Experimental and Computational Probes of the Nature of Halogen Bonding: Complexes of Bromine-Containing Molecules with Bromide Anions

Construction of intermolecular σ-hole interactions in rare earth metallocene complexes using a 2,3,4,5-tetraiodopyrrolyl anion

The generation of noncovalent intermolecular interactions represents a powerful method to control molecular vibrations and rotations. Combining these with the axial ligand field enforced by the metallocene ligand scaffold provides a dual-pronged approach in controlling the magnetic-relaxation pathways for dysprosium-based single-molecule magnets (SMMs). Here, we present the first implementation of 2,3,4,5-tetraiodopyrrole (TIPH) in its anionic form [TIP]- as a ligand in three isostructural rare-earth metal complexes Cp*2RE(TIP) (1-RE, RE = Y, Gd, and Dy; Cp* = pentamethylcylopentadienyl), where the TIP ligand binds through the nitrogen and one iodine atom κ2(N,I) to the metal centre. The shallow potential energy surface of the intermolecular σ-hole interaction yields distortions of the interatomic distances at elevated temperatures which were investigated by variable-temperature SCXRD. 1-RE constitute the first crystallographically characterized molecules containing TIP as a ligand for any metal ion, and 1-Dy is the first SMM that employs the TIP ligand. The structural dependence on temperature allowed the mechanism of magnetic relaxation to be explored through ab initio calculations at different temperatures. The electronic influence of the coordinated iodine substituent was probed via magnetometry and cw-EPR spectroscopy on 1-Gd. To further scrutinize the impact of the iodine substituents on the physical properties, a second set of new complexes Cp*2RE(DMP) (2-RE, RE = Y, and Dy) where DMP = 2,5-dimethylpyrrolyl were synthesized. Here, the DMP ligand binds similarly to the TIP ligand and represents an all-hydrocarbon analogue to 1-RE. 2-Dy constitutes the first SMM bearing a DMP ligand.

Intermolecular Metal-Involving Pnictogen Bonding: The Case of σ-(SbIII)-Hole···dz2[PtII] Interaction

Cocrystallizations of trans-[PtX'2(NCNR2)2] (R2 = Me2, X' = Cl 1a, Br 1b, I 1c; R2 = (CH2)5, X' = I 2c) with SbX3 (X = Cl, Br, I) gave 1:2 cocrystals 1a·2SbCl3, 1b·2SbBr3, 1c·2SbCl3, 1c·2SbBr3, 1c·2SbI3, and 2c·2SbI3. In all six X-ray structures, the association of the molecular coformers is achieved mainly by SbIII···dz2[PtII] metal-involving intermolecular pnictogen bonding. Density functional theory (DFT) calculations (based on experimentally determined geometries) using both gas-phase and solid-state approximations revealed that a σ-(Sb)-hole interacts with an area of negative potential associated with the dz2-orbital of the positively charged platinum(II) sites, thus forming a pnictogen bond whose energy falls in the range between -7.3 and -16.9 kcal/mol.

Catalytic activation via π-backbonding in halogen bonds

The role of halogen bonding (XB) in chemical catalysis has largely involved using XB donors as Lewis acid activators to modulate the reactivity of partner Lewis bases. We explore a more uncommon scenario, where a Lewis base modulates reactivity via a spectator halogen bond interaction. Our computational studies reveal that spectator halogen bonds may play an important role in modulating the rate of SN2 reactions. Most notably, π acceptors such as PF3 significantly decrease the barrier to substitution by decreasing electron density in the very electron rich transition state. Such π-backbonding represents an example of a heretofore unexplored situation in halogen bonding: the combination of both σ-donation and π-backdonation in this "non-covalent" interaction. The broader implications of this observation are discussed.

