Solvent Exchange on Lanthanides and Actinides

The coordination numbers of the Ln3+ ions in water are now well established from different experimental techniques (214-221). The lighter La3+-Nd3+ ions are predominantly nine-coordinate, Pm3+ Eu3+ exist in equilibria between nine- and eight-coordinate states and the heavier Gd3+-Lu3+ are predominantly eight-coordinate. The change in coordination number is also reflected in the absolute partial molar volumes, U°bs, of several Ln3+ ions determined in aqueous solutions (222,223). 

Rate Constants and Activation Parameters for Water Exchange on Lanthanide Aqua Ions 

Cyclic ligands like DOTA or DOTAM are known to exist in two isomeric forms in solution, usually termed M for the major and m for the minor isomer (Fig. 11). They interconvert slowly on the NMR timescale (251-253). It was demonstrated by NMR that the m-isomer exchanges its 1st shell water much faster than the M-isomer, most probably due to steric effects (238,244). 

Apromising new class of stable Gd3+ complexes based onTREN-Me- 3,2 -HOPO shows relatively fast water exchange rates. The water soluble tris(2-hydroxymethyl)-TREN-Me-3,2-HOPO complex is eight-coordinate with two water molecules in the first coordination sphere (234). The water exchange rate is more than one order of magnitude faster than on DTPA and DOTA complexes and the mechanism is proposed to be a-activated. 

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