A surprising host-guest relationship between 1,2-dichloroethane and the cesium complex of tetrabenzo-24-crown-8

The structure of the complex [Cs(tetrabenzo-24-crown-8)(1,2- dichloroethane)₂](NO₃)·H₂O was shown by X-ray crystallography to involve an unprecedented bidentate coordination of two 1,2-dichloroethane solvent molecules to the Cs⁺ cation via the four chlorine atoms. The coordination of the solvent molecules occurs within two clefts between facing benzo groups, one pair of benzo groups related to the other pair by an improper noncrystallographic 90°rotation. Resembling the seam on a tennis ball, the crown ether envelops the metal cation within a cagelike arrangement of eight crown ether oxygen atoms. Good geometric and electronic complementarity characterizes the apparent host-guest relationship between the cleft environment and the solvent molecules. The complete encapsulation of the cation by the crown ether and two solvent molecules explains well the speciation behavior observed in liquid-liquid extraction of CsNO₃ or CsClO₄ from aqueous solution to 1,2-dichloroethane solutions of the alkylated analogues 4,4_- or 4,5_-bis(tert-octylbenzo)dibenzo. 24-crown-8. In the extraction process studied at 25 °C, simple 1:1 metal/crown complexes form in the solvent phase, as modeled by the program SXLSQI. The complex cation and counteranion are present both as ion-pairs, postulated to be ligand- separated ion-pairs as suggested by the crystal structure, and as dissociated ions. In agreement with a theoretical treatment of ion-pairing, the ion-pairs possess unusually low stability and exhibit no discrimination between the anions, largely ascribed to the large effective radius of the complex metal cation. Values of log K(f) corresponding to the formation of the complex cations Cs[bis(tert-octylbenzo)-dibenzo-24-crown-8]⁺ in 1,2-dichloroethane at 25 °C average 10.5 ± 0.2 for both positional isomers of the crown ether and for their 3:2 mixture. Overall, these results provide insight into the role of clefts as host environments for inclusion of neutral molecules and show how even solvent molecules with exceptionally weak donor-acceptor properties may participate in supramolecular assemblies. In addition, the results are unique in enabling a clear assessment of the effect of the encapsulation of the metal cation on the ion-pairing tendency of the metal complex and implications for anion selectivity.