Mechanism of the oxidative addition of hypervalent iodonium salts to palladium(II) pincer-complexes

Oxidative addition of hypervalent iodonium salts to palladium pincer-complexes was studied to rationalize the mechanism of the key-step in related Pd(II)/Pd(IV) based catalytic processes. It was found that this oxidative addition is an exothermic process with a relatively low activation barrier. The activation energy is highly dependent on the organic substituents of the iodoniun salts. For example, an ethynyl group is transferred with a considerably lower activation barrier from iodine to palladium, than a phenyl functionality. We have compared the reaction profiles of the oxidative addition of hypervalent iodonium salts and phenyl iodide. The most important difference between the two processes is that the oxidative addition of phenyl iodide is highly endothermic, as the reductive elimination of phenyl iodide from the Pd(IV) species requires a very low barrier. In contrast, the formation of the Pd(IV) species using hypervalent iodonium salts is an irreversible process, which allows ligand exchange and trasmetallation reactions generating a productive catalytic cycle. The studies indicate that the most important MO's involved in the oxidation is the antibonding σ* orbital of the iodonium salt and a non-bonding Pd(4d) orbital, which is perpendicular to the coordination plane of the palladium atom.

DFT modeling studies were performed for oxidative addition of hypervalent iodonium salts to Pd(II) pincer-complexes. The focus was directed to steric and electronic effects, which are important in the key-step of the Pd(II)/Pd(IV) redox cycle based catalytic reactions.Figure optionsDownload high-quality image (51 K)Download as PowerPoint slide