Biphasic hydrogenation of olefins by functionalized ionic liquid-stabilized palladium nanoparticles

Article abstract of DOI:10.1002/adsc.200800290

Palladium nanoparticles in the size range of 5-6 nm were prepared conveniently by reducing palladium(II) with atmospheric pressure hydrogen and stabilized by 2,2′-dipyridylamine-functionalized imidazolium cations according to our approach. The efficient catalytic conversion of cyclohexene into cyclohexane by the functionalized ionic liquid-stabilized palladium nanoparticles has been performed under very mild hydrogen pressure (0.1 MPa) and at 35°C. It was found that the concentration of palladium and the reaction temperature considerably affected the size and degree of aggregation of Pd nanoparticles in ionic liquid, which further changed the performance of the catalyst activity. The synthesized nanocatalysts can be recycled at least five times without any loss of the activity. Finally, the scope of substrates was also investigated. The excellent catalytic activity of the present system can be attributed to good stabilization and high dispersion of palladium nanoparticles.

Full text of DOI:10.1002/adsc.200800290

FULL PAPERS
DOI: 10.1002/adsc.200800290
Biphasic Hydrogenation of Olefins by Functionalized Ionic
Liquid-Stabilized Palladium Nanoparticles
Yu Hu,a Yinyin Yu,a Zhenshan Hou,a,* Huan Li,a Xiuge Zhao,a and Bo Fenga
a
Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and
Technology, Shanghai 200237, Peopleꢀs Republic of China
Fax : (+86)-21-6425-3372; phone: (+86)-21-6425-1686; e-mail: houzhenshan@ecust.edu.cn
Received: May 11, 2008; Revised: July 18, 2008; Published online: August 19, 2008
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800290.
Abstract: Palladium nanoparticles in the size range ticles in ionic liquid, which further changed the per-
of 5–6 nm were prepared conveniently by reducing formance of the catalyst activity. The synthesized
palladium(II) with atmospheric pressure hydrogen nanocatalysts can be recycled at least five times with-
and stabilized by 2,2’-dipyridylamine-functionalized out any loss of the activity. Finally, the scope of sub-
imidazolium cations according to our approach. The strates was also investigated. The excellent catalytic
efficient catalytic conversion of cyclohexene into cy- activity of the present system can be attributed to
clohexane by the functionalized ionic liquid-stabi- good stabilization and high dispersion of palladium
lized palladium nanoparticles has been performed nanoparticles.
under very mild hydrogen pressure (0.1 MPa) and at
358C. It was found that the concentration of palladi-
um and the reaction temperature considerably affect- Keywords: hydrogenation; ionic liquid; nanoparti-
ed the size and degree of aggregation of Pd nanopar- cles; olefins; palladium
Introduction
example, the electron delocalization of metal nano-
particles can change the extent of the decomposition
Recently, the application of transition-metal nanopar- and solubility in the solvents, thereby offering the
ticles has attracted large interest because metal nano- possibility that these metal nanoparticles act as homo-
particles can act as the highly efficient catalysts. It has geneous catalysts which are also called ꢁsolubleꢀ ana-
been suggested that the small size of the metal nano- logues of heterogeneous catalysts and have been char-
particles leads to high surface/volume ratio and conse- acterized by a higher activity and selectivity.[15]
quently high numbers of potential active sites avail-
Over the last few years, ionic liquids, as successful
able to the substrates, as a result of large enhance- alternative solvents for homogeneous biphasic cataly-
ment in their catalytic activity while they are used as sis, have been designed for immobilizing catalysts, fa-
catalysts.[1,2] However, nanocatalysts are thermody- cilitating product isolation and recycling the catalyst
namically not stable due to their high excess surface- system. These neoteric solvents have been extensively
energy, therefore they have a tendency to aggregate applied in different reactions such as acylation, hydro-
unless they are stabilized. The methods for stabilizing formylation, Diels–Alder reaction, olefin dimerization
nanoparticles mainly involve electrostatic stabiliza- or oligomerization, hydrogenation, oxidation and bio-
tion, steric protection and coordination stabilization catalysis.[16–23] Ionic liquids have also been named as
by the use of polymers, quaternary ammonium salts, the ꢁdesigner solventsꢀ, which offer us a much easier
dendrimers, surfactants, polyoxoanions or ligands way to find a better ionic liquid with special proper-
etc.[3–14]
ties suitable for given reactions. Very recently, Wel-
Among these approaches, particular attention has tonꢀs group has employed the designability of the
been paid to the ligand stabilization of metal nanopar- ionic liquids to optimize the nucleophilic aromatic
ticles. By coordinating with the ligand, the entrapped substitution reaction.[24]
metal nanoparticles can undergo some potential
It has been reported that he imidazolium ionic
changes of their physical and chemical properties. For liquid (IL) is an interesting medium for the formation
Adv. Synth. Catal. 2008, 350, 2077 – 2085
ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2077

Yu Hu et al.
