Catalysis with Pnictogen, Chalcogen, and Halogen Bonds

Facile Access to Organostibines via Selective Organic Superbase Catalyzed Antimony-Carbon Protonolysis

The selective formation of antimony-carbon bonds via organic superbase catalysis under metal- and salt-free conditions is reported. This novel approach utilizes electron-deficient stibine, Sb(C6F5)3, to give upon base-catalyzed reactions with weakly acidic aromatic and heteroaromatic hydrocarbons access to a range of new aromatic and heteroaromatic stibines, respectively, with loss of C6HF5. Also, the significantly less electron-deficient stibines, Ph2SbC6F5 and PhSb(C6F5)2 smoothly underwent base-catalyzed exchange reactions with a range of terminal alkynes to generate the stibines of formulae PhSb(C≡CPh)2, and Ph2SbC≡CR [R=C6H5, C6H4-NO2, COOEt, CH2Cl, CH2NEt2, CH2OSiMe3, Sb(C6H5)2], respectively. These formal substitution reactions proceed with high selectivity as only the C6F5 groups serve as a leaving group to be liberated as C6HF5 upon formal proton transfer from the alkyne. Kinetic studies of the base-catalyzed reaction of Ph2SbC6F5 with phenyl acetylene to form Ph2SbC≡CPh and C6HF5 suggested the empirical rate law to exhibit a first-order dependence with respect to the base catalyst, alkyne and stibine. DFT calculations support a pathway proceeding via a concerted σ-bond metathesis transition state, where the base catalyst activates the Sb-C6F5 bond sequence through secondary bond interactions.

Telluronium-Catalyzed Halogenation Reactions: Chalcogen-Bond Activation of N-Halosuccinimides and Catalysis

The ability of triaryltelluronium salts to interact with N-halosuccinimides (NXS) through chalcogen bonding (ChB) in the solid state and in solution is demonstrated herein. Cocrystals of the triaryltelluronium bearing two CF3 electron-withdrawing groups per aryl ring with N-chloro-, N-bromo- and N-iodosuccinimide (respectively NCS, NBS and NIS) were analyzed by X-ray diffraction, evidencing a ChB between tellurium and the carbonyl group of NXS. This ChB was confirmed in solution by NMR spectroscopy, especially by 125Te NMR titration experiment, which allowed the determination of the association constant (Ka) between the telluronium and NBS. The so-obtained Ka value of 17.3±0.6 M-1 indicated a moderate interaction in solution because of the competitive role of the solvent. The strength of the Te⋅⋅⋅O ChB was however sufficient enough to promote the catalytic halofunctionalization of aromatics and of alkenes such as the intra- and intermolecular haloalkoxylation and haloesterification of alkenes.

A geminal antimony(iii)/phosphorus(iii) frustrated Lewis pair

The geminal Lewis pair (F5C2)2SbCH2P(tBu)2 (1) was prepared by reacting (F5C2)2SbCl with LiCH2P(tBu)2. Despite its extremely electronegative pentafluoroethyl substituents, the neutral 1 exhibits a relatively soft acidic antimony function according to the HSAB concept (hard-soft acid-base). These properties lead to a reversibility in the binding of CS2 to 1, as observed by VT-NMR spectroscopy, while no reaction with CO2 is observed. The reaction behaviour towards heterocumulenes and the specific interaction situation in the CS2 adduct were analysed by quantum chemical calculations. The FLP-type reactivity of 1 has also been demonstrated by reaction with a variety of small molecules (SO2, PhNCO, PhNCS, (MePh2P)AuCl). The reactions of 1 with PhNCO and PhNCS led to different types of cyclic addition products: PhNCO adds with its N[double bond, length as m-dash]C bond and PhNCS adds preferentially with its C[double bond, length as m-dash]S bond. The reaction of 1 with (MePh2P)AuCl gave an adduct {[(F5C2)2SbCH2(tBu)2P]2Au}+ with a clamp-like structure binding a chloride anion by its two antimony atoms in chelate mode. Compound 1 and its adducts have been characterised by X-ray diffraction experiments, multinuclear NMR spectroscopy, elemental analyses and computational calculations (DFT, QTAIM, IQA).

Halogen, Chalcogen, Pnictogen, and Tetrel Bonding in Non-Covalent Organocatalysis: An Update

The use of noncovalent interactions based on electrophilic halogen, chalcogen, pnictogen, or tetrel centers in organocatalysis has gained noticeable attention. Herein, we provide an overview on the most important developments in the last years with a clear focus on experimental studies and on catalysts which act via such non-transient interactions.

Intermolecular Metal-Involving Pnictogen Bonding: The Case of σ-(SbIII)-Hole···dz2[PtII] Interaction

Cocrystallizations of trans-[PtX'2(NCNR2)2] (R2 = Me2, X' = Cl 1a, Br 1b, I 1c; R2 = (CH2)5, X' = I 2c) with SbX3 (X = Cl, Br, I) gave 1:2 cocrystals 1a·2SbCl3, 1b·2SbBr3, 1c·2SbCl3, 1c·2SbBr3, 1c·2SbI3, and 2c·2SbI3. In all six X-ray structures, the association of the molecular coformers is achieved mainly by SbIII···dz2[PtII] metal-involving intermolecular pnictogen bonding. Density functional theory (DFT) calculations (based on experimentally determined geometries) using both gas-phase and solid-state approximations revealed that a σ-(Sb)-hole interacts with an area of negative potential associated with the dz2-orbital of the positively charged platinum(II) sites, thus forming a pnictogen bond whose energy falls in the range between -7.3 and -16.9 kcal/mol.

