Advanced crystallisation methods for small organic molecules

Host-Guest Cocrystallization of Phenanthrene[2]arene Macrocycles Facilitating Structure Determination of Liquid Organic Molecules

Single-crystal X-ray diffraction analysis has emerged as the most reliable method for determining the structures of organic molecules. However, numerous analytes, such as liquid organic molecules, pose challenges in crystallization, making their structures directly elusive via X-ray crystallography methods. Herein, we introduced the rapid cocrystallization of a macrocycle named phenanthrene[2]arene (PTA, host) with 15 liquid organic molecules (guests). The guest liquid organic molecules were successively cocrystallized with the aid of the PTA host. Moreover, the chemical structures of the liquid organic molecules could be determined through single-crystal X-ray diffraction analysis. PTA exhibited high adaptivity and was capable of encapsulating liquid organic molecules without forming covalent bonds or strong directional interactions. The results revealed that the adaptive crystals of PTA exhibited excellent cocrystallization capacity. Weak noncovalent interactions between the host and guest molecules were crucial for organizing the guests in an ordered pattern.

A Fluorinated Chaperone Gives X-ray Crystal Structures of Acyclic Natural Product Derivatives up to 338 Molecular Weight

Crystallizing molecules with long flexible chains is a challenge, making it difficult to perform X-ray crystallography. Chaperones can assist in the crystallization of compounds that do not crystallize by themselves by producing solvate crystals that contain the analyte in their three-dimensional lattices. Among the most versatile chaperones for liquid analytes are tetraaryladamantanes (TAAs), but the size of the compounds that can be encapsulated is limited, and attempts to surpass this limit with known TAAs were unsuccessful. Here we report that 1,3,5,7-tetrakis(2-fluoro-4-methoxyphenyl)adamantane (TFM) is a crystallization chaperone for acyclic molecules up to the molecular weight of phytyl acetate (338 g/mol). Encapsulation of such a large acyclic compound was achieved when the analyte was esterified and when a two-step temperature protocol was used for crystallization. Exploratory work indicates that a drop to -20 °C allows for encapsulation of squalene (Mr 411 g/mol), albeit with positional disorder of the analyte. Our X-ray crystal structures of solvates with flexible analytes shed light on how crystalline order can be imposed on large acyclic analytes. The new, fluorinated TAA gives access to crystal structures that were inaccessible thus far.

High-throughput nanoscale crystallization of dihydropyridine active pharmaceutical ingredients

Single-crystal X-ray diffraction analysis of small molecule active pharmaceutical ingredients is a key technique in the confirmation of molecular connectivity, including absolute stereochemistry, as well as the solid-state form. However, accessing single crystals suitable for X-ray diffraction analysis of an active pharmaceutical ingredient can be experimentally laborious, especially considering the potential for multiple solid-state forms (solvates, hydrates and polymorphs). In recent years, methods for the exploration of experimental crystallization space of small molecules have undergone a `step-change', resulting in new high-throughput techniques becoming available. Here, the application of high-throughput encapsulated nanodroplet crystallization to a series of six dihydropyridines, calcium channel blockers used in the treatment of hypertension related diseases, is described. This approach allowed 288 individual crystallization experiments to be performed in parallel on each molecule, resulting in rapid access to crystals and subsequent crystal structures for all six dihydropyridines, as well as revealing a new solvate polymorph of nifedipine (1,4-dioxane solvate) and the first known solvate of nimodipine (DMSO solvate). This work further demonstrates the power of modern high-throughput crystallization methods in the exploration of the solid-state landscape of active pharmaceutical ingredients to facilitate crystal form discovery and structural analysis by single-crystal X-ray diffraction.

Pyridine 3,5-dicarboxylate-based metal-organic frameworks as an active electrode material for battery-supercapacitor hybrid energy storage devices

Efficient energy storage and conversion is crucial for a sustainable society. Battery-supercapacitor hybrid energy storage devices offer a promising solution, bridging the gap between traditional batteries and supercapacitors. In this regard, metal-organic frameworks (MOFs) have emerged as the most versatile functional compounds owing to their captivating structural features, unique properties, and extensive diversity of applications in energy storage. MOF properties are governed by the structure and topological characteristics, which are influenced by the types of ligands and metal nodes. Herein, MOFs based on pyridine 3,5-dicarboxylate (PYDC) ligand in combination with copper and cobalt are electrochemically analyzed. Owing to the promising initial characterization of Cu-PYDC-MOF, a battery supercapacitor hybrid device was fabricated, comprising Cu-PYDC-MOF and activated carbon (AC) electrodes. The device showcased energy and power density of 17 W h kg -1 and 2550 W kg -1, respectively. Dunn's model was employed to gain deeper insights into the capacitive and diffusive contributions of the device. With their performance and versatility, the PYDC-based MOFs stand at the forefront of energy technology, ready to power a brighter future for upcoming generations.

