Chalcogen bonding in synthesis, catalysis and design of materials

Formation of chalcogen-bonding interactions and their role in the trans-trans conformation of thiourea

The chalcogen-bonding interactions formed at both sides of the thiocarbonyl group in thiourea were investigated. In particular, the role of these chalcogen-bonding interactions in the trans-trans conformation of thiourea was evaluated via single-crystal X-ray diffraction analysis and DFT calculations. The obtained results indicate that the Se⋯S⋯Se dual chalcogen-bonding interactions play a stronger role in controlling the planar structure than the S⋯S⋯S interactions.

Elucidating the Binding Mode of Sulfur- and Selenium-Based Cationic Chalcogen-Bond Donors

For a comparison of the interaction modes of various chalcogen-bond donors, 2-chalcogeno-imidazolium salts have been designed, synthesized, and studied by single crystal X-ray diffraction, solution NMR and DFT as well as for their ability to act as activators in an SN1-type substitution reaction. Their interaction modes in solution were elucidated based on NMR diffusion and chemical shift perturbation experiments, which were supported by DFT-calculations. Our finding is that going from lighter to the heavier chalcogens, hydrogen bonding plays a less, while chalcogen bonding an increasingly important role for the coordination of anions. Anion-π interactions also show importance, especially for the sulfur and selenium derivatives.

Tunable electronic and photoelectric properties of Janus group-III chalcogenide monolayers and based heterostructures

Janus group-III chalcogenide monolayers and based heterostructures with breaking vertical structural symmetry offer additional prospects in the upcoming high-performance photoelectric devices. We studied the geometrical, electronic, and photoelectric properties of Janus group-III chalcogenide monolayers and heterostructures. The most energy favorable stacking design of ten vertical heterostructures are considered. The results showed that the Janus Se-In-Ga-S and S-In-Ga-Se monolayers exhibit semiconducting characteristics with the band gaps of 1.295 eV and 1.752 eV, respectively. Furthermore, the different stacking configurations and surface termination at interface can realize the transition of band alignment between type I and type II due to the interlayer coupling. Moreover, we systematically investigated the photoelectric properties of Janus group-III chalcogenide heterostructures and predicated an optimized power conversion efficiency of 16.2%. These findings can aid in comprehending the customized characteristics of Janus group-III chalcogenide heterostructures, offering theoretical guidance for creating innovative photoelectric devices.

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.

Carbon-chalcogen wires: alkynyltellurolatocarbynes

The reactions of [W(CBr)(CO)2(Tp*)] (Tp* = tris(dimethylpyrazolyl)borate) with LiTeCCR (R = SiMe3, SiiPr3, iPr, nBu, tBu, Ph, C6H4Me-4, methylimidazol-2-yl) afford the first alkynyltellurolatocarbynes [W(CTeCCR)(CO)2(Tp*)]. Both the WC and CC multiple bonds are prone to metal addition as exemplified by treatment with [MCl(SMe2)] (M = Cu, Au) to afford the hexametallic complex [W2Cu4(μ-CTeCCSiiPr3)2Cl4(CO)4(Tp*)2] and [WAu(μ-CTeCCSiMe3)Cl(CO)2-(Tp*)] which evolves to the unusual hypervalent [WAu(μ-CTeCl4)(SMe2)(CO)2(Tp*)].

σ-Hole Site-Based Interactions within Hypervalent Pnicogen, Halogen, and Aerogen-Bearing Molecules with Lewis Bases: A Comparative Study