Polydentate chalcogen bonding: anion trapping with a water-stable host compound carrying Se-CF3 functions

Bidentate and tetradentate chalcogen bonding host systems with SeCF3 functions as σ-hole donors in close proximity at the alkyne functions of 1,8-diethynylanthracene and its syn-dimer were prepared in quantitative yield by tin-selenium exchange reactions of the corresponding trimethylstannyl precursors with ClSeCF3. The bidentate system shows chalcogen bonding interactions with THF, but does not bind halide ions. The tetradentate system cooperatively chelates chloride, bromide and iodide ions with its four CC-SeCF3 units by rotating the four σ-holes towards the halide ion. The structures of these halide ion adducts were determined by X-ray diffraction. The hydrobromide and -iodide salts of the ethyl derivative of Schwesinger's phosphazene superbase served as halide salts with very weakly coordinating cations.

Oxidation-dependent Lewis acidity in chalcogen adducts of Sb/P frustrated Lewis pairs

The reactions of the frustrated Lewis pair (F5C2)2SbCH2P(tBu)2 with oxygen, sulphur, selenium and tellurium led to the mono-oxidation products (F5C2)2SbCH2P(E)(tBu)2 (E = O, S, Se, Te). Further oxidation of these chalcogen adducts with tetrachloro-ortho-benzoquinone (o-chloranil) gave (F5C2)2Sb(CH2)(μ-E)P(tBu)2·CatCl (CatCl = o-O2C6Cl4) with a central four-membered ring heterocycle for E = O, S, and Se. For E = Te the elimination of elemental tellurium led to an oxidation product with two equivalents of o-chloranil, (F5C2)2SbCH2P(tBu)2·2CatCl, which is also accessible by reaction of (F5C2)2SbCH2P(tBu)2 with o-chloranil. The synthesised compounds were characterised by NMR spectroscopy and X-ray structure analyses, and the structural properties were analysed in the light of the altered Lewis acidity due to the oxidation of the antimony atoms.

Pressure Induces Six-fold Coordination for the Lighter Pnictides Phosphorus and Arsenic Triiodide

In this study, we employ an evolutionary algorithm in conjunction with first-principles density functional theory (DFT) calculations to comprehensively investigate the structural transitions, electronic properties, and chemical bonding behaviors of XI3 compounds, where X denotes phosphorus (P) and arsenic (As), across a range of elevated pressures. Our computational analyses reveal a distinctive phenomenon occurring under compression, wherein the initially trigonal structures of PI3 (P 63) and AsI3 (R-3) undergo an intriguing transformation, leading to the emergence of six-coordinated monoclinic phases (C2/m) at 6 GPa and 2 GPa for PI3 and AsI3, respectively. These high-pressure phases exhibit their stability up to 10 GPa for PI3 and 12 GPa for AsI3. Notably, the resulting structures at elevated pressures bear striking resemblance to the widely recognized six-coordinated octahedral BiI3 crystal configuration observed at ambient conditions. Our investigation further underscores the pivotal role of pressure-induced reactivity of the lone-pair electrons in PI3 and AsI3, facilitating their enhanced stereochemical reactivity and thereby enabling higher six-fold coordination. Complementary analyses employing electron localization function (ELF) and density of states (DOS) effectively delineate the progression towards augmented coordination in PI3 and AsI3 with increasing pressure. While the phenomenon of heightened coordination is conventionally associated with heavier pnictide iodides such as SbI3 and BiI3 under ambient conditions due to heightened ionic character and relativistic effects in bismuth (Bi) and antimony (Sb), our findings accentuate that analogous structural transformations can also be induced in lighter elements like phosphorus (P) and arsenic (As) under the influence of pressure.

Phenotellurazine redox catalysts: elements of design for radical cross-dehydrogenative coupling reactions

Redox active phenotellurazine catalysts have been recently utilized in two different cross-dehydrogenative coupling reactions. In this study, we revisit the design of the phenotellurazine redox catalysts. In particular, we investigate the level of cooperativity between the Te- and N-centers, the effect of secondary versus tertiary N-centers, the effect of heterocyclic versus non-heterocyclic structures, and the effect of substitution patterns on the redox catalytic activity.

Pnictogen bonding in imide derivatives for chiral folding and self-assembly

Pnictogen bonding (PnB) is an attraction interaction that originates from the anisotropic distribution of electron density of pnictogen elements, which however has been rarely found in nitrogen atoms. In this work, for the first time, we unveil the general presence of N-involved PnB in aromatic or aliphatic imide groups and reveal its implications in chiral self-assembly of folding. This long-neglected interaction was consolidated by Cambridge structural database (CSD) searching as well as subsequent computational studies. Though the presence of PnB has limited effects on spectroscopic properties in the solution phase, conformation locking effects are sufficiently expressed in the chiral folding and self-assembly behavior. PnB anchors the chiral conformation to control the emergence and inversion of chiroptical signals, while intramolecular PnB induces the formation of supramolecular tilt chirality. It also enables the chiral folding of imide-containing amino acid or peptide derivatives, which induces the formation of unique secondary structural sequences such as β-sheets. Finally, the effects of PnB in directing folded helical structures were revealed. Examples of cysteine and cystine derivatives containing multiple N⋯O and N⋯S PnBs constitute an α-helix like secondary structure with characteristic circular dichroism. This work discloses the comprehensive existence of imide-involved PnB, illustrates its important role in folding and self-assembly, and sheds light on the rational fabrication of conformation-locked compounds and polymers with controllable chiroptical activities.

