Self‐Assembly and (Hydro)gelation Triggered by Cooperative π–π and Unconventional CH⋅⋅⋅X Hydrogen Bonding Interactions

Weak C? H???X hydrogen bonds are important stabilizing forces in crystal engineering and anion recognition in solution. In contrast, their quantitative influence on the stabilization of supramolecular polymers or gels has thus far remained unexplored. Herein, we report an oligophenyleneethynylene (OPE)‐based amphiphilic Pt^II complex that forms supramolecular polymeric structures in aqueous and polar media driven by π–π and different weak C‐H???X (X=Cl, O) interactions involving chlorine atoms attached to the Pt^II centers as well as oxygen atoms and polarized methylene groups belonging to the peripheral glycol chains. A collection of experimental techniques (UV/Vis, 1D and 2D NMR, DLS, AFM, SEM, and X‐Ray diffraction) demonstrate that the interplay between different weak noncovalent interactions leads to the cooperative formation of self‐assembled structures of high aspect ratio and gels in which the molecular arrangement is maintained in the crystalline state.     

Recurrence of carboxylic acid-pyridine supramolecular synthon in the crystal structures of some pyrazinecarboxylic acids.

X-ray crystal structures of pyrazinic acid 1 and isomeric methylpyrazine carboxylic acids 2-4 are analyzed to examine the occurrence of carboxylic acid-pyridine supramolecular synthon V in these heterocyclic acids. Synthon V, assembled by (carboxyl)O-H...N(pyridine) and (pyridine)C-H...O(carbonyl) hydrogen bonds, controls self-assembly in the crystal structures of pyridine and pyrazine monocarboxylic acids. The recurrence of acid-pyridine heterodimer V compared to the more common acid-acid homodimer I in the crystal structures of pyridine and pyrazine monocarboxylic acids is explained by energy computations in the RHF 6-31G* basis set. Both the O-H.N and the C-H...O hydrogen bonds in synthon V result from activated acidic donor and basic acceptor atoms in 1-4. Pyrazine 2,3- and 2,5-dicarboxylic acids 10 and 11 crystallize as dihydrates with a (carboxyl)O-H...O(water) hydrogen bond in synthon VII, a recurring pattern in the diacid structures. In summary, the carboxylic acid group forms an O-H...N hydrogen bond in pyrazine monocarboxylic acids and an O-H...O hydrogen bond in pyrazine dicarboxylic acids. This structural analysis correlates molecular features with supramolecular synthons in pyridine and pyrazine carboxylic acids for future crystal engineering strategies.     

Herringbone and Helical Self‐Assembly of π‐Conjugated Molecules in the Solid State through CH/π Hydrogen Bonds

Self‐organization of organic molecules through weak noncovalent forces such as CH/π interactions and creation of large hierarchical supramolecular structures in the solid state are at the very early stage of research. The present study reports direct evidence for CH/π interaction driven hierarchical self‐assembly in π‐conjugated molecules based on custom‐designed oligophenylenevinylenes (OPVs) whose structures differ only in the number of carbon atoms in the tails. Single‐crystal X‐ray structures were resolved for these OPV synthons and the existence of long‐range multiple‐arm CH/π interactions was revealed in the crystal lattices. Alignment of these π‐conjugated OPVs in the solid state was found to be crucial in producing either right‐handed herringbone packing in the crystal or left‐handed helices in the liquid‐crystalline mesophase. Pitch‐ and roll‐angle displacements of OPV chromophores were determined to trace the effect of the molecular inclination on the ordering of hierarchical structures. Furthermore, circular dichroism studies on the OPVs were carried out in the aligned helical structures to prove the existence of molecular self‐assembly. Thus, the present strategy opens up new approaches in supramolecular chemistry based on weak CH/π hydrogen bonding, more specifically in π‐conjugated materials.     

Hydrogen Bonding Assembled 3D Supramolecular Structures Formed by 5-Amino-2,4,6-triiodoisophthalic Acid and N-Heterocyclic Aromatic Ligands

Three salts constructed by 5-amino-2,4,6-triiodoisophthalic acid(ATIPA) with N-heterocycles aromatic coformers such as pyridine tetrazolium, tetramethylpyraziiie and cyanuric acid were synthesized by slowing evaporation of solvent. X-Ray single crystal analysis shows that hydrogen protons of the carboxyl groups transfer to nitrogen atoms of the N-heterocyclic coformers to form N-H…0 hydrogen bonds in all the three compounds. A huge amount of H-bonds play significant role in tlie construction of these compounds and all of them generate 3D structures through strong O-H…N, O-H…O, N-H…O and weak C-H…O hydrogen bonds. Moreover, solvent water molecules are indispensable in the formation of compounds 1 and 3, which constitutes different supramolecular synthons to bridge individual molecules and chains to form stable structures. In addition, these crystal structures were further characterized by themiogravimetric analysis and infrared spectroscopy.     

