Radial Behavior of Electrostatic Potentials of Atoms and Ions Revisited: Isotropy and Anisotropy

In this paper we revisit earlier work relating to monoatomic atoms and ions published by pioneers in the area of electrostatic potentials. We include plots of the radial distributions of the electrostatic potentials for spherically symmetric atoms and cations, and for singly, doubly and triply negative anions. For atoms with anisotropy in their densities and electrostatic potentials, such as the halonium cations, it is shown how the molecular surface approach for plotting electrostatic potentials complements that achieved by directional radial distributions.

Halogen-Bonded Organic Frameworks (XOFs) Based on N⋅⋅⋅Br+⋅⋅⋅N Bonds: Robust Organic Networks Constructed by Fragile Bonds

Organic frameworks face a trade-off between the framework stability and the bond dynamics, which necessitates the development of innovative linkages that can generate stable frameworks without hindering efficient synthesis. Although iodine(I)-based halogen-bonded organic frameworks (XOFs) have been developed, constructing XOFs based on bromine(I) is desirable yet challenging due to the high sensitivity of bromine(I) species. In this work, we present the inaugural construction of stable bromine(I)-bridged two-dimensional (2D) halogen-bonded organic frameworks, XOF(Br)-TPy-BF4/OTf, based on sensitive [N⋅⋅⋅Br⋅⋅⋅N]+ halogen bonds. The formation of XOF(Br)-TPy-BF4/OTf was monitored by 1H NMR, XPS, IR, SEM, TEM, HR-TEM, SEAD. Their framework structures were established by the results from PXRD, theoretical simulations and SAXS. More importantly, XOF(Br) displayed excellent chemical and thermal stabilities. They exhibited stable two-dimensional framework structures in various organic solvents and aqueous media, even over a wide pH range (pH 3-12), while the corresponding model compounds BrPy2BF4/OTf decomposed quickly even in the presence of minimal water. Furthermore, the influence of the counterions were investigated by replacing BF4 with OTf, which improved the stability of XOF(Br). This characteristic enabled XOF(Br) to serve as an efficient oxidizing reagent in aqueous environments, in contrast with the sensitivity of BrPy2BF4/OTf, which performed well only in organic media. This study not only deepens our fundamental understanding of organic frameworks but also opens new avenues for the development and application of multifunctional XOFs.

Anionic Coordination Control in Building Cu-Based Electrocatalytic Materials for CO2 Reduction Reaction

Renewable energy-driven conversion of CO2 to value-added fuels and chemicals via electrochemical CO2 reduction reaction (CO2RR) technology is regarded as a promising strategy with substantial environmental and economic benefits to achieve carbon neutrality. Because of its sluggish kinetics and complex reaction paths, developing robust catalytic materials with exceptional selectivity to the targeted products is one of the core issues, especially for extensively concerned Cu-based materials. Manipulating Cu species by anionic coordination is identified as an effective way to improve electrocatalytic performance, in terms of modulating active sites and regulating structural reconstruction. This review elaborates on recent discoveries and progress of Cu-based CO2RR catalytic materials enhanced by anionic coordination control, regarding reaction paths, functional mechanisms, and roles of different non-metallic anions in catalysis. Finally, the review concludes with some personal insights and provides challenges and perspectives on the utilization of this strategy to build desirable electrocatalysts.

Triphenylphosphine Oxide: A Versatile Covalent Functionality for Carbon Nanotubes

Broadening the scope of functionalities that can be covalently bound to single-walled carbon nanotubes (SWCNTs) is crucial for enhancing the versatility of this promising nanomaterial class in applied settings. Here we report the covalent linkage of triphenylphosphine oxide [Ph3P(O)] to SWCNTs, a hitherto overlooked surface functionality. We detail the synthesis and structural characterization of a new family of phosphine oxide-functionalized diaryliodonium salts that can facilitate direct Ph3P(O) transfer and afford novel SWCNTs with tunable Ph3P(O) content (SWCNT-P). The molecularly-distributed and robust nature of the covalent Ph3P(O) attachment in SWCNT-P was supported by a combination of characterization methods including Raman, infrared, UV/Vis-NIR and X-ray photoelectron spectroscopies coupled with thermogravimetric analysis. Electron microscopy further revealed the effectiveness of the Ph3P(O) moiety for de-bundling SWCNTs to yield SWCNT-P with superior dispersibility and processability. Finally, electrochemical studies established that SWCNT-P is sensitive to the presence of Li+, Na+ and K+ wherein the Gutmann-Beckett Lewis acidity parameters of the ions were quantitatively transduced by Ph3P(O) to electrochemical responses. This work hence presents a synthetic, structural, spectroscopic and electrochemical foundation for a new phosphorus-enriched responsive nanomaterial platform featuring the Ph3P(O) functionality.

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.

Pyridine Iodine(I) Cations: Kinetic Trapping as a Sulfonate Complexes

Seven pyridine iodine(I) sulfonate complexes were prepared and isolated at low temperatures and characterized by X-ray diffraction analysis. The inherently instable pyridine iodine(I) cations are stabilized by an oxygen of sulfonate anions via the I⋅⋅⋅O halogen bond. In these complexes, the iodine atom of the pyridine iodine(I) cation acts as an electron acceptor and the sulfonate oxygen as the electron donor. These complexes are stable enough in the crystalline state, yet decompose rapidly under ambient conditions, also being unstable in solution. The (pyridine)N-I bond lengths [2.140(3)-2.197(2) Å] and the I⋅⋅⋅O halogen bonds [2.345(6)-2.227(3) Å] are analogous to (imide)N-I⋅⋅⋅O-N-pyridine uncharged halogen-bonded complexes formed from N-haloimides and pyridine N-oxides, thus confirming the existence of elusive pyridine iodine(I) cation.

