The role of the carboxylic group in the copper(II) mixed-ligand complexes of dl-aspartic acid-β-hydroxamic acid and polyamines

Equilibrium studies of the mixed-ligand complexes of the Cu2+ ion with polyamines {ethylenediamine (1,2-diaminoethane, en), diethylenetriamine (bis(2-aminoethyl)amine, dien) or pentamethyldiethylenetriamine (N,N,N′,N″,N″-pentamethyl-[bis(2-aminoethyl)amine], Me5dien)} as a primary ligand A, and dl-aspartic acid-β-hydroxamic acid (N-hydroxyasparagine, Asp-β-ha) as a secondary ligand L were performed by potentiometric titration and UV–Vis and EPR spectroscopy. The results show that the parent and two mixed-ligand complexes, [CuA(HL)] and [CuA(L)], are formed. In the first [CuA(HL)] species the amino nitrogen and carboxylate oxygen of Asp-β-ha are the coordinating donor atoms (glycine-like coordination) and in the second [CuA(L)] species the amino and hydroxamate nitrogens are the coordinating donor atoms of this ligand. The first manner of coordination predominates in acidic and neutral solutions while the second one in basic solution.Absorption (UV–Vis) and EPR spectra at various pH values indicate a five-coordinate structure of the formed complexes with a geometry close to a square-pyramid in the case of dien and trigonal bipyramid in the case of Me5dien. In the binary copper(II)–aspartic acid-β-hydroxamic acid system the occurrence of a pentanuclear complex was confirmed.

Graphical abstractStability of the mixed-ligand complexes with tridentate amines as a primary ligand and dl-aspartic acid-β-hydroxamic acid as a secondary ligand, their absorption and EPR spectra indicate a five-coordinate structure of the formed complexes. H-Asp-β-ha coordinates to [Cu(A)]2+ by the carboxylate group in addition to the amino nitrogen (glycine-like coordination) in acidic and neutral pH. At higher pH the hydroxamate ligand is coordinated by Namino and Nhydrox. donor atoms.Figure optionsDownload full-size imageDownload as PowerPoint slide