A Water-Stable Boronate Ester Cage.

The reversible condensation of catechols and boronic acids to boronate esters is a paradigm reaction in dynamic covalent chemistry. However, facile backward hydrolysis is detrimental for stability and has so far prevented applications for boronate-based materials. Here, we introduce cubic boronate ester cages 6 derived from hexahydroxy tribenzotriquinacenes and phenylene diboronic acids with ortho-t-butyl substituents. Due to steric shielding, dynamic exchange at the Lewis acidic boron sites is feasible only under acid or base catalysis but fully prevented at neutral conditions. For the first time, boronate ester cages 6 tolerate substantial amounts of water or alcohols both in solution and solid state. The unprecedented applicability of these materials under ambient and aqueous conditions is showcased by efficient encapsulation and on-demand release of ß-carotene dyes and heterogeneous water oxidation catalysis after the encapsulation of ruthenium catalysts.

Size Determination of Organic Cages by Diffusion NMR Spectroscopy.

Reliable structure elucidation of covalent organic cage compounds remains challenging as routine analysis might leave ambiguities. Diffusion-ordered NMR spectroscopy (DOSY) allows insight into the molecular size and mass of the species present in solution, but a systematic evaluation of the diffusion behavior for cage assemblies is rarely considered. Here we report the synthesis of four series of covalent organic cages based on tribenzotriquinacenes and diboronic acids with varying geometry and exohedral substituents. We provide a guideline for the consistent measurement of diffusion coefficients from 1 H-DOSY NMR spectroscopy, which was utilized to study the diffusion behavior for the whole set of cages and selected examples from the literature. For structurally similar cages, a linear correlation between the solvodynamic volume and the molecular mass allows precise size determination. For more complex systems, multiple parameters, such as window size or rigid exohedral functionalization. further modulate cage diffusion in solution.

Folding-Induced Promotion of Proton-Coupled Electron Transfers via Proximal Base for Light-Driven Water Oxidation.

Proton-coupled electron-transfer (PCET) processes play a key role in biocatalytic energy conversion and storage, for example, photosynthesis or nitrogen fixation. Here, we report a series of bipyridine-containing di- to tetranuclear Ru(bda) macrocycles 2 C-4 C (bda: 2,2'-bipyridine-6,6'-dicarboxylate) to promote O-O bond formation. In photocatalytic water oxidation under neutral conditions, all complexes 2 C-4 C prevail in a folded conformation that support the water nucleophilic attack (WNA) pathway with remarkable turnover frequencies of up to 15.5 s-1 per Ru unit respectively. Single-crystal X-ray analysis revealed an increased tendency for intramolecular π-π stacking and preorganization of the proximal bases close to the active centers for the larger macrocycles. H/D kinetic isotope effect studies and electrochemical data demonstrate the key role of the proximal bipyridines as proton acceptors in lowering the activation barrier for the crucial nucleophilic attack of H2 O in the WNA mechanism.

Self-assembled Ru(bda) Coordination Oligomers as Efficient Catalysts for Visible Light-Driven Water Oxidation in Pure Water.

Water-soluble multinuclear complexes based on ruthenium 2,2'-bipyridine-6,6'-dicarboxylate (bda) and ditopic bipyridine linker units are investigated in three-component visible light-driven water oxidation catalysis. Systematic studies revealed a strong enhancement of the catalytic efficiency in the absence of organic co-solvents and with increasing oligomer length. In-depth kinetic and morphological investigations suggest that the enhanced performance is induced by the self-assembly of linear Ru(bda) oligomers into aggregated superstructures. The obtained turnover frequencies (up to 14.9 s-1 ) and turnover numbers (more than 1000) per ruthenium center are the highest reported so far for Ru(bda)-based photocatalytic water oxidation systems.

A Covalent Organic Framework for Cooperative Water Oxidation.

