The Nature of Halogen⋅⋅⋅Halogen Synthons: Crystallographic and Theoretical Studies

Competition vs. Cooperativity of I⋅⋅⋅Omorpholinyl and I⋅⋅⋅Cl-M Halogen Bonds in Cocrystals of Zinc(II) and Copper(II) Coordination Compounds Carrying Multiple Acceptor Sites

In order to explore a strategy for synthesizing halogen-bonded metal-organic cocrystals by utilizing metal complexes whose pendant chloride group and the morpholinyl oxygen atom enables halogen bonding, we have synthesized four pentacoordinated Cu(II) and Zn(II) complexes of the MCl2L general formula (L=imines prepared by the condensation reaction of 4-aminoethylmorpholine with 2-pyridinecarboxyaldehide or 2-acetylpyridine). The prepared metal complexes were further cocrystallized with selected iodoperfluorinated benzenes. Out of 20 combinations, 14 experiments yielded crystals suitable for single-crystal X-ray diffraction. Structural analysis revealed that in 7 cocrystals halogen bonds are formed both with morpholinyl oxygen as well as with chloride atoms. In 6 cocrystals only I⋅⋅⋅Cl halogen bonds are present, while only one cocrystal exclusively featured I⋅⋅⋅Omorpholinyl halogen bonds. We observed 5 halogen bonding motifs to the MCl2 moiety, in which each chloride atom can be an acceptor of one halogen bond, two, or none at all. The most common motif in our work (6 cocrystals) is where one chlorine atom is an acceptor of one halogen bond, while the other chlorine atom does not participate in halogen bonding. The crystal packing in the prepared cocrystals is directed by halogen-bonded architectures which are either zero-, one- or two-dimensional.

Mol-ecular structure of tris-[(6-bromo-pyridin-2-yl)meth-yl]amine

Coordination compounds of polydentate nitro-gen ligands with metals are used extensively in research areas such as catalysis, and as models of complex active sites of enzymes in bioinorganic chemistry. Tris(2-pyridyl-meth-yl)amine (TPA) is a tripodal tetra-dentate ligand that is known to form coordination compounds with metals, including copper, iron and zinc. The related compound, tris-[(6-bromo-pyridin-2-yl)meth-yl]amine (TPABr3), C18H15Br3N4, which possesses a bromine atom on the 6-position of each of the three pyridyl moieties, is also known but has not been heavily investigated. The mol-ecular structure of TPABr3 as determined by X-ray diffraction is reported here. The TPABr3 molecule belongs to the triclinic, P space group and displays interesting intermolecular Br⋯Br interactions that provide a stabilizing influence within the molecule.

Leveraging Halogen Interactions for a Supramolecular Nanotube

We demonstrate the formation of supramolecular nanotubes from molecular triangles in a single crystal by balancing the hydrogen bonds and halogen interactions between individual macrocycles. Thereby, we template the supramolecular nanotube growth by intermolecular interactions encoded directly in the macrocycles instead of those provided by the crystallization solvent. Ultimately, we show that replacing bromines for iodines in the macrocycle is necessary to achieve this supramolecular organization by enhancing the strength of the halogen interactions and concomitant reduction of the detrimental hydrogen bonds. We investigated the nature and the interplay of the individual intermolecular interactions by analysis of the experimental single crystal data and quantum chemical calculations. This work enriches the available toolbox of supramolecular interactions and will aid and abet the development of rationally-designed materials with a long-range 1D tubular organization.

Impact of Halogen Termination and Chain Length on π-Electron Conjugation and Vibrational Properties of Halogen-Terminated Polyynes

We explored the optoelectronic and vibrational properties of a new class of halogen-terminated carbon atomic wires in the form of polyynes using UV-vis, infrared absorption, Raman spectroscopy, X-ray single-crystal diffraction, and DFT calculations. These polyynes terminate on one side with a cyanophenyl group and on the other side, with a halogen atom X (X = Cl, Br, I). We focus on the effect of different halogen terminations and increasing lengths (i.e., 4, 6, and 8 sp-carbon atoms) on the π-electron conjugation and the electronic structure of these systems. The variation in the sp-carbon chain length is more effective in tuning these features than changing the halogen end group, which instead leads to a variety of solid-state architectures. Shifts between the vibrational frequencies of samples in crystalline powders and in solution reflect intermolecular interactions. In particular, the presence of head-to-tail dimers in the crystals is responsible for the modulation of the charge density associated with the π-electron system, and this phenomenon is particularly important when strong I··· N halogen bonds occur.