Molecular Insights into Water-Chloride and Water-Water Interactions in the Supramolecular Architecture of Aconine Hydrochloride Dihydrate

Despite the previous preparation of aconine hydrochloride monohydrate (AHM), accurate determination of the crystal's composition was hindered by severely disordered water molecules within the crystal. In this study, we successfully prepared a new dihydrate form of the aconine hydrochloride [C25H42NO9+Cl-·2(H2O), aconine hydrochloride dihydrate (AHD)] and accurately refined all water molecules within the AHD crystal. Our objective is to elucidate both water-chloride and water-water interactions in the AHD crystal. The crystal structure of AHD was determined at 136 K using X-ray diffraction and a multipolar atom model was constructed by transferring charge-density parameters to explore the topological features of key short contacts. By comparing the crystal structures of dihydrate and monohydrate forms, we have observed that both AHD and AHM exhibit identical aconine cations, except for variations in the number of water molecules present. In the AHD crystal, chloride anions and water molecules serve as pivotal connecting hubs to establish three-dimensional hydrogen bonding networks and one-dimensional hydrogen bonding chain; both water-chloride and water-water interactions assemble supramolecular architectures. The crystal packing of AHD exhibits a complete reversal in the stacking order compared to AHM, thereby emphasizing distinct disparities between them. Hirshfeld surface analysis reveals that H···Cl- and H···O contacts play a significant role in constructing the hydrogen bonding network and chain within these supramolecular architectures. Furthermore, topological analysis and electrostatic interaction energy confirm that both water-chloride and water-water interactions stabilize supramolecular architectures through electrostatic attraction facilitated by H···Cl- and H···O contacts. Importantly, these findings are strongly supported by the existing literature evidence. Consequently, navigating these water-chloride and water-water interactions is imperative for ensuring storage and safe processing of this pharmaceutical compound.

Anisotropies in electronic densities and electrostatic potentials of Halonium Ions: focus on Chlorine, Bromine and Iodine

CONTEXT

Why are the halonium cations so effective in forming strongly-bound complexes? We directed our research to address this question and we present electrostatic potential data for the valence-state halogen atoms X and halonium cations X+, where X = Cl, Br, I. The electron densities and electrostatic potentials of the halonium cations show considerably greater anisotropy than do the valence state halogens. The distances from the electrostatic potential surface maxima to the halogen nuclei are about 0.5 Å smaller than the distances from the electrostatic potential surface minima to the nuclei, giving the halonium cations each a more disk-like shape than the corresponding neutral valence state halogens. Their surface electrostatic potentials are totally consistent with the directionalities of halonium cations in complexes and the strengths of their interactions. To add perspective to this brief report, we have included calculations of the isotropic cation K+ and noble gas Kr.

METHODS

The calculations of the electrostatic potentials of the valence states of the halogen atoms Cl, Br and I and the halonium cations Cl+, Br+ and I+, as well as K+ and Kr, on 0.001 au contours of their electronic densities were carried out with Gaussian O9 and the Wave Function Analysis - Surface Analysis Suite (WFA-SAS) at the M06-2X/6-31 + G(d,p) and M06-2X/3-21G* levels.

Intrinsic dynamic and static natures of APn--X+--BPn σ(3c-4e) type interactions (APn = BPn = N, P, As and Sb; X = H, F, Cl, Br and I) in bicyclo[3.3.3] and bicyclo[4.4.4] systems and their behaviour, elucidated with QTAIM dual functional analysis

The intrinsic dynamic and static natures of APn--X+--BPn (APn = BPn: N, P, As and Sb; X = H, F, Cl, Br and I) in 1a+-8c+ were elucidated with the quantum theory of atoms-in-molecules dual functional analysis (QTAIM-DFA). Species 1a+-8c+ were formed by incorporating X+ between APn and BPn of APn(CH2CH2CH2)3BPn (1-4) and APn(CH2CH2CH2CH2)3BPn (5-8). The relative stabilities between the symmetric and nonsymmetric structures along with their transition states were investigated. Various natures from typical hydrogen bonds (t-HB) to classical covalent bonds were predicted for the APn-X/BPn-X interactions in APn--X+--BPn with QTAIM-DFA. The secondary interactions of H-H and X-C were also detected. The vdW to molecular complexes through charge transfer natures were predicted for them. Natural bond orbital analysis clarified that the CT terms were caused by not only n(APn)→ σ*(X-BPn) but also σ(APn-C)→σ*(X-BPn), σ(APn-C/BPn-C)→np(X+) and n(X)→ns(Pn+). The direction and magnitude of the p-character of n(APn) were the factors that determined the types of donor-acceptor interactions. Estimating the order of the interaction strengths was attempted. The σ(3c-4e) characters of APn--X+--BPn were also examined by analysing the charge distributions on APn--X+--BPn. These results would provide fundamentally important insight into designing molecules with high functionality containing X+ in symmetric and nonsymmetric structures.

Halogen Bonds of Halogen(I) Ions─Where Are We and Where to Go?

