Labile imidazolium salt protected palladium nanoparticles

Article abstract of DOI:10.1021/la501569e

An imidazolium (Im) salt with two long alkyl substituents at N atoms is employed to prepare cubelike palladium nanoparticles (PdNPs). The bilayer nature of the capped Im salts is characterized by thermogravimetric analysis and NMR studies. These capped Im salts are labile, as evidenced by their displacement reaction with dimethylaminopyridine, and the observation of fast exchange between those free and capped Im salts on the NMR time scale. NMR results also show that these capped Im salts exhibit different diffusion rates, and interesting spinning rate dependent chemical shifts. These cubelike PdNPs could catalyze the Suzuki coupling of aryl chlorides and boronic acids with high yields in 10 min, even at room temperature.

Full text of DOI:10.1021/la501569e

Article
pubs.acs.org/Langmuir
Labile Imidazolium Salt Protected Palladium Nanoparticles
Jung-Tang Lu, Joseph C. Y. Lin, Meng-Che Lin, Nageshwar D. Khupse, and Ivan J. B. Lin*
Department of Chemistry, National Dong-Hwa University, Shoufeng, Hualien 974, Taiwan
S
* Supporting Information
ABSTRACT: An imidazolium (Im) salt with two long alkyl
substituents at N atoms is employed to prepare cubelike
palladium nanoparticles (PdNPs). The bilayer nature of the
capped Im salts is characterized by thermogravimetric analysis
and NMR studies. These capped Im salts are labile, as
evidenced by their displacement reaction with dimethylami-
nopyridine, and the observation of fast exchange between
those free and capped Im salts on the NMR time scale. NMR
results also show that these capped Im salts exhibit different diffusion rates, and interesting spinning rate dependent chemical
shifts. These cubelike PdNPs could catalyze the Suzuki coupling of aryl chlorides and boronic acids with high yields in 10 min,
even at room temperature.
the sample is spun at the magic angle (54.7° relative to the
direction of the main magnetic field), the line-broadening
effects, due to the magnetic susceptibilities and chemical shifts
anisotropy, are significantly diminished.34 Additionally, 2D
diffusion-ordered spectroscopy (DOSY) NMR, which com-
bines both chemical shift and diffusion coefficient scale, is able
to obtain different diffusion rates of mixtures with highly
overlapped NMR spectra35 and to discriminate the organization
of the orgnaic ligands in colloidal metal NPs.36−38
In this work, we demonstrate a simple method to prepare
cubelike colloidal PdNPs, which are capped by 1,3-di(dodecyl)-
imidazolium chloride, [C12,C12−Im]Cl. Thermogravimetric
analysis (TGA), HRMAS, and DOSY NMR are used to
characterize the structure of Im salts on the PdNPs. Various
interactions between Im salts and PdNPs could be differ-
entiated. The labile nature of this capping agent is also studied
by the displacement reaction with dimethylaminopyridine.
These PdNPs show excellent catalytic activity toward Suzuki
cross-coupling reaction under ambient conditions.
INTRODUCTION
■
Pd-catalyzed Suzuki cross-coupling reaction between aryl halide
and boronic acid is an important tool in organic synthesis.1−5
Over the past decade, colloidal PdNPs are considered to be a
promising catalyst for the Suzuki reaction.6−8 It has been well
recognized that the catalytic activity of a nanomaterial depends
primarily on the particle size and shape.8−11 How these cross-
coupling reactions occur in the presence of PdNPs is also an
important issue that has been intensively discussed. The
reactions have been proposed to operate at the NP surface,11,12
and on the other hand, homogeneous reactions occurring in
solution via leaching palladium(0) or dissolution of the
activated palladium(II) species from PdNPs have also been
proposed.13
Moreover, the influence of capping agents on the catalytic
activity has been noticed and reviewed.14,15 An inverse
correlation of stability and catalytic activity is often observed,
and therefore, it is crucial to acquire a balance between the
stability and catalytic activity in performing optimum nano-
particle catalysis.13 Various capping agents have been used to
stabilize PdNPs for C−C coupling reactions,7 including the use
of Im salts for PdNP-catalyzed Suzuki coupling reaction.16−26
Calculations have been performed to understand the mode and
energy of the interaction between metal nanoparticales and Im
salts. Results suggest that it is the ring head of the Im salt that
sits on the metal surface,27,28 and the interaction energy
between the Im salt and metal surface is smaller than that
between the quaternary ammonium salt and metal surface.
