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Benexate, a drug used clinically as a defensive type anti-ulcer agent, has poor solubility and a bitter taste. To improve its solubility, a crystal engineering approach was proposed with the formation of novel salts using an artificial sweetener as a salt co-former. This was also expected to address the bitter taste of the drug. In this work, we report on the preparation and evaluation of the physicochemical properties of the novel salts benexate saccharinate monohydrate and benexate cyclamate whose crystal structures were determined by single-crystal X-ray structure analysis. These novel salts showed higher solubility and faster dissolution profiles that were associated with the occurrence of local layered-like structures. They also showed better moisture uptake profiles and were classified as non-hygroscopic materials. Therefore, benexate saccharinate monohydrate and benexate cyclamate expedited the development of sweet pharmaceutical salts of benexate with improved performances.

In the coming years, multipurpose catalysts for delivering different products under the same chemical condition will be required for developing smart devices for industrial or household use. In order to design such multipurpose devices with two or more specific roles, we need to incorporate a few independent but externally controllable catalytically active centers. Through space crystal engineering, such an externally controllable multipurpose MOF-based photocatalyst could be designed. In a chemical system, a few mutually independent secondary reaction cycles nested within the principal reaction cycle can be activated externally to yield different competitive products. Each reaction cycle can be converted into a time crystal, where the time consuming each reaction step could be converted as an event and all the reaction steps or events could be connected by a circle to build a time crystal. For fractal reaction cycles, a time polycrystal can be generated. By activating a certain fractal event based nested time crystal branch, we can select one of the desired competitive products according to our needs. This viewpoint intends to bring together the ideas of (spatial) crystal engineering and time crystal engineering in order to make use of the time–space arrangement in reaction–catalysis systems and introduce new aspects to futuristic chemical engineering technology.

A one-dimensional (1D) coordination polymer [Cu2(bpba)(CH3COO)4] (1) and a two-dimensional (2D) coordination polymer [Cu(bpba)2(H2O)(NO3)](NO3)∙2H2O∙MeOH (2) were synthesized by the reaction between Cu(CH3COO)2∙H2O/Cu(NO3)2∙3H2O and bis(4-pyridyl)benzylamine (bpba). The Cu(II) ions of 1 and 2 have distorted-square pyramidal coordination with a paddle-wheel structure and an octahedral geometry, respectively. By coordinating the Cu(II) ions and bpba ligands, 1 and 2 formed zigzag 1D and puckered 2D coordination polymers, respectively. Polymer 1 exhibits strong emissions at 355 and 466 nm, whereas polymer 2 exhibits strong emissions only at 464 nm. The emissions are strongly dependent on the geometry of the Cu(II) ions linked by the bpba and anionic ligands. Polymer 1 exhibits a very strong antiferromagnetic interaction within the paddle-wheel dimer, whereas polymer 2 exhibits a very weak antiferromagnetic interaction through the bpba linkers and/or space.

Biopharmaceutics Classification System (BCS) Class II and IV drugs suffer from poor aqueous solubility and hence low bioavailability. Most of these drugs are hydrophobic and cannot be developed into a pharmaceutical formulation due to their poor aqueous solubility. One of the ways to enhance the aqueous solubility of poorlywater-soluble drugs is to use the principles of crystal engineering to formulate cocrystals of these molecules with water-soluble molecules (which are generally called coformers). Many researchers have shown that the cocrystals significantly enhance the aqueous solubility of poorly water-soluble drugs. In this review, we present a consolidated account of reports available in the literature related to the cocrystallization of poorly water-soluble drugs. The current practice to formulate new drug cocrystals with enhanced solubility involves a lot of empiricism. Therefore, in this work, attempts have been made to understand a general framework involved in successful (and unsuccessful) cocrystallization events which can yield different solid forms such as cocrystals, cocrystal polymorphs, cocrystal hydrates/solvates, salts, coamorphous solids, eutectics and solid solutions. The rationale behind screening suitable coformers for cocrystallization has been explained based on the rules of five i.e., hydrogen bonding, halogen bonding (and in general non-covalent bonding), length of carbon chain, molecular recognition points and coformer aqueous solubility. Different techniques to screen coformers for effective cocrystallization and methods to synthesize cocrystals have been discussed. Recent advances in technologies for continuous and solvent-free production of cocrystals have also been discussed. Furthermore, mechanisms involved in solubilization of these solid forms and the parameters influencing dissolution and stability of specific solid forms have been discussed. Overall, this review provides a consolidated account of the rationale for design of cocrystals, past efforts, recent developments and future perspectives for cocrystallization research which will be extremely useful for researchers working in pharmaceutical formulation development.

