Anion recognition by neutral macrocyclic amides

Although amides often serve as anchoring groups in natural and synthetic anion receptors, the structure-affinity relationship studies of amide-based macrocyclic receptors are still very limited. Therefore, we decided to investigate the influence of the size of the macroring on the strength and selectivity of anion binding by uncharged, amide-based receptors. With this aim, we synthesized a series of macrocyclic tetraamides derived from 2,6-pyridinedicarboxylic acid and aliphatic alpha,omega-diamines of different lengths. X-ray analysis shows that all ligands studied adopt expanded conformations in the solid state with the convergent arrangement of all four hydrogen-bond donors. 1H NMR titrations in DMSO solution revealed a significant effect of the ring size on the stability constants of anion complexes; the 20-membered macrocyclic tetraamide 2 is a better anion receptor than its both 18- and 24-membered analogues. This effect cannot be interpreted exclusively in terms of matching between anion diameter and the size of macrocyclic cavity, because 2 forms the most stable complexes with all anions studied, irrespective of their sizes. However, geometric complementarity manifests in extraordinarily high affinity of 2 towards the chloride anion. The results obtained for solutions were interpreted in the light of solid-state structural studies. Taken together, these data suggest that anion binding by this family of macrocycles is governed by competitive interplay between their ability to adjust to a guest, requiring longer aliphatic spacers, and preorganization, calling for shorter spacers. The 20-membered receptor 2 is a good compromise between these factors and, therefore, it was selected as a promising leading structure for further development of anion receptors. Furthermore, the study of an open chain analogue of 2 revealed a substantial macrocyclic effect. X-ray structure of the acyclic model 14 suggests that this may be due to its ill-preorganized conformation, stabilized by two intramolecular hydrogen bonds.     

Anion recognition with porphyrin-bottomed tetraurea receptors

Three tetraurea-appended porphyrin-derived anion receptors were synthesized and were found to efficiently encapsulate various anions with high association constants of up to 1.1×10⁸ L/mol.     

A new macrocycle demonstrates ditopic recognition properties

The synthesis and binding properties of a new macrocycle is reported. The host, comprised of three basic pyridines, four hydrogen bond accepting carbonyls, and two hydrogen bond donating amide groups, binds mono-alkyl ammonium salts in a manner that is dependent on the counter-ion of the ammonium guest.     

含酚羟基Schiff碱化合物的阴离子识别研究

近年来,超分子化学及其应用研究越来越受到科学家的重视[1].许多氢键供体已成功应用于阴离子识别研究中,如酰胺[2]、硫脲[3]及吡咯大环[4]化合物等.含有酚羟基的Schiff碱型化合物常被用于阳离子识别体系中[5,6],而以其为阴离子识别位点的研究相对较少.作为研究超分子化合物的合成、识别性能等工作的一部分[7],我们设计合成了三种具有多位点识别且含有酚羟基的Schiff 碱新型受体.考察了主-客体配合物中所形成的氢键数目对阴离子选择性识别的影响.     

The synthesis and recognition properties of colorimetric fluoride receptors bearing sulfonamide

Two artificial receptors, 1,2-bis-p-substituted phenyl-sulfonamido-4,5-bis-nitrobenzene, have been designed and synthesized. The interactions of these receptors with halide anions are determined by UV-vis and ¹H NMR titration experiments. Results indicate that two receptors have strong sensitivity and selectivity for fluoride among halide anions. In addition, the visible color changes upon the addition of fluoride anion can make the receptors as convenient detection tools for fluoride anion.     

基于二吡咯甲烷的双腙类阴离子受体性质研究

设计合成了一种含腙基单元的二吡咯甲烷类阴离子受体1,通过UV-vis和1H NMR滴定波谱研究了1对不同阴离子的识别作用。测试结果表明1在DMSO中对Ac O-和F-表现出很好的选择性识别作用,对它们的结合常数分别为(1.19±0.14)×104和(5.31±0.33)×103L·mol–1。Job法证明1与Ac O-形成了1∶1型配合物,1H NMR滴定实验则表明1与Ac O-之间建立了分子间氢键作用。     

Anion recognition through hydrogen bonding by adamantane-dipyrromethane receptors

