Soluble Zintl Phases <i>A</i><sub>14</sub>ZnGe<sub>16</sub> (<i>A</i> = K, Rb) Featuring [(η<sup>3</sup>‑Ge<sub>4</sub>)Zn(η<sup>2</sup>‑Ge<sub>4</sub>)]<sup>6–</sup> and [Ge<sub>4</sub>]<sup>4–</sup> Clusters and the Isolation of [(MesCu)<sub>2</sub>(η<sup>3</sup>,η<sup>3</sup>‑Ge<sub>4</sub>)]<sup>4–</sup>: The Missing Link in the Solution Chemistry of Tetrahedral Group 14 Element Zintl Clusters
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The number of Zintl phases containing polyhedral clusters of tetrel elements that are accessible for chemical reactions of the main-group element clusters is rather limited. The synthesis and structural characterization of two novel ternary intermetallic phases A14ZnGe16 (A = K, Rb) are presented, and their chemical reactivity is investigated. The compounds can be rationalized as Zintl phases with 14 alkali metal cations A+ (A = K, Rb), two tetrahedral [Ge4]4– Zintl anions, and one anionic heterometallic [(Ge4)Zn(Ge4)]6– cluster per formula unit. The Zn–Ge cluster comprises two (Ge4) tetrahedra which are linked by a Zn atom, with one (Ge4) tetrahedron coordinating with a triangular face (η3) and the other one with an edge (η2). [(η3-Ge4)Zn(η2-Ge4)]6– is a new isomer of the [(Ge4)Zn(Ge4)]6– anion in Cs6ZnGe8. The phases dissolve in liquid ammonia and thus represent rare examples of soluble Zintl compounds with deltahedral units of group 14 element atoms. Compounds with tetrahedral [E4]4– species have previously been isolated from solution for E = Si, Sn, and Pb, and the current investigation provides the “missing link” for E = Ge. Reaction of an ammonia solution of K14ZnGe16 with MesCu (Mes = 2,4,6-(CH3)3C6H2) in the presence of [18]-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) yielded crystals of the salt [K([18]-crown-6)]2K2[(MesCu)2Ge4](NH3)7.5 with the polyanion [(MesCu)2Ge4]4–. This MesCu-stabilized tetrahedral [Ge4]4– cluster also completes the series of [(MesCu)2Si4–xGex]4– clusters, which have previously been isolated from solution for x = 0 and 0.7, as the end member with x = 4. The electronic structures of [(Ge4)Zn(Ge4)]6– and [(MesCu)2Ge4]4– were investigated in terms of a molecular orbital description and analyses of the electron localization functions. The results are compared with band structure calculations for the A14ZnGe16 phases (A = K, Rb).