Multicomponent Crystal of Metformin and Barbital: Design,Crystal Structure Analysis and Characterization

The formation of most multicomponent crystals relies on the interaction of hydrogen bonds between the components, so rational crystal design based on the expected hydrogen-bonded supramolecular synthons was employed to establish supramolecular compounds with desirable properties. This theory was put into practice for metformin to participate in more therapeutic fields to search for a fast and simple approach for the screening of candidate crystal co-formers. The prediction of intermolecular synthons facilitated the successful synthesis of a new multicomponent crystal of metformin (Met) and barbital (Bar) through an anion exchange reaction and cooling crystallization method. The single crystal X-ray diffraction analysis demonstrated the hydrogen bond-based ureide/ureide and guanidine/ureide synthons were responsible for the self-assembly of the primary structural motif and extended into infinite supramolecular heterocatemeric structures.     

Herringbone and Helical Self-Assembly of π-Conjugated Molecules in the Solid State through CH/π Hydrogen Bonds

Self-organization of organic molecules through weak noncovalent forces such as CH/π interactions and creation of large hierarchical supramolecular structures in the solid state are at the very early stage of research. The present study reports direct evidence for CH/π interaction driven hierarchical self-assembly in π-conjugated molecules based on custom-designed oligophenylenevinylenes (OPVs) whose structures differ only in the number of carbon atoms in the tails. Single-crystal X-ray structures were resolved for these OPV synthons and the existence of long-range multiple-arm CH/π interactions was revealed in the crystal lattices. Alignment of these π-conjugated OPVs in the solid state was found to be crucial in producing either right-handed herringbone packing in the crystal or left-handed helices in the liquid-crystalline mesophase. Pitch- and roll-angle displacements of OPV chromophores were determined to trace the effect of the molecular inclination on the ordering of hierarchical structures. Furthermore, circular dichroism studies on the OPVs were carried out in the aligned helical structures to prove the existence of molecular self-assembly. Thus, the present strategy opens up new approaches in supramolecular chemistry based on weak CH/π hydrogen bonding, more specifically in π-conjugated materials.     

Crystal structure prediction of aminols: advantages of a supramolecular synthon approach with experimental structures

The supramolecular synthon approach to crystal structure prediction (CSP) takes into account the complexities inherent in crystallization. The synthon is a kinetically favored unit, and through analysis of commonly occurring synthons in a group of related compounds, kinetic factors are implicitly invoked. The working assumption is that while the experimental structure need not be at the global minimum, it will appear somewhere in a list of computationally generated structures so that it can be suitably identified and ranked upward using synthon information. These ideas are illustrated with a set of aminophenols, or aminols. In the first stage, a training database is created of the 10 isomeric methylaminophenols. The crystal structures of these compounds were determined. The prototypes 2-, 3-, and 4-aminophenols were also included in the training database. Small and large synthons in these 13 crystal structures were then identified. Small synthons are of high topological but low geometrical value and are used in negative screens to eliminate computationally derived structures that are chemically unreasonable. Large synthons are more restrictive geometrically and are used in positive screens ranking upward predicted structures that contain these more well-defined patterns. In the second stage, these screens are applied to CSP of nine new aminols carried out in 14 space groups. In each space group, up to 10 lowest energy structures were analyzed with respect to their synthon content. The results are encouraging, and the predictions were classified as good, unclear, or bad. Two predictions were verified with actual crystal structure determinations.     

A systematic study of ligand intermolecular interactions in crystals of copper(II) complexes of bidentate guanidino derivatives

The synthesis and X-ray structures of four neutral copper(II) complexes and one cationic complex incorporating bidentate alkyl N-(4-oxo-5,5-diphenyl-4,5-dihydro-1H-imidazol-2-yl)imidocarbamate ligands are reported. The neutral complexes, which possess potential doublet (DA) hydrogen bonding motifs, form supramolecular structures based on synthons involving hydrogen bonding or phenyl embraces. The formation of sheets within the crystal through combination of these synthons, and the occurrence of guest molecules trapped in cavities between the sheets, are described. The cationic complex forms an extended hydrogen-bonded structure that incorporates nitrate ions. The structures of the five complexes are compared with others reported previously for complexes of related ligands.     

