Making Use of the Direct Process Residue: Synthesis of Bifunctional Monosilanes.

The industrial production of monosilanes Men SiCl4-n (n=1-3) through the Müller-Rochow Direct Process generates disilanes Men Si2 Cl6-n (n=2-6) as unwanted byproducts ("Direct Process Residue", DPR) by the thousands of tons annually, large quantities of which are usually disposed of by incineration. Herein we report a surprisingly facile and highly effective protocol for conversion of the DPR: hydrogenation with complex metal hydrides followed by Si-Si bond cleavage with HCl/ether solutions gives (mostly bifunctional) monosilanes in excellent yields. Competing side reactions are efficiently suppressed by the appropriate choice of reaction conditions.

Lewis Base Catalyzed Selective Chlorination of Monosilanes.

A preparatively facile, highly selective synthesis of bifunctional monosilanes R2 SiHCl, RSiHCl2 and RSiH2 Cl is reported. By chlorination of R2 SiH2 and RSiH3 with concentrated HCl/ether solutions, the stepwise introduction of Si-Cl bonds is readily controlled by temperature and reaction time for a broad range of substrates. In a combined experimental and computational study, we establish a new mode of Si-H bond activation assisted by Lewis bases such as ethers, amines, phosphines, and chloride ions. Elucidation of the underlying reaction mechanisms shows that alcohol assistance through hydrogen-bond networks is equally efficient and selective. Remarkably, formation of alkoxysilanes or siloxanes is not observed under moderate reaction conditions.

Thermal Synthesis of Perchlorinated Oligosilanes: A Fresh Look at an Old Reaction.

A combined experimental and theoretical study of the high-temperature reaction of SiCl4 and elemental silicon is presented. The nature and reactivity of the product formed upon rapid cooling of the gaseous reaction mixture is investigated by comparison with the defined model compounds cyclo-Si5 Cl10 , n-Si5 Cl12 and n-Si4 Cl10 . A DFT assessment provides mechanistic insight into the oligosilane formation. Experimental 29 Si NMR investigations, supported by quantum-chemical 29 Si NMR calculations, consistently show that the reaction product is composed of discrete molecular perchlorinated oligosilanes. Low-temperature chlorination is an unexpectedly selective means for the transformation of cyclosilanes to acyclic species by endocyclic Si-Si bond cleavage, and we provide a mechanistic rationalization for this observation. In contrast to the raw material, the product obtained after low-temperature chlorination represents an efficient source of neo-Si5 Cl12 or the amine-stabilized disilene EtMe2 N⋅SiCl2 Si(SiCl3 )2 through reaction with aliphatic amines.

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

A detailed quantum-chemical study on the amine-induced disproportionation reaction of perchlorinated silanes to neo-Si5 Cl12 is reported. The key intermediate in the resulting mechanistic scenario is a dichlorosilylene amine adduct, which is in tune with recent experimental findings. Yet, at variance with the generally accepted notion of silicon-chain growth by concerted silylene insertion into Si-Cl bonds of lower silanes, the formation of neo-Si5 Cl12 follows more complex pathways. The reactivity is dominated by the Lewis-base character of the dichlorosilylene amine adduct and characterized by three elementary steps that bear close resemblance to the key elementary steps identified earlier for the chloride-induced disproportionation of Si2 Cl6 . NBO and QTAIM analyses of the key reactive species SiCl2 ⋅NMe3 and SiCl3 (-) provide a rationale for these striking similarities.

Chloride-induced aufbau of perchlorinated cyclohexasilanes from Si2Cl6: a mechanistic scenario.

A surprisingly simple preparative procedure, addition of Si2Cl6 to a solution of [nBu4N]Cl in CH2Cl2, leads to the formation of the chloride-complexed cyclic dianions [Si6Cl12⋅2Cl](2-), [(SiCl3)Si6Cl11⋅2Cl](2-), or [1,y-(SiCl3)2Si6Cl10⋅2Cl](2