Charged Si₉ Clusters in Neat Solids and the Detection of [H₂Si₉]²⁻ in Solution: A Combined NMR, Raman, Mass Spectrometric, and Quantum Chemical Investigation

Abstract

Polyanionic silicon clusters are provided by the Zintl phases K₄ Si₄ , comprising [Si₄ ]^4- units, and K₁₂ Si₁₇ , consisting of [Si₄ ]^4- and [Si₉ ]^4- clusters. A combination of solid-state MAS-NMR, solution NMR, and Raman spectroscopy, electrospray ionization mass spectrometry, and quantum-chemical investigations was used to investigate four- and nine-atomic silicon Zintl clusters in neat solids and solution. The results were compared to ²⁹ Si isotope-enriched samples. ²⁹ Si-MAS NMR and Raman shifts of the phase-pure solids K₄ Si₄ and K₁₂ Si₁₇ were interpreted by quantum-chemical calculations. Extraction of [Si₉ ]^4- clusters from K₁₂ Si₁₇ with liquid ammonia/222crypt and their transfer to pyridine yields in a red solid containing Si₉ clusters. This compound was characterized by elemental and EDX analyses and ²⁹ Si-MAS NMR and Raman spectroscopy. Charged Si₉ clusters were detected by ²⁹ Si NMR in solution. ²⁹ Si and ¹ H NMR spectra reveal the presence of the [H₂ Si₉ ]^2- cluster anion in solution.