A novel framework of N—H...Br hydrogen bonds forming Br(4,4′‐bipyridinium)4 supramolecular synthons: bis(4,4′‐bipyridinium) tris[tetrabromidoferrate(III)] bromide

In the asymmetric unit of the title compound, (C₁₀H₁₀N₂)₂[FeBr₄]₃Br, the Fe atoms are in a distorted tetrahedral environment. The crystal structure contains a novel arrangement of Br(4,4′‐bipyridinium)₄ supramolecular synthons assembled via short N—H...Br hydrogen bonds (H...Br = 2.55, 2.40, 2.38 and 2.55 Å), where four cations surround one nonbonded bromide ion in a tetrahedral arrangement. These synthons are further connected by hydrogen bonds using the remaining terminal NH hydrogens in each cation and the Br⁻ ions to form an adamantoid‐like network and thus produce a three‐dimensional supramolecular architecture with the [FeBr₄]⁻ ions located in the cavities. The structure shows no significant intermolecular Br...Br, Br...aryl or aryl–aryl interactions.     

Crystals at a Carrefour on the Way through the Phase Space: A Middle Path

Multiple supramolecular functionalities of cyclic α-alkoxy tellurium-trihalides (including Te---O, Te---X (X = Br, I) and Te---π(C=C) supramolecular synthons) afford rich crystal packing possibilities, which consequently results in polymorphism or Z’ > 1 crystal structures. Example of three crystal forms of cyclohexyl-ethoxy-tellurium-trihalides, one of which combines the packing of two others, affords a unique model to observe the supramolecular synthon evolution at the early stages of crystallization, when crystals on the way find themself at a carrefour between the evolutionally close routes, but fail to choose between two energetically close packing patterns, so taking the “middle path”, which incorporates both of them (and results in two crystallographically independent molecules). In general, this allows a better understanding of the existing structures, and an instrument to search for the new polymorphic forms.     

Secondary Bonding,C═H…O Hydrogen Bonding Assisted Supramolecular Associations and Charge Transfer (CT) Complexes of Organotelluriums and their Nonlinear Optical Properties

Abstract

A library of supramolecular assemblies of acyclic- and cyclic organotelluriums assisted by intermolecular Te… X (X = Cl, Br, I, O, S) secondary bonds has been synthesized and X-ray characterized. In each case the immediate coordination geometry around the central Te atom is pseudotrigonal bipyramidal in which two methylene carbon atoms (attached to Te) in cyclic organotelluriums and methyl carbon atoms in acyclic organotelluriums and the stereochemically active electron lone pair occupy equatorial positions whereas the axial positions are occupied by halogen, oxygen or sulphur. They exists either as (a) ordered oligomers (trimeric, tetrameric, octameric aggregates) (b) cross linked chains, (c) zig-zag ?2 dimensional ribbons and stairs, and (d) 3-dimensional supramolecular networks. It is observed that the supramolecular associations assisted by Te…O and Te…S secondary bonds are modified whereas those assisted by Te…halogen remain more or less the same vis-à-vis the supramolecular associations present in their precursors in the solid state. The first detection of C─H…O hydrogen bonds in organotellurium compounds has been done and their use in the synthesis of tellurium essential and ligand essential supramolecular assemblies is demonstrated. Tetraorganotelluroxanes obtained by easy and efficient routes represent the examples of cooperative participation of intermolecular and intramolecular Te…O secondary bonds and C─H…O hydrogen bonds. Hypervalent Te─I (formed through n → σ* orbital interactions) bonds in cyclic telluranes act as potential synthons for the formation of CT complexes possessing unusual structures. The utility of organotelluriums in the serendipitous synthesis of the first triphenyl methyl phosphonium salts of [C₄H₈TeI₄]^2? and [TeI₆]^2? anions is shown. The second harmonic generation (SHG) efficiency of some of these new supramolecular assemblies of organotelluriums indicates that the presence of C─H…O hydrogen bonds enhances their non linear optical (NLO) properties.     

Topological classification and supramolecular chirality of 21-helical ladder-type hydrogen-bond networks composed of primary ammonium carboxylates: bundle control in 21-helical assemblies

The supramolecular chirality of 1D ladder-type hydrogen-bond networks composed of primary ammonium carboxylates was determined based on topological considerations. Chirality in such networks is based on the absolute configuration of the primary ammonium cation, which arises from discrimination between the two oxygen atoms of the carboxylate anion. The configurations of the cations and anions generate topological diversity in the networks, which are classified into six subgroups. In the Cambridge Structural Database, salts based on ladder type 1 constitute over 70 % of salts with a 1D-ladder-type network. Ladder type 1, based on a 2(1)-axis, is not superimposable on its mirror image, which leads to the first definition of right- or left-handedness of 2(1)-helicity on the basis of supramolecular tilt chirality. Helical assemblies of 2(1)-type with triaxial chirality can be assembled in various ways to yield chiral bundles and crystals. On the basis of these considerations, we constructed clay mimic structures with several bundle patterns by connecting the hydrogen-bond networks by using bifunctional molecules. These results open up the possibility of in-depth crystal engineering based on hydrogen-bond topology.     

