Synthesis, structure, and extraction behavior of 4,5′,4″,5‴-tetra-tert-butyltetrabenzo-24-crown-8

Isomerically pure 4,5′,4″,5‴-tetra-tert-butyltetrabenzo-24-crown-8 was synthesized by a route that eliminates any need for isomer separation. Its structure was determined using single crystal X-ray diffraction methods. Since the observed saddle-shaped conformation of the crown ether is the same as previously reported for the closely related tetrabenzo-24-crown-8, the presence of the peripheral tert-butyl groups has no apparent effect on ring structure. This structure experiences few intermolecular interactions beyond London forces, suggesting it represents a low energy conformation. Liquid-liquid distribution behavior of CsNO₃ and CsClO₄ between water and 1,2-dichloroethane (1,2-DCE) containing 4,5′,4″,5‴-tetra-tert-butyltetrabenzo)-24-crown-8 was investigated. Equilibrium constants corresponding to the extraction of ion pairs and dissociated ions, formation of the caesium complex, and dissociation of the ion pairs in water-saturated 1,2-DCE at 25°C were obtained from equilibrium modeling using the SXLSQI program. 4,5′,4″,5‴-Tetra-tert-butyltetrabenzo-24-crown-8 exhibits stronger caesium binding and a similarly low tendency towards ion-pairing, in comparison with the previously characterized isomeric 4,4′- and 4,5′-di-tert-octyltetrabenzo-24-crown-8 ethers.