Cooperative effects in catalytic hydrogenation regulated by both the cation and anion of an ionic liquid

Article abstract of DOI:10.1002/chem.201202707

The use of transition-metal nanoparticles/ionic liquid (IL) as a thermoregulated and recyclable catalytic system for hydrogenation has been investigated under mild conditions. The functionalized ionic liquid was composed of poly(ethylene glycol)-functionalized alkylimidazolium as the cation and tris(meta-sulfonatophenyl)phosphine ([P(C₆H₄-m-SO ₃)₃]^3-) as the anion. Ethyl acetate was chosen as the thermomorphic solvent to avoid the use of toxic organic solvents. Due to a cooperative effect regulated by both the cation and anion of the ionic liquid, the nanocatalysts displayed distinguished temperature-dependent phase behavior and excellent catalytic activity and selectivity, coupled with high stability. In the hydrogenation of α,β-unsaturated aldehydes, the ionic-liquid-stabilized palladium and rhodium nanoparticles exhibited higher selectivity for the hydrogenation of the C=C bonds than commercially available catalysts (Pd/C and Rh/C). We believe that the anion of the ionic liquid, [P(C₆H₄-m-SO₃)₃]^3-, plays a role in changing the surrounding electronic characteristics of the nanoparticles through its coordination capacity, whereas the poly(ethylene glycol)-functionalized alkylimidazolium cation is responsible for the thermomorphic properties of the nanocatalyst in ethyl acetate. The present catalytic systems can be employed for the hydrogenation of a wide range of substrates bearing different functional groups. The catalysts could be easily separated from the products by thermoregulated phase separation and efficiently recycled ten times without significant changes in their catalytic activity. Copyright

Full text of DOI:10.1002/chem.201202707

FULL PAPER
DOI: 10.1002/chem.201202707
Cooperative Effects in Catalytic Hydrogenation Regulated by both the
Cation and Anion of an Ionic Liquid
[
a]
[a]
[b]
[a]
[a]
Wenwen Zhu, Yinyin Yu, Hanmin Yang, Li Hua, Yunxiang Qiao,
[
a]
[a]
Xiuge Zhao, and Zhenshan Hou*
Abstract: The use of transition-metal
nanoparticles/ionic liquid (IL) as
havior and excellent catalytic activity
and selectivity, coupled with high sta-
bility. In the hydrogenation of a,b-un-
saturated aldehydes, the ionic-liquid-
stabilized palladium and rhodium
nanoparticles exhibited higher selectiv-
ity for the hydrogenation of the C=C
bonds than commercially available cat-
alysts (Pd/C and Rh/C). We believe
that the anion of the ionic liquid,
changing the surrounding electronic
characteristics of the nanoparticles
through its coordination capacity,
whereas the poly(ethylene glycol)-func-
tionalized alkylimidazolium cation is
responsible for the thermomorphic
properties of the nanocatalyst in ethyl
acetate. The present catalytic systems
can be employed for the hydrogenation
of a wide range of substrates bearing
different functional groups. The cata-
lysts could be easily separated from the
products by thermoregulated phase
separation and efficiently recycled ten
times without significant changes in
their catalytic activity.
a
thermoregulated and recyclable cata-
lytic system for hydrogenation has
been investigated under mild condi-
tions. The functionalized ionic liquid
was composed of poly(ethylene glycol)-
functionalized alkylimidazolium as the
cation and tris(meta-sulfonatophenyl)-
3
À
phosphine ([P(C H -m-SO ) ] ) as the
6
4
3 3
3À
anion. Ethyl acetate was chosen as the
thermomorphic solvent to avoid the
use of toxic organic solvents. Due to a
cooperative effect regulated by both
the cation and anion of the ionic liquid,
the nanocatalysts displayed distinguish-
ed temperature-dependent phase be-
[P(C H -m-SO ) ] , plays a role in
6 4 3 3
Keywords: hydrogenation
· ionic
liquids · nanoparticles · thermo-
regulated phase separation · transi-
tion metals
Introduction
products. To solve this problem, some liquid/liquid biphasic
catalytic systems have been constructed to improve catalyst
[2]
Catalytic hydrogenation performed by transition-metal
nanoparticles (NPs) has attracted great interest in both aca-
demic and industrial research due to the high activity of
such catalysts and the wide applications of the reduction re-
action. It is known that transition-metal NPs have unique
electronic, optical, and catalytic properties due to quantum
size effects and their high surface-to-volume ratios. Catalysis
by soluble NPs is considered as “quasi-homogeneous cataly-
sis”, which is at the frontier between homogeneous and het-
erogeneous catalysis. However, a disadvantage of soluble
NPs is that it is difficult to separate them from the reaction
recoverability, such as aqueous/organic biphasic and ionic
[3]
liquid (IL)/organic biphasic systems. Although the separa-
tion of catalyst was thereby made relatively simple, a bipha-
sic system introduces kinetic barriers and mass-transfer limi-
tations.
