Palladium(II) complexes bearing methylene and ethylene bridged pyrido-annelated N-heterocyclic carbene ligands as active catalysts for Heck and Suzuki–Miyaura cross-coupling reactions

The synthesis and characterization of new bidentate N-heterocyclic carbene ligands is described. The ligands are derived from methylene (me) and ethylene (et) bridged imidazo[1,5-a]pyridine-3-ylidenes (impy) and can be synthesized readily from imidazo[1,5-a]pyridine and the respective dihaloalkanes. Palladium(II) dihalide complexes bearing these et(impy)2 or me(impy)2 ligands were prepared and also structurally characterized. The angle of the impy plane vs. the C–Pd–C plane is depending on the bridging unit as well as the halide ligands. In the solid state the me(impy)2PdBr2 complex forms a dimer by weak intermolecular Pd-hydrogen bridges. The activity of the Pd-complexes as catalysts for the Heck and Suzuki–Miyaura reactions was tested under various conditions. The catalysts show good activity at 120 °C in Heck and at 85 °C in Suzuki–Miyaura reactions.