Chalcogen Bonding and Hydrophobic Effects Force Molecules into Small Spaces

Benzoselenadiazole-Functionalized H-Bonded Arylamide Foldamers: Solvent-Responsive Properties and Helix Self-Assembly Directed by Chalcogen Bonding in Solid State

In this study, a series of H-bonded arylamide foldamers bearing benzoselenadiazole ends with solvent-responsive properties have been synthesized. In dichloromethane or dimethyl sulfoxide solvents, the molecules exhibit meniscus or linear structures, respectively, which can be attributed to the unique intramolecular hydrogen bonding behavior evidenced by 1D 1H NMR and 2D NOESY spectra. UV-vis spectroscopy experiments show that the absorption wavelength of H-bonded arylamide foldamers are significantly red-shifted due to the presence of benzoselenadiazole group. In addition, the crystal structures reveal that effective intermolecular dual Se ⋅ ⋅ ⋅ N interactions between benzoselenadiazole groups induce further assembly of the monomers. Remarkably, supramolecular linear and double helices structures are constructed under the synergistic induction of intramolecular hydrogen bonding and intermolecular chalcogen bonding. Additionally, 2D DOSY diffusion spectra and theoretical modelling based on density functional theory (DFT) are performed to explore the persistence of intermolecular Se ⋅ ⋅ ⋅ N interactions beyond the crystalline state.

Multiple non-covalent-interaction-directed supramolecular double helices: the orthogonality of hydrogen, halogen and chalcogen bonding

In this study, a benzoselenadiazole- and pyridine-bifunctionalized hydrogen-bonded arylamide foldamer was synthesized. A co-crystallization experiment with 1,4-diiodotetrafluorobenzene showed that a new type of supramolecular double helices, which were induced by three orthogonal interactions, namely, three-center hydrogen bonding (O⋯H⋯O), I⋯N halogen bonding and Se⋯N chalcogen bonding, have been constructed in the solid state. This work presents a novel instance of multiple non-covalent interactions that work together to construct supramolecular architectures.

Dynamic Chalcogen Squares for Material and Topological Control over Macromolecules

Herein we introduce chalcogen squares via selenadiazole motifs as a new class of dynamic supramolecular bonding interactions for the modification and control of soft matter materials. We showcase selenadiazole motifs in supramolecular networks of varying primary chain length prepared through polymerization using tandem step-growth/Passerini multicomponent reactions (MCRs). Compared to controls lacking the selenadiazole motif, these networks display increased glass transition temperatures and moduli due to the chalcogen bonding linkages formed between chains. These elastomeric networks were shown to autonomously heal at room temperature, retaining up to 83 % of the ultimate tensile strength. Lastly, we use post-polymerization modification via the Biginelli MCR to add selenadiazole motifs to narrowly dispersed polymers for controlled topology in solution. Chalcogen squares via selenadiazoles introduce an exciting exchange mechanism to the realm of dynamic materials.

Selective recognition and extraction of iodide from pure water by a tripodal selenoimidazol(ium)-based chalcogen bonding receptor

A selenium-based tripodal chalcogen bond (ChB) donor TPI-3Se is demonstrated for the recognition and extraction of I- from 100% water medium. NMR and ITC studies with the halides reveal that the ChB donor selectively binds with the large, weakly hydrated I-. Interestingly, I- crystallizes out selectively in the presence of other halides supporting the superiority of the selective recognition of I-. The X-ray structure of the ChB-iodide complex manifests both the μ1 and μ2 coordinated interactions, which is rare in the C-Se···I chalcogen bonding. Furthermore, to validate the selective I- binding potency of TPI-3Se in pure water, comparisons are made with its hydrogen and halogen bond donor analogs. The computational analysis also provides the mode of I- recognition by TPI-3Se. Importantly, this receptor is capable of extracting I- from pure water through selenium sigma-hole and I- interaction with a high degree of efficiency (∼70%).

Light-Controlled Switching of Perylene Bisimide Assemblies

Light-driven changes in supramolecular interactions in perylene bisimides (PBIs) with pendant sulfur-containing functional groups at the bay position are demonstrated. In the ground state, a noncovalent S···X interaction between the σ-hole on sulfur and a heteroatom, X (X = O, N, S), of a neighboring molecule is the main driving force for intermolecular interactions, while in the excited state it is the π-π interaction between PBI scaffolds which drives assembly. The presence of heteroatoms in the solvent results in acceleration of the π-stacking process via the formation of a PBI-solvent complex. The excited-state dynamics involved in the assembly process were revealed via time-resolved fluorescence and transient absorption spectroscopies, while steady-state spectroscopy was used to evaluate the structure of the supramolecular assembly.

