Applications of Supramolecular Anion Recognition

Ion mobility mass spectrometry - an efficient tool for the analysis of conformational switch of macrocyclic receptors upon anion binding

Interactions between anions and synthetic macrocyclic receptors belong to the extensively explored area of research due to the particularly important functions of anions in biological and environmental sciences. Structures of anion-macrocycle complexes are closely related to their function, highlighting the importance of structural analysis of the complexes. Here, we discuss the application of ion mobility mass spectrometry (IM-MS) and theoretical calculations to the structural analysis of tetralactam macrocycles (M) with varying flexibility and structural properties, and their complexes with anions [M + X]^-. Collision cross section (CCS) values obtained from both direct drift tube (DT) and indirect using traveling-wave (TW) IM-MS measurements supplemented by theoretical calculations were successfully used to describe the structural properties of various macrocycle-anion complexes, proving the suitability of the IM-MS approach for sensitive, selective, and fast detection of anion complexes and characterization of their structures and conformations.

Solvent Effects in Halogen and Hydrogen Bonding Mediated Electrochemical Anion Sensing in Aqueous Solution and at Interfaces

Sensing anionic species in competitive aqueous media is a well-recognised challenge to long-term applications across a multitude of fields. Herein, we report a comprehensive investigation of the electrochemical anion sensing performance of novel halogen bonding (XB) and hydrogen bonding (HB) bis-ferrocene-(iodo)triazole receptors in solution and at self-assembled monolayers (SAMs), in a range of increasingly competitive aqueous organic solvent media (ACN/H2 O). In solution, the XB sensor notably outperforms the HB sensor, with substantial anion recognition induced cathodic voltammetric responses of the ferrocene/ferrocenium redox couple persisting even in highly competitive aqueous solvent media of 20 % water content. The response to halides, in particular, shows a markedly lower sensitivity to increasing water content associated with a unique halide selectivity at unprecedented levels of solvent polarity. The HB sensor, in contrast, generally displayed a preference towards oxoanions. A significant surface-enhancement effect was observed for both XB/HB receptive films in all solvent systems, whereby the HB sensor generally displayed larger responses towards oxoanions than its halogen bonding analogue.

Highly efficient coumarin-derived colorimetric chemosensors for sensitive sensing of fluoride ions and their applications in logic circuits

In this study, three new compounds 6-substituted-3-acetylcoumarin 4-(2'-isocamphanyl)thiosemicarbazones (3a, 3b, and 3c) were facilely synthesized and employed as colorimetric chemosensors for fluoride ions. The recognition behaviors of receptors 3a, 3b, and 3c toward F^- were investigated by using UV-vis absorption spectroscopy. Among them, the receptor 3c displayed more superior sensitivity and rapid response time toward F^- with a swift naked-eye color change from colorless to purple, and its detection limit was as low as 6.3 × 10^-7 M, and the binding constant was calculated to be 2.58 × 10⁴ M^-1. Furthermore, the interaction mechanism between the receptor 3c and F^- was studied by ¹H NMR, HRMS, and density functional theory (DFT) studies, suggesting that initial formation of a hydrogen-bonded host-guest complex and the subsequent deprotonation of receptor 3c upon the addition of excess F^-, which was responsible for the remarkable changes in the absorption spectra of 3c. Besides, a combinatorial logic circuit of IMPLICATION and INHIBITION gates at the molecular level was fabricated using the reversibility of receptor 3c toward F^- and Mg²⁺. Finally, the test strips coated with receptor 3c revealed a good sensitivity for F^- in an aqueous medium. Apart from that, the excellent performance of receptor 3c for detecting F^- was reconfirmed by grinding them with KBr powder.

A Fluorescence Sensor Array Based on Zinc(II)-Carboxyamidoquinolines: Toward Quantitative Detection of ATP*

The newly prepared fluorescent carboxyamidoquinolines (1-3) and their Zn(II) complexes (Zn@1-Zn@3) were used to bind and sense various phosphate anions utilizing a relay mechanism, in which the Zn(II) ion migrates from the Zn@1-Zn@3 complexes to the phosphate, namely adenosine 5'-triphosphate (ATP) and pyrophosphate (PPi), a process accompanied by a dramatic change in fluorescence. Zn@1-Zn@3 assemblies interact with adenine nucleotide phosphates while displaying an analyte-specific response. This process was investigated using UV-vis, fluorescence, and NMR spectroscopy. It is shown that the different binding selectivity and the corresponding fluorescence response enable differentiation of adenosine 5'-triphosphate (ATP), adenosine 5'-diphosphate (ADP), pyrophosphate (PPi), and phosphate (Pi). The cross-reactive nature of the carboxyamidoquinolines-Zn(II) sensors in conjunction with linear discriminant analysis (LDA) was utilized in a simple fluorescence chemosensor array that allows for the identification of ATP, ADP, PPi, and Pi from 8 other anions including adenosine 5'-monophosphate (AMP) with 100 % correct classification. Furthermore, the support vector machine algorithm, a machine learning method, allowed for highly accurate quantitation of ATP in the range of 5-100 μM concentration in unknown samples with error <2.5 %.