Investigating Recurrent Matere Bonds in Pertechnetate Compounds

In this manuscript we evaluate the X-ray structure of five new pertechnetate derivatives of general formula [M(H2O)4(TcO4)2], M=Mg, Co, Ni, Cu, Zn (compounds 1-5) and one perrhenate compound Zn(H2O)4(ReO4)2 (6). In these complexes the metal center exhibits an octahedral coordination with the pertechnetate units as axial ligands. All compounds exhibit the formation of directional Tc⋅⋅⋅O Matere bonds (MaBs) that propagate the [M(H2O)4(TcO4)2], into 1D supramolecular polymers in the solid state. Such 1D polymers are linked, generating 2D layers, by combining additional MaBs and hydrogen bonds (HBs). Such concurrent motifs have been analyzed theoretically, suggesting the noncovalent σ-hole nature of the MaBs. The interaction energies range from weak (~ -2 kcal/mol) for the MaBs to strong (~ -30 kcal/mol) for the MaB+HB assemblies, where HB dominates. In case of M=Zn, the corresponding perrhenate Zn(H2O)4(ReO4)2 complex, has been also synthesized for comparison purposes, resulting in the formation of an isostructural X-ray structure, corroborating the structure-directing role of Matere bonds.

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.

Tunable Pnictogen Bonding at the Service of Hydroxide Transport across Phospholipid Bilayers

Our growing interest in the design of pnictogen-based strategies for anion transport has prompted an investigation into the properties of three simple triarylcatecholatostiboranes (1-3) of the general formula (o-C6Cl4O2)SbAr3 with Ar = Ph (1), o-tolyl (2), and o-xylyl (3) for the complexation and transport of hydroxide across phospholipid bilayers. A modified hydroxypyrene-1,3,6-trisulfonic acid (HPTS) assay carried out in artificial liposomes shows that 1 and 2 are potent hydroxide transporters while 3 is inactive. These results indicate that the steric hindrance imposed by the three o-xylyl groups prevents access by the hydroxide anion to the antimony center. Supporting this interpretation, 1 and 2 quickly react with TBAOH·30 H2O ([TBA]+ = [nBu4N]+) to form the corresponding hydroxoantimonate salts [nBu4N][1-OH] and [nBu4N][2-OH], whereas 3 resists hydroxide coordination and remains unperturbed. Moreover, the hydroxide transport activities of 1 and 2 are correlated to the +V oxidation state of the antimony atom as the parent trivalent stibines show no hydroxide transport activity.

Green and sustainable approaches for the Friedel-Crafts reaction between aldehydes and indoles

The synthesis of indoles and their derivatives, more specifically bis(indolyl)methanes (BIMs), has been an area of great interest in organic chemistry, since these compounds exhibit a range of interesting biological and pharmacological properties. BIMs are naturally found in cruciferous vegetables and have been shown to be effective antifungal, antibacterial, anti-inflammatory, and even anticancer agents. Traditionally, the synthesis of BIMs has been achieved upon the acidic condensation of an aldehyde with indole, utilizing a variety of protic or Lewis acids. However, due to the increased environmental awareness of our society, the focus has shifted towards the development of greener synthetic technologies, like photocatalysis, organocatalysis, the use of nanocatalysts, microwave irradiation, ball milling, continuous flow, and many more. Thus, in this review, we summarize the medicinal properties of BIMs and the developed BIM synthetic protocols, utilizing the reaction between aldehydes with indoles, while focusing on the more environmentally friendly methods developed over the years.

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.

Chalcogen bonding catalysis

Catalysts play a major role in chemical synthesis, and catalysis is considered to be a green and economic process. Catalysis is dominated by covalent interactions between the catalyst and substrate. The design of non-covalent catalysts came into limelight only recently. Hydrogen bonding (HB) catalysts are well established among non-covalent catalysts, including asymmetric HB catalysts. Though halogen bonding (XB) catalysis and its asymmetric version are gaining admiration, non-covalent chalcogen bonding catalysis (ChB) is in the budding stage. This tutorial review will focus on the recently evolved chalcogen bonding catalysis and emphasis will be given to the chalcogen bonding of chiral molecules. Since successful enantioselective chalcogen bonding catalysis is yet to be reported, this review will focus on the basics of non-covalent bonding catalysis, chalcogen bonding catalysis, chiral chalcogenide synthesis, rigidification of transition states by ChB, stabilization of cations by chiral chalcogens, details of unsuccessful asymmetric chalcogen bonding catalysis, enantioseparation of racemic molecules using ChB, and the existence of ChB in chiral biomolecules.

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.

