Multifold Post-Modification of Macrocycles and Cages by Isocyanate-Induced Azadefluorination Cyclisation

We present the multiple post-modification of organic macrocycles and cages, introducing functional groups into two- and three-dimensional supramolecular scaffolds bearing fluorine substituents, which opens up new possibilities in multi-step supramolecular chemistry employing the vast chemical space of readily available isocyanates. The mechanism and scope of the reaction that proceeds after isocyanate addition to the benzylamine motif via an azadefluorination cyclisation (ADFC) were investigated using DFT calculations, and a series of aromatic isocyanates with different electronic properties were tested. The compounds show excellent chemical stability and were fully characterised. They can be used for subsequent cross-coupling reactions, and ADFC can be used directly to generate cross-linked membranes from macrocycles or cages when using ditopic isocyanates. Single-crystal X-ray (SC-XRD) analysis shows the proof of the formation of the desired supramolecular entity together with the connectivity predicted by calculations and from 19F NMR shifts, allowing the late-stage functionalisation of self-assembled macrocycles and cages by ADFC.

The Marriage of Sierpiński Triangles and Platonic Polyhedra

Fractal structures with self-similarity are of fundamental importance in the fields of aesthetic, chemistry and mathematics. Here, by taking advantage of constructs the rational geometry-directed precursor design, we report the construction of two fascinating Platonic solids, the Sierpiński tetrahedron ST-T and the Sierpiński octahedron ST-O, in which each possesses a fractal Sierpiński triangle on their independent faces. These two discrete complexes are formed in near-quantitative yield from the multi-component self-assembly of truncated Sierpiński triangular kernel L¹ with tribenzotriquinacene-based hexatopic and anthracene-based tetratopic terpyridine ligands (L³ and L⁴ ) in the presence of metal ions, respectively. The enhanced stabilities of the 3D discrete structures were investigated by gradient tandem mass spectrometry (gMS² ). This work provides new constructs for the imitation of complex virus assemblies and for the molecular encapsulation of giant guest molecules.

Donor-Acceptor Tribenzotriquinacene-Based Molecular Wizard Hats Bearing Three ortho-Benzoquinone Units

Two π-extended bay-bridged tribenzotriquinacenes ("TBTQ wizard hats") 12 and 16 bearing three mutually conjugated, alternating veratrole-type and ortho-benzoquinone units were synthesized. The electronic properties of these complementarily arranged, nonplanar push-pull systems are affected by the fusion with the rigid, C₃ -symmetric TBTQ core to a different extent, as revealed by X-ray structural analysis, UV-vis spectroscopy and cyclovoltammetry. The combination of three quinone units within the original TBTQ core and three veratrole-type bay bridging units in 12 gives rise to a more efficiently π-conjugated chromophore, as reflected by the shallower shape of wizard hat and its absorption in the visible up to 750 nm in comparison to 16. Congener 12 contains an aromatic 18-π electron system in contrast to the cross-conjugated analog 16. X-ray structure analysis of the precursor dodecaether 15 revealed the formation of a cage-like supramolecular dimer, in which the peripheral dioxane-type ether groups interlace by twelve noncovalent C-H⋅⋅⋅⋅⋅O bonds.

3D-Boronic Ester Architectures: Synthesis, Host-Guest Chemistry, Dynamic Behavior, and Supramolecular Catalysis

Boronic esters are known to be formed simply by mixing boronic acids and alcohols under neutral conditions, and the equilibrium is in favor of the boronic esters when 1,2- or 1,3-diols are employed as alcohols. By utilizing the dynamic nature of the boronic ester formation, our group successfully constructed unique boron-containing 3D structures, such as ring-shaped macrocycles, cages, and tubes, based on the boronic ester formation of various aromatic di-, tri-, or hexaboronic acids with an originally designed tetrol 1 containing two sets of fixed 1,2-diol units oriented on the same face of an indacene framework. Various functions of the obtained boronates were further pursued to disclose the characteristic features of this system. This personal account describes our self-assembled boronate system using tetrol 1 including synthesis, host-guest chemistry, kinetic connection, characteristic dynamic behaviors, and supramolecular catalysis.

