Cone Calix[4]arene Diethyl Ester Strapped Calix[4]pyrrole: A Selective Receptor for the Fluoride Anion

N-Confused strapped calix[4]pyrrole: the missing member of calix[4]pyrrole chemistry

The first example of N-confused strapped calix[4]pyrrole 5 is presented. The structural integrity of 5 and its regular isomer 4 was unambiguously confirmed by single crystal X-ray diffraction analysis. Anion binding studies using 1H NMR titration carried out in CDCl3 revealed a small but detectable tendency of 5 to interact with an anion. Conversely, the isomeric regular strapped calix[4]pyrrole 4 displayed high selectivity for fluoride anions under similar experimental conditions. The high fluoride selectivity of 4 and unexpectedly low anion affinity of 5 were ascribed to the presence of intramolecular hydrogen bonds within strapping subunits.

Does Larger Cavity-Size Really Help Bigger Anions to Bind? A Scrutiny on Core-Expanded Calix[4]pyrroles and Their Properties

Calix[4]pyrroles are an important class of oligopyrrolic macrocycles and have found applications in many diverse fields including anion recognition. To modulate the properties of the calix[4]pyrrole, several structural modifications are realized. The core-expansion has attracted extra attention as it provides larger cavity-size compared to parent calix[4]pyrrole(s). This review highlights the synthetic development of various core-expanded calix[4]pyrroles and their applications in anion-binding properties. Emphasis is given to the changes in the binding properties observed with expanded versions of calix[4]pyrrole(s) in both solution and the solid states. The expanded versions of calix[4]pyrrole do not always show higher binding affinities for larger anions as anticipated. Rather, they display reduced affinities with the anions. The truncated form or asymmetric nature of the expanded versions of calix[4]pyrrole does not probably allow to access all the available binding sites for the anions and hence reduced binding affinities are observed. The receptors which contain a greater number of binding sites and are somehow rigid or preorganized apparently show enhanced binding affinities for anions. The relative binding constants for halide series indicate that the enlarged molecules are more beneficial for largest iodide among others. However, most of the receptors show selectivity towards smallest fluoride over other anions studied.

Diphenylpyrrole-Strapped Calix[4]pyrrole Extractant for the Fluoride and Chloride Anions

The anion binding features of diphenylpyrrole-strapped calix[4]pyrrole 1 have been investigated by means of 1 H NMR spectroscopy and ITC (isothermal titration calorimetry), as well as single crystal X-ray diffraction analyses. Receptor 1 bearing an auxiliary pyrrolic NH donor and solubilizing phenyl groups on the strap was found to bind F- , Cl- , and Br- as their tetrabutylammonium salts with high affinity in DMSO-d6 . In addition, receptor 1 was found to extract the fluoride anion (as both its tetraethylammonium (TEA+ ) and tetrabutylammonium (TBA+ ) salts), as well as the chloride anion into chloroform-d from an aqueous source phase. Cation metathesis using TBAI or the use of a dual host approach involving crown ethers enabled receptor 1 to extract simple alkali metal fluoride or chloride salts from water. Quantitative binding of NaF by receptor 1 was observed in 20 % D2 O-DMSO-d6 allowing for the direct determination of the NaF concentration in an unknown sample.

Tying a knot between crown ethers and porphyrins

Porphyrins and crown ether hybrids have emerged as a promising class of molecules composed of elements of a tetrapyrrole macrocycle and an oligo(ethylene glycol) segment. These hybrid systems constitute a broad group of compounds, including crowned porphyrins, crownphyrins, and calixpyrrole-crown ether systems forming Pacman complexes with transition metals. Their unique nature accustoms them as excellent ligands and hosts capable of binding guest molecules/ions, but also to undergo unusual transformations, such as metal-induced expansion/contraction. Depending on the design of the particular hybrid, they present unique features involving intriguing redox chemistry, interesting optical properties, and reactivity towards transition metals. In this perspective article, the overview of both the early designs of porphyrin-crown ether hybrids, as well as the most recent advances in the synthesis and characterisation of this remarkable group of macrocyclic systems, are addressed. The discussion covers the strategies employed in synthesising these systems, including cyclisation reactions, self-assembly, and their remarkable reactivity. The potential applications of porphyrin-crown ether hybrids are also highlighted. Moreover, the discussion identifies the challenges associated with synthesising and characterising hybrids, outlining the possible future directions.

