Chelating properties exhibited by a series of monohydroxamic acids toward copper(II) and iron(III) ions were studied by pH-metric, spectrophotometric and EPR methods. The ligands can be divided into three groups: (i) ligands with alkyl substituents either on the hydroxamate carbon atom (acetohydroxamic acid, Aha; propanohydroxamic acid, Pha; and hexanohydroxamic acid, Hha) or on both the carbon and nitrogen atoms (N-methyl-acetohydroxamic acid, MAha; N-isopropyl-acetohydroxamic acid, iPAha) (ii) ligands with aryl substituents (benzohydroxamic acid, Bha; N-phenyl-acetohydroxamic acid, PhAha; and N-phenyl-benzohydroxamic acid, PhBha); (iii) cyclic derivatives (the natural 2,4-dihydroxy-2H-1,4-benzoxazin-3-(4H)-on-glucoside, DIBOA-gl; and 2-hydroxypyridine-N-oxide,PYRha). In addition to the complexes with the well-known hydroxamate type chelate(s), 1:2 species containing one or both of the coordinated ligands in hydroximato (RC-CONO2-) form, have been found in the copper(II)-Aha and copper(II)-Bha systems. Complex formation with iron(III) starts at a very acidic pH and in the most systems, if the ligand excess is high enough, the 1:3 species solely exists in the pH range ca. 4-8. Hydroxo complexes are generally formed above pH 8-8.5. However, in the cases of iron(III)-iPAha or -Hha, where the ligands have quite large bulky groups, the hydrolysis starts at somewhat lower pH if the metal to ligand ratios are below 1:5 and precipitation starts to form in iron(III)-DIBOA-gl system at ca. pH 5-5.5. In all systems, the stability constants were determined only for the complexes formed below hydrolytic regions. Evaluation of calculated stability constants show that they are determined by a combination of different substituent effects (electronic, resonance and steric effects). The most significant effects are due to substituents on the nitrogen atom in the hydroxamate moieties. The phenyl ring on carbon atom results in somewhat higher stabilities of the complexes.
- Hydroxamic acids
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry
- Materials Chemistry