Conformational Dimorphism of 1,1,3,3,5,5‐Hexachloro‐1,3,5‐trigermacyclohexane: Solvent‐Induced Crystallization of a Metastable Polymorph Containing Boat‐Shaped Molecules

Two crystalline modifications of 1,1,3,3,5,5‐hexachloro‐1,3,5‐trigermacyclohexane have been experimentally obtained as phase pure products and studied by single‐crystal X‐ray diffraction. The six‐membered heterocycles adopt a chair conformation in the α‐phase; this polymorph is accessible by crystallisation from solution and from the melt. In contrast, the β‐form is built up from boat‐shaped molecules; it can exclusively be crystallised from n ‐hexane. At the molecular level, formation energies of the 1,1,3,3,5,5‐hexachloro‐1,3,5‐trigermacyclohexane conformers have been compared by using molecular mechanics, semiempirical and ab‐initio quantum mechanical calculations. Possible reasons for the selective formation of the α‐ or β‐phase in specific solvents have been considered. Formation of the metastable phase is suggested to occur via a hypothetical intermediate of composition [(GeCl 2 CH 2 ) 3 ]⋅0.5 C 6 H 14 . For such an in‐silico solvate, a crystal structure of favourable lattice energy, closely related to the experimentally observed β‐modification, has been found through global energy minimisation. Elimination of the n‐ hexane molecules from this computer‐generated solid and subsequent simulated annealing resulted in a crystal structure that corresponds to the experimentally observed β‐phase within the limits of the force field calculations. This scenario implies solvent directed crystallisation of a metastable polymorphic molecular crystal.