Theoretical, Solid-State, and Solution Quantification of the Hydrogen Bond-Enhanced Halogen Bond

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

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

From Polyester Plastics to Diverse Monomers via Low-Energy Upcycling

Polyester plastics, constituting over 10% of the total plastic production, are widely used in packaging, fiber, single-use beverage bottles, etc. However, their current depolymerization processes face challenges such as non-broad spectrum recyclability, lack of diversified high-value-added depolymerization products, and crucially high energy consumption. Herein, an efficient strategy is developed for dismantling the compact structure of polyester plastics to achieve diverse monomer recovery. Polyester plastics undergo swelling and decrystallization with a low depolymerization energy barrier via synergistic effects of polyfluorine/hydrogen bonding, which is further demonstrated via density functional theory calculations. The swelling process is elucidated through scanning electron microscopy analysis. Obvious destruction of the crystalline region is demonstrated through X-ray crystal diffractometry curves. PET undergoes different aminolysis efficiently, yielding nine corresponding high-value-added monomers via low-energy upcycling. Furthermore, four types of polyester plastics and five types of blended polyester plastics are closed-loop recycled, affording diverse monomers with exceeding 90% yields. Kilogram-scale depolymerization of real polyethylene terephthalate (PET) waste plastics is successfully achieved with a 96% yield.

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

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

Combination of Hydrogen and Halogen Bonds in the Crystal Structures of 5-Halogeno-1H-isatin-3-oximes: Involvement of the Oxime Functionality in Halogen Bonding

Various functional groups have been considered as acceptors for halogen bonds, but the oxime functionality has received very little attention in this context. In this study, we focus on the analysis of the hydrogen and halogen bond preferences observed in the crystal structures of 5-halogeno-1H-isatin-3-oximes. These molecules can be involved in various non-covalent interactions, and the competition between these interactions has a decisive influence on their self-organization. In particular, we were interested to see whether the crystal structures of 5-halogeno-1H-isatin-3-oximes, especially bromine- and iodine-substituted ones, are characterized by the presence of halogen bonds formed with the oxime functionality. The oxime group proved its ability to compete with the other strong donor and acceptor sites by participating in the formation of cyclic hydrogen-bonded heterosynthons oxime∙∙∙amide and Ooxime∙∙∙Br/I halogen bonds.

Elucidating the Role of Electron-Donating Groups in Halogen Bonding

Computational quantum chemical techniques were utilized to systematically examine how electron-donating groups affect the electronic and spectroscopic properties of halogen bond donors and their corresponding complexes. Unlike the majority of studies on halogen bonding, where electron-withdrawing groups are utilized, this work investigates the influence of electron-donating substituents within the halogen bond donors. Statistical analyses were performed on the descriptors of halogen bond donors in a prescribed set of archetype, halo-alkyne, halo-benzene, and halo-ethynyl benzene halogen bond systems. The σ-hole magnitude, binding and interaction energies, and the vibrational X···N local force constant (where X = Cl, Br, I, and At) were found to correlate very well in a monotonic and linear manner with all other properties studied. In addition, enhanced halogen bonds were found when the systems contained electron-donating groups that could form intramolecular hydrogen bonds with the electronegative belt of the halogen atom and adjacent linker features.

The interplay between hydrogen and halogen bonding: substituent effects and their role in the hydrogen bond enhanced halogen bond

The hydrogen bond enhanced halogen bond (HBeXB) has recently been used to effectively improve anion binding, organocatalysis, and protein structure/function. In this study, we present the first systematic investigation of substituent effects in the HBeXB. NMR analysis confirmed intramolecular HBing between the amine and the electron-rich belt of the XB donor (N-H⋯I). Gas-phase density functional theory studies showed that the influence of HBing on the halogen atom is more sensitive to substitution on the HB donor ring (R1). The NMR studies revealed that the intramolecular HBing had a significant impact on receptor performance, resulting in a 50-fold improvement. Additionally, linear free energy relationship (LFER) analysis was employed for the first time to study the substituent effect in the HBeXB. The results showed that substituents on the XB donor ring (R2) had a competing effect where electron donating groups strengthened the HB and weakened the XB. Therefore, selecting an appropriate substituent on the adjacent HB donor ring (R1) could be an alternative and effective way to enhance an electron-rich XB donor. X-ray crystallographic analysis demonstrated that intramolecular HBing plays an important role in the receptor adopting the bidentate conformation. Taken together, the findings imply that modifying distal substituents that affect neighboring noncovalent interactions can have a similar impact to conventional para substitution substituent effects.

