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

Crystal structure of 4,4′-(ethane-1,2-diyl)bis(2,6-dibromoaniline)

The 4,4′-(ethane-1,2-di­yl)bis­(2,6-di­bromo­aniline) mol­ecule lies across an inversion center and hence the benzene rings are strictly planar. Mol­ecules are linked by N—H⋯N and weak N—H⋯Br hydrogen bonds, forming a two-dimensional network parallel to (101). Type II Br⋯Br inter­actions complete a three-dimensional supra­molecular network.

In the title compound, C₁₄H₁₂Br₄N₂, the mol­ecule lies across an inversion center and hence the benzene rings are strictly coplanar. In the crystal, mol­ecules are linked by N—H⋯N and weak N—H⋯Br hydrogen bonds, forming a two-dimensional network parallel to (101). In addition, type II Br⋯Br inter­actions [3.625 (4) Å] complete a three-dimensional supra­molecular network.

Synthesis, crystal structures and magnetic behaviour of four coordination compounds constructed with a phosphinic amide-TEMPO radical and [M(hfac)2] (M = Cu(II), Co(II) and Mn(II))

In the present work we describe the synthesis, crystal structures and magnetic properties of four coordination compounds obtained by assembling a new phosphinic amide containing the TEMPO moiety, 1-piperidinyloxy-4-[(diphenylphosphinyl)amino]-2,2,6,6-tetramethyl radical (dppnTEMPO), with [M(hfac)2] building blocks (M = Cu(II), Co(II), Mn(II)). The crystal structures of the coordination compounds revealed the usefulness of the functionalized radical to provide discrete or extended architectures. In the copper compound () the ligand is coordinated through the oxygen atom of the NP[double bond, length as m-dash]O linkage to the metal, which exists in a square pyramidal or octahedral geometry. For the cobalt and manganese complexes (), both the phosphinic amide and the nitroxide oxygen atoms are involved in the coordination to the metal leading to one dimensional systems. In the cobalt complex () an interesting spin topology in the zig-zag chain was obtained due to the oxygen atom of the phosphinic amide group being μ2 coordinated to two cobalt(ii) ions. The magnetic behaviour of the coordination compounds shows overall antiferromagnetic interactions involving the metal ion and the organic radical. DFT calculations were performed in order to assign the main path for the magnetic interactions.

Halogen bonds in crystal engineering: like hydrogen bonds yet different

The halogen bond is an attractive interaction in which an electrophilic halogen atom approaches a negatively polarized species. Short halogen atom contacts in crystals have been known for around 50 years. Such contacts are found in two varieties: type I, which is symmetrical, and type II, which is bent. Both are influenced by geometric and chemical considerations. Our research group has been using halogen atom interactions as design elements in crystal engineering, for nearly 30 years. These interactions include halogen···halogen interactions (X···X) and halogen···heteroatom interactions (X···B). Many X···X and almost all X···B contacts can be classified as halogen bonds. In this Account, we illustrate examples of crystal engineering where one can build up from previous knowledge with a focus that is provided by the modern definition of the halogen bond. We also comment on the similarities and differences between halogen bonds and hydrogen bonds. These interactions are similar because the protagonist atoms-halogen and hydrogen-are both electrophilic in nature. The interactions are distinctive because the size of a halogen atom is of consequence when compared with the atomic sizes of, for example, C, N, and O, unlike that of a hydrogen atom. Conclusions may be drawn pertaining to the nature of X···X interactions from the Cambridge Structural Database (CSD). There is a clear geometric and chemical distinction between type I and type II, with only type II being halogen bonds. Cl/Br isostructurality is explained based on a geometric model. In parallel, experimental studies on 3,4-dichlorophenol and its congeners shed light on the nature of halogen···halogen interactions and reveal the chemical difference between Cl and Br. Variable temperature studies also show differences between type I and type II contacts. In terms of crystal design, halogen bonds offer a unique opportunity in the strength, atom size and interaction gradation; this may be used in the design of ternary cocrystals. Structural modularity in which an entire crystal structure is defined as a combination of modules is rationalized on the basis of the intermediate strength of a halogen bond. The specific directionality of the halogen bond makes it a good tool to achieve orthogonality in molecular crystals. Mechanical properties can be tuned systematically by varying these orthogonally oriented halogen···halogen interactions. In a further development, halogen bonds are shown to play a systematic role in organization of LSAMs (long range synthon aufbau module), which are bigger structural units containing multiple synthons. With a formal definition in place, this may be the right time to look at differences between halogen bonds and hydrogen bonds and exploit them in more subtle ways in crystal engineering.