Cooperative Recognition of Ni2+ Triggered by Fluoride Ions in Naturally Occurring α-Hydroxyquinone Derivatives

Perezone is a naturally occurring hydroxyquinone that has been deeply studied from different chemical aspects, such as therapeutics, electrochemistry, physical-chemical properties, or synthetic approaches that turn it an attractive template for new semisynthetic derivatives with a wide range of purposes. Herein, we describe a facile synthetic pathway to obtain new perezone derivatives by the addition of a pyrrole moiety that can be used for ion recognition. Compounds 2-4 showed the capability to interact with several anions and M2+ cations as separate events that result in colorimetric changes. Moreover, the compounds can behave as heteroditopic receptors. Besides, a previous interaction between fluoride ions and perezone derivatives triggered a successful recognition of M2+ ions, remarking Ni2+ as the most interesting phenomenon. These results project the compounds as potential colorimetric receptors for nickel ions in complex solutions.

Sulphonamidic Groups as Electron-Withdrawing Units in Ureido-Based Anion Receptors: Enhanced Anion Complexation versus Deprotonation

A sulphonamidic moiety was utilized as an electron-withdrawing group for enhancement of anion complexation features of urea-based receptors. A series of receptors varying in acidity of sulphonamidic and urea NH groups was synthesized and thoroughly tested. The individual complexation properties reflect deprotonation/complexation equilibrium in a given molecule as a function of the substitution. The receptors containing electron-donating groups in conjugation to the sulphonamidic moiety showed higher association constants towards H₂ PO₄ ^- and carboxylate anions, while those containing electron-withdrawing groups inclined to deprotonation of sulphonamidic NH. The deprotonation issue can be avoided by alkylation at the early step of receptor synthesis or it can be utilized for insertion of suitable groups that enable its anchoring on various substrates to form more elaborated receptor structures.

Anion Chelation via Double Chalcogen Bonding: The Case of a Bis-telluronium Dication and Its Application in Electrophilic Catalysis via Metal-Chloride Bond Activation

Telluronium cations have long been known to engage their counteranions via secondary interactions. Yet, this property has rarely been exploited for anion binding. Motivated by such an application, we have now synthesized a bis-telluronium dication ([3]²⁺) that was obtained as a tetrafluoroborate salt by reaction of 2,7-di-tert-butyl-9,9-dimethylxanthene-4,5-diboronic acid with phenoxatellurine difluoride and BF₃·OEt₂. As confirmed by the formation of Te-(μ-BF₄)-Te bridges in the structure of [3][BF₄]₂, [3]²⁺ functions as a bidentate Lewis acid toward anions. [3][BF₄]₂ has also been converted into the more exposed [3][BArF₂₄]₂ ([BArF₂₄]^- = [B(3,5-(CF₃)₂C₆H₃)₄]^-). The latter, which readily ionizes Ph₃CCl, displays a chloride anion binding constant that exceeds that of a monofunctional model compound by almost 4 orders of magnitude. The unique properties of this new bis-telluronium dication are further highlighted by its ability to activate Ph₃PAuCl and cis-(Ph₃P)₂PtCl₂, leading to catalytic systems highly active in the cycloisomerization of propargylamide or enyne substrates.

Transport of Anions across the Dialytic Membrane Induced by Complexation toward Dendritic Receptors

A novel approach to inducing anion transport over the dialytic membrane was proposed and successfully tested using the dihydrogen phosphate anion. The anion receptor based on isophthalamide was anchored on a dendritic skeleton, resulting in a macromolecular structure with a limited possibility to cross the dialytic membrane. The dendritic receptor was placed in a compartment separated from a mother anion solution by a membrane. The resulting anion complexation reduced the actual concentration of the anion and induced the anion transfer across the membrane. The anion concentration in mother solution decreased, while it was found to be increased in the compartment with the dendritic receptor. This phenomenon was observed using dendritic receptors with four and eight complexation sites. A detailed analysis of a series of dialytic experiments by ¹H NMR spectroscopy enabled an assessment of the complexation behavior of both receptors and an evaluation of the dendritic effect on the anion complexation.