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

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

Anisotropies in electronic densities and electrostatic potentials of Halonium Ions: focus on Chlorine, Bromine and Iodine

CONTEXT

Why are the halonium cations so effective in forming strongly-bound complexes? We directed our research to address this question and we present electrostatic potential data for the valence-state halogen atoms X and halonium cations X+, where X = Cl, Br, I. The electron densities and electrostatic potentials of the halonium cations show considerably greater anisotropy than do the valence state halogens. The distances from the electrostatic potential surface maxima to the halogen nuclei are about 0.5 Å smaller than the distances from the electrostatic potential surface minima to the nuclei, giving the halonium cations each a more disk-like shape than the corresponding neutral valence state halogens. Their surface electrostatic potentials are totally consistent with the directionalities of halonium cations in complexes and the strengths of their interactions. To add perspective to this brief report, we have included calculations of the isotropic cation K+ and noble gas Kr.

METHODS

The calculations of the electrostatic potentials of the valence states of the halogen atoms Cl, Br and I and the halonium cations Cl+, Br+ and I+, as well as K+ and Kr, on 0.001 au contours of their electronic densities were carried out with Gaussian O9 and the Wave Function Analysis - Surface Analysis Suite (WFA-SAS) at the M06-2X/6-31 + G(d,p) and M06-2X/3-21G* levels.

Bispidine as a Versatile Scaffold: From Topological Hosts to Transmembrane Transporters

The development of designer topological structures is a synthetically challenging endeavor. We present herein bispidine as a platform for the design of molecules with various topologies and functions. The bispidine-based acyclic molecule, which shows intriguing S-shape topology, is discussed. Single-crystal X-ray diffraction studies revealed that this molecule exists in the solid state as two conformational enantiomers. In addition, bispidine-based designer macrocycles were synthesized and investigated for ionophoric properties. Patch clamp experiments revealed that these macrocycles transport both anions and cations non-specifically with at least tenfold higher chloride conductance over the cations under the given experimental conditions. Ultramicroscopy and single-crystal X-ray crystallographic studies indicated that the self-assembling macrocycle forms a tubular assembly. Our design highlights the use of unconventional dihydrogen interactions in nanotube fabrication.

A nickel(ii)-based one-dimensional organic-inorganic halide perovskite ferroelectric with the highest Curie temperature

Organic-inorganic halide perovskites (OIHPs) are very eye-catching due to their chemical tunability and rich physical properties such as ferroelectricity, magnetism, photovoltaic properties and photoluminescence. However, no nickel-based OIHP ferroelectrics have been reported so far. Here, we designed an ABX3 OIHP ferroelectric (3-pyrrolinium)NiCl3, where the 3-pyrrolinium cations are located on the voids surrounded by one-dimensional chains composed of NiCl6-face-sharing octahedra via hydrogen bonding interactions. Such a unique structure enables the (3-pyrrolinium)NiCl3 with a high spontaneous polarization (P s) of 5.8 μC cm-2 and a high Curie temperature (T c) of 428 K, realizing dramatic enhancement of 112 and 52 K compared to its isostructural (3-pyrrolinium)MCl3 (M = Cd, Mn). To our knowledge, remarkably, (3-pyrrolinium)NiCl3 should be the first case of nickel(ii)-based OIHP ferroelectric to date, and its T c of 428 K (35 K above that of BaTiO3) is the highest among all reported one-dimensional OIHP ferroelectrics. This work offers a new structural building block for enriching the family of OIHP structures and will inspire the further exploration of new nickel(ii)-based OIHP ferroelectrics.

Halogen bond and polymorphism in trans-bis(2-iodo-5-halopyridine)dihalocopper(ii) complexes: crystallographic, theoretical and magnetic studies

As the halogen atom on position 5 of the 2I5YP ligand gets heavier the probability of crystallizing the syn-conformer increases; 2I5Cl-Cl crystallizes as the anti-conformer whereas 2I5Br-Cl crystallizes as syn- and anti-conformers.