Exploring the role of halogen bonding in iodonium ylides: insights into unexpected reactivity and reaction control

Halogen bonding is commonly found with iodine-containing molecules, and it arises when Lewis bases interact with iodine's σ-holes. Halogen bonding and σ-holes have been encountered in numerous monovalent and hypervalent iodine-containing compounds, and in 2022 σ-holes were computationally confirmed and quantified in the iodonium ylide subset of hypervalent iodine compounds. In light of this new discovery, this article provides an overview of the reactions of iodonium ylides in which halogen bonding has been invoked. Herein, we summarize key discoveries and mechanistic proposals from the early iodonium ylide literature that invoked halogen bonding-type mechanisms, as well as recent reports of reactions between iodonium ylides and Lewis basic nucleophiles in which halogen bonding has been specifically invoked. The reactions discussed herein are organized to enable the reader to build an understanding of how halogen bonding might impact yield and chemoselectivity outcomes in reactions of iodonium ylides. Areas of focus include nucleophile σ-hole selectivity, and how ylide structural modifications and intramolecular halogen bonding (e.g., the ortho-effect) can improve ylide stability or solubility, and alter reaction outcomes.

The Dark Side of Lead-Free Metal Halide Nanocrystals: Substituent-Modulated Photocatalytic Activity in Benzyl Bromide Reduction

We illustrate here the high photocatalytic activity of sustainable lead-free metal halide nanocrystals (NCs), namely, Cs₃Sb₂Br₉ NCs, in the reduction of p-substituted benzyl bromides in the absence of a cocatalyst. The electronic properties of the benzyl bromide substituents and the substrate affinity to the NC surface determine the selectivity in C-C homocoupling under visible light irradiation. This photocatalyst can be reused for at least three cycles and preserves its good performance with a turnover number of ca. 105,000.

Computationally rational design of metal-involving halogen bonds with π-covalency: Structures and bonding analysis

Traditional π-covalent interactions have been proved in the non-metal halogen bond adducts formed by chloride and halogenated triphenylamine-based radical cations. In this study, we have rationally designed two metal-involving halogen bond adducts with π-covalency property, such as [L1-Pd···I-PTZ]⁺ (i.e., 1) and [L2-Pd···I-PTZ]⁺ (i.e., 2), in which the square-planar palladium complexes serve as halogen bond acceptor and 3,7-diiodo-10H-phenothiazine radical cation (i.e., [I-PTZ]^•+ ) acts as halogen bond donor. Noncovalent interaction analysis and quantum theory of atoms in molecules analysis revealed that there are notable halogen bond interactions along the Pd···I direction without genuine chemical bond formed in both designed adducts. Energy decomposition analysis together with natural orbital for chemical valence calculations were performed to gain insight into their bonding nature, which demonstrated the presence of remarkable π-covalent interactions and σ-covalent interactions in both 1 and 2. We therefore proposed a new strategy for building the metal-involving halogen bonds with π-covalency property, which will help the further development of new types of halogen bonds.

Hydrogen Bonding versus Halogen Bonding: Spectroscopic Investigation of Gas-Phase Complexes Involving Bromide and Chloromethanes

Hydrogen bonding and halogen bonding are important non-covalent interactions that are known to occur in large molecular systems, such as in proteins and crystal structures. Although these interactions are important on a large scale, studying hydrogen and halogen bonding in small, gas-phase chemical species allows for the binding strengths to be determined and compared at a fundamental level. In this study, anion photoelectron spectra are presented for the gas-phase complexes involving bromide and the four chloromethanes, CH₃ Cl, CH₂ Cl₂ , CHCl₃ , and CCl₄ . The stabilisation energy and electron binding energy associated with each complex are determined experimentally, and the spectra are rationalised by high-level CCSD(T) calculations to determine the non-covalent interactions binding the complexes. These calculations involve nucleophilic bromide and electrophilic bromine interactions with chloromethanes, where the binding motifs, dissociation energies and vertical detachment energies are compared in terms of hydrogen bonding and halogen bonding.

