The road to aryl CHanion binding was paved with good intentions: fundamental studies, host design, and historical perspectives in CH hydrogen bonding

Anion recognition using meta-substituted ureidocalix[4]arene receptors

Calix[4]arenes bearing urea units at the meta position(s) of the upper rim of the macrocyclic skeleton were prepared by the reaction of the corresponding amines with aryl isocyanates. As shown by the 1H NMR and UV/vis titration experiments, these systems are capable of effectively complexing selected anions even in a highly competitive environment (such as DMSO-d6). While the monoureido derivatives showed approximately the same complexation ability irrespective of the substitution (para vs. meta isomers), the bisureas at the upper rim demonstrated interesting differences in complexation. The meta,meta and para,para isomers were shown to prefer 2 : 1 complexes (anion : receptor) regardless of the anion tested, while the analogous meta,para isomer formed 1 : 1 complexes with strongly coordinated anions (e.g. H2PO4-) based on synchronous complexation by both ureido groups. This suggests that the regioselective introduction of urea units into the upper rim of calix[4]arene brings with it the possibility of "tuning" the complexation properties depending on the substitution pattern of the functional groups.

Interplay between anion-receptor and anion-solvent interactions in halide receptor complexes characterized with ultrafast infrared spectroscopies

The competition between host-guest binding and solvent interactions is a crucial factor in determining the binding affinities and selectivity of molecular receptor species. The interplay between these competing interactions, however, have been difficult to disentangle. In particular, the development of molecular-level descriptions of solute-solvent interactions remains a grand experimental challenge. Herein, we investigate the prototypical halide receptor meso-octamethylcalix[4]pyrrole (OMCP) complexed with either chloride or bromide anions in both dichloromethane (DCM) and chloroform (trichloromethane, TCM) solvent using ultrafast infrared transient absorption and 2D IR spectroscopies. OMCP·Br- complexes in both solvents display slower vibrational relaxation dynamics of the OMCP pyrrole NH stretches, consistent with weaker H-bonding interactions with OMCP compared to chloride and less efficient intermolecular relaxation to the solvent. Further, OMCP·Br- complexes show nearly static spectral diffusion dynamics compared to OMCP·Cl-, indicating larger structural fluctuations occur within chloride complexes. Importantly, distinct differences in the vibrational spectra and dynamics are observed between DCM and TCM solutions. The data are consistent with stronger and more perturbative solvent effects in TCM compared to DCM, despite DCM's larger dielectric constant and smaller reported OMCP·X- binding affinities. These differences are attributed to the presence of weak H-bond interactions between halides and TCM, in addition to competing interactions from the bulky tetrabutylammonium countercation. The data provide important experimental benchmarks for quantifying the role of solvent and countercation interactions in anion host-guest complexes.

A new biphenol-dipicolylamine based ligand and its dinuclear Zn2+ complex as fluorescent sensors for ibuprofen and ketoprofen in aqueous solution

In this work, the study of the new ligand 3,3'-bis[N,N-bis(pyridine-2-ylmethyl)aminomethyl]-2,2'-dihydroxybiphenyl (L) is reported, where a central 2,2'-biphenol (BPH) fluorophore was functionalized at 3,3'-positions with two dipicolylamine (DPA) side arms as receptor units. Following the synthesis and full chemical-physical characterization, the acid-base and Zn2+-coordination abilities of L were investigated through a combination of potentiometric, UV-Vis, fluorescence, NMR, XRD and DFT measurements. The optical properties of the ligand turned out to be strongly dependent on the pH, being straightforwardly associated with the protonation state of the BPH moiety, whereas its peculiar design allowed to form stable mono and dinuclear Zn2+ complexes. In the latter species, the presence of two Zn2+ ions coordinatively unsaturated and placed at close distance to each other, prompted us to test their usefulness as metallo-receptors for two environmental pollutants of great relevance, ibuprofen and ketoprofen. Potentiometric and fluorescence investigations evidenced that these important non-steroidal anti-inflammatory drugs (NSAIDs) are effectively coordinated by the metallo-receptors and, of relevance, both the stability and the fluorescence properties of the resulting ternary adducts are markedly affected by the different chemical architectures of the two substrates. This study aims at highlighting the promising perspectives arising from the use of polyamino phenolic ligands as chemosensors for H+/Zn2+ and other additional anionic targets in their metal-complexed forms.