A Two-Dimensional Metallacycle Cross-Linked Switchable Polymer for Fast and Highly Efficient Phosphorylated Peptide Enrichment

The selective and efficient capture of phosphopeptides is critical for comprehensive and in-depth phosphoproteome analysis. Here we report a new switchable two-dimensional (2D) supramolecular polymer that serves as an ideal platform for the enrichment of phosphopeptides. A well-defined, positively charged metallacycle incorporated into the polymer endows the resultant polymer with a high affinity for phosphopeptides. Importantly, the stimuli-responsive nature of the polymer facilitates switchable binding affinity of phosphopeptides, thus resulting in an excellent performance in phosphopeptide enrichment and separation from model proteins. The polymer has a high enrichment capacity (165 mg/g) and detection sensitivity (2 fmol), high enrichment recovery (88%), excellent specificity, and rapid enrichment and separation properties. Additionally, we have demonstrated the capture of phosphopeptides from the tryptic digest of real biosamples, thus illustrating the potential of this polymeric material in phosphoproteomic studies.

Distiboranes based on ortho-phenylene backbones as bidentate Lewis acids for fluoride anion chelation

As part of our efforts in the chemistry of main group platforms that support anion sensing and transport, we are now reporting the synthesis of anitmony-based bidentate Lewis acids featuring the o-C6F4 backbone. These compounds can be easily accessed by reaction of the newly synthesized o-C6F4(SbPh2)2 (5) with o-chloranil or octafluorophenanthra-9,10-quinone, affording the corresponding distiboranes 6 and 7 of general formula o-C6F4(SbPh2(diolate))2 with diolate = tetrachlorocatecholate for 6 and octafluorophenanthrene-9,10-diolate for 7, respectively. While 6 is very poorly soluble, its octafluorophenanthrene-9,10-diolate analog 7 readily dissolves in CH2Cl2 and undergoes swift conversion into the corresponding fluoride chelate complex [7-μ2-F]- which has been isolated as a [nBu4N]+ salt. The o-C6H4 analog of 7, referred to as 8, has also been prepared. Although less Lewis acidic than 7, 8 also forms a very stable fluoride chelate complex ([8-μ2-F]-). Altogether, our experiental results, coupled with computational analyses and fluoride anion affinity calculations, show that 7 and 8 are some of the strongest antimony-based fluoride anion chelators prepared to date. Another notable aspect of this work concerns the use of the octafluorophenanthrene-9,10-diolate ligand and its ablity to impart advantageous solubility and Lewis acidity properties.

Recent progress in the development of MOF-based optical sensors for Fe3

Fe(iii) is a common pollutant released into our ecosystem from various industrial and anthropogenic activities which when in excess interferes with human health. A plethora of sensors based on various designs and working principles are being continuously synthesized and improvised for its facile detection. In the present review, we have provided a brief overview of the developments made in the field of metal organic framework (MOF) based optical sensors for Fe3+. MOFs have exponentially emerged in the field of research due to their high porosity, modular construction and easy tunability. These inorganic-organic hybrid porous materials are being essentially promoted as optical sensors because of their unique photophysical properties and potential sensing applications.

Carbazole-Based Colorimetric Anion Sensors

Owing to their strong carbazole chromophore and fluorophore, as well as to their powerful and convergent hydrogen bond donors, 1,8-diaminocarbazoles are amongst the most attractive and synthetically versatile building blocks for the construction of anion receptors, sensors, and transporters. Aiming to develop carbazole-based colorimetric anion sensors, herein we describe the synthesis of 1,8-diaminocarbazoles substituted with strongly electron-withdrawing substituents, i.e., 3,6-dicyano and 3,6-dinitro. Both of these precursors were subsequently converted into model diamide receptors. Anion binding studies revealed that the new receptors exhibited significantly enhanced anion affinities, but also significantly increased acidities. We also found that rear substitution of 1,8-diamidocarbazole with two nitro groups shifted its absorption spectrum into the visible region and converted the receptor into a colorimetric anion sensor. The new sensor displayed vivid color and fluorescence changes upon addition of basic anions in wet dimethyl sulfoxide, but it was poorly selective; because of its enhanced acidity, the dominant receptor-anion interaction for most anions was proton transfer and, accordingly, similar changes in color were observed for all basic anions. The highly acidic and strongly binding receptors developed in this study may be applicable in organocatalysis or in pH-switchable anion transport through lipophilic membranes.

Doubling the Carbonate-Binding Capacity of Nanojars by the Formation of Expanded Nanojars

Anion binding and extraction from solutions is currently a dynamic research topic in the field of supramolecular chemistry. A particularly challenging task is the extraction of anions with large hydration energies, such as the carbonate ion. Carbonate-binding complexes are also receiving increased interest due to their relevance to atmospheric CO2 fixation. Nanojars are a class of self-assembled, supramolecular coordination complexes that have been shown to bind highly hydrophilic anions and to extract even the most hydrophilic ones, including carbonate, from water into aliphatic solvents. Here we present an expanded nanojar that is able to bind two carbonate ions, thus doubling the previously reported carbonate-binding capacity of nanojars. The new nanojar is characterized by detailed single-crystal X-ray crystallographic studies in the solid state and electrospray ionization mass spectrometric (including tandem MS/MS) studies in solution.

