The curious case of proton migration under pressure in the malonic acid and 4,4'-bipyridine cocrystal.

In the search for new active pharmaceutical ingredients, the precise control of the chemistry of cocrystals becomes essential. One crucial step within this chemistry is proton migration between cocrystal coformers to form a salt, usually anticipated by the empirical ΔpKa rule. Due to the effective role it plays in modifying intermolecular distances and interactions, pressure adds a new dimension to the ΔpKa rule. Still, this variable has been scarcely applied to induce proton-transfer reactions within these systems. In our study, high-pressure X-ray diffraction and Raman spectroscopy experiments, supported by DFT calculations, reveal modifications to the protonation states of the 4,4'-bipyridine (BIPY) and malonic acid (MA) cocrystal (BIPYMA) that allow the conversion of the cocrystal phase into ionic salt polymorphs. On compression, neutral BIPYMA and monoprotonated (BIPYH+MA-) species coexist up to 3.1 GPa, where a phase transition to a structure of P21/c symmetry occurs, induced by a double proton-transfer reaction forming BIPYH22+MA2-. The low-pressure C2/c phase is recovered at 2.4 GPa on decompression, leading to a 0.7 GPa hysteresis pressure range. This is one of a few studies on proton transfer in multicomponent crystals that shows how susceptible the interconversion between differently charged species is to even slight pressure changes, and how the proton transfer can be a triggering factor leading to changes in the crystal symmetry. These new data, coupled with information from previous reports on proton-transfer reactions between coformers, extend the applicability of the ΔpKa rule incorporating the pressure required to induce salt formation.

Pressure-Induced Structural Effects in the Square Lattice (sql) Topology Coordination Network Sql-1-Co-NCS·4OX.

A high-pressure study of a switching coordination network of square lattice topology (sql) loaded with o-xylene (OX), [Co(4,4'-bipyridine)2(NCS)2] n ·4nC8H10 (sql-1-Co-NCS·4OX), was conducted up to approximately 1 GPa to investigate pressure-induced structural changes. Previous reports revealed that sql-1-Co-NCS exhibits multiple phases thanks to its ability to switch between closed (nonporous) and several open (porous) phases in the presence of various gases, vapors, and liquids. Networks of such properties are of topical interest because they can offer high working capacity and improved recyclability for gas adsorption. The monoclinic crystal structure of sql-1-Co-NCS·4OX at 100 K was previously reported to show an increase in interlayer separation of more than 100% compared to the corresponding closed phase, sql-1-Co-NCS, when exposed to gases or vapors under ambient conditions. Herein, a tetragonal crystal form of sql-1-Co-NCS·4OX (space group I4/mmm, Phase I) that exists at 0.1 MPa/303 K is reported. Exposure of Phase I to high pressure using penetrable pressure transmitting media (OX and 1:1 vol MeOH/EtOH) did not result in further separation of the sql networks. Rather, compression of the crystals and release of adsorbed OX molecules occurred. These pressure-induced changes are discussed in terms of structural voids, framework conformation, and molecular packing of the sql layers. Although Phase I retained tetragonal symmetry throughout the investigated pressure range, the interlayer voids occupied by OX molecules were significantly reduced between 0.3 and 0.5 GPa; further compression above 0.5 GPa induced structural disorder. Additionally, analysis of the electron count present in the pores of sql-1-Co-NCS confirmed the multistep evacuation of OX molecules from the crystal, and two intermediate phases, Ia and Ib, differing in the OX loading level, are postulated.

Polymorphism in Ionic Cocrystals Comprising Lithium Salts and l-Proline.

