A Robust and Efficient Pd3Cluster Catalyst for the Suzuki Reaction and Its Odd Mechanism

The palladium-catalyzed Suzuki–Miyaura coupling reaction is one of the most versatile and powerful tools for constructing synthetically useful unsymmetrical aryl–aryl bonds. In designing a Pd cluster as a candidate for efficient catalysis and mechanistic investigations, it was envisaged to study a case intermediate between, although very different from, the “classic” Pd(0)L n and Pd nanoparticle families of catalysts. In this work, the cluster Pd 3 Cl(PPh 2 ) 2 (PPh 3 ) 3 + SbF 6 − (abbreviated Pd 3 Cl ) was synthesized and fully characterized as a remarkably robust framework that is stable up to 170 °C and fully air-stable. Pd 3 Cl was found to catalyze the Suzuki–Miyaura C–C cross-coupling of a variety of aryl bromides and arylboronic acids under ambient aerobic conditions. The reaction proceeds while keeping the integrity of the cluster framework all along the catalytic cycle via the intermediate Pd 3 Ar, as evidenced by mass spectrometry and quick X-ray absorption fine structure. In the absence of the substrate under the reaction conditions, the Pd 3 OH species was detected by mass spectrometry, which strongly favors the “oxo-Pd” pathway for the transmetalation step involving substitution of the Cl ligand by OH followed by binding of the OH ligand with the arylboronic acid. The kinetics of the Suzuki–Miyaura reaction shows a lack of an induction period, consistent with the lack of cluster dissociation. This study may provide new perspectives for the catalytic mechanisms of C–C cross-coupling reactions catalyzed by metal clusters.