Enhancement of Halogen Bond Strength by Intramolecular H-Bonds

Quantum calculations study the potential of an intramolecular H-bond between the halogen atom (X) of a halobenzene and a substituent placed ortho to it, to amplify the ability of X to engage in a halogen bond (XB) with a Lewis base. H-bonding substituents NH₂, CH₂CH₂OH, CH₂OH, OH, and COOH were added to halobenzenes (X = Cl, Br, I). The amino group had little effect, but those containing OH increased the CX···N XB energy to a NH₃ nucleophile by about 0.5 kcal/mol; the increment associated with COOH is larger, nearly 2 kcal/mol. These energy increments were approximately doubled if two such H-bonding substituents are present. Combining a pair of ortho COOH groups with an electron-withdrawing NO₂ group in the para position has a particularly large effect, raising the XB energy by about 4 kcal/mol, which can amount to as much as a 4-fold magnification.

Keggin-Type Anions as Halogen Bond Acceptors

To study the potential of Keggin-type polyoxometalate anions to act as halogen bond acceptors, we have prepared a series of 10 halogen-bonded compounds starting from phosphomolybdic and phosphotungstic acid and halogenopyridinium cations as halogen (and hydrogen) bond donors. In all the structures, the cations and the anions were interconnected by halogen bonds, more often with terminal M=O oxygen atoms than with bridging oxygen atoms as acceptors. In four structures comprising protonated iodopyridinium cations capable of forming both hydrogen and halogen bonds with the anion, the halogen bond with the anion is apparently favored, whereas hydrogen bonds preferentially involve other acceptors present in the structure. In three obtained structures derived from phosphomolybdic acid, the corresponding oxoanion has been found in its reduced state [Mo₁₂PO₄₀]^4-, which has also led to a decrease in halogen bond lengths as compared to the fully oxidated [Mo₁₂PO₄₀]^3-. The electrostatic potential on the three types of anions involved in the study ([Mo₁₂PO₄₀]^3-, [Mo₁₂PO₄₀]^4-, and [W₁₂PO₄₀]^3-) has been calculated for optimized geometries of the anions, and it has been shown that the terminal M=O oxygen atoms are the least negative sites of the anions, indicating that they act as halogen bond acceptors primarily due to their steric availability.

η6 -Metalated Aryl Iodides in Diels-Alder Cycloaddition Reactions: Mode of Activation and Catalysis

The potential application of η⁶ -metalated aryl iodides as organocatalyst has been explored by means of computational methods. It is found that the enhanced halogen bonding donor ability of these species, in comparison with their demetalated counterparts, translates into a significant acceleration of the Diels-Alder cycloaddition reaction involving cyclohexadiene and methyl vinyl ketone. The factors behind this acceleration, the endo-exo selectivity of the process and the influence of the nature of the transition metal fragment in the activity of these species are quantitatively explored in detail by means of the combination of the Activation Strain Model of reaction and the Energy Decomposition Analysis methods.

Ability of Peripheral H Bonds to Strengthen a Halogen Bond

Quantum calculations study the manner in which the involvement of a halogen atom as a proton acceptor in one or more H bonds (HBs) affects the strength of the halogen bond (XB) it can form with a nucleophile aligned with the X σ-hole. A variety of Lewis acids wherein X = F, Cl, Br, and I are attached to a tetrel atom C or Ge engaged in a XB with nucleophile NH₃. One, two, and three HF molecules were positioned perpendicular to the XB axis so that they could form a HB to the X atom. Each such HB strengthened the XB by an increment of 1 kcal/mol or more that does not attenuate as each new HB is added, potentially increasing the interaction energy manyfold. Additionally, the presence of one or more HBs facilitates the formation of a XB by molecules which are reluctant to engage in such a bond in the absence of these auxiliary interactions. Even the F atom, which avoids such a XB, can be coaxed to participate in a XB of moderate strength by one or more of these external HBs.

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.

Isolation and crystal and molecular structures of [(C5H2Br3)2Fe], [(C5HBr4)2Fe] and [(C5Br5)(C5Br4HgBr)Fe]

The reaction of [(C5H3Br2)2Fe] with lithium tetramethylpiperidinide (LiTMP) in a 1:10 molar ratio in tetrahydrofuran yields, after quenching with C2H2Br4, a mixture of the polybromoferrocenes [C10H10-nBrnFe] with n = 4-9, from which single crystals of bis(1,2,3-tribromocyclopentadienyl)iron(II), [Fe(C5H2Br3)2], and bis(1,2,3,4-tetrabromocyclopentadienyl)iron(II), [Fe(C5HBr4)2Fe], were obtained by a combination of chromatography and fractional crystallization. Treatment of `[C10(HgOAc)10Fe]' with KBr3 yields a mixture of polybromoferrocenes [C10H10-nBrnFe] with n = 8-10 and bromomercurioferrocenes [C10H9-nBrn(HgBr)Fe] with n = 7-9, from which single crystals of (1-bromomercurio-2,3,4,5-tetrabromocyclopentadienyl)(1,2,3,4,5-pentabromocyclopentadienyl)iron(II), [FeHgBr(C5Br4)(C5Br5)], were obtained by fractional crystallization. The crystal structures of all the compounds show Br...Br, Br...H and sometimes Br...Cp...π (Cp is a ring centroid) interactions, as well as π-π interactions. The findings are supported by Hirshfeld analyses.

