Halogen Bonding in Brominated BODIPY Crystals: a Crystallographic and Computational Study

The study of halogen bonds (XBs) has been a subject of great interest in recent years due to its clear application in catalysis, liquid crystals, and crystal engineering. In this study, we analyzed the intermolecular interactions, in particular halogen bonds in BODIPYs with an increasing number of bromine atoms. The computational study included analyses through three different methods: the first approach of close contacts provided by mercury, then the expanded approach of the electron density partition of the molecules in the crystals provided by the analysis of Hirshfeld surfaces, and finally, the approach of the Quantum Theory of Atoms in Molecules (QT-AIM) to characterize the non-covalent interactions through finding electron density critical points between atoms and between neighboring molecules. The use of different computational methods allowed to gain insight into the interactions directing the crystal packing as the number of bromine atoms increased in the BODIPY moiety. Monocoordinated and bifurcated halogen bonds involving halide/halide were found. The penta-brominated BODIPY showed four-center cyclic nodes where each node is linked via XBs. This kind of motif can be useful in supramolecular chemistry and self-assembly.

Structure and Bonding of Halonium Compounds

The geometrical parameters and the bonding in [D···X···D]⁺ halonium compounds, where D is a Lewis base with N as the donor atom and X is Cl, Br, or I, have been investigated through a combined structural and computational study. Cambridge Structural Database (CSD) searches have revealed linear and symmetrical [D···X···D]⁺ frameworks with neutral donors. By means of density functional theory (DFT), molecular electrostatic potential (MEP), and energy decomposition analyses (EDA) calculations, we have studied the effect of various halogen atoms (X) on the [D···X···D]⁺ framework, the effect of different nitrogen-donor groups (D) attached to an iodonium cation (X = I), and the influence of the electron density alteration on the [D···I···D]⁺ halonium bond by variation of the R substituents at the N-donor upon the symmetry, strength, and nature of the interaction. The physical origin of the interaction arises from a subtle interplay between electrostatic and orbital contributions (σ-hole bond). Interaction energies as high as 45 kcal/mol suggest that halonium bonds can be exploited for the development of novel halonium transfer agents, in asymmetric halofunctionalization or as building blocks in supramolecular chemistry.

The Halogen Bonding Proclivity of the sp3 Sulfur Atom as a Halogen Bond Acceptor in Cocrystals of Tetrahydro-4H-thiopyran-4-one and Its Derivatives

In this work, we present a systematic study of the capability of the sp³ hybridized sulfur atom for halogen bonding both in a small building block, tetrahydro-4H-thiopyran-4-one, and two larger ones derived from it, Schiff bases with a morpholine fragment on the other end of the molecule. These three building blocks were cocrystallized with six perhalogenated aromates: 1,4-diiodotetrafluorobenzene, 1,3,5-triiodotrifluorobenzene, 1,3-diiodotetrafluorobenzene, 1,2-diiodotetrafluorobenzene, iodopentafluorobenzene, and 1,4-dibromotetrafluorobenzene. Out of the 18 combinations, only 7 (39%) yielded cocrystals, although with a high occurrence of the targeted I···S halogen bonding motif in all cocrystals (71%), and in imine cocrystals the I···O_morpholine motif (100%) as well as, surprisingly, the I···N_imine motif (100%). The I···S halogen bonds presented in this work feature lower relative shortening values than those for other types of sulfur atoms; however, the sp³ sulfur atom could potentially be more specific an acceptor for halogen bonding.

Halogen Bond Motifs in Cocrystals of N,N,O and N,O,O Acceptors Derived from Diketones and Containing a Morpholine or Piperazine Moiety

In this study, we investigate the halogen bond acceptor potential of oxygen and nitrogen atoms of morpholine and piperazine fragments when they are peripherally located on N,O,O or N,N,O acceptor molecules. We synthesized four acceptor molecules derived from either acetylacetone or benzoylacetone and cocrystallized them with 1,4-diiodotetrafluorobenzene and 1,3,5-triiodotrifluorobenzene. This resulted in eight cocrystals featuring different topicities and geometric dispositions of donor atoms. In all cocrystals, halogen bonds are formed with either the morpholinyl oxygen atom or the terminal piperazine nitrogen atom. The I···O_morpholine halogen bonds feature lower relative shortening values than I···N_terminal, I···O_carbonyl, and I···N_proximal halogen bonds. The N and O halogen bond acceptor sites were evaluated through calculations of molecular electrostatic potential values.

