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

CrystEngComm Pub Date: 2016-04-04 DOI: 10.1039/C6CE00438E

Abstract

Two polymorphic modifications of 1,4-dibromo-2,5-bis(bromomethyl)benzene have been discovered and structurally characterized; their thermodynamic relationships and transformation behaviour have been investigated. Form I crystallizes in the triclinic space group P[1 with combining macron], whereas form II crystallizes monoclinic in space group P21/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.

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