How to Twist, Split and Warp a σ-Hole with Hypervalent Halogens

Halogen Bond via an Electrophilic π-Hole on Halogen in Molecules: Does It Exist?

This study reveals a new non-covalent interaction called a π-hole halogen bond, which is directional and potentially non-linear compared to its sister analog (σ-hole halogen bond). A π-hole is shown here to be observed on the surface of halogen in halogenated molecules, which can be tempered to display the aptness to form a π-hole halogen bond with a series of electron density-rich sites (Lewis bases) hosted individually by 32 other partner molecules. The [MP2/aug-cc-pVTZ] level characteristics of the π-hole halogen bonds in 33 binary complexes obtained from the charge density approaches (quantum theory of intramolecular atoms, molecular electrostatic surface potential, independent gradient model (IGM-δginter)), intermolecular geometries and energies, and second-order hyperconjugative charge transfer analyses are discussed, which are similar to other non-covalent interactions. That a π-hole can be observed on halogen in halogenated molecules is substantiated by experimentally reported crystals documented in the Cambridge Crystal Structure Database. The importance of the π-hole halogen bond in the design and growth of chemical systems in synthetic chemistry, crystallography, and crystal engineering is yet to be fully explicated.

Yet another perspective on hole interactions, part II: lp-hole vs. lp-hole interactions

Most of the experimental and theoretical work in hole interactions (HIs) is mainly focused on exploiting the nature and characteristics of σ and π-holes. In this perspective, we focus our attention on understanding the origin and properties of lone-pair holes. These holes are present on an atom opposite to its lone-pair region. Utilizing some new and old examples, such as X₃N/P⋯F^- (X = F/Cl/Br/I), F-Cl/Br/I⋯H₃P⋯NCH and H₃B-NBr₃ along with other molecular systems, we explored to what extent these lp-holes participate in lp-hole interactions, if they participate at all.

Sterically crowded 1,4-diiodobenzene as a precursor to difunctional hypervalent iodine compounds

A bulky 1,4-di-iodobenzene having four adjacent para-^tBu-C₆H₄ group (Ar') substituents (1) was used to prepare the di-hypervalent iodine compound 1,4-[I(OAc)₂]₂-2,3,5,6-Ar'₄-C₆ (2). Despite the steric encumbrance of the iodine center by the flanking aryl substituents, compound 2 undergoes ready cyclization under mild conditions (excess CF₃COOH at 55 °C, 30 min) to afford a dicyclic di-iodonium di-triflate salt 3. The single crystal structures of compounds 2 and 3 were examined and compared to the formerly characterized precursor 1. The para-tert-butyl groups on these compounds also render the compounds more soluble than multifunctional hypervalent iodine (HVI) compounds. HVI compounds having multiple iodine(III) centers are increasingly of interest for applications as recyclable reagents, materials precursors, and as Lewis acids.

Charge neutral halogen bonding tetradentate-iodotriazole macrocycles capable of anion recognition and sensing in highly competitive aqueous media

A series of neutral tetradentate halogen bonding (XB) macrocycles, comprising of two bis-iodotriazole XB donors were synthesised in 60-70% yields via a stepwise CuAAC-mediated cyclisation strategy. Extensive ¹H NMR anion titration experiments reveal halide binding affinities are critically dependent on the substitution pattern of the xylyl spacer unit. The meta-substituted macrocycle remarkably displays cooperative tetradentate XB-halide anion recognition in highly competitive 40% aqueous-organic D₂O/acetone-d₆ (40 : 60, v/v) solvent mixtures. Integration of para-xylyl and naphthyl spacer units generates extended macrocyclic cavities, capable of selective oxalate recognition. Furthermore, preliminary fluorescence exeperiments reveal dicarboxylate specific sensing can be achieved through monitoring of the naphthylene centred emission.

Yet another perspective on hole interactions

Hole interactions are known by different names depending on the key atom of the bond (halogen bond, chalcogen bond, hydrogen bond, etc.), and the geometry of the interaction (σ if in line, π if perpendicular to the Lewis acid plane). However, its origin starts with the creation of a Lewis acid by an underlying covalent bond, which forms an electrostatic depletion and a virtual antibonding orbital, which can create non-covalent interactions with Lewis bases. In this (maybe subjective) perspective, we will claim that hole interactions must be defined via the molecular orbital origin of the molecule. Under this premise we can better explore the richness of such bonding patterns. For that, we will study old, recent and new systems, trying to pinpoint some misinterpretations that are often associated with them. We will use as exemplars the triel bonds, a couple of metal complexes, a discussion on convergent σ-holes, and many cases of anti-electrostatic hole interactions.

