Abnormal NHC Palladium(II) Complex Containing CNN Pincer Skeleton and Its Application to Suzuki-Miyaura Coupling of Aryl Chlorides in Water

Abstract

The excellent catalytic activities of abnormal NHC carbene complexes, where the C2 position remains unaffected, and coordination occurs at the C4 or C5 position of the imidazolium ring, is still a fascinating but scarce area of research. This present work reports a palladium complex (LPdBr) bearing abnormal NHC with CNN pincer architecture. It is synthesized from imidazolium-based quinoline ligand (LH) with Pd(OAc)2 via -NH and carbene deprotonation. The complex was fully characterized by NMR spectroscopy (1D and 2D) and high-resolution mass spectrometry (HRMS). The structure and abnormal coordination modes of the complex are further confirmed with the help of single-crystal X-ray diffraction. The studies revealed the abnormal coordination mode in LPdBr where C5 carbon of the imidazolium ring participates in bonding, leaving the C2 position unaffected. The catalytic potential of complex LPdBr was investigated in the Pd-catalyzed Suzuki-Miyaura coupling of various aryl/heteroaryl chlorides with different aryl/naphthyl/phenanthryl boronic acids. The catalyst LPdBr successfully activated various less-activated aryl chloride substrates. LPdBr also exhibited good catalytic activity at 1 mol% Pd loading under microwave irradiation and in green solvent water. The mechanism of the catalytic cycle is investigated and proposed by HRMS.