The roles of charge transfer and polarization in non-covalent interactions: a perspective from ab initio valence bond methods

Noncovalent interactions are ubiquitous and have been well recognized in chemistry, biology and material science. Yet, there are still recurring controversies over their natures, due to the wide range of noncovalent interaction terms. In this Essay, we employed the Valence Bond (VB) methods to address two types of interactions which recently have drawn intensive attention, i.e., the halogen bonding and the CH‧‧‧HC dihydrogen bonding. The VB methods have the advantage of interpreting molecular structures and properties in the term of electron-localized Lewis (resonance) states (structures), which thereby shed specific light on the alteration of the bonding patterns. Due to the electron localization nature of Lewis states, it is possible to define individually and measure both polarization and charge transfer effects which have different physical origins. We demonstrated that both the ab initio VB method and the block-localized wavefunction (BLW) method can provide consistent pictures for halogen bonding systems, where strong Lewis bases NH₃, H₂O and NMe₃ partake as the halogen bond acceptors, and the halogen bond donors include dihalogen molecules and XNO₂ (X = Cl, Br, I). Based on the structural, spectral, and energetic changes, we confirm the remarkable roles of charge transfer in these halogen bonding complexes. Although the weak C-H∙∙∙H-C interactions in alkane dimers and graphene sheets are thought to involve dispersion only, we show that this term embeds delicate yet important charge transfer, bond reorganization and polarization interactions.

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

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

"Anti-electrostatic" Halogen Bonding between Ions of Like Charge

Halogen bonding occurs between molecules featuring Lewis acidic halogen substituents and Lewis bases. It is often rationalized as a predominantly electrostatic interaction and thus interactions between ions of like charge (e. g., of anionic halogen bond donors with halides) seem counter-intuitive. Herein, we provide an overview on such complexes. First, theoretical studies are described and their findings are compared. Next, experimental evidences are presented in the form of crystal structure database analyses, recent examples of strong "anti-electrostatic" halogen bonding in crystals, and the observation of such interactions also in solution. We then compare these complexes to select examples of "counter-intuitive" adducts formed by other interactions, like hydrogen bonding. Finally, we comment on key differences between charge-transfer and electrostatic polarization.

Noncovalent Bonding Caught in Action: From Amorphous to Cocrystalline Molecular Thin Films

We demonstrate the solvent-free amorphous-to-cocrystalline transformations of nanoscale molecular films. Exposing amorphous films to vapors of a haloarene results in the formation of a cocrystalline coating. This transformation proceeds by gradual strengthening of halogen-bonding interactions as a result of the crystallization process. The gas-solid diffusion mechanism involves formation of an amorphous metastable phase prior to crystallization of the films. In situ optical microscopy shows mass transport during this process, which is confirmed by cross-section analysis of the final structures using focused ion beam milling combined with scanning electron microscopy. Nanomechanical measurements show that the rigidity of the amorphous films influences the crystallization process. This surface transformation results in molecular arrangements that are not readily obtained through other means. Cocrystals grown in solution crystallize in a monoclinic centrosymmetric space group, whereas the on-surface halogen-bonded assembly crystallizes into a noncentrosymmetric material with a bulk second-order nonlinear optical response.

Examining a Transition from Supramolecular Halogen Bonding to Covalent Bonds: Topological Analysis of Electron Densities and Energies in the Complexes of Bromosubstituted Electrophiles

The transition from weak (noncovalent) interactions to fully developed covalent bonds is examined using the quantum theory of atoms in molecules in a series of halogen-bonded (XB) complexes of bromosubstituted electrophiles, RBr, with 1,4-diazabicyclo[2.2.2]octane (DABCO) and Cl^- and Br^- anions. The gradual decrease in the XB lengths in these associations, d _Br···Y (where Y = Cl^-, Br^-, or N), was accompanied by the exponential increase in the binding energies and charge transfer, as well as electron densities and magnitudes of the kinetic and potential energy densities at the bond critical points (BCPs) on the Br···Y bond path. These indices, as well as characteristics of the adjacent bonds in the XB donor, followed remarkably close trend lines when plotted against the normalized XB length R _BrY = d _Br···Y/(r _Br + r _Y) (where r _Br and r _Y are the van der Waals radii) regardless of the methods [MP2/6-311+G(d,p) or M062X/6-311+G(d,p)], media (gas phase or dichloromethane), and nucleophiles (Cl^-, Br^-, or DABCO). In the systems with an R _BrY higher than about 0.78, the energy densities H(r) at BCPs at the Br···Y bond path were small and positive, and XBs did not substantially affect the characteristics of the adjacent R-Br covalent bond in the XB donor. Accordingly, the XB can be identified as noncovalent in this range. In the complexes with R _BrY values between about 0.67 and 0.78, energy densities H(r) at Br···Y BCPs were negative, and their magnitudes increased with the decrease in the Br···Y separation. In this range, formation of XBs was accompanied by the increase in the R-Br bond length in the XB donor and the decrease in the magnitude of the (negative) H(r) values at the BCPs of the R-Br bonds. XBs can be classified as partially covalent in this R _BrY range. At an R _BrY less than about 0.67, electron densities were larger, and energy densities were more negative at BCPs of the Br···Y bond than those at BCPs of the R-Br bond in the XB donor. This indicates that Br···Y bonds were stronger than R-Br bonds, and these (Br···Y) XBs can be regarded as essentially covalent. The synchronous change of a variety of (R-Br and Br···Y) bonding characteristics with R _BrY suggests that the normalized XB bond length can be used as a basic parameter in the identification of the type of intermolecular interaction. A continuity of these characteristics suggests an inherent relationship between limiting (covalent and noncovalent) types of XBs and thus an onset of molecular-orbital interactions in the weaker bonds.

