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Keywords:

  • Cluster compounds;
  • Heteroatomic Zintl anions;
  • Zintl phases;
  • ­Silicon;
  • Germanium;
  • Solid-state NMR spectroscopy

Abstract

The quaternary phases NaRb7(Si4–xGex)2 (x = 1–3) crystallize with an NaRb7Ge8-type structure and contain tetrahedral four-atom clusters that consist of Si and Ge atoms. The quaternary phases possess larger cell volumes with increasing Ge amount. The formation of heteroatomic [Si4–xGex]4– clusters is indicated by chemically different Si environments, which were analyzed by means of solid-state 29Si MAS (magic-angle spinning) NMR spectroscopy of a 29Si-enriched NaRb7(Si4–xGex)2 sample with x = 0.5 as well as quantum chemical calculations of the NMR coupling parameters. NaRb7(Si4–xGex)2 represents another rare example of a phase with exclusively tetrahedral clusters that readily dissolves in liquid ammonia. The nature of the heteroatomic clusters was further investigated by dissolution of NaRb7(Si4–xGex)2 with x = 2 in liquid ammonia. In the presence of MesCu (Mes = mesityl) and 18-crown-6 as a sequestering agent, crystals with the composition [Rb(18-crown-6)]2Rb2[(MesCu)2(Si4–xGex)](NH3)11 were isolated with x = 2.2(1). [(MesCu)2(Si4–xGex)]4– represents another isomer of a MesCu-stabilized tetrahedral anion and supports the observation that Zintl phases, which contain solely tetrahedranide subunits, represent a new class of soluble Zintl phases.