Dinuclear Palladium(II) Complexes of N-(imidazo[1,5-a]pyridin-1yl)picolinimidamides: Highly Active Catalysts for C-C and C-P Bond Formation

Abstract

This report describes the facile synthesis and characterization of five dinuclear palldium(II) complexes featuring substituted imidazo [1,5-a]pyridinylpicolinimidamides. Molecular structure of ligands and complexes was established using single crystal X-ray diffraction technique. The geometry of complexes revealed that ligands coordinated with palladium center through pseudo-pincer type mode where Pd(II) possessed distorted square planar geometry and an eight-membered metallic cavity was observed. Computational study was 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 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 the photophysical evaluation was carried out for 3p and the results were supported by DFT/TDDFT calculations. Notably, the intermediates of Sonogashira coupling were detected using HRMS which revealed the role of free amine group in stabilizing the intermediates and facilitated oxidative addition at both palladium centers. Also, these complexes (0.5 mol%) were employed as catalyst in C-P bond formation reaction using diethylphosphite as phosphorus coupling partner and excellently produced phosphonates. High catalytic activity of these complexes was accredited to cooperativity and synergic effect of two Pd(II) center present within optimum proximity.