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

Solvent Acts as the Referee in a Match-Up Between Charged and Preorganized Receptors

The prevalence of anion-cation contacts in biomolecular recognition under aqueous conditions suggests that ionic interactions should dominate the binding of anions in solvents across both high and low polarities. Investigations of this idea using titrations in low polarity solvents are impaired by interferences from ion pairing that prevent a clear picture of binding. To address this limitation and test the impact of ion-ion interactions across multiple solvents, we quantified chloride binding to a cationic receptor after accounting for ion pairing. In these studies, we created a chelate receptor using aryl-triazole CH donors and a quinolinium unit that directs its cationic methyl inside the binding pocket. In low-polarity dichloromethane, the 1 : 1 complex (log K1 : 1 ~ 7.3) is more stable than neutral chelates, but fortuitously comparable to a preorganized macrocycle (log K1 : 1 ~ 6.9). Polar acetonitrile and DMSO diminish stabilities of the charged receptor (log K1 : 1 ~ 3.7 and 1.9) but surprisingly 100-fold more than the macrocycle. While both receptors lose stability by dielectric screening of electrostatic stability, the cationic receptor also pays additional costs of organization. Thus even though the charged receptor has stronger binding in apolar solvents, the uncharged receptor has more anion affinity in polar solvents.

Influence of Triazole Substituents of Bis-Heteroleptic Ru(II) Probes toward Selective Sensing of Dihydrogen Phosphate

A series of seven new bis-heteroleptic Ru(II) probes (1[PF₆]₂-7[PF₆]₂) along with two previously reported probes (8[PF₆]₂ and 9[PF₆]₂) containing a similar anion binding triazole unit (hydrogen bond donor) functionalized with various substituents are employed in a detailed comparative investigation for the development of superior selective probes for H₂PO₄^-. Various solution- and solid-state studies, such as ¹H-DOSY NMR, dynamic light scattering (DLS), single-crystal X-ray crystallography, and transmission electron microscopy (TEM), have established that the selective sensing of H₂PO₄^- by this series of probes is primarily due to supramolecular aggregation driven enhancement of ³MLCT emission. Intestingly, 1[PF₆]₂ and 7[PF₆]₂, having an electron-deficient (π-acidic) aromatic pentafluorophenyl substituent are found to be superior probes for H₂PO₄^- in comparison to the other aryl- and polyaromatic-substituted analogues (2[PF₆]₂-6[PF₆]₂, 8[PF₆]₂, and 9[PF₆]₂), in terms of a higher enhancement of the ³MLCT emission band, a greater binding constant, and a lower detection limit. The superiority of 1[PF₆]₂ and 7[PF₆]₂ could be due to better supramolecular aggregation properties in the cases of pentafluorophenyl analogues via both hydrogen bonding and anion-fluorine/anion-π noncovalent interactions.

Macrocyclic Transmembrane Anion Transporters via a One-Pot Condensation Reaction

Synthetic chloride transporters are potential therapeutic agents for cystic fibrosis and cancer. Reported herein are macrocyclic transmembrane chloride transporters prepared by a one-pot condensation reaction. The most efficient macrocycle possesses a fine balance of hydrophobicity for membrane permeation and hydrophilicity for ion recognition. The macrocycle transports chloride ions by forming channels in the membrane. Hydrogen bonds and anion-π interactions assist chloride transport.

A fluorescent "ON-OFF-ON" switch for the selective and sequential detection of Hg2+ and I- with applications in imaging using human AGS gastric cancer cells

A new fluorescent "on-off-on" probe (BIPQ) is designed and developed which selectively binds with Hg²⁺; its emission intensity is quenched almost 40-fold at 455 nm without interference from other metal cations. On gradual addition of I^- to the solution of BIPQ-Hg²⁺, the emission reverts to its original intensity. The limits of detection of BIPQ for Hg²⁺ and I^- are found to be on the order of 3.12 × 10^-9 and 5.48 × 10^-8 M, respectively, which shows clearly that BIPQ can sense Hg²⁺ at a very minute level. DFT and TDDFT studies are conducted with the probe to establish similarity between theoretical and experimental outcomes. Finally, to demonstrate its practical benefit in biological fields, live cell imaging experiments with BIPQ are carried out to detect Hg²⁺ in human AGS gastric cancer cell lines.

