Mesoscale Nanostructures as a Bridge Between Homogeneous and Heterogeneous Catalysis

The environment that surrounds catalytically active metallic nanoclusters has an important role in tuning their catalytic reactivity and selectivity and can initiate novel reaction routes. In this review we will demonstrate two different approaches for utilization of the environment in bi-functional, mesoscale catalysts. In these catalytic systems, the molecules that surround the metallic nanoclusters have an active role as co-catalysts. In the first bi-functional system, the steric effects of metal-adsorbed ligands have been exploited for regulating the adsorption orientation of reactants. By favoring specific orientation of the reactants, the products selectivity was widely tuned. In the second bi-functional catalytic system, the catalytic properties of the metallic nanoclusters were controlled by their encapsulation within a polymeric matrix. The oxidation state, catalytic reactivity and stability of metallic nanoclusters were tuned by their encapsulation in polyamidoamine dendrimer molecules. Oxidation of dendrimer-encapsulated nanoclusters into highly oxidized metal-ions activated the catalyst toward a variety of reactions which were previously catalyzed by homogeneous catalysts. Moreover, by modifying the properties of the polymeric matrix, enhanced chemo-, diastereo- and enantio-selectivity were obtained. These two examples of mesoscale catalysts indicate the important role of the surrounding environment in tuning the catalytic reactivity and selectivity. In addition, it is demonstrated that these catalysts can function as a bridge between homogeneous and heterogeneous catalysis.