Halogen Bonding in Solution: Anion Recognition, Templated Self-Assembly, and Organocatalysis

Key-to-lock halogen bond-based tetragonal pyramidal association of iodonium cations with the lacune rims of beta-octamolybdate

The structure-directing "key-to-lock" interaction of double σ-(IIII)-hole donating iodonium cations with the O-flanked pseudo-lacune rims of [β-Mo8O26]4- gives halogen-bonded iodonium-beta-octamolybate supramolecular associates. In the occurrence of their tetragonal pyramidal motifs, deep and broad σ-(IIII)-holes of a cation recognize the molybdate backbone, which provides an electronic pool localized around the two lacunae. The halogen-bonded I⋯O linkages in the structures were thoroughly studied computationally and classified as two-center, three-center bifurcated, and unconventional "orthogonal" I⋯O halogen bonds. In the latter, the O-atom approaches orthogonally the C-IIII-C plane of an iodonium cation and this geometry diverge from the IUPAC criteria for the identification of the halogen bond.

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.

Taming Molecular Folding: Anion-Templated Foldamers with Tunable Quaternary Structures

Higher-order foldamers represent a unique class of supramolecules at the forefront of molecular design. Herein we control quaternary folding using a novel approach that combines halogen bonding (XBing) and hydrogen bonding (HBing). We present the first anion-templated double helices induced by halogen bonds (XBs) and stabilized by "hydrogen bond enhanced halogen bonds" (HBeXBs). Our findings demonstrate that the number and orientation of hydrogen bond (HB) and XB donors significantly affect the quaternary structure and guest selectivity of two similar oligomers. This research offers new design elements to engineer foldamers and tailor their quaternary structure for specific guest binding.

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.

Halogen Bonding Tripodal Metallo-Receptors for Phosphate Recognition and Sensing in Aqueous-Containing Organic Media

The anion recognition and electrochemical anion-sensing properties of halogen-bonding (XB) tripodal zinc(II) receptors strategically designed and constructed for tetrahedral anion guest binding are described. The XB tris(iodotriazole)-containing hosts exhibit high affinities and selectivities for inorganic phosphate over other more basic, mono-charged oxoanions such as acetate and the halides in a competitive CD3 CN/D2 O (9 : 1 v/v) aqueous solvent mixture. 1 H NMR anion binding and electrochemical voltammetric anion sensing studies with redox-active ferrocene functionalised metallo-tripodal receptor analogues, reveal each of the XB tripods as superior anion complexants when compared to their tris(prototriazole)-containing, hydrogen bonding (HB) counterparts, not only exemplifying the halogen bond as a strong alternative interaction to the traditional hydrogen bond for molecular recognition but also providing rare evidence of the ability of XB receptors to preferentially bind the "harder" phosphate oxoanion over the "softer" and less hydrated halides in aqueous containing media.

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.

Fine-Tuning Substrate-Catalyst Halogen-Halogen Interactions for Boosting Enantioselectivity in Halogen-Bonding Catalysis

A new approach towards highly enantioselective halogen-bonding catalysis has been developed. To circumvent the intrinsic issues of the nature of the halogen-bond (XB) and the resultant unresolved limitations in asymmetric catalysis, fine-tuned halogen-halogen interactions between the substrate and XB-donor were designed to preorganize the substrate in the catalyst's cavity and boost enantiocontrol. The present strategy exploits both the electron cloud (Lewis base site) and the sigma (σ)-hole site of the halogen substituent of the substrates to form a tight catalyst-substrate-counteranion chiral complex, thus enabling a controlled induction of high levels of chirality transfer. Remarkable enantioselectivities of up to 95 : 5 e.r. (90 % ee) have been achieved in a model dearomatization reaction of halogen-substituted (iso)quinolines with tetrakis-iodotriazole multidentate anion-binding catalysts.