Probing Ion-Receptor Interactions in Halide Complexes of Octamethyl Calix[4]Pyrrole

Ion receptors are molecular hosts that bind ionic guests, often with great selectivity. The interplay of solvation and ion binding in anion host-guest complexes in solution governs the binding efficiency and selectivity of such ion receptors. To gain molecular-level insight into the intrinsic binding properties of octamethyl calix[4]pyrrole (omC4P) host molecules with halide guest ions, we performed cryogenic ion vibrational spectroscopy (CIVS) of omC4P in complexes with fluoride, chloride, and bromide ions. We interpret the spectra using density functional theory, describing the infrared spectra of these complexes with both harmonic and anharmonic second-order vibrational perturbation theory (VPT2) calculations. The NH stretching modes of the pyrrole moieties serve as sensitive probes of the ion binding properties, as their frequencies encode the ion-receptor interactions. While scaled harmonic spectra reproduce the experimental NH stretching modes of the chloride and bromide complexes in broad strokes, the high proton affinity of fluoride introduces strong anharmonic effects. As a result, the spectrum of F-·omC4P is not even qualitatively captured by harmonic calculations, but it is recovered very well by VPT2 calculations. In addition, the VPT2 calculations recover the intricate coupling of the NH stretching modes with overtones and combination bands of CH stretching and NH bending modes and with low-frequency vibrations of the omC4P macrocycle, which are apparent for all of the halide ion complexes investigated here. A comparison of the CIVS spectra with infrared spectra of solutions of the same ion-receptor complexes in d3-acetonitrile and d6-acetone shows how ion solvation changes the ion-receptor interactions for the different halide ions.

Tandem catalysis enables chlorine-containing waste as chlorination reagents

Chlorinated compounds are ubiquitous. However, accumulation of chlorine-containing waste has a negative impact on human health and the environment due to the inapplicability of common disposal methods, such as landfill and incineration. Here we report a sustainable approach to valorize chlorine-containing hydrocarbon waste, including solids (chlorinated polymers) and liquids (chlorinated solvents), based on copper and palladium catalysts with a NaNO3 promoter. In the process, waste is oxidized to release the chlorine in the presence of N-directing arenes to afford valuable aryl chlorides, such as the FDA-approved drug vismodegib. The remaining hydrocarbon component is mineralized to afford CO, CO2 and H2O. Moreover, the CO and CO2 generated could be further utilized directly. Thus, chlorine-containing hydrocarbon waste, including mixed waste, can serve as chlorination reagents that neither generate hazardous by-products nor involve specialty chlorination reagents. This tandem catalytic approach represents a promising method for the viable management of a wide and diverse range of chlorine-containing hydrocarbon wastes.

Remote control of anion binding by CH-based receptors

We show that the substitution of tetra(benzimidazole)resorcin[4]arenes with electron withdrawing groups on the upper rim enhances anion binding at the opposite edge by more than three orders of magnitude. Moreover, selective anion binding at either the OH/NH or CH binding sites is demonstrated.

Tripodal Organic Cages with Unconventional CH···O Interactions for Perchlorate Remediation in Water

Perchlorate anions used in industry are harmful pollutants in groundwater. Therefore, selectively binding perchlorate provides solutions for environmental remediation. Here, we synthesized a series of tripodal organic cages with highly preorganized Csp3-H bonds that exhibit selectively binding to perchlorate in organic solvents and water. These cages demonstrated binding affinities to perchlorate of 105-106 M-1 at room temperature, along with high selectivity over competing anions, such as iodide and nitrate. Through single crystal structure analysis and density functional theory calculations, we identified unconventional Csp3-H···O interactions as the primary driving force for perchlorate binding. Additionally, we successfully incorporated this cage into a 3D-printable polymer network, showcasing its efficacy in removing perchlorate from water.

Synthetic Receptors with Micromolar Affinity for Chloride in Water

A water-soluble coordination cage was obtained by reaction of Pd(NO₃ )₂ with a 1,3-di(pyridin-3-yl)benzene ligand featuring a short PEG chain. The cavity of the metal-organic cage contains one nitrate anion, which is readily replaced by chloride. The apparent association constant for chloride binding in buffered aqueous solution is K_a =1.8(±0.1)×10⁵  M^-1 . This value is significantly higher than what has been reported for other macrocyclic chloride receptors. The heavier halides Br^- and I^- compete with binding or self-assembly, but the receptor displays very good selectivity over common anions such as phosphate, acetate, carbonate, and sulfate. A further increase of the chloride binding affinity by a factor of 3 was achieved using a fluorinated dipyridyl ligand.