Indeed a better catalytic efficiency for the Im salt stabilized Au
nanorods has been found for the reduction of nitro
compounds.26
RESULTS AND DISCUSSION
■
[C12,C12−Im]Cl (structure formula in Scheme 1) is a near-
room temperature ionic liquid crystal having a melting
temperature of 47 °C and is an amphiphile, soluble in both
aqueous and many polar nonaqueous solvents.39 Previously, we
reported that the reaction of [C12,C12−Im]Cl with PdCl2 in a
molar ratio of 2:1 could produce [C12,C12−Im]2[PdCl4], a
liquid crystal compound, in which the imidazolium cations
adopt a U-shape interdigitated bilayer packing in the solid
state.40 Although this palladate salt could be used as precursor
to prepare PdNPs, a colloidal solution of PdNPs is produced
easily through reduction of a mixture of [C12,C12−Im]Cl and
High-resolution magic angle spinning spectroscopy
(HRMAS) NMR has been shown as reliable in resolving the
inhomogeneous system with liquidlike dynamics,29 and
successful in the characterization of bionanomaterials,30,31
gold nanoparticles,32 and magnetic nanoparticles.27,33 When
Received: April 27, 2014
Revised: August 6, 2014
© XXXX American Chemical Society
A
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Scheme 1. Proposed Form of the Im Salts Capped on the Surface of PdNPs
PdCl2 (5:1 molar ratio) in deionized (DI) water/p-dioxane (1:1
by volume) by hydrogen gas. The slight excess of [C12,C12−Im]
Cl employed in the process is to enhance the stability of the
PdNPs, and the use of this solvent system is because it has been
commonly used in the Suzuki cross coupling reactions.41,42
Solid sample of PdNPs could be obtained via centrifugation
from the DI water/p-dioxane solution and is denoted as
Pd@[C12,C12−Im]Cl. The size and morphology of this solid
sample as observed under TEM are shown in Figure 1. These
To examine the surface structure of the capped Im salts,
three different sample treatments were performed. The first
sample denoted as PdNPs-1 was generated via rinsing the
freshly obtained solid Pd@[C12,C12−Im]Cl with DI water,
ethanol, and acetone sequentially. This process was repeated
twice. To prepare PdNPs-2, Pd@[C12,C12−Im]Cl was
dissolved in dichloromethane (CH2Cl2), then washed several
times with DI water to remove the free Im salts. After that, the
CH2Cl2 solvent was removed under vacuum. For PdNPs-3, a
CH2Cl2 solution of Pd@[C12,C12−Im]Cl was first treated with
ethanol, followed by centrifugation. The structure of the
capping agents on PdNPs 1−3 was first examined via TGA,
which has been often employed to investigate the surface
structure of surfactants capped on metal nanoparticles.44−47
Plots of TGA and their derivatives are given in Figure 2. The
Figure 1. A TEM image of the cubelike Pd@[C12,C12−Im]Cl with an
average length of ca. 7 nm. The inset (upper right) shows the particle
size distribution from the experiment of DLS. The upper left inset
shows the ED pattern taken from an individual PdNP.
particles display cubelike morphology with an average length of
ca.7 nm, consistent with the size distribution obtained via
dynamic light scattering (DLS) study (upper right inset Figure
1). The {100} facets of the cubelike PdNPs are evidenced by
the electron diffraction pattern shown in the upper left inset in
Figure 1 and further by high-resolution TEM and powder X-ray
diffraction in Figure S1 in the Supporting Information. For
comparison, [C1,C12−Im]Cl, which has only one long alkyl
chain, was also employed to study its ability to stabilize PdNPs.
Under similar reaction conditions, agglomeration occurred in a
short time. Very recently, [C6,C6−Im] cation with two hexyl
substituents was used to stabilize the PdNPs and no long-term
stability was reported.43 In the present study, the Im salt with
two symmetrical dodecyl chains was found to be a good
protecting agent for the fabrication of PdNPs.
