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.