Anion Recognition in Water by Charge-Neutral Halogen and Chalcogen Bonding Foldamer Receptors

Abstract

A novel strategy for the recognition of anions in water using charge-neutral σ-hole halogen bonding and chalcogen bonding acyclic hosts is demonstrated for the first time. Exploiting the intrinsic hydrophobicity of halogen and chalcogen bond donor atoms integrated into a foldamer structural molecular framework containing hydrophilic functionalities, a series of water-soluble receptors were constructed for anion recognition investigation. Isothermal titration calorimetry (ITC) binding studies with a range of anions revealed the receptors to display very strong and selective binding of large, weakly hydrated anions such as Iand ReO4-. This is achieved through the formation of 2:1 host-guest stoichiometric complex assemblies, resulting in an encapsulated anion stabilized by cooperative, multidentate convergent σ-hole donors, as shown by molecular dynamics simulations carried out in water. Importantly, the combination of multiple σ-hole-anion interactions and hydrophobic collapse results in Iaffinities in water that exceed all known σ-hole receptors, including cationic systems (β2 up to 1.68 × 10 11 M-2). Furthermore, the anion binding affinities and selectivity trends of the first example of an all-chalcogen bonding anion receptor in pure water are compared with halogen bonding and hydrogen bonding receptor analogues. These results further advance and establish halogen and chalcogen bond donor functions as new tools for overcoming the challenging goal of anion recognition in pure water.

Recent Advances in Luminescent Recognition and Chemosensing of Iodide in Water

“…These two sensing approaches include 1) Cation displacement assays where transition-metal-complexes of Hg 2 + , Ag + and Cu + 2 , with a modest emission, irreversibly react with I À ions to form its corresponding salts (MI n ) along with release a luminescent ligand. This displacement reaction is an indirect measure of binding and generates a fluorescence "Turn-on" response ( Figure 1 Among these strategies two approaches that are specifically good for I À include (a) the use of luminescent complexes of Hg 2 + and Pt 2 + due to the strong affinity of the anion for these metals [46][47][48]59] and (b) chemosensors based on organic receptors containing ionic H-bond donors [66][67][68][69][70] of the kind (CÀ H) + or halogen donors (CÀ I) [80][81][83][84] in combination with hydrophobic moieties because these fragments can generate a binding site with a low degree of hydration which favors the formation of the supramolecular complex. The lower hydration enthalpy of I À can also make it easier the binding with the organic receptor compared to oxoanions and lighter halides which are better hydrogen bond acceptors but with higher hydration energies ( Table 1).…”

Anion Recognition by Neutral Chalcogen Bonding Receptors: Experimental and Theoretical Investigations

“…However, the application of ChB to anion recognition in solution is clearly an underdeveloped area of host–guest chemistry . However, a recent investigation reported by Beer and collaborators demonstrated the ability of halogen and chalcogen bonding based receptors to recognize anions in water . The lack of progress in this area is likely due to the chemical instability of compounds containing the heavier chalcogen atoms, which are the ones that exhibit more positive σ‐holes.…”

“…[41] However,a recent investigation reported by Beer and collaborators demonstrated the ability of halogen and chalcogen bonding based receptors to recognize anions in water. [42] The lack of progress in this area is likely due to the chemical instability of compounds containing the heavier chalcogen atoms, which are the ones that exhibit more positive s-holes. Herein,w er eport the design and synthesis of severals ulfur,s elenium, and telluriumbased ChB binding motifs,w hich has allowed us to analyze the association constants for ChB halide anion binding in the polar aprotico rganics olventT HF.T he energetic and geometric characteristics of the host-guest complexes have also been studied theoretically by using DFT calculations and molecular electrostatic potential surface analysis.…”

Anion receptors with 1,3,5‐triazacyclohexane and 1,4,7,10‐tetraazacyclododecane scaffolds

“…[22][23][24] Although foldamers interact mainly via hydrogen bond interactions, recently, many foldamers with halogen and chalcogen bonds have been reported. [25,26] It has been found that the anion receptors with large halogens (I, Br) as acceptor site, interact more strongly with anions. Also, the interaction energies of the halogen bonds for the halides are as F − > Cl − > Br − > I − because of the higher charge density of the smaller anions.…”