Selective Binding and Isomerization of Oximes in a Self-Assembled Capsule

A series of straight-chain (C7-C13) alkyl-O-methyl aldoximes (R-C(H)═NOMe) were synthesized with various functional groups at the remote ends (alkenes, halogen, -COOH, and NH₂). Their isomers about the C═N bond showed ∼60-40% E-Z-ratio in organic solutions. Surprisingly, their confinement in a water-soluble capsule with benzoselenodiazole walls shows high selectivity for the cis-/Z-isomer. Their relative affinities for the chalcogen-bonded capsule at room temperature depend mainly on the guest chain length and functional groups. A chain length of 14 heavy atoms showed especially high E- to Z-isomer selectivity (>99%) and was used in separation. The E-Z isomerization occurred only in the capsular cavity at room temperature and was accelerated 10-fold by sonication. The Z-isomer selective binding, separation, and E-Z isomerization are supported by NMR, DOSY, and computational studies.

Competition Between the Two σ-Holes in the Formation of a Chalcogen Bond

A chalcogen atom Y contains two separate σ-holes when in a R₁ YR₂ molecular bonding pattern. Quantum chemical calculations consider competition between these two σ-holes to engage in a chalcogen bond (ChB) with a NH₃ base. R groups considered include F, Br, I, and tert-butyl (tBu). Also examined is the situation where the Y lies within a chalcogenazole ring, where its neighbors are C and N. Both electron-withdrawing substituents R₁ and R₂ act cooperatively to deepen the two σ-holes, but the deeper of the two holes consistently lies opposite to the more electron-withdrawing group, and is also favored to form a stronger ChB. The formation of two simultaneous ChBs in a triad requires the Y atom to act as double electron acceptor, and so anti-cooperativity weakens each bond relative to the simple dyad. This effect is such that some of the shallower σ-holes are unable to form a ChB at all when a base occupies the other site.

Visible-Light-Switchable Tellurium-Based Chalcogen Bonding: Photocontrolled Anion Binding and Anion Abstraction Catalysis

Exploring new noncovalent bonding motifs with reversibly tunable binding affinity is of fundamental importance in manipulating the properties and functions of supramolecular self-assembly systems and materials. Herein, for the first time, we demonstrate a unique visible-light-switchable telluro-triazole/triazolium-based chalcogen bonding (ChB) system in which the Te moieties are connected by azobenzene cores. The binding strengths between these azo-derived ChB receptors and the halide anions (Cl^- , Br^- ) could be reversibly regulated upon irradiation by visible light of different wavelengths. The cis-bidentate ChB receptors exhibit enhanced halide anion binding ability compared to the trans-monodentate receptors. In particular, the telluro-triazolium-based ChB receptor can achieve both high and significantly photoswitchable binding affinities for halide anions, which enable it to serve as an efficient photocontrolled organocatalyst for ChB-assisted halide abstraction in a Friedel-Crafts alkylation benchmark reaction.

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.

Chiral Water-Soluble Molecular Capsules With Amphiphilic Interiors

We present the synthesis of new chiral water-soluble dimeric capsules by the multicomponent Mannich reaction between charged amino acids (glutamic acid or arginine), resorcinarene, and formaldehyde and by subsequent self-assembly. The zwitterionic character of the backbones enables electrostatic interactions between arms and induces self-assembly of dimeric capsules, namely, (L-ArgR)₂ and (L-GluR)₂, in water with a wide range of pH, as demonstrated by NMR, diffusion coefficient measurement, and circular dichroism. The assembly/disassembly processes are fast on the NMR timescale. This mode of dimerization leaves side chains available for additional interactions and creates chiral cavities of mixed hydrophobic/hydrophilic character. According to this characteristic, capsules do not bind fully nonpolar or fully polar guests but effectively encapsulate a variety of chiral molecules with mixed polar/apolar characters (aliphatic and aromatic aldehydes, epoxides, alcohols, carboxylic acids, amines, and amino acids) with moderate strength. We also demonstrate the formation of heterocapsules (GluR) (ArgR) (homo- and heterochiral) that utilize additional interactions between charged acidic and basic side chains and have better encapsulation properties than those of the homodimers.