Balancing Activity and Stability in Halogen-Bonding Catalysis: Iodopyridinium-Catalyzed One-Pot Synthesis of 2,3-Dihydropyridinones

A one-pot cascade reaction for 2,3-dihydropyridinone synthesis was accomplished with 3-fluoro-2-iodo-1-methylpyridinium triflate as the halogen bond catalyst. The desired [4+2] cycloaddition products, bearing aryl, heteroaryl, alkyl, and alicyclic substituents, were successfully furnished in 28-99% yields. Mechanistic investigations proved that a strong halogen-bonding interaction forged between the iodopyridinium catalyst and imine intermediate was essential to dynamically masking the vulnerable C-I bond on the catalyst and accelerating the following aza-Diels-Alder reaction.

Rational Supramolecular Strategy via Halogen Bonding for Effective Halogen Recognition in Molecular Imprinting

Halogen bonding is a highly directional interaction and a potential tool in functional material design through self-assembly. Herein, we describe two fundamental supramolecular strategies to synthesize molecularly imprinted polymers (MIPs) with halogen bonding-based molecular recognition sites. In the first method, the size of the σ-hole was increased by aromatic fluorine substitution of the template molecule, enhancing the halogen bonding in the supramolecule. The second method involved sandwiching hydrogen atoms of a template molecule between iodo substituents, which suppressed competing hydrogen bonding and enabled multiple recognition patterns, improving the selectivity. The interaction mode between the functional monomer and the templates was elucidated by ¹H NMR, ¹³C NMR, X-ray absorption spectroscopy, and computational simulation. Finally, we succeeded in the effective chromatographic separation of diiodobenzene isomers on the uniformly sized MIPs prepared by multi-step swelling and polymerization. The MIPs selectively recognized halogenated thyroid hormones via halogen bonding and could be applied to screening endocrine disruptors.

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.

2D conjugated microporous polyacetylenes synthesized via halogen-bond-assisted radical solid-phase polymerization for high-performance metal-ion absorbents

The paper reports the first free-radical solid-phase polymerization (SPP) of acetylenes. Acetylene monomers were co-crystalized using halogen bonding, and the obtained cocrystals were polymerized. Notably, because of the alignment of acetylene monomers in the cocrystals, the adjacent C≡C groups were close enough to undergo radical polymerization effectively, enabling the radically low-reactive acetylene monomers to generate high-molecular-weight polyacetylenes that are unattainable in solution-phase radical polymerizations. Furthermore, the SPP of a crosslinkable diacetylene monomer yielded networked two-dimensional conjugated microporous polymers (2D CMPs), where 2D porous polyacetylene nanosheets were cumulated in layer-by-layer manners. Because of the porous structures, the obtained 2D CMPs worked as highly efficient and selective adsorbents of lithium (Li⁺) and boronium (B³⁺) ions, adsorbing up to 312 mg of Li⁺ (31.2 wt%) and 196 mg of B³⁺ (19.6 wt%) per 1 g of CMP. This Li⁺ adsorption capacity is the highest ever record in the area of Li⁺ adsorption.

A Quantum-chemical Analysis on the Lewis Acidity of Diarylhalonium Ions

Cyclic diaryliodonium compounds like iodolium derivatives have increasingly found use as noncovalent Lewis acids in the last years. They are more stable toward nucleophilic substitution than acyclic systems and are markedly more Lewis acidic. Herein, this higher Lewis acidity is analyzed and explained via quantum-chemical calculations and energy decomposition analyses. Its key origin is the change in energy levels and hybridization of iodine's orbitals, leading to both more favorable electrostatic interaction and better charge transfer. Both of the latter seem to contribute in similar fashion, while hydrogen bonding as well as steric repulsion with the phenyl rings play at best a minor role. In comparison to iodolium, bromolium and chlorolium are less Lewis acidic the lighter the halogen, which is predominantly based on less favorable charge-transfer interactions.

