Synthesis of Pd nanocrystals in phosphonium ionic liquids without any external reducing agents

Article abstract of DOI:10.1039/c0gc00851f

Trihexyl(tetradecyl)phosphonium ionic liquids (ILs) are found to support the formation of Pd(0) nanoparticles without the addition of reducing agents such as NaBH₄. The resulting particles are highly crystalline and have been characterized by a range of techniques including TEM and XRD. Their sizes are in the range 7 ± 2 nm. The particle formation process is highly dependent on the anion of the IL with no nanoparticles observed when the chloride derivative was used. Particle shape could be controlled through varying the anion of the IL and the Pd(ii) precursor used. Palladium chloride reagents gave truncated octahedron shaped particles in trihexyl(tetradecyl)phosphonium dodecylbenzenesulfonate, whereas palladium acetate under the same conditions afforded a mixture of shapes including triangular plates. We propose that the presence of chloride anions in the reaction mixtures led to oxidative etching of the particles and that this process can be reduced in the presence of certain anion partners in the IL including bis(2,4,4-trimethylpentyl)phosphinate. Two possible reduction mechanisms based on reactions of the phosphonium cation are suggested for the formation of the Pd(0) species. The resulting nanoparticles are effective in catalyzing the Suzuki reaction of 4-bromotoluene and benzene boronic acid. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0gc00851f

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PAPER
Synthesis of Pd nanocrystals in phosphonium ionic liquids without any
external reducing agents†
Hassan A. Kalviri and Francesca M. Kerton*
Received 25th November 2010, Accepted 25th January 2011
DOI: 10.1039/c0gc00851f
Trihexyl(tetradecyl)phosphonium ionic liquids (ILs) are found to support the formation of Pd(0)
nanoparticles without the addition of reducing agents such as NaBH4. The resulting particles are
highly crystalline and have been characterized by a range of techniques including TEM and
XRD. Their sizes are in the range 7 2 nm. The particle formation process is highly dependent on
the anion of the IL with no nanoparticles observed when the chloride derivative was used. Particle
shape could be controlled through varying the anion of the IL and the Pd(II) precursor used.
Palladium chloride reagents gave truncated octahedron shaped particles in
trihexyl(tetradecyl)phosphonium dodecylbenzenesulfonate, whereas palladium acetate under the
same conditions afforded a mixture of shapes including triangular plates. We propose that the
presence of chloride anions in the reaction mixtures led to oxidative etching of the particles and
that this process can be reduced in the presence of certain anion partners in the IL including
bis(2,4,4-trimethylpentyl)phosphinate. Two possible reduction mechanisms based on reactions of
the phosphonium cation are suggested for the formation of the Pd(0) species. The resulting
nanoparticles are effective in catalyzing the Suzuki reaction of 4-bromotoluene and benzene
boronic acid.
them an inherent risk. Indeed, there has been growing interest
Introduction
in ‘greening’ nanoparticle synthesis. One way of doing this is
Pd nanostructures have found many applications in science and
by using naturally sourced and benign reducing agents and
engineering. Because of an increased surface area to volume
this has been particularly successful in the formation of Au
ratio, nanostructures have more active sites compared with bulk
or Ag nanoparticles. For example, HAuCl4 has been reduced
materials. Therefore, nanomaterials are often more efficient in
to Au(0) using glucose.13 Another interesting example is the
their applications, whether that be as catalysts,1 sensors or in
synthesis of Au nanoparticles in a continuous fashion using
other fields. For example, Pd nanostructures have been employed
ascorbic acid as the reducing agent.14 Silver nanoparticles have
for hydrogen gas detection based on a change in their resistivity.2
been prepared in aqueous solution using Capsicum annuum L.