H-bonds superstructures built by aquacomplexes and an azapurine derivative: a case of molecular recognition

The crystal structures of six compounds involving the divalent cations of Mn, Zn and Cd, the anionic form of the heterocycle 4,6-dimethyl-1,2,3-triazolo[4,5-d]pyrimidin-5,7-dione (Hdmax) and bipyridyl based spacer ligands are reported. The most important feature of these structures is the presence, in all cases, of a topologically identical 1-D polymeric superstructure (tape), involving tetra- or hexaaqua complex cations and triazolopyrimidine anions, built through hydrogen bonds. Adding these results to others previously published, we may consider these tapes as robust supramolecular synthons where a defined/clear case of molecular recognition between these two moieties takes place.     

Crystal engineering for topochemical polymerization of muconic esters using halogen-halogen and CH/pi interactions as weak intermolecular interactions

We now report the molecular and crystal structure design of muconic ester derivatives on the basis of crystal engineering using halogen-halogen contacts and CH/pi interactions. The solid-state photoreaction pathway of the dibenzyl (Z,Z)-muconates as the 1,3-diene dicarboxylic acid monomers depends on the structure of the ester groups. The substitution of a halogen atom for the aromatic hydrogen of a benzyl group induces topochemical polymerization to produce stereoregular polymers in a crystalline form, whereas the unsubstituted benzyl derivative isomerizes to yield the corresponding E,E isomer under similar conditions. The topochemical polymerization process is directly confirmed by the fact that the single-crystal structures before and after the polymerization are very similar to each other. From the crystal structure analysis for a series of substituted benzyl (Z,Z)- and (E,E)-muconates, it has been revealed that the planar diene moieties are closely packed to form a columnar structure in the crystals. The stacking of the polymerizable monomers is characterized by a stacking distance of 4.9-5.2 A along the columns. This structure is supported by a halogen-halogen interaction between the chlorine or bromine atoms introduced at the p position of the benzyl groups in addition to an aromatic stacking due to the CH/pi interaction between the benzylic methylene hydrogens and aromatic rings. The design of a monomer packing corresponds to the type and position of the introduced halogen atom and also the polymorphs. To make a stacking distance of 5 A using both halogen-halogen and CH/pi interactions as supramolecular synthons is important for the molecular design of muconic ester derivatives appropriate for topochemical polymerization.     

Double Chalcogen Bonds: Crystal Engineering Stratagems via Diffraction and Multinuclear Solid-State Magnetic Resonance Spectroscopy

Group 16 chalcogens potentially provide Lewis-acidic σ-holes, which are able to form attractive supramolecular interactions with electron rich partners through chalcogen bonds. Here, a multifaceted experimental and computational study of a large series of novel chalcogen-bonded cocrystals, prepared using the principles of crystal engineering, is presented. Single-crystal X-ray diffraction studies reveal that dicyanoselenadiazole and dicyanotelluradiazole derivatives work as promising supramolecular synthons with the ability to form double chalcogen bonds with a wide range of electron donors including halides and oxygen- and nitrogen-containing heterocycles. Extensive ⁷⁷Se and ¹²⁵Te solid-state nuclear magnetic resonance spectroscopic investigations of cocrystals establish correlations between the NMR parameters of selenium and tellurium and the local chalcogen bonding geometry. The relationships between the electronic environment of the chalcogen bond and the ⁷⁷Se and ¹²⁵Te chemical shift tensors were elucidated through a natural localized molecular orbital density functional theory analysis. This systematic study of chalcogen-bond-based crystal engineering lays the foundations for the preparation of the various multicomponent systems and establishes solid-state NMR protocols to detect these interactions in powdered materials.     