A Tetranuclear Copper(II)/Calcium(II) Complex as Dual Chemosensor for Colorimetric and Fluorescent Detection of Non-Steroidal Anti-Inflammatory Drugs

The tetranuclear Cu²⁺ /Ca²⁺ /Ca2⁺ /Cu²⁺ complex based on Malten ligand has been investigated as a platform for anion binding. Simple organic carboxylates and non-steroidal anti-inflammatory drugs (NSAIDs) have been tested, revealing the ability of the platform to bind them. The receiving platform hosts at least two guests in solution although a third anion can be bound, as suggested by X-ray diffraction analysis. The addition of the anions is accompanied by a color change of the solution, making the system a colorimetric sensor for carboxylates (LOD values comprised between 3.6 and 20.7 ppm). A fluorescent system consisting of the 2-(3-oxido-6-oxoxanthen-9-yl)benzoate (fluorescein anion) linked to the tetranuclear platform has been also prepared and used in a chemosensing ensemble approach to signal the presence of the selected anions (Log K between 2.6 and 5.6 for the addition of two guests). The latter also works in a paper strip test, offering the chemosensor a possible practical application.

Protonophoric and mitochondrial uncoupling activity of aryl-carbamate substituted fatty acids

Aryl-urea substituted fatty acids are protonophores and mitochondrial uncouplers that utilise a urea-based synthetic anion transport moiety to carry out the protonophoric cycle. Herein we show that replacement of the urea group with carbamate, a functional group not previously reported to possess anion transport activity, produces analogues that retain the activity of their urea counterparts. Thus, the aryl-carbamate substituted fatty acids uncouple oxidative phosphorylation and inhibit ATP production by collapsing the mitochondrial proton gradient. Proton transport proceeds via self-assembly of the deprotonated aryl-carbamates into membrane permeable dimeric species, formed by intermolecular binding of the carboxylate group to the carbamate moiety. These results highlight the anion transport capacity of the carbamate functional group.

Noncovalent Stabilization of Radical Intermediates in the Enantioselective Hydroamination of Alkenes with Sulfonamides

Noncovalent interactions (NCIs) are critical elements of molecular recognition in a wide variety of chemical contexts. While NCIs have been studied extensively for closed-shell molecules and ions, very little is understood about the structures and properties of NCIs involving free radical intermediates. In this report, we describe a detailed mechanistic study of the enantioselective radical hydroamination of alkenes with sulfonamides and present evidence suggesting that the basis for asymmetric induction in this process arises from attractive NCIs between a neutral sulfonamidyl radical intermediate and a chiral phosphoric acid (CPA). We describe experimental, computational, and data science-based evidence that identifies the specific radical NCIs that form the basis for the enantioselectivity. Kinetic studies support that C-N bond formation determines the enantioselectivity. Density functional theory investigations revealed the importance of both strong H-bonding between the CPA and the N-centered radical and a network of aryl-based NCIs that serve to stabilize the favored diastereomeric transition state. The contributions of these specific aryl-based NCIs to the selectivity were further confirmed through multivariate linear regression analysis by comparing the measured enantioselectivity to computed descriptors. These results highlight the power of NCIs to enable high levels of enantioselectivity in reactions involving uncharged open-shell intermediates and expand our understanding of radical-molecule interactions.

Three modes of interactions between anions and phenolic macrocycles: a comparative study

Macrocyclic polyphenolic compounds such as resorcin[4]arenes can be considered as multidentate anion receptors. In the current work, we combine new experimental data and reports from the previous literature (solution data and deposited crystal structures from the CCDC) to systematically analyze binding motifs between resorcin[4]arene derivatives and anions, determine the role of supporting interactions from CH donors, ion pairing and estimate their relative strength. We have found that in medium polarity solvents (THF) anion binding is a main driving force for the formation of complexes between resorcinarenes and Alk₄NX salts. Three binding modes have been detected using ¹H NMR and DOSY, depending on the type of additional interactions. Mode I was observed for upper-rim unsubstituted resorcinarenes, which use OH groups and aromatic CH from the upper rim as hydrogen bond donors to form multidentate and multivalent binding sites at the upper rim. Mode II was observed for upper-rim halogenated resorcinarenes (tetrabromo- and tetraiodo-derivatives), which use OH groups and aliphatic CH atoms from the bridges to support the chelation of anions between aromatic units. This binding mode is also multidentate and multivalent, but weaker and more anion-selective than mode I (works effectively for chlorides but not for bromides). For O-substituted derivatives, mode III is observed, with anions bound in a nest formed by aromatic CH atoms in the lower rim (multidentate but monovalent binding). The relative strength of these three binding modes, their solvent-dependence, and emergence in the crystal structures (CCDC) have been evaluated.

