13C NMR study of halogen bonding of haloarenes: measurements of solvent effects and theoretical analysis

Redox-Responsive Halogen Bonding as a Highly Selective Interaction for Electrochemical Separations

Leveraging specific noncovalent interactions can broaden the mechanims for selective electrochemical separations beyond solely electrostatic interactions. Here, we explore redox-responsive halogen bonding (XB) for selective electrosorption in nonaqueous media, by taking advantage of directional interactions of XB alongisde a cooperative and synergistic ferrocene redox-center. We designed and evaluated a new redox-active XB donor polymer, poly(5-iodo-4-ferrocenyl-1-(4-vinylbenzyl)-1H-1,2,3-triazole) (P(FcTS-I)), for the electrochemically switchable binding and release of target organic and inorganic ions at a heterogeneous interface. Under applied potential, the oxidized ferrocene amplifies the halogen binding site, leading to significantly enhanced uptake and selectivity towards key inorganic and organic species, including chloride, bisulfate, and benzenesulfonate, compared to the open-circuit potential or the hydrogen bonding donor analog. Density functional theory calculations, as well as spectroscopic analysis, offer mechanistic insight into the degree of amplification of σ-holes at a molecular level, with selectivity modulated by charge transfer and dispersion interactions. Our work highlights the potential of XB in selective electrosorption by uniquely leveraging noncovalent interactions for redox-mediated electrochemical separations.

Unveiling the electronic structure peculiarities of phosphine selenides as NMR probes for non-covalent interactions: an experimental and theoretical study

In this work, R3PSe (R = Me, Et, n-Bu, t-Bu and Ph) were studied experimentally using NMR spectroscopy in solution and the solid-state in combinaton with quantum chemical methods. The study shows that the NMR parameters of these phosphine selenides, such as δP, δSe, and 1JPSe, are sensitive to subtle changes in the electronic environment of the P and Se atoms. Consequently, phosphine selenides R3PSe can serve as promising spectral probes for the detection and quantitative investigation of various non-covalent interactions. Additionally, the variations of R in phosphine selenides influence the observed NMR spectral parameters, primarily through effects such as π-backdonation and hyperconjugation, which have been observed experimentally and confirmed theoretically.

The Halogen Bond in Weakly Bonded Complexes and the Consequences for Aromaticity and Spin-Orbit Coupling

The halogen bond complexes CF3X⋯Y and C2F3X⋯Y, with Y = furan, thiophene, selenophene and X = Cl, Br, I, have been studied by using DFT and CCSD(T) in order to understand which factors govern the interaction between the halogen atom X and the aromatic ring. We found that PBE0-dDsC/QZ4P gives an adequate description of the interaction energies in these complexes, compared to CCSD(T) and experimental results. The interaction between the halogen atom X and the π-bonds in perpendicular orientation is stronger than the interaction with the in-plane lone pairs of the heteroatom of the aromatic cycle. The strength of the interaction follows the trend Cl < Br < I; the chalcogenide in the aromatic ring nor the hybridization of the C−X bond play a decisive role. The energy decomposition analysis shows that the interaction energy is dominated by all three contributions, viz., the electrostatic, orbital, and dispersion interactions: not one factor dominates the interaction energy. The aromaticity of the ring is undisturbed upon halogen bond formation: the π-ring current remains equally strong and diatropic in the complex as it is for the free aromatic ring. However, the spin-orbit coupling between the singlet and triplet π→π* states is increased upon halogen bond formation and a faster intersystem crossing between these states is therefore expected.

An accurate vibrational signature in halogen bonded molecular crystals

The far infrared (FIR) and Raman fingerprints of the halogen bond in two representative 1D halogen bonded networks based on the recognition of TFIB, tetrafluorodiiodobenzene, with piperazine or azopyridine, have been accurately identified. It was demonstrated that the signature of the halogen bonding in the solid state, especially the N⋯I signal can be simply and directly evidenced in the far infrared region. The DFT theoretical calculations identified the N⋯I interaction in the molecular crystals and allowed estimation of the corresponding energies and distances of the involved halogen bonds, in accordance with the cristallographic data.

