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Palladium(II) Complexes with Small N-Heterocyclic Carbene Ligands as Highly Active Catalysts for the Suzuki–Miyaura Cross-Coupling Reaction



Four new palladium(II) complexes of the type [Pd(NHC)2X2] with N-heterocyclic carbene (NHC) ligands of relatively small steric hindrance were prepared and characterized by using spectroscopic and X-ray methods. For [Pd(bmim-y)2Br2] (bmim-y=1-butyl-3-methylimidazol-2-ylidene), crystals of both cis and trans isomers were obtained. All the studied complexes demonstrated very high activity in Suzuki–Miyaura cross-coupling in ethylene glycol, which yielded turnover numbers of up to 760 000. High activity was also observed if NaBPh4 was used instead of PhB(OH)2, and the best results (turnover number=580 000) were obtained with [Pd(emim-y)2Cl2] (emim-y=1-ethyl-3-methylimidazol-2-ylidene). In the reaction mixture, different forms containing [Pdx(NHC)y]+ fragments (x=1–4, y=2–5) were identified by using ESI-MS. In the presence of Suzuki–Miyaura reaction substrates, catalytic palladium intermediates with aryl groups—[Pd(NHC)2Ph]+ and [Pd3(NHC)4Ph]+—were detected. Additional mechanistic investigations, such as TEM observations and mercury poisoning experiments, substantiated the formation of nanoparticles as a catalyst resting state. These heterogeneous particles serve as a reservoir for soluble palladium species—atoms or clusters that function as homogeneous catalysts for the Suzuki–Miyaura reaction.