Low-melting, low-viscous, hydrophobic ionic liquids: N-alkyl(alkyl ether)-N-methylpyrrolidinium perfluoroethyltrifluoroborate

Article abstract of DOI:10.1246/cl.2004.1636

A series of new hydrophobic ionic liquids comprising N-alkyl (alkyl ether)-N-methylpyrrolidinium and perfluoroethyltrifluoroborate were prepared and characterized. The new [C₂F₅BF₃] ^--based salts show lower melting points than the corresponding [BF₄]^--based ones. Of these new salts, five are liquids at room temperature and show very low viscosities (37-71 cP at 25°C), high ionic conductivities (3.0-6.8 mScm^-1) and wide electrochemical windows. Copyright

Full text of DOI:10.1246/cl.2004.1636

1
636
Chemistry Letters Vol.33, No.12 (2004)
Low-melting, Low-viscous, Hydrophobic Ionic Liquids:
N-Alkyl(alkyl ether)-N-methylpyrrolidinium Perfluoroethyltrifluoroborate
ꢀ
Zhi-Bin Zhou, Hajime Matsumoto, and Kuniaki Tatsumi
Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology,
1-8-31 Midorigaoka Ikeda, Osaka 563-8577
(Received September 10, 2004; CL-041067)
A series of new hydrophobic ionic liquids comprising N-
alkyl (alkyl ether)-N-methylpyrrolidinium and perfluoroethyl-
To depress halide impurities in the resultant ILs, the new
pyrrolidinium perfluoroethyltrifluoroborate salts (3) were pre-
pared by neutralization of aqueous solution of pyrrolidinium hy-
droxide (2) with aqueous solution of H[C2F5BF3] (Scheme 1),
followed by an evaporation. The pyrrolidinium hydroxide (2) so-
lution was prepared by passing the corresponding bromide salts
(1) through a column filled with basic anion-exchange resin
(Mitsubishi Chem., ion exchange capacity >2:0 mequiv./mL),
while the aq. H[C2F5BF3] was prepared by cation exchange of
trifluoroborate were prepared and characterized. The new
ꢁ
[
corresponding [BF4] -based ones. Of these new salts, five are
C2F5BF3] -based salts show lower melting points than the
ꢁ
liquids at room temperature and show very low viscosities
ꢂ
ꢁ1
(37–71 cP at 25 C), high ionic conductivities (3.0–6.8 mScm
and wide electrochemical windows.
)
7
K[C2F5BF3], as described in the literature. The bromide salts
Ionic liquids (ILs) are being extensively investigated as new
reaction media in chemistry and as electrolytes in electrochem-
istry because of their desirable properties including nonvolatili-
(1) were synthesized by quaternizing N-methylpyrrolidine with
equimolar n-alkyl (alkyl ether) bromide in acetone at room tem-
2
b
perature and purified by recrystallization. Since the salts (3) are
immiscible with water, the water content in the liquid salts was
1
ty, nonflammability, and high thermal stability. Of the various
ILs reported, the ILs based on saturated quaternary ammonium
ꢂ
<50 ppm after vacuum drying at 100 C.
The structures and compositions of these new salts were
1
b,2-4
(
One of the reasons for this growing interest is that the QA salts
QA) have attracted increasing attention in recent years.
1
19
11
confirmed by NMR ( H, F, and B), FAB-MS, and elemental
8
ꢁ
with electrochemically stable anions, such as [(CF3SO2)2N]
analysis. The thermal properties of these salts were examined
ꢂ ꢁ1
ꢁ
(
TFSI ), have much larger electrochemical windows than the
–4
by DSC on heating (heating rate: 10 C min ) and TGA (heat-
ꢂ
2
ꢁ1
more popular 1,3-dialkylimidazolium ones, which may allow
them as possible safety electrolytes for high-energy storage de-
ing rate: 10 C min ). The data for the physicochemical proper-
ties of these salts are summarized in Table 1.
5
vices, such as Li batteries.
