Hetero-dicarbene Complexes of Palladium(II): Syntheses and Catalytic Activities

A series of Pd­(II) dibromido complexes 2–6 bearing cis-chelating hetero-dicarbenes, which contain two different types of NHCs linked by a propylene chain, have been synthesized. In most cases, the N-methylbenzimidazolin-2-ylidene moiety was kept as one NHC donor, while the other one varies with different heterocyclic backbones. As an exception, the hetero-diNHC in complex 8 is derived by combining 1,2,4-triazole and indazole precursors instead. Analogous complexes 9–17, carrying more labile CF₃CO₂ ^– or CH₃CN ligands, were synthesized by reacting the aforementioned bromido complexes with AgO₂CCF₃ or AgOTf in CH₃CN. A systematic catalytic comparison of 9–17 in the direct arylation of pentafluorobenzene with 4-chlorobromobenzene was carried out, and complexes that contain bulkier and less electron-donating ligands were found to be more active. Complex 12, carrying the mesitylimidazolin-2-ylidene unit, proved to be the most efficient, and its activity was also tested in the direct arylation of tetrafluorobenzenes.