The Structure of a Complex Open-Framework Germanate Obtained by Combining Powder Charge-Flipping and Simulated Annealing

A novel open-framework germanate, denoted as SU-74-MPMD, was prepared by hydrothermal synthesis using 2-methylpentamethylenediamine (MPMD) as the organic structure directing agent (SDA). The structure of SU-74-MPMD was determined from synchrotron X-ray powder diffraction (XRPD) data using the charge-flipping algorithm, and is one of the most complicated germanates solved from X-ray powder diffraction. The structure is built of layers of Ge10O26(OH)2 (Ge10) clusters that are connected by additional GeO4 tetrahedra to form a three-dimensional (3D) framework with 10- and 12-rings. The organic structure directing agent MPMD in the pores was located by simulated annealing. SU-74-MPMD: space group P21/c, a = 12.62726(4) Å, b = 17.35186 (5) Å, c = 16.17343(5) Å, β = 115.5509(2)° and Z = 4. SU-74 was later synthesized using 1-(2-aminoethyl)­piperazine (AEP) as the SDA (denoted as SU-74-AEP), and its structure was solved by single crystal synchrotron X-ray diffraction. The framework structure of SU-74-AEP is the same as that of SU-74-MPMD. The locations of the organic SDAs in the pores are different for SU-74-MPMD and SU-74-AEP, suggesting a space-filling role of the SDAs. The Ge10 clusters follow the fcu-11-P21/c net. The underlying topology of SU-74 is compared to those of other Ge10 structures containing additional GeO4 tetrahedra. The presence of additional tetrahedra can significantly affect the framework topology through the addition of edges between the Ge10 nodes.