Weaving a 2D net of hydrogen and halogen bonds: cocrystal of a pyrazolium bromide with tetrafluorodiiodobenzene

Hydrohalides of Lewis bases may act as halogen bond (XB) acceptors and combine two directional interactions, namely, hydrogen bonds (HB) and XBs in the same solid. 3-(1,3,5-Trimethyl-1H-pyrazol-4-yl)acetylacetone (C11H16N2O2, HacacMePz) was protonated with HX (X = Cl or Br) to afford the hydrohalides, C11H17N2O2+·X- or H2acacMePz+·X- (1, X = Cl; 2, X = Br). Hydrohalides 1 and 2 are isomorphous and adopt a classical dipole packing. Consistent with the observation for most β-diketones, the enol form with an intramolecular HB is observed. Additional noteworthy interactions are HBs of the protonated pyrazolium towards the X- anion at donor-acceptor distances of 2.9671 (17) Å for 1 and 3.159 (4) Å for 2. Cocrystallization of hydrobromide 2 with the XB donor tetrafluorodiiodobenzene (TFDIB) leads to the adduct C11H17N2O2+·Br-·0.5C6F4I2·H2O or (H2acacMePz+·Br-)2·(H2O)2·TFDIB (3), in which the XB donor TFDIB is situated on a crystallographic centre of inversion. Classical HBs link organic cations, water molecules and Br- anions into chains along [010]. Almost orthogonal to this interaction, XBs with Br...I = 3.2956 (4) Å connect neighbouring chains along [102] into two-dimensional sheets in the (10-2) plane. Assisted by their negative charge, halide anions represent particularly good nucleophiles towards XB donors.

Structural Characterization of Multicomponent Crystals Formed from Diclofenac and Acridines

Multicomponent crystals containing diclofenac and acridine (1) and diclofenac and 6,9-diamino-2-ethoxyacridine (2) were synthesized and structurally characterized. The single-crystal XRD measurements showed that compound 1 crystallizes in the triclinic P-1 space group as a salt cocrystal with one acridinium cation, one diclofenac anion, and one diclofenac molecule in the asymmetric unit, whereas compound 2 crystallizes in the triclinic P-1 space group as an ethanol solvate monohydrate salt with one 6,9-diamino-2-ethoxyacridinium cation, one diclofenac anion, one ethanol molecule, and one water molecule in the asymmetric unit. In the crystals of the title compounds, diclofenac and acridines ions and solvent molecules interact via N–H⋯O, O–H⋯O, and C–H⋯O hydrogen bonds, as well as C–H⋯π and π–π interactions, and form heterotetramer bis[⋯cation⋯anion⋯] (1) or heterohexamer bis[⋯cation⋯ethanol⋯anion⋯] (2). Moreover, in the crystal of compound 1, acridine cations and diclofenac anions interact via N–H⋯O hydrogen bond, C–H⋯π and π–π interactions to produce blocks, while diclofenac molecules interact via C–Cl⋯π interactions to form columns. In the crystal of compound 2, the ethacridine cations interact via C–H⋯π and π–π interactions building blocks, while diclofenac anions interact via π–π interactions to form columns.

Halogen Bonding in N-Alkyl-3-halogenopyridinium Salts

We performed a structural study of N-alkylated halogenopyridinium cations to examine whether choice of the N-substituent has any considerable effect on the halogen bonding capability of the cations. For that purpose, we prepared a series of N-ethyl-3-halopyridinium iodides and compared them with their N-methyl-3-halopyridinium analogues. Structural analysis revealed that N-ethylated halogenopyridinium cations form slightly shorter C−X⋯I− halogen bonds with iodide anion. We have also attempted synthesis of ditopic symmetric bis-(3-iodopyridinium) dications. Although successful in only one case, the syntheses have afforded two novel ditopic asymmetric monocations with an iodine atom bonded to the pyridine ring and another on the aliphatic N-substituent. Here, the C−I⋯I− halogen bond lengths involving pyridine iodine atom were notably shorter than those involving an aliphatic iodine atom as a halogen bond donor. This trend in halogen bond lengths is in line with the charge distribution on the Hirshfeld surfaces of the cations—the positive charge is predominantly located in the pyridine ring making the pyridine iodine atom σ-hole more positive than the one on the alkyl chan.

