Halogen Bond Acceptor Research Articles

Organometallic halogen bond acceptors: directionality, hybrid cocrystal precipitation, and blueshifted CO ligand vibrational band

The halogen bonding (XB) in metal carbonyls blueshifts the νCO band and XB directionality is dictated by XB-accepting atom hybridization.

Cooperative halogen bonding and polarized π-stacking in the formation of coloured charge-transfer co-crystals

Matched electron rich halogen bond acceptors and donor have been synthesized and the halogen bonded charge transfer cocrystals characterized.

Bamboo-like Chained Cavities and Other Halogen-Bonded Complexes from Tetrahaloethynyl Cavitands with Simple Ditopic Halogen Bond Acceptors

Halogen bonding provides a useful complement to hydrogen bonding and metal-coordination as a tool for organizing supramolecular systems. Resorcinarenes, tetrameric bowl-shaped cavitands, have been previously shown to function as efficient scaffolds for generating dimeric capsules in both solution and solid-phase, and complicated one-, two-, and three-dimensional frameworks in the solid phase. Tetrahaloethynyl resorcinarenes (bromide and iodide) position the halogen atoms in a very promising “crown-like” orientation for acting as organizing halogen-bond donors to help build capsules and higher-order networks. Symmetric divalent halogen bond acceptors including bipyridines, 1,4-dioxane, and 1,4-diazabicyclo[2.2.2]octane are very promising halogen bond accepting partners for creating these systems. This report describes the complex structures arising from combining these various systems including self-included dimers, herringbone-packed architectures enclosing medium (186 Å3) cavities, and a very intriguing bamboo-like one-dimensional rod with large (683 Å3) cavities between adjacent dimeric units. These various structures, all organized through host–host, host–acceptor, and host–solvent interactions highlight the emergent complexity of these types of complexes. As halogen bonds are weaker than hydrogen-bonds, the resulting architectures are harder to predict, and these results provide additional insight into the parameters requiring consideration when designing crystalline supramolecular systems using halogen-bonds as the core organizing principle.

Connectivity and Topology Invariance in Self-Assembled and Halogen-Bonded Anionic (6,3)-Networks.

We report here that the halogen bond driven self-assembly of 1,3,5-trifluorotriiodobenzene with tetraethylammonium and -phosphonium bromides affords 1:1 co-crystals, wherein the mutual induced fit of the triiodobenzene derivative and the bromide anions (halogen bond donor and acceptors, respectively) elicits the potential of these two tectons to function as tritopic modules (6,3). Supramolecular anionic networks are present in the two co-crystals wherein the donor and the acceptor alternate at the vertexes of the hexagonal frames and cations are accommodated in the potential empty space encircled by the frames. The change of one component in a self-assembled multi-component co-crystal often results in a change in its supramolecular connectivity and topology. Our systems have the same supramolecular features of corresponding iodide analogues as the metric aspects seem to prevail over other aspects in controlling the self-assembly process.

Two-Dimensional Arrangements of Bis(haloethynyl)benzenes Combining Halogen and Hydrogen Interactions

The electronic distribution of some haloethynylbenzene derivatives may favor the formation of two-dimensional organizations by combining halogen and hydrogen bonds. In order to highlight this strategy, we have prepared seven cocrystals and analyzed their structures. 1,4-Bis(iodoethynyl)benzene, 1,4-bis(bromoethynyl)benzene, and 1,3-bis(iodoethynyl)benzene were used as halogen bond donors and 1,2-bis(4-pyridyl)ethylene, pyridazine, propanone, hexamethylenetetramine, and 2,8-dimethyl-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine (Troger’s base) were employed as halogen bond acceptors. The crystal structures of seven halogen-bonded complexes show C–X···Y (X = I, Br; Y = N, O) distances shorter than the sum of the van der Waals radii, and six of them contain the edge-to-edge C–H···X (X = I, Br) supramolecular hydrogen bond synthon. The stabilization energies with basis set superposition error correction of hydrogen bond synthons have been determined by DFT calculations, and they are in the range 2.9 to 5.7 kc...

