Isomeric 2,6-Pyridino-Cryptands Based on Dibenzo-24-crown-8

Cryptands <b>4</b> and <b>5</b> were synthesized from <i>cis-</i> and <i>trans</i>-bis(hydroxymethylbenzo)-24-crown-8 by reaction with pyridine-2,6-dicarboxylic acid chloride in 42 and 48% yields, respectively. The new cryptands form pseudorotaxanes with the paraquat derivative <i>N</i>,<i>N</i>‘-bis(β-hydroxyethyl)-4,4‘-bipyridinium bis(hexafluorophosphate) (“paraquat diol”, <b>6</b>):  <i>K</i><sub>a</sub> = 1.0 × 10<sup>4</sup> and 1.4 × 10<sup>4</sup> M<sup>-1</sup>, respectively. The cryptands also form complexes with ammonium hexafluorophosphate. Formation of the paraquat/cryptand-based pseudorotaxanes can be switched off and on in a controllable manner on the basis of the cryptands' abilities to complex KPF<sub>6</sub> strongly, providing a new mechanism for control of molecular shuttles. K<sup>+</sup> displaces paraquat diol from the cryptands, converting yellow-orange solutions to colorless; however, addition of 18-crown-6 binds the KPF<sub>6</sub> and allows the colored cryptand−paraquat complex to reform. Crystal structures are reported for both cryptands, both paraquat diol-based pseudorotaxanes, both NH<sub>4</sub>PF<sub>6</sub> complexes, and both KPF<sub>6</sub> complexes.