Electrochemical, Photophysical, Morphological and DFT Study of Polymorphic Sn(IV)-Porphyrins Containing Fluorinated Axial Ligand

In this study, we report the polymorphism of six coordinated Sn(IV)- tetrabromophenyl porphyrins axially armed with fluorine-substituted phenolate ligands (structural formula [Sn(TBrPP)2+(A-)2], where A is the axial ligand = 3,5-difluoro phenol, compound 1). One form stabilizes in triclinic system (namely, 1α), and the other stabilizes in monoclinic system (namely, 1β). The two 1α and 1β polymorphs display distinct photophysical and morphological properties in the solid state. X-ray diffraction study reveals that these polymorphs 1α and 1β significantly differ in their supramolecular architecture, different axial phenolate conformations, and noncovalent interactions, which are responsible for their distinct solid-state properties. The crystal packing of these polymorphs dominates by intermolecular C-H···F, C-H···π and C-Br···F interhalogen interactions. Furthermore, the solid-state emission spectra of 1α showed red-shifted emission bands with respect to 1β, in addition the redox behavior of 1α is slightly different in comparison to 1β. Complementary theoretical studies with Hirshfeld surface analysis show the definite role of Br···F interhalogen interactions in the overall stability. Mapping the electrostatic potential isosurfaces with the aid of density functional theory in compound 1 clearly shows the presence of σ-hole, a requisite feature to show halogen interactions in the crystalline state. In addition, lattice energy and single point energy calculation shows that 1α was found to be energetically more favorable and thermodynamically more stable compare to 1β.

Halogen Bond Mediated Self-Assembly of Mononuclear Lanthanide Complexes: Perception of Supramolecular Interactions, Slow Magnetic Relaxation, and Photoluminescence Properties

Five new mononuclear lanthanide complexes, [LnL₂][Et₃NH]·THF/H₂O (Ln = Nd, Tb, Dy) (H₂L^Cl = 2-bis(2-hydroxy-3,5-dichloro benzyl)aminomethyl]pyridine), Ln = Nd (1), Tb (2), and Dy (3), and (H₂L^Br = 2-bis(2-hydroxy-3,5-dibromo benzyl)aminomethyl]pyridine), Ln = Nd (4, H₂O) and Tb (5), were synthesized and structurally characterized by single-crystal X-ray diffraction analyses. Being isostructural in all the five cases, the metal center is octa-coordinated with a triangular dodecahedron (D_2d symmetry) geometry, and it is independent of the halogen substitution (Cl/Br). This close similarity is due to the composite interplay of hydrogen/halogen bond interactions that control the overall crystal packing, yet notable differences in association patterns among the individual ones arise from the subtle stereo-electronic requirement of individual molecules in the three-dimensional (3D) architecture. Hirshfeld surface and density functional theory (DFT) calculations clearly vouch for the importance of the hydrogen bond and halogen bond interactions observed in the structure. Detailed magnetic measurements using direct-current and alternating-current susceptibility measurements show slow magnetic relaxation in 3, a characteristic feature of the single-molecule magnets (SMMs), which is not shown by 1 and 2. Steady-state and time-resolved photoluminescence of Tb(III) complexes shows a strong ligand-to-metal energy transfer that can be modulated by changing the substitution on phenolic ligands. The results from these analyses indicate that it may be advantageous to consider the subtle role of hydrogen bond (HB)/halogen bond (XB) intermolecular interactions judiciously for the design of SMMs and luminescent materials based on halogen-substituted ligands.

Type I-IV Halogen⋯Halogen Interactions: A Comparative Theoretical Study in Halobenzene⋯Halobenzene Homodimers

In the current study, unexplored type IV halogen⋯halogen interaction was thoroughly elucidated, for the first time, and compared to the well-established types I−III interactions by means of the second-order Møller−Plesset (MP2) method. For this aim, the halobenzene⋯halobenzene homodimers (where halogen = Cl, Br, and I) were designed into four different types, parodying the considered interactions. From the energetic perspective, the preference of scouted homodimers was ascribed to type II interactions (i.e., highest binding energy), whereas the lowest binding energies were discerned in type III interactions. Generally, binding energies of the studied interactions were observed to decline with the decrease in the σ-hole size in the order, C6H5I⋯IC6H5 > C6H5Br⋯BrC6H5 > C6H5Cl⋯ClC6H5 homodimers and the reverse was noticed in the case of type IV interactions. Such peculiar observations were relevant to the ample contributions of negative-belt⋯negative-belt interactions within the C6H5Cl⋯ClC6H5 homodimer. Further, type IV torsional trans → cis interconversion of C6H5X⋯XC6H5 homodimers was investigated to quantify the π⋯π contributions into the total binding energies. Evidently, the energetic features illustrated the amelioration of the considered homodimers (i.e., more negative binding energy) along the prolonged scope of torsional trans → cis interconversion. In turn, these findings outlined the efficiency of the cis configuration over the trans analog. Generally, symmetry-adapted perturbation theory-based energy decomposition analysis (SAPT-EDA) demonstrated the predominance of all the scouted homodimers by the dispersion forces. The obtained results would be beneficial for the omnipresent studies relevant to the applications of halogen bonds in the fields of materials science and crystal engineering.

