Variable Intercalation of Calcium Ions in Bilayers of Partially Deprotonated p-Phosphonic Acid Calix[4]arene

Calix[4]arene bearing phosphonic acid groups with different degrees of deprotonation in the para-positions bind Ca²⁺ in the solid state, forming complexes with a 2:5 or 1:1 calixarene:metal ion ratio for complexes 1 and 2, respectively. The cone-shaped calixarenes are organized into bilayers of similar packing ilk, with the nature of the calixarene interplay mapped out using Hirshfeld surface analysis. The cavities of the calixarenes are directed normal to the plane of the bilayers, with the bilayers bridged by metal ions bound to phosphonate O-centers, with additional coordination involving water molecules. Calixarenes from adjacent bilayers in complex 1 are aligned as “molecular capsules” with metal ions around their equatorial seam.