Supramolekulare Kapseln: starke und schwache Chalkogenbrücken im Vergleich

Supramolecular Handshakes: Characterization of Urea-Carboxylate Interactions Within Calixarene Frameworks

The research of molecular capsules offers high application potential and numerous benefits in various fields. With the aim of forming supramolecular capsules which can be reversibly assembled and dissociated by simple external stimuli, we studied interactions between calixarenes containing urea and carboxylate moieties. To this end two ureido-derivatives of p-tert-butylcalix[4]arene comprising phenylureido-moieties and diacetate-calix[4]arenes were prepared. The binding of acetate by ureido-derivatives of calixarene in acetonitrile was characterized, revealing high affinity of ureido-calixarenes for carboxylates. This suggested high potential for uniting the complementary calix[4]arenes via H-bonds between carboxylic groups and urea moieties. The assembly of calixarenes was examined in detail by means of UV, 1H NMR, ITC, DOSY, MS, and conductometry providing insight in the structure-stability relationship. The tetraureido-calixarene derivative formed the most stable heterodimers with diacetate-calix[4]arenes featuring practically quantitative association upon mixing the two calixarene counterparts. The possibility of controlling the formation of the heterodimer by protonating the carboxylates, thereby hindering the interactions critical for capsule assembly, was investigated. Indeed, the reversibility of breaking and re-forming the heterodimer by addition of an acid and base to the solution containing urea- and carboxylate-derivative calix[4]arene was demonstrated using NMR spectroscopy.

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.

Supramolecular Chalcogen‐Bonded Semiconducting Nanoribbons at Work in Lighting Devices

This work describes the design and synthesis of a π‐conjugated telluro[3,2‐β][1]‐tellurophene‐based synthon that, embodying pyridyl and haloaryl chalcogen‐bonding acceptors, self‐assembles into nanoribbons through chalcogen bonds. The ribbons π‐stack in a multi‐layered architecture both in single crystals and thin films. Theoretical studies of the electronic states of chalcogen‐bonded material showed the presence of a local charge density between Te and N atoms. OTFT‐based charge transport measurements showed hole‐transport properties for this material. Its integration as a p‐type semiconductor in multi‐layered Cu^I‐based light‐emitting electrochemical cells (LECs) led to a 10‐fold increase in stability (38 h vs. 3 h) compared to single‐layered devices. Finally, using the reference tellurotellurophene congener bearing a C−H group instead of the pyridyl N atom, a herringbone solid‐state assembly is formed without charge transport features, resulting in LECs with poor stabilities (<1 h).

Topochemical, Single‐Crystal‐to‐Single‐Crystal [2+2] Photocycloadditions Driven by Chalcogen‐Bonding Interactions

The face‐to‐face association of (E)‐1,2‐di(4‐pyridyl)ethylene (bpen) molecules into rectangular motifs stabilized for the first time by chalcogen bonding (ChB) interactions is shown to provide photoreactive systems leading to cyclobutane formation through single‐crystal‐to‐single‐crystal [2+2] photodimerizations. The chelating chalcogen bond donors are based on original aromatic, ortho‐substituted bis(selenocyanato)benzene derivatives 1–3, prepared from ortho‐diboronic acid bis(pinacol) ester precursors and SeO₂ and malononitrile in 75–90 % yield. The very short intramolecular Se⋅⋅⋅Se distance in 1–3 (3.22–3.24 Å), a consequence of a strong intramolecular ChB interaction, expands to 3.52–3.54 Å in the chalcogen‐bonded adducts with bpen, a distance (<4 Å) well adapted to the face‐to‐face association of the bpen molecules into the reactive position toward photochemical dimerization.

Noncovalent Interactions Involving Group 6 in Biological Systems: The Case of Molybdopterin and Tungstopterin Cofactors

In this study we propose to coin the term Wolfium bond (WfB) to refer to a net attractive force (noncovalent interaction) between any element of group 6 and electron donor atoms (neutral molecules or anions) and to differentiate it from a coordination bond (metal‐ligand interaction). We provide evidence of the existence of this interaction by inspecting the X‐ray crystal structure of proteins containing Molybdopterin and Tungstopterin cofactors from the Protein Data Bank (PDB). The plausible biological role of the interaction as well as its physical nature (antibonding Wf‐Ligand orbital involved) are also analyzed by means of ab initio calculations (RI‐MP2/def2‐TZVP level of theory), Atoms in Molecules (AIM), Natural Bond Orbital (NBO) and Noncovalent Interactions plot (NCIplot) analyses.

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.

