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, H-1 NMR, C-13 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. (C) 2010 Elsevier B.V. All rights reserved.