Isomeric Imidazopyridine-Based NHC Ligands: Enhanced Catalytic Activity of an Electron-Donating Remote NHC in a PEPPSI-Type Palladium Complex

We report the synthesis and characterization of a new series of imidazopyridine-based <i>N</i>-heterocyclic carbene (NHC) ligands, including three unique binding modes: less atom-stabilized (C5-bound), remote normal (C7-bound), and remote mesoionic (C8-bound) configurations. These distinct ligands were incorporated into palladium-(II) complexes of both mixed phosphine/NHC and PEPPSI-type structures to investigate the ligands' electron-donating properties and their catalytic properties. Spectroscopic and computational analyses revealed that the C7-bound remote NHC ligand exhibits the highest electron-donating ability, with a unique balance of σ-donating and π-accepting characteristics that significantly enhance catalytic performance. The catalytic efficiency of the C7-bound NHC ligand, as demonstrated in the Mizoroki-Heck coupling of aryl chlorides, achieved near-quantitative yields and displayed rapid activation. This study underscores the potential of imidazopyridine-based NHC ligands for advancing catalyst design, especially for applications demanding high electron density and stability. These findings open new avenues to explore remote NHC ligands for challenging catalytic transformations, paving the way to improved reaction efficiency in synthetic organic chemistry.