KINETICALLY STABLE COMPLEXES OF ALKALI CATIONS WITH CALIXSPHERANDS - AN EVALUATION OF SHIELDING

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-te rphenyl (7-9) and subsequently alkylated. H-1 NMR spectroscopy and X-r ay crystallography showed that all the complexes are in a partial cone conformation. All the calixspherands form kinetically stable complexe s with Na+, K+, and Rb+. The kinetic stability was determined both by H-1 NMR spectroscopy, in CDCl3 saturated with D2O, and by a new method based on the exchange of radioactive rubidium or sodium in the comple xes for nonradioactive sodium in different solvents. Both methods show ed that the kinetic stability of the different complexes is strongly i ncreased when the size of the group on the central aromatic ring of th e m-terphenyl is increased. This effect is most pronounced for the rub idium complexes. The half-life times for decomplexation, in CDCl3 satu rated 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 excha nge of rubidium in the complex for sodium present in solution. These r esults can be explained in terms of increased shielding of the cavity from solvent molecules. The kinetic stabilities of the complex of 3 wi th Na+, K+, and Rb+ are the highest ever reported.