When Molecules Meet in Water-Recent Contributions of Supramolecular Chemistry to the Understanding of Molecular Recognition Processes in Water

Molecular recognition processes in water differ from those in organic solvents in that they are mediated to a much greater extent by solvent effects. The hydrophobic effect, for example, causes molecules that only weakly interact in organic solvents to stay together in water. Such water‐mediated interactions can be very efficient as demonstrated by many of the synthetic receptors discussed in this review, some of which have substrate affinities matching or even surpassing those of natural binders. However, in spite of considerable success in designing such receptors, not all factors determining their binding properties in water are fully understood. Existing concepts still provide plausible explanations why the reorganization of water molecules often causes receptor‐substrate interactions in water to be strongly exothermic rather than entropically favored as predicted by the classical view of the hydrophobic effect.

Solid-State Assembly by Chelating Chalcogen Bonding in Quinodimethane Tetraesters Fused with a Chalcogenadiazole

In contrast to p-quinodimethane tetraesters, which undergo facile polymerization due to their diradical character, newly synthesized 1 and 2 consisting of a chalcogenadiazole fused to a p-naphthoquinodimethane tetraester are thermodynamically stable due to butterfly-shaped deformation. Such a folded molecular structure is also favorable for chalcogen bond (ChB) formation through intermolecular close contacts between a chalcogen atom (E: Se or S) and the oxygen atoms of ester groups in a crystal. The less-explored chelating-ChB through a C=O⋅⋅⋅E⋅⋅⋅O=C contact [Se⋅⋅⋅O: 2.94-3.37 Å] is the key supramolecular synthon for the formation of a one-dimensional rod-like assembly in a crystal, which is commonly observed in selenadiazole-tetraesters (1) with OMe, OEt, and OiPr groups. The formation of inclusion cavities between the rods shows that 1 could serve as solid-state host molecules for clathrate formation, as found in a hexane-solvated crystal. In contrast, thiadiazole-tetraesters (2) are less suitable for the formation of a rod-like assembly since the ChB involving S is less effective, and thus is overwhelmed by weak hydrogen bonds through C-H⋅⋅⋅O contacts.

Incorporating Selenium into Heterocycles and Natural Products─From Chemical Properties to Pharmacological Activities

Selenium (Se)-containing compounds have emerged as potential therapeutic agents for the treatment of a range of diseases. Through tremendous effort, considerable knowledge has been acquired to understand the complex chemical properties and biological activities of selenium, especially after its incorporation into bioactive molecules. From this perspective, we compiled extensive literature evidence to summarize and critically discuss the relationship between the pharmacological activities and chemical properties of selenium compounds and the strategic incorporation of selenium into organic molecules, especially bioactive heterocycles and natural products. We also provide perspectives regarding the challenges in selenium-based medicinal chemistry and future research directions.

Chalcogen-bonded donor–acceptor complexes of 5,6-dicyano[1,2,5]selenadiazolo[3,4-b]pyrazine with halide ions

With halides X− (X = Cl, Br, I) 5,6-dicyano-[1,2,5]selenadiazolo[3,4-b]pyrazine 1 forms chalcogen-bonded complexes [1–X]− structurally defined by XRD. UV/Vis spectra of [1–X]− feature red-shifted charge-transfer bands in the Vis part.

Rigidified Cavitand Hosts in Water: Bent Guests, Shape Selectivity, and Encapsulation

We report the synthesis and characterization of two water-soluble container compounds (cavitand hosts) with rigidified open ends. One cavitand uses four (CH₂)₄'s as spacers to bridge the adjacent walls, while another cavitand uses four CH₂CH₂OCH₂CH₂'s bridges and features a wider open end. The spacers preorganize the deep cavitands into vase-like, receptive shapes and prevent their unfolding to the unreceptive kite-like conformation. Cycloalkane guests (C6-C8) and small n-alkanes (C5-C7) form 1:1 complexes with the cavitands and move freely in the cavitands' spaces. Hydrophilic compounds 1,4-dioxane, tetrahydrofuran, tetrahydropyran, pyridine, and 1-methylimidazole also showed good binding affinity to the new cavitands. Longer alkanes (C11-C14) and n-alcohols (C11-C16) are taken up with a -CH₃ group fixed at the bottom of the cavity and the groups near the rim in compressed conformations. The methylene bridges appear to divide the cavitand into a narrow hydrophobic compartment and a broader space with exposure to the aqueous medium. Longer alkane guests (C15-C18), N,N-dimethyldioctylammonium, and dioctylamine induce the formation of capsules (2:1 host:guest complexes). The new cavitands showed selectivity for p/m-cresol isomers and xylene isomers. The cavitand with CH₂CH₂OCH₂CH₂ bridges bound long-chain α,ω-diols (C13-C15) and diamines in folded, U-shaped conformations with polar functions exposed to the aqueous medium. It was used to separate o-xylene from its isomers by using simple extraction procedures.

