Roles of Hydrogen, Halogen Bonding and Aromatic Stacking in a Series of Isophthalamides

The synthesis and spectroscopic characterisation of six bis(5-X-pyridine-2-yl)isophthalamides (X = H, F, Br, Cl, I, NO2) are reported, together with five crystal structure analyses (for X = H, F to I). The isophthalamides span a range of conformations as syn/anti (H-DIP; I-DIP), anti/anti- (F-DIP; Br-DIP) and with both present in ratio 2:1 in Cl-DIP. The essentially isostructural F-DIP and Br-DIP molecules (using strong amide…amide interactions) aggregate into 2D molecular sheets that align with either F/H or Br atoms at the sheet surfaces (interfaces), respectively. Sheets are linked by weak C-H⋯F contacts in F-DIP and by Br⋯Br halogen bonding interactions as a ‘wall of bromines’ at the Br atom rich interfaces in Br-DIP. Cl-DIP is an unusual crystal structure incorporating both syn/anti and anti/anti molecular conformations in the asymmetric unit (Z’ = 3). The I-DIP•½(H2O) hemihydrate structure has a water molecule residing on a twofold axis between two I-DIPs and has hydrogen and N⋯I (Nc = 0.88) halogen bonding. The hydrate is central to an unusual synthon and involved in six hydrogen bonding interactions/contacts. Contact enrichment analysis on the Hirshfeld surface demonstrates that F-DIP, Cl-DIP and Br-DIP have especially over-represented halogen···halogen interactions. With the F-DIP, Cl-DIP and Br-DIP molecules having an elongated skeleton, the formation of layers of halogen atoms in planes perpendicular to the long unit cell axis occurs in the crystal packings. All six DIPs were analysed by ab initio calculations and conformational analysis; comparisons are made between their minimized structures and the five crystal structures. In addition, physicochemical properties are compared and assessed.

Polymorphs with Remarkably Distinct Physical and/or Chemical Properties

Polymorphism in crystals is known since 1822 and the credit goes to Mitscherlich who realized the existence of different crystal structures of the same compound while working with some arsenate and phosphate salts. Later on, this phenomenon was observed also in organic crystals. With the advent of different technologies, especially the easy availability of single crystal XRD instruments, polymorphism in crystals has become a common phenomenon. Almost 37 % of compounds (single component) are polymorphic to date. As the energies of the different polymorphic forms are very close to each other, small changes in crystallization conditions might lead to different polymorphic structures. As a result, sometimes it is difficult to control polymorphism. For this reason, it is considered to be a nuisance to crystal engineering. It has been realized that the property of a material depends not only on the molecular structure but also on its crystal structure. Therefore, it is not only of interest to academia but also has widespread applications in the materials science as well as pharmaceutical industries. In this review, we have discussed polymorphism which causes significant changes in materials properties in different fields of solid-state science, such as electrical, magnetic, SHG, thermal expansion, mechanical, luminescence, color, and pharmaceutical. Therefore, this review will interest researchers from supramolecular chemistry, materials science as well as medicinal chemistry.

Isostructural Halogen Exchange and Halogen Bonds: The Case of N-(4-Halogenobenzyl)-3-halogenopyridinium Halogenides

Six N-(4-halogenobenzyl)-3-halogenopyridinium cations were prepared by reacting meta-halogenopyridines (Cl, Br, and I) with (4-halogenobenzyl) bromides (Br and I) and were isolated as bromide salts, which were further used to obtain iodides and chlorides. Sixteen compounds (out of 18 possible cation/anion combinations) were obtained; two crystallized as hydrates and 14 as solvent free salts, 11 of which belonged to one isostructural series and 3 to another. All crystal structures comprise halogen-bonded chains, with the anion as an acceptor of two halogen bonds, with the pyridine and the benzyl halogen substituents of two neighboring cations. The halogen bonds with the pyridine halogen show a linear correlation between the relative halogen bond length and angle, which primarily depend on the donor halogen. The parameters of the other halogen bonds vary with all three halogens, indicating that the former halogen bond is the dominant interaction. This is also in accord with the calculated electrostatic potential in the σ-holes of the halogens and the thermal properties of the solids. The second isostructural group comprises combinations of the best halogen bond donors and acceptors, and features a more favorable halogen bond geometry of the dominant halogen bond, reaffirming its significance as the main factor in determining the structure.

