Anion-Based Self-assembly of Resorcin[4]arenes and Pyrogallol[4]arenes

Phenol[4]arenes: Excellent Macrocyclic Precursors for Constructing Chiral Porous Organic Cages

The development of new chiral building blocks for constructing complex chiral architectures, such as macrocycles and cages, is both crucial and challenging. Although concave-shaped calixarenes have been established as versatile building blocks for the synthesis of cage compounds, there are no reports on cages constructed from chiral calix[4]arene derivatives. Herein, we present a straightforward and effective method for gram-scale synthesis of a new member of chiral calix[4]arene macrocycle enantiomers, namely, phenol[4]arene (PC[4]A). As a proof of concept, we functionalized these enantiomers into tetraformylphenol[4]arene (PC[4]ACHO) derivatives via the Duff reaction to construct chiral porous organic cages (CPOCs) using polyamine synthons. Specifically, we employ two fluorescent amine synthons, bis(4-aminophenyl)phenylamine and tris(4-aminophenyl)amine, to assemble with PC[4]ACHO enantiomers, resulting in [2 + 4] lantern-shaped and [6 + 8] truncated octahedral CPOCs, respectively. These structures have been unambiguously characterized by single-crystal X-ray diffraction and circular dichroism (CD) spectroscopy. Notably, the [6 + 8] truncated CPOCs exhibit internal diameters of approximately 3.1 nm, a cavity volume of around 5300 Å3, and high specific surface areas of up to 1300 m2 g-1 after desolvation, making them among the largest CPOCs reported. Additionally, investigations into their chiral sensing performance demonstrate that these PC[4]A-based CPOCs enable the enantioselective recognition of amino acids and their derivatives. This work strongly suggests that PC[4]A can serve as an excellent building block for the rational design of chiral materials with practical applications.

Internal and External Pockets in Pillar[n]arene Sheets and Their Host-Guest Binding Beyond Cavity Volume Limitations

Constructing binding pockets by hierarchically assembling tailored building blocks and understanding structure-property relationships are challenging goals. Herein, amphiphilic pillar[5]arene and pillar[6]arene were prepared and used to construct 2D sheets, which consisted of well-defined hydrophobic and hydrophilic interlayers. In the hydrophobic interlayers, internal hydrophobic pockets were created by packing pairs of pillar[n]arenes, and external hydrophobic pockets were simultaneously generated from gaps between pillar[n]arenes due to electrostatic attractions. Aromatic hydrocarbons were accommodated in these hydrophobic pockets by ball milling. Due to the external pockets, bulky guests larger than the pillar[n]arene cavity sizes were also captured in the sheets.

Mechanochemical Synthesis of Molecular Chemoreceptors

The design of environmentally friendly methods for synthesizing molecular receptors is an expanding area within applied organic chemistry. This work systematically summarizes advances in the mechanochemical synthesis of molecular chemoreceptors. It discusses key achievements related to the synthesis of chemoreceptors containing azine, Schiff base, thiosemicarbazone, hydrazone, rhodamine 6G, imide, or amide moieties. Additionally, it highlights the application potential of mechanochemically synthesized molecular chemoreceptors in the recognition of ions and small molecules, along with a discussion of the mechanisms of detection processes.

Thermodynamics of Anion Binding by (Thio)ureido-calix[4]arene Derivatives in Acetonitrile

