Compared Catalytic Efficiency of Click-Dendrimer-Stabilized Late Transition Metal Nanoparticles in 4-Nitrophenol Reduction

Transition metal nanoparticles (TMNPs) are very efficient catalysts even without ligands for a large number of reactions. On the other hand, comparison between catalytic efficiencies of TMNPs is key to mechanistic understanding. Here late TMNPs stabilized by a click dendrimer containing an arene core and 27 triethylene glycol (TEG) termini have been synthesized for Fe, Ru, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag and Au by NaBH4 reduction in water of TM cations that are coordinated to the dendritic triazole groups. All these late TMNPs have been characterized by transmission electron microcopy with core sizes being between 1.4 and 5.2 nm and by UV-vis spectroscopy. The catalytic activities of these freshly synthesized TMNPs under N-2 have been compared for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by NaBH4, a standard reference reaction. Kinetic studies and induction times for this reaction have been recorded. They show that the catalytic activity is all the better as the metal atoms are more electron rich and that the click dendrimer frame sterically inhibits the rate-limiting approach of the substrates to the TMNP surface. These stereoelectronic trends are in agreement with Ballauffs' mechanism involving NP restructuration as rate limiting. Catalysis of 4-nitrophenol reduction by NaBH4 by eleven click dendrimer-stabilized late yransition-metal nanoparticles is compared. A remarkable increase of catalytic activity has been disclosed in the periodic table upon increasing the TM atomic number both from the first row down to the third-raw TMs and within the same row of transition metals. [GRAPHICS] .