Mixed Si/Ge
Nine-Atom Zintl Clusters: ESI Mass Spectrometric Investigations and
Single-Crystal Structure Determination of Paramagnetic [Si<sub>9–<i>x</i></sub>Ge<sub><i>x</i></sub>]<sup>3–</sup>
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Abstract
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 K<sub>12</sub>Si<sub>17–<i>x</i></sub>Ge<sub><i>x</i></sub> (<i>x</i> = 9, 12) and Rb<sub>12</sub>Si<sub>17–<i>x</i></sub>Ge<sub><i>x</i></sub> (<i>x</i> = 9). Electrospray ionization (ESI) mass spectrometry demonstrates
the presence of mixed Si<sub>9–<i>x</i></sub>Ge<sub><i>x</i></sub> clusters in acetonitrile solution. From
ammonia solutions of the ternary phases, four compounds that contain
3-fold negatively charged [Si<sub>9–<i>x</i></sub>Ge<sub><i>x</i></sub>]<sup>3–</sup> clusters are
obtained. The paramagnetic behavior is confirmed by EPR spectroscopy.
[E<sub>9</sub>]<sup>3–</sup> Zintl clusters are considered
as intermediate structures in the stepwise oxidation of [E<sub>9</sub>]<sup>4–</sup> clusters to novel element allotropes (E = Si–Pb).
The structure of Rb[Rb-crypt]<sub>2</sub>[Si<sub>2.3(1)</sub>Ge<sub>6.7(1)</sub>](NH<sub>3</sub>)<sub>7</sub> and the isostructural structures
of [Rb-crypt]<sub>3</sub>[Si<sub>2.2(1)</sub>Ge<sub>6.8(1)</sub>](NH<sub>3</sub>)<sub>8</sub>, [K-crypt]<sub>3</sub>[Si<sub>2.4(1)</sub>Ge<sub>6.6(1)</sub>](NH<sub>3</sub>)<sub>8.5</sub>, and [K-crypt]<sub>3</sub>[Si<sub>4.6(1)</sub>Ge<sub>4.4(1)</sub>](NH<sub>3</sub>)<sub>8.5</sub> 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
