Hydroxamato ligands

Naturally occurring and synthetically accessible siderophores (iron carriers) contain predominantly bidentate pyrocatechinato- or hydroxamato ligands and are of special interest because of their high affinity towards trivalent metal ions, especially towards iron(III) ions [5, 118-127]. The methyl ( )-2-(l-alkyl/aryl-4,5-dihydro-l//-tetrazol-5-ylidene)-2-cyanoacetates 41a-c, first prepared by us [128], also appeared to be suitable as siderophores. 

A multiple-path mechanism has been elaborated for dissociation of the mono- and binuclear tris(hydroxamato)-iron(III) complexes with dihydroxamate ligands in aqueous solution. " Iron removal by edta from mono-, bi-, and trinuclear complexes with model desferrioxamine-related siderophores containing one, two, or three tris-hydroxamate units generally follows first-order kinetics though biphasic kinetics were reported for iron removal from one of the binuclear complexes. The kinetic results were interpreted in terms of discrete intrastrand ferrioxamine-type structures for the di-iron and tri-iron complexes of (288). " Reactivities for dissociation, by dissociative activation mechanisms, of a selection of bidentate and hexadentate hydroxamates have been compared with those of oxinates and salicylates. ... 

Most substitutions at iron(III) are fast, and are therefore discussed elsewhere in this report (see Chapter 9), but several are slow enough to monitor by conventional techniques and are therefore mentioned here (though pentacyanoferrate(III) complexes are in Section 8.3.1). The first system bridges this and the preceding sections, for it involves relatively slow fac mer isomerization for tris-hydroxamato complexes of iron(II) and of iron(III). These complexes containing ligand (36) have been known for some time, but isomer details have only been sorted out in the course of the present kinetic study. Kinetics of formation of several iron(III)-hydroxamate complexes have also been reported. ... 

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