Chalcogen bonding in materials chemistry

Chalcogen bonded metal-organic frameworks: insights from X-ray analysis and theoretical calculations

The use of chalcogen bonding in the design of building blocks of metal-organic frameworks was demonstrated for the first time by experimental and theoretical methods. The supramolecular mode of the coordinated ligand as well as the chalcogen bond parameters (strengths and directionality) between the tectons are dependent on the metal centres.

Competition between Hydrogen and Chalcogen Bonding in Homodimers of Chalcogen Hydrides (H2X)2, X = O, S, Se, Te

The structural and chemical bonding motifs manifested by the competition between hydrogen and chalcogen bonding in the homodimers of chalcogen hydrides (H2X)2, where X = O, S, Se, Te have been characterized using high-level electronic structure calculations and analysis of the electron density based on Quasi-atomic orbital (QUAO) and the Symmetry-adapted perturbation theory (SAPT) methods. The QUAO analysis clearly identifies a three-center interaction responsible for either hydrogen or chalcogen bonds: in the former, the σ-bond connecting the donor and hydrogen atom participating in the hydrogen bond interacts with the lone pair on the nucleophile acceptor via the hydrogen atom, while in the latter this same σ-bond interacts with the nucleophile lone pair via the donor chalcogen. The number of minimum energy structures increase dramatically from one for (H2O)2, three for (H2S)2, four for (H2Se)2, and finally six for (H2Te)2. The emergence of the chalcogen-bonded arrangements appears for (H2S)2 with their subsequent energetic stabilization over the hydrogen-bonded minima manifesting in (H2Se)2 and (H2Te)2. In particular, one of the (H2S)2 , two of the (H2Se)2, and three of the (H2Te)2 dimers are chalcogen bonded. Induction plays a small but important role in stabilizing hydrogen over chalcogen-bonded structures, while dispersion is more important for chalcogen bonds.

Using Energetic Information Quantities from Density Functional Theory to Simultaneously Identify Both Covalent and Noncovalent Interactions

Covalent bonding and noncovalent interactions are important chemical concepts and how to identify them has been of current interest in the literature. Within the framework of density functional theory (DFT), we recently proposed a few qualitative descriptors to categorize different types of interactions with Pauli energy and its derivatives. In this work, we expand the scope by including the quantities derived from energetic information, which were recently proposed and thoroughly investigated by us from the framework of information-theoretic approach (ITA) in DFT. To that end, six energetic information quantities stemmed from the steric energy (Es), including Shannon entropyS S E s ${S_S^{E_s } }$ , Fisher informationI F E s ${I_F^{E_s } }$ , information gainI G E s ${I_G^{E_s } }$ , alternative Fisher informationI F ' E s ${I_F^{{^\prime}E_s } }$ , relative Fisher informationF r I E s ${{}_F^r I^{E_s } }$ , and relative alternative Fisher informationF r I ' E s ${{}_F^r I^{{^\prime}E_s } }$ are examined for the purpose. A strong linear correlation of Es or its topological analysis results with different covalent bond orders is established. We also unveil that signature isosurfaces of different categories of interactions fromI G E s ${I_G^{E_s } }$ andF r I ' E s ${{}_F^r I^{{^\prime}E_s } }$ can be employed to simultaneously identify single, double, triple, and quadruple covalent bonds, ionic and metallic bonds, and van der Waals interactions. This work provides another pathway for us to use density-based quantities to simultaneously identify covalent and noncovalent interactions.

Light-Induced Increase in Bond Strength─from Chalcogen Bond to Three-Electron σ Bond upon Excitation

Chalcogen bonds are σ hole interactions between a chalcogen center and a Lewis base center and have been applied in recent years as an alternative to hydrogen bonds in supramolecular chemistry and catalysis. While the electronic interactions of chalcogen bonds in the ground state have been intensively analyzed, there is barely any knowledge about the electron structure in the excited state. This is despite the fact that in some cases photoswitches containing chalcogen bonds exhibit exceptional switching behavior. Here, we investigate the effect of light absorption on chalcogen bonds containing divalent chalcogen centers. Quantum chemical calculations reveal that in the excited S1 state the noncovalent chalcogen bond converts to a covalent three-electron σ bond. The bond between the chalcogen center and the Lewis base center is thus significantly reinforced by light excitation. This change in bond type explains the previously experimentally observed nonswitchability of some tellurium-containing azo compounds. Furthermore, we were able to demonstrate that the switchability of certain selenium-containing compounds is temperature-dependent, whereby the ratio of the less stable cis compound is higher for higher temperatures. These results highlight the potential for designing responsive materials and dynamic molecular systems based on light-induced chalcogen bond modulation.

A Potent Bis-Heteroleptic Ruthenium(II) Complex-Based Chalcogen Bonding Receptor for Selective Sensing of Phosphates

The incorporation of a selenoimidazolium-based chalcogen bond (ChB) donor into a bis-heteroleptic Ru(II) complex (Ru-Se) has been designed for the first time to explore its anion-sensing properties and understand its selectivity to specific classes of anions. Photophysical studies demonstrate the receptor's selectivity toward phosphates, while 1H NMR displays its ability to recognize both I- and H2PO4- among the different halides and oxoanions through ChB interaction in CH3CN and dimethyl sulfoxide-d6 solvents, respectively. Additionally, microscopic studies such as DLS and TEM reveal that the selective turn-on sensing of H2PO4- and HP2O73- compared to I- is driven by supramolecular aggregation behavior. Hence, the successful fabrication of a selenium ChB-based Ru(II) complex makes it a promising candidate for anion monitoring in supramolecular chemistry.

