Studies on nickel(II) complexes with amide-based ligands: Syntheses, structures, electrochemistry and oxidation chemistry

The present work discusses the nickel chemistry in a set of amide-based open-chain ligands with subtle differences in the backbone or terminal amine substituents. The ligands coordinate to the Ni2+ ion through the N-amide and N-amine atoms maintaining a square-planar geometry. Absorption spectra and NMR studies reveal that the solid-state square-planar geometry is retained in solution. The electrochemical results suggest that the Ni-III/Ni-II redox couple primarily depends on the N-4 donors, which is composed of two Namide and two N-amine atoms and not on the peripheral substituents. All four ligands with variable backbone and substituents are equally competent in stabilizing the Ni-III state. On the basis of electrochemical findings, chemical oxidations were carried out, and they reveal generation of the Ni-III state in two cases, whereas decomposition was observed in others. Preliminary alkene epoxidation reactions suggest that the present nickel complexes transiently stabilize the higher oxidation state of the nickel ion that possibly participates in the oxidation of the substrates. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).