σ-Hole site-based interactions in the trigonal bipyramidal geometrical structure of hypervalent pnicogen, halogen, and aerogen-bearing molecules with pyridine and NCH Lewis bases (LBs) were comparatively examined. In this respect, the ZF5···, XF3O2···, and AeF2O3···LB complexes (where Z = As, Sb; X = Br, I; Ae = Kr, Xe; and LB = pyridine and NCH) were investigated. The electrostatic potential (EP) analysis affirmations outlined the occurrence of σ-holes on the systems under consideration with disparate magnitudes that increased according to the following order: AeF2O3 < XF3O2 < ZF5. In line with EP outcomes, the proficiency of σ-hole site-based interactions increased as the atomic size of the central atom increased with a higher favorability for the pyridine-based complexes over NCH-based ones. The interaction energy showed the most favorable negative values of -35.97, -44.53, and -56.06 kcal/mol for the XeF2O3···, IF3O2···, and SbF5···pyridine complexes, respectively. The preferentiality pattern of the studied interactions could be explained as a consequence of (i) the dramatic rearrangement of ZF5 molecules from the trigonal bipyramid geometry to the square pyramidal one, (ii) the significant and tiny deformation energy in the case of the interaction of XF3O2 molecules with pyridine and NCH, respectively, and (iii) the absence of geometrical deformation within the AeF2O3···pyridine and ···NCH complexes other than the XeF2O3···pyridine one. Quantum theory of atoms in molecules and noncovalent interaction index findings reveal the partially covalent nature of most of the investigated interactions. Symmetry-adapted perturbation theory affirmations declared that the electrostatic component was the driving force beyond the occurrence of the considered interactions. The obtained findings will help in improving our understanding of the effect of geometrical deformation on intermolecular interactions.

Chalcogen- and Halogen-Bond-Donating Cyanoborohydrides Provide Imine Hydrogenation

Sulfonium, selenonium, telluronium, and iodonium cyanoborohydrides have been synthesized, isolated, and fully characterized by various methods, including single-crystal X-ray diffraction (XRD) analysis. The quantum theory of atoms in molecules' analysis based on the XRD data indicated that the hydride···σ-hole short contacts observed in the crystal structures of each compound have a purely noncovalent nature. The telluronium and iodonium cyanoborohydrides provide a significantly higher rate of the model reaction of imine hydrogenation compared with sodium and tetrabutylammonium cyanoborohydrides. Based on the NMR and high-resolution electrospray ionization mass spectrometry data indicating that the reaction progress is accompanied by the cation reduction, a mechanism involving intermediate formation of elusive onium hydrides has been proposed as an alternative to conventional electrophilic activation of the imine moiety by its ligation to the cation's σ-hole.

Spodium bonding in bis(alkynyl)mecurials

The new bis(alkynyl)mercurial Hg{CCSeCW(CO)2(Tp*)}2 (Tp* = tris(dimethylpyrazolyl)borate) forms adducts with fluoride and phenathroline, the structures of which are interpreted in the context of two-coordinate mercury presenting a σ-torroid for spodium bonding.

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 selenium-based chalcogen bond catalyzed cationic polymerization of p-methoxystyrene

The application of selenium-based non-covalent bond catalysis in living cationic polymerization has rarely been reported. In this work, the cationic polymerization of p-methoxystyrene (pMOS) was performed using a bidentate selenium bond catalyst - a new water-tolerant Lewis acid catalyst. A polymer with controllable molecular weight and narrow molecular weight distribution can be obtained at room temperature, with a maximum molecular weight of 23.3 kDa. This selenium bond compound can also catalyze the controllable cationic polymerization of p-methoxy styrene under environmental conditions. By changing the monomer feeding ratio, a secondary feeding experiment and DFT analysis, it is shown that the selenium bond catalyst can induce polymer chain growth by reversibly activating dormant covalent bonds (C-OH).

Influence of donor point modifications on the assembly of chalcogen-bonded organic frameworks

Incremental, single-atom substitutions of Se-based chalcogen bond (Ch-bond) donors with stronger donating Te centers were implemented in two new triptycene tris(1,2,5-chalcogenadiazole) tectons. The appreciably more favorable Ch-bonding ability of the Te-based donors promotes assembly of low-density networks and more stable Ch-bonded organic frameworks (ChOFs).

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.