Plenates: Anion-Dependent Self-Assembly of the Pyrrole Cage Encapsulating Silver(I) Clusters

The self-assembly of 2,5-diformylpyrrole, tris(2-aminoethyl)amine, and silver(I) yielded, depending on the size and basicity of the anion, new cascade complexes or plenates, that is, cryptates incorporating Ag_n ⁿ⁺ clusters. The nature of the product was counterion-dependent, and its formation was either driven by cascade anion binding or by argentophilic interactions stabilizing the cluster within the cavity. The reaction of plenates with tetrabutylammonium halides resulted in the protonation-coupled replacement of the Ag₃ ³⁺ with anion(s), yielding cascade cryptates.

Aryl- and Superaryl-Extended Calix[4]pyrroles: From Syntheses to Potential Applications

The incorporation of aryl substituents at the meso-positions of calix[4]pyrrole (C4P) scaffolds produces aryl-extended (AE) and super-aryl-extended (SAE) calix[4]pyrroles. The cone conformation of the all-α isomers of "multi-wall" AE-C4Ps and SAE-C4Ps displays deep aromatic clefts or cavities. In particular, "four-wall" receptors feature an aromatic polar cavity closed at one end with four convergent pyrrole rings and fully open at the opposite end. This makes AE- and SAE-C4P scaffolds effective receptors for the molecular recognition of negatively charged ions and neutral guest molecules with donor-acceptor and hydrogen bonding motifs. In addition, adequately functionalized all-α isomers of multi wall AE- and SAE-C4P scaffolds self-assemble into uni-molecular and supra-molecular aggregates displaying capsular and cage-like structures. The self-assembly process requires the presence of template ions or molecules that lock the C4P cone conformation and complementing the inner polar functions and volumes of their cavities. We envisioned performing an in-depth revision of AE- and SAE-C4P scaffolds owing to their importance in different domains such as supramolecular chemistry, biology, material sciences and pharmaceutical chemistry. Herewith, besides the synthetic details on the elaboration of their structures, we also draw attention to their diverse applications. The organization of this review is mainly based on the number of "walls" present in the AE-C4P derivatives and their structural modifications. The sections are further divided based on the C4P functions and applications. The authors are convinced that this review will be of interest to researchers working in the general area of supramolecular chemistry as well as those involved in the study of the binding properties and applications of C4P derivatives.

Triazolated calix[4]semitubes: assembling strategies towards long multicalixarene architectures

Cone and 1,3-alternate calix[4]arenes bearing pairs of 2-azidoethyl or propargyl groups, and 1,3-alternate calix[4]arenes having four 2-azidoethyl, four propargyl groups or pairs of 2-azidoethyl and silylated propargyl groups were explored...

1,2-Phenylene-Incorporated Smallest Expanded Calix[4]pyrrole via One-Step Synthesis of Tetrapyrrane: A Fluorescent Host for Fluoride Ion

Synthesis of tetrapyrrane 8 from acetone and pyrrole via one-step condensation was achieved for the first time along with a much-improved yield of the tripyrrane 9. Diborylation of the tetrapyrrane and subsequent "1 + 1" cyclocoupling with 1,2-diiodobenzene following the Suzuki protocol generated novel o-phenylene incorporated macrocycle belonging to the smallest meso-expanded calix[4]pyrrole family. The latter macrocycle displays exclusive turn-on fluorescence sensing of fluoride ion upon complexation via a unique partial cone conformation supported by DFT analysis in acetonitrile solvent.

Benzenebistriazole-strapped calix[4]pyrrole: a neutral anion receptor with CH and NH donor groups that exhibits high sulfate binding affinity and selectivity in aqueous solutions

A calix[4]pyrrole strapped by benzenebistriazole has been prepared as an artificial anion binding receptor. This neutral anion receptor shows high sulfate binding affinity and selectivity in an aqueous solution. In solid state, the receptor binds the sulfate anion in a chair-like 3D cavity via multiple N-H and C-H hydrogen bonds.