Crystal structure, Hirshfeld surface analysis and crystal voids of 4-nitro-benzo[c][1,2,5]selena-diazole

The title mol-ecule, C6H3N3O2Se, is almost planar. In the crystal, inter-molecular C-H⋯O hydrogen bonds link the mol-ecules into a network structure, enclosing R 2 2(7) and R 3 3(8) ring motifs, parallel to the bc plane. There are π-π inter-actions present with centroid-to-centroid distances of 3.746 (3) and 3.697 (3) Å. A Hirshfeld surface analysis of the crystal structure indicates that the most important contributions for the crystal packing are from H⋯O/O⋯H (19.6%), H⋯N/N⋯H (11.0%), H⋯Se/Se⋯H (8.5%), O⋯Se/Se⋯O (8.2%), H⋯H (7.4%), C⋯N/N⋯C (7.3%) and N⋯Se/Se⋯N (7.2%) inter-actions. Hydrogen bonding and van der Waals inter-actions are the dominant inter-actions in the crystal packing. The volume of the crystal voids and the percentage of free space were calculated to be 25.60 Å3 and 3.73%, showing that there is no large cavity in the crystal.

Unraveling the Strength and Nature of Se∙∙∙O Chalcogen Bonds: A Comparative Study of SeF2 and SeF4 Interactions with Oxygen-Bearing Lewis Bases

Chalcogen bonds (ChBs) involving selenium have attracted substantial scholarly interest in past years owing to their fundamental roles in various chemical and biological fields. However, the effect of the valency state of the electron-deficient selenium atom on the characteristics of such ChBs remains unexplored. Herein, we comparatively studied the σ-hole-type Se∙∙∙O ChBs between SeF2/SeF4 and a series of oxygen-bearing Lewis bases, including water, methanol, dimethyl ether, ethylene oxide, formaldehyde, acetaldehyde, acetone, and formic acid, using ab initio computations. The interaction energies of these chalcogen-bonded heterodimers vary from -5.25 to -11.16 kcal/mol. SeF2 participates in a shorter and stronger ChB than SeF4 for all the examined heterodimers. Such Se∙∙∙O ChBs are closed-shell interactions, exhibiting some covalent character for all the examined heterodimers, except for SeF4∙∙∙water. Most of these chalcogen-bonded heterodimers are predominantly stabilized through orbital interactions between the lone pair of the O atom in Lewis bases and the σ*(Se-F) antibonding orbitals of Lewis acids. The back-transfer of charge from the lone pair of selenium into the σ* or π* antibonding orbitals of Lewis bases is also observed for all systems. Energy decomposition analysis reveals that the electrostatic component significantly stabilizes the targeted heterodimers, while the induction and dispersion contributions cannot be ignored.

Surface modification of graphene and fullerene with Sulfur (S), Selenium (Se), and Oxygen (O): DFT Simulation for enhanced zidovudine delivery in HIV treatment

HIV is one of the most threatening health conditions with a highly increasing rate, affecting millions of people globally, and from its time of discovery until now, its potential cure cannot be explicitly defined. This challenge of having no/low effective drugs for the subjected virus has called for serious attention in the scientific world of virus disease therapeutics. Most of these drugs yields low effectiveness due to poor delivery; hence, there is a need for novel engineering methods for efficient delivery. In this study, two nanomaterilas (graphene; GP, and fullerene; C60) were modelled and investigated with sulfur (S), selenium (Se), and oxygen (O) atoms, to facilitate the delivery of zidovudine (ZVD). This investigation was computationally investigated using the density functional theory (DFT), calculated at B3LYP functional and Gd3bj/Def2svp level of theory. Results from the frontier molecular orbital (FMO), revealed that the GP/C60_S_ZVD complex calculated the least energy gap of 0.668 eV, thus suggesting a favourable interactions. The study of adsorption energy revealed chemisorption among all the interacting complexes wherein GP/C60_S_ZVD complex (-1.59949 eV) was highlighted as the most interacting system, thereby proving its potential for the delivery of ZVD. The outcome of this research urges that a combination of GP and C60 modified with chalcogen particularly, O, S, and Se can aid in facilitating the delivery of zidovudine.

Expanding chalcogen bonds in thiophenes to interactions with halogens

Compounds containing the thiophene moiety find several applications in physics and chemistry, such as electrical conduction, which depends on specific conformations to properly exhibiting the desired properties. In turn, chalcogen bonding has found to modulate the conformation of some N-thiophen-2-ylfomamides. Since halogens participate in a kin interaction (halogen bonding) and are abundant in agrochemicals, pharmaceuticals, and materials, we have quantum-chemically explored the interaction between organic halogen and thiophene as a conformational modulator in some model compounds. Although such interaction indeed appears, as demonstrated by atoms in molecules and natural bond orbital analysis, it is inefficient to control the conformational equilibrium. An energy decomposition analysis scheme demonstrated that halomethane and thiophene tend to move away from one another due to a core component (Pauli repulsion and exchange), which is mainly due to a deformation term. Therefore, chalcogen bonds with halogens appear weaker than with other chalcogens.

Intramolecular chalcogen bonding activated SuFEx click chemistry for efficient organic-inorganic linking

SuFEx click chemistry demonstrates remarkable molecular assembly capabilities. However, the effective utilization of alkyl sulfonyl fluoride hubs in SuFEx chemistry, particularly in reactions with alcohols and primary amines, presents considerable challenges. This study pioneers an intramolecular chalcogen bonding activated SuFEx (S-SuFEx) click chemistry employing alkyl sulfonyl fluorides with γ-S as the activating group. The ChB-activated alkyl sulfonyl fluorides can react smoothly with phenols, alcohols, and amines, exhibiting enhanced reactivity compared to SO2F2. Excellent yields have been achieved with all 75 tested substrates. Pioneering the application of S-SuFEx chemistry, we highlight its immense potential in organic-inorganic linking, considering the critical role of interfacial covalent bonding in material fabrication. The S-SuFEx hub 1c, incorporating a trialkoxy silane group has been specifically designed and synthesized for organic-inorganic linking. In a simple step, 1c efficiently anchors various organic compounds onto surfaces of inorganic materials, forming functionalized surfaces with properties such as antibacterial activity, hydrophobicity, and fluorescence.

