Palladium nanoparticles stabilized by aqueous vesicles self-assembled from a PEGylated surfactant ionic liquid for the chemoselective reduction of nitroarenes

Article abstract of DOI:10.1016/j.catcom.2017.04.051

Vesicles self-assembled from an aqueous PEGylated surfactant ionic liquid solution can be applied for stabilizing palladium nanoparticles, which prove to be an efficient catalytic system for chemoselective hydrogen transfer of nitroarenes using hydrazine hydrate as a hydrogen source. The particle sizes of vesicles are decreased with the increase of ionic liquid's concentrations and relatively small particle sizes are beneficial to the reduction. Moreover, the aqueous catalytic system still stays in reactor by simple extraction, and is reused without further treatment.

Full text of DOI:10.1016/j.catcom.2017.04.051

CatalysisCommunications99(2017)57–60
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Palladium nanoparticles stabilized by aqueous vesicles self-assembled from
a PEGylated surfactant ionic liquid for the chemoselective reduction of
nitroarenes
Zhu-bing Xu, Guo-ping Lu⁎, Chun Cai
School of Chemical Engineering, Nanjing University of Science and Technology, Xiaolingwei 200, Nanjing 210094, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Aqueous vesicles
PEGylated surfactant ionic liquid
Palladium nanoparticles
Hydrogen transfer
Aromatic amine
Vesicles self-assembled from an aqueous PEGylated surfactant ionic liquid solution can be applied for stabilizing
palladium nanoparticles, which prove to be an efficient catalytic system for chemoselective hydrogen transfer of
nitroarenes using hydrazine hydrate as a hydrogen source. The particle sizes of vesicles are decreased with the
increase of ionic liquid's concentrations and relatively small particle sizes are beneficial to the reduction.
Moreover, the aqueous catalytic system still stays in reactor by simple extraction, and is reused without further
treatment.
1. Introduction
micelles formed by ILs and micellar catalysis in aqueous-ionic liquid
systems [11–13], only a few examples of vesicles have been observed in
Metal nanoparticles stabilized in ionic liquids (MNPs@ILs) are
promising catalysts for transformations [1–2]. Although MNPs@ILs can
be recycled by simple extraction, ILs are relatively expensive solvents
than other solvents that are widely applied in industry. Moreover, the
environmental fate of ionic liquids is a complex situation, such as their
modes of toxicity, biodegradation pathways and behavior concerning
biosorption [3]. Thus, the use of catalytic amount of ILs as stabilizers
for the formation of MNPs in a cheap, green solvent is a greener al-
ternative to applying ILs as both stabilizers and solvents. ILs can be
supported on solid materials via covalent bonding for immobilization of
MNPs [4–5]. In these approaches, a small amount of ILs is required to
stabilize the MNPs, and the properties of ILs enhance their catalytic
performance. Like other supported MNPs [6], these MNPs can be easily
reused by simple filtration but washing steps and extra solutions (or-
ganic solvents or acidic/basic aqueous solutions) are always the norm.
Additionally, the reaction systems (mediums and additives) are often
discarded in these strategies.
Ideally, the catalyst solution remains in the reactor and is reused
with a fresh batch of reactants without further treatment [7]. The use of
water as a solvent opens the way to biphasic extraction of products and
recycling of the catalytic systems [8]. Lipshutz's group has reported that
recyclable aqueous TPGS-750-M micelles can stabilize the MNPs (Pd, Ni
and Fe) for catalyzing organic synthesis [9,10]. Meanwhile, there has
been a tremendous increase of interest in the behavior of surfactant
ionic liquids in water. Although there are several reports on aqueous
aqueous-ionic liquid systems [14,15] and no report has applied them as
reaction medium. Actually, vesicles may have better stabilization and
catalytic activity than micelles in some transformations [16–18].
Therefore, we reason that the metal NPs can be stabilized by aqueous
vesicles generated by surfactant ILs with a combination of steric and
electronic stabilization, which may be an efficient catalyst for organic
reactions.
Functionalized arylamines that play an important role in agro-
chemicals, pharmaceuticals, fine chemicals, dyes, pigments, material
science and biotechnology [19], are mainly produced by chemoselec-
tive hydrogenation of the corresponding nitroarenes [20–22]. The use
of active metals or sulfides results in serious pollution [23,24], and
hydrogen reduction methods suffer from the requirement of specialized
equipment [25,26]. The reductions by hydrazine monohydrate are one
of valid solutions to these issues, but high temperature is always re-
quired [27–29]. Based on these results, we disclosed a recyclable aqu-
eous catalytic system for the chemoselective reduction of nitroarenes
using hydrazine monohydrate as the reducer under relatively mild
conditions, in which the Pd NPs are stabilized in vesicles fabricated by
self-assembly of a PEGylated surfactant IL. To the best of our knowl-
edge, this is the first example that aqueous vesicles self-assembled from
a PEGylated surfactant ionic liquid are employed for stabilizing Pd NPs
and as the reaction medium. Although the recyclability of the catalyst is
less satisfactory than supported MNPs, but the catalyst still has some
advantages compared with supported MNPs, including easy to
⁎
Corresponding author.
E-mail address: glu@njust.edu.cn (G.-p. Lu).
http://dx.doi.org/10.1016/j.catcom.2017.04.051
Received 8 December 2016; Received in revised form 9 March 2017; Accepted 15 April 2017
Availableonline26May2017
1566-7367/©2017ElsevierB.V.Allrightsreserved.