Tuning band gaps of BN nanosheets and nanoribbons via interfacial dihalogen bonding and external electric field

Density functional theory computations with dispersion corrections (DFT-D) were performed to investigate the dihalogen interactions and their effect on the electronic band structures of halogenated (fluorinated and chlorinated) BN bilayers and aligned halogen-passivated zigzag BN nanoribbons (BNNRs). Our results reveal the presence of considerable homo-halogen (FF and ClCl) interactions in bilayer fluoro (chloro)-BN sheets and the aligned F (Cl)-ZBNNRs, as well as substantial hetero-halogen (FCl) interactions in hybrid fluoro-BN/chloro-BN bilayer and F-Cl-ZBNNRs. The existence of interfacial dihalogen interactions leads to significant band-gap modifications for the studied BN nanosystems. Compared with the individual fluoro (chloro)-BN monolayers or pristine BNNRs, the gap reduction in bilayer fluoro-BN (B-FF-N array), hybrid fluoro-BN/chloro-BN bilayer (N-FCl-N array), aligned Cl-ZBNNRs (B-ClCl-N alignment), and hybrid F-Cl-ZBNNRs (B-FCl-N alignment) is mainly due to interfacial polarizations, while the gap narrowing in bilayer chloro-BN (N-ClCl-N array) is ascribed to the interfacial nearly-free-electron states. Moreover, the binding strengths and electronic properties of the interactive BN nanosheets and nanoribbons can be controlled by applying an external electric field, and extensive modulation from large-gap to medium-gap semiconductors, or even metals can be realized by adjusting the direction and strength of the applied electric field. This interesting strategy for band gap control based on weak interactions offers unique opportunities for developing BN nanoscale electronic devices.

Anion-π and halide-halide nonbonding interactions in a new ionic liquid based on imidazolium cation with three-dimensional magnetic ordering in the solid state

We present the first magnetic phase of an ionic liquid with anion-π interactions, which displays a three-dimensional (3D) magnetic ordering below the Néel temperature, TN = 7.7 K. In this material, called Dimim[FeBr4], an exhaustive and systematic study involving structural and physical characterization (synchrotron X-ray, neutron powder diffraction, direct current and alternating current magnetic susceptibility, magnetization, heat capacity, Raman and Mössbauer measurements) as well as first-principles analysis (density functional theory (DFT) simulation) was performed. The crystal structure, solved by Patterson-function direct methods, reveals a monoclinic phase (P21 symmetry) at room temperature with a = 6.745(3) Å, b = 14.364(3) Å, c = 6.759(3) Å, and β = 90.80(2)°. Its framework, projected along the b direction, is characterized by layers of cations [Dimim](+) and anions [FeBr4](-) that change the orientation from layer to layer, with Fe···Fe distances larger than 6.7 Å. Magnetization measurements show the presence of 3D antiferromagnetic ordering below TN with the existence of a noticeable magneto-crystalline anisotropy. From low-temperature neutron diffraction data, it can be observed that the existence of antiferromagnetic order is originated by the antiparallel ordering of ferromagnetic layers of [FeBr4](-) metal complex along the b direction. The magnetic unit cell is the same as the chemical one, and the magnetic moments are aligned along the c direction. The DFT calculations reflect the fact that the spin density of the iron ions spreads over the bromine atoms. In addition, the projected density of states (PDOS) of the imidazolium with the bromines of a [FeBr4](-) metal complex confirms the existence of the anion-π interaction. Magneto-structural correlations give no evidence for direct iron-iron interactions, corroborating that the 3D magnetic ordering takes place via superexchange coupling, the Fe-Br···Br-Fe interplane interaction being defined as the main exchange pathway.