N-Heterocyclic Iod(az)olium Salts - Potent Halogen-Bond Donors in Organocatalysis

This article describes the application of N-heterocyclic iod(az)olium salts (NHISs) as highly reactive organocatalysts. A variety of mono- and dicationic NHISs are described and utilized as potent XB-donors in halogen-bond catalysis. They were benchmarked in seven diverse test reactions in which the activation of carbon- and metal-chloride bonds as well as carbonyl and nitro groups was achieved. N-methylated dicationic NHISs rendered the highest reactivity in all investigated catalytic applications with reactivities even higher than all previously described monodentate XB-donors based on iodine(I) and (III) and the strong Lewis acid BF3 .

Hydrogen-Bonded and Halogen-Bonded: Orthogonal Interactions for the Chloride Anion of a Pyrazolium Salt

In the hydrochloride of a pyrazolyl-substituted acetylacetone, the chloride anion is hydrogen-bonded to the protonated pyrazolyl moiety. Equimolar co-crystallization with tetrafluorodiiodobenzene (TFDIB) leads to a supramolecular aggregate in which TFDIB is situated on a crystallographic center of inversion. The iodine atom in the asymmetric unit acts as halogen bond donor, and the chloride acceptor approaches the σ-hole of this TFDIB iodine subtending an almost linear halogen bond, with Cl···I = 3.1653(11) Å and Cl···I-C = 179.32(6)°. This contact is roughly orthogonal to the N-H···Cl hydrogen bond. An analysis of the electron density according to Bader's Quantum Theory of Atoms in Molecules confirms bond critical points (bcps) for both short contacts, with ρbcp = 0.129 for the halogen and 0.321eÅ-3 for the hydrogen bond. Our halogen-bonded adduct represents the prototype for a future class of co-crystals with tunable electron density distribution about the σ-hole contact.

Iodine(III)-Based Halogen Bond Donors: Properties and Applications

Halogen bonding, the non-covalent interaction of Lewis bases with an electron-deficient region of halogen substituents, received increased attention recently. Consequently, the design and evaluation of numerous halogen-containing species as halogen bond donors have been subject to intense research. More recently, organoiodine compounds at the iodine(III) state have been receiving growing attention in the field. Due to their electronic and structural properties, they provide access to unique binding modes. For this reason, our groups have been involved in the development of such compounds, in the quantification of their halogen bonding strength (through the evaluation of their Lewis acidities), as well as in the evaluation of their activities as catalysts in several model reactions. This account will describe these contributions.

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

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

Mixed Noble-Gas Compounds of Krypton(II) and Xenon(VI); [F5 Xe(FKrF)AsF6 ] and [F5 Xe(FKrF)2 AsF6 ]

The coordination chemistry of KrF₂ has been limited in contrast with that of XeF₂ , which exhibits a far richer coordination chemistry with main-group and transition-metal cations. In the present work, reactions of [XeF₅ ][AsF₆ ] with KrF₂ in anhydrous HF solvent afforded [F₅ Xe(FKrF)AsF₆ ] and [F₅ Xe(FKrF)₂ AsF₆ ], the first mixed krypton/xenon compounds. X-ray crystal structures and Raman spectra show the KrF₂ ligands and [AsF₆ ]^- anions are F-coordinated to the xenon atoms of the [XeF₅ ]⁺ cations. Quantum-chemical calculations are consistent with essentially noncovalent ligand-xenon bonds that may be described in terms of σ-hole bonding. These complexes significantly extend the XeF₂ -KrF₂ analogy and the limited chemistry of krypton by introducing a new class of coordination compound in which KrF₂ functions as a ligand that coordinates to xenon(VI). The HF solvates, [F₅ Xe(FH)AsF₆ ] and [F₅ Xe(FH)SbF₆ ], are also characterized in this study and they provide rare examples of HF coordinated to xenon(VI).