Adamantane-dipyrromethane (AdD) receptors [di(pyrrole-2-yl)methyladamantane (1), 2,2-di(pyrrole-2-yl)adamantane (2), 1,3-bis[di(pyrrole-2-yl)methyl]adamantane (3), 2,2,6,6-tetra(pyrrole-2-yl)adamantane (4)] form complexes with F⁻, Cl⁻, Br⁻, AcO⁻, NO₃⁻, HSO₄⁻, and H₂PO₄⁻. The association constants of the complexes were determined by ¹H NMR titrations, whereas the geometries of complexes 1·F⁻ (2:1), 2·F⁻ (2:1), 2·Cl⁻ (2:1), 2·AcO⁻ (2:1), and 4·F⁻ (1:1) were determined by X-ray structural analysis. The most stable complexes are of 2:1 stoichiometry with F⁻ and AcO⁻. The stability constants are in accordance with the anion basicity and the ability of AdD receptors to place the hydrogen bonding donor groups in a tetrahedral fashion around anions. The binding energies of the complexes between receptors 1-4 and F⁻ anion are calculated using quantum chemical methods. The calculated results show that the solvent polarity is important for the complexation of fluoride ion with AdD receptors 1-4.     

Anion recognition in water with use of a neutral uranyl-salophen receptor

A new water-soluble uranyl-salophen complex incorporating two glucose units has been synthesized. This neutral derivative shows noteworthy binding affinity for fluoride in water thanks to the Lewis acid-base interaction occurring between the metal and the anion. Such interaction is strong enough to overcome the high hydration enthalpy of fluoride. Moreover this complex effectively binds hydrogen phosphate and exhibits remarkably strong association for nucleotide polyanions ADP(3-) and ATP(4-).     

Anion recognition by 1,2,3-triazolium receptors: application of click chemistry in anion recognition

Cyclic and acyclic bile acid-based 1,2,3-triazolium receptors which show remarkable ability to recognize anions through C-H...X- hydrogen bond interactions have been synthesized using click chemistry.     

Anion receptors: a new class of amide/quaternized amine macrocycles and the chelate effect

A new class of tetraamide macrocyclic receptors for anions with two quaternized amine functionalities exhibited higher affinities for anions compared with the corresponding neutral amides. In two crystal structures of halide complexes of the prototypes with phenyl and pyridine spacers, the anions are held by hydrogen bonding with the amide hydrogens. The pyridine analogues display higher affinities in general than the phenyl systems, a phenomenon which is attributed to the anion version of the chelate effect.     

Anion recognition using preorganized thiourea functionalized [3]polynorbornane receptors

[structure: see text] A new family of [3]polynorbornane frameworks exhibiting conformationally preorganized aromatic thiourea (cleft-like) receptors have been designed and synthesized for anion recognition. These show excellent affinity for the biologically relevant dihydrogenphosphate (H(2)PO(4)(-)) and dihydrogenpyrophosphate (H(2)P(2)O(7)(2)(-)) anions (among others), which are bound in 1:1 and 2:1 (host:anion) ratio, respectively. Moreover, visually striking color changes accompany guest binding, enabling this family to act as colorimetric anion sensors.     

Investigating the effects of structure on sulfate recognition by neutral dipeptide receptors

A small library of neutral peptide-based anion receptors was synthesised, where changes were made to the scaffold structure to investigate the effect these structural features have on the anion binding ability of these receptors. These changes included shortening the peptide side chain lengths, increasing the number of electron withdrawing substituents present on the squaramide phenyl substituents and increasing the length and flexibility of the peptide backbone. An effort was also made to increase the aqueous solubility of these receptors by functionalising the N-terminus of the peptide with a hydrophilic moiety. All the receptors displayed strong affinity and selectivity for sulfate in 20% v/v H₂O/DMSO-d₆ and a 5-fold increase in the affinity of the thiourea receptors was observed upon shortening the side chains by one methylene unit. Overall, the squaramide derivatives displayed much stronger association, in this competitive media, than the thiourea based receptors.     

