hz, 8H), 3.48 (q, J = 5.6 Hz, 8H);
13C NMR (100 MHz, CD3CN): ¤ = 27.0, 30.5, 46.2, 62.5,
70.3, 72.0.
1
t-C5-OH: Yield: 78%. H NMR (400 MHz, CD3CN): ¤ =
1.37 (m, 8H), 1.46-1.58 (m, 16H), 2.74 (t, J = 5.4 Hz, 4H),
3.31 (s, 8H), 3.37 (t, J = 6.4 Hz, 8H), 3.49 (q, J = 6.0 Hz, 8H);
13C NMR (100 MHz, CD3CN): ¤ = 23.4, 30.2, 33.4, 46.3, 62.6,
70.2, 72.1.
1
t-C6-OH: Yield: 75%. H NMR (400 MHz, CD3CN): ¤ =
1
t-C2-O-THP: Yield: 85%. H NMR (400 MHz, CD3CN):
1.27-1.38 (m, 16H), 1.42-1.56 (m, 16H), 2.67 (t, J = 5.2 Hz,
4H), 3.28 (s, 8H), 3.34 (t, J = 6.2 Hz, 8H), 3.46 (q, J = 6.2 Hz,
8H); 13C NMR (100 MHz, CD3CN): ¤ = 26.5, 26.9, 30.4, 33.6,
46.3, 62.6, 70.2, 72.0.
¤ = 1.40-1.82 (m, 24H), 3.30-3.56 (m, 24H), 3.66-3.86 (m,
8H), 4.58 (t, J = 3.4 Hz, 4H); 13C NMR (100 MHz, CD3CN):
¤ = 20.3, 26.3, 31.5, 46.6, 62.6, 67.2, 70.5, 71.8, 99.6.
Bull. Chem. Soc. Jpn. 2020, 93, 1218–1225 | doi:10.1246/bcsj.20200148
© 2020 The Chemical Society of Japan | 1219

t-EG2-OH: Yield: 73%. 1H NMR (400 MHz, CD3CN): ¤ =
3.16 (t, J = 5.2 Hz, 4H), 3.38 (s, 8H), 3.47-3.62 (m, 32H);
13C NMR (100 MHz, CD3CN): ¤ = 46.3, 62.0, 70.8, 70.9, 71.9,
73.3.
t-EG3-OH: Yield: 83%. 1H NMR (400 MHz, CD3CN):
¤ = 3.24 (t, J = 5.8 Hz, 4H), 3.40 (s, 8H), 3.47-3.64 (m, 48H);
13C NMR (100 MHz, CD3CN): ¤ = 46.2, 61.9, 70.9, 70.9, 71.0,
71.1, 71.8, 73.3.
General Procedure of t-R1-I Synthesis.
t-R1-Ts (5.9
mmol) and NaI (0.12 mol) were dissolved in acetone (100 mL).
The reaction mixture was stirred at 60 °C for 60 h under N2
atmosphere. After removing the solvent by evaporation, the
residue was partitioned between CH2Cl2 (150 mL) and water
(150 mL). The organic layer was recovered, dried with MgSO4,
filtered and concentrated by evaporation. The crude prod-
uct was purified by column chromatography (SiO2, CH2Cl2/
AcOEt = 9/1).
General Procedure of t-R1-Ts Synthesis.
t-R1-OH
1
(6.4 mmol), triethylamine (TEA, 43 mmol), and 4-dimethyl-
amino-pyridine (DMAP, 4.1 mmol) were dissolved in CH2Cl2/
THF mixed solvent (50 mL/50 mL). p-Toluenesulfonyl chlo-
ride (TsCl, 42 mmol) was added to the solution. The reac-
tion mixture was stirred for 16 h at room temperature under
N2 atmosphere. The solution was washed with 1 M HCl aque-
ous solution. The solution was dried with MgSO4, filtered
and concentrated by evaporation. The crude product was puri-
fied by column chromatography (SiO2, CH2Cl2/AcOEt =
4/1).
t-C2-I: Yield: 94%. H NMR (400 MHz, CD3CN): ¤ =
3.30 (t, J = 6.0 Hz, 8H), 3.45 (s, 8H), 3.67 (t, J = 6.0 Hz, 8H);
13C NMR (100 MHz, CD3CN): ¤ = 5.5, 46.8, 69.6, 72.5.
t-C3-I: Yield: 98%. 1H NMR (400 MHz, CDCl3): ¤ = 2.04
(m, 8H), 3.27 (t, J = 6.8 Hz, 8H), 3.37 (s, 8H), 3.44 (t, J =
6.8 Hz, 8H); 13C NMR (100 MHz, CDCl3): ¤ = 3.9, 33.6, 45.6,
69.6, 70.6.
