Intermolecular Halogen Research Articles

Computational insights into the photocyclization of diclofenac in solution: effects of halogen and hydrogen bonding.

The effects of noncovalent interactions, namely halogen and hydrogen bonding, on the photochemical conversion of the photosensitizing drug diclofenac (DCF) in solution were investigated computationally. Both explicit and implicit solvent effects were qualitatively and quantitatively assessed employing the DFT/6-31+G(d) and SQM(PM7) levels of theory. Full geometry optimizations were performed in solution for the reactant DCF, hypothesized radical-based intermediates, and the main product at both levels of theories. Notably, in good agreement with previous experimental results concerning the intermolecular halogen bonding of DCF, the SQM(PM7) method revealed different values for d(ClO, Å) and ∠(C-ClO, °) for the two chlorine-substituents of DCF, with values of 2.63 Å/162° and 3.13 Å/142° for the trans and cis orientations, respectively. Employing the DFT/6-31+G(d) method with implicit solvent effects was not conclusive; however, explicit solvent effects confirmed the key contribution of hydrogen and halogen bonding in stabilizing/destabilizing the reactant and hypothesized intermediates. Interestingly, the obtained results revealed that a protic solvent such as water can increase the rate of photocyclization of DCF not only through hydrogen bonding effects, but also through halogen bonding. Furthermore, the atomic charges of atoms majorly involved in the photocyclization of DCF were calculated using different methods, namely Mulliken, Hirshfeld, and natural bond orbital (NBO). The obtained results revealed that in all cases there is a notable nonequivalency in the noncovalent intermolecular interactions of the two chlorine substituents of DCF and the radical intermediates with the solvent, which in turn may account for the discrepancy of their reactivity in different media. These computational results provide insight into the importance of halogen and hydrogen bonding throughout the progression of the photochemical conversion of DCF in solution.

The constructions and applications of purely organic phosphorescent material

Phosphorescence is among the many functional features of the excited organic molecules. Organic molecules with long-lived triplet excited states enable exciton migration over long distance, which is essential in a variety of optoelectronic applications such as photovoltaics, photocatalytic reactions, display and lighting system, molecular sensing and biological imaging. In general, phosphorescence materials involve the organometallic compounds that are expensive, such as Pt, Ir and so on. Owing to the presence of metal, the phosphorescence materials is toxic and are also intrinsically unstable in the case of high-energy blue emitters. Therefore the purely organic materials that show room-temperature phosphorescence (RTP) are attractive alternatives to organometallic phosphors. The triplet excitons are not commonly generated in organic materials and are mostly consumed through radiationless processes, such as vibrational dissipation and oxygen-mediated quenching, under ambient conditions. Recently, a variety of methods from molecular design to material design to access organic room temperature phosphorescence have been explored. This review focus on the recent advancements in the field of organic phosphorescence, classifying the strategies based on material design. The outlook of organic room temperature phosphorescence is also discussed. The basic concepts for RTP process are introduced to demonstrate what’s the key factors to obtain efficient RTP. They are: (1) the promotion of both singlet-to-triplet and triplet-to-singlet intersystem crossing, and (2) suppression of the nonradiative quenching processes from the triplet to the ground state. Comparisons between RTP and delayed fluorescence are also made in the review. The triplet excited states of organic molecules can be stabilized by interactions of host molecules and guest molecules, rigid solid-crystallized structures and the chromophore embedded in polymer, which can suppress the nonradiative deactivation pathways and rates of triplet excitons by restricting the vibrational dissipation of long-lived triplets, and considerably increase the possibility of radiative decay. The directed intermolecular halogen bonding can also promote the intersystem-crossing processes by enhancing spin-orbit coupling, and improve the radiative decay. In most studies, various methods are combined to obtain phosphorescence materials of long-lived triplet excited states with high quantum yield.

Stereodynamics in the Collisional Autoionization of Water, Ammonia, and Hydrogen Sulfide with Metastable Rare Gas Atoms: Competition Between Intermolecular Halogen and Hydrogen Bonds.