Currently, the synthesis and application of chemically and
electrochemically stable QA-based ILs are almost devoted to
The melting points (T ’s) of these new salts are lower than
m
65 C (Table 1). Of these nine salts, five (Entries 3b, 3f–3i) are
liquids at room temperature. As expected, for a given cation, the
ꢂ
ꢁ
ꢁ
the anion TFSI , because this fluoroanion combines low sym-
T ’s of the salts with lower symmetry [C F BF ] are all signif-
m
2
5
3
ꢁ
metry, good flexibility and weakly coordinating nature, all of
which are beneficial to lowering the melting point and viscosity
of the ILs.2 With the rapidly expanding applications of QA-
based ILs in various fields, new robust anions that can form
low-melting and/or low-viscous ILs with QA cations are cer-
tainly needed.
icantly lower than those with higher symmetry [BF ] , e.g.
4
ꢂ
ꢂ
3
PY [C F BF ] (22 C) vs PY [BF ] (138 C). As seen in
1
4
2
5
3
14
4
,4,5
Table 1, replacing the N-alkyl in the cation with an isoelectronic
alkyl ether group generally result in a decrease of T , as a con-
m
sequence of better flexibility of alkyl ether group (Entry 3a vs 3f,
3b vs 3g, 3c vs 3h).
Most recently, a new class of chemically and electrochemi-
cally stable fluoroanions, namely perfluoroalkyltrifluoroborate
The liquid salts (Entries 3b, 3f–3i) exhibit very low viscos-
ꢂ
ities (37–71 cP at 25 C) and high ionic conductivities (3.0–6.8
ꢂ
ꢁ
ꢁ1
(
introduced into the ILs field. Compared with the representative
[RFBF3] , RF = n-CmF2mþ1, m ¼ 1{4), have been successfully
mScm at 25 C). All the salts containing an alkyl ether group
(Entries 3f–3i) in the cation are more fluid than PY [C F BF ]
7
1
4
2
5
3
ꢁ
ꢁ
anion [BF4] , [RFBF3] have the obvious advantages of (1) bet-
ter charge distribution and lower symmetry, thus their ILs gen-
erally exhibit very low melting points and viscosities; and (2)
much better stability toward hydrolysis because of their hydro-
(Entry 3b), indicating that the viscosity of the former is more de-
termined by the flexibility of the alkyl ether chain than by van
ion exchange
aq.H[C F BF ]
2 5 3
6
ꢁ
Br
OH
[C F BF ]
2 5 3
phobicity. Moreover, among the series of [RFBF3] , perfluor-
N
N
N
ꢁ
oethyltrifluoroborate ([C2F5BF3] ) has a medium size and usu-
ally form the most fluid ILs with good thermal stability.6 Of the
R
R
R
d
1
2
3
QA cations, N-alkyl-N-methylpyrrolidiniums have been demon-
R
Cation
PY13
PY14
PY15
PY16
PY17
R
CH
CH
C
CH
Cation
PY1.1O1
PY1.1O2
PY1.2O2
strated to produce low-viscous ILs due to the quasi-flat shape of
pyrrolidinium core.2b Additionally, having an alkyl ether group
in the QA cation also tends to lower the melting point and viscos-
n-C
n-C
n-C
n-C
n-C
3
H
7
3
OCH
O(CH
2
4
5
6
7
H
H
H
H
9
3
2
)
2
ity of the ILs.4
a,6c
11
13
15
2
H
5
O(CH
2
)
2
On the base of above known knowledge, we
therefore try to prepare new low-melting and/or low-viscous
3
O(CH
2
)
2
O(CH
)
2 2
PY1.1O2O2
QA-based ILs by combining asymmetric N-alkyl (alkyl ether)-
ꢁ
N-methylpyrrolidinium with [C2F5BF3] .
Scheme 1.