1
t-C4-I: Yield: 94%. H NMR (400 MHz, DMSO-d6): ¤ =
1.56 (m, 8H), 1.82 (m, 8H), 3.22-3.31 (m, 16H), 3.35 (t, J =
6.2 Hz, 8H); 13C NMR (100 MHz, DMSO-d6): ¤ = 8.7, 29.9,
30.2, 44.8, 69.0, 69.6.
1
t-C2-Ts: Yield: 92%. H NMR (400 MHz, CD3CN): ¤ =
2.41 (s, 12H), 3.03 (s, 8H), 3.43 (t, J = 4.2 Hz, 8H), 4.04 (t,
J = 4.2 Hz, 8H), 7.40 (d, J = 8.4 Hz, 8H), 7.75 (d, J = 8.4 Hz,
8H); 13C NMR (100 MHz, CD3CN): ¤ = 21.6, 46.2, 69.6, 69.9,
70.9, 128.7, 131.0, 133.9, 146.3.
t-C5-I: Yield: 98%. 1H NMR (400 MHz, CDCl3): ¤ = 1.46
(m, 8H), 1.56 (m, 8H), 1.84 (m, 8H), 3.19 (t, J = 7.0 Hz, 8H),
3.34-3.40 (m, 16H); 13C NMR (100 MHz, CDCl3): ¤ = 7.3,
27.5, 28.7, 33.4, 45.5, 69.9, 71.2.
1
t-C3-Ts: Yield: 92%. H NMR (400 MHz, CD3CN): ¤ =
t-C6-I:
Yield: 95%. 1H NMR (400 MHz, CDCl3): ¤ =
1.78 (m, 8H), 2.43 (s, 12H), 3.03 (s, 8H), 3.27 (t, J = 5.6 Hz,
8H), 4.05 (t, J = 6.4 Hz, 8H), 7.43 (d, J = 8.4 Hz, 8H), 7.78
(d, J = 8.4 Hz, 8H); 13C NMR (100 MHz, CD3CN): ¤ = 22.6,
30.6, 46.9, 68.2, 70.1, 70.8, 129.6, 131.9, 134.8, 147.2.
1.28-1.44 (m, 16H), 1.52 (q, J = 7.0 Hz, 8H), 1.81 (q, J =
7.0 Hz, 8H), 3.17 (t, J = 7.2 Hz, 8H), 3.30-3.38 (m, 16H);
13C NMR (100 MHz, CDCl3): ¤ = 7.1, 25.2, 29.4, 30.3, 33.5,
45.4, 69.8, 71.2.
1
1
t-C4-Ts: Yield: 90%. H NMR (400 MHz, CD3CN): ¤ =
t-EG2-I: Yield: 93%. H NMR (400 MHz, CD3CN): ¤ =
1.44 (m, 8H), 1.62 (m, 8H), 2.41 (s, 12H), 3.10 (s, 8H), 3.21 (t,
J = 6.2 Hz, 8H), 4.01 (t, J = 6.2 Hz, 8H), 7.40 (d, J = 8.4 Hz,
8H), 7.75 (d, J = 8.4 Hz, 8H); 13C NMR (100 MHz, CD3CN):
¤ = 21.7, 26.2, 26.7, 46.0, 70.1, 71.2, 72.0, 128.7, 131.0,
134.0, 146.2.
3.29 (t, J = 6.4 Hz, 8H), 3.39 (s, 8H), 3.53 (t, J = 4.8 Hz, 8H),
3.58 (t, J = 4.8 Hz, 8H), 3.70 (t, J = 6.4 Hz, 8H); 13C NMR
(100 MHz, CD3CN): ¤ = 5.2, 46.4, 70.6, 70.6, 71.9, 72.4.
1
t-EG3-I: Yield: 98%. H NMR (400 MHz, CD3CN): ¤ =
3.32 (t, J = 6.4 Hz, 8H), 3.39 (s, 8H), 3.52-3.64 (m, 32H), 3.73
(t, J = 6.4 Hz, 8H); 13C NMR (100 MHz, CD3CN): ¤ = 5.0,
46.4, 70.5, 70.9, 71.1, 71.2, 71.9, 72.4.
1
t-C5-Ts: Yield: 94%. H NMR (400 MHz, CD3CN): ¤ =
1.31 (m, 8H), 1.42 (m, 8H), 1.61 (m, 8H), 2.44 (s, 12H), 3.21
(s, 8H), 3.26 (t, J = 6.2 Hz, 8H), 4.01 (t, J = 6.2 Hz, 8H), 7.43
(d, J = 7.6 Hz, 8H), 7.78 (d, J = 7.6 Hz, 8H); 13C NMR (100
MHz, CD3CN): ¤ = 21.7, 22.9, 29.2, 29.6, 46.2, 70.1, 71.6,
72.0, 128.7, 131.0, 134.0, 146.2.
