Date Published: May 01, 2020
Publisher: International Union of Crystallography
Author(s): Andrew D. Bond, Chris L. Truscott.
The dimethyl sulfoxide solvate of 2,3,5,6-tetrafluoro-1,4-diiodobenzene is polymorphic; one polymorph undergoes a phase transformation on cooling, associated with re-orientation of the dimethyl sulfoxide molecules.
The molecule 2,3,5,6-tetrafluoro-1,4-diiodobenzene (TFDIB) is a common halogen-bond donor (Metrangolo & Resnati, 2001 ▸; Cavallo et al., 2016 ▸). The work described in this article originated from a cocrystal screening, where TFDIB was combined with a series of potential halogen-bond acceptors in dimethyl sulfoxide (DMSO) solution. Crystals of TFDIB·DMSO (see Scheme) were quite commonly obtained from these experiments, some of which were found to be different from a previously reported crystal structure at 297 K [Britton, 2003 ▸; Cambridge Structural Database (CSD; Groom et al., 2016 ▸) refcode IKIFOX]. We refer to the previously reported structure (IKIFOX) as form I and the newly obtained polymorph as form II. The structures are similar and we suspected at first that form II might have arisen from a phase transformation on cooling of form I in the N2 cryostream during single-crystal data collection. We therefore obtained crystals of form I and measured them at various temperatures. We did not find any transformation of form I to form II, but instead observed re-orientation of the DMSO molecules in form I to give a further new structure measured at 120 K. We describe herein the various crystal structures of TFDIB·DMSO and the application of dispersion-corrected DFT and PIXEL calculations (Gavezzotti, 2002 ▸, 2003 ▸, 2011 ▸) to examine the DMSO re-orientation on cooling of form I.
The existence of TFDIB·DMSO form II and the variation of the form I structure as a function of temperature shows that the layered arrangement of TFDIB molecules can exhibit significant flexibility in the crystalline state. This flexibility accommodates several orientations for the DMSO molecules between the layers, with apparently little variation in the DMSO–TFDIB interaction energies. The DMSO molecules are consistently anchored by accepting I⋯O halogen bonds, but their orientation can vary relative to the TFDIB molecules and relative to each other. The PIXEL calculations suggest no clear preference for orientations A or B in form I, consistent with the observed disorder in the structure at 297 K, but they show clearly why orientation C is preferred in the structure at 120 K. Optimization of the interactions between neighbouring DMSO molecules locks in an ordered arrangement, which accounts for the observed changes in the unit-cell parameters and space group on cooling of form I below ca 220 K. The applied combination of temperature-dependent X-ray diffraction measurements and intermolecular energy calculations provides a clear picture of the temperature-dependent phase transformation in this case