Nickel(II) and palladium(II) complexes with chelating N-heterocyclic carbene amidate ligands: Interplay between normal and abnormal coordination modes

A series of six Ni(II) and Pd(II) complexes of two bidentate and two tetradentate N-heterocyclic carbene (NHC)/amidate ligands have been prepared. The complexes are uncharged, with square-planar coordination geometries, and the ligands are bound via the NHC groups and the deprotonated amide nitrogen atoms. Pd(II) complexes were prepared for the bidentate ligands, and in each case, two chelating bidentate ligands were bound to the metal center, yielding cis/trans geometric isomeric forms. The Pd(II) complexes of the tetradentate ligands were obtained as a series of constitutional isomeric forms that were separable by fractional crystallization. The constitutional isomers differed in the coordination mode of the NHC groups, which were bound as either "normal" (nNHC) or "abnormal" (aNHC) carbenes. Density functional theory (DFT) studies show that the energies of the isomeric forms increase in the order nNHC/nNHC <nNHC/aNHC <aNHC/aNHC and suggest that the "abnormal" NHC coordination mode occurs in kinetic rather than thermodynamic reaction products. The Ni(II) complexes of the tetradentate ligand showed only "normal" NHC coordination, suggesting that the mechanism by which aNHC binding occurs is metal dependent. The Ni(II) and Pd(II) complexes with nNHC donors displayed distorted-square-planar coordination geometries and axial chirality. © 2013 American Chemical Society.