Halogenbrücken in Lösung: Anionenerkennung, Templat‐gestützte Selbstorganisation und Organokatalyse

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.

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.

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.

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.

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 .

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.

Halogen Complexes of Anionic N-Heterocyclic Carbenes

The lithium complexes [(WCA-NHC)Li(toluene)] of anionic N-heterocyclic carbenes with a weakly coordinating anionic borate moiety (WCA-NHC) reacted with iodine, bromine, or CCl4 to afford the zwitterionic 2-halogenoimidazolium borates (WCA-NHC)X (X=I, Br, Cl; WCA=B(C6 F5 )3 , B{3,5-C6 H3 (CF3 )2 }3 ; NHC=IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene, or NHC=IMes=1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene). The iodine derivative (WCA-IDipp)I (WCA=B(C6 F5 )3 ) formed several complexes of the type (WCA-IDipp)I⋅L (L=C6 H5 Cl, C6 H5 Me, CH3 CN, THF, ONMe3 ), revealing its ability to act as an efficient halogen bond donor, which was also exploited for the preparation of hypervalent bis(carbene)iodine(I) complexes of the type [(WCA-IDipp)I(NHC)] and [PPh4 ][(WCA-IDipp)I(WCA-NHC)] (NHC=IDipp, IMes). The corresponding bromine complex [PPh4 ][(WCA-IDipp)2 Br] was isolated as a rare example of a hypervalent (10-Br-2) system. DFT calculations reveal that London dispersion contributes significantly to the stability of the bis(carbene)halogen(I) complexes, and the bonding was further analyzed by quantum theory of atoms in molecules (QTAIM) analysis.

A Bidentate Iodine(III)-Based Halogen-Bond Donor as a Powerful Organocatalyst*

In contrast to iodine(I)-based halogen bond donors, iodine(III)-derived ones have only been used as Lewis acidic organocatalysts in a handful of examples, and in all cases they acted in a monodentate fashion. Herein, we report the first application of a bidentate bis(iodolium) salt as organocatalyst in a Michael and a nitro-Michael addition reaction as well as in a Diels-Alder reaction that had not been activated by noncovalent organocatalysts before. In all cases, the performance of this bidentate XB donor distinctly surpassed the one of arguably the currently strongest iodine(I)-based organocatalyst. Bidentate coordination to the substrate was corroborated by a structural analysis and by DFT calculations of the transition states. Overall, the catalytic activity of the bis(iodolium) system approaches that of strong Lewis acids like BF3 .

Insights into the Topology and the Formation of a Genuine ppσ Bond: Experimental and Computed Electron Densities in Monoanionic Trichlorine [Cl3 ]. Angew Chem Int Ed Engl 2021;60:2569-2573. [PMID: 33151006 PMCID: PMC7898528 DOI: 10.1002/anie.202013727]

So far, several publications have discussed the bonding concepts in polyhalides on a theoretical basis. In particular, the trichlorine monoanion is of great interest because its structure should be symmetrical and show two equidistant Cl-Cl bonds. However, apart from matrix-isolation studies, only asymmetric trichlorine anions have been reported so far. Herein, the trichlorine monoanions in 2-chloroethyltrimethylammonium trichloride [NMe₃ EtCl][Cl₃ ], 1, tetramethylammonium trichloride [NMe₄ ][Cl₃ ], 2, and tetrapropylammonium trichloride [NnPr₄ ][Cl₃ ], 3, are analysed. High-resolution X-ray structures and experimental charge density analyses supported by periodic quantum-chemical calculations provide insight into the influence of the crystalline environment on the structure of these [Cl₃ ]^- anions as well as into the progress of the bond formation between a dichlorine molecule and a Cl^- anion.

Neutral Chiral Tetrakis-Iodo-Triazole Halogen-Bond Donor for Chiral Recognition and Enantioselective Catalysis

Halogen bonding represents a powerful tool in the field of noncovalent interactions. However, applications in enantioselective recognition and catalysis remain almost nonexistent, due in part to the distinct features of halogen bonds, including long covalent and noncovalent bond distances and high directionality. Herein, this work presents a novel chiral tetrakis-iodo-triazole structure as a neutral halogen bond donor for both chiral anion-recognition and enantioinduction in ion-pair organocatalysis. NMR-titration studies revealed significant differences in anion affinity between the halogen bonding receptor and its hydrogen bonding parent. Selective recognition of chiral dicarboxylates and asymmetric induction in a benchmark organocatalytic reaction were demonstrated using the halogen bond donor. Inversions in the absolute sense of chiral recognition, enantioselectivity, and chiroptical properties relative to the related hydrogen donor were observed. Computational modeling suggested that these effects were the result of distinct anion-binding modes for the halogen- versus hydrogen-bond donors.