Halenium ions, X+, are particularly strong halogen-bond donors that interact with two Lewis bases simultaneously to form linear [D···X···D]+-type halonium complexes. Their three-center, four-electron halogen bond is both fundamentally interesting and technologically valuable as it tames the reactivity of halogen(I) ions, opening up new horizons in a variety of fields including synthetic organic and supramolecular chemistry. Understanding this bonding situation enables the development of improved halogen(I) transfer reactions and of advanced functional materials. Following a decade of investigations of basic principles, the range of applications is now rapidly widening. In this Perspective, we assess the status of the field and identify its key advances and the main bottlenecks. Clearing common misunderstandings that may hinder future progress, we aim to inspire and direct future research efforts.

Halogenated Antimicrobial Agents to Combat Drug-Resistant Pathogens

Antimicrobial resistance presents us with a potential global crisis as it undermines the abilities of conventional antibiotics to combat pathogenic microbes. The history of antimicrobial agents is replete with examples of scaffolds containing halogens. In this review, we discuss the impacts of halogen atoms in various antibiotic types and antimicrobial scaffolds and their modes of action, structure-activity relationships, and the contributions of halogen atoms in antimicrobial activity and drug resistance. Other halogenated molecules, including carbohydrates, peptides, lipids, and polymeric complexes, are also reviewed, and the effects of halogenated scaffolds on pharmacokinetics, pharmacodynamics, and factors affecting antimicrobial and antivirulence activities are presented. Furthermore, the potential of halogenation to circumvent antimicrobial resistance and rejuvenate impotent antibiotics is addressed. This review provides an overview of the significance of halogenation, the abilities of halogens to interact in biomolecular settings and enhance pharmacological properties, and their potential therapeutic usages in preventing a postantibiotic era. SIGNIFICANCE STATEMENT: Antimicrobial resistance and the increasing impotence of antibiotics are critical threats to global health. The roles and importance of halogen atoms in antimicrobial drug scaffolds have been established, but comparatively little is known of their pharmacological impacts on drug resistance and antivirulence activities. This review is the first to extensively evaluate the roles of halogen atoms in various antibiotic classes and pharmacological scaffolds and to provide an overview of their ability to overcome antimicrobial resistance.

The Impact of ortho-substituents on Bonding in Silver(I) and Halogen(I) Complexes of 2-Mono- and 2,6-Disubstituted Pyridines: An In-Depth Experimental and Theoretical Study

The coordination nature of 2-mono- and 2,6-disubstituted pyridines with electron-withdrawing halogen and electron-donating methyl groups for [N-X-N]+ (X=I, Br) complexations have been studied using 15 N NMR, X-ray crystallography, and Density Functional Theory (DFT) calculations. The 15 N NMR chemical shifts reveal iodine(I) and bromine(I) prefer to form complexes with 2-substituted pyridines and only 2,6-dimethylpyridine. The crystalline halogen(I) complexes of 2-substituted pyridines were characterized by using X-ray diffraction analysis, but 2,6-dihalopyridines were unable to form stable crystalline halogen(I) complexes due to the lower nucleophilicity of the pyridinic nitrogen. In contrast, the halogen(I) complexes of 2,6-dimethylpyridine, which has a more basic nitrogen, are characterized by X-crystallography, which complements the 15 N NMR studies. DFT calculations reveal that the bond energies for iodine(I) complexes vary between -291 and -351 kJ mol-1 and for bromine between -370 and -427 kJ mol-1 . The bond energies of halogen(I) complexes of 2-halopyridines with more nucleophilic nitrogen are 66-76 kJ mol-1 larger than those of analogous 2,6-dihalopyridines with less nucleophilic nitrogen. The experimental and DFT results show that the electronic influence of ortho-halogen substituents on pyridinic nitrogen leads to a completely different preference for the coordination bonding of halogen(I) ions, providing new insights into bonding in halogen(I) chemistry.

Halogen Bonding and/or Covalent Bond: Analogy of 3c-4e N···I···X (X = Cl, Br, I, and N) Interactions in Neutral, Cationic, and Anionic Complexes

X-ray structural measurements and computational analysis demonstrated the similarity of the geometries and electronic structures of the X-I···N (X = Cl, Br, I, and N) bonding in strong halogen-bonded (HaB) complexes and in the anionic or cationic halonium ions. In particular, I···N bond lengths in the solid-state associations formed by strong HaB donors (e.g., I2, IBr, ICl, and N-iodosuccinimide) and acceptors (e.g., quinuclidine or pyridines) were in the same range of 2.3 ± 0.1 Å as those in the halonium ions [e.g., the bis(quinuclidine)iodonium cation or the 1,1'-iodanylbis(pyrrolidine-2,5-dione) anion]. In all cases, bond lengths were much closer to those of the N-I covalent bond than to the van der Waals separations of these atoms. The strong N···I bonding in the HaB complexes led to a substantial charge transfer, lengthening and weakening of the I···X bonds, and polarization of the HaB donors. As a result, the central iodine atoms in the strong HaB complexes bear partial positive charges akin to those in the halonium ions. The energies and Mayer bond orders for both N···I and I···X bonds in such associations are also comparable to those in the halonium ions. The similarity of the bonding in such complexes and in halonium ions was further supported by the analysis of electron densities and energies at bond critical (3, -1) points in the framework of the quantum theory of atoms in molecules and by the density overlap region indicator. Overall, all these data point out the analogy of the symmetric N···I···N bonding in the halonium ions and the asymmetric X···I···N bonding in the strong HaB complexes, as well as the weakly covalent character of these 3c-4e interactions.

