Alkali Metal Halide Ion‐Pair Binding in Conformationally Dynamic Halogen Bonding Heteroditopic [2]Rotaxanes
Hui Min Tay, Andrew Docker, Yuen Cheong Tse, Paul D. Beer
Abstract
Abstract 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.