(1,2-DAPH₂)₂Ge₉(OH)₄O₁₈ · 2H₂O: A New Microporous Germanate Based on the Interconnection of Ge₉O₁₈ Clusters Showing Reversible Water Emission and Uptake

Treatment of germanium dioxide with a 33% aqueous solution of 1,2-diaminopropane (DAP) under solvothermal conditions leads to the formation of colourless octahedral crystals of the novel germanate (1,2-DAPH₂)₂Ge₉(OH)₄O₁₈ · 2 H₂O. The compound crystallises in the orthorhombic space group Pbca, a = 14.4155(10), b = 12.9384(9), c = 14.5417(8) Å,V =2712.2 (3) ų; Z =4. The structure consists of an anionic [Ge₉(OH)₄O₁₈]⁴⁻ framework with isolated [1,2-DAPH₂]²⁺ cations and water molecules in the channels of the structure. The primary building units of the anionic framework are GeO₄ tetrahedra, GeO₅ trigonal bipyramids, and GeO₆ octahedra. The asymmetric unit consists of two GeO₅ units sharing a common edge to form a Ge₂O₈ unit. This Ge₂O₈ unit is corner-linked to two GeO₄ tetrahedra, and finally one GeO₄ tetrahedron is connected to a GeO₆ octahedron by cornersharing. The connectivity yields a chain-like Ge₅O₁₉ fragment as a secondary building unit (SBU). The chain fragments are interconnected with each other forming the three-dimensional framework. Three types of channels with diameters ranging from 5.98 to 8.025 Å intersect the three-dimensional germanate network. Upon heating the compound decomposes in three steps starting at about 125˚C losing the water and the 1,2-diaminopropane molecules.

In situ X-ray diffraction experiments show that the water molecules are removed retaining the integrity of the skeleton of the material. The removal of H₂O is accompanied with an anisotropic shrinkage of the structure. The original lattice parameters were obtained after the uptake of water.