Comparison of Bifurcated Halogen with Hydrogen Bonds

Bifurcated halogen bonds are constructed with FBr and FI as Lewis acids, paired with NH₃ and NCH bases. The first type considered places two bases together with a single acid, while the reverse case of two acids sharing a single base constitutes the second type. These bifurcated systems are compared with the analogous H-bonds wherein FH serves as the acid. In most cases, a bifurcated system is energetically inferior to a single linear bond. There is a larger energetic cost to forcing the single σ-hole of an acid to interact with a pair of bases, than the other way around where two acids engage with the lone pair of a single base. In comparison to FBr and FI, the H-bonding FH acid is better able to participate in a bifurcated sharing with two bases. This behavior is traced to the properties of the monomers, in particular the specific shape of the molecular electrostatic potential, the anisotropy of the orbitals of the acid and base that interact directly with one another, and the angular extent of the total electron density of the two molecules.

Reliable Comparison of Pnicogen, Chalcogen, and Halogen Bonds in Complexes of 6-OXF2-Fulvene (X = As, Sb, Se, Te, Be, I) With Three Electron Donors

The pnicogen, chalcogen, and halogen bonds between 6-OXF₂-fulvene (X = As, Sb, Se, Te, Br, and I) and three nitrogen-containing bases (FCN, HCN, and NH₃) are compared. For each nitrogen base, the halogen bond is strongest, followed by the pnicogen bond, and the chalcogen bond is weakest. For each type of bond, the binding increases in the FCN < HCN < NH₃ pattern. Both FCN and HCN engage in a bond with comparable strengths and the interaction energies of most bonds are < -6 kcal/mol. However, the strongest base NH₃ forms a much more stable complex, particularly for the halogen bond with the interaction energy going up to -18 kcal/mol. For the same type of interaction, its strength increases as the mass of the central X atom increases. These bonds are different in strength, but all of them are dominated by the electrostatic interaction, with the polarization contribution important for the stronger interaction. The presence of these bonds changes the geometries of 6-OXF₂-fulvene, particularly for the halogen bond formed by NH₃, where the F-X-F arrangement is almost vertical to the fulvene ring.

Factors Impacting σ- and π-Hole Regions as Revealed by the Electrostatic Potential and Its Source Function Reconstruction: The Case of 4,4'-Bipyridine Derivatives

Positive electrostatic potential (V) values are often associated with σ- and π-holes, regions of lower electron density which can interact with electron-rich sites to form noncovalent interactions. Factors impacting σ- and π-holes may thus be monitored in terms of the shape and values of the resulting V. Further precious insights into such factors are obtained through a rigorous decomposition of the V values in atomic or atomic group contributions, a task here achieved by extending the Bader-Gatti source function (SF) for the electron density to V. In this article, this general methodology is applied to a series of 4,4'-bipyridine derivatives containing atoms from Groups VI (S, Se) and VII (Cl, Br), and the pentafluorophenyl group acting as a π-hole. As these molecules are characterized by a certain degree of conformational freedom due to the possibility of rotation around the two C-Ch bonds, from two to four conformational motifs could be identified for each structure through conformational search. On this basis, the impact of chemical and conformational features on σ- and π-hole regions could be systematically evaluated by computing the V values on electron density isosurfaces (V_S) and by comparing and dissecting in atomic/atomic group contributions the V_S maxima (V_S,max) values calculated for different molecular patterns. The results of this study confirm that both chemical and conformational features may seriously impact σ- and π-hole regions and provide a clear analysis and a rationale of why and how this influence is realized. Hence, the proposed methodology might offer precious clues for designing changes in the σ- and π-hole regions, aimed at affecting their potential involvement in noncovalent interactions in a desired way.

Hypervalent Iodine(III) Compounds as Biaxial Halogen Bond Donors

“Hypervalent” iodine(III) derivatives have been established as powerful reagents in organic transformations, but so far only a handful of studies have addressed their potential use as halogen-bonding noncovalent Lewis acids. In contrast to “classical” halogen-bond donors based on iodine(I) compounds, iodine(III) salts feature two directional electrophilic axes perpendicular to each other. Herein we present the first systematic investigation on biaxial binding to such Lewis acids in solution. To this end, hindered and unhindered iodolium species were titrated with various substrates, including diesters and diamides, via ¹H NMR spectroscopy and isothermal titration calorimetry. Clear evidence for biaxial binding was obtained in two model systems, and the association strengths increased by 2 orders of magnitude. These findings were corroborated by density functional theory calculations (which reproduced the trend well but underestimated the absolute binding constants) and a cocrystal featuring biaxial coordination of a diamide to the unhindered iodolium compound.

