Synthesis, Characterization, Crystal Structures, and Catalytic C–C Coupling and Hydrosilylation Reactions of Palladium(II) Complexes Derived from CNC Pincer‐Type N‐Heterocyclic Carbenes

Abstract

AbstractA series of pyridine linker containing bis(benz)imidazolium salts serving as precursors to CNC pincer‐type N‐heterocyclic carbene (NHC) ligands were prepared by successive N‐alkylation of the azole core. Binuclear NHC complexes of silver(I) were synthesized by in situ deprotonation of the corresponding (benz)imidazolium salt with silver(I) oxide in acetonitrile at elevated temperature in the dark. Subsequently, pincer‐type palladium(II)–NHC complexes were prepared by transmetalation using silver(I)–NHC complexes in acetonitrile at elevated temperatures. All compounds were fully characterized by elemental analysis, FTIR spectroscopy, and 1H and 13C NMR spectroscopy. X‐ray diffraction studies on single crystals of three compounds confirmed the distorted square‐planar geometry around the palladium(II) center. All complexes are monomeric in nature, and the bis‐NHC ligand chelates in the CNC mode. The effective catalytic potentials of the palladium complexes were demonstrated by their high activities in aqueous‐phase Suzuki–Miyaura and Heck–Mizoroki cross‐coupling reactions of phenylboronic acid and n‐butyl acrylate with a range of functionalized bromobenzene derivatives. The former coupling reaction afforded biphenyls in yields of 71–99 %, whereas the latter reaction evidenced low yields of the n‐butyl cinnamates. Further, all the palladium complexes were studied for their potential in a catalytic hydrosilylation reaction.