Bridging the Gap from Mononuclear PdII Precatalysts to Pd Nanoparticles: Identification of Intermediate Linear [Pd3(XPh3)4]2+ Clusters as Catalytic Species for Suzuki-Miyaura Couplings (X = P, As)

Tripalladium clusters of the type [Pd-3(PPh3)(4)](2+), wherein three linearly connected Pd atoms are stabilized by phosphine and arsine ligands, have been detected and isolated as intermediates during the reduction of well-defined mononuclear [Pd(OTf)(2)(XPh3)(2)] (X = P and X = As, respectively) to Pd nanoparticles (PdNPs). The isolated [Pd-3(PPh3)(4)](2+) cluster isomerizes on broad-band UV irradiation to form an unexpected photoisomer, produced by a remarkable change in conformation at one of the bridging PPh3 ligands. A catalytic role for these [Pd-3(XPh3)(4)](2+) species is exemplified in Suzuki-Miyaura cross-coupling (SMCC) reactions, with high activity seen in the arylation of a brominated heterocyclic 2-pyrone. Use of the [Pd-3(PPh3)(4)](2+) cluster enables a switch in site selectivity for SMCC reactions involving 2,4-dibromopyridine from the typical C2-bromide site (seen previously for mononuclear Pd catalysts) to the atypical C4-bromide site, thereby mirroring recently reported cyclic Pd-3 clusters and PdNPs. We have further determined that the thermal isomer and photoisomer of [Pd-3(PPh3)(4)](2+) are similarly catalytically active in the Pd-catalyzed hydrogenation of phenylacetylene to give styrene. Our findings link the evolution of mononuclear Pd(II) salts to PdNPs via the intermediacy of linear [Pd-3(XPh3)(4)](2+) clusters.