Supramolecular axial chirality in [N-I-N]+-type halogen bonded dimers

Axial chiral molecules are extensively used as skeletons in ligands for asymmetric catalysis and as building blocks of chiroptical materials. Designing axial chirality at the supramolecular level potentially endows a material with dynamic tunability and adaptivity. In this work, for the first time, we have reported a series of halogen-bonded dimeric complexes with axial chirality that were formed by noncovalent bonds. The [N-I-N]+-type halogen bond is highly directional and freely rotatable with good linearity and ultra-high bond energy; this bond was introduced to couple quinoline moieties with chiral substitutes. The resultant dimers were stable in solutions with thermo-resistance. Prominent steric effects from the 2' chiral pendant allowed the chirality to be transferred to aryl skeletons with induced preferred axial chirality and optical activities. Halogen-bonded complexation presented visible emissions to afford luminescent axial chiral materials, whereby circularly polarized fluorescence and phosphorescence were achieved. The [N-I-N]+-type halogen bond performed as a powerful tool to construct functional axial chiral compounds, enriching the toolbox for asymmetric synthesis and optics.

Linear bis-Coordinate Silver(I) and Iodine(I) Complexes with R3 R2 R1 N Tertiary Amines

Homoleptic [L-I-L]+ iodine(I) complexes (where L is a R3 R2 R1 N tertiary amine) were synthesized via the [L-Ag-L]+ → [L-I-L]+ cation exchange reaction. In solution, the amines form [R3 R2 R1 N-Ag-NR1 R2 R3 ]+ silver(I) complexes, which crystallize out from solution as the meso-[L-Ag-L]+ complexes, as characterized by X-ray crystallography. The subsequent [L-I-L]+ iodine(I) analogues were extremely reactive and could not be isolated in the solid state. Density functional theory (DFT) calculations were performed to study the Ag+ -N and I+ -N interaction energies in silver(I) and iodine(I) complexes, with the former ranging from -80 to -100 kJ mol-1 and latter from -260 to -279 kJ mol-1 . The X-ray crystal structures revealed Ag+ ⋅⋅⋅Cπ and Ag+ ⋅⋅⋅H-C short contacts between the silver(I) cation and flexible N-alkyl/N-aryl groups, which are the first of their kind in such precursor complexes.

N-X⋅⋅⋅O-N Halogen Bonds in Complexes of N-Haloimides and Pyridine-N-oxides: A Large Data Set Study

N-X⋅⋅⋅- O-N+ halogen-bonded systems formed by 27 pyridine N-oxides (PyNOs) as halogen-bond (XB) acceptors and two N-halosuccinimides, two N-halophthalimides, and two N-halosaccharins as XB donors are studied in silico, in solution, and in the solid state. This large set of data (132 DFT optimized structures, 75 crystal structures, and 168 1 H NMR titrations) provides a unique view to structural and bonding properties. In the computational part, a simple electrostatic model (SiElMo) for predicting XB energies using only the properties of halogen donors and oxygen acceptors is developed. The SiElMo energies are in perfect accord with energies calculated from XB complexes optimized with two high-level DFT approaches. Data from in silico bond energies and single-crystal X-ray structures correlate; however, data from solution do not. The polydentate bonding characteristic of the PyNOs' oxygen atom in solution, as revealed by solid-state structures, is attributed to the lack of correlation between DFT/solid-state and solution data. XB strength is only slightly affected by the PyNO oxygen properties [(atomic charge (Q), ionization energy (Is,min ) and local negative minima (Vs,min )], as the σ-hole (Vs,max ) of the donor halogen is the key determinant leading to the sequence N-halosaccharin>N-halosuccinimide>N-halophthalimide on the XB strength.

Iodine(I) and Silver(I) Complexes Incorporating 3-Substituted Pyridines

Building upon the first report of a 3-acetaminopyridine-based iodine(I) complex (1b) and its unexpected reactivity toward ^tBuOMe, several new 3-substituted iodine(I) complexes (2b-5b) have been synthesized. The iodine(I) complexes were synthesized from their analogous silver(I) complexes (2a-5a) via a silver(I) to iodine(I) cation exchange reaction, incorporating functionally related substituents as 3-acetaminopyridine in 1b; 3-acetylpyridine (3-Acpy; 2), 3-aminopyridine (3-NH₂py; 3), and 3-dimethylaminopyridine (3-NMe₂py; 4), as well as the strongly electron-withdrawing 3-cyanopyridine (3-CNpy; 5), to probe the possible limitations of iodine(I) complex formation. The individual properties of these rare examples of iodine(I) complexes incorporating 3-substituted pyridines are also compared to each other and contrasted to their 4-substituted counterparts which are more prevalent in the literature. While the reactivity of 1b toward etheric solvents could not be reproduced in any of the functionally related analogues synthesized herein, the reactivity of 1b was further expanded to a second etheric solvent. Reaction of bis(3-acetaminopyridine)iodine(I) (1b) and ⁱPr₂O gave [3-acetamido-1-(3-iodo-2-methylpentan-2-yl)pyridin-1-ium]PF₆ (1d), which demonstrated potentially useful C-C and C-I bond formation under ambient conditions.

