Lanthanide chelates, hydration

In addition to a detailed discussion of hydration dynamics of the lanthanides, Lincoln s review also covers the kinetics of solvation in nonaqueous media and complexation kinetics. As our focus is on the aqueous chemistry of the lanthanides, we will not discuss recent developments in nonaqueous lanthanide solution chemistry. The intervening four years have seen the publication of a handful of studies of the kinetics of lanthanide chelate dissociation kinetics. In the following section we will discuss the best of these results. 

The hydration state of lanthanide(III) chelates can be assessed by different techniques. Luminescence studies are widely used for Eu111 and Tb111 chelates (see Chapter 9.21).17 18 170 NMR chemical shift measurements in solution of lanthanide(III) (most often Dy or Gd) complexes can also give information on q.19 These techniques in the context of MRI contrast agent research have been reviewed in 2001.1... 

The hydration state of lanthanide(III) chelates can be assessed by different techniques which have been reviewed in details (1,2). 

The lanthanide elements, in their complexes with jS-diketones, tend to adopt interesting, higher coordination geometries. These compounds frequently crystallize as hydrates from which water removal without decomposition of the compound is difficult. Some structural information is summarized in Table 1. 2,2,6,6-Tetramethylheptanedionate chelates of the lighter lanthanides (La to Dy) can be obtained in nonsolvated form, crystallize in the monoclinic system and contain dimer units whereas the heavier analogues (Ho to Lu) tend to be orthorhombic with isolated six-coordinate monomers.128... 

Information on the hydration state of the Gd(III) chelate in solution is indispensable for the analysis of its proton relaxivity Several methods exist to determine q, though they are mostly applicable for other lanthanides than Gd(III). In the case of Eu(III) and Tb(III) complexes, the difference of the luminescence lifetimes measured in D20 and H20 can be related to the hydration number [15, 16]. For Dy(III) chelates, the lanthanide induced 170 chemical shift of the bulk water is proportional to the hydration number [17]. Different hydration states of the same chelate may also coexist in solution giving rise to a hydration equilibrium. Such an equilibrium can be assessed by UV-Vis measurements on the Eu(III) complex [18-20]. These techniques have been recently discussed [21]. 

Choppin [24] examined some aspects of lanthanide-organic ligand interaction in aqueous solutions. An interpretation of thermodynamic parameters (AG, AH and AS) of complexation have been given in terms of hydration, inner versus outer sphere character, stability vs. chelate ring size and ligand charge polarization. 

The preparation, characterization, aqueous stability, and photophysical properties of NIR emitting lanthanide complexes with tetradentate chelating ligands 36 and 37 were described by Raymond and coworkers [61, 62]. In aqueous solution, the chelating ligand 36 or 37 forms stable complexes with Ln(III) ions, and sensitized NIR lanthanide luminescence was detected for the complexes with Pr(III), Nd(III), Ho(III), or Yb(III) ions. For [Ln(36)2] complexes, the luminescence decay curves were biexponential due to partial hydrolysis of the complexes or alternately the presence of a slowly exchanging equilibrium mixture with a hydrated form of the complexes. For [Ho(37)2] , the NIR band due to Fs -> I transition of the Ho(III)... 

Zone chromatography is a variant of the zone melting method, in which the mixture being separated is introduced into a column with a solid solvent and a molten zone is passed repeatedly along the length of the column to separate mixtures into separate bands of their components. Zone chromatography has been used for the separation of mixtures of lanthanides for preparative and analytical purposes The chelates used were mixtures of hydrated / -diketonates and their adducts with 2,2 -bipyridyl (bipy) and acety-lacetonimines. The distribution coefficients of different chelates and binary mixtures have been determined . 

A number of volatile rare earth chelates containing the ligands, l,l,l,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedi-one, [H(fod)], and 2,2,6,6-tetramethyl-3,5-heptanedione, [H(thd)] have been synthesized and investigated. The fod complexes are more volatile than other known compounds of the lanthanide elements. The fod complex of Sc(III) and the lanthanide thd compounds are anhydrous. The fod chelates of Y(III) and the lanthanides are isolated as hydrates but can easily be dehydrated in vacuo over P Oio. Gas chromatographic and thermo gravimetric data reveal that the volatilities of the complexes increase as the ionic radii of the metal ions become smaller. 

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