Reactivity and Controlled Redox Reactions of Salt-like Intermetallic Compounds in Imidazolium-Based Ionic Liquids

Substituted imidazolium ionic liquids (ILs) were investigated for their reactivity towards Na₁₂ Ge₁₇ as a model system containing redox-sensitive Zintl cluster anions. The ILs proved widely inert for imidazolium cations with a 1,2,3-trisubstitution at least by alkyl groups, and for the anion bis(trifluoromethylsulfonyl)azanide (TFSI). A minute conversion of Na₁₂ Ge₁₇ observed on long-time contact with such ILs was not caused by dissolution of the salt-like compound, and did thus not provide dissolved Ge clusters. Rather, a cation exchange led to the transfer of Na⁺ ions into solution. In contrast, by using benzophenone as an oxidizer, heterogeneous redox reactions of Na₁₂ Ge₁₇ were initiated, transferring a considerable part of Na⁺ into solution. At optimized conditions, an X-ray amorphous product NaGe_6.25 was obtained, which was thermally convertible to the crystalline type-II clathrate Na_24-δ Ge₁₃₆ with almost completely Na-filled polyhedral cages, and α-Ge. The presented method thus provides unexpected access to Na_24-δ Ge₁₃₆ in bulk quantities.