Clˉ as the halogen bond acceptor: studies on strong halogen bonds

The Clˉ anion as the halogen bond acceptor, the diiodotetrafluoroethane I(CF2)2I and its derivatives I(MF2)nI (M = C, Si, Ge, Sn) as the halogen bond donor, and the strong halogen bonds could be formed. The halogen bonds between I(MF2)nI and Clˉ have been designed and investigated by Moller–Plesset perturbation/aug-cc-pVDZ calculations together with the aug-cc-pVDZ-pp basis set for iodine and stannum. The halogen bonds in the I(MF2)nI∙∙∙Clˉ complexes are strong, which are apparently related to the group IV elements, becoming stronger along the sequence of M = Si, C, Ge, Sn. Accompanied with increasing number (n) of MF2 unit, the halogen bonds (M = Si, Ge, Sn) also become stronger. The energy decomposition analyses reveal that the exchange energy contributes most in forming these halogen-bonded interactions. In the meantime, the electrostatic energy is also a significant factor for the I∙∙∙Clˉ interactions. The halogen bonds of I(MF2)nI∙∙∙Clˉ(M = C, Ge, Sn) belong to partial-covalent interactions, while they are noncovalent interactions when M = Si.

Regioselective monobromination of aromatics via a halogen bond acceptor-donor interaction of catalytic thioamide and N-bromosuccinimide

Regioselective monobromination of various aromatics was achieved at room temperature using N-bromosuccinimide and 5 mol% of thioamides in acetonitrile. With thiourea as catalyst, activated aromatics, such as anisole, acetanilide, benzamide and phenol analogues containing electron donating or withdrawing groups, were brominated with high regioselectivity. Room temperature brominations of weakly activated aromatics and deactivated 9-fluorenone were accomplished by 5 mol% thioacetamide, higher substrates concentrations and longer reaction times. A backbonding of the bromine lone pairs with the π*of CS group and a halogen bond between the halogen bond donor bromine and the halogen bond acceptor sulfur of the thioamide are thought to be the principal interactions and cause of N-bromosuccinimide activation.

Halogen-bond-mediated atom transfer radical addition of perfluoroalkyl iodides to alkynes under visible light irradiation

A halogen-bond-promoted atom transfer radical addition of alkynes with perfluoroalkyl iodides under visible light irradiation has been reported. Fluoroalkyl radicals can be generated by a visible-light-induced single electron transfer (SET) process with fluorinated iodides as halogen bond donors and organic bases as halogen bond acceptors. Subsequently fluoroalkyl radicals add to terminal alkynes to afford the corresponding iodoperfluoroalkylated products with high yields and broad substrate scope at room temperature. The resulting perfluoroalkylated alkenyl iodides can be further functionalized by cross-coupling reactions.

The Role of Halogen Bonding in Controlling Assembly and Organization of Cu(II)-Acac Based Coordination Complexes

In order to explore the use of non-covalent interactions in the deliberate assembly of metal-supramolecular architectures, a series of β-diketone based ligands capable of simultaneously acting as halogen-bond donors and chelating ligands were synthesized. The three ligands, L1, L2, and L3, carry ethynyl-activated chlorine, bromine, and iodine atoms, respectively and copper(II) complexes of all three ligands were crystallized from different solvents, acetonitrile, ethyl acetate, and nitromethane in order to study specific ligand-solvent interaction. The free ligands L2 and L3, with more polarizable halogen atoms, display C-X⋯O halogen bonds in the solid state, whereas the chloro-analogue (L1) does not engage in halogen bonding. Both acetonitrile and ethyl acetate act as halogen-bond acceptors in Cu(II)-complexes of L2 and L3 whereas nitromethane is present as a ‘space-filling’ guest without participating in any significant intermolecular interactions in Cu(II)-complexes of L2. L3, which is decorated with an iodoethynyl moiety and consistently engages in halogen-bonds with suitable acceptors. This systematic structural analysis allows us to rank the relative importance of a variety of electron-pair donors in these metal complexes.

