Investigations of the phase transitions in thiourea inclusion compounds with cycloheptane, cyclooctane, and cyclooctanone

Variable temperature single-crystal X-ray diffraction and solid-state NMR experiments have been performed on the thiourea cycloheptane (TC7), cyclooctane (TC8), and cyclooctanone (TCO8) inclusion compounds. Three different phases have been evidenced for TC7 and four for TC8, while only one phase change at high temperature has been found for TCO8. At room temperature, the cyclooctane and cycloheptane adducts exhibit the classical rhombohedral lattice (space group R (3) over barc). In these structures, the well-resolved host thiourea structure is composed of one-dimensional nonintersecting channels running along the c axis. The guest molecules, located in the center of the channels, are highly disordered as confirmed by the solid-state C-13 NMR spectra. Lowering temperature leads to a monoclinic phase resulting from the distortion of the room-temperature lattice where the guest molecules still have some degrees of disorder. Refinement has been performed with a model involving two cycloalkane rings. For TC8, a supplementary phase with a doubling of the b axis is observed below 200 K, having two apparently ordered cyclooctane rings with the same conformation. The cyclooctanone adduct, TCO8, is already monoclinic at room temperature with an ordered guest molecule stabilized within the thiourea channels by a hydrogen bond. Upon heating, a phase change can be detected by solid-sate NMR and X-ray powder diffraction just before decomposition.