FULL PAPERS
and stabilization of catalytically active transition-
metal nanoparticles. The pre-organized ꢁsupermolecu-
larꢀ structures of imidazolium ionic liquids can create
an external layer around the metal nanoparticles to
control the growth of the particle size and affect the
mass transportation, however, simple imidazolium
ionic liquids cannot provide an effective protection to
the nanoparticles against aggregation under the cata-
lytic conditions.[21,25] Therefore, there have been some
reports about addition of ligands to stabilize metal
nanoparticles for biphasic reactions in ionic liq-
uids.[26,27,28] Even if the catalyst/ligand dissolves well in
the ionic liquid, there is often a dramatic decrease in
its activity after several runs. The main cause of this
decrease is that the metal component and/or the li-
gands might leach successively in the recycling experi-
ments. In addition, the nanocatalyst-ligand complexes
are not stable enough in the process of separation,
which will also lead to an aggregation of nanoparticles
and then a decrease of the catalytic activity,[29,30,31]
thus the cations of ionic liquids designed for immobi-
lizing metal nanoparticles have been reported.[32,33]
Recently, Hanꢀs group has also reported a kind of
guanidinium ionic liquid containing N-donor ligands
to stabilize palladium nanoparticles on molecular
Figure 1. Structures of [BMMIM] [PF6], [BMIM] [PF6],
[BMMDPA] [PF6], PCDPA and [BMMDPA] [Cl].
sieves which showed synergistic effects to enhance the Results and Discussion
activity and durability of the catalyst.[34]
To the best of our knowledge, there is no such The synthetic route to the functionalized ionic liquid
report on ionic liquid cations tethered to a bidentate [BMMDPA] [PF6] is shown in Scheme 1. Briefly, it
nitrogen ligand to stabilize palladium nanoparticles was synthesized according to a conventional method
for catalysis. In this work, the N-donor bidentate by quaternarization of an imidazole with the corre-
ligand (2,2’-dipyridylamine) was covalently attached
to the imidazolium cation to obtain a novel ionic
liquid
– 2,3-dimethyl-1-[3-N,N-bis(2-pyridyl)propyl-
ACHTREUNG
DPA] [PF6], Figure 1) in order to prevent aggregation
of palladium nanoparticles, on the other hand, the
linker (2,2’-dipyridylamine) could also change the sur-
rounding electronic characteristics of the palladium
nanoparticles, which may influence the hydrogenation
activity of palladium nanocatalysts. 1-n-Butyl-2,3-di-
methylimidazolium hexafluorophosphate ([BMMIM]
[PF6], Figure 1) instead of 1-n-butyl-3-methylimidazo-
lium hexafluorophosphate ([BMIM] [PF6], Figure 1)
was intentionally selected as the reaction media for
the hydrogenation of various olefins because an N-
heterocyclic carbene might form by C-2 deprotona-
À
tion or oxidative addition of the C-2 H bond of 1,3-
di
A
of the palladium catalyst and decrease of the activi-
ty.[35,36] We disclose herein that indeed Pd(0) nanopar-
ticles stabilized by the bidentate nitrogen ligand at-
tached to the imidazolium cations are highly efficient
multiphase catalysts for the hydrogenation of various
olefins in up to 100% yield. Additionally, the effects
of palladium concentration and reaction temperature
on catalytic activity were investigated.
Scheme 1. Synthesis of 2, 3-dimethyl-1-[3-N,N-bis(2-pyridyl)-
propylamido)imidazolium
hexafluorophosphate
[BMMDPA] [PF6].
2078
asc.wiley-vch.de
ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 2077 – 2085

Biphasic Hydrogenation of Olefins
FULL PAPERS
sponding alkyl halide, followed by the step of ion ex-
change. However, in the step of the synthesis of N,N-
bis(pyridyl)-3-chloropropanamide (PCDPA, Figure 1),
G
it was found that the reaction of amide formation nec-
essarily started at À308Cand then was slowly
warmed to room temperature. Thermogravimetric
analysis (TGA) measurements of the ionic liquids
showed that [BMMDPA] [PF6] started to decompose
from 1908Cand was almost completely decomposed
at 3258C, while [BMMDPA] [Cl] started to decom-
pose from 1258Cand was degraded completely at
2508C. These results indicated that the thermal stabil-
ity of [BMMDPA] [Cl] was somewhat poorer than
that of [BMMDPA] [PF6] (see Supporting Informa-
tion, Figures S1 and S2).
