Synthesis of novel palladium allyl complexes bearing heteroditopic NHC–S ligands. Kinetic study on the carbene exchange between bis-carbene palladium allyl complexes

We have synthesized several novel palladium allyl and 1,1-dimethylallyl complexes bearing different heteroditopic NHC–S ligands giving rise to a five-membered chelate ring with the metal center. We were able to synthesize some homoleptic bis-carbene allyl derivatives by taking advantage of the hemilability of the thioetheric sulfur. Attempts at preparing mixed bis-carbene complexes bearing two different heteroditopic carbenes (i.e. NHC–S and NHC–Py) simultaneously coordinated to the palladium center lead to a carbene transmetalation with the formation of a statistically distributed equilibrium mixture of the two pure homoleptic and of the mixed bis-carbene palladium allyl complexes in solution. In two different cases the rate of the equilibrium reaction was measured and a mechanistic hypothesis provided. Finally, we have determined the solid state structures of a complex bearing only one NHC–S heteroditopic carbene and of the bis-carbene (NHC–S, NHC–Py) palladium allyl derivatives.

Novel palladium allyl complexes with NHC–S ligands were synthesized. Preparation of mixed bis-carbene complexes bearing two different carbenes simultaneously coordinated to the palladium center yield the formation of an equilibrium mixture of the two homoleptic and the mixed bis-carbene species. The reaction rates were measured and a mechanistic hypothesis advanced.Figure optionsDownload as PowerPoint slideHighlights
► Novel allyl palladium complexes bearing two carbene ligands were synthetized.
► The kinetics of carbene exchange were studied in two different cases.
► The solid state structures of two palladium allyl carbene complexes were determined.