An Insight into Non-Covalent Interactions on the Bicyclo[1.1.1]pentane Scaffold

Bicyclo[1.1.1]pentane (BCP) is studied extensively as a bioisosteric component of drugs. Not found in nature, this molecular unit approximates the distance of a para-disubstituted benzene which is replaced in medicines as a method of improving treatments. Predicting interactions of these drugs with specific active sites requires knowledge of the non-covalent interactions engaged by this subunit. Structure determinations and computational analysis (Hirshfeld analysis, 2D fingerprint plots, DFT) of seven BCP derivatives chosen to probe specific and directional interactions. X-ray analysis revealed the presence of various non-covalent interactions including I ⋅⋅⋅ I, I ⋅⋅⋅ N, N-H ⋅⋅⋅ O, C-H ⋅⋅⋅ O, and H-C ⋅⋅⋅ H-C contacts. The preference of halogen bonding (I ⋅⋅⋅ I or I ⋅⋅⋅ N) in BCP 1-4 strictly depends upon the electronic nature and angle between bridgehead substituents. The transannular distance in co-crystals 2 and 4 was longer as compared to monomers 1 and 3. Stronger N-H ⋅⋅⋅ O and weaker C-H ⋅⋅⋅ O contacts were observed for BCP 5 while the O ⋅⋅⋅ H interaction was a prominent contact for BCP 6. The presence of 3D BCP units prevented the π ⋅⋅⋅ π stacking between phenyl rings in 3, 4, and 7. The BCP skeleton was often rotationally averaged, indicating fewer interactions compared to bridgehead functional groups. Using DFT analysis, geometries were optimized and molecular electrostatic potentials were calculated on the BCP surfaces. These interaction profiles may be useful for designing BCP analogs of drugs.

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.

Morpholino-Substituted BODIPY Species: Synthesis, Structure and Electrochemical Studies

Functionalization of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) chromophores at the 2,6-positions with iodo substituents and morpholino-substituted α-methyl groups affords molecules with strong absorbance in the visible spectrum. The effect of such substitution on the solid-state arrangements, absorption, fluorescence and electronic properties of these dye molecules is reported. The spectroscopic and spectroelectrochemical measurements display intense absorptions in the UV-visible spectrum with bathochromic shifts, in comparison to unfunctionalized BODIPY, and a positive shift in redox potentials due to functionalisation of the BODIPY core. Halogen bonds are observed in the solid-state structures of both halogenated BODIPY species, which in one case leads to the formation of an unusual halogen bonded framework.

Two single-enantiomer amidophosphoesters: a database study on the chirality of (O)2P(O)(N)-based structures

The crystal structures of two single-enantiomer amidophosphoesters with an (O)₂P(O)(N) skeleton, i.e. diphenyl [(R)-(+)-α-methylbenzylamido]phosphate, (I), and diphenyl [(S)-(-)-α-methylbenzylamido]phosphate, (II), both C₂₀H₂₀NO₃P, are reported. In both structures, chiral one-dimensional hydrogen-bonded architectures, along [010], are mediated by N-H...OP interactions. The statistically identical assemblies include the noncentrosymmetric graph-set motif C(4) and the compounds crystallize in the chiral space group P2₁. As a result of synergistic co-operation from C-H...O interactions, a two-dimensional superstructure is built including a noncentrosymmetric R₄⁴(22) hydrogen-bonded motif. A Cambridge Structural Database survey was performed on (O)₂P(O)(N)-based structures in order to review the frequency of space groups observed in this family of compounds; the hydrogen-bond motifs in structures with chiral space groups and the types of groups inducing chirality are discussed. The ^2,3J_X-P (X = H or C) coupling constants from the NMR spectra of (I) and (II) have been studied. In each compound, the two diastereotopic C₆H₅O groups are different, which is reflected in the different chemical shifts and some coupling constants.

How Meaningful Is the Halogen Bonding in 1-Ethyl-3-methyl Imidazolium-Based Ionic Liquids for CO2 Capture?

