Rational Modification of the Hierarchy of Intermolecular Interactions in Molecular Crystal Structures by Using Tunable Halogen Bonds

A family of 16 isomolecular salts (3‐XpyH)₂[MX′₄] (3‐XpyH=3‐halopyridinium; M=Co, Zn; X=(F), Cl, Br, (I); X′=Cl, Br, I) each containing rigid organic cations and tetrahedral halometallate anions has been prepared and characterized by X‐ray single crystal and/or powder diffraction. Their crystal structures reflect the competition and cooperation between non‐covalent interactions: N? H???X′? M hydrogen bonds, C? X???X′? M halogen bonds and π–π stacking. The latter are essentially unchanged in strength across the series, but both halogen bonds and hydrogen bonds are modified in strength upon changing the halogens involved. Changing the organic halogen (X) from F to I strengthens the C? X???X′? M halogen bonds, whereas an analogous change of the inorganic halogen (X′) weakens both halogen bonds and N? H???X′? M hydrogen bonds. By so tuning the strength of the putative halogen bonds from repulsive to weak to moderately strong attractive interactions, the hierarchy of the interactions has been modified rationally leading to systematic changes in crystal packing. Three classes of crystal structure are obtained. In type A (C? F???X′? M) halogen bonds are absent. The structure is directed by N? H???X′? M hydrogen bonds and π‐stacking interactions. In type B structures, involving small organic halogens (X) and large inorganic halogens (X′), long (weak) C? X???X′? M interactions are observed with type I halogen–halogen interaction geometries (C? X???X′ ≈ X???X′? M ≈155°), but hydrogen bonds still dominate. Thus, minor but quite significant perturbations from the type A structure arise. In type C, involving larger organic halogens (X) and smaller inorganic halogens (X′), stronger halogen bonds are formed with a type II halogen–halogen interaction geometry (C? X???X′ ≈180°; X???X′? M ≈110°) that is electrostatically attractive. The halogen bonds play a major role alongside hydrogen bonds in directing the type C structures, which as a result are quite different from type A and B.     

Multidentate,V-Shaped Pyridine Building Blocks as Tectons for Crystal Engineering

The formation of supramolecular structural units through self-assembly is a powerful method to design new architectures and materials endowed with specific properties. With the aim of adding a group of versatile tectons to the toolkit of crystal engineers, we have devised and synthesised four new V-shaped building blocks characterised by an aryl acetylene scaffold comprising three substituted pyridine rings connected by two triple bonds. The judicious choice of different substituents on the pyridine rings provides these tectons with distinctive steric, electrostatic and self-assembly properties, which influence their crystal structures and their ability to form co-crystals. Co-crystals of the tectons with tetraiododifluorobenzene were obtained both via traditional and mechanochemical crystallisation strategies, proving their potential use in crystal engineering. The energetic contributions of the supramolecular interactions at play in the crystal lattice have also been evaluated to better understand their nature and strength and to rationalise their role in designing molecular crystals.     

Thermotropic Liquid Crystals Formed by Intermolecular Hydrogen Bonding Interactions

The role of hydrogen bonding in the formation or stabilization of liquid crystalline phases has only recently been appreciated. Following the first, wellestablished examples of liquid crystal formation from the dimerization of aromatic carboxylic acids, through hydrogen bonding, several classes of compounds have recently been synthesized, the liquid crystalline behavior of which is also dependent on intermolecular hydrogen bonds between similar or dissimilar molecules. In this review the main classes of compounds exhibiting liquid crystallinity due to hydrogen bonding are presented to show the diversity of organic compounds that can be used as building elements in liquid crystals. The molecules are either of the rigid-rod anisotropic or amphiphilic types such as molecules appropriately functionalized with pyridyl and carboxyl groups, whose interaction leads to the formation of liquid crystals; amphiphilic carbohydrates and amphiphilic and bolaamphiphilic compounds with multiple hydroxyl groups whose dimerization or association is indispensable for the formation of liquid crystals; and certain amphiphilic carboxylic acids with monomeric or polymeric mesogens and amphiphilic-type compounds bearing different moieties, whose interaction may lead to the formation of mesomorphic compounds. Associated with the macroscopic display of liquid crystalline phases is the supramolecular structure, and therefore rather extended discussion of these structures are included in this review.     

Exploring Orthogonal Hydrogen Bonding towards Designing Organic‐Salt‐Based Supramolecular Gelators: Synthesis,Structures, and Anticancer Properties

A series of primary ammonium monocarboxylate (PAM) salts derived from β‐alanine derivatives of pyrene and naphthalene acetic acid, along with the parent acids, were explored to probe the plausible role of orthogonal hydrogen bonding resulting from amide???amide and PAM synthons on gelation. Single‐crystal X‐ray diffraction (SXRD) studies were performed on two parent acids and five PAM salts in the series. The data revealed that orthogonal hydrogen bonding played an important role in gelation. Structure–property correlation based on SXRD and powder X‐ray diffraction data also supported the working hypothesis upon which these gelators were designed. 3‐(4,5‐Dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide (MTT) and cell migration assay on a highly aggressive human breast cancer cell line, MDA‐MB‐231, revealed that one of the PAM salts in the series, namely, PAA.B2 , displayed anticancer properties, and internalization of the gelator salt in the same cell line was confirmed by cell imaging.     

