Research Article: Polymorphism and phase transformation in the dimethyl sulfoxide solvate of 2,3,5,6-tetra­fluoro-1,4-di­iodo­benzene

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.

Partial Text

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




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