Crystal Structure at the Origin of the Thermal Stability and Large Enthalpy of Fusion and Sublimation Values of Calixarenes

Abstract

The high melting point in the synthetic description of calixarenes has been described as a common characteristic of these compounds. In order to explore the origin of the high thermal stability of calixarenes, a thermochemical and single-crystal X-ray diffraction study for solvent-free p-tert-butyl-calix[4]arene (PTBC4), p-tert-butyl-calix[5]arene (PTBC5), and p-tert-butyl-calix[6]arene (PTBC6) was performed, along with the analysis of the intermolecular interactions in the crystal phase through the Hirshfeld surface (HS) calculations. The experimental values obtained and trend of thermodynamic properties of temperature of fusion, as well as enthalpies of fusion and sublimation, are well supported by the single-crystal X-ray structure analysis. This dual combination allowed us to rationalize the chemical behavior of the calixarenes under study. The study of the intermolecular interactions in these calixarenes also includes the available crystallographic information for analogues PTBC4 and PTBC5 previously published in the literature. A comparative study is reported, together with the novel crystallographic structure of PTBC6. The synthetic procedure of a pure sample of PTBC6 is also described. Differential scanning calorimetry has shown through the melting point that the thermal stability of the crystals of calixarenes PTBC4, PTBC5, and PTBC6 is over 580 K. Comparison of the temperature of fusion of the corresponding oligomers reveals a macrocyclic effect, which can be attributed to the thermal stability of the three calixarenes. The values and trend for the enthalpies of sublimation are correctly supported by the crystal growth analysis.