General Procedure of t-R1-R2Im¢Tf2N Synthesis. t-R1-I
(2.0 mmol) and R2Im (42 mmol) were mixed. The reaction
mixture was stirred at 100 °C for 72 h under N2 atmosphere.
The reaction mixture was diluted with distilled water (150 mL).
The aqueous solution was washed with CH2Cl2 (100 mL © 4)
and diethyl ether (100 mL © 3). Lithium bis(trifluoromethane-
sulfonyl)imide (Li¢Tf2N) aqueous solution (10 g in 10 mL
water) was added to the aqueous solution. The solution was
stirred for 1 h at room temperature. The aqueous solution was
concentrated to 100 mL by evaporation. The turbid aqueous
solution was subjected to centrifugation (6000 rpm © 10 min).
The supernatant was removed by decantation. The precipitate
was dissolved in acetone and dried with MgSO4, filtrated and
concentrated by evaporation.
1
t-C6-Ts: Yield: 91%. H NMR (400 MHz, CD3CN): ¤ =
1.17-1.28 (m, 16H), 1.40 (q, J = 7.0 Hz, 8H), 1.57 (q, J = 7.0
Hz, 8H), 2.41 (s, 12H), 3.20-3.28 (m, 16H), 3.98 (t, J = 6.4
Hz, 8H), 7.40 (d, J = 8.4 Hz, 8H), 7.75 (d, J = 8.4 Hz, 8H);
13C NMR (100 MHz, CD3CN): ¤ = 21.7, 25.8, 26.3, 29.4, 30.1,
46.2, 70.2, 71.8, 72.0, 128.7, 131.0, 134.1, 146.2.
1
t-EG2-Ts: Yield: 92%. H NMR (400 MHz, CD3CN): ¤ =
2.41 (s, 12H), 3.27 (s, 8H), 3.40 (t, J = 4.4 Hz, 8H), 3.44 (t,
J = 4.4 Hz, 8H), 3.58 (t, J = 4.4 Hz, 8H), 4.08 (t, J = 4.4 Hz,
8H), 7.40 (d, J = 8.4 Hz, 8H), 7.75 (d, J = 8.4 Hz, 8H);
13C NMR (100 MHz, CD3CN): ¤ = 21.7, 46.3, 69.2, 70.3, 70.9,
71.0, 71.7, 128.7, 131.0, 133.9, 146.3.
t-C2-EtIm¢Tf2N:
Yield: 89%. 1H NMR (400 MHz,
CD3CN): ¤ = 1.45 (t, J = 7.4 Hz, 12H), 3.23 (s, 8H), 3.56 (t,
J = 5.0 Hz, 8H), 4.12-4.26 (m, 16H), 7.34 (t, J = 1.8 Hz, 4H),
7.41 (t, J = 1.8 Hz, 4H), 8.43 (s, 4H); 13C NMR (100 MHz,
CD3CN): ¤ = 15.6, 46.0, 46.2, 50.5, 69.8, 70.3, 120.9 (q, J =
318.4 Hz), 123.0, 124.0, 136.3.
1
t-EG3-Ts: Yield: 90%. H NMR (400 MHz, CD3CN): ¤ =
2.45 (s, 12H), 3.34 (s, 8H), 3.44-3.54 (m, 32H), 3.61 (t, J = 4.6
Hz, 8H), 4.12 (t, J = 4.6 Hz, 8H), 7.44 (d, J = 8.4 Hz, 8H),
7.79 (d, J = 8.4 Hz, 8H); 13C NMR (100 MHz, CD3CN): ¤ =
21.7, 46.4, 69.2, 70.5, 71.0, 71.0, 71.1, 71.2, 71.9, 128.8,
131.0, 133.8, 146.3.
t-C3-EtIm¢Tf2N:
Yield: 84%. 1H NMR (400 MHz,
CD3CN): ¤ = 1.49 (t, J = 7.2 Hz, 12H), 2.07 (m, 8H), 3.38
1220 | Bull. Chem. Soc. Jpn. 2020, 93, 1218–1225 | doi:10.1246/bcsj.20200148
© 2020 The Chemical Society of Japan

(s, 8H), 3.43 (t, J = 6.0 Hz, 8H), 4.16-4.23 (m, 16H), 7.39
(t, J = 1.8 Hz, 4H), 7.44 (t, J = 1.8 Hz, 4H), 8.47 (s, 4H);
13C NMR (100 MHz, CD3CN): ¤ = 15.4, 30.7, 46.0, 46.2, 48.0,
68.2, 70.4, 120.9 (q, J = 318.4 Hz), 123.1, 123.7, 136.0.
