Effect of colloidal catalysis on the nanoparticle size distribution: Dendrimer-Pd vs PVP-Pd nanoparticles catalyzing the Suzuki coupling reaction

A comparison of the stability and catalytic activity of PAMAM-OH generation 4 dendrimer-Pd nanoparticles (1.3 +/- 0.1 nm) with the previously studied PVP-Pd nanoparticles (2.1 +/- 0.1 nm) in the Suzuki coupling reaction between phenylboronic acid and iodobenzene is conducted. After the first cycle, the average size of the PVP-Pd nanoparticles increases by 38% and the dendrimer-Pd nanoparticles increases by 54%. After the second cycle, the PVP-Pd nanoparticles decrease in size by 24% whereas the dendrimer-Pd nanoparticles continue to increase in size by 35%. The strong encapsulating action of the PAMAM-OH generation 4 dendrimer-Pd nanoparticles could make the rate of conversion to the full nanoparticle size slow, resulting in a large excess Pd metal atom concentration in solution, resulting in the continuous growth of the nanoparticles during the catalytic reaction. The effect of the individual reactants on the stability of the dendrimer-Pd nanoparticles has also been investigated and found to be similar to that observed for the PVP-Pd nanoparticles previously. It was found that the nanoparticle size growth occurs while refluxing in the presence of only the solvent, sodium acetate, or iodobenzene. However, the presence of phenylboronic acid is found to inhibit the particle growth, suggesting that it acts as a capping agent. Thus, the reaction mechanism must involve the adsorption of phenylboronic acid to the nanoparticle surface, which subsequently reacts with the iodobenzene in solution. This is similar to the mechanism found previously on PVP-Pd nanoparticles, suggesting that the mechanism is insensitive to the capping material used. The ratio of the yield of biphenyl formed in the second cycle to that in the first cycle is higher for the dendrimer-Pd nanoparticles catalyzed reaction than for the PVP-Pd nanoparticles. This could be due to the greater stability of the dendrimer-Pd nanoparticles and the increase in its size during the reaction. The larger PVP-Pd nanoparticles studied previously is believed to aggregate and precipitate out of solution during the second cycle. The presence of excess dendrimer is found to severely diminish the catalytic activity of the dendrimer-Pd nanoparticles and also diminishes the change in the Pd nanoparticle size during the catalysis.