Figure 2. Thermogravimetric profiles of (a) the PdNPs-1, (b) PdNPs-
2, (c) PdNPs-3, and their derivatives (DTGA).
thermogram of PdNPs-1 shows broad peaks between 150 and
200 °C (a total weight loss of 15%), followed by two
consecutive major peaks of roughly equal weight losses at 211
°C (20%) and 230 °C (23%). The TGA profile of PdNPs-2
shows a trace of weight loss around 178 °C, together with two
major equal weight loss peaks at 212 °C (29%) and 233 °C
B
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Figure 3. 1H-HRMAS spectra for (a) free [C12,C12−Im]Cl, (b) PdNPs-1, (c) PdNPs-2, and (d) PdNPs-3 at MAS rates of 3 kHz in benzene-d6. The
asterisks indicate the residual solvents and spinning side bands.
1
(30%). Because both the profiles of PdNPs-1 and -2 have a set
of two major equal weight loss peaks between 210 and 240 °C,
and these two sets of peaks happen at almost identical
temperatures, we propose that these desorption events are from
the bilayer [C12,C12−Im]Cl capped on the PdNPs with an
equal molar ratio for the outer and inner salts. Those
desorption peaks occurring at lower temperatures (150−200
°C) are attributed to the suspended Im salts, which sit topside
and interact weakly with the bilayer. The sample of PdNPs-1
has 15% of the suspended salts, but only a trace for the PdNPs-
2. The structure of Im salts in PdNPs-1 is similar to that
observed for the cetyltrimethylammonium bromide (CTAB)
capped on gold nanorod, in which three types of surfactants are
observed: the inner and outer layers of the bilayer surfactants
and the suspended surfactants on top of the bilayer.44 On the
other hand, the sample of PdNPs-3 has only a major
desorption peak between 150 and 185 °C (8%), presumably
due to the monolayered [C12,C12−Im]Cl on the PdNP surface;
the desorption band at ∼100 °C could be due to the trapped
solvent. The small percentage of the monocapped [C12,C12−
Im]Cl in PdNPs-3 suggests that the PdNP surface is not
completely covered by the stabilizer. The long-term stability of
PdNPs-3 is expected to be poor. Indeed dispersion of PdNPs-3
in CH2Cl2 is only stable for 4 h.
HRMAS NMR was also utilized to study these PdNPs.
Spectra of the free [C12,C12−Im]Cl and PdNPs 1−3 in
benzene-d6 are given in Figure 3. We will first compare the
spectra between the free [C12,C12−Im]Cl and PdNPs-1, shown
in Figure 3, a and b, respectively. In Figure 3b the signals for
the Im salts in PdNPs-1 are slightly broad and shifted upfield
relative to that of the free salts in Figure 3a. The relative
intensity of the signals in Figure 3b needs additional comments.
First, we notice that the intensities of the aliphatic protons are
higher than that of the ring protons. For example, the ratio of
the signals of the two terminal CH3 protons to that of the ring
C2H proton is 7.0:1.0. This value is compared to that of the
6.0:1.0 found and expected in the free [C12,C12−Im]Cl (Table
1). Further, the intensity of the NCβH2 signal is greater than
Table 1. Relative Intensities of H-HRMAS NMR of
Imidazolium-Based PdNPs at 3 kHz in Benzene-d6 by
Taking the Signal Integration of C2−H as 1.0
aromatic
C2H
aromatic
C4,5
aliphatic terminal
NCαH NCβH
CH3
sample
H
[C12,C12-Im] 1.0
Cl
2.0
4.0
4.0
6.0
PdNPs-1
PdNPs-2
PdNPs-3
1.0
2.0
2.1
4.0
4.0
4.3
5.1
7.7
8.2
−
1.0
not found
−
−
−
that of the NCαH2 (4.3 to 4.0), which should otherwise be
equal. The spectrum of PdNPs-2 in Figure 3c indicates that the
C2,4,5,α,β protons are further shifted upfield and broadened, and
an increase in the intensity of (1) the aliphatic to ring protons
(8.2:1.0, Table 1), and (2) the NCβH2 to NCαH2 signals (5.1 to
4.0). For PdNPs-3, the protons at the aromatic ring cannot be
observed, while those of the alkyl chains are very broad. To
explain these observations we will begin from the simple
PdNPs-3, which has a monolayer of [C12,C12−Im]Cl. NMR
studies suggest that the monolayered [C12,C12−Im]Cl has its
