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.

Automated summary

Tripalladium clusters [Pd-3(PPh3)(4)](2+), stabilized by phosphine and arsine ligands, are detected and isolated as intermediates during the reduction of mononuclear Pd complexes to Pd nanoparticles. These clusters exhibit unexpected photoisomerization under UV irradiation and show catalytic activity in Suzuki-Miyaura cross-coupling reactions. Furthermore, they can alter site selectivity in reactions involving 2,4-dibromopyridine, showcasing their potential as catalysts for various transformations.