J-aggregation strategy of organic dyes for near-infrared bioimaging and fluorescent image-guided phototherapy

With the continuous development of organic materials for optoelectronic devices and biological applications, J-aggregation has attracted a great deal of interest in both dye chemistry and supramolecular chemistry. Except for the characteristic red-shifted absorption and emission, such ordered head-to-tail stacked structures may be accompanied by special properties such as enhanced absorption, narrowed spectral bandwidth, improved photothermal and photodynamic properties, aggregation-induced emission enhancement (AIEE) phenomenon, and so forth. These excellent properties add great potential to J-aggregates for optical imaging and phototherapy in the near-infrared (NIR) region. Despite decades of development, the challenge of rationally designing the molecular structure to adjust intermolecular forces to induce J-aggregation of organic dyes remains significant. In this review, we discuss the formation of J-aggregates in terms of intermolecular interactions and summarize some recent studies on J-aggregation dyes for NIR imaging and phototherapy, to provide a clear direction and reference for designing J-aggregates of near-infrared organic dyes to better enable biological applications. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease.

Abnormalities of the Halogen Bonds in the Complexes between Y2CTe (Y = H, F, CH3) and XF (X = F, Cl, Br, I)

In this work, the hydrogen bonds and halogen bonds in the complexes between Y2CTe (Y = H, F, CH3) and XF (X = F, Cl, Br, I) have been studied by quantum chemical calculations. We found three interesting abnormalities regarding the interactions. Firstly, the strength of halogen bonds increases in the order of IF < BrF < ClF < F2. Secondly, the halogen bonds formed by F2 are very strong, with an interaction energy in the range between −199.8 and −233.1 kJ/mol. Thirdly, all the halogen bonds are stronger than the hydrogen bonds in the systems we examined. All these results are against the general understanding of halogen bonds. These apparent abnormal properties are reconciled with the high polarizability of the Te atom and the strong inducing effect of F on the Te atom of Y2CTe. These findings provide a new perspective on halogen bonds. Additionally, we also proposed bonding distance-based methods to compare the strength of halogen/hydrogen bonds formed between different donor atoms and the same acceptor atom.

A Theoretical Study of the Halogen Bond between Heteronuclear Halogen and Benzene

Halogen bonds play an important role in many fields, such as biological systems, drug design and crystal engineering. In this work, the structural characteristics of the halogen bond between heteronuclear halogen XD (ClF, BrCl, IBr, ICl, BrF and IF) and benzene were studied using density functional theory. The structures of the complexes between heteronuclear halogen and benzene have Cs symmetry. The interaction energies of the complexes between heteronuclear halogen XD (ClF, BrCl, IBr, ICl, BrF and IF) and benzene range from -27.80 to -37.18 kJ/mol, increasing with the increases in the polarity between the atoms of X and D, and are proportional to the angles of a between the Z axis and the covalent bond of heteronuclear halogen. The electron density (ρ) and corresponding Laplacian (∇²ρ) values indicate that the interaction of the heteronuclear halogen and benzene is a typical long-range weak interaction similar to a hydrogen bond. Independent gradient model analysis suggests that the van der Waals is the main interaction between the complexes of heteronuclear halogen and benzene. Symmetry-adapted perturbation theory analysis suggests that the electrostatic interaction is the dominant part in the complexes of C₆H₆⋯ClF, C₆H₆⋯ICl, C₆H₆⋯BrF and C₆H₆⋯IF, and the dispersion interaction is the main part in the complexes of C₆H₆⋯BrCl, C₆H₆⋯IBr.

