ROLE OF THE NH3+ MOIETY IN IRON(III)-AMINOHYDROXAMATE, ALUMINUM(III)-AMINOHYDROXAMATE AND GALLIUM(III)-AMINOHYDROXAMATE INTERACTIONS

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(lll) with alpha-alaninehydroxamic acid (alpha-Alaha), beta-alaninehydroxamic acid (beta-Alaha), aspartic acid-beta-hydroxamic acid (Asp-beta-ha) and glutamic acid-gamma-hydroxamic acid (Glu-gamma-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, alpha- and beta-Alaha, while Asp-beta-ha and Glu-gamma-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 M(3+) ion.