Ion Pair Recognition of Quaternary Ammonium and Iminium Salts by Uranyl−Salophen Compounds in Solution and in the Solid State

Efficient ditopic receptors for quaternary ammonium and iminium salts have been obtained upon
functionalization of the uranyl−salophen unit with conformationally flexible side arms bearing phenyl or
β-naphthyl substituents. Binding affinities in chloroform solution have been measured for a large number
of quaternary salts comprising tetramethylammonium (TMA), tetrabutylammonium (TBA), acetylcholine
(ACh), N-methylpyridinium (NMP), and N-methylisoquinolinium (NmiQ) cations. Recognition of the anion
partner is ensured by coordination to the hard Lewis acidic uranyl center, whereas cation−π/CH−π
interactions of the quaternary ions are established with the aromatic pendants. The role of the cation−anion interactions on the dynamics of exchange between the free and complexed species is discussed.
Solid-state structures have been obtained for a few salt−receptor combinations. In the solid state, side-armed receptor molecules form assemblies that enclose ion pair aggregates of varying composition and
structure, including AChCl dimers, two different kinds of tetrameric (TMA)Cl clusters, and unidimentional
salt strips of (NMP)Br. The lack of side arms as preferential binding sites for the polar quaternary cations
prevents association patterns of the kinds formed with the side-armed receptors, as shown by the crystal
structure of the complex of (TMA)Cl with the parent uranyl−salophen receptor.