A Robust Supramolecular Heterosynthon Assembled by a Hydrogen Bond and a Chalcogen Bond

The 1:1 and 2:1 cocrystals of isophthalic acid and 2,1,3-benzoselenadiazole have been successfully synthesized and resolved; the noncovalent interactions in the crystal structures have been studied in detail by quantum chemical calculations. In both of the crystal structures, isophthalic acid and 2,1,3-benzoselenadiazole are bound together by a cyclic supramolecular heterosynthon assembled by an O–H···N hydrogen bond and a N–Se···O chalcogen bond. The crystal structures of the 1:1 and 2:1 cocrystals of isophthalic acid and 2,1,3-benzoselenadiazole and the crystal structure of pure isophthalic acid are very similar, which indicates that the [COOH]···[Se−N] cyclic heterosynthon can be an effective alternative to the strong [COOH]2 cyclic homosynthon. The quantum theory of atoms in molecules further recognizes the existence of the hydrogen bond and chalcogen bond. The results of quantum chemical calculations show that the strengths of the π···π stacking interactions in the 1:1 cocrystals of isophthalic acid and 2,1,3-benzoselenadiazole are almost the same as those in the 2:1 cocrystals of isophthalic acid and 2,1,3-benzoselenadiazole, and the strengths of the [COOH]···[Se−N] cyclic heterosynthons (about 9.00 kcal/mol) are less than the strengths of the much stronger [COOH]2 cyclic homosynthons (14.00 kcal/mol). These calculated results are in good agreement with those experimentally observed, demonstrating that, although not as strong as the [COOH]2 cyclic homosynthon, the [COOH]···[Se−N] cyclic heterosynthon can also play a key role in the crystal growth and design.

The role of electric field, peripheral chains, and magnetic effects on significant 1H upfield shifts of the encapsulated molecules in chalcogen-bonded capsules

The chalcogen-bonded homo-cavitand and hetero-cavitand AY+AY' capsules (Y, Y' = Se, Te), as well as their encapsulated complexes with one or two guest molecules have been studied theoretically via density functional theory (DFT), while the ¹H NMR spectra of the homo-cavitand encapsulated complexes (in ASe+ASe) have been measured experimentally. There is excellent agreement between theoretical and experimental spectra. In all cases, we found significant ¹H upfield shifts which are more intense in the ASe+ASe cage compared to the ATe+ATe and ASe+ATe cages. The non-uniform electron distribution which gives rise to an inherent electric field and a non-zero electric dipole moment of the encapsulated complexes, the induced electric field effects, the magnetic anisotropy which is enhanced due to the polarizability of chalcogen atoms, and the peripheral chains, which are responsible for the solubility of the cages, increase the upfield shifts of ¹H of the encapsulated molecules; the peripheral chains lead to an increase of the upfield shifts by up to 1.8 ppm for H of the rim and up to 1.2 ppm for the terminal H in the interior of the cage. Hence, substantial ¹H upfield chemical shifts of the guests in these capsules are consequences of (i) the enhanced aromaticity of the walls of the capsules due to the polarizability of chalcogen atoms, (ii) the induced and inherent electric field effects, and (iii) the peripheral chains.

Solution NMR of synthetic cavity containing supramolecular systems: what have we learned on and from?

NMR has been instrumental in studies of both the structure and dynamics of molecular systems for decades, so it is not surprising that NMR has played a pivotal role in the study of host-guest complexes and supramolecular systems. In this mini-review, selected examples will be used to demonstrate the added value of using (multiparametric) NMR for studying macrocycle-based host-guest and supramolecular systems. We will restrict the discussion to synthetic host systems having a cavity that can engulf their guests thus restricting them into confined spaces. So discussion of selected examples of cavitands, cages, capsules and their complexes, aggregates and polymers as well as organic cages and porous liquids and other porous materials will be used to demonstrate the insights that have been gathered from the extracted NMR parameters when studying such systems emphasizing the information obtained from somewhat less routine NMR methods such as diffusion NMR, diffusion ordered spectroscopy (DOSY) and chemical exchange saturation transfer (CEST) and their variants. These selected examples demonstrate the impact that the results and findings from these NMR studies have had on our understanding of such systems and on the developments in various research fields.

