Metallocenium Ionic Liquids

Article abstract of DOI:10.1246/cl.2010.572

Ionic liquids have been prepared from simple metallocenium cations and bis(trifluoromethanesulfonyl)amide anion (TFSA). Their properties were tunable by the choice of metals and substituents; the ferrocenium salts were deep-blue paramagnetic liquids, which are readily prepared by a one-step solventless reaction, and the cobaltocenium salts were orange diamagnetic liquids.

Full text of DOI:10.1246/cl.2010.572

doi:10.1246/cl.2010.572
572
Published on the web May 8, 2010
Metallocenium Ionic Liquids
Takashi Inagaki and Tomoyuki Mochida*
Department of Chemistry, Graduate School of Science, Kobe University, Rokkodai, Nada-ku, Kobe 657-8501
(Received February 18, 2010; CL-100168; E-mail: tmochida@platinum.kobe-u.ac.jp)
Ionic liquids have been prepared from simple metallocenium
cations and bis(trifluoromethanesulfonyl)amide anion (TFSA).
Their properties were tunable by the choice of metals and
substituents; the ferrocenium salts were deep-blue paramagnetic
liquids, which are readily prepared by a one-step solventless
reaction, and the cobaltocenium salts were orange diamagnetic
liquids.
Ionic liquids (ILs) are intriguing materials from the view-
points of fundamental physical chemistry and green chemistry,
as well as their technological applications, and their chemistry
and physics have attracted special attention.1 The majority of
the ILs thus far investigated have been onium salts, such as
alkylimidazolium salts. The conferring of functionality on ILs is
an interesting approach toward materials and reaction chemis-
try.2 From this viewpoint, imidazolium ILs with magnetic
anions3 and organometallic substituents,4 and ILs containing
metal complexes,2,5 have been developed.
Figure 2. Metallocenium ILs: 2a (M = Fe, left) and 8a (M = Co,
right).
R1
R2
R1
R2
+
Fe
AgTFSA
Fe
TFSA
In this study, we employed metallocenium cations as
components of ILs with the aim of developing functional
liquids.6 Metallocenium cations exhibit a variety of chemical
functions and physical properties,7 the intriguing magnetic
properties of metallocenium charge-transfer salts have been well
documented.8 Thus it was expected that producing ILs from
metallocenium cations, or lowering the melting points of the
metallocenium salts, might open a route to novel materials
chemistry. Here we report for the first time the preparation and
properties of metallocenium ILs (Figure 1), which are charac-
terized by their simple formulae and tunable liquid properties.
We found that the combination of substituted metallocenium
cations (M = Fe and Co) with TFSA and related fluorinated
anions afford ILs generally. Figure 2 shows the appearance of
the typical metallocenium ILs 1,1¤-diethylferrocenium bis(tri-
fluoromethanesulfonyl)amide (2a) and 1,1¤-diethylcobaltoceni-
um bis(trifluoromethanesulfonyl)amide (8a), which are deep
blue and orange, respectively. Changing the metal atom leads to
the remarkable color change.
Scheme 1. Preparation of ferrocenium bis(trifluoromethanesulfonyl)-
amide.
Ferrocenium ILs with low melting points (1a-4a, and 7a)
were readily prepared by a one-step solventless reaction
(Scheme 1); grinding substituted ferrocenes and silver bis(tri-
fluoromethanesulfonyl)amide (AgTFSA) with an agate and
mortar for 5 min, followed by filtration via a syringe filter to
remove silver, gave the deep blue ionic liquids in quantitative
yield.13 Ferrocenium salts with higher melting points were
prepared by adding dichloromethane as a solvent to make the
filtration process easier. The cobaltocenium ILs 8a-8d were
prepared by a metathesis reaction of 1,1¤-diethylcobaltocenium
hexafluorophosphate with LiTFSA or other salts.13 The ferroce-
nium ILs were unstable under air, reacting rapidly with oxygen,
while the cobaltocenium ILs were stable under air. The
decomposition temperature of 8a was 430 °C, exhibiting high
thermal stability comparable to [bmim][TFSA] (dec. at 423 °C).9
The ILs were miscible with polar solvents (EtOH, acetone,
CH2Cl2, etc.) but not with nonpolar solvents (hexane, Et2O,
toluene, etc.). Miscibility with water was dependent on the
cation substituent.
The melting points of the metallocenium salts are summa-
rized in Table 1. The metallocenium salts with ethyl and longer
substituents were all ILs, while unsubstituted ferrocene, meth-
ylferrocene, and 1,1¤-dimethylferrocene did not give ILs; the
melting points of their TFSA salts were higher than 100 °C. The
melting points of salts with branched alkyl substituents 5a and
6a were much higher than those with linear substituents, in
accordance with the smaller degree of motional freedom.
Among the ILs in Table 1, it is notable that the melting point
of iodoferrocenium bis(trifluoromethanesulfonyl)amide (7a) was
33 °C; the introduction of a simple substituent to ferrocenium
R1
M
X
R2
1: M = Fe, R1 = Et, R2 = H
2: M = Fe, R1 = R2 = Et
a: X = N(SO2CF3)2 (TFSA)
b: X = N(SO2C2F5)2
c: X = N(SO2C3F7)2
d: X = N(SO2C4F9)2
3: M = Fe, R1 = n-Bu, R2 = H
4: M = Fe, R1 = R2 = n-Bu
5: M = Fe, R1 = t-Bu, R2 = H
6: M = Fe, R1 = CH2C(CH3)3, R2 = H
1 = I, R2 = H
7: M = Fe, R
8: M = Co, R1 = R2 = Et
Figure 1. Structural formulae of metallocenium ILs.
Chem. Lett. 2010, 39, 572-573
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