Identification of an Overlooked Halogen-Bond Synthon and Its Application in Designing Fluorescent Materials

Research on new supramolecular synthons facilitates the progress of materials design. Herein, the ability of sp² carbonyl oxygen atoms to act as halogen-bond acceptors was established through cocrystallization. Four sets of carbonyl compounds, including aldehydes, ketones, esters, and amides, were selected as halogen-bond acceptors. In the absence of strong hydrogen bonds, 14 out of 16 combinations of halogen-bond donors and acceptors could form cocrystals, whereby the supramolecular synthon C=O ⋅⋅⋅ X acts as the main interaction. Further, the geometric parameters of the C=O ⋅⋅⋅ X interaction were statistically revealed on the basis of the crystallographic database. The bifurcated interaction mode that has been observed in other halogen-bond synthons rarely occurs in the case of C=O ⋅⋅⋅ X. The robustness of C=O ⋅⋅⋅ X makes its application in crystal engineering possible and opens up new opportunities in designing multicomponent fluorescent materials, as indicated by multicolor emission of cocrystals D through C=O ⋅⋅⋅ X interactions.     

On crystal versus fiber formation in dipeptide hydrogelator systems

Naphthalene dipeptides have been shown to be useful low-molecular-weight gelators. Here we have used a library to explore the relationship between the dipeptide sequence and the hydrogelation efficiency. A number of the naphthalene dipeptides are crystallizable from water, enabling us to investigate the comparison between the gel/fiber phase and the crystal phase. We succeeded in crystallizing one example directly from the gel phase. Using X-ray crystallography, molecular modeling, and X-ray fiber diffraction, we show that the molecular packing of this crystal structure differs from the structure of the gel/fiber phase. Although the crystal structures may provide important insights into stabilizing interactions, our analysis indicates a rearrangement of structural packing within the fibers. These observations are consistent with the fibrillar interactions and interatomic separations promoting 1D assembly whereas in the crystals the peptides are aligned along multiple axes, allowing 3D growth. This observation has an impact on the use of crystal structures to determine supramolecular synthons for gelators.     

Co-crystallization of glycine anhydride with the hydroxybenzoic acids: Controlled formation of dimers via synthons cooperation and structural characterization

The results of crystallographic analyses on 1:1 and 1:4 well-defined co-crystals formed between glycine anhydride and each of 4-hydroxybenzoic acid and 3,5-dihydroxybenzoic acid are described. Neutral molecules are connected via heteromeric O-H···O and N-H···O contacts leading to different packing arrangements of supramolecular chains. On the basis of the molecular structures of glycine anhydride and carboxylic acid guests, the hydrogen bonds are arranged to give centrosymmetric synthons V and VII which are noteworthy for their robustness. Hydrogen-bond interactions between glycine anhydride and aromatic acid provide sufficient driving force to direct molecular recognition and crystal packing. Utilization of the orientation of functional groups of the building blocks, the acidity, and weak interactions provides a route for the creation of novel supra- molecular architectures in the crystal lattice. Both two co-crystals contain the expected hydrogen-bonded motifs, and there has been no proton transfer from either of the two carboxylic acids to the aza compound moiety. This demonstrates that glycine anhydride is very capable of affecting the construction of binary co-crystals in a predictable and rationale manner. It is noted that synthons VⅢ and IX are fairly large, but the real challenge in crystal engineering is to find a big enough synthon that occurs often enough. Thermal stability of these compounds has been investigated by thermogravimetric analysis (TGA) of mass loss.     

Stereochemical activity of lone pairs of electrons and supramolecular aggregation patterns based on secondary interactions involving tellurium in its 1,1-dithiolate structures

A survey of the crystallographic literature of tellurium(II)/(IV) 1,1-dithiolates (dithiocarbamate, xanthate, dithiophosphate, or dithiophosphinate) is presented. Coordination numbers range from a low of three in some organotellurium(II) 1,1-dithiolates to a high of eight in the binary tellurium(IV) dithiocarbamates. The coordination geometries are rich and varied due to the stereochemical influence exerted by up to two lone pairs of electrons and the penchant of tellurium to increase its coordination number by forming secondary Te?X interactions, where X = sulphur, halide, tellurium, oxygen, and, in one case, a π system defined by a four-membered TeS₂C chelate. Stereochemical roles of the lone pairs of electrons are always evident in the tellurium(II) structures. By contrast, a stereochemical position is not always evident for the lone pair of electrons in the tellurium(IV) derivatives, in particular in circumstances where the tellurium centre has a high coordination number. Supramolecular aggregation mediated by Te?X secondary interactions often leads to the formation of dimeric aggregates but sometimes to supramolecular polymers, and rarely three-dimensional networks. Comparisons between closely related structures clearly indicate that the dithiocarbamate ligand is a more effective chelating ligand compared with the other 1,1-dithiolate ligands covered in this survey. This difference in coordinating ability is clearly correlated with the observation that non-dithiocarbamate structures are more likely to form high-dimensional supramolecular architectures.