Differences in thermal expansion and motion ability for herringbone and face-to-face π-stacked solids

A series of aromatic organic molecules functionalized with different halogen atoms (I/ Br), motion-capable groups (olefin, azo or imine) and molecular length were designed and synthesized. The molecules self-assemble in the solid state through halogen bonding and exhibit molecular packing sustained by either herringbone or face-to-face π-stacking, two common motifs in organic semiconductor molecules. Interestingly, dynamic pedal motion is only achieved in solids with herringbone packing. On average, solids with herringbone packing exhibit larger thermal expansion within the halogen-bonded sheets due to motion occurrence and molecular twisting, whereas molecules with face-to-face π-stacking do not undergo motion or twisting. Thermal expansion along the π-stacked direction is surprisingly similar, but slightly larger for the face-to-face π-stacked solids due to larger changes in π-stacking distances with temperature changes. The results speak to the importance of crystal packing and intermolecular interaction strength when designing aromatic-based solids for organic electronics applications.

Chameleonic Metal-bound Isocyanides: π-Donating CuI-center Imparts a Nucleophilicity to the Isocyanide Carbon toward Halogen Bonding

In the structures of the isostructural cocrystals [CuI3(CNXyl)3]·CHX3 (X = Br, I), two adjacent CuI-bound isocyanide groups, whose carbon lone pairs are blocked by the ligation, exhibit nucleophilic properties induced...

The Isocyanide Complexes cis-[MCl2(CNC6H4-4-X)2] (M = Pd, Pt; X = Cl, Br) as Tectons in Crystal Engineering Involving Halogen Bonds

The isocyanide complexes cis-[MCl2(CNC6H4-4-X)2] (M = Pd; X = Cl, Br; M = Pt; X = Br) form isomorphous crystal structures exhibiting the Cl/Br and Pd/Pt exchanges featuring 1D chains upon crystallisation. Crystal packing is supported by the C–X···X–C halogen bonds (HaBs), C–H···X–C hydrogen bonds (HB), X···M semicoordination, and C···C contacts between the C atoms of aryl isocyanide ligands. The results of DFT calculations and topological analysis indicate that all the above contact types belong to attractive noncovalent interactions. A projection of the electron localization function (ELF) and an inspection of the electron density (ED) and the electrostatic potential (ESP) reveal the amphiphilic nature of X atoms playing the role of HaB donors, HaB and HB acceptors, and a nucleophilic partner in X···M semicoordination.

Synthesis and Structural Characterization of Isostructural 4-(4-Aryl)-2-(5-(4-fluorophenyl)-3-(1-(4-fluorophenyl)-5-methyl-1H-1,2,3-triazol-4-yl)-4,5-dihydro-1H-pyrazol-1-yl)thiazoles

4-(4-Chlorophenyl)-2-(5-(4-fluorophenyl)-3-(1-(4-fluorophenyl)-5-methyl-1H-1,2,3-triazol-4-yl)-4,5-dihydro-1H-pyrazol-1-yl)thiazole (4) and 4-(4-fluorophenyl)-2-(5-(4-fluorophenyl)-3-(1-(4-fluorophenyl)-5-methyl-1H-1,2,3-triazol-4-yl)-4,5-dihydro-1H-pyrazol-1-yl)thiazole (5) have been synthesized in high yields. Crystallization of 4 and 5 from dimethylformamide solvent produced samples suitable for structure determination by single crystal diffraction. The materials are isostructural with triclinic, PĪ and symmetry and comprise two independent molecules in the asymmetric unit. The two independent molecules in the asymmetric unit assume similar conformation. The molecule is essentially planar apart from one of the two fluorophenyl groups, which is oriented roughly perpendicular to the plane of the rest of the molecule.

Supramolecular architectures sustained by delocalised C–I⋯π(arene) interactions in molecular crystals and the propensity of their formation

A survey of delocalised C–I⋯π(chelate ring) interactions is presented.

Modulation of luminescence properties for [cyclometalated]-PtII(isocyanide) complexes upon co-crystallisation with halosubstituted perfluorinated arenes

C–X⋯Cl–Pt Halogen bonds and πh⋯d_z2[Pt^II] contacts led to the 2-fold increase of phosphorescence quantum yields for [cyclometalated]-Pt^II(isocyanide) complexes upon co-crystallisation with halosubstituted perfluorinated arenes.

Controlling thermal expansion within mixed cocrystals by tuning molecular motion capability

Thermal expansion behavior is tuned by incorporating motion-capable or -incapable molecules into organic solids.

Controlling thermal expansion (TE) behaviors of organic materials is challenging because several mechanisms can govern TE, such as noncovalent interaction strength and structural motions. Here, we report the first demonstration of tuning TE within organic solids by using a mixed cocrystal approach. The mixed cocrystals contain three unique molecules, two of which are present in variable ratios. These two molecules either lack or exhibit the ability to undergo molecular motion in the solid state. Incorporation of higher ratios of motion-capable molecules results in larger, positive TE along the motion direction. Addition of a motion-incapable molecule affords solids that undergo less TE. Fine-tuned TE behavior was attained by systematically controlling the ratio of motion-capable and -incapable molecules in each solid.