Water and the Cation-π Interaction

The cation-π interaction and the hydrophobic effect are important intermolecular forces in chemistry and play major roles in controlling recognition in biological systems. We compared their relative contributions to the binding of molecular "dumbbell" guests in synthetic container hosts in water. The guests offered direct, intramolecular competition between trimethylammonium groups, -N⁺(CH₃)₃, and tert-butyl groups, -C(CH₃)₃, for the internal surfaces (aromatic panels) of the containers. In contrast with previous studies, the container molecules consistently preferred binding to the uncharged tert-butyl groups, regardless of the presence of anionic, cationic, or zwitterionic groups on the container peripheries. This preference is determined by solvation of the polar trimethylammonium group in water, which outcompetes the attraction between the positive charge and the π-surfaces in the container. The synthetic container complexes provide a direct measure of the relative strengths of cation-π interactions and desolvation in water. Interactions with the uncharged tert-butyl group are more than 12 kJ mol^-1 more favorable than the cation-π interactions with the trimethylammonium group in these cavitand complexes.

Self-Assembled Open Porous Nanoparticle Superstructures

Imparting porosity to inorganic nanoparticle assemblies to build up self-assembled open porous nanoparticle superstructures represents one of the most challenging issues and will reshape the property and application scope of traditional inorganic nanoparticle solids. Herein, we discovered how to engineer open pores into diverse ordered nanoparticle superstructures via their inclusion-induced assembly within 1D nanotubes, akin to the molecular host-guest complexation. The open porous structure of self-assembled composites is generated from nonclose-packing of nanoparticles in 1D confined space. Tuning the size ratios of the tube-to-nanoparticle enables the structural modulation of these porous nanoparticle superstructures, with symmetries such as C₁, zigzag, C₂, C₄, and C₅. Moreover, when the internal surface of the nanotubes is blocked by molecular additives, the nanoparticles would switch their assembly pathway and self-assemble on the external surface of the nanotubes without the formation of porous nanoparticle assemblies. We also show that the open porous nanoparticle superstructures can be ideal candidate for catalysis with accelerated reaction rates.

The Balance between Hydrogen Bonds, Halogen Bonds, and Chalcogen Bonds in the Crystal Structures of a Series of 1,3,4-Chalcogenadiazoles

In order to explore how specific atom-to-atom replacements change the electrostatic potentials on 1,3,4-chalcogenadiazole derivatives, and to deliberately alter the balance between intermolecular interactions, four target molecules were synthesized and characterized. DFT calculations indicated that the atom-to-atom substitution of Br with I, and S with Se enhanced the σ-hole potentials, thus increasing the structure directing ability of halogen bonds and chalcogen bonds as compared to intermolecular hydrogen bonding. The delicate balance between these intermolecular forces was further underlined by the formation of two polymorphs of 5-(4-iodophenyl)-1,3,4-thiadiazol-2-amine; Form I displayed all three interactions while Form II only showed hydrogen and chalcogen bonding. The results emphasize that the deliberate alterations of the electrostatic potential on polarizable atoms can cause specific and deliberate changes to the main synthons and subsequent assemblies in the structures of this family of compounds.

Hydrophobic and Metal-Coordinated Confinement Effects Trigger Recognition and Selectivity

We report the synthesis and characterization of a new water-soluble cavitand 1. The container features 2-aminobenzimidazole panels at the "rim" and pyridiniums at the "feet". In the solid state, a single-crystal X-ray structure of the organic-soluble precursor 2 showed a stable vase form. The structure is stabilized by hydrogen-bonded bridges between adjacent panels through solvents and ions. In aqueous solution, binding of hydrophobic and amphiphilic guest molecules to 1 was investigated using ¹H NMR. Alkanes, alcohols, acids, diols, and diacids formed 1:1 host-guest complexes, and the guest conformations were deduced from characteristic chemical shift changes. In the presence of [Pd(ethylenediamine)(H₂O)₂·2NO₃], cavitand 1 formed a complex incorporating two metals. The metal-coordinated cavitand also bound hydrophobic linear alkanes and difluorobenzene isomers in aqueous medium. The metallo-cavitand showed shape and size selectivity and was used to separate o-difluorobenzene from its isomers as observed by ¹⁹F NMR spectroscopy. The primary amino function of the cavitands offers possibilities for further elaboration to covalent clusters of these container compounds.