Poly-pnictogen bonding: trapping halide ions by a tetradentate antimony(iii) Lewis acid

A highly halide affine, tetradentate pnictogen-bonding host-system based on the syn-photodimer of 1,8-diethynylanthracene was synthesized by a selective tin-antimony exchange reaction. The host carries four C[triple bond, length as m-dash]C-Sb(C2F5)2 units and has been investigated regarding its ability to act as a Lewis acidic host component for the cooperative trapping of halide ions (F-, Cl-, Br-, I-). The chelating effect makes this host-system superior to its bidentate derivative in competition experiments. It represents a charge-reversed crown-4 and has the ability to dissolve otherwise poorly soluble salts like tetra-methyl-ammonium chloride. Its NMR-spectroscopic properties make it a potential probe for halide ions in solution. Insights into the structural properties of the halide adducts by X-ray diffraction and computational methods (DFT, QTAIM, IQA) reveal a complex interplay of attractive pnictogen bonding interactions and Coulomb repulsion.

Electrocatalytic hydrogen evolution mediated by an organotelluroxane macrocycle stabilized through secondary interactions

A discrete liphophilic organotelluroxane macrocycle has been found to catalyse the hydrogen evolution reaction (HER) by proton reduction efficiently. The macrocycle is synthesized via chloride abstraction from bis(p-methoxyphenyl) tellurium dichloride (p-MeOC6H5)2TeCl2 (1) by silver salts AgMX4 (MX4 = BF4-, and ClO4-) resulting in in situ generated di-cationic tetraorganoditelluroxane units; two such units are held together by two weak anions μ2-MX4, bridging to form 12-membered di-cationic macrocycles [((p-MeO-C6H4)2Te)2(μ-O)(μ2-F2BF2)2]2+ (2) and [((p-MeO-C6H4)2Te)2(μ-O)(μ2-O2ClO2)2]2+ (3) stabilized via Te-(μ2-BF4/ClO4), with secondary interactions. The charge is balanced by the presence of two more anions, one above and another below the plane of the macrocycle. Similar reaction at higher temperatures leads to the formation of telluronium salts R3TeX [X = BF4- (4), ClO4- (5)] as a major product. The BF4- anion containing macrocycle and telluronium salt were monitored using 19F NMR. HRMS confirmed the structural stability of all the compounds in the solution state. The organotelluroxane macrocycle 2 has been found to act as an efficient electrocatalyst for proton reduction in an organic medium in the presence of p-toluene sulfonic acid as a protic source.

A Bidentate Antimony Pnictogen Bonding Host System

A bidentate pnictogen bonding host-system based on 1,8-diethynylanthracene was synthesized by a selective tin-antimony exchange reaction and investigated regarding its ability to act as a Lewis acidic host component for the complexation of Lewis basic or anionic guests. In this work, the novel C≡C-Sb(C2 F5 )2 unit was established to study the potential of antimony(III) sites as representatives for the scarcely explored pnictogen bonding donors. The capability of this partly fluorinated host system was investigated towards halide anions (Cl- , Br- , I- ), dimethyl chalcogenides Me2 Y (Y=O, S, Se, Te), and nitrogen heterocycles (pyridine, pyrimidine). Insights into the adduct formation behavior as well as the bonding situation of such E⋅⋅⋅Sb-CF moieties were obtained in solution by means of NMR spectroscopy, in the solid state by X-ray diffraction, by elemental analyses, and by computational methods (DFT, QTAIM, IQA), respectively.

Synthesis, X-ray characterization and DFT analysis of dicyanidoaurate telluronium salts: on the importance of charge assisted chalcogen bonds

In this manuscript we report the synthesis and X-ray characterization of two cyanidoaurate telluronium salts, namely (3-fluorophenyl)(methyl)(phenyl)telluronium dicyanidoaurate [(3-F-Ph)(Me)(Ph)Te][Au(CN)2] (1) and methyldiphenyltelluronium dicyanidoaurate [(Me)(Ph)2Te][Au(CN)2] (2). In the solid state, the tellurium atom establishes three concurrent and directional chalcogen bonds (ChBs) with the adjacent anions, in both compounds. These charge-assisted ChBs (CAChBs) have been analyzed using DFT calculations and several computational tools. The MEP surface analysis discloses the existence of three σ-holes at the Te-atoms capable of establishing strong CAChBs with the counter-ions. In addition, significant charge transfer from the lone pair orbital at the N-atom of the anion to the antibonding σ*(Te-C) orbital of the cation is observed in some cases.

