Alkali Metal Halide Ion‐Pair Binding in Conformationally Dynamic Halogen Bonding Heteroditopic [2]Rotaxanes

Abstract

AbstractA 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 1H 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 1H 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.