[1]
Recently, numerous thermoregulated biphasic catalytic
systems have been developed, which combine the advantage
of monophasic catalysis with the simple method of biphasic
catalyst/product separation, thereby eliminating the mass-
[4,5]
transfer problem.
Fluorous biphasic systems consisting of
a fluorinated solvent and/or a fluorous catalyst exhibit mark-
edly temperature-dependent phase behavior, which facili-
[6]
tates catalyst/product separation and catalyst reuse. How-
ever, some drawbacks hinder the industrial utilization of flu-
orous catalysis, such as cost, solvent leaching, and environ-
mental hazards. As a result, thermomorphic systems based
on polymer-supported catalysts have been studied during
the last decades, which show low catalyst leaching and are
[
a] W. Zhu, Y. Yu, L. Hua, Y. Qiao, X. Zhao, Prof. Dr. Z. Hou
Key Laboratory for Advanced Materials
Research Institute of Industrial Catalysis
East China University of Science and Technology
Shanghai 200237 (P. R. China)
Fax : (+86)21-64251686
E-mail: houzhenshan@ecust.edu.cn
[5,7]
compatible with common solvents.
In such studies, vari-
[
b] Prof. Dr. H. Yang
ous thermoregulated ligands containing poly(ethylene
oxide) chains capable of phase separation on the basis of
critical solution temperature have been used to stabilize
transition-metal catalysts. In analogy to this concept, some
thermoregulated phase-separable ionic-liquid biphasic sys-
tems have been devised and employed in catalytic reac-
Key Laboratory of Catalysis and Materials Science of the
State Ethnic Affairs Commission and Ministry of Education
Hubei Province, South-Central University for Nationalities
Wuhan, 430074 (P. R. China)
[8]
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202707.
Chem. Eur. J. 2013, 19, 2059 – 2066
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2059

[
9–13]
tions.
Although rhodium NPs stabilized by a thermo-
ing, whereas other conventional imidazolium-based ILs con-
taining a shorter alkyl chain, such as [BMMIM]Cl, are in-
soluble in EtOAc over the entire temperature range. In ad-
dition, poly(ethylene glycol)-2000 (PEG-2000)-functional-
ized 1-dodecyl-2-methylimidazolium chloride ([PEG-2000-
C MIM]Cl ), coupled with EtOAc as the solvent, exhibits
regulated ionic liquid/organic biphasic system have been re-
ported to catalyze the hydrogenation of simple olefins, such
[14]
as styrene, 1-octene, and 1-dodecene, the selective hydro-
genation of a,b-unsaturated carbonyl compounds in a ther-
moregulated system has rarely been studied up to now. Ad-
ditionally, the thermomorphic solvents required are highly
specific for temperature-dependent phase behavior in cata-
lytic reactions. In other words, the choice of solvent is cru-
cial. From the viewpoint of green catalysis, more attention
needs to be paid to the use of environmentally benign
media. In many catalytic hydrogenation systems involving
metal nanoparticles in an ionic liquid, the cation and anion
of the ionic liquid have respective functions that serve to in-
fluence the physicochemical properties of the metal nano-
particles, including their size, shape, and catalytic perform-
1
2
2
strong thermoregulated behavior. However, imidazolium
cations functionalized with lower molecular weight PEG-
300 or PEG-800 do not display such thermomorphic proper-
ties. To obtain stable transition-metal nanoparticles for use
3À
in catalytic reactions, the choice of [P(C H -m-SO ) ]
6
4
3 3
(tppt) as the anion of the IL was crucial. Thus, the present
thermoregulated IL was composed of poly(ethylene glycol)-
functionalized alkylimidazolium cations and tris(meta-sulfo-
3À
natophenyl)phosphine ([P(C H -m-SO ) ] ) anions. The cat-
6
4
3 3
ions of the IL provide the thermoregulated property while
the anions play a pivotal role in stabilizing and modifying
nanoparticles in ethyl acetate, so that both are crucial.