3. Results and Discussion
3.1 Synthesis of Tetra-Cationic Ionic Liquids. Scheme 1
shows the synthetic route used to prepare the tetra-cationic ionic
liquids. The Williamson ether synthesis was used to graft HO-
R1-O-THP to the pentaerythrityl core. R1 with 2-6 alkylene
carbons or 2-3 ethylenedioxy units were selected as spacers.
After removing the THP groups, each compound was trans-
formed into the corresponding tetra-iodide through tosylation
followed by a Finkelstein reaction. The tetra-iodide was then
reacted with excess 1-alkylimidazole in the absence of solvent.
Methyl, ethyl, and n-butyl groups were selected as tail units
(R2). After ion-metathesis, tetra-cations with Tf2N counter ions
were successfully obtained as pale yellow liquids. The chem-
t-C4-EtIm¢Tf2N:
Yield: 93%. 1H NMR (400 MHz,
CD3CN): ¤ = 1.43-1.54 (m, 20H), 1.86 (m, 8H), 3.32 (s,
8H), 3.36 (t, J = 6.2 Hz, 8H), 4.09-4.20 (m, 16H), 7.37 (t, J =
1.8 Hz, 4H), 7.40 (t, J = 1.8 Hz, 4H), 8.45 (s, 4H); 13C NMR
(100 MHz, CD3CN): ¤ = 15.4, 27.0, 27.8, 45.9, 46.3, 50.5,
70.7, 71.4, 120.9 (q, J = 318.5 Hz), 123.2, 123.4, 135.9.
t-C5-EtIm¢Tf2N:
Yield: 83%. 1H NMR (400 MHz,
CD3CN): ¤ = 1.35 (m, 8H), 1.48 (m, 12H), 1.55 (m, 8H),
1.86 (m, 8H), 3.30 (s, 8H), 3.35 (t, J = 6.4 Hz, 8H), 4.12 (t,
J = 7.2 Hz, 8H), 4.18 (q, J = 7.2 Hz, 8H), 7.39 (t, J = 1.8 Hz,
4H), 7.42 (t, J = 1.8 Hz, 4H), 8.47 (s, 4H); 13C NMR (100
MHz, CD3CN): ¤ = 15.4, 23.6, 29.7, 30.4, 45.9, 46.3, 50.5,
70.4, 71.8, 120.9 (q, J = 318.5 Hz), 123.1, 123.4, 135.8.
1
ical structures of the tetra-cations were confirmed by H and
13C NMR spectroscopy (see the Supporting Information), which
revealed perfect imidazolium functionalization. In our previous
study, we attempted to introduce the imidazolium unit directly
onto the pentaerythrityl core without a spacer, but this strategy
failed, presumably due to steric hindrance.39 The spacer units
contribute to relieving steric crowding around the core. The
names of ten tetra-cations are summarized in Scheme 1, along
with their chemical structures of the R1 and R2 units.
t-C6-EtIm¢Tf2N:
Yield: 90%. 1H NMR (400 MHz,
CD3CN): ¤ = 1.25-1.40 (m, 16H), 1.42-1.54 (m, 20H), 1.81
(q, J = 7.4 Hz, 8H), 3.28 (s, 8H), 3.32 (t, J = 6.6 Hz, 8H), 4.10
(t, J = 7.2 Hz, 8H), 4.16 (q, J = 7.3 Hz, 8H), 7.37 (t, J =
1.8 Hz, 4H), 7.40 (t, J = 1.8 Hz, 4H), 8.46 (s, 4H); 13C NMR
(100 MHz, CD3CN): ¤ = 15.4, 26.3, 26.6, 30.2, 30.5, 45.9,
46.3, 50.6, 70.4, 72.0, 120.9 (q, J = 318.5 Hz), 123.1, 123.4,
135.9.
