Crystallization of Organic Salts and Co-crystals by Sublimation: The Effect of Experimental Conditions

The selective growth of either organic salts or co-crystals from the gas phase has been achieved by varying specific conditions in a series of sublimation experiments. The effect of vacuum pressure, sublimation time, sublimation temperature and temperature gradient, physical separation of components, and mass scale on the sublimation of two acid/base systems, namely succinic acid/hexamethylenetetramine and oxalic acid/4,4′-bipyridine, was investigated. For both systems, salt formation was favored at lower pressures and higher temperatures, and correspondingly co-crystal was selectively formed at lower temperatures and higher pressures

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

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

Cyanocalix[4]arenes: synthesis, crystal structures and reactivity studies

<p>Herein, we describe an improved method to synthesise mono-, di- and tetra-cyanocalix[4]arene and report their crystal structure determinations. We also report our attempts to further functionalise the cyanocalix[4]arenes into dithiadiazolyl-calix[4]arenes, and propose a hypothesis as to why the cyano group on a calix[4]arene is an extremely challenging group to modify.</p

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

Cobalt Porphyrin–Thiazyl Radical Coordination Polymers: Toward Metal–Organic Electronics

Herein we delineate an unusual one-dimensional coordination polymer (CP), <b>3</b>, prepared from <i>S</i> = 1/2 Co­(TPP), <b>1</b> (TPP = 5,10,15,20-tetraphenylporphyrin dianion), and <i>S</i> = 1/2 4-(4′-pyridyl)-1,2,3,5-dithiadiazolyl (py-DTDA) radical, <b>2</b>. The atypically long S–S distance for CP <b>3</b> (2.12 Å) reflects fractional electron transfer from the formally Co­(II) ion into the antibonding π-SOMO of the metal-bound py-DTDA bridging ligand. The bonding in solid CP <b>3</b> involves noninteger redox states in a resonance hybrid repeat unit best formulated as [Co­(TPP)]^0.5+ hemication (Co^2.5+) bound to a dithiadiazolide hemianion (py-DTDA^0.5–). DFT calculations confirm the metal to ligand charge transfer (MLCT) character of the low-lying electronic states (641, 732, and 735 nm) observed for CP <b>3</b> and show that oligomer chains of length ≥14 repeat units tend toward a band structure with a limiting band gap energy of 0.669(6) eV. In dichloromethane, the reaction between radicals <b>1</b> and <b>2</b> involves coordination of the Co­(II) ion by a py-DTDA ring sulfur atom, orbitally favored spin-pairing, and the formation of the thermodynamically favored diamagnetic five-coordinate S-bound adduct, Co­(TPP)­(<i>S</i>-py-DTDA), <b>3a</b>. Polymerization and crystallization of <b>3a</b> affords diamagnetic CP <b>3</b>. Dissolution of CP <b>3</b> in DMSO favors Co–S bond heterolysis, yielding the diamagnetic six-coordinate purple N-bound Co^III(TPP)­(<i>N</i>-py-DTDA^–)­(OSMe₂) complex (λ_max, 436 nm). However, monomerization of CP <b>3</b> in dry 1,2-dichloroethane affords bright green diamagnetic Co^III(TPP)­(<i>N</i>-py-DTDA^–), <b>3b</b>, with multiple MLCT bands in the 800–1100 nm NIR region and a red-shifted Soret band (λ_max, 443 nm). Implications for the use of CP <b>3</b> in electronic devices are discussed based on its density of states

Cobalt Porphyrin–Thiazyl Radical Coordination Polymers: Toward Metal–Organic Electronics

Herein we delineate an unusual one-dimensional coordination polymer (CP), <b>3</b>, prepared from <i>S</i> = 1/2 Co­(TPP), <b>1</b> (TPP = 5,10,15,20-tetraphenylporphyrin dianion), and <i>S</i> = 1/2 4-(4′-pyridyl)-1,2,3,5-dithiadiazolyl (py-DTDA) radical, <b>2</b>. The atypically long S–S distance for CP <b>3</b> (2.12 Å) reflects fractional electron transfer from the formally Co­(II) ion into the antibonding π-SOMO of the metal-bound py-DTDA bridging ligand. The bonding in solid CP <b>3</b> involves noninteger redox states in a resonance hybrid repeat unit best formulated as [Co­(TPP)]^0.5+ hemication (Co^2.5+) bound to a dithiadiazolide hemianion (py-DTDA^0.5–). DFT calculations confirm the metal to ligand charge transfer (MLCT) character of the low-lying electronic states (641, 732, and 735 nm) observed for CP <b>3</b> and show that oligomer chains of length ≥14 repeat units tend toward a band structure with a limiting band gap energy of 0.669(6) eV. In dichloromethane, the reaction between radicals <b>1</b> and <b>2</b> involves coordination of the Co­(II) ion by a py-DTDA ring sulfur atom, orbitally favored spin-pairing, and the formation of the thermodynamically favored diamagnetic five-coordinate S-bound adduct, Co­(TPP)­(<i>S</i>-py-DTDA), <b>3a</b>. Polymerization and crystallization of <b>3a</b> affords diamagnetic CP <b>3</b>. Dissolution of CP <b>3</b> in DMSO favors Co–S bond heterolysis, yielding the diamagnetic six-coordinate purple N-bound Co^III(TPP)­(<i>N</i>-py-DTDA^–)­(OSMe₂) complex (λ_max, 436 nm). However, monomerization of CP <b>3</b> in dry 1,2-dichloroethane affords bright green diamagnetic Co^III(TPP)­(<i>N</i>-py-DTDA^–), <b>3b</b>, with multiple MLCT bands in the 800–1100 nm NIR region and a red-shifted Soret band (λ_max, 443 nm). Implications for the use of CP <b>3</b> in electronic devices are discussed based on its density of states

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