Phosphonated calixarenes: structural and self-assembly behaviour governed by the cavity size

This study presents a crystallographic investigation of phosphonated calix[4]-, calix[5]- and calix[6]-arenes, with complementary analysis of their brominated calix[6] intermediates. The influence of upper-rim and lower-rim substituents on molecular conformation and self-assembly is examined. A survey of the Cambridge Structural Database (CSD) confirms a limited number of phosphonated calix[6]arene, calix[5]arene and calix[4]arene structures, stressing a gap in the structural characterization. To address this gap, we report new crystal structures of phosphonated calixarenes bearing various lower-rim alkoxy groups, with brominated calix[6] derivatives included for comparison as key synthetic intermediates. The results suggest that decreasing the ring size enhances structural rigidity, thereby imposing greater constraints on hydrogen-bond organization and packing arrangements. Hirshfeld surface analysis was employed to quantitatively assess intermolecular interactions and packing features across the studied structures.