Im ring head sitting parallel on the PdNP surface, while with
the alkyl chains tilt away from the surface to form a U-shaped
geometry (Scheme 1). The anisotropic nature of the adsorbed
molecule causes the absence of the ring proton signals and gives
broad alkyl chain signals.48−50 Further, because the NCαH2
groups are closer to the NP surface than those of the NCβH2
groups, the relative intensity of NCαH2/ NCβH2 would be
lower than the expected value of 1. This proposed mode of
interaction is consistent with those reported based on
theoretical25 and experimental studies.51−53 In PdNPs-2, a
bilayer of interdigitated U-shaped Im salts is proposed to act on
the surface (Scheme 1). The geometry of this bilayer structure
is very similar to that found in [C12,C12−Im]2[PdCl4].40 Since
the outer layer of the salts has an ionic head exposed outward,
these ring protons are less influenced by the PdNP, and thus
show slightly broadened signals. Also shown in Scheme 1 for
PdNPs-1, some suspended Im salts locate further away from
C
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Figure 4. Selected region of 1H-HRMAS spectra for (a) free [C12,C12−Im]Cl and (b−d) PdNPs-1 at MAS rates of 3, 9, and 12 kHz in benzene-d6,
respectively. (e−g) Sample PdNPs-2 at spinning rates of 3, 9, and 12 kHz, respectively.
the PdNP surface. Therefore, the Im ring signals in PdNPs-1
are sharper and shifted more downfield. In addition, the
presence of only one set of signals suggests that the exchange
between the suspended and bilayer Im salts is fast on the NMR
time scale.
fully understood at present. One possible explanation is that
there is a structural change upon increasing the spinning rate, a
scenario similar to thermally treated liquid crystals. For
example, the liquid crystalline [C12,C12−Im]2[PdCl4] has an
interdigitated U-shaped [C12,C12−Im]+ cation at the crystal
phase, which changes to a none or partially interdigitated
structure upon melting to mesophase.40 Another possibility is
that the exchange between the suspended and bilayer Im salts is
suppressed under high spinning rates such that they become
differentiable.
Further information could be obtained from DOSY study,
which provides a two-dimensional spectrum correlating the
chemical shift with the logarithmic diffusion coefficient (log D)
(Figure 5). The DOSY spectrum of [C12,C12−Im]Cl is given in
Figure 5a, in which a diffusion coefficient with a log D value of
−8.4 is found. The spectrum for PdNPs-1 (Figure 5b) exhibits
a diffusion coefficient of an averaged log D value of −9.5. The
distinct difference between these two species suggests a large
disparity in the overall size and mass. As expected, simple Im
1H-HRMAS NMR spectroscopy operating at different
spinning rates also provides interesting insights about the Im
salts anchored on the PdNPs. In Figure 4, the spectra taken at
different MAS rates for the free [C12,C12−Im]Cl, PdNPs-1, and
PdNPs-2 in the region of chemical shift from 6 to 11 ppm are
shown (the ring protons of PdNPs-3 are not observable,
therefore will not be discussed). Panel (a) shows that the
chemical shifts of the C2 and C4,5 ring protons for the free
[C12,C12−Im]Cl appear at 10.60 and 8.01 ppm, respectively,
and are independent of the MAS rates. For PdNPs-1, at the
spinning rate of 3 kHz, the C2 and C4,5 proton signals are at
9.82 and 7.62 ppm, respectively (panel (b)). Under the
spinning rate of 9 kHz, the C2 proton signal splits to two signals
at ca. 9.92 and 9.81 ppm, while those of the C4,5 protons are at
7.68 and 7.60 ppm (panel (c)). At the spinning rate of 12 kHz,
the split signals are further apart: 10.02 and 9.89 ppm for the C2
proton, and 7.74 and 7.62 ppm for the C4,5 protons (panel (d)).
In panel (e), the spectrum of PdNPs-2 taken at 3 kHz shows
that the corresponding Im ring proton signals are at 9.89 and
7.44 ppm, and are relatively broad compared to the free salt.