The Pd(0) nanoparticles were simply prepared by
stirring a mixture of Pd(OAc)2, the functionalized
E
ionic liquid [BMMDPA] [PF6] and [BMMIM] [PF6] in
acetone at room temperature in a stainless steel auto-
clave for 1 h, then evaporating the solvent under
vacuum, followed by reduction of Pd(II) with molecu-
lar hydrogen (0.1 MPa) at 608Cfor 10 min, which af-
forded a dark brownish ꢁsolutionꢀ and then adjusting
Figure 2. X-ray diffraction pattern of isolated Pd(0) nano-
particles after three consecutive recycles of cyclohexene hy-
drogenation. The peaks are labelled with the hkl of the
planes for the corresponding Bragg angles.
the concentration of atomic Pd to 2.5 mmolLÀ1. After face of palladium nanoparticles even if the samples
three consecutive recycles of cyclohexene hydrogena- have undergone the previous isolation procedures.
tion, a black powder could be isolated from the black This was further verified by a TGA curve of the sepa-
ꢁsolutionꢀ by adding acetone and then centrifuging rated palladium nanoparticles after three consecutive
(4000 rpm, 5 min, 3 times). Washed three times with cycles of hydrogenation of cyclohexene. To our sur-
acetone and dried under reduced pressure, the isolat- prise, the separated palladium nanoparticles lost ca.
ed powder was analyzed by X-ray diffraction (XRD), 23% weight within the temperature range studied
thermogravimetric analysis (TGA), transmission elec- (40–6008C) (Supporting Information, Figure S3),
tron microscopy (TEM) and energy dispersive spec- which further proved that the isolated nanoparticles
troscopy (EDS).
still contained residues of the ILs even if the isolation
The XRD pattern (Figure 2) of the isolated materi- procedure had been repeated for several times. The
al confirmed the presence of crystalline Pd(0). The Dupont group has also observed a similar phenomen-
most representative reflections of Pd(0) were indexed on previously by XPS analysis of the isolated nano-
as face-centered cubic (fcc) with a unit cell parameter particles.[25]
a=0.39026 nm. The Bragg reflections at 39.9808,
The functionalized ionic liquid [BMMDPA] [PF6]-
46.5008, 67.5998, 81.6008, and 86.7278 corresponded to stabilized Pd nanoparticles were prepared in situ in
the indexed planes of the crystals of Pd(0) (111), [BMMIM] [PF6] and employed directly as catalysts.
(200), (220), (311) and (222).
The catalytic activities under different reaction condi-
The previously isolated [BMMDPA] [PF6]-stabi- tions are shown in Figure 6, Table 1 and Table 2. Cy-
lized palladium nanoparticles were redispersed in ace- clohexane was the only product in the hydrogenation
tonitrile, placed as a thin film in a carbon coated of cyclohexene. The hydrogenation of styrene or ethyl
copper grid and characterized by TEM. Analysis of acrylate produced only ethylbenzene or ethyl propio-
the micrographs displayed nearly spherical Pd(0) nate, respectively. Most reactions were carried out at
nanoparticles and a nearly monomodal distribution 358Cunder 0.1 MPa of hydrogen pressure for 3 h
with an average diameter of 5–6 nm when reaction unless indicated otherwise. The yield of the product
was performed at 2.5 mmolLÀ1 palladium concentra- was obtained based on the conversion of the sub-
tion and 358C(Figure 3), but the obvious aggregation
strates and the selectivity of the product determined
of the Pd particles was visually observed when the by gas chromatographic analysis.
catalyst was exposed under higher palladium concen-
As shown in Figure 6, the effect of Pd(0) concentra-
tration (10 mmolLÀ1) or higher temperature (508C), tion on the catalyst activity was firstly investigated.
as shown in Figure 4. Moreover, the presence of N The activity of the catalyst decreased slightly as the
and Pd in the sample by EDS (Figure 5) spectra palladium concentration increased from 1 mmolLÀ1 to
proved that [BMMDPA] [PF6] and/or [BMMIM] 10 mmolLÀ1 at 358C. The TEM images demonstrated
[PF6] might coordinate or adsorb strongly on the sur- that the higher concentration of palladium
Adv. Synth. Catal. 2008, 350, 2077 – 2085
ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2079

Yu Hu et al.
FULL PAPERS
Figure 3. TEM (left) and HR-TEM (right) images of twinned palladium nanoparticles after three consecutive recycles of hy-
drogenation of cyclohexene under the conditions of 2.5 mmolLÀ1 palladium concentration and 358Creaction temperature,
supported on a carbon-coated grid.