Among the ionic liquids listed in Table S1 in the Supporting
[1c]
ance.
It is worthwhile to note that cooperative effects
regulated by both the cation and anion of an ionic liquid
could have more far-reaching implications for catalysis re-
search and application.
Information, only [PEG-2000-C MIM] ACHTUNGTERNNGN[U tppt] displayed
1.5
1
2
both strong stabilization of the nanoparticles and thermo-
regulated properties. One can envisage a cooperative effect
derived from the respective functional moieties of the cation
and anion. Furthermore, the thermomorphic ionic liquid is
intentionally protected at the C2 position of the imidazoli-
um cation by a methyl group, because otherwise an N-
Results and Discussion
Herein, we report a new temperature-dependent IL/organic
biphasic catalytic system that consists of a recoverable ther-
moregulated IL, stabilized transition-metal NPs, and ethyl
acetate, and its application in selective catalytic hydrogena-
hetero ACHUTNGRENNUGc yclic carbene might form by deprotonation at C2 or
oxidative addition of the C2ÀH bond of the imidazolium
tion under very mild conditions (H at atmospheric pressure,
cation, leading to poisoning of the metal catalyst and hence
2
[
15,16]
4
58C). Compared with all previously used thermomorphic
a decrease in catalytic activity.
0
solvents, such as fluorinated solvents, hydrocarbons, di-
chloromethane, and toluene, ethyl acetate is clearly a more
environmentally benign medium for the reactions.
Pd /IL nanocatalysts were prepared by adding aqueous
NaBH solution to a mixture of Pd(OAc) and IL ([PEG-
AHCTUNGTRENNUNG
4
2
2000-C MIM] ACHTGNUTERNN[UNG tppt]) in dichloromethane and stirring for
1.5
1
2
Initially, different ILs were employed to modify the prop-
erties of Pd nanoparticles (Scheme 1). Concerning the cat-
2 h, then washing the organic phase several times with distil-
led water, and finally evaporating the solvent under
vacuum. In this way, a dark-
brownish solid (catalyst 1) was
obtained at room temperature,
which was characterized by
XPS, XRD, TEM and NMR
methods.
XPS analysis showed a bind-
ing energy for the Pd 3d5/2 elec-
trons of 335.2 eV, which corre-
0
sponds to Pd (Figure 1).
High-resolution transmission
electron microscopy (HRTEM)
Scheme 1. The structures of the functionalized ionic liquids used in this work.
was used to confirm the forma-
tion of Pd particles in ethyl ace-
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
ionic moiety of the IL, the length of the N-alkyl chain and
tate. Figure 2a shows the Pd particles obtained, and it can
be seen that the diameter of most of these Pd particles was
2.3 nm with a standard deviation of 0.2 (as determined from
the measurement of about 300 particles). In addition, there
was no diffraction peak for Pd nanoparticles in the XRD
pattern in the wide-angle range (Figure S1 in the Supporting
Information), which confirmed that the Pd particles were
very small.
the numbers of ethylene oxide units per chain were meticu-
lously screened to ensure desirable temperature-dependent
phase behavior.
It is evident from the data in Table S1 in the Supporting
Information that the ionic liquid 1-dodecyl-2,3-dimethyl
idazolium chloride ([DMMIM]Cl) is almost insoluble in
EtOAc at room temperature but becomes soluble on heat-
ACHTUNGERTNNUNGi m-
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2060
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Thermoregulated Ionic-Liquid-Stabilized Nanocatalyst
FULL PAPER
Figure 1. X-ray photoelectron spectrum of catalyst 1.
Figure 3. The thermomorphic IL ([PEG-2000-C12MIM]1.5
lized/modified palladium nanoparticles.
ACHTUNGTRENNU[GN tppt])-stabi-
groups have demonstrated that imidazolium-based ILs pos-
sess pre-organized structures, which can create an external
layer around the metal nanoparticles to avoid aggregation
and/or agglomeration though both steric and electronic pro-
[19]
tection.
