Ca 2 Ga 4 Ge 6 and Ca 3 Ga 4 Ge 6 : Synthesis, Structure, and Electronic Properties

Abstract

During the search for transition metal-free alkyne hydrogenation catalysts, two new ternary Ca−Ga−Ge phases, Ca2Ga4Ge6 (Cmc21, a=4.1600(10) Å, b=23.283(5) Å, c=10.789(3) Å) and Ca3Ga4Ge6 (C2/m, a=24.063(2) Å, b=4.1987(4) Å, c=10.9794(9) Å, β=91.409(4)°), were discovered. These compounds are isostructural to the previously established Yb2Ga4Ge6 and Yb3Ga4Ge6 analogues, and according to Zintl-Klemm counting rules, consist of anionic [Ga4Ge6]4− and [Ga4Ge6]6− frameworks in which every Ga and Ge atom would have a formal octet with no Ga−Ga or Ga−Ge π-bonding. These compounds are metallic, based on temperature dependent electrical resistivity and thermopower measurements for Ca3Ga4Ge6, along with density functional theory calculations for both phases. Unlike the highly active 13-layer trigonal CaGaGe phase, these new compounds exhibit minimal activity in the semi/full alkyne hydrogenation of phenylacetylene, which is consistent with previous observations that the lack of a formal octet for framework atoms is essential for catalysis in these Zintl-Klemm compounds.