573
Table 1. Melting pointsa, glass transition temperaturesa, and viscos-
ities of metallocenium salts
The ferrocenium ILs were paramagnetic liquids; the
magnetic susceptibilities (»T values) of 2a and 4a at around
room temperature were 0.68 and 0.59 emu K mol¹1, respectively,
which are typical for ferrocenium salts.8 In contrast, the
cobaltocenium salts were diamagnetic liquids.
In conclusion, we have shown that metallocenes with
various substituents, anions, and metal ions afford metal-
containing functional ILs which are distinguished by the absence
of an onium framework, simple preparation methods, and
tunable liquid properties such as color, magnetism, and melting
point. The strategy presented here may be extended to other
highly functional metallocenes, which might be a promising
approach toward the development of various functional liquids.
b
Tm
Tg
©
Tm(precursor)
25°C
ILs
/°C
/°C
/mPa s
/°C
1a
2a
3a
4a
5a
6a
7a
8a
24.5
3.8
®
®
®
26.6
45.0
112.3
94.3
¹1.3
¹36.2
7.4
¹18.4
9.4
¹80.7
¹87.4
®
1.0
79.7
60.0
33.1
14.52
6.3
®
9.1
44.8
¹58.5
®
®
57.3
107.5
182.6
386.7
8b
8c
8d
14.5
6.1
¹84.2
¹64.1
This work was supported by Grants-in-Aid for Scientific
Research (Nos. 19655053 and 21350077) from the Japan
Society for the Promotion of Science (JSPS) and was carried
out as part of the “High-Tech Research Center” Project (Toho
University) 2005-2009 from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT). We thank Dr.
Chikahide Kanadani and Prof. Toshiaki Saito (Toho University)
for SQUID measurements, and Yoshito Furuie (Kobe Univer-
sity) for elemental analysis.
Non-ILs
[FcH][TFSA]
[MeFc][TFSA]
[Me2Fc][TFSA]
131.5
104.5
104.1
174.6c
38.8d
38.4c
aDetermined by means of differential scanning calorimetry with a
¹1
scan rate of 10 K min (Ref. 13). bMelting points of neutral
c
d
ferrocene derivatives. Ref. 10. Ref. 11.
bis(trifluoromethanesulfonyl)amide (mp: 131.5 °C) resulted in a
remarkable decrease in the melting point. Comparison of 2a
(M = Fe, mp: 3.8 °C) and 8a (M = Co, mp: 14.2 °C) shows the
effect of the different metal ions; the higher melting point of the
latter is probably ascribable to its smaller molecular volume.
Comparison of 8a-8d indicates that the melting points are nearly
independent of the length of the -CF2- chain in the anion. This
contrasts with the effect of the cation substituents as noted
above. The use of TFSA-type anions is essential for obtaining
metallocenium ILs, in view of the high melting point of 1,1¤-
diethylcobaltocenium hexafluorophosphate (150.1 °C).
The melting points of the precursors of the salts®i.e., the
neutral ferrocene derivatives®are also included in Table 1. It
was found that precursors with lower melting points tend to give
ILs with lower melting points; melting points of ferrocenium ILs
with linear substituents 1a, 2a, and 4a are higher than those of
their precursors by about 30 K. An exception was 7a (mp:
33 °C), which exhibits a lower milting point than iodoferrocene
(44.8 °C). In metallocenium ILs, the properties of the neutral
precursors and the ILs can be compared without changing the
molecular framework, which may provide important information
on fundamental aspects of ILs.
Viscosities were measured at 25 °C for some of the salts
(Table 1). The viscosities of TFSA salts with ethyl groups 1a, 2a,
and 8a were comparable to those of typical imidazolium ILs such
as [ethylmethylimidazolium][TFSA] (36.5 mPa s) and [butyl-
methylimidazolium][TFSA] (50.5 mPa s),12 but the values in-
creased with increasing substituent length in both the cation and
the anion. In particular, elongation of the -CF2- chain in the
anion resulted in a remarkable increase in viscosity (8a-8d).
Furthermore, a reasonable correlation was found between
viscosity and the thermal behavior of the salts (Table 1). Upon
cooling, the ILs exhibited either (i) crystallization (1a, 2a, 8a,
and 8b), (ii) glass transition (3a), or (iii) either of these
depending on the cooling conditions (4a, 8c, and 8d); the ILs
with high viscosities (>90 mPa s) exhibited glass transitions,
while less viscous ILs (<60 mPa s) exhibited only crystallization.
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Chem. Lett. 2010, 39, 572-573
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/