In this work, we developed (thio)ureido-calix[4]arene derivatives and thoroughly explored their anion-binding properties in acetonitrile. A series of anions, including important inorganic ones (Cl-, HSO4 -, H2PO4 -, and HP2O7 3-) and several ever-present carboxylates (acetate, benzoate, and fumarate), were studied. All systems were investigated by several methods (NMR, ITC, and UV) used in a synergistic fashion, providing their comprehensive thermodynamic description. Acidities of the receptors were determined prior to the anion-binding studies and considered in the data-handling procedures. Complexes of various stoichiometries were detected and the driving force for their formation elucidated. The correlation of the anion structural features and H-bond acceptor properties with the stoichiometries and complexation thermodynamics parameters was rationalized. Generally, stability of the complexes followed the trend defined by the basicity of anions. Thiourea and urea analogues exhibited similar affinities for anion binding except for the H2PO4 - and HP2O7 3-, which interacted with the thiourea analogue more strongly. The hosts endowed with 4 (thio)urea groups formed species containing two receptor molecules bridged by a fumarate or hydrogen pyrophosphate anion. Thermodynamic information provided in this work is applicable in further design of supramolecular systems, whereas the presented approach to data handling will aid researchers when dealing with multiple coexisting equilibria.

HFIP as a versatile solvent in resorcin[n]arene synthesis

Herein, we present 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) as an efficient solvent for synthesizing resorcin[n]arenes in the presence of catalytic amounts of HCl at ambient temperature and within minutes. Remarkably, resorcinols with electron-withdrawing groups and halogens, which are reported in the literature as the most challenging precursors in this cyclization, are tolerated. This method leads to a variety of 2-substituted resorcin[n]arenes in a single synthetic step with isolated yields up to 98%.

Window[1]resorcin[3]arenes: A Novel Macrocycle Able to Self-Assemble to a Catalytically Active Hexameric Cage

The hexameric resorcin[4]arene capsule has been utilized as one of the most versatile supramolecular capsule catalysts. Enlarging its size would enable expansion of the substrate size scope. However, no larger catalytically active versions have been reported. Herein, we introduce a novel class of macrocycles, named window[1]resorcin[3]arene (wRS), that assemble to a cage-like hexameric host. The new host was studied by NMR, encapsulation experiments, and molecular dynamics simulations. The cage is able to bind tetraalkylammonium ions that are too large for encapsulation inside the hexameric resorcin[4]arene capsule. Most importantly, it retained its catalytic activity, and the accelerated conversion of a large substrate that does not fit the closed hexameric resorcin[4]arene capsule was observed. Thus, it will help to expand the limited substrate size scope of the closed hexameric resorcin[4]arene capsule.

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.

A Cationic Catechol Derivative Binds Anions in Competitive Aqueous Media

A simple dihydroxy isoquinolinium molecule (3+ ) was prepared by a modification of a literature procedure. Interestingly, during optimisation of the synthesis a small amount of the natural product pseudopalmatine was isolated, and characterised for the first time by X-ray crystallography. Compound 3+ contains a catechol motif and positive charge on the same scaffold and was found to be a potent anion receptor, binding sulfate strongly in 8 : 2 d6 -acetone:D2 O and 7 : 3 d6 -acetone:D2 O (Ka >104 and 2,100 M-1 , respectively). Unsurprisingly, chloride binding was much weaker, even in the less polar solvent mixture 9 : 1 d6 -acetone:D2 O. The sulfate binding is remarkably strong for such a simple molecule, however anion binding studies were complicated by the tendency of the molecule to react with BPh4 - or BF4 - species during anion metathesis reactions. This gave two unusual zwitterions containing tetrahedral boronate centres, which were both characterised by X-ray crystallography.

Encapsulation in Charged Droplets Generates Distorted Host-Guest Complexes

The ability of various hydrogen-bonded resorcinarene-based capsules to bind α,ω-alkylbisDABCOnium (DnD) guests of different lengths was investigated in solution and in the gas-phase. While no host-guest interactions were detected in solution, encapsulation could be achieved in the charged droplets formed during electrospray ionisation (ESI). This included guests, which are far too long in their most stable conformation to fit inside the cavity of the capsules. A combination of three mass spectrometric techniques, namely, collision-induced dissociation, hydrogen/deuterium exchange, and ion-mobility mass spectrometry, together with computational modelling allow us to determine the binding mode of the DnD guests inside the cavity of the capsules. Significant distortions of the guest into horseshoe-like arrangements are required to optimise cation-π interactions with the host, which also adopt distorted geometries with partially open hydrogen-bonding seams when binding longer guests. Such quasi "spring-loaded" capsules can form in the charged droplets during the ESI process as there is no competition between guest encapsulation and ion pair formation with the counterions that preclude encapsulation in solution. The encapsulation complexes are sufficiently stable in the gas-phase - even when strained - because non-covalent interactions significantly strengthen in the absence of solvent.

