Dimorphism of 1,4-dibromo-2,5-bis(bromomethyl)benzene: crystallographic and physico-chemical investigations

Abstract

Two polymorphic modifications of 1,4-dibromo-2,5-bisIJbromomethyl)benzene have been discovered and structurally characterized; their thermodynamic relationships and transformation behaviour have been investigated. Form I crystallizes in the triclinic space group P1, whereas form II crystallizes monoclinic in space group P2 1 /c, both with imposed inversion symmetry of the molecule. Their crystal structures involve layers, in which the molecules are linked by intermolecular Br⋯Br interactions to form similar systems of linked rings. Initial studies involved batches consisting of pure form I or a mixture of I with traces of II (as obtained by chance from the synthesis), but solvent-mediated conversion experiments in various solvents clearly prove that form II is the thermodynamically stable form at room temperature. Thermomicroscopic and heating-rate-dependent DSC measurements show that the melting point of form I is slightly higher than that of form II and that the higher melting polymorph exhibits the lower heat of fusion. Therefore, form I becomes thermodynamically stable at higher temperatures and both forms are related by enantiotropism. This is also in agreement with the density rule, because the low-temperature form II exhibits the higher density. Isothermal annealing of both modifications at different temperatures reveals a thermodynamic transition temperature of about 135°C, which is in excellent agreement with that of 137.5°C calculated from the melting temperatures and the heat of fusion of both forms. The high-temperature form can easily be prepared pure by solidification of the melt, which is in agreement with Ostwald's step rule, because form I crystallizes at a temperature where it is thermodynamically metastable. A qualitative energy/ temperature diagram is presented.

Using Solid Catalysts in Disulfide‐Based Dynamic Combinatorial Solution‐ and Mechanochemistry

“…Where this was not possible, the geometry was extracted from a cocrystal containing the desired molecule. For this study, geometries were taken from: cyclam, [37] 4‐OH‐Piperidine, [38] DBU, [39] TBD, [40] piperazine, [41] 4Ph‐4OH‐piperidine, [42] DABCO hydrate, [43] and quinuclidine carbon tetrabromide [44] . The structure for 3OH‐piperidine was prepared manually.…”

Characterising Supramolecular Architectures in Crystals Featuring I⋯Br Halogen Bonding: Persistence of X⋯X’ Secondary-Bonding in Their Congeners

“…In the first, the bromide atoms were replaced by chloride, and in the isostructural crystal, the chain features I• • • Cl interactions [60]. In the isostructural all-bromide congener [85], Br• • • Br contacts are apparent, see Table S2. Apparent are longer Br• • • Br contacts beyond the sum of the van der Waals radii.…”

“…Apparent are longer Br• • • Br contacts beyond the sum of the van der Waals radii. There is a polymorph of the isostructural all-bromide congener [85] in which each independent Br• • • Br interaction identified in the isostructural crystal now occurs at separations less than the sum of the van der Waals radii, resulting in a two-dimensional array with a distinctive zigzag topology, as represented in the plan view of Figure 13. In the case of 21, the iodide atom was replaced by bromide in both known congeners and in one of these, the phenyl-bound bromide was substituted by chloride [61].…”

Four Crystalline Forms of (Tetrahydrothiophene)trichloridogold(III): Polymorphism and Reversible Low‐temperature Phase Transitions

“…During our ongoing investigations of phosphine chalcogenide complexes of gold halides, we recently determined the structure of another form II of compound 2 at –173 °C, which crystallizes in the monoclinic space group P 2 1 / n with Z = 8. Again this was fortuitous (compound 2 crystallized as an impurity, and the relationship between the cells of I and II was not immediately recognized), but in retrospect it prompted us, in view of our interest in polymorphism, to investigate the crystal forms of 2 more thoroughly. Crystallization of pure 2 from dichloromethane / petroleum ether yielded a sample consisting of yellow prisms and plates (Figure ).…”