C-H⋯S hydrogen bonding interactions

The short C-H⋯S contacts found in available structural data for both small molecules and larger biomolecular systems suggest that such contacts are an often overlooked yet important stabilizing interaction. Moreover, many of these short C-H⋯S contacts meet the definition of a hydrogen bonding interaction. Using available structural data from the Cambridge Structural Database (CSD), as well as selected examples from the literature in which important C-H⋯S contacts may have been overlooked, we highlight the generality of C-H⋯S hydrogen bonding as an important stabilizing interaction. To uncover and establish the generality of these interactions, we compare C-H⋯S contacts with other traditional hydrogen bond donors and acceptors as well as investigate how coordination number and metal bonding affect the preferred geometry of interactions in the solid state. This work establishes that the C-H⋯S bond meets the definition of a hydrogen bond and serves as a guide to identify C-H⋯S hydrogen bonds in diverse systems.

Nonspherical anion sequestration by C-H hydrogen bonding

Macrocyclic arenes laid the foundations of supramolecular chemistry and their study established the fundamentals of noncovalent interactions. Advancing their frontier, here we designed rigidified resorcin[4]arenes that serve as hosts for large nonspherical anions. In one synthetic step, we vary the host's anion affinity properties by more than seven orders of magnitude. This is possible by engineering electropositive aromatic C–H bond donors in an idealized square planar geometry embedded within the host's inner cavity. The hydrogen atom's electropositivity is tuned by introducing fluorine atoms as electron withdrawing groups. These novel macrocycles, termed fluorocages, are engineered to sequester large anions. Indeed, experimental data shows an increase in the anion association constant (K_a) as the number of F atoms increase. The observed trend is rationalized by DFT calculations of Hirshfeld Charges (HCs). Most importantly, fluorocages in solution showed weak-to-medium binding affinity for large anions like [PF₆]⁻ (10²< K_a <10⁴ M⁻¹), and high affinity for [MeSO₃]⁻ (K_a >10⁶).

Fluorocages: new class of rigidified host utilizing nontraditional C–H hydrogen bonds to capture the nonspherical anions.

Pushing the Limits of the Hydrogen Bond Enhanced Halogen Bond —The Case of the C–H Hydrogen Bond

C–H hydrogen bonds have remarkable impacts on various chemical systems. Here we consider the influence of C–H hydrogen bonds to iodine atoms. Positioning a methyl group between two iodine halogen bond donors of the receptor engendered intramolecular C–H hydrogen bonding (HBing) to the electron-rich belt of both halogen bond donors. When coupled with control molecules, the role of the C–H hydrogen bond was evaluated. Gas-phase density functional theory studies indicated that methyl C–H hydrogen bonds help bias a bidentate binding conformation. Interaction energy analysis suggested that the charged C–H donors augment the halogen bond interaction—producing a >10 kcal mol⁻¹ enhancement over a control lacking the C–H⋯I–C interaction. X-ray crystallographic analysis demonstrated C–H hydrogen bonds and bidentate conformations with triflate and iodide anions, yet the steric bulk of the central functional group seems to impact the expected trends in halogen bond distance. In solution, anion titration data indicated elevated performance from the receptors that utilize C–H Hydrogen Bond enhanced Halogen Bonds (HBeXBs). Collectively, the results suggest that even modest hydrogen bonds between C–H donors and iodine acceptors can influence molecular structure and improve receptor performance.

C–H hydrogen bonds to iodine halogen bond donors are shown to improve halogen bonding and molecular preorganization.

Mining anion-aromatic interactions in the Protein Data Bank

Mutual positioning and non-covalent interactions in anion–aromatic motifs are crucial for functional performance of biological systems. In this context, regular, comprehensive Protein Data Bank (PDB) screening that involves various scientific points of view and individual critical analysis is of utmost importance. Analysis of anions in spheres with radii of 5 Å around all 5- and 6-membered aromatic rings allowed us to distinguish 555 259 unique anion–aromatic motifs, including 92 660 structures out of the 171 588 structural files in the PDB. The use of a scarcely exploited (x, h) coordinate system led to (i) identification of three separate areas of motif accumulation: A – over the ring, B – over the ring-substituent bonds, and C – roughly in the plane of the aromatic ring, and (ii) unprecedented simultaneous comparative description of various anion–aromatic motifs located in these areas. Of the various residues considered, i.e. aminoacids, nucleotides, and ligands, the latter two exhibited a considerable tendency to locate in region Avia archetypal anion–π contacts. The applied model not only enabled statistical quantitative analysis of space around the ring, but also enabled discussion of local intermolecular arrangements, as well as detailed sequence and secondary structure analysis, e.g. anion–π interactions in the GNRA tetraloop in RNA and protein helical structures. As a purely practical issue of this work, the new code source for the PDB research was produced, tested and made freely available at https://github.com/chemiczny/PDB_supramolecular_search.