On the basis of the above considerations, aggregation of
the IL was to be expected when catalyst 1 was dispersed in
ethyl acetate. This was confirmed by the observation of in-
creasingly bulky aggregates of nanometer dimensions by dy-
namic light scattering (DLS) measurements on increasing
the concentration of catalyst 1 in ethyl acetate (Figure S3 in
the Supporting Information). Therefore, we believe that the
ionic liquid can form a protective layer of pre-organized
structure to stabilize the metal nanoparticles through a co-
operative effect of both the cations and anions. Subsequent-
ly, we found that catalyst 1 exhibited temperature-depend-
ent phase behavior in ethyl acetate. Catalyst 1 is insoluble in
ethyl acetate at low temperature, but becomes miscible at
slightly higher temperature, whereupon the system becomes
monophasic with a brown color (the critical solution temper-
ature is about 308C). After reaction, on cooling to around
08C, the catalyst precipitates completely from ethyl acetate
and the organic phase reverts to colorless (Figure 4).
Figure 2. High-resolution transmission electron microscopy images of 1:
a,b) before and c,d) after reaction. Scale bars: a,c) 2 nm, b,d) 10 nm.
The coordination chemistry of phosphine ligands towards
0
[17]
Pd has been well documented, which allowed us to inves-
tigate the coordination ability of the phosphine ligand used
3
1
here (the anion of the IL) by P NMR spectroscopy. The
pure IL exhibited a peak at d=À5.94 ppm, whereas catalyst
1
showed a single peak at d=29.05 ppm (Figure S2 in the
Supporting Information), indicating that coordination be-
tween the phosphine ligand of the IL and a Pd nanoparticle
[18]
was accompanied by a remarkable downfield shift.
The
absence of free ligand in the catalyst is noteworthy. Due to
the strong coordination capacity of the ionic liquid anion,
the Pd NPs were well dispersed and highly stable in the
thermoregulated phase-separable catalyst. In light of these
À
results, it was deduced that the [P(C H -m-SO ) ] ions
6
4
3 3
formed a layer around the surface of the Pd NPs, leading to
a sphere of negative charge, and then the cations of the IL
became arranged as an outer layer for charge conservation
(
Figure 3). Indeed, ionic liquids are by definition described
Figure 4. The process of thermoregulated phase-separable catalysis:
a) catalyst 1, ethyl acetate, and substrate at 08C; b) reaction at 458C;
c) complete phase-separation after cooling to 08C.
as supramolecular fluids due to the formation of self-organ-
ized structures on the nanomolecular scale. Recently, several
Chem. Eur. J. 2013, 19, 2059 – 2066
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2061

Z. Hou et al.
Vis absorption spectrum of catalyst 1 in ethyl acetate
showed a peak in the range 225–300 nm. It can be seen that
the absorption band of the supernatant ethyl acetate solu-
tion after reaction was very weak and barely observable in
comparison with that under the reaction conditions. This in-
dicated that almost no catalyst remained in the organic
phase after thermoregulated phase separation, and that the
IL-stabilized nanoparticles could be efficiently separated
from the ethyl acetate.
In addition, conductance measurements were used to
monitor the content changes of catalyst 1 in ethyl acetate as
the temperature was reduced. The electrical conductivity de-
creased sharply as the temperature was decreased from 25
to 08C, and the value was close to zero at 08C (Figure S4 in
the Supporting Information). It was further found that the
amount of leaching from catalyst 1 was negligible by ele-
mental and ICP-AES analyses in consecutive catalytic
cycles. The catalyst could be reused by simple decantation
of the products in the supernatant organic phase after reac-
tion. The procedure was repeated ten times and the results
indicated that catalyst 1 could be recycled without any loss
of catalytic activity (Figure 7).
0
Figure 5. Reaction profiles for hydrogenation of styrene and Hg poison-
ing of the catalyst. Reaction conditions: catalyst 1 (20 mg), styrene (1.5ꢁ
À3
1
0
2
mol; styrene/Pd=1000), ethyl acetate (2.0 mL), 458C, H at atmo-
ACHTUNGTRENNUNGs pheric pressure.
We first chose styrene as a model substrate to explore the
catalytic performance of 1 under very mild conditions.
Figure 5 shows the effect of reaction time on the product
yield. The hydrogenation of styrene produced only ethylben-
zene in this system. It was found that the yield increased
almost linearly with time, showing quasi-zeroth-order kinet-
ics with respect to styrene concentration. Although HRTEM
measurements showed the presence of Pd nanoparticles, the
possibility that the active hydrogenation species were Pd
0
complexes could not be excluded. Hg is a well-known
poison for metal nanoparticles due to the formation of an
amalgam, whereas molecular metal complexes remain unal-
[20]
tered. Complete suppression of catalytic activity upon the
0
addition of Hg to a reaction mixture provides strong evi-
dence for catalysis by metal nanoparticles. In the present
case, no further catalytic activity was seen when the catalytic
0
system had been vigorously stirred with Hg for 2 h
(
Figure 5). This result clearly suggested that Pd nanoparti-
cles were the true catalytically active species.
