Complex-forming properties of L-α-alaninehydroxamic acid (2-amino-N-hydroxypropanamide)

Abstract

The systems of manganese(II), cobalt(II), nickel(II), copper(II), zinc(II), cadmium(II), iron(III), and aluminium(III) with L-alaninehydroxamic acid (2-amino-N-hydroxypropanamide)(ahpr) have been investigated in the metal–ligand ratio range 1:1–1:10 by means of pH-metric and spectrophoto-metric methods. Formation constants and assumptions concerning the bonding modes in the species present in aqueous solution are reported. There is appreciable complex formation between iron (III) and ahpr even below pH 3. Mixed hydroxo complexes can be proposed in addition to the binary iron(III)–ahpr complexes. The aluminium(III)–ahpr complexes are formed in the range pH ca. 3–9. There is no significant complex formation below pH 3, and [Al(OH)₄]^– is the only species present in measurable concentration above pH 9. Co-ordination of the hydroxamate oxygens is suggested in both the iron(III)- and aIuminium(III)–ahpr systems. Of the metal ions in the 2+ oxidation state, copper(II) forms the most stable complexes with ahpr. It can be postulated that both the oxygen and the nitrogen donor atoms of the ligand (H₂A⁺) are involved in the co-ordination complexes [CuA₂]⁺ and [Cu₂A₂H_–1]⁺, but in the species [CuA₂] and [CuA₂H_–1]^– only the nitrogens are co-ordinated. Co-ordination of the nitrogen atoms is proposed in the planar complexes present in the nickel(II)–ahpr system. Complexes of medium stability are formed in the cobalt(II)– and zinc(II)–ahpr systems in the range pH 5.0–8.5. Complex formation only occurs to a small extent with manganese(II) and cadmium(II) ions.