In catalysis, Pd nanostructures have been extensively employed
extract. It was proposed that the proteins within the extract acted
in different fields. Pd nanowire arrays have been used as effective
as the reducing agent.15 However, in terms of Pd nanoparticle
electrocatalysts for ethanol oxidation in direct alcohol fuel cells.3
In organic synthesis, Pd nanoparticles are being used widely in
catalytic C–C and C–N bond formation reactions.4–8
Noble metal nanostructures have been synthesized using a
wide range of reducing agents including organic solvents such
preparation, the most widely used reagents are sources of ‘active’
hydrogen, e.g. NaBH4. Therefore, in order to reduce the risk of
handling such reagents for the preparation of nanoparticles to
be used in catalytic reactions, the nanoparticles are often formed
in situ through reduction by reagents or solvent,16 but this can
as DMF, ethylene glycol or ethanol and inorganic reagents like
lead to less control over the particle size and distribution.
hydrogen, NaBH4 and carbon monoxide.9–12 Clearly, from a
In order to reduce the number of components (metal pre-
‘green’ perspective, some of these reducing agents carry with
cursor, solvent, stabilizer, reducing agent) in the preparation of
nanoparticles, in line with the principles of green chemistry,
there is growing interest in the use of ionic liquids (ILs) as
Centre for Green Chemistry and Catalysis, Department of Chemistry,
stabilizing agents and reaction media in their synthesis.6,11,12,17
Memorial University of Newfoundland, St. John’s, Newfoundland and
Modified ILs17 have been prepared that contain a reducing
Labrador, Canada A1B 3X7. E-mail: fkerton@mun.ca; Fax: +1 709
8643702; Tel: +1 709 8648089
moiety, however, to the best of our knowledge there is no report
† Electronic supplementary information (ESI) available: UV-vis, MS and
of using commercially available phosphonium ILs as reducing
NMR spectra, TGA plot, additional TEM images and particle size
agents in the synthesis of metal nanoparticles.
distribution histograms, Fig. S1–S13. See DOI: 10.1039/c0gc00851f
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Fig. 1 Characterization of Pd nanocrystals prepared in 1. (a) TEM image. (b) X-Ray diffraction pattern. (c) and (d) Selected region of FT-IR spectra
of 1 and the nanocrystals, respectively.
In this article, we report the synthesis of Pd nanocrystals
with different shapes in phosphonium ILs composed of
source, was dissolved in a capped vial containing [P66614]DBS
1. The initially orange-coloured solution was stirred at room
temperature and after 1 h it had turned brown. UV-vis data
for a sample showed good agreement with literature spectra for
Pd nanoparticles (Fig. S1, ESI†).9 Acetone was added to the
solution and a black precipitate was separated by centrifuge.
The precipitate was washed four times with acetone to remove
excess 1 and dried, therefore any remaining organic groups
were incorporated into the nanoparticle or on its surface. TEM
images showed that the particles were truncated octahedrons
(Fig. 1a and S2). Their powder XRD spectrum (Fig. 1b) matched
well to face-centered cubic (fcc) Pd (JCPDS card 05-0681)
and the selected area electron diffraction pattern (Fig. S2)
confirmed the fcc crystalline structure of the product. ATR
FT-IR spectra of the dry powder comprised of weak bands
(symmetric and asymmetric C–H stretches) related to 1 (Fig. 1c
and d). These results suggested that the IL was working as
both the reducing and capping agent in the formation of the
a
trihexyl(tetradecyl)phosphonium ([P66614]) cation and
different anions without any external reducing agents.18
The ILs studied are: [P66614]dodecylbenzenesulfonate 1,
[P66614]bis(trifluoromethylsulfonyl)amide 2, [P66614]bis(2,4,4-
trimethylpentyl)phosphinate 3, [P66614]hexafluorophosphate 4
and [P66614]chloride 5. Phosphonium halide ILs have been used
previously as media for Pd-catalyzed bond-forming reactions,
however the presence of Pd nanoparticles was not reported.19
We also report that the nanoparticles we have prepared can be
used as effective catalysts in Suzuki cross-coupling reactions.