The future of water-derived hydrogen as the "sustainable energy source" straightaway bets on the success of the sluggish oxygen-generating half-reaction. The endeavor to emulate the natural photosystem II for efficient water oxidation has been extended across the spectrum of organic and inorganic combinations. However, the achievement has so far been restricted to homogeneous catalysts rather than their pristine heterogeneous forms. The poor structural understanding and control over the mechanistic pathway often impede the overall development. Herein, we have synthesized a highly crystalline covalent organic framework (COF) for chemical and photochemical water oxidation. The interpenetrated structure assures the catalyst stability, as the catalyst's performance remains unaltered after several cycles. This COF exhibits the highest ever accomplished catalytic activity for such an organometallic crystalline solid-state material where the rate of oxygen evolution is as high as ∼26,000 µmol L-1 s-1 (second-order rate constant k ≈ 1650 µmol L s-1 g-2). The catalyst also proves its exceptional activity (k ≈ 1600 µmol L s-1 g-2) during light-driven water oxidation under very dilute conditions. The cooperative interaction between metal centers in the crystalline network offers 20-30-fold superior activity during chemical as well as photocatalytic water oxidation as compared to its amorphous polymeric counterpart.

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.

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 covalent organic cage compound acting as a supramolecular shadow mask for the regioselective functionalization of C60.

A trigonal-bipyramidal covalent organic cage compound serves as an efficient host to form stable 1 : 1-complexes with C60 and C70. Fullerene encapsulation has been comprehensively studied by NMR and UV/Vis spectroscopy, mass spectrometry as well as single-crystal X-ray diffraction. Exohedral functionalization of encapsulated C60 via threefold Prato reaction revealed high selectivity for the symmetry-matched all-trans-3 addition pattern.

Scaffold-Induced Diketopyrrolopyrrole Molecular Stacks in a Covalent Organic Framework.

In recent years, covalent organic frameworks (COFs) have attracted considerable attention due to their crystalline and porous nature, which positions them as intriguing candidates for diverse applications such as catalysis, sensing, or optoelectronics. The incorporation of dyes or semiconducting moieties into a rigid two-dimensional COF can offer emergent features such as enhanced light harvesting or charge transport. However, this approach can be challenging when dealing with dye molecules that exhibit a large aromatic backbone, since the steric demand of solubilizing side chains also needs to be integrated into the framework. Here, we report the successful synthesis of DPP2-HHTP-COF consisting of diketopyrrolopyrrole (DPP) diboronic acid and hexahydroxytriphenylene (HHTP) building blocks. The well-known boronate ester coupling motif guides the formation of a planar and rigid backbone and long-range molecular DPP stacks, resulting in a highly crystalline and porous material. DPP2-HHTP-COF exhibits excellent optical properties including strong absorption over the visible spectral range, broad emission into the NIR and a singlet lifetime of over 5 ns attributed to the formation of molecular stacks with J-type interactions between the DPP subcomponents in the COF. Electrical conductivity measurements of crystalline DPP2-HHTP-COF pellets revealed conductivity values of up to 10-6 S cm-1.

Covalent Organic Frameworks and Cage Compounds: Design and Applications of Polymeric and Discrete Organic Scaffolds.

Porous organic materials are an emerging class of functional nanostructures with unprecedented properties. Dynamic covalent assembly of small organic building blocks under thermodynamic control is utilized for the intriguingly simple formation of complex molecular architectures in one-pot procedures. In this Review, we aim to analyze the basic design principles that govern the formation of either covalent organic frameworks as crystalline porous polymers or covalent organic cage compounds as shape-persistent molecular objects. Common synthetic procedures and characterization techniques will be discussed as well as more advanced strategies such as postsynthetic modification or self-sorting. When appropriate, comparisons are drawn between polymeric frameworks and discrete organic cages in terms of their underlying properties. Furthermore, we highlight the potential of these materials for applications ranging from gas storage to catalysis and organic electronics.

Microtubular Self-Assembly of Covalent Organic Frameworks.

Despite significant progress in the synthesis of covalent organic frameworks (COFs), reports on the precise construction of template-free nano- and microstructures of such materials have been rare. In the quest for dye-containing porous materials, a novel conjugated framework DPP-TAPP-COF with an enhanced absorption capability up to λ=800 nm has been synthesized by utilizing reversible imine condensations between 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (TAPP) and a diketopyrrolopyrrole (DPP) dialdehyde derivative. Surprisingly, the obtained COF exhibited spontaneous aggregation into hollow microtubular assemblies with outer and inner tube diameters of around 300 and 90 nm, respectively. A detailed mechanistic investigation revealed the time-dependent transformation of initial sheet-like agglomerates into the tubular microstructures.

Metal-Based Diversity for Crystalline Metal-Fullerene Frameworks.

