Chemistry Letters 2000
1431
The physical properties of TFSI-based RTMS in this study
are also summarized in Table 1. The melting point of
TES–TFSI (–35 °C) was much lower than QA-based RTMS
such as trimethylpropylammonium (TMPA)–TFSI and the
value was close to the cyclic aliphatic ammonium cation of N-
methyl-n-butylpyrrolidinium (P14)–TFSI (–18 °C)6 and a cyclic
aromatic EMI–TFSI. This fact suggests that the melting points
of TFSI salts might be lowered with the use of planar cations in
the case of TFSI system.
In conclusion, we have succeeded in the synthesis of
RTMS based on symmetric sulfonium cation with the use of
TFSI. As we expected, the conductivity of the planar sulfoni-
um based RTMS was much higher than that of QA–TFSI melts.
Though the cathodic limit of trialkylsulfonium-based melt were
not so much negative comparing with QA-based RTMS, it
should be noted that the symmetric cation could also form
RTMS. Currently, the preparation and investigation of asym-
metric or cyclic sulfonium-based melts are in progress to eluci-
date the relationship between the ionic strucuture and the physi-
cal properties such as viscosity, conductivity, and melting point.
The conductivities of TES–TFSI and TBS–TFSI are shown
in Figure 1. This figure also shows the other cyclic planar sys-
tem such as EMI–TFSI, P14–TFSI for the comparison. The
conductivity of TES–TFSI is much higher than that of
P14–TFSI, which was previously reported as the highest con-
ductivity of all the QA-based RTMS.7 However not only the
conductivity but also the molar conductivity of TES–TFSI was
a little lower than that of EMI–TFSI though the viscosity of
TES–TFSI was almost the same as that of EMI–TFSI (Table 1).
The ionic conductivity varies in proportion to the number of
carrier ion and ionic mobility. If ionic mobility would be sim-
ply proportional to the viscosity in the case of such ionic liq-
uids, the difference of the number of an ionic carrier caused the
difference of molar conductivity between TES–TFSI and
EMI–TFSI at almost the same viscosity. Considering that the
number of ionic carrier contributed to the ionic conductivity
might decrease with increasing the number of associated ion
pair, such difference of the conductivity between TES–TFSI
and EMI–TFSI might attribute to the degree of association
between cation and anion. It means that the degree of associa-
tion of TES and TFSI might be a little stronger than that of EMI
and TFSI. However, further investigation will be necessary to
clear the relationships between the ionic structure and the phys-
ical properties of RTMS.
References and Notes
1
J. S. Wilkes and M. J. Zaworotko, J. Chem. Soc., Chem.
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P. Bonhôte, A. P. Dias, N. Papageorgiou, K.
Kalyanasundaram, and M. Grätzel, Inorg. Chem., 35, 1168
(1996).
3
N. Papageorgiou, Y. Athanassov, M. Armand, P. Bonhôte,
H. Pettersson, A. Azam, and M. Grätzel, J. Electrochem.
Soc., 143, 3099 (1996).
4
5
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7
8
9
J. Fuller, R. T. Carlin, and R. A. Osteryoung, J.
Electrochem. Soc., 144, 3881 (1997).
R. Hagiwara, T. Hirashige, T. Tsuda, and Y. Ito, J.
Fluorine Chem., 99, 1 (1999).
J. Sun, D. R. MacFarlane, and M. Forsyth, Ionics, 3, 356
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H. Matsumoto, M. Yanagida, K. Tanimoto, M. Nomura, Y.
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Fullbier, Electrochim. Acta, 37, 379 (1992).
As previously reported, QA-based RTMS exhibited high
electrochemical stability comparing with aromatic cations such
as EMI.8 However, in the case of trialkylsulfonium-based
RTMS, the cathodic limits were about 1.0 V positively shifted
from the cathodic limit of QA-based RTMS as shown in Figure
2. The cathodic limit of trialkylsulfonium-based RTMS were
almost the same as that of EMI-based RTMS. This must be due
to the poor cathodic stability of sulfonium cations comparing
with QA.
10 Results of elemental analysis. Found: C, 16.5 ; H, 2.4 ; N,
3.8 ; F, 31.8 ; S,27.8%. Calcd for TMS–TFSI: C, 16.8; H,
2.5; N, 3.9; F, 31.9; S, 26.9%. Found: C, 23.9; H, 3.8; N,
3.5; F, 28.7; S, 24.6%. Calcd for TES–TFSI: C, 24.1; H,
3.8; N, 3.5; F, 28.5; S, 24.1%. Found: C, 34.6; H, 5.6; N,
2.9; F, 23.6; S, 20.3%. Calcd for TBS–TFSI: C, 34.8; H,
5.6; N, 2.9; F, 23.6; S, 19.9%.