We have studied the roles and behavior of typical resin- and
silica-bound
thiol scavengers in the removal of
palladium (Pd) residues and in the determination of the true catalytic species in the Heck coupling of
bromobenzene and
styrene. The results of Pd scavenging and catalyst
poisoning by elemental analysis (EA) and transmission electron microscopy (TEM) indicate that
silica-bound
thiols have an advantage over resin-bound
thiols in residual Pd removal from a Heck reaction
solution and that all of these scavengers
poison effectively the catalytic species but hardly scavenge Pd clusters, even as small as 1 nm from
solution presumably because of the steric barrier. Because of a smaller proportion of soluble Pd clusters, using a molecular Pd precatalyst results in a much higher Pd scavenging efficiency than using a supported Pd particle precatalyst. With the aid of catalyst
poisoning by the scavengers, filtration testing and TEM studies further illustrate that Pd clusters are inactive for the Heck reaction over Pd(0)/SiO(2), with molecular Pd(0) being solely active. Studies through EA and thermogravimetric analysis suggest that the bound
thiols are leached from the scavengers to a different extent at reaction temperatures of 90-135 °C, probably owing to base-catalyzed decomposition or based-promoted dissociation of functional groups from the surfaces, leading to interaction between leached
thiols and a solid Pd precatalyst. Meanwhile, the effect of solid-bound
thiol binding to soluble Pd on the efficiency of Pd scavenging and the impact of a scavenger on the Pd leaching from supported Pd particles are discussed.