Interaction between iron(II) and hydroxamic acids: oxidation of iron(II) to iron(III) by desferrioxamine B under anaerobic conditions.

Interaction between iron(II) and acetohydroxamic acid (Aha), alpha-alaninehydroxamic acid (alpha-Alaha), beta-alaninehydroxamic acid (beta-Alaha), hexanedioic acid bis(3-hydroxycarbamoyl-methyl)amide (Dha) or desferrioxamine B (DFB) under anaerobic conditions was studied by pH-metric and UV-Visible spectrophotometric methods. The stability constants of complexes formed with Aha, alpha-Alaha, beta-Alaha and Dha were calculated and turned out to be much lower than those of the corresponding iron(II) complexes. Stability constants of the iron(II)-hydroxamate complexes are compared with those of other divalent 3d-block metal ions and the Irving-Williams series of stabilities was found to be observed. Above pH 4, in the reactions between iron(II) and desferrioxamine B, the oxidation of the metal ion to iron(III) by the ligand was found. The overall reaction that resulted in the formation of the tris-hydroxamato complex [Fe(HDFB)]+ and monoamide derivative of DFB at pH 6 is: 2Fe2+ + 3H4DFB+ = 2[Fe(HDFB)]+ + H3DFB-monoamide+ + H2O + 4H+. Based on these results, the conclusion is that desferrioxamine B can uptake iron in iron(III) form under anaerobic conditions.

Some factors affecting metal ion-monohydroxamate interactions in aqueous solution.

The chelating properties exhibited by a series of monohydroxamic acids (propanohydroxamic acid (Pha), hexanohydroxamic acid (Hha), benzohydroxamic acid (Bha), N-methyl-acetohydroxamic acid (MAha), N-phenyl-acetohydroxamic acid (PhAha) and 2-hydroxypyridine-N-oxide (PYRha)) towards copper(II), nickel(II), zinc(II), calcium(II), magnesium(II) and aluminium(III) ions were studied by pH-metric, spectrophotometric and, in one case, by 27Al NMR methods. The results were compared with the corresponding data for metal ion-acetohydroxamate (Aha) and metal ion-desferrioxamine B (DFB) complexes. Changes of the substituents either on the carbon or on the nitrogen of the hydroxamate moiety caused a measurable effect on the chelate stability only in the case of aluminium(III) complexes. The aromatic derivative, PYRha, formed significantly more stable complexes than expected on the basis of the ligand basicity. The higher complex-forming ability of DFB compared to monohydroxamic acids diminishes in the case of the largest calcium(II) ion.