Unraveling the Amine-Induced Disproportionation Reaction of Perchlorinated Silanes-A DFT Study

Challenges in the Synthesis and Processing of Hydrosilanes as Precursors for Silicon Deposition

Hydrosilanes are highly attractive compounds, which can be processed as liquids with printing technology to amorphous silicon films on nearly any solid substrate. The silicon layers can be processed for electronic devices like transistors or thin-film solar cells. The endothermic character of hydrosilanes with their positive enthalpies of formation results in favorable properties for processing. The larger the molecules, the lower their decomposition temperature and the higher their photoactivity. Cyclic hydrosilanes such as cyclopentasilane and cyclohexasilane can be easily deposited. The branched neopentasilane is more difficult to deposit but yields better-quality films after processing. The key challenge is the complex synthesis of the precursors and the hydrosilanes. The available preparative methods are presented in this review and their advantages and disadvantages are evaluated. The following synthesis methods are presented and discussed in this article: Wurtz coupling and other reductive coupling processes, dehydrogenative coupling of silanes, plasma synthesis of chlorinated polysilanes, amine- or chloride-induced disproportionations, and transformation of monosilane to higher silanes. Plasma synthesis is already carried out today as a continuous industrial process. The most effective synthesis methods in the laboratory are currently amine- and chloride-induced disproportionations. There is a great need to further optimize the syntheses of hydrosilanes and to develop new simple synthesis variants.

Siemens Reloaded: Chloride-Assisted Selective Hydrodechlorination of SiCl4 to HSiCl3

Trichlorosilane is the key intermediate for the large-scale production of polycrystalline silicon in the Siemens and Union Carbide processes. Both processes, however, are highly inefficient, and over two thirds of the trichlorosilane employed is converted to unwanted silicon tetrachloride accumulating in millions of tons per year on a global scale. In this combined experimental and theoretical study we report an energetically and environmentally benign synthetic protocol for the highly selective conversion of SiCl₄ to HSiCl₃ using organohydridosilanes as recyclable hydrogen transfer reagents in combination with onium chlorides as efficient catalysts. We put the same protocol to further use demonstrating the quantitative conversion of higher oligosilane residues, which form as another unwanted and potentially hazardous byproduct of Siemens processes, to HSiCl₃ in a low-temperature recycling step.

Syntheses and Molecular Structures of Liquid Pyrophoric Hydridosilanes

Trisilane, isotetrasilane, neopentasilane, and cyclohexasilane have been prepared in gram scale. In-situ cryo crystallization of these pyrophoric liquids in sealed capillaries on the diffractometer allows access to the single crystal structures of these compounds. Structural parameters are discussed and compared to gas-phase electron diffraction structures from literature and with the results from quantum chemical calculations. Significantly higher packing indices are found for the silanes compared to the corresponding alkanes. Radiation with ultraviolet light (365 nm) and parallel ESR (EPR) measurement shows that cyclohexasilane is easily split into radicals, which subsequently leads to the formation of branched and chain-like oligomers. The other compounds form no radicals under these conditions. NMR spectra of all four compounds have been recorded.

Disilane Cleavage with Selected Alkali and Alkaline Earth Metal Salts

The industry-scale production of methylchloromonosilanes in the Müller-Rochow Direct Process is accompanied by the formation of a residue, the direct process residue (DPR), comprised of disilanes Me_n Si₂ Cl_6-n (n=1-6). Great research efforts have been devoted to the recycling of these disilanes into monosilanes to allow reintroduction into the siloxane production chain. In this work, disilane cleavage by using alkali and alkaline earth metal salts is reported. The reaction with metal hydrides, in particular lithium hydride (LiH), leads to efficient reduction of chlorine containing disilanes but also induces disproportionation into mono- and oligosilanes. Alkali and alkaline earth chlorides, formed in the course of the reduction, specifically induce disproportionation of highly chlorinated disilanes, whereas highly methylated disilanes (n>3) remain unreacted. Nearly quantitative DPR conversion into monosilanes was achieved by using concentrated HCl/ether solutions in the presence of lithium chloride.

New Condensation Polymer Precursors Containing Consecutive Silicon Atoms-Decaisopropoxycyclopentasilane and Dodecaethoxyneopentasilane-And Their Sol⁻Gel Polymerization

The sol-gel polymerization of alkoxysilanes is a convenient and widely used method for the synthesis of silicon polymers and silicon-organic composites. The development of new sol-gel precursors is very important for obtaining new types of sol-gel products. New condensation polymer precursors containing consecutive silicon atoms-decaisopropoxycyclopentasilane (CPS) and dodecaethoxyneopentasilane (NPS)-were synthesized for the preparation of polysilane-polysiloxane material. The CPS and NPS xerogels were prepared by the sol-gel polymerization of CPS and NPS under three reaction conditions (acidic, basic and neutral). The CPS and NPS xerogels were characterized using N2 physisorption measurements (Brunauer-Emmett-Teller; BET and Brunauer-Joyner-Halenda; BJH), solid-state CP/MAS (cross-polarization/magic angle spinning) NMRs (nuclear magnetic resonances), TEM, and SEM. Their porosity and morphology were strongly affected by the structure of the precursors, and partial oxidative cleavage of Si-Si bonds occurred during the sol-gel process. The new condensation polymer precursors are expected to expand the choice of approaches for new polysilane-polysiloxane.

Silicon chemistry in zero to three dimensions: from dichlorosilylene to silafullerane

As one of the simplest examples of functionalized Si(ii) species, the SiCl₂/[SiCl₃]⁻ system is not only fundamentally interesting, but also an important starting point for the assembly of oligosilane chains, rings, and clusters.

The Weakly Coordinating Tris(trichlorosilyl)silyl Anion

Closely following the procedure for the preparation of the base-stabilized dichlorosilylene complex NHC^Dipp ⋅SiCl₂ reported by Roesky, Stalke, and co-workers (Angew. Chem. Int. Ed. 2009, 48, 5683-5686), a few crystals of the salt [NHC^Dipp -H⋅⋅⋅Cl⋅⋅⋅H-NHC^Dipp ]Si(SiCl₃ )₃ were isolated, aside from the reported byproduct [NHC^Dipp -H⁺ ⋅⋅⋅Cl^- ], and characterized by X-ray crystallography (NHC^Dipp =N,N-di(2,6-diisopropylphenyl)imidazo-2-ylidene). They contain the weakly coordinating anion Si(SiCl₃ )₃^- , which was also obtained in high yields upon deprotonation of the conjugate Brønsted acid HSi(SiCl₃ )₃ with NHC^Dipp or PMP (PMP=1,2,2,6,6-pentamethylpiperidine). The acidity of HSi(SiCl₃ )₃ was estimated by DFT calculations to be substantially higher than those of other H-silanes. Further DFT studies on the electronic structure of Si(SiCl₃ )₃^- , including the electrostatic potential and the electron localizability, confirmed its low basicity and nucleophilicity compared with other silyl anions.