Chalcogen bonds: Hierarchical ab initio benchmark and density functional theory performance study

We have performed a hierarchical ab initio benchmark and DFT performance study of D2 Ch•••A- chalcogen bonds (Ch = S, Se; D, A = F, Cl). The ab initio benchmark study is based on a series of ZORA-relativistic quantum chemical methods [HF, MP2, CCSD, CCSD(T)], and all-electron relativistically contracted variants of Karlsruhe basis sets (ZORA-def2-SVP, ZORA-def2-TZVPP, ZORA-def2-QZVPP) with and without diffuse functions. The highest-level ZORA-CCSD(T)/ma-ZORA-def2-QZVPP counterpoise-corrected complexation energies (ΔECPC ) are converged within 1.1-3.4 kcal mol-1 and 1.5-3.1 kcal mol-1 with respect to the method and basis set, respectively. Next, we used the ZORA-CCSD(T)/ma-ZORA-def2-QZVPP (ΔECPC ) as reference data for analyzing the performance of 13 different ZORA-relativistic DFT approaches in combination with the Slater-type QZ4P basis set. We find that the three-best performing functionals are M06-2X, B3LYP, and M06, with mean absolute errors (MAE) of 4.1, 4.2, and 4.3 kcal mol-1 , respectively. The MAE for BLYP-D3(BJ) and PBE amount to 8.5 and 9.3 kcal mol-1 , respectively.

Anion-Anion Interactions in Aerogen-Bonded Complexes. Influence of Solvent Environment

Ab initio calculations are applied to the question as to whether a AeX₅^- anion (Ae = Kr, Xe) can engage in a stable complex with another anion: F^-, Cl^-, or CN^-. The latter approaches the central Ae atom from above the molecular plane, along its C₅ axis. While the electrostatic repulsion between the two anions prevents their association in the gas phase, immersion of the system in a polar medium allows dimerization to proceed. The aerogen bond is a weak one, with binding energies less than 2 kcal/mol, even in highly polar aqueous solvent. The complexes are metastable in the less polar solvents THF and DMF, with dissociation opposed by a small energy barrier.

A Quantitative Molecular Orbital Perspective of the Chalcogen Bond

We have quantum chemically analyzed the structure and stability of archetypal chalcogen-bonded model complexes D2 Ch⋅⋅⋅A- (Ch = O, S, Se, Te; D, A = F, Cl, Br) using relativistic density functional theory at ZORA-M06/QZ4P. Our purpose is twofold: (i) to compute accurate trends in chalcogen-bond strength based on a set of consistent data; and (ii) to rationalize these trends in terms of detailed analyses of the bonding mechanism based on quantitative Kohn-Sham molecular orbital (KS-MO) theory in combination with a canonical energy decomposition analysis (EDA). At odds with the commonly accepted view of chalcogen bonding as a predominantly electrostatic phenomenon, we find that chalcogen bonds, just as hydrogen and halogen bonds, have a significant covalent character stemming from strong HOMO-LUMO interactions. Besides providing significantly to the bond strength, these orbital interactions are also manifested by the structural distortions they induce as well as the associated charge transfer from A- to D2 Ch.

Recognition of Hydrophilic Cyclic Compounds by a Water-Soluble Cavitand

A water-soluble deep cavitand bearing amides on the upper rim and trimethyl ammonium groups on the feet was synthesized. The open-ended cavity is stabilized by the intramolecular hydrogen bonds formed between the adjacent amides, and the introduction of trimethylammonium imparts to the cavitand good solubility in water. The cavitand exhibits high binding affinity and selectivity to hydrophilic molecules in water. With certain guests, such as cyclohexyl alcohols, amines and acids, the recognition involves the synergistic action of hydrogen bonding with hydrophobic effects. The binding phenomena are interpreted in terms of a fixed solvent cage presented by the host to the guest.

Selective Macrocycle Formation in Cavitands

The traditional end-to-end cyclization of long-chain linear precursors is difficult and often unpredictable because the unfavorable entropy of macrocyclic closure allows undesired intermolecular reactions to compete. Here, we apply cavitands to the selective intramolecular aldol/dehydration reaction of long-chain α,ω-dialdehydes in aqueous solution. Hydrophobic forces drive the dialdehydes into the cavitands in folded conformations and favor macrocyclization reactions over intermolecular reactions observed in bulk solution. The macrocyclic aldol reaction products are isolated in good yields (30-85%) over a wide range (11 to 17-membered rings). Unlike conventional templates that become guests inside their assembled hosts, cavitands reverse the roles and resemble the situation in biological catalysis-the templates are hosts for guests undergoing the assisted reaction processes.

