First Incorporation of the Tetrahedral [Sn4]4- Cluster into aDiscrete Solvate Na4[Sn4](NH3)13 from Solutions of Na4Sn4 in Liquid Ammonia

The solution chemistry of nine-atomic group 14 Zintl clusters [E9] (E = Si–Pb) shows a fascinating diversity. Mild oxidation or oxidative coupling of these clusters lead to free radicals or dimeric, oligomeric and polymeric structures of the elements [1 – 5]. Further oxidation even results in new element allotropes with tetrahedrally connected tetrel atoms [6], and structures that are exclusively built up from E9 clusters are predicted to be stable [7, 8]. Additionally, E9 clusters can serve as ligands in transition metal complexes or form more complex intermetalloidal clusters [1 – 5, 9, 10]. Regarding the chemistry of the tetrahedral group 14 Zintl clusters [E4], the number of species isolated from solution-based reactions is rather limited, even though A4E4 Zintl phases (A = alkali metal) with tetrahedral units have been known since 1953 [11]. [Pb4] is the only known tetrahedral cluster anion that has been extracted from an A4E4 phase. Crystals of Rb4Pb4(NH3)2 were obtained from ammonia solutions of Rb4Pb4 [12]. Recently tetrahedral [Si4] and [Ge4] clusters as well as heteroatomic clusters with the composition [Si4−xGex] have been isolated from A12E17 – a Zintl phase that simultaneously contains four-atomic [E4] and nine-atomic [E9] clusters – as CuMes-stabilized complexes [E4(CuMes)2] (Mes = mesityl) [13 – 15]. Bare [Sn4] clusters are further accessible from reactions of alkali metals and elemental Sn or Ph4Sn in liquid ammonia [12]. Related reactions of alkali metals and elemental Sn in molten 18-crown-6, in contrast, lead to the formation of nine-atomic [Sn9] clusters [16]. The isolation of a tetrahedral tetrastannide from an ammonia solution of A4Sn4 has not yet been described in the literature. In the course of our investigations on soluble tetrahedral E4 clusters we report here on results of reactions of A4Sn4 (A = Na, K) phases and CuMes in liquid ammonia.