Guest Exchange in a Robust Hydrogen-Bonded Organic Framework: Single-Crystal to Single-Crystal Exchange and Kinetic Studies

The salt 3,4-lutidinium pamoate crystallizes as its hemihydrate, forming a hydrogen-bonded organic framework with tetrahydrofuran (THF) as a guest in channels in the structure (<b>1·THF</b>). Extensive investigation has shown this framework to be highly robust: the THF in the channels can be exchanged for 20 different compounds, with 13 of these exchanges occurring in a single-crystal to single-crystal manner. The THF can also be exchanged for the volatile solids pyrazine or iodine, both via single-crystal to single-crystal transformations. Stepwise exchange of solvents is also possible, with a sequence of five exchanges occurring before the crystals begin to deteriorate. Investigation of the kinetics of exchange in <b>1·THF</b> revealed that exchange occurs according to a deceleratory kinetic model for contracting volume

A Series of Polymorphs of Hexakis(4-fluorophenoxy)cyclotriphosphazene

A series of conformational polymorphs of hexakis­(4-fluorophenoxy)­cyclotriphosphazene have been identified and characterized. Three of the four polymorphs are previously unreported. Thermal analysis coupled with variable temperature powder X-ray diffraction has shown that conversion between polymorphs occurs at well-defined temperatures. The high temperature form is metastable at room temperature and is never obtained from solution. Two forms are observed in solution crystallization experiments, a monoclinic form and a triclinic form. Slurry experiments revealed that the triclinic form is the most stable at room temperature. The fourth polymorph is observed only at low temperatures. Interconversion between three of the polymorphs occurs in a single-crystal to single-crystal manner

Selectivity Behavior of a Robust Porous Organic Salt Based on the Pamoate Ion

The selectivity of a porous hydrogen-bonded framework, <b>1</b>, toward various solvent mixtures has been investigated. Framework <b>1</b>, which is the hydrate of 3,4-lutidinium pamoate, crystallizes as its THF solvate, <b>1·THF</b>. Solvent exchange can take place either by exposing crystals of <b>1·THF</b> to solvent vapors or by immersing the crystals in a solvent. Framework <b>1</b> shows preferential inclusion of particular solvents when exposed to mixed solvent vapors, or to mixed solvents in the liquid form. Analysis showed that solvents are included in the same ratios when exposed to mixed solvents in the liquid or vapor phase, despite the significant differences in mole ratios between the liquid and vapor phases due to the different vapor pressures of the solvents investigated. In some cases, when immersed in solvent mixtures containing acetonitrile or acetone, <b>1·THF</b> recrystallizes as an acetonitrile or acetone solvate of 3,4-lutidinium pamoate. The crystal structures of these two new solvates are also reported

A Series of Polymorphs of Hexakis(4-fluorophenoxy)cyclotriphosphazene

A series of conformational polymorphs of hexakis­(4-fluorophenoxy)­cyclotriphosphazene have been identified and characterized. Three of the four polymorphs are previously unreported. Thermal analysis coupled with variable temperature powder X-ray diffraction has shown that conversion between polymorphs occurs at well-defined temperatures. The high temperature form is metastable at room temperature and is never obtained from solution. Two forms are observed in solution crystallization experiments, a monoclinic form and a triclinic form. Slurry experiments revealed that the triclinic form is the most stable at room temperature. The fourth polymorph is observed only at low temperatures. Interconversion between three of the polymorphs occurs in a single-crystal to single-crystal manner

Polymorphic Behavior of Two Organic Zwitterions: Two Rare Cases of No Observable Conversion between Polymorphs

The solid-state behavior of two organic zwitterions, (<i>Z</i>)-3-carboxy-2-(4-cyanopyridin-1-ium-1-yl)-acrylate (<b>1</b>) and (<i>Z</i>)-1-(3-carboxy-1,1-dihydroxyprop-2-en-1-ide-2-yl)-pyridin-1-ium (<b>2</b>), has been extensively investigated. Variation of crystallization conditions resulted in the identification of two conformational polymorphs of <b>1</b>, a kinetic and thermodynamic form. Three polymorphs of <b>2</b>, where the formation of a particular polymorph depends entirely on solvent of crystallization, were also identified. The relationship between the polymorphs of both <b>1</b> and <b>2</b> has been extensively studied, and attempts were made to interconvert between forms to determine their relative stability. No interconversion between polymorphs is observed in either <b>1</b> or <b>2</b>. In the case of <b>1</b>, this is likely due to restrictions on rotation of the carboxylic group in the solid state, which is required for conversion between polymorphs

Iron(III) Protoporphyrin IX Complexes of the Antimalarial <i>Cinchona</i> Alkaloids Quinine and Quinidine

The antimalarial properties of the <i>Cinchona</i> alkaloids quinine and quinidine have been known for decades. Surprisingly, 9-epiquinine and 9-epiquinidine are almost inactive. A lack of definitive structural information has precluded a clear understanding of the relationship between molecular structure and biological activity. In the current study, we have determined by single crystal X-ray diffraction the structures of the complexes formed between quinine and quinidine and iron­(III) protoporphyrin IX (Fe­(III)­PPIX). Coordination of the alkaloid to the Fe­(III) center is a key feature of both complexes, and further stability is provided by an intramolecular hydrogen bond formed between a propionate side chain of Fe­(III)­PPIX and the protonated quinuclidine nitrogen atom of either alkaloid. These interactions are believed to be responsible for inhibiting the incorporation of Fe­(III)­PPIX into crystalline hemozoin during its <i>in vivo</i> detoxification. It is also possible to rationalize the greater activity of quinidine compared to that of quinine

Solid-State Supramolecular Chemistry of a Benzylpyridine-Functionalized Zwitterion: Polymorphism, Interconversion, and Porosity

The reaction between acetylenedicarboxylic acid and 4-benzylpyridine yields two products: a salt and a zwitterion. The relationship between these two products has been investigated, revealing that the salt is the kinetic product of the reaction and the zwitterion is the thermodynamic product. The zwitterion has four polymorphs and three solvates. Interconversion between these forms has been studied and was shown to be solvent-mediated when it occurs; conversion between most polymorphic forms cannot be achieved using thermal methods. The temperature at which the reaction to form the zwitterion is carried out and the solvent system used are crucial in determining which polymorphic form crystallizes from the reaction. In addition, polymorphic Form I is shown to be porous to dioxane

Solid-State Supramolecular Chemistry of a Benzylpyridine-Functionalized Zwitterion: Polymorphism, Interconversion, and Porosity

The reaction between acetylenedicarboxylic acid and 4-benzylpyridine yields two products: a salt and a zwitterion. The relationship between these two products has been investigated, revealing that the salt is the kinetic product of the reaction and the zwitterion is the thermodynamic product. The zwitterion has four polymorphs and three solvates. Interconversion between these forms has been studied and was shown to be solvent-mediated when it occurs; conversion between most polymorphic forms cannot be achieved using thermal methods. The temperature at which the reaction to form the zwitterion is carried out and the solvent system used are crucial in determining which polymorphic form crystallizes from the reaction. In addition, polymorphic Form I is shown to be porous to dioxane