Halogen-Bonded Co-Crystals of Aromatic N-oxides: Polydentate Acceptors for Halogen and Hydrogen Bonds

Seventeen new halogen-bonded co-crystals characterized by single crystal X-ray analysis are presented from 8 × 4 combinations using methyl-substituted pyridine N-oxides and 1,ω-diiodoperfluoroalkanes. The N−O group in six of 17 co-crystals is monodentate and 11 have μ-O,O bidentate halogen bond acceptor modes. Remarkably, the N−O group in co-crystals of 3-methyl-, 4-methyl- and 3,4-dimethylpyridineN-oxides with octafluoro-1,4-diiodobutane acted as a μ-O,O,O,O halogen and hydrogen bond acceptor, while acting as a μ-O,O,O acceptor in the co-crystal of 2,5-dimethylpyridineN-oxide and tetrafluoro-1,2-diiodoethane. The C−H···O−N hydrogen bonds demonstrated the polydentate cooperativity of the N−O group as a mixed halogen-hydrogen bond acceptor. The co-crystal of 2,4,6-trimethylpyridineN-oxide and dodecafluoro-1,6-diiodohexane exhibited C−I···O−−N+ halogen bonds with RXB value 0.76, the shortest of its kind compared to previously reported structures. The RXB values between 0.76 and 0.83 suggested that the C−I···O−−N halogen bonds are moderately strong compared to our previously studied N−−I···O−−N system, with RXB in the order 0.66.

Testing the tools for revealing and characterizing the iodine-iodine halogen bond in crystals.

To understand what tools are really suitable to identify and classify the iodine-iodine non-covalent interactions in solid organic polyiodides, we have examined the anisotropy of the electron density within the iodine atomic basin along and across the iodine-iodine halogen bond using the Laplacian of electron density, one-electron potential and electron localization function produced by Kohn-Sham calculations with periodic boundary conditions. The Laplacian of electron density exhibits the smallest anisotropy and yields a vague picture of the outermost electronic shells. The one-electron potential does not show such a deficiency and reveals that the valence electron shell for the halogen-bond acceptor iodine is always wider than that for the halogen-bond donor iodine along its σ-hole direction. We have concluded that the one-electron potential is the most suitable for classification of the iodine-iodine bonds and interactions in complicated cases, while the electron localization function allows to distinguish the diiodine molecule bonded with the monoiodide anion from the typical triiodide anion.

Toward the Classical Description of Halogen Bonds: A Quantum Based Generalized Empirical Potential for Fluorine, Chlorine, and Bromine.

The electronic effects in supramolecular systems are a great challenge for computational chemistry, and the understanding of ligand-protein interactions driven by halogen bonds can be limited by molecular mechanics point of view. In fact, the variations of the halogen bond acceptors, such as an aromatic ring and electrons lone pairs, restrict the classical approximations even more. Our work enhances the statement that halogen bonds are led mainly by orbital interactions via σ*C-X. Nonetheless, we have pointed a straight relationship between the maximum ESP value on the σ-hole and the LUMO energy levels of the halogen bond donor. In line with this scenario, the current work introduces a new promising empirical potential based on quantum parametrizations able to describe general halogen bonded systems. The new parameters allow force fields to detect variations on the molecular electronic structure of halogenated organic compounds to improve the description of fluorine, chlorine, and bromine in halogen bonds.

1,3,5-Tri(iodoethynyl)-2,4,6-trifluorobenzene: halogen-bonded frameworks and NMR spectroscopic analysis.