Tripodal, Squaramide-Based Ion Pair Receptor for Effective Extraction of Sulfate Salt

Combining three features-the high affinity of squaramides toward anions, cooperation in ion pair binding and preorganization of the binding domains in the tripodal platform-led to the effective receptor 2. The lack of at least one of these key elements in the structures of reference receptors 3 and 4 caused a lower affinity towards ion pairs. Receptor 2 was found to form an intramolecular network in wet chloroform, which changed into inorganic-organic associates after contact with ions and allowed salts to be extracted from an aqueous to an organic phase. The disparity in the binding mode of 2 with sulfates and with other monovalent anions led to the selective extraction of extremely hydrated sulfate anions in the presence of more lipophilic salts, thus overcoming the Hofmeister series.

Selective sensing of adenosine monophosphate (AMP) over adenosine diphosphate (ADP), adenosine triphosphate (ATP), and inorganic phosphates with zinc(II)-dipicolylamine-containing gold nanoparticles

Mixed monolayer-protected gold nanoparticles containing surface-bound triethylene glycol and dipicolylamine groups aggregated in water/methanol, 1 : 2 (v/v) in the presence of nucleotides, if the solution also contained zinc(ii) nitrate to convert the dipicolylamine units into the corresponding zinc complexes. Nanoparticle aggregation could be followed with the naked eye by the colour change of the solution from red to purple followed by nanoparticle precipitation. The sensitivity was highest for adenosine triphosphate (ATP), which could be detected at concentrations >10 μM, and decreased over adenosine diphosphate (ADP) to adenosine monophosphate (AMP), consistent with the typically higher affinity of zinc(ii)-dipicolylamine-derived receptors for higher charged nucleotides. Inorganic sodium diphosphate and triphosphate interfered in the assay by also inducing nanoparticle aggregation. However, while the nucleotide-induced aggregates persisted even at higher analyte concentrations, the nanoparticles that were precipitated with inorganic salts redissolved again when the salt concentration was increased. The thus resulting solutions retained their ability to respond to nucleotides, but they now preferentially responded to AMP. Accordingly, AMP could be sensed selectively at concentrations ≥50 μM in an aqueous environment, even in the presence of other nucleotides and inorganic anions. This work thus introduces a novel approach for the sensing of a nucleotide that is often the most difficult analyte to detect with other assays.

Making Waxy Salts in Water: Synthetic Control of Hydrophobicity for Anion-Induced and Aggregation-Enhanced Light Emission

We show that multipodal polycationic receptors function as anion-responsive light-emitters in water. Prevailing paradigms utilize rigid holes and cavities for ion recognition. We instead built open amphiphilic scaffolds that trigger polar-to-nonpolar environment transitions around cationic fluorophores upon anion complexation. This ion-pairing and aggregation event produces a dramatic enhancement in the emission intensity, as demonstrated by perchlorate as a non-spherical hydrophobic anion model. A synergetic interplay of C-H⋅⋅⋅anion hydrogen bonding and tight anion-π⁺ contacts underpins this supramolecular phenomenon. By changing the aliphatic chain length, we demonstrate that the response profile and threshold of this signaling event can be controlled at the molecular level. With appropriate molecular design, inherently weak, ill-defined, and non-directional van der Waals interaction enables selective, sensitive, and tunable recognition in water.

A sensitive and selective BINOL based ratiometric fluorescence sensor for the detection of cyanide ions

A highly selective, novel BINOL based sensor BBCN has been developed for the fluorescent ratiometric detection of cyanide ions (CN⁻). The optical study revealed that BBCN exhibited unique spectral changes only with cyanide ions in the presence of other competing ions. Besides, an apparent fluorescent colour change from green to blue was observed. A clear linear relationship was observed between the fluorescence ratiometric ratio of BBCN and the concentration of CN⁻ with a reasonably low detection limit (LOD) of 189 nM (507 ppb). The optical response was due to the nucleophilic addition of CN⁻ to the dicyanovinyl group of the sensor, which compromises the probe's intramolecular charge transfer. This mechanism was well confirmed by Job's plot, ¹H-NMR and ESI-MS studies. BBCN showed immediate spectral response towards (1 second) CN⁻ and detection could be realized in a broad pH window. Furthermore, the practical utility of BBCN was studied by test paper-based analysis and the detection of CN⁻ in various water resources.

A highly selective, novel BINOL based sensor BBCN has been developed for the fluorescent ratiometric detection of cyanide ions (CN⁻).