In the next step, the isolation efficiency of the catalyst
was assessed by recording the UV/Vis spectrum of the or-
ganic phase at the end of the reaction (Figure 6). The UV/
Figure 7. Recyclability of 1 for hydrogenation of styrene. Reaction condi-
tions: catalyst 1 (20 mg), styrene (1.5ꢁ10 mol; styrene/Pd=1000), ethyl
acetate (2.0 mL), 458C, H at atmospheric pressure.
2
À3
In short, the process of thermoregulated phase-separable
catalysis offers clear advantages in product isolation and cat-
alyst recycling. Moreover, no apparent aggregation or size
change of the Pd nanoparticles was observed after the reac-
tion, as shown by the HRTEM images in Figure 2c,d. This
result can be explained in terms of the strong coordination
stability of the IL [PEG-2000-C MIM] ACHTUNGTERNNGN[U tppt] toward the
1.5
1
2
palladium center, which hinders the aggregation and growth
of Pd nanoparticles. The scope of the substrates was also es-
tablished for a wide range of functionalized alkenes, and in
all instances the reaction was completed under atmospheric
pressure at 458C with excellent conversion for the hydroge-
nation of C=C double bonds (Table 1).
The chemoselective reduction of a,b-unsaturated alde-
hydes is of considerable importance in the synthesis of fine
Figure 6. UV/Vis spectra of the ethyl acetate phase: a) during the reac-
tion, b) after phase separation.
2062
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Thermoregulated Ionic-Liquid-Stabilized Nanocatalyst
FULL PAPER
[
a]
Table 1. Hydrogenation of various olefins by catalyst 1.
[
b]
Substrate
Product
t
[
Conv.
[%]
Yield
[%]
h]
1
2
3
4
5
6
cyclohexene
1-hexene
cinnamyl alcohol
ethyl acrylate
benzalacetone
crotonaldehyde
cyclohexane
hexane
3-phenyl-1-propanol
ethyl propionate
benzylacetone
butyraldehyde
4
5
3
2.5
4
100
100
100
100
99
100
100
100
100
99
6
100
100
[
a] Reaction conditions: catalyst 1 (20 mg), ethyl acetate (2.0 mL), sub-
À3
strate (1.5ꢁ10 mol; substrate/Pd=1000), 458C, H
2
at atmospheric pres-
Figure 8. High-resolution transmission electron microscopy images of cat-
alyst 2: a) before and b) after reaction. Scale bars: 10 nm.
sure. [b] GC yield.
chemicals, particularly intermediates in the fragrance and
XPS analysis revealed the surface state of the Rh nano-
particles (Figure 9a). Medium intensity Rh 3d5/2 and Rh
3d3/2 signals with binding energies of 307.4 and 312.1 eV,
[21]
pharmaceutical industries. It has been found that simple
olefins such as styrene, cyclohexene, and 1-hexene (Figure 4;
entries 1 and 2 in Table 1), as well as a,b-unsaturated alco-
hols, esters, ketones, and aldehydes, can be efficiently con-
verted into the corresponding saturated CÀC single-bond
[22]
characteristic of metallic Rh, were observed. As shown in
Figure 9b, the XRD pattern of the obtained Rh particles
featured characteristic reflections corresponding to the
0
compounds. The hydroxyl (-OH) and carbonyl group (C=O)
(111), (200), (220), and (311) planes of crystals of Rh
remain almost completely intact in these transformations
(JCPDS 22-0700). In short, the XPS, XRD, and TEM results
(
entries 3–6 in Table 1). Although catalyst 1 proved to be
indicated that the rhodium AHCTUNGTRENNUN(G III) species was completely re-
more selective in the hydrogenation of conjugated C=C
bonds of a,b-unsaturated aldehydes than commercially
available Pd/C catalysts (entries 1 vs. 3, 5 vs. 7 in Table 2);
duced to rhodium NPs by NaBH , and that these rhodium
NPs could be protected by the IL without any change of va-
lence.
4
It was demonstrated that cat-
alyst 2 is more effective for the
[
a]
Table 2. Comparison of four different catalysts for the hydrogenation of various olefins.
selective hydrogenation of the
C=C bonds of a,b-unsaturated
aldehydes than catalyst 1, Pd/C,
or Rh/C under the same condi-
tions (entries 1–8 in Table 2).