Frustrated Lewis Pairs Based on Carbon⋅⋅⋅Carbon+ Tetrel Bonds: A DFT Study

The ability of Triangulenium (TA⁺ ) compounds to form Frustrated Lewis Pairs (FLPs) with N-HeteroCycle Carbenes (NHCs) is analysed in this manuscript at the PBE0-D3/def2-TZVP level of theory. We have used six TA⁺ -based moieties, three presenting similar bridging groups (O (trioxo), -CH₂ (triaryl) and -NH (triaza)) and another three mixing, O, -CH₂ and NH moieties. In addition, several aryl-substituted NHCs have been used as electron donor moieties to undergo carbon⋅⋅⋅carbon⁺ tetrel bonds with the TA⁺ derivatives. More precisely, -Me,-iPr, -tBu and -Ph groups were used. Finally, we have used Bader's quantum theory of "atoms in molecules" (QTAIM) and Natural Bonding Analysis (NBO) to characterize the carbon⋅⋅⋅carbon⁺ tetrel bonds described herein. We expect the results gathered herein will be useful for further exploitation of carbon⋅⋅⋅carbon⁺ bonds in the formation of FLPs as well as to expand the current knowledge of tetrel bonds to the fields of synthetic chemistry and catalysis.

Bioinspired Ether Cyclizations within a π-Basic Capsule Compared to Autocatalysis on π-Acidic Surfaces and Pnictogen-Bonding Catalysts

While the integration of supramolecular principles in catalysis attracts increasing attention, a direct comparative assessment of the resulting systems catalysts to work out distinct characteristics is often difficult. Herein is reported how the broad responsiveness of ether cyclizations to diverse inputs promises to fill this gap. Cyclizations in the confined, π-basic and Brønsted acidic interior of supramolecular capsules, for instance, are found to excel with speed (exceeding general Brønsted acid and hydrogen-bonding catalysts by far) and selective violations of the Baldwin rules (as extreme as the so far unique pnictogen-bonding catalysts). The complementary cyclization on π-acidic aromatic surfaces remains unique with regard to autocatalysis, which is shown to be chemo- and diastereoselective with regard to product-like co-catalysts but, so far, not enantioselective.

Competing Effects of Chlorination on the Strength of Te⋅⋅⋅O Chalcogen Bonds Select the Structure of Mixed Supramolecular Macrocyclic Aggregates of Iso-Tellurazole N-Oxides

Chlorination of 3-methyl-5-phenyl-1,2-tellurazole-2-oxide yielded the λ⁴ Te dichloro derivative. Its crystal structure demonstrates that the heterocycle retains its ability to autoassociate by chalcogen bonding (ChB) forming macrocyclic tetramers. The corresponding Te⋅⋅⋅O ChB distances are 2.062 Å, the shortest observed to date in aggregates of this type. DFT-D3 calculations indicate that while the halogenated molecule is stronger as a ChB donor it also is a weaker ChB acceptor; the overall effect is that the ChBs in the chlorinated homotetramer are not significantly stronger. However, partial halogenation or scrambling selectively yield the 2 : 2 heterotetramer with alternating λ⁴ Te and λ² Te centers, which calculations identified as the thermodynamically preferred arrangement.

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.

Strong σ-Hole Activation on Icosahedral Carborane Derivatives for a Directional Halide Recognition

Crystal engineering based on σ-hole interactions is an emerging approach for realization of new materials with higher complexity. Neutral inorganic clusters derived from 1,2-dicarba-closo-dodecaborane, substituted with -SeMe, -TeMe, and -I moieties on both skeletal carbon vertices are experimentally demonstrated herein as outstanding chalcogen- and halogen-bond donors. In particular, these new molecules strongly interact with halide anions in the solid-state. The halide ions are coordinated by one or two donor groups (μ₁ - and μ₂ -coordinations), to stabilize a discrete monomer or dimer motifs to 1D supramolecular zig-zag chains. Crucially, the observed chalcogen bond and halogen bond interactions feature remarkably short distances and high directionality. Electrostatic potential calculations further demonstrate the efficiency of the carborane derivatives, with V_s,max being similar or even superior to that of reference organic halogen-bond donors, such as iodopentafluorobenzene.

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.

The Nature of Strong Chalcogen Bonds Involving Chalcogen-Containing Heterocycles

Chalcogen bonds are σ hole interactions and have been used in recent years as an alternative to hydrogen bonds. In general, the electrostatic potential at the chalcogen atom and orbital delocalization effects are made responsible for the orientation of the chalcogen bond. Here, we were able to show by means of SAPT calculations that neither the induction (orbital delocalization effects) nor the electrostatic term is causing the spatial orientation of strong chalcogen bonds in tellurium-containing aromatics. Instead, steric interactions (Pauli repulsion) are responsible for the orientation. Against chemical intuition the dispersion energies of the examined tellurium-containing aromatics are far less important for the net attractive forces compared to the energies in the corresponding sulfur and selenium compounds. Our results underline the importance of often overlooked steric interactions (Pauli repulsion) in conformational control of σ hole interactions.