Figure 4. TEM micrographs of Pd(0) nanoparticles after three consecutive recycles of hydrogenation of cyclohexene, sup-
ported on a carbon coated grid under different reaction conditions: a (left) 2.5 mmolLÀ1 palladium and 508Creaction tem-
perature; b (right) 10 mmolLÀ1 palladium and 358Creaction temperature.
(10 mmolLÀ1) induced the aggregation of palladium the concentration of palladium changed from
nanoparticles (Figure 4b), leading to a decrease of 1 mmolLÀ1 to 2.5 mmolLÀ1. This phenomenon illus-
the activity. The similar results have also been report- trated that the present palladium nanocatalysts were
ed by Finkꢀs and Reetzꢀs groups, respectively.[37,38] No highly effective even if the concentration of Pd de-
significant difference in activity was observed when creased to a very low level since the number of cata-
2080
asc.wiley-vch.de
ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 2077 – 2085

Biphasic Hydrogenation of Olefins
FULL PAPERS
Figure 5. Energy dispersion spectrum (EDS) of the sample as employed for Figure 3 confirming the presence of palladium,
[BMMDPA] [PF6] and/or [BMMIM] [PF6] in the sample.
Figure 6. Influence of the concentration of Pd(0) in [BMMIM] [PF6] on catalytic activity. cyclohexene/Pd=500 (molar ratio),
0.1 MPa H2; reaction temperature: 358C; reaction time: 3 h.
lytic sites of Pd nanoparticles were enough for the suspended impressively in IL ([BMMIM] [PF6]) with
complete conversion of cyclohexene under present increasing reaction temperature (>358C) or Pd con-
conditions.
centration (>5 mmolLÀ1), which revealed that the co-
From Table 1, we see that the activity decreased alescence of palladium nanoparticles into bulk metal
when the reaction temperature was higher than 358C particles was accelerated when the unsuitable reaction
(entries 1, 2; 4, 5). Furthermore, the activity of the conditions were adopted, thus leading to lower activi-
catalyst decreased gradually in the consecutive cata- ty towards the conversion of cyclohexene to cyclohex-
lytic recycles at a relatively higher temperature (en- ane correspondingly. However, the nano-sized palladi-
tries 2 and 5). Interestingly, the lower concentration um particles were still in existence (Figure 4a and Fig-
of Pd (2.5 mmolLÀ1) was beneficial to increasing the ure 4b) even if some of Pd nanoparticles had aggre-
durability of the catalysts (entries 1 and 3). It was vis- gated and grown up under these conditions. It is
ually observed that more bulk metal particles were worth noting that the low temperature not only pro-
Adv. Synth. Catal. 2008, 350, 2077 – 2085
ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2081

Yu Hu et al.
FULL PAPERS
Table 1. Hydrogenation of olefins catalyzed by functionalized ionic liquid-stabilized Pd(0) nanoparticles in [BMMIM]
[PF6].[a]
Entry
T [oC]
Substrate
Substrate/Pd (molar ratio)
CPd(0) [mmolLÀ1][b]
GCYield [%]
2nd run
1st run
3rd run
1
2
3
4
5
6
7
8
35
50
35
35
50
35
35
35
Cyclohexene
Cyclohexene
Cyclohexene
Cyclohexene
Cyclohexene
Cyclohexene
Styrene
500
500
500
2000
2000
4000
500
10
10
95
77
100
96
81
50
100
100
90
65
100
95
75
90
58
100
95
72
2.5
2.5
2.5
2.5
2.5
2.5
100
100
100
100
Ethyl acrylate
500
[a]
Reaction conditions: 0.1 MPa H2, reaction time: 3 h.
CPd(0) denotes the Pd(0) concentration.
[b]
Table 2. Influence of different ligands on the activity of cyclohexene hydrogenation.[a]
Entry
Ligands
[BMMDPA] [PF6]
[BMMDPA] [PF6]
2,2’-bispyridineamine
2,2’-bispyridineamine
Ionic Liquid
GCYield [%]
1
2
3
4
G
A
96
5
83
8
ACHTREUNG
G
AHCTREUNG
[a]
Reaction conditions: Pd (2.5 mmolLÀ1), Pd/Ligands=2:1 (molar ratio), cyclohexene/Pd=2000 (molar ratio), [BMMIM]
[PF6] or [BMIM] [PF6] (1 mL), 0.1 MPa H2, reaction temperature: 358C, reaction time: 3 h.
moted the formation and dispersion of Pd nanoparti- even during three catalytic recycles without the loss
cles, but also favored the exothermic hydrogenation of any activity. Actually, this Pd nanocatalyst was far
reaction. In consideration of the melting point, viscos- more active than we expected, when the molar radio
ity of the ionic liquid and the efficient stirring speed, of substrate to Pd increased from 500 to 2000, the GC
we optimized most of the reaction temperatures at yield of cyclohexane could still reach 95% after the
358C.
two consecutive recycles of hydrogenation (entry 4).