Selective confinement of potassium, rubidium, or caesium ions in a non-covalent hydroxyproline octamer cage stabilized by cis-hydroxyl locks

While numerous studies have focused on the impact of chirality on some magic amino acid clusters, this article investigates the effects of steric isomerization using 4-hydroxyproline octamers as a model system. Through mass spectrometry, infrared photodissociation spectroscopy, and theoretical calculation, it was demonstrated that the cis-4-hydroxy-L-proline octamer can selectively cage potassium, rubidium, or caesium ions through stable cis-hydroxyl locks, while the trans-form cannot. The results highlight the importance of hydroxyl group orientation in designing biocompatible membrane transporters with high ion-selectivity.

Nanocapsules of unprecedented internal volume seamed by calcium ions

The inception of an unprecedented class of voluminous Platonic solids displaying hierarchical geometry based on pyrogallol[4]arene moieties seamed by divalent calcium ion is described. Single-crystal X-ray structural determination has established the highly conserved geometry of two original Ca2+-seamed nanocapsules to be essentially cubic in shape with C-ethylpyrogallol[4]arene units located along the twelve edges of the cube which are then bridged by metallic polyatomic cations ([Ca4Cl]7+ or [Ca(HCO2)Na4]5+) at the six cube faces. The accessible volume of the nanocapsules is ca. 3500 Å3 and 2500 Å3 and is completely isolated from the exterior of the capsules. These remarkable nanocapsule discoveries cast a spotlight on a marginalized area of synthetic materials chemistry and encourage future exploration of diversiform supramolecular assemblies, networks, and capsules built on calcium, with clear benefits deriving from the intrinsic biocompatibility of calcium. Finally, a proof-of-concept is demonstrated for fluorescent reporter encapsulation and sustained release from the calcium-seamed nanocapsules, suggesting their potential as delivery vehicles for drugs, nutrients, preservatives, or antioxidants.

Noncovalent tailoring of coordination complexes by resorcin[4]arene-based supramolecular hosts

Molecular recognition of guest molecules in a confined cavity is one of the important phenomena in biological and artificial molecular systems. When the guest is trapped within an artificial nano-space, its conformation is fixed in an unusual fashion by noncovalent interactions with host frameworks, and also the guest is kept away from the bulk solvent by the steric effect of the host. Therefore, host-guest formations lead to the effective modulation of the chemical and physical properties of guests via noncovalent interactions. In contrast to the many examples of organic guests, the examples of host-guest formation using coordination complex guests have been less explored. This is simply due to the size and shape complementarity problem between small hosts and large coordination complex guests. Resorcin[4]arene-based supramolecular hosts have been shown to provide internal cavities that are large enough to fully accommodate coordination complexes within the internal spaces via effective molecular interactions. In this article, we focus on supramolecular strategies to control the chemical and physical properties of the coordination complex guests within resorcin[4]arene-based supramolecular hosts. By the careful selection of the host and guest complexes, these combinations can produce a new supramolecular system, showing unusual structures, redox, catalytic, and photophysical properties derived from the entrapped coordination complexes.

The "Nitrogen Effect": Complexation with Macrocycles Potentiates Fused Heterocycles to Form Halogen Bonds in Competitive Solvents

Weak intermolecular forces are typically very difficult to observe in highly competitive polar protic solvents as they are overwhelmed by the quantity of competing solvent. This is even more challenging for three-component ternary assemblies of pure organic compounds. In this work, we overcome these complications by leveraging the binding of fused aromatic N-heterocycles in an open resorcinarene cavity to template the formation of a three-component halogen-bonded ternary assembly in a protic polar solvent system.

