Synthetic and structural studies of (NHC)Pd(allyl)Cl complexes (NHC = N-heterocyclic carbene)

The synthesis and structural characterization of a series of palladium complexes bearing N-heterocyclic carbenes (NHC) as supporting ligands are described. The reaction of commercially available [Pd(allyl)Cl](2) and isolated or in situ generated NHC leads to monomeric palladium complexes where one NHC is bound to the metal center, as indicated by spectroscopic and single-crystal X-ray diffraction studies. The relative reactivity trend for these complexes as catalysts in aryl amination is discussed in terms of ligand steric properties, which vary as a function of imidazole-nitrogen substituents and perturbation resulting in modulation of ring planarity. The concept of buried volume is used to quantify the steric demand of each NHC in the corresponding complexes.