An unsymmetrical calix[4]-bis-crown ether having both conventional crown-6
and dibenzocrown-6 rings in a fixed 1,3-alternate conformation was synthesi
zed with good yield by the reaction of a monocyclic calix[4]crown-6 with di
benzodimesylate in the presence of cesium carbonate. The cesium ion selecti
vity among alkali metal ions increased compared to symmetrical calix[4]-bis
-crown-6. The solid state structure of the ligand-cesium complex illustrate
s a 1:2 complex ratio. On the contrary, in solution, e.g., extraction equil
ibrium and H-1 NMR experiment gave a 1:1 complex ratio. From the result of
the chemical shift change upon metal ion complex, the cesium ion seems to p
refer the dibenzocrown loop to the conventional crown-6 ring.