Controllable Crystallization of Two-Dimensional Bi Nanocrystals with Morphology-Boosted CO2 Electroreduction in Wide pH Environments

Two-dimensional low-melting-point (LMP) metal nanocrystals are attracting increasing attention with broad and irreplaceable applications due to their unique surface and topological structures. However, the chemical synthesis, especially the fine control over the nucleation (reduction) and growth (crystallization), of such LMP metal nanocrystals remains elusive as limited by the challenges of low standard redox potential, low melting point, poor crystalline symmetry, etc. Here, a controllable reduction-melting-crystallization (RMC) protocol to synthesize free-standing and surfactant-free bismuth nanocrystals with tunable dimensions, morphologies, and surface structures is presented. Especially, ultrathin bismuth nanosheets with flat or jagged surfaces/edges can be prepared with high selectivity. The jagged bismuth nanosheets, with abundant surface steps and defects, exhibit boosted electrocatalytic CO2 reduction performances in acidic, neutral, and alkaline aqueous solutions, achieving the maximum selectivity of near unity at the current density of 210 mA cm-2 for formate evolution under ambient conditions. This work creates the RMC pathway for the synthesis of free-standing two-dimensional LMP metal nanomaterials and may find broader applicability in more interdisciplinary applications.

Baloxavir Marboxil Shows Anomalous Conversion of Crystal Forms from Stable to Metastable through Formation of Specific Solvate Form

Baloxavir marboxil is a novel cap-dependent endonuclease inhibitor of influenza. This study aimed to identify its polymorphs and their relationship with crystal engineering. Polymorph screening by evaporation gave forms I-III and solvate forms IV and V. Heating enabled the conversion of form III to form II, but did not enable that of forms I and II. The solvent-mediated transformation of the forms I-III by magnetic stirring in various solvents resulted in the formation of form I. These results indicate that form I is the stable form. However, all crystal forms transformed to form II after magnetic stirring in a 50% acetonitrile aqueous solution, which was not obtained from water or acetonitrile. The suspension in a 50% acetonitrile aqueous solution exhibited a novel X-ray diffraction pattern as shown in form VI. The measurement of the suspension by solid-state ¹³C-nuclear magnetic resonance revealed that the spectra of forms II and VI were similar. From these results, we conclude that the drug forms a solvate with both water and acetonitrile and spontaneously transforms to form II upon rapid desolvation under ambient conditions. This study elucidates the mechanism of unexpected convergence to a metastable form in a specific solvent and contributes to the crystal engineering of baloxavir marboxil.

Differences in Coformer Interactions of the 2,4-Diaminopyrimidines Pyrimethamine and Trimethoprim

The identification and study of supramolecular synthons is a fundamental task in the design of pharmaceutical cocrystals. The malaria drug pyrimethamine (pyr) and the antibiotic trimethoprim (tmp) are both 2,4-diaminopyrimidine derivatives, providing the same C-NH2/N=C/C-NH2 and C-NH2/N=C interaction sites. In this article, we analyze and compare the synthons observed in the crystal structures of tmp and pyr cocrystals and molecular salts with sulfamethazine (smz), α-ketoglutaric acid (keto), oxalic acid (ox), sebacic acid (seb), and azeliac acid (az). We show that the same coformer interacts with different binding sites of the 2,4-diaminopyrimidine ring in the respective tmp and pyr cocrystals or binds at the same site but gives H bonding patterns with different graph set notions. Pyr·smz·CH3OH is the first crystal structure in which the interaction of the sulfa drug at the C-NH2/N=C/C-NH2 site with three parallel NH2···N, N···NHsulfonamide, and NH2···O=S H bonds is observed. The main synthon in (tmp+)(keto-).0.5H2O and (tmp+)2(ox2-)·2CH3OH is the motif of fused R 2 1(6) and R 1 2(5) rings instead of the R 2 2(8) motif typically observed in tmp+ and pyr+ carboxylates. Tmp/az is a rare example of cocrystal-salt polymorphism where the two solid-state forms have the same composition, stoichiometry, and main synthon. Theoretical calculations were performed to understand the order of stability, which is tmp·az cocrystal > (tmp+)(az-) salt. Finally, two three-component tmp/sulfa drug/carboxylate cocrystals with a unique ternary synthon are described.

Importance of Nuclear Quantum Effects for Molecular Cocrystals with Short Hydrogen Bonds

Many efforts have been recently devoted to the design and investigation of multicomponent pharmaceutical solids, such as salts and cocrystals. The experimental distinction between these solid forms is often challenging. Here, we show that the transformation of a salt into a cocrystal with a short hydrogen bond does not occur as a sharp phase transition but rather a smooth shift of the positional probability of the hydrogen atoms. A combination of solid-state NMR spectroscopy, X-ray diffraction, and diffuse reflectance measurements with density functional theory calculations that include nuclear quantum effects (NQEs) provides evidence of temperature-induced hydrogen atom shift in cocrystals with short hydrogen bonds. We demonstrate that for the predictions of the salt/cocrystal solid forms with short H-bonds, the computations have to include NQEs (particularly hydrogen nuclei delocalization) and temperature effects.

