Lanthanide complexes aqueous media

Chitry, F. et al., Lanthanides(ttt)/actinides(III) separation by nano-filtration-complexation in aqueous medium, in Proceedings of International Conference, Nuclear waste From Research to Industrial Maturity, Montpellier, Oct. 2-4, 2000. 

In an aqueous medium, cations are solvated by some number of water molecules, the number of water molecules being determined primarily by the charge and size of the cation. The size of the solvent sheath carried by the cation and its complexes is clearly of significance in predicting the relative mobility of lanthanide ions as they traverse an analytical column, as transport properties are proportional to the fit of the analyte into the normal solvent structure. In the following paragraphs, we will explore a few of the more interesting aspects of these phenomena. 

Furthermore, lanthanides form stable complexes with polydentate chelators like DTPA, which exhibit a noncyclic structure. Two structures are depicted in Scheme 2. The following examples are only representatives for the variety of analyte molecules that can be determined by these kind of lanthanide complexes. Structure 9 employs a quinolyl ligand both as chromophore and acceptor for Zn ". The emission of the europium ion is strongly enhanced upon binding of Zn " and showing distinct selectivity over other biologically relevant metal cations in aqueous solution at neutral pH [29]. The luminescence of the chelate 10 is efficiently quenched by Cu " ions in aqueous medium [30]. The presence of Fe ", Co ", Ni ", Cr ", and Mn " interferes with the determination of Cu, although to a relatively small extent, whereas the ions Zn ", Cd ", Hg, and Pb do not interact with probe 10. 

Cation exchange resins in general resemble the sulfonic acid extractants most closely. Separation factors for cation exchange separations of the trivalent cations are extremely small. Successful lanthanide/actinide separation using cation exchange resins depend primarily on the use of aqueous complexants and/or alteration of the aqueous medium for the separation. 

The potential of diluent modification in enhancing lanthanide/actinide separation factors has barely been tapped. The absence of a reliable predictive theory of solvation of both the extractant molecules and the extracted complexes hampers the development of this area. In view of the small energies required to reverse extraction order (a few hundred joules), subtle alteration of the organic diluent (or the aqueous medium in ion exchange procedures) also has the potential for significantly improving (at least) group separations. 

The inorganic lanthanide triflate complexes Ln(OTf)3 (made in aqueous solution) have been shown by Kobayashi to be efficient Lewis-acid catalysts for hydroxy-methylation (using commercial aqueous formaldehyde solutions) of silicon enolates in aqueous medium (water -i- THF) or even in water alone in the presence of a surfactant. In these reactions, activation proceeds by coordination of the aldehyde oxygen atom by the Ln center that is a strong Lewis acid due to its hard character. Among the lanthanide triflates, ytterbium triflate was found to be the most active catalyst, but scandium triflate can sometimes also be efficiently used. Enantio-selective versions are also known in the presence of chiral macrocyclic ligands. The water-soluble catalyst is recovered in water after extraction of the organic products. 

Extraction data of alkali metals with the picrate method have shown that pyridinyl compounds are better ionophores than their A-oxide counterparts (deleterious role of hydrogen bonding between A-oxide functionalities and water molecules). The highest phase-transfer values are observed for cone conformers, where selectivity follows the order Na" " > K" " > Rb" " > Cs" " > Li. In aprotic solvents A-oxide ligands are much stronger complexers, and alkali and lanthanide metal complexes have been prepared and characterized. The Eu(III) and Tb(III) complexes of tetra-A-oxide cone conformer are fluorescent upon UV light excitation at 312 nm. Unfortunately, these complexes totally lose their luminescence upon addition of water, indicating their modest stability in aqueous medium [56,57]. 

The cellulose phosphate was widely applied in the removal process of divalent and trivalent (Cu ", Zn ", Co, Ni, Pb ", Mn ", Fe, Fe , Cr ) ions because of the rapid adsorption of the metals ions in comparison with the synthetic polymers. Also it was observed that the cellulose phosphate functions as an ion exchanger for lanthanide ion removal from aqueous solution. In order to evaluate the adsorbent characteristics of each support in the removal process of metal ions from aqueous solution, the most widely used method by researchers has been the batchwise one. In any adsorption process the most important parameter is the pH of the medium, which depends on the nature of the support used and also on which metal ions are to be removed. In most cases, researchers worked with solutions that have an acid pH value in order to avoid the possible precipitation of the studied metal ions. A complex study of the pH influence upon the efficiency of Fe ", Cu ", Mn, Zn, Co ", and lanthanide(III) ions removal from aqueous solutions with PBC was made by Oshima et al. Their conclusion was that the lanthanide ions are adsorbed when the pH value of the aqueous solution is less than 3, and in the case of transition metal ions the adsorption percentage increases with increasing aqueous pH and reaches over 90% at a pH value of around 4.5. 

An additional phenomenon related to the anion in the aqueous phase is a so-called perchlorate effect (Gmelin 1983). It has been often observed that extraction of metal ions from perchlorate media is greater than that from equivalent nitrate or chloride solutions. Marcus reports (Gmelin 1983) the effect in two different systems involving acidic extractant molecules. The enhanced extraction of metal ions is also observed in systems based on neutral molecules. In the latter case, formation of aqueous complexes of stoichiometry 1 3 (R L) is required for neutrality in the extracted complex. As lanthanide perchlorate ion pairs normally do not exist in aqueous solution, it is something of a dichotomy that extraction from perchlorate medium should be more readily achieved than from more strongly complexing nitrate (or even chloride) solutions. An explanation for the phenomenon, and for the relative ease of extraction of metal ions from salt solutions (relative to that from equivalent acids), may lie in the effect of the solutes on water structure. 

The interaction between bromide and lanthanide ions was also studied, but experimental data are available for aqueous methanol solutions only (Kozachenko et al. 1973). Using a spectrophotometric method, the formation of rather weak outer-sphere bromo complexes was evidenced, and their stability constants for Pr, Nd, Sm, and Ho were determined in water and in 50% and 90% methanol (table 3). For solutions in 50% methanol, the stability of the outer-sphere bromo complexes is larger for Pr, Nd, and Sm Ki = 1.3—1.9) compared to Ho (0.97) and Er (0.70). Kozachenko et al. (1973) explained this behaviour as reflecting a higher stability for the solvates of the heavier lanthanide ions. A similar trend was observed in the stability constants of the chloro complexes in absolute methanol vide supra). Finally, the stability of the bromo complexes of the lanthanides increases as the dielectric constant of the medium is reduced. 

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