Dynamic Nucleophilic Aromatic Substitution of Tetrazines

A dynamic nucleophilic aromatic substitution of tetrazines (SN Tz) is presented herein. It combines all the advantages of dynamic covalent chemistry with the versatility of the tetrazine moiety. Indeed, libraries of compounds or sophisticated molecular structures can be easily obtained, which are susceptible to post-functionalization by inverse electron demand Diels-Alder (IEDDA) reaction, which also locks the exchange. Additionally, the structures obtained can be disassembled upon the application of the right stimulus, either UV irradiation or a suitable chemical reagent. Moreover, SN Tz is compatible with the imine chemistry of anilines. The high potential of this methodology has been proved by building two responsive supramolecular systems: A macrocycle that displays a light-induced release of acetylcholine; and a truncated [4+6] tetrahedral shape-persistent fluorescent cage, which is disassembled by thiols unless it is post-stabilized by IEDDA.

Isoreticular Crystallization of Highly Porous Cubic Covalent Organic Cage Compounds*

Modular frameworks featuring well-defined pore structures in microscale domains establish tailor-made porous materials. For open molecular solids however, maintaining long-range order after desolvation is inherently challenging, since packing is usually governed by only a few supramolecular interactions. Here we report on two series of nanocubes obtained by co-condensation of two different hexahydroxy tribenzotriquinacenes (TBTQs) and benzene-1,4-diboronic acids (BDBAs) with varying linear alkyl chains in 2,5-position. n-Butyl groups at the apical position of the TBTQ vertices yielded soluble model compounds, which were analyzed by mass spectrometry and NMR spectroscopy. In contrast, methyl-substituted cages spontaneously crystallized as isostructural and highly porous solids with BET surface areas and pore volumes of up to 3426 m2  g-1 and 1.84 cm3  g-1 . Single crystal X-ray diffraction and sorption measurements revealed an intricate cubic arrangement of alternating micro- and mesopores in the range of 0.97-2.2 nm that are fine-tuned by the alkyl substituents at the BDBA linker.

Self-Assembly of a Purely Covalent Cage with Homochirality by Imine Formation in Water

Self-assembly of host molecules in aqueous media via metal-ligand coordination is well developed. However, the preparation of purely covalent counterparts in water has remained a formidable task. An anionic tetrahedron cage was successfully self-assembled in a [4+4] manner by condensing a trisamine and a trisformyl in water. Even although each individual imine bond is rather labile and apt to hydrolyze in water, the tetrahedron is remarkably stable or inert due to multivalence. The tetrahedral cages, as well as its neutral counterparts dissolved in organic solvent, have homochirality, namely that their four propeller-shaped trisformyl residues adopt the same rotational conformation. The cage is able to take advantage of hydrophobic effect to accommodate a variety of guest molecules in water. When a chiral guest was recognized, the formation of one enantiomer of the cage became more favored relative to the other. As a consequence, the cage could be produced in an enantioselective manner. The tetrahedron is able to maintain its chirality after removal of the chiral guest-probably on account of the cooperative occurrence of intramolecular forces that restrict the intramolecular flipping of phenyl units in the cage framework.

Soluble Congeners of Prior Insoluble Shape-Persistent Imine Cages

One of the most applied reaction types to synthesize shape‐persistent organic cage compounds is the imine condensation reaction and it is assumed that the formed cages are thermodynamically controlled products due to the reversibility of the imine condensation. However, most of the synthesized imine cages reported are formed as precipitate from the reaction mixture and therefore rather may be kinetically controlled products. There are even examples in literature, where resulting cages are not soluble at all in common organic solvents to characterize or study their formation by NMR spectroscopy in solution. Here, a triptycene triamine containing three solubilizing n‐hexyloxy chains has been used to synthesize soluble congeners of prior insoluble cages. This allowed us to study the formation as well as the reversibility of cage formation in solution by investigating exchange of building blocks between the cages and deuterated derivatives thereof.