From Chalcogen Bonding to S-π Interactions in Hybrid Perovskite Photovoltaics

The stability of hybrid organic-inorganic halide perovskite semiconductors remains a significant obstacle to their application in photovoltaics. To this end, the use of low-dimensional (LD) perovskites, which incorporate hydrophobic organic moieties, provides an effective strategy to improve their stability, yet often at the expense of their performance. To address this limitation, supramolecular engineering of noncovalent interactions between organic and inorganic components has shown potential by relying on hydrogen bonding and conventional van der Waals interactions. Here, the capacity to access novel LD perovskite structures that uniquely assemble through unorthodox S-mediated interactions is explored by incorporating benzothiadiazole-based moieties. The formation of S-mediated LD structures is demonstrated, including one-dimensional (1D) and layered two-dimensional (2D) perovskite phases assembled via chalcogen bonding and S-π interactions. This involved a combination of techniques, such as single crystal and thin film X-ray diffraction, as well as solid-state NMR spectroscopy, complemented by molecular dynamics simulations, density functional theory calculations, and optoelectronic characterization, revealing superior conductivities of S-mediated LD perovskites. The resulting materials are applied in n-i-p and p-i-n perovskite solar cells, demonstrating enhancements in performance and operational stability that reveal a versatile supramolecular strategy in photovoltaics.

On-Surface Molecular Recognition Driven by Chalcogen Bonding

Chalcogen bonding interactions (ChBIs) have been widely employed to create ordered noncovalent assemblies in solids and liquids. Yet, their ability to engineer molecular self-assembly on surfaces has not been demonstrated. Here, we report the first demonstration of on-surface molecular recognition solely governed by ChBIs. Scanning tunneling microscopy and ab initio calculations reveal that a pyrenyl derivative can undergo noncovalent chiral dimerization on the Au(111) surface through double Ch···N interactions involving Te- or Se-containing chalcogenazolo pyridine motifs. In contrast, reference chalcogenazole counterparts lacking the pyridyl moiety fail to form regular self-assemblies on Au, resulting in disordered assemblies.

Hysteretic Room-Temperature Magnetic Bistability of the Crystalline 4,7-Difluoro-1,3,2-Benzodithiazolyl Radical

The title radical R⋅, synthesized by reduction of the corresponding cation R+, is thermally stable up to ~380 K in the crystalline state under anaerobic conditions. With SQUID magnetometry, single-crystal and powder XRD, solid-state EPR and TG-DSC, reversible spin-Peierls transition between diamagnetic and paramagnetic states featuring ~10 K hysteretic loop is observed for R⋅ in the temperature range ~310-325 K; ΔH=~2.03 kJ mol-1 and ΔS=~6.23 J mol-1 K-1. The transition is accompanied by mechanical movement of the crystals, i. e., by thermosalient behavior. The low-temperature diamagnetic P-1 polymorph of R⋅ consists of R⋅2 π-dimers arranged in (…R⋅2…)n π-stacks; whereas the high-temperature paramagnetic P21/c polymorph, of uniform (…R⋅…)n π-stacks. With the XRD geometries, CASSCF and broken-symmetry DFT jointly suggest strong antiferromagnetic (AF) interactions within R⋅2 and weak between R⋅2 for the (…R⋅2…)n stacks; and moderate AF interactions between R⋅ for the (…R⋅…)n stacks. The fully hydrocarbon archetype of R⋅ does not reveal the aforementioned properties. Thus, the fluorinated 1,3,2-benzodithiazolyls pave a new pathway in the design and synthesis of metal-less magnetically-bistable materials.

Unveiling the Nature and Strength of Selenium-Centered Chalcogen Bonds in Binary Complexes of SeO2 with Oxygen-/Sulfur-Containing Lewis Bases: Insights from Theoretical Calculations

Among various non-covalent interactions, selenium-centered chalcogen bonds (SeChBs) have garnered considerable attention in recent years as a result of their important contributions to crystal engineering, organocatalysis, molecular recognition, materials science, and biological systems. Herein, we systematically investigated π-hole-type Se∙∙∙O/S ChBs in the binary complexes of SeO2 with a series of O-/S-containing Lewis bases by means of high-level ab initio computations. The results demonstrate that there exists an attractive interaction between the Se atom of SeO2 and the O/S atom of Lewis bases. The interaction energies computed at the MP2/aug-cc-pVTZ level range from -4.68 kcal/mol to -10.83 kcal/mol for the Se∙∙∙O chalcogen-bonded complexes and vary between -3.53 kcal/mol and -13.77 kcal/mol for the Se∙∙∙S chalcogen-bonded complexes. The Se∙∙∙O/S ChBs exhibit a relatively short binding distance in comparison to the sum of the van der Waals radii of two chalcogen atoms. The Se∙∙∙O/S ChBs in all of the studied complexes show significant strength and a closed-shell nature, with a partially covalent character in most cases. Furthermore, the strength of these Se∙∙∙O/S ChBs generally surpasses that of the C/O-H∙∙∙O hydrogen bonds within the same complex. It should be noted that additional C/O-H∙∙∙O interactions have a large effect on the geometric structures and strength of Se∙∙∙O/S ChBs. Two subunits are connected together mainly via the orbital interaction between the lone pair of O/S atoms in the Lewis bases and the BD*(OSe) anti-bonding orbital of SeO2, except for the SeO2∙∙∙HCSOH complex. The electrostatic component emerges as the largest attractive contributor for stabilizing the examined complexes, with significant contributions from induction and dispersion components as well.

Substitution-pattern- and counteranion-dependent ion-pairing assemblies of heteroporphyrin-based π-electronic cations

Metal complexation and peripheral modifications of thiaporphyrins have been investigated for preparing polarized π-electronic cations with anion-dependent ion-pairing assembling modes, including charge-segregated structures exhibiting electric conductive properties.