Chalcogen-Bonding Interactions in Telluroether Heterocycles [Te(CH2 )m ]n (n=1-4; m=3-7)

The Te⋅⋅⋅Te secondary bonding interactions (SBIs) in solid cyclic telluroethers were explored by preparing and structurally characterizing a series of [Te(CH2 )m ]n (n=1-4; m=3-7) species. The SBIs in 1,7-Te2 (CH2 )10 , 1,8-Te2 (CH2 )12 , 1,5,9-Te3 (CH2 )9 , 1,8,15-Te3 (CH2 )18 , 1,7,13,19-Te4 (CH2 )20 , 1,8,15,22-Te4 (CH2 )24 and 1,9,17,25-Te4 (CH2 )28 lead to tubular packing of the molecules, as has been observed previously for related thio- and selenoether rings. The nature of the intermolecular interactions was explored by solid-state PBE0-D3/pob-TZVP calculations involving periodic boundary conditions. The molecular packing in 1,7,13,19-Te4 (CH2 )20 , 1,8,15,22-Te4 (CH2 )24 and 1,9,17,25-Te4 (CH2 )28 forms infinite shafts. The electron densities at bond critical points indicate a narrow range of Te⋅⋅⋅Te bond orders of 0.12-0.14. The formation of the shafts can be rationalized by frontier orbital overlap and charge transfer.

Orthogonal Stability and Reactivity of Aryl Germanes Enables Rapid and Selective (Multi)Halogenations

While halogenation is of key importance in synthesis and radioimaging, the currently available repertoire is largely designed to introduce a single halogen per molecule. This report makes the selective introduction of several different halogens accessible. Showcased here is the privileged stability of nontoxic aryl germanes under harsh fluorination conditions (that allow selective fluorination in their presence), while displaying superior reactivity and functional-group tolerance in electrophilic iodinations and brominations, outcompeting silanes or boronic esters under rapid and additive-free conditions. Mechanistic experiments and computational studies suggest a concerted electrophilic aromatic substitution as the underlying mechanism.

Facile and Reliable Emission-Based Nanomolar Anion Sensing by Luminescent Iridium Receptors Featuring Chelating Halogen-Bonding Sites

An anion sensor is presented that combines a bidentate hydrogen‐ (HB) or halogen‐bonding (XB) site with a luminescent monocationic Ir fragment for strong binding of common anions (K_a up to 6×10⁴  m⁻¹) with diagnostic emission changes. A new emission‐based protocol for fast and reliable detection was derived on the basis of correction for systematic but unspecific background effects. Such a simple correction routine circumvents the hitherto practical limitations of systematic emission‐based analysis of anion binding with validated open‐source software (BindFit). The anticipated order of K_a values was obeyed according to size and basicity of the anions (Cl>Br=OAc) as well as the donor atom of the receptor (XB: 6×10⁴  m⁻¹ > HB: 5×10³  m⁻¹), and led to submicromolar limits of detection within minutes. The results were further validated by advanced NMR techniques, and corroborated by X‐ray crystallographic data and DFT analysis, which reproduced the structural and electronic features in excellent agreement. The results suggest that corrected emission‐based sensing may become a complementary, reliable, and fast tool to promote the use of XB in various application fields, due to the simple and fast optical determination at high dilution.

Recent Advances in Luminescent Recognition and Chemosensing of Iodide in Water

This Minireview covers the latest developments of chemosensors based on transition-metal receptors and organic fluorophores with specific binding sites for the luminescent detection and recognition of iodide in aqueous media and real samples. In all selected examples within the last decade (made-post 2010), the iodide sensing and recognition is probed by monitoring real-time changes of the fluorescence or phosphorescence properties of the chemosensors. This review highlights effective strategies to iodide sensing from a structural approach where the iodide recognition/sensing process, through supramolecular interactions as coordination bonds, hydrogen bonds, halogen bonds and electrostatic interactions, is transduced into an optical change easily measurable. The selective iodide sensing is an active field of research with global interest due to the importance of iodide in biological, medicinal, industrial, environmental and chemical processes.