Selection of isomerization pathways of multistep photoswitches by chalcogen bonding

Multistep photoswitches are able to engage in different photoisomerization pathways and are challenging to control. Here we demonstrate a multistep sequence of E/Z isomerization and photocyclization/cycloreversion of photoswitches via manipulating the strength and mechanism of noncovalent chalcogen bonding interactions. The incorporation of chalcogens and the formyl group on open ethene bridged dithienylethenes offers a versatile skeleton for single photochromic molecules. While bidirectional E/Z photoswitching is dominated by neutral tellurium arising from enhanced resonance-assisted chalcogen bonding, the creation of cationic telluronium enables the realization of photocyclization/cycloreversion. The reversible nucleophilic substitution reactions further allow interconversion between neutral tellurium and cationic telluronium and selection of photoisomerization mechanisms on purpose. By leveraging unique photoswitching patterns and dynamic covalent reactivity, light and pH stimuli-responsive multistate rewritable materials were constructed, triggered by an activating reagent for additional control. The results should provide ample opportunities to molecular recognition, intelligent switches, information encryption, and smart materials.

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.

Dual Chalcogen-Bonding Interactions for the Conformational Control of Urea

Dual chalcogen-bonding interactions is proposed as a novel means for the conformational control of urea derivatives. The formation of a chalcogen-bonding interaction at both sides of the urea carbonyl group was unambiguously confirmed by X-ray diffraction as well as computational studies including non-covalent interaction (NCI) plot index analysis, quantum theory of atoms in molecules (QTAIM) analysis, and natural bond orbital (NBO) analysis via DFT calculations. By virtue of this dual interaction, urea derivatives that bear chalcogen atoms (X=S and Se) adopt a planar structure via the carbonyl oxygen (O) with an X⋅⋅⋅O⋅⋅⋅X arrangement on the same side of the molecule. The rigidity of the conformational lock was evaluated using the molecular arrangement in the crystal and the rotational barrier of benzochalcogenophene ring, which indicated a stronger conformational lock in benzoselenophene than in benzothiophene urea derivatives. Furthermore, the acidity of the urea derivatives increases according to the Lewis-acidic properties of the chalcogen-bonding interactions, whereby benzoselenophene urea is more acidic than benzothiophene urea. Tweezer-shaped urea derivatives were prepared, and their stereostructure proved the viability of the conformational control for defining the location of the substituents on the urea framework.

Preparation of Carbon-Based Solid Acid Catalyst from High-Sulfur Petroleum Coke with Nitric Acid and Ball Milling, and a Computational Evaluation of Inherent Sulfur Conversion Pathways

A series of petroleum coke (petcoke)-derived solid acid catalysts were prepared via nitric acid treatment with or without ball milling pretreatment. The inherent sulfur in petcoke was converted to sulfonic groups, which were active sites for the esterification of octanoic acid and methanol at 60 °C, with ester yields of 14-43%. More specifically, samples without ball milling treated at 120 °C for 3 h had a total acidity of 4.67 mmol/g, which was 1.6 times that of the samples treated at 80 °C, despite their -SO3H acidities being similar (~0.08 mmol/g). The samples treated for 24 h had higher -SO3H (0.10 mmol/g) and total acidity (5.25 mmol/g) but not increased catalytic activity. Ball milling increased the defects and exposed aromatic hydrogen groups on petcoke, which facilitated further acid oxidation (0.12 mmol -SO3H/g for both materials and total acidity of 5.18 mmol/g and 5.01 mmol/g for BP-N-3/120 and BP-N-8/90, respectively) and an increased ester yield. DFT calculations were used to analyze the pathways of sulfonic acid group formation, and the reaction pathway with NO2• was the most thermodynamically and kinetically favourable. The activities of the prepared catalysts were related to the number of -SO3H acid sites, the total acidity, and the oxygen content, with the latter two factors having a negative impact.

