Anion-Induced Synthesis and Combinatorial Selection of Polypyrrolic Macrocycles

Dynamic Covalent Self-Assembly of Chloride- and Ion-Pair-Templated Cryptates

While supramolecular hosts capable of binding and transporting anions and ion pairs are now widely available, self‐assembled architectures are still rare, even though they offer an inherent mechanism for the release of the guest ion(s). In this work, we report the dynamic covalent self‐assembly of tripodal, urea‐based anion cryptates that are held together by two orthoester bridgeheads. These hosts exhibit affinity for anions such as Cl⁻, Br⁻ or I⁻ in the moderate range that is typically advantageous for applications in membrane transport. In unprecedented experiments, we were able to dissociate the Cs⋅Cl ion pair by simultaneously assembling suitably sized orthoester hosts around the Cs⁺ and the Cl⁻ ion.

Paraoxonase Mimic by a Nanoreactor Aggregate Containing Benzimidazolium Calix and l-Histidine: Demonstration of the Acetylcholine Esterase Activity

An anion-mediated preorganization approach was used to design and synthesize the benzimidazolium-based calix compound R1⋅2 ClO₄ ^- . X-ray crystallography analysis revealed that the hydrogen-bonding interactions between the benzimidazolium cations and N,N-dimethylformamide (DMF) helped R1⋅2 ClO₄ ^- encapsulate DMF molecule(s). A nanoreactor, with R1⋅2 ClO₄ ^- and l-histidine (l-His) as the components, was fabricated by using a neutralization method. The nanoreactor could detoxify paraoxon in 30 min. l-His played a vital role in this process. Paraoxonase is a well-known enzyme used for pesticide degradation. The Ellman's reagent was used to determine the percentage inhibition of the acetylcholinesterase (AChE) activity in the presence of the nanoreactor. The results indicated that the nanoreactor inhibited AChE inhibition.

Advances in anion supramolecular chemistry: from recognition to chemical applications

Since the start of this millennium, remarkable progress in the binding and sensing of anions has been taking place, driven in part by discoveries in the use of hydrogen bonding, as well as the previously under-exploited anion-π interactions and halogen bonding. However, anion supramolecular chemistry has developed substantially beyond anion recognition, and now encompasses a diverse range of disciplines. Dramatic advance has been made in the anion-templated synthesis of macrocycles and interlocked molecular architectures, while the study of transmembrane anion transporters has flourished from almost nothing into a rapidly maturing field of research. The supramolecular chemistry of anions has also found real practical use in a variety of applications such as catalysis, ion extraction, and the use of anions as stimuli for responsive chemical systems.

Anion-templated supramolecular C3 assembly for efficient inclusion of charge-dispersed anions into hydrogen-bonded networks

The binding properties and conformational adaptability of a known nitrate/sulfate receptor N,N'-3-azapentane-1,5-bis[3-(1-aminoethylidene)-6-methyl-3H-pyran-2,4-dione] (L) toward various charge-dispersed monoanions (HSO(3)(-), ClO(4)(-), IO(4)(-), PF(6)(-), and SbF(6)(-)) are considered. These anions template the folding of three HL(+) species through a self-assembly process into a new hollow supramolecular trication. During the self-assembly, all strong hydrogen-bond donors of the podand become coordinatively saturated by interactions with the oxo functionalities from other HL(+) molecules. In that way, only the weak hydrogen-bond-donating groups in the exterior part of the receptor are accessible for anion binding. The investigated anions are accommodated in the hydrophobic pockets of the isomorphous hydrogen-bonded frameworks, which serve as a basis for selective crystallization from the highly competitive anion/solvent systems. This behavior is discussed in terms of size and geometry of the anions as well as the receptor's coordination capabilities to provide the most favorable surroundings for guest inclusion both in solution and in the solid state.

Rotaxanes capable of recognising chloride in aqueous media

A new, versatile chloride-anion-templating synthetic pathway is exploited for the preparation of a series of eight new [2]rotaxane host molecules, including the first sulfonamide interlocked system. (1)H NMR spectroscopic titration investigations demonstrate the rotaxanes' capability to selectively recognise the chloride anion in competitive aqueous solvent media. The interlocked host's halide binding affinity can be further enhanced and tuned through the attachment of electron-withdrawing substituents and by increasing its positive charge. A dicationic rotaxane selectively binds chloride in 35% water, wherein no evidence of oxoanion binding is observed. NMR spectroscopy, X-ray structural analysis and computational molecular dynamics simulations are used to account for rotaxane formation yields, anion binding strengths and selectivity trends.

Protonated macrobicyclic hosts containing pyridine head units for anion recognition

In this paper, we report two macrobicyclic receptors containing pyridine head units derived from 1,10-diaza-15-crown[5] (L1) or 4,13-diaza-18-crown[6] (L2) that can be protonated in MeCN and used for anion recognition. The interaction of these protonated lateral macrobicycles with different anions has been investigated by means of spectrophotometric titrations in MeCN. The association constants for the complexes of halide anions with the protonated macrobicycles follow the sequences Cl(-)>Br(-)>I(-)>F(-) (L1) and Cl(-)>F(-)>I(-)>Br(-) (L2), whereby an increase of more than two logarithmic units is observed from F(-) to Cl(-) for the binding constants of the receptor derived from L1. The association constants also indicate an important degree of selectivity of these macrobicyclic receptors for Cl(-) over Br(-) or I(-). The X-ray crystal structure analyses of the chloride and bromide complexes confirms the formation of the envisaged supramolecular complexes. Moreover, the binding constants indicate that these receptors present a high sulfate-to-nitrate binding selectivity. The stability trend observed for the recognition of halide anions by the macrobicycles presented herein as well as the sulfate-to-nitrate binding selectivity have been rationalised by means of DFT calculations at the B3LYP/LanL2DZ level. These studies indicate that the especially high binding selectivity for Cl(-) is the result of the optimum fit between the protonated macrobicyclic cavity and the size of the anion, whereas the sulfate-to-nitrate selectivity results from shape complementarity between the hydrogen-binding acceptor sites on sulfate and the hydrogen-bond donors of the macrobicycle.

Amide-Based Ligands for Anion Coordination

Anion recognition is an active area of research in supramolecular chemistry. The rapidly increasing amount of structural data now allows anion coordination chemistry to be formalized in terms of coordination numbers and geometries based on hydrogen-bonding interactions between the host (ligand) and the guest (anion). This Minireview targets just one class of anion receptors, namely, amide-based ligands. The structural data for a series of five anion shapes are compiled according to coordination number, and distinct commonalities are observed within a given anion topology. The results also indicate a number of similarities between the coordination of anions and transition-metal ions.