Copper(ii) 12-metallacrown-4 complexes of α-, β- and γ-aminohydroxamic acids: a comparative thermodynamic study in aqueous solution

Abstract

A complete thermodynamic study of the protonation and Cu^II complex formation equilibria of a series of α- and β-aminohydroxamic acids in aqueous solution was performed. The thermodynamic parameters obtained for the protonation of glycine-, (S)-α-alanine-, (R,S)-valine-, (S)-leucine-, β-alanine- and (R)-aspartic-β-hydroxamic acids were compared with those previously reported for γ-amino- and (S)-glutamic-γ-hydroxamic acids. The enthalpy/entropy parameters calculated for the protonation microequilibria of these three types of ligands are in very good agreement with the literature values for simple amines and hydroxamic acids. The pentanuclear complexes [Cu₅L₄H₋₄]²⁺ contain the ligands acting as (NH₂,N⁻)-(O,O⁻) bridging bis-chelating and correspond to 12-metallacrown-4 (12-MC-4) which are formed by self-assembly between pH 4 and 6 with α-aminohydroxamates (HL), while those with β- and γ-derivatives exist in a wider pH range (4–11). The stability order of these metallomacrocycles is β- ≫ α- > γ-aminohydroxamates. The formation of 12-MC-4 with α-aminohydroxamates is entropy-driven, and that with β-derivatives is enthalpy-driven, while with γ-GABAhydroxamate both effects occur. These results are interpreted on the basis of specific enthalpies or entropy contributions related to chelate ring dimensions, charge neutralization and solvation-desolvation effects. The enthalpy/entropy parameters of 12-MC-4 with α-aminohydroxamic acids considered are also dependent on the optical purity of the ligands. Actually, that with (R,S)-valinehydroxamic acid presents an higher entropy and a lower enthalpy value than those of enantiopure ligands, although the corresponding stabilities are almost equivalent. Moreover, DFT calculations are in agreement with a more exothermic enthalpy found for metallacrowns with enantiomerically pure ligands.