Unexpected Stability of CO-Coordinated Palladacycle in Bidentate Auxiliary Directed C(sp3)–H Bond Activation: A Combined Experimental and Computational Study

We have thoroughly studied four kinds of different palladacycles with 8-aminoquinoline as the auxiliary and carbon monoxide (CO), acetonitrile (CH3CN), pyridine, and dimethyl sulfoxide (DMSO) as the ligands, respectively, using crystallographic analysis, cyclic voltammetry (CV), stoichiometric reactions, and computational calculations. A higher oxidation potential in CV and lower chemical reactivities to react with weaker electrophilic reagents are presented for CO-coordinated palladacycles, which has been supported by DFT studies that the CO-coordinated Pd complex has the highest ΔG value for the initial ligand exchange process. Meanwhile, a rigid structure can be observed and relatively high activation energy (ΔG⧧ = 54.2 kcal/mol) is needed for further migratory insertion. All this evidence suggests that the CO-coordinated Pd complex is a kinetically and thermodynamically stable intermediate, which accounts for its reluctance for migratory insertion and the formation of the corresponding carbonylation product.