The occurrence of polymorphism in ionic cocrystals formed by two lithium salts, lithium salicylate (LIS) and lithium 4-methoxybenzoate (L4M), and l-proline (PRO) has been investigated. The previously reported monoclinic form of the 1:1 cocrystal of LIS and PRO, LISPRO(α), and a new thermodynamically stable orthorhombic polymorph, LISPRO(ß), were prepared and characterized. The two polymorphs form square grid, sql, topology coordination networks and differ mainly in the conformation of the salicylate ions and positioning of the sql nets. LISPRO(α) was observed to transform to LISPRO(ß) under slurry conditions. The 1:1 ionic cocrystal of L4M and PRO (L4MPRO) was found to form three polymorphs. Apart from the previously reported orthorhombic crystal form, L4MPRO(α), two new monoclinic crystal forms, L4MPRO(ß) and L4MPRO(γ), were obtained by modifying crystallization conditions. The new polymorphs were found to be metastable, undergoing transformations to L4MPRO(α) upon exposure to humidity. Experimental conditions that induce transformations between the polymorphs of LISPRO and L4MPRO are detailed, and the structural differences between the polymorphs are discussed in the broader context of polymorphism.

NUIG4: A biocompatible pcu metal-organic framework with an exceptional doxorubicin encapsulation capacity.

Metal-organic frameworks (MOFs) are promising multifunctional porous materials for biomedical and environmental applications. Here, we report synthesis and characterization of a new MOF based on the tetrahedral secondary building unit [Zn4O(CBAB)3]n (NUIG4), where CBABH2 = 4-((4-carboxybenzylidene)amino)benzoic acid. NUIG4 belongs to the family of MOFs with primitive cubic pcu topology, being a rare example with 4-fold interpenetration. The pore architecture enables unprecedentedly high doxorubicin (DOX) loading capacity (1955 mg DOX/g NUIG4) with pH-controlled release. Solid-state NMR and ab initio modeling confirmed formation of aromatic π-π stacking interactions between DOX and the framework. Preliminary cell-line experiments suggested a protective effect of NUIG4 on healthy HDF cells against DOX toxicity. NUIG4 also displays potential for adsorptive small-molecule gas separation, with a BET surface area of 1358 m2 g-1 and high selectivity of 2.75 for C2H2 over CO2.

Toward an Understanding of the Propensity for Crystalline Hydrate Formation by Molecular Compounds. Part 2.

The propensity of molecular organic compounds to form stoichiometric or nonstoichiometric crystalline hydrates remains a challenging aspect of crystal engineering and is of practical relevance to fields such as pharmaceutical science. In this work, we address the propensity for hydrate formation of a library of eight compounds comprised of 5- and 6-membered N-heterocyclic aromatics classified into three subgroups: linear dipyridyls, substituted Schiff bases, and tripodal molecules. Each molecular compound studied possesses strong hydrogen bond acceptors and is devoid of strong hydrogen bond donors. Four methods were used to screen for hydrate propensity using the anhydrate forms of the molecular compounds in our library: water slurry under ambient conditions, exposure to humidity, aqueous solvent drop grinding (SDG), and dynamic water vapor sorption (DVS). In addition, crystallization from mixed solvents was studied. Water slurry, aqueous SDG, and exposure to humidity were found to be the most effective methods for hydrate screening. Our study also involved a structural analysis using the Cambridge Structural Database, electrostatic potential (ESP) maps, full interaction maps (FIMs), and crystal packing motifs. The hydrate propensity of each compound studied was compared to a compound of the same type known to form a hydrate through a previous study of ours. Out of the eight newly studied compounds (herein numbered 4-11), three Schiff bases were observed to form hydrates. Three crystal structures (two hydrates and one anhydrate) were determined. Compound 6 crystallized as an isolated site hydrate in the monoclinic space group P21/a, while 7 and 10 crystallized in the monoclinic space group P21/c as a channel tetrahydrate and an anhydrate, respectively. Whereas we did not find any direct correlation between the number of H-bond acceptors and either hydrate propensity or the stoichiometry of the resulting hydrates, analysis of FIMs suggested that hydrates tend to form when the corresponding anhydrate structure does not facilitate intermolecular interactions.

Crystal engineering of a rectangular sql coordination network to enable xylenes selectivity over ethylbenzene.