Four different three-dimensional metal-fullerene frameworks were synthesized through polymerization of two C60 -derived dodecaacids with varying alkyl spacers in the presence of Ca2+ , Cu2+ or Cd2+ ions. Structural analysis of the frameworks was performed by single-crystal X-ray diffraction and porosity of the materials was investigated by sorption measurements.

Supramolecular frameworks based on [60]fullerene hexakisadducts.

[60]Fullerene hexakisadducts possessing 12 carboxylic acid side chains form crystalline hydrogen-bonding frameworks in the solid state. Depending on the length of the linker between the reactive sites and the malonate units, the distance of the [60]fullerene nodes and thereby the spacing of the frameworks can be controlled and for the most elongated derivative, continuous channels are obtained within the structure. Stability, structural integrity and porosity of the material were investigated by powder X-ray diffraction, thermogravimetry and sorption measurements.

Apical Functionalization of Tribenzotriquinacenes.

The introduction of one alkyne moiety at the central carbon atom of the tripodal tribenzotriquinacene scaffold allows easy access to a great variety of apically functionalized derivatives. The spatially well-separated arrangement of different functional units on the convex face and outer rim was further proven by single-crystal X-ray studies. Subsequent modifications that feature a general protecting group-free strategy for the demethylation of protected catechols in the presence of a terminal alkyne group, an azide-alkyne Huisgen cycloaddition, and Sonogashira cross-coupling reactions showcase the high synthetic potential of this modular approach for tribenzotriquinacene derivatization.

Three-Dimensional Metal-Fullerene Frameworks.

Hexakis-substituted [60]fullerene adducts with icosahedral symmetry provide an unprecedented scaffold for the spatial arrangement of twelve functional groups with high geometric precision. This unique molecular symmetry identifies such polyfunctional organic building blocks as potential highly connective linkers for coordination polymer and metal-organic framework synthesis. Hereby, the linker exhibits a higher connectivity than the metal ions and with the main connectivity based on the ligand, this can create a new type of inversely cross-linked framework. Two hexakis adducts bearing either twelve glycolic acid or 3-hydroxypropionic acid side chains attached to its malonate units were incorporated as organic connectivity centers in the first fullerene-containing three-dimensional frameworks by coordination with Zn(2+) .

Reversible Assembly of a Supramolecular Cage Linked by Boron-Nitrogen Dative Bonds.

Boron-nitrogen dative bonds provide a suitable motif for reversible, yet strong and directed interactions, leading to the highly efficient self-assembly of small organic building blocks into supramolecular cage structures. A bipyramidal [2+3] assembly, as the first example of a supramolecular cage mediated by BN dative bonds that exists as a discrete species in solution, is quantitatively obtained from a tribenzotriquinacene-based trisboronate ester and 1,4-diazabicyclo[2.2.2]octane. Thermodynamic equilibria of cage formation are investigated by isothermal titration calorimetry and fully reversible cage opening can be observed at elevated temperatures.

Shape-Controlled Synthesis and Self-Sorting of Covalent Organic Cage Compounds.

The directional bonding approach is a powerful tool to rationally control both shape and stoichiometry of three-dimensional objects built from rigid building blocks under dynamic covalent conditions. Co-condensation of catechol-functionalized tribenzotriquinacene derivatives which have 90° angles between the reactive sites and diboronic acids with bite angles of 60°, 120°, and 180°, led to the efficient formation of, respectively, bipyramidal, tetrahedral, or cubic covalent organic cage compounds in a predictable manner. Investigations on the self-sorting of ternary mixtures containing two competitive boronic acids revealed either narcissistic or social self-sorting depending on the stability of the segregated cages relative to feasible three-component assemblies.

Dynamic covalent assembly of tribenzotriquinacenes into molecular cubes.

Molecular cubes constructed from catechol-functionalized tribenzotriquinacenes and 1,4-phenylene diboronic acids were synthesized in a one-pot procedure by crosslinking 20 individual components through a dynamic covalent approach. Structural identity of the nanocubes was confirmed by mass spectrometry and (1)H-NMR spectroscopy.

Dynamic covalent templated-synthesis of [c2]daisy chains.

A couple of [c2]daisy chains have been assembled in each case from four components in quantitative yields at room temperature in acetonitrile as a result of the self-templated clippings of their [24]crown-8 rings by reversible imine bond formation around secondary dialkylammonium recognition sites in their stalks.