Thermally Stable Self‐assembling Open‐Frameworks: Isostructural Cs+ and (CH3)4N+ Iron Germanium Sulfides

Abstract

AbstractHere we report on investigations that have revealed for the first time that the Cs+ ion templates the same metal germanium sulfide open‐framework as (CH3)4N+ (TMA+), and that metal complexing agents enhance crystal size by at least two orders of magnitude. The synthesis, structures and thermal properties of Cs2FeGe4S10 ·× H2O and TMA2FeGe4S10 are also described. Both have 3D zinc blende‐type open‐framework structures. These materials have the same connectivity as TMA2MnGe4S10. The tetrahedral sites in the lattice are alternately substituted by pseudo‐tetrahedral Fe2+ and adamantanoid Ge4S104‐ building blocks, covalently linked together by Fe(μ‐S)Ge bridge bonds, to give a tetragonal unit cell. The charge‐balance of the anionic framework [Fe‐Ge4S10]2‐ is maintained by either Cs+ or TMA+ ions in the cavity spaces. Synthesis of these materials demonstrates an interesting example of a self‐assembly process in which a 3D framework is built from molecular precursors. Water adsorption‐desorption cycling from room temperature to 200 °C reveals framework flexibility between larger and smaller tetragonal unit cell 14 isotypes. The compound TMA2FeGe4S10 is stable in nitrogen at 350 °C and under vacuum at 450 °C. The corresponding temperatures for Cs2FeGe4S10 are 530 °C and 630°C; it is stable on cooling to room temperature under vacuum, and after subsequent exposure to air. Six hundred thirty degrees celsius is the highest recorded temperature at which the integrity of a non‐oxide framework has been maintained. The framework stability and flexibility of “all‐inorganic” Cs2FeGe4S10 provides an encouraging example for researchers interested in developing sulfide‐based framework materials with practical applications.