Structural Type and Thermal Stability of Tris (5-acetyl-3-thienyl) Methane Inclusion Compounds: a Combined 13C CP/MAS NMR, Powder X-Ray Diffraction and Differential Scanning Calorimetric Study

Tris (5-acetyl-3-thienyl) methane (TATM) inclusion compounds with a wide variety of guest molecules have been studied by the techniques of solid-state 13 C cross polarization/magic angle spinning nuclear magnetic resonance spectroscopy (CP/MAS NMR), powder X-ray diffraction and differential scanning calorimetry (DSC). The approach taken was to study guests with the same hydrocarbon backbone but with different halogen substituents (F, Cl, Br, I) at selected sites on that backbone. From the multiplicities of the host signals in the 13 C CP/MAS NMR spectra, the number of TATM molecules in the crystallographic asymmetric unit was determined. Powder X-ray diffraction experiments allowed us to investigate structural isomorphism of TATM inclusion compounds with different guests. In most cases, TATM forms isomorphic structures with guests that have the same hydrocarbon backbone, but with different halogen substituents. This was illustrated in the case of cyclohexyl halide and methylene halide guests. On the other hand, TATM will form one type of structure with a guest that has smaller halogen substituents (e.g., F, Cl, Br), but a completely different structure with a guest that has larger halogen substituents (e.g., I). This was illustrated in the case of monohalobenzene, 2-halobutane and 1,3-dihalopropane guests. Differential scanning calorimetry experiments allow one to investigate thermal stabilities of inclusion compounds with different guests. The enthalpy of decomposition data show that TATM forms more stable inclusion compounds with guests that have smaller, more electronegative halogen substituents (for a given hydrocarbon backbone), and the stability decreases as the size of the halogen substituent increases.