Crystal engineering: a holistic view

Crystal engineering, the design of molecular solids, is the synthesis of functional solid-state structures from neutral or ionic building blocks, using intermolecular interactions in the design strategy. Hydrogen bonds, coordination bonds, and other less directed interactions define substructural patterns, referred to in the literature as supramolecular synthons and secondary building units. Crystal engineering has considerable overlap with supramolecular chemistry, X-ray crystallography, materials science, and solid-state chemistry and yet it is a distinct discipline in itself. The subject goes beyond the traditional divisions of organic, inorganic, and physical chemistry, and this makes for a very eclectic blend of ideas and techniques. The purpose of this Review is to highlight some current challenges in this rapidly evolving subject. Among the topics discussed are the nature of intermolecular interactions and their role in crystal design, the sometimes diverging perceptions of the geometrical and chemical models for a molecular crystal, the relationship of these models to polymorphism, knowledge-based computational prediction of crystal structures, and efforts at mapping the pathway of the crystallization reaction.     

Liquid‐Crystalline Supramolecular Polymers Formed through Complementary Nucleobase‐Pair Interactions

We report how the placement of nucleobase units, thymine, or N ⁶‐(4‐methoxybenzoyl)adenine, onto the ends of a mesogenic core, bis‐4‐alkoxy‐substituted bis(phenylethynyl)benzene, affects the properties of these materials. We show that addition of these bulky polar groups significantly reduces the range of liquid‐crystalline behavior of these compounds. However, mixing two complementary nucleobase‐containing AA‐ and BB‐type monomer units together does result in the formation of stable, thermotropic liquid‐crystalline (LC) phases. Hydrogen bonding is shown to play an important role in the formation of these LC phases, consistent with the formation of oligomeric or polymeric hydrogen‐bonded aggregates. X‐ray analyses of these mixed materials are consistent with the formation of smectic C phases.     

Conformational Flexibility of Tetralactam Macrocycles and Their Intermolecular Hydrogen‐Bonding Patterns in the Solid State

Flexible rigidity : Tetralactam macrocycles of the Hunter type bear a rigid scaffold (see space‐filling representation), but can still widely adapt to the properties of a guest molecule inside their cavities. X‐ray crystal structures of a series of differently substituted macrocycles reveal a remarkably broad variety of intermolecular hydrogen‐bonding patterns organizing the macrocycles in the crystals in intriguingly different ways.

     

Luminescence Properties of C‐Diazaborolyl‐ortho‐Carboranes as Donor–Acceptor Systems

Seven derivatives of 1,2‐dicarbadodecaborane (ortho‐carborane, 1,2‐C₂B₁₀H₁₂) with a 1,3‐diethyl‐ or 1,3‐diphenyl‐1,3,2‐benzodiazaborolyl group on one cage carbon atom were synthesized and structurally characterized. Six of these compounds showed remarkable low‐energy fluorescence emissions with large Stokes shifts of 15100–20260 cm^?1 and quantum yields (Φ_F) of up to 65 % in the solid state. The low‐energy fluorescence emission, which was assigned to a charge‐transfer (CT) transition between the cage and the heterocyclic unit, depended on the orientation (torsion angle, ψ) of the diazaborolyl group with respect to the cage C? C bond. In cyclohexane, two compounds exhibited very weak dual fluorescence emissions with Stokes shifts of 15660–18090 cm^?1 for the CT bands and 1960–5540 cm^?1 for the high‐energy bands, which were assigned to local transitions within the benzodiazaborole units (local excitation, LE), whereas four compounds showed only CT bands with Φ_F values between 8–32 %. Two distinct excited singlet‐state (S₁) geometries, denoted S₁(LE) and S₁(CT), were observed computationally for the benzodiazaborolyl‐ortho‐carboranes, the population of which depended on their orientation (ψ). TD‐DFT calculations on these excited state geometries were in accord with their CT and LE emissions. These C‐diazaborolyl‐ortho‐carboranes were viewed as donor–acceptor systems with the diazaborolyl group as the donor and the ortho‐carboranyl group as the acceptor.     

Intermolecular Interactions in Photodamaged DNA from Density Functional Theory Symmetry‐Adapted Perturbation Theory

The intermolecular interactions of the photodamaged cyclobutane pyrimidine dimer (CPD) lesion with adjacent nucleobases in the native intrahelical DNA double strand are investigated at the level of density functional theory symmetry‐adapted perturbation theory (DFT‐SAPT) and compared to the original (or repaired) case with pyrimidines (TpT) instead of CPD. The CPD aggregation is on average destabilized by about 6 kcal mol^?1 relative to that involving TpT. The effect of destabilization is asymmetric, that is, it involves a single H‐bonding (Watson–Crick (WC) type) base‐pair interaction.     

