A New Fluorescent Calix Crown Ether: Synthesis and Complex Formation with Alkali Metal Ions

We synthesised a new N‐benzylaza‐21‐crown‐7 ether 5 with a dihydroxy coumarin as a fluorescence sensor and investigated the binding behaviour towards alkali metal cations in methanol by fluorescence titrations. The association constants are within one order of magnitude, with the exception of sodium. Potassium is the preferred binding partner (KNa=330 M−1; KK=8600 M−1; KRb=8200 M−1; KCs=4400 M−1). The corresponding aza‐21‐crown‐7 ether (6) was attached by a methylene unit to a resorcarene to give fluorescent calix crown ether 12. The binding abilities of the calix crown ether towards alkali metal ions in methanol have also been investigated, and an increasing complex stability, distinct for potassium and rubidium in comparison with 5, was found: KNa=440 M−1; KK=110 000 M−1; KRb=63 000 M−1; KCs=20 000 M−1. Like bis(crown ether)s, a cooperative complexation of the crown ether and the cavitand scaffold can be assumed. The proposed complex geometry is supported by Kohn–Sham DFT calculations for the potassium and caesium complexes. Building a sensor: A fluorescent calix crown ether based on a cavitand scaffold (see figure) was synthesised and the association constants of alkali metal ion complexes in solution were determined by means of fluorescence titration. The results indicate cooperative effects in both the resorcarene and the crown ether in complex formation.