The comprehensive analysis of non-redundant PDB macromolecular structures investigating anion distributions around all aromatic molecules in available biosystems is presented.

Centroid⋯centroid and hydrogen bond interactions as robust supramolecular units for crystal engineering: X-ray crystallographic, computational and urease inhibitory investigations of 1,2,4-triazolo[3,4-a]phthalazines

The antiparallel π-stacked dimer of compound 6 (left) presenting a large dimerization energy (ΔE5 = −11.2 kcal mol−1) and confirming its relevance in the solid state of compound 6. 3D binding mode of 6 (right) docked in the catalytic domain of urease.

Syntheses, crystal structures and supramolecular assemblies of two Cu(ii) complexes based on a new heterocyclic ligand: insights into C–H⋯Cl and π⋯π interactions

A new heterocyclic ligand, N3L [4-(1-methylimidazole)-2,6-di(pyrazinyl)pyridine] and two Cu(ii) complexes have been synthesized and characterized by several spectroscopic and DFT methods.

Accurate Quantum Chemical Prediction of Gas-Phase Anion Binding Affinities and Their Structure-Binding Relationships

This paper systematically examines the performance of contemporary wavefunction and density functional theory methods to identify robust and cost-efficient methods for predicting gas-phase anion binding energies. This includes the local coupled cluster LNO-CCSD(T) and DLPNO-CCSD(T), as well as double-hybrid DSD-PBEP86-D3(BJ) and various hybrid functionals M06-2X, B3LYP-D3(BJ), ωB97M-V, and ωB97X-V. The focus is on dual-hydrogen-bonding anion receptors that are commonly found in supramolecular chemistry and organocatalysis, namely, (thio)ureas, deltamides, (thio)squaramides, and croconamides as well as the yet-to-be-explored rhodizonamides. Of the methods examined, M06-2X emerged as the overall best performing method as the other functionals including DSD-PBEP86-D3(BJ) and the local coupled cluster DLPNO-CCSD(T) method displayed systematic errors that increase with the degree of carbonylation of the receptors. Hybrid ONIOM models that employed semiempirical methods (PM7, GFN1-xTB, and GFN2-xTB) and "threefold"-corrected small-basis set potentials (HF-3c, B97-3c, and PBEh-3c) were explored, and the best models resulted in 50- to 500-fold reduction in CPU time compared to W1-local. These calculations provide important insight into the structure-binding relationships where there is a direct correlation between Brønsted acidity and anion binding affinity, though the strength of the correlation also depends on other factors such as hydrogen-bonding geometry and the geometrical distortion that the receptor needs to undergo to bind the anion.

Being positive is not everything - experimental and computational studies on the selectivity of a self-assembled, multiple redox-state, receptor that binds anions with up to picomolar affinities

The interaction of the self-assembled trinuclear ruthenium bowl 1 3+ , that displays three other accessible oxidation states, with oxo-anions is investigated. Using a combination of NMR and electrochemical experimental data, estimates of the binding affinities of 1 4+ , 1 5+ , and 1 6+ for both halide and oxo-anions were derived. This analysis revealed that, across the range of oxidation states of the host, both high anion binding affinities (>10 9 M -1 for specific guests bound to 1 6+ ) and high selectivities (a range of >10 7 M -1 ) were observed. As the crystal structure of binding of the hexafluorophosphate anion revealed that the host has two potential binding sites (named the α and β pockets), the host-guest properties of both putative binding sites of the bowl, in all of its four oxidation states, were investigated through detailed quantum-based computational studies. These studies revealed that, due to the interplay of electrostatically assisted hydrogen-bonding and anion-π interactions, binding to the α pocket is generally preferred, except for the case of the relatively large and lipophilic hexafluorophosphate anionic guest and the host in the highest oxidation states, where the β pocket becomes relatively favourable. This analysis confirms that host-guest interactions involving structurally complex supramolecular architectures are driven by a combination of non-covalent interactions and, even in the case of charged binding pairs, electrostatics alone cannot accurately define these recognition processes.

Molecular Recognition of Nerve Agents and Their Organophosphorus Surrogates: Toward Supramolecular Scavengers and Catalysts

Nerve agents are tetrahedral organophosphorus compounds (OPs) that were developed in the last century to irreversibly inhibit acetylcholinesterase (AChE) and therefore impede neurological signaling in living organisms. Exposure to OPs leads to a rapid development of symptoms from excessive salivation, nasal congestion and chest pain to convulsion and asphyxiation which if left untreated may lead to death. These potent toxins are prepared on a large scale from inexpensive staring materials, making it feasible for terrorist groups or states to use them against military and civilians. The existing antidotes provide limited protection and are difficult to apply to a large number of affected individuals. While new prophylactics are currently being developed, there is still need for therapeutics capable of both preventing and reversing the effects of OP poisoning. In this review, we describe how the science of molecular recognition can expand the pallet of tools for rapid and safe sequestration of nerve agents.