Separation of the C8 aromatic isomers, p-xylene (PX), m-xylene (MX), o-xylene (OX) and ethylbenzene (EB), is relevant thanks to their widespread application as chemical feedstocks but challenging because of their similar boiling points and close molecular dimensions. Physisorptive separation could offer an energy-efficient solution to this challenge but sorbents which exhibit strong selectivity for one of the isomers remain a largely unmet challenge despite recent reports of OX or PX selective sorbents with high uptake capacity. For example, the square lattice, sql, topology coordination network [Co(bipy)2(NCS)2] n (sql-1-Co-NCS) exhibits the rare combination of high OX selectivity and high uptake capacity. Herein we report that a crystal engineering approach enabled isolation of the mixed-linker sql coordination network [Co(bipy)(bptz)(NCS)2] n (sql-1,3-Co-NCS, bipy = 4,4'-bipyridine, bptz = 4,4'-bis(4-pyridyl)tetrazine) and study of its C8 vapour and liquid sorption properties. sql-1,3-Co-NCS was found to exhibit high adsorption capacity from liquid xylenes (∼37 wt%) and is to our knowledge the first sorbent to exhibit high selectivity for each of xylene isomer over EB (S OX/EB, S MX/EB, S PX/EB > 5). Insights into the performance of sql-1,3-Co-NCS are gained from structural studies which reveal stacking interactions between electron-deficient bptz linkers and the respective xylenes. sql-1,3-Co-NCS is the first N-donor mixed-linker sql coordination network studied for its gas/vapour sorption properties and represents a large and diverse class of understudied coordination networks.

A new high-pressure benzocaine polymorph - towards understanding the molecular aggregation in crystals of an important active pharmaceutical ingredient (API).

Benzocaine (BZC), an efficient and highly permeable anaesthetic and an active pharmaceutical ingredient of many commercially available drugs, was studied under high pressure up to 0.78 GPa. As a result, new BZC polymorph (IV) was discovered. The crystallization of polymorph (IV) can be initiated by heating crystals of polymorph (I) at a pressure of at least 0.45 GPa or by their compression to 0.60 GPa. However, no phase transition from polymorph (I) to (IV) was observed. Although polymorph (IV) exhibits the same main aggregation motif as in previously reported BZC polymorphs (I)-(III), i.e. a hydrogen-bonded ribbon, its molecular packing and hydrogen-bonding pattern differ considerably. The N-H...N hydrogen bonds joining parallel BZC ribbons in crystals at ambient pressure are eliminated in polymorph (IV), and BZC ribbons become positioned at an angle of about 80°. Unfortunately, crystals of polymorph (IV) were not preserved on pressure release, and depending on the decompression protocol they transformed into polymorph (II) or (I).

Vitrification and New Phases in the Water:Pyrimidine Binary Eutectic System.

The binary diagram for pyrimidine:water mixtures has been determined by differential scanning calorimetry, in situ single-crystal, and powder X-ray diffraction experiments. The eutectic point has been located near the 1:4 n/n ratio at 234.5 K. The eutectic and nearly eutectic mixtures easily vitrify, and the vitrification could be kinetically induced for 1:3 n/n mixtures, too. Depending on the cooling rate, the 1:4 mixture freezes in the glass state, as a conglomerate of the glass and crystalline phases, or as the eutectic mixture of pyrimidine phase I and hexagonal ice Ih. When heated above 160 K, the glass phase transforms to a novel crystalline phase, tentatively identified as a pyrimidine hydrate, which in turn at ca. 200-210 K transforms into a eutectic mixture of pyrimidine phase I and hexagonal ice Ih. The pyrimidine-water binary diagram and novel crystalline and amorphous phases are relevant to the thermodynamic behavior of hydrophilic pyrimidine and its natural and synthetic derivatives in humid environments. The presently determined binary diagram can be straightforwardly applied for assessing the contents of water in highly hygroscopic pyrimidine samples.

