1,2-Bis[N-(N′-alkylimidazolium)]ethane salts as new guests for crown ethers and cryptands

1,2-Bis[N-(N′-alkylimidazolium)ethane salts form complexes presumed to be pseudorotaxanes with crown ether and cryptand hosts. The association constants of 1,2-bis[N-(N′-butylimidazolium)]ethane bis(hexafluorophosphate) with dibenzo-24-crown-8 and a dibenzo-24-crown-8-based pyridyl cryptand were estimated as 24 (±1) and 348 (±30) M⁻¹, respectively, in acetonitrile at 25 °C. The pseudorotaxane-like structure of the 1:2 complex of the N′-methyl analog with the cryptand was observed by X-ray crystallography. Replacement of the ethylene spacer with propylene and butylene spacers resulted in K_a values an order of magnitude smaller.