Increasing the spinning rate to 9 kHz, signals for the ring
protons become sharper and are downfield shifted to 9.96 and
7.52 ppm (panel (f)). At a spinning rate of 12 kHz, the
corresponding signals are further downfield shifted to 10.34 and
7.66 ppm (panel (g)). These results are reversible: decreasing
the spinning rate, spectra (b) and (e) could be restored. The
nature of the interaction between the inner layered Im salts and
PdNPs is presumably electrostatic as often proposed.7,15 On the
other hand, the interaction between outer and inner layers is
van der Waals force through chains. The suspended Im salts
interact weakly mainly via electrostatic forces with the outer Im
salts. For PdNPs-2, a higher spinning rate tends to force the
outer layer of the Im salts to be pulled away, thus the ring
protons are shifted to lower fields and peaks sharpened. For
PdNPs-1, the observation of two sets of ring protons could be
attributed to the poorly interacting outer and suspended Im
salts. This spinning rate dependent behavior has never been
reported. The exact cause of this interesting observation is not
Figure 5. 1H DOSY 2D spectra of (a) [C12,C12−Im]Cl and (b)
PdNPs-1 acquired in benzene-d6. Inset: the corresponding DOSY
spectra to C2 and C4,5 protons. D is expressed in m2 s−1.
D
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Figure 6. 1H NMR spectra of PdNPs-2 and its reaction with DMAP in D2O: (a) PdNPs-2, (b) free DMAP, (c) 10 min after adding 100 μL of 1.5 M
DMAP to PdNPs-2, (d) 20 min, and (e) 30 min. Enlarged signal for C4,5 proton of Im salt is given on top of each of the original signals. Inset:
Schematic drawing for displacement of PdNPs-2 by DMAP with labelings.
salt diffuses faster, while those in PdNPs-1 diffuse slower.
Interesting results could be further obtained via careful
inspection of the spectrum of PdNPs-1. There are two major
sets of C2 protons with averaged log D values of −9.3 and −9.8.
So is true for those of C4,5 protons: −9.4 and −9.5. We propose
that the observation of two sets of diffusion coefficients arises
from the suspended and outer [C12,C12−Im]Cl. The faster
diffusion rate is presumably attributed to the suspended salts.
This observation is consistent with the HRMAS experiments.
Although DOSY NMR has been employed to evaluate the
interaction between NPs and ligand,34,35 this is the first report
of using this technique to differentiate two different stabilizer
on the NPs.
on PdNPs albeit slowly. The use of DMAP for displacing the
capping agent of tetraoctylammonium bromide on gold and
palladium NPs in order to transfer NPs from organic solvent to
water has been reported.54 Further, during the displacement
reaction, while two sets of DMAP signals corresponding to the
free and bound DMAPs are observed, only one set of C4,5
proton signal is found. This observation additionally suggests
that the exchange between the free and capped DMAP is slow
on the NMR time scale, whereas the exchange between the
capped and those displaced Im salts is fast. In short, (1) DMAP
has a greater affinity toward PdNPs than that of the Im salt, and
(2) the salt of [C12,C12−Im]Cl on the PdNPs-2 is labile.
The results therefore indicate that [C12,C12−Im]Cl on the
PdNP surface is labile but is capable of stabilizing PdNPs in
solution. This feature should be advantageous for use as a
catalyst.55 Therefore, Pd@[C12,C12−Im]Cl prepared in situ in a
mixed solvent of water/p-dioxane (1:1 by volume) was
employed to catalyze Suzuki cross-coupling reaction of aryl
halide and phenylboronic acid at room temperature with
K2CO3 as a base. The progress of the reaction was followed by
thin layer chromatography (TLC) and gas chromatography
(GC), and the results are shown in Tables 2 and 3.
To evaluate the lability of [C12,C12−Im]Cl in PdNPs-2, we
studied its displacement reaction by dimethylaminopyridine
(DMAP). The displacement reaction was carried out via adding
100 μL solution of 1.5 M of DMAP in D2O to PdNPs-2 (0.017
1
mg, 2 mL of D2O), and the reaction was monitored by H
NMR at a time interval of 10 min. Since for a free Im salt, the
C2 proton at the Im ring undergoes facile H/D exchange
reaction in D2O and is silent in the spectrum, only regions
covering the chemical shifts of the C4,5 protons of PdNPs-2 and
the ring protons of DMAP were monitored as shown in Figure
6.
1H NMR spectrum of PdNPs-2 in D2O shows that the C4,5
protons is at 7.52 ppm (Figure 6a); this is compared to the
value of 8.01 ppm found for the free [C12,C12−Im]Cl in D2O.
