Liquid Ammonia: More than an Innocent Solvent for Zintl Anions.
Stefanie Gärtner1, Michael Witzmann1, Corinna Lorenz-Fuchs1
1Institute of Inorganic Chemistry, University of Regensburg, 93053 Regensburg, Germany.
Inorganic Chemistry
|August 9, 2024
Summary
Liquid ammonia enables the study of Zintl anions, acting as either an innocent solvent or participating in reactions. This research explores its role in synthesizing and characterizing novel ammoniate crystals and co-crystals.
Area of Science:
- Inorganic Chemistry
- Materials Science
- Solution Chemistry
Background:
- Liquid ammonia was the traditional solvent for Zintl anions but has largely been replaced by more versatile alternatives.
- Advances in crystallography and nuclear magnetic resonance (NMR) spectroscopy now allow for detailed structural investigations of Zintl anions in both solid ammoniate crystals and solutions.
- Liquid ammonia can act as an inert solvent or actively participate in chemical reactions, leading to diverse outcomes.
Purpose of the Study:
- To summarize and contextualize recent literature on Zintl chemistry in liquid ammonia.
- To present and analyze newly characterized ammoniate crystals.
- To explore the multifaceted role of liquid ammonia as a solvent in Zintl anion chemistry.
Main Methods:
- Literature review and synthesis of Zintl compounds.
- Crystallographic analysis of ammoniate crystals.
- Nuclear magnetic resonance (NMR) spectroscopy for solution-phase investigations.
Main Results:
- Innocent dissolution of Zintl anions was observed in several ammoniate crystals, including K2.9Rb5.1[Si4][Si9]·15NH3, Cs4Sn9·12NH3, Cs4Pb9·5NH3, and [Rb@[18]crown-6]2[Rb@[2.2.2]crypt]Rb[Ge9]·4NH3.
- Formal protonation of the [Ge4]4- anion led to the crystallization of [Na@[2.2.2]crypt]2[H2Ge4]·3NH3.
- Novel co-crystals involving triphenylphosphine (PPh3) were synthesized for Sn52- and Pb52- anions, as well as the HSi93- anion, which are not accessible as binary solid-state materials.
Conclusions:
- Liquid ammonia remains a valuable, albeit specialized, solvent for Zintl anion chemistry, offering unique reaction pathways and stabilization of kinetically controlled species at low temperatures.
- The solvent's ability to facilitate both simple dissolution and chemical transformations (e.g., protonation, redox reactions) provides access to a wider range of Zintl structures.
- The successful synthesis of co-crystals demonstrates a strategy to isolate and study Zintl anions that are otherwise difficult to obtain.
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