Selective sensing of sulfate anions in water with cyclopeptide-decorated gold nanoparticles

The interaction of cyclopeptides bound to the surface of mixed monolayer-protected gold nanoparticles with sulfate anions causes the crosslinking and concomitant precipitation of the nanoparticles from aqueous solutions even in presence of an excess of competing anions, thus allowing the naked eye detection of sulfate in water.

Ground- and excited-state dynamic control of an anion receptor by hydrostatic pressure

Stimulus-responsive supramolecular architectures have become an attractive alternative to conventional ones for many applications in sensing, drug-delivery and switchable memory systems. Herein, we used an anion receptor (H: host) as a hydrostatic-pressure-manipulatable fluorescence foldamer and halide anions as chiral (binaphthylammonium) and achiral (tetrabutylammonium) ion pairs (SS or RR ·X and TBA·X; X = Cl, Br), and then investigated their (chir)optical properties and molecular recognition behavior under hydrostatic pressures. The conformational changes and optical properties of H in various organic solvents were revealed by UV/vis absorption and fluorescence spectra and fluorescence lifetimes upon hydrostatic pressurization. The anion-recognition abilities of H upon interactions with SS or RR·X and TBA·X at different pressure ranges were determined by hydrostatic-pressure spectroscopy to quantitatively afford the binding constant (K anion) and apparent reaction volume changes . The results obtained indicate that hydrostatic pressure as well as solvation plays significant roles in the dynamic control of the present supramolecular system in the ground and excited states. This work will provide a new guideline for further developing hydrostatic-pressure-responsive foldamers and supramolecular materials.

Optical Anion Receptors with Urea/Thiourea Subunits on a TentaGel Support

Two simple chemosensors with urea (L1) and thiourea (L2) groups were synthesized and studied by different spectroscopic techniques. Both receptors can sense acetate (Ac^-), dihydrogen phosphate (H₂PO₄ ^-), and fluoride (F^-) anions, accompanied by changes in UV-vis and ¹H NMR spectra, and an optical response is observed as a color change of the solutions due to deprotonation and hydrogen-bonding processes. Also, L1 and L2 were supported on TentaGel resins (R1 and R2), and their fluoride-sensing properties in DMSO and water solutions were studied. Interestingly, R2 can sense fluoride ions in sample solutions of 100% water.

Nanopore-Based Electrodes for Quinotrione Detection: Host-Guest-Induced Electrochemical Signal Switching

Nanopore-based detection techniques, with a wide range of transport properties, exhibit impressive selectivity and sensitivity for analytes. To expand the application of nanoporous sensors, real-time and fast detection of targets, all within a portable device, is highly desired for nanopore-based sensors. In addition, to improve the accuracy of the output signal, more appropriate readout methods also need to be explored. In this manuscript, we describe a nanopore-based electrode, regarded as NAC-P6-PC@AuE, prepared by coupling a pillararene-based nanoporous membrane with an electrochemical impedance measurement method. The fabricated device is demonstrated by exposing pillararene-based receptors to trace amounts of pesticide molecules. NAC-P6-PC@AuE devices exhibit distinguished selectivity to quinotrione, as well as the ability to quantify quinotrione with a limit of quantitation (LOQ) of 10 nM. The mechanism that allows sensing was verified using finite-element simulations and may be explained as host-guest-induced surface charge shielding, which influences the electrochemical response of probe molecules. The applications of this nanopore-based electrode may be extended toward other target molecules by decorating the nanopore surfaces with specifically chosen receptors.

A chiral spirobifluorene-based bis(salen) zinc(ii) receptor towards highly enantioselective binding of chiral carboxylates

We have designed a new chiral receptor based on two salen zinc(ii) complex units connected with a spirobifluorene framework. The chiral receptor is proven to enantioselectively bind chiral carboxylate guests and the differences between the binding constants of enantiomeric guests were up to more than one order of magnitude.

Simple acyclic molecules containing a single charge-assisted O-H group can recognize anions in acetonitrile : water mixtures

Hydroxypyridinium and hydroxyquinolinium compounds containing acidic O-H groups attached to a cationic aromatic scaffold were synthesized, i.e. N-methyl-3-hydroxypyridinium (1⁺) and N-methyl-8-hydroxyquinolinium (2⁺). These very simple compounds are capable of binding to chloride very strongly in CD₃CN and with moderate strength in 9 : 1 CD₃CN : D₂O. Comparison with known association constants reveals that 1⁺ and 2⁺ bind chloride in CD₃CN or CD₃CN : D₂O with comparable affinities to receptors containing significantly more hydrogen bond donors and/or higher positive charges. Crystal structures of both compounds with coordinating anions were obtained, and feature short O-Hanion hydrogen bonds. A receptor containing two hydroxyquinolinium groups was also prepared. While the low solubility of this compound caused difficulties, we were able to demonstrate chloride binding in a competitive 1 : 1 CD₃CN : CD₃OD solvent mixture. Addition of sulfate to this compound results in the formation of a crystallographically-characterised solid state anion coordination polymer.

