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

Abstract

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 [Pd_x(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.