Besides these ordinary sub-
strates, the biomass-derived
substrate 4-(2-furyl)-3-butene-2-
one could also be selectively
converted into 4-(2-furyl)-2-bu-
tanone by catalysts 1 and 2 (en-
tries 9 and 10 in Table 2). Evi-
dently, the catalyst 2 system
also offered the property of
thermoregulated phase-separa-
[
b]
Cat.
Substrates
Products
t
[
Conv.
[%]
Yield
[%]
h]
1
2
3
4
5
6
7
8
9
0
1
2
3
4
1
2
cinnamaldehyde
cinnamaldehyde
cinnamaldehyde
cinnamaldehyde
citral
citral
citral
citral
4-(2-furyl)-3-butene-2-one
4-(2-furyl)-3-butene-2-one
4-vinylpyridine
4-vinylpyridine
1-(1-cyclohexen-1-yl)pyrrolidine
1-(1-cyclohexen-1-yl)pyrrolidine
3-phenylpropionaldehyde
3-phenylpropionaldehyde
3-phenylpropionaldehyde
3-phenylpropionaldehyde
citronellal
citronellal
citronellal
citronellal
4-(2-furyl)-2-butanone
4-(2-furyl)-2-butanone
4-ethylpyridine
4-ethylpyridine
1-cyclohexylpyrrolidine
1-cyclohexylpyrrolidine
4
7
2
6
5
99
97
98
92
99
98
98
99
98
82
93
68.5
66.2
88
Pd/C
Rh/C
1
2
Pd/C
Rh/C
1
2
1
2
1
2
12
4
95
72.3
71.2
94.5
95
100
100
100
100
6
3
3.5
2
2
1
1
1
1
1
99
100
100
100
100
3
4
[
a] Reaction conditions: catalyst 1 (20 mg), catalyst 2 (40 mg), Pd/C (10 wt%, 1.6 mg), or Rh/C (5 wt%,
2
0 mg), ethyl acetate (2.0 mL), 458C, H
2
at atmospheric pressure; substrate/M=250 (M=Pd or Rh). [b] GC ble catalysis. The results clearly
yield.
showed that the ionic liquid
played a very important role in
modifying the metal nanoparti-
higher selectivity towards saturated aldehydes was difficult
to achieve with 1 (entries 1 and 5 in Table 2).
cles so that they could catalyze the selective hydrogenation
of various functionalized alkenes. We believe that the
À
On the basis of the above results, we examined the use of
rhodium nanoparticles as active species and thereby extend-
ed the application scope of the thermoregulated catalytic
[P(C H -m-SO ) ] ions of the IL change the surface proper-
6
4
3 3
ties of the metal NPs by coordination, such as the surround-
ing electronic characteristics, which enhances the selectivity
0
[23]
system. Rh /IL (catalyst 2) was synthesized in a similar
of the nanocatalysts for hydrogenation
compared with
manner to 1, as described in the Experimental Section. The
sizes of the Rh particles in the ethyl acetate solution (before
and after reaction) were determined by transmission elec-
tron microscopy (TEM), and the diameters of most of these
particles were 4.2 and 5 nm, respectively (Figure 8).
conventional Pd/C and Rh/C catalysts. In addition, it is note-
worthy that selective hydrogenation of substrates containing
an N atom also proceeded efficiently, and almost quantita-
tive yields were achieved with both catalyst systems (en-
tries 11–14 in Table 2).
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2063

Z. Hou et al.
fered the unique advantage of complete phase-separation
simply by adjusting the temperature.
Conclusion
We have prepared a new type of thermoregulated ionic-
liquid-stabilized transition-metal nanocatalyst that is capable
of catalyzing various hydrogenation reactions under very
mild conditions. The cation and anion of the ionic liquid
appear to play cooperative roles in the thermoregulated
phase separation and stabilization/modification of the metal
nanoparticles in the catalytic process. Moreover, the catalyt-
ic system displayed excellent activity and recyclability in the
selective hydrogenation of alkenes by virtue of its excellent
dispersion in ethyl acetate and the highly efficient separa-
tion of the nanocatalyst from the reaction mixture. Addi-
tionally, this catalytic system could be employed for the hy-
drogenation of a wide range of the substrates bearing differ-
ent functional groups. Through the selection of different
transition metal nanoparticles, it is envisaged that this ther-
moregulated IL-stabilized nanocatalyst system may be ex-
tended to other reactions while still retaining excellent activ-
ity coupled with clean separation of the nanocatalyst.