Supramolecular Alloys from Fluorinated Hybrid Tri4 Di6 Imine Cages

To create innovative materials, efficient control and engineering of pore sizes and their characteristics, crystallinity and stability is required. Eight hybrid Tri4 Di6 imine cages with a tunable degree of fluorination and one fully fluorinated Tri4 Di6 imine cage are investigated. Although the fluorinated and the non-fluorinated building blocks used herein differ vastly in reactivity, it was possible to gain control over the outcome of the self-assembly process, by carefully controlling the feed ratio. This represents the first hybrid material based on fluorinated/hydrogenated porous organic cages (POCs). These cages with unlimited miscibility in the solid state were obtained as highly crystalline samples after recrystallization and even showed retention of the crystal lattice, forming alloys. All mixtures and the fully fluorinated Tri4 Di6 imine cage were analyzed by MALDI-MS, single-crystal XRD, powder XRD and in regard to thermal stability (TGA).

Self-Sorting in Dynamic Combinatorial Libraries Leads to the Co-Existence of Foldamers and Self-Replicators

Nature segregates fundamental tasks such as information storage/transmission and catalysis between two different compound classes (e.g. polynucleotides for replication and folded polyamides for catalysis). This division of labor is likely a product of evolution, raising the question of how simpler systems in which replicators and folded macromolecules co-exist may emerge in the transition from chemistry to biology. In synthetic systems, achieving co-existence of replicators and foldamers in a single molecular network remains an unsolved problem. Previous work on dynamic molecular networks has given rise to either self-replicating fibers or well-defined foldamer structures (or completely un-sorted complex systems). We report a system in which two cross-reactive dithiol (nucleobase- and peptide-based) building blocks self-sort into a replicator fiber and foldamer that both emerge spontaneously and co-exist. The self-sorting behavior remains prevalent across different building block ratios as two phases of emergence occur: replicator growth followed by foldamer formation. This is attributed to the autocatalytic formation of the replicator fiber, followed by enrichment of the system in the remaining building block, which is subsequently incorporated into a foldamer.

Chiral Self-sorting of Giant Cubic [8+12] Salicylimine Cage Compounds

Chiral self-sorting is intricately connected to the complicated chiral processes observed in nature and no artificial systems of comparably complexity have been generated by chemists. However, only a few examples of purely organic molecules have been reported so far, where the self-sorting process could be controlled. Herein, we describe the chiral self-sorting of large cubic [8+12] salicylimine cage compounds based on a chiral TBTQ precursor. Out of 23 possible cage isomers only the enantiopure and a meso cage were observed to be formed, which have been unambiguously characterized by single crystal X-ray diffraction. Furthermore, by careful choice of solvent the formation of meso cage could be controlled. With internal diameters of din =3.3-3.5 nm these cages are among the largest organic cage compounds characterized and show very high specific surface areas up to approx. 1500 m2  g-1 after desolvation.

Endohedral Hydrogen Bonding Templates the Formation of a Highly Strained Covalent Organic Cage Compound*

A highly strained covalent organic cage compound was synthesized from hexahydroxy tribenzotriquinacene (TBTQ) and a meta-terphenyl-based diboronic acid with an additional benzoic acid substituent in 2'-position. Usually, a 120° bite angle in the unsubstituted ditopic linker favors the formation of a [4+6] cage assembly. Here, the introduction of the benzoic acid group is shown to lead to a perfectly preorganized circular hydrogen-bonding array in the cavity of a trigonal-bipyramidal [2+3] cage, which energetically overcompensates the additional strain energy caused by the larger mismatch in bite angles for the smaller assembly. The strained cage compound was analyzed by mass spectrometry and 1 H, 13 C and DOSY NMR spectroscopy. DFT calculations revealed the energetic contribution of the hydrogen-bonding template to the cage stability. Furthermore, molecular dynamics simulations on early intermediates indicate an additional kinetic effect, as hydrogen bonding also preorganizes and rigidifies small oligomers to facilitate the exclusive formation of smaller and more strained macrocycles and cages.

A Giant [8+12] Boronic Ester Cage with 48 Terminal Alkene Units in the Periphery for Postsynthetic Alkene Metathesis

Dynamic covalent chemistry (DCC) is a powerful synthetic tool to construct large defined molecules in one step from rather simple precursors. The advantage of the intrinsic dynamics of the applied reversible reaction steps is a self‐correction under the chosen conditions, to achieve high yields of the target compound. To date, only a few examples are known, in which DCC was used to build up a molecular defined but larger product that was chemically transferred to a more stable congener in a second (irreversible) step. Here, we present a nanometer‐sized [8+12] boronic ester cage containing 48 peripheral terminal alkene units which allows to put a hydrocarbon exoskeleton around the cage via alkene metathesis.