In summary, low reaction temperature and low con- Even if we continued to increase the molar radio of
centration of Pd favored better dispersion of Pd nano- substrate to Pd to 4000 (entry 6), the yield of cyclo-
particles which were the actual catalytic active sites. hexane could still reach up to 50%.
This was the reason why the catalyst showed the best
We also investigated the roles of [BMMDPA] [PF6]
activity at 358Cand 1–2.5 mmol LÀ1 Pd concentration. in the present catalytic system. Some control experi-
As shown clearly in Table 1 (entries 3 and 4), the ments have been conducted for the hydrogenation of
total consumption of cyclohexene took less than 3 h cyclohexene. It was visually observed that the addi-
by using the present catalyst which also can be reused tion of unfunctionalized ligand (2,2’-bispyridinea-
at least five times without loss of the activity (not mine) can also effectively stabilize the palladium
listed) when the molar ratio of cyclohexene to Pd nanoparticles, but it was much less active than the ad-
atom was 500, and the palladium nanoparticles were dition of [BMMDPA] [PF6] (Table 2, entries 1 and 3).
still in an almost monomodal distribution with an We speculate that the unfunctionalized ligand 2,2’-bis-
average diameter of 5–6 nm (Figure 3) during the cat- pyridineamine has limited solubility in [BMMIM]
alytic recycles. From high-resolution transmission [PF6] and/or too strong a coordinating ability com-
electron microscopic (HR-TEM) images, it seems that pared with [BMMDPA] [PF6], then the active surface
the present Pd nanoparticles (5–6 nm) were aggre- atoms of Pd(0) nanoparticles are strongly coordinated
gates of smaller Pd crystallites (2–3 nm), and the and poisoned,[39] which inhibits the hydrogenation of
defect in the Pd(0) crystalline lattice was also found. olefins.
Note that these twin defects are typical for small par-
Finally, we testified the effect of the possible forma-
ticles which may be the actual active sites towards the tion of carbene on the catalytic reactivity. To our sur-
catalytic hydrogenation. The present system was also prise, under the same conditions, after we displaced
very active to other substrates such as the hydrogena- [BMMIM] [PF6] with [BMIM] [PF6] as the reaction
tion of styrene or ethyl acrylate (entry 7 and entry 8), medium, and either [BMMDPA] [PF6] or 2,2’-bispyri-
2082
asc.wiley-vch.de
ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 2077 – 2085

Biphasic Hydrogenation of Olefins
FULL PAPERS
formed under high-purity nitrogen purge (100 mLminÀ1) by
dineamine was added as the ligand, the yield of cyclo-
hexane was only 5% or 8% even if we repeated the
experiments three times in order to avoid the error of
the experiments (Table 2, entries 2 and 4). After the
reaction, the ionic liquid phase was still slightly
yellow-brownish which specified that the Pd(II) com-
plex in the present system was hard to be reduced to
Pd(0) species under 0.1 MPa hydrogen pressure at
608Cfor 10 min. This accounts for the fact that the
formation of carbene may hinder the reduction of
Pd(II) complexes under the present conditions, a simi-
lar conclusion has been drawn by the Trzeciak
group[40] and the Yokoyama group.[41]
heating the samples to 4008C, 5008Cand 600 8Cseparately
À1
at a rate of 108Cmin
(Figures S1–S3). The XRD analysis
was performed in a D/MAX 2550 VB/PCusing a graphite
crystal as monochromator. The TEM analyses were per-
formed in a JEOL JEM 2010 transmission electron micro-
scope operating at 200 kV with nominal resolution of
0.25 nm.
Preparation of N,N-Bis(pyridyl)-3-chloropropan-
R
amide (PCDPA)
The reaction was performed under a nitrogen atmosphere
by standard Schlenk techniques. To a solution of 2, 2’-bispyr-
idineamine (0.50 g, 2.92 mmol) and triethylamine (0.30 g,
2.96 mmol) in dry dichloromethane (15 mL) was added 3-
chloropropionyl chloride (1.48 g, 11.7 mmol) at À308C . The
mixture was slowly warmed to room temperature and stirred
overnight. For work-up, the mixture was poured into water
and extracted with dichloromethane. The dichloromethane
extracts were washed with saturated aqueous sodium bicar-
bonate solution (until the aqueous layer was alkaline), de-
ionized water, following by drying over sodium sulfate, then
the solvent was removed under vacuum to gave PCDPA;
yield: 0.67 g (88%). The oily product was used in next step
without any further purification.
