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 CD 3 CN:CDCl 3 solvent mixtures revealed a notable "switch-on" of halide anion binding in the presence of a cobound 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.

Recent Advancements in Ion‐Pair Receptors

“…[92] On the other hand, Beer and co-workers have prepared a series of novel heteroditopic [2]rotaxane type receptors, displaying lithium salt ion-pair recognition, via triazole based halogen as well as hydrogen bonding interactions in both the axle and macrocycle units (Figure 16). [94] From the analysis of spectroscopy and DFT studies, it was clearly observed that the MIM systems 127-129 displayed higher binding affinity with lithium halides as compared to the non-interlocked macrocyclic systems of same components. Interestingly, it was found that the binding of Li + was found to polarize the halogen bond donors and preorganize the anion binding sites, thereby leading to the cation-induced switch on of the cooperative halide recognition.…”

Harnessing ion–dipole interactions: a simple and effective approach to high-performance lithium receptors

“…The design and synthesis of high-performance receptors for Li + have traditionally focused on developing macrocyclic receptors, featuring small binding cavities meticulously tailored 19,20,23–30 to selectively bind Li + while excluding competing ions such as sodium (Na + ) and potassium (K + ). A pioneering work in this domain was exemplified 31–36 by Donald Cram's spherands, which displayed an extraordinary binding affinity exceeding 10 16 M −1 for Li + due to their rigid and preorganized binding cavities.…”

“…[9][10][11][12][13][14][15] Consequently, the design and implementation of molecular receptors capable of effectively recognizing, sensing, extracting, and purifying this critical mineral have become paramount in addressing this pressing challenge. [16][17][18][19][20][21][22] The design and synthesis of high-performance receptors for Li + have traditionally focused on developing macrocyclic receptors, featuring small binding cavities meticulously tailored 19,20,[23][24][25][26][27][28][29][30] to selectively bind Li + while excluding competing ions such as sodium (Na + ) and potassium (K + ). A pioneering work in this domain was exemplied [31][32][33][34][35][36] by Donald Cram's spherands, which displayed an extraordinary binding affinity exceeding 10 16 M −1 for Li + due to their rigid and preorganized binding cavities.…”

“…A pioneering work in this domain was exemplied [31][32][33][34][35][36] by Donald Cram's spherands, which displayed an extraordinary binding affinity exceeding 10 16 M −1 for Li + due to their rigid and preorganized binding cavities. In alignment with this foundational principle, numerous macrocyclic scaffolds have been devised for Li + binding, employing molecular design strategies based on crown ether derivatives, 21,27,28,[37][38][39][40][41][42][43] ion pair receptors, 16,25,25,[44][45][46][47][48] mechanically interlocked molecules, 26,[49][50][51][52] and organometallic macrocycles. 18,19,29,53 However, despite the success in achieving strong binding affinity for Li + , the majority of these macrocyclic receptors exhibit limited Li + /Na + selectivity (ratio <1000) and necessitate high-dilution conditions during synthesis.…”

Allosteric and Electrostatic Cooperativity in a Heteroditopic Halogen Bonding Receptor System

“…[17] We and others have demonstrated that anion recognition strategies employing exotic sigma-hole based interactions, [18][19][20][21][22][23] including halogen bonding (XB), [24][25][26][27][28][29][30][31] engender considerable advantages over more traditional approaches, such as hydrogen bonding (HB). Furthermore, while rare, the few reports [32][33][34][35][36][37] of XB integration in heteroditopic receptor design have also revealed unique behavior. In particular, XB mediated anion binding potency can be effectively 'switched on' by an inductive effect of a co-bound alkali metal cation, polarizing a proximal CÀ I XB donor, which constitutes a novel and interesting mechanism of cooperativity.…”