Electronically Tunable Imidazo[1,2- a ]pyridine Mesoionic Carbenes for PEPPSI-type Palladium Catalysts

Abstract

A series of PEPPSI-type palladium complexes, trans-PdI2(NHC)(pyridine), incorporating unsymmetrical imidazo[1,2-a]pyridine-derived mesoionic carbene (MIC) ligands, was synthesized and characterized by single-crystal X-ray diffraction and DFT analysis. The C3-aryl substituents significantly modulate the electronic structure, whereas the associated steric effects are comparatively small and largely directional. Frontier-orbital, NBO, CDA, and ETS-NOCV analyses show that Pd–NHC bonding is dominated by σ-donation with a smaller but significant π-backbonding contribution, and while wingtip substitution primarily influences the π-acceptor ability of the MIC ligands. The p-CF3-substituted complex affords the strongest overall (σ + π) Pd–C interactions in the Pd0 models and exhibits a lower calculated oxidative-addition barrier than the normal NHC analogue, providing a mechanistic rationale for its improved catalytic performance. Steric analysis using %Vbur further reveals that the ostensibly bulky anthracenyl substituent imposes the smallest buried volume because of its perpendicular orientation, underscoring the importance of steric directionality. In direct C–H arylation of 1,2-dimethylimidazole, all MIC-supported PEPPSI complexes outperform trans-PdI2(IMes)(py), with the p-CF3-substituted complex showing the highest efficiency at low catalyst loading. These findings establish imidazo[1,2-a]pyridine-derived MIC ligands as electronically tunable ligands for highly active PEPPSI-type palladium catalysts.