The anion binding properties of neutral receptors bearing nitro group,phenanthroline or ruthenium(II) metal

A series of neutral cyclohexadiamine anion receptors containing nitro, phenanthroline or ruthenium(II) have been designed and synthesized. Their u.v.–vis spectroscopy investigations reveal that the receptor bearing nitro group displays the strongest affinities for F⁻, AcO⁻, H₂PO ₄ ⁻ and can be used as an efficient detection tool for the above anions. Results indicate that the anion affinities can be enhanced through appending nitro group and ruthenium(II) metal compared with phenanthroline moiety.     

Anion recognition by d-ribose-based receptors

Anion recognition properties of d-ribose-based receptors α- and β-1 were measured by ¹H NMR in CDCl₃ and MeCN-d₃. Receptor β-1 showed effective binding with anions by cooperative hydrogen bonds of cis-diol. The anomeric isomer α-1 is a less effective anion receptor which has similar cis-diol as a recognition site, indicating that the stereo configuration of the anomeric position is of significant influence on the anion recognition ability.     

New redox anion receptors based on calix[4]pyrrole bearing ferrocene amide

Two redox anion receptors based on calix[4]pyrrole and ferrocene have been synthesized. The electrochemical investigation revealed that these compounds can be response to the anions with different shifts of Fc/Fc⁺ couple. With the ¹H NMR titration study, the selectivity to F⁻ and AcO⁻ ions in CD₃CN solution was confirmed. The conformations of the mono-aromatic meso-substituted calix[4]pyrroles, which were the synthetical intermediate of the ferrocene based receptors, and their anion complexes in the solid state have also been studied by single X-ray crystallography, and the rationality of the crystal conformations was proved by theoretical study.     

Anion effects on the recognition of ion pairs by calix[4]arene-based heteroditopic receptors

A novel heteroditopic receptor (5) based on a rigid calix[4]arene cavity bearing at the upper rim four arylsulfonamido binding sites has been synthesized. The binding abilities of this new host have been investigated in apolar solvents toward a series of tetramethylammonium salts (tosylate, chloride, acetate, trifluoroacetate, and picrate) and compared with those of monotopic and heteroditopic calix[4]arene-bis(crown-3)-based receptors 1 and 2 in order to evaluate the role of the anion on ion-pair recognition. While monotopic host 1 shows an efficiency order toward the different salts that increases when the anion is less interactive (Hofmeister trend), an opposite role of the counteranion on the recognition process was observed with host 5 (anti-Hofmeister trend). A more complex behavior is experienced by host 2, which shows a high and leveled efficiency for all the anions tested. The results obtained were explained on the basis of the different types of ion pairs present in the recognition process. Further information on the role of the anion were obtained by the "dual host" strategy utilizing the tri-n-butylthioureido derivative of tren 7, which forms a stable complex with chloride anion. The very high efficiency shown by these heteroditopic hosts opens new routes in supramolecular projects and is a very interesting tool in the molecular recognition of ion-pairs and its applications.     

Anion induced conformational switch of a macrocyclic amide receptor

Isophthalic acid-based macrocyclic tetraamide 4 shows considerable conformational change during anion binding. In the solid state and in solution the free receptor exists in nonbonding, closed conformation stabilized by two intramolecular hydrogen bonds. Upon anion complexation, the receptor switches to a conformation with convergent arrangement of hydrogen bond donors. The conformational switch is evidenced by 2D NMR and X-ray analyses of the free ligand and its Cl⁻ complex.     

Anion,cation and ion-pair recognition by bis-urea based receptors containing a polyether bridge

Two bis-urea type receptors were synthesized containing a polyether bridge and two 4-nitrophenyl groups as chromogenic units, R1 and R2. Molecular recognition studies of receptors towards different tetraalkylammonium and alkali metal salts were carried out in DMSO by UV-Vis and ¹H-NMR spectroscopy. The receptors were found to have high affinity for diverse anions and ion-pairs, showing the highest affinities towards the tetramethylammonium and sodium salts. The cation binding ability of the receptors was evidenced by means ¹H NMR, mass spectrometric ESI⁺ studies and the crystal structures of some precursors. Additionally, the molecular modelling at the DFT level of the tetramethylammonium acetate complexes illustrates the potential ion-pair binding ability of the receptors: the anion is recognized through strong hydrogen bonds from the NH─ groups from the two urea sites, while the cation is bound by a combination of cation─π, C-H···O and C-H···π interactions.