Remote Anion and Cation Binding Sites

One of the first ion pair receptors was the 21-membered crown ether 5.12, prepared by M. T. Reetz of the University of Marburg, Germany in 1991, which possesses a Lewis acidic boronate centre. Ligand 5.12 is capable of dissolving stoichiometric amounts of KF in dichloromethane at room temperature to give a KF adduct in which the K+ ion is bound by the crown ether ring while the F anion forms a dative bond with the boron centre out of the plane of the crown ether. Ligand 5.12 fails to dissolve either KCl or KBr as a consequence of the weaker nature of the B-Cl and B-Br bonds. 

KI and KSCN are dissolved by 5.12 but without complexation of the boron atom. The stabilisation of the K+ ion by the crown ether moiety is apparently sufficient in these cases. Similarly, the 18-membered phenolic crown ether analogue can be metalated (= exchange of a proton for a metal ion in this case the OH proton) with trimethylaluminium to give 5.13 which forms a ditopic complex with LiCl in solution and in the solid state. 

---