Tetrabromoethane as σ-Hole Donor toward Bromide Ligands: Halogen Bonding between C2H2Br4 and Bromide Dialkylcyanamide Platinum(II) Complexes

The complexes trans-[PtBr2(NCNR2)2] (R2 = Me21, (CH2)52) were cocrystallized with 1,1,2,2-tetrabromoethane (tbe) in CH2Cl2 forming solvates 1·tbe and 2·tbe, respectively. In both solvates, tbe involved halogen bonding, viz. the C–Br···Br–Pt interactions, were detected by single-crystal X-ray diffractions experiments. Appropriate density functional theory calculations (M06/def2-TZVP) performed for isolated molecules and complex-tbe clusters, where the existence of the interactions and their noncovalent nature were confirmed by electrostatic potential surfaces (ρ = 0.001 a.u.) for isolated molecules, topology analysis of electron density, electron localization function and HOMO-LUMO overlap projections for clusters.

Halogen Bonding: An Odd Chemistry?

Halogen bonding is a flourishing field of research, but has for long been little recognized. The same goes for its scientific hero, Odd Hassel, who laid the foundations for all current developments. The crystallographic observation of halogen-oxygen interatomic distances shorter than the sum of the van der Waals radii of the involved atoms, and the interpretation of this phenomenon as a charge-transfer interaction, have been ground-breaking. Today, charge-transfer to a polarized halogen is not any longer seen as "odd", but is commonly referred to as halogen bonding, and is widely exploited in chemistry. Despite the recognition of Hassel's work with a Nobel prize in 1969, surprisingly little appreciation is given to date to the devoted scientist, who established a world-leading laboratory during one of the darkest eras of history. Herein, we wish to revive the legacy and highlight the impact of Odd Hassel's ground-breaking discoveries.

Role of Charge Transfer in Halogen Bonding

Halogen bonding has received intensive attention recently for its applications in the construction of supramolecular assemblies and crystal engineering and its implications and potentials in chemical and biological processes and rational drug design. Peculiarly, in intermolecular interactions, halogen atoms are known as electron-donating groups carrying partial negative charges in molecules due to its high electronegativity, but they can counterintuitively act as Lewis acids and bind with Lewis bases in the form of a halogen bond. The unsettling issue regarding the nature of the halogen bonding is whether the electrostatics or charge transfer interaction dominates. The recently proposed σ-hole concept nicely reinforces the role of electrostatic attraction. Also, good correlations between the halogen bonding strength and the interaction energy from the simple point-charge model have been found. This leads to the claim that there is no need to invoke the charge transfer concept in the halogen bond. But there is alternative evidence supporting the importance of charge transfer interaction. Here, we visited a series of prominent halogen bonded complexes of the types Y₃C-X···Z (X = Br, I; Y = F, Cl, Br; Z = F^-, Cl^-, Br^-, I^-, NMe₃) with the block-localized wave function (BLW) method at the M06-2X-D3/6-311+G(d,p) (def2-SVP for iodine) level of theory. As the simplest variant of ab initio valence bond (VB) theory, the BLW method is unique in the strict localization of electrons within interacting moieties, allowing for quantitative evaluation of the charge transfer effect on geometries, spectral properties, and energetics in halogen bonding complexes. By comparing the halogen bonding complexes with and without the charge transfer interaction, we proved that the charge transfer interaction significantly shortens the X···Z bonding distance and stretches the C-X bonds. But the shortening of the halogen bonding results in the less favorable steric effect, which is composed of Pauli repulsion, electrostatics, and electron correlation. There are approximate linear correlations between the charge transfer effect and binding energy and between bonding distance and binding energy. These correlations may lead to the illusion that the charge transfer interaction is unimportant or irrelevant, but further analyses showed that the inclusion of charge transfer is critical for the proper description of the halogen bonding, as considering only electrostatics and polarization leads to only about 45-60% of the binding strengths and much elongated bonding distances.