Functional polydiynes prepared by metathesis cyclopolymerization of 1,7-dihalogen-1,6-heptadiyne derivatives

The MCP route used for the polymerization of 1,6-heptadiynes was successfully applied to the polymerization of 1,7-dihalogen-1,6-heptadiynes, and the target polymers were obtained in high yield with high molecular weight and unique UCST behavior.

Direct matrix isolation IR spectroscopic evidence of halogen bonding from a comparative study of complexes of CBr4 and CCl4with acetone and formic acid

Binary complexes of acetone and formic acid with tetrahalomethanes CBr₄ and CCl₄ have been isolated in argon matrix. Spectral shifts in the characteristic ν_C=O region of acetone, as well as in the fingerprint regions, are unambiguously assigned to the formation of halogen bond involving one of the halogen atoms on CBr₄/CCl₄ as donor, and the carbonyl oxygen of acetone as acceptor. The higher magnitude of shifts of ν_C=O and the fingerprint vibrations for the CBr₄ complex, as compared to the CCl₄ complex, is consistent with theoretical predictions of higher value of positive electrostatic potential in the "σ-hole" region of the former, and hence its higher susceptibility to halogen bonding. The formation of halogen bonded complexes involving formic acid as acceptor and CBr₄/CCl₄ as donors is also being reported for the first time. In this case too, distinct shifts are obtained in the ν_C=O as well as ν_C-O regions of formic acid, which again are significantly larger in magnitude for the CBr₄ complex, as compared to the CCl₄ complex. Electronic structure calculations have been carried out using different theoretical methods to identify the various possible structural isomers of the halogen bonded complexes, and to obtain relevant information regarding their energies and intermolecular geometrical parameters. In addition, NBO and AIM analysis have been carried out to understand the role of local interactions at the halogen bonded interface. Such predicted data are found to be consistent with experimental observations, and re-assert the stronger nature of CBr₄ as halogen bond donor, as compared to CCl₄.

5-Iodo-1-Arylpyrazoles as Potential Benchmarks for Investigating the Tuning of the Halogen Bonding

5-Iodo-1-arylpyrazoles are interesting templates for investigating the halogen bond propensity in small molecules other than the already well-known halogenated molecules such as tetrafluorodiiodobenzene. Herein, we present six compounds with different substitution on the aryl ring attached at position 1 of the pyrazoles and investigate them in the solid state in order to elucidate the halogen bonding significance to the crystallographic landscape of such molecules. The substituents on the aryl ring are generally combinations of halogen atoms (Br, Cl) and various alkyl groups. Observed halogen bonding types spanned by these six 5-iodopyrazoles included a wide variety, namely, C–I⋯O, C–I⋯π, C–I⋯Br, C–I⋯N and C–Br⋯O interactions. By single crystal X-ray diffraction analysis combined with the descriptive Hirshfeld analysis, we discuss the role and influence of the halogen bonds among the intermolecular interactions.

NMR Quantification of Halogen-Bonding Ability To Evaluate Catalyst Activity

Quantification of halogen-bonding abilities is described for a series of benzimidazolium-, imidazolium- and bis(imidazolium) halogen-bond donors (XBDs) using ³¹P NMR spectroscopy. The measured Δδ(³¹P) values correlate with calculated activation free energy ΔG^‡ and catalytic activity for a Friedel-Crafts indole addition. This rapid method also serves as a sensitive indicator for Brønsted acid impurities.