Excited State Intramolecular Proton Transfer (ESIPT)-Based Sensor for Ion Detection

C-2 and C-5 substituted imidazole skeleton was synthesized through a one-pot two-step strategy. Synthesized molecule emits the light on ESIPT (excited-state intramolecular proton transfer). This molecule was utilized for its proton donor ability, and we have observed that fluoride and cyanide ions can be detected selectively. Different cations and anions were selected to observe the response of the synthesized molecule. However, there were not any fluorometric and colorimetric response except for fluoride and cyanide ions. Detection limits of fluoride and cyanide ions were found to be 9.22 μM and 11.48 μM, respectively. ¹H-NMR spectra for the solution of the sensor and TBAF (tetrabuthylammoniumfluoride) were used for the identification of [L]^-[HF₂]^- species. 3 equiv. TBAF saturated the solution of the sensor in d₆-DMSO, and some of the proton resonances shifted to upfield due to the through-bond effect. The disappearance of NH proton with 0.5 equiv. TBAF or TBACN (tetrabuthylammoniumcyanide) showed that there was a proton abstraction by fluoride and cyanide ions, instead of the hydrogen bond. Solid-state application was utilized, and paper test strips were applied. Emission differences emerged when the sensor loaded strips were reacted with TBAF. Time resolved experiments revealed that solution of the sensor and TBAF in DMSO have multiexponential decay, and one of the lifetime was measured as 13.4 ns.

Bambusuril Macrocycles as Mediators of Supramolecular Interactions: Application to the Europium Cage Helicate

Herein, it is shown how bambusurils can be used for tuning and/or characterizing supramolecular systems. Indeed, the addition of bambusurils as anion scavengers to metal-mediated self-assemblies allows manipulation of the subtle equilibria in the given system. This is demonstrated for the case of the tetranuclear europium helical cage, which is well suited to different applications. Among the reported results, experimental evidence is provided showing that perchlorate and triflate anions act as a molecular template for the cage assembly. The complexation of inorganic anions with neutral bambusurils resulted in bulky non-coordinating counterions that may trigger the self-assembly process or stimulate specific interactions between components. Moreover, bambusuril was able to selectively remove coordinating nitrates from the mixture with non-coordinating anions, enabling the regeneration of the helical cage.

Solution and Solid State Studies of Urea Derivatives of DITIPIRAM Acting as Powerful Anion Receptors

Herein, we present the synthesis and anion binding studies of a family of homologous molecular receptors 4–7 based on a DITIPIRAM (8-propyldithieno-[3,2-b:2′,3′-e]-pyridine-3,5-di-amine) platform decorated with various urea para-phenyl substituents (NO₂, F, CF₃, and Me). Solution, X-ray, and DFT studies reveal that the presented host–guest system offers a convergent array of four urea NH hydrogen bond donors to anions allowing the formation of remarkably stable complexes with carboxylates (acetate, benzoate) and chloride anions in solution, even in competitive solvent mixtures such as DMSO-d₆/H₂O 99.5/0.5 (v/v) and DMSO-d₃/MeOH-d₃ 9:1 (v/v). The most effective derivatives among the series turned out to be receptors 5 and 6 containing electron-withdrawing F- and -CF₃para-substituents, respectively.

Chain Entropy Beats Hydrogen Bonds to Unfold and Thread Dialcohol Phosphates inside Cyanostar Macrocycles To Form [3]Pseudorotaxanes

The recognition of substituted phosphates underpins many processes including DNA binding, enantioselective catalysis, and recently template-directed rotaxane synthesis. Beyond ATP and a few commercial substrates, however, little is known about how substituents effect organophosphate recognition. Here, we examined alcohol substituents and their impact on recognition by cyanostar macrocycles. The organophosphates were disubstituted by alcohols of various chain lengths, dipropanol, dihexanol, and didecanol phosphate, each accessed using modular solid-phases syntheses. Based on the known size-selective binding of phosphates by π-stacked dimers of cyanostars, threaded [3]pseudorotaxanes were anticipated. While seen with butyl substituents, pseudorotaxane formation was disrupted by competitive OH···O- hydrogen bonding between both terminal hydroxyls and the anionic phosphate unit. Crystallography also showed formation of a backfolded propanol conformation resulting in an 8-membered ring and a perched cyanostar assembly. Motivated by established entropic penalties accompanying ring formation, we reinstated [3]pseudorotaxanes by extending the size of the substituent to hexanol and decanol. Chain entropy overcomes the enthalpically favored OH···O- contacts to favor random-coil conformations required for seamless, high-fidelity threading of dihexanol and didecanol phosphates inside cyanostars. These studies highlight how chain length and functional groups on phosphate's substituents can be powerful design tools to regulate binding and control assembly formation during phosphate recognition.