Halogen Bond-Assisted Supramolecular Dimerization of Pyridinium-Fused 1,2,4-Selenadiazoles via Four-Center Se2N2 Chalcogen Bonding

The synthesis and structural characterization of α-haloalkyl-substituted pyridinium-fused 1,2,4-selenadiazoles with various counterions is reported herein, demonstrating a strategy for directed supramolecular dimerization in the solid state. The compounds were obtained through a recently discovered 1,3-dipolar cycloaddition reaction between nitriles and bifunctional 2-pyridylselenyl reagents, and their structures were confirmed by the X-ray crystallography. α-Haloalkyl-substituted pyridinium-fused 1,2,4-selenadiazoles exclusively formed supramolecular dimers via four-center Se···N chalcogen bonding, supported by additional halogen bonding involving α-haloalkyl substituents. The introduction of halogens at the α-position of the substituent R in the selenadiazole core proved effective in promoting supramolecular dimerization, which was unaffected by variation of counterions. Additionally, the impact of cocrystallization with a classical halogen bond donor C6F3I3 on the supramolecular assembly was investigated. Non-covalent interactions were studied using density functional theory calculations and topological analysis of the electron density distribution, which indicated that all ChB, XB and HB interactions are purely non-covalent and attractive in nature. This study underscores the potential of halogen and chalcogen bonding in directing the self-assembly of functional supramolecular materials employing 1,2,4-selenadiazoles derived from recently discovered cycloaddition between nitriles and bifunctional 2-pyridylselenyl reagents.

Exploring the fluorescence properties of tellurium-containing molecules and their advanced applications

This review article explores the fascinating realm of fluorescence using organochalcogen molecules, with a particular emphasis on tellurium (Te). The discussion encompasses the underlying mechanisms, structural motifs influencing fluorescence, and the applications of these intriguing phenomena. This review not only elucidates the current state of knowledge but also identifies avenues for future research, thereby serving as a valuable resource for researchers and enthusiasts in the field of fluorescence chemistry with a focus on Te-based molecules. By highlighting challenges and prospects, this review sparks a conversation on the transformative potential of Te-containing compounds across different fields, ranging from environmental solutions to healthcare and materials science applications. This review aims to provide a comprehensive understanding of the distinct fluorescence behaviors exhibited by Te-containing compounds, contributing valuable insights to the evolving landscape of chalcogen-based fluorescence research.

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%).

The Missing Chalcogen Bonding Donor: Strongly Polarized Oxygen of Water

A biscyclen molecular cabin, synthesized by connecting two cyclen macrocycles with four linkages, entraps a Li+···H2O···Li+ trimer with a water molecule clamped by two Li+ ions. This configuration results in strongly polarized water, characterized by a water proton resonance shift of up to 10.00 ppm. The arrangement facilitates unprecedented O-centered chalcogen bonds between the lone pairs of pyridinyl nitrogen atoms and polarized water oxygen, as confirmed by X-ray crystallography, NMR spectroscopy, and theoretical calculations. Further observation of O-centered chalcogen bonding in a H2O·(LiCl)2 cluster suggests its widespread presence in hydrated salt systems.

Bidentate Ligand Driven Intramolecularly Te…O Bonded Organotellurium Cations from Synthesis, Stability to Catalysis

A new series of unsymmetrical phenyl tellurides derived from 2-N-(quinolin-8-yl) benzamide ligand has been synthesized in a practical manner by the copper-catalyzed method by using diaryl ditelluride and Mg as a reductant at room temperature. In order to augment the Lewis acidity of these newly formed unsymmetrical monotellurides, these have been transformed into corresponding unsymmetrical 2-N-(quinolin-8-yl)benzamide tellurium cations. Subsequently, these Lewis acidic tellurium cations were used as chalcogen bonding catalysts, enabling the synthesis of various substituted 1,2-dihydroquinolines by activating ketones with anilines under mild conditions. Moreover, the synthesized 2-N-(quinolin-8-yl)benzamide phenyl tellurium cation has also catalyzed the formation of β-amino alcohols in high regioselectivity by effectively activating epoxides at room temperature. Mechanistic insight by 1 H and 19 F NMR study, electrostatic surface potential (ESP map), control reaction in which tellurium cation reacted explosively with epoxide, suggested that the enhanced Lewis acidity of tellurium center seems responsible for efficient catalytic activities under mild conditions enabling β-amino alcohols with excellent regioselectivity and 1,2-dihydroquinolines with trifluoromethyl, nitro, and pyridylsubstitution, which were difficult to access.

Computational Insight into the Nature and Strength of the π-Hole Type Chalcogen∙∙∙Chalcogen Interactions in the XO2∙∙∙CH3YCH3 Complexes (X = S, Se, Te; Y = O, S, Se, Te)

In recent years, the non-covalent interactions between chalcogen centers have aroused substantial research interest because of their potential applications in organocatalysis, materials science, drug design, biological systems, crystal engineering, and molecular recognition. However, studies on π-hole-type chalcogen∙∙∙chalcogen interactions are scarcely reported in the literature. Herein, the π-hole-type intermolecular chalcogen∙∙∙chalcogen interactions in the model complexes formed between XO2 (X = S, Se, Te) and CH3YCH3 (Y = O, S, Se, Te) were systematically studied by using quantum chemical computations. The model complexes are stabilized via one primary X∙∙∙Y chalcogen bond (ChB) and the secondary C-H∙∙∙O hydrogen bonds. The binding energies of the studied complexes are in the range of -21.6~-60.4 kJ/mol. The X∙∙∙Y distances are significantly smaller than the sum of the van der Waals radii of the corresponding two atoms. The X∙∙∙Y ChBs in all the studied complexes except for the SO2∙∙∙CH3OCH3 complex are strong in strength and display a partial covalent character revealed by conducting the quantum theory of atoms in molecules (QTAIM), a non-covalent interaction plot (NCIplot), and natural bond orbital (NBO) analyses. The symmetry-adapted perturbation theory (SAPT) analysis discloses that the X∙∙∙Y ChBs are primarily dominated by the electrostatic component.

