Acridinylresorcinol as a self-complementary building block of robust hydrogen-bonded 2D nets with coordinative saturation. Preservation of crystal structures upon guest alteration, guest removal, and host modification

Solvophobicity-directed assembly of microporous molecular crystals

Dense packing is a universal tendency of organic molecules in the solid state. Typical porous crystals utilize reticular strong intermolecular bonding networks to overcome this principle. Here, we report a solvophobicity-based methodology for assembling discrete molecules into a porous form and succeed in synthesizing isostructural porous polymorphs of an amphiphilic aromatic molecule Py₆Mes. A computational analysis of the crystal structure reveals the major contribution of dispersion interaction as the driving force for assembling Py₆Mes into a columnar stacking while the columns are sterically salient and form nanopores between them. The porous packing is facilitated particularly in solvents with weak dispersion interaction due to the solvophobic effect. Conversely, solvents with strong dispersion interaction intercalate between Py₆Mes due to the solvophilic effect and provide non-porous inclusion crystals. The solvophobicity-directed polymorphism is further corroborated by the polymorphs of Py₆Mes-analogues, m-Py₆Mes and Ph₆Mes.

Ad Hoc Adjustment of Photoredox Properties by the Late-Stage Diversification of Acridinium Photocatalysts

The steadily growing interest in substituting precious-metal photoredox catalysts with organic surrogates is vibrantly sustained by emerging methodologies to vary their photochemical behavior. Herein, we report an ad hoc approach for the preparation of acridinium salts with a particularly broad range of photoredox properties. The method involves an aryne-imine-aryne coupling to a linchpin tetrafluoro acridinium salt for a late-stage diversification by nucleophilic aromatic substitution reactions to form diaminoacridinium and undescribed aza-rhodol photocatalysts. The different functionalities and redox properties of the organic acridinium photocatalysts render them suitable for bifunctional photoredox catalysis and organocatalytic photochemical C-N cross-couplings.

Formation of sandwich, macrocyclic and box supramolecular assemblies that were controlled by the distance of two oxygen atoms in hydrogen bonding donors

Sandwich, macrocyclic and box supramolecular assemblies were synthesized. They can further self assemble to form a double-layer supramolecular polymer, nanotubes and one-dimensional “iron chain type” supramolecular polymer.

An Expandable Hydrogen-Bonded Organic Framework Characterized by Three-Dimensional Electron Diffraction

A molecular crystal of a 2-D hydrogen-bonded organic framework (HOF) undergoes an unusual structural transformation after solvent removal from the crystal pores during activation. The conformationally flexible host molecule, ABTPA, adapts its molecular conformation during activation to initiate a framework expansion. The microcrystalline activated phase was characterized by three-dimensional electron diffraction (3D ED), which revealed that ABTPA uses out-of-plane anthracene units as adaptive structural anchors. These units change orientation to generate an expanded, lower density framework material in the activated structure. The porous HOF, ABTPA-2, has robust dynamic porosity (SA_BET = 1183 m² g^-1) and exhibits negative area thermal expansion. We use crystal structure prediction (CSP) to understand the underlying energetics behind the structural transformation and discuss the challenges facing CSP for such flexible molecules.

A H-shaped heterometallic Sn4Au4 system with guest-tuneable multicolour and selective luminescence sensing properties

A H-shaped heterometallic Sn₄Au₄ system displays guest-dependent optical properties and its stimuli-responsive methylred-rich crystals are sensitive towards pH variations.

Pharmaceutical cocrystals: along the path to improved medicines

Cocrystals, a long known but understudied class of crystalline solids, have attracted interest from crystal engineers and pharmaceutical scientists in the past decade and are now an integral part of the preformulation stage of drug development.

Crystal-to-crystal structural transformation of hydrogen-bonding molecular crystals of (imidazolium)(3-hydroxy-2-quinoxalinecarboxylate) through H2O adsorption–desorption

Crystal-to-crystal structural transformation was observed following H₂O adsorption–desorption of hydrogen-bonding molecular crystals.

Synthesis, Structures, and Conformations of Linked Bis-Glyoxylamides Derived from Bis-Acylisatins

A series of bis-glyoxylamides possessing hydrophobic alkyl chains was successfully synthesised by ring opening of bis-acylisatins with amines or amino acid alkyl esters. The crystal structures revealed the interplay of intra- and intermolecular interactions (NH···O and C=O···C=O interactions) and the conformations of these long molecules.

Pyrazolo[3,4-d]pyrimidines as inhibitor of anti-coagulation and inflammation activities of phospholipase A 2 : insight from molecular docking studies

Phospholipase A2 (PLA2), isolated from Daboia russelli pulchella (Russell's viper), is enzymatically active as well as induces several pharmacological disorders including neurotoxicity, myotoxicity, cardiotoxicity, anti-coagulant, hemolytic, and platelet effects. Indomethacin reduces the effects of anti-coagulant and pro-inflammatory actions of PLA2. Pyrazolo[3,4-d]pyrimidines constitute a class of naturally occurring fused uracils that posses diverse biological activities. The in-silico docking studies of nine pyrazolo[3,4-d]pyrimidine molecules have been carried out with the X-ray crystal structure of Russell's viper PLA2 (PDB ID: 3H1X) to predict the binding affinity, molecular recognition, and to explicate the binding modes, using AUTODOCK and GLIDE (Standard precision and Extra precision) modules, respectively. Docking results through each method make obvious that pyrazolo[3,4-d]pyrimidine molecules with trimethylene linker can bind with both anti-coagulation and enzymatic regions of PLA2.