Chalcogen bonding in copper(II)-mediated synthesis

The chalcogen bond (ChB) is a noncovalent attraction between an electrophilic chalcogen atom and a nucleophilic (Nu) region in the same (intramolecular) or another (intermolecular) molecular entity: R-Ch⋯Nu (Ch = O, S, Se or Te; R = substituents; Nu = nucleophile). ChB is comparable to the hydrogen and halogen bonds both in terms of strengths and directionality. However, in contrast to the monovalent halogen atoms, usually the divalent or tetravalent chalcogen atoms are able to display more than one electrophilic centre (on account of the existence of two or three species bonded to the chalcogen atom), which provides an additional opportunity in the use of this type of noncovalent binding in synthetic operations. In this work, the role of ChB at the secondary coordination sphere of metal complexes through copper(II)-mediated activation of dioxygen or of one nitrile group of a 1,2,5-selenadiazole-3,4-dicarbonitrile ligand to form a carbimidate or an imino-carboxylic acid is demonstrated. DFT calculations allowed evaluation of the strength of the ChBs and proved their relevant structure directing role in the solid state architectures. The effect of metal-coordination on the σ-hole opposite to the coordinated SeO bond has been analysed using molecular electrostatic potential (MEP) surfaces and explains the greater ability of the coordinated selenoxide derivatives to form strong ChBs.

Ion-Pairing Assemblies of Dithienylnitrophenol-Based π-Electronic Anions Stabilized by Intramolecular Interactions

Dithienylnitrophenols were synthesized as precursors of π-electronic anions, which were stabilized by intramolecular chalcogen bonding, forming various ion pairs in combination with cations. The modes of solid-state charge-by-charge assemblies, along with solution-state stacking and photoinduced electron transfer behaviors, were modulated by the constituent ionic species.

Sulfur-mediated chalcogen versus hydrogen bonds in proteins: a see-saw effect in the conformational space

Divalent sulfur (S) forms a chalcogen bond (Ch-bond) via its σ-holes and a hydrogen bond (H-bond) via its lone pairs. The relevance of these interactions and their interplay for protein structure and function is unclear. Based on the analyses of the crystal structures of small organic/organometallic molecules and proteins and their molecular electrostatic surface potential, we show that the reciprocity of the substituent-dependent strength of the σ-holes and lone pairs correlates with the formation of either Ch-bond or H-bond. In proteins, cystines preferentially form Ch-bonds, metal-chelated cysteines form H-bonds, while methionines form either of them with comparable frequencies. This has implications for the positioning of these residues and their role in protein structure and function. Computational analyses reveal that the S-mediated interactions stabilise protein secondary structures by mechanisms such as helix capping and protecting free β-sheet edges by negative design. The study highlights the importance of S-mediated Ch-bond and H-bond for understanding protein folding and function, the development of improved strategies for protein/peptide structure prediction and design and structure-based drug discovery.

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.

Solid-state molecular structures of Se(IV) and Te(IV) dihalides X2Se(CH3)(C6F5) and the gas-phase structure of Se(CH3)(C6F5)

In a systematic study the Se(IV) and Te(IV) dihalides F2E(CH3)(C6F5), Cl2E(CH3)(C6F5) and Br2E(CH3)(C6F5) (E = Se, Te) have been synthesized and their crystal and molecular structures been investigated by X-ray diffraction and computational methods. The solid-state structures of all compounds show significant correlations between the lengths of the E–C1 bond and the intermolecular E···X (X = F, Cl and Br) contacts, indicating the presence of σ-hole interactions. For comparison, the crystal and gas phase (electron diffraction) structures of Se(CH3)(C6F5) are presented as well. They show very similar structural parameters in both phases. The structures of the single molecules X2E(CH3)(C6F5) have been analyzed by quantum-chemical methods in terms of their surface potentials. They show significant similarities of their molecular electrostatic-potential topologies (V s,max). The magnitude of V s,max correlates with the aggregation pattern.

