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

Abstract

Abstract Here we report on investigations that have revealed for the first time that the Cs + ion templates the same metal germanium sulfide open‐framework as (CH 3 ) 4 N + (TMA + ), and that metal complexing agents enhance crystal size by at least two orders of magnitude. The synthesis, structures and thermal properties of Cs 2 FeGe 4 S 10 ·× H 2 O and TMA 2 FeGe 4 S 10 are also described. Both have 3D zinc blende‐type open‐framework structures. These materials have the same connectivity as TMA 2 MnGe 4 S 10 . The tetrahedral sites in the lattice are alternately substituted by pseudo‐tetrahedral Fe 2+ and adamantanoid Ge 4 S 10 4‐ 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‐Ge 4 S 10 ] 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 TMA 2 FeGe 4 S 10 is stable in nitrogen at 350 °C and under vacuum at 450 °C. The corresponding temperatures for Cs 2 FeGe 4 S 10 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” Cs 2 FeGe 4 S 10 provides an encouraging example for researchers interested in developing sulfide‐based framework materials with practical applications.