Recent experiments on the title subject, performed with a high-resolution crossed-beam apparatus, have provided the total ionization cross sections as a function of the collision energy between noble gas atoms, electronically excited in their metastable states (Ng*), and H2 O, H2 S, and NH3 reagents, as well as the emitted electron energy spectra. This paper presents a rationalization of all the experimental findings in a unifying picture to cast light on the basic chemical properties of Ng* under conditions of great relevance both from a fundamental and from an applied point of view. The importance of this investigation is that it isolates the selective role of the intermolecular halogen and hydrogen bonds, to assess their anisotropic effects on the stereodynamics of the promoted ionization reactions, and to model energy transfer and reactivity in systems of applied interest, such as planetary atmospheres, plasmas, lasers, and flames.

Crystal structure of 8-iodo-quinolinium tetra-chlorido-aurate(III).

The structure of the title salt, (C9H7IN)[AuCl4], is comprised of planar 8-iodo­quinolinium cations (r.m.s. deviation = 0.05 Å) and square-planar tetra­chlorido­aurate(III) anions. The asymmetric unit contains one 8-iodo­quinolinium cation and two halfs of [AuCl4]− anions, in each case with the central AuIII atom located on an inversion center. Inter­molecular halogen–halogen contacts were found between centrosymmetric pairs of I [3.6178 (4) Å] and Cl atoms [3.1484 (11), 3.3762 (13), and 3.4935 (12) Å]. Inter­molecular N—H⋯Cl and C—H⋯Cl hydrogen bonding is also found in the structure. These inter­actions lead to the formation of a three-dimensional network. Additionally, there is an intra­molecular N—H⋯I hydrogen bond between the aromatic iminium and iodine. There are no aurophilic inter­actions or short contacts between I and Au atoms, and there are no notable π-stacking inter­actions between the aromatic cations.

The stereo-dynamics of collisional autoionization of ammonia by helium and neon metastable excited atoms through molecular beam experiments.

A combined analysis of both new (energy spectra of emitted electrons) and previously published (ionization cross sections) experimental data, measured under the same conditions and concerning electronically excited lighter noble gas -NH3 collisional autoionization processes, is carried out. Such an analysis, performed by exploiting a formulation of the full potential energy surface both in the real and imaginary parts, provides direct information on energetics, structure, and lifetime of the intermediate collision complex over all the configuration space. The marked anisotropy in the attraction of the real part, driving the approach of reagents, and the selective role of the imaginary component, associated to the charge transfer coupling between entrance and exit channels, suggests that reactive events occur almost exclusively in the molecular hemisphere containing the nitrogen lone pair. Crucial details on the stereo-dynamics of elementary collisional autoionization processes are then obtained, in which the open shell nature of the disclosed ionic core of metastable atom plays a crucial role. The same analysis also suggests that the strength of the attraction and the anisotropy of the interaction increases regularly along the series Ne*((3)P), He*((3)S), He*((1)S)-NH3. These findings can be ascribed to the strong rise of the metastable atom electronic polarizability (deformability) along the series. The obtained results can stimulate state of the art ab initio calculations focused on specific features of the transition state (energetics, structure, lifetime, etc.) which can be crucial for a further improvement of the adopted treatment and to better understand the nature of the leading interaction components which are the same responsible for the formation of the intermolecular halogen and hydrogen bond.