Conclusions
In conclusion, we have first synthesized a new ionic
liquid ([BMMDPA] [PF6]) functionalized by 2,2’-bis-
pyridineamine, and then prepared Pd nanoparticles in
situ in ionic liquid ([BMMIM] [PF6]) media. It was
found that Pd(0) nanoparticles were stabilized effec-
tively by the bidentate nitrogen ligands attached cova-
lently to the imidazolium cations. The nanocatalyst
was a highly efficient multiphase catalyst for the hy-
drogenation of olefins in up to 100% yield. Further-
more, we have demonstrated that the higher concen-
tration of palladium and reaction temperature did not
favor the stabilization of the palladium nanoparticles,
which also led to the decrease of catalytic activity in
our present catalytic system. We are currently investi-
gating how such ligand-stabilized nanocatalysts cata-
lyze selective hydrogenation and other reactions in
the ionic liquid medium.
Preparation of 2,3-Dimethyl-1-[3-N,N-bis(2-pyridyl)-
propylamido]imidazolium Chloride ([BMMDPA]
[Cl])
A mixture of PCDPA (0.67 g, 2.56 mmol) and 1, 2-dimethy-
limidazole (0.27 g, 2.82 mmol) in acetonitrile (15 mL) was
refluxed overnight. The solvent was evacuated and the mix-
ture was poured into deionized water (20 mL), washed with
small volumes of toluene three times, dried under a vacuum
to give the product as a highly viscous orange oil at room
1
temperature; yield: 0.81 g (88%). H NMR (D2O, 500 MHz):
d=2.55 (s), 2.70 (t), 3.70 (s), 4.30 (t), 7.20 (m), 7.25 (s), 7.32
(s), 7.98 (m), 8.25 (m); 13CNMR (D 2O, 125 MHz): d=177.2,
151.3, 144.7, 143.1, 140.5, 122.5, 120.9, 118.2, 114.1, 45.2,
36.2, 34.5, 9.0.
Experimental Section
Materials
Preparation of 2,3-Dimethyl-1-[3-N,N-bis(2-pyridyl)-
propylamido]imidazolium Hexafluorophosphate
([BMMDPA] [PF6])
All manipulations involving air-sensitive materials were car-
ried out using standard Schlenk line techniques under an at-
mosphere of nitrogen. CH2Cl2, C HCN (A.R. grade) and
3
olefins (A.R. grade) were distilled from CaH2. Acetone
(A.R. grade) was distilled from molecular sieves. Palladium
acetate, 2,2’-bispyridineamine, silver hexafluorophosphate
and hexafluorophosphoric acid (60 wt% solution in water)
were obtained from Aldrich and used without further purifi-
cation. The ionic liquids ([BMMIM] [PF6] and [BMIM]
[PF6]) were synthesized using the procedure reported by
other authors.[42] All ionic liquids were dried under a
vacuum at 608Cfor 4 h prior to reaction. The chloride con-
tent of the ionic liquids was checked by the AgNO3 test and
Silver hexafluorophosphate (0.20 g, 0.788 mmol) was added
to 10 mL of deionized water and stirred for half an hour.
The clear solution was poured into
a solution of
[BMMDPA] [Cl] (0.28 g, 0.788 mmol) in deionized water
(10 mL) and stirred for half an hour. The suspension was fil-
tered to remove the silver chloride precipitate, washed with
small volumes of water. The filtrate was collected and then
carefully evaporated under a vacuum and dried for 3 h
which afforded the salt as a highly viscous brownish oil at
room temperature; yield: 0.35 g (95%). 1H NMR (D2O,
500 MHz): d=2.70 (s), 3.00 (t), 3.80 (s), 4.47 (t), 7.52 (m),
7.56 (s), 7.60 (s), 8.19 (m), 8.42 (m).
1
purity by H NMR spectroscopy. The GCanalyses were per-
formed in a GC112 A with FID detector equipped with an
HP-5 column (30 m, 0.25 mm i.d.). A Perkin–Elmer Pyris
Diamond was used in the current study for the thermogravi-
metric analysis (TGA). TGA measurements of the ionic liq-
uids and the separated palladium nanoparticles were per-
Adv. Synth. Catal. 2008, 350, 2077 – 2085
ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2083

Yu Hu et al.
FULL PAPERS
References
General Preparation of Catalysts: Ionic Liquid
([BMMDPA] [PF6]) Stabilized Palladium(0)
Catalysts
[1] M. N. Vargaftik, V. P. Zagorodnikov, I. P. Stolarov, I. I.