Halogen-Bond-Mediated Self-Assembly of Polymer-Resorcinarene Complexes

A new supramolecular system based on halogen-bonded macromolecular substances is presented. Binding and complex formation between a halogen bond acceptor N-benzyl ammonium resorcinarene bromide and a library of polymeric halogen bond donors based on iodotetrafluorophenoxy functionality is shown. The complex formation was confirmed in liquid state by dynamic light scattering and transmission electron microscopy. Spectroscopic measurements in the solid state verify the halogen bonding. In particular, the study shows that both homopolymers and polyethylene glycol block copolymers act as effective halogen bond donors leading to polymer-architecture-dependent complex morphologies.

Dual Geometry Schemes in Tetrel Bonds: Complexes between TF₄ (T = Si, Ge, Sn) and Pyridine Derivatives

When an N-base approaches the tetrel atom of TF₄ (T = Si, Ge, Sn) the latter molecule deforms from a tetrahedral structure in the monomer to a trigonal bipyramid. The base can situate itself at either an axial or equatorial position, leading to two different equilibrium geometries. The interaction energies are considerably larger for the equatorial structures, up around 50 kcal/mol, which also have a shorter R(T··N) separation. On the other hand, the energy needed to deform the tetrahedral monomer into the equatorial structure is much higher than the equivalent deformation energy in the axial dimer. When these two opposite trends are combined, it is the axial geometry which is somewhat more stable than the equatorial, yielding binding energies in the 8–34 kcal/mol range. There is a clear trend of increasing interaction energy as the tetrel atom grows larger: Si < Ge < Sn, a pattern which is accentuated for the binding energies.

Halogen bonding in 5-iodo-1-arylpyrazoles investigated in the solid state and predicted by solution13C-NMR spectroscopy

Halogen bonding as important directional forces in the supramolecular structure of iodinated 1-arylpyrazoles.

Comparative Strengths of Tetrel, Pnicogen, Chalcogen, and Halogen Bonds and Contributing Factors

Ab initio calculations are employed to assess the relative strengths of various noncovalent bonds. Tetrel, pnicogen, chalcogen, and halogen atoms are represented by third-row atoms Ge, As, Se, and Br, respectively. Each atom was placed in a series of molecular bonding situations, beginning with all H atoms, then progressing to methyl substitutions, and F substituents placed in various locations around the central atom. Each Lewis acid was allowed to engage in a complex with NH₃ as a common nucleophile, and the strength and other aspects of the dimer were assessed. In the context of fully hydrogenated acids, the strengths of the various bonds varied in the pattern of chalcogen > halogen > pnicogen ≈ tetrel. Methyl substitution weakened all bonds, but not in a uniform manner, resulting in a greatly weakened halogen bond. Fluorosubstitution strengthened the interactions, increasing its effect as the number of F atoms rises. The effect was strongest when the F atom lay directly opposite the base, resulting in a halogen > chalcogen > pnicogen > tetrel order of bond strength. Replacing third-row atoms by their second-row counterparts weakened the bonds, but not uniformly. Tetrel bonds were weakest for the fully hydrogenated acids and surpassed pnicogen bonds when F had been added to the acid.

Quantifying the Effects of Halogen Bonding by Haloaromatic Donors on the Acceptor Pyrimidine

The effects of intermolecular interactions by a series of haloaromatic halogen bond donors on the normal modes and chemical shifts of the acceptor pyrimidine are investigated by Raman and NMR spectroscopies and electronic structure computations. Halogen-bond interactions with pyrimidine's nitrogen atoms shift normal modes to higher energy and upfield shift ¹ H and ¹³ C NMR peaks in adjacent nuclei. This perturbation of vibrational normal modes is reminiscent of the effects of hydrogen bonded networks of water, methanol, or silver on pyrimidine. The unexpected observation of vibrational red shifts and downfield ¹³ C NMR shifts in some complexes suggests that other intermolecular forces such as π interactions are competing with halogen bonding. Natural bond orbital analyses indicate a wide range of charge transfer is possible from pyrimidine to different haloaromatic donors and computed halogen bond binding energies can be larger than a typical hydrogen bond. These results emphasize the importance in strategic selection of substituents and electron withdrawing groups in developing supramolecular structures based on halogen bonding.

