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

Abstract

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 YNa 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.