Intramolecular halogen bonds in 1,2-aryldiyne molecules: a theoretical study

Intramolecular halogen bonds have been the subject of several current experimental and theoretical studies. In this work, intramolecular halogen bonds in a series of 1,2-aryldiyne molecules were investigated using density functional theory calculations at the M06-2x level of theory. For comparison, some dimeric complexes between halogenated aryldiynes and quinolinyl compounds were also considered. The calculated interatomic distances and interaction angles of intramolecular halogen bonds compare fairly well with those determined experimentally, and the triangle motifs retain almost perfectly planar in all the studied molecules. Many of the well-known properties of conventional halogen bonds are reproduced in intramolecular halogen bonds: the interaction strength tends to increase with the enlargement of the atomic radius of halogens (I > Br > Cl); the attachment of electron-withdrawing moieties to halogens leads to much stronger intramolecular halogen bonds; the X···N (quinolinyl) interactions are stronger than the X···O (carbonyl) halogen bonds. On the basis of the shorter interatomic distances and the larger values of electron densities at the bond critical points, intramolecular halogen bonds become stronger in strength than corresponding intermolecular halogen bonds. However, these interactions have similar structural, energetic, atoms in molecules (AIM), and noncovalent interaction index (NCI) characteristics to traditional halogen bonds. Therefore, these interactions can be recognized as halogen bonds that are primarily electrostatic in nature. Particularly, the formation of intramolecular halogen bonds gives rise to the essential coplanarity of the molecules, whereas the two subunits in the dimeric complexes deviate from planarity to a large degree. In addition, a small number of crystal structures containing intramolecular halogen bonds were retrieved from the Cambridge Structural Database (CSD), to provide more insights into these interactions in crystals. This work not only will extend the knowledge of noncovalent interactions involving halogens as electrophilic centers but also could be very useful in molecular design and synthetic chemistry.

Concerted Halogen-Bonded Networks with N-Alkyl Ammonium Resorcinarene Bromides: From Dimeric Dumbbell to Capsular Architectures.

N-Alkyl ammonium resorcinarene bromides and 1,4-diiodooctafluorobutane via multiple intermolecular halogen bonds (XB) form different exotic supramolecular architectures through subtle changes of the upper rim substituents. Dimeric dumbbell-like assembly with encapsulated guest molecules is generated with N-benzyl substituents. The N-hexyl groups engender an XB-induced polymeric pseudocapsule and an XB-induced dimeric capsule with entrapped 1,4-dioxane guest molecules. The N-propyl and N-cyclohexyl groups generate deep cavity cavitands. The deep cavity cavitands possess cavities for self-inclusion leading to polymeric herringbone arrangement in one direction and that pack into 3D polymeric arrangement resembling egg crate-like supramolecular networks. These assemblies are studied in solution via NMR spectroscopy and in the solid state via X-ray crystallography.

Catching the role of anisotropic electronic distribution and charge transfer in halogen bonded complexes of noble gases.

The systems studied in this work are gas-phase weakly bound adducts of the noble-gas (Ng) atoms with CCl4 and CF4. Their investigation was motivated by the widespread current interest for the intermolecular halogen bonding (XB), a structural motif recognized to play a role in fields ranging from elementary processes to biochemistry. The simulation of the static and dynamic behaviors of complex systems featuring XB requires the formulation of reliable and accurate model potentials, whose development relies on the detailed characterization of strength and nature of the interactions occurring in simple exemplary halogenated systems. We thus selected the prototypical Ng-CCl4 and Ng-CF4 and performed high-resolution molecular beam scattering experiments to measure the absolute scale of their intermolecular potentials, with high sensitivity. In general, we expected to probe typical van der Waals interactions, consisting of a combination of size (exchange) repulsion with dispersion/induction attraction. For the He/Ne-CF4, the analysis of the glory quantum interference pattern, observable in the velocity dependence of the integral cross section, confirmed indeed this expectation. On the other hand, for the He/Ne/Ar-CCl4, the scattering data unravelled much deeper potential wells, particularly for certain configurations of the interacting partners. The experimental data can be properly reproduced only including a shifting of the repulsive wall at shorter distances, accompanied by an increased role of the dispersion attraction, and an additional short-range stabilization component. To put these findings on a firmer ground, we performed, for selected geometries of the interacting complexes, accurate theoretical calculations aimed to evaluate the intermolecular interaction and the effects of the complex formation on the electron charge density of the constituting moieties. It was thus ascertained that the adjustments of the potential suggested by the analysis of the experiments actually reflect two chemically meaningful contributions, namely, a stabilizing interaction arising from the anisotropy of the charge distribution around the Cl atom in CCl4 and a stereospecific electron transfer that occurs at the intermolecular distances mainly probed by the experiments. Our model calculations suggest that the largest effect is for the vertex geometry of CCl4 while other geometries appear to play a minor to negligible role.