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.

Anion-Selective Electrodes Based On a CH-Hydrogen Bonding Bis-macrocyclic Ionophore with a Clamshell Architecture

CH-hydrogen bonding provides access to new building blocks for making macrocyclic ionophores with high degrees of preorganization and selective anion recognition. In this study, an anion-binding ionophore in the shape of a clamshell (ClS) was employed that is composed of two cyanostar (CNstar) macrocycles with preorganized cavities linked with a 12-carbon chain. This ionophore allows for anion complexation by CH-hydrogen bonding. The potentiometric performance of membrane-based ion-selective electrodes incorporating this ionophore was evaluated. Different membrane compositions were prepared to determine the optimum concentrations of the ionophore and lipophilic additive in the membrane. The optimized electrode had a slope of -58.2 mV/decade and demonstrated an anti-Hofmeister selectivity pattern toward iodide with a nanomolar detection limit. Electrospray ionization mass spectrometry was employed to study the relative association strengths of ClS with various anions. The observed mass peaks of the ion-ionophore complexes were found to be consistent with the potentiometric selectivity pattern of the corresponding electrodes. Overall, the selectivity of the electrode could be altered by using an ionophore in which the two CNstar macrocycles are linked together with a flexible 12-carbon chain to control the molecularity of the binding event.

Carbazole sulfonamide-based macrocyclic receptors capable of selective complexation of fluoride ion

Two carbazole sulfonamide-based macrocycles 1 and 2 were facilely synthesized and carefully evaluated for their anion recognition properties. The obtained results revealed that macrocycle 1 with a 1,3-xylyl linker was able to bind fluoride ion more strongly and selectively in acetonitrile medium than its strong competitors (like acetate and dihydrogen phosphate anions), with a large binding constant (K_a) of 50 878 M⁻¹. More importantly, an exclusive fluoride recognition was achieved for macrocycle 1 in the more polar DMSO-d₆ solution, albeit with a moderate affinity of K_a = 147 M⁻¹. Compared with macrocycle 1, macrocycle 2 bearing a 2,6-lutidinyl linkage exhibited a remarkable change not only in the anion affinity but also in the anion selectivity, although with only a slight difference in their molecular structures.

Two carbazole sulfonamide-based macrocycles 1 and 2 were facilely synthesized and carefully evaluated for their anion recognition properties.

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 Strained Ion Pair Permits Carbon Dioxide Fixation at Atmospheric Pressure by C-H H-Bonding Organocatalysis

The cycloadditions of carbon dioxide into epoxides to afford cyclic carbonates by H-bond donor (HBD) and onium halide (X) cocatalysis have emerged as a key strategy for CO₂ fixation. However, if the HBD is also a halide receptor, the two will quench each other, decreasing the catalytic activity. Here, we propose a strained ion pair tris(alkylamino)cyclopropenium halide (TAC·X), in which TAC repels X. TAC possesses a positively charged cyclopropenium core that makes the vicinal C-H or N-H a nonclassical HBD. The interionic strain within TAC·X makes TAC a more electrophilic HBD, allowing it to activate the oxygen of the epoxide and making X more nucleophilic and better able to attack the methylene carbon of the epoxide. NMR titration spectra and computational studies were employed to probe the mechanism of the cycloaddition of CO₂ to epoxides reactions under the catalysis of TAC·X. The ¹H and ¹³C{¹H}NMR titration spectra of the catalyst with the epoxide substrate unambiguously confirmed H-bonding between TAC and the epoxide. DFT computational studies identified the transition states in the ring-opening of the epoxide (TS1) and in the ring-closure of the cyclic carbonate (TS2).

A [15]paracyclophenone and its fluorenone-containing derivatives: syntheses and binding to nerve agent mimics via aryl-CH hydrogen bonding interactions

A [1₅]paracyclophenone and its fluorenone-containing derivatives were synthesized. The novel macrocyclic host I binds nerve agent mimics through the ‘non-traditional’ aryl-CH hydrogen bonding interactions.

Synthesis and crystal structure of the simultaneous binding of Ni(ii) cation and chloride by the protonated 2,4,6 tris-(2-pyridyl)-1,3,5 triazine ligand: theoretical investigations of anion⋯π, π⋯π and hydrogen bonding interactions

A new octahedral Ni(ii) complex of a protonated 2,4,6-tris(2-pyridyl)-1,3,5-triazine ligand has been synthesized and X-ray analysis exhibited cooperative anion–π and H-bonding interactions.