Controlled and Regioselective Ring-Opening Polymerization for Poly(disulfide)s by Anion-Binding Catalysis

Poly(disulfide)s are an emerging class of sulfur-containing polymers with applications in medicine, energy, and functional materials. However, the constituent dynamic covalent S-S bond is highly reactive in the presence of the sulfide (RS-) anion, imposing a persistent challenge to control the polymerization. Here, we report an anion-binding approach to arrest the high reactivity of the RS- chain end to control the synthesis of linear poly(disulfide)s, realizing a rapid, living ring-opening polymerization of 1,2-dithiolanes with narrow dispersity and high regioselectivity (Mw/Mn ∼ 1.1, Ps ∼ 0.85). Mechanistic studies support the formation of a thiourea-base-sulfide ternary complex as the catalytically active species during the chain propagation. Theoretical analyses reveal a synergistic catalytic model where the catalyst preorganizes the protonated base and anionic chain end to establish spatial confinement over the bound monomer, effecting the observed regioselectivity. The catalytic system is amenable to monomers with various functional groups, and semicrystalline polymers are also obtained from lipoic acid derivatives by enhancing the regioselectivity.

Chalcogen Bond Catalysis with Telluronium Cations for Bromination Reaction: Importance of Electrostatic and Polarization Effects

Recently, chalcogen bond catalysts with telluronium cations have garnered considerable attention in organic reactions. In this work, chalcogen bond catalysis on the bromination reaction of anisole with N-bromosuccinimide (NBS) with the telluronium cationic catalysts has been explored with density functional theory (DFT). The catalytic reaction is divided into two stages: the bromine transfer step and the proton transfer step. Based on the computational results, one can find the rate-determining step is the bromine transfer step. Moreover, the present study elucidates that a stronger chalcogen bond between catalysts and NBS will give better catalytic performance. Additionally, this work also clarified the importance of the electrostatic and polarization effects in the chalcogen bond between the oxygen atom of NBS and the Te atom of the catalyst in this bromination reaction. The electrostatic and polarization effects are significantly influenced by the electron-withdrawing ability of the substitution groups on the catalysts. Moreover, the structure-property relationship between the strength of chalcogen bond, electrostatic effect, polarization effect and catalytic performance are established for the design of more efficient chalcogen bond catalysts.

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.

Halogen bonding-driven chiral amplification of a bimetallic gold-copper cluster through hierarchical assembly

Understanding the fundamentals and applications of chirality relies substantially on the amplification of chirality through hierarchical assemblies involving various weak interactions. However, a notable challenge remains for metal clusters chiral assembly driven by halogen bonding, despite their promising applications in lighting, catalysis, and biomedicine. Here, we used halogen bonding-driven assembly to achieve a hierarchical degree of achiral emissive Au2Cu2 clusters. From single crystals to one-dimensional ribbons and then to helixes, the morphologies were primarily modulated by intermolecular halogen bonding that evoked by achiral or/and chiral iodofluorobenzene (IFBs) molecules. Concomitantly, the luminescence and circularly polarized luminescence (CPL) changed a lot, ultimately leading to a substantial increase in the luminescence dissymmetry g-factor (glum) of 0.036 in the supramolecular helix. This work opens an avenue for hierarchical assemblies using predesigned metal clusters as building blocks though directional halogen bonding. This achievement marks a noteworthy advancement in the field of nanosized inorganic functional blocks.

N-Coordinated tellurenium(II) and telluronium(IV) cations: synthesis, structure and hydrolysis

A set of N-coordinated tellurium(II) compounds containing either C,N-chelating ligands CNR (where CN = 2-(RNCH)C6H4, R = tBu or Dipp; Dipp = 2,6-iPr2C6H3) or N,C,N pincer ligands NCNR (where NCN = 2,6-(RNCH)2C6H4, R = tBu or Dipp) were synthesized. In the case of C,N-chelated compounds, the reaction of CNDippLi with Te(dtc)2 (where dtc = Et2NCS2) in a 1 : 1 molar ratio smoothly provided the carbamate CNDippTe(dtc) which upon treatment with 2 eq. of HCl provided the chloride CNDippTeCl. In contrast, the analogous conversion of NCNRLi with Te(dtc)2 surprisingly furnished ionic bromides [NCNRTe]Br as a result of the exchange of dtc by Br coming from nBuBr present in the reaction mixture. Furthermore, the reaction of CNDippTeCl or [NCNRTe]Br with silver salts AgX (X = OTf or SbF6) provided the expected tellurenium cations [CNDippTe]SbF6 and [NCNRTe]X. To further increase the Lewis acidity of the central atom, the oxidation of selected compounds with 1 eq. of SO2Cl2 was examined yielding stable compounds [CNtBuTeCl2]X and [NCNtBuTeCl2]X. The oxidation of the Dipp substituted compounds proved to be more challenging and an excess of SO2Cl2 was necessary to obtain the oxidized products [CNDippTeCl2]SbF6 and [NCNDippTeCl2]SbF6, which could solely be characterized in solution. Compounds [CNtBuTeCl2]OTf and [NCNtBuTeCl2]OTf were shown to undergo a controlled hydrolysis to the corresponding telluroxanes. All compounds were studied by multinuclear NMR spectroscopy in solution and for selected compounds solid state 125Te NMR spectroscopy and single-crystal X-ray diffraction analysis were performed. The Lewis acidity of the studied cations was examined by the Gutmann-Beckett method using Et3PO as the probing agent. The Te-N chalcogen bonding situation of selected compounds has also been examined computationally by a set of real-space bonding indicators.