Halogen Interactions in Macrocyclic Thiacalix[4]arene Systems

The crystalline supramolecular assemblies of brominated thiacalix[4]arene alkyl ethers were studied by single-crystal X-ray diffraction and computational analysis. Eight compounds were synthesized that exhibited linear alkyl groups of varying lengths projecting from the thiacalixarene lower rims (−CH3: 4, −CH2CH3: 5, −(CH2)4CH3) to –(CH2)9CH3): 6–11) in order to study the intermolecular halogen interactions between the macrocycles. These crystals displayed remarkably different assemblies, which was rationalized as an effect of varying alkyl chain length on the stabilization afforded by the halogen interactions. The two compounds with the shortest alkyl chains, 4 and 5, assembled primarily through the influence of halogen−π interactions between distinct molecules. In contrast, the compounds with the longest chains, 8–11, assembled primarily through the influence of halogen–sulfur halogen bonding. Oddly, 6 and 7 did not follow this pattern in that they showed no major assembly motivated by halogen interactions. Finally, the supramolecular assemblies were found to be stabilized by additional intermolecular interactions, including hydrogen bonding, S-π, CH-π, and Br–H interactions.

Self-assembled two-dimensional nanoporous molecular arrays and photoinduced polymerization of 4-bromo-4'-hydroxybiphenyl on Ag(111).

Self-assembled two-dimensional molecular arrays and photoinduced polymerization of 4-bromo-4'-hydroxybiphenyl on Ag(111) were studied using low-temperature scanning tunneling microscopy combined with density functional theory calculations. Square-like self-assembled structures of 4-bromo-4'-hydroxybiphenyl stabilized by intermolecular hydrogen and halogen bonds were transformed into hexagonal nanopores of biphenyl biradicals by 266 nm UV laser irradiation at 80 K. The biradicals further coupled to each other and formed covalently linked polyphenylene polymer chains at room temperature.

A Br-substituted phenanthroimidazole derivative with aggregation induced emission from intermolecular halogen-hydrogen interactions.

A unique aggregation induced emission (AIE) active emitter based on a Br-substituted phenanthroimidazole derivative was reported. The yellowish green emission with high quantum yield and a large Stokes shift (∼150 nm) in the aggregated state is proposed to be obtained from weak interactions especially C-H···Br interactions.

Thermodynamic parameters for binding of some halogenated inhibitors of human protein kinase CK2

The interaction of human CK2α with a series of tetrabromobenzotriazole (TBBt) and tetrabromobenzimidazole (TBBz) analogs, in which one of the bromine atoms proximal to the triazole/imidazole ring is replaced by a methyl group, was studied by biochemical (IC50) and biophysical methods (thermal stability of protein–ligand complex monitored by DSC and fluorescence). Two newly synthesized tri-bromo derivatives display inhibitory activity comparable to that of the reference compounds, TBBt and TBBz, respectively. DSC analysis of the stability of protein–ligand complexes shows that the heat of ligand binding (Hbind) is driven by intermolecular electrostatic interactions involving the triazole/imidazole ring, as indicated by a strong correlation between Hbind and ligand pKa. Screening, based on fluorescence-monitored thermal unfolding of protein–ligand complexes, gave comparable results, clearly identifying ligands that most strongly bind to the protein. Overall results, additionally supported by molecular modeling, confirm that a balance of hydrophobic and electrostatic interactions contribute predominantly, relative to possible intermolecular halogen bonding, in binding of the ligands to the CK2α ATP-binding site.