Moiseev, J. Mol. Catal. 1989, 53, 315.
[2] A. Roucoux, J. Schulz, H. Patin, Chem. Rev. 2002, 102,
A mixture of 2.5 mmolLÀ1 Pd
(OAc)2 solution in acetone
A
(1 mL), 1.25 mmolLÀ1 [BMMDPA] [PF6] solution in acetone
(1 mL), and [BMMIM] [PF6] (1 mL) was stirred at room
temperature in a stainless steel autoclave (50 mL) for 1 h,
then the solvent was evaporated under vacuum, followed by
reduction with molecular hydrogen (0.1 MPa) at 608Cfor
10 min which afforded a dark brownish ꢁsolutionꢀ. The cata-
lysts with the different concentrations of Pd in the ionic
liquid were prepared in a similar way.
3757.
[3] Y. M. A. Yamada, T. Arakawa, H. Hocke, Y. Uozumi,
Angew. Chem. Int. Ed. 2006, 45, 1.
[4] A. Nowicki, V. L. Boulaire, A. Roucoux, Adv. Synth.
Catal. 2007, 349, 2326.
[5] R. P. Andres, J. D. Bielefeld, J. I. Henderson, D. B.
Janes, V. R. Kolagunta, C. P. Kubink, W. Mahoney,
R. G. Osifchin, R. Reifenberger, Science 1996, 273,
1690.
[6] J. D. Aiken, R. G. Finke, J. Am. Chem. Soc. 1999, 121,
8803.
[7] J. A. Widegren, R. G. Finke, Inorg. Chem. 2002, 41,
1558.
[8] M. Zhao, L. Sun, R. M. Crooks, J. Am. Chem. Soc.
1998, 120, 4877.
After three consecutive recycles of hydrogenation of cy-
clohexene, to the phase of the ionic liquid was added ace-
tone (5 mL), and then the black powder was isolated by cen-
trifugation (4000 rpm, 5 min, 3 times). The isolated powder
was washed with acetone (3”5 mL), dried under reduced
pressure, and then analyzed by X-ray diffraction (XRD),
thermogravimetric analysis (TGA), transmission electron
microscopy (TEM) and energy dispersive spectroscopy
(EDS).
[9] R. W. J. Scott, C. Sivadinarayana, O. M. Wilson, Z.
Yan, D. W. Goodman, R. M. Crooks, J. Am. Chem. Soc.
2005, 127, 1380.
General Procedure for Hydrogenation under
Hydrogen Pressure
[10] H. Ohde, C. M. Wai, H. Kim, M. Ohde, J. Am. Chem.
Soc. 2002, 124, 4540.
[11] P. Meric, K. M. K. Yu, S. C. Tsang, Catal. Lett. 2004, 95,
39.
[12] F. Lu, J. Ruiz, D. Astruc, Tetrahedron Lett. 2004, 45,
9443.
[13] S. Jansat, M. Gomez, K. Philippot, G. Muller, E. Guiu,
C. Claver, S. Castillon, B. Chaudret, J. Am. Chem. Soc.
2004, 126, 1592.
The stainless steel autoclave containing previously prepared
[BMMDPA] [PF6]-stabilized Pd(0) catalysts was charged
with the appropriate substrate (1.25”10À3 mol, 500 equiv.),
then hydrogen was admitted to the system at a constant
pressure up to 0.1 MPa. The mixture was heated to 358C
and stirred for 3 h. After cooling to ambient temperature,
the products were isolated by liquid-liquid extraction with
diethyl ether and analyzed by gas chromatography. In all ex-
periments, the molar ratio of Pd to [BMMDPA] [PF6] was
kept at 2:1 and 1 mL of [BMMIM] [PF6] was used as solvent
unless described otherwise.
For the recycling procedure, after reaction for 3 h and
then cooling back to ambient temperature, the products
were extracted by liquid-liquid extraction with diethyl ether
for three times. After evaporating the diethyl ether under a
vacuum, the substrate was charged into the autoclave for
the next recycling.
[14] D. Astruc, F. Lu, J. R. Aranzaes, Angew. Chem. Int. Ed.
2005, 44, 7852.
[15] G. Schmid, Chem. Rev. 1992, 92, 1709.
[16] M. J. Earle, U. Hakala, C. Hardacre, J. Karkkainen,
B. J. McAuley, D. W. Rooney, K. R. Seddon, J. M.
Thompson, K. Wähäla, Chem. Commun. 2005, 903.
[17] P. B. Webb, M. F. Sellin, T. E. Kunene, S. Williamson,
A. M. Z. Slawin, D. J. Cole-Hamilton, J. Am. Chem.
Soc. 2003, 125, 15577.
