Enantiotropic Serendipity: Using Dsc to Assist in the Data Acquisition of Otherwise Routine Single Crystal Diffraction Experiments

An enantiotropic phase transition requires two polymorphs that are each thermodynamically stable over a range of temperatures and pressures, where variation of either will lead to a phase transition to the other polymorph. A series of phase transitions most often starts with a nominal Z’=1, non-twinned crystal structure that descends to lower symmetry stepwise by transforming to a lower symmetry space group with twinning and/or undergoing an order-disorder transition. The resultant structure generally has higher Z’ than the starting structures, more pseudo-symmetry, and less disorder as the structure is cooled to a lower temperature. We have found differential scanning calorimetry (DSC) inspection of samples prior to crystallographic data acquisition very useful in planning the crystallographic experimental design. A number of low-melting organic salts (or ionic liquids) form enantiotropic phase transition series. For example, the material [N(CH3)4][IM14], where IM14 is CF3SO2NSO2CF2CF2CF2CF3, exhibits two first-order phase transitions by DSC at 168 K and 193 K before melting at 390 K. Phase I is found in space group P21/m where the IM14 anion is disordered over the crystallographic mirror and the N(CH3)4 is bisected by it. In phase II the unit cell doubles as the crystallographic mirror transforms to a glide plane as the space group P21/n emerges. The IM14 anion is disordered in both phases I and II, but not in III. The space group for phase III is P21/n, as is phase II, however the order-disorder phase transition causes a major reconstruction of the contents and the aand c-axes. Enantiotropic phase transitions happen serendipitously in the course of standard service crystallography work. One recent example is that of CrCl3L, where L = 2(dimethylamino)ethyl)-N-(diphenylphosphino)benzimidamide). A crystal structure determination of this material at 173 K is found in space group P21 with Z=8 (Z’=4) and twinned by non-merohedry. Analysis of this structure suggested the four unique