Actinide complexes crown ethers

Table 48 Some Complexes of Actinide(IV) Compounds with Crown Ethers and Cryptands...

In the second project, gathering nine teams from six EC countries, more than 150 new extractants were prepared and studied, and the target was reached for the decategorization of waste. Dialkoxy calix[4]arene-crown-6 for cesium, octaamide calix[8] arenes, and CMPO-like calixarenes for actinides display much higher complexing and extracting abilities than other classical extractants, crown ethers, or dicarbollides proposed and sometimes used for this purpose. 

In the early 1990s, there existed several classes of extractants for actinides (CMPO), for cesium and more generally alkali cations, and for strontium and alkaline earth cations (crown ethers and cosan). The combination of these extractants and the grafting of these functions on calixarene platforms have led to new classes of extremely efficient and selective extractants, in particular calixarene-crown, which are presently applied in the United States to treat the huge amounts of waste at the SRS. Calixarenes/ CMPO, crown ethers/cosan, CMPO/cosan, and more recently calixarenes/CMPO/ cosan are promising compounds. It is desirable that these studies, conducted at the international level, continue in particular to obtain a better understanding of the complex mechanisms of extraction of these compounds.127187... 

Membrane-based separation methods are popular in chemical industry for effluent treatment, desalination, and gas purification. However, their application for separation of actinides relevant to nuclear industry is still at infancy stage. The past few decades saw the use of exotic reagents such as crown ethers, calrxarenes for the selective complexation and removal of actinides... 

As in the presence of aqua complexes, the presence of crown ethers facilitates the crystallization of actinide halide solvate complexes in this manner ThCl4(EtOH)3(H20) 18-crown-6 and ThCl4(MeOH)2(H20)2 15-crown-5 have been isolated. " ... 

Crystallization of complex halides from solution containing crown ethers results not in the complexation of the actinide by the ether oxygen atoms, but rather in the isolation of [AnXg] " salts in which the crown appears to act as a crystallization aid. " ... 

In many cases, synergists are added to HTTA extraction systems to enhance the separation of actinide ions. One example is the addition of the crown ethers (CE) dibenzo-18-crown-6, dicyclo-hexyl-18-crown-6, dibenzyl 24-crown-8, and benzyl-15-crown-5. These crown ethers have been shown to synergistically enhance extraction into benzene and the increase follows Eu > 1102 " > Th". The extraction equilibrium for crown ether/HTTA systems for the separation of Th is shown in Equation (23). The binding of the crown ether in the extracted complex seems to be a function of crown ether basicity and steric effects2 ... 

The extraction of trivalent actinides with /3-diketones in the presence of crown ethers has also been investigated. Trifluoroacetylacetone(HTFA), HTTA, benzoyltrifluoroacetone(HBFy, and 2-napthoyltrifluoroacetone (HNFA) have all been used with 18-crown-6 to remove Am and Cm from perchlorate media into 1,2-dichloroethane. It has been shown that the mechanism of extraction for all of the /3-diketones (HA) is the formation of a cationic complex via a synergistic ion-pair extraction (SIPE) as shown in Equation (66) ... 

M. The results suggest that the crown ethers are not complexing the actinide ions, nor are they responsible for any observed increase in D value other than the effect they have on system composition. The distribution ratios are better correlated with thermodynamic hydration parameters. 

Liquid clathrates as crystallization media have resulted in many new structures. Crystals of crown ether, cryptand, and tetramethylethylenediammonium and triphenylphosphonium complexes were prepared. Presented in Table 3 is a short list of some oxonium ion crown-ether crystal complexes which deposited from the liquid clathrate mixture. Oxonium ion complexes with crown ethers from a liquid clathrate media were formed of a suitable size and shape to crystallize lanthanides and actinides. 

Hassaballa. H. Steed. J.W. Junk. P.C. Elsegood, M.R.J. Formation of lanthanide and actinide oxonium ion complexes with crown ethers from a liquid clathrate medium. Inorg. Chern. 1998. 37. 4666-4671. 

Typical first coordination sphere hydration numbers for the lanthanides and actinides are 8-10 (Nash and Sullivan 1991), 9 is selected here for illustrative purposes only]. For the extraction of the metal ion by solvating extractants (requiring neutral or anionic complexes), dehydration in reaction (2) must be near complete. This reaction sequence is also relevant for chelating extractants which exhibit significant miscibility in the aqueous phase (e.g., acac and various crown ethers). For ligands which form more hydrophilic complexes with the metal ions, complete dehydration is not observed, and the complex-formation reaction effectively opposes phase transfer. This characteristic is often exploited in lanthanide/actinide separation. 

Gas-phase binding energies calculated for the penta aquo complexes and crown ether complexes of the actinides studied show that there is no intrinsic preference, or better fit, for actinyl(V) cations as compared to actinyl(Vt) ones. Rather, the ability of Np02" (Np-V) to form in-cavity 18-crown-6 complexes in water is traced to solvation effects in polar solvents. Thus, the effective screening of the charge provided by the macrocycle leads to destabilization of the An(VI) crown complexes relative to their An(V) counterparts." ... 

The replacement of the oxygen atoms present in classic Pederson-type crown ethers by S atoms results in the formation of thia-crown hgands. These latter hgands are characterized by a tendency to form complexes with softer ions, including those of the transition, lanthanide, and actinide series. This stands in contrast to what is true for oxygen-based crown ethers, for which a preference for hard cations, such as alkali metal ions, is well established. One of the earliest examples of ISEs based on a thia-crown ether was reported... 

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