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

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 (CH3),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,FeGe4Slo . x H20 and TMA2FeGe4Slo are also described. Both have 3D zinc blende-type open-framework structures. These materials have the same connectivity a s TMA2MnGe,Slo. The tetrahedral sites in the lattice are alternately substituted by pseudo-tetrahedral Fez+ and adamantanoid Ge&; building blocks, covalently linked together by Fe(p-S)Ge bridge bonds, to give a tetragonal unit cell. The charge-balance of the anionic framework [FeGe4S,o]2p is maintained by either Cst 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 TMA2FeGe,Slo is stable in nitrogen at 350°C and under vacuum at 450°C. The corresponding temperatures for Cs2FeGe,SIo 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 a t which the integrity of a non-oxide framework has been maintained. The framework stability and flexibility of "all-inorganic" CszFeGe4Slo provides an encouraging example for researchers interested in developing sulfide-based framework materials with practical applications. The Cs+ and (CH3)4N+ (TMA+) templatc mediated synthesis, single crystal XRD structure deterinination and thermal properties of CszFeGe4Slo x H20 and TMA2FeGe4Slo are reported here for the first time. These materials have isostructural zinc blende-type iron, germanium, and sulfide open-frameworks; they exhbit impressive framework stability, and in the case of Cs2FeGe4Slo . x HZO, framework flexibility. In a procedure similar to that reported for TMA2MnGe4Slo, they were prepared by a two step reaction sequence [*]. Elemental Ge (99.99%). Aldrich) and S (99.999%, Aldrich), in stoichiometric amounts, were hydrothermally digested in a 5% excess of aqueous TMAOH or CsOH (H20/Ge = 50; 150°C, tumbled, 16 hours). From the resultant solution, TMA4Ge4Slo or Cs4Ge4Slo was precipitated by adding acetonelethanol. lf was then recovered by filtration or centrifugation, and purified and characterized (pXRD, Ranan). Monoclinic salts of Cs4Ge4Slo . 3 HZO and Cs4Ge4Slo . 4 H 2 0 were previously reported, along with their vibrational ~pectra['~,'~]. Yields of greater than 95% were readily achieved for the TMA4Ge4Slo material with greater than 99.99% purity [']. Use of TMA4Ge4Slo or Cs4Ge4Sio as synthetic precursors avoided inhomogeneous and insoluble GeS2, which can often contain impurities deleterious to the crystallization of metal cha...

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