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.12 (2004)
Table 1. Synthesis yields and physicochemical properties of ionic liquids (water content: <50 ppm)
1637
a
Tgb
c
Ts-sd
e
f
g
h
i
j
Yield /
%
/
Tc /
/
Tm / Td /
d /
g mL
ꢁ /
cP
ꢂ /
mScm
ꢃ /
2
Entry
Salts
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢁ1
ꢁ1
ꢁ1
C
C
C
C
C
Scm mol
½
kꢄ
3
3
3
3
3
3
3
3
3
a
a
b
c
d
e
f
g
h
i
PY13[C2F5BF3]
PY14[C2F5BF3]
PY15[C2F5BF3]
PY16[C2F5BF3]
PY17[C2F5BF3]
PY1.1O1[C2F5BF3]
PY1.1O2[C2F5BF3]
PY1.2O2[C2F5BF3]
PY1.1O2O2[C2F5BF3]
87
89
93
93
95
70
75
81
82
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
ꢁ93
—
63
22
36
58
52
26
ꢁ3
ꢁ6
—
312
311
307
307
311
299
289
290
297
solid
1.30
solid
solid
solid
1.39
1.36
1.33
1.34
c
solid
71
solid
solid
solid
37
52
49
54
solid
3.5
solid
solid
solid
6.8
4.5
3.7
3.0
solid
0.89
solid
solid
solid
1.55
1.10
0.96
0.84
—
ꢁ64, ꢁ58
ꢁ51, ꢁ16
ꢁ116
ꢁ13
ꢁ108 ꢁ53
ꢁ98
—
—
b
Based on the amount of H[C2F5BF3]; Glass transition temperature determined by DSC; Crystallization temperature determined
by DSC; Solid–solid transition temperature determined by DSC; Melting point determined by DSC; Decomposition temperature
determined by TGA; Density measured by weighting 1.0 mL of ionic liquid at 25 C; Viscosity measured at 25 C. Specific con-
ductivity at 25 C; Molar conductivity (ꢃ ¼ ꢂM=1000 d, M is formula weight of the salt) at 25 C. Not detected.
d
e
f
g
ꢂ
h
ꢂ
i
ꢂ
j
ꢂ
k
References and Notes
PY14[C F BF ]
1
a) P. Wasserscheid and W. Keim, Angew. Chem., Int. Ed., 39,
3772 (2000). b) D. MacFarlane and M. Forsyth, Adv. Mater.,
13, 957 (2001). c) R. Hagiwara and Y. Ito, J. Fluorine Chem.,
9
6
3
0
2
5
3
PY1.1O2[C F BF ]
2
2
5
3
PY1.1O1[C
F BF ]
5 3
EMI[C F BF ]
2
5
3
105, 221 (2000).
2
a) J. Sun, M. Forsyth, and D. R. MacFarlane, J. Phys. Chem.
B, 102, 8858 (1998). b) D. R. MacFarlane, P. Meakin, J. Sun,
N. Amini, and M. Forsyth, J. Phys. Chem. B, 103, 4164
(1999). c) D. MacFarlane, J. Huang, and M. Forsyth, Nature,
402, 792 (1999).
-
3
6
9
-
-
-4
-3
-2
-1
0
1
2
3
+
V / V vs Fc/Fc
3
4
S. Forsyth, J. Golding, and D. R. MacFarlane, and M. Forsyth,
Electrochim. Acta, 46, 1753 (2001).
a) H. Matsumoto, M. Yanagida, K. Tanimoto, M. Nomura, Y.
Kitagawa, and Y. Miyazaki, Chem. Lett., 2000, 922. b) H.
Matsumoto, H. Kageyama, and Y. Miyazaki, Chem. Lett.,
Figure 1. Linear sweep voltammogram of ionic liquids on a
ꢁ
3
ꢁ2
glassy carbon electrode (surface area: 7:85 ꢅ 10 cm ) in
ꢁ
1
the first scan; scan rate: 50 mV s ; counter electrode: Pt wire;
potential (V) was referred to ferrocene (Fc)/ferrocenium (Fc )
redox couple in each salt.
þ
2
001, 182.
a) H. Sakaebe and H. Matsumoto, Electrochem. Commun., 5,
94 (2003). b) P. C. Howlett, D. R. MacFarlane, and A. F.
5
6
5
der Waals interactions.
The electrochemical stability of three salts (Entries 3b, 3f,
Hollenkamp, Electrochem. Solid-State Lett., 7, A97 (2004).
a) Z. B. Zhou, M. Takeda, and M. Ue, J. Fluorine Chem., 125,
6
a
and 3g) was comparatively studied with that of EMI[C2F5BF3]
(
471 (2004). b) Z. B. Zhou, H. Matsumoto, and K. Tatsumi, Chem.
þ
EMI = 1-ethyl-3-methylimidazolium) by linear sweep vol-
Lett., 33, 680 (2004). c) Z. B. Zhou, H. Matsumoto, and K.
