Selective Synthesis of Ni(II) and Pd(II) Complexes with either Normal' or Abnormal' N-Heterocyclic Carbene Coordination Modes

A family of Ni(II) and Pd(II) complexes of one bidentate and three tetradentate, N-heterocyclic carbene (NHC) / amidate ligands have been prepared. The imidazolium salt pro-ligands used in the synthesis of these complexes were designed to facilitate the abnormal' NHC coordination mode, through the introduction of either (1) methyl blocking group at the C2 position of the imidazolium units or (2) sterically demanding tert-butyl imidazolium wingtip substituents or (3) the ligand was macrocyclic in nature. In all cases, the desired complexes were uncharged, with square-planar coordination geometries about the metal centres. These studies showed that in the case of Ni(II), abnormal NHC complexes could be formed for the C2-blocked tetradentate ligands only. In contrast for Pd(II), abnormal' NHC complexes could be prepared from C2-blocked bidentate and tetradentate ligands with sterically demanding tert-butyl NHC wingtip substituents. For the macrocyclic tetradentate ligand, Ni(II) and Pd(II) complexes displaying the normal' NHC coordination mode were obtained.