Capture and displacement-based release of the bicarbonate anion by calix[4]pyrroles with small rigid straps

Two-phenoxy walled calix[4]pyrroles 1 and 2 strapped with small rigid linkers containing pyridine and benzene, respectively, have been synthesized. 1H NMR spectroscopic analyses carried out in CDCl3 revealed that both of receptors 1 and 2 recognize only F- and HCO3 - among various test anions with high preference for HCO3 - (as the tetraethylammonium, TEA+ salt) relative to F- (as the TBA+ salt). The bound HCO3 - anion was completely released out of the receptors upon the addition of F- (as the tetrabutylammonium, TBA+ salt) as a result of significantly enhanced affinities and selectivities of the receptors for F- once converted to the TEAHCO3 complexes. Consequently, relatively stable TEAF complexes of receptors 1 and 2 were formed via anion metathesis occurring within the receptor cavities. By contrast, the direct addition of TEAF to receptors 1 and 2 produces different complexation products initially, although eventually the same TEAF complexes are produced as via sequential TEAHCO3 and TBAF addition. These findings are rationalized in terms of the formation of different ion pair complexes involving interactions both inside and outside of the core receptor framework.

Highly Selective Fluoride Recognition by a Small Tris-Urea Covalent Cage

A highly selective recognition of fluoride was achieved through the design of a small hemicryptophane cage (3) presenting a southern tris-urea hosting moiety. The resulting host-guest complex has been characterized by electrospray ionization-high-resolution mass spectrometry, ¹H and ¹⁹F NMR, and X-ray diffraction techniques. In particular, X-ray diffraction analysis of [3·F^-] reveals that the encapsulation of one fluoride, within 3, occurs through NH···F^- H-bonding with the six NH residues of the tris-urea ligand. An association constant of 1200 M^-1 was extracted from ¹H NMR titration experiments, indicating that efficient fluoride binding also occurs in solution. Finally, in sharp contrast with previously reported urea-based hemicryptophane hosts, the small preorganized cavity found in 3 allows for an exclusive selectivity for fluoride over other competing halides.

Strapped calix[4]pyrroles: from syntheses to applications

Supramolecular chemistry is a central topic in modern chemistry. It touches on many traditional disciplines, such as organic chemistry, inorganic chemistry, physical chemistry, materials chemistry, environmental chemistry, and biological chemistry. Supramolecular hosts, inter alia macrocyclic hosts, play critical roles in supramolecular chemistry. Calix[4]pyrroles, non-aromatic tetrapyrrolic macrocycles defined by sp³ hybridized meso bridges, have proved to be versatile receptors for neutral species, anions, and cations, as well as ion pairs. Compared to the parent system, octamethylcalix[4]pyrrole and its derivatives bearing simple appended functionalities, strapped calix[4]pyrroles typically display enhanced binding affinities and selectivities. In this review, we summarize advances in the design and synthesis of strapped calix[4]pyrroles, as well as their broad utility in molecular recognition, supramolecular extraction, separation technology, ion transport, and as agents capable of inhibiting cancer cell proliferation. Future challenges within this sub-field are also discussed.

Phenanthroline-strapped calix[4]pyrroles: anion receptors displaying affinity reversal as a function of solvent polarity

Calix[4]pyrroles 1 and 2, diametrically strapped with a phenanthroline via ester and amide linkages, respectively, have been synthesized as anion receptors.

Small triiminopyrrolic molecular cage with high affinity and selectivity for fluoride

A small molecular cage (4) with high affinity and complete selectivity for fluoride to the limit of detection over other competing small anions was synthesized. Cage 4 was also found to retain the encapsulated fluoride anion within its cavity even after one or two pyrrolic NH protons were subject to deprotonation.

