Mixed Si/Ge nine-atom zintl clusters: ESI mass spectrometric investigations and single-crystal structure determination of paramagnetic [Si _9-xGe_x]^3-

Mixed Si/Ge compounds are of special interest as potential materials for photovoltaic applications. In order to evaluate the usage of soluble precursor compounds, we investigated the synthesis of heteroatomic nine-atom clusters that consist of Si and Ge atoms through dissolution of the ternary Zintl phases K₁₂Si_17-xGe_x (x = 9, 12) and Rb ₁₂Si_17-xGe_x (x = 9). Electrospray ionization (ESI) mass spectrometry demonstrates the presence of mixed Si _9-xGe_x clusters in acetonitrile solution. From ammonia solutions of the ternary phases, four compounds that contain 3-fold negatively charged [Si_9-xGe_x]^3- clusters are obtained. The paramagnetic behavior is confirmed by EPR spectroscopy. [E₉] ^3- Zintl clusters are considered as intermediate structures in the stepwise oxidation of [E₉]^4- clusters to novel element allotropes (E = Si-Pb). The structure of Rb[Rb-crypt]₂[Si _2.3(1)Ge_6.7(1)](NH₃)₇ and the isostructural structures of [Rb-crypt]₃[Si_2.2(1)Ge _6.8(1)](NH₃)₈, [K-crypt]₃[Si _2.4(1)Ge_6.6(1)](NH₃)_8.5, and [K-crypt]₃[Si_4.6(1)Ge_4.4(1)](NH ₃)_8.5 are investigated by single-crystal X-ray diffraction (crypt = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]-hexacosane). The Si/Ge ratio of the products correlates with the composition of the ternary precursor phases.