Insights into the Suzuki-Miyaura Reaction Catalyzed by Novel Pd−Carbene Complexes. Are Palladium−Tetra- carbene Entities the Key Active Species?

Abstract

The assumption that the real active species involved in the Suzuki-Miyaura reaction are homogeneous, heterogeneous or both is often proposed. However a lack in the characterization of true catalytic entities and their monitoring makes assumptions somewhat elusive. Here, three families of palladium(II) complexes bearing bisNHC, bispyridyl and bisphosphine ligands were synthesized in order to get new insights into the formation of active species in the Suzuki-Miyaura reaction. Their comparative catalytic study reveals that the nature of the ligands as well as their spacer lengths are pivotal parameters governing the performance. All complexes evolve to Pd NPs under reaction conditions, and an orthogonal behavior is observed for two bisNHC complexes that form homoleptic tetracarbene species. Notably, these species are presumably involved in a new catalytic modus operandi. This ligand-controlled reactivity and the formation of tetracarbene species open new routes towards the design of novel cross-coupling catalysts via the mastering of highly-active catalytic species.