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Chemistry Letters Vol.33, No.12 (2004)
Discovery of a Magnetic Ionic Liquid [bmim]FeCl4
Satoshi Hayashi and Hiro-o Hamaguchiꢀ
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
(Received September 27, 2004; CL-041131)
A magnetic ionic liquid, which shows a strong response to
magnetic field, has been discovered. We synthesized an ionic
liquid by mixing 1-butyl-3-methylimidazolium chloride
([bmim]Cl) and FeCl3, in anticipation of unique magnetic prop-
erties due to the possible local ordering of the magnetic anions.
The visible absorption spectroscopy has shown that this liquid
contains the high spin FeCl4ꢁ as the anion. Raman spectroscopy
has indicated that it contains bmimþ as the cation. It has thus
been confirmed that the prepared liquid is [bmim]FeCl4. The liq-
uid [bmim]FeCl4 responded strongly to magnetic field. The
SQUID measurements have shown that it is paramagnetic hav-
a
b
c
Figure 1. Pictures showing the response of [bmim]FeCl4 (Liq-
uid 2) to a magnet.
brown liquid (Liquid 2) was obtained endothermically. Liquid
2 is immiscible with water and the reaction ended with two lay-
ers of liquids, Liquid 2 and water. Both Liquid 1 and Liquid 2
show strong response to a magnet. Pictures in Figure 1 show
how Liquid 2 responds to a magnet. Without a magnet
(Figure 1a), the layer of Liquid 2 spreads beneath the water lay-
er. As a magnet approaches, the Liquid 2 layer is attracted to-
ward the magnet (Figure 1b) and is finally distorted to make a
concave curve (Figure 1c).
In order to spectroscopically characterize Liquids 1 and 2,
we first measured their visible absorption spectra. A Hitachi
U-3500 spectrometer was used. The results are given in
Figure 2. Both Liquid 1 (full line) and Liquid 2 (dotted line)
show three absorption peaks at 534, 619, and 688 nm. These
peaks are well known as characteristic peaks of FeCl4
We therefore conclude that both Liquid 1 and 2 contain the
FeCl4 ion.
We then measured the Raman spectra of Liquids 1 and 2 so
as to obtain information about the cation structure. A laboratory
made near-infrared multichannel Raman spectrometer9 was
used. The excitation wavelength was 1064 nm. The use of
near-infrared excitation enabled the collection of high S/N
Raman spectra from dark colored liquids. The Raman spectra
of Liquids 1 and 2 agree very well with each other. Figure 3
ing a large magnetic susceptibility of 40:6 ꢂ 10ꢁ6 emu gꢁ1
.
The present discovery of a magnetic ionic liquid has opened
up a new research area of the magnetism of liquids.
Liquids that are composed solely of ions are called ionic liq-
uids. In contrast to ordinary organic liquids, in which the dipolar
or higher order multipolar interactions are dominating, the Cou-
lomb interaction plays a major role in ionic liquids. Because of
the long-range nature of the Coulomb interaction, ionic liquids
may possibly have higher structural ordering than organic liq-
uids. In fact, our recent studies on the crystal and liquid struc-
tures of the 1-butyl-3-methylimidazolium salts ([bmim]X, where
X is Cl, Br, I, BF4, and PF6) have revealed a few unexpected fea-
tures of these ionic liquids that suggest the existence of a local
ordering of the constituent ions.1–4 X-ray crystallography has
shown that, in [bmim]Cl and [bmim]Br crystals, the bmimþ cat-
ions form columns extending along the a axis and the halogen
anions are accommodated in the channels surrounded by four
cation columns to form linear chains.1,5 The Raman spectra of
the bmimþ cation in crystalline and liquid [bmim]X highly re-
semble each other, indicating that their structures in the liquid
state are similar to those in the crystalline state.2,3 Most recent
wide-angle X-ray scattering experiment on [bmim]I has shown
a few clear peaks in the radial distribution. This finding is indi-
cative of the local ordering of the iodide anions.4 If the ions are
locally ordered as suggested by these results, it will give rise to
many unique physical properties of ionic liquids. With this ex-
pectation in mind, we started the preparation and the character-
ization of [bmim]X ionic liquids having magnetic anions. In the
present letter, we report on the first compound [bmim]FeCl4.
The synthesis of [bmim]FeCl4 has already been reported in
the literature.6 In the present study, we prepared [bmim]FeCl4 by
mixing crystal powder of [bmim]Cl with two ferric chlorides,
ꢁ
8
.
ꢁ
.
anhydrous FeCl3 and FeCl3 6H2O. Crystalline [bmim]Cl was
prepared according to the standard procedure7 followed by re-
peated recrystalization from dry acetonitrile. By mixing equi-
molar amount of [bmim]Cl (1.16 g) and FeCl3 (1.07 g) in a dry
box with N2 atmosphere, a dark brown liquid (Liquid 1) was ob-
tained as a result of an exothermic solid-state reaction. Similarly,
by mixing [bmim]Cl (1.46 g) and FeCl36H2O (2.31 g), a dark
Figure 2. Visible absorption spectra of [bmim]FeCl4 (Liquids 1
and 2).
Copyright Ó 2004 The Chemical Society of Japan
Hayashi, Satoshi
