6
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M. Cao et al. / Journal of Catalysis 321 (2015) 62–69
NCs have been sinter into large ones (TEM image in Fig. S4), leading
to the catalytic activity decreased dramatically to 3% for the second
run (black curve with square in Fig. 1). It is well known that the
stabilization of the nanocatalyst against agglomeration during
the catalytic procedure is of special importance, since the aggrega-
tion results in complete loss of catalytic activity. The as-prepared
CB[6]-Pd NCs still preserve nearly 90% of its original reactivity
for the fifth cycle, indicating that the CB[6] protected Pd NCs
catalyst is highly stable and recyclable under the harsh reaction
conditions. Such excellent activity and good recyclability success-
fully overcome the reusable problem for Pd salt as homogeneous
catalyst and the loss of catalytic activity problem for commercial
Pd/C caused by agglomeration during the catalytic reaction.
Simultaneously, the good catalytic performance of the reusable
Pd NCs catalyst greatly decreases the overall cost and improves
the efficiency of the synthetic process for practical application.
3.3. Catalytically active species
2 2
Fig. 1. (a) Recycling of the as-prepared Pd NCs, Pd/C, PdCl , and Pd(OAc) for direct
arylation of pentafluorobenzene with bromobenzene.
To identify the catalytically active species from the as-prepared
Pd NCs catalyzed system, Hg-poisoning and hot-filtration experi-
ment test were carried out. A model reaction was added a drop
of Hg(0) after 8 h and then heated more 16 h. No more products
were detected during the subsequent process with Hg(0) added
(Fig. S5). The result indicates that the amalgamation is formed on
the surface of the Pd NCs, resulting in catalyst poisoning and the
reaction ceasing. This suggests that the nature of catalytically
active species is Pd(0) in this work [37–39]. Hot-filtration experi-
ment was carried out for two hours for the model reaction to
remove all solid materials. The filtrate was further reacted and
monitored after fresh base and additive added. The filtrate gave
no products during the additional reaction time, suggesting that
all active catalytic species were removed by hot filtration. Such
results suggest that the catalytic process may occur on the surface
of Pd NCs, i.e., the direct arylation catalyzed by the as-prepared Pd
NCs is heterogeneous reaction pathways.
The strong interaction of CB[n] (n = 5, 7, 8) with substrate might
hinder the full interaction between substrate and Pd NCs, resulting
a lower catalytic performance for CB[n]-Pd NCs (n = 5, 7, 8) com-
pared to CB[6]-Pd NPs. Such phenomenon also has been reported
in the case of aromatic ring hydrogenation reactions by RaMe-
cyclodextrins supported Ru(0) NCs [36]. Therefore, as evidenced
from the above data and analysis, CB[6], which features the most
stable symmetric structure, moderate cavity size and suitable vol-
ume size among common stable CB[n] (n = 5–8), should be the
most suitable nanocatalyst stabilizer for harsh reactive conditions
such as C–H bond activation.
With the optimized reaction conditions established, the sub-
strate scope of the direct arylation of fluoroarenes with various aryl
halides was investigated, and the results are summarized in
Table 2. As depicted in Table 2, this reaction catalyzed by the as-
prepared Pd NCs exhibits broad scope with respect to both the aryl
halide and the fluoroarenes components. Pentafluorobenzene can
be directly arylated by both aryl bromides and aryl iodides. Both
electron-rich and electron-deficient aryl halides are reactive
3.4. Characterization of the catalyst
In order to address the structural features of the as-prepared Pd
NCs catalyst during the catalytic reaction, TEM and HR-TEM
images of the Pd NCs before and after catalysis have been studied
(Fig. 2c–f). Typical TEM image of the as-synthesized Pd NCs is
shown in Fig. 2a, along with the obtained size-distribution histo-
gram (3.8 ± 0.3 nm), which is homogeneously dispersed. HR-TEM
image of an individual nanoparticle shows clear lattice fringes with
an interplanar distance of approximately 0.23 nm, corresponding
to Pd(111) planes (Fig. 2b). There are clearly many structural
defects on the surface of the as-prepared Pd NCs, indicated by
the corresponding fast Fourier transform (FFT) diffraction pattern
of the HR-TEM image (Fig. 2b, insert). Numerous researches
demonstrated experimentally and computationally that small NCs
possess a high percentage of surface atoms. The smaller the NCs,
the more abundant of step edges and corners or other defect sites
are on the surface. Surface defects, featured coordinative unsatura-
tion, have an excellent reactivity for the large adsorption energy to
preferentially adsorb the substrates and the strong bonding ability
with one or two atoms of the adsorbate, thereby enhancing the cat-
alytic performance [40–42]. Hence, the uniform dispersed small Pd
NCs catalyst exhibits unique excellent catalytic properties in this
work. As shown in Fig. 2c, most of the Pd NCs after catalysis main-
tain the uniform dispersion without obviously agglomeration, but
increase in size to about 10 nm caused by Ostwald ripening. The
HR-TEM images clearly show the distinct twin planes and stacking
faults, demonstrating that the defects on the surface of the Pd NCs
are maintained after catalysis (Fig. 2d and f). This demonstrates
(
Entries 3b–d and 3e–h in Table 1, respectively). Functional groups
are also tolerated including ester, carbonyl, cyano, and trifluoro-
methyl (Entries 3e–h, Table 2). Bromonaphthalene also gives good
yield of 85% (Entry 3i, Table 2). Tetrafluoro- and trifluoroarenes are
found to be efficient for the direct arylated reaction with bromo-
benzene as coupling partner. For example, 1,2,4,5-tetrafloroben-
zene and 1,2,3,5-tetrafluorobenzene can be obtained in good
yields of 66% and 75%, respectively (Entries 3j and 3l, Table 2). Both
of these two tetrafluorobenzenes possess two potential reaction
sites, but the monoarylation products are preferentially generated
in the as-prepared Pd NCs-catalyzed system. Additionally, 1,3,5-
trifluorobenzene as less acidic trifluorobenzene also affords mono-
arylated product preferentially in moderate yields (Entry 3m–o,
Table 2).
The recyclability of the as-prepared Pd NCs catalyst was studied
by using the direct arylation of 1a with 2a as a model reaction. The
as-prepared CB[6]-Pd NCs catalyst showed good recyclability over
multiple cycles. As illustrated in Fig. 1, a sample of the Pd NCs has
been recycled five times without significant loss in catalytic activ-
2 2
ity. Both PdCl and Pd(OAc) could catalyze the model reaction,
giving 67% and 62% yields, respectively. However, neither of the
two Pd salts is recyclable for the model reaction, like most of other
homogeneous catalysts (red curve with dot and blue curve with
triangle in Fig. 1). The commercial Pd/C only obtained 48% yield
for the first run, which significantly underperformed the as-pre-
pared Pd NCs catalyst. It is supposed that the commercial Pd/C