Lanthanide EDTA complexes

As with (105), ligand (106, dota) also forms strong complexes with a range of both non-transition and transition metal ions (Stetter Frank, 1976 Delgado da Silva, 1982 Spirlet, Rebizant, Desreux Loncin, 1984) which are often more stable than the corresponding edta complexes. In particular, the calcium complex shows extremely high stability and very stable complexes are also formed with the trivalent lanthanides (Desreux, 1980 Spirlet, Rebizant, Desreux Loncin, 1984). 

EDTA complexes of trivalent metals can be extracted successively with liquid anion exchangers such as Aliquat 336-S by careful pH control. Mixtures of lanthanides can be separated by exploiting differences in their EDTA complex formation constants. 

Averaged values for the separation factors for adjacent pairs of lanthanides eluted with EDTA and its homologues are given in Table 1.21. The separation factors are in the range of 1.5 to 3.5 showing considerable improvement in separation factors. In the majority of cases, the separation factors with EDTA as an eluant are greater than the values obtained in methanol-nitric acid medium, which is expected based on the stability constant data for rare earth EDTA complexes. 

Lanthanide-EDTA and lanthanide-NTA complexes, where Ln = Nd, Ho, Er at various Ln L ratios in a broad pH range were studied by spectroscopy. Analysis of the data on oscillator strengths and 7 parameters provided some clues on the nature of bonding in these complexes. Further 7) and T(> parameters showed a dependence on the ligand concentration. However, these studies could identify only one species [Nd(NTA)2]3 in solution [260]. 

S.A. Cotton, Lanthanides and Actinides, Macmillan, 1991, p. 64 (structure of La EDTA complex). 

Improvements in modes of complexing by Marsh (1950), who separated lanthanides by fractional crystallisation of the ethylenediaminetetra-acetic acid (EDTA) complexes, have been coupled with the ion exchange technique by Vickery (1952). EDTA proved an even more satisfactory complexing agent than citric acid and gave purer specimens than any other method. 

EDTA has also found application in the cation-exchange separations. Erom an EDTA solution at pH 2.1, Dowex 50 cation-exchanger retains the REE elements, whereas Th is eluted as the more stable EDTA complex. The lighter lanthanides have been separated by eluting them from a cation-exchanger with EDTA or NTA solution. 

The relative ability of the transition metal ions to form complex ions is Mn2+ < Fe2+ < Co2+ < Ni2+ < Cu2+ > Zn2+ for the divalent cations and Cr3+ — Mn3+ > Fe3+ < Co3+ for the trivalent cations. The strongest complexing divalent cation is Cu(II). Fe(III) is the weakest complexing trivalent transition metal ion, but is stronger than other trivalent cations such as Al3+ and the lanthanides. The heats of hydration (Table 3.2), strengths of EDTA complexes (Table 3.5), and solubility products of metal hydroxyoxides (Table 3.3) also follow this general order, with water, EDTA, and OH" as the respective ligands. Stability constants less than I09 indicate the weaker ion-ion interaction of ion pairs. 

In view of the structural and functional relationship between amino acids and polyaminopolycarboxylic acids, the coordination of some lanthanide ions (La, Pr, Er) with ethylenediaminetetraacetate (EDTA) has also been examined under much higher pH conditions. The formation of lanthanide-hydroxo complexes formulated as K2R(0H)EDTA 4H20 was reported as early as in 1955 (Djordjevic and Vuletic, 1980), but it is not until recent studies that the first crystal structure of a lanthanide-EDTA hydroxo species was reported. Efforts in this field were also stimulated by the recognition that the R(OH2)s array present in previously known mononuclear EDTA complexes is dimensionally close to the R(/is-0H)3 fragment of [R4(/i3-OH)4] + clusters and by the desire to understand the mechanism of formation of the clusters. A proposal for the latter is outlined in Figure 90. 

