A series of heteroditopic halogen bonding (XB) [2]rotaxanes were prepared via a combination of passive and active metal template-directed strategies. The ability of the [2]rotaxanes to bind alkali metal halide ion-pairs was investigated by extensive 1 H NMR titration studies, wherein detailed analysis of cation, anion and ion-pair affinity measurements indicate dramatic positive cooperative enhancements in halide anion association upon either Na+ or K+ pre-complexation. This study demonstrates that careful consideration of multiple, parallel and competing binding equilibria is essential when interpreting observed 1 H NMR spectral changes in ion-pair receptor systems, especially those which exhibit dynamic behaviour. Importantly, in comparison to XB [2]catenane analogues, these neutral XB heteroditopic [2]rotaxane host systems demonstrated that despite their relatively weaker cation and anion binding affinities, they exhibit a notably higher level of positive cooperativity for alkali metal halide ion-pair binding, highlighting the role of greater co-conformational adaptive behaviour in mechanically-bonded hosts for the purposes of charged species recognition.
Keywords: conformational dynamism; halogen bonding; heteroditopic receptors; ion-pair recognition; mechanically interlocked molecules.
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