Results and discussion
Preliminary investigations
During our study of etherification reactions in ILs,20 upon
changing the IL from those containing imidazolium cations
to [P66614]DBS (DBS = dodecylbenzenesulfonate) we discovered
that the Pd(II)-IL mixtures would change colour over time
in the absence of any reagents. We sought to discover the
origin of this colour change. Pd(CH3CN)2Cl2, as the Pd(II)
◦
nanoparticles. Furthermore, a weight loss event below 400 C
was observed via TGA and is further proof for the presence of
a capping agent on the surface of the nanocrystals. However,
subsequent weight gain and loss at above 530 ◦C is likely related
to the oxidation and reduction of Pd (Fig. S3).21 Although
682 | Green Chem., 2011, 13, 681–686
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electrospray ionization mass spectra (both positive and negative
modes) did not show any considerable differences for 1 before
and after the reaction (Fig. S4 and S5), 31P-NMR reveals a
small 0.20 ppm downfield shift and a slight broadening of the
phosphorus peak after the reaction which may be significant
(Fig. S6). Size distribution histograms of the images show that
the majority of the particles have sizes smaller than 9 nm (Fig.
S7).
presence was noted during the preparation of the nanoparticles.
Although, it should also be noted that both trialkyl phosphines
and phosphine oxides can be used as stabilizing (or capping)
agents in nanoparticle syntheses. If the phosphorus-containing
species has become absorbed onto the Pd nanoparticle surface
(as potentially indicated by TGA and IR data above), we would
not expect to see a corresponding resonance in the solution
NMR spectra of the IL after its use (Fig. S6). Another possible
mechanism, given the microscale reversibility of many reactions,
is the dehydrogenation of alkyl chains in the presence of
palladium, Scheme 1B, and subsequently the hydrogen formed
acts to reduce the Pd(II). Mechanism (B) may lead to the slight
broadening of the resonance that we see in the 31P NMR of 1
after formation of nanoparticles but at this stage mechanism (A)
or other possible mechanisms (e.g. simultaneous routes such as
A + B) cannot be ruled out.
Mechanism
As no external inorganic or organic reagent was added to the
ionic liquid solutions containing Pd(II) salts, the ionic liquid
must be playing the dual role of reducing and stabilizing agent.
Its role as a stabilizing agent is evident from the IR spectra
of the particles and the additional defects seen in the particles
when the solution is diluted with a typical organic solvent (see
below) and thereby contains less stabilizing agent. However, the
possible reduction mechanism is still open to debate. We propose
two possible routes. Several groups have previously studied the
electrochemical stability of phosphonium salts.22 The presence
of current shoulders (+3.2 V vs. Fc/Fc+) has been observed
in their cyclic voltammograms.22c These have previously been
assigned to either the presence of water or a methoxy functional
group in the IL. We also observe an irreversible current shoulder
(oxidation) at +1.73 V vs. Fc/Fc+ for 1 in CH3CN (+2.20 V vs.
SCE, Fig. S8). This demonstrates that 1 is less electrochemically
stable compared with the shorter alkyl chain phosphonium
ILs studied by others, and that the shoulder observed in
the cyclic voltammograms of some phosphonium salts is an
inherent property of these species (not water or functional group
dependent) and likely represents an oxidation process occurring
within the alkyl chains. It is also worth noting that when the
electrochemical oxidation of 1 is repeated in the presence of
Pd(CH3CN)2Cl2 this region of the voltammogram changes in
appearance. Therefore, researchers should be aware that not
all very similar ILs, e.g. phosphonium ILs with varying alkyl
chain lengths, have the same useful electrochemical window and
each one needs to be tested. Chu and co-workers have shown
that [P66614]Cl 5 can be deprotonated and undergo ‘non-classical’
Wittig reactions upon reaction with a suitable nucleophile.23
A first step in this process was the formation of a trialkyl
phosphine, Scheme 1A. In organometallic chemistry, it is well
known that reduced palladium can be formed via oxidation of
phosphine ligands by oxygen from an anion (e.g. acetate or in
our case DBS) or adventitious water or oxygen.24 Furthermore,
via GC-MS analysis of the volatile components from reaction
mixtures, we have observed the formation of hydrocarbon
chains. However, we have not observed the presence of the
intermediate phosphines or product phosphine-oxides by 31P
NMR. Also, no distinctive odour indicative of phosphine
Effect of varying the palladium precursor
Under the same reaction conditions, Pd(OAc)2 and PdCl2
also afforded nanocrystals (Fig. S2 and S9). Interestingly,
Pd(OAc)2 yielded particles with a variety of different crystalline
forms including octahedrons or triangular plates in addition
to truncated octahedrons (Fig. S2, bottom). Since in Pd(OAc)2
there is no chloride ion source, less oxidative etching can occur
and this results in the formation of more diverse nanocrystalline
shapes.