Halide Complexes of 5,6-Dicyano-2,1,3-Benzoselenadiazole with 1 : 4 Stoichiometry: Cooperativity between Chalcogen and Hydrogen Bonding

The [M4 -Hal]- (M=the title compound; Hal=Cl, Br, and I) complexes were isolated in the form of salts of [Et4 N]+ cation and characterized by XRD, NMR, UV-Vis, DFT, QTAIM, EDD, and EDA. Their stoichiometry is caused by a cooperative interplay of σ-hole-driven chalcogen (ChB) and hydrogen (HB) bondings. In the crystal, [M4 -Hal]- are connected by the π-hole-driven ChB; overall, each [Hal]- is six-coordinated. In the ChB, the electrostatic interaction dominates over orbital and dispersion interactions. In UV-Vis spectra of the M+[Hal]- solutions, ChB-typical and [Hal]- -dependent charge-transfer bands are present; they reflect orbital interactions and allow identification of the individual [Hal]- . However, the structural situation in the solutions is not entirely clear. Particularly, the UV-Vis spectra of the solutions are different from the solid-state spectra of the [Et4 N]+ [M4 -Hal]- ; very tentatively, species in the solutions are assigned [M-Hal]- . It is supposed that the formation of the [M4 -Hal]- proceeds during the crystallization of the [Et4 N]+ [M4 -Hal]- . Overall, M can be considered as a chromogenic receptor and prototype sensor of [Hal]- . The findings are also useful for crystal engineering and supramolecular chemistry.

Halogen bonds, chalcogen bonds, pnictogen bonds, tetrel bonds and other σ-hole interactions: a snapshot of current progress

We report here on the status of research on halogen bonds and other σ-hole interactions involving p-block elements in Lewis acidic roles, such as chalcogen bonds, pnictogen bonds and tetrel bonds. A brief overview of the available literature in this area is provided via a survey of the many review articles that address this field. Our focus has been to collect together most review articles published since 2013 to provide an easy entry into the extensive literature in this area. A snapshot of current research in the area is provided by an introduction to the virtual special issue compiled in this journal, comprising 11 articles and entitled `Halogen, chalcogen, pnictogen and tetrel bonds: structural chemistry and beyond.'

A Structural Approach to the Strength Evaluation of Linear Chalcogen Bonds

The experimental structural features of chalcogen bonding (ChB) interactions in over 34,000 linear fragments R–Ch⋯A (Ch = S, Se, Te; R = C, N, O, S, Se, Te; A = N, O, S, Se, Te, F, Cl, Br, I) were analyzed. The bond distances dR–Ch and the interaction distances dCh⋯A were investigated, and the functions δR–Ch and δCh⋯A were introduced to compare the structural data of R–Ch⋯A fragments involving different Ch atoms. The functions δR−Ch and δCh⋯A were calculated by normalizing the differences between the relevant bond dR–Ch and ChB interaction dCh⋯A distances with respect to the sum of the relevant covalent (rcovR + rcovCh) and the van der Waals (vdW) radii (rvdWCh + rvdWA), respectively. A systematic comparison is presented, highlighting the role of the chalcogen involved, the role of the R atoms covalently bonded to the Ch, and the role of the A species playing the role of chalcogen bond acceptor. Based on the results obtained, an innovative approach is proposed for the evaluation and categorization of the ChB strength based on structural data.

Tuning Molecular Electron Affinities against Atomic Electronegativities by Spatial Expansion of a π-System

2,1,3-Benzochalcogenadiazoles C₆ R₄ N₂ E (E/R; E=S, Se, Te; R=H, F, Cl, Br, I) and C₆ H₂ R₂ N₂ E (E/R'; E=S, Se, Te; R=Br, I) are 10π-electron hetarenes. By CV/EPR measurements, DFT calculations, and QTAIM and ELI-D analyses, it is shown that their molecular electron affinities (EAs) increase with decreasing Allen electronegativities and electron affinities of the E and non-hydrogen R (except Cl) atoms. DFT calculations for E/R+e⋅^- →[E/R]⋅^- electron capture reveal negative ΔG values numerically increasing with increasing atomic numbers of the E and R atoms; positive ΔS has a minor influence. It is suggested that the EA increase is caused by more effective charge/spin delocalization in the radical anions of heavier derivatives due to contributions from diffuse (a real-space expanded) p-AOs of the heavier E and R atoms; and that this counterintuitive effect might be of the general character.

Chalcogen bond-assisted syn-locked scaffolds: DFT analysis and biological implications of novel tubulin inhibitors

A series of new tubulin inhibitors containing chalcogen bonds have been discovered. Density functional theory (DFT) analysis of the O-C-C-S torsion profile shows a preference of 0.8 kcal/mol for the syn-conformer over the anti-conformer. Besides, the O-S natural bond orbital (NBO) analysis reveals that the O_LP ∼ C-S_BD∗ energy potential is 0.62 kcal/mol. Further pharmacochemical screening of several series of (4-arylthiophen-2-yl)(3,4,5-trimethoxyphenyl)methanones identified IPO-10 as a highly effective tubulin inhibitor with an IC₅₀ of 23 nm for MCF-7.

Robust Supramolecular Dimers Derived from Benzylic-Substituted 1,2,4-Selenodiazolium Salts Featuring Selenium⋯π Chalcogen Bonding

The series of benzylic-substituted 1,2,4-selenodiazolium salts were prepared via cyclization reaction between 2-pyridylselenyl chlorides and nitriles and fully characterized. Substitution of the Cl anion by weakly binding anions promoted the formation supramolecular dimers featuring four center Se₂N₂ chalcogen bonding and two antiparallel selenium⋯π interactions. Chalcogen bonding interactions were studied using density functional theory calculations, molecular electrostatic potential (MEP) surfaces, the quantum theory of atoms-in-molecules (QTAIM), and the noncovalent interaction (NCI) plot. The investigations revealed fundamental role of the selenium⋯π contacts that are stronger than the Se⋯N interactions in supramolecular dimers. Importantly, described herein, the benzylic substitution approach can be utilized for reliable supramolecular dimerization of selenodiazolium cations in the solid state, which can be employed in supramolecular engineering.