Activating Chalcogen Bonding (ChB) in Alkylseleno/Alkyltelluroacetylenes toward Chalcogen Bonding Directionality Control

Activation of a deep electron-deficient area on chalcogen atoms (Ch=Se, Te) is demonstrated in alkynyl chalcogen derivatives, in the prolongation of the (C≡)C-Ch bond. The solid-state structures of 1,4-bis(methylselenoethynyl)perfluorobenzene (1Se) show the formation of recurrent chalcogen-bonded (ChB) motifs. Association of 1Se and the tellurium analogue 1Te with 4,4'-bipyridine and with the stronger Lewis base 1,4-di(4-pyridyl)piperazine gives 1:1 co-crystals with 1D extended structures linked by short and directional ChB interactions, comparable to those observed with the corresponding halogen bond (XB) donor, 1,4-bis(iodoethynyl)-perfluorobenzene. This "alkynyl" approach for chalcogen activation provides the crystal-engineering community with efficient, and neutral ChB donors for the elaboration of supramolecular 1D (and potentially 2D or 3D) architectures, with a degree of strength and predictability comparable to that of halogen bonding in iodoacetylene derivatives.

Resonance Assisted Chalcogen Bonding as a New Synthon in the Design of Dyes

Intramolecular chalcogen bonding in arylhydrazones of sulfamethizole is strengthened by conjugation in the π-system of a noncovalent five-membered ring. The S⋅⋅⋅O distance in the sulfamethizole moiety of these compounds ranges from 2.698(3) to 2.806(15) Å, which indicates its strong dependence on the attached arylhydrazone fragments. Information on the nature of the intramolecular chalcogen bond was afforded by DFT calculations.

The Carbonyl⋅⋅⋅Tellurazole Chalcogen Bond as a Molecular Recognition Unit: From Model Studies to Supramolecular Organic Frameworks

In the last years, chalcogen bonding, the noncovalent interaction involving chalcogen centers, has emerged as interesting alternative to the ubiquitous hydrogen bonding in many research areas. Here, we could show by means of high-level quantum chemical calculations that the carbonyl⋅⋅⋅tellurazole chalcogen bond is at least as strong as conventional hydrogen bonds. Using the carbonyl⋅⋅⋅tellurazole binding motif, we were able to design complex supramolecular networks in solid phase starting from tellurazole-substituted cyclic peptides. X-ray analyses reveal that the rigid structure of the cyclic peptides is caused by hydrogen bonds, whereas the supramolecular network is held together by chalcogen bonding. The type of the supramolecular network depends on peptide used; both linear wires and a honeycomb-like supramolecular organic framework (SOF) were observed. The unique structure of the SOF shows two channels filled with different types of solvent mixtures that are either locked or freely movable.

A Regulable Internal Cavity inside a Resorcinarene-Based Hemicarcerand

Covalent organic capsules, such as carcerands and hemicarcerands, are an interesting class of molecular hosts. These container molecules have confined spaces capable of hosting small molecules, although the fact that the size of the inner cavities cannot be changed substantially limits the scope of their applications. The title covalently linked container was produced by metal-directed dimerization of a resorcinarene-based cavitand having four 2,2'-bipyridyl arms on the wide rim followed by olefin metathesis at the vertices of the resulting capsule with a second-generation Grubbs catalyst. The covalently linked bipyridyl arms permit expansion of the inner cavity by demetalation. This structural change influences the molecular recognition properties; the metal-coordinated capsule recognizes only 4,4'-diacetoxybiphenyl, whereas the metal-free counterpart can encapsulate not only 4,4'-diacetoxybiphenyl, but also 2,5-disubstituted-1,4-bis(4-acetoxyphenylethynyl)benzene, which is 9.4 Å longer than the former guest. Molecular mechanics calculations predict that the capsule expands the internal cavity to encapsulate the long guest by unfolding the folded conformation of the alkyl chains, which demonstrates the flexible and regulable nature of the cavity. Guest competition experiments show that the preferred guest can be switched by metalation and demetalation. This external-stimuli-responsive guest exchange can be utilized for the development of functional supramolecular systems controlling the uptake, transport, and release of chemicals.

Double Chalcogen Bonds: Crystal Engineering Stratagems via Diffraction and Multinuclear Solid-State Magnetic Resonance Spectroscopy

Group 16 chalcogens potentially provide Lewis-acidic σ-holes, which are able to form attractive supramolecular interactions with electron rich partners through chalcogen bonds. Here, a multifaceted experimental and computational study of a large series of novel chalcogen-bonded cocrystals, prepared using the principles of crystal engineering, is presented. Single-crystal X-ray diffraction studies reveal that dicyanoselenadiazole and dicyanotelluradiazole derivatives work as promising supramolecular synthons with the ability to form double chalcogen bonds with a wide range of electron donors including halides and oxygen- and nitrogen-containing heterocycles. Extensive ⁷⁷ Se and ¹²⁵ Te solid-state nuclear magnetic resonance spectroscopic investigations of cocrystals establish correlations between the NMR parameters of selenium and tellurium and the local chalcogen bonding geometry. The relationships between the electronic environment of the chalcogen bond and the ⁷⁷ Se and ¹²⁵ Te chemical shift tensors were elucidated through a natural localized molecular orbital density functional theory analysis. This systematic study of chalcogen-bond-based crystal engineering lays the foundations for the preparation of the various multicomponent systems and establishes solid-state NMR protocols to detect these interactions in powdered materials.