Embedding and Positioning of Two FeII4L4 Cages in Supramolecular Tripeptide Gels for Selective Chemical Segregation

An unreported d,l ‐tripeptide self‐assembled into gels that embedded Fe^II₄L₄ metal–organic cages to form materials that were characterized by TEM, EDX, Raman spectroscopy, rheometry, UV/Vis and NMR spectroscopy, and circular dichroism. The cage type and concentration modulated gel viscoelasticity, and thus the diffusion rate of molecular guests through the nanostructured matrix, as gauged by ¹⁹F and ¹H NMR spectroscopy. When two different cages were added to spatially separated gel layers, the gel–cage composite material enabled the spatial segregation of a mixture of guests that diffused into the gel. Each cage selectively encapsulated its preferred guest during diffusion. We thus present a new strategy for using nested supramolecular interactions to enable the separation of small molecules.     

Controlling the Host–Guest Interaction Mode through a Redox Stimulus

A proof‐of‐concept related to the redox‐control of the binding/releasing process in a host–guest system is achieved by designing a neutral and robust Pt‐based redox‐active metallacage involving two extended‐tetrathiafulvalene (exTTF) ligands. When neutral, the cage is able to bind a planar polyaromatic guest (coronene). Remarkably, the chemical or electrochemical oxidation of the host–guest complex leads to the reversible expulsion of the guest outside the cavity, which is assigned to a drastic change of the host–guest interaction mode, illustrating the key role of counteranions along the exchange process. The reversible process is supported by various experimental data (¹H NMR spectroscopy, ESI‐FTICR, and spectroelectrochemistry) as well as by in‐depth theoretical calculations performed at the density functional theory (DFT) level.     

Fluorometric Recognition of Nucleotides within a Water‐Soluble Tetrahedral Capsule

The design of aqueous probes and binders for complex, biologically relevant anions presents a key challenge in supramolecular chemistry. Herein, a tetrahedral assembly with cationic faces and corners is reported that is capable of discriminating between anionic and neutral guests in water. Electrostatic repulsion between subcomponents can be overcome by the addition of an anionic template, or generating a robust covalent framework by incorporating tris(2‐aminoethyl)amine (TREN). The resultant TREN‐capped, water‐soluble, fluorescent cage binds mono‐ and poly‐phosphoric esters, including nucleotides. Its covalent skeleton renders it stable at micromolar concentrations in water, enabling the fluorometric detection of biologically relevant guests in an aqueous environment. Selective supramolecular encapsulants, such as 1 , could enable new sensing applications, such as recognition of toxins and drugs, under biological conditions.     

Chiral Self‐Discrimination and Guest Recognition in Helicene‐Based Coordination Cages

Chiral nanosized confinements play a major role for enantioselective recognition and reaction control in biological systems. Supramolecular self‐assembly gives access to artificial mimics with tunable sizes and properties. Herein, a new family of [Pd₂L₄] coordination cages based on a chiral [6]helicene backbone is introduced. A racemic mixture of the bis‐monodentate pyridyl ligand L¹ selectively assembles with Pd^II cations under chiral self‐discrimination to an achiral meso cage, cis‐[Pd₂ L^1P ₂ L^1M ₂]. Enantiopure L¹ forms homochiral cages [Pd₂ L^1P/M ₄]. A longer derivative L² forms chiral cages [Pd₂ L^2P/M ₄] with larger cavities, which bind optical isomers of chiral guests with different affinities. Owing to its distinct chiroptical properties, this cage can distinguish non‐chiral guests of different lengths, as they were found to squeeze or elongate the cavity under modulation of the helical pitch of the helicenes. The CD spectroscopic results were supported by ion mobility mass spectrometry.     

