Dinuclear palladium(ii) complexes of N-(imidazo[1,5-a]pyridin-1-yl)picolinimidamides: highly active catalysts for C–C and C–P bond formation

Abstract

This report describes the facile synthesis and characterization of five dinuclear palladium(II) complexes featuring substituted imidazo[1,5-a]pyridinylpicolinimidamides. The molecular structures of the ligands and complexes were established using the single crystal X-ray diffraction technique. The geometry of the complexes revealed that the ligands coordinated with the palladium center through a pseudo-pincer type mode, where Pd(II) possessed a distorted square planar geometry and an eight-membered metallic cavity was observed. Computational studies were also performed on all ligands and Pd(II) complexes, which supported the experimental findings. The complexes effectively catalyzed copper-free Sonogashira coupling reactions with only 0.5 mol% catalyst loading under mild reaction conditions with excellent functional group tolerance. Using catalyst 1 under the Sonogashira protocol, the catalytic system was used to synthesize π-extended triarylamines 3o and 3p. The molecular structure of 3o was established by X-ray diffraction analysis, while photophysical evaluation was carried out for 3p and the results are supported by DFT/TDDFT calculations. Notably, the intermediates of Sonogashira coupling were detected using HRMS, which revealed the role of the free amine group in stabilizing the intermediates and facilitating oxidative addition at both palladium centers. Also, these complexes (0.5 mol%) were employed as catalysts in a C–P bond formation reaction using diethylphosphite as the phosphorus coupling partner and excellently produced phosphonates. The high catalytic activity of these complexes was accredited to the cooperativity and synergic effect of the two Pd(II) centers present within optimum proximity.