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.

Utilizing bis(imino)dihydroacridanide pincer ligands in p-block chemistry: synthesis and catalysis of an antimony monocation salt

We present the synthesis and characterization of an Sb(III) monocation salt stabilized by a bulky bis(imino)dihydroacridanide pincer ligand. The Lewis acidity of the Sb cation is quantified using the Guttmann-Beckett method and confirmed by its reaction with 4-dimethylaminopyridine, which forms a Lewis acid-base adduct. This Sb cation exhibits catalytic activity in the cyanosilylation of arylketones. The electronic structure of the Sb cation as well as the mechanism of the catalytic transformation are explored by density functional theory computations.

Cross-Electrophile C-PIII Coupling of Chlorophosphines with Organic Halides: Photoinduced PIII and Aminoalkyl Radical Generation Enabled by Pnictogen Bonding

Pnictogen bonding (PnB) has gained recognition as an appealing strategy for constructing novel architectures and unlocking new properties. Within the synthetic community, the development of a straightforward and much simpler protocol for cross-electrophile C-PIII coupling remains an ongoing challenge with organic halides. In this study, we present a simple strategy for photoinduced PnB-enabled cross-electrophile C-PIII couplings using readily available chlorophosphines and organic halides via merging single electron transfer (SET) and halogen atom transfer (XAT) processes. In this photomediated transformation, the PnB formed between chlorophosphines and alkyl amines facilitates the photogeneration of PIII radicals and α-aminoalkyl radicals through SET. Subsequently, the resulting α-aminoalkyl radicals activate C-X bonds via XAT, leading to the formation of carbon radicals. This methodology offers operational simplicity and compatibility with both aliphatic and aromatic chlorophosphines and organic halides.

Chalcogen Bonding Catalysis: Tellurium, the Last Frontier?

Chalcogen bonding (ChB) is the non-covalent interaction occurring between chalcogen atoms as Lewis acid sites and atoms or groups of atoms able to behave as Lewis bases through their lone pair or π electrons. Analogously to its sister halogen bonding, the high directionality of this interaction was implemented for precise structural organizations in the solid state and in solution. Regarding catalysis, ChB is now accepted as a new mode of activation as demonstrated by the increased number of examples in the last five years. In the family of ChB catalysts, those based on tellurium rapidly appeared to overcome their lighter sulfur and selenium counterparts. In this review, we highlight the Lewis acid properties of tellurium-based derivatives in solution and summarize the start-of-the-art of their applications in catalysis.

Binding, Sensing, And Transporting Anions with Pnictogen Bonds: The Case of Organoantimony Lewis Acids

Motivated by the discovery of main group Lewis acids that could compete or possibly outperform the ubiquitous organoboranes, several groups, including ours, have engaged in the chemistry of Lewis acidic organoantimony compounds as new platforms for anion capture, sensing, and transport. Principal to this approach are the intrinsically elevated Lewis acidic properties of antimony, which greatly favor the addition of halide anions to this group 15 element. The introduction of organic substituents to the antimony center and its oxidation from the + III to the + V state provide for tunable Lewis acidity and a breadth of applications in supramolecular chemistry and catalysis. The performances of these antimony-based Lewis acids in the domain of anion sensing in aqueous media illustrate the favorable attributes of antimony as a central element. At the same time, recent advances in anion binding catalysis and anion transport across phospholipid membranes speak to the numerous opportunities that lie ahead in the chemistry of these unique main group compounds.

2-Benzamide Tellurenyl Iodides: Synthesis and Their Catalytic Role in CO2 Mitigation

Benzamide-derived organochalcogens (chalcogen=S, Se, and Te) have shown promising interest in biological and synthetic chemistry. Ebselen molecule derived from benzamide moiety is the most studied organoselenium. However, its heavier congener organotellurium is under-explored. Here, an efficient copper-catalyzed atom economical synthetic method has been developed to synthesize 2-phenyl-benzamide tellurenyl iodides by inserting a tellurium atom into carbon-iodine bond of 2-iodobenzamides in one pot with 78-95 % yields. Further, the Lewis acidic nature of Te center and Lewis basic nature of nitrogen of the synthesized 2-Iodo-N-(quinolin-8-yl)benzamide tellurenyl iodides enabled them as pre-catalyst for the activation of epoxide with CO2 at 1 atm for the preparation of cyclic carbonates with TOF and TON values of 1447 h-1 and 4343, respectively, under solvent-free conditions. In addition, 2-iodo-N-(quinolin-8-yl)benzamide tellurenyl iodides have also been used as pre-catalyst for activating anilines and CO2 to form a variety of 1,3-diaryl ureas up to 95 % yield. The mechanistic investigation for CO2 mitigation is done by 125 Te NMR and HRMS studies. It seems that the reaction proceeds via formation of catalytically active Te-N heterocycle, an ebtellur intermediate which is isolated and structurally characterized.