Halogen bonding is the non-covalent interaction between the region of positive electrostatic potential associated with a covalently bonded halogen atom, named the σ-hole, and a Lewis base. Single-crystal X-ray diffraction structures are reported for a series of seven halogen-bonded cocrystals featuring 1,3,5-tris(iodoethynyl)-2,4,6-trifluorobenzene (1) as the halogen-bond donor, and bromide ions (as ammonium or phosphonium salts) as the halogen-bond acceptors: (1)·MePh3PBr, (1)·EtPh3PBr, (1)·acetonyl-Ph3PBr, (1)·Ph4PBr, (1)·[bis(4-fluorophenyl)methyl]triphenylphosphonium bromide, and two new polymorphs of (1)·Et3BuNBr. The cocrystals all feature moderately strong iodine-bromide halogen bonds. The crystal structure of pure [bis(4-fluorophenyl)methyl]triphenylphosphonium bromide is also reported. The results of a crystal engineering strategy of varying the size of the counter-cation are explored, and the features of the resulting framework materials are discussed. Given the potential utility of (1) in future crystal engineering applications, detailed NMR analyses (in solution and in the solid state) of this halogen-bond donor are also presented. In solution, complex 13C and 19F multiplets are explained by considering the delicate interplay between various J couplings and subtle isotope shifts. In the solid state, the formation of (1)·Et3BuNBr is shown through significant 13C chemical shift changes relative to pure solid 1,3,5-tris(iodoethynyl)-2,4,6-trifluorobenzene.

Halogen bonding-assisted adsorption of iodoperfluoroarenes on a strong anion exchanger and its potential application in solid-phase extraction

The adsorption of iodoperfluoroarenes (IPFArs) by solid-liquid interfacial halogen bonding was investigated and preliminarily applied to solid-phase extraction (SPE). Based on 19F NMR titration experiments, UV spectrophotometric titrations, and chemical computations, silica gel functionalized with trimethylaminopropyl chloride (SAX) groups was chosen as a strong anion-exchanging adsorbent, since the solution-phase association ability of Cl− as halogen bond acceptor is superior to that of Br− or I−. Further explorations indicated that the adsorption of IPFArs is mainly driven by halogen bonding. The adsorption isotherms of 1,4-diiodotetrafluorobenzene (1,4-DITFB) and iodoperfluorobenzene (IPFB) were fitted by the Freundlich equation, while that of 1,2-diiodotetrafluorobenzene (1,2-DITFB) was fitted by the Langmuir equation. Such alternative adsorption models can meet the requirements of different self-assembly systems based on halogen bonding. In SPE applications, the adsorption efficiency of IPFArs on SAX is as follows: 1,4-DITFB≈1,2-DITFB>IPFB, with no significant adsorption of bromoperfluoroarenes. These results indicate promising applications of halogen bonding in the separation and enrichment of IPFArs.

13C and 19F solid-state NMR and X-ray crystallographic study of halogen-bonded frameworks featuring nitrogen-containing heterocycles.

Halogen bonding is a noncovalent interaction between the electrophilic region of a halogen (σ-hole) and an electron donor. We report a crystallographic and structural analysis of halogen-bonded compounds by applying a combined X-ray diffraction (XRD) and solid-state nuclear magnetic resonance (SSNMR) approach. Single-crystal XRD was first used to characterize the halogen-bonded cocrystals formed between two fluorinated halogen-bond donors (1,4-diiodotetrafluorobenzene and 1,3,5-trifluoro-2,4,6-triiodobenzene) and several nitrogen-containing heterocycles (acridine, 1,10-phenanthroline, 2,3,5,6-tetramethylpyrazine, and hexamethylenetetramine). New structures are reported for the following three cocrystals, all in the P21/c space group: acridine-1,3,5-trifluoro-2,4,6-triiodobenzene (1/1), C6F3I3·C13H9N, 1,10-phenanthroline-1,3,5-trifluoro-2,4,6-triiodobenzene (1/1), C6F3I3·C12H8N2, and 2,3,5,6-tetramethylpyrazine-1,3,5-trifluoro-2,4,6-triiodobenzene (1/1), C6F3I3·C8H12N2. 13C and 19F solid-state magic-angle spinning (MAS) NMR is shown to be a convenient method to characterize the structural features of the halogen-bond donor and acceptor, with chemical shifts attributable to cocrystal formation observed in the spectra of both nuclides. Cross polarization (CP) from 19F to 13C results in improved spectral sensitivity in characterizing the perfluorinated halogen-bond donor when compared to conventional 1H CP. Gauge-including projector-augmented wave density functional theory (GIPAW DFT) calculations of magnetic shielding constants, along with optimization of the XRD structures, provide a final set of structures in best agreement with the experimental 13C and 19F chemical shifts. Data for carbons bonded to iodine remain outliers due to well-known relativistic effects.