Anti‐Hofmeister Anion Selectivity via a Mechanical Bond Effect in Neutral Halogen‐Bonding [2]Rotaxanes

Exceptionally strong halogen bonding (XB) donor-chloride interactions are exploited for the chloride anion template synthesis of neutral XB [2]rotaxane host systems which contain perfluoroaryl-functionalised axle components, including a remarkably potent novel 4,6-dinitro-1,3-bis-iodotriazole motif. Halide anion recognition properties in aqueous-organic media, determined via extensive ¹ H NMR halide anion titration experiments, reveal the rotaxane host systems exhibit dramatically enhanced affinities for hydrophilic Cl^- and Br^- , but conversely diminished affinities for hydrophobic I^- , relative to their non-interlocked axle counterparts. Crucially, this mechanical bond effect induces a binding selectivity which directly opposes Hofmeister bias. Free-energy analysis of this mechanical bond enhancement demonstrates anion recognition by neutral XB interlocked host systems as a rare and general strategy to engineer anti-Hofmeister bias anion selectivity in synthetic receptor design.

The Distance between Minima of Electron Density and Electrostatic Potential as a Measure of Halogen Bond Strength

In this study, we present results of a detailed topological analysis of electron density (ED) of 145 halogen-bonded complexes formed by various fluorine-, chlorine-, bromine-, and iodine-containing compounds with trimethylphosphine oxide, Me3PO. To characterize the halogen bond (XB) strength, we used the complexation enthalpy, the interatomic distance between oxygen and halogen, as well as the typical set of electron density properties at the bond critical points calculated at B3LYP/jorge-ATZP level of theory. We show for the first time that it is possible to predict the XB strength based on the distance between the minima of ED and molecular electrostatic potential (ESP) along the XB path. The gap between ED and ESP minima exponentially depends on local electronic kinetic energy density at the bond critical point and tends to be a common limiting value for the strongest halogen bond.

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.

Solid-state multinuclear magnetic resonance and X-ray crystallographic investigation of the phosphorus...iodine halogen bond in a bis(dicyclohexylphenylphosphine)(1,6-diiodoperfluorohexane) cocrystal

Halogen bonding to phosphorus atoms remains uncommon, with relatively few examples reported in the literature. Here, the preparation and investigation of the cocrystal bis(dicyclohexylphenylphosphine)(1,6-diiodoperfluorohexane) by X-ray crystallography and solid-state multinuclear magnetic resonance spectroscopy is described. The crystal structure features two crystallographically unique C-I...P halogen bonds [d_I...P = 3.090 (5) Å, 3.264 (5) Å] and crystallographic disorder of one of the 1,6-diiodoperfluorohexane molecules. The first of these is the shortest and most linear I...P halogen bond reported to date. ¹³C, ¹⁹F, and ³¹P magic angle spinning solid-state NMR spectra are reported. A ³¹P chemical shift change of -7.0 p.p.m. in the cocrystal relative to pure dicyclohexylphenylphosphine, consistent with halogen bond formation, is noted. This work establishes iodoperfluoroalkanes as viable halogen bond donors when paired with phosphorus acceptors, and also shows that dicyclohexylphenylphosphine can act as a practical halogen bond acceptor.

Modular Generation of (Iodinated) Polyarenes Using Triethylgermane as Orthogonal Masking Group

While the modular construction of molecules from suitable building blocks is a powerful means to more rapidly generate a diversity of molecules than through customized syntheses, the further evolution of the underlying coupling methodology is key to realize widespread applications. We herein disclose a complementary modular coupling approach to the widely employed Suzuki coupling strategy of boron containing precursors, which relies on organogermane containing building blocks as key orthogonal functionality and an electrophilic (rather than nucleophilic) unmasking event paired with air‐stable Pd^I dimer based bond construction. This allows to significantly shorten the reaction times for the iterative coupling steps and/or to close gaps in the accessible compound space, enabling straightforward access also to iodinated compounds.