[18] S. Doherty, P. Goodrich, C. Hardacre, H. K. Luo, D. W.
Rooney, K. R. Seddon, P. Styring, Green Chem. 2004, 6,
63.
[19] M. Picquet, I. Tkatchenko, I. Tommasi, P. Wassersc-
heid, J. Zimmermann, Adv. Synth. Catal. 2003, 345, 959.
[20] A. J. Carmichael, D. M. Haddelton, S. A. F. Bon, K. R.
Seddon, Chem. Commun. 2000, 1237.
Supporting Information
TGA curves of ionic liquids and the Pd nanoparticles are
given in the Supporting Information.
[21] A. P. Umpierre, G. Machado, G. H. Fecher, J. Morais, J.
Dupont, Adv. Synth. Catal. 2005, 347, 1404.
[22] J. Muzart, Adv. Synth. Catal. 2006, 348, 275.
[23] F. van Rantwijk, R. A. Sheldon, Chem. Rev. 2007, 107,
2757.
[24] I. Newington, J. M. Perez-Arlandis, T. Welton, Org.
Lett. 2007, 9, 5247.
Acknowledgements
[25] C. W. Scheeren, G. Machado, S. R. Teixeira, J. Morais,
J. B. Domingos, J. Dupont, J. Phys. Chem. B. 2006, 110,
13011.
[26] J. Huang, T. Jiang, B. Han, H. Gao, Y. Chang, G. Zhao,
W. Wu, Chem. Commun. 2003, 1654.
The authors are grateful for the support from the National
Natural Science Foundation of China (No. 20773037),
ECUST (No. YJ0142136), and the Commission of Science
and Technology of Shanghai Municipality (No. 07J14023),
China. The authors would also like to express their thanks to
Prof. Walter Leitner for helpful suggestions and Dr. Nils
Theyssen for experimental assistance.
[27] B. LØger, A. Denicourt-Nowicki, A. Roucoux, H. Olivi-
er-Bourbigou, Adv. Synth. Catal. 2008, 350, 153.
2084
asc.wiley-vch.de
ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 2077 – 2085

Biphasic Hydrogenation of Olefins
FULL PAPERS
[28] V. Calò, A. Nacci, A. Monopoli, J. Mol. Cat. A: Chem.
2004, 214, 45.
[29] J. Li, B. Tang, L. Tao, Y. Xie, Y. Liang, M. Zhang, J.
[35] L. S. Ott, M. L. Cline, M. Deetlefs, K. R. Seddon, R. G.
Finke, J. Am. Chem. Soc. 2005, 127, 5758.
[36] L. S. Ott, S. Campbell, K. R. Seddon, R. G. Finke,
Inorg. Chem. 2007, 46, 10335.
Org. Chem. 2006, 71, 7488.
[37] C. Besson, E. E. Finney, R. G. Finke, J. Am. Chem.
Soc. 2005, 127, 8179.
[38] M. T. Reetz, J. G. de Vries, Chem. Commun. 2004,
1559.
[39] R. ClØrac, F. A. Cotton, K. R. Dunbar, C. A. Murillo, I.
Pascual, X. Wang, Inorg. Chem. 1999, 38, 2655.
[40] W. Zawartka, A. M. Trzeciak, J. J. Ziółkowski, T. Lis,
Z. Ciunik, J. Pernak, Adv. Synth. Catal. 2006, 348, 1689.
[41] Y. Kume, K. Qiao, D. Tomida, C. Yokoyama, Catal.
Commun. 2008, 9, 369.
[30] A. J. Bruss, M. A. Gelesky, G. Machado, J. Dupont, J.
Mol. Cat. A: Chem. 2006, 252, 212.
[31] J. Wang, J. Feng, R. Qin, H. Fu, M. Yuan, H. Chena, X.
Li, Tetrahedron: Asymmetry 2007, 18, 1643.
[32] F. Fernµndez, B. Cordero, J. Durand, G. Muller, F. Mal-
bosc, Y. Kihn, E. Teuma, M. Gómez, Dalton Trans.
2007, 5572.
[33] D. Zhao, Z. Fei, T. J. Geldbach, R. Scopelliti, P. J.
Dyson, J. Am. Chem. Soc. 2004, 126, 15876.
[34] J. Huang, T. Jiang, H. Gao, B. Han, Z. Liu, W. Wu, Y.
Chang, G. Zhao, Angew. Chem. Int. Ed. 2004, 43, 1397.
[42] J. G. Huddleston, H. D. Willauer, R. P. Swatloski, A. E.
Visser, R. D. Rogers, Chem. Commun. 1998, 1765.
Adv. Synth. Catal. 2008, 350, 2077 – 2085
ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2085