Xenon Derivatives as Aerogen Bond-Donating Catalysts for Organic Transformations: A Theoretical Study on the Metaphorical "Spherical Cow in a Vacuum" Provides Insights into Noncovalent Organocatalysis

Computations indicate that cationic and noncharged xenon derivatives should exhibit higher catalytic activity than their iodine-based noncovalent organocatalytic congeners. Perfluorophenyl xenonium(II) is expected to demonstrate the best balance between catalytic activity and chemical stability for use in organocatalysis. Comparing its catalytic activity with that of isoelectronic perfluoroiodobenzene indicates that the high catalytic activity of cationic noncovalent organocatalysts is predominantly attributed to the electrostatic interactions with the reaction substrates, which cause the polarization of ligated species during the reaction progress. In contrast, the electron transfer and covalent contributions to the bonding between the catalyst and substrate have negligible effects. The dominant effect of electrostatic interactions results in a strong negative correlation between the calculated Gibbs free energies of activation for the modeled reactions and the highest potentials of the σ-holes on the central atoms of the catalysts. No such correlation is observed for noncharged catalysts.

Square Planar Pt(II) Ion as Electron Donor in Pnictogen Bonding Interactions

It has been proposed that late transition metals with low coordination numbers (square planar or linear) can act as nucleophiles and participate in σ-hole interactions as electron donors. This is due to the existence, in this type of metal complexes, of a pair of electrons located at high energy d-orbitals (dz2 or dx2-y2), which are adequate for interacting with antibonding σ-orbitals [σ*(X–Y)] where Y is usually an electron withdrawing element and X an element of the p-block. This type of d[M]→σ*(X–Y) interaction has been reported for metals of groups 9–11 in oxidation states +1 and +2 (d8 and d10) as electron donors and σ-holes located in halogen and chalcogen atoms as electron acceptors. To our knowledge, it has not been described for σ-holes located in pnictogen atoms. In this manuscript, evidence for the existence of pnictogen bonding involving the square planar Pt(II) metal as the electron donor and Sb as the electron acceptor is provided by using an X-ray structure retrieved from the Cambridge Structural Database (CSD) and theoretical calculations. In particular, the quantum theory of atoms in molecules (QTAIM), the noncovalent interaction plot (NCIPlot) and molecular electrostatic potential (MEP) methods were used. Moreover, to further confirm the nature of the Sb···Pt(II) contact, a recently developed method was used where the electron density (ED) and electrostatic potential (ESP) distribution were compared along the Sb···Pt(II) bond path.

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.

Non-covalent interactions (NCIs) in π-conjugated functional materials: advances and perspectives

The design and development of functional materials with real-life applications are highly demanding. Understanding and controlling inter- and intra-molecular interactions provide opportunities to design new materials. A judicious manipulation of the molecular structure significantly alters such interactions and can boost selected properties and functions of the material. There is burgeoning evidence of the beneficial effects of non-covalent interactions (NCIs), showing that manipulating NCIs may generate functional materials with a wide variety of physical properties leading to applications in catalysis, drug delivery, crystal engineering, etc. This prompted us to review the implications of NCIs on the molecular packing, optical properties, and applications of functional π-conjugated materials. To this end, this tutorial review will cover different types of interactions (electrostatic, π-interactions, metallophilic, etc.) and their impact on π-conjugated materials. Attempts have also been made to delineate the effects of weak interactions on opto-electronic (O-E) applications.

Selenoxides as Excellent Chalcogen Bond Donors: Effect of Metal Coordination

The chalcogen bond has been recently defined by the IUPAC as the attractive noncovalent interaction between any element of group 16 acting as an electrophile and any atom (or group of atoms) acting as a nucleophile. Commonly used chalcogen bond donor molecules are divalent selenium and tellurium derivatives that exhibit two σ-holes. In fact, the presence of two σ-hole confers to the chalcogen bonding additional possibilities with respect to the halogen bond, the most abundant σ-hole interaction. In this manuscript, we demonstrate that selenoxides are good candidates to be used as σ-hole donor molecules. Such molecules have not been analyzed before as chalcogen bond donors, as far as our knowledge extends. The σ-hole opposite to the Se=O bond is adequate for establishing strong and directional ChBs, as demonstrated herein using the Cambridge structural database (CSD) and density functional theory (DFT) calculations. Moreover, the effect of the metal coordination of the selenoxide to transition metals on the strength of the ChB interaction has been analyzed theoretically. The existence of the ChBs has been further supported by the quantum theory of atoms in molecules (QTAIM) and the noncovalent interaction plot (NCIPlot).

