Ion Template Effects on the Conformation of Water-Soluble Calixarenes

The template effects of alkali metal ions and organic ammonium ions on the rate of calixarene ring inversion were investigated. In the temperature-dependent ¹H NMR spectra in D₂O, the coalescence temperature (T_c) of p-sulfonatocalix[4]arene (1₄H) was enhanced up to 17-26 °C in the presence of alkali metal ions and up to 50-65 °C in the presence of ammonium ions from 9 °C in the absence of these ions. The temperature rise, which means the suppression of the ring-inversion rate, was explained as such that these ions have the template effects on the calixarene ring inversion. Organic ammonium ions, in comparison to alkali metal ions, exerted large template effects. In particular, di- and tricationic ammonium ions (e.g., m- and p-N,N′-hexamethylphenylenediammonium ions) were very effective. This is rationalized in terms of the electrostatic bridge occurring on the cavity edge. From plots of ¹H NMR chemical shift versus [1_nH]/[guest], the association constants for these template molecules were estimated. The possible relations between the binding mode and the association ability and the mechanisms of ring inversion were discussed.