Exploring halogen⋯halogen interactions in supramolecular self-assemblies of BODIPY networks

In this study, the efficiency of halogen⋯halogen interactions to control supramolecular assemblies of boron dipyrromethene (BODIPY) (B1–B5) derivatives was explored.

Binding of Gold(III) from Solutions by the [Ag6{S2CN(CH2)6}6] Cluster: Synthesis, Thermal Behavior, and Self-Organization of the Supramolecular Structure of the Double Complex [Au{S2CN(CH2)6}2]2[AgCl2]Cl·2CHCl3 (Role of Secondary Au⋅⋅⋅Cl, Ag⋅⋅⋅S, and Cl⋅⋅⋅Cl Interactions)

Abstract

The capability of silver(I) cyclo-hexamethylenedithiocarbamate to concentrate gold(III) from solutions characterized by a high level of salinity (5.15 M NaCl) into the solid phase has been established. The double chloroform-solvated Au(III)–Ag(I) complex [Au{S2CN(CH2)6}2]2[AgCl2]Cl·2CHCl3 (I) was preparatively isolated as an individual form of binding of [AuCl4]– anions. The composition of the ionic structural units of compound I indicates that gold(III) binding from a solution to the solid phase is accompanied by the complete redistribution of the HmDtc ligands between the coordination spheres of Ag(I) and Au(III). Complex I characterized by IR spectroscopy, simultaneous thermal analysis, and X-ray structure analysis (CIF file CCDC no. 2051654) exhibits the supramolecular structure containing two oppositely charged pseudo-polymeric subsystems. Complex cations [Au{S2CN(CH2)6}2]+ and anions [AgCl2]– (in a ratio of 2 : 1) form a complicatedly organized cation-anionic pseudo-polymeric ribbon ({[Au(HmDtc)2]⋅⋅⋅[AgCl2]⋅⋅⋅[Au(HmDtc)2]}+)n due to secondary interactions Ag⋅⋅⋅S (3.2613 Å) and Au⋅⋅⋅Cl (3.2765 Å). The pseudo-polymeric ribbon consists of two rows of cations and a row of anions. The outer-sphere chloride ions combine the solvate chloroform molecules by two equivalent hydrogen bonds Cl⋅⋅⋅H–C yielding anion-molecular triads [Cl3CH⋅⋅⋅Cl⋅⋅⋅HCCl3]–, which are involved in the formation of the supramolecular ribbon due to the secondary Cl⋅⋅⋅Cl interactions (3.4058 Å) between the nonequivalent chlorine atoms of the nearest solvate molecules. The study of the thermal behavior of complex I makes it possible to determine the character of thermolysis and conditions for the quantitative regeneration of bound gold.

Halogen-Halogen Nonbonded Interactions

Halogen-halogen nonbonded interactions were studied for methyl halides and phenyl halides using both B3LYP and MP2 along with 6-311+G* and aug-cc-pVTZ. With the methyl halides, the linear approach was found to lead to little stabilization, whereas the "90°" approach gave 1-2 kcal/mol. This modest stabilization was due to long-range electron correlation effects. The lowest-energy arrangement had the molecules side-by-side, with the major stabilization being derived from halogen-hydrogen interactions. The results for methyl bromide were quite similar. Chlorobenzene dimer with the 90° orientation gave a small stabilization energy, but the best arrangement had the two benzene rings oriented over each other. The meta orientation of the chlorines had a lower energy than ortho or para. The dimerization energy was larger than that for two benzene rings sitting directly above each other, suggesting that whereas Cl···Cl interaction is not very important, the effect of the halogen on the electron distribution does have an effect. This suggests that much of the crystallographic results for these compounds may not be due to halogen-halogen interactions but rather the interaction between the substituted benzene rings along with crystal forces.