Halogen Bonding in the Complexes of Brominated Electrophiles with Chloride Anions: From a Weak Supramolecular Interaction to a Covalent Br–Cl Bond

The wide-range variation of the strength of halogen bonds (XB) not only facilitates a variety of applications of this interaction, but it also allows examining the relation (and interconversion) between supramolecular and covalent bonding. Herein, the Br…Cl halogen bonding in a series of complexes of bromosubstituted electrophiles (R-Br) with chloride anions were examined via X-ray crystallographic and computational methods. Six co-crystals showing such bonding were prepared by evaporation of solutions of R-Br and tetra-n-propylammonium chloride or using Cl− anions released in the nucleophilic reaction of 1,4-diazabicyclo[2.2.2]octane with dichloromethane in the presence of R-Br. The co-crystal comprised networks formed by 3:3 or 2:2 halogen bonding between R-Br and Cl−, with the XB lengths varying from 3.0 Å to 3.25 Å. Analysis of the crystallographic database revealed examples of associations with substantially longer and shorter Br…Cl separations. DFT computations of an extended series of R–Br…Cl− complexes confirmed that the judicious choice of brominated electrophile allows varying halogen Br…Cl bond strength and length gradually from the values common for the weak intermolecular complexes to that approaching a fully developed covalent bond. This continuity of halogen bond strength in the experimental (solid-state) and calculated associations indicates a fundamental link between the covalent and supramolecular bonding.

Halogen Bonding Between Anions: Association of Anion Radicals of Tetraiodo-p-benzoquinone with Iodide Anions

Halogen bonding between two negatively charged species, tetraiodo-p-benzoquinone anion radicals (I₄ Q^-. ) and iodide anions, was observed and characterized for the first time. X-ray structural and EPR/UV-Vis spectral studies revealed that the anion-anion bonding led to the formation of crystals comprising 2D layers of I₄ Q^-. anion radicals linked by iodides and separated by Et₄ N⁺ counter-ions. Computational analysis suggested that the seemingly antielectrostatic halogen bonds in these systems were formed via a combination of several factors. First, an attenuation of the interionic repulsion by the solvent facilitated close approach of the anions leading to their mutual polarization. This resulted in the appearance of positively charged areas (σ-holes) on the surface of the iodine substituents in I₄ Q^-. responsible for the attractive interaction. Finally, the solid-state associations were also stabilized by multicenter (4:4) halogen bonding between I₄ Q^-. and iodide.

π covalency in the halogen bond

Halogen bonds are a highly directional class of intermolecular interactions widely employed in chemistry and chemical biology. This linear interaction is commonly viewed to be analogous to the hydrogen bond because hydrogen bonding models also intuitively describe the σ-symmetric component of halogen bonding. The possibility of π-covalency in a halogen bond is not contemplated in any known models. Here we present evidence of π-covalency being operative in halogen bonds formed between chloride and halogenated triphenylamine-based radical cations. We reach this conclusion through computational analysis of chlorine K-edge X-ray absorption spectra recorded on these halogen bonded pairs. In light of this result, we contend that halogen bonding is better described by analogy to metal coordination bonds rather than hydrogen bonds. Our revised description of the halogen bond suggests that these interactions could be employed to influence the electronic properties of conjugated molecules in unique ways.