3.2 Thermal Properties of Tetra-Cationic Ionic Liquids.
The thermal properties of the tetra-cationic ionic liquids were
examined by differential scanning calorimetry (Table 1). The
thermograms of the tetra-cations did not show melting peaks,
but heat-capacity changes due to glass transitions (Figure S18
in the Supporting Information). Figure 2 summarizes the glass
transition temperatures (Tgs) of the tetra-cations, which reveals
that all samples have Tgs that are below ¹43 °C, which is much
lower than those reported for tetra-cations without spacer units
(ca. ¹35 °C, Figure 1a).39 In cases of the tetra-cations with
alkylene spacers (t-Cn-EtIm¢Tf2N), the Tg values decreased
drastically with increasing the spacer length from C2 to C4,
while the Tg values of the tetra-cations with C4, C5, and C6
spacers were almost the same. Although a longer spacer de-
creases the Tg value due to higher degree of conformational
freedom, it can cause larger molecular interactions between
alkylene chains to increase the Tg value. Therefore, there is an
optimum chain length to give the lowest Tg.23,25,33 The Tg values
of the tetra-cations with EG3 spacer were lower than those with
EG2 spacer. As for the effect of the tail structure, the ethyl group
provided lower Tgs than the methyl group, but the Tgs of t-EG2-
BuIm¢Tf2N and t-EG2-EtIm¢Tf2N were the same.
The thermal-decomposition behavior of the tetra-cations was
characterized by thermogravimetric analysis (Figure S19 in the
Supporting Information). The 5% weight-loss temperatures
(Td5) ranged between 346 °C and 403 °C (Table 1). It must be
emphasized that the spacer could improve thermal stability,
since the Td5 values of tetra-cations without spacer units in our
previous study (Figure 1a) were all below 310 °C.39 The spacer
unit relieves mechanical stresses around the pentaerythrityl
core, concurrently increasing the distance between the imid-
azole units. These effects suppress thermal reactions between
the imidazoles.
t-EG2-MeIm¢Tf2N:
Yield: 83%. 1H NMR (400 MHz,
CD3CN): ¤ = 3.34 (s, 8H), 3.51 (m, 8H), 3.57 (m, 8H), 3.79
(t, J = 4.8 Hz, 8H), 3.85 (s, 12H), 4.27 (t, J = 4.8 Hz, 8H), 7.35
(t, J = 1.8 Hz, 4H), 7.41 (t, J = 1.8 Hz, 4H), 8.45 (s, 4H);
13C NMR (100 MHz, CD3CN): ¤ = 36.9, 46.4, 50.5, 69.4, 70.5,
70.9, 71.7, 120.9 (q, J = 318.5 Hz), 123.9, 124.3, 137.2.
t-EG2-EtIm¢Tf2N:
Yield: 91%. 1H NMR (400 MHz,
CD3CN): ¤ = 1.46 (t, J = 7.2 Hz, 12H), 3.32 (s, 8H), 3.48 (m,
8H), 3.56 (m, 8H), 3.78 (t, J = 5.2 Hz, 8H), 4.17 (q, J =
7.2 Hz, 8H), 4.25 (t, J = 5.2 Hz, 8H), 7.40 (t, J = 1.8 Hz, 4H),
7.41 (t, J = 1.8 Hz, 4H), 8.48 (s, 4H); 13C NMR (100 MHz,
CD3CN): ¤ = 15.5, 45.9, 46.4, 50.5, 69.4, 70.6, 71.0, 71.8,
120.9 (q, J = 318.5 Hz), 122.8, 124.1, 136.3.
t-EG2-BuIm¢Tf2N:
Yield: 90%. 1H NMR (400 MHz,
CD3CN): ¤ = 0.93 (t, J = 7.4 Hz, 12H), 1.31 (m, 8H), 1.81 (m,
8H), 3.32 (s, 8H), 3.47 (m, 8H), 3.55 (m, 8H), 3.77 (t, J =
5.0 Hz, 8H), 4.13 (t, J = 7.4 Hz, 8H), 4.25 (t, J = 5.0 Hz, 8H),
7.38 (t, J = 1.8 Hz, 4H), 7.41 (t, J = 1.8 Hz, 4H), 8.46 (s, 4H);
13C NMR (100 MHz, CD3CN): ¤ = 13.6, 20.0, 32.6, 46.4, 50.4,
50.5, 69.4, 70.6, 70.9, 71.8, 120.9 (q, J = 318.5 Hz), 123.1,
124.0, 136.6.
t-EG3-MeIm¢Tf2N:
Yield: 86%. 1H NMR (400 MHz,
CD3CN): ¤ = 3.35 (s, 8H), 3.48-3.62 (m, 32H), 3.79 (t, J =
5.2 Hz, 8H), 3.85 (s, 8H), 4.28 (t, J = 5.2 Hz, 8H), 7.36 (t, J =
1.8 Hz, 4H), 7.44 (t, J = 1.8 Hz, 4H), 8.51 (s, 4H); 13C NMR
(100 MHz, CD3CN): ¤ = 36.9, 46.4, 50.5, 69.3, 70.5, 70.9,
70.0, 71.1, 71.9, 120.9 (q, J = 318.5 Hz), 124.0, 124.3, 137.4.