Halogen Bonding Involving Isomeric Isocyanide/Nitrile Groups

2,3,5,6-Tetramethyl-1,4-diisocyanobenzene (1), 1,4-diisocyanobenzene (2), and 1,4-dicyanobenzene (3) were co-crystallized with 1,3,5-triiodotrifluorobenzene (1,3,5-FIB) to give three cocrystals, 1·1,3,5-FIB, 2·2(1,3,5-FIB), and 3·2(1,3,5-FIB), which were studied by X-ray diffraction. A common feature of the three structures is the presence of I···Cisocyanide or I···Nnitrile halogen bonds (HaBs), which occurs between an iodine σ-hole and the isocyanide C-(or the nitrile N-) atom. The diisocyanide and dinitrile cocrystals 2·2(1,3,5-FIB) and 3·2(1,3,5-FIB) are isostructural, thus providing a basis for accurate comparison of the two types of noncovalent linkages of C≡N/N≡C groups in the composition of structurally similar entities and in one crystal environment. The bonding situation was studied by a set of theoretical methods. Diisocyanides are more nucleophilic than the dinitrile and they exhibit stronger binding to 1,3,5-FIB. In all structures, the HaBs are mostly determined by the electrostatic interactions, but the dispersion and induction components also provide a noticeable contribution and make the HaBs attractive. Charge transfer has a small contribution (<5%) to the HaB and it is higher for the diisocyanide than for the dinitrile systems. At the same time, diisocyanide and dinitrile structures exhibit typical electron-donor and π-acceptor properties in relation to the HaB donor.

N-Heterocyclic Nitrenium-Catalyzed Photohomolysis of CF3SO2Cl for Alkene Trifluoromethylation

N-Heterocyclic nitreniums (NHNs) have been utilized as Lewis acid catalysts to activate substrates with lone pairs. Alternative to their conventional applications, we have discovered that NHNs can also serve as charge transfer complex catalysts. Herein, we present another potential of NHNs by utilizing a weak interaction between NHNs and CF3SO2Cl. The method promotes CF3SO2Cl to undergo photohomolysis, resulting in the CF3 radical. Mechanistic studies suggested that the weak interaction could be due to the π-hole effect of NHNs.

Analyzing Fluoride Binding by Group 15 Lewis Acids: Pnictogen Bonding in the Pentavalent State

We report the results of a computational investigation into fluoride binding by a series of pentavalent pnictogen Lewis acids: pnictogen pentahalides (PnX5), tetraphenyl pnictogeniums (PnPh4+), and triphenyl pnictogen tetrachlorocatecholates (PnPh3Cat). Activation strain and energy decomposition analyses of the Lewis adducts not only clearly delineate the electrostatic and orbital contributions to these acid-base interactions but also highlight the importance of Pauli repulsion and molecular flexibility in determining relative Lewis acidity among the pnictogens.

Solution Studies of a Water-Stable, Trivalent Antimony Pnictogen Bonding Anion Receptor with High Binding Affinities for CN-, OCN-, and OAc. Inorg Chem 2023. [PMID: 37499143 DOI: 10.1021/acs.inorgchem.3c01887] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The solution phase anion binding behavior of a water-stable bidentate pnictogen bond donor was studied. A modest change in the visible absorption spectrum allowed for the determination of the binding constants. High binding constants were observed with cyanide, cyanate, and acetate, and these were corroborated with density functional theory (DFT) calculations. The receptor could be recovered free from the anion following treatment with methyl triflate, confirming that it remains intact. The tight binding of cyanide and water stability were exploited to use this system as a supramolecular catalyst in a phase-transfer Strecker reaction, further demonstrating the utility of pnictogen bonding as a tool in noncovalent catalysis.

New Complexes of Antimony(III) with Tridentate O,E,O-Ligands (E = O, S, Se, Te, NH, NMe) Derived from N-Methyldiethanolamine

We synthesized a series of new antimony(III) compounds by reaction of Sb(OEt)3 with organic ligands of the type E(CH2-CH2-OH)2, with E = NH, NMe, O, S, Se, and Te. The synthesized compounds have the general composition [E(CH2-CH2-O)2]Sb(OEt). For comparison, the compound (O-CH2-CH2-S)Sb(OEt) was prepared. All compounds are characterized using NMR, IR, and Raman spectroscopy. The molecular structures of the products reveal the formation of chelate complexes, wherein the ligand molecules coordinate as tridentate O,E,O-ligands to the antimony atom. Dimer formation in the solid state allows the antimony atoms to reach pentacoordination. Quantum chemical calculations including topological analysis of electron density reveal that there are polar shared bonds between antimony and the oxygen atoms bound to antimony. The interactions between the donor atom E and the Sb atom and the interactions in the dimers can be characterized as Van der Waals interactions. The reactivity of [MeN(CH2-CH2-O)2]Sb(OEt) was investigated as an example. For this purpose, the compound reacted with a range of organic compounds such as carboxylic acids and carboxylic anhydrides and small molecules like CO2 and NH3. This study establishes a new and easy accessible class of antimony(III) compounds, provides new insights into the chemistry of antimony compounds and opens up new opportunities for further research in this field.