[Si(O2 C6 F4 )2 ]14 : Self-Assembly of a Giant Perfluorinated Macrocyclic Host by Low-Barrier Si-O Bond Metathesis

The dynamic covalent self-assembly of 14 units of bis(perfluorocatecholato)silane leads to [Si(O2 C6 F4 )2 ]14 -the first giant perfluorinated macrocycle. The oligomerization process is monitored spectroscopically, and the macrocycle analyzed by single-crystal X-ray diffraction. The molecule forms a rigid cavity that can host two o-closo-dodecacarboranes. Computations rationalize the consistent and reproducible formation of the 14mer and disclose a non-catalyzed Si-O/ Si-O σ-bond metathesis with an exceptionally low energetic barrier. For the first time, the most prevalent linker in our geosphere-SiO4 -is disposed to construct a shape-defined crystalline macromolecule.

A Robust Porous Quinoline Cage: Transformation of a [4+6] Salicylimine Cage by Povarov Cyclization

Porous shape-persistent organic cages have become the object of interest in recent years because they are soluble and thus processable from solution. A variety of cages can be achieved by applying dynamic covalent chemistry (DCC), but they are less chemically stable. Here the transformation of a salicylimine cage into a quinoline cage by a twelve-fold Povarov reaction as the key step is described. Besides the chemical stability of the cage over a broad pH regime, it shows a unique absorption and emission depending on acid concentration. Furthermore, thin films for the vapor detection of acids were investigated, showing color switches from pale-yellow to red, and characteristic emission profiles.

Boronate Covalent and Hybrid Organic Frameworks Featuring PIII and P=O Lewis Base Sites

Two covalent organic frameworks comprising Lewis basic PIII centers and Lewis acidic boron atoms were prepared by poly-condensation reactions of newly obtained tris(4-diisopropoxyborylphenyl)phosphine with 2,3,6,7,10,11-hexahydroxytriphenylene and 2,3,6,7-tetrahydroxy-9,10-dimethylanthracene. Obtained materials exhibit significant sorption of dihydrogen (100 cm3  g-1 at 1 bar at 77 K), methane (20 cm3  g-1 at 1 bar at 273 K) and carbon dioxide (50 cm3  g-1 at 1 bar at 273 K). They were exploited as solid-state ligands for coordination of Pd0 centers. Alternatively, in a bottom-up approach, boronated phosphine was treated with Pd2 dba3 and poly-condensated, yielding hybrid materials where the polymer networks are formed by means of covalent boronate linkages and coordination P-Pd bonds. In addition, the analogous materials based on phosphine oxide were synthesized. The DFT calculations on framework-guest interactions revealed that the behavior of adjacent boron and phosphorus/phosphine oxide centers is reminiscent of that found in Frustrated Lewis Pairs and may improve sorption of selected molecules.

Inducing Social Self-Sorting in Organic Cages To Tune The Shape of The Internal Cavity

Many interesting target guest molecules have low symmetry, yet most methods for synthesising hosts result in highly symmetrical capsules. Methods of generating lower symmetry pores are thus required to maximise the binding affinity in host-guest complexes. Herein, we use mixtures of tetraaldehyde building blocks with cyclohexanediamine to access low-symmetry imine cages. Whether a low-energy cage is isolated can be correctly predicted from the thermodynamic preference observed in computational models. The stability of the observed structures depends on the geometrical match of the aldehyde building blocks. One bent aldehyde stands out as unable to assemble into high-symmetry cages-and the same aldehyde generates low-symmetry socially self-sorted cages when combined with a linear aldehyde. We exploit this finding to synthesise a family of low-symmetry cages containing heteroatoms, illustrating that pores of varying geometries and surface chemistries may be reliably accessed through computational prediction and self-sorting.

