Crystal structure, redox and spectral properties of copper(II) complexes with macrocyclic ligands incorporating both oxamido and imine groups

The electrochemical and spectral behaviors of three copper(n) complexes [CuL¹] 1, [CuL²] 2 and [CuL³] 3, where L¹, L² and L³ are the dianions of macrocyclic oxamido Schiff bases, have been investigated by cyclic voltammetry, electronic and ESR spectra. These macrocyclic complexes, which incorporate both oxamido and imine groups, can undergo quasi-reversible reduction (Cu^II→Cu¹) and oxidation (Cu^II→Cu^III). The crystal structures of 1 and 2 have been determined by single-crystal X-ray analysis. The difference in ring size leads to significant differences in molecular and crystal structure, electronic and ESR spectra and redox behavior. In the [14]N₄ macrocyclic complex, 2, the CuN₄ chromophore assumes a nearly square-planar co-ordination geometry, but the geometry in the [15]N₄ macrocyclic complex, 1, is distorted towards tetrahedral. The results from spectroscopic and redox studies are consistent with the crystallographic results and perfectly related to each other. The main factors that determine the relative stability of Cu^I and Cu^III in these complexes are the size, geometry and flexibility of the co-ordination cavity.