Redox-Switchable Chalcogen Bonding for Anion Recognition and Sensing

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

Probing the Electronic Properties and Interaction Landscapes in a Series of N-(Chlorophenyl)pyridinecarboxamides

A 3 × 3 isomer grid of nine N-(chlorophenyl)pyridinecarboxamides (NxxCl) is reported with physicochemical studies and single crystal structures (Nx = pyridinoyl moiety; xCl = aminochlorobenzene ring; x = para-/meta-/ortho-), as synthesized by the reaction of the substituted p-/m-/o-pyridinecarbonyl chlorides (Nx) with p-/m-/o-aminochlorobenzenes (xCl). Several of the nine NxxCl crystal structures display structural similarities with their halogenated NxxX and methylated NxxM relatives (x = p-/m-/o-substitutions; X = F, Br; M = methyl). Indeed, five of the nine NxxCl crystal structures are isomorphous with their NxxBr analogues as the NpmCl/Br, NpoCl/Br, NmoCl/NmoBr, NopCl/Br, and NooCl/Br pairs. In the NxxCl series, the favored hydrogen bonding mode is aggregation by N-H···N_pyridine interactions, though amide···amide intermolecular interactions are noted in NpoCl and NmoCl. For the NoxCl triad, intramolecular N-H···N_pyridine interactions influence molecular planarity, whereas NppCl·H₂O (as a monohydrate) exhibits O-H···O, N-H···O1W, and O1W-H···N interactions as the primary hydrogen bonding. Analysis of chlorine-containing compounds on the CSD is noted for comparisons. The interaction environments are probed using Hirshfeld surface analysis and contact enrichment studies. The melting temperatures (T _m) depend on both the lattice energy and molecular symmetry (Carnelley's rule), and the melting points can be well predicted from a linear regression of the two variables. The relationships of the T _m values with the total energy, the electrostatic component, and the strongest hydrogen bond components have been analyzed.

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

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

Diaryliodoniums as Hybrid Hydrogen- and Halogen-Bond-Donating Organocatalysts for the Groebke-Blackburn-Bienaymé Reaction

Dibenziodolium and diphenyliodonium triflates display high catalytic activity for the multicomponent reaction that leads to a series of imidazopyridines. Density functional theory (DFT) calculations indicate that both the salts can play the role of hybrid hydrogen- and halogen-bond-donating organocatalysts, which electrophilically activate the carbonyl and imine groups during the reaction process. The ortho-H atoms in the vicinal position to the I atom play a dual role: forming additional noncovalent bonds with the ligated substrate and increasing the maximum electrostatic potential on the σ-hole at the iodine atom owing to the effects of polarization. Dibenziodolium triflate exhibits higher catalytic activity, and the results obtained from ¹H nuclear magnetic resonance (NMR) titrations, in conjunction with those from DFT calculations, indicate that this could be explained in terms of the additional energy required for the rotation of the phenyl ring in the diphenyliodonium cation during ligation of the substrate.

Synthesis and cationic polymerization of halogen bonding vinyl ether monomers

Halogen bonding is rapidly becoming recognized as a viable and useful intermolecular interaction in supramolecular chemistry. While various monomers amenable to radical polymerization methods containing halogen bonding donors have been developed, this study aims to expand the type of monomers that incorporate this intermolecular interaction to facilitate use of cationic polymerization by developing three novel vinyl ether monomers containing halogen bonding donor moieties: 2,3,5,6-tetrafluoro-4-iodophenoxyethyl vinyl ether (C2I), 2,3,5,6-tetrafluoro-4-iodophenoxybutyl vinyl ether (C4I), and 2-(2,3,5,6-tetrafluoro-4-iodophenoxyethoxy)ethyl vinyl ether (O3I). Well controlled cationic polymerization is achievable through the use of a proton trap, 2,6-di-tert-butylpyridine. The use of SnCl₄ as a co-Lewis acid was found to accelerate the reaction. Between the three monomers, the difference in the chain length is shown to influence the reaction rate, with the longest chain demonstrating the fastest polymerization. Initial studies of the halogen bonding ability shows that halogen bonding exists for all three monomers but is most pronounced in C4I. The polymerized vinyl ethers also exhibit halogen bonding. Due to the ease of synthesis and polymerization, these are promising new monomers to increase functionality available for polymers synthesized using cationic polymerization.