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

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

Symmetry of Hydrogen Bonds: Application of NMR Method of Isotopic Perturbation and Relevance of Solvatomers

Short, strong, symmetric, low-barrier hydrogen bonds (H-bonds) are thought to be of special significance. We have been searching for symmetric H-bonds by using the NMR technique of isotopic perturbation. Various dicarboxylate monoanions, aldehyde enols, diamines, enamines, acid-base complexes, and two sterically encumbered enols have been investigated. Among all of these, we have found only one example of a symmetric H-bond, in nitromalonamide enol, and all of the others are equilibrating mixtures of tautomers. The nearly universal lack of symmetry is attributed to the presence of these H-bonded species as a mixture of solvatomers, meaning isomers (or stereoisomers or tautomers) that differ in their solvation environment. The disorder of solvation renders the two donor atoms instantaneously inequivalent, whereupon the hydrogen attaches to the less well solvated donor. We therefore conclude that there is no special significance to short, strong, symmetric, low-barrier H-bonds. Moreover, they have no heightened stability or else they would have been more prevalent.

Structure and Bonding of Halonium Compounds

The geometrical parameters and the bonding in [D···X···D]⁺ halonium compounds, where D is a Lewis base with N as the donor atom and X is Cl, Br, or I, have been investigated through a combined structural and computational study. Cambridge Structural Database (CSD) searches have revealed linear and symmetrical [D···X···D]⁺ frameworks with neutral donors. By means of density functional theory (DFT), molecular electrostatic potential (MEP), and energy decomposition analyses (EDA) calculations, we have studied the effect of various halogen atoms (X) on the [D···X···D]⁺ framework, the effect of different nitrogen-donor groups (D) attached to an iodonium cation (X = I), and the influence of the electron density alteration on the [D···I···D]⁺ halonium bond by variation of the R substituents at the N-donor upon the symmetry, strength, and nature of the interaction. The physical origin of the interaction arises from a subtle interplay between electrostatic and orbital contributions (σ-hole bond). Interaction energies as high as 45 kcal/mol suggest that halonium bonds can be exploited for the development of novel halonium transfer agents, in asymmetric halofunctionalization or as building blocks in supramolecular chemistry.

Halogen bonds with carbenes acting as Lewis base units: complexes of imidazol-2-ylidene: theoretical analysis and experimental evidence

ωB97XD/aug-cc-pVDZ and ωB97XD/aug-cc-pVTZ calculations were performed on complexes of imidazol-2-ylidene that are linked by halogen bonds. This singlet carbene acts as the Lewis base through a lone electron pair located at the carbon centre. The XCCH, XCN and X₂ units were chosen here as those that interact through the X Lewis acid halogen centre (X = Cl, Br and I); if X = F the complexes are linked by interactions which are not classified as halogen bonds. The properties of interactions that occur in complexes are analyzed using the results of DFT calculations which are supported by parameters derived from the Quantum Theory of Atoms in Molecules, QTAIM, and the Natural Bond Orbital, NBO, approaches. The energy decomposition analysis, EDA, applied here provided additional characteristics of interactions linking complexes analyzed. The majority of complexes are linked by the medium in strength and strong halogen bonds which often possess characteristics typical for covalent bonds. Searches through the Cambridge Structural Database were also performed and structures analogues to complexes analyzed theoretically were found, and these structures are also discussed in this study.

Protocol for efficient dearomatization of N-heteroaromatics with halogen(I) complex catalyst

In this protocol, we describe the application of a halogen(I) complex as a highly active non-metallic complex catalyst. Specifically, we present a detailed guide to synthesize the halogen(I) complex catalyst and utilize it as an anion-binding catalyst for the Mukaiyama-Mannich-type reaction of N-heteroaromatics such as pyridines. By utilizing a simple catalyst preparation approach and relatively low catalyst loading, the steps outlined in this protocol contribute to the rapid development of useful substances such as pharmaceuticals and functional materials. For complete details on the use and execution of this protocol, please refer to Oishi et al. (2022).1.

Biomimetic Cationic Cyclopropanation Enables an Efficient Chemoenzymatic Synthesis of 6,8-Cycloeudesmanes

Cationic cyclopropanation involves the γ-elimination at carbocations to form a new σ-C-C bond through proton loss. While exceedingly rare in bulk solution, it is recognized as one of the main biosynthetic cyclopropanation pathways. Despite the rich history of bioinspired synthetic chemistry, cationic cyclopropanation has not been appropriated for the synthetic toolbox, likely due to the preference of carbocations to undergo competing E1 β-elimination pathways. Here, we present an in-depth synthetic and computational study of cationic cyclopropanation, focusing on the 6,8-cycloeudesmanes as a platform for this investigation. We were able to apply biomimetic cationic cyclopropanation to the synthesis of several 6,8-cycloeudesmanes and non-natural analogues─in doing so, we showcase the power of this transformation in the preparation of complex cyclopropanes.

On pyridine chloronium cations

We present the first solid-state structural evidence of mono- and bis(pyridine)chloronium cations. The latter was synthesized from a mixture of pyridine, elemental chlorine and sodium tetrafluoroborate in propionitrile at low temperatures. The mono(pyridine) chloronium cation was realized with the less reactive pentafluoropyridine, using ClF, AsF₅, and C₅F₅N in anhydrous HF. During the course of this study, we also investigated pyridine dichlorine adducts and found a surprising disproportionation reaction of chlorine that depended on the substitutional pattern of the pyridine. Electron richer dimethylpyridine (lutidine) derivatives favor full disproportionation into a positively and a negatively charged chlorine atom which forms a trichloride monoanion, while unsubstituted pyridine forms a 1 : 1 py·Cl₂ adduct.