Highly Selective, High-Capacity Separation of o-Xylene from C8 Aromatics by a Switching Adsorbent Layered Material.

Purification of the C8 aromatics (xylenes and ethylbenzene) is particularly challenging because of their similar physical properties. It is also relevant because of their industrial utility. Physisorptive separation of C8 aromatics has long been suggested as an energy efficient solution but no physisorbent has yet combined high selectivity (>5) with high adsorption capacity (>50 wt %). Now a counterintuitive approach to the adsorptive separation of o-xylene from other C8 aromatics involves the study of a known nonporous layered material, [Co(bipy)2 (NCS)2 ]n (sql-1-Co-NCS), which can reversibly switch to C8 aromatics loaded phases with different switching pressures and kinetics, manifesting benchmark o-xylene selectivity (SOX/EB ≈60) and high saturation capacity (>80 wt %). Structural insight into the observed selectivity and capacity is gained by analysis of the crystal structures of C8 aromatics loaded phases.

Solvent-directed control over the topology of entanglement in square lattice (sql) coordination networks.

We report herein that the mode of entanglement in square lattice, sql, coordination networks formed by an extended bis-imidazole ligand, L, can be controlled by the solvent used during solvothermal synthesis. [ML2(NO3)2]n (M = Co, Ni) exhibit both parallel 2D → 2D interpenetration and, unusually, mixed entanglement (parallel-inclined 2D → 3D polycatenation).

A dynamic and multi-responsive porous flexible metal-organic material.

Stimuli responsive materials (SRMs) respond to environmental changes through chemical and/or structural transformations that can be triggered by interactions at solid-gas or solid-liquid interfaces, light, pressure or temperature. SRMs span compositions as diverse as organic polymers and porous inorganic solids such as zeolites. Metal-organic materials (MOMs), sustained by metal nodes and organic linker ligands are of special interest as SRMs. SR-MOMs have thus far tended to exhibit only one type of transformation, e.g. breathing, in response to one stimulus, e.g. pressure change. We report [Zn2(4,4'-biphenyldicarboxylate)2(4,4'-bis(4-pyridyl)biphenyl)]n, an SR-MOM, which exhibits six distinct phases and four types of structural transformation in response to various stimuli. The observed structural transformations, breathing, structural isomerism, shape memory effect, and change in the level of interpenetration, are previously known individually but have not yet been reported to exist collectively in the same compound. The multi-dynamic nature of this SR-MOM is mainly characterised by using in-situ techniques.

Recyclable switching between nonporous and porous phases of a square lattice (sql) topology coordination network.

A nonporous square lattice (sql) coordination network [Co(bipy)2(NCS)2]n (sql-1-Co-NCS) exhibits recyclable switching induced by CO2. The sorption isotherms are stepped with moderate hysteresis, temperature controlled and saturation uptake is fixed. Such switching, which has rarely been observed, offers the promise of exceptional working capacity for gas storage.

Impact of partial interpenetration in a hybrid ultramicroporous material on C2H2/C2H4 separation performance.

Phases of a 2-fold pcu hybrid ultramicroporous material (HUM), SIFSIX-14-Cu-i, exhibiting 99%, 93%, 89%, and 70% partial interpenetration have been obtained. 1 : 99 C2H2/C2H4 gas separation studies reveal that as the proportion of interpenetrated component decreases, so does the separation performance.

Pressure inverse solubility and polymorphism of an edible γ-cyclodextrin-based metal-organic framework.

A very exceptional effect of pressure-induced dissolution has been revealed for an edible metal-organic framework, γ-CD-MOF-1, formed using γ-cyclodextrin and KOH base. In addition, a new polymorph of γ-CD-MOF-1 has been obtained. The trigonal structure is a symmetry sub-group modification of the cubic form. The pressure-induced dissolution of γ-CD-MOF-1 and its polymorphism are shown to be closely related and regulated by adsorption in the pores, as well as the guest framework interactions.