Modular and Selective Arylation of Aryl Germanes (C-GeEt3 ) over C-Bpin, C-SiR3 and Halogens Enabled by Light-Activated Gold Catalysis

Selective Csp 2 -Csp 2 couplings are powerful strategies for the rapid and programmable construction of bi- or multiaryls. To this end, the next frontier of synthetic modularity will likely arise from harnessing the coupling space that is orthogonal to the powerful Pd-catalyzed coupling regime. This report details the realization of this concept and presents the fully selective arylation of aryl germanes (which are inert under Pd0 /PdII catalysis) in the presence of the valuable functionalities C-BPin, C-SiMe3 , C-I, C-Br, C-Cl, which in turn offer versatile opportunities for diversification. The protocol makes use of visible light activation combined with gold catalysis, which facilitates the selective coupling of C-Ge with aryl diazonium salts. Contrary to previous light-/gold-catalyzed couplings of Ar-N2 + , which were specialized in Ar-N2 + scope, we present conditions to efficiently couple electron-rich, electron-poor, heterocyclic and sterically hindered aryl diazonium salts. Our computational data suggest that while electron-poor Ar-N2 + salts are readily activated by gold under blue-light irradiation, there is a competing dissociative deactivation pathway for excited electron-rich Ar-N2 + , which requires an alternative photo-redox approach to enable productive couplings.

Halogen-Bonded Assemblies of Arylene Imides and Diimides: Insight from Electronic, Structural, and Computational Studies

Halogen-bonding interactions in electron-deficient π scaffolds have largely been underexplored. Herein, the halogen-bonding properties of arylene imide/diimide-based electron-deficient scaffolds were studied. The influence of scaffold size, from small (phthalimide) to moderately sized (pyromellitic diimide or naphthalenediimides) to large (perylenediimide), axial-group modification, and number of halo substituents on the halogen bonding and its self-assembly was probed in a set of nine compounds. The structural modification leads to tunable optical and redox properties. The first reduction potential E 1 / 2 1 ranges between -1.09 and -0.17 V (vs. SCE). Two of the compounds, that is, 6 and 9, have deep-lying LUMOs with values reaching -4.2 eV. Single crystals of all nine systems were obtained, which showed Br⋅⋅⋅O, Br⋅⋅⋅Br, or Br⋅⋅⋅π halogen-bonding interactions, and a few systems are capable of forming all three types. These interactions lead to halogen-bonded rings (up to 12-membered), which propagate to form stacked 1D, 2D, or corrugated sheets. A few outliers were also identified, for example, molecules that prefer C-H⋅⋅⋅O hydrogen bonding over halogen bonding, or noncentrosymmetric rather than centrosymmetric organization. Computational studies based on Atoms in Molecules and Natural Bond Orbital analysis provided further insight into the halogen-bonding interactions. This study can lead to a predictive design tool-box to further explore related systems on surfaces reinforced by these weak directional forces.

Nanoarchitectonics beyond Self-Assembly: Challenges to Create Bio-Like Hierarchic Organization

Incorporation of non-equilibrium actions in the sequence of self-assembly processes would be an effective means to establish bio-like high functionality hierarchical assemblies. As a novel methodology beyond self-assembly, nanoarchitectonics, which has as its aim the fabrication of functional materials systems from nanoscopic units through the methodological fusion of nanotechnology with other scientific disciplines including organic synthesis, supramolecular chemistry, microfabrication, and bio-process, has been applied to this strategy. The application of non-equilibrium factors to conventional self-assembly processes is discussed on the basis of examples of directed assembly, Langmuir-Blodgett assembly, and layer-by-layer assembly. In particular, examples of the fabrication of hierarchical functional structures using bio-active components such as proteins or by the combination of bio-components and two-dimensional nanomaterials, are described. Methodologies described in this review article highlight possible approaches using the nanoarchitectonics concept beyond self-assembly for creation of bio-like higher functionalities and hierarchical structural organization.