Synthesis of functionalised isochromans: epoxides as aldehyde surrogates in hexafluoroisopropanol

The oxa-Pictet-Spengler reaction is arguably the most straightforward and modular way to construct the privileged isochroman motif, but its scope is largely limited to benzaldehyde derivatives and to electron-rich β-phenylethanols that lack substitution along the aliphatic chain. Here we describe a variant of this reaction starting from an epoxide, rather than an aldehyde, that greatly expands the scope and rate of the reaction (<1 h, 20 °C). Besides facilitating the initial Meinwald rearrangement, the use of hexafluoroisopropanol (HFIP) as a solvent expands the electrophile scope to include partners equivalent to ketones, aliphatic aldehydes, and phenylacetyl aldehydes, and the nucleophile scope to include modestly electronically deactivated and highly substituted β-phenylethanols. The products could be easily further derivatised in the same pot by subsequent ring-opening, reductions, and intra- and intermolecular Friedel-Crafts reactions, also in HFIP. Finally, owing to the high pharmacological relevance of the isochroman motif, the synthesis of drug analogues was demonstrated.

Chalcogen-Bond Catalysis: Telluronium-Catalyzed [4+2]-Cyclocondensation of (in situ Generated) Aryl Imines with Alkenes

In the chalcogen series, tellurium species exhibit the strongest chalcogen bonding (ChB) interaction with electron-rich atom. This property explains the renewed interested toward tellurium-based derivatives and their use in different applications, such as organocatalysis. In this context, the catalytic activity of telluronium salts in the Povarov reaction is presented herein. Different dienophiles, as well as imines of variable electronic nature, efficiently react in the presence of catalytic amount of either diarylmethyltelluronium or triaryltelluronium salts. Both catalysts could also readily perform the three-component Povarov reaction starting from aldehyde, aniline and dihydrofuran. The reactivity of telluroniums towards imines and aldehydes was confirmed in the solid state by the ability of Te atom to interact through ChB with the oxygen carbonyl of acetone, and in solution with significant shift variations of the imine proton and of the tellurium atom in ¹ H and ¹²⁵ Te NMR spectroscopy. For the most active telluronium catalysts bearing CF₃ groups, association constants (K) with N-phenyl phenylmethanimine in the range 22-38 M^-1 were measured in dichloromethane.

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.

Unravelling the Mechanism and Governing Factors in Lewis Acid and Non-Covalent Diels-Alder Catalysis: Different Perspectives

In the current literature, many non-covalent interaction (NCI) donors have been proposed that can potentially catalyze Diels-Alder (DA) reactions. In this study, a detailed analysis of the governing factors in Lewis acid and non-covalent catalysis of three types of DA reactions was carried out, for which we selected a set of hydrogen-, halogen-, chalcogen-, and pnictogen-bond donors. We found that the more stable the NCI donor-dienophile complex, the larger the reduction in DA activation energy. We also showed that for active catalysts, a significant part of the stabilization was caused by orbital interactions, though electrostatic interactions dominated. Traditionally, DA catalysis was attributed to improved orbital interactions between the diene and dienophile. Recently, Vermeeren and co-workers applied the activation strain model (ASM) of reactivity, combined with the Ziegler-Rauk-type energy decomposition analysis (EDA), to catalyzed DA reactions in which energy contributions for the uncatalyzed and catalyzed reaction were compared at a consistent geometry. They concluded that reduced Pauli repulsion energy, and not enhanced orbital interaction energy, was responsible for the catalysis. However, when the degree of asynchronicity of the reaction is altered to a large extent, as is the case for our studied hetero-DA reactions, the ASM should be employed with caution. We therefore proposed an alternative and complementary approach, in which EDA values for the catalyzed transition-state geometry, with the catalyst present or deleted, can be compared one to one, directly measuring the effect of the catalyst on the physical factors governing the DA catalysis. We discovered that enhanced orbital interactions are often the main driver for catalysis and that Pauli repulsion plays a varying role.

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.

Are Ar3SbCl2 Species Lewis Acidic? Exploration of the Concept and Pnictogen Bond Catalysis Using a Geometrically Constrained Example

As part of our investigations into the Lewis acidic behavior of antimony derivatives, we have decided to study the properties of 5-phenyl-5,5-dichloro-λ5-dibenzostibole (1), a dichlorostiborane with an antimony atom confined to a five-membered heterocycle. Our work shows that the resulting geometrical constraints elevate the Lewis acidity of the antimony atom, as confirmed by the crystal structure of 1-THF and the solution study of the interaction of 1 with Ph3PO. The enhanced Lewis acidic properties of 1, which exceed those of simple dichlorostiboranes such as Ph3SbCl2, also become manifest in pnictogen bonding catalysis experiments involving the reductions of imines with Hantzsch ester. The influence of geometrical constraints in the chemistry of this compound is also supported by a computational activation strain analysis as well as by an energy decomposition analysis of a model Me3PO adduct.

