Synthesis, crystal structure and thermal properties of Ca₆(C₁₂H₁₄O₄)₄(CO₃)(OH)₂(H₂O)_x – a 3D inorganic hybrid material

The inorganic–organic compound Ca6(1,3-adamantanedicarboxylate)4(CO3)(OH)2(H2O)x with 0 <x <15.2 was synthesized by hydrothermal methods. The crystal structure was determined on the basis of high resolution synchrotron powder diffraction data and poly-crystal measurements. The crystal structure of Ca6(C12H14O4)4(CO3)(OH)2(H2O)14 is tetragonal, space group I41/amd (141) with a = 29.12 Å, c = 15.85 Å, V = 13 440 Å3 and Z = 8. The compound is classified as a 3D inorganic hybrid material with a 3-dimensional inorganic framework consisting of Ca and O, connected to 1,3-adamantanedicarboxylate anions. The structure shows hydrophilic channels in a diamond-like network. In between the channels there exist hydrophobic pores with surfaces defined by adamantane cages. The shortest distance between hydrogen atoms from different molecules in these pores is 3.6 Å. The largest hydrophilic cavity has a diameter of 10 Å and the pores connecting the channels have a diameter of 5 Å. In the as-synthesised state these channels are filled with water molecules. Reversible dehydration–rehydration occurs. The dehydrated compound easily takes up water from ambient air.