Palladium N-heterocyclic carbene complexes: Synthesis, characterization and catalytic properties in amination

Transmetalation has proved to be a promising procedure to obtain NHC–metal complexes, which typically involves treatment of the imidazolium salt with Ag2O to form the Ag–NHC complex, followed by transmetalation to a species such as [PdCl2(CH3CN)2] gave the palladium complex in which the N-heterocyclic carbene was bound to the metal center. New bis(NHC)–Pd complexes were synthesized and characterized by elemental analysis, 1H NMR, 13C NMR, and IR spectroscopy. The crystal and molecular structure of the trans-dichlorobis{1-(2,3,5,6-tetramethylbenzyl)-3-(2,3,4,5,6-pentamethylbenzyl)imidazolidin-2-yliden}palladium(II) complex was determined by single-crystal X-ray diffraction. The activity of the Pd(II) complexes in the coupling reaction of anilines or amines with bromobenzene was investigated. A preliminary catalytic study showed that these bis(NHC)–Pd complexes were highly active in the direct synthesis of triarylamines and secondary amines in a single step.