Triangular Interchalcogen Interactions: A Joint Crystallographic Data Analysis and Theoretical Study

Noncovalent chalcogen-chalcogen interactions are being actively investigated from both crystallographic and theoretical viewpoints in recent years. According to our search of the Cambridge Structural Database (CSD), a huge number of crystal structures containing triangular Ch₃ synthons were extracted. On the basis of the results of the CSD survey, a series of trimeric complexes of organic divalent chalcogen molecules were selected to model such interaction motifs. Similar to that in conventional chalcogen bonds, triangular interchalcogen interactions become gradually stronger along the sequence of Ch = S, Se, Te. Particularly, hydrogen bonds between the chalcogen centers and the H atoms in the substituents occur, which play a significant role in stabilizing the Ch₃ motifs in the trimers. Through multibody energy calculations, the complexes under study exhibit no or only weak cooperativity. Finally, the differences between the Ch₃ interaction motifs and the well-studied windmill X₃ bonding (X means halogen and this is halogen bond) patterns were elucidated.

Crystal engineering strategies towards halogen-bonded metal–organic multi-component solids: salts, cocrystals and salt cocrystals

This highlight presents an overview of the current advances in the preparation of halogen bonded metal–organic multi-component solids, including salts and cocrystals comprising neutral and ionic constituents.

Unconventional Type III Halogen···Halogen Interactions: A Quantum Mechanical Elucidation of σ-Hole···σ-Hole and Di-σ-Hole Interactions

Herein, two unconventional type III halogen···halogen interactions, namely, σ-hole···σ-hole and di-σ-hole interactions, were reported in a series of halogenated complexes. In type III, the A-halogen···halogen angles are typically equal to 180°, and the occurrence of σ-hole on halogen atoms is mandatory. Using diverse quantum mechanical calculations, it was demonstrated that the occurrence of such interactions with binding energies varied from -0.35 to -1.30 kcal/mol. Symmetry-adapted perturbation theory-based energy decomposition analysis (SAPT-EDA) revealed that type III interactions are dominated by dispersion forces, while electrostatic forces are unfavorable. Cambridge Structure Database (CSD) survey unveiled the experimental evidence for the manifestation of σ-hole···σ-hole interactions in crystal structures. This work might be deemed as a foundation for a vast number of forthcoming crystal engineering and materials science studies.

Hydrogen and Halogen Bond Mediated Coordination Polymers of Chloro-Substituted Pyrazin-2-Amine Copper(I) Bromide Complexes

A new class of six mono- (1; 3-Cl-, 2; 5-Cl-, 3; 6-Cl-) and di-(4; 3,6-Cl, 5; 5,6-Cl-, 6; 3,5-Cl-) chloro-substituted pyrazin-2-amine ligands (1–6) form complexes with copper (I) bromide, to give 1D and 2D coordination polymers through a combination of halogen and hydrogen bonding that were characterized by X-ray diffraction analysis. These Cu(I) complexes were prepared indirectly from the ligands and CuBr2 via an in situ redox process in moderate to high yields. Four of the pyrazine ligands, 1, 4–6 were found to favor a monodentate mode of coordination to one CuI ion. The absence of a C6-chloro substituent in ligands 1, 2 and 6 supported N1–Cu coordination over the alternative N4–Cu coordination mode evidenced for ligands 4 and 5. These monodentate systems afforded predominantly hydrogen bond (HB) networks containing a catenated (μ3-bromo)-CuI ‘staircase’ motif, with a network of ‘cooperative’ halogen bonds (XB), leading to infinite polymeric structures. Alternatively, ligands 2 and 3 preferred a μ2-N,N’ bridging mode leading to three different polymeric structures. These adopt the (μ3-bromo)-CuI ‘staircase’ motif observed in the monodentate ligands, a unique single (μ2-bromo)-CuI chain, or a discrete Cu2Br2 rhomboid (μ2-bromo)-CuI dimer. Two main HB patterns afforded by self-complimentary dimerization of the amino pyrazines described by the graph set notation R22(8) and non-cyclic intermolecular N–H∙∙∙N’ or N–H∙∙∙Br–Cu leading to infinite polymeric structures are discussed. The cooperative halogen bonding between C–Cl∙∙∙Cl–C and the C–Cl∙∙∙Br–Cu XB contacts are less than the sum of the van der Waals radii of participating atoms, with the latter ranging from 3.4178(14) to 3.582(15) Å. In all cases, the mode of coordination and pyrazine ring substituents affect the pattern of HBs and XBs in these supramolecular structures.