Three novel vinyl ether monomers containing halogen bonding moieties were synthesized, polymerized via cationic polymerization and exhibited clear halogen bonding.

Halogen bond-directed self-assembly in bicomponent blends at the solid/liquid interface: Effect of the alkyl chain substitution position

The fabrication of well-organised molecular assemblies on surfaces is fundamental for the creation of functional molecular systems applicable to nanoelectronics and molecular devices. In this study, we investigated the effect...

Charge neutral halogen bonding tetradentate-iodotriazole macrocycles capable of anion recognition and sensing in highly competitive aqueous media

A series of neutral tetradentate halogen bonding (XB) macrocycles, comprising of two bis-iodotriazole XB donors were synthesised in 60-70% yields via a stepwise CuAAC-mediated cyclisation strategy. Extensive ¹H NMR anion titration experiments reveal halide binding affinities are critically dependent on the substitution pattern of the xylyl spacer unit. The meta-substituted macrocycle remarkably displays cooperative tetradentate XB-halide anion recognition in highly competitive 40% aqueous-organic D₂O/acetone-d₆ (40 : 60, v/v) solvent mixtures. Integration of para-xylyl and naphthyl spacer units generates extended macrocyclic cavities, capable of selective oxalate recognition. Furthermore, preliminary fluorescence exeperiments reveal dicarboxylate specific sensing can be achieved through monitoring of the naphthylene centred emission.

Halogen bonding regulated functional nanomaterials

Non-covalent interactions have gained increasing attention for use as a driving force to fabricate various supramolecular architectures, exhibiting great potential in crystal and materials engineering and supramolecular chemistry. As one of the most powerful non-covalent bonds, the halogen bond has recently received increasing attention in functional nanomaterial design. The present review describes the latest studies based on halogen bonding induced self-assembly and its applications. Due to the high directionality and controllable interaction strength, halogen bonding can provide a facile platform for the design and synthesis of a myriad of nanomaterials. In addition, both the fundamental aspects and the real engineering applications are discussed, which encompass molecular recognition and sensing, organocatalysis, and controllable multifunctional materials and surfaces.

Metal Centers as Nucleophiles: Oxymoron of Halogen Bond-Involving Crystal Engineering

This review highlights recent studies discovering unconventional halogen bonding (HaB) that involves positively charged metal centers. These centers provide their filled d‐orbitals for HaB, and thus behave as nucleophilic components toward the noncovalent interaction. This role of some electron‐rich transition metal centers can be considered an oxymoron in the sense that the metal is, in most cases, formally cationic; consequently, its electron donor function is unexpected. The importance of Ha⋅⋅⋅d‐[M] (Ha=halogen; M is Group 9 (Rh, Ir), 10 (Ni, Pd, Pt), or 11 (Cu, Au)) interactions in crystal engineering is emphasized by showing remarkable examples (reported and uncovered by our processing of the Cambridge Structural Database), where this Ha⋅⋅⋅d‐[M] directional interaction guides the formation of solid supramolecular assemblies of different dimensionalities.

Exploiting non-covalent interactions in selective carbohydrate synthesis

Non-covalent interactions (NCIs) are a vital component of biological bond-forming events, and have found important applications in multiple branches of chemistry. In recent years, the biomimetic exploitation of NCIs in challenging glycosidic bond formation and glycofunctionalizations has attracted significant interest across diverse communities of organic and carbohydrate chemists. This emerging theme is a major new direction in contemporary carbohydrate chemistry, and is rapidly gaining traction as a robust strategy to tackle long-standing issues such as anomeric and site selectivity. This Review thus seeks to provide a bird's-eye view of wide-ranging advances in harnessing NCIs within the broad field of synthetic carbohydrate chemistry. These include the exploitation of NCIs in non-covalent catalysed glycosylations, in non-covalent catalysed glycofunctionalizations, in aglycone delivery, in stabilization of intermediates and transition states, in the existence of intramolecular hydrogen bonding networks and in aggregation by hydrogen bonds. In addition, recent emerging opportunities in exploiting halogen bonding and other unconventional NCIs, such as CH-π, cation-π and cation-n interactions, in various aspects of carbohydrate chemistry are also examined.