77Se and 125Te solid-state NMR and X-ray diffraction structural study of chalcogen-bonded 3,4-dicyano-1,2,5-chalcogenodiazole cocrystals

Three novel chalcogen-bonded cocrystals featuring 3,4-dicyano-1,2,5-selenodiazole (C₄N₄Se) or 3,4-dicyano-1,2,5-tellurodiazole (C₄N₄Te) as chalcogen-bond donors and hydroquinone (C₆H₆O₂), tetraphenylphosphonium chloride (C₂₄H₂₀P⁺·Cl^-) or tetraethylphosphonium chloride (C₈H₂₀P⁺·Cl^-) as chalcogen-bond acceptors have been prepared and characterized by single-crystal X-ray diffraction (XRD), powder X-ray diffraction and ⁷⁷Se/¹²⁵Te magic-angle spinning solid-state NMR spectroscopy. The single-crystal XRD results show that the chalcogenodiazole molecules interact with the electron donors through two σ-holes on each of the chalcogen atoms, which results in highly directional and moderately strong chalcogen bonds. Powder XRD confirms that the crystalline phases are preserved upon moderate grinding of the samples for solid-state NMR experiments. Measurement of ⁷⁷Se and ¹²⁵Te chemical shift tensors via magic-angle spinning solid-state NMR spectroscopy confirms the number of magnetically unique chalcogen sites in each asymmetric unit and reveals the impact of chalcogen-bond formation on the local electronic structure. These NMR data are further assessed in the context of analogous data for a wider range of crystalline chalcogen-bonded systems.

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 CuI -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).

Ion-pairing assemblies of heteroporphyrin-based π-electronic cation with various counteranions

Various counteranions of the thiaporphyrin-Ni^II complex as a π-electronic cation were exchanged for preparing stable ion pairs. The ion-pairing assembling modes, which included contributions of charge-by-charge and charge-segregated modes, and properties depended on the geometries and electronic states of the counteranions.

Synthesis of cis-thiiranes as diastereoselective access to epoxide congeners via 4π-electrocyclization of thiocarbonyl ylides

Organochalcogen heterocycles are ubiquitously present and widely utilized in various fields. Among them, oxirane has been extensively studied, and all of the stereoisomeric forms are readily available. In contrast, synthetic studies on thiirane were rarely reported, and thus the useful sulfur-congener of oxirane has been difficult to access in a stereodefined form. In this research, a general stereoselective synthesis of cis-thiiranes is accomplished by taking advantage of stereospecific electrocyclization of trans-thiocarbonyl ylides, which are generated in situ from readily available E,E-aldazine N-oxides upon treatment with Lawesson's reagent. This newly developed practical method provides a variety of cis-1,2-diarylthiiranes as essentially single diastereomers in high yields under mild reaction conditions. The intermediacy of trans-thiocarbonyl yilde is confirmed by mechanistic experiments, and the excellent stereocontrol is rationalized by DFT calculation.

Towards Anion Recognition and Precipitation with Water-Soluble 1,2,4-Selenodiazolium Salts: Combined Structural and Theoretical Study

The synthesis and structural characterization of a series of supramolecular complexes of bicyclic cationic pyridine-fused 1,2,4-selenodiazoles with various anions is reported. The binding of trifluoroacetate, tetrachloroaurate, tetraphenylborate, perrhenate, and pertechnetate anions in the solid state is regarded. All the anions interact with selenodiazolium cations exclusively via a pair of “chelating” Se⋯O and H⋯O non-covalent interactions, which make them an attractive, novel, non-classical supramolecular recognition unit or a synthon. Trifluoroacetate salts were conveniently generated via novel oxidation reaction of 2,2′-dipyridyl diselenide with bis(trifluoroacetoxy)iodo)benzene in the presence of corresponding nitriles. Isolation and structural characterization of transient 2-pyridylselenyl trifluoroacetate was achieved. X-ray analysis has demonstrated that the latter forms dimers in the solid state featuring very short and strong Se⋯O and Se⋯N ChB contacts. 1,2,4-Selenodiazolium trifluoroacetates or halides show good solubility in water. In contrast, (AuCl₄)⁻, (ReO₄)⁻, or (TcO₄)⁻ derivatives immediately precipitate from aqueous solutions. Structural features of these supramolecular complexes in the solid state are discussed. The nature and energies of the non-covalent interactions in novel assembles were studied by the theoretical methods. To the best of our knowledge, this is the first study that regards perrhenate and pertechnetate as acceptors in ChB interactions. The results presented here will be useful for further developments in anion recognition and precipitation involving cationic 1,2,4-selenodiazoles.

Redox-Switchable Chalcogen Bonding for Anion Recognition and Sensing

Inspired by the success of its related sigma-hole congener halogen bonding (XB), chalcogen bonding (ChB) is emerging as a powerful noncovalent interaction with a plethora of applications in supramolecular chemistry and beyond. Despite its increasing importance, the judicious modulation of ChB donor strength remains a formidable challenge. Herein, we present, for the first time, the reversible and large-scale modulation of ChB potency by electrochemical redox control. This is exemplified by both the switching-ON of anion recognition via ChB oxidative activation of a novel bis(ferrocenyltellurotriazole) anion host and switching-OFF reductive ChB deactivation of anion binding potency with a telluroviologen receptor. The direct linking of the redox-active center and ChB receptor donor sites enables strong coupling, which is reflected by up to a remarkable 3 orders of magnitude modulation of anion binding strength. This is demonstrated through large voltammetric perturbations of the respective receptor ferrocene and viologen redox couples, enabling, for the first time, ChB-mediated electrochemical anion sensing. The sensors not only display significant anion-binding-induced electrochemical responses in competitive aqueous-organic solvent systems but can compete with, or even outperform similar, highly potent XB and HB sensors. These observations serve to highlight a unique (redox) tunability of ChB and pave the way for further exploration of the reversible (redox) modulation of ChB in a wide range of applications, including anion sensors as well as molecular switches and machines.

