Role of the NH3+ moiety in iron(III)–, aluminium(III)– and gallium(III)–aminohydroxamate interactions

Abstract

Stability constants have been determined and the bonding modes and effects caused by the side-chain NH₃⁺ moiety in aminohydroxamic acids evaluated for complexes formed in aqueous solution in between iron(III), aluminium(III) and gallium(III) with α-alaninehydroxamic acid (α-Alaha), β-alaninehydroxamic acid (β-Alaha), aspartic acid-β-hydroxamic acid (Asp-β-ha) and glutamic acid-γ-hydroxamic acid (Glu-γ-ha). The iron(III)–, aluminium(III)– and gallium(III)–acetohydroxamic acid (aha) systems were studied as models. Co-ordination of hydroxamate oxygens occurs in the cases of aha, α- and β-Alaha, while Asp-β-ha and Glu-γ-ha are co-ordinated via their hydroxamate and carboxylate oxygens. The OH^– ion was found to be an effective ligand in these systems (especially for Ga^III) causing the formation of both binary and ternary hydroxo complexes. The presence of NH₃⁺ in the hydroxamic acids favours the hydrolysis to an extent which depends on the distance between the hydroxamate moiety and NH₃⁺. These findings can be explained by the electron-withdrawing effect of NH₃⁺ and electrostatic repulsion between it and the co-ordinating M³⁺ ion.