Hydroxamic acids stability constants

A large volume of work64 has been published on the determination of stability constants for complexes of hydroxamic acids, e.g. acetohydroxamic acid.65 The stability of 3d transition metal ions (Mn2+ to Zn2+) with salicylhydroxamic and 5-methyl-, 5-chloro-, 5-bromo-, 5-nitro-, 4-chloro-, 4-bromo- and 3-chloro-salicylhydroxamic acids,66 as well as with methyltolylbenzohydroxamic acid,67 has been studied potentiometrically. Stability constants of iron(III) with a number of hydroxamic acids have been determined by redox potential studies.68... 

Narula and Gupta76 have measured the thermodynamic stepwise stability constants for the formation of complexes of dialkyltin(TV) ions, R2Sn2+ (R = Me or Bu ), with hydroxamic acids, RCONHOH (R = Ph, Me2CH, Me) potentiometrically in 75% dioxane-water media. This has been followed by determination of formation constant of Af-arylhydroxamic acids-dialkyltin(IV) ions.77... 

A remarkable property of the hydroxamates is the pronounced specificity for ferric iron and even very simple model compounds like acethy-droxamic acid have stability constants approaching 1030 (Table 1). Ferrous iron is bound only relatively weakly and this discrepancy in the avidity for the two oxidation states is powerful evidence that the ferric hydroxamates do not function biologically, as do the hemes, by alternate oxidation and reduction. On the other hand these properties provide a simple means of pick up, transfer and release of iron at the point of demand in living cells. 

Stability constants for the complexation of Cd by edta (H L) to yield [Cd(HL)] and [CdL] have been determined, as well as data on the complexation of zinc with propylenediaminetetra-acetic acid, zinc and cadmium with trans-1,2-cyclohexanediaminetetra-acetic acid, ° and mercury with diamino derivatives of succinic and malonic acids. Substituted hydroxamic acids have also been used as chelates in the complexation of zinc and cadmium. ... 

Research in the chelation of iron is sponsored by various Cooley s Anemia Fmmdations, the National Institute of Arthritis, Diabetes, and Digestive, and Kidney Diseases of the National Institutes of Health. Much of effort has been directly toward mimicking the naturally occurring siderophores such as desferrioxamine by inclusion of hydroxamic acids, catechols, and phenols into a variety of structures. This gives compoimds that have vary high stability constants for iron but are nontoxic and active in the human body. 

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