Symmetry-breaking host-guest assembly in a hydrogen-bonded supramolecular system

Bio-inspired self-assembly is invaluable to create well-defined giant structures from small molecular units. Owing to a large entropy loss in the self-assembly process, highly symmetric structures are typically obtained as thermodynamic products while formation of low symmetric assemblies is still challenging. In this study, we report the symmetry-breaking self-assembly of a defined C₁-symmetric supramolecular structure from an O_h-symmetric hydrogen-bonded resorcin[4]arene capsule and C₂-symmetric cationic bis-cyclometalated Ir complexes, carrying sterically demanding tertiary butyl (^tBu) groups, on the basis of synergistic effects of weak binding forces. The flexible capsule framework shows a large structural change upon guest binding to form a distorted resorcin[4]arene hexameric capsule, providing an asymmetric cavity. Location of the chiral guest inside the anisotropic environment leads to modulation of its Electric Dipole (ED) and Magnetic Dipole (MD) transition moments in the excited state, causing an increased emission quantum yield, longer emission lifetime, and enhancement of the dissymmetry factor (g_lum) in the circularly polarized luminescence.

Recognition-Induced Enhanced Emission of Core-Fluorescent ESIPT-type Macrocycles

Core-fluorescent cavitands based on 2-(2'-resorcinol)benzimidazole fluorophores (RBIs) merged with the resorcin[4]arene skeleton were designed and synthesized. The cavitands, due to the presence of intramolecular hydrogen bonds and increased acidity, show excited state intramolecular proton transfer (ESIPT) and readily undergo deprotonation to form dianionic cavitands, capable of strong binding to organic cations. The changes in fluorescence are induced by deprotonation and binding events and involve huge Stokes shifts (due to emission from anionic double keto tautomers) and cation-selective enhancement of emission originating from the restriction of intramolecular motion (RIR) upon recognition in the cavity. Ab initio calculations indicate that the macrocyclic scaffold stabilizes the ground state tautomeric forms of the fluorophores that are not observed for non-macrocyclic analogs. In the excited state, the emitting forms for both macrocyclic scaffolds and non-macrocyclic analogs are anionic double keto tautomers, which are the result of excited state intramolecular proton transfer (ESIPT) or excited state double proton transfer (ESDPT).

Diffusion NMR Reveals the Structures of the Molecular Aggregates of Resorcin[4]arenes and Pyrogallol[4]arenes in Aromatic and Chlorinated Solvents

The hexameric assemblies of resorcinarenes and pyrogallolarenes are fascinating structures that can serve as nanoreactors in which new chemistry and catalysis occur. Recently, it was suggested based on SANS or SAXS that C₁₁-resorcin[4]arene (1) forms octameric aggregates of a micellar rather than capsular structure in toluene. Here, using NMR spectroscopy, diffusion NMR, and DOSY performed on solutions of C₁₁-resorcin[4]arene (1), C₁₁-pyrogallol[4]arene (2), and mixtures thereof in protonated and deuterated solvents, we found that, in benzene and toluene, 1 primarily formed hexameric capsules accompanied by a minor product with diffusion characteristics consistent with an octameric assembly. In chloroform, 1 formed hexameric capsules. In toluene, 2D NMR revealed two populations of encapsulated toluene molecules in the same capsule of 1. The addition of tetrahexylammonium bromide to the assemblies of 1 in aromatic solvents drove the equilibrium toward the formation of the hexameric capsules. Interestingly, 2 formed only hexameric capsules in all solvents tested.

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.

An electron-deficient supramolecular macrocyclic host for the selective separation of aromatics and cyclic aliphatics

Host 1 with an electron-deficient nanometer-sized cavity were synthesized in two steps. Moreover, 1 has been successfully used as a gas chromatographic stationary phase to prepare a packed column for the separation of PhH/Cy and Tol/MCy.