Co-crystallization of an organic solid and a tetraaryladamantane at room temperature

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.

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.

Benchmarking Guanidinium Organosulfonate Hydrogen-Bonded Frameworks for Structure Determination of Encapsulated Guests

Single crystal X-ray diffraction (SCXRD) is arguably the most definitive method for molecular structure determination, but it is often challenged by compounds that are liquids or oils at room temperature or do not form crystals adequate for analysis. Our laboratory previously reported a simple, cost-effective, single-step crystallization method based on guanidinium organosulfonate (GS) hydrogen bonded frameworks for structure determination of a wide range of encapsulated guest molecules, including assignment of the absolute configuration of chiral centers. Herein, we expand on those results and report a head-to-head comparison of the GS method with adamantoid "molecular chaperones", which have been reported to be useful hosts for structure determination. Inclusion compounds limited to only two GS hosts are characterized by low R1 values and Flack parameters, infrequent disorder of the host and guest, and manageable disorder when it does exist. The structures of some target molecules that were not included or resolved using the adamantoid chaperones were successfully included and resolved by the GS hosts, and vice versa. Of the 32 guests attempted by the GS method, 31 inclusion compounds afforded successful guest structure solutions, a 97% success rate. The GS hosts and adamantoid chaperones are complementary with respect to guest inclusion, arguing that both should be employed in the arsenal of methods for structure determination. Furthermore, the low cost of organosulfonate host components promises an accessible route to molecular structure determination for a wide range of users.

Advanced crystallisation methods for small organic molecules

Molecular materials based on small organic molecules often require advanced structural analysis, beyond the capability of spectroscopic techniques, to fully characterise them. In such cases, diffraction methods such as single crystal X-ray diffraction (SCXRD), are one of the most powerful tools available to researchers, providing molecular and structural elucidation at atomic level resolution, including absolute stereochemistry. However SCXRD, and related diffraction methods, are heavily dependent on the availability of suitable, high-quality crystals, thus crystallisation often becomes the major bottleneck in preparing samples. Following a summary of classical methods for the crystallisation of small organic molecules, this review will focus on a number of recently developed advanced methods for crystalline material sample preparation for SCXRD. This review will cover two main areas of modern small organic molecule crystallisation, namely the inclusion of molecules within host complexes (e.g., "crystalline sponge" and tetraaryladamantane based inclusion chaperones) and the use of high-throughput crystallisation, employing "under-oil" approaches (e.g., microbatch under-oil and ENaCt). Representative examples have been included for each technique, together with a discussion of their relative advantages and limitations to aid the reader in selecting the most appropriate technique to overcome a specific analytical challenge.

Structure and absolute configuration of liquid molecules based on adamantane derivative cocrystallization

Liquid molecules are difficult to crystallize, and their structures and absolute configurations cannot be directly determined by X-ray crystallography. We herein report the rapid cocrystallization of tetraaryladamantanes with liquid molecules. The structure of the liquid small molecules can be obtained by determining the crystal structure of the cocrystallized compound. The absolute configuration of chiral molecules can also be assigned, which cannot be accomplished by other methods such as nuclear magnetic resonance. In this paper, liquid compounds such as phenylethanol and phenylpropanol derivatives were selected. 1,3,5,7-Tetrakis(2,4-diethoxyphenyl)adamantane (TEO) powder was heated and dissolved in liquid molecules and allowed to stand overnight to undergo cocrystallization. The results show that the single-crystal structures and the absolute configurations of 16 liquid molecules were determined by cocrystallization, and the homochiral natures of chiral compounds were confirmed by solid circular dichroism spectral measurements.

Liquid molecules are difficult to crystallize, and their structures and absolute configurations cannot be directly determined by X-ray crystallography.