Room-temperature monoclinic and low-temperature triclinic phase-transitionstructures of meso-octamethylcalix[4]pyrrole-dimethyl sulfoxide (1/1)

Crystals of the title complex, C28H36N4.C2H6OS, undergo a phase transition between room temperature and 198 K, as determined by X-ray diffraction tech niques. A monoclinic form is observed at room temperature, while a triclini c modification is found at 198 K, with Z' changing from 1 to 2. Differentia l scanning calorimetry (DSC) of the calixpyrrole- dimethyl sulfoxide comple x revealed a series of phase changes between 273 and 243 K. The transition from the room-temperature monoclinic form to the low-temperature triclinic form is reversible, as determined by changes in the cell dimensions from re measuring selected reflections at room temperature and at temperatures belo w 223 K. The uncomplexed calix[4]pyrrole molecule shows no phase changes oc curring between room temperature and 233 K, the low-temperature limit of th e DSC.