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 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 degrees 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 MoO42- 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. Mo no(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 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 uncoordinated carboxylates in bis(hydroxamato) dioxomolybdenum(VI)
can protonate below pH 3. The tridentate coordination mode of aspartic acid-P-hydroxamic
acid (Asp-P-ha) via the hydroxamate and carboxylate oxygens 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.
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