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The Effect of Ligand Architecture on the Structure and Properties of Nickel and Copper Complexes of Amide‐Based Macrocycles

The present work demonstrates the influence of electron-donating and electron-withdrawing substituents on the structural, chemical and redox properties of nickel(II) and copper(II) complexes in a set of 12-membered macrocyclic ligands. Four macrocyclic ligands, H 2 L H , H 2 L Cl , H 2 L Me , and H 2 L OMe , in their doubly deprotonated form, have been used to synthesize the Ni II and Cu II complexes. The crystallographic studies reveal that whereas the nickel ion is in an N 4 square-planar environment, the Cu" complexes have a square-pyramidal geometry with an N 4 basal plane and the O amide atom in the apical position. Furthermore, the Cu-N amide bonds are shorter than the Ni-N amide bonds. The Ni" and Cu" complexes are capable of undergoing one- and two-electron oxidations that are most likely metal- and ligand-centered. The electrochemical results indicate that the Ni III/II potentials are more positive than the Cu III/II potentials. Electron-donating substituents on the ring shift the redox-potential value towards less positive values and better stabilize a higher oxidation state of the metal ion. The Ni III and Cu III species were generated in solution both electrochemically as well as chemically and are shown to have rich spectroscopic features. The absorption and anisotropic EPR spectra of the nickel complexes suggest that the Ni III species is in a square-planar geometry. The spectroscopic data for the nickel(III) and copper(III) species bearing an OMe group on the ligand suggest a metal complex with a semiquinone-type ligand-based radical.