Interchangeability and Disorder in the Solid-State Structures of "Two Wall" Calix[4]pyrroles Equipped with Iodine and Ethynyl para-Substituents

Herein, the synthesis and X-ray structures of three α,β "two wall" aryl-extended calix[4]pyrroles having either identical (symmetrically substituted) or different (non-symmetrically substituted) meso-aryl substituents (aryl=4-ethynylphenyl and 4-iodophenyl) are reported. The X-ray structure of the co-crystal formed by the two symmetrically substituted calix[4]pyrroles is also described. In the solid state, all studied α,β-calix[4]pyrroles exhibit a 1,3-alternate conformation with two co-crystallized acetonitrile solvent molecules H-bonded to adjacent cis-pyrrole rings. Remarkably, the 1,3-conformer of the non-symmetrically substituted iodophenyl/ethynylphenyl compound is intrinsically chiral. The two enantiomers are present in the average asymmetric unit in a 65 : 35 occupancy ratio displaying a head-to-tail directional disorder. This is due to the functional complementarity and the isosteric and isoelectronic properties of the para-substituents: iodo and ethynyl. That is, the negative belt of iodine is similar to the negative π-system of the C≡C triple bond and the σ-hole in the iodine atom is similar to the positive proton at the C≡C-H group.

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.

Pnictogen-Bridged Diphenyl Sulfones as Photoinduced Pnictogen Bond Forming Emission Motifs

In this study, pnictogen (Pn)-bridged diphenyl sulfones were synthesized as motifs for photoinduced dynamic rearrangement. The newly synthesized sulfones exhibited dual fluorescence at 298 K. Density functional theory calculations revealed that the longer-wavelength fluorescence was derived from the geometries after structural relaxation through photo-driven pnictogen bond formation between the O atom lone pair of the sulfonyl moiety and the antibonding orbital of the Pn-C bond. This is the first report on emission dynamics driven by pnictogen bond formation upon photoexcitation.

Stereochemistry Controls Dihydrogen Bonding Strengths in Chiral Amine Boranes Adducts

The growing interest in exploiting novel concepts of non-covalent interactions in catalysts and supramolecular chemistry made us revisit a special kind of hydrogen bonding: the dihydrogen bond (DHB), formed between a classical hydrogen bond donor and a hydridic hydrogen as acceptor. Herein, we investigate how the strength of the N-H^δ+ ⋅⋅⋅^δ- H-B interaction and hence the DHB-driven self-aggregation of amine-borane adducts is governed by steric effects by comparing the structures and binding enthalpies of various chiral derivatives. For a diastereomeric pair of amine-boranes prepared from a chiral secondary amine, we show that the stereochemistry at the nitrogen has significant influence on the interaction enthalpy. Based on this finding, N-chiral amine boranes can be envisioned to become interesting building blocks in supramolecular chemistry to fine-tune the formation dynamics of assemblies.

Shedding Light on the Vibrational Signatures in Halogen-Bonded Graphitic Carbon Nitride Building Blocks

The relative contributions of halogen and hydrogen bonding to the interaction between graphitic carbon nitride monomers and halogen bond (XB) donors containing C-X and C≡C bonds were evaluated using computational vibrational spectroscopy. Conventional probes into select vibrational stretching frequencies can often lead to disconnected results. To elucidate this behavior, local mode analyses were performed on the XB donors and complexes identified previously at the M06-2X/aVDZ-PP level of theory. Due to coupling between low and high energy C-X vibrations, the C≡C stretch is deemed a better candidate when analyzing XB complex properties or detecting XB formation. The local force constants support this conclusion, as the C≡C values correlate much better with the σ-hole magnitude than their C-X counterparts. The intermolecular local stretching force constants were also assessed, and it was found that attractive forces other than halogen bonding play a supporting role in complex formation.

Adjusting the balance between hydrogen and chalcogen bonds

A complex is assembled which pairs a carboxyl group of X₁COOH with a 1,2,5-chalcogenadiazole ring containing substituents on its C atoms. The OH of the carboxyl group donates a proton to a N atom of the ring to form a OH⋯N H-bond (HB), while its carbonyl O engages in a Y⋯O chalcogen bond (ChB) with the ring in which Y = S, Se, Te. The ChB is strengthened by enlarging the size of the Y atom from S to Se to Te. Placement of an electron-withdrawing group (EWG) X₁ on the acid strengthens the HB while weakening the ChB; the reverse occurs when EWGs are placed on the ring. By selection of the proper substituents on the two units, it is possible to achieve a near perfect balance between the strengths of these two bonds. These bond strengths are also reflected in the NMR spectroscopic properties of the chemical shielding of the various atoms and the coupling between the nuclei directly involved in each bond.

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.

Anion···Anion [AuI4]-···[AuI2]- Complex Trapped in the Solid State by Tetramethylammonium Cations

A discrete π-hole···σ-hole dimer is synthesized and X-ray characterized. It presents a perfect thumbtack geometry where the σ-hole of the linear [AuI₂]^- anion points to the π-hole located above the central Au-atom of the [AuI₄]^- anion. Such discrete π-hole···σ-hole dimers are unprecedented in literature, since all mixed-valence gold(I/III) iodide compounds reported to date form infinite ···([AuI₄]^-···[AuI₂]^-) _n ·· chains in the solid state. If an excess of iodine is used for the synthesis, triiodide [I₃]^- ions are partially incorporated into the [AuI₂]^- sites, forming infinite chains. The nature of the anion···anion interaction has been studied considering two possibilities: (i) a π-hole coinage bond or (ii) σ-hole halogen bond using high-level density functional theory calculations, the quantum theory of atoms in molecules, and the noncovalent interaction plot index.