Why much of Chemistry may be indisputably non-bonded?

In this compendium, the wide scope of all intermolecular interactions ever known has been revisited, in particular giving emphasis the capability of much of the elements of the periodic table to form non-covalent contacts. Either hydrogen bonds, dihydrogen bonds, halogen bonds, pnictogen bonds, chalcogen bonds, triel bonds, tetrel bonds, regium bonds, spodium bonds or even the aerogen bond interactions may be cited. Obviously that experimental techniques have been used in some works, but it was through the theoretical methods that these interactions were validate, wherein the QTAIM integrations and SAPT energy partitions have been useful in this regard. Therefore, the great goal concerns to elucidate the interaction strength and if the intermolecular system shall be total, partial or non-covalently bonded, wherein this last one encompasses the most majority of the intermolecular interactions what leading to affirm that chemistry is debatably non-bonded.

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.

Halogen-bonded shape memory polymers

Halogen bonding (XB), a non-covalent interaction between an electron-deficient halogen atom and a Lewis base, is widely adopted in organic synthesis and supramolecular crystal engineering. However, the roadmap towards materials applications is hindered by the challenges in harnessing this relatively weak intermolecular interaction to devise human-commanded stimuli-responsive soft materials. Here, we report a liquid crystalline network comprising permanent covalent crosslinks and dynamic halogen bond crosslinks, which possess reversible thermo-responsive shape memory behaviour. Our findings suggest that I···N halogen bond, a paradigmatic motif in crystal engineering studies, enables temporary shape fixation at room temperature and subsequent shape recovery in response to human body temperature. We demonstrate versatile shape programming of the halogen-bonded polymer networks through human-hand operation and propose a micro-robotic injection model for complex 1D to 3D shape morphing in aqueous media at 37 °C. Through systematic structure-property-performance studies, we show the necessity of the I···N crosslinks in driving the shape memory effect. The halogen-bonded shape memory polymers expand the toolbox for the preparation of smart supramolecular constructs with tailored mechanical properties and thermoresponsive behaviour, for the needs of, e.g., future medical devices.

[N···I···N]+ Type Halogen-Bonding-Driven Supramolecular Helical Polymers with Modulated Chirality

The [N···I···N]⁺ type halogen bond has been utilized to synthesize supramolecular architectures, while the applications in constructing helical motifs and modulating supramolecular chirality have been unexplored so far. In this work, the [N···I···N]⁺ halogen bond was introduced to drive the formation of supramolecular helical polymers via a Ag(I) coordination intermediate, showing tunable supramolecular chirality. Pyridine segments were conjugated to the asymmetric ferrocene skeleton, which show "open" and "closed" geometry depending on the sp² N positions. Coordination with Ag(I) generated one-dimensional (1D) double helices and 2D helicates featured the [Ag(O)···I···Ag(O)]⁺ bond, which further stacked into 3D porous frameworks with chiral channels and adjustable pore sizes. Ionic exchange afforded 1D supramolecular helical polymers in solution phases driven by the [N···I···N]⁺ type halogen bonds, which was evidenced by the experimental results and density functional theory calculation. Fc2 exclusively demonstrated tunable supramolecular chirality in the formation of coordinated and halogen bonded polymers. In addition, solvent change would further inverse the helicity of halogen bonded supramolecular helical polymers depending on the rotation of the ferrocenyl core whose "closed" and "open" states were accompanied by the breakage of intramolecular hydrogen bonds. This work introduces a [N···I···N]⁺ type ionic halogen bond to prepare supramolecular helical polymers, providing multiple protocols in regulating helicity by ion exchange and solvent environments.

Construction of Halogen-Bonded Organic Frameworks (XOFs) as Novel Efficient Iodinating Agents

The structural diversity and the various applications of organic frameworks have attracted much attention in recent years. Recently, halogen-bonded organic frameworks (XOFs) became a novel member of these materials, thereby facilitating the exploration of the interesting structures as well as functions. Here we present two types of [N···I⁺···N] connected XOFs (XOF-TPy and XOF-TPEB) with two tridentate ligands as building blocks. XOF-TPy and XOF-TPEB were characterized by ¹H NMR, UV-vis, X-ray photoelectron spectroscopy (XPS), IR, SEM, and HR-TEM. Two-dimensional (2D) structural models were established based on powder X-ray diffraction (PXRD) data and theoretical simulations. Further experiment showed that these XOFs were excellent iodinating agents for the substituted arylboronic acids with either the electron-donating or electron-withdrawing groups upon heating without any catalyst. This research not only brings further understanding to the XOFs but also extends the applications of XOFs.

Carbonyl hypoiodites from pivalic and trimesic acid and their silver(I) intermediates

The first tris(O-I-N) carbonyl hypoiodites have been synthesised based on trimesic acid and pyridine or 4-methylpyridine, with their structures definitively confirmed by single crystal X-ray diffraction (SCXRD). The more soluble carbonyl hypoiodites based on pivalic acid have also been studied via NMR, SCXRD, and computational analyses, enabling the study of the direct silver(I) precursor and intermediates of the resulting carbonyl hypoiodites generated using a range of substituted pyridines.