4-Bromo-N-(2-nitrophenyl)benzamide

The title nitro­phenyl benzamide, C₁₃H₉BrN₂O₃, with two mol­ecules in the asymmetric unit, has dihedral angles of 16.78 (15) and 18.87 (14)° between the benzene rings. An intra­molecular N—H⋯O hydrogen bond is observed in each mol­ecule. In the crystal, the molecules are linked by weak C—H⋯O inter­actions; halogen–halogen inter­actions are also observed [Br⋯Br = 3.4976 (7) Å]. These inter­actions form R²₂(10), R²₂(15) and R⁶₆(32) edge-fused rings along [010].

Competition between hydrogen and halogen bonding in the structures of 5,10-dihydroxy-5,10-dihydroboranthrenes

X-ray crystallographic and computational studies are reported for a series of boranthrenes, substituted with halogen atoms. The role of competitive hydrogen (O—H...O, O—H...F, C—H...O) and halogen (Cl...Cl, O...Br, F...F) bonding interactions on the molecular arrangement in the crystal structures is discussed. The structural analysis and calculations reveal that the O—H...O hydrogen bond in the unsubstituted derivative 5,10-dihydroxy-5,10-dihydroboranthrene, C12H10B2O2, is of moderate strength (ca−20 kJ mol−1), but weaker than that in the related thiophene derivative 4,8-dihydro-4,8-dihydroxy-p-diborino[2,3-b:5,6-b]dithiophene, C8H6B2O2S2(ca−40 kJ mol−1). This is due to shielding of the OH group by the H atoms in the β-position of the boranthrene unit. Structural diversity derived from the flexibility of the O—H...O hydrogen bond facilitates the occurrence of other competitive interactions. For instance, in the 1,6-difluoro derivative, C12H8B2F2O2, the crystal packing results from O—H...F and F...F interactions. In turn, the 1,6-dibromo derivative, C12H8B2Br2O2, is dominated by Br...O halogen-bond interactions. In the most interesting case, the 1,6-dichloro derivative, C12H8B2Cl2O2, molecular disorder leads to the formation of two different supramolecular arrangements co-existing in the crystal lattice, one based on the Cl...Cl and C—H...O bonds, and the other stabilized by O—H...O hydrogen bonds. Calculations performed with density-functional theory (DFT;CRYSTAL09) andPIXELmethodologies show that both lattices are characterized by similar energy values (ca−100 kJ mol−1). A mixed arrangement with random or short-range-ordered molecular orientations can also be expected.

Halogen bonding of electrophilic bromocarbons with pseudohalide anions

Spectral, thermodynamic and structural features of the complexes of bromocarbons with polydentate azide, cyanate or thiocyanate anions are presented. They suggest a significant role of the molecular-orbital interactions in formation of these halogen-bonded associates.

Influence of the coligand in the magnetic properties of a series of copper(ii)–phenylmalonate complexes

The magnetic properties of a series of copper(ii)–phenylmalonate complexes are tuned by the pyridine-type coligand.

The nature of the C–Cl⋯Cl–C intermolecular interactions found in molecular crystals: a general theoretical-database study covering the 2.75–4.0 Å range

An exhaustive study of the nature of the C–Cl⋯Cl–C interactions found in crystals has been carried out at the MP2/aug-cc-pVDZ level using model dimers and a set of 45 dimers with Cl⋯Cl distances in the 2.75–4 Å range.

Intramolecular pnicogen interactions in phosphorus and arsenic analogues of proton sponges

A computational study of the intramolecular pnicogen bond in 1,8-bis-substituted naphthalene derivatives (ZXH and ZX₂ with Z = P, As and X = H, F, Cl, and Br), structurally related to proton sponges, has been carried out.