Halogen Bonding in Isostructural Co(II) Complexes with 2-Halopyridines

Three complexes [Co(2-XPy)2Cl2] (X = Cl, Br, and I) were prepared and characterized, representing a rare case of isostructurality within the Cl-Br-I row. The nature of halogen bonding (XB) in a solid state was studied by DFT calculations, revealing a tendency of XB energy growth for heavier halogens.

Influence of molecular width on the thermal expansion in solids

It has been shown that the thermal expansion would be higher in a direction along which the molecular width is shorter and it would be smaller if the molecular width is longer along that direction.

Probing halogen⋯halogen interactions via thermal expansion analysis

The magnitude of the thermal expansion of the 1-D chain coordination polymer Cu(MeCN)₂[AuX₂(CN)₂] (X = Cl, Br, I) is significantly reduced in the presence of interchain halogen⋯halogen interactions.

Thermomechanical effect in molecular crystals: the role of halogen-bonding interactions

The design and synthesis of mechanically responsive materials is interesting because they are potential candidates to convert thermal energy into mechanical work. Reported in this paper are thermosalient effects in a series of halogen derivatives of salinazids. The chloro derivative, with higher electronegativity and a weaker inter-halogen bond strength (Cl⋯Cl) exhibits an excellent thermal response, whereas the response is weaker in the iodo derivative with stronger I⋯I halogen bonding. 3,5-Di-chloro-salinazid (Compound-A) exists in three polymorphic forms, two room-temperature polymorphs (Forms I and II) and one high-temperature modification (Form III). The transformation of Form I to Form III upon heating at 328-333 K is a reversible thermosalient transition, whereas the transformation of Form II to Form III is irreversible and non-thermosalient. 3,5-Di-bromo- (Compound-B) and 3-bromo-5-chloro- (Compound-C) salinazid are both dimorphic: the Form I to Form II transition in Compound-B is irreversible, whereas Compound-C shows a reversible thermosalient effect (362-365 K). In the case of 3,5-di-iodo-salinazid (Compound-D) and 3,5-di-fluoro-salinazid (Compound-E), no phase transitions or thermal effects were observed. The thermosalient behaviour of these halosalinazid molecular crystals is understood from the anisotropy in the cell parameters (an increase in the a axis and a decrease in the b and c axes upon heating) and the sudden release of accumulated strain during the phase transition. The di-halogen salinazid derivatives (chlorine to iodine) show a decrease in thermal effects with an increase in halogen-bond strength. Interestingly, Compound-B shows solid-state photochromism in its polymorphs along with the thermosalient effect, wherein Form I is cyan and Form II is light orange.

Achieving dynamic behaviour and thermal expansion in the organic solid state via co-crystallization

Molecular motion of an azo functional group is ‘unlocked’ via co-crystallizations.

Thermal expansion involves a response of a material to an external stimulus that typically involves an increase in a crystallographic axis (positive thermal expansion (PTE)), although shrinking with applied heat (negative thermal expansion (NTE)) is known in rarer cases. Here, we demonstrate a means to achieve dynamic molecular motion and thermal expansions in organic solids via co-crystallizations. One co-crystal component is known to exhibit dynamic behaviour in the solid state while the second, when varied systematically, affords co-crystals with linear thermal expansion coefficients that range from colossal to nearly zero. Two co-crystals exhibit rare NTE. We expect the approach to guide the design of molecular solids that enable predesigned motion related to thermal expansion processes.

Influence of noncovalent interactions on the structures of metal–organic hybrids based on a [VO2(2,6-pydc)]− tecton with cations of imidazole, pyridine and its derivatives

Hydrogen-bonding has a profound effect on topologies, and various 1D (band, pillar or chain), 2D and 3D hydrogen bonding networks have been observed.

Can the thermal expansion be controlled by varying the hydrogen bond dimensionality in polymorphs?

A higher dimensional (1-D) hydrogen bonded form shows smaller thermal expansion than a lower dimensional (0-D) hydrogen bonded form of 2-butynoic acid.

A thermal expansion investigation of the melting point anomaly in trihalomesitylenes

The order of melting point is tribromomesitylene > triiodomesitylene ≈ trichloromesitylene > hexamethylbenzene and the order of thermal expansion is tribromomesitylene < triiodomesitylene ≈ trichloromesitylene < hexamethylbenzene.