Reversible Dehydration Behavior Reveals Coordinatively Unsaturated Metal Sites in Microporous Aluminum Phosphonates

Incorporation of the same ligand into three different aluminum phenylenediphosphonates (Al­(H2O)­(O3PC6H4PO3H) (1), Al4(H2O)2­(O3PC6H4PO3)3 (2), and Al4(H2O)4­(O3PC6H4PO3)2.84­(OH)0.64 (3)) was accomplished by varying the synthetic conditions. The compounds have different sorption properties; however, all exhibit reversible dehydration behavior. The structures of the hydrated and dehydrated phases were determined from powder X-ray diffraction data. Compounds 2 and 3 were found to be microporous, while compound 1 was found to be nonporous. The stability of the dehydrated phase and the resulting porosity was found to be influenced by the change in the structure upon loss of water.