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) Å3; Z = 4. The structure consists of an anionic [Ge9(OH)4O18]4− framework with isolated [1,2-DAPH2] 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.