"Anti-Electrostatic" Halogen Bonding

Halogen bonding is often described as being driven predominantly by electrostatics, and thus adducts between anionic halogen bond (XB) donors (halogen-based Lewis acids) and anions seem counterintuitive. Such "anti-electrostatic" XBs have been predicted theoretically but for organic XB donors, there are currently no experimental examples except for a few cases of self-association. Reported herein is the synthesis of two negatively charged organoiodine derivatives that form anti-electrostatic XBs with anions. Even though the electrostatic potential is universally negative across the surface of both compounds, DFT calculations indicate kinetic stabilization of their halide complexes in the gas phase and particularly in solution. Experimentally, self-association of the anionic XB donors was observed in solid-state structures, resulting in dimers, trimers, and infinite chains. In addition, co-crystals with halides were obtained, representing the first cases of halogen bonding between an organic anionic XB donor and a different anion. The bond lengths of all observed interactions are 14-21 % shorter than the sum of the van der Waals radii.

Diversity and uniformity in anion-π complexes of thiocyanate with aromatic, olefinic and quinoidal π-acceptors

Despite the progress in the study of anion-π interactions, there are still inconsistencies in the use of this term and the experimental data about factors affecting the strength of such bonding are limited. To shed light on these issues, we explored supramolecular associations between NCS^- anions and a series of aromatic, olefinic or quinoidal π-acceptors. Combined experimental and computational studies revealed that all these complexes were formed by an attraction of the anion to the face of the π-system, and the arrangements of thiocyanate followed the areas of the most positive potentials on the surfaces of the π-acceptors. The stabilities of the complexes increased with the π-acceptor strength (reflected by their reduction potentials), and were essentially independent of the magnitudes of the maximum electrostatic potentials on their surfaces. The complexes showed intense absorption bands in the UV-Vis range, and the energies of these bands were correlated with the difference of the redox potentials of the anions and π-acceptors. Such features, as well as results of atoms-in-molecules and non-covalent index analyses suggested that besides electrostatics, molecular orbital interactions play a substantial role in the formation of these complexes. The unified trends in variations of the characteristics of the complexes between thiocyanate and a variety of π-acceptors point to their common nature. To embrace diversity and uniformity of the anion-π associates, we suggest (following the halogen bond's definition) that anion-π bonding occurs when there is evidence of a net attraction between the anions and the face of the electrophilic π-system.

Cobaloximes as Building Blocks in Halogen-Bonded Cocrystals

In this work, we explore the halogen-bonded cocrystallization potential of cobaloxime complexes in the synthesis of cocrystals with perhalogenated benzenes. We demonstrate a strategy for synthesizing halogen-bonded metal–organic cocrystals by utilizing cobaloximes whose pendant bromide group and oxime oxygen enable halogen bonding. By combining three well-known halogen bond donor molecules differing in binding geometry and composition with three cobaloxime units, we obtained a total of four previously unreported cocrystals. Single crystal X-ray diffraction experiments showed that the majority of obtained cocrystals exhibited the formation of the targeted I···O and I···Br motives. These results illustrate the potential of cobaloximes as halogen bond acceptors and indicate that this type of halogen bond acceptors may offer a novel route to metal–organic halogen-bonded cocrystals.

In Situ Assessment of Intrinsic Strength of X-I⋯OA-Type Halogen Bonds in Molecular Crystals with Periodic Local Vibrational Mode Theory

Periodic local vibrational modes were calculated with the rev-vdW-DF2 density functional to quantify the intrinsic strength of the X-I⋯OA-type halogen bonding (X = I or Cl; OA: carbonyl, ether and N-oxide groups) in 32 model systems originating from 20 molecular crystals. We found that the halogen bonding between the donor dihalogen X-I and the wide collection of acceptor molecules OA features considerable variations of the local stretching force constants (0.1–0.8 mdyn/Å) for I⋯O halogen bonds, demonstrating its powerful tunability in bond strength. Strong correlations between bond length and local stretching force constant were observed in crystals for both the donor X-I bonds and I⋯O halogen bonds, extending for the first time the generalized Badger’s rule to crystals. It is demonstrated that the halogen atom X controlling the electrostatic attraction between the σ-hole on atom I and the acceptor atom O dominates the intrinsic strength of I⋯O halogen bonds. Different oxygen-containing acceptor molecules OA and even subtle changes induced by substituents can tweak the n→σ∗(X-I) charge transfer character, which is the second important factor determining the I⋯O bond strength. In addition, the presence of the second halogen bond with atom X of the donor X-I bond in crystals can substantially weaken the target I⋯O halogen bond. In summary, this study performing the in situ measurement of halogen bonding strength in crystalline structures demonstrates the vast potential of the periodic local vibrational mode theory for characterizing and understanding non-covalent interactions in materials.

