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

Andrew D Bond,Chris L Truscott

doi:10.1107/s2053229620005690

Andrew D Bond, Chris L Truscott

Open Access

https://doi.org/10.1107/s2053229620005690

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Abstract

A new polymorph (form II) is reported for the 1:1 dimethyl sulfoxide solvate of 2,3,5,6-tetrafluoro-1,4-diiodobenzene (TFDIB·DMSO or C6F4I2·C2H6SO). The structure is similar to that of a previously reported polymorph (form I) [Britton (2003). Acta Cryst. E59, o1332-o1333], containing layers of TFDIB molecules with DMSO molecules between, accepting I...O halogen bonds from two TFDIB molecules. Re-examination of form I over the temperature range 300-120 K shows that it undergoes a phase transformation around 220 K, where the DMSO molecules undergo re-orientation and become ordered. The unit cell expands by ca 0.5 Å along the c axis and contracts by ca 1.0 Å along the a axis, and the space-group symmetry is reduced from Pnma to P212121. Refinement of form I against data collected at 220 K captures the (average) structure of the crystal prior to the phase transformation, with the DMSO molecules showing four distinct disorder components, corresponding to an overlay of the 297 and 120 K structures. Assessment of the intermolecular interaction energies using the PIXEL method indicates that the various orientations of the DMSO molecules have very similar total interaction energies with the molecules of the TFDIB framework. The phase transformation is driven by interactions between DMSO molecules, whereby re-orientation at lower temperature yields significantly closer and more stabilizing interactions between neighbouring DMSO molecules, which lock in an ordered arrangement along the shortened a axis.

Highlights

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 dimethyl sulfoxide (DMSO) molecules lie on the crystallographic mirror planes at x,14,z and x,34,z, accepting IÁ Á ÁO halogen bonds from two TFDIB molecules either side of the mirror plane [I1Á Á ÁO1 = 2.847 (2) A ]
The existence of TFDIBÁDMSO 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

Summary

Introduction

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 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 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. TFDIB and an anticipated coformer were dissolved in DMSO, and crystals were produced by vapour diffusion of water into the solution under ambient conditions. The structure of form II was measured at 180 K, whereby the crystals were plunged directly from ambient conditions into a cold N2 stream. Analysis of form I was made by placing the crystal initially into the N2 stream at room temperature, followed by slow cooling as described in x3.2

Refinement

Computational details

Structure of form II

Temperature-dependent structure of form I

Conclusion