Microhydration of a Carbonyl Group: How does the Molecular Electrostatic Potential (MESP) Impact the Formation of (H2O)n:(R2C═O)Complexes?

The presence of a carbonyl group in a molecule usually leads to the identification of a π-hole on the molecular electrostatic potential (MESP) of the species. How does this electrophilic site influence the formation of microhydrated complexes? To address this point, a panel of R₂CO solutes with various MESPs was selected, and we identified the structures and properties of several complexes containing one, two, three and six water molecules. The following solutes were considered in the present study: H₂CO, F₂CO, Cl₂CO,(NC)₂CO and H₂C═CO. Geometry optimizations and frequency calculations were carried out at the LC-ωPBE/6-311++G(d,p) level, with the GD3BJ empirical correction for dispersion. For a number of n water molecules around the R₂CO solute, the structure and the features of the most stable (H₂O)_n:(R₂CO) complexes are highly dependent on the MESP of the isolated R₂CO solute. The formation of pi-hole bondings appears to play a decisive role in the initiation of a three-dimensional organization of water molecules around the solute.

The many flavours of halogen bonds - message from experimental electron density and Raman spectroscopy

Experimental electron-density studies based on high-resolution diffraction experiments allow halogen bonds between heavy halogens to be classified. The topological properties of the electron density in Cl...Cl contacts vary smoothly as a function of the interaction distance. The situation is less straightforward for halogen bonds between iodine and small electronegative nucleophiles, such as nitrogen or oxygen, where the electron density in the bond critical point does not simply increase for shorter distances. The number of successful charge-density studies involving iodine is small, but at least individual examples for three cases have been observed. (a) Very short halogen bonds between electron-rich nucleophiles and heavy halogen atoms resemble three-centre-four-electron bonds, with a rather symmetric heavy halogen and without an appreciable σ hole. (b) For a narrow intermediate range of halogen bonds, the asymmetric electronic situation for the heavy halogen with a pronounced σ hole leads to rather low electron density in the (3,-1) critical point of the halogen bond; the properties of this bond critical point cannot fully describe the nature of the associated interaction. (c) For longer and presumably weaker contacts, the electron density in the halogen bond critical point is only to a minor extent reduced by the presence of the σ hole and hence may be higher than in the aforementioned case. In addition to the electron density and its derived properties, the halogen-carbon bond distance opposite to the σ hole and the Raman frequency for the associated vibration emerge as alternative criteria to gauge the halogen-bond strength. We find exceptionally long C-I distances for tetrafluorodiiodobenzene molecules in cocrystals with short halogen bonds and a significant red shift for their Raman vibrations.

Ping-Pong Tunneling Reactions: Can Fluoride Jump at Absolute Zero?

In a recent study, Scheiner designed a double-germanium-based fluoride receptor that binds the halogen by means of strong tetrel bonds (Chem. Eur. J. 2016, 22, 18850). In this system the F^- binds to the germanium atoms in an asymmetric fashion, thereby producing a double-well potential in which the fluoride can jump from one germanium to the other as in a ping-pong game. Herein we prove through the use of computational tools that at cryogenic temperatures this rearrangement occurs by heavy-atom quantum mechanical tunneling. The inductive strength of the substituents and the polarity of the solvent can modify the barrier and the tunneling rate. But the strongest effect is observed upon modification of the geometry of the molecule by specific substitutions that affect the barrier width, the most critical factor in a tunneling mechanism. We postulate two experimental tests, one by microwave spectroscopy and one by cryogenic NMR spectroscopy, that can prove the predicted fluoride tunneling.