Carbon Nitrides from Supramolecular Crystals: From Single Atoms to Heterojunctions and Advanced Photoelectrodes

Carbon nitride materials (CN) have become one of the most studied photocatalysts within the last 15 years. While CN absorbs visible light, its low porosity and fast electron-hole recombination hinder its photoelectric performance and have motivated the research in the modification of its physical and chemical properties (such as energy band structure, porosity, or chemical composition) by different means. In this Concept we review the utilization of supramolecular crystals as CN precursors to tailor its properties. We elaborate on the features needed in a supramolecular crystal to serve as CN precursor, we delve on the influence of metal-free crystals in the morphology and porosity of the resulting materials and then discuss the formation of single atoms and heterojunctions when employing a metal-organic crystal. We finally discuss the performance of CN photoanodes derived from crystals and highlight the current standing challenges in the field.

Effect of Extra-Framework Anion Substitution on the Properties of a Chiral Crystalline Sponge

Chiral metal-organic materials, CMOMs, are of interest as they can offer selective binding sites for chiral guests. Such binding sites can enable CMOMs to serve as chiral crystalline sponges (CCSs) to determine molecular structure and/or purify enantiomers. We recently reported on the chiral recognition properties of a homochiral cationic diamondoid, dia, network {[Ni(S-IDEC)(bipy)(H2O)][NO3]}n (S-IDEC = S-indoline-2-carboxylicate, bipy = 4,4'-bipyridine), CMOM-5[NO3]. The modularity of CMOM-5[NO3] means there are five feasible approaches to fine-tune structures and properties via substitution of one or more of the following components: metal cation (Ni2+); bridging ligand (S-IDEC); linker (bipy); extra-framework anion (NO3-); and terminal ligand (H2O). Herein, we report the effect of anion substitution on the CCS properties of CMOM-5[NO3] by preparing and characterizing {[Ni(S-IDEC)(bipy)(H2O)][BF4]}n, CMOM-5[BF4]. The chiral channels in CMOM-5[BF4] enabled it to function as a CCS for determination of the absolute crystal structures of both enantiomers of three chiral compounds: 1-phenyl-1-butanol (1P1B); methyl mandelate (MM); ethyl mandelate (EM). Chiral resolution experiments revealed CMOM-5[BF4] to be highly selective toward the S-isomers of MM and EM with enantiomeric excess, ee, values of 82.6 and 78.4%, respectively. The ee measured for S-EM surpasses the 64.3% exhibited by [DyNaL(H2O)4] 6H2O and far exceeds that of CMOM-5[NO3] (6.0%). Structural studies of the binding sites in CMOM-5[BF4] provide insight into their high enantioselectivity.

Host-guest system of a phosphorylated macrocycle assisting structure determination of oily molecules in single-crystal form

X-ray crystallography is the most reliable method for structure elucidation and absolute configuration determination of organic molecules based on their single-crystal forms. However, many analytes are hard to crystallize because of their low melting points (an oily state at room temperature) or conformational flexibility. Here, we report the crystallization of a macrocycle, CTX[P(O)Ph] (host), which is a cyclotrixylohydroquinoylene (CTX) derivative, with 26 oily organic molecules (guests), which is applied for the structural determination of the guest with X-ray crystallography. With the aid of the host, CTX[P(O)Ph], the guest molecules were well-ordered with full occupancy in crystal structures. In most cases, at least one guest structure without any disorder could be observed; solvent masking was not necessary for the single crystal X-ray structural analysis, and thus the structures of the guests could be successfully determined, and the absolute configuration could be assigned reliably for chiral guests with this method. The crystallization mechanism was further discussed from theoretical and experimental perspectives, suggesting that the negative electrostatic potential surface of CTX[P(O)Ph] and noncovalent interactions between the host and guest were crucial for the ordered arrangements of the guest.

Rapid single crystal growth via guest displacement from host-guest complexes

The preparation of single crystals of sparingly soluble polycyclic aromatic compounds in MeCN is facilitated by solubilizing ionic host-guest complexation under otherwise poor solvent conditions. The guest is then crystallized within minutes through controlled guest displacement from the host via competitor equilibria, or over days through direct crystallization of the host-guest complex itself.

Hydrogen atoms in supramolecular chemistry: a structural perspective. Where are they, and why does it matter?

Hydrogen bonding interactions are ubiquitous across the biochemical and chemical sciences, and are of particular interest to supramolecular chemists. They have been used to assemble hydrogen bonded polymers, cages and frameworks, and are the functional motif in many host-guest systems. Single crystal X-ray diffraction studies are often used as a key support for proposed structures, although this presents challenges as hydrogen atoms interact only weakly with X-rays. In this Tutorial Review, we discuss the information that can be gleaned about hydrogen bonding interactions through crystallographic experiments, key limitations of the data, and emerging techniques to overcome these limitations.

A systematic study of the interplay between guest molecule structure and intermolecular interactions in crystalline sponges

Utilization of the crystalline sponge {[(ZnI2)3(tpt)2·x(solvent)]n} method has enabled characterization of a novel family of synthetic organic oils. The systematic structural differences and diversity of functional groups offered by 13 related molecular adsorbates provide a detailed quantitative understanding of the relationship between the guest structure, its conformation, and the type of intermolecular interactions adopted with neighbouring guests and the host framework. This analysis is extended to assess the connection of these factors to the resulting quality indicators for a particular molecular structure elucidation.