Liquid Ammonia: More than an Innocent Solvent for Zintl Anions

Liquid ammonia as the original solvent for Zintl anions has been replaced by easier to handle or more versatile solvents in most recent Zintl chemistry. However, methodological advances have made it possible to structurally investigate the anions in ammoniate crystals via crystallography or in the solutions themselves via nuclear magnetic resonance. While in some cases liquid ammonia acts as an innocent solvent, it also provides different possibilities of direct involvement in chemical reactions. In addition to simple dissolution without changes to the anions observed in the solid starting materials, protonation of the anion, incongruent dissolution involving redox processes, and further oxidation and reduction products have been observed. The use of the solvent liquid ammonia under ambient pressure is limited to low temperatures, which in turn allows the monitoring of kinetically stabilized species, some of which cannot be accessed at higher temperatures. In this work, the available literature reports are summarized or referenced, and compounds that have been characterized as new ammoniate crystals are presented and contextualized. Innocent dissolution is observed for clusters involved in K_2.9Rb_5.1[Si₄][Si₉]·15NH₃, Cs₄Sn₉·12NH₃, Cs₄Pb₉·5NH₃, and [Rb@[18]crown-6]₂[Rb@[2.2.2]crypt]Rb[Ge₉]·4NH₃. Formal protonation of [Ge₄]^4- results in the crystallization of [Na@[2.2.2]crypt]₂[H₂Ge₄]·3NH₃. Tt₅^2- (Tt = Sn or Pb) and HSi₉^3- cannot be accessed in a binary solid state material but can be crystallized in co-crystals of PPh₃ in [Rb@[2.2.2]crypt]₂[Sn₅][PPh₃]₂·NH₃, [Rb@[2.2.2]crypt]₂[Pb₅][PPh₃]₂·NH₃, and [K@[2.2.2]crypt]₃[HSi₉][PPh₃]·5NH₃.