1,5-Bis[(2-meth-oxy-eth-oxy)meth-yl]-1,5-naphthyridine-4,8(1H,5H)-dione

The complete mol-ecule of the title compound, C(16)H(22)N(2)O(6), is generated by crystallographic inversion symmetry. The conformation of the N-C-O-C fragment of the side chain is approximately gauche [torsion angle = -74.84 (17)°]. In the crystal, weak C-H⋯O inter-actions link the mol-ecules.

Soft porous crystals

The field of host-guest complexation is intensely attractive from diverse perspectives, including materials science, chemistry and biology. The uptake and encapsulation of guest species by host frameworks are being investigated for a wide variety of purposes, including separation and storage using zeolites, and recognition and sensing by enzymes in solution. Here we focus on the concept of the cooperative integration of 'softness' and 'regularity'. Recent developments on porous coordination polymers (or metal-organic frameworks) have provided the inherent properties that combine these features. Such soft porous crystals exhibit dynamic frameworks that are able to respond to external stimuli such as light, electric fields or the presence of particular species, but they are also crystalline and can change their channels reversibly while retaining high regularity. We discuss the relationship between the structures and properties of these materials in view of their practical applications.

Nitrogen and hydrogen adsorption by an organic microporous crystal

Quick on the uptake: Following its identification during a targeted search, the intriguing crystal structure of 3,3',4,4'-tetra(trimethylsilylethynyl)biphenyl was investigated. Simple removal of the included solvent provides an organic crystal with an open microporous structure that has a striking similarity to that of zeolite A (see picture). Reversible adsorption of nitrogen and hydrogen gases at 77 K confirms that the microporosity is permanent.

Ultralong carbon-carbon bonds in dispirobis(10-methylacridan) derivatives with an acenaphthene, pyracene, or dihydropyracylene skeleton

Acenapthalene, pyracene, and dihydropyracylene attached to two units of spiroacridan are a novel class of hexaphenylethane (HPE) derivatives that have an ultralong Csp3-Csp3 bond (1.77-1.70 A). These sterically challenged molecules were cleanly prepared by C-C bond formation through two-electron reduction from the less-hindered dications. These ultralong bonds were realized based on several molecular-design concepts including enhanced "front strain" through "multiclamping" by means of fusing or bridging aryl groups in the HPE molecule. The lengths of these ultralong bonds and their relation to the conformation (torsional angle) were also validated by means of theoretical calculations. Bond-fission experiments revealed that the bonds are more easily cleaved than standard covalent bonds to produce the corresponding dication upon oxidation with an increase in the length of the C-C bond.

Challenges and breakthroughs in recent research on self-assembly

The controlled fabrication of nanometer-scale objects is without doubt one of the central issues in current science and technology. However, existing fabrication techniques suffer from several disadvantages including size-restrictions and a general paucity of applicable materials. Because of this, the development of alternative approaches based on supramolecular self-assembly processes is anticipated as a breakthrough methodology. This review article aims to comprehensively summarize the salient aspects of self-assembly through the introduction of the recent challenges and breakthroughs in three categories: (i) types of self-assembly in bulk media; (ii) types of components for self-assembly in bulk media; and (iii) self-assembly at interfaces.

Two-Component Supramolecular Helical Architectures: Creation of Tunable Dissymmetric Cavities for the Inclusion and Chiral Recognition of the Third Components

The inclusion and chiral recognition of racemic arylalkanols by supramolecular helical architectures consisting of enantiopure primary amines and achiral carboxylic acids were thoroughly studied. Among the architectures examined, a supramolecular helical architecture composed of the salt of enantiopure erythro-2-amino-1,2-diphenylethanol (1 b) and benzoic acid (2 a) was found to include a wide variety of racemic arylalkanols with recognition of their chirality. The helical architecture gave a dissymmetric 1D groove in the salt crystal, and the arylalkanols were enantioselectively included in the groove. The size and shape of the groove were tunable by proper selection of the achiral carboxylic acid component. The origin of the chiral recognition with the combination 1 b/2 a is discussed on the basis of X-ray crystallographic analyses.

Three-Dimensional Six-Connecting Organic Building Blocks Based on Polychlorotriphenylmethyl Units—Synthesis, Self-Assembly, and Magnetic Properties

The synthesis of a three-dimensional, six-connecting, organic building block based on a robust, rigid, and open-shell polychlorotriphenylmethyl (PTM) unit (radical 1) is reported, and its self-assembly properties are described in detail. The tendencies of this highly polar molecule and its hydrogenated precursor, compound 4, to form hydrogen bonds with oxygenated solvents ([1THF(6)] and [4THF(6)]) were reduced by replacing THF with diethyl ether in the crystallization process to yield two-dimensional (2D) hydrogen-bonded structures ([1(Et(2)O)(3)] and [4(Et(2)O)(3)]). The presence of direct hydrogen bonds between the radicals in the latter phase of 1 gives rise to very weak ferromagnetic intermolecular interactions at low temperatures, whereas when the radicals are isolated by THF molecules these interactions are antiferromagnetic and very weak. The role played by the carboxylic groups not only in the self-assembly properties but also in the transmission of the magnetic interactions has been illustrated by determination of the crystal structure and measurement of the magnetic properties of the corresponding hexaester radical 6, in which the close packing of molecular units gives rise to weak antiferromagnetic intermolecular interactions. Attempts to avoid solvation of the molecules in the solid state and to increase the structural and magnetic dimensionality were pursued by recrystallization of both compounds 1 and 4 from concentrated nitric acid, affording two three-dimensional (3D) robust hydrogen-bonded structures. While the structure obtained with compound 4 is characterized by the presence of polar channels and boxes containing water guest molecules along the c axis, radical 1 was oxidized to the corresponding fuchsone 10, which presented a completely different close-packed, guest-free structure.