Investigation on the chain-to-chain and chain-to-open-framework transformations of two one-dimensional aluminophosphate chains

Using ethylenediamine as a template, two one-dimensional (1-D) aluminophosphate compounds [AlP₂O₈H][NH₃CH₂ CH₂NH₃] (1) and [AlP₂O₈][NH₃CH₂ CH₂NH₃][NH₄] (2) have been prepared from a gel system: 1.0:x:y:44 Al(i-PrO)₃-H₃PO₄-en-EG (x = 3.0-9.0, y = 1.0-11.0) Compound 1 consists of edge-sharing four-membered ring (MR) chains, denoted as AlPO-ESC, and compound 2 consists of corner-sharing 4-MR chains, denoted as AlPO-CSC. The molar ratio of en: H₃PO₄ in the starting gel has an important influence on the final product. If en: H₃PO₄ ≥ 1, AlPO-CSC is obtained, while if en: H₃PO₄ < 1, AlPO-ESC is formed. These two chains can transform to each other upon addition of some extra amount of en or H₃PO₄ to the reaction mixture in which AlPO-ESC or AlPO-CSC is crystallized. On the basis of XRD and ²⁷Al and ³¹P MAS NMR analyses, a possible chain-to-chain transformation mechanism is proposed. The corner-sharing 4-MR chains of AlPO-CSC, as well as the edge-sharing 4-MR chains of AlPO-ESC can be assembled to 3-D open-framework compound NiAlP₂O₈·C₂N₂H₉ through Ni²⁺ cations. It is noted that AlPO-CSC remains in the structure of NiAlP₂O₈·C₂N ₂H₉. It is believed that AlPO-ESC might be first transformed to AlPO-CSC followed by the assembly to 3-D open-framework of NiAlP₂O₈·C₂N₂H₉ through Ni²⁺ cations.