Halogen–halogen interactions and halogen bonding in thiacalixarene systems

5,11,17,23-tetraiodo-25,26,27,28-tetrapropoxythiacalix[4]arene exhibited I⋯I interactions which are ca. 2% shorter than the sum of their respective van der Waals atomic radii. 5,11,17,23-tetraiodo-25,26,27,28-tetrabutoxythiacalix[4]arene were identified to have S⋯I interactions that are ca. 4.5% shorter than the sum of their respective van der Waals atomic radii. We have elucidated both interactions by computational approaches.

Conformational analysis of some N,N-diethyl-2-[(4'-substituted) phenylthio] acetamides

The conformational analysis of some N,N-diethyl-2[(4'-substituted)phenylthio]acetamides bearing the substituents OMe 1, Me 2, H 3, Cl 4, Br 5 and NO26, was performed by νCO IR analysis, along with B3LYP/6-311++G(d,p) and Polarisable Continuum Model (PCM) calculations, as well as NBO analysis for 1, 3, and 6 and X-ray diffraction for 4. The results of the calculations indicated the existence of two stable conformation pairs, i.e. gauche (anti; syn) (most stable) and cis (anti; syn) in the gas phase. The gauche conformers were less polar with respect to the cis ones for 1 and 3, but more polar for 6. The most intense IR carbonyl doublet component observed at the lower frequency can be ascribed to the gauche conformers g(anti; syn) for 3-6 in n-C6H14, which is in agreement with the gauche and cis relative stabilities and frequencies resulting from the PCM calculations. Similarly, the single IR band for 1 and 2 in n-hexane may be attributed to the gauche conformers. The PCM calculations compared well with the IR data for the compounds in solution, showing that there is a progressive increase of the cis/gauche population ratio as the solvent polarity increases. The NBO analysis indicated that the gauche(anti; syn) conformation in the gas phase was stabilized by the relevant LPS4→πC2O1(∗),πC2O1→σC3S4(∗),σC3S4→πC2O1(∗),πC2O1(∗)→σC3S4(∗), and LPO1→σ(∗)C11H28 orbital interactions, which were absent in the cis(anti; syn) conformer. On the contrary, the cis conformer for derivatives 1, 3, and 6 were stabilized by the σC3-S4(∗)→σ(∗)C2N5 orbital interaction (through bond coupling), along with the additional LPO1→σ(∗)S4C10 interaction for 6. Moreover, the electrostatic repulsion between the C(δ+)S(δ-) and C(δ+)O(δ-) dipoles (Repulsive Field Effect) contributed to both the larger destabilization and increase of the νCO frequency of the cis conformer with respect to the gauche conformer. X-ray single crystal analysis indicates that compound 4 assumes the c2(anti) conformation in the solid state, which is the conformation obtained by compound 6 in the gas phase. To obtain the largest energy gain, the molecules were arranged in the crystal in a six-molecules synthon mediated by CH⋯O and Cl⋯Cl interactions, where the chlorine atoms were related by a crystallographic inversion center.

Cl···Cl interactions in molecular crystals: insights from the theoretical charge density analysis

The structure, IR harmonic frequencies and intensities of normal vibrations of 20 molecular crystals with the X-Cl···Cl-X contacts of different types, where X = C, Cl, and F and the Cl···Cl distance varying from ~3.0 to ~4.0 Å, are computed using the solid-state DFT method. The obtained crystalline wave functions have been further used to define and describe quantitatively the Cl···Cl interactions via the electron-density features at the Cl···Cl bond critical points. We found that the electron-density at the bond critical point is almost independent of the particular type of the contact or hybridization of the ipso carbon atom. The energy of Cl···Cl interactions, E(int), is evaluated from the linking E(int) and local electronic kinetic energy density at the Cl···Cl bond critical points. E(int) varies from 2 to 12 kJ/mol. The applicability of the geometrical criterion for the detection of the Cl···Cl interactions in crystals with two or more intermolecular Cl···Cl contacts for the unique chlorine atom is not straightforward. The detection of these interactions in such crystals may be done by the quantum-topological analysis of the periodic electron density.

