INTERACTION BETWEEN MO(VI) AND SIDEROPHORE MODELS IN AQUEOUS-SOLUTION

A series of dihydroxamic acids (HORNOC-(CH2)(n)-CONROH, where if R = H - then n = 2, 5-7 and if R = CH3- then n = 4, 5) and two new dihydrox amate-based siderophore models, hexanedioic acid bis (3-hydroxycarbamo yl-methyl)amide (DhaI) and hexanedioic acid bis(3-hydroxycarbamoyl-pro pyl)amide (DhaII) have been characterized in terms of chelating proper ties toward molybdenum(VI). For comparison, the molybdenum(VI)-acetohy droxamic acid (Aha) and molybdenum(VI)-aminohydroxamic acid systems ha ve also been studied. Potentiometric and spectrophotometric studies at ionic strength of 0.2 mol/dm(3) (KCl) and at 25 degrees C have been p erformed and the equilibrium constants have been determined. It has be en found, that of the dihydroxamic acids, only the DhaI and DhaII form water soluble complexes with molybdenum(VI). Polynuclear complexes mo st probably precipitate with the other dihydroxamic acids. Complexes a re formed up to ca. the neutral pH in all systems. Above this pH MoO42 - and the free ligands exist. Although, very stable complexes are form ed especially with DhaII, none of the studied ligands form a single bi s(hydroxamato)dioxomolybdenum(VI) species. Mo no(hydroxamato) trioxomo lybdenum(VI) species are also formed, containing the uncoordinated moi ety of the DhaI or DhaII in its protonated form. Out of aminohydroxami c acids, the beta-alaninehydroxamic acid (beta-Alaha) shows ''Aha-like '' coordination properties as the glutamic acid-gamma-hydroxamic acid (Glu-gamma-ha) does. The small differences with this latter ligand are possibly due to weak coordination of the carboxylate which makes the mono(hydroxamato)trioxomolybdenum(VI) species more stable and the unco ordinated carboxylates in bis(hydroxamato) dioxomolybdenum(VI) can pro tonate below pH 3. The tridentate coordination mode of aspartic acid-P -hydroxamic acid (Asp-P-ha) via the hydroxamate and carboxylate oxygen s results in the formation of a dinuclear complex, [Mo2O5(LH)(2)](2-) in addition to [MoO3(LH)](-) (the protons are on the amino groups) in the pi-I-range 2.5-7.0. (C) 1998 Elsevier Science Inc. All rights rese rved.