Chiral anion recognition using calix[4]arene-based ureido receptors in a 1,3-alternate conformation

The introduction of chiral alkyl substituents into the lower rim of calix[4]arene immobilised in the 1,3-alternate conformation led to a system possessing a preorganised ureido cavity hemmed with chiral alkyl units in the near proximity. As shown by the ¹H NMR titration experiments, these compounds can be used as receptors for chiral anions in DMSO-d ₆. The chiral recognition ability can be further strengthened by the introduction of another chiral moiety directly onto the urea N atoms. The systems with double chiral units being located around the binding ureido cavity showed better stereodiscrimination, with the highest selectivity factor being 3.33 (K _L/K _D) achieved for N-acetyl-ʟ-phenylalaninate. The structures of some receptors were confirmed by single crystal X-ray analysis.

Enhancing the selectivity of optical sensors using synthetic transmembrane ion transporters

Herein, we introduce a new method to optimize the properties of optical sensors, coined the transporter-liposome-fluorophore (TLF) approach. It is shown that this approach can greatly improve the selectivity of the sensor, increase the dynamic range and maintain the sensitivity of the original fluorophore.

Molecular recognition and sensing of dicarboxylates and dicarboxylic acids

The recognition and detection of dicarboxylic acids and dicarboxylates is of significance for a wide variety of applications, including medical diagnosis, monitoring of health and of environmental contaminants, and in industry. Hence small molecule receptors and sensors for dicarboxylic acids and dicarboxylates have great potential for applications in these fields. This review outlines the challenges faced in the recognition and detection of these species, strategies that have been used to obtain effective and observable interactions with dicarboxylic acids and dicarboxylates, and progress made in this field in the period from 2014 to 2020.

Iminoguanidines: from anion recognition and separation to carbon capture

Iminoguanidines, first reported in 1898, have received renewed attention in the last 5 years due to their ability to recognize and separate anions from competitive aqueous environments. Iminoguanidines display high recognition abilities towards hydrophilic oxyanions (e.g., sulfate, chromate, carbonate) through strong and complementary hydrogen bonding from the guanidinium groups. This feature article reviews the fundamental anion recognition chemistry of iminoguanidines, as well as real-world applications including sulfate removal from seawater and CO₂ capture for climate change mitigations.

Dramatic Enhancement of Binding Affinities Between Foldamer-Based Receptors and Anions by Intra-Receptor π-Stacking

As a synthetic model for intra-protein interactions that reinforce binding affinities between proteins and ligands, the energetic interplay of binding and folding was investigated using foldamer-based receptors capable of adopting helical structures. The receptors were designed to have identical hydrogen-bonding sites for anion binding but different aryl appendages that simply provide additional π-stacking within the helical backbones without direct interactions with the bound anions. In particular, the presence of electron-deficient aryl appendages led to dramatic enhancements in the association constant between the receptor and chloride or nitrate ions, by up to three orders of magnitude. Extended stacking within the receptor contributes to the stabilization of the entire folding structure of complexes, thereby enhancing binding affinities.

A neutral porous organic polymer host for the recognition of anionic dyes in water

Neutral hosts for the recognition of anionic guests in water remain underdeveloped due to the inherent thermodynamic barrier for desolvation. To address this challenge, we have repurposed crosslinked porous organic polymers (POPs) as hosts. This polymer architecture affords a hydrophobic environment with a densely packed array of urea hydrogen bond donors to cooperatively promote anion desolvation and recognition in water. Using the principles of supramolecular design, we demonstrate through adsorption assays that the resulting Urea-POP-1 can recognize structurally different dyes containing phosphonate, sulfonate, and carboxylate anions in water. Moreover, when compared to Methyl-POP-1, a control POP lacking hydrogen bond donors, we find that the driving force for desolvation and adsorption of each dye is achieved through hydrophobic interactions with the POP backbone and, more importantly, cooperative hydrogen bonding interactions with the urea sidechains. This starting point sets the stage to exploit the modularity of our design to build a family of neutral polymer hosts with tunable pore sizes and anion preferences for fundamental investigations and targeted applications.