t-EG3-EtIm¢Tf2N:
Yield: 81%. 1H NMR (400 MHz,
CD3CN): ¤ = 1.49 (t, J = 7.4 Hz, 12H), 3.35 (s, 8H), 3.47-3.62
(m, 32H), 3.79 (t, J = 4.8 Hz, 8H), 4.19 (q, J = 7.4 Hz, 8H),
4.28 (t, J = 4.8 Hz, 8H), 7.42 (t, J = 1.8 Hz, 4H), 7.45 (t, J =
1.8 Hz, 4H), 8.54 (s, 4H); 13C NMR (100 MHz, CD3CN): ¤ =
15.5, 45.9, 46.4, 50.5, 69.2, 70.5, 70.9, 71.1, 71.9, 120.9 (q,
J = 318.5 Hz), 122.7, 124.1, 136.4.
3.3 Density and Viscosity of Tetra-Cationic Ionic Liquids.
The densities of the tetra-cations were measured by pycnom-
etry. The results are summarized in Table 1. Densities were
Bull. Chem. Soc. Jpn. 2020, 93, 1218–1225 | doi:10.1246/bcsj.20200148
© 2020 The Chemical Society of Japan | 1221

Scheme 1. Synthesis of tetra-branched tetra-cationic ionic liquids. Reaction conditions: a) NaH, DMF, 100 °C, 24 h; b) PPTS, MeOH,
70 °C, 16 h; c) TEA, DMAP, TsCl, THF, r.t., 16 h; d) NaI, acetone, 60 °C, 60 h; e) R2-imidazole, 100 °C, 72 h then Li¢Tf2N, water,
r.t., 1 h. Chemical structures and sample names are summarized at bottom.
Table 1. Physical properties of tetra-cationic ionic liquids
a
b
c
Mw
Tg
Td5
Densityd
g cm
Viscositye
Tetra-cation
¹1
¹3
g mol
°C
°C
Pa s
t-C2-EtIm¢Tf2N
t-C3-EtIm¢Tf2N
t-C4-EtIm¢Tf2N
t-C5-EtIm¢Tf2N
t-C6-EtIm¢Tf2N
t-EG2-MeIm¢Tf2N
t-EG2-EtIm¢Tf2N
t-EG2-BuIm¢Tf2N
t-EG3-MeIm¢Tf2N
t-EG3-EtIm¢Tf2N
1749.4
1805.5
1861.7
1917.8
1973.9
1869.5
1925.7
2037.9
2045.8
2101.9
¹43
¹45
¹51
¹51
¹52
¹47
¹52
¹52
¹50
¹53
403
393
346
361
375
382
387
382
370
364
1.52
1.50
1.46
1.43
1.40
1.52
1.47
1.39
1.48
1.45
4.4
3.4
2.5
2.8
3.0
2.9
1.8
2.2
2.1
1.6
aMolecular weight. bGlass transition temperature. c5 wt% loss temperature. dMeasured using
e
pycnometer at 25 °C. Measured using a rolling-ball viscometer at 25 °C.
lower density than that with shorter spacer, and the densities
of the tetra-cations with ethylenedioxy spacers are somewhat
higher than those with alkylene spacers.
The viscosities of the tetra-cations were measured using a
rolling-ball viscometer (Table 1). The results are summarized
in Figure 4. Among the tetra-cations with alkylene spacer (t-
Cn-EtIm¢Tf2N), t-C4-EtIm¢Tf2N exhibited the lowest vis-
cosity of 2.5 Pa s. The relatively short spacers (C2, C3, and C4)
are effective to lower the molecular interaction between the
pentaerythrityl cores to decrease the viscosity. On the other
hand, the alkylene spacer longer than C4 causes larger molec-
ular interaction between alkylene chains to increase the vis-
cosity.23,25,33 In cases of the tetra-cations with ethylenedioxy
spacer (t-EGn-R2Im¢Tf2N), longer spacers gave lower vis-
cosities. The flexibility of the ethylenedioxy spacer is consid-
ered to contribute to lowering the viscosity. As for the tail unit,
the ethyl-substituted imidazoles were found to be less viscous
than the methyl- and n-butyl-substituted imidazoles.
Figure 2. Effects of spacer and tail structure on glass
transition temperature.
found to range between 1.39 and 1.52 g cm¹3. These values
are comparable to those of conventional ionic liquids.17 The
relationship between molecular weight and density was plot-
ted (Figure 3). Tetra-cation with longer spacer generally gives
The lowest viscosity of 1.6 Pa s was observed for t-EG3-
EtIm¢Tf2N. The viscosity of the representative mono-cationic
1222 | Bull. Chem. Soc. Jpn. 2020, 93, 1218–1225 | doi:10.1246/bcsj.20200148
© 2020 The Chemical Society of Japan

Figure 3. Relationship between molecular weight and
density. Temperature: 25 °C.