Intermolecular iodine–iodine interactions in bis(pyridine-3-carboxylato)[tetrakis(4-iodophenyl)porphyrinato]tin(IV) and bis(pyrimidine-5-carboxylato)[tetrakis(4-iodophenyl)porphyrinato]tin(IV)

Crystals of bis(pyridine-3-carboxylato)[tetrakis(4-iodophenyl)porphyrinato]tin(IV) dimethylformamide sesquisolvate, [Sn(C44H24I4N4)(C6H4NO2)2]·1.5C3H7NO, (I), and bis(pyrimidine-5-carboxylato)[tetrakis(4-iodophenyl)porphyrinato]tin(IV) dimethylformamide sesquisolvate, [Sn(C44H24I4N4)(C5H3N2O2)2]·1.5C3H7NO, (II), exhibit interporphyrin iodine–iodine halogen bonds, which direct the supramolecular assembly of the porphyrin entities into halogen-bonded layers. Each molecule interacts with its four nearest neighbours within the layerviaeight I...I interactions at approximately 3.8 and 4.0 Å. The two structures are isomorphous and isometric, with the metalloporphyrin complexes located on centres of crystallographic inversion.

1,3,5-Tris(bromo-meth-yl)-2,4,6-trimeth-oxy-benzene

There are three independent mol­ecules in the asymmetric unit of the title compound, C₁₂H₁₅Br₃O₃, two of which have approximate trigonal symmetry, the third being conformationally different as it adopts near mirror symmetry. The crystal structure features C—H⋯Br inter­actions, a weak C—H⋯O hydrogen bond, π–π inter­actions [minimum ring centroid separation = 3.4927 (18) Å] and a short Br⋯Br contact [3.5894 (5) Å], resulting in a three-dimensional supramolecular network.

4-Formyl-2-nitro-phenyl 4-bromo-benzoate

In the title compound, C14H8BrNO5, the benzene rings form a dihedral angle of 62.90 (7)°. The central ester group is twisted away from the nitro-substituted and bromo-substituted rings by 71.67 (7) and 8.78 (15)°, respectively. The nitro group forms a dihedral angle of 7.77 (16)° with the benzene ring to which it is attached. In the crystal, mol-ecules are linked by weak C-H⋯O inter-actions, forming C(12) chains which run along [001]. Halogen-halogen inter-actions [Br⋯Br = 3.523 (3) Å] within the chains stabilized by C-H⋯O inter-actions are observed.

1,3-Diiodo-azulene-2-carbonitrile

In the title compound, C11H5I2N, the two iodine-atom substitutents with their large atomic sizes lead to short intra-molecular I⋯H distances (3.01 Å). In the crystal, the tris-ubstituted azulene system forms π-stacks [centroid-centroid distance = 3.6343 (11) Å] along the a-axis direction, showing the characteristic azulene inter-action mode between the electron-rich five-membered ring and the electron-deficient seven-membered ring. I⋯I [3.9129 (2) Å] non-covalent contacts are observed along with weak C-H⋯N and C-H⋯π. bonds.

Intramolecular pnicogen interactions in PHF-(CH2)(n)-PHF (n=2-6) systems

A computational study of the intramolecular pnicogen bond in PHF-(CH2)n-PHF (n=2-6) systems was carried out. For each compound, two different conformations, (R,R) and (R,S), were considered on the basis of the chirality of the phosphine groups. The characteristics of the closed conformers, in which the pnicogen interaction occurs, were compared with those of the extended conformer. In several cases, the closed conformations are more stable than the extended conformations. The calculated interaction energies of the pnicogen contact, by means of isodesmic reactions, provide values between -3.4 and -26.0 kJ mol(-1). Atoms in molecules and electron localization function analysis of the electron density showed that the systems in the closed conformations with short P···P distances have a partial covalent character in this interaction. The calculated absolute chemical shieldings of the P atoms showed an exponential relationship with the P···P distance. In addition, a search in the Cambridge crystallographic database was carried out to detect those compounds with a potential intramolecular pnicogen bond in the solid phase.