N-Heterocyclic Iod(az)olium Salts - Potent Halogen-Bond Donors in Organocatalysis

This article describes the application of N-heterocyclic iod(az)olium salts (NHISs) as highly reactive organocatalysts. A variety of mono- and dicationic NHISs are described and utilized as potent XB-donors in halogen-bond catalysis. They were benchmarked in seven diverse test reactions in which the activation of carbon- and metal-chloride bonds as well as carbonyl and nitro groups was achieved. N-methylated dicationic NHISs rendered the highest reactivity in all investigated catalytic applications with reactivities even higher than all previously described monodentate XB-donors based on iodine(I) and (III) and the strong Lewis acid BF3 .

Predicting the catalytic activity of azolium-based halogen bond donors: an experimentally-verified theoretical study

This report demonstrates the successful application of electrostatic surface potential distribution analysis for evaluating the relative catalytic activity of a series of azolium-based halogen bond donors. A strong correlation (R² > 0.97) was observed between the positive electrostatic potential of the σ-hole on the halogen atom and the Gibbs free energy of activation of the model reactions (i.e., halogen abstraction and carbonyl activation). The predictive ability of the applied approach was confirmed experimentally. It was also determined that the catalytic activity of azolium-based halogen bond donors was generally governed by the structure of the azolium cycle, whereas the substituents on the heterocycle had a limited impact on the activity. Ultimately, this study highlighted four of the most promising azolium halogen bond donors, which are expected to exhibit high catalytic activity.

The Chloroazaphosphatrane Motif for Halogen Bonding in Solution

Chloroazaphosphatranes, the corresponding halogenophosphonium cations of the Verkade superbases, were evaluated as a new motif for halogen bonding (XB). Their modulable synthesis allowed for synthetizing chloroazaphosphatranes with various substituents on the nitrogen atoms. The binding constants determined from NMR titration experiments for Cl^-, Br^-, I^-, AcO^-, and CN^- anions are comparable to those obtained with conventional iodine-based monodentate XB receptors. Remarkably, the protonated azaphosphatrane counterparts display no affinity for anions under the same conditions. The strength of the XB interaction is, to some extent, related to the basicity of the corresponding Verkade superbase. The halogen bonding abilities of this new class of halogen donor motif were also revealed by the Δδ(³¹P) NMR shift observed in CD₂Cl₂ solution in the presence of triethylphosphine oxide (TEPO). Thus, chloroazaphosphatranes constitute a new class of halogen bond donors, expanding the repertory of XB motifs mainly based on C_Ar-I bonds.

In silico characterization and prediction of thiourea-like neutral bidentate halogen bond catalysts

Preorganization is a common strategy to align halogen bond (XB) donors to form two or more halogen bonds simultaneously. Previous approaches have utilized various non-covalent interactions such as steric interactions, ππ stacking, and hydrogen bond interactions. However, some of the introduced aligning interactions may compete with halogen bond interactions if the donors are employed in catalysis. To achieve thiourea-like properties, we have designed in silico several neutral bidentate halogen bond donors in whose structures the donor moieties are connected via covalent bonds. Compared to previous XB catalyst designs, the new design does not involve other potentially competitive non-covalent interactions such as hydrogen bonds. One of the designed XB donors can deliver strong halogen bonds, with a O-I distance as short as 2.64 Å. Density functional theory (DFT) calculations predicted that our designed catalysts may catalyze important organic reactions on their own, particularly for those reactions that involve (developing) soft anions such as thiolates.

