Hydroxamates, ligands

Another factor that relates complex stability and siderophore architecture is the chelate effect. The chelate effect is represented by an increase in complex stability for a multidentate ligand when compared to complexes with homologous donor atoms of lower denticity. The effect can be observed when comparing the stability of complexes of mono-hydroxamate ligands to their tris-hydroxamate analogs, such as ferrichrome (6) or desferrioxamine B (4). However, the increase in stability alone is not sufficient to explain the preponderance of hexadentate siderophores over tetradentate or bidentate siderophores in nature, and the chelate effect is not observed to a great extent in some siderophore structures (10,22,50,51). 

Bodenant B, Fages F, Delville MH (1998) Metal-induced self-assembly of a pyrene-tethered hydroxamate ligand for the generation of multichromophoric supramolecular systems. The pyrene excimer as switch for iron(III)-driven intramolecular fluorescence quenching. J Am Chem Soc 120 7511-7519... 

The high specificity of siderophore iron coordination has been extensively explored in iron-chelation therapy for various medical applications, including iron overload diseases, control of iron in specific brain tissues , arresting the growth and proliferation of malaria parasite within their host , as well as arresting the proliferation of cancer cells . Other directions for metal ligation involve enzyme inhibition, which have been demonstrated by the inhibition of urease by coordination of hydroxamate ligand to nickel ions and zinc coordination in matrix metalloprotease (MMP) inhibition by primary hydroxamates. ... 

Such a stabilization of the palladium catalyst can also be achieved in homogeneous liquid phase by the use of appropriate ligands. Thus, it has recently been shown that palladium(II) hydroxamates are effective catalysts for the acetoxylation of ethylene with high selectivity and a high turnover (>200) (equation (162), whereas Pd(OAc)2 rapidly becomes deactivated and precipitates in the form of metallic palladium.419 It is probable that the bidentate hydroxamate ligand stabilizes the hydride Pd—H species and prevents palladium from precipitating. 

Reaction of Os(NO)2(PPh3)2 with trifluoroacetic acid gives Os(NO)(ON= C(0)CF3)(0C0CF3)(PPh3) (Scheme 7), in which both coordinated trifluoroacetate and 0,0-tri-fluoroacetato-hydroxamate ligands are present (see p. 548 for X-ray structure).224-229 There are also two metal-metal-bonded species for which X-ray crystal structures have been determined (p. 548) ... 

Siderophores,hexadentate tris-catecholate and tris-hydroxamate ligands, are produced by many microorganisms to facihtate iron uptake. Several synthetic models, such as trencam (35) and licams (36), for the tris-catecholate enterobactin (37) do not quite match its affinity for iron(ni) (Table 8). Similar tris-hydroxamate ligands model naturally occurring ferrichromes and desferrioxamines (e g. dfo, (38)). 

Table IX. Second-order rate constants for hydroxamate/hydroxamate ligand exchange in siderophore...

As an alternative to Ti-based systems for allylic oxidation, the groups of Bregeault [81] and Yamamoto [178] have reported V-based catalysts. Bregeault uses a O=V(OC3H7)3 catalyst which achieves high TOP. Yamamoto employs V catalysts with chiral Zzz s-hydroxamic acid (foz s-hydroxam) ligands of C2 symmetry which allow use of lower catalyst concentrations and the use of aqueous TBHP rather than the use of anhydrous reagent (Table 1.7). 

Fig. 16. Catechol and hydroxamate ligand binding to ferric iron...

The asymmetry of the hydroxamate ligand leads to the possibility of the isolation of geometrical (and optical) isomers in the tris complexes. However, for high-spin Fe" , expected to be kinetically labile, all structures so far determined have the cis configuration. Thus, for example, the most stable crystalline form of tris(benzohydroxamato)iron(III) has the cis configuration, as have also ferri-chrome and ferric A, iV, iV"-triacetylfusarinine. However, for the inert Cr" complexes, on... 

Figure 28 is-bidentate hydroxamate ligands lie on the edges of a [Ga4Ls] tetrahedron. (View down the... 

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