On the Power of Geometry over Tetrel Bonds

Tetrel bonds are noncovalent interactions formed by tetrel atoms (as σ-hole carriers) with a Lewis base. Here, we present a computational and molecular orbital study on the effect of the geometry of the substituents around the tetrel atom on the σ-hole and on the binding strengths. We show that changing the angles between substituents can dramatically increase bond strength. In addition, our findings suggest that the established Sn > Ge > Si order of binding strength can be changed in sufficiently distorted molecules due to the enhancement of the charge transfer component, making silicon the strongest tetrel donor.

Dual function of the boron center of BH(CO)2/BH(N2)2 in halogen- and triel-bonded complexes with hypervalent halogens

The complexes between BH(CO)₂/BH(N₂)₂ and XF₃/XF₅ are stabilized by a halogen bond and a triel bond. The MEP analyses of BH(CO)₂/BH(N₂)₂ indicate that there are both a region with negative MEPs on the B atom in the vertical direction of the molecular plane and a σ-hole at the B-H bond end. Therefore, the boron atom in BH(CO)₂/BH(N₂)₂ plays a dual role of a Lewis base and an acid in the halogen bond and triel bond, respectively. The halogen and triel bonds are stronger in order of IF₃< BrF₃< ClF₃, IF₅< BrF₅< ClF₅, and BH(CO)₂< BH(N₂)₂ in most complexes. These complexes have large stability since the interaction energy varies from -5 to -115 kcal/mol. The halogen bond belongs to a covalent interaction or a partially covalent interaction in most complexes. The subsystems in these complexes have prominent deformation, accompanied with big charge transfer and large polarization.

Coordination of Be and Mg Centres by HCN Ligands - Be…N and Mg…N Interactions

ωB97XD/aug-cc-pVTZ calculations were performed for clusters of Z2+ cations (Z=Be and Mg) and HCN molecules (up to six molecules). The clusters of Be(CH3 )2 and Mg(CH3 )2 with HCN species were also calculated to analyse the influence of the Be/Mg-C formally covalent bonds on interactions of Be or Mg centre with ligands. The beryllium and magnesium centres possess different areas of a positive electrostatic potential that depend on a number of HCN ligands in the cluster considered. Numerous correlations between geometrical, energetic and topological parameters of the clusters considered are discussed since various theoretical approaches are applied; Quantum Theory of 'Atoms in Molecules', Natural Bond Orbital method and decomposition of the energy of interaction. The Be/Mg…N interactions classified as beryllium and magnesium bonds possess numerous characteristics which are known for the hydrogen bonds. Different types of coordination of Be and Mg centres analysed here exist also in crystal structures.

On the σ, π and δ hole interactions: a molecular orbital overview

With molecular orbital theory it is possible to distinguish and design σ, π and the elusive δ electrostatic holes.

Insight into trifluoromethylation – experimental electron density for Togni reagent I

The experimentally observed electron density for the “Togni reagent” explains the interaction of the hypervalent iodine atom with a nucleophile.

New Type of Halogen Bond: Multivalent Halogen Interacting with π- and σ-Electrons

MP2/aug-cc-pVTZ calculations were performed for complexes of BrF₃ and BrF₅ acting as Lewis acids through the bromine centre, with species playing a role of Lewis base: dihydrogen, acetylene, ethylene, and benzene. The molecular hydrogen donates electrons by its σ-bond, while in remaining moieties—in complexes of hydrocarbons; such an electron transfer follows from π-electrons. The complexes are linked by a kind of the halogen bond that is analyzed for the first time in this study, i.e., it is the link between the multivalent halogen and π or σ-electrons. The nature of such a halogen bond is discussed, as well as various dependencies and correlations are presented. Different approaches are applied here, the Quantum Theory of Atoms in Molecules, Natural Bond Orbital method, the decomposition of the energy of interaction, the analysis of electrostatic potentials, etc.

Hydrogen bonds, and σ-hole and π-hole bonds – mechanisms protecting doublet and octet electron structures

For various interactions electron charge shifts try to protect the former doublet or octet electronic structure of the Lewis acid centre.

IF5 affects the final stage of the Cl–F exchange fluorination in the synthesis of pentafluoro-λ6-sulfanyl-pyridines, pyrimidines and benzenes with electron-withdrawing substituents

SF₅-(hetero)arenes having electron-deficient groups were efficiently synthesized by oxidative chlorotetrafluorination followed by a S_Ni-type Cl–F exchange reaction using IF₅.