Iodine(III)-Based Halogen Bond Donors: Properties and Applications

Halogen bonding, the non-covalent interaction of Lewis bases with an electron-deficient region of halogen substituents, received increased attention recently. Consequently, the design and evaluation of numerous halogen-containing species as halogen bond donors have been subject to intense research. More recently, organoiodine compounds at the iodine(III) state have been receiving growing attention in the field. Due to their electronic and structural properties, they provide access to unique binding modes. For this reason, our groups have been involved in the development of such compounds, in the quantification of their halogen bonding strength (through the evaluation of their Lewis acidities), as well as in the evaluation of their activities as catalysts in several model reactions. This account will describe these contributions.

Radical-pairing-induced molecular assembly and motion

Radical-pairing interactions between conjugated organic π-radicals are relative newcomers to the inventory of molecular recognition motifs explored in supramolecular chemistry. The unique electronic, magnetic, optical and redox-responsive properties of the conjugated π-radicals render molecules designed with radical-pairing interactions useful for applications in various areas of chemistry and materials science. In particular, the ability to control formation of radical cationic or anionic species, by redox stimulation, provides a flexible trigger for directed assembly and controlled molecular motions, as well as a convenient means of inputting energy to fuel non-equilibrium processes. In this Review, we provide an overview of different examples of radical-pairing-based recognition processes and of their emerging use in (1) supramolecular assembly, (2) templation of mechanically interlocked molecules, (3) stimuli-controlled molecular switches and, by incorporation of kinetic asymmetry in the design, (4) the creation of unidirectional molecular transporters based on pumping cassettes powered by fuelled switching of radical-pairing interactions. We conclude the discussion with an outlook on future directions for the field.

A Hypervalent Cyclic Dibenzoiodolium Salt as a Halogen-Bond-Donor Catalyst for the [4+2] Cycloaddition of 2-Alkenylindoles

A stable, hypervalent cyclic dibenzoiodolium salt acted as a strong halogen bonding (XB)-donor catalyst for [4+2] cycloaddition of 2-alkenylindoles, and not as an oxidizing agent. The cross-[4+2] cycloaddition of 2-vinylindoles with 2-alkenylindoles was catalyzed smoothly by the hypervalent cyclic dibenzoiodolium triflate catalyst to give the tetrahydrocarbazoles in up to 99 % yield with 17 : 1 diastereoselectivity. The hypervalent cyclic dibenzoiodolium salt was also applicable to the Povarov reaction of 2-vinylindole with N-p-methoxyphenyl (PMP) imine to give the indolyl-tetrahydroquinoline in 83 % yield.

Characterising Supramolecular Architectures in Crystals Featuring I⋯Br Halogen Bonding: Persistence of X⋯X’ Secondary-Bonding in Their Congeners

The Cambridge Structural Database was surveyed for crystals featuring I⋯Br secondary-bonding in their supramolecular assemblies occurring independently of other obvious supramolecular synthons and devoid of other halogen bonding interactions. In all, 41 crystals satisfied these criteria, with nine examples of zero-dimensional aggregation (uniformly two-molecule aggregates) and 30 one-dimensional chains of varying topology (linear, zigzag and helical). There is one example each of two- and three-dimensional patterns. Type-I, type-II and intermediate bonding situations are apparent; for type-II bonding, the ratio of iodide:bromide functioning as the electrophile is 2:1. Most molecules participated, on average, in one I⋯Br contact, although smaller numbers of half (zero-dimensional) or two contacts (two- and three-dimensional) were observed. The propensity of the formation of related halogen bonding interactions in congeners of the 41 investigated crystals was also studied. Congeners were apparent for 11 crystals, with seven of these exhibiting isostructural relationships, in terms of space-group symmetry and unit-cell parameters. Isostructural relationships do not ensure the formation of analogous aggregation patterns, particularly and in accord with expectation, for the lighter halides. When formed, often distinct aggregation patterns are observed despite the isostructural relationships. Hetero-atomic halogen bonding offers surprises and opportunities in crystal engineering endeavours.