Supramolecular networks by imine halogen bonding

Halogen bonding of neutral donors using imine groups of porous organic cage compounds as acceptors leads to the formation of halogen-bonded frameworks. We report the use of two different imine cages, in combination with three electron-poor halogen bond donors. Four resulting solid-state structures elucidated by single-crystal X-ray analysis are presented and analysed for the first time by plane-wave DFT calculations and QTAIM-analyses of the entire unit cells, demonstrating the formation of halogen bonds within the networks. The supramolecular frameworks can be obtained either from solution or mechanochemically by liquid-assisted grinding.

t BuOLi-promoted terminal alkyne functionalizations by aliphatic thiols and alcohols

Selective radical addition to terminal alkynes is always a difficult task to achieve because it gives a mixture of stereo- and regioisomers. Herein we describe the selective addition of aliphatic thiols or alcohols to N-phenylpropiolamides (terminal alkynes) using lithium tert-butoxide (^tBuOLi) in ethanol as a promoter. Mechanistically, it has been shown that the reaction proceeded through the generation of a thiyl radical intermediate, and the amide group in N-phenylpropiolamide could help in the activation of the alkyne, which led to thioacetalization via the formation of a (Z)-selective anti-Markovnikov vinyl sulfide. The (Z)-selectivity during the formation of vinyl sulfides was controlled by an intramolecular sulfur⋯oxygen interaction.

Dynamic [1]rotaxanes via a reversible covalent bond and host-guest anion recognition

Bambus[6]uril-based [1]rotaxanes were formed quantitatively, utilizing a bis(acyloxy)iodate(I) reversible covalent bond and host-guest anion recognition. These novel [1]rotaxanes exhibited a dynamic nature facilitating carboxylate component exchange reactions in acetonitrile.

Pushing the Limits of Characterising a Weak Halogen Bond in Solution

Detection and characterisation of very weak, non-covalent interactions in solution is inherently challenging. Low affinity, short complex lifetime and a constant battle against entropy brings even the most sensitive spectroscopic methods to their knees. Herein we introduce a strategy for the accurate experimental description of weak chemical forces in solution. Its scope is demonstrated by the detailed geometric and thermodynamic characterisation of the weak halogen bond of a non-fluorinated aryl iodide and an ether oxygen (0.6 kJ mol-1 ). Our approach makes use of the entropic advantage of studying a weak force intramolecularly, embedded into a cooperatively folding system, and of the combined use of NOE- and RDC-based ensemble analyses to accurately describe the orientation of the donor and acceptor sites. Thermodynamic constants (ΔG, ΔH and ΔS), describing the specific interaction, were derived from variable temperature chemical shift analysis. We present a methodology for the experimental investigation of remarkably weak halogen bonds and other related weak forces in solution, paving the way for their improved understanding and strategic use in chemistry and biology.

Do 2-coordinate iodine(I) and silver(I) complexes form Nucleophilic Iodonium Interactions (NIIs) in solution?

The interaction of a [bis(pyridine)iodine(I)]+ cation with a [bis(pyridine)silver(I)]+ cation, in which an iodonium ion acts as nucleophile by transferring electron density to the silver(I) cation, is reinvestigated herein. No...

Co-crystallization studies of the syn- and anti-atropisomers of triphenyl-based perfluorinated halogen bond donors with halides

Highly fluorinated halogen bond donors (based on meta- or para-terphenyl backbones) with two iodine substituents were co-crystallized with halides. Significant differences in the interaction motifs of the syn- and anti-atropisomers were found.

Dimeric iodine(i) and silver(i) cages from tripodal N-donor ligands via the [N–Ag–N]+ to [N–I–N]+ cation exchange reaction

Complexation of tripodal ligands with silver(I) salts generated M3L2 cage complexes that encapsulated anions within their cavities. Subsequent [N–Ag–N]+ to [N–I–N]+ cation exchange with I2 resulted in the corresponding halogen-bonded iodine(I) cages.

Ligand Exchange Among Iodine(I) Complexes

A detailed investigation of ligand exchange between iodine(I) ions in [N···I···N]+ halogen-bonded complexes is presented. Ligand exchange reactions were conducted to successfully confirm whether iodine(I) complex formation, via the classical...

Iodine(I) and Silver(I) Complexes of Benzoimidazole and Pyridylcarbazole Derivatives

The synthesis of iodine(I) complexes with either benzoimidazole or carbazole-derived sp² N-containing Lewis bases is described, as well as their corresponding silver(I) complexes. The addition of elemental iodine to the linear two-coordinate Ag(I) complexes produces iodine(I) complexes with a three-center four-electron (3c-4e) [N-I-N]⁺ bond. The ¹ H and ¹ H-¹⁵ N HMBC NMR studies unambiguously confirm the formation of the complexes in all cases via the [N-Ag-N]⁺ →[N-I-N]⁺ cation exchange, with the ¹⁵ N NMR chemical shift change between 94 to 111 ppm when compared to the free ligand. The single crystal X-ray crystallographic studies on eight I⁺ complexes revealed highly symmetrical [N-I-N]⁺ bonds with I-N bond distances of 2.21-2.26 Å and N-I-N angles of 177-180°, whilst some of the corresponding Ag⁺ complexes showed a clear deviation from linearity with N-Ag-N angles of ca. 150° and Ag-N bond distances of 2.09-2.18 Å.