We report on several parameters that can be used to describe the 1-ethyl-3-methyl-4,5-(X₂)imidazolium cations (where X = H, Br, and I) within the Canongia-Lopez and Padua Force Field (CL&P) framework. Geometrical parameters like intramolecular distances and radial distribution functions are close to the experimental structure. Density values obtained with our force field are within the expected ones from CL&P calculations in related systems. This information is used to simulate through molecular dynamics the solubilization of CO₂ by these ILs. For pure ILs, the addition of halides in position 4 and 5 promotes an enhanced hydrogen bond interaction at position 2 with the oxygen atoms in the anion. It is found that CO₂ should be in the interstices of the anion-cation 3D network with longer distances than those found in other reports at ab initio levels, suggesting that halogen bond, if present, may be not the driving force interaction in these systems. Therefore, it seems that CO₂ interacts linearly via an oxygen atom with the cation and with the anion through a π-stacking or hydrogen-bonded fashions. Solvation enthalpies compare well with the experimental data, thereby suggesting that halogenated ILs dissolve more efficiently in CO₂ than C₂C₁Im⁺ derivatives. This result suggests that halogenated ILs can be considered as reliable candidates for CO₂ capture.

Co-crystal synthesis: fact, fancy, and great expectations

Some strategies for driving co-crystal synthesis using a variety of competing non-covalent interactions are presented.

Relationship between crystal structures and photochromic properties of N-salicylideneaminopyridine derivatives

Photochromic properties of the N-salicylideneaminopyridine (SAP) crystals were related to both the molecular conformation and the packing efficiency.

Cooperativity between hydrogen- and halogen bonds: the case of selenourea

A combined experimental/theoretical investigation on the cooperativity between hydrogen- and halogen bonds is presented. In this study, it is shown that selenourea can interact at the same time with a benzoate anion and a polarized iodine, with the two interactions influencing and enhancing each other.

Solid-State Versatility of the Molecular Salts/Cocrystals of 2-Chloro-4-nitrobenzoic Acid: A Case Study on Halogen Bonds

2-Chloro-4-nitrobenzoic acid (2c4n) is an antiviral agent used for the treatment of HIV infection and to boost the immune response in immune deficiency diseases. In the present study, a series of eight molecular salts of 2c4n with pyridyl and benzoic acid derivatives have been synthesized by a crystal engineering approach and were characterized structurally by various spectroscopic, thermal, and X-ray diffraction techniques. Crystal structures of all synthesized molecular salts were determined by single-crystal X-ray diffraction techniques. In all synthesized molecular salts, the charge-assisted acid···pyridine/amine heterosynthon was found to be the primary supramolecular synthon. The synthesized salts, namely, 2c4n.g and 2c4n.h salts were found to be isostructural. Further, in the current work, the occurrence of weak halogen bonds in the presence of strong hydrogen bonds in the synthesized and in the reported molecular salts/cocrystals of 2c4n has been investigated. A detailed inspection of the crystal structures of salts/cocrystals of 2c4n was carried out to demonstrate the importance of halogen bonds in these crystal structures. It was found that 4 out of 8 synthesized molecular salts and 12 out of 24 reported molecular adducts of 2c4n were found to exhibit halogen bonds in their crystal structures. A similar kind of conformational change was observed for molecular salts exhibiting halogen bonds in their crystal structures; however, the conformations were found to be slightly different in other molecular salts. It was observed that two-point primary supramolecular synthon and stronger intramolecular Cl···O halogen bonds in the molecular adducts of 2c4n are found to be more susceptible to exhibit halogen bonds in their crystal structures. Halogen bond interactions played a vital role in the crystal stabilization of these molecular adducts.