Z.-b. Xu et al.
CatalysisCommunications99(2017)57–60
preparation and simple recycling process.
2. Experimental section
Table 1
Optimization of reaction conditionsa.
2.1. The reduction of nitroarenes catalyzed by Pd NPs
To an oven-dried reaction flask with Teflon coated stir bar purged
with argon, Pd(OAc)2 0.01 mmol, IL4 0.08 mmol, KOH (or K2CO3)
0.30 mmol, nitroarene 1 1.0 mmol and degassed HPLC grade water
1.5 mL were added, and the mixture was stirred for 5 min under argon
at room temperature. Then, 0.5 mL of hydrazine hydrate aqueous so-
lution (~5.0 mmol) was syringed into the flask at the same tempera-
ture. After stirring for additional 5 min, the reaction was heat to 50 °C,
and stirring for 8 h under argon. After completion, the reaction mixture
was extracted by methyl tertiary butyl ether (MTBE) (3 × 2 mL), and
the organic layer was collected and filtered through a bed of silica gel
layered over Celite. The volatiles were removed in vacuo to afford the
product 2. In some cases, further column chromatography on silica gel
was required to afford the pure desired products.
Entry
1c
2c
3c
4c
5
IL
Base (x)
/
/
/
/
/
/
Yield (%)b
IL1
IL2
IL3
IL4
IL4
IL4
IL4
0c
0c
11c
26c
54
88
99
6
7
K2CO3 (0.30)
2.2. Recycling aqueous catalytic system
8
9
IL4
IL4
IL4
IL4
IL4
IL4
IL4
IL4
/
IL4
IL4
IL4
IL4
IL4
/
29
48
trace
75
K2CO3 (0.25)
LiOH (0.25)
NaOH (0.25)
KOH (0.25)
KOH (0.30)
KOH (0.30)
KOH (0.30)
KOH (0.30)
KOH (0.40)
t-BuONa (0.25)
NEt3 (0.25)
KHMDS (0.25)
KOH (0.30)
10
11
12
13
14
15
16
17
18
19
20
21
After reaction completion, the mixture was then extracted with
MTBE (3 × 1 mL). The organic layer was collected by syringes and
filtered through a bed of silica gel layered over Celite. The volatiles
were removed in vacuo to afford the product 2b. To the IL4-palladium
mixture (aqueous phase), 1b 1.0 mmol was added, followed by hy-
drazine hydrate (3.0 mmol, 80 wt% aqueous solution) at room tem-
perature and the reaction stirred under argon for 8 h at 50 °C. The ex-
traction cycle was then repeated for the separation of 2b.
92
96, 78d, 45e
90f
68c
28
98
53
39
77
71g
3. Results and discussion
a
Reaction conditions: 1a or 1b 1.0 mmol, ionic liquid 0.08 mmol, base × mmol,
Initially, four PEGylated imidazole-based ILs were synthesized by a
three-step process (See SI). The contrast experiments were performed
by selecting the reduction of 4-nitrotoluene in the presence of Pd(OAc)2
and hydrazine hydrate in IL1-4 aqueous solutions (Table 1, entries
1–4), and IL4 provided the best result. A good yield could be obtained
by increasing the amount of hydrazine hydrate and heating to 50 °C
(entry 6). However, only a poor yield of p-methoxyaniline 2b was
