A comparison between the chelating properties of some dihydroxamic acids, desferrioxamine B and acetohydroxamic acid

The complexes of hexanedioic acid bis( 3-hydroxycarbamoyl-methyl)amide (Dha1) and hexanedioic acid bis(3-hydroxycarbamoylpropyl)amide (Dha2) with cobalt(II), nickel(II), copper(II), zinc(II), iron(III), calcium(II) and magnesium(II) have been studied by pH-metric and spectrophotometric methods. All the complexes formed with Dha2 are soluble in water, but a very insoluble complex is formed in the copper(ll)-Dha1 system. Besides the 1:I species complexes with 2:3 stoichiometry are also formed in the cobalt(II)-, nickel(ll)-, zinc(II)- and iron(III)-containing systems. Dha2 generally forms more stable complexes than those of Dha1 (e.g. log beta values for their iron(IU) I:I complexes are 17.9 and 19.1, respectively). A comparison of the data with those on the complexes of a simple monohydroxamate, (acetohydroxamate, Aha), nonano-dihydroxamate (Dha3) and the natural trihydroxamate-based siderophore, desferrioxamine B (DFB) revealed that the stability sequence of the complexes is generally: DFB>Dha2 greater than or equal to Dha3 similar to Dha1>Aha. The shorter but more flexible connecting chain of Dha3 results in the ca. same stability of complexes of Dha1 and Dha3. The above sequence, however, did not hold for copper(II) allowing the coordination of at most two hydroxamates and for calcium(II). In this tatter case, Dha2, containing the longest connecting chain, formed the most stable complexes. (C) 1999 Elsevier Science Ltd. All rights reserved.