Interaction between Mo(VI) and siderophore models in aqueous solution

Abstract

A series of dihydroxamic acids (HORNOC-(CH 2) n -CONROH, where if R=H– then n=2,5–7 and if R=CH 3– then n=4,5) and two new dihydroxamate-based siderophore models, hexanedioic acid bis (3-hydroxycarbamoyl-methyl)amide (DhaI) and hexanedioic acid bis(3-hydroxycarbamoyl-propyl)amide (DhaII) have been characterized in terms of chelating properties toward molybdenum(VI). For comparison, the molybdenum(VI)–acetohydroxamic acid (Aha) and molybdenum(VI)–aminohydroxamic acid systems have also been studied. Potentiometric and spectrophotometric studies at ionic strength of 0.2 mol/dm 3 (KCl) and at 25 ∘C have been performed and the equilibrium constants have been determined. It has been found, that of the dihydroxamic acids, only the DhaI and DhaII form water soluble complexes with molybdenum(VI). Polynuclear complexes most probably precipitate with the other dihydroxamic acids. Complexes are formed up to ca. the neutral pH in all systems. Above this pH MoO 4 2− and the free ligands exist. Although, very stable complexes are formed especially with DhaII, none of the studied ligands form a single bis(hydroxamato)dioxomolybdenum(VI) species. Mono(hydroxamato)trioxomolybdenum(VI) species are also formed, containing the uncoordinated moiety of the DhaI or DhaII in its protonated form. Out of aminohydroxamic acids, the β-alaninehydroxamic acid ( β-Alaha) shows “Aha-like” coordination properties as the glutamic acid- γ-hydroxamic acid (Glu- γ-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 uncoordinated carboxylates in bis(hydroxamato) dioxomolybdenum(VI) can protonate below pH 3. The tridentate coordination mode of aspartic acid- β-hydroxamic acid (Asp- β-ha) via the hydroxamate and carboxylate oxygens results in the formation of a dinuclear complex, [Mo 2O 5(LH) 2] 2− in addition to [MoO 3(LH)] − (the protons are on the amino groups) in the pH-range 2.5–7.0.