Computational insight into the halogen bonded self-assembly of hexa-coordinated metalloporphyrins

We demonstrate herein a computational study probing the influence of metalloporphyrins on intermolecular halogen bonding (XB) during supramolecular self-assembly. The results demonstrate that porphyrin aromatic rings can activate or deactivate halogen bonding interactions, especially those on axial ligands, and further influence the preference type of halogen···halogen bonding during the supramolecular self-assembly. Calculations show that the halogen atom present at the equatorial position has a higher sigma hole potential (V_S,max) than that at the axial position. The computational analysis and our observations from the X-ray structure analysis are in good agreement. From structural analysis it is clear that equatorial halogen atoms prefer to participate in Type-II XB interactions whereas the axial halogen atoms either participate in Type-I XB interaction or reluctant to participate in XB interactions due to the decrease of their sigma hole potential. Thus, we demonstrate, herein, for the first time a computational study probing the direct influence of the porphyrin's ring current on the sigma hole potential (V_S,max) of the halogen atoms and subsequently the effects of the supramolecular self-assembly.

Columnar Stacking of Partially Fluorinated [4]Helicenes: C-H⋅⋅⋅F Interactions Change the Stacking Orientation

The partial fluorination of polycyclic aromatic hydrocarbons often produces a layered crystal packing, where fluorinated aromatic surfaces are stacked over nonfluorinated aromatic surfaces. Herein, we report the synthesis and crystal packing of partially fluorinated [4]helicenes with steric congestion resulting from H and F atoms in the fjord region. F₆ -[4]Helicene forms head-to-tail columnar stacks consisting of an alternate arrangement of perfluorinated and nonfluorinated naphthalene moieties. With decreasing fluorine content, aromatic stacking switched from arene-fluoroarene (Ar^H -Ar^F ) hetero-stacking to Ar^H -Ar^H /Ar^F -Ar^F homo-stacking with the help of intermolecular C-H⋅⋅⋅F contacts in the fjord region. As a result, head-to-head columnar stacks appear. Therefore, the conventional Ar^H -Ar^F stacking motif is not always applicable to F_n -[4]helicenes with twisted π-surfaces.

Spin-orbit coupling is the key to unraveling intriguing features of the halogen bond involving astatine

The effect of spin-orbit coupling (SOC) on the halogen bond involving astatine has been investigated using state-of-the-art two- and four-component relativistic calculations. Adducts between Cl-X (X = Cl, Br, I and At) and ammonia have been selected to establish a trend on going down the periodic table. The SOC influence has been explored not only on the geometric and energetic features that can be used to characterize the halogen bond strength but also on the three main contributions to it that are the charge transfer, the "σ-hole" (i.e. the localized region with a net positive electrostatic potential at the halogen site) and the "polar flattening" (which is related to the effective shape of the halogen site). A surprisingly large increase of the Cl-At dipole moment, due to the inclusion of SOC, has been worked out using four-component CCSD(T) reference calculations, indicating that this bond is significantly more ionic than one may predict. Due to the SOC effect, which induces a peculiar charge accumulation on the At side in the Cl-At dimer, a weakening of the astatine-mediated halogen bond occurs arising from the (i) reduced amount of charge transfer, (ii) decrease of the polar flattening and (iii) lowering of the short-range Coulomb potential. The analysis of the electronic structure of the Cl-At moiety allows for a rationalization of the SOC effects on all the considered features of the halogen bond, including an unprecedented unsymmetrical charge back-donation from Cl-At to ammonia.

Theoretical Description of R-X⋯NH3 Halogen Bond Complexes: Effect of the R Group on the Complex Stability and Sigma-Hole Electron Depletion

In the present work, a number of R–X⋯NH₃ (X = Cl, Br, and I) halogen bonded systems were theoretical studied by means of DFT calculations performed at the ωB97XD/6-31+G(d,p) level of theory in order to get insights on the effect of the electron-donating or electron-withdrawing character of the different R substituent groups (R = halogen, methyl, partially fluorinated methyl, perfluoro-methyl, ethyl, vinyl, and acetyl) on the stability of the halogen bond. The results indicate that the relative stability of the halogen bond follows the Cl < Br < I trend considering the same R substituent whereas the more electron-withdrawing character of the R substituent the more stable the halogen bond. Refinement of the latter results, performed at the MP2/6-31+G(d,p) level showed that the DFT and the MP2 binding energies correlate remarkably well, suggesting that the Grimme’s type dispersion-corrected functional produces reasonable structural and energetic features of halogen bond systems. DFT results were also observed to agree with more refined calculations performed at the CCSD(T) level. In a further stage, a more thorough analysis of the R–Br⋯NH₃ complexes was performed by means of a novel electron localization/delocalization tool, defined in terms of an Information Theory, IT, based quantity obtained from the conditional pair density. For the latter, our in-house developed C++/CUDA program, called KLD (acronym of Kullback–Leibler divergence), was employed. KLD results mapped onto the one-electron density plotted at a 0.04 a.u. isovalue, showed that (i) as expected, the localized electron depletion of the Br sigma-hole is largely affected by the electron-withdrawing character of the R substituent group and (ii) the R–X bond is significantly polarized due to the presence of the NH₃ molecule in the complexes. The afore-mentioned constitutes a clear indication of the dominant character of electrostatics on the stabilization of halogen bonds in agreement with a number of studies reported in the main literature. Finally, the cooperative effects on the [Br—CN]_n system (n = 1–8) was evaluated at the MP2/6-31+G(d,p) level, where it was observed that an increase of about ~14.2% on the complex stability is obtained when going from n = 2 to n = 8. The latter results were corroborated by the analysis of the changes on the Fermi-hole localization pattern on the halogen bond zones, which suggests an also important contribution of the electron correlation in the stabilization of these systems.