Carbon-chalcogen bond formation initiated by [Al(NONDipp)(E)]- anions containing Al-E{16} (E{16} = S, Se) multiple bonds

Multiply-bonded main group metal compounds are of interest as a new class of reactive species able to activate and functionalize a wide range of substrates. The aluminium sulfido compound K[Al(NONDipp)(S)] (NONDipp = [O(SiMe2NDipp)2]2-, Dipp = 2,6-iPr2C6H3), completing the series of [Al(NONDipp)(E)]- anions containing Al-E{16} multiple bonds (E{16} = O, S, Se, Te), was accessed via desulfurisation of K[Al(NONDipp)(S4)] using triphenylphosphane. The crystal structure showed a tetrameric aggregate joined by multiple K⋯S and K⋯π(arene) interactions that were disrupted by the addition of 2.2.2-cryptand to form the separated ion pair, [K(2.2.2-crypt)][Al(NONDipp)(S)]. Analysis of the anion using density functional theory (DFT) confirmed multiple-bond character in the Al-S group. The reaction of the sulfido and selenido anions K[Al(NONDipp)(E)] (E = S, Se) with CO2 afforded K[Al(NONDipp)(κ2 E,O-EC{O}O)] containing the thio- and seleno-carbonate groups respectively, consistent with a [2 + 2]-cycloaddition reaction and C-E bond formation. An analogous cycloaddition reaction took place with benzophenone affording compounds containing the diphenylsulfido- and diphenylselenido-methanolate ligands, [κ2 E,O-EC{O}Ph2]2-. In contrast, when K[Al(NONDipp)(E)] (E = S, Se) was reacted with benzaldehyde, two equivalents of substrate were incorporated into the product accompanied by formation of a second C-E bond and complete cleavage of the Al-E{16} bonds. The products contained the hitherto unknown κ2 O,O-thio- and κ2 O,O-seleno-bis(phenylmethanolate) ligands, which were exclusively isolated as the cis-stereoisomers. The mechanisms of these cycloaddition reactions were investigated using DFT methods.

Assessing the persistence of chalcogen bonds in solution with neural network potentials

Non-covalent bonding patterns are commonly harvested as a design principle in the field of catalysis, supramolecular chemistry and functional materials to name a few. Yet, their computational description generally neglects finite temperature and environment effects, which promote competing interactions and alter their static gas-phase properties. Recently, neural network potentials (NNPs) trained on Density Functional Theory (DFT) data have become increasingly popular to simulate molecular phenomena in condensed phase with an accuracy comparable to ab initio methods. To date, most applications have centered on solid-state materials or fairly simple molecules made of a limited number of elements. Herein, we focus on the persistence and strength of chalcogen bonds involving a benzotelluradiazole in condensed phase. While the tellurium-containing heteroaromatic molecules are known to exhibit pronounced interactions with anions and lone pairs of different atoms, the relevance of competing intermolecular interactions, notably with the solvent, is complicated to monitor experimentally but also challenging to model at an accurate electronic structure level. Here, we train direct and baselined NNPs to reproduce hybrid DFT energies and forces in order to identify what are the most prevalent non-covalent interactions occurring in a solute-Cl$^-$-THF mixture. The simulations in explicit solvent highlight competition with chalcogen bonds formed with the solvent and the short-range directionality of the interaction with direct consequences for the molecular properties in the solution. The comparison with other potentials (e.g., AMOEBA, direct NNP and continuum solvent model) also demonstrates that baselined NNPs offer a reliable picture of the non-covalent interaction interplay occurring in solution.

Organotelluroxane molecular clusters assembled via Te⋯X- (X = Cl-, Br-) chalcogen bonding anion template interactions

The synthesis and characterisation of two novel molecular organotelluroxane clusters, comprising of an inorganic Te₈O₆X₄ (X = Cl, Br) core structure are described. The integration of highly electron withdrawing 3,5-bis-trifluoromethylphenyl groups to the constituent Te(IV) centres is determined to be crucial in the chalcogen bonding (ChB) halide template directed assembly. Characterised by multi-nuclear ¹H, ¹²⁵Te, ¹⁹F NMR, UV-Vis, IR spectroscopies and X-ray crystal structure analysis, the discrete molecular clusters exhibit excellent organic solvent solubility and remarkable chemical stability. Furthermore, preliminary fluorescence investigations reveal the telluroxanes exhibit aggregation induced emission (AIE) behaviour in organic aqueous solvent mixtures.

Exploring Supramolecular Assembly Space of Cationic 1,2,4-Selenodiazoles: Effect of the Substituent at the Carbon Atom and Anions

Chalcogenodiazoles have been intensively studied in recent years in the context of their supramolecular chemistry. In contrast, the newly discovered cationic 1,2,4-selenodiazole supramolecular building blocks, which can be obtained via coupling between 2-pyridylselenyl halides and nitriles, are virtually unexplored. A significant advantage of the latter is their facile structural tunability via the variation of nitriles, which could allow a fine tuning of their self-assembly in the solid state. Here, we explore the influence of the substituent (which derives from the nitrile) and counterions on the supramolecular assembly of cationic 1,2,4-selenodiazoles via chalcogen bonding.

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.

Large interaction energy for the homodimer and the heterodimer extracted from the supramolecular chain of a bent trinuclear zinc(ii) complex with a reduced Schiff base ligand

The coordinated amino groups in a trinuclear zinc complex participate in strong H-bonding interactions that have been analysed. The dimerization energy is very large for both the homodimer and the heterodimer in the 1D supramolecular chain.

Heterovalent chalcogen bonding: supramolecular assembly driven by the occurrence of a tellurium(ii)⋯Ch(i) (Ch = S, Se, Te) linkage

The revealed heterovalent TeII⋯ChI (Ch = S, Se, Te) chalcogen bonding was used for targeted noncovalent integration of two Ch centers in different oxidation states.