The interaction of CCl4 with Ng (Ng = He, Ne, Ar), O2, D2O and ND3: rovibrational energies, spectroscopic constants and theoretical calculations

This investigation generated rovibrational energies and spectroscopic constants for systems of CCl₄ with Ng (Ng = He, Ne, Ar), O₂, D₂O and ND₃ from scattering experimental data, and the results presented are of interest for microwave spectroscopy studies of small halogenated molecules. The rovibrational spectra were obtained through two different approaches (Dunham and DVR) within the improved Lennard Jones (ILJ) model. Spectra were also generated within ordinary Lennard Jones and deviations suggest that the ILJ model should be preferred due to interactions beyond dispersion forces presented in these systems. Data from the literature and additional high level quantum mechanical calculations presented in this work show that these systems should not be considered as van der Waals complexes due to halogen bonding (HB) interactions, and this is especially true for the CCl₄-D₂O and CCl₄-ND₃ complexes. The charge displacement from the latter systems are one order of magnitude higher than the values from literature for CCl₄ and He, Ne, Ar and O₂ systems, and show significant deviations between DFT and Hartree-Fock values not previously reported in the literature.

Direct Synthesis of Symmetrical Azines from Alcohols and Hydrazine Catalyzed by a Ruthenium Pincer Complex: Effect of Hydrogen Bonding

Azines (2,3-diazabuta-1,3-dienes) are a widely used class of compounds with conjugated C=N double bonds. Herein, we present a direct synthesis of azines from alcohols and hydrazine hydrate. The reaction, catalyzed by a ruthenium pincer complex, evolves dihydrogen and can be run in a base-free version. The dehydrogenative coupling of benzylic and aliphatic alcohols led to good conversions and yields. Spectroscopic evidence for a hydrazine-coordinated dearomatized ruthenium pincer complex was obtained. Isolation of a supramolecular crystalline compound provided evidence for the important role of hydrogen bonding networks under the reaction conditions.

Magnitude and Directionality of Halogen Bond of Benzene with C6F5X, C6H5X, and CF3X (X = I, Br, Cl, and F)

Geometries of benzene complexes with C6F5X, C6H5X, and CF3X (X is I, Br, Cl, and F) were optimized, and their interaction energies were evaluated. The CCSD(T) interaction energies at the basis set limit (Eint) of C6F5X (X is I, Br, Cl, and F) with benzene were -3.24, -2.88, -2.31, and -0.92 kcal mol(-1). Eint of C6H5X (X is I, Br, and Cl) with benzene were -2.31, -1.97, and -1.48 kcal mol(-1). The fluorination of halobenzenes slightly enhances the attraction. Eint of CF3X (X is I, Br, Cl, and F) with benzene (-3.11, -2.74, -2.22, and -0.71 kcal mol(-1)) were very close to Eint of corresponding C6F5X with benzene. In contrast to the halogen bond of iodine and bromine with pyridine (n-type halogen bond acceptor) where the main cause of the attraction is the electrostatic interactions, that of halogen bond with benzene (p-type acceptor) is dispersion interaction. In the halogen bonds with p-type acceptors (halogen-π interactions), the electrostatic interactions and induction interactions are small. The overall orbital-orbital interactions are repulsive. The directionality of halogen bonds with p-type acceptors is very weak, owing to the weak electrostatic interactions, in contrast to the strong directionality of the halogen bonds with n-type acceptors and hydrogen bonds.