Free DMAP exhibits two doublets at δ = 7.93 and 6.55 ppm,
corresponding to ring protons of Ha and Hb, respectively, as in
Figure 6b. At the first 10 min of reaction, as shown in Figure 6c,
two new doublets appear at δ = 7.77 ppm (as Ha′) and δ = 6.42
ppm (as Hb′), assignable to DMAP capped. At the same time,
the chemical shift for the C4,5 protons is moved downfield to
7.54 ppm. The integration ratio for Ha/Ha′ (or Hb/Hb′) is 3.6,
meaning that the molar ratio of the free and absorbed DMAP is
3.6. Subsequently, after 20 min (Figure 6d), there is a decrease
in the integration ratio of Ha/Ha′ (or Hb/Hb′) to 1.3, whereas
the C4,5 ring proton signal shifts to 7.56 ppm. After 30 min
(Figure 6e), the ratio further decreases to 0.5, and the C4,5 ring
proton signal further increases to 7.58 ppm. The gradual
decrease on the signal intensity of the free DMAP concomitant
with an increase of the bounded ones in a duration of 30 min
suggests that these PdNPs interact stronger with DMAP than
with Im salt, and therefore, DMAP could displace those Im salts
Table 2. Suzuki Coupling Reaction of Chlorobenzene and
Phenylboronic Acid Catalyzed by Pd@[C12,C12−Im]Cl
time
yield
ab
TOF
(h−1
,
entry
1
catalyst (mol %)
(min)
(%)
99
TON
15.8
)
10.0 mol % Pd@[C12,C12-
Im]Cl
10
95.0
180.5
22.6
2
3
4
5
5.0 mol % Pd@[C12,C12-
Im]Cl
10
94
53
17
9
30.1
33.9
27.2
28.8
2.5 mol % Pd@[C12,C12-
Im]Cl
90
1.0 mol % Pd@[C12,C12-
Im]Cl
120
150
13.6
0.5 mol % Pd@[C12,C12-
Im]Cl
11.5
a
The reaction was carried out with chlorobenzene (1.0 equiv),
phenylboronic acid (1.5 equiv), K2CO3 (2.0 equiv), and
Pd@[C12,C12−Im]Cl in 2.0 mL of H2O/p-dioxane (1:1) at room
b
temperature. Yield determined by GC.
E
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in 5 min (entry 9, Table 3). In the literature, examples of
PdNPs-catalyzed Suzuki reactions with high yields under
ambient conditions are known.13,56−58 However, most of
them are aryl bromide or iodide with prolonged reaction
time.13,56−58 In a report, thiolate-stabilized PdNPs have also
been used for the coupling of chlorobenzene and phenyl-
boronic acid at 20 °C to produce a modest 52% yield after 5
days.56
Table 3. Suzuki Coupling Reactions Using 5.0 mol %
Pd@[C12,C12−Im]Cl
a
time
(min)
yield
b
TOF
(h−1
entry
X
R
(%)
TON
)
1
2
3
4
5
6
7
Cl
Cl
Cl
Cl
Cl
Cl
Cl
H
10
15
15
12
40
40
40
94
91
90
91
72
83
77
30.1
29.1
28.8
29.1
23.0
26.6
24.6
180.5
116.5
115.2
145.6
34.6
Despite the high catalytic activity found in entry 9, the
colloidal system of Pd@[C12,C12−Im]Cl is good for only two
runs (see entry 9; 95, 90, and 26% for the first, second, and
third runs, respectively). To understand this poor recyclability,
morphologies of the PdNPs after each run are monitored by
TEM. After the first run, cubelike PdNPs with some truncations
at corners and edges are observed (Figure S2(a) in the
Supporting Information). Thus, before and after the first run,
there is a transformation from cube to truncated cube. It
indicates the dissolution of the more reactive sites of the
corners and edges during the reaction, and supports the
possible participation of the homogeneous nature in our PdNP-
catalyzed Suzuki reactions. In the second run, the slight
decrease of the product yield is consistent with the diminishing
corners and edges of the PdNPs. After the second run, a small
amount of black precipitate is formed, whereas in the remaining
colloidal solution, fused NPs are observed (Figure S2(b)). The
sudden decrease in the catalytic activity in the third run (26%
yield) is also consistent with the change of morphology. It has
been noticed that in a Suzuki cross-coupling reaction a shape
transformation for the PtNP catalyst from tetrahedron to
sphere rendered a sudden decrease in the catalytic activity in
the second cycle.59 Thus, while the labile nature of the cubelike
PdNPs could lead to a high catalytic activity, it could also cause
the aggregation of NPs and lose its activity. The catalytic
activity of this work appears to be limited by approximately 30
turnovers, as suggested by the TON of 30 often observed in
this work. Although the catalytic system prepared in this work
is only good for two cycles and the Pd loading is high, this
system is capable to catalyze the coupling reaction of aryl
chloride and boronic acids at room temperature with high yield
in a short time.