Nature of fluorine interactions in ‘wheel and axle’ topology based hexa-coordinated Sn(iv)-porphyrins: an experimental and theoretical analysis

The experimental and theoretical investigations on Sn(iv)-tetrapyridylporphyrins demonstrate that ‘Gulliver effect’ has to be taken into consideration in explaining the genesis of F-based intermolecular interactions.

Seed-triggered solid-to-solid transformation between color polymorphs: striking differences between quasi-isomorphous crystals of dichloro-substituted salicylideneaniline regioisomers

The title compound exhibited two color polymorphs, of which the yellow form transformed to the orange form during heating, and this peculiar phase transition behavior was explained in relation to the pseudo-symmetry of its molecular structure.

Polymorphism of 1,3-X-adamantanes (X = Br, OH, CH3) and the crystal plastic phase formation ability

The polymorphism of 1,3-dimethyladamantane (13DMA), 1,3-adamantanediol (13DOHA) and 1,3-dibromoadamantane (13DBrA) has been studied by X-ray powder diffraction, density measurements and differential scanning calorimetry at normal and high-pressure.

The effect of vicinal di-halo substituents on the organogelling properties of aromatic supramolecular gelators and their application as soft templates

Vicinal di-halo substituents have a determinant effect on the supramolecular self-assembly and properties of aromatic physical gelators with application as soft templates.

Supramolecular assembly of lanthanide-2,3,5,6-tetrafluoroterephthalic acid coordination polymers via fluorine⋯fluorine interactions: a platform for luminescent detection of Fe3+ and nitroaromatic compounds

F⋯F interactions stabilize {[Ln(TFTA)_1.5(H₂O)₂]·H₂O}_n 2D coordination polymers which selectively detect Fe³⁺ and p-nitrophenols.

Halogen Bonds in 2,5-Dihalopyridine-Copper(I) Halide Coordination Polymers

Two series of 2,5-dihalopyridine-Cu(I)A (A = I, Br) complexes based on 2-X-5-iodopyridine and 2-X-5-bromopyridine (X = F, Cl, Br and I) are characterized by using single-crystal X-ray diffraction analysis to examine the nature of C2-X2···A-Cu and C5-X5···A-Cu halogen bonds. The reaction of the 2,5-dihalopyridines and Cu(I) salts allows the synthesis of eight 1-D coordination polymers and a discrete structure. The resulting Cu(I)-complexes are linked by C-X···A-Cu halogen bonds forming 3-D supramolecular networks. The C-X···A-Cu halogen bonds formed between halopyridine ligands and copper(I)-bound halide ions are stronger than C-X···X'-C interactions between two 2,5-dihalopyridine ligands. The C5-I5···I-Cu and C5-Br5···Br-Cu halogens bonds are shorter for C2-fluorine than C2-chlorine due to the greater electron-withdrawing power of fluorine. In 2,5-diiodopyridine-Cu(I)Br complex, the shorter C2-I2···Br-Cu [3.473(5) Å] distances are due to the combined polarization of C2-iodine by C2-I2···Cu interactions and para-electronic effects offered by the C5-iodine, whilst the long halogen bond contacts for C5-I5···Br-Cu [3.537(5) Å] are indicative that C2-iodine has a less para-electronic influence on the C5-iodine. In 2-fluoro-5-X-pyridine-Cu(I) complexes, the C2-fluorine is halogen bond passive, while the other C2-halogens in 2,5-dihalopyridine-Cu(I), including C2-chlorine, participate in halogen bonding interactions.