Experimental Section
Materials: Palladium acetate, rhodium chloride, Pd/C, Rh/C, and tris(m-
Figure 9. a) XPS spectrum of Rh 3d in catalyst 2, b) X-ray diffraction pat-
tern of isolated Rh NPs from catalyst 2.
sulfonatophenyl)phosphine trisodium (Na
Aldrich. Poly(ethylene glycol)-2000, 2-methylimidazole, 1-bromododec-
ane, various substrates, and all other materials were obtained from
3
TPPTS) were purchased from
AHCTUNGTRENNUNG
SCRC (Sinopharm Chemical Reagent Co., Ltd., Shanghai).
Because processes for the selective hydrogenation of ni-
trobenzene and substituted nitrobenzenes by metal nanopar-
Synthesis of the thermoregulated ionic liquid: All manipulations involv-
ing air-sensitive materials were carried out by using standard Schlenk
line techniques under an atmosphere of nitrogen and all solvents were
dried by standard methods prior to use. 1-Dodecyl-2-methylimidazole
[24]
ticles have seen remarkable growth in recent years,
we
also investigated the hydrogenation of aromatic nitro com-
pounds over the thermoregulated phase-separable catalyst 2.
As shown in Table 3, catalyst 2 showed comparable activity
and selectivity to those achieved with a commercially availa-
ble Rh/C catalyst in the selective reduction of nitrobenzene
derivatives under very mild conditions, but additionally of-
[
25]
was prepared according to our previously reported procedure. 1-Butyl-
,3-dimethylimidazolium chloride ([BMMIM]Cl) and 1-dodecyl-2,3-di-
methylimidazolium chloride ([DMMIM]Cl) were synthesized by a similar
2
A
H
U
G
R
N
U
G
ACHTUNGTRENNUNG
A
H
U
G
R
N
U
G
ACHTUNGTRENNUNG
[
26]
method to that reported previously.
1
A
H
U
G
R
N
U
G
3
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(H,H)=
(H,H)=
), 1.80 (m, 2H;
ACHTUNGTREN(NGNU H,H)=7.3 Hz, 3H;
1.98 Hz, 1H; CH), 7.56 (d, J
.4 Hz, 2H; CH
ACHTUNGTRENNUNG
7
), 1.38 (m, 2H; CH
).
[
a]
Table 3. Hydrogenation of aromatic nitro compounds by catalyst 2.
1
[
b]
A
H
U
T
E
N
N
[DMMIM]Cl: H NMR (400 MHz, CDCl
3
, TMS): d=7.80 (d,
(H,H)=1.98 Hz, 1H;
), 3.92 (s, 3H; CH ),
), 1.30–1.26 (m, 18H;
(H,H)=7.3 Hz, 3H; CH ).
: The poly(ethylene glycol)-functional-
ized dicationic IL [PEG-2000-C12MIM]Cl was synthesized ac-
cording to our previously reported procedure.
400 MHz, CDCl , TMS): d=8.05 (s, 2H; CH), 7.37 (s, 2H;
CH), 4.62 (t, J(H,H)=7.4 Hz, 4H; CH ), 4.15 (t, J(H,H)=
.4 Hz, 4H; CH ), 3.92 (t, J(H,H)=7.3 Hz, 4H; CH ), 3.46–
.81 (m; PEG), 2.80 (s, 6H; CH ), 1.78 (m, 4H; CH ), 1.33–
.25 (m, 36H; CH ), 0.88 ppm (t, J(H,H)=6.7 Hz, 6H; CH );
Cl : C 57.75, H
Cat.
