Structural robustness of crystalline sulfonated calix[4]arene complexes incorporating organic and metal cations

Six different sulfonated calix[4]arene-based compounds incorporating diverse metal ions (Na⁺, Y³⁺, Gd³⁺), organic guests (1-ethyl-3-methylimidazolium, EMIM⁺, tetraphenylphosphonium, PPh₄⁺), and solvents were synthesized and structurally characterized. Single-crystal X-ray analysis showed that the calixarene scaffold consistently adopts the typical cone conformation, stabilized by multiple weak intramolecular interactions. Subtle variations in metal cation coordination (e.g., mono- versus hetero-metallic assemblies) drive minor shifts in the coordination geometry, leading to previously unreported polymorphic forms and coordination motifs. In metal-only frameworks, sodium ions bridge sulfonate groups to form tightly packed bilayers devoid of guest inclusion, while heterometallic Y[sbnd]Na networks enhance structural stability through complementary coordination and supramolecular forces. The introduction of bulky PPh₄⁺ cations expands the unit cell sixfold involving C-H···π interactions without disrupting primary metal-ligand coordination interactions. In contrast, complex with both EMIM⁺ and PPh₄⁺ exhibit mixed host-guest behaviour where EMIM⁺ is included in the calixarene cavity, while PPh₄⁺ molecules fill the interstices/voids between the bilayers. A new PPh₄⁺ inclusion polymorph displays unique mixed phenyl embraces and significantly increased hydration relative to known structures. Collectively, these findings reveal how minimal compositional adjustments govern crystal packing and host-guest inclusion, providing valuable insights for the design of customised calixarene-based materials.