Top7 is a de novo designed protein whose amino acid sequence has no evolutional trace. Such a property makes Top7 a suitable scaffold for studying the pure nature of protein and protein engineering applications. To use Top7 as an engineering scaffold, we initially attempted structure determination and found that crystals of our construct, which lacked the terminal hexahistidine tag, showed weak diffraction in X-ray structure determination. Thus, we decided to introduce surface residue mutations to facilitate crystal structure determination. The resulting surface mutants, Top7sm1 and Top7sm2, crystallized easily and diffracted to the resolution around 1.7 Å. Despite the improved data, we could not finalize the structures due to high R values. Although we could not identify the origin of the high R values of the surface mutants, we found that all the structures shared common packing architecture with consecutive intermolecular β-sheet formation aligned in one direction. Thus, we mutated the intermolecular interface to disrupt the intermolecular β-sheet formation, expecting to form a new crystal packing. The resulting mutant, Top7sm2-I68R, formed new crystal packing interactions as intended and diffracted to the resolution of 1.4 Å. The surface mutations contributed to crystal packing and high resolution. We finalized the structure model with the R/Rfree values of 0.20/0.24. Top7sm2-I68R can be a useful model protein due to its convenient structure determination.

The crystal structures of salts 6–9 prepared from (R)-2-methoxy-2-(1-naphthyl)propanoic acid [(R)-MαNP acid, (R)-1] and (R)-1-arylethylamines [salt 6, (R)-1-(4-methoxyphenyl)ethylamine∙(R)-1; salt 7, (R)-1-(4-fluorophenyl)ethylamine∙(R)-1; salt 8, (R)-1-(4-chlorophenyl)ethylamine∙(R)-1; and salt 9, (R)-1-(3-chlorophenyl)ethylamine∙(R)-1] were elucidated by X-ray crystallography. The solid-state associations and conformations of the MαNP salts were defined using the concepts of supramolecular- and planar chirality, respectively, and the crystal structures of salts 6–9 were interpreted as a three-step hierarchical assembly. The para-substituents of the (R)-1-arylethylammonium cations were found on sheet structures consisting of 21 columns. Thus, salts possessing smaller para-substituents, that is, salt 7 (p-F) and salt 9 (p-H), and larger para-substituents, that is, salt 6 (p-OMe) and salt 8 (p-Cl), crystallized in the space groups P21 and C2, respectively. Additionally, weak intermolecular interactions, that is, aromatic C–H···π, C–H···F, and C–H···O interactions, were examined in crystalline salts 6–9.

[Fe(dppOH)2]2+ (dppOH = 2,6-di(pyrazol-1-yl)-4-(hydroxymethyl)pyridine) is known to show spin crossover (SCO) behavior and light-induced excited spin state transitions (LIESST). Here, we show that the SCO properties of the [Fe(dppOH)2]2+ complex can be altered by a crystal engineering approach employing counter anion exchange with polyoxometalate (POM) anions. Using this strategy, two new composite materials (TBA)[Fe(dppOH)2][PMo12O40] (1) and [Fe(dppOH)2]3[PMo12O40]2 (2) (TBA = tetra-n-butylammonium) have been isolated and studied by single crystal X-ray diffraction and magnetic susceptibility measurements. 1 was found to be in a high spin state at 300 K and showed no spin crossover behavior due to a dense packing structure induced by hydrogen bonding between the hydroxyl group of the dppOH ligands and the POM anions. Conversely, 2 contains two crystallographically unique Fe centers, where one is in the low spin state whilst the other is locked in a high spin state in a manner analogous to 1. As a result, 2 was found to show partial spin crossover behavior around 230 K with a decrease in the χmT value of 1.9 emu·mol−1·K. This simple approach could therefore provide a useful method to aid in the design of next generation spin crossover materials.

