Polyoxomolybdate Layered Crystals Constructed from a Heterocyclic Surfactant: Syntheses, Pseudopolymorphism and Introduction of Metal Cations

Crystals with layered structures are crucial for the construction of functional materials exhibiting intercalation, ionic conductivity, or emission properties. Polyoxometalate crystals hybridized with surfactant cations have distinct layered packings due to the surfactants which can form lamellar structures. Introducing metal cations into such polyoxometalate-surfactant hybrid crystals is significant for the addition of specific functions. Here, polyoxomolybdate-surfactant hybrid crystals were synthesized as single crystals, and unambiguously characterized by X-ray structure analyses. Octamolybdate ([Mo₈O₂₆]^4-, Mo₈) and heterocyclic surfactant of 1-dodecylpyridinium (C₁₂py) were employed. The hybrid crystals were composed of α-type and β-type Mo₈ isomers. Two crystalline phases containing α-type Mo₈ were obtained as pseudopolymorphs depending on the crystallization conditions. Crystallization with the presence of rubidium and cesium cations caused the formation of metal cation-introduced hybrid crystals comprising β-Mo₈ (C₁₂py-Rb-Mo₈ and C₁₂py-Cs-Mo₈). The yield of the C₁₂py-Rb-Mo₈ hybrid crystal was almost constant within crystallization temperatures of 279-303 K, while that of C₁₂py-Cs-Mo₈ decreased over 288 K. This means that the C₁₂py-Mo₈ hybrid crystal can capture Rb⁺ and Cs⁺ from the solution phase into the solids as the C₁₂py-Rb-Mo₈ and C₁₂py-Cs-Mo₈ hybrid crystals. The C₁₂py-Mo₈ hybrid crystals could be applied to ion-capturing materials for heavy metal cation removal.