Edta-related lanthanide complexes are of course coordinatively unsaturated in the absence of other ligands. In water, they are hydrated and the water of hydration may be displaced by other species. The kinetics of such a process, the reaction of europium 1,2-diaminocyclohexane-tetraacetate, [Eu(dcta)aq] with iminodiacetate ion, ida , to give [Eu(dcta)(ida)] ", have been studied by the elegant method of selective tunable laser excitation of the transition in... 

Hydration of lanthanide complexes. X-ray diffraction studies of the solid complexes of KLn(EDTA)(H20),c showed the number of water molecules in the coordination sphere to be three for the lighter lanthanides and two for the heavier ones for total coordination numbers of nine and eight, respectively, since EDTA is hexadentate (Hoard et al. 1967). Ots (1973) measured a maximum at europium for the heat capacity change AC° for the formation of lanthanide-EDTA complexes. This maximum was taken as a strong evidence for hydration equilibrium between the complexed species,... 

Ligand replacements for which kinetic data have been reported include [Ni(dgen)] + plus edta (parallel dissociative and associative paths), [Pb(edta)] plus R-( —)-pdta, polyether complexes of lead(n) plus nitrogen macrocycles, metal(ii)-oxine complexes plus edta, and a variety of analogous reactions involving lanthanide and actinide complexes (see Chapter 10). Kinetic data are also available for ligand exchange between trien and tetren complexes of cad-mium(ii) and the edta complex of copper(ii). ... 

Recent kinetic studies of metal ion exchanges include those of europium(iii) with its cydta complex and of various lanthanides(iii) with their respective dtpa complexes. Kinetic studies of metal ion replacement include reactions of nta, edda, heedta, egta, cydta, and dtpa complexes of zinc(ii) with copper(ii), of edta nedta, cydta, and dtpa complexes of lead(ii) with cobalt(ii), of the edta complex of nickel(ii) with indium(iii), and of ttha complexes of cadmium(ii) with lanthanides(in). There are again several systems mainly concerned with 3 + and 4+ ions, for example those involving edta and dtpa complexes of lanthanides. In systems containing ions of high charge, it seems to be easier to demonstrate the existence of dinuclear intermediates of the type invoked in associative (cf. above) pathways. 

It is important to note that the kinetics of lanthanide complexation reactions in general involve rapid association and dissociation reactions, except for structurally complex ligands like edta. Generally, lanthanide complexation kinetics in aqueous media can be considered sufficiently rapid as to have minimal effect on separations. Phase-transfer rates may be important in some systems, and should be considered in the optimization of an analytical separation procedure. The kinetics of lanthanide complexation reactions has been discussed in a previous report (Nash and Sulhvan 1991). There has been some consideration of kinetics-based separations for f-elements (Nash 1994, Merciny et al. 1986), but no useful analytical applications based solely on differences in lanthanide kinetics are known. 

If ytterbium(lll) complexes are to be used in in vivo cellular imaging instead of in in vitro diagnostics, the potential toxicity [30] of lanthanide ions becomes an issue, and the complexes should display low dissociation constants in order to limit the amount of free intracellular lanthanide ions. For instance, the dissociation constant of the Yb -fluorexon complex was found to be comparable to that of the corresponding EDTA complex [7], being higher than DTPA and DOTA complexes, but still limiting the presence of free Yb. However, the cytotoxicity of this luminescent lanthanide complex and its derivatives is not known. 

Figure 10. Crystal structures of lanthanide EDTA complexes (Adapted with permission from ref. 24. Copyright 1979 International Union of Crystallography)...

In studies on nucleotides, Williams group has employed edta complexes of lanthanide ions (La +, Pr +, Eu +, and Gd +) to probe the conformations of adenosine and cytidine 5 -phosphates at pH 7.5. The conformation of cytidine 5 -phosphate at pH 2.0 was also investigated using lanthanide ions, and relaxation studies with Gd + cations confirmed the findings derived from the shift data. ... 

The rates of ternary complex formation between the EDTA complexes of the lanthanides and pyridine-2-carboxylate or 8-hydroxyquinoline-5-sulfonate have been reported. 

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