Harnessing Noncovalent Interactions for a Directed Evolution of a Six-Component Molecular Crystal

Building up on weak orthogonal interactions in supramolecular chemistry, a six-component crystal is designed. Using five distinctly different noncovalent forces, namely, hydrogen bonding, halogen bonding, cation-π, anion-π, and ion-pair interactions, three six-component crystals were designed with crown-ether (I), thiourea (II), 2,3,5,6-tetrafluoro-1,4-dibromobenzene (III), lone-pair donating anion (IV), ammonium cation (V), and electron-rich aromatic ring (VI). The M06-2X functional which is highly suitable in describing other weak interactions fails for ion-pairs. Tuned range-separated (RS)-DFT calculations are found to be capable in describing the ionic interactions in molecular solids. Molecular dynamics simulations show that the predicted multicomponent crystals are stable at room temperature and reducing the ionic charges for the ion-pairs destabilizes them. The strong electrostatic interactions between the three ion-pairs, NH₄⁺···ClO₄^-, NH₄⁺···HSO₄^-, and NH₄⁺···HCO₃^- is the primary driving force for the stabilization of the six-component crystal. Using a hybrid of strong and weak intermolecular interactions, one may generate exotic molecular complexity like n-component crystals.

Unexpected Salt/Cocrystal Polymorphism of the Ketoprofen-Lysine System: Discovery of a New Ketoprofen-l-Lysine Salt Polymorph with Different Physicochemical and Pharmacokinetic Properties

Ketoprofen–l-lysine salt (KLS) is a widely used nonsteroidal anti-inflammatory drug. Here, we studied deeply the solid-state characteristics of KLS to possibly identify new polymorphic drugs. Conducting a polymorph screening study and combining conventional techniques with solid-state nuclear magnetic resonance, we identified, for the first time, a salt/cocrystal polymorphism of the ketoprofen (KET)–lysine (LYS) system, with the cocrystal, KET–LYS polymorph 1 (P1), being representative of commercial KLS, and the salt, KET–LYS polymorph 2 (P2), being a new polymorphic form of KLS. Interestingly, in vivo pharmacokinetics showed that the salt polymorph has significantly higher absorption and, thus, different pharmacokinetics compared to commercial KLS (cocrystal), laying the basis for the development of faster-release/acting KLS formulations. Moreover, intrinsic dissolution rate (IDR) and electronic tongue analyses showed that the salt has a higher IDR, a more bitter taste, and a different sensorial kinetics compared to the cocrystal, suggesting that different coating/flavoring processes should be envisioned for the new compound. Thus, the new KLS polymorphic form with its different physicochemical and pharmacokinetic characteristics can open the way to the development of a new KET–LYS polymorph drug that can emphasize the properties of commercial KLS for the treatment of acute inflammatory and painful conditions.

Mechanochemistry: A Green Approach in the Preparation of Pharmaceutical Cocrystals

Mechanochemistry is considered an alternative attractive greener approach to prepare diverse molecular compounds and has become an important synthetic tool in different fields (e.g., physics, chemistry, and material science) since is considered an ecofriendly procedure that can be carried out under solvent free conditions or in the presence of minimal quantities of solvent (catalytic amounts). Being able to substitute, in many cases, classical solution reactions often requiring significant amounts of solvents. These sustainable methods have had an enormous impact on a great variety of chemistry fields, including catalysis, organic synthesis, metal complexes formation, preparation of multicomponent pharmaceutical solid forms, etc. In this sense, we are interested in highlighting the advantages of mechanochemical methods on the obtaining of pharmaceutical cocrystals. Hence, in this review, we describe and discuss the relevance of mechanochemical procedures in the formation of multicomponent solid forms focusing on pharmaceutical cocrystals. Additionally, at the end of this paper, we collect a chronological survey of the most representative scientific papers reporting the mechanochemical synthesis of cocrystals.

A new monohydrated molecular salt of GABA with l-tartaric acid: the structure-forming role of water

A novel monohydrated molecular salt of GABA with l-tartaric acid was crystallized and investigated.

Crystallisation of organic salts by sublimation: salt formation from the gas phase

Co-sublimation of two neutral components yields crystals of salts and co-crystals. Experiments show that during sublimation of salts, proton transfer occurs after molecules enter the gas phase.

Robust bulk preparation and characterization of sulfamethazine and saccharine salt and cocrystal polymorphs

The complex between sulfamethazine and saccharine (SMT–SAC) can exist in two polymorphs, one is a cocrystal and the other is a salt.

Combining two distinctive intermolecular forces in designing ternary co-crystals and molecular salts of 1,3,5-trinitrobenzene, 9-anthracenecarboxylic acid and ten substituted pyridines

Coloured three component complexes are made using both charge transfer and hydrogen bonding intermolecular interactions.

Crystal structure of a 1:1 salt of 4-amino-benzoic acid (vitamin B10) with pyrazinoic acid

The title 1:1 salt, C7H8NO2 +·C5H3N2O2 - (systematic name: 4-carb-oxy-anilinium pyrazine-2-carboxyl-ate), was synthesized successfully by slow evaporation of a saturated solution from water-ethanol (1:1 v/v) mixture and characterized by X-ray diffraction (SCXRD, PXRD) and calorimetry (DSC). The crystal structure of the salt was solved and refined at 150 and 293 K. The salt crystallizes with one mol-ecule of 4-amino-benzoic acid (PABA) and one mol-ecule of pyrazinoic acid (POA) in the asymmetric unit. In the crystal, the PABA and POA mol-ecules are associated via COOH⋯Narom heterosynthons, which are connected by N-H⋯O hydrogen bonds, creating zigzag chains. The chains are further linked by N-H⋯O hydrogen bonds and π-π stacking inter-actions along the b axis [centroid-to-centroid distances = 3.7377 (13) and 3.8034 (13) Å at 150 and 293 K, respectively] to form a layered three-dimensional structure.