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Published on the web June 19, 2010
Extraction of Cd2+ and Am3+ Ions into Organic and Fluorous Solvents
with a TPEN Chelating Agent bearing a Fluoroalkyl Substituent
Tatsuya Kida,1 Yusuke Inaba,1,2 Wataru Watanabe,1 Yasutaka Nakajima,1
Sachio Fukuoka,1 Kenji Takeshita,*2 and Atsunori Mori*1
1Department of Chemical Science and Engineering, Kobe University, Rokkodai, Nada, Kobe, Hyogo 657-8501
2Integrated Research Institute, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8503
(Received April 19, 2010; CL-100376; E-mail: amori@kobe-u.ac.jp)
OCH2CF3
Introduction of polyfluorinated alkyl groups into TPEN
F
F
F
(N,N,N¤,N¤-tetrakis(2-pyridylmethyl)ethylenediamine) deriva-
tives provides soft metal ion extracting agents with high acid
tolerance. The TPEN derivative bearing a pentafluoropropoxy
substituent efficiently extracts Cd2+ and Am3+ into organic and
fluorous solvents.
O
N
H2N
NH2
+
4
Cl
O
N
2
3
N
N
NaOH, KI
16H33(CH3)3NCl (40 mol%)
F
C
F
F
N
N
THF/H2O, rt, 87 h
75%
F
F
N
O
F
O
F
F
Extraction of metal ions with a chelating agent has recently
attracted considerable attention. Much effort has been devoted to
the development of effective multidentate chelating agents of
metal ions1 and thereby precise design of organic molecules that
bind to a specific ion highly efficiently is intriguing in organic
synthesis. In particular, extraction of metal ions such as d-block,
lanthanide, and actinide metals is a major concern in the
environment, resources, and nuclear electric power genera-
tion.1a,1b TPEN (1), N,N,N¤,N¤-tetrakis(2-pyridylmethyl)ethyl-
enediamine (Chart 1), which possesses six nitrogen donors to
capture several metal ions such as cadmium, iron, copper,
lanthanides, and actinides, has been a potential chelating agent.2
However, a major drawback of TPEN has been water solublity
and easy protonation and therefore the use of TPEN as a
extracting agent of metal ions particularly in an acidic aqueous
solution has been considered to be unlikely. We envisaged that
introduction of hydrophobic substituents onto the pyridine rings
of TPEN would be a solution of such problems concerning the
extraction of metal ions and found that TPEN derivatives bearing
alkoxy groups somewhat improved the extraction performance of
cadmium(II) ion from an acidic aqueous solution.2a,2b However,
the acid tolerance of hydrophobic TPEN was found to be far from
satisfactory and we recognized that further improvement of
hydrophobic characteristics of TEPN derivatives with molecular
design based on organic synthesis is necessary.
4
F
Scheme 1.
tion of fluorine-containing molecules induces highly hydro-
phobic and fluorophilic characteristics. Accordingly, introduc-
tion of fluoroalkyl substituents onto TPEN derivatives is
intriguing for the development of a practical extraction of metal
ions. Herein, we report syntheses and properties of TPEN
derivatives bearing fluoroalkyl substituents on the pyridine rings.
Synthesis of TPEN derivatives bearing a fluoroalkyl
substituent on the pyridine ring was first carried out with the
reaction of the corresponding 2-(chloromethyl)pyridine 2 and
1,2-ethylenediamine (3) as shown in Scheme 1. When 2-
chloromethyl-3-methyl-4-(2,2,2-trifluoroethoxy)pyridine (2) that
was commercially available as a hydrochloride was employed,
TPEN derivative 4 was obtained in an excellent yield.4 The
obtained TPEN derivative was found to be dissolved freely in
several organic solvents, while hardly soluble in water.
With the obtained TPEN derivative bearing fluoroalkyl
groups 4, extraction of cadmium(II) ion that serves as a typical
soft metal ion was examined.2a A nitric acid solution of
Cd(NO3)2 (1 mM) was subjected to the extraction with a
chloroform solution of 4 (1 mM). The experiments were carried
out under various pH values of Cd2+ solutions. The percent
extraction of Cd was estimated with ICP-AES5 by the concen-
tration of the Cd2+ ion remaining in an aqueous phase after
extraction.
Table 1 shows results of the extraction with TPEN 1, and
the fluorinated derivative 4. In contrast to the TPEN showing
little extraction of Cd2+ in the pH range of 1-3, fluorinated
TPEN derivative 4 showed remarkable performance in the pH
range of ca. 2-3. However, the percent extraction decreased
when the aqueous solution became highly acidic (pH <2). The
result suggests that such high extraction performance is achieved
by the introduction of fluoroalkyl group showing remarkably
high hydrophobic characteristics that inhibit protonation of the
nitrogen atoms by the contact with acid in the aqueous phase.
Encouraged by the above results, we envisaged further
molecular design of hydrophobic TPEN by the introduction of a
longer fluoroalkyl chain. The TPEN derivatives were synthe-
On the other hand, fluorocarbons attract much interest in a
wide range of fields in organic chemistry, biochemistry, and
materials science.3 A number of organic molecules as well as
polymer materials are shown to change their performance by the
introduction of fluorine-substituted organic groups into the core
structure. In particular, remarkably low intermolecular interac-
N
N
N
N
N
N
TPEN (1)
Chart 1.
Chem. Lett. 2010, 39, 774-776
© 2010 The Chemical Society of Japan
Kida, Tatsuya
