The ubiquitous P(o-tol)3 ligand promotes formation of catalytically-active higher order palladacyclic clusters

Abstract

The Herrmann–Beller catalyst, Pd[(C^P)(μ₂-OAc)]₂, is readily formed by reaction of the cyclic trimer of ‘Pd(OAc)₂’ with P(o-tol)₃. In the presence of hydroxide, Pd(C^P)(μ₂-OAc)]₂ converts to [Pd(C^P)(μ₂-OH)]₂. Here, we report how this activated Pd precatalyst species, and related species, serve as a conduit for formation of higher order Pd_n clusters containing multiple cyclopalladated P(o-tol)₃ ligands. The catalytic competency of a Pd₄-palladacyclic cluster is demonstrated in an arylated Heck cross-coupling, which is comparable to the base-activated form of Herrmann's catalyst, namely [Pd(C^P)(μ₂-OH)]₂. The findings show that ‘simple’ ubiquitous phosphine ligands can promote higher order Pd speciation, moving beyond well-known phosphine-ligated Pd₁ and Pd₂ complexes. The findings challenge the status quo in the field, in that phosphine ligands can ligate higher order Pd_n species which are catalytically competent species in cross-coupling reactions.