Bimetallic gold-palladium alloy nanoclusters: an effective catalyst for Ullmann coupling of chloropyridines under ambient conditions

An efficient method for the Ullmann coupling of chloropyridines catalyzed by poly(N-vinylpyrrolidone) (PVP)-stabilized bimetallic Au-Pd alloy nanoclusters (NCs) under ambient conditions is demonstrated. The reaction does not occur with either gold or palladium single-metal clusters alone, nor with a physical mixture of the two metals. The experimental results indicate that the inclusion of Au as a nearest heteroatom is crucial to initiate the coupling and its composition up to 50% is essential to accelerate the reaction. Unlike the conventional transition metal catalysis, 2-chloropyridine was found to be highly reactive as compared to 2-bromopyridine. From the UV-vis and ICP-AES measurements, a significant amount of leached Pd(II) was observed in the coupling with 2-bromopyridine as compared with 2-chloropyridine, indicating that the leaching process might be a crucial factor in diminishing the reactivity.