Ternary Cocrystals with Large Soft Cavities: A 1,4-diiodotetrafluorobenzene (DITFB)⋅4-Biphenylpyridine N-oxide (BPNO) Host Assembled by Inclusion of Planar Aromatic Guests

A large soft-cavity host composed of 1,4-diiodotetrafluorobenzene (DITFB) and 4-biphenylpyridine N-oxide (BPNO) is assembled under the mediation of a planar aromatic guest molecule (pyrene or perylene) through C-I⋅⋅⋅^- O-N⁺ halogen bonds and π-hole⋅⋅⋅π bonds. Single-crystal X-ray diffraction reveals that guest molecules can be completely encapsulated in the four-layer host cavity to assemble ternary host-guest cocrystals; namely, Pyr@DITFB ⋅ BPNO and Per@DITFB ⋅ BPNO. The luminescence of these ternary cocrystals originates from their discrete guest molecules, which exhibit pure-blue and yellow emissions, respectively, that are localized at 425 nm and in the range of 485 to 578 nm, respectively. In addition, the contribution of different fragments to the stabilization of the crystal structure is estimated by computational chemistry. These cocrystals have significant potential for use in optical applications or materials, such as photonics or organic light-emitting diodes, respectively, that require to avoid the aggregation between luminophores.

Dimeric capsules self-assembled through halogen and chalcogen bonding

Supramolecular capsules are nanoscale containers useful for the study of molecular behavior in confined spaces. They offer practical applications in catalysis, molecular transport, drug delivery, and materials science. Self-assembly has proven to be an effective approach in constructing supramolecular capsules and a variety of well-known noncovalent interactions including hydrogen bonding, metal coordination and ionic interactions have been deployed. Bowl-shaped cavitand structures built up from resorcinarenes have particular advantages for the construction of supramolecular capsules, and this highlight discusses the unconventional self-assembly of molecular capsules held together by halogen and chalcogen bonds.

Molecular Recognition in Water Using Macrocyclic Synthetic Receptors

Molecular recognition in water using macrocyclic synthetic receptors constitutes a vibrant and timely research area of supramolecular chemistry. Pioneering examples on the topic date back to the 1980s. The investigated model systems and the results derived from them are key for furthering our understanding of the remarkable properties exhibited by proteins: high binding affinity, superior binding selectivity, and extreme catalytic performance. Dissecting the different effects contributing to the proteins' properties is severely limited owing to its complex nature. Molecular recognition in water is also involved in other appreciated areas such as self-assembly, drug discovery, and supramolecular catalysis. The development of all these research areas entails a deep understanding of the molecular recognition events occurring in aqueous media. In this review, we cover the past three decades of molecular recognition studies of neutral and charged, polar and nonpolar organic substrates and ions using selected artificial receptors soluble in water. We briefly discuss the intermolecular forces involved in the reversible binding of the substrates, as well as the hydrophobic and Hofmeister effects operating in aqueous solution. We examine, from an interdisciplinary perspective, the design and development of effective water-soluble synthetic receptors based on cyclic, oligo-cyclic, and concave-shaped architectures. We also include selected examples of self-assembled water-soluble synthetic receptors. The catalytic performance of some of the presented receptors is also described. The latter process also deals with molecular recognition and energetic stabilization, but instead of binding ground-state species, the targets become elusive counterparts: transition states and other high-energy intermediates.

Participation of S and Se in hydrogen and chalcogen bonds

The heavier chalcogen atoms S, Se, and Te can each participate in a range of different noncovalent interactions. They can serve as both proton donor and acceptor in H-bonds. Each atom can also act as electron acceptor in a chalcogen bond.

Chemistry Evolves, Terms Evolve, but Phenomena Do Not Evolve: From Chalcogen-Chalcogen Interactions to Chalcogen Bonding

Chalcogen bonding is important in numerous aspects of chemistry, both in the solid state and in solution. Surveying the literature, it becomes clear that during its rebranding from chalcogen-chalcogen interactions, some parts of the community have somewhat neglected to recall its discovery and the initial studies referring to it in its previous guise. In this Viewpoint, we trace the path of research into this phenomenon, from its discovery, through its renaming, and to some of the varied and interesting chemistry it has led to so far, ranging from crystal engineering through supramolecular assembly to modern catalysis.

