Paper
Catalysis Science & Technology
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4. Conclusion
In summary, we have demonstrated an anomalous catalytic
activity of bimetallic Au–Pd alloyed nanoclusters in Ullmann
coupling of chloropyridines under ambient conditions. The
corresponding monometallic Au or Pd NCs did not catalyze
the reaction. Therefore, our reaction protocol is one of the
useful examples to demonstrate the unique alloy effect of a
bimetallic catalyst. The current methodology provides a con-
venient reaction system and avoids harsher conditions that are
required for Cu or Pd-catalyzed reactions. The modified elec-
tronic structure of the catalyst, resulting from a synergistic
effect of the presence of alloy phases, plays a crucial role in
generating the unusual activity. Furthermore, we have reported
the opposite reaction trend to the conventional Pd-catalyzed
coupling reaction in the activation of the C–X bond in the
Ullmann coupling reaction. Thanks to the strong coordinating
ability of the 2,20-bipyridine (2a, a coupling product), we were
able to observe by UV-vis spectroscopy and ICP-AES measure-
ments that a significant amount of leached Pd was observed
during the reaction more with 2-bromopyridine than with
2-chloropyridine, which is likely a crucial factor in diminishing
the reactivity. However, it is necessary to observe the structural
changes of the bimetallic catalyst before and after reaction with
1a and 1k for understanding the generality and mechanism
of the leaching process. Including the above observations,
further investigation to obtain more details of the mechanism
for Au–Pd-catalyzed Ullmann coupling experimentally as well as
by computational analysis is underway in our laboratory.
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Acknowledgements
29 I. Blakey, Z. Merican, L. Rintoul, Y.-M. Chuang, K. S. Jack
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30 Monometallic Au and Pd show the diffraction features
appearing at 2Y as 38.81 and 39.82 correspond to the (111)
plane of the standard cubic phase of Au and Pd respectively.
31 The crystallite size of the Au–Pd core was also estimated
from peak width of the diffraction peak of alloy Au–Pd (111)
plane and found 5.7 and 5.4 nm after used with 1a and 1k,
respectively. The estimated core size of Au–Pd alloy before
used was found to be 4.1 nm.
This work was supported by ACT-C (JST). We are grateful to Ms
Setsiri Haesuwannakij for assistance with TEM measurements.
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c
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