Figure 5. Ionic conductivities as functions of reciprocal
temperature. Dashed curves are fitting using eq (1).
Table 2. Ionic conductivity data and VFT parameters
a
b
b
·
·
Bb
K
T0
DC
¨
Tetra-cation
¹1
¹1
mS cm
S cm
K
t-C2-EtIm¢Tf2N
t-C3-EtIm¢Tf2N
t-C4-EtIm¢Tf2N
t-C5-EtIm¢Tf2N
t-C6-EtIm¢Tf2N
t-EG2-MeIm¢Tf2N
t-EG2-EtIm¢Tf2N
t-EG2-BuIm¢Tf2N
t-EG3-MeIm¢Tf2N
t-EG3-EtIm¢Tf2N
0.17
0.15
0.19
0.15
0.14
0.17
0.28
0.16
0.15
0.24
0.1660
0.1825
0.1986
0.2528
0.1755
0.1653
0.1469
0.1863
0.1238
0.1967
634.0
677.6
706.8
782.1
767.2
609.3
574.6
713.3
595.1
639.4
205.7
202.5
195.6
191.7
190.1
208.8
205.9
196.1
208.4
201.6
Figure 4. Effects of spacer and tail structure on viscosity.
Temperature: 25 °C.
aDC conductivity at 25 °C obtained from impedance measure-
b
ment. VFT parameters obtained from the fitting using eq (1).
ionic liquid (1-Ethyl-3-methylimidazolium Tf2N, EMIm¢
Tf2N) was reported to be 28 mPa s (26 °C).17 Much higher
viscosity of the tetra-cation is attributable to larger molecular
weight of the tetra-cation.
conductivities of the tetra-cations with and without spacers, the
changes in the chemical structures of the spacer and tail units
make little difference in ionic conductivity. Among the tetra-
cations with alkylene spacer (t-Cn-EtIm¢Tf2N), t-C4-EtIm¢
3.4 Ionic Conductivity of Tetra-Cationic Ionic Liquids.
The ionic conductivity of each tetra-cation was determined by
impedance spectroscopy under anhydrous conditions.40 The
direct current conductivity (·DC) was obtained from the plateau
region of the real component of the conductivity vs. frequency
plot (Figure S20 in the Supporting Information),41,42 and the
relationships between ionic conductivity and reciprocal temper-
ature are shown in Figure 5. Ionic conductivities were observed
to follow a Vogel-Fulcher-Tammann-type (VFT-type) temper-
ature dependence, which reflects ion diffusion in viscous
media.41 The data were fitted using the following equation:
¹4
Tf2N exhibited the highest ionic conductivity (1.9 © 10
S cm¹1). The tetra-cations with ethylenedioxy spacer afforded
better ionic conductivities than those with alkylene spacers.
As for the tail units, the ethyl-substituted imidazole exhibited
higher ionic conductivity than the methyl- and n-butyl-
imidazoles.
¹1
The highest ionic conductivity of 2.8 © 10¹4 S cm was
observed for t-EG2-EtIm¢Tf2N. It should be noted that t-EG3-
EtIm¢Tf2N is less ionically conductive than t-EG2-EtIm¢
Tf2N despite the former being less viscous than the latter. Since
the ion concentration decreases with increasing the spacer
length, it is considered that there is an optimal spacer length
that provides the highest ionic conductivity. The ionic conduc-
tivity of the representative mono-cationic ionic liquid (EMIm¢
·
¼ ·1 ꢀ expðꢁB=ðT ꢁ T0ÞÞ
ð1Þ
DC
where ·¨, B, and T0 are constants,43 the values of which were
obtained by data fitting and are summarized in Table 2.
Figure 6 summarizes the ionic conductivities of the tetra-
¹1
¹1
cations at 25 °C. All of the tetra-cations showed 10¹4 S cm
Tf2N) was reported to be 8.4 mS cm (26 °C).17 Much lower
order ionic conductivity, which is two orders of magnitude
higher than those reported for the tetra-cations without spacer
units (ca. 4.7-7.6 © 10¹6 S cm¹1).39 These results indicate that
the introduction of the spacer unit effectively improves ionic
conductivity. Compared to the large differences in the ionic
ionic conductivity of the tetra-cation is attributable to higher
viscosity of the tetra-cation.