Traditional hydrogen bonding donors controlled colorimetric selective anion sensing in tripodal receptors: First-naked-eye detection of cyanide by a tripodal receptor via fluoride displacement assay

Here in we report tris (3-aminopropyl) amine based tripodal receptors L, L¹ and L² which were functionalized with 4-nitrophenyl moieties having thio-urea, amide and sulfonamide as hydrogen bonding moieties respectively, shows a strong selectivity towards cyanide. A competitive colorimetric assay with L in the presence of fluoride ion suggests that the cyanide ion is much capable of displacing the bound fluoride, showing a sharp distinguishable color change. To the best of our knowledge, this is the first example of a naked-eye detection of cyanide via fluoride displacement assay by a tripodal receptor and such a displacement phenomenon is not observes in the cases of L¹ and L², instead the receptor L¹ binds nitrate and cyanide; L² binds dihydrogen phosphate and cyanide. Using this assay, we have proposed an AND logic gate using L·F^- and CN^-.

Supramolecular Self-Assembly Driven Selective Sensing of Phosphates

A new bis-heteroleptic Ru^II complex (1[PF₆]₂) with iodotriazole as the anion binding group along with the attached pyrene moiety is developed to investigate anion sensing properties and the origin of its selectivity toward a particular class of anions. Selective sensing of phosphates over other anions in both the solution and solid states by 1[PF₆]₂ is clearly evident from the perturbation of the absorption band and a large degree of amplification of ³MLCT emission band in the presence of phosphates. Importantly, macroscopic investigation such as Scanning Electron Microscopy (SEM) and Dynamic Light Scattering (DLS) indicated the formation of supramolecular architecture in the presence of dihydrogen phosphate via halogen bonding interaction and π-π stacking of pyrene moieties. Such macroscopic property is further corroborated by solution and solid state spectroscopic studies, e.g., ¹H-DOSY NMR, single crystal X-ray crystallography, and solid state photoluminescence (PL) spectroscopy.

Conformational Selection in Anion Recognition: cGMP-Selective Binding by a Naphthalimide-Functionalized Amido-Amine Macrocycle

Amido-amine macrocycles with two and four naphthalimide dyes were designed to bind nucleoside monophosphates and oligonucleotides in an aqueous buffered solution. Anion-templated synthesis was used to direct the macrocyclization reaction to the [2+2] product, while high dilution conditions favored the formation of the [4+4] macrocycle with an unprecedented geometry, as revealed from the X-ray analysis. The [2+2] product was found to exhibit a remarkable binding strength and fluorescence response for cyclic guanosine monophosphate (cGMP) in an aqueous solution. To our knowledge, this is the first synthetic receptor for cGMP, which also demonstrates a high preference to bind guanine-rich sequences accomplished by a strong fluorescence quenching. The receptor conformation is very sensitive to the guest structure in an aqueous solution, thus modeling the adaptive behavior of proteins. The study of synthetic systems with a detectable conformational equilibrium represents a great potential for understanding highly specific and tightly regulated interactions in biological systems.