Regio‐ and Enantioselective Photodimerization within the Confined Space of a Homochiral Ruthenium/Palladium Heterometallic Coordination Cage

The photoinduced regio‐ and enantioselective coupling of naphthols and derivatives thereof is achieved in the confined chiral coordination space of a Ru^II metalloligand based cage. The racemic or enantiopure cages encapsulate naphthol guests, which then undergo a regiospecific 1,4‐coupling, rather than the normal 1,1‐coupling, to form 4‐(2‐hydroxy‐1‐naphthyl)‐1,2‐napthoquinones; moderate stereochemical control is achieved with homochiral cages. The photoreactions proceed under both aerobic and anaerobic conditions but through distinct pathways that nevertheless involve the same radical intermediates. This unusual dimerization constitutes a very rare example of asymmetric induction in biaryl coupling by making use of coordination cages with dual functionality—photoredox reactivity and stereoselectivity.     

A Self‐Sorting Scheme Based on Tetra‐Urea Calix[4]arenes

Size and shape do matter : When dimerized in nonpolar solvents, an equimolar mixture of eleven tetra‐urea calix[4]arenes with different wide‐rim substituents self‐sorts into only six out of 35 different homo‐ and heterodimers (see picture). Since the calixarene scaffold and the four urea units are the same in all cases, the self‐sorting process is driven only by the cooperative action of steric requirements and stoichiometry.

     

Catalytic Hydrogenation of meso‐Octamethylporphyrinogen (Calix[4]pyrrole)

Hydrogenation of meso‐octamethylporphyrinogen (calix[4]pyrrole) with a number of heterogeneous catalysts under different experimental conditions has been investigated. GC‐MS analyses of the reaction mixtures showed the formation of one to four products in low to moderate yields: three of them were diastereoisomers of the product derived from half‐hydrogenation of the substrate, and displayed alternating pyrrolidine and pyrrole rings, while the fourth was the all‐cis saturated product. An acidic medium was necessary to achieve hydrogenation. However, the use of too strongly acidic solvents or additives was detrimental to the stability of the substrate and/or the catalyst.     

Transformation of a [4+6] Salicylbisimine Cage to Chemically Robust Amide Cages

In recent years, interest in shape‐persistent organic cage compounds has steadily increased, not least because dynamic covalent bond formation enables such structures to be made in high to excellent yields. One often used type of dynamic bond formation is the generation of an imine bond from an aldehyde and an amine. Although the reversibility of the imine bond formation is advantageous for high yields, it is disadvantageous for the chemical stability of the compounds. Amide bonds are, in contrast to imine bonds much more robust. Shape‐persistent amide cages have so far been made by irreversible amide bond formations in multiple steps, very often accompanied by low yields. Here, we present an approach to shape‐persistent amide cages by exploiting a high‐yielding reversible cage formation in the first step, and a Pinnick oxidation as a key step to access the amide cages in just three steps. These chemically robust amide cages can be further transformed by bromination or nitration to allow post‐functionalization in high yields. The impact of the substituents on the gas sorption behavior was also investigated.     

Coordination‐Driven Switching of a Preorganized and Cooperative Calix[4]pyrrole Receptor

The study of preorganization in receptors, particularly in cooperative receptors, and their reversible control by external stimuli is important for elucidating design strategies that can lead to increased sensitivity and external control of molecular recognition. In this work we present the design, synthesis, and operation of an asymmetric tetrathiafulvalene (TTF)–calix[4]pyrrole receptor appended with a pyridine moiety. ¹H NMR spectroscopy was employed to demonstrate that intramolecular complexation between the receptor and the pyridine moiety leads to a preorganized receptor. Absorption and ¹H NMR spectroscopy along with a computational investigation were used to demonstrate the ability of the receptor to complex the substrate 1,3,5‐trinitrobenzene (TNB) and that the receptor can be reversibly modulated between negative and positive cooperativity by employing external stimuli in the form of Zn^II. Fitting procedures incorporating multiple datasets and fitting to multiple equilibria simultaneously have been employed to quantitatively determine the preorganization effects.