Back Cover: Tailoring Intermolecular Interactions for Efficient Room‐Temperature Phosphorescence from Purely Organic Materials in Amorphous Polymer Matrices (Angew. Chem. Int. Ed. 42/2014)

Rationally designed strong intermolecular hydrogen and halogen bonds between a novel phosphor and a poly(vinyl alcohol) (PVA) matrix led to bright room-temperature phosphorescence with a quantum yield of 24 %. J. Kim and co-workers show in their Communication on page 11177 ff. that modulation of the strength of halogen and hydrogen bonding in the purely organic phosphor–PVA system by water enabled reversible switching between phosphorescence (green) and fluorescence (blue).

Intermolecular Hydrogen and Halogen Bonds in Fluorinated Benzimidazoles

After hydrogen bonds (HB) the most studied of related weak interactions are the halogen bonds (XB). The competition between these two interactions as well as their interplay to determine the crystal packing of organic derivatives is a subject of interest. Most the studies related to XB concerns the heaviest halogen atoms, I and Br, less Cl and much less F, because the interaction energy decreases in this order. [1,2] We present here our studies on the structure of five tetrafluorinated benzazoles by X-ray crystallography and solid-state NMR, a powerful synergic mixture of techniques. The compounds are 4,5,6,7-tetrafluoro-1H-benzimidazole (1), 4,5,6,7-tetrafluoro-2-(trifluoromethyl)-1H-benzimidazole (2), 4,5,6,7-tetrafluoro-1H-benzimidazole-2(3H)-one (3), 4,5,6,7-tetrafluoro-1-methyl-1H-benzimidazole-2(3H)-one (4), and 4,5,6,7-tetrafluoro-1,3-dimethyl-1H-benzimidazole-2(3H)-one (5). As a common general feature, these compounds are quite planar due to the presence of the two aromatic rings. The existence of the methyl groups as substituent in the nitrogen atoms does not modify the planarity of the molecule. The bond distances and angles are in agreement with the expected ones for this kind of compounds. The presence of the carbonyl group induces some electronic changes in the imidazole ring that is resembled in the lengthening of the C2N3 bond distance and in a higher deviation of C2 atom from the molecular plane. Moreover, all the compounds show one or more interactions by strong linear hydrogen bonds, which lead to the formation of chains that, in some cases, can exhibit additional interactions via pi-pi and/or F···F contacts spreading out the dimensionality of the structure in the crystal. [3] The distribution of the F···F contacts in compounds 1-5 is similar to those of the literature and includes for 4,5,6,7-tetrafluoro-1H-benzimidazole-2(3H)-one (3) one of the shortest F···F distances ever reported [2.596(3) Å]. Two polymorphs of compound 4 are identified.

Toward the development of the potential with angular distortion for halogen bond: a comparison of potential energy surfaces between halogen bond and hydrogen bond.

As noncovalent intermolecular interactions, hydrogen bond (HB) and halogen bond (XB) are attracting increasing attention. In this work, the potential energy surfaces (PESs) of hydrogen and halogen bonds are compared. Twelve halogen-bonded and three hydrogen-bonded models are scanned for analysis using the MP2 level of theory. This work indicates that potential energy surfaces of both HB and XB have angular distortion. The potential well of XB is narrower than that of HB. With the elongation of the bond length, the potential energy surfaces get flatter. The best fitting functions for angular distortion and the flattening character of angular terms are also combined into a modified Buckingham potential. The testing results show that the essential features of the PES, including angular distortion and flattening character, have been reproduced. These results provide a better understanding of halogen and hydrogen bonds and the optimization of halogen bond force fields.

