An Enantiotropic Disorder-Partial Order Solid-State Transformation in a Molecular Solid Involving a Phase with Z' = 12.

A structural study of the complex aminated stereotriads [(tert-butyldimethylsilyl)oxy]-4-ethynyl-4-(1-fluorohexyl)-1,2,3-oxathiazocane-2,2-dione (1) and its reduced analogue [(tert-butyldimethylsilyl)oxy]-4-ethenyl-4-(1-fluorohexyl)-1,2,3-oxathiazocane-2,2-dione (2) revealed a k12-type second-order phase transition in 1 and phase stability of 2 over the temperature range 100-298 K. The observed phase change in 1 has been linked to the partial ordering of the extensive positional disorder of the high temperature phase upon cooling as revealed by single-crystal X-ray diffraction. The ordering breaks the translational symmetry, introduces differences in the molecular volumes of the packed species, and results in a low-temperature crystal structure with Z' = 12. The in situ powder X-ray diffraction studies suggest that the transition occurs between 237 and 180 K upon cooling, but complete amorphization in the range of 244-274 K during heating complicated the assignment of the transition temperature. The observed loss of crystallinity has been linked to the phase transition and implies that the loss of long-range order is due to the crystallites undergoing the transformation independently from one another. [ABSTRACT FROM AUTHOR]