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

Cryptands 4 and 5 were synthesized from cis- and trans-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 N,N‘-bis(β-hydroxyethyl)-4,4‘-bipyridinium bis(hexafluorophosphate) (“paraquat diol”, 6):  K_a = 1.0 × 10⁴ and 1.4 × 10⁴ M^-1, 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 KPF₆ strongly, providing a new mechanism for control of molecular shuttles. K⁺ displaces paraquat diol from the cryptands, converting yellow-orange solutions to colorless; however, addition of 18-crown-6 binds the KPF₆ and allows the colored cryptand−paraquat complex to reform. Crystal structures are reported for both cryptands, both paraquat diol-based pseudorotaxanes, both NH₄PF₆ complexes, and both KPF₆ complexes.