In nuclear waste treatment processes there is a need for improved ligands for the separation of actinides (An(III)) and lanthanides (Ln(III)). Several research groups are involved in the design and synthesis of new An(III) ligands and in the confinement of these and existing An(III) ligands onto molecular platforms giving multicoordinate ligands. The preorganization of ligands considerably improves the An(III) extraction properties, which are largely dependent on the solubility and rigidity of the platform. This tutorial review summarizes the most important An(III) ligands with emphasis on the preorganization strategy using (macrocyclic) platforms.

Multicoordinate ligands for actinide/lanthanide separations.

Tetrafunctionalized resorcinarene cavitands (4−6) have been synthesized starting from tetrakis(bromomethyl)cavitand 3. New cavitand-based cation ligands with (carbamoylmethyl)phosphonate (CMP) and -phosphine oxide (CMPO) moieties (8a,b and 9a,b) were prepared via two different routes in good (45−86%) overall yields. The ligands (8a,b and 9b) are very effective europium extractants. Ph2CMPON(Pr)-cavitand 8b has the highest extraction constant, determined with radiotracer experiments, for 1:1 complexation with Eu(picrate)3 (Kex1 = 2.7 × 1012 M-4). The steric preorganization of the four CMP(O) moieties on the resorcinarene cavitand improves the efficiency and selectivity [of Eu(III) over UO2(II) and Fe(III)] of the metal extraction processes, compared to simple CMP(O) extractants.

Novel Resorcinarene Cavitand-Based CMP(O) Cation Ligands: Synthesis and Extraction Properties.

This review discusses different types of artificial tripodal receptors for the selectiverecognition and sensing of cations and anions. Examples on the relationship between structure andselectivity towards cations and anions are described. Furthermore, their applications as potentiometricion sensing are emphasised, along with their potential applications in optical sensors or optodes.

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Tripodal Receptors for Cation and Anion Sensors

A new series of boron substituted cobalt bis(dicarbollide)(1−) ion (1) derivatives of the general formula [(8-CMPO-(CH2-CH2O)2-1,2-C2B9H10)(1′,2′-C2B9H11)-3,3′-Co]−
(CMPO = Ph2P(O)-CH2C(O)NR, R = C4H9
(3b), -C12H25
(4b), -CH2-C6H5
(5b)) was prepared by ring cleavage of the 8-dioxane-cobalt bis(dicarbollide)
(2) bi-polar compound by the respective primary amines and by subsequent reaction of the resulting amino derivatives (3a–5a) with the nitrophenyl ester of diphenylphosphorylacetic acid. The compounds were synthesized with the aim to develop a new class of more efficient extraction agents for liquid–liquid extraction of polyvalent cations, i.e. lanthanides and actinides, from high-level activity nuclear waste. All compounds were characterized by a combination of 11B NMR, 1H high field NMR, Mass Spectrometry with Electrospray and MALDI TOF ionisation, HPLC and other techniques. The molecular structure of the supramolecular Ln3+ complex of the anion 5b was determined by single crystal X-ray diffraction analysis. Crystallographic results proved that the Ln(III) atom is bonded to three functionalised cobalt bis(dicarbollide) anions in a charge compensated complex. The cation is tightly coordinated by six oxygen atoms of the CMPO terminal groups (two of each ligand) and by three water molecules completing the metal coordination number to 9. Atoms occupying the primary coordination sphere form a tri-capped trigonal prismatic arrangement. Very high liquid–liquid extraction efficiency of all anionic species was observed. Moreover, less polar toluene can be applied as an auxiliary solvent replacing the less environmentally friendly nitro- and chlorinated solvents used in the current dicarbollide liquid–liquid extraction process. The extraction coefficients are sufficiently high for possible technological applications.

Cobalt bis(dicarbollide) ions with covalently bonded CMPO groups as selective extraction agents for lanthanide and actinide cations from highly acidic nuclear waste solutions

A ligand system containing three preorganized carbamoylmethylphosphine oxide (CMPO) moieties anchored onto a rigid C(3)-symmetric triphenoxymethane platform has been developed for facile metal complexation and subsequent extraction from aqueous acidic nuclear waste streams. Intended to mimic the 3:1 CMPO-actinide stoichiometry of the extracted species in the TRUEX nuclear waste treatment process, the CMPO arms on this ligand are oriented such that all three CMPO moieties can cooperatively bind a metal ion. Extractions of simulated nuclear waste streams (10(-4) M metal in 1 M nitric acid) with solutions of this ligand in methylene chloride (10(-3) M) reveal a high affinity for the actinide thorium and a very low, but constant, affinity for the lanthanides across the series. Thorium and five lanthanide (lanthanum, cerium, neodymium, europium, and ytterbium) nitrate complexes of this ligand have been synthesized and fully characterized by X-ray crystallography, (1)H and (31)P NMR spectra, and FT-ICR-MS to elucidate the mechanism of this unique actinide selectivity. All six oxygen donors from the three CMPO arms of the ligand and one or two nitrate counterions coordinate these metals to afford 2+ cationic complexes in every case. Because of the large size of the ligand, both the thorium and lanthanide complexes present similarly charged and sized surfaces to the extraction solvents, but the thorium complex is extracted quantitatively over the lanthanide complexes. A possible rationale for this extraction behavior difference is presented and further illustrated by the extraction properties of this ligand system for the alkali metals (lithium, sodium, potassium, rubidium, and cesium) as picrate salts and by the solid- and solution-state structures of its lithium picrate complex.

Enhanced Selectivity for Actinides over Lanthanides with CMPO Ligands Secured to a C3-Symmetric Triphenoxymethane Platform

Various calix[4]arene tetraalkyl ethers and a calix[5]arene pentaalkyl ether substituted at the upper rim by the CMPO-like functional group –NH–C(O)–CH2–P(O)Ph2 as well as some linear model compounds have been synthesized for the first time. Their ability to extract europium, thorium, neptunium, plutonium and americium from 1 mol dm–3 aqueous HNO3 into methylene chloride or o-nitrophenyl hexyl ether was studied under various conditions. All the new oligomeric ligands, but especially the calixarene derivatives, are tremendously strong extractants compared with the commonly used CMPO [(N,N-diisobutylcarbamoylmethyl)octylphenylphosphine oxide]. Also in transport studies through supported liquid membranes their efficiency is higher than that of CMPO which shows lower transport rates even at a 10-fold concentration.

Olivier Mauprivez

Papers

Calixarenes with diphenylphosphoryl acetamide functions at the upper rim. A new class of highly efficient extractants for lanthanides and actinides