p-CH3
p-CH3O
p-CH3CO
m-O2N
c
p-OH
39.8
p-
COOH
37.0
c
8
9
2-
20
5
95
30.4
91.2
chlorothiophene
Br
H
95
30.4
28.8
8.3
364.8
345.6
99.8
d
90
e
26
a
Reactions were carried out at room temperature with 5.0 mol %
Pd@[C12,C12−Im]Cl, 1.0 equiv aryl halide, 1.5 equiv phenylboronic
b
acid, 2.0 equiv K2CO3 in H2O/p-dioxane (1:1). Isolated yield (GC
c
d
e
yield). 2.5 equiv K2CO3 was used. Second cycle. Third cycle.
Cross-coupling between the least reactive chlorobenzene and
phenylboronic acid is examined first. Here we investigate the
effect of loading of Pd@[C12,C12−Im]Cl on the promotion of
the Suzuki reaction. With respect to chlorobenzene, a loading
of 10.0 mol % of Pd@[C12,C12−Im]Cl (based on the mol of
PdCl2 used) gives the coupling product of biphenyl with a 99%
yield in 10 min at room temperature (Table 2). Subsequently,
at a loading of 5.0, 2.5, 1.0, and 0.5 mol % of PdNPs catalyst,
yields decrease to 94% (10 min), 53% (90 min), 17% (120
min), and 9% (150 min), respectively (Table 2). Thus,
increasing the loading of Pd@[C12,C12−Im]Cl leads to an
increase in the product yield. Remarkably, the least reactive aryl
chloride produces the high yield over short reaction time even
at room temperature, albeit at high Pd@[C12,C12−Im]Cl
loading.
Aryl chlorides with various electron-donating and -with-
drawing substituents are used to investigate the scope of the
Suzuki reaction. The results are summarized in Table 3. All the
reactions are carried out using 5.0 mol % of Pd@[C12,C12−
Im]Cl, since this catalyst loading is sufficient to catalyze the
reaction effectively. In general, substituted aryl chloride gives
lower yields than that of the unsubstituted chlorobenzene. Aryl
chloride with substituents of p-CH3 and p-CH3O gives 91% and
90% product yields, respectively (entries 2 and 3, Table 3). The
observation of high yields of the cross-coupling reaction
products suggests that the homocoupling reaction is negligible.
Aryl chloride with a weak electron-withdrawing group of
p-CH3CO yields 91% of the coupling product (entry 4, Table 3).
A strong electron withdrawing group −NO2 at meta position
gives a 72% yield of the coupling product (entry 5, Table 3).
Aryl chlorides with p-OH and p-COOH substituents are acidic;
at higher quantity of K2CO3, they react with boronic acid to
give 83% and 77% yields of the coupling product (entries 6 and
7, Table 3), respectively. Further, 2-chlorothiophene also reacts
smoothly and has promising yields (95% yield, entry 8, Table
3). As expected, aryl bromides react more effectively than aryl
chlorides. Indeed, reaction of bromobenzene with boronic acid
at room temperature gives a 95% yield of the coupling product
CONCLUSION
■
Here we report the use of [C12,C12−Im]Cl to fabricate cubelike
PdNPs. These PdNPs are capped by a U-shaped bilayer of
[C12,C12−Im]Cl with interdigitated alkyl chains and could be
slowly displaced by DMAP. The techniques of HRMAS and
DOSY NMR are useful to differentiate the outer layer and
suspended Im salts in this system. The {100} surface of the Pd
nanocubes, and the labile yet stable Im salt capped PdNPs,
allow them to show high activities for Suzuki coupling
reactions. Im salts are easy to prepare and modify. They
therefore are potentially very useful in the preparation of metal
nanoparticles with various properties to suit for different
purposes. Preliminary study also shows that stable colloidal
PtNPs could be prepared with [C12,C12−Im]Cl. We expect that
the use of Im salt in nanoscience would flourish in the near
future, and the HRMAS and DOSY NMR analyses would be
very helpful to understand the nature of the capping agent.
