A Chiral [2+3] Covalent Organic Cage Based on 1,1'-Bi-2-naphthol (BINOL) Units

A [2+3] chiral covalent organic cage is produced through a dynamic covalent chemistry approach by mixing two readily available building units, viz. an enantiopure 3,3'-diformyl 2,2'-BINOL compound (A) with a triamino spacer (B). The two enantiomeric (R,R,R) and (S,S,S) forms of the cage C are formed nearly quantitatively thanks to the reversibility of the imine linkage. The X-ray diffraction analysis of cage (S,S,S)-C highlights that the six OH functions of the BINOL fragments are positioned inside the cage cavity. Upon reduction of the imine bonds of cage C, the amine cage D is obtained. The ability of the cage D to host the 1-phenylethylammonium cation (EH+) as a guest is evaluated through UV, CD and DOSY NMR studies. A higher binding constant for (R)-EH+ cation (Ka=1.7 106±10 % M-1) related to (S)-EH+ (Ka=0.9 106±10 % M-1) is determined in the presence of the (R,R,R)-D cage. This enantiopreference is in close agreement with molecular dynamics simulation.

Purely Covalent Molecular Cages and Containers for Guest Encapsulation

Cage compounds offer unique binding pockets similar to enzyme-binding sites, which can be customized in terms of size, shape, and functional groups to point toward the cavity and many other parameters. Different synthetic strategies have been developed to create a toolkit of methods that allow preparing tailor-made organic cages for a number of distinct applications, such as gas separation, molecular recognition, molecular encapsulation, hosts for catalysis, etc. These examples show the versatility and high selectivity that can be achieved using cages, which is impossible by employing other molecular systems. This review explores the progress made in the field of fully organic molecular cages and containers by focusing on the properties of the cavity and their application to encapsulate guests.

Water-stable hydrazone-linked porous organic cages

Although porous organic cages (POCs), particularly imine-linked (C[double bond, length as m-dash]N) ones, have advanced significantly over the last few decades, the reversible nature of imine linkages makes them prone to hydrolysis and structural collapse, severely limiting their applications under moist or water conditions. Herein, seven water-stable hydrazone-linked (C[double bond, length as m-dash]N-N) POCs are prepared through a simple coupling of the same supramolecular tetraformylresorcin[4]arene cavitand with different dihydrazide linkers. Their structures are all determined by single-crystal X-ray crystallography, demonstrating rich structural diversity from the [2 + 4] lantern, [3 + 6] triangular prism, and unprecedented [4 + 8] square prism to the extra-large [6 + 12] octahedron. In addition, they respectively exhibit tunable window diameters and cavity volumes ranging from about 5.4 to 11.1 nm and 580 to 6800 ų. Moreover, their application in the water environment for pollutant removal was explored, indicating that they can effectively eliminate various types of contaminants from water, including radionuclide waste, toxic heavy metal ions, and organic micropollutants. This work demonstrates a convenient method for rationally constructing versatile robust POCs and presents their great application potentialities in water medium.

Efficient ethylene purification by a robust ethane-trapping porous organic cage

The removal of ethane (C₂H₆) from its analogous ethylene (C₂H₄) is of paramount importance in the petrochemical industry, but highly challenging due to their similar physicochemical properties. The use of emerging porous organic cage (POC) materials for C₂H₆/C₂H₄ separation is still in its infancy. Here, we report the benchmark example of a truncated octahedral calix[4]resorcinarene-based POC adsorbent (CPOC-301), preferring to adsorb C₂H₆ than C₂H₄, and thus can be used as a robust absorbent to directly separate high-purity C₂H₄ from the C₂H₆/C₂H₄ mixture. Molecular modelling studies suggest the exceptional C₂H₆ selectivity is due to the suitable resorcin[4]arene cavities in CPOC-301, which form more multiple C–H···π hydrogen bonds with C₂H₆ than with C₂H₄ guests. This work provides a fresh avenue to utilize POC materials for highly selective separation of industrially important hydrocarbons.