Multiphotocatalyst Cascades: From Furans to Fused Butyrolactones and Substituted Cyclopentanones

High value oxygenated polycycles have been rapidly and efficiently accessed from simple precursors in one pot processes. The reported methodology relies on a new and mild method for butenolide synthesis mediated by thiols. The initial photooxygenation and butenolide synthesis have been merged with subsequent photoredox reactions to achieve rare dual-photocatalyst cascades affording various fused butyrolactones. Ground state Lewis acid activity for methylene blue has been unveiled and then exploited in the synthesis of substituted cyclopentanones.

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.

Probing the Directionality of S···O/N Chalcogen Bond and Its Interplay with Weak C-H···O/N/S Hydrogen Bond Using Molecular Electrostatic Potential

The directionality of the chalcogen bond (Ch-bond) formed by S and its interplay with other weak interactions have important chemical and biological implications. Here, dimers made of CH₃-S-X and O/N containing nucleophiles are studied and found to be stabilized by coexisting S···O/N and C-H···O/N interactions. Based on experimentally accessible electron density and molecular electrostatic potentials (MESPs), we showed that reciprocity between S···O/N and C-H···O/N interactions in the stability of cumulative molecular interaction (ΔE) was dependent on the strength of the σ-hole on S (V_s,max). Direct correlation between ΔE of dimers with V_s,max of S supports the electrostatic nature of the Ch-bond. Such interplay of the Ch-bond is necessary for its directionality in complex nucleophiles (carbonyl groups) with multiple electron-rich centers, which is explained using MESP. A correlation between the MESP minima in the π-region and the strength of the S-π interaction explains the directional selectivity of the Ch-bond.

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).

Halonium, chalconium, and pnictonium salts as noncovalent organocatalysts: a computational study on relative catalytic activity

This theoretical study sheds light on the relative catalytic activity of pnictonium, chalconium, and halonium salts in reactions involving elimination of chloride and electrophilic activation of a carbonyl group. DFT calculations indicate that for cationic aromatic onium salts, values of the electrostatic potential on heteroatom σ-holes gradually increase from pnictogen- to halogen-containing species. The higher values of the potential on the halogen atoms of halonium salts result in the overall higher catalytic activity of these species, but in the case of pnictonium and chalconium cations, weak interactions from the side groups provide an additional stabilization effect on the reaction transition states. Based upon quantum-chemical calculations, the catalytic activity of phosphonium(V) and arsenonium(V) salts is expected to be too low to obtain effective noncovalent organocatalytic compounds, whereas stibonium(V), telluronium(IV) and iodonium(III) salts exhibit higher potential in application as noncovalent organocatalysts.

CBr4 catalyzed activation of α,β-unsaturated ketones

We have shown here that weak interactions such as halogen bonding (XB) can be used to activate the carbonyl group of α,β-unsaturated ketones. Carbon tetrabromide (CBr₄) has been used as the sole reagent for the selective synthesis of flavanones and aza-flavanones from the corresponding 2'-hydroxy- and 2'-aminochalcones under metal-free and additive-free conditions. DFT calculations support the catalytic role of XB between the oxygen of chalcones and CBr₄ in these reactions.

Catalytic Activation of Imines by Chalcogen Bond Donors in a Povarov [4+2] Cycloaddition Reaction

Recently, chalcogen bonding has been investigated in more detail in organocatalysis and the scope of activated functionalities continues to increase. Herein, the activation of imines in a Povarov [4+2] cycloaddition reaction with bidentate cationic chalcogen bond donors is presented. Tellurium‐based Lewis acids show superior properties compared to selenium‐based catalysts and inactive sulfur‐based analogues. The catalytic activity of the chalcogen bonding donors increases with weaker binding anions. Triflate, however, is not suitable due to its participation in the catalytic pathway. A solvent screening revealed a more efficient activation in less polar solvents and a pronounced effect of solvent (and catalyst) on endo : exo diastereomeric ratio. Finally, new chiral chalcogen bonding catalysts were applied but provided only racemic mixtures of the product.