3.5 Comparison with Other Multi-Cations. Tetra-cationic
ionic liquids are considered to be meso-class electrolytes that
lie between di-cationic ionic liquids (the simplest multi-
Bull. Chem. Soc. Jpn. 2020, 93, 1218–1225 | doi:10.1246/bcsj.20200148
© 2020 The Chemical Society of Japan | 1223

alkylene spacer. The spacer structure-property relationship
observed for the tetra-cations is similar to that of the poly-
cations. In cases of poly-cations, the ethylenedioxy spacers
reduce interactions between main chains, resulting in lower
glass-transition temperatures. At the same time, the ethyl-
enedioxy spacer increases the mobility of the cationic units.19
In a similar way, the ethylenedioxy spacer is considered to
reduce molecular interaction between the pentaerythritol-based
cores and increase cation-unit mobility. Although the structure-
property relationship of the tetra-cations is similar to that of the
poly-cations, the physical parameters of the tetra-cations are
closer in value to those of the di-cations rather than the poly-
cations. These results highlight the unique physical properties
of the tetra-branched tetra-cations, which are different from
those of the di-cations and poly-cations. The cationic units of
the tetra-cations without spacers (Figure 1a) are considered to
become a part of the rigid core;39 as a consequence, molecular
interaction between the rigid cores increases and cation-unit
mobility decreases, which is the reason why the introduction
of the spacer units decreases viscosity and increases ionic
conductivity.
Figure 6. Effects of spacer and tail structure on ionic
conductivity under anhydrous condition at 25 °C.
4. Conclusion
Ten tetra-branched tetra-cationic ionic liquids with various
spacers and tail units were successfully synthesized. The intro-
duction of spacer units was confirmed to effectively decrease
the glass transition temperature and increase the ionic con-
ductivity compared to the tetra-cations without spacer units. As
for the effect of the spacer structure on the physical properties
of the tetra-cationic ionic liquids, the ethylenedioxy spacers
were confirmed to afford lower viscosities and higher ionic
conductivities than the alkylene spacers. While the tri-ethylene-
dioxy spacer was the best for lowering viscosity, the di-
ethylenedioxy spacer was the best for improving ionic conduc-
tivity. As for the effect of the tail structure, the ethyl group was
better for lowering viscosity and improving ionic conductiv-
ities than the methyl or n-butyl groups. The cationic units of
tetra-cations without spacer units are considered to become a
part of the rigid core, which leads to larger molecular inter-
action between the rigid cores. On the other hand, the spacer
units are considered to reduce molecular interaction between
pentaerythritol-based cores, which is the reason why the
introduction of spacer units decreases viscosity and increases
ionic conductivity.
Figure 7. Chemical structures of di-cations and poly-
cations.
Table 3. Physical properties of di-cations and poly-cations.
b
c
Tg
°C
25, 31 ¹63
Viscosity
Pa s
·
DC
Multi-cation
Refa
¹1
mS cm
d-C5-MeIm¢Tf2N
0.74
>1.5
®
0.60
0.52
3.0 © 10
1.0 © 10
d-EG2-MeIm¢Tf2N 24, 32 ¹49
¹5
¹2
p-C3-MeIm¢Tf2N
p-EG3-MeIm¢Tf2N
44
45
16
¹18
®
b
c
aReference. Glass transition temperature. DC conductivity at
25 °C.
cations) and poly-cations. It is interesting to compare the
physical properties of the tetra-cations in this study with those
of di- and poly-cations. The multi-cations shown in Figure 7
were selected as representative di-cationic ionic liquids24,25,31,32
and poly-cations;44,45 the physical properties of these multi-
cations are summarized in Table 3.
The tetra-cations are more viscous and less ionically con-
ductive than the di-cations, which is mainly attributable to the
higher molecular weights of the tetra-cations than the di-
cations. A larger molecular weight results in stronger molecular
interactions and a lower ion concentration, which leads to
higher viscosity and lower ionic conductivity. Despite unfav-
orable conditions for the tetra-cation, it should be noted that the
viscosity of t-EG3-EtIm¢Tf2N is as low as that of d-EG2-
MeIm¢Tf2N. In case of di-cations, changing the spacer from
alkylene to ethylenedioxy chains resulted in increases in Tg and
viscosity; consequently, decrease in ionic conductivity. This
observation is attributable to increased molecular interactions
due to ether oxygen atoms. On the other hand, poly-cation with
ethylenedioxy spacer exhibited lower Tgs than that with
This work was financially supported by Grant-in-Aids for
Scientific Research C, 18K04762 (JSPS) and M-Cube project
(NIMS). T.I. thanks NIMS Molecule & Material Synthesis
Platform for NMR measurements. T.I. would like to thank
Editage (www.editage.com) for English language editing.
Supporting Information
Materials, NMR spectra, DSC and TGA charts, Impedance
data. This material is available on https://doi.org/10.1246/bcsj.
20200148.
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