Actinide crown-complexes

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." ... 

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. 

Distribution ratios and transport were carried out on real HAW arising from dissolution of a mixed oxide of uranium and plutonium (MOX) fuel (burnup 34,650 MW d/tU), where uranium and plutonium have been previously extracted by TBP.86 The experiments were performed in the CARMEN hot cell of CEA Fontenay aux Roses with two dialkoxy-calix[4]arene-crown-6 derivatives (diisopropoxy and dini-trophenyl-octyloxy). High cesium distribution ratios were obtained (higher than 50) by contacting the HAW solution with diisopropoxy calix[4]arene-crown-6 (0.1 M in NPHE). Moreover, the high selectivity observed with the simulated waste was confirmed for most of the elements and radionuclides (actinides or fission products Eu, Sb, Ce, Mo, Zr, and Nd). The residual concentration or activity of elements, other than cesium, was less than 1% in the stripping solution, except for iron (2%) and ruthenium (8%) the extraction of these two cations, probably under a complexed... 

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... 

Recently, the first U(V) heteroleptic cyclo-octatetraene-dithiolene complex 992 was synthesized by AgBPh4 oxidation in THF of the [(COT)UIV(dddt)2][Na(18-crown-6)]2 precursor, which was synthesized by reacting [(COT)UX2(THF)J (X = BH4, n = 0 X = I, n = 2) with Na2dddt [490]. In general, despite the relatively small number of reported actinide S-containing complexes, they have a potentially rich coordination chemistry of such compounds of hard /-elements with soft S-ligands [489] and possible applications [491]. 

Hexavalent. Nitrate complexation with actinide ions is very weak, and the determination of the formation constants for aqueous nitrate solution species is extremely difficult. Under aqueous conditions with high nitric acid concentrations, complexes of the form An02(N03)(H20)x+, An02(N03)2(H20)2, and An02(N03)3 (An = U, Np, Pu) are likely to be present. Solids of the anionic trisnitrato complex have been isolated for U and Np however, minimal structural data have been obtained. Solid uranyl nitrate, U02(N03)2-xH20, is obtained as the orthorhombic hexahydrate from dilute nitric acid solutions and as the trihydrate from concentrated acid. The Np analog can be precipitated from a mixed aqneons HNO3 and MeCN solntion by the addition of 18-crown-6. Multiple structural determinations have been made for the hexavalent uranium nitrate complexes, and all show the common formula unit of... 

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. 

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