Germanium tetrafluoride (GeF₄) is used, e.g., for the preparation of Si-Ge thin films and could have applications in semiconductor technology. Its fluorination product GeF₅^–, in contrast, had not been isolated so far and only substituted analogues such as [(C₂F₅)₃GeF₂]⁻ and [(CF₃)₃GeF₂]⁻ had been characterized. There have been attempts to transform GeF₄ to GeF₅^– using fluorinating agents, e.g., XeF₆ or ClO₂F, however, the resulting product polymerizes due to insufficient stabilization.

Gašper Tavčar, Jožef Stefan Institute, Ljubljana, Slovenia, and colleagues have synthesized and characterized discrete GeF₅^– anions. The team used bulky 1,3-bis(2,6-diisopropylphenyl)imidazolium fluoride ([(L^Dipp)H]F) as a fluorinating agent for GeF₄ in acetonitrile. The resulting salt [(LDipp)H][GeF₅] was obtained in quantitative yield and characterized using X-ray crystallography as well as Raman and ¹⁹F NMR spectroscopy. The researchers successfully performed the same reaction with SiF₄ to give [(LDipp)H][SiF₅].

The monomeric MF₅^– anions (M = Si, Ge) in the products have a trigonal pyramidal structure (pictured). The sterically demanding imidazolium cation stabilizes the discrete anions and protects them from polymerization. In addition, according to quantum chemical calculations, the electrostatic interactions with such large ions favor the trigonal pyramidal structure of GeF₅^– over the octahedral structure in the polymerized product. According to the researchers, the bulky fluorinating agent used in these syntheses could find widespread use in exploring other areas of fluorine chemistry.

• Discrete GeF₅^– Anion Structurally Characterized with a Readily Synthesized Imidazolium Based Naked Fluoride Reagent,
  Blaž Alič, Melita Tramšek, Anton Kokalj, Gašper Tavčar,
  Inorg. Chem. 2017.
  DOI: 10.1021/acs.inorgchem.7b01606