Angewandte
Communications
Chemie
Ionic Liquids
Tuning the Hydrophilicity and Hydrophobicity of the Respective
Cation and Anion: Reversible Phase Transfer of Ionic Liquids
Abstract: The separation and recycling of catalyst and
cocatalyst from the products and solvents are of critical
importance. In this work, a class of functionalized ionic liquids
(ILs) were designed and synthesized, and by tuning the
hydrophilicity and hydrophobicity of cation and anion,
respectively, these ILs could reversibly transfer between water
and organics triggered upon undergoing a temperature change.
From a combination of multiple spectroscopic techniques, it
was shown that the driving force behind the transfer was
originated from a change in conformation of the PEG chain of
the IL upon temperature variation. By utilizing the novel
property of this class of ILs, a highly efficient and controllable
CuI-catalyzed cycloaddition reaction was achieved wherein the
IL was used to entrain, activate, and recycle the catalyst, as well
as to control the reaction.
water phase to a mixture of THF and CHCl3 by adjusting the
pH value of the solution.
In the last two decades, ionic liquids (ILs) have been
found widespread applications in chemical synthesis,[5] mate-
rials preparation,[6] biomass dissolution,[7] gas capture,[8]
energy production,[9] and separation science[10] because of
their unique properties,[11] such as ultralow vapor pressure,
wide liquid temperature range, nonflammability, high stabil-
ity, and tunable structure and property. Many efforts have
been made to realize product separation from IL-based
solvents by means of pressure,[12] temperature,[13] and CO2.[14]
For example, Leitner et al.[12a] and Cheng et al.[12b] found that
a pressure-controlled supercritical CO2/IL system provides
superior advantages in product separation, catalyst recycling,
and reuse of the reaction media over traditional organic
solvents. Davis et al.[13a] and Wang et al.[13b] reported thermo-
driven liquid-solid separation by using an alkane sulfonic acid
IL as the solvent for esterification. Phase transfer of block
copolymer micelles,[15] nanogels,[16] and carbon nanotubes[17]
between H2O and hydrophobic ILs, triggered by temperature,
was also developed. ILs which exhibit phase separation in
molecular solvents, depending on temperature, were also
reported.[18] However, the separation and recycling of a cata-
lyst and co-catalyst from the products and IL-related solvents
still remain a key challenge. Therefore, searching for task-
specific ILs for sustainable processes of chemical reactions,
product separation, and catalyst and IL recycling is a long-
standing topic. Considering the fact that the most interesting
feature of ILs is that their structures and properties can be
fine-tuned by a judicious variation of the cation and anion, we
were interested in the creating of a class of thermo-responsive
ILs which can reversibly transfer between water and organic
phases to achieve easy separation and recycling of catalyst.
It is known that the bis(trifluoromethanesulfonyl)imide
anion ([NTf2]À) is a strongly hydrophobic ion,[19] whereas
poly(ethylene glycol) (PEG) is hydrophilic and has the ability
to dissolve ILs,[20] as well as to coordinate with metallic
organic salts[21] and inorganic salts.[22] In this work, a PEG unit
was functionalized on both ends with methylimidazolium
cations, and then paired with [NTf2]À to form PEG-function-
alized ILs. By tuning the hydrophilicity of the cation with the
molecular weight of PEG against the hydrophobicity of the
anion, these ILs can reversibly transfer between water and
organic solvents triggered by temperature changes. Through
a combination of multiple spectroscopic techniques, the
mechanism for the phase transfer of the ILs between water
and ethyl acetate (EA) was investigated. In particular, this
unique phase behavior of the IL was applied to a CuI-
catalyzed alkyne–azide cycloaddition. Thus a highly efficient
P
hase-transfer processes for two immiscible liquid phases
have been widely employed for various applications, such as
recycling of catalysts,[1] transfer of nanoparticles from a prep-
aration medium to an application medium,[2] and transport
across biological membranes.[3] Indeed, it is more important
to develop novel strategies for the use of an external stimulus
to induce the reversible phase transfer of a substance. For
example, Cole-Hamilton et al.[1a] reported that a rhodium
catalyst could move back and forth between aqueous and
toluene phases using CO2 as a trigger, and thus simplifying the
process for recycling and reusing the catalyst. Tan et al.[1d]
demonstrated a reversible phase transfer of a-cyclodextrin-
coated gold nanoparticles between aqueous and toluene
phases, and it could be controlled by UV-visible light. Tang
et al.[2b] reported a reversible phase transfer of CdTe nano-
particles between aqueous and toluene phases driven by
temperature, and Ludwigs et al.[4] found that polythiophenes
with a carboxylic acid group could reversibly transfer from the
[*] W. Yao, Dr. H. Wang, Dr. G. Cui, Z. Li, Dr. A. Zhu, Prof. J. Wang
Henan Key Laboratory of Green Chemistry, Collaborative Innovation
Center of Henan Province for Green Manufacturing of Fine
Chemicals, Key Laboratory of Green Chemical Media and Reactions,
Ministry of Education, School of Chemistry and Chemical Engineer-
ing, Henan Normal University, Xinxiang, Henan 453007 (P.R. China)
E-mail: Jwang@htu.cn
Prof. S. Zhang
Beijing Key Laboratory of Ionic Liquids Clean Process
Institute of Process Engineering, Chinese Academy of Sciences
Beijing 100190 (P.R. China)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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