Non-covalent C-Cl…π interaction in acetylene-carbon tetrachloride adducts: Matrix isolation infrared and ab initio computational studies

Non-covalent halogen-bonding interactions between π cloud of acetylene (C2H2) and chlorine atom of carbon tetrachloride (CCl4) have been investigated using matrix isolation infrared spectroscopy and quantum chemical computations. The structure and the energies of the 1:1 C2H2-CCl4 adducts were computed at the B3LYP, MP2 and M05-2X levels of theory using 6-311++G(d,p) basis set. The computations indicated two minima for the 1:1 C2H2-CCl4 adducts; with the C-Cl…π adduct being the global minimum, where π cloud of C2H2 is the electron donor. The second minimum corresponded to a C-H…Cl adduct, in which C2H2 is the proton donor. The interaction energies for the adducts A and B were found to be nearly identical. Experimentally, both C-Cl…π and C-H…Cl adducts were generated in Ar and N2 matrixes and characterized using infrared spectroscopy. This is the first report on halogen bonded adduct, stabilized through C-Cl…π interaction being identified at low temperatures using matrix isolation infrared spectroscopy. Atoms in Molecules (AIM) and Natural Bond Orbital (NBO) analyses were performed to support the experimental results. The structures of 2:1 ((C2H2)2-CCl4) and 1:2 (C2H2-(CCl4)2) multimers and their identification in the low temperature matrixes were also discussed.

The Halogen Bond

The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design.

Cooperative halogen bonds in V-shaped H3N·X1X2·X3Y (X1, X2, X3 = Cl and Br; Y = F, Cl and Br) complexes

The dihalogen molecule can simultaneously interact with NH₃ and another dihalogen molecule, forming a V-shaped trimer via cooperative halogen bonds.

Investigations into the Nature of Halogen Bonding Including Symmetry Adapted Perturbation Theory Analyses

In recent years it has been recognized that, because of their unique properties, halogen bonds have tremendous potential in the development of new pharmaceutical compounds and materials. In this study we investigate the phenomenon of halogen bonding by carrying out ab initio calculations on the halomethane-formaldehyde complexes as well as the fluorine substituted FnH3-nCX···OCH2 dimers, where the halogen bonding halogens (X) are chlorine, bromine, and iodine. Coupled cluster (CCSD(T)/aug-cc-pVTZ) calculations indicate that the binding energies for these type of interactions lie in the range between -1.05 kcal/mol (H3CCl···OCH2) and -3.72 kcal/mol (F3CI···OCH2). One of the most important findings in this study is that, according to symmetry adapted perturbation theory (SAPT) analyses, halogen bonds are largely dependent on both electrostatic and dispersion type interactions. As the halogen atom involved in halogen bonding becomes larger the interaction strength for this type of interaction also gets larger and, interestingly, more electrostatic (and less dispersive) in character. Halogen bonding interactions also become stronger and more electrostatic upon substitution of (the very electronegative) fluorines onto the halomethane molecule.

Solution and solid-phase halogen and C-H hydrogen bonding to perrhenate

(1)H NMR spectroscopic and X-ray crystallographic investigations of a 1,3-bis(4-ethynyl-3-iodopyridinium)benzene scaffold with perrhenate reveal strong halogen bonding in solution, and bidentate association in the solid state. A nearly isostructural host molecule demonstrates significant C-H hydrogen bonding to perrhenate in the same phases.

Identification of unusual C–Cl⋯π contacts in 2-(alkylamino)-3-chloro-1,4-naphthoquinones: effect of N-substituents on crystal packing, fluorescence, redox and anti-microbial properties

XRD study demonstrates the opening of unusual C–Cl⋯π synthon in 2-(alkylamino)-3-chloro-1,4-naphthoquinone. Notably, compound holding N-pyridylmethyl exhibits enhanced activity against S. aureus and proved to be more potent than ciprofloxacin.

Investigation of hydrogen bonding patterns in a series of multi-component molecular solids formed by tetrabromoterephthalic acid with selected N-heterocycles

Hydrogen bonding patterns in a series of multi-component molecules constructed by tetrabromoterephthalic acid with N-heterocycles are discussed in this study.

