An electron paramagnetic resonance study of copper(II)-β-substituted β-amino acid systems by the two-dimensional simulation method: First evidence of primarily steric effects of substituents on equilibria of metal complexes

We have studied the complex equilibria of copper(II) with a series of β-substituted β-amino acids (R:  H, Me, Et, iBu, iPr, cHex, 1-EtPr, and tBu) in aqueous solution by pH potentiometry and electron paramagnetic resonace (EPR) spectroscopy in the range pH = 2−8 at various metal and ligand concentrations. The basicities of the corresponding donor groups differed only slightly in the series of ligands. A purely mathematical method, the matrix rank analysis carried out on the EPR spectrum package recorded in the presence of copper(II), indicated the formation of 6 independent paramagnetic species. Accordingly, Cu2+ (aqua complex) and the complexes [CuLH]2+, [CuL]+, [CuL2H2]2+, [CuL2H]+, and [CuL2] were considered in the subsequent analysis of series of spectra, and also two isomers of [CuL2] were identified. The formation constants and the EPR parameters, e.g. the isotropic g-factors and the copper and nitrogen hyperfine couplings for the above species, were determined in the same optimization procedure by ...