Effect of varying the anion of the ionic liquid
Several other phosphonium ILs were screened for Pd nanocrys-
tal formation using Pd(CH3CN)2Cl2 as the Pd precursor.
[P66614]bis(trifluoromethylsulfonyl)amide 2 yielded truncated oc-
tahedrons (Fig. S10) with a narrow size range. In con-
trast, under the same room temperature reaction conditions
no colour change was observed when using [P66614]bis(2,4,4-
trimethylpentyl)phosphinate 3 and [P66614]PF6 4, however, after
5 h at 100 ◦C octahedrons and triangular plates of Pd were the
◦
major products in 3 (Fig. 2) and after 15 h at 80 C a mixture
of truncated octahedrons and octahedrons of Pd were formed
in 4 (Fig. S11).‡ For a fcc structure octahedron or tetrahedron
shapes have the lowest total surface energy. This is despite a
truncated octahedron, which has a semispherical shape, having
a smaller surface area with more {111} and {100} facets and
lower interfacial free energy.9,25
Oxidative etching of Pd can happen in the presence of O2
and Cl-, and twinned particles (octahedrons) are known to be
etched faster than single crystals (truncated octahedrons).26,27
Therefore truncated octahedron structures are those most likely
to form under our reaction conditions, unless a stabilizing agent
can attach to {111} facets (in the case of an octahedron) and
prevent the oxidative etching from occurring (Fig. 2). As when
3 is used, octahedron-shaped structures are seen, the bis(2,4,4-
trimethylpentyl)phosphinate anion must also be involved in the
‡ The need to heat 3 and 4 to achieve nanoparticle formation is due to
the comparatively high viscosity of 3 and because 4 is a solid at room
temperature. 4 melts and 3 becomes less viscous upon heating and thus
allows palladium seed particles to migrate through the IL phase and
form nanocrystals.
Scheme 1 Some possible reactions of trihexyl(tetradecyl)phosphonium
ionic liquids.
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Table 1 Suzuki coupling reaction of 4-bromotoluene and benzene
boronic acid in [P66614]DBS using Pd nanocrystals and different bases
Base/Bromotoluene
(mole ratio)
Entry Base
Time/min Yield (%)a
1
2
3
4
5
6
KOH
KOH
Triethylamine 1.1
1.1
1.1
10
15
10
15
15
50
60
9
78
90
Fig. 2 TEM image of the Pd nanocrystals prepared in 3 and the
corresponding selected area electron diffraction pattern.
K2CO3·H2O
1.6
1.8
2.0
KOH
Cs2CO3
15 (2 ¥ 15) 43 (75)
stabilizing process and prevents etching. Etching may also be the
reason why no nanoparticles were observed when the reaction
was performed in [P66614]Cl 5 even at elevated temperatures, while
the anions in the other ILs can stabilize the nanocrystal seeds
and slow down any oxidative etching.
a Yields were calculated relative to 4-bromotoluene using 1H-NMR
spectroscopy with acetophenone as the internal standard.
