Tetranuclear Palladium Complexes of Abnormal <i>N</i>‐Heterocyclic Carbene Ligands and their Catalytic Activities in Mizoroki‐Heck Coupling Reaction of Electron‐Rich Aryl Chlorides

Abstract

Abstract Based on the ligand scaffold of an imidazolyl/benziimidazolyl moiety and a N ‐CH 2 C(=O)NHPh substituent, two series of ligand precursors with ortho hydroxy groups incorporated on the N ‐phenyl rings were prepared. The structural fine tuning of the ligand scaffold allowed the synthesis of tetranuclear palladium complexes with abnormal N ‐heterocyclic carbene ( a NHC) ligands. For precursors with C2‐methyl blocking groups, pyridine‐assisted C−H bond activation led to the formation of mononuclear tridentate palladium a NHC complexes or tetranuclear complexes with tridentate CNO donors. Representative mononuclear and tetranuclearpalladium a NHC complexes were structurally characterized by X‐ray diffraction studies, revealing very short Pd−C bond distances. The tetranuclear palladium a NHC complexes were very effective in catalyzing Mizoroki‐Heck coupling reaction, and were capable of employing a range of aryl chlorides including deactivated substrates with low palladium loading of 0.2 mol%. magnified image