Triple Self-Sorting in Constitutional Dynamic Networks: Parallel Generation of Imine-Based CuI , FeII , and ZnII Complexes

Three imine-based metal complexes, having no overlap in terms of their compositions, have been simultaneously generated from the self-sorting of a constitutional dynamic library (CDL) containing three amines, three aldehydes, and three metal salts. The hierarchical ordering of the stability of the three metal complexes assembled and the leveraging of the antagonistic and agonistic relationships existing between the constituents within the constitutional dynamic network corresponding to the CDL were pivotal in achieving the sorting. Examination of the process by NMR spectroscopy showed that the self-sorting of the FeII and ZnII complexes depended on an interplay between the thermodynamic driving forces and a kinetic trap involved in their assembly. These results also exemplify the concept of "simplexity"-the fact that the output of a self-assembling system may be simplified by increasing its initial compositional complexity-as the two complexes could self-sort only in the presence of the third pair of organic components, those of the CuI complex.

Organic Cage Dumbbells

Molecular dumbbells with organic cage capping units were synthesised via a multi-component imine condensation between a tri-topic amine and di- and tetra-topic aldehydes. This is an example of self-sorting, which can be rationalised by computational modelling.

Solvent-Controlled Racemic Resolution of C3-Symmetric Trihydroxytribenzotriquinacenes

A racemic C₃-symmetric trihydroxytribenzotriquinacene was resolved on a large scale by fractional crystallization of the corresponding (1S)-camphanic esters, achieving both enantiopure enantiomers (>99% ee) in 35% and 32% yields. The method relies on a distinct solvent-controlled discrimination process between the diastereomers. The enantiopure trihydroxytribenzotriquinacenes were converted into four other enantiopure building blocks, which are valuable precursors for supramolecular and materials chemistry to illustrate the utility of the synthesized compounds.

Organic Imine Cages: Molecular Marriage and Applications

Imine condensation has been known to chemists for more than a century and is used extensively to synthesize large organic cages of defined shapes and sizes. Surprisingly, in the context of the synthetic methods for organic imine cages (OICs), a self-sorting/self-selection (molecular marriage) process has been overlooked over the years. Such processes are omnipresent in nature, from the creation of galaxies to the formation of the smallest building blocks of life (the cell). Such processes have the incredible ability to guide a system toward the formation of a specific product or products out of a collection of equally probable multiple possibilities. This Minireview sheds light on new opportunities in cage design offered by the self-sorting/self-selection protocol in OICs. Recent efforts to explore organic cages for various exciting new applications are discussed; for example, for detection of harmful small organic molecules, as templates for nucleation of metal nanoparticles (MNPs), and as proton-conducting materials.

Dynamic Covalent Self-Assembly Based on Oxime Condensation

Oxime, whose dynamic nature was reported to be switchable between ON/OFF by tuning the acidity, is employed in a novel type of dynamic covalent approach that is amenable to use in water for self-assembly of purely organic molecules with complex topology. In strongly acidic conditions, the dynamic nature of oxime is turned ON, allowing occurrence of error-checking and therefore a catenane and a macrocycle self-assembled in high yields. In neutral conditions, oxime ceases to be dynamic, which helps to trap the self-assembled products even when the driving forces of their formation are removed. We envision that this switchable behaviour might help, at least partially, to resolve a commonly encountered drawback of dynamic covalent chemistry, namely that the intrinsic stability of the self-assembled products containing dynamic bonds, such as imine or hydrazone, are often jeopardized by their reversible nature.

Post-Assembly Reactivity of N-Aryl Iminoboronates: Reversible Radical Coupling and Unusual B-N Dynamic Covalent Chemistry

Post-assembly reaction of a dynamic covalent iminoboronate system following addition of Cp2 Co resulted in the formation of a series of new reductively coupled dianionic dimers via C-C bond formation. The dimers formed as a mixture of BN-containing isomeric products: diastereomers rac5 and meso5, with coupled five-membered rings, and enantiomeric rac6, with a fused six-membered ring bicyclic system from C-C bond formation and rearrangement of the B-N bonds. Each isomer was identified using 1 H NMR spectroscopy in combination with single crystal X-ray structure determination. Interestingly, interconversion between the coupled five-membered rings (rac5 ) and fused bicyclic systems (rac6 ) was found to occur through an unprecedented breaking and reforming of the B-N covalent bond. Further, the coupled products could be converted quantitatively back to their iminoboronate precursors with addition of the electron abstractor Ph3 C+ .