EXPERIMENTAL SECTION
Chemicals. PdCl2 were purchased from Aldrich and used as
received. Solvents, boronic acids, aryl halides, and K2CO3 were
purchased either from Aldrich, Lancaster, or local commercial sources
■
F
dx.doi.org/10.1021/la501569e | Langmuir XXXX, XXX, XXX−XXX

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Article
and were used as received. All solvent purchased from local
commercial sources and were used as received. All reactions were
carried out with double DI water.
Dong Hwa University Nanotechnology Research Center for
providing research facilities.
Preparation of PdNPs. [C12,C12−Im]Cl (176 mg, 0.40 mmol)
was mixed with PdCl2 (14 mg, 0.08 mmol) in DI water/1,4-dioxane
(10 mL of each) and was stirred for 10 min. Dihydrogen gas was then
charged into the solution for 10 min (until the color of solution turned
from reddish brown to gray colorless). Thus prepared, the colloidal
solution could be used for the Suzuki reaction. Centrifugation (8000
rpm, 20 min) of the colloidal solution produced solid Pd@[C12,C12−
Im]Cl. Washing these residues with DI water (1 mL), ethanol (1 mL),
and then acetone (1 mL) twice gave PdNPs-1. For PdNPs-2, the
virgin solid Pd@[C12,C12−Im]Cl was redissolved in 1 mL of CH2Cl2,
and 3 mL of DI water was added to extract the free Im salts. The
CH2Cl2 was then removed by an evaporator. The process was repeated
three times. PdNPs-3 was obtained by dissolving the Pd@[C12,C12−
Im]Cl in 1 mL of CH2Cl2, followed by addition of 3 mL of EtOH and
then centrifugation (8000 rpm, 20 min). This process was repeated
twice.
Characterization of PdNPs. The nanoparticles were studied by
TEM, JEOL 3010. TEM samples were prepared by placing a drop of
the solution on carbon-coated Cu grids and allowed to air-dry. The
particle size and standard deviation were determined by DLS using a
Malvern Zetasizer Nano ZS. TGA was performed on a Mettler Toledo
TGA851. 1H NMR spectra were recorded on an Avance DPX300
BRUKER and the chemical shifts (δ) were given in ppm and
referenced to residual solvent signals. All the 1H HRMAS NMR
experiments were acquired on a BRUKER 600 UltraShield equipped
with a 4 mm HRMAS 1H/13C Z-GRD probe. Samples were first
dispersed or dissolved in deuterated solvents and then loaded to a 50
μL zirconia rotor. HRMAS experiments were accomplished under the
spinning rate of up to 12 kHz. 2D DOSY NMR experiments were
acquired at 298 K on a BRUKER 600 UltraShield equipped with a 5
mm BBO inverseprobe. All DOSY experiments were performed with a
longitudinal eddy-current delay (LED) and two spoil gradients pulse
sequence (ledbpgp2s). 32 spectra were acquired using a linear gradient
ramp from 2 to 95% using sine-shaped gradients. A gradient recovery
delay of 100 μs and an eddy current delay of 5 ms were used. The
diffusion time (Δ) was set between 50 to 55 ms and the magnetic field
pulse gradient (δ) was set between 0.9 to 1.6 ms depending on the
sample.
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ASSOCIATED CONTENT
* Supporting Information
■
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Further experimental data, including PXRD pattern and image
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AUTHOR INFORMATION
Corresponding Author
*E-mail: ijblin@mail.ndhu.edu.tw.
Notes
■
The authors declare no competing financial interest.
(21) Fernan
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ez, M. Palladium Catalyzed Suzuki C-C
ACKNOWLEDGMENTS
■
Kihn, Y.; Teuma, E.; Gom
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We thank the National Science Council (NSC 102-2113-M-
259-002; Taiwan, ROC) for financial support and National
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