Mechanical Bond Enhanced Lithium Halide Ion-Pair Binding by Halogen Bonding Heteroditopic Rotaxanes

Abstract

A family of novel halogen bonding (XB) and hydrogen bonding (HB) heteroditopic [2]rotaxane host systems constructed by active metal template (AMT) methodology, were studied for their ability to cooperatively recognise lithium halide (LiX) ion-pairs. ¹ H NMR ion-pair titration experiments in CD₃ CN:CDCl₃ solvent mixtures revealed a notable "switch-on" of halide anion binding in the presence of a co-bound lithium cation, with rotaxane hosts demonstrating selectivity for LiBr over LiI. The strength of halide binding was shown to greatly increase with increasing number of halogen bond donors integrated into the interlocked cavity, where an all-XB rotaxane was found to be the most potent host for LiBr. DFT calculations corroborated these findings, determining the mode of LiX ion-pair binding. Notably, ion-pair binding was not observed with the corresponding XB/HB macrocycles alone, highlighting the cooperative, heteroditopic, rotaxane axle-macrocycle component mechanical bond effect as an efficient strategy for ion-pair recognition in general.

Keywords: Ion-pair; halogen bond; heteroditopic rotaxane; hydrogen bond; lithium halide.

Figure 1

Schematic representation of the ion‐pair bound rotaxane.

Scheme 1

Synthesis of a) macrocycle 5 and 6, b) heteroditopic rotaxanes 9, 10, and 11.* 3.8 equiv. of Cu(MeCN)₄PF_6, at 30 °C for 5 days.

Figure 2

Crystal structure of XB macrocycle hydrochloride 6.HCl. Colour code of atoms: O‐red, N‐blue, I‐purple, H⁺‐white, Cl⁻‐green, C‐teal. Non‐covalent host‐guest interactions are shown in yellow.

Figure 3

Truncated ¹H NMR spectra in 1 : 9, CD₃CN:CDCl₃, of a)Rotaxane 10 b) Rotaxane 10 in the presence of 1 equivalent of LiClO₄ c) Rotaxane 10 in the presence of 1 equivalent of LiClO₄ and TBABr.

Figure 4

Anion binding isotherms generated by monitoring perturbations of the H₁₀ triazole proton signal of lithium precomplexed rotaxane 9 and H₂ internal tert‐butyl benzene proton signal of lithium precomplexed rotaxane 10 and 11 upon addition of TBABr and TBAI, 298 K, 500 MHz; a) in 1 : 9 CD₃CN:CDCl₃ solvent mixture, b) in 3 : 7 CD₃CN:CDCl₃ solvent mixture.

Figure 5

DFT optimised structure of LiBr bound rotaxane 10. H atoms are omitted for clarity. Colour code of atoms: O‐red, N‐blue, I‐purple, Li‐pink, Br‐ brown, C_(axle) ‐gray C_(MC)‐teal. Non‐covalent host‐guest interactions are shown in yellow.