Insight into π-hole interactions containing the inorganic heterocyclic compounds S2N2/SN2P2

Similar to σ-hole interactions, the π-hole interaction has attracted much attention in recent years. According to the positive electrostatic potentials above and below the surface of inorganic heterocyclic compounds S₂N₂ and three SN₂P₂ isomers (heterocyclic compounds 1-4), and the negative electrostatic potential outside the X atom of XH₃ (X = N, P, As), S₂N₂/SN₂P₂⋯XH₃ (X = N, P, As) complexes were constructed and optimized at the MP2/aug-cc-pVTZ level. The X atom of XH₃ (X = N, P, As) is almost perpendicular to the ring of the heterocyclic compounds. The π-hole interaction energy becomes greater as the trend goes from 1⋯XH₃ to 4⋯XH₃. These π-hole interactions are weak and belong to "closed-shell" noncovalent interactions. According to the energy decomposition analysis, of the three attractive terms, the dispersion energy contributes more than the electrostatic energy. The polarization effect also plays an important role in the formation of π-hole complexes, with the contrasting phenomena of decreasing electronic density in the π-hole region and increasing electric density outside the X atom of XH₃ (X = N, P, As). Graphical abstract Computed density difference plots for the complexes 3⋯NH ₃ (a ₁), 3⋯PH ₃ (b ₁), 3⋯AsH ₃ (c ₁) and electron density shifts for the complexes 3⋯NH ₃ (a ₂), 3⋯PH ₃ (b ₂),3⋯AsH ₃ (c ₂) on the 0.001 a.u. contour.

Development of a pharmaceutical cocrystal with solution crystallization technology: Preparation, characterization, and evaluation of myricetin-proline cocrystals

Myricetin shows low oral bioavailability (<10%) in rats due to poor aqueous solubility, although it has demonstrated various pharmacological activities such as those related to anticancer, anti-diabetes, and hepatic protection. To overcome this issue, in this study, pharmaceutical cocrystals were designed to efficiently deliver myricetin by oral administration. A 1:2 stoichiometric cocrystal of myricetin with proline was prepared successfully by solution crystallization based on the ternary phase diagram (TPD) principle, and it is presented as a new sphericity-like crystalline phase characterized by differential scanning calorimetry (DSC), powder X-ray diffraction (PXRD), and scanning electron microscopy (SEM). The formation of myricetin-proline cocrystals was a spontaneous and exothermic process, probably due to the supramolecular interactions between themselves, which were determined by Fourier transform-infrared spectroscopy (FT-IR). Consequently, the dissolution efficiency of myricetin from cocrystals was increased 7.69-fold compared with that of coarse myricetin, and the oral bioavailability of myricetin cocrystals in rats was enhanced by approximately 3.03 times compared with that of pure myricetin. The present study provides useful information for the potential application of cocrystal technology for water-insoluble drugs, especially flavonoid compounds.

Insights into halogen bond-driven enantioseparations

Although the halogen bond (XB) has been so far mainly studied in silico and in the solid state, its potential impact in solution is yet to be fully understood. In this study, we describe the first systematic investigation on the halogen bond in solvated environment by high-performance liquid chromatography (HPLC). Thirty three atropisomeric polyhalogenated-4,4'-bipyridines (HBipys), containing Cl, Br and I as substituents, were selected and used as potential XB donors (XBDs) on two cellulose-based chiral stationary phases (CSPs) containing potential XB acceptors (XBAs). The impact of the halogens on the enantiodiscrimination mechanism was investigated and iodine showed a pivotal role on the enantioseparation in non-polar medium. Electrostatic potentials (EPs) were computed to understand the electrostatic component of CSP-analyte interaction. Moreover, van't Hoff studies for ten HBipys were performed and the thermodynamic parameters governing the halogen-dependent enantioseparations are discussed. Finally, a molecular dynamic (MD) simulation is proposed to model halogen bond in polysaccharide-analyte complexes by inclusion of a charged extra point to represent the positive 'σ-hole' on the halogen atom. On the basis of both experimental results and theoretical data, we have profiled the halogen bond as a chemo-, regio-, site- and stereoselective interaction which can work in HPLC environment besides other known interactions based on the complementarity between selector and selectand.

Quaternary cocrystals: combinatorial synthetic strategies based on long-range synthon Aufbau modules (LSAM)

A synthetic strategy is outlined whereby a binary cocrystal may be developed in turn into a ternary and finally into a quaternary cocrystal. The strategy hinges on the concept of the long-range synthon Aufbau module (LSAM) which is a large supramolecular synthon containing more than one type of intermolecular interaction. Modulation of these interactions may be possible with the use of additional molecular components so that higher level cocrystals are produced. We report six quaternary cocrystals here. All are obtained as nearly exclusive crystallization products when four appropriate solid compounds are taken together in solution for crystallization.