Tatsumi, Chem. Lett., 33, 886 (2004). d) Z. B. Zhou, H.
Matsumoto, and K. Tatsumi, Chem.—Eur. J., (2004), in press.
Z. B. Zhou, M. Takeda, and M. Ue, J. Fluorine Chem., 123, 127
tammetry (LSV) under the same conditions. Figure 1 shows
the polarization curves of these four salts, which mainly mani-
fests the impact of the cation species on the cathodic and anodic
stability. If the cathodic and anodic limits are defined as the
7
8
(
2003).
Selected characterization data: 3a. Anal. Calcd. for C10H18BF8N:
C, 38.12; H, 5.76; N, 4.45%. Found: C, 37.48; H, 5.65; N, 4.41%.
ꢁ2
potential at which the current density reached 1 mA cm , the
electrochemical windows for these salts show the order [P14]-
3b. Anal. Calcd. for C11H20BF8N: C, 40.15; H, 6.13; N, 4.26%.
[
2
C2F5BF3] (ꢁ3:41 to 2.24 V) > [P1.1O1][C2F5BF3] (ꢁ3:03 to
Found: C, 39.94; H, 5.82; N, 4.20%. 3c. Anal. Calcd. for
C H BF N: C, 42.01; H, 6.46; N, 4.08%. Found: C, 41.72;
.24 V) ꢃ [P1.1O2][C2F5BF3] (ꢁ2:99 to 2.24 V) > [EMI]-
1
2
22
8
[
C2F5BF3] (ꢁ2:50 to 2.15 V). This result indicates that (1) these
H, 6.37; N, 4.14%. 3d. Anal. Calcd. for C13H24BF8N: C,
43.72; H, 6.77; N, 3.92%. Found: C, 43.57; H, 6.61; N, 3.86%.
three pyrrolidinium salts show much better cathodic and anodic
stability than EMI[C2F5BF3]; and (2) having an alkyl ether
group in the cation tends to reduce the cathodic stability.
In conclusion, a new series of hydrophobic ILs, N-alkyl (al-
kyl ether)-N-methylpyrrolidinium perfluoroethyltrifluoroborate,
have been prepared and characterized. These new salts show
3e. Anal. Calcd. for C14H26BF8N: C, 45.30; H, 7.06; N, 3.77%.
Found: C, 45.21; H, 6.81; N, 3.72%. 3f. Anal. Calcd. for
C9H16BF8NO: C, 34.10; H, 5.09%; N, 4.42. Found: C, 33.84;
H, 5.11; N, 4.39%. 3g. Anal. Calcd. for C10H18BF8NO: C,
36.28%; H, 5.48; N, 4.23. Found: C, 36.10; H, 5.47; N, 4.24%.
3h. Anal. Calcd. for C H BF NO: C, 38.29%; H, 5.84; N,
1
1
20
8
ꢂ
low melting points (<65 C). Among them, five salts are liquids
4.06. Found: C, 38.02; H, 5.58; N, 4.06%. 3i. Anal. Calcd. for
C12H22BF8NO2: C, 38.42; H, 5.91%; N, 3.73. Found: C, 38.12;
H, 5.60; N, 3.66%. All above salts have the same anion [C2F5-
at room temperature and exhibit very low viscosities, high con-
ductivities, as well as wide electrochemical windows. All these
promising properties may support them as new media for chem-
istry and electrochemistry.
ꢁ
BF3] and show an equivalent chemical shift value and coupling
1
9
11
19
constant in the respective F and B NMR. F NMR (376
MHz/acetone-d6, ꢀppm relative to external CCl3F): ꢀ ꢁ83 (s,
11
2
1
CF3), ꢁ136 (q, CF2, JBF ¼ 19:6 Hz), ꢁ153 (q, BF3, JBF ¼
This work was supported by R&D project for Li batteries by
METI and NEDO.
3
9:9 Hz). B NMR (128 MHz/acetone-d6, ꢀppm relative to
.
1 2
BF
external BF3
Et O): 0.21 (qt, J ¼ 40:9 Hz, J ¼ 19:1 Hz).
2
BF
Published on the web (Advance View) November 20, 2004; DOI 10.1246/cl.2004.1636