A Multidirectional Superelastic Organic Crystal by Versatile Ferroelastical Manipulation

Mechanical twinning changes atomic, molecular, and crystal orientations along with directions of the anisotropic properties of the crystalline materials while maintaining single crystallinity in each domain. However, such deformability has been less studied in brittle organic crystals despite their remarkable anisotropic functions. Herein we demonstrate a direction‐dependent mechanical twinning that shows superelasticity in one direction and ferroelasticity in two other directions in a single crystal of 1,3‐bis(4‐methoxyphenyl)urea. The crystal can undergo stepwise twinning and ferroelastically forms various shapes with multiple domains oriented in different directions, thereby affording a crystal that shows superelasticity in multiple directions. This adaptability and shape recoverability in a ferroelastic and superelastic single crystal under ambient conditions are of great importance in future applications of organic crystals as mechanical materials, such as in soft robotics.     

Organic chlorine as a hydrogen-bridge acceptor: evidence for the existence of intramolecular O--H...Cl--C interactions in some gem-alkynols

The acceptor capabilities of "organic" halogen, CX (X=F, Cl, Br, I), with respect to hydrogen bonding are controversial, and unactivated organic chlorine is generally deemed to be a poor acceptor. Hydrogen bridges of the type O--H...Cl--C are uncommon and occur mainly in an intramolecular situation when the donor group is sterically hindered, so that the formation of intermolecular interactions is difficult. In this paper, intramolecular O--H...Cl--C interactions in a series of chloro-substituted gem-alkynols are studied. We describe various features of this interaction using crystallographic, spectroscopic and computational methods. The O--H...Cl--C interaction occurs in five of the six compounds under consideration here (CDDA, 14DDDA, 15DDDA, 18DDDA, 15MKA). Solution (1)H NMR spectroscopy shows that the interaction is intramolecular and that it is a true hydrogen bond. DFT calculations give a stabilisation energy around 4.0 kcal mol(-1). In the crystal structures of the compounds studied, the intramolecular O--H...Cl--C interactions fit into the overall scheme of cooperative interactions. These structures may be derived from that of the unsubstituted compound DDA by means of synthon exchange and the O--H...Cl--C interaction fares surprisingly well in the presence of competing stronger acceptors. The crystal structures show an unusual degree of modularity for compounds that generally form interactions that are weak and variable. It is noteworthy that the so-called "weak" acceptor, organic chlorine, is able to sustain a good intramolecular hydrogen bridge that is of an attractive and stabilizing nature and which is of potential importance in crystal engineering and supramolecular chemistry.     

Spontaneously organizing metal connectors as supramolecular structure directors of two- and three-dimensional organometallic assemblies

The supramolecular interplay of Me(3)Sn(+) and [M(CN)(2n)](n-) ions (n=3 and 4) with either 4,4'-bipyridine (bpy), trans-bis(4-pyridyl)ethene (bpe) or 4cyanopyridine (cpy) in the presence of H(2)O has been investigated for the first time. Crystal structures of the six novel assemblies: [(Me(3)Sn)(4)Mo(IV)(CN)(8).2 H(2)O.bpy] (8) and [(Me(3)Sn)(4)Mo(IV)(CN)(8).2 H(2)O.bpe] (8 a; isostructural), [(Me(3)Sn)(3)Fe(III)(CN)(6).4 H(2)O.bpy] (9), [(Me(3)Sn)(3)Co(III)(CN)(6).3 H(2)O.3/2 bpy] (10), [(Me(3)Sn)(4)Fe(II)(CN)(6).H(2)O.3/2 bpy] (11), and [(Me(3)Sn)(4)Ru(II)(CN)(6).2 H(2)O.3/2 cpy] (12) are presented. H(2)O molecules are usually coordinated to tin atoms and involved in two significant O-H.N hydrogen bonds, wherein the nitrogen atoms belong either to bpy (bpe, cpy) molecules or to M-coordinated cyanide ligands. Extended supramolecular assemblies such as -CN-->Sn(Me(3))<--O(H.)H.N(L)N.HO(H.)-->Sn(Me(3))<--NC- (L=bpy, bpe or cpy) function as efficient metal connectors (or spacers) in the structures of all six compounds. Only in the three-dimensional framework of 11, one third of all bpy molecules is involved in coordinative N-->Sn bonds. The supramolecular architecture of 9 involves virtually non-anchored (to cyanide N atoms), Me(3)Sn(+) units with a strictly planar SnC(3) skeleton, and two zeolitic H(2)O molecules. Pyrazine (pyz) is surprisingly reluctant to afford assemblies similar to 8-12, however, the genuine host-guest systems [(Me(3)Sn)(4)Mo(CN)(8).0.5pyz] and [(Me(3)Sn)(4)Mo(CN)(8).pym] (pym=pyrimidine) could be isolated and also structurally characterized.