Hydrogen and halogen bonding in a concerted act of anion recognition: F− induced atmospheric CO2 uptake by an iodophenyl functionalized simple urea receptor

Halogen bonding plays a key role in the fixation of atmospheric CO₂ because air-stable crystals of HCO₃⁻ dimer by a simple urea based para-iodo substituted acyclic receptor in the presence of n-TBA salt of F⁻ in MeCN solution.

Exploring the Molecular Basis of Qo bc1 Complex Inhibitors Activity to Find Novel Antimalarials Hits

Cytochrome bc1 complex is a crucial element in the mitochondrial respiratory chain, being indispensable for the survival of several species of Plasmodia that cause malaria and, therefore, it is a promising target for antimalarial drug development. We report a molecular docking study building on the most recently obtained X-ray structure of the Saccharomyces cerevisiae bc1 complex (PDB code: 3CX5) using several reported inhibitors with experimentally determined IC50 values against the Plasmodium falciparum bc1 complex. We produced a molecular docking model that correlated the calculated binding free energy with the experimental inhibitory activity of each compound. This Qo model was used to search the drug-like database included in the MOE package for novel potential bc1 complex inhibitors. Twenty three compounds were chosen to be tested for their antimalarial activity and four of these compounds demonstrated activity against the chloroquine-resistant W2 strain of P. falciparum. The most active compounds were also active against the atovaquone-resistant P. falciparum FCR3 strain and S. cerevisiae. Our study suggests the validity of the yeast bc1 complex structure as a model for the discovery of new antimalarial hits.

Structures, properties and nature of DMSO-XY (XY=ClF and BrF) complexes: redshift and blueshift of S=O stretch

The DMSO-XY (XY=ClF and BrF) complexes have been investigated with quantum chemical calculations. In general, two minima complexes were found, one with an O···X halogen bond and the other one with a S···X halogen bond. The former is more stable than the latter. Additionally, one first order saddle point complex was also observed. The interaction energies in the S complexes suffer a prominent influence from the calculation methods. At the CCSD(T)/aug-cc-pVDZ level, the interaction energies are calculated to be -9.19 and -12.73 kcal/mol for the Cl and Br global minima, respectively. Both complexes have also been evidenced to be stable at room temperature. The SO stretch vibration exhibits a red shift at the global minimum but a blue shift at the local minimum, whereas the CSC and CH stretch vibrations move to high frequency in both cases. The energy decomposition analyses indicate that the electrostatic interaction plays a dominant role in stabilizing these halogen-bonded complexes.

Synthesis and physico-chemical properties in aqueous medium of all possible isomeric bromo analogues of benzo-1H-triazole, potential inhibitors of protein kinases

In ongoing studies on the role of the individual bromine atoms of 4,5,6,7-tetrabromobenzotriazole (TBBt) in its relatively selective inhibition of protein kinase CK2α, we have prepared all the possible two mono-, four di-, and two tri-bromobenzotriazoles and determined their physicochemical properties in aqueous medium. They exhibited a general trend of a decrease in solubility with an increase in the number of bromines on the benzene ring, significantly modulated by the pattern of substitution. For a given number of attached bromines, this was directly related to the electronic effects resulting from different sites of substitution, leading to marked variations of pK(a) values for dissociation of the triazole proton. Experimental data (pK(a), solubility) and ab initio calculations demonstrated that hydration of halogenated benzotriazoles is driven by a subtle balance of hydrophobic and polar interactions. The combination of QM-derived free energies for solvation and proton dissociations was found to be a reasonably good predictor of inhibitory activity of halogenated benzotriazoles vs CK2α. Since the pattern of halogenation of the benzene ring of benzotriazole has also been shown to be one of the determinants of inhibitory potency vs some viruses and viral enzymes, the present comprehensive description of their physicochemical properties should prove helpful in efforts to elucidate reaction mechanisms, including possible halogen bonding, and the search for more selective and potent inhibitors.