Supersensitive Detection of Anions in Pure Organic and Aqueous Media by Amino Acid Conjugated Ellman's Reagent

The last few decades witnessed a remarkable advancement in the field of molecular anion receptors. A variety of anion binding motifs have been discovered, and large number of designer molecular anion receptors with high selectivity are being reported. However, anion detection in an aqueous medium is still a formidable challenge as evident from only a miniscule of synthetic systems available in the literature. We, herein, report 5,5'-dithio-bis(2-nitrobenzoic acid) (Ellman's reagent) appended with amino acids as supersensitive anion sensors that can detect F^- and H₂PO₄^- ions in both aqueous as well as organic media. Interestingly, the sensors showed a dual response to anions, viz., chromogenic response in organic medium and electrochemical response in aqueous solutions. Various spectroscopic techniques such as UV-vis and ¹H NMR are used to investigate the binding studies in acetonitrile, whereas electrochemical methods such as cyclic voltammetry (CV) and differential pulse voltammetry (DPV) are employed to explore the anion binding in water. The host-guest complex stoichiometry and binding constants are calculated using the BindFit software. The geometry of host-guest complex has been optimized by the density functional theory (DFT) method. These molecules are versatile sensors since these function in both water and acetonitrile with extremely low limit of detection (LOD) up to 0.07 fM and limit of quantification (LOQ) up to 0.23 fM. To our knowledge, the present system is the first example of a sensor that can detect the lowest concentration of anions in water quantitatively. The minimalistic design strategy presented here opens up the innumerable possibilities for designing dual anion sensors in a one fell swoop.

Conventional vs. Microwave- or Mechanically-Assisted Synthesis of Dihomooxacalix[4]arene Phthalimides: NMR, X-ray and Photophysical Analysis

Direct O-alkylation of p-tert-butyldihomooxacalix[4]arene (1) with N-(bromopropyl)- or N-(bromoethyl)phthalimides and K₂CO₃ in acetonitrile was conducted under conventional heating (reflux) and using microwave irradiation and ball milling methodologies. The reactions afforded mono- and mainly distal di-substituted derivatives in the cone conformation, in a total of eight compounds. They were isolated by column chromatography, and their conformations and the substitution patterns were established by NMR spectroscopy (¹H, ¹³C, COSY and NOESY experiments). The X-ray structures of four dihomooxacalix[4]arene phthalimide derivatives (2a, 3a, 3b and 5a) are reported, as well as their photophysical properties. The microwave (MW)-assisted alkylations drastically reduced the reaction times (from days to less than 45 min) and produced higher yields of both 1,3-di-substituted phthalimides (3a and 6a) with higher selectivity. Ball milling did not reveal to be a good method for this kind of reaction.

Development of effective potassium acetate extractant

Carboxylates are commonly used in the food and pharmaceutical industry and due to their extensive use, carboxylates present a significant environmental burden. In this context, valine based, heteroditopic receptor 1 was prepared and its ability to bind simultaneously potassium cation and acetate anion in water containing CH₃CN solutions was demonstrated. Under liquid–liquid extraction conditions the receptor 1 was capable of extracting hydrophilic AcOK salt from aqueous solution and was proved to be nearly ten times more effective than the equimolar mixture of monotopic receptors. Furthermore, compound 1 could extract one of the most popular nonsteroidal anti-inflammatory drugs, ibuprofen (IbuOK), from relatively dilute aqueous solutions.

Structurally simple, heteroditopic receptor is capable of extracting hydrophilic potassium acetate and other carboxylate salts from water to organic phase.

Phthalocyanine-Functionalized Magnetic Silica Nanoparticles as Anion Chemosensors

Anionic species are one of the most common pollutants in residual and freshwaters. The presence of anthropogenic anions in water drastically increases the toxicity to living beings. Here, we report the preparation of a new optical active material based on tri(tosylamino)phthalocyanines grafted to ferromagnetic silica nanoparticles for anion detection and removal. The new unsymmetrical phthalocyanines (Pcs) proved to be excellent chemosensors for several anions (AcO−, Br−, Cl−, CN−, F−, H₂PO₄−, HSO₄−, NO₂−, NO₃−, and OH−) in dimethyl sulfoxide (DMSO). Furthermore, the Pcs were grafted onto magnetic nanoparticles. The resulting novel hybrid material showed selectivity and sensitivity towards CN−, F−, and OH− anions in DMSO with limit of detection (LoD) of ≈4.0 µM. In water, the new hybrid chemosensor demonstrated selectivity and sensitivity for CN− and OH− anions with LoD of ≈0.2 µM. The new hybrids are easily recovered using a magnet, allowing recyclability and reusability, after acidic treatment, without losing the sensing proprieties.

