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action was followed by UV/Vis spectrophotometry. The second
cycle was complete in 13 min (Figure 9b). Similarly, we ran the
third and fourth cycles. The third and fourth cycles were com-
plete in 20 and 26 min, respectively, with quantitative transfor-
mations (Figure 9c,d). Completion of the fourth cycle in
Conclusion
A novel organic–inorganic trihybrid material was synthesized
by in situ generation of palladium nanoparticles (PdNPs) in
aqueous media in a hybrid gel matrix consisting of a self-as-
sembled vesicular network of the naturally occurring triterpe-
noid arjunolic acid and the leaf extract of C. cainito. According
to our knowledge, this is the first report of the synthesis of
a PdNP trihybrid material based on nontoxic and renewable
precursors in aqueous media. The dried trihybrid material con-
sisting of in situ generated PdNPs was shown to be an active
catalyst for CÀC coupling reactions such as the Heck and
Suzuki reactions under phosphane-free conditions and for the
sodium borohydride reduction reactions of 3-nitrophenol and
4-nitrophenol in aqueous media in excellent yields. For both
the CÀC coupling and reduction reactions, the catalyst was
reused up to four cycles without a significant loss in the cata-
lytic activities. The trihybrid material synthesized by in situ gen-
erated PdNPs was catalytically more efficient than the trihybrid
material generated from presynthesized PdNPs. As the renewa-
ble raw materials could be easily extracted from plants and the
trihybrid materials could be synthesized easily under environ-
mentally friendly conditions, the method described herein can
be extended to generate newer hybrid materials for novel util-
ities, some of which are currently under investigation in our
laboratory and will be reported in due course.
26 min confirmed the efficacy of the material as a recyclable
heterogeneous catalyst. The decreasing intensity of the band
for 4-nitrophenolate at l=400 nm with time provided a rea-
sonable tool to calculate the rate constant for each cycle. As
NaBH was present in a large excess amount in each cycle with
4
respect to 4-nitrophenol, the rate constant was calculated for
each cycle by considering the reaction to be a pseudo-first-
order reaction. From the plot of ln(A /A ) versus time for each
t
0
cycle, the rate constants for the first, second, third, and fourth
À1
cycles were calculated to be 0.985, 0.3, 0.185, and 0.105 min ,
respectively (Figure S13). For each cycle, a good correlation be-
tween ln(A /A ) and time was found.
t
0
Similarly, the NaBH reduction of 4-nitrophenol was also per-
4
formed in the presence of trihybrid material 2 (0.3 mg,
0.17 mmol). The first cycle was complete in 4 min (Figure S14),
and then the catalyst was reused over three more cycles. The
second, third, and fourth cycles were complete in 35, 90, and
1
45 min, respectively. From the plot of ln(A /A ) versus time for
t 0
each cycle, the rate constants for first, second, third, and
fourth cycles were calculated to be 1.465, 0.08, 0.03, and
À1
0
.016 min , respectively (Figure S15). For each cycle, a good
correlation between ln(A /A ) and time was found. These re-
t
0
sults clearly indicate that trihybrid material 1 was a more effi-
cient catalyst than trihybrid material 2 for this transformation.
The efficacy of the catalyst depended on how efficiently the
reactant molecules could be adsorbed on the surfaces of the
Experimental Section
Materials
[
10]
PdNPs. In our work, trihybrid material 1 contained very small
9 nm) in situ generated discrete PdNPs that were dispersed
Arjunolic acid was extracted from the heavy wood of Termina-
lia arjuna. The crude arjunolic acid was purified by following
a previously reported procedure developed in our laborato-
(
throughout the trihybrid material. Because of the very small
size, these discrete PdNPs possessed a very large surface area
to volume ratio, which facilitated the adsorption of the reac-
[34]
ry. The leaves of Chrysophyllum cainito were collected from
the north central part of Thailand. PdCl was purchased from
[42]
2
tant molecules onto the surfaces of the PdNPs (Figure 7). As
Alfa Aesar Company, and phenyl boronic acid was purchased
from Sisco Research Laboratory Pvt. Ltd. (SRL), India. Iodoben-
zene was purchased from Spectrochem Pvt. Ltd., India. Methyl
acrylate and 3-nitrophenol were purchased from Loba Chemie
Pvt. Ltd., India. 4-Nitrophenol was purchased from Merck. All
these chemicals were used without further purification. All
commercial-grade solvents were purified by distillation before
use.
À
a result, very fast hydride transfer occurred from BH4 to the
nitrophenolate molecule to reduce it to aminophenolate. As
the PdNPs in trihybrid material 2 were larger in size (24 nm),
the surface to volume ratio of these nanoparticles was lower
than that of trihybrid material 1. For that reason, the reactant
molecules adsorbed less efficiently on the surface of trihybrid
material 2 than on the surface of trihybrid 1. As a result, trihy-
brid material 1 was a more efficient catalyst than trihybrid ma-
terial 2. In addition, trihybrid 1 as a recyclable catalyst was
more efficient than trihybrid 2 probably because the support
provided by the arjunolic acid–LECC hybrid matrix for the
in situ generated PdNPs was more effective in trihybrid 1. The
reaction rate observed for the reduction of 3-nitrophenol was
higher than that for the reduction of 4-nitrophenol, and this is
probably due to the fact that the nitro group of 3-nitropheno-
Methods
Preparation of the leaf extract of C. Cainito (LECC): Dried and
finely powdered leaves of C. cainito (6.0 g) were suspended in
methanol (50 mL) and stirred magnetically at 508C for 2 h and
then filtered. The filtrate was centrifuged for 10 min in an ultra-
centrifuge machine. Volatiles of the greenish filtrate were re-
moved under reduced pressure to afford a greenish crystalline
solid (0.974 g). The greenish crystalline solid (0.005 g) was sus-
pended in distilled water (10 mL) and sonicated in an ultrasoni-
cator bath for 10 min to afford a semitransparent light yellow-
[
43]
late is more reactive than that of 4-nitrophenolate.
Chem. Asian J. 2016, 00, 0 – 0
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