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.

Noncovalent interactions between benzochalcogenadiazoles and nitrogen bases

A theoretical study has been carried out on the intermolecular interactions between tetrafluoro-benzochalcogenadiazoles (chalcogen = S, Se, Te) and a series of nitrogen bases (FCN, ClCN, NP, trans-N₂H₂, pyridine, pyrazole, imidazole) at the B97-D3/def2-TZVP level, to obtain a better insight into the nature and strength of Ch···N chalcogen bond and secondary interaction in the binary and 1:2 ternary complexes. The dispersion force plays a prominent role on the stability of the sulfur complexes, and the electrostatic effect enhanced for the heavier chalcogen complexes. Most of intermolecular bonds display the characters of closed-shell and noncovalent interaction. For the complexes involving pyridine and imidazole, chalcogen bond is stronger than hydrogen bond, while the strength of chalcogen bond is equivalent to the secondary interaction for other complexes. With the addition of nitrogen base in the 1:2 complexes, chalcogen bond is weakened, while the secondary interaction remains unchanged. In the 1:2 complexes formed by pyridine and imidazole, stronger chalcogen bond results in larger negative cooperativity than that of other complexes.

Stereoselective Processes Based on σ-Hole Interactions

The σ-hole interaction represents a noncovalent interaction between atoms with σ-hole(s) on their surface (such as halogens and chalcogens) and negative sites. Over the last decade, significant developments have emerged in applications where the σ-hole interaction was demonstrated to play a key role in the control over chirality. The aim of this review is to give a comprehensive overview of the current advancements in the use of σ-hole interactions in stereoselective processes, such as formation of chiral supramolecular assemblies, separation of enantiomers, enantioselective complexation and asymmetric catalysis.

Sulfonium and Selenonium Salts as Noncovalent Organocatalysts for the Multicomponent Groebke-Blackburn-Bienaymé Reaction

Sulfonium and selenonium salts, represented by S-aryl dibenzothiophenium and Se-aryl dibenzoselenophenium triflates, were found to exhibit remarkable catalytic activity in the model Groebke-Blackburn-Bienaymé reaction. Kinetic analysis and density functional theory (DFT) calculations indicated that their catalytic effect is induced by the ligation of the reaction substrates to the σ-holes on the S or Se atom of the cations. The experimental data indicated that although 10-fold excess of the chloride totally inhibits the catalytic activity of the sulfonium salts, the selenonium salt remains catalytically active, which can be explained by the experimentally found lower binding constant of the selenonium derivative to chloride in comparison with the sulfonium analogue. Both types of salts exhibit lower catalytic activity in the model reaction than dibenziodolium species.

Two-photon absorption of 28-hetero-2,7-naphthiporphyrins: expanded carbaporphyrinoid macrocycles

The one- and two-photon absorption (1PA and 2PA) properties of three expanded aceneporphyrinoids, 28-thia-, 28-selena- and 28-tellura-2,7-naphthiporphyrin, have been studied. The open-aperture Z-scan technique was used to determine two-photon absorption cross-sections in the near infrared range using an amplified femtosecond laser system. The maximum values of the cross sections were found to be 99, 200 and 650 GM at 900 nm and 1, 13 and 31 GM at 1400 nm for the three investigated compounds, respectively. These results demonstrate enhanced 2PA properties compared with well-known porphyrin photosensitizers, such as Foscan®, showing the potential of porphyrin core modification for optimizing infrared nonlinear absorbers.

The one- and two-photon absorption (1PA and 2PA) properties of three expanded aceneporphyrinoids, 28-thia-, 28-selena- and 28-tellura-2,7-naphthiporphyrin, have been studied.