The aqueous dependent sensing of hydrazine and phosphate anions using a bis-heteroleptic Ru(II) complex with a phthalimide-anchored pyridine-triazole ligand

Selective turn-on luminescence properties are shown by a non-luminescent metalloreceptor upon the addition of phosphate anions in CH3CN and hydrazine in CH3CN/H2O (6/4, v/v). The non-luminescent metalloreceptors [RuII(phen)2(TpH)]2PF6- (RtpH) and [RuII(Phen)2(TpI)]2PF6- (RtpI) {phen = 1,10-phenanthroline; TpH = 2-(2-(4-(pyridin-2-yl)-1H-1,2,3-triazol-1-yl)ethyl)isoindoline-1,3-dione; and TpI = 2-(2-(5-iodo-4-(pyridin-2-yl)-1H-1,2,3-triazol-1-yl)ethyl)isoindoline-1,3-dione} were synthesized and characterized. Both metalloreceptors have excellent sensing properties for phosphate anions (H2PO4- and H2P2O72-) over other anions in CH3CN. The limit of detection (LOD) values were calculated to be 79 nM and 48 nM for H2PO4- upon addition to RtpH and RtpI, respectively. Noncovalent interactions play a key role in the sensing of phosphate anions, among which the halogen-anion interaction showed superior recognition properties over the hydrogen-anion interaction. Comparative electrochemical experiments, 1H NMR titration, 31P NMR titration, and lifetime studies also show that RtpI has better sensing properties, as evidenced by its more drastic emission response to H2PO4- anions compared with RtpH. Moreover, the metalloreceptors showed a remarkable fluorescence increase (at ∼584 nm) upon the addition of hydrazine, without the interference of other amines in CH3CN/H2O (6/4, v/v). Interestingly, fluorescence enhancement was observed within live HeLa cells upon hydrazine addition, which is caused by the efficient photoinduced electron transfer process.

A naphthalimide-linked new pyridylazo phenol derivative for selective sensing of cyanide ions (CN-) in sol-gel medium

Napthalimide-linked pyridylazo derivatives 1 and 2 have been designed and synthesized. Compound 1 acts as a gelator in DMF-H₂O (1 : 1, v/v). The brown gel is photostable and shows good viscoelastic properties. The value of G' is almost 10 times higher than that of G'' over the entire range of frequencies at a constant strain of 1%. The SEM image shows the presence of densely stacked flakes. In comparison, compound 2, devoid of free phenolic -OH, does not show gelation properties under identical conditions. However, the brown gel of 1 shows selective sensing of CN^- ions over a series of anions involving phase change through the deprotonation mechanism. While the brown gel of 1 is selectively ruptured in the presence of CN^- to the sol, compound 1 in solution shows measurable UV-vis and emission changes in the presence of CN^- over the other anions and validates the visual sensing of CN^-. In the test-kit application, the yellow paper strip turned into pinkish-red upon contact with CN^-.

Supramolecular Gold Stripping from Activated Carbon Using α-Cyclodextrin

We report the molecular recognition of the Au(CN)₂^- anion, a crucial intermediate in today's gold mining industry, by α-cyclodextrin. Three X-ray single-crystal superstructures-KAu(CN)₂⊂α-cyclodextrin, KAu(CN)₂⊂(α-cyclodextrin)₂, and KAg(CN)₂⊂(α-cyclodextrin)₂-demonstrate that the binding cavity of α-cyclodextrin is a good fit for metal-coordination complexes, such as Au(CN)₂^- and Ag(CN)₂^- with linear geometries, while the K⁺ ions fulfill the role of linking α-cyclodextrin tori together as a result of [K⁺···O] ion-dipole interactions. A 1:1 binding stoichiometry between Au(CN)₂^- and α-cyclodextrin in aqueous solution, revealed by ¹H NMR titrations, has produced binding constants in the order of 10⁴ M^-1. Isothermal calorimetry titrations indicate that this molecular recognition is driven by a favorable enthalpy change overcoming a small entropic penalty. The adduct formation of KAu(CN)₂⊂α-cyclodextrin in aqueous solution is sustained by multiple [C-H···π] and [C-H···anion] interactions in addition to hydrophobic effects. The molecular recognition has also been investigated by DFT calculations, which suggest that the 2:1 binding stoichiometry between α-cyclodextrin and Au(CN)₂^- is favored in the presence of ethanol. We have demonstrated that this molecular recognition process between α-cyclodextrin and KAu(CN)₂ can be applied to the stripping of gold from the surface of activated carbon at room temperature. Moreover, this stripping process is selective for Au(CN)₂^- in the presence of Ag(CN)₂^-, which has a lower binding affinity toward α-cyclodextrin. This molecular recognition process could, in principle, be integrated into commercial gold-mining protocols and lead to significantly reduced costs, energy consumption, and environmental impact.