Effect of Aromatic System Expansion on Crystal Structures of 1,2,5-Thia- and 1,2,5-Selenadiazoles and Their Quaternary Salts: Synthesis, Structure, and Spectroscopic Properties

Rational manipulation of secondary bonding interactions is a crucial factor in the construction of new chalcogenadiazole-based materials. This article reports detailed experimental studies on phenanthro[9,10-c][1,2,5]chalcogenadiazolium and 2,1,3-benzochalcogenadiazolium salts and their precursors. The compounds were synthesized, characterized employing NMR and UV-Vis spectroscopy. TD-DFT calculations were also performed. The influence of the size of the aromatic system on the molecular motifs formed by the compounds in the solid state has been studied by means of single-crystal X-ray diffraction. In case of the salts, the nature of an anion was also taken into consideration. The results show that cyclic [E···N]2 supramolecular synthon connects neighboring molecules of phenanthro[9,10-c][1,2,5]chalcogenadiazoles, with a relatively large aromatic system, in dimers regardless of the chalcogen atom in the molecule. Both N-methyl-2,1,3-benzothiadiazolium and N-methylphenanthro[9,10-c][1,2,5]chalcogenadiazolium cations have a strong affinity for triflate and iodide anions, therefore the formation of S···N or Se···N secondary bonding interactions is observed only in two out of the eight quaternary salts. Less coordinating anions must be used to enable the building blocks studied to form cyclic [E···N]2 synthons. Moreover, for two of the triflate salts, which are isostructural, a new supramolecular motif has been observed.

Aromaticity and Chemical Bonding of Chalcogen-Bonded Capsules Featuring Enhanced Magnetic Anisotropy

We present a theoretical study of chalcogen bonded container capsules (A_X +A_X ) where X=O, S, Se, and Te, and their encapsulation complexes with n-C₉ H₂₀ (n-C₉ H₂₀ @A_X +A_X ). Both Se and Te encapsulation complexes have significant experimental and computed binding energies, analogous to the hydrogen bonded counterparts, while the S and O capsules and their encapsulation complexes show only weak binding energies, which are attributed to different types of bonding: chalcogen S⋅⋅⋅N bonds for S-capsules and π-π stacking and weak hydrogen bonds for the O case. All A_X +A_X and C₉ H₂₀ @A_X +A_X present unusually high magnetic anisotropies in their interiors. The ¹ H NMR spectra of the encapsulation complexes display the proton signals of the encapsulated n-nonane highly upfield shifted, in agreement with the available experimental data for the Se capsule. We found that different factors contribute to the observed magnetic anisotropy of the capsule's interior: for the Te capsule the most important factor is Te's large polarizability; for the O analogue the inductive effects produced by the electronegative nature of the O and N heteroatoms; and for the S and Se capsules, the polarizability of the heteroatoms combines with electric field effects.

Solution Self-Assembly of Chalcogen-Bonding Polymer Partners

Chalcogen bonding is regarded as a form of noncovalent interaction; however, harnessing chalcogen bonds to drive macromolecular self-assembly remains unexplored. Here we report two classes of chalcogen-bonding partner polymers and their unique self-assembly behaviors in THF/H₂O solution. Using simple poly(4-vinylphenyl chalcogenide) (P4VCh, Ch = Te or Se) as the donor polymer and poly(ethylene oxide)-block-poly(4-vinylpyridine N-oxide) (PEO-b-P4VO) as the acceptor polymer, they can form donor-acceptor noncovalent complexes and further aggregate into tubular and spherical assemblies through interchain Te···O and Se···O chalcogen-chalcogen interactions. The small distinction in binding affinity of chalcogen bonds can dictate the assembly of different geometries. Moreover, mixing the strong and weak chalcogen-bonding pairs in various ratios can allow us to obtain ergodic phase evolution with tunable dimensionality and morphology.

Binding selectivity and separation of p-functionalized toluenes with a metallo-cavitand in water

A metallo-cavitand (1-2Pd) showed unprecedented binding selectivity and sequestration of p-functionalized toluene isomers in water. The host-guest complexation was studied using 1H and COSY NMR methods and xylene-isomer complexes were examined by using DFT calculations. A liquid-liquid extraction scheme was developed for the separation of p-functionalized toluenes.

Confined molecules: experiment meets theory in small spaces

The behavior of molecules confined to small spaces is fascinating chemistry and lies at the heart of signaling processes in biology. Our approach to confinement is through reversible encapsulation of small molecules in synthetic containers. We show that confinement leads to amplified reactivities in bimolecular reactions, stabilization of otherwise reactive species, and limitation in motions that create new stereochemical arrangements. The isolation of molecules from solvent makes for manageable computations and has stimulated theorist to examine reaction details in the limited space. Transition states for reactions and rearrangements can be calculated, the effects of (de)solvation can be evaluated and the magnetic properties of the containers can be compared with experimental observations. Finally, we outline several potential applications, including entanglement chemistry and the use of isomers in data storage.