Dendrimer-templated Pd nanoparticles and Pd nanoparticles synthesized by reverse microemulsions as efficient nanocatalysts for the Heck reaction: A comparative study.

Palladium nanoparticles (NPs) were prepared using a dendrimer-templated method using G4, G5 and G6 PAMAM-OH dendrimers as well as a reverse microemulsion method using the water/dioctyl sulfosuccinate sodium salt (aerosol-OT, AOT) surfactant/isooctane system with water to surfactant ratios (ω0) of 5, 10 and 13. These 6 catalysts were characterized by UV-Vis spectroscopy, TEM, EDX, and XRD. TEM micrographs showed that the average sizes of 2.74-3.32nm with narrower size distribution were achieved by using dendrimer-templated synthetic methods, whereas the reverse microemulsion method resulted in broad size distribution with an average size of 3.87-5.06nm. The influence of various reaction parameters such as base, catalyst dosing, alkene, aryl halide and temperature on the Heck C-C coupling reaction was evaluated. The activation parameters were derived from the reaction rate of each catalyst obtained at various temperatures. A correlation of catalytic activity, enthalpy of activation and particle size is discussed. Particle size changes of each catalyst were investigated after the catalytic reaction. Overall results indicated that dendrimer-templated Pd NP catalysts showed superior activity as compared to the Pd NPs synthesized by reverse microemulsions, with the dendrimer-templated G5-OH(Pd80) showing the best activity. These catalysts were also reusable for 3 cycles, retaining high yield and showing excellent yields under mild conditions. Therefore, the dendrimer-templated Pd NPs are efficient catalyst systems for the ligand-free Heck C-C coupling reaction.