Anion Complexation Studies of 3-Nitrophenyl-Substituted Tripodal Thiourea Receptor: A Naked-Eye Detection of Sulfate via Fluoride Displacement Assay

A thiourea-based tripodal receptor L substituted with 3-nitrophenyl groups has been synthesized, and the binding affinity for a variety of anions has been studied by ¹H NMR titrations and nuclear Overhauser enhancement spectroscopy experiments in dimethyl sulfoxide-d₆. As investigated by ¹H NMR titrations, the receptor binds an anion in a 1:1 binding mode, showing the highest binding and strong selectivity for sulfate anion. A competitive colorimetric assay in the presence of fluoride suggests that the sulfate is capable of displacing the bound fluoride, showing a sharp visible color change. The strong affinity of L for sulfate was further supported by UV-vis titrations and density functional theory (DFT) calculations. Time-dependent DFT calculations indicate that the fluoride complex possesses a different optical absorption spectrum (due to charge transfer between the fluoride and the surrounding ligand) than the sulfate complex, reflecting the observed colorimetric change in these two complexes. The receptor was further tested for its biocompatibility on primary human foreskin fibroblasts and HeLa cells, exhibiting an excellent cell viability up to 100 μM concentration.

Catching the chloride: searching for non-Hofmeister selectivity behavior in systematically varied polyamide macrocyclic receptors

Searching for regularities in the large set of structurally diverse macrocyclic probes allowed us to determine the structural requirements for the selective recognition of chloride over more basic anions such as H₂PO₄⁻ or RCO₂⁻ by a putative anion receptor.

Synthesis and structural characterization of anion complexes with azacalix[2]dipyrrolylmethane: effect of anion charge on the conformation of the macrocycle

Tetrahomodiazacalix[2]dipyrrolylmethane 1, [-CH2(C4H3N)CR2(C4H3N)CH2N(Me)-]2 an expanded version of the calix[4]pyrrole system, has the tendency to change its ring conformation (1,3-alternate) upon anion binding analogous to calix[4]pyrrole. However, owing to its tertiary amine nitrogen atoms in the ring, it can be protonated and becomes a powerful cationic receptor for anions, besides its inherent hydrogen bonding features. Macrocycle 1 binds with a series of monoanions BF4(-), Cl(-), PhCOO(-) and ClO4(-), and their X-ray structures showed that the ring conformation constitutes the 1,2-alternate form. Upon binding with dianions SO4(2-), CrO4(2-), SiF6(2-) and S2O3(2-), the ring conformation changes to the cone conformation. The intermediate partial cone conformation results for complexes with NO3(-) and Cr2O7(2-) ions. The change in the orientation of the pyrrole NH groups depending on the charge of the anion demonstrates the flexibility of the macrocycle and the dicationic macrocycle stabilizes the anions via both hydrogen bonding and electrostatic interactions.

A mercuric ensemble based on a cycloruthenated complex as a visual probe for iodide in aqueous solution

A new water-soluble cycloruthenated complex Ru(bthiq)(dcbpy)2(+) (1, Hbthiq=1-(2-benzo[b]thiophenyl)isoquinoline, dcbpy=4,4'-dicarboxylate-2,2'-bipyridine) was designed and synthesized to form its mercuric ensemble (1-Hg(2+)) to achieve visual detection of iodide anions. The binding constant of 1-Hg(2+) is calculated to be 2.40×10(4)M(-1), which is lower than that of HgI2. Therefore, the addition of I(-) to the aqueous solution of 1-Hg(2+)lead to significant color changes from yellow to deep-red by the release of 1. The results showed that iodide anions could be easily detected by the naked eyes. The detection limit of iodide anion is calculated as 0.77μM. In addition, an easily-prepared test strip of 1-Hg(2+) was obtained successfully to detect iodide anions.

A case of oxoanion recognition based on combined cationic and neutral C–H hydrogen bond interactions

A bidentate bis-(benzimidazolium) receptor containing pyrene as fluorescent signaling units recognizes sulphate and hydrogenpyrophosphate in a competitive water–DMSO medium through combinations of cationic and neutral C–H hydrogen bonding.