Close contacts and noncovalent interactions in crystals

Close contacts, defined as interatomic separations less than the sum of the respective van der Waals radii, are commonly invoked to identify attractive nonbonded interactions in crystal lattices. While this is often effective, it can also be misleading because (a) there are significant uncertainties associated with van der Waals radii, and (b) it may not be valid to attribute the interactions solely to specific pairs of atoms. The interactions within crystal lattices are Coulombic, and the strongest positive and/or negative regions do not always correspond to the positions of atoms; they are sometimes located between atoms. Examples of both types are given and discussed, focusing in particular upon σ-hole interactions.

Cooperative Effects in Weak Interactions: Enhancement of Tetrel Bonds by Intramolecular Hydrogen Bonds

A series of silyl and germanium complexes containing halogen atoms (fluorine and chlorine atoms) and exhibiting tetrel bonds with Lewis bases were analyzed by means of Møller-Plesset computational theory. Binding energies of germanium derivatives were more negative than silicon ones. Amongst the different Lewis bases utilized, ammonia produced the strongest tetrel bonded complexes in both Ge and Si cases, and substitution of the F atom by Cl led to stronger complexes with an ethylene backbone. However, with phenyl backbones, the fluorosilyl complexes were shown to be less stable than the chlorosilyl ones, but the opposite occurred for halogermanium complexes. In all the cases studied, the presence of a hydroxyl group enhanced the tetrel bond. That effect becomes more remarkable when an intramolecular hydrogen bond between the halogen and the hydrogen atom of the hydroxyl group takes places.

Solving the enigma of weak fluorine contacts in the solid state: a periodic DFT study of fluorinated organic crystals

The nature and strength of weak interactions with organic fluorine in the solid state are revealed by periodic density functional theory (periodic DFT) calculations coupled with experimental data on the structure and sublimation thermodynamics of crystalline organofluorine compounds. To minimize other intermolecular interactions, several sets of crystals of perfluorinated and partially fluorinated organic molecules are considered. This allows us to establish the theoretical levels providing an adequate description of the metric and electron-density parameters of the C–F⋯F–C interactions and the sublimation enthalpy of crystalline perfluorinated compounds. A detailed comparison of the C–F⋯F–C and C–H⋯F–C interactions is performed using the relaxed molecular geometry in the studied crystals. The change in the crystalline packing of aromatic compounds during their partial fluorination points to the structure-directing role of C–H⋯F–C interactions due to the dominant electrostatic contribution to these contacts. C–H⋯F–C and C–H⋯O interactions are found to be identical in nature and comparable in energy. The factors that determine the contribution of these interactions to the crystal packing are revealed. The reliability of the results is confirmed by considering the superposition of the electrostatic potential and electron density gradient fields in the area of the investigated intermolecular interactions.

The nature and strength of weak C–H⋯F–C and C–F⋯F–C interactions and their role in organofluorine molecular crystals were studied using periodic DFT coupled with CSD data mining and experimental sublimation enthalpies.

Metal–ligand ring aromaticity in a 2D coordination polymer used as a photosensitive electronic device

14-Membered metal–ligand ring present in a photosensitive 2D coordination polymer, [Zn₂(fum)₂(4-phpy)₄(H₂O)₂] (H₂fum = fumaric acid and 4-phpy = 4-phenyl pyridine) shows aromatic character as evident by the nucleus-independent chemical shifts (NICS) calculation.

Wheel-and-axle topology-driven halogen bonds: formation of ladder, 1D and 2D networks in hexa-coordinated Sn(iv) porphyrins

Wheel-and-axle topology-driven halogen bonded supramolecular networks in six-coordinated Sn(iv)-porphyrins.

The nature of interactions of benzene with CF3I and CF3CH2I

Weak though structure determining interactions exist between benzene and F₃CI or F₃CCH₂I; their natures are quite different and lead to different types of networks.

Bis-heterocycles. Part I: tetrahydro-5,5′- bi(1,2,4-triazin-6-ones)

Selected sets of tetrahydro-5,5′-bi(1,2,4-triazines) (1–3) appended with acetyl, benzoyl, and ester moieties at C-3 position and N-1 (p-substituted)phenyl ring have been prepared and characterized by spectral (IR, NMR, MS) data and X-ray diffraction for compound 3a. Their synthesis was achieved in high yield via the reaction of diethyl aminomalonate with various N-(aryl)hydrazonoyl chlorides in the presence of triethylamine.