Coordination-Dependent ESR Spectra of Copper(II) Complexes with a CuN₄ Type Cooridnation Mode : Relationship between ESR Parameters and Stability Constants or Redox Potentials of the Complexes

Abstract

ESR is useful for determining the unpaired electronic state around paramagnetic metal ions such as, copper(II), iron(III) and manganese(II). The coordination structure of Cu(II) complexes was examined by ESR. ESR spectra were measured for a series of polyamine Cu(II) complexes, whose coordination modes are square-planar, axially-coordinate square-planar, tetrahedral and distored square-planar (copper ion displaced from N₄ plane). ESR parameters, g-values and hyperfine coupling constants, A-values, were found to be correlated with the coordination mode of the complex, in which g vs. A plots gave important information on coordination structure of the complex. Moreover, linear relationships between ESR parameters and stability constants or halfwave potentials of the Cu(II) complexes were observed. Based on these results, the coordination structure around the Cu(II) in copper complexes as well as copper proteins was predicted by ESR spectra and associated parameters.