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.