Studies on the structure and properties of nickel complexes in a set of amide-based 13-membered macrocyclic ligands

This work reports a systematic investigation to understand the structural, spectroscopic and redox properties of Ni(II) ion in a set of 13-membered amide-based macrocyclic ligands. Four macrocyclic ligands containing e−-donating/withdrawing substituents and their Ni(II) complexes have been synthesized and characterized. Structural analysis shows that the macrocyclic ligands create a square-planar environment and nicely accommodate the Ni(II) ion. Electrochemical results suggest that the complexes are capable of undergoing metal-centered oxidation. The electron-donating substituents on ligand lowers the redox potentials and better stabilizes the +3 oxidation state of metal. The electrochemically generated NiIII species are shown to have rich spectroscopic features. For majority of complexes, the oxidized species are concluded to be NiIII by their anisotropic EPR spectra typical for NiIII ion in square-planar geometry. The absorption and EPR spectra for nickel complex bearing an –OMe group on the ligand; however, suggest a Ni(II) complex with a ligand-based radical.