Water Confined in Sub‐Nanochannels of Supramolecular Crystals Exhibiting Ice‐Water‐Like Phase Transition near Room Temperature

Abstract Water is a remarkable yet enigmatic substance with a unique array of anomalous properties, and nanoconfined water exhibits behavior starkly distinct from its bulk counterpart. Herein, two new isomorphic supramolecular polar crystals, [M(18‐crown‐6)] ₃ [Al(ox) ₃ ]·9H ₂ O (M ⁺ = K ⁺ for 1 ; NH ₄ ⁺ for 2 ) are reported, self‐assembled from the cake‐shaped [M(18‐crown‐6)] ⁺ cations (18‐crown‐6 = 1,4,7,10,13,16‐hexaoxacyclooctadecane) and Werner‐type [Al(ox) ₃ ] ³⁻ coordination anions (ox ²⁻ = oxalate). The crystals contain tetranuclear supramolecular clusters and double‐stranded helical water chains, undergoing an isostructural phase transition near room temperature. This transition stems from volume‐conserving rotational motion of crown ether rings and an order‐to‐disorder transformation of lattice water molecules. Strikingly, 1 and 2 exhibits colossal negative thermal expansion behavior ( β = −345.3 m K ⁻¹ for 1 and −261.8 m K ⁻¹ for 2 ) between 293–333 K, linked to the disordering of lattice water molecules, a process analogous to the structural transition from hexagonal ice to liquid water. This is the rare example of water confined within sub‐nanochannels exhibiting characteristics of an ice‐to‐water transition. The findings deepen insights into the functional significance of nanoconfined water in biological systems and propose pathways to modulate physical properties in supramolecular materials.