In a series of Hofmann-like spin crossover complexes, two new compounds, Fe(3-F-4-Methyl-py)2[Au(CN)2]2 (1) and Fe(3-Methyl-py)2[Au(CN)2]2 (2) (py = pyridine) are described. The series maintains a uniform 2-dimentional (2-D) layer structure of Fe[Au(CN)2]2. The layers are combined with another layer by strong aurophilic interactions, which results in a bilayer structure. Both coordination compounds 1 and 2 at 293 K crystallize in the centrosymmetric space groups P21/c. The asymmetric unit contains two pyridine derivative ligands, one type of Fe2+, and two types of crystallographically distinct [Au(CN)2]− units. Compound 1 undergoes a complete two-step spin transition. On the other hand, 2 maintains the characteristic of the high-spin state. The present compounds and other closely related bilayer compounds are compared and discussed in terms of the cooperativity and critical temperature. The bilayer structure is able to be further linked by substituent-substituent contact resulting in 3-dimentional (3-D) network cooperativity.

Two-photon excited near-infrared fluorescence materials have garnered considerable attention because of their superior optical penetration, higher spatial resolution, and lower optical scattering compared with other optical materials. Herein, a convenient and efficient supramolecular approach is used to synthesize a two-photon excited near-infrared emissive co-crystalline material. A naphthalenediimide-based triangular macrocycle and coronene form selectively two co-crystals. The triangle-shaped co-crystal emits deep-red fluorescence, while the quadrangle-shaped co-crystal displays deep-red and near-infrared emission centered on 668 nm, which represents a 162 nm red-shift compared with its precursors. Benefiting from intermolecular charge transfer interactions, the two co-crystals possess higher calculated two-photon absorption cross-sections than those of their individual constituents. Their two-photon absorption bands reach into the NIR-II region of the electromagnetic spectrum. The quadrangle-shaped co-crystal constitutes a unique material that exhibits two-photon absorption and near-infrared emission simultaneously. This co-crystallization strategy holds considerable promise for the future design and synthesis of more advanced optical materials. Two-photon excited near-infrared fluorescence materials have garnered considerable attention because of their superior optical properties compared with other optical materials. Here, the authors use a convenient and efficient supramolecular approach to synthesize a two-photon excited near-infrared emissive co-crystalline material.

Polymers possessing helical conformation in the solid state are in high demand. We report a helical peptide-polymer via the topochemical ene-azide cycloaddition (TEAC) polymerization. The molecules of the designed Gly-Phe–based dipeptide, decorated with ene and azide, assemble in its crystals as β-sheets and as supramolecular helices in two mutually perpendicular directions. While the NH...O H-bonding facilitates β-sheet–like stacking along one direction, weak CH...N H-bonding between the azide-nitrogen and vinylic-hydrogen of molecules belonging to the adjacent stacks arranges them in a head-to-tail manner as supramolecular helices. In the crystal lattice, the azide and alkene of adjacent molecules in the supramolecular helix are suitably preorganized for their TEAC reaction. The dipeptide underwent regio- and stereospecific polymerization upon mild heating in a single-crystal-to-single-crystal fashion, yielding a triazoline-linked helical covalent polymer that could be characterized by single-crystal X-ray diffraction studies. Upon heating, the triazoline-linked polymer undergoes denitrogenation to aziridine-linked polymer, as evidenced by differential scanning calorimetry, thermogravimetric analysis, and solid-state NMR analyses.