Soluble Zintl Phases A14ZnGe16 (A = K, Rb) Featuring ​[(η3‑Ge4)Zn(η2‑Ge4)]6– and [Ge4]4– Clusters and the Isolation of [(MesCu)2(η3,η3‑Ge4)]4–: The Missing Link in the Solution Chemistry of Tetrahedral Group 14 Element Zintl Clusters

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).