Supramolecular Encapsulation of Tetrahedrally Hydrated Guests in a Tetrahedron Host

Anthracene-Based Amido-Amine Cage Receptor for Anion Recognition under Neutral Aqueous Conditions

A new amido-amine cage receptor, which combines 1,8-anthracene diacarboxamide subunit and a polyammonium azamacrocycle, is reported. Bearing both the hydrogen bond donor and the acceptor binding sites, the receptor is able to bind phosphate selectively under neutral (pH 7.2) aqueous conditions. The recognition events for phosphate and dicarboxylates are accomplished by a fluorescence enhancement in the anthracene emission. As revealed by experimental and theoretical studies, phosphate and oxalate show different recognition modes. Phosphate demonstrates hydrogen bond acceptor properties, while the coordination of oxalate favours the protonation of the receptor.

Macrocycle-Directed Construction of Tetrahedral Anion-π Receptors for Nesting Anions with Complementary Geometry

Manipulation of the emerging anion-π interactions in a highly cooperative manner through sophisticated host design represents a very challenging task. In this work, unprecedented tetrahedral anion-π receptors have been successfully constructed for complementary accommodation of tetrahedral and relevant anions. The synthesis was achieved by a macrocycle-directed approach by using large macrocycle precursors bearing four reactive sites, which enabled a kinetic-favored pathway and afforded the otherwise inaccessible tetrahedral cages in considerable yields. Crystal structure suggested that the tetrahedral cages have an enclosed three-dimensional cavity surrounded by four electron-deficient triazine faces in a tetrahedral array. The complementary accommodation of a series of tetrahedral and relevant anions including BF₄ ^- , ClO₄ ^- , H₂ PO₄ ^- , HSO₄ ^- , SO₄ ^2- and PF₆ ^- was revealed by ESI-MS and DFT calculations. Crystal structures of ClO₄ ^- and PF₆ ^- complexes showed that the anion was nicely encapsulated within the tetrahedral cavity with up to quadruple cooperative anion-π interactions by an excellent shape and size match. The strong anion-π binding was further confirmed by negative ion photoelectron spectroscopy measurements.

Selective Anion Extraction and Recovery Using a FeII4 L4 Cage

Selective anion extraction is useful for the recovery and purification of valuable chemicals, and in the removal of pollutants from the environment. Here we report that Fe^II₄L₄ cage 1 is able to extract an equimolar amount of ReO₄⁻, a high‐value anion and a nonradioactive surrogate of TcO₄⁻, from water into nitromethane. Importantly, the extraction was efficiently performed even in the presence of 10 other common anions in water, highlighting the high selectivity of 1 for ReO₄⁻. The extracted guest could be released into water as the cage disassembled in ethyl acetate, and then 1 could be recycled by switching the solvent to acetonitrile. The versatile solubility of the cage also enabled complete extraction of ReO₄⁻ (as the tetrabutylammonium salt) from an organic phase into water by using the sulfate salt of 1 as the extractant.

Hydrogen Peroxide Insular Dodecameric and Pentameric Clusters in Peroxosolvate Structures

Peroxosolvates of 2-aminonicotinic acid (I) and lidocaine N-oxide (II) including the largest insular hydrogen peroxide clusters were isolated and their crystal structures were determined by single-crystal X-ray diffraction. An unprecedented dodecameric hydrogen peroxide insular cluster was found in I. An unusual cross-like pentameric cluster was observed in the structure of II. The topology of the (H₂ O₂ )₁₂ assembly was never observed for small-molecule clusters. In I and II new double and triple cross-orientational disorders of H₂ O₂ were found. Cluster II is the first example of a peroxosolvate crystal structure containing H₂ O₂ molecules with a homoleptic hydrogen peroxide environment. In II, a hydrogen bond between an H₂ O₂ molecule and a peptide group -CONH⋅⋅⋅O₂ H₂ was observed for the first time.

Peripheral Templation Generates an M(II) 6 L4 Guest-Binding Capsule

Pseudo‐octahedral M^II₆L₄ capsules result from the subcomponent self‐assembly of 2‐formylphenanthroline, threefold‐symmetric triamines, and octahedral metal ions. Whereas neutral tetrahedral guests and most of the anions investigated were observed to bind within the central cavity, tetraphenylborate anions bound on the outside, with one phenyl ring pointing into the cavity. This binding configuration is promoted by the complementary arrangement of the phenyl rings of the intercalated guest between the phenanthroline units of the host. The peripherally bound, rapidly exchanging tetraphenylborate anions were found to template an otherwise inaccessible capsular structure in a manner usually associated with slow‐exchanging, centrally bound agents. Once formed, this cage was able to bind guests in its central cavity.

A unique fluoride nanocontainer: porous molecular capsules can accommodate an unusually high number of "rather labile" fluoride anions

The present work refers to the challenging issue of fluoride anion recognition/binding in water by taking advantage of the unique possibilities offered by the porous molecular nanocontainers of the {Mo132} Keplerate type allowing the study of a variety of new phenomena. Reaction of the highly reactive carbonate-type capsule with aqueous HF results in the release of carbon dioxide and integration of an unprecedentedly large number of fluoride anions--partly as coordinated ligands at both the pentagonal units and the linkers, partly as a disordered water/fluoride assembly inside the cavity. The internal assembly and some of the fluoride ligands are easily released, which provides interesting options for future studies regarding coordination chemistry and catalysis under confined conditions.

Molecular recognition in chemical and biological systems

Structure-based ligand design in medicinal chemistry and crop protection relies on the identification and quantification of weak noncovalent interactions and understanding the role of water. Small-molecule and protein structural database searches are important tools to retrieve existing knowledge. Thermodynamic profiling, combined with X-ray structural and computational studies, is the key to elucidate the energetics of the replacement of water by ligands. Biological receptor sites vary greatly in shape, conformational dynamics, and polarity, and require different ligand-design strategies, as shown for various case studies. Interactions between dipoles have become a central theme of molecular recognition. Orthogonal interactions, halogen bonding, and amide⋅⋅⋅π stacking provide new tools for innovative lead optimization. The combination of synthetic models and biological complexation studies is required to gather reliable information on weak noncovalent interactions and the role of water.