Well-Defined Pd_n Clusters for Cross-Coupling and Hydrogenation Catalysis: New Opportunities for Catalyst Design

In recent studies it has been demonstrated that the privileged reactivity of higher-order metal clusters can be exploited in widely applied catalytic processes, particularly cross-coupling reactions and hydrogenative transformations. Relatively small, well-defined Pdn clusters have been known since the 1960s. Unique reactivity, reaction (product) selectivity, and catalyst behavior have been recently uncovered, from which there is much potential in catalyst design and application. Ligated Pdn clusters of a smaller size (where n is less than 6) may form upon degradation of mononuclear Pd species en route to larger particulate Pd (from <5 nm particles to large moribund forms in the >1 μm range). This review presents the catalytic applications of Pdn clusters. We pay particular attention to the underlying structure of the Pdn clusters, linked to their reactivity. A hypothesis that ligated Pdn clusters may constitute a mechanism by which higher-order Pd species may form (as a bridging point for monoligated Pd species through to PdNPs) is further discussed. Where appropriate, we mention other catalytic reaction processes that complement the discussion focused on cross-coupling and hydrogenation processes.