Hg2+-induced deprotonation of an anthracene-based chemosensor: set–reset flip-flop at the molecular level using Hg2+ and I− ions

A simple chemosensor 1 shows a “turn-off” sensing of Hg²⁺ ions, whereas the in situ formed Hg²⁺ complex can be used for “turn-on” sensing with I⁻ ions. These fluorescence changes have been used for the construction of a set–reset flip-flop circuit.

Multifunctional Macrocyclic Receptors as Templates for Aromatic Amino Acids: A Rare Example of a Highly Selective Multi-Input Multi-Output Chemo-"Logic Gate"

Proton-bound [M⋅H⋅G]⁺ diastereomeric complexes between some chiral aromatic amino acids or dipeptides (G) and a chiral multifunctional macrocyclic receptor (M=Chirabite-A) undergo, in the gas phase, highly selective substitution and addition reactions by amines, such as 2-aminobutane and piperidine. All the [M⋅H⋅G]⁺ complexes follow time-dependent monoexponential decays. In some cases, the kinetic curves exhibit a plateau revealing the presence of unreactive [M⋅H⋅G]⁺ structures. In them, the amino acid is accommodated in the macrocycle cavity in the zwitterionic form by sharing its acidic hydrogen atoms with the pyridine nitrogen atoms of the host. The same interactions are structurally inaccessible to G=dipeptides or monofunctional amines, which then can be readily released from [M⋅H⋅G]⁺ . When the amino acid interacts with the amidocarbonyl oxygen atoms pointing outside the macrocycle cavity, it saves the canonical structure and can be readily displaced by the amine. The Chirabite-A may act as an efficient template for aromatic amino acids by releasing them or not depending upon the amino acid configuration and the basicity of the amine. These unique properties confer to the gas-phase diastereomeric [M⋅H⋅G]⁺ complexes the features of multi-input multi-output chemo-"logic gates".

Thiazole-containing conjugated polymer as a visual and fluorometric sensor for iodide and mercury

A neutral conjugated polymer poly-p-phenylene (PPP) derivative, poly(1,4-bis-(8-(4-phenylthiazole-2-thiol)-octyloxy)-benzene) (PPT), was prepared using a simple and economical method of oxidative polymerization reaction. This newly synthesized polymer PPT was characterized by means of Fourier transform infrared spectroscopy (FT-IR), (1)H nuclear magnetic resonance ((1)H NMR), ultraviolet-visible (UV-Vis), and fluorescence spectroscopy. PPT displays fluorescence "turn-off/turn-on" characteristics and colorimetric responses to I(-) and Hg(2+). The UV-Vis and fluorescence spectra of the PPT showed a significant shift in λmax via the addition of iodides and mercury. A colorless PPT solution turns to deep yellow in the presence of iodide salts, which subsequently becomes colorless again on addition of Hg(2+) salts that could be easily detected visually by the naked eye. The Stern-Volmer constant (Ksv) value obtained for the detection of iodide is 0.13 × 10(5) M(-1), confirming very high sensitivity of this polymer for iodide salts. The detection limit of Hg(2+) salt using the PPT polymer was found to be 2.1 nM in water. The detection of both iodide and mercury was also possible in solid state by using a membrane film prepared by mixing 1% PPT in polystyrene. This membrane changes color in the presence of iodide as well as mercury salts. These results confirm that the PPT polymer can be applied for the colorimetric as well as fluorometric sensing of I(-) and Hg(2+) ions in a competent environment in solution, as well as in the solid state, using a membrane film rapidly.

