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 SuzukiMiyaura cross-coupling in ethylene glycol, which yielded turnover numbers of up to 760000. High activity was also observed if NaBPh4 was used instead of PhB(OH)2, and the best results (turnover number=580000) 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=14, y=25) were identified by using ESI-MS. In the presence of SuzukiMiyaura 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 speciesatoms or clusters that function as homogeneous catalysts for the SuzukiMiyaura reaction.