Abstract
Bimetallic Ni-Pd alloy nanoparticles with high Ni to Pd atomic ratios (99 : 1 or 95 : 5) were prepared, and the catalytic performances of these nanoparticle catalysts were explored for C-C coupling reactions (Suzuki-Miyaura, Heck and Sonogashira reactions) under moderate reaction conditions. In contrast to their monometallic counterparts, significantly enhanced catalytic activity was achieved with the studied Ni-Pd nanoparticle catalysts for the C-C coupling reactions, and products were obtained in moderate to high yields. The turnover number (TON) increases with the increase in the Ni to Pd atomic ratio for Ni-Pd nanoparticle catalysts and can reach 3.6 x 10(3) for Ni0.99Pd0.01 nanoparticle catalysed Suzuki-Miyaura reaction of aryl bromides with arylboronic acid at 50 degrees C. Advantageously, such Ni-Pd nanoparticle catalysts with high Ni to Pd atomic ratios not only show significantly enhanced catalytic activity but are also stable (ICP-AES analysis showed only marginal or no Pd leaching) and retain their catalytic activities for several catalytic runs (> 90% conversion even at the 7th catalytic run). Experimental and relevant theoretical calculations (net charge localization using first principles calculations) suggested a substantial Ni to Pd charge transfer which resulted in a highly negatively charged Pd centre, a favourable site for facile oxidative addition of aryl halides, and hence enhanced catalytic activity for Ni-Pd nanoparticle catalysts.