A case for molecular recognition in nuclear separations: sulfate separation from nuclear wastes

In this paper, we present the case for molecular-recognition approaches for sulfate removal from radioactive wastes via the use of anion-sequestering systems selective for sulfate, using either liquid-liquid extraction or crystallization. Potential benefits of removing sulfate from the waste include improved vitrification of the waste, reduced waste-form volume, and higher waste-form performance, all of which lead to potential cleanup schedule acceleration and cost savings. The need for sulfate removal from radioactive waste, especially legacy tank wastes stored at the Hanford site, is reviewed in detail and primarily relates to the low solubility of sulfate in borosilicate glass. Traditional methods applicable to the separation of sulfate from radioactive wastes are also reviewed, with the finding that currently no technology has been identified and successfully demonstrated to meet this need. Fundamental research in the authors' laboratories targeting sulfate as an important representative of the class of oxoanions is based on the hypothesis that designed receptors may provide the needed ability to recognize sulfate under highly competitive conditions, in particular where the nitrate anion concentration is high. Receptors that have been shown to have promising affinity for sulfate, either in extraction or in crystallization experiments, include hexaurea tripods, tetraamide macrocycles, cyclo[8]pyrroles, calixpyrroles, and self-assembled urea-lined cages. Good sulfate selectivity observed in the laboratory provides experimental support for the proposed molecular-recognition approach.

A Quinoline Based bis-Urea Receptor for Anions: A Selective Receptor for Hydrogen Sulfate

A dipodal bis-urea receptor has been synthesized from the reaction of 8-amino quinoline and 1,4-phenylene diisocyanate in dichloromethane, and the anion binding ability of the receptor has been studied using fluoride, chloride, bromide, iodide, perchlorate, nitrate, dihydrogen phosphate and hydrogen sulfate by UV-Vis titrations in DMSO. The results show that the receptor binds each of the anions with a 1:1 stoichiometry, showing high affinity and moderate selectivity for hydrogen sulfate among the anions studied. Ab initio calculations based on density functional theory (DFT) suggest that an anion (X-) is bonded within the cleft formed by the two arms of the receptor through two NH…X- and two aromatic CH…X- interactions. The results from solution and theoretical studies suggest that binding is predominantly influenced by hydrogen bonding interactions and the basicity of anions.

Cryptand-like anion receptors

The design of supramolecular hosts for anions began with simple diaza bicycles, named katapinands, and has evolved over the last 40 years to a number of elegantly designed receptors capable of binding many different anions. About the same time the term cryptand appeared in reference to another bicyclic compound that was selective for alkaline-earth ions. Since the first report these simple bicycles, a vast arena of hosts has appeared, including acyclic, monocyclic, and other multicyclic supramolecular receptors. Studies of these systems have revealed considerable information about anion coordination chemistry, including the fact that many of these complexes mimic their transition-metal corollaries in terms of coordination numbers. However, for anions interactions occur via H-bonding most often, rather than the coordinate covalent or dative bonds observed in transition-metal coordination. This critical review examines the design of enclosed, primarily bicyclic cryptands as hosts for anions, with a small scattering of higher polyhedra when deemed appropriate to the discussion. In order to show the development (evolution) of the field, key examples of early work will be noted and compared with more recent developments (136 references).

A new hexaaminomacrocycle for ditopic binding of bromide

A hexaaminomacrocycle L, containing four secondary and two tertiary amines has been synthesized and crystallized with hydrobromic acid. Structural analysis of the bromide complex suggests that the ligand in its tetrtaprotonated form, is involved in coordinating two bromides from both sides via hydrogen bonding interactions with N…Br(-) distance of 3,351 Å, forming a ditopic complex. The other two bromides are outside the cavity, and singly bonded to the macrocycle. The molecules are packed showing layer structures in which the internal bromides are locked between the layers of macrocycles. The bromide anions are coordinated alternatively by one and two hydrogen bonds with the protonated amines from the two adjacent macrocycles.

Recognition and sensing of fluoride anion

Fluoride anion recognition is attracting a mounting interest in the scientific community due to its duplicitous nature. It is a useful chemical for many industrial applications, and it has been used in human diet, but, recently it has been accused for several human pathologies. Here we describe the ample panorama of different approaches the chemists world-wide have employed to face the challenge of fluoride binding, and we outline some of the research which in our view can contribute to the development of this field, especially when fluoride binding has to be achieved in highly competitive protic solvents and water.