Theoretical study on L-H+-L with identical donors: short strong hydrogen bond or not?

Short strong hydrogen bonds (SSHBs) play crucial role in many chemical processes. Recently, as the representative of SSHBs, [F-H-F]- was experimentally observed. [F-H-F]- has a symmetric structure, which can be described as a H+ acid shared by two terminal F- donors (F--H+-F-). To explore whether two identical donors are bound to result in SSHBs, we performed theoretical studies on a series of compounds (L-H+-L) with two identical electron donors (L corresponds to donors containing group 14, 15, 16 and 17 elements). The results show that identical donors do not definitely lead to SSHBs. Instead, typical hydrogen bonds also exist. We found that both electronegativity and basicity contribute to the patterns of hydrogen bonds, where more electronegative and weaker donors benefit to SSHBs. Besides, it was found that zero-point energies also respond to the hydrogen bonding systems. This systemic work is expected to provide more insights into SSHBs.

Halogenated Baicalein as a Promising Antiviral Agent toward SARS-CoV-2 Main Protease

The coronavirus disease pandemic is a constant reminder that global citizens are in imminent danger of exposure to emerging infectious diseases. Therefore, developing a technique for inhibitor discovery is essential for effective drug design. Herein, we proposed fragment molecular orbital (FMO)-based virtual screening to predict the molecular binding energy of potential severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) main protease inhibitors. The integration of quantum mechanical approaches and trajectory analysis from a microsecond molecular dynamics simulation was used to identify potential inhibitors. We identified brominated baicalein as a potent inhibitor of the SARS-CoV-2 main protease and confirmed its inhibitory activity in an in vitro assay. Brominated baicalein did not demonstrate significant toxicity in either in vitro or in vivo studies. The pair interaction energy from FMO-RIMP2/PCM and inhibitory constants based on the protease enzyme assay suggested that the brominated baicalein could be further developed into novel SARS-CoV-2 protease inhibitors.

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

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

Pushing the Limits of the Hydrogen Bond Enhanced Halogen Bond —The Case of the C–H Hydrogen Bond

C–H hydrogen bonds have remarkable impacts on various chemical systems. Here we consider the influence of C–H hydrogen bonds to iodine atoms. Positioning a methyl group between two iodine halogen bond donors of the receptor engendered intramolecular C–H hydrogen bonding (HBing) to the electron-rich belt of both halogen bond donors. When coupled with control molecules, the role of the C–H hydrogen bond was evaluated. Gas-phase density functional theory studies indicated that methyl C–H hydrogen bonds help bias a bidentate binding conformation. Interaction energy analysis suggested that the charged C–H donors augment the halogen bond interaction—producing a >10 kcal mol⁻¹ enhancement over a control lacking the C–H⋯I–C interaction. X-ray crystallographic analysis demonstrated C–H hydrogen bonds and bidentate conformations with triflate and iodide anions, yet the steric bulk of the central functional group seems to impact the expected trends in halogen bond distance. In solution, anion titration data indicated elevated performance from the receptors that utilize C–H Hydrogen Bond enhanced Halogen Bonds (HBeXBs). Collectively, the results suggest that even modest hydrogen bonds between C–H donors and iodine acceptors can influence molecular structure and improve receptor performance.

C–H hydrogen bonds to iodine halogen bond donors are shown to improve halogen bonding and molecular preorganization.

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.

Enantioselective Michael addition to vinyl phosphonates via hydrogen bond-enhanced halogen bond catalysis

An asymmetric Michael addition of malononitrile to vinyl phosphonates was accomplished by hydrogen bond-enhanced bifunctional halogen bond (XB) catalysis. NMR titration experiments were used to demonstrate that halogen bonding, with the support of hydrogen-bonding, played a key role in the activation of the Michael acceptors through the phosphonate group. This is the first example of the use of XBs for the activation of organophosphorus compounds in synthesis. In addition, the iodo-perfluorophenyl group proved to be a better directing unit than different iodo- and nitro-substituted phenyl groups. The developed approach afforded products with up to excellent yields and diastereoselectivities and up to good enantioselectivities.

Nature of the Hydrogen Bond Enhanced Halogen Bond

The factors responsible for the enhancement of the halogen bond by an adjacent hydrogen bond have been quantitatively explored by means of state-of-the-art computational methods. It is found that the strength of a halogen bond is enhanced by ca. 3 kcal/mol when the halogen donor simultaneously operates as a halogen bond donor and a hydrogen bond acceptor. This enhancement is the result of both stronger electrostatic and orbital interactions between the XB donor and the XB acceptor, which indicates a significant degree of covalency in these halogen bonds. In addition, the halogen bond strength can be easily tuned by modifying the electron density of the aryl group of the XB donor as well as the acidity of the hydrogen atoms responsible for the hydrogen bond.