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

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

A Crystallographic Charge Density Study of the Partial Covalent Nature of Strong N⋅⋅⋅Br Halogen Bonds

The covalent nature of strong N-Br⋅⋅⋅N halogen bonds in a cocrystal (2) of N-bromosuccinimide (NBS) with 3,5-dimethylpyridine (lut) was determined from X-ray charge density studies and compared to a weak N-Br⋅⋅⋅O halogen bond in pure crystalline NBS (1) and a covalent bond in bis(3-methylpyridine)bromonium cation (in its perchlorate salt (3). In 2, the donor N-Br bond is elongated by 0.0954 Å, while the Br⋅⋅⋅acceptor distance of 2.3194(4) is 1.08 Å shorter than the sum of the van der Waals radii. A maximum electron density of 0.38 e Å^-3 along the Br⋅⋅⋅N halogen bond indicates a considerable covalent contribution to the total interaction. This value is intermediate to 0.067 e Å^-3 for the Br⋅⋅⋅O contact in 1, and approximately 0.7 e Å^-3 in both N-Br bonds of the bromonium cation in 3. A calculation of the natural bond order charges of the contact atoms, and the σ*(N1-Br) population of NBS as a function of distance between NBS and lut, have shown that charge transfer becomes significant at a Br⋅⋅⋅N distance below about 3 Å.

Complexes of Diiodine with Heteroaromatic N-Oxides: Effects of Halogen-Bond Acceptors in Halogen Bonding

Halogen bonding (XB) in complexes of diiodine with heteroaromatic N-oxides was examined via a combination of UV-vis spectral and X-ray structural measurements, as well as computational analysis. While all of these associates were formed by analogous I···O bonds, they showed considerable variations of formation constants (5-1500 M^-1) and intermolecular I···O bond length (2.3-3.2 Å). In the solid state, both atoms of I₂ molecules were involved in XB, and the I···O separations were determined by the electron-donor abilities of N-oxides and the strength of the bonding on the opposite side of the ditopic XB donor. The solution-phase formation constants of 1:1 complexes, K, as well as magnitudes of the calculated interaction energies, ΔE, increased with the shift of the values of the most negative potentials on the surfaces of N-oxides' oxygen atoms, V_min, toward more negative values. Yet, the interatomic contacts consistently deviated from the locations of V_min. Instead, the structures of complexes were well suited for highest occupied molecular orbital/lowest unoccupied molecular orbital interactions of reactants. The values of K, ΔE, and the intermolecular distances d_I···O in the calculated complexes were highly correlated with the charge-transfer interaction energies derived from the natural bond orbital analysis. This indicated that, besides electrostatic, molecular orbital interactions play a substantial role in XB between diiodine and N-oxides. This conclusion was supported by the analysis of the complexes using the quantum theory of atoms in molecules, noncovalent interaction index, and density overlap region indicator, which showed that the covalent character of I···O bonding increases with the rise of interaction energies in the complexes.

