Cation exchange macrocyclic ligands

With the cylindrical cryptands, each macrocycle may bind one cation so that both mono- and dinuclear cryptates may be formed. Although the 12-membered (N202) macrocycles of ligand 5 are too small to bind two cations within each of the macrocycles, variable temperature 13C-NMR measurements have revealed intramolecular cation exchange between identical sites at the top and bottom of this cryptand, for Ca2+, Sr2+, and Ba2+. Cation jump between the two sites is fast with respect to intermolecular cation exchange, modeling the elementary jump processes of cations between binding sites in membrane channels (91). 

Preliminary kinetic results on cation exchange in cryptates [ligand (21)] show some correlation with thermodynamic parameters, but only go a small way towards the complete elucidation of the mechanism. Metal replacement in complexes of macrocyclic ligands is also illustrated by studies of several metalloporphyrins. ... 

FIGURE 7 Proposed interlayer arrangements of exchangeable cations and associated macrocyclic ligands in complexes with montmorillonite and vermiculite. = radius of the macrocyclic cavity r, = radius of cations Mi = interlayer distance. 

In solvent of low permittivity, where the cation will be very weakly solvated, there is an effective competition between a macrocyclic ligand, C, and the counter anion, A , for the metal, M. From ultrasonics and IR spectra on the Na /18-crown-6 system in propylene carbonate, which show a dependence on the nature as well as the concentration of the anion present, it has been proposed that such a competition process between the anion and the macrocycle is also relevant in solvents with a much higher permittivity it is suggested that this could account for the bimolecular mechanism proposed as a path for metal exchange and complex dissociation, given by Eq. (1). 

Aime, S., Barge, A., Bruce, J.I., et al. (1999) NMR, relaxometric, and structural smdies of the hydration and exchange dynamics of cationic lanthanide complexes of macrocyclic tetraamide ligands. Journal of the American Chemical Society, 121, 5762-5771. 

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