NaRb sub(7)(Si sub(4-x)Ge sub(x)) sub(2) - Soluble Zintl Phases Containing Heteroatomic Tetrahedral -[Si sub(4-x)Ge sub(x)] super(4-) Clusters

The quaternary phases NaRb sub(7)(Si sub(4-x)Ge sub(x)) sub(2) (x= 1-3) crystallize with an NaRb sub(7)Ge sub(8)-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 [Si sub(4-x)Ge sub(x)] super(4-) clusters is indicated by chemically different Si environments, which were analyzed by means of solid-state super(29)Si MAS (magic-angle spinning) NMR spectroscopy of a super(29)Si-enriched NaRb sub(7)(Si sub(4-x)Ge sub(x)) sub(2) sample with x= 0.5 as well as quantum chemical calculations of the NMR coupling parameters. NaRb sub(7)(Si sub(4-x)Ge sub(x)) sub(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 NaRb sub(7)(Si sub(4-x)Ge sub(x)) sub(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)] sub(2)Rb sub(2)[ (MesCu) sub(2)(Si sub(4-x)Ge sub(x))](NH sub(3)) sub(11) were isolated with x= 2.2(1). [(MesCu) sub(2)(Si sub(4-x)Ge sub(x))] super(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. Dissolving Zintl phases with tetrahedral tetrelides: The solid solution NaRb sub(7)(Si sub(4-x)Ge sub(x)) sub(2) enables the isolation of an MesCu-stabilized heteroatomic tetrahedranide by dissolution of the phases in liquid ammonia. Thus, a new soluble Zintl phase containing exclusively tetrel-element tetrahedra is identified, and solid-state NMR spectroscopy reveals the presence of mixed clusters.