The removal of ethane from ethylene is of importance in the petrochemical industry, but similar physicochemical properties of these molecules makes separation a challenging task. Here, the authors demonstrate that a robust octahedral calix[4]resorcinarene-based porous organic cage can separate high-purity ethylene from ethane/ethylene mixtures.

Olive-Shaped Organic Cages: Synthesis and Remarkable Promotion of Hydrazone Condensation through Encapsulation in Water

Two organic cages have been prepared in situ in water through the 2 + 3 hydrazone coupling of two pyridinium-derived trialdehydes and oxalohydrazide. The highly water-soluble cages encapsulate and solubilize linear neutral molecules. Such encapsulation has been applied for the promotion of both two- or three-component hydrazone condensation in water. For two-component reactions, the yields of the resulting monohydrazones are increased from 5-10 to 90-96%. For three-component reactions of hydrazinecarbohydrazide with 11 aromatic aldehydes, in the presence of the organic cages, the bihydrazone products can be produced in 88-96% yields. In contrast, without the promotion of the organic cages, 9 of the reactions do not afford the corresponding dihydrazone product.

Concentration-Dependent Self-Assembly of an Unusually Large Hexameric Hydrogen-Bonded Molecular Cage

The sizes of available self-assembled hydrogen-bond-based supramolecular capsules and cages are rather limited. The largest systems have volumes of approximately 1400-2300 ų . Herein, we report a large, hexameric cage based on intermolecular amide-amide dimerization. The unusual structure with openings, reminiscent of covalently linked cages, is held together by 24 hydrogen bonds. With a diameter of 2.3 nm and a cavity volume of ∼2800 ų , the assembly is larger than any previously known capsule/cage structure relying exclusively on hydrogen bonds. The self-assembly process in chlorinated, organic solvents was found to be strongly concentration dependent, with the monomeric form prevailing at low concentrations. Additionally, the formation of host-guest complexes with fullerenes (C₆₀ and C₇₀ ) was observed.

Reticular Chemistry in the Construction of Porous Organic Cages

Reticular chemistry offers the possibility of systematic design of porous materials with different pores by varying the building blocks, while the emerging porous organic cage (POC) system remains generally unexplored. A series of new POCs with dimeric cages with odd-even behaviors, unprecedented trimeric triangular prisms, and the largest recorded hexameric octahedra have been prepared. These POCs are all constructed from the same tetratopic tetraformylresorcin[4]arene cavitand by simply varying the diamine ligands through Schiff-base reactions and are fully characterized by X-ray crystallography, gas sorption measurements, NMR spectroscopy, and mass spectrometry. The odd-even effects in the POC conformation changes of the [2 + 4] dimeric cages have been confirmed by density functional theory calculations, which are the first examples of odd-even effects reported in the cavitand-based cage system. Moreover, the "V" shape phenylenediamine linkers are responsible for the novel [3 + 6] triangular prisms. The window size and environment can be easily functionalized by different groups, providing a promising platform for the construction of multivariate POCs. Use of linear phenylenediamines led to record-breakingly large [6 + 12] truncated octahedral cages, the maximum inner cavity diameters and volumes of which could be readily modulated by increasing the spacer length of the phenylenediamine linkers. This work can lead to an understanding of the self-assembly behaviors of POCs and also sheds light on the rational design of POC materials for practical applications.

Catechol[4]arene: The Missing Chiral Member of the Calix[4]arene Family

A missing, inherently chiral member of the calix[4]arene family denoted "catechol[4]arene" was synthesized. Its properties were studied and compared to the ones of its close relatives resorcin[4]arene and pyrogallol[4]arene. This novel supramolecular host exhibits binding capabilities that are superior to its sister molecules in polar media. The enantiomerically pure forms of the macrocycle display modest recognition of chiral ammonium salts.

