Kinetically stable complexes of alkali cations with calixspherands: An evaluation of shielding

Article abstract of DOI:10.1021/ja00080a015

Three new calixspherands (2-4) were synthesized in good yields (>60%) via a new method; p-tert-butylcalix-[4]arene (6) is bridged with a m-terphenyl (7-9) and subsequently alkylated. ¹H NMR spectroscopy and X-ray crystallography showed that all the complexes are in a partial cone conformation. All the calixspherands form kinetically stable complexes with Na⁺, K⁺, and Rb⁺. The kinetic stability was determined both by ¹H NMR spectroscopy, in CDCl₃ saturated with D₂O, and by a new method based on the exchange of radioactive rubidium or sodium in the complexes for nonradioactive sodium in different solvents. Both methods showed that the kinetic stability of the different complexes is strongly increased when the size of the group on the central aromatic ring of the m-terphenyl is increased. This effect is most pronounced for the rubidium complexes. The half-life times for decomplexation, in CDCl₃ saturated with D₂O, increased from 2.8 h for [1·Rb]⁺ to 139 h and 180 days for [2·Rb]⁺ and [3·Rb]⁺, respectively. The 'exchange method' shows that the rate of decomplexation is the rate-limiting step in the exchange of rubidium in the complex for sodium present in solution. These results can be explained in terms of increased shielding of the cavity from solvent molecules. The kinetic stabilities of the complex of 3 with Na⁺, K⁺, and Rb⁺ are the highest ever reported.