Palladium nanoclusters in Sonogashira cross-coupling: A true catalytic species?

The question of whether palladium nanoclusters are the actual catalysts in the so-called 'cluster-catalyzed Sonogashira cross-coupling' is investigated, using the coupling of phenylacetylene with 4-bromobenzonitrile as a model reaction. By combining a detailed kinetic analysis with transmission electron microscopy (TEM), we show that a soluble species must be present in the system when Pd nanoclusters are used as catalysts. Various Pd clusters show similar kinetic profiles to that of a homogeneous Pd(dba)(2) complex. Most importantly, TEM analysis of samples taken before, during, and after the reaction shows that the cluster size decreases during the reaction. Based on these findings, we present a possible two-path mechanism for Sonogashira cross-coupling reactions in the presence of Pd nanoclusters.