Increased halide recognition strength by enhanced intercomponent preorganisation in triazolium containing [2]rotaxanes

Three triazolium-based [2]rotaxanes containing different sized axle and macrocycle components were synthesised in good yields (40-57 %) through chloride anion templation. The anion recognition properties of the interlocked receptor systems were investigated using ¹H NMR titration experiments: all three rotaxanes display impressive selectivities for halide anions over the more basic oxoanion acetate. The rotaxanes incorporating shorter, more rigid axle components with aryl-substituted triazolium groups display substantially higher anion binding affinities than those with longer, bis-alkyl-substituted heterocycles, which is attributed to the increased intercomponent preorganisation afforded by the smaller axle component. Computational DFT and molecular dynamics simulations composed of unconstrained and umbrella sampling simulations corroborate the experimental observations. Bigger is not always better! A series of triazolium-containing rotaxanes with varying axle and macrocycle components have been prepared by halide anion templation. The rotaxane containing the shortest axle displays the strongest anion recognition properties, which is demonstrated to be the result of improved intercomponent preorganisation.