Lanthanides water exchange rates

EXAFS study on Eu2+ and Sr2+ in both solid state and aqueous solution gave coordination numbers of 8.0 for strontium(II) and 7.2 for europium(II) (228). The water exchange rate measured on the divalent europium aqua ion is the fastest ever measured by 170 NMR (Table XVI) (2). The activation volume is much more negative (—11.7 cm3 mol-1) than those determined on trivalent lanthanide aqua ions clearly indicating an a-activation mechanism which is most probably a limiting... 

Water exchange on cationic lanthanide chelates can also be influenced by the nature of the counter-anions (170,171). Anions like halides, sulfate, nitrate, acetate, and fluoroacetate impose different order on the second coordination shell around the chelate by influencing the hydrogen bond network. Anions with a high charge density like CU and S04 can break up the hydrogen bond network between water molecules around the metal center and by that, slow down the water exchange rate of the inner shell water molecule (171). 

The change in structure along the lanthanide series, going from La3+ to Lu3+, can be ascribed to the decrease in ionic radius of the metal center which causes an increase of the rigidity and in the steric constraint on the water binding site [35,36]. For this reason the 9-coordinate Gd complex shows a remarkably high water exchange rate. 

Neutral N-derivatized octadentate ligands based on cyclen (1,4,7,10-tetraazacy-clododecane) form tripositive cationic complexes with the trivalent lanthanides [116-124]. The N-substituted tetraamide derivatives have proven useful in understanding the relationship between the solution structure of the Ln3+ complex and its water exchange rate, a critical issue in attaining optimal relaxation efficiency of CA s [125-131]. 

Trivalent lanthanide ions are known as being kinetically very labile, with water exchange rates in the range 107—109 s 1. Complexation reactions with non-cyclic, simple, ligands... 

Now that we have data for water exchange rates, the complex formation reactions of lanthanides in aqueous media can be considered. Complex formation reactions of lanthanides usually involve initial formation of an outer-sphere complex followed by a loss of water molecule and the ligand taking the position of the leaving water molecule. The reaction scheme is as follows ... 

In the absence of data on water exchange rates for lighter lanthanides, it is difficult to say whether water exchange rates will be similar to sulphate complex formation rates for the lighter lanthanides. 

Another approach is to design homogeneous Lewis acids which are water-compatible. For example, triflates of Sc, Y and lanthanides can be prepared in water and are resistant to hydrolysis. Their use as Lewis acid catalysts in aqueous media was pioneered by Kobayashi and coworkers [144-146]. The catalytic activity is dependent on the hydrolysis constant (Kh) and water exchange rate constant (WERC) for substitution of inner sphere water ligands of the metal cation [145]. Active catalysts were found to have pKh values in the range 4-10. Cations having a pKh of less than 4 are easily hydrolyzed while those with a pKh greater than 10 display only weak Lewis acidity. 

Zech, S.G, Eldredge, H.B., Lowe, M.P, and Caravan, P. (2007) Protein binding to lanthanide(III) complexes can reduce the water exchange rate at the lanthanide. Inorganic Chemistry, 46, 3576-3584. 

The composition of the primary hydration sphere of the lanthanide ions continues to be a matter of intense interest from both experimental and theoretical points of view. Lincoln (1986) discussed both hydration numbers and water exchange rates for the lanthanides, ending with the conclusion that the most probable hydrated lanthanide ion is R(H20)9 for the entire series, with the qualifier that new data could alter this opinion. 

It has also been demonstrated by NMR spectroscopy that water exchange rates arc slower for lanthanide complexes than in the corresponding aquo ions (Powell et al. 1995). 

The catalytic activities of these lanthanide triflates were found to be mainly dependent on two parameters, the hydrolysis constant and the water exchange rate constant. Moreover,... 

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