The halogen bonding proclivity of the ortho-methoxy–hydroxy group in cocrystals of o-vanillin imines and diiodotetrafluoro-benzenes

Novel halogen bondedo-hydroxy imine cocrystals with 1,2-, 1,3- and 1,4-diiodotetrafluorobenzene have been synthesized. We present an insight into the halogen bond acceptor potential of theortho-methoxy–hydroxy group of theo-vanillin moiety.

Halogen-bonded solvates of tetrahaloethynyl cavitands

Tetrahaloethynyl cavitands as multitopic halogen bond donors form a multitude of solvate structures with halogen bond acceptor solvents, such as DMF and DMSO, depending on the structure of the cavitand and the nucleophilicity of the solvent used.

Strength, character, and directionality of halogen bonds involving cationic halogen bond donors.

Halogen bonds involving cationic halogen bond donors and anionic halogen bond acceptors have recently been recognized as being important in stabilizing the crystal structures of many salts. Theoretical characterization of these types of interactions, most importantly in terms of their directionality, has been limited. Here we generate high-quality symmetry adapted perturbation theory potential energy curves of a H3N-C[triple bond, length as m-dash]C-Br+Cl- model system in order to characterize halogen bonds involving charged species, in terms of contributions from electrostatics, exchange, induction, and dispersion, with special emphasis on analyzing contributions that are most responsible for the directionality of these interactions. It is found that, as in the case of neutral halogen bonds, exchange forces are important contributors to the directionality of charged halogen bonds, however, it is also found that induction effects, which contribute little to the stability and directionality of neutral halogen bonds, play a large role in the directionality of halogen bonds involving charged species. Potential energy curves based on the ωB97X-D/def2-TZVP/C-PCM method, which includes an implicit solvation model in order to mimic the effects of the crystal medium, are produced for both the H3N-C[triple bond, length as m-dash]C-Br+Cl- model system and for the 4-bromoaniliniumCl- dimer, which is based on the real 4-bromoanilinium chloride salt, whose crystal structure has been determined experimentally. It is found that, within a crystal-like medium, charged halogen bond are significantly weaker than in the gas phase, having optimum interaction energies up to approximately -20 kcal mol-1.

Tridentate C–I⋯O−–N+ halogen bonds

N-Oxides can act as tridentate halogen bond acceptors, or as tetradentate ligands in a pseudo-μ4 mode with silver(i).

Electron-transfer reactions of halogenated electrophiles: a different look into the nature of halogen bonding.

The rates of oxidation of ferrocene derivatives by brominated molecules R-Br (CBr3CN, CBr4, CBr3NO2, CBr3COCBr3, CBr3CONH2, CBr3F, and CBr3H) were consistent with the predictions of the outer-sphere dissociative electron-transfer theory. The similar redox-reactions of the R-Br electrophiles with the typical halogen-bond acceptors tetramethyl-p-phenylenediamine (TMPD) or iodide were much faster than calculated using the same model. The fast redox-processes in these systems were related to the involvement of the transient halogen-bonded [R-Br, TMPD] or [R-Br, I-] complexes in which barriers for electron transfer were lowered by the strong electronic coupling of reactants. The Mulliken-Hush treatment of the spectral and structural characteristics of the [R-Br, TMPD] or [R-Br, I-] complexes corroborated the values of coupling elements, Hab, of 0.2-0.5 eV implied by the kinetic data. The Natural Bond Orbital analysis of these complexes indicated a noticeable donor/acceptor charge transfer, Δq, of 0.03-0.09 ē. The Hab and Δq values in the [R-Br, TMPD] and [R-Br, I-] complexes (which are similar to those in the traditional charge-transfer associates) indicate significant contribution of charge-transfer (weakly-covalent) interaction to halogen bonding. The decrease of the barrier for electron transfer between the halogen-bonded reactants demonstrated in the current work points out that halogen bonding should be taken into account in the mechanistic analysis of the reactions of halogenated species.