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.