Preferred formation of the carboxylic acid–pyridine heterosynthon in 2-anilinonicotinic acids

Preferred formation of carboxylic acid–pyridine heterosynthon in 2-(phenylamino)nicotinic acid was realized through structural modification of the N bridge.

Effect of cooperativity in lithium bonding on the strength of halogen bonding and tetrel bonding: (LiCN)n···ClYF3 and (LiCN)n···YF3Cl (Y = C, Si and n = 1-5) complexes as a working model

This paper reports results of cooperativity in lithium bonding on the strength of halogen bonding and tetrel bonding in complexes pairing CF3Cl and SiF3Cl with (LiCN)n complexes, where n varies from 1 to 5. Molecular geometries and stabilization energies of title complexes are calculated at the MP2 level with 6-311++G(d,p) basis set. Cooperative effects are found in terms of structural and energetic properties when lithium, halogen, and tetrel bonds are present in these complexes simultaneously. Our results reveal that strength of halogen and tetrel bondings are enhanced due to cooperativity of Li···N interactions in lithium bonded complexes. Good linear correlations between cooperativity parameters and electronic properties of complexes were established in the present study.

The enhancing effects of group V σ-hole interactions on the F···O halogen bond

The σ-hole interaction, which occurs between the covalent IV-VII atoms and nucleophilic substances, has become a hot issue of weak interaction. In this work, NCF···O=PX3···(NCF)n (X = F, Cl, Br, H, CH3·; n = 0, 1, 2) complexes were constructed and studied based on the second-order Møller-Plesset perturbation theory (MP2) calculations to investigate the enhancing effects of group V σ-hole interactions on the F···O halogen bond. With increasing n, the FO halogen bond becomes stronger, indicating that the group V σ-hole interactions could enhance the F···O halogen bond. As the capacity of donating electrons of X increases, the most negative electrostatic potentials outside the oxygen atom of O=PX3···(NCF)n (n = 0, 1, 2) become more negative, resulting in a stronger F···O halogen bond. In the formation of a F···O halogen bond, along the sequence of X = F, Cl, Br, H, CH3 of the negative sites O=PX3, the electric field of the lone pair of oxygen becomes greater and causes a larger decrease in electron density outside the fluorine atom. On the other hand, with increasing n from 0 to 2, the group V σ-hole interactions also increase the electric field of lone pair of oxygen and results in a larger decrease in electron density outside the fluorine atom.

Cocrystals of 6-chlorouracil and 6-chloro-3-methyluracil: exploring their hydrogen-bond-based synthon motifs with several triazine and pyrimidine derivatives

In order to obtain complexes held together by hydrogen as well as halogen bonds, 6-chlorouracil [6-chloropyrimidin-2,4(1H,3H)-dione; 6CU] and its 3-methyl derivative [6-chloro-3-methylpyrimidin-2,4(1H,3H)-dione; M6CU] were cocrystallized with 2,4,6-triaminopyrimidine and the three triazine derivatives 2,4,6-triamino-1,3,5-triazine (melamine), 2,4-diamino-6-methyl-1,3,5-triazine and 2-chloro-4,6-diamino-1,3,5-triazine, which all offer complementary hydrogen-bonding sites. Three of these compounds form cocrystals with 6CU; however, melamine yielded only a new pseudopolymorph with 6CU, but formed a cocrystal with M6CU. All six cocrystals contain solvent molecules (N,N-dimethylformamide,N,N-dimethylacetamide orN-methylpyrrolidin-2-one), whose intermolecular interactions contribute significantly to the stabilization of the crystal packing. Each of these structures comprises chains, which are primarily formed by strong hydrogen bonds with a basic framework built byR22(8) hydrogen bonds of either pure N—H...N or mixed patterns. Solvent molecules are aligned to the border of these chainsviaN—H...O hydrogen bonds. Two of the reported crystal structures containing 6CU show additional Cl...O halogen bonds, which connect the chains to two-dimensional layers, while one weak and one strong Cl...Cl interaction are observed in the two structures in which molecules of M6CU are present.

Halogen bonding interactions in ion pairs versus conventional charge-assisted and neutral halogen bonds: a theoretical study based on imidazolium species

The properties of three different types of halogen bonds in the complexes of imidazolium species are characterized.

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.