Organic Cages as Building Blocks for Mechanically Interlocked Molecules: Towards Molecular Machines

The research on systems able to perform controllable motions under external stimuli arises great interest in the scientific community. Over the years, a library of innovative devices has been produced, classified in different categories according to the molecular or supramolecular level of motion. This minireview aims to highlight some representative studies, in which organic cages are used as building blocks for mechanically interlocked molecules, and in which intramolecular motions are triggered by external input. However, the application of organic cages in the construction of molecular machines is hardly achieved. A good compromise must actually be reached, between flexibility and rigidity of the cage's framework for an effective control of the intra- and/or intermolecular motion in the final mechanical device. Our final goal is to stimulate researchers' curiosity towards cage-like molecules, so that they take on the challenge of converting a cage into a molecular machine.

Multicomponent Metallo-Supramolecular Nanocapsules Assembled from Calix[4]resorcinarene-Based Terpyridine Ligands

Tetrafunctionalized calix[4]resorcinarene cavitands commonly serve as supramolecular scaffolds for construction of coordination-driven self-assembled capsules. However, due to the calix-like shape, the structural diversity of assemblies is mostly restricted to dimeric and hexameric capsules. Previously, we reported a spontaneous heteroleptic complexation strategy based on a pair of self-recognizable terpyridine-based ligands and Cd^II ions. Building on this complementary ligand pairing system, herein three types of nanocapsules, including a dimeric capsule, a Sierpiński triangular prism, and a cubic star, could be readily obtained through dynamic complexation reactions between a tetratopic cavitand-based ligand and various multitopic counterparts in the presence of Cd^II ions. The dimeric capsular assemblies display the spacer-length-dependent self-sorting behavior in a four-component system. Moreover, the precise multicomponent self-assembly of a Sierpiński triangular prism and a cubic star possessing three and six cavitand-based motifs, respectively, demonstrates that such self-assembly methodology is able to efficiently enhance architectural complexity for calix[4]resorcinarene-containing metallo-supramolecules.

Efficient syntheses of macrocycles ranging from 22-28 atoms through spontaneous dimerization to yield bis-hydrazones

Acid treatment of a triazine displaying both a tethered acetal and BOC-protected hydrazine group leads to spontaneous condensation to yield macrocyclic dimers in excellent yields and purity. The bis-triazinyl hydrazones that form are characterized by ¹H-NMR, ¹³C-NMR, ¹H-COSY spectroscopy, X-ray diffraction, and mass spectrometry. By varying the length of the tether-the condensation product of an amino acid and amino acetal-rings comprising 22-28 atoms can be accessed. Glycine and β-alanine were used for the amino acid. The amino acetal comprised 2, 3 or 4 carbon atoms in the backbone. High-performance liquid chromatography (HPLC) was employed to assess purity as well as to fingerprint the six homodimeric products. By combining the protected monomers and subjecting them to acid, mixtures of homodimers and heterodimers are obtained. When all six protected monomers are combined, at least 14 of the 21 theoretical dimeric products are observed by HPLC. Single crystal X-ray diffraction and solution NMR studies reveal the diversity of shapes available to these molecules.

A propanol-seamed C-methylcalix[4]resorcinarene hexamer accessible via solution crystallization, liquid-assisted grinding and vapour sorption

A propanol-seamed C-methylcalix[4]resorcinarene hexamer formed from a bilayer structure by unprecedented, exposure to solvent in the vapour phase.

Template-directed self-assembly of dynamic covalent capsules with polar interiors

Chiral polyimine molecular capsules with polar interiors have been prepared through template covalent dynamic self-assembly. An aryl-extended tetraaldehyde calix[4]pyrrole scaffold was condensed with suitable diamines as linkers using templates for efficient self-assembly. The capsular complexes were characterized in solution, gas phase and the solid-state. Unprecedented transfer of asymmetry was observed from a chiral diamine linker to the resulting supramolecular capsular assembly.