Macrocycles as Ion Pair Receptors

Cation and anion recognition have both played central roles in the development of supramolecular chemistry. Much of the associated research has focused on the development of receptors for individual cations or anions, as well as their applications in different areas. Rarely is complexation of the counterions considered. In contrast, ion pair recognition chemistry, emerging from cation and anion coordination chemistry, is a specific research field where co-complexation of both anions and cations, so-called ion pairs, is the center of focus. Systems used for the purpose, known as ion pair receptors, are typically di- or polytopic hosts that contain recognition sites for both cations and anions and which permit the concurrent binding of multiple ions. The field of ion pair recognition has blossomed during the past decades. Several smaller reviews on the topic were published roughly 5 years ago. They provided a summary of synthetic progress and detailed the various limiting ion recognition modes displayed by both acyclic and macrocyclic ion pair receptors known at the time. The present review is designed to provide a comprehensive and up-to-date overview of the chemistry of macrocycle-based ion pair receptors. We specifically focus on the relationship between structure and ion pair recognition, as well as applications of ion pair receptors in sensor development, cation and anion extraction, ion transport, and logic gate construction.

Molecular Engineering of Free-Base Porphyrins as Ligands-The N-H⋅⋅⋅X Binding Motif in Tetrapyrroles

The core N-H units of planar porphyrins are often inaccessible to forming hydrogen-bonding complexes with acceptor molecules. This is due to the fact that the amine moieties are "shielded" by the macrocyclic system, impeding the formation of intermolecular H-bonds. However, methods exist to modulate the tetrapyrrole conformations and to reshape the vector of N-H orientation outwards, thus increasing their availability and reactivity. Strategies include the use of porpho(di)methenes and phlorins (calixphyrins), as well as saddle-distorted porphyrins. The former form cavities due to interruption of the aromatic system. The latter are highly basic systems and capable of binding anions and neutral molecules via N-H⋅⋅⋅X-type H-bonds. This Review discusses the role of porphyrin(oid) ligands in various coordination-type complexes, means to access the core for hydrogen bonding, the concept of conformational control, and emerging applications, such as organocatalysis and sensors.

Triazole-bearing calixpyrroles: strong halide binding affinities through multiple N–H and C–H hydrogen bonds

Triazole-bearing calixpyrroles (TCPs) were synthesized as artificial anion binding receptors.

Development of Ion Chemosensors Based on Porphyrin Analogues

Sensing of metal ions and anions is of great importance because of their widespread distribution in environmental systems and biological processes. Colorimetric and fluorescent chemosensors based on organic molecular species have been demonstrated to be effective for the detection of various ions and possess the significant advantages of low cost, high sensitivity, and convenient implementation. Of the available classes of organic molecules, porphyrin analogues possess inherently many advantageous features, making them suitable for the design of ion chemosensors, with the targeted sensing behavior achieved and easily modulated based on their following characteristics: (1) NH moieties properly disposed for binding of anions through cooperative hydrogen-bonding interactions; (2) multiple pyrrolic N atoms or other heteroatoms for selectively chelating metal ions; (3) variability of macrocycle size and peripheral substitution for modulation of ion selectivity and sensitivity; and (4) tunable near-infrared emission and good biocompatibility. In this Review, design strategies, sensing mechanisms, and sensing performance of ion chemosensors based on porphyrin analogues are described by use of extensive examples. Ion chemosensors based on normal porphyrins and linear oligopyrroles are also briefly described. This Review provides valuable information for researchers of related areas and thus may inspire the development of more practical and effective approaches for designing high-performance ion chemosensors based on porphyrin analogues and other relevant compounds.

A nano-sized container for specific encapsulation of isolated water molecules

This calix[4]arene-based molecular box is able to encapsulate specifically two isolated water molecules in both non-protic and protic solvents.

A turn-on fluorescent sensor for relay recognition of two ions: from a F−-selective sensor to highly Zn2+-selective sensor by tuning electronic effects

A simple ion sensor bearing quinoline and an amide group was designed and synthesized, which showed both colorimetric detection for F⁻ and a fluorescence turn-on response for Zn²⁺. Moreover, sensor L2 can distinguish F⁻ and Zn²⁺via different sensing mechanisms.

A hybrid macrocyclic anion receptor exploiting the pyrrole-2,5-diacetamide unit

Macrocyclic compound built on pyrrole-2,5-diacetic bisamide was found to be an efficient receptor for H₂PO₄⁻ and PhCOO⁻.

Remarkably selective, non-linear allosteric regulation of anion binding by a tetracationic calix[4]pyrrole homodimer

A covalently coupled, dimeric tetra-cationic calix[4]pyrrole homodimer bearing anthracene linkers displayed distinctive cooperativity and fluoride selectivity with large positive allosterism.