A Porous Chalcogen-Bonded Organic Framework

The manner of bonding between constituent atoms or molecules invariably influences the properties of materials. Perhaps no material family is more emblematic of this than porous frameworks, wherein the namesake modes of connectivity give rise to discrete subclasses with unique collections of properties. However, established framework classes often display offsetting advantages and disadvantages for a given application. Thus, there exists no universally applicable material, and the discovery of alternative modes of framework connectivity is highly desirable. Here we show that chalcogen bonding, a subclass of σ-hole bonding, is a viable mode of connectivity in low-density porous frameworks. Crystallization studies with the triptycene tris(1,2,5-selenadiazole) molecular tecton reveal how chalcogen bonding can template high-energy lattice structures and how solvent conditions can be rationalized to obtain molecularly programmed porous chalcogen-bonded organic frameworks (ChOFs). These results provide the first evidence that σ-hole bonding can be used to advance the diversity of porous framework materials.

The First Selenoanhydride in the Series of Chlorophyll a Derivatives, Its Stability and Photoinduced Cytotoxicity

In this work, we obtained the first selenium-containing chlorin with a chalcogen atom in exlocycle E. It was shown that the spectral properties were preserved in the target compound and the stability increased at two different pH values, in comparison with the starting purpurin-18. The derivatives have sufficiently high fluorescence and singlet oxygen quantum yields. The photoinduced cytotoxicity of sulfur- and selenium-anhydrides of chlorin p₆ studied for the first time in vitro on the S37 cell line was found to be two times higher that of purpurin-18 and purpurinimide studied previously. Moreover, the dark cytotoxicity increased four-fold in comparison with the latter compounds. Apparently, the increase in the dark cytotoxicity is due to the interaction of the pigments studied with sulfur- and selenium-containing endogenous intracellular compounds. Intracellular distributions of thioanhydride and selenoanhydride chlorin p₆ in S37 cells were shown in cytoplasm by diffusion distribution. The intracellular concentration of the sulfur derivative turned out to be higher and, as a consequence, its photoinduced cytotoxicity was higher as well.

Design of Azobenzene beyond Simple On-Off Behavior

Azobenzenes are without a doubt the most widely used light-induced switching units, and there is a plethora of application examples ranging from supramolecular chemistry to material science and biological chemistry. Here, we present a smart azobenzene, in which the photoswitching capability of the azobenzene moiety can be reversibly switched on and off using a second unit (redox switch). This second switching unit is based on the variation of the strength of a chalcogen bond between the azo group and a Te-Ph unit in ortho position to the azo group. This allows the selective switching of only one azobenzene unit in the presence of other azobenzene switches. The entire double-switch is a very simple, small system that can also be easily synthesized. As a result, this double-switch can be used as a smarter replacement for the established azobenzene system in the future. For example, in contrast to the latter this double-switch could be employed to store state information analogous to a flip-flop in digital electronic systems.

Iso-Tellurazolium-N-Phenoxides: A Family of Te···O Chalcogen-Bonding Supramolecular Building Blocks

Formal substitution of the oxygen atom of an iso-tellurazole N-oxide with deprotonated (ortho, meta, and para)-hydroxyphenyl groups generated molecules that readily aggregate through Te···O chalcogen bonding (ChB) interactions. The molecules undergo autoassociation in solution, as shown by variable temperature (VT) ¹H NMR experiments and paralleling the behavior of iso-tellurazole N-oxides. Judicious adjustment of crystallization conditions enabled the isolation of either polymeric or macrocyclic aggregates. Among the latter, the ortho compound assembled a calixarene-like trimer, while the para isomer built a macrocyclic tetramer akin to a molecular square. The Te···O ChB distances in these structures range from 2.13 to 2.17 Å, comparable to those in the structures of iso-tellurazole N-oxides. DFT calculations estimate that the corresponding Te···O ChB energies are between -122 and -195 kJ mol^-1 in model dimers and suggest that macrocyclic aggregation enhances these interactions.

Metal-Involving Chalcogen Bond. The Case of Platinum(II) Interaction with Se/Te-Based σ-Hole Donors

Platinum(II) complexes exhibiting an expressed d_z²-nucleophilicity of the positively charged metal centers, namely, [Pt(ppy)(acac)] (1; acacH is acetylacetone; ppyH is 2-Ph-pyridine) and [Pt(ppy)(tmhd)] (2; tmhdH is 2,2,6,6-tetramethylheptanedione-3,5), were cocrystallized with the chalcogen bond donors (4-NC₅F₄)₂Ch (Ch = Se, Te) to form two isostructural cocrystals 1·¹/₂(4-NC₅F₄)₂Ch, and 2·²/₃(4-NC₅F₄)₂Se and 2·(4-NC₅F₄)₂Te. The X-ray data for these cocrystals and appropriate theoretical DFT calculations (PBE0-D3BJ) allowed the recognition of the metal-involving chalcogen bond, namely, Ch···d_z²-Pt^II (its energy spans from -7 to -12 kcal/mol). In 1·¹/₂(4-NC₅F₄)₂Ch, Ch···d_z²-Pt^II bonding is accompanied by the C···d_z²-Pt^II interaction, representing a three-center bifurcate, whereas in 2·(4-NC₅F₄)₂Te the chalcogen bond Te···d_z²-Pt^II is purely two-centered and is stronger than that in 1·¹/₂(4-NC₅F₄)₂Ch because of more efficient orbital overlap. The association of 2 with (4-NC₅F₄)₂Te and the structure of the formed adduct in CDCl₃ solutions was studied by using ¹H, ¹³C, ¹⁹F, ¹⁹⁵Pt, ¹²⁵Te NMR, ¹⁹F-¹H HOESY, and diffusion NMR methods. The ¹⁹⁵Pt and ¹²⁵Te NMR titration and the isothermal titration calorimetry results revealed a 1:1 association of 2 with (4-NC₅F₄)₂Te.