Tetrel Bonding as a Vehicle for Strong and Selective Anion Binding

Tetrel atoms T (T = Si, Ge, Sn, and Pb) can engage in very strong noncovalent interactions with nucleophiles, which are commonly referred to as tetrel bonds. The ability of such bonds to bind various anions is assessed with a goal of designing an optimal receptor. The Sn atom seems to form the strongest bonds within the tetrel family. It is most effective in the context of a -SnF₃ group and a further enhancement is observed when a positive charge is placed on the receptor. Connection of the -SnF₃ group to either an imidazolium or triazolium provides a strong halide receptor, which can be improved if its point of attachment is changed from the C to an N atom of either ring. Aromaticity of the ring offers no advantage nor is a cyclic system superior to a simple alkyl amine of any chain length. Placing a pair of -SnF₃ groups on a single molecule to form a bipodal dicationic receptor with two tetrel bonds enhances the binding, but falls short of a simple doubling. These two tetrel groups can be placed on opposite ends of an alkyl diamine chain of any length although SnF₃⁺NH₂(CH₂)nNH₂SnF₃⁺ with n between 2 and 4 seems to offer the strongest halide binding. Of the various anions tested, OH− binds most strongly: OH− > F− > Cl− > Br− > I−. The binding energy of the larger NO₃− and HCO₃− anions is more dependent upon the charge of the receptor. This pattern translates into very strong selectivity of binding one anion over another. The tetrel-bonding receptors bind far more strongly to each anion than an equivalent number of K⁺ counterions, which leads to equilibrium ratios in favor of the former of many orders of magnitude.

The atomic level mechanism of white phosphorous demolition by di-iodine

A detailed mechanism of the I₂-induced transformation of white phosphorus into PI₃ emerges from a DFT analysis. This multi-step process implies that at any stage one P-P and two I-I bonds cleavages, associated with the formation of two P-I bonds plus an in situ generated brand new I₂ molecule. Significant electron transfer between the atoms is observed at any step, but the reactions are better defined as concerted rather than redox. Along the steepest descent to the product, no significant barrier is encountered except for the very first P₄ activation, which costs +14.6 kcal mol^-1. At the atomic level, one first I₂ molecule, a typical mild oxidant, is first involved in a linear halogen bonding interaction (XB) with one P donor, while its terminal I atom is engaged in an additional XB adduct with a second I₂. Significant electron transfer through the combined diatomics allows the external I atom of the dangling I₃ grouping to convey electrons into the σ* level of one P-P bond with its consequent cleavage. This implies at some point the appearance of a six-membered ring, which alternatively switches its bonding and no-bonding interactions. The final transformation of the P₂I₄ diphosphine into two PI₃ phosphines is enlightening also for the specific role of the I substituents. In fact, it is proved that an organo-diphosphine analogue hardly undergoes the separation of two phosphines, as reported in the literature. This is attributable to the particularly high donor power of the carbo-substituted P atoms, which prevents the concertedness of the reaction but favors charge separation in an unreactive ion pair.

Continuum of covalent to intermolecular bonding in the halogen-bonded complexes of 1,4-diazabicyclo[2.2.2]octane with bromine-containing electrophiles

The Br⋯N bonds in a series of halogen-bonded complexes change gradually from the typical intermolecular to the traditional covalent bond.

Resolving the halogen vs. hydrogen bonding dichotomy in solutions: intermolecular complexes of trihalomethanes with halide and pseudohalide anions

Simultaneous (multivariable) treatments of UV-vis and NMR data yielded formation constants of halogen- and hydrogen-bonded complexes coexisting in solutions.

Structural preferences in strong anion–π and halogen-bonded complexes: π- and σ-holes vs. frontier orbitals interaction

Intermolecular contacts in strong anion–π and halogen-bonded complexes follow frontier orbitals (instead of most positive or negative surface potentials) of reactants.

Characterizing the interplay of Pauli repulsion, electrostatics, dispersion and charge transfer in halogen bonding with energy decomposition analysis

Variational energy decomposition analysis establishes charge-transfer as the origin of halogen bond strength differences that go against electrostatics.

Polymorphism of a widely used building block for halogen-bonded assemblies: 1,3,5-trifluoro-2,4,6-triiodobenzene

After reporting the structure of a new polymorph of 1,3,5-trifluoro-2,4,6-triiodobenzene (denoted BzF3I3), C₆F₃I₃, (I), which crystallized in the space group P2₁/c, we perform a comparative analysis with the already reported P2₁/n polymorph, (II) [Reddy et al. (2006). Chem. Eur. J. 12, 2222-2234]. In polymorph (II), type-II I...I halogen bonds and I...π interactions connect molecules in such a way that a three-dimensional structure is formed; however, the way in which molecules are connected in polymorph (I), through type-II I...I halogen bonds and π-π interactions, gives rise to an exfoldable lamellar structure, which looks less tightly bound than that of (II). In agreement with this structural observation, both the melting point and the melting enthalpy of (I) are lower than those of (II).