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 K Si , comprising [Si ] units, and K Si , consisting of [Si ] and [Si ] 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 Si isotope-enriched samples. Si-MAS NMR and Raman shifts of the phase-pure solids K Si and K Si were interpreted by quantum-chemical calculations. Extraction of [Si ] clusters from K Si with liquid ammonia/222crypt and their transfer to pyridine yields in a red solid containing Si clusters. This compound was characterized by elemental and EDX analyses and Si-MAS NMR and Raman spectroscopy. Charged Si clusters were detected by Si NMR in solution. Si and H NMR spectra reveal the presence of the [H Si ] cluster anion in solution.

Anionic Siliconoids from Zintl Phases: R3Si9− with Six and R2Si92− with Seven Unsubstituted Exposed Silicon Cluster Atoms (R=Si(tBu)2H)

“…Recently, the mono‐protonated cluster HSi 9 3− was obtained upon the isolation of crystals of A 12 Si 17 ( A =K, Rb) from liquid ammonia with the help of sequestering agents. A transfer of the extraction products from liquid ammonia to pyridine yielded the doubly‐protonated species H 2 Si 9 2− . A similar procedure allowed for the structural characterization of the iso‐valence‐electronic cluster with mixed tetrel elements H 2 Si 9− x Ge x 2− ( x= 4.1) .…”

“…A similar procedure allowed for the structural characterization of the iso‐valence‐electronic cluster with mixed tetrel elements H 2 Si 9− x Ge x 2− ( x= 4.1) . Both, HSi 9 3− and H 2 Si 9 2− , were fully characterized by 1 H and 29 Si NMR spectroscopy in solution. The formation of well‐defined solutions that contain nine‐atom silicon clusters after activation with liquid ammonia finally paved the way for further conversions of these clusters in solution with appropriate reactants.…”

“…The 29 Si NMR resonance at 18.00 ppm is attributed to the three equivalent Si( t Bu) 2 H groups, whereas the strongly high‐field shifted signals at −175.16 and −358.81 ppm arise from the three substituted and six unsubstituted Si vertex atoms in the Si 9 polyhedron, respectively. The experimental 29 Si NMR signal γ (−358.81 ppm) of the unsubstituted vertex atoms shows a stronger upfield shift, as usually observed for prominent siliconoid species, and is comparable to the signals for the protonated Si 9 cluster species HSi 9 3− (unsubstituted vertex atoms: −359 ppm) and to the experimental and calculated 29 Si NMR average values for H 2 Si 9 2− (exp. : −346 ppm; calcd: −348 ppm).…”

“…, [19] were fully characterized by 1 Ha nd 29 Si NMR spectroscopy in solution.T he formation of well-defined solutions that contain nine-atom silicon clusters after activation with liquid ammonia finally paved the way for further conversionso ft hese clusters in solution with appropriate reactants. Theoretical studies support the formation of siliconoids by the attachment of sp 3 -Si linkers, [20] and triggered us to investigate the reactivity of Si 9 4À clusters obtained from K 12 Si 17 and chloro-silanes with the ambition to synthesize larger siliconoid units.…”

“…At ransfer of the extractionp roducts from liquid ammonia to pyridine yielded the doubly-protonated species H 2 Si 9 2À . [19] As imilarp rocedure allowed for the structural characterizationo ft he iso-valence-electronic cluster with mixed tetrel elements H 2 Si 9Àx Ge x 2À (x = 4.1). [18a] Both, HSi 9 3À [18b] and H 2 Si 9…”