gained under identical conditions (entry 8). Thus, various bases were
screened to further improve the unsatisfactory results (entries 7, 9–12,
18–20), because base can enhance the solubility of Pd(OAc)2 in water
and may capture the C-2 hydrogen of imidazole ring in IL4 to form N-
heterocyclic carbene palladium complex [30,31]. KOH proved to be the
best choice and 0.30 equiv. was the optimized amount of usage (entry
13). The reaction was inhibited in the absence of IL4 since the stability
of Pd NPs may be weakened (entries 16). The temperature (entries
13–15, 21) and the amount of hydrazine hydrate (entry 13) were also
optimized, and 50 °C and 5 equiv. of hydrazine hydrate were the best
option in the reduction of 1b.
N2H4·H2O y mmol, H2O 2 mL, Ar, 8 h.
Yields were determined by GC with 1,3-dimethoxybenzene (100 μL) as the internal
standard, which was added to the mixture after the reaction was complete.
b
c
At room temperature.
3 equiv of N2H4·H2O is used.
1 equiv of N2H4·H2O is used.
At 40 °C.
d
e
f
g
At 80 °C.
To investigate the relationship between the aqueous PEGylated
imidazole-based ILs' solutions and the reaction yields, the average
particle sizes of aggregates formed by ILs in water were detected by
dynamic light scattering (DLS). The particle sizes are inversely pro-
portional to the alkyl chain's length of ILs [16–18,32] (Fig. S1 in SI) and
the concentrations of aqueous IL4 solutions (Fig. 1). The aggregates
may be vesicles owing to the relatively large range of their particle sizes
[14,15]. Meanwhile, the reaction yields were enhanced with the de-
creasing of the particle sizes (Fig. 1), presumably due to the larger
specific surface area and the better stabilization for Pd NPs [33].
Transmission electron microscopy (TEM) allowed us to further
confirm the structure of aggregates formed by IL4 (40 mM) (Fig. 2a)
and the combination mode between Pd nanoparticles and vesicles
(Fig. 2b). The TEM specimens were prepared by dipping a polymer-
coated copper grid into aqueous solutions of IL4 (40 mM). The TEM
Fig. 1. The average particle sizes of vesicles formed by different concentrations of aqu-
eous IL4 solutions (10–70 mM) determined by DLS. (b) The yields (GC yields) of the
reduction of 1b in different concentrations of aqueous IL4 solutions.
grid was dried for 0.5 h at room temperature, and then subjected to
TEM observation. A lot of water is in the inner core of aggregates (dark
areas in Fig. 2a) and a hydrophobic region around the core is also ob-
served (gray areas in Fig. 2a), so the aggregates formed by IL4 were
vesicles [16–18]. The Pd NPs may be in vesicles or enwrapped by IL4
58

