Addressing Reversibility of R–NHC Coupling on Palladium: Is Nano-to-Molecular Transition Possible for the Pd/NHC System?

• Published in: Inorganic chemistry 2019-09, Vol.58 (18), p.12218-12227
• Main Authors: Denisova, Ekaterina A, Eremin, Dmitry B, Gordeev, Evgeniy G, Tsedilin, Andrey M, Ananikov, Valentine P
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.9b01630

It has recently been shown that palladium-catalyzed reactions with N-heterocyclic carbene (NHC) ligands involve R–NHC coupling accompanied by transformation of the molecular catalytic system into the nanoscale catalytic system. An important question appeared in this regard is whether such a change in the catalytic system is irreversible. More specifically, is the reverse nano-to-molecular transformation possible? In view of the paramount significance of this question to the area of catalyst design, we studied the capability of 2-substituted azolium salts to undergo the breakage of C–C bond and exchange substituents on the carbene carbon with corresponding aryl halides in the presence of Pd nanoparticles. The study provides important experimental evidence of possibility of the reversible R–NHC coupling. The observed behavior indicates that the nanosized metal species are capable of reverse transition to molecular species. Such an option, known for phosphine ligands, was previously unexplored for NHC ligands. The present study for the first time demonstrates bidirectional dynamic transitions between the molecular and nanostructured states in Pd/NHC systems. As a unique feature, surprisingly small activation barriers (