On the Mechanism of the Oxidative Cleavage of N‐Heterocyclic‐Carbene–Palladium Bonds with Iodine

• Published in: European journal of inorganic chemistry Vol. 2017; no. 13; pp. 2058 - 2067
• Main Authors: Lee, Eunsung, Lee, Jaechul, Yandulov, Dmitry V.
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 03.04.2017
• Subjects: Bonding; Carbene ligands; Chemical bonds; Cleavage; Iodides; Iodine; Isomers; Ligand iodination; Oxidation; Oxidative cleavage; Palladium; Rubber; Stability
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.201700034

The excellent performance of N‐heterocyclic carbenes (NHCs) in various palladium‐catalyzed reactions has been explained by their strong donor character, which grants remarkable stability to palladium complexes often under extreme conditions and imparts adjustable steric demand. However, despite such a strong stability, the oxidative cleavage of palladium–carbene bonds by iodine was observed in NHC–palladium complexes. The oxidative cleavage of a palladium(II) iodide complex supported by the NHC with the smallest steric profile, 1,3‐dimethylimidazol‐2‐ylidene (IMe), is demonstrated here. The syntheses and characterization of various palladium –IMe complexes are reported along with a study of the mechanism of the oxidative cleavage of Pd–IMe bonds with iodine. Owing to the small steric profile of IMe, the important features of structure and bonding in these complexes are preserved, but reactivity with the relatively weak oxidant iodine is facilitated. Further computational mechanistic insights for this oxidative cleavage of the Pd–NHC bond were demonstrated, and both dissociative oxidative cleavage and metathesis of the Pd–NHC bond with iodine are feasible on the basis of DFT calculations. The cis and trans isomers of PdI2IMe2 (IMe = 1,3‐dimethylimidazol‐2‐ylidene) show different reactivity toward I2 in CH2Cl2. The trans isomer cleaves the Pd–IMe bond through direct electrophilic attack of I2 or simple dissociation. However, cis‐PdI2IMe2 forms cis‐PdI(I3)IMe2. In dimethyl sulfoxide (DMSO), both isomers iodide to form [PdI(DMSO)IMe2](I3) in equilibrium with [PdI(I3)IMe2].