Substrates
Products
t
Conv. Yield
J
A
H
U
G
R
N
U
G
ACHTUNGTRENNUNG
[
h] [%]
[%]
A
H
U
G
R
N
U
G
2
3
1
2
3
4
5
6
7
8
9
0
2
nitrobenzene
aniline
aniline
4
4
5
5
9
7
6
6
5
5
100
100
100
100
92
99
98
99
99
100
99
99
99
92
99
98
99
99
99
2
CH
3
2
Rh/C nitrobenzene
2
Rh/C para-methylnitrobenzene para-methylaniline
2
A
H
N
T
E
N
N
3
para-methylnitrobenzene para-methylaniline
[
PEG-2000-C12MIM]Cl
2
2
2
para-chloronitrobenzene
Rh/C para-chloronitrobenzene
para-nitrophenol
Rh/C para-nitrophenol
para-nitroaniline
Rh/C para-nitroaniline
para-chloroaniline
para-chloroaniline
para-aminophenol
para-aminophenol
1,4-benzenediamine
1,4-benzenediamine
[25]
1
H NMR
(
3
2
A
H
U
G
R
N
U
G
2
ACHTUNGTRENNUNG
7
3
1
2
A
H
U
G
R
N
U
G
2
2
3
2
1
99
2
A
H
U
T
N
U
G
3
[
a] Reaction conditions: catalyst 2 (40 mg), Rh/C (5 wt%, 20 mg), substrate/Rh=250,
elemental analysis calcd (%) for C122
H
240
O
44
N
4
2
ethyl acetate (2.0 mL), 458C, H at atmospheric pressure. [b] GC yield.
2
9.47, N 2.21; found: C 56.16, H 9.06, N 1.87.
2064
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 2059 – 2066

Thermoregulated Ionic-Liquid-Stabilized Nanocatalyst
FULL PAPER
[
2
PEG-2000-C12MIM]1.5
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[tppt]: The thermoregulated ionic liquid [PEG-
vern NanoZetaSizer. GC analyses were performed on a GC112A appara-
tus equipped with an HP-5 column (30 m, 0.25 mm i.d.) and an FID de-
tector.
000-C12MIM]1.5[tppt] was obtained by threefold ion-exchange with tris-
ACHTUNGTRENNUNG
meta-sulfonato-phenylphosphine anions in dichloromethane at room tem-
perature. Briefly, excess Na
solution of [PEG-2000-C12MIM]Cl
À4
3
TPPTS (0.30 g, 5.3ꢁ10 mol) was added to a
À4
2
(1.8ꢁ10 mol) in dichloromethane
(
10.0 mL) and the mixture was stirred for 3 h. The suspension was fil-
tered to remove the precipitate. The filtrate was collected and subjected
to two further exchanges with fresh Na TPPTS solution. Finally, the fil-
trate was carefully concentrated in vacuo and the residue was dried for
h, which afforded a slightly yellow solid at room temperature. No
traces of chloride ions were detected by a silver nitrate test. [BMMIM]
[tppt] and [DMMIM] [tppt] were synthesized in a similar manner. [PEG-
Acknowledgements
3
The authors are grateful for support from the National Natural Science
Foundation of China (No. 21073058), the Research Fund for the Doctoral
Program of Higher Education of China (20100074110014), and the Fun-
damental Research Funds for the Central Universities. The authors
would also like to express their thanks to Prof. Walter Leitner for his val-
uable suggestion.
3
3
-
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
3
ACHTUNGTRENNUNG
+
1
2+
2
[
000-C12MIM]1.5
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[tppt]: MS (ESI ): m/z: 1101.3 [ =
2
M
] (n=39), 1123.3
1
2+
2+
2+
2+
1
2+
2+
2+
1
2+
=
2
M
M
M
M
] (n=40), 1145.3 [ =
2
M
M
] (n=41), 1167.3 [ =
2
M
M
M
] (n=42), 1189.3
] (n=45), 1255.3
] (n=48), 1321.4
1
1
1
2+
2+
[
[
[
[
=
2
] (n=43), 1211.3 [ =
2
] (n=44), 1233.3 [ =
2
1
1
1
=
2
] (n=46), 1277.3 [ =
2
M
] (n=47), 1299.4 [ =
2
1
1
2+
À
=
2
]
(n=49), 1343.4 [ =
2
M
]
(n=50); MS (ESI ): m/z: 500.96
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A
H
U
G
E
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(OAc)
2
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À4
12MIM]1.5
A
C
H
T
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E
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N
U
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G
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4
(
C
1.0 mL) was rapidly added to RhCl
3
·3H
2
O (0.24 mmol) and [PEG-2000-
À4
12MIM]1.5
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
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extract the catalyst 2 from the mixture. After stirring for several minutes,
the dichloromethane phase turned black, while the aqueous phase
became colorless. The dichloromethane phase was washed three times
with water and then concentrated to dryness in vacuo to leave a black
solid (catalyst 2).
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Received: July 28, 2012
1
088.
Published online: December 19, 2012
2066
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 2059 – 2066