Substitution effect on the intermolecular halogen and hydrogen bonds of the σ‐bonded fluorinated pyridine···XY/HX complexes (XY = F2, Cl2, ClF; HX = HF, HCl)

Optimal structures, electronic and thermodynamic properties of the title complexes are presented. The stability of the hydrogen bonded systems is enhanced by the increasing dipole moments whereas in the halogen bonded systems it is also affected by the atom size in the diatomics. The consecutive addition of fluorine atoms to the pyridine moiety results in the decrease of the interaction energy for both types of the investigated bonds. The substitution on the meta sites in pyridine leads to more stable complexes than the substitution in the ortho position. The role of substitution on electric polarization and electrostatic forces is estimated by the symmetry‐adapted perturbation theory energy decomposition. The predicted Gibbs free energies of the complexes of mono fluorinated pyridines with HCl, HF, and ClF are from −12 to −22 kJ mol−1 at 200 K. The possible experimental identification of the complexes with respect to the vibrational modes is discussed. © 2014 Wiley Periodicals, Inc.

Two-point halogen bonding between 3,6-dihalopyromellitic diimides

The syntheses of several 3,6-dihalopyromellitic diimides have been achieved. Two-point intermolecular halogen–oxygen interactions—reminiscent of carboxylic acid dimers—are observed in the solid-state superstructures of (i) these molecules and (ii) their charge-transfer complexes with electron-rich naphthalenes.

Kinetic Control of Chlorine Packing in Crystals of a Precisely Substituted Polyethylene. Toward Advanced Polyolefin Materials

The crystallization of a polyethylene with precise chlorine substitution on each and every 15th backbone carbon displays a drastic change in crystalline structure in a narrow interval of crystallization temperatures. The structural change occurs within one degree of undercooling and is accompanied by a sharp increase in melting temperature, a change in WAXD patterns, and a dramatic increase in TG conformers around the Cl substitution while the main CH2 sequence remains with the all-trans packing. These changes correlate with the formation of two different polymorphs characterized by a different packing and distribution of Cl atoms in the crystallites. Under fast crystallization kinetics, the chains assemble in an all-trans planar packing (form I) with a layered Cl distribution that presents some longitudinal disorder, while slower crystallization rates favor a more structured intermolecular halogen staggering consistent with a herringbone-like nonplanar structure (form II). The drastic change in morpholog...

Mixed CoN4Cl2 and CoCl4 units versus two CoN2Cl3 units in [Co2(Htbi)2Cl4] isomers

The solvothermal reaction of CoCl2 and 2-(4-thiazolyl)-benzimidazole (Htbi) in ethanol solution at 160°C and 72h gave rise to a majority of green crystals of [Co2(Htbi)2Cl4] 1 and minor isomeric blue crystals of 2. Two isomers show quite different isolated dinuclear structures bridged by two chlorides. Complex 1 consists of edge-sharing CoN4Cl2 octahedron and CoCl4 tetrahedron, while 2 is composed of two edge-sharing CoN2Cl3 square pyramids. The intra- and inter-molecular halogen hydrogen bonds play an important role at the stabilization in the solid state. Density functional theory calculations manifest that the optimized geometric parameters match very well with the experimental ones for both isomers. The difference of calculated Gibbs free energies demonstrates that intrinsic stability of 1 is superior to 2, in agreement with their relative yields under experimental conditions. The experimental UV–vis absorption spectra in N, N-dimethyl acetamide for 1 and 2 show the same absorption bands and match the computed results. Magnetic studies of 1 reveal that strong antiferromagnetic interactions were detected between octahedral and tetrahedral Co(II) atoms.

Complexation of two macrocycles for amide, saccharide, and halide derivatives: the capacity of 1,2,3-triazole as hydrogen and halogen bonding acceptors

Two 1,2,3-triazole- and amide-incorporated macrocycles have been prepared by 1,3-dipolar cycloaddition of the corresponding dialkyne and diazide precursors. Intramolecular C–H⋯O hydrogen bonding is introduced to lock the C5–H atoms of the 1,2,3-triazole rings. The binding of the two macrocycles to amide, monosaccharide, and halide derivatives in chloroform or dichloromethane has been investigated. It is revealed that the amide units dominate their binding toward the amide and monosaccharide guests through forming intermolecular hydrogen bonding and 1,2,3-triazole is as weak as an intermolecular hydrogen bonding acceptor, but it forms intermolecular halogen bonding when cooperative effect exists.