Synthesis of late transition-metal nanoparticles by Na naphthalenide reduction of salts and their catalytic efficiency

Abstract

The investigation of new methods of facile synthesis of salt-free, catalytically efficient nanoparticles is challenging. Pd, Cu, Ag and Au nanoparticles (NPs) have been synthesized here by Na naphthalenide reduction of commercial transition-metal salts in THF or MeCN in the presence of poly(ethylene glycol). These NPs have been characterized by UV-vis. and XPS spectroscopy showing the presence of zero-valent metals and by TEM showing their small sizes. They have been purified by double extraction benefiting from the salting-out effect. The catalytic properties of these NPs have been compared for the classic 4-nitrophenol reduction by excess NaBH₄. The small Pd and Au NPs (1.7 resp. 2.0 nm) are the most efficient catalysts without retention times for this reaction, whereas Cu and Ag nanoparticles that are larger (3.3 resp. 4 nm) showed retention times and lower rate constants. NPs purified by the salting-out effect were more efficient than non-purified ones. PdNPs were also very active catalysts for the Suzuki–Miyaura reaction between bromobenzene and phenylboronic acid (95% yield with 100 ppm Pd) and for the Sonogashira reaction between phenyl acetylene and iodobenzene. CuNPs reported using this method were more efficient “click” catalysts as Cu₂ONPs, and these NPs were used in organometallic “click” synthesis and Sonogashira co-catalysis. For the latter PdNP-catalyzed reaction, the yield was slightly improved in the presence of supported Cu₂ONP on SBA-15 (90% vs. 81%). Supporting the PdNP catalyst on SBA-15 even proceeded without a large NP size change, provided improved reaction yields and afforded several recycling cycles without significant Pd leaching.