Dendrimer-stabilized Pd nanoparticles are used to catalyse carbon–carbon bond-forming reactions and show increasing activity as their concentration is decreased
It is often desirable to reduce the amount of metal-based catalysts used in chemical reactions, because these compounds are generally expensive and can often be toxic to humans or the environment. Transition-metal nanoparticles, however, are an attractive alternative to other types of catalysts because their very high surface-area-to-volume ratio means that proportionately lower loadings can be equally effective.
Dendrimers, which are highly branched macromolecules, are effective templates for the formation of nanoparticles with controlled sizes and numbers of metal atoms. The ability to tailor these parameters in such a precise fashion means that the influence of nanoparticle size on properties such as stability and catalytic activity can be studied in greater detail. Now, Didier Astruc and colleagues1 from the Université Bordeaux I in France have made dendrimer-stabilized palladium nanoparticles and investigated their activity towards the so-called Suzuki reaction — a well-known carbon–carbon bond-forming reaction that is catalysed with palladium.
It was found that the rate and yield of the reaction increased as the concentration of the dendrimer–nanoparticle complex decreased — a somewhat counterintuitive observation, in that less catalyst was better than more. Through careful analysis, however, Astruc and colleagues were able to determine that the improved performance at lower loadings occurred because the catalyst wasn't deactivated as quickly when diluted. Furthermore, it is proposed that the mechanism for the Suzuki reaction under these conditions, which is a controversial subject, involves Pd-atom leaching from the nanoparticle surface after reaction with one of the reagents.
Diallo, A. K. Ornelas, C. Salmon, L. Aranzaes, J. R. & Astruc, D. "Homeopathic" catalytic activity and atom-leaching mechanism in Miyaura-Suzuki reactions under ambient conditions with precise dendrimer-stabilised Pd nanoparticles. Angew. Chem. Int. Edn 10.1002/anie.200703067 (2007).
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Moxham, G. Less is more. Nature Nanotech (2007). https://doi.org/10.1038/nnano.2007.375