Preliminary results using phosphonium-stabilized Pd
nanoparticles as catalysts
Effect of oxygen levels
Although Cl- was present in most of the reaction mixtures,
reactions were repeated under oxygen-free conditions in order to
assess any oxidative etching effects. For example, two reaction
mixtures were prepared using 1 and Pd(CH3CN)2Cl2. One of
the samples was transferred to a glove box under a nitrogen
atmosphere and the other was kept in a capped vial on
the benchtop. The sample in the glove box changed colour
much faster than the sample outside (Fig. S12) indicating
that nanocrystal formation is inhibited by the presence of O2.
However, it should be noted that suspensions of nanoparticles
in 1–3 are stable (showing no evidence of precipitate formation)
upon storage in capped vials on a benchtop for at least 3 months.
As an important use of Pd nanoparticles is in catalysis, a Suzuki
reaction was carried out using in situ prepared Pd nanocrystals
in 1 under microwave irradiation. A high yield was obtained
when 4-bromotoluene was reacted with benzene boronic acid
in the presence of potassium hydroxide (Table 1). TEM images
of the Pd nanocrystals after the Suzuki reaction showed that
the reaction does not affect the shapes and the sizes of the
nanoparticles (Fig. 4). However, no tests were performed in situ
on the catalytic reaction mixtures to definitely prove the nature
of the active catalytic species. The active species might not be the
nanocrystals themselves but the nanocrystals may be acting as a
reservoir of soluble Pd. Nanocrystals with different shapes based
on their crystal facets can have different catalytic activities and
selectivities in a chemical reaction.9,26 Therefore, future work will
involve preparing Pd nanocrystals with different shapes based
on this method in order to investigate the relationship between
shape and reactivity.
Effect of IL concentration
As ILs are quite expensive, we wanted to see if smaller amounts
of ILs could be used in solution to yield nanoparticles. A
Pd(CH3CN)2Cl2 solution composed of a 1 : 1 mole ratio of
1 and THF yielded ‘merged’ Pd nanocrystals (Fig. 3 and
S13). Since coalescence of particles in the less viscous reaction
solution (THF+IL) is more probable and also there is a lower
concentration of stabilizer, more stacking faults were observed
in TEM images of these samples (one of these faults has been
highlighted by an arrow in Fig. 3a).
Fig. 3 (a) TEM and (b) HRTEM images, respectively, of the Pd
nanocrystals prepared from Pd(CH3CN)2Cl2 in a 1 : 1 mole ratio of
1 and THF at room temperature.
Fig. 4 TEM image of the Pd nanocrystals after the Suzuki reaction.
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Preparation of Pd nanocrystals
Conclusions
Pd(CH3CN)2Cl2 (0.0072 g, 0.028 mmol) was dissolved in
degassed IL (0.500 g) in a vial capped under nitrogen flow. For
1 or 2, the solution was stirred for 18 h at room temperature.
For ILs 3 and 4, the reaction mixtures were heated to 100 ◦C for
5 h and 80 ◦C for 15 h, respectively. The reaction mixtures were
monitored using UV-Vis spectroscopy for the disappearance of
Pd2+. Acetone (3 ¥ 3 mL) was added to the brown reaction
mixture to precipitate the Pd nanoparticles and remove the
IL. The resulting fine black powder was isolated from acetone
via centrifuge and dried under vacuum for 18 h prior to analysis
using FT-IR, TGA and XRD. Nanoparticles were redispersed
in acetone for TEM sample preparation. The ILs were recovered
from the acetone washings and analysed via ESI-MS and
NMR spectroscopy. ESI-MS and NMR experiments were also
performed on the reaction mixtures themselves.
We have described a simple method for preparing Pd nanocrys-
tals with different shapes without using any external reducing
agents. In this method, phosphonium ILs have been used as
both reducing and stabilizing agents. This work provides further
evidence of the useful nature of phosphonium-derived ILs in
reduction chemistry.29 This method circumvents the need for
additional reagents in the formation of the particles as the IL acts
as the medium, reducing agent and stabilizer for the process. This
in some way reduces the risk and hazards associated with the
preparation of Pd nanoparticles. However, it cannot be ignored
that phosphonium ILs themselves are known to be toxic,28 and
therefore the quest for truly benign syntheses of Pd nanoparticles
continues. The in situ prepared Pd nanocrystals showed good
catalytic activity in a Suzuki cross-coupling reaction.
