Leaching Mechanism of Different Palladium Surface Species in Heck Reactions of Aryl Bromides and Chlorides

Since the development of solid palladium catalysts for the Heck coupling reaction, many studies focused on the differentiation between homogeneous and heterogeneous reaction pathways. It is generally accepted now that also in heterogeneous catalytic systems, the reaction is catalyzed by molecular, dissolved Pd(0) complexes. In recent time, the mechanism of leaching and, in particular, the dynamics of dissolution and redeposition of metal species came into the focus of research reports. In this context, the specific approach of this study is the comparison of different solid palladium precatalysts regarding their activity in the Heck reaction and the corresponding relation to the dissolution process of palladium in particular with demanding substrates. The potential correlation of leaching and reaction kinetics has been studied for bromobenzene and chlorobenzene. The contemporaneous determination of the conversion and concentration of palladium in solution and separate temperature-programmed Pd dissolution experiments revealed differing leaching mechanisms comparing supported palladium oxide catalysts with isolated palladium surface complexes.