Ligand-Free Deltahedral Clusters of Silicon in Solution: Synthesis, Structure, and Electrochemistry of Si₉^2-

Deltahedral nine-atom clusters of silicon, Si92-, were synthesized by mild oxidation of a liquid ammonia solution of K12Si17 with Ph3GeCl in the presence of 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) or 2,2,2-crypt (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane). The clusters were structurally characterized in [K(18-crown-6)]2Si9·C5H5N (yellow; orthorhombic, Pnma; a = 14.013(1), b = 18.108 (1), c = 18.320 (1) Å; Z = 4) crystallized from a pyridine solution of the product of the aforementioned reaction in liquid ammonia. Si92- is the first unequivocally characterized nine-atom cluster of group 14 with a charge of 2−. In addition to pyridine, the product from the reaction in liquid ammonia is also soluble in DMF, and the Si92- clusters were characterized by mass spectrometry in such a solution. The more reduced clusters Si93- have also been crystallized from pyridine solution. Cyclic voltammetry in both pyridine and DMF solutions clearly shows the Si92-/Si93- redox couple as one-electron reversible process. The structural similarities and differences between Si93- and Si92- are discussed herein.