Halogen bonding in water results in enhanced anion recognition in acyclic and rotaxane hosts

Abstract

Halogen bonding (XB), the attractive interaction between an electron-deficient halogen atom and a Lewis base, has undergone a dramatic development as an intermolecular force analogous to hydrogen bonding (HB). However, its utilization in the solution phase remains underdeveloped. Furthermore, the design of receptors capable of strong and selective recognition of anions in water remains a significant challenge. Here we demonstrate the superiority of halogen bonding over hydrogen bonding for strong anion binding in water, to the extent that halide recognition by a simple acyclic mono-charged receptor is achievable. Quantification of iodide binding by rotaxane hosts reveals the strong binding by the XB-rotaxane is driven exclusively by favourable enthalpic contributions arising from the halogen-bonding interactions, whereas weaker association with the HB-rotaxanes is entropically driven. These observations demonstrate the unique nature of halogen bonding in water as a strong alternative interaction to the ubiquitous hydrogen bonding in molecular recognition and assembly.

Iodide Recognition and Sensing in Water by a Halogen‐Bonding Ruthenium(II)‐Based Rotaxane

“…The permethylated β‐cyclodextrin‐stoppered axle component features a 3,5‐bis‐iodotriazole pyridinium bi‐dentate XB‐donor motif, which was prepared from 3,5‐diethynyl pyridine and the appropriate mono‐functionalized azido permethylated β‐cyclodextrin derivative using CuAAC “click” chemistry 28. A 4,4′‐bis‐amide‐2,2′‐bipyridyl motif is incorporated into the macrocycle component of the initial precursor rotaxane, to facilitate subsequent metalation with a photo‐active [Ru II (bipy) 2 ] motif.…”

“…However, in the past few years, a number of studies have demonstrated that XB anion receptors exhibit superior and contrasting recognition properties in comparison to their hydrogen‐bonding (HB) analogues 26. 27 For instance, we very recently reported that XB is remarkably effective for halide‐anion binding in water, and results in significantly enhanced recognition in acyclic and rotaxane‐based receptors compared to the analogous hydrogen‐bonding systems 28…”

Anion Recognition in Water: Recent Advances from a Supramolecular and Macromolecular Perspective

“…We later published a seminal study to quantify halogen‐bonding interactions in 100 % water, and demonstrated that the intermolecular interaction is superior to hydrogen bonding for anion recognition in aqueous solution 46. Anion‐binding titrations in D 2 O with a series of acyclic and rotaxane‐based receptors containing the bidentate 3,5‐bisiodotriazole pyridinium halogen bond donor motif, and solubilized with permethylated β‐cyclodextrin derivatives, revealed a remarkable enhancement in the binding affinity for halide anions when compared to analogous C−H and amide N−H hydrogen‐bond donors.…”

Selective Nitrate Recognition by a Halogen‐Bonding Four‐Station [3]Rotaxane Molecular Shuttle

“…The starting binding scenarios of 5⋅(Cl) 2 and 5⋅NO 3 − were built assembling the two macrocycles and the bis‐iodo‐triazolium axle central motif in an interlocked orthogonal binding arrangement, in agreement with the structures of analogous XB [2]rotaxane hosts 50, 63. In addition, in 5⋅(Cl) 2 , the two chloride anions together with the two macrocycles were initially disposed in a parallel manner with each anion establishing two hydrogen bonds with a single isophthalamide binding cleft and one halogen bond with a iodo‐triazolium XB binding unit, as shown in Figure 4.…”

“…Of the relatively few examples of XB anion receptors reported to date, it is noteworthy that all display promising, and significantly contrasting, anion recognition behaviour when compared to HB analogues, by virtue of their comparable bond strengths and more strict linear geometry preference 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42. Importantly in a significant step forward for highlighting the potential importance of halogen bonding in anion supramolecular chemistry, we have recently demonstrated the first examples of solution phase halogen bonding being exploited to control and facilitate the anion‐templated assembly of interlocked structures43, 44, 45, 46, 47 and demonstrated that the incorporation of halogen bond donor atoms into a [2]rotaxane host cavity dramatically improves the anion recognition capabilities of the XB interlocked receptor 48, 49, 50, 51…”