Evaluation of Halogenopyridinium Cations as Halogen Bond Donors

We have performed a database survey and a structural and computational study of the potential and the limitations of halogenopyridinium cations as halogen bond donors. The database survey demonstrated that adding a positive charge on a halogenopyridine ring increases the probability that the halogen atom will participate in a halogen bond, although for chloropyridines it remains below 60%. Crystal structures of both protonated and N-methylated monohalogenated pyridinium cations revealed that the iodo- and bromopyridinium cations always form halogen-bonding contacts with the iodide anions shorter than the sum of the vdW radii, while chloropyridinium cations mostly participate in longer contacts or fail to form halogen bonds. Although a DFT study of the electrostatic potential has shown that both protonation and N-methylation of halogenopyridines leads to a considerable increase in the ESP of the halogen σ-hole, it is generally not the most positive site on the cation, allowing for alternate binding sites.

Macrocyclic complexes based on [N⋯I⋯N]+ halogen bonds

New 1-2 nm macrocyclic iodine(I) complexes prepared VIA a simple ligand exchange reaction manifest rigid 0.5-1 nm cavities that bind the hexafluorophosphate anion in the gas phase. The size of the cavities and the electrostatic interactions with the iodine(I) cations influence the anion binding properties of these macrocyclic complexes.

Are bis(pyridine)iodine(I) complexes applicable for asymmetric halogenation?

Enantiopure halogenated molecules are of tremendous importance as synthetic intermediates in the construction of pharmaceuticals, fragrances, flavours, natural products, pesticides, and functional materials. Enantioselective halofunctionalizations remain poorly understood and generally applicable procedures are lacking. The applicability of chiral trans-chelating bis(pyridine)iodine(i) complexes in the development of substrate independent, catalytic enantioselective halofunctionalization has been explored herein. Six novel chiral bidentate pyridine donor ligands have been designed, routes for their synthesis developed and their [N–I–N]⁺-type halogen bond complexes studied by ¹⁵N NMR and DFT. The chiral complexes encompassing a halogen bond stabilized iodenium ion are shown to be capable of efficient iodenium transfer to alkenes; however, without enantioselectivity. The lack of stereoselectivity is shown to originate from the availability of multiple ligand conformations of comparable energies and an insufficient steric influence by the chiral ligand. Substrate preorganization by the chiral catalyst appears a necessity for enantioselective halofunctionalization.

The enantioselectivity of the iodine(i) transfer process from chiral bis(pyridine)iodine(i) complexes to alkenes is explored.

The Influence of Secondary Interactions on the [N-I-N]+ Halogen Bond

[Bis(pyridine)iodine(I)]⁺ complexes offer controlled access to halonium ions under mild conditions. The reactivity of such stabilized halonium ions is primarily determined by their three-center, four-electron [N-I-N]⁺ halogen bond. We studied the importance of chelation, strain, steric hindrance and electrostatic interaction for the structure and reactivity of halogen bonded halonium ions by acquiring their ¹⁵ N NMR coordination shifts and measuring their iodenium release rates, and interpreted the data with the support of DFT computations. A bidentate ligand stabilizes the [N-I-N]⁺ halogen bond, decreasing the halenium transfer rate. Strain weakens the bond and accordingly increases the release rate. Remote modifications in the backbone do not influence the stability as long as the effect is entirely steric. Incorporating an electron-rich moiety close by the [N-I-N]⁺ motif increases the iodenium release rate. The analysis of the iodine(I) transfer mechanism highlights the impact of secondary interactions, and may provide a handle on the induction of stereoselectivity in electrophilic halogenations.

Carbonyl Hypoiodites as Extremely Strong Halogen Bond Donors

Neutral halogen-bonded O-I-N complexes were prepared from in situ formed carbonyl hypoiodites and aromatic organic bases. The carbonyl hypoiodites have a strongly polarized iodine atom with larger σ-holes than any known uncharged halogen bond donor. Modulating the Lewis basicity of the selected pyridine derivatives and carboxylates leads to halogen-bonded complexes where the classical O-I⋅⋅⋅N halogen bond transforms more into a halogen-bonded COO- ⋅⋅⋅I-N+ ion-pair (salt) with an asymmetric O-I-N moiety. X-ray analyses, NMR studies, and calculations reveal the halogen bonding geometries of the carbonyl hypoiodite-based O-I-N complexes, confirming that in the solid-state the iodine atom is much closer to the N-atom of the pyridine derivatives than its original position at the carboxylate O-atom.

A-X⋯σ Interactions-Halogen Bonds with σ-Electrons as the Lewis Base Centre

CCSD(T)/aug-cc-pVTZ//ωB97XD/aug-cc-pVTZ calculations were performed for halogen-bonded complexes. Here, the molecular hydrogen, cyclopropane, cyclobutane and cyclopentane act as Lewis base units that interact through the electrons of the H–H or C–C σ-bond. The FCCH, ClCCH, BrCCH and ICCH species, as well as the F₂, Cl₂, Br₂ and I₂ molecular halogens, act as Lewis acid units in these complexes, interacting through the σ-hole localised at the halogen centre. The Quantum Theory of Atoms in Molecules (QTAIM), the Natural Bond Orbital (NBO) and the Energy Decomposition Analysis (EDA) approaches were applied to analyse these aforementioned complexes. These complexes may be classified as linked by A–X···σ halogen bonds, where A = C, X (halogen). However, distinct properties of these halogen bonds are observed that depend partly on the kind of electron donor: dihydrogen, cyclopropane, or another cycloalkane. Examples of similar interactions that occur in crystals are presented; Cambridge Structural Database (CSD) searches were carried out to find species linked by the A–X···σ halogen bonds.