Recognition of Alkali Metal Halide Contact Ion Pairs by Uranyl−Salophen Receptors Bearing Aromatic Sidearms. The Role of Cation−π Interactions

Hard anions have long been known to bind strongly to the uranium of uranyl−salophen complexes.
Upon functionalization of the salophen framework with one or two benzyloxy substituents, efficient ditopic
receptors for alkali metal ions are obtained. The solid-state structures of complexes formed by the two-armed receptor 1 with CsF and with the chlorides of K+, Rb+, and Cs+ reported here reveal the existence
of dimeric supramolecular assemblies in which two receptor units assemble into capsules fully enclosing
(MX)2 ion quartets. In addition to the strong coordinative binding of the anion to the uranyl center and to
electrostatic cation−anion interactions, stabilizing interactions arise from coordination of each cation to six
oxygens, three from each receptor, and most importantly, to two aromatic sidearms belonging to different
receptors. There are marked differences in organization at the supramolecular level in the CsCl complex
of the one-armed receptor 3, in that four uranyl−salophen units instead of two are assembled in a capsule-like arrangement housing a (CsCl)2 ion quartet. However, both receptors achieve the common goal of
having each metal cation in close contact with carbon atoms of two aromatic rings. 1H NMR data provide
strong evidence that cation−π(arene) interactions with the sidearms participate in binding also in solution.