An Electron Paramagnetic Resonance Study of Copper(II)-(beta)-Substituted (beta)-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 (beta)-substituted (beta)-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, Cu^2+ (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 the simultaneous evaluation of spectra. The ligands "LH" are suggested to bind in equatorial positions through their carboxylate groups, while the amino acids in the L protonation state are likely to occupy two equatorial sites via the amino and carboxylate groups. For the isomers of [CuL2], the donors of the same kind are in the cis or trans position. As far as we know, this is the first reported case in which a strong correlation has been found between the steric effects of substituents characterized by Meyerʹs steric parameter V^a and the protonation constants of metal complexes. The observed trend for the preference for nonprotonated complexes [CuL]^+ and [CuL2] to increase with the steric demand of the substituent was explained by the increasing shielding effect of the substituent hindering protonation of the nonprotonated complex.