Z.-b. Xu et al.
CatalysisCommunications99(2017)57–60
Fig. 2. (a) TEM micrograph of vesicles formed by IL4 in
water (40 mM).a (b) TEM micrograph of vesicles formed by
IL4 with Pd NPs in water (40 mM).a (c) TEM micrograph of
Pd NPs in aqueous vesicles of IL4 (40 mM) using Pd(OAc)2
as the precursor and hydrazine hydrate as the reducing
a
agent. On a polymer-coated copper grid, stained with an
aqueous solution of phosphotungstic acid (2 wt%).
because the dark areas that may be water or Pd NPs are only found in
vesicles and the irregular aggregates of IL4 (gray areas in Fig. 2b).
Therefore, there may be three stabilizing effects of IL4 on Pd NPs,
including (i) electrostatic stabilization (cations and anions of ILs) [34];
(ii) steric protection (PEG chain) [35]; (iii) the formation of N-hetero-
cyclic carbene palladium complex [30,31]. The TEM images (Fig.2c)
showed the Pd NPs were generated and stabled by the aqueous vesicles
of IL4. The TEM images (Fig. S2f in SI) showed Pd NPs in vesicles are
aggregated obviously after reusing four times. Obvious aggregation of
Pd NPs is also found in aqueous vesicles of IL4 (10 mM) (Fig. S2 g in SI),
so the larger particle sizes of vesicles may enhance the aggregation of
Pd NPs, which goes against the reactivity of Pd NPs (more details on
TEM images see Fig. S2 in SI). SEM was also used to further confirm the
vesicles. Some spherical particles can be observed, which may be ve-
sicles (more details on SEM images see Fig. S3 in SI).
With the optimized conditions in hands, various nitroarenes were
chosen to establish the scope and generality of this protocol (Scheme 1).
The steric hindrance effects of nitrobenzenes had few influences on the
reaction. Meanwhile, nitrobenzenes containing both electron-withdraw
and electron-donating groups could provide satisfactory yields. No de-
halogenation occurred in the cases of 2d, 2e and 2o which could be
attributed to the mild conditions and weak leaving abilities of F and Cl
atoms, but a 18% of dehalogenation product was detected in the case of
2f. Slight lower isolated yields were gained using nitroarenes with OH
group (2g, 2h) as starting materials because aminophenols have better
solubility in water than other anilines. Nitroarenes substituted with
easily reducible functional groups such ester, cyano and vinyl groups
(2k, 2l, 2n) were also selectively reduced to the corresponding anilines
with excellent yields. Different substituted heterocyclic nitroarenes
were reduced to the corresponding anilines in good to excellent yields
without affecting the heterocyclic ring (2p–2r).
Studies were also conducted to assess the potential for recycling of
the reaction medium and transfer hydrogenation of 4-methoxyl ni-
trobenzene 1b was selected as the model reaction. After completion of
reaction, the product undergoes in-flask extraction with minimum
amounts of an organic solvent (MTBE). Remaining in the water are the
PEGylated surfactant ionic liquid, KOH, N2H4·H2O and the palladium
catalyst. Addition of fresh N2H4·H2O (3.0 equiv, 80 wt% aqueous so-
lution) leads to an active catalyst ready for re-introduction of the
starting material. The process could be repeated 4 times without an
obvious change in yields, but fresh Pd(OAc)2 (0.5 mol%) and KOH (0.1
equiv) was added to retain the catalytic activity of the system since the
third recycle (Fig. 3).
4. Conclusions
In summary, we disclose the first example that Pd NPs are stabilized
in aqueous vesicles derived by a new PEGylated surfactant ionic liquid
for the chemoselective transfer hydrogenation of nitroarenes using
hydrazine hydrate as a hydrogen donor. The particle sizes of vesicles
are inversely proportional to the concentrations of IL4 in water and
smaller particle sizes are beneficial to the reaction. The newly
a
Fig. 3. Recycle studies. Conditions: 1b 1.0 mmol, IL4 0.08 mmol, KOH 0.30 mmol,
N2H4·H2O 5.0 mmol, H2O 2.0 mL, Ar, 8 h. IL4 was recycled in all runs, and fresh
N2H4·H2O (3.0 equiv) was added in each recycle. b Isolated yields. c The yields (blue) were
obtained without employing additional Pd(OAc)2. The yields (red) were obtained by
adding fresh Pd(OAc)2 (0.5 mol%) and KOH (0.1 equiv). (For interpretation of the re-
ferences to colour in this figure legend, the reader is referred to the web version of this
article.)
Scheme 1. The reduction of nitroarenes in aqueous vesicles of IL4.a,b Conditions: 1
1.0 mmol, IL4 0.08 mmol, KOH 0.30 mmol, N2H4·H2O 5.0 mmol, H2O 2 mL, Ar, 50 °C,
b
c
8 h. Isolated yields. The use of K2CO3 (0.30 mmol) instead of KOH.
59

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