Intertwined Detection and Recognition Roles of Tetrazine in Synergistic Anion-π and H-bond Based Anion Receptor

The intrinsic properties of tetrazine as a π-anion receptor and as an on/off recognition probe merged with H-bond ability of an urea motif into a single architecture constitutes a new generation of well-defined anion receptors. Complexation properties directly benefit from the dual and synergistic contribution of tetrazine and urea. In this study, we report on the synthesis and assessment of binding properties to anions of diverse geometries. Association constants have been predicted by theoretical calculations and evaluated by multiple and complementary experimental techniques including electrospray-mass tandem spectroscopy, NMR, UV-visible, steady state fluorescence spectroscopies and time resolved fluorescence. These results provide the basis for a better understanding of both the complexation and the anion-dependent quenching mechanism.

Hydrogen bond design principles

Hydrogen bonding principles are at the core of supramolecular design. This overview features a discussion relating molecular structure to hydrogen bond strengths, highlighting the following electronic effects on hydrogen bonding: electronegativity, steric effects, electrostatic effects, π-conjugation, and network cooperativity. Historical developments, along with experimental and computational efforts, leading up to the birth of the hydrogen bond concept, the discovery of nonclassical hydrogen bonds (C-H…O, O-H…π, dihydrogen bonding), and the proposal of hydrogen bond design principles (e.g., secondary electrostatic interactions, resonance-assisted hydrogen bonding, and aromaticity effects) are outlined. Applications of hydrogen bond design principles are presented.

Tuning Supramolecular Selectivity for Hydrosulfide: Linear Free Energy Relationships Reveal Preferential C-H Hydrogen Bond Interactions

Supramolecular anion receptors can be used to study the molecular recognition properties of the reactive yet biologically critical hydrochalcogenide anions (HCh^-). Achieving selectivity for HCh^- over the halides is challenging but necessary for not only developing future supramolecular probes for HCh^- binding and detection, but also for understanding the fundamental properties that govern these binding and recognition events. Here we demonstrate that linear free energy relationships (LFERs)-including Hammett and Swain-Lupton plots-reveal a clear difference in sensitivity to the polarity of an aryl C-H hydrogen bond (HB) donor for HS^- over other HCh^- and halides. Analysis using electrostatic potential maps highlights that this difference in sensitivity results from a preference of the aryl C-H HB donor for HS^- in this host scaffold. From this study, we demonstrate that LFERs are a powerful tool to gain interpretative insight into motif design for future anion-selective supramolecular receptors and highlight the importance of C-H HB donors for HS^- recognition. From our results, we suggest that aryl C-H HB donors should be investigated in the next generation of HS^- selective receptors based on the enhanced HS^- selectivity over other competing anions in this system.

Highly Selective Anion Template Effect in the Synthesis of Constrained Pseudopeptidic Macrocyclic Cyclophanes

Herein, we report the synthesis of a novel family of constrained pseudopeptidic macrocyclic compounds containing the hexahydropyrrolo[3,4-f]isoindolocyclophane scaffold and involving four coupled substitution reactions in the macrocyclization process. Although the increase in the number of steps involved in the macrocyclization could lead to a larger number of possible side products, the optimization of the methodology and the study of the driving forces have made it possible to obtain the desired macrocycles in excellent yields. A thorough computational study has been carried out to understand the macrocyclization process, and the results obtained nicely agree with experimental data. Moreover, the bromide anion had a clear catalytic template effect in the macrocyclization reaction, and surprisingly, the chloride anion had a negative template effect in opposition to the results obtained for analogous macrocycles. The parameters responsible for the specific kinetic template effect observed have been studied in detail.

Electrochemical Anion Sensing: Supramolecular Approaches

Anions play a vital role in a broad range of environmental, technological, and physiological processes, making their detection/quantification valuable. Electroanalytical sensors offer much to the selective, sensitive, cheap, portable, and real-time analysis of anion presence where suitable combinations of selective (noncovalent) recognition and transduction can be integrated. Spurred on by significant developments in anion supramolecular chemistry, electrochemical anion sensing has received considerable attention in the past two decades. In this review, we provide a detailed overview of all electroanalytical techniques that have been used for this purpose, including voltammetric, impedimetric, capacititive, and potentiometric methods. We will confine our discussion to sensors that are based on synthetic anion receptors with a specific focus on reversible, noncovalent interactions, in particular, hydrogen- and halogen-bonding. Apart from their sensory properties, we will also discuss how electrochemical techniques can be used to study anion recognition processes (e.g., binding constant determination) and will furthermore provide a detailed outlook over future efforts and promising new avenues in this field.

Supramolecular and theoretical perspectives of 2,2′:6′,2′′-terpyridine based Ni(ii) and Cu(ii) complexes: on the importance of C–H⋯Cl and π⋯π interactions

In this manuscript we report the synthesis and X-ray characterization of two new Ni(ii) and Cu(ii) complexes using 2,2′:6′,2′′-terpyridine as a ligand and chloride as a coligand.