Gold/Palladium Alloy for Carbon-Halogen Bond Activation: An Unprecedented Halide Dependence.
New catalytic activity of gold/palladium alloy nanoclusters (NCs) for carbon-halogen bond activation is demonstrated. In the case of an aryl chloride, the inclusion of gold in a bimetallic catalyst is indispensable to achieve the coupling reactions. Gold has the unique effect of stabilizing palladium, such that Pd(2+) leached from clusters by means of spillover of chloride during oxidative addition. The thus-formed spillover intermediate further reacts heterogeneously in both Ullmann and Suzuki-type coupling reactions through a new type of mechanism. In the case of an aryl bromide, Ullmann coupling occurs through the spillover of bromide, similar to that of aryl chloride. However, a significant fraction of palladium also leached, which diminished the Ullmann coupling activity of the aryl bromide and, as a result, the order of reactivity was ArCl>ArBr. With regard to the activation of the C-Br bond towards a Suzuki-type reaction, the inclusion of a higher gold content in gold/palladium clusters stabilized palladium to prevent the leaching of Pd(2+) from the clusters by means of spillover of bromide. The spillover intermediate reacts heterogeneously with PhB(OH)2, palladium-rich gold/palladium, or pure palladium clusters; the oxidative addition of ArBr favors the extraction of palladium from the clusters, yielding Pd(2+) intermediates. The extracted intermediates react homogenously (Pd(2+/)Pd(0) catalysis) with PhB(OH)2, which results in the higher selectivity of the cross-coupling product. An initial step to observe such unprecedented halide dependency, together with the dynamic behavior of palladium on the surface of gold is the oxidative addition of Ar-X. A detailed insight into the first oxidative addition process was also examined by quantum chemical calculations.
Division of Applied Chemistry, School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka, 565-0871, Japan.
Raghu Nath
RN
Dhital
Department of Chemistry, Faculty of Science and Technology, Rajamangala University of Technology Thanyaburi, Thanyaburi, Pathum Thani, 12110, Thailand.
Karan
K
Bobuatong
Institute for Molecular Science, Research Center for Computational Science, Myodaiji, Okazaki, 444-8585, Japan. ehara@ims.ac.jp.|Japan Science and Technology Agency, ACT-C 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan. ehara@ims.ac.jp.
Masahiro
M
Ehara
Division of Applied Chemistry, School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka, 565-0871, Japan. hsakurai@chem.eng.osaka-u.ac.jp.|Japan Science and Technology Agency, ACT-C 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan. hsakurai@chem.eng.osaka-u.ac.jp.
Hidehiro
H
Sakurai
C−X activation; alloys; cluster compounds; gold; palladium