Experimental for the Suzuki reactions
Experimental
In a typical reaction, Pd(CH3CN)2Cl2 (0.0058 g, 2.7 mol%) was
dissolved in degassed [P66614]DBS 1 (0.563 g) in a vial capped
under nitrogen flow. The solution was stirred until it turned deep
brown in colour indicating that Pd nanocrystals had formed.§
4-Bromotoluene (0.141 g, 0.825 mmol), benzeneboronic acid
(0.130 g, 1.07 mmol) and KOH (0.082 g, 1.47 mmol) was added
to the reaction mixture (KOH was ground to a powder before
addition). This reaction mixture was capped again and sealed
under a nitrogen blanket. The reaction was heated in a Biotage
MW system for 15 min at 100 ◦C (the absorptivity on the MW
was set at “high”). The vial was opened and the product isolated
from the [P66614]DBS/Pd/Base mixture by vacuum distillation.
(Note: For other ionic liquids, solvent extraction of the products
could be performed.)
General information and instrumentation
PdCl2 was purchased from Precious Metals Online. The ionic
liquids [P66614]DBS (Cyphos phosphonium IL 202) and [P66614]Cl
(Cyphos phosphonium IL 101) were gift samples from Cytec
Industries and the other ILs were purchased from Sigma-
Aldrich. Nanoparticles were prepared using ILs as they were
received and ILs purified using a literature method,29 but the
same results were obtained using purified and ‘as received’
batches of ILs.
TEM images were obtained using a JEOL 2011 high reso-
lution (200 kV) Scanning Transmission Electron Microscope
(STEM) with EDAX (Genesis) Energy Dispersive X-ray system
at the University of New Brunswick. Samples were shipped to
New Brunswick as suspensions in acetone that had been re-
constituted from nanoparticulate dry powders. The suspensions
were sonicated before sampling in order to have a homogeneous
dispersion of particles. TEM images were taken by putting a
drop of nanoparticle suspension in acetone on a carbon coated
copper grid. ImageJ software was used to obtain size distribution
histograms.
Infrared spectra were obtained using a Bruker TENSOR
27 infrared spectrometer equipped with a MIRacle ATR (At-
tenuated Total Reflection). 31P-NMR spectra were obtained
on a Bruker AVANCE II 600 spectrometer (referenced with
respect to 85% H3PO4 at 0.00 ppm). An Ocean Optics UV-vis
spectrophotometer was used for UV-vis spectroscopy and the
IL-nanoparticle suspensions were diluted in dichloromethane.
X-Ray diffraction analyses were performed on a Rigaku Ultima
IV with cross-beam optics (CBO). Bragg-Brentano mode was
used for the analysis. Cu was used as the X-ray source (40 kV
and 44 mA). For the TGA analysis, a TA Instruments Q500
was used and the sample was loaded onto a platinum pan. The
experiment was performed under 60.◦0 mL min-1 nitrogen flow
with a temperature increase rate of 20 C min-1. An Agilent 1100
LC/MSD instrument was used for the electrospray ionization
spectrometry (ESI-MS) and all samples were diluted in acetone
prior to analysis. A Biotage Initiator 2.5 microwave reactor was
employed for the Suzuki reactions. 0.5–2 mL microwave vials
with PTFE seals were used for all the experiments.
Acknowledgements
We acknowledge support from NSERC and Canada Foundation
for Innovation in the form of a Discovery Grant, a Research
Tools and Instrument Grant and a Leaders Opportunity Fund
Award (F. M. K.). We would also like to thank Memorial
University for funding and Cytec Industries Inc. for samples of
[P66614]DBS and [P66614]Cl. Dr Louise Weaver (University of New
Brunswick, Canada) is thanked for obtaining TEM images and
Prof. Peter Pickup and Mr Rodney Smith (Memorial University)
for access to electrochemical equipment.
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