Actinide hydroxamates

Hydroxamate. Similar to trivalent hydroxamates, pentavalent actinide hydroxamate complexes are generally unstable relative to tetravalent and/or hexavalent complexes. Pu or Pu hydroxamate complexes can be prepared however, at near-neutral and basic pH they rapidly reduce to Pu complexes. ... 

As with the actinide catecholates, we are interested in determining the optimum structure of actinide hydroxamates for use in the design of an octadentate actinide sequestering agent. 

Most of the naturally occurring chelating agents are substituted hydroxamates which are produced by a variety of protista so that iron(III) subsequently becomes available for biochemical processes. Neilands (73) has suggested that the hydroxamates facilitate the transport of iron across cell membranes. The distribution of hydroxamates in the biosphere appears limited. However, if there was a wider distribution of hydroxamates in the environment then the management of actinide wastes could become a problem of horrifying dimensions if these chelators facilitated the transport of actinides across cell membranes. 

Hydroxamates have been observed in the water in the Bay of Quinte, a eutrophic bay of Lake Ontario, and are believed to be produced by blue-green algae (76). Simpson and Neilands (77) have identified schizokinen, a hydroxamic acid derivative of citric acid as an extracellular product of the blue-green algae, Anabaena sp. However, not all Anabaena produce hydroxamates, Walsby (78) has shown that Anabaena cylindrica releases a large pigmented, peptide-containing material which complexes iron. As yet these peptides have not been examined for ability to complex the actinides. 

It is significant that oat plants, which are known to contain the Fe3+ complexor, 2,4-dihydroxy-7-methoxy-l,4-benzoxazin-3-one (128), do not show any significant accumulation of plutonium, or the other actinides. It is possible that this complexing agent is located within plant cells which do not come into contact with the cation transporting mechanisms. Although there is evidence of the existence of microbial hy-droxamates in soil and that hydroxamates do become concentrated in plants (129), there has been no evidence presented yet that hydroxamates are the agents responsible for plutonium uptake into plants. On the other hand there is evidence that EDTA and DTPA can stimulate actinide concentration in plants (See Table 6). 

The known complexes, MIVL4 (Table 43), precipitate from aqueous solutions of actinide(IV) compounds on addition of the ligand. Somewhat surprisingly, one hydroxamic add, (PhCO)NHOH, behaves as a neutral ligand, and the complex Th(N03)4-2(PhC0)NH0H... 

Table 43 Actinide(IV) Complexes with Hydroxamates, Cupferron and Related ligands ...

TBP is a sufficiently powerful extractant for actinides that it may be used in diluted form. Dilution improves the hydrodynamic properties of the solvent, allowing more complete and rapid phase disengagement. Typically concentrations of 20-30 v/o TBP in OK are used in process flowsheets. Although TBP is relatively stable as an extractant, radiolysis does lead to the formation of some acidic phosphate esters, HDBP and H2MBP, which can impair process performance.289 An aqueous alkali wash of the recycled solvent is usually carried out to remove those by products. Radiolytic degradation of the diluent in the presence of nitric acid can result in the formation of hydroxamic acids,290 which can lead to fission product retention by the organic phase. Again the solvent wash is used to prevent the accumulation of such species. A comprehensive account of the industrial utilization of TBP has recently been published.291... 

Hydroxamates and their monothio and dithio analogues form chelate complexes usually. They have been much studied because hydroxamates are present in sidero-phores, the microbial iron transport systems (Section 17-E-10) and also because of their use in sequestering agents for actinide ions and in analytical studies. 

Hydroxamate. Hydroxamate complexes of trivalent actinides can be prepared directly in aqueous solution and other polar solvents and extracted into organic solvents, but due to the high thermodynamic stability of the corresponding tetravalent actinide complexes they are rapidly oxidized. They can also be prepared in solution via electrochemical reduction of the tetravalent complexes. These complexes have been studied for their role in separating high and low valent actinides in nuclear fuel processing schemes. ... 

Hydroxamates, cupferron, and related ligands. As anionic oxygen donor ligands, hydroxamates have a strong affinity for the oxophilic tetravalent actinides, with solution complex formation constants generally greatest for Pu and decreasing as follows Pu > Np > > U. ... 

A significant effort has been made to prepare ligands with high specific affinity and selectivity for actinides that could be used for mammalian chelation therapy or as a specific extractant. A biomimetic approach to such ligand design, based on naturally occurring hydroxamate and catecholate siderophores and hydroxypyridinoate moieties, has been the most vital. The actinide... 

Table 4. Formation constants for some actinide (IV) hydroxamates and catecholates...

Table II. Formation Constants for Some Actinide(IV) Hydroxamates ...

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