Activation of a Metal-Halogen Bond by Halogen Bonding

In recent years, the non-covalent interaction of halogen bonding (XB) has found increasing application in organocatalysis. However, reports of the activation of metal-ligand bonds by XB have so far been limited to a few reactions with elemental iodine or bromine. Herein, we present the activation of metal-halogen bonds by two classes of inert halogen bond donors and the use of the resulting activated complexes in homogenous gold catalysis. The only recently explored class of iodolium derivatives were shown to be effective activators in two test reactions and their activity could be modulated by blocking of the Lewis acidic sites. Bis(benzimidazolium)-based halogen bonding activators provided even more rapid conversion, while the non-iodinated reference compound showed little activity. The role of halogen bonding in the activation of metal-halogen bonds was further investigated by NMR experiments and DFT calculations, which support the mode of activation occurring via halogen bonding.

Dramatic Enhancement of Binding Affinities Between Foldamer-Based Receptors and Anions by Intra-Receptor π-Stacking

As a synthetic model for intra-protein interactions that reinforce binding affinities between proteins and ligands, the energetic interplay of binding and folding was investigated using foldamer-based receptors capable of adopting helical structures. The receptors were designed to have identical hydrogen-bonding sites for anion binding but different aryl appendages that simply provide additional π-stacking within the helical backbones without direct interactions with the bound anions. In particular, the presence of electron-deficient aryl appendages led to dramatic enhancements in the association constant between the receptor and chloride or nitrate ions, by up to three orders of magnitude. Extended stacking within the receptor contributes to the stabilization of the entire folding structure of complexes, thereby enhancing binding affinities.

Bidentate Chiral Bis(imidazolium)-Based Halogen-Bond Donors: Synthesis and Applications in Enantioselective Recognition and Catalysis

Even though halogen bonding-the noncovalent interaction between electrophilic halogen substituents and Lewis bases-has now been established in molecular recognition and catalysis, its use in enantioselective processes is still very rarely explored. Herein, we present the synthesis of chiral bidentate halogen-bond donors based on two iodoimidazolium units with rigidly attached chiral sidearms. With these Lewis acids, chiral recognition of a racemic diamine is achieved in NMR studies. DFT calculations support a 1:1 interaction of the halogen-bond donor with both enantiomers and indicate that the chiral recognition is based on a different spatial orientation of the Lewis bases in the halogen-bonded complexes. In addition, moderate enantioselectivity is achieved in a Mukaiyama aldol reaction with a preorganized variant of the chiral halogen-bond donor. This represents the first case in which asymmetric induction was realized with a pure halogen-bond donor lacking any additional active functional groups.

Two Orthogonal Halogen-Bonding Interactions Directed 2D Crystalline Supramolecular J-Dimer Lamellae

Dye assemblies exhibit fascinating properties and performances, both of which depend critically on the mutual packing arrangement of dyes and on the supramolecular architecture. Herein, we engineered, for the first time, an intriguing chlorosome-mimetic 2D crystalline J-dimer lamellar structure based on halogenated dyes in aqueous media by employing two distinct orthogonal halogen-bonding (XB) interactions. As the only building motif, antiparallel J-dimer was formed and stabilized by single π-stacking and dual halogen⋅⋅⋅π interactions. With two substituted halogen atoms acting as XB donors and the other two acting as acceptors, the constituent J-dimer units were linked by quadruple highly-directional halogen⋅⋅⋅halogen interactions in a staggered manner, resulting in unique 2D lamellar dye assemblies. This work champions and advances halogen-bonding as a remarkably potent tool for engineering dye aggregates with a controlled molecular packing arrangement and supramolecular architecture.

Solid-Phase Radical Polymerization of Halogen-Bond-Based Crystals and Applications to Pre-Shaped Polymer Materials

Liquid vinyl monomers were converted into solid crystals via halogen bonding. They underwent solid-phase radical polymerizations through heating at 40 °C or ultraviolet photo-irradiation (365 nm). The X-ray crystallography analysis showed the high degree of monomer alignment in the crystals. The polymerizations of the solid monomer crystals yielded polymers with high molecular weights and relatively low dispersities because of the high degree of the monomer alignment in the crystal. As a unique application of this system, the crystalized monomers were assembled to pre-determined structures, followed by solid-phase polymerization, to obtain a two-layer polymer sheet and a three-dimensional house-shaped polymer material. The two-layer sheet contained a unique asymmetric pore structure and exhibited a solvent-responsive shape memory property and may find applications to asymmetric membranes and polymer actuators.

Is There a Single Ideal Parameter for Halogen-Bonding-Based Lewis Acidity?

Halogen-bond donors (halogen-based Lewis acids) have now found various applications in diverse fields of chemistry. The goal of this study was to identify a parameter obtainable from a single DFT calculation that reliably describes halogen-bonding strength (Lewis acidity). First, several DFT methods were benchmarked against the CCSD(T) CBS binding data of complexes of 17 carbon-based halogen-bond donors with chloride and ammonia as representative Lewis bases, which revealed M05-2X with a partially augmented def2-TZVP(D) basis set as the best model chemistry. The best single parameter to predict halogen-bonding strengths was the static σ-hole depth, but it still provided inaccurate predictions for a series of compounds. Thus, a more reliable parameter, Ωσ* , has been developed through the linear combination of the σ-hole depth and the σ*(C-I) energy, which was further validated against neutral, cationic, halogen- and nitrogen-based halogen-bond donors with very good performance.

Four-Center Nodes: Supramolecular Synthons Based on Cyclic Halogen Bonding

The isocyanide trans-[PdBr₂ (CNC₆ H₄ -4-X')₂ ] (X'=Br, I) and nitrile trans-[PtX₂ (NCC₆ H₄ -4-X')₂ ] (X/X'=Cl/Cl, Cl/Br, Br/Cl, Br/Br) complexes exhibit similar structural motif in the solid state, which is determined by hitherto unreported four-center nodes formed by cyclic halogen bonding. Each node is built up by four Type II C-X'⋅⋅⋅X-M halogen-bonding contacts and include one Type I M-X⋅⋅⋅X-M interaction, thus giving the rhombic-like structure. These nodes serve as supramolecular synthons to form 2D layers or double chains of molecules linked by a halogen bond. Results of DFT calculations indicate that all contacts within the nodes are typical noncovalent interactions with the estimated strengths in the range 0.6-2.9 kcal mol^-1 .

A Potent Halogen-Bonding Donor Motif for Anion Recognition and Anion Template Mechanical Bond Synthesis

The covalent attachment of electron deficient perfluoroaryl substituents to a bis-iodotriazole pyridinium group produces a remarkably potent halogen bonding donor motif for anion recognition in aqueous media. Such a motif also establishes halogen bonding anion templation as a highly efficient method for constructing a mechanically interlocked molecule in unprecedented near quantitative yield. The resulting bis-perfluoroaryl substituted iodotriazole pyridinium axle containing halogen bonding [2]rotaxane host exhibits exceptionally strong halide binding affinities in competitive 50 % water containing aqueous media, by a factor of at least three orders of magnitude greater in comparison to a hydrogen bonding rotaxane host analogue. These observations further champion and advance halogen bonding as a powerful tool for recognizing anions in aqueous media.

Iodide Discrimination by Tetra-Iodotriazole Halogen Bonding Interlocked Hosts

Whilst the exploitation of interlocked host frameworks for anion recognition is widely established, examples incorporating halogen bond donor groups are still relatively rare. Through the integration of a novel tetra(iodotriazole)-pyridinium motif into macrocycle and axle components, a family of halogen bonding catenane and rotaxanes are constructed for anion recognition studies in a competitive aqueous-organic solvent mixture. Importantly, the degree of anion selectivity displayed is dictated by the topological nature and charged state of the respective interlocked host cavity. All the interlocked hosts exhibit iodide anion selectivity over other halides and sulfate, with the level of discrimination being the greatest with the mono-cationic rotaxane. Arising from greater electrostatic interactions working in tandem with halogen bonding and hydrogen bonding, the di-cationic rotaxane displays stronger anion association at the expense of a relatively lower degree of iodide selectivity.

A Long-Lived Halogen-Bonding Anion Triple Helicate Accommodates Rapid Guest Exchange

Anion-templated helical structures are emerging as a dynamic and tractable class of supramolecules that exhibit anion-switchable self-assembly. We present the first kinetic studies of an anion helicate by utilizing halogen-bonding m-arylene-ethynylene oligomers. These ligands formed high-fidelity triple helicates in solution with surprisingly long lifetimes on the order of seconds even at elevated temperatures. We propose an associative ligand-exchange mechanism that proceeded slowly on the same timescale. In contrast, intrachannel anion exchange occurred rapidly within milliseconds or faster as determined by stopped-flow visible spectroscopy. Additionally, the helicate accommodated bromide in solution and the solid state, while the thermodynamic stability of the triplex favored larger halide ions (bromide≈iodide≫chloride). Taken together, we elucidate a new class of kinetically stable helicates. These anion-switchable triplexes maintain their architectures while accommodating fast intrachannel guest exchange.