Hybrid Open Frameworks. 8. Hydrothermal Synthesis, Crystal Structure, and Thermal Behavior of the First Three-Dimensional Titanium(IV) Diphosphonate with an Open Structure:  Ti₃O₂(H₂O)₂(O₃P−(CH₂)−PO₃)₂·(H₂O)₂, or MIL-22

Ti3O2(H2O)2(O3P−(CH2)−PO3)2·(H2O)2, or MIL-22, was prepared in a pure form under hydrothermal conditions (4 days, 493 K, autogenous pressure). Its structure was determined from single-crystal X-ray diffraction data. This compound is monoclinic (space group P21/c (no. 14)), with a = 6.3845(2) Å, b = 11.1816(4) Å, c = 11.7003(5) Å, β = 99.994(1)°, V = 822.60(5) Å3, and Z = 2. Its three-dimensional structure is built up from corner-shared trimeric units of titanium(IV) octahedra linked together via diphosphonate groups. This delimits a three-dimensional network with cross-linked 10-, 7-, and 6-membered ring tunnels along a, b, and c, respectively, at the intersection of which free water molecules are interacting with the terminal water bound to the titanium atoms. The thermal behavior, deduced from TGA and X-ray thermodiffractometry, is described.