A general method for the determination of the kinetic stability of macrocyclic alkali-metal complexes with rates of decomplexation below 10–3s–1

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Abstract
A general method has been developed for the determination of kinetic stabilities of macrocyclic alkali-metal complexes with rates of decomplexation (Kd) below 10–3 s–1, by use of radioactive isotopes. This method offers the possibility to study the influence of the solvent polarity and of the salt concentration in solution on the rate of decomplexation of macrocyclic metal complexes. Further advantages are the small amounts of ligand required for these determinations and the simplicity of the method. Furthermore, it is possible by this method to study the ‘degenerate’ exchange of sodium for sodium and of rubidium for rubidium. By this method the kinetic stabilities of the sodium and rubidium complexes of calixspherands 1–4 were determined. Calixspherand 3 forms kinetically very stable complexes with sodium and rubidium cations in acetone and Me2SO in the presence of high concentrations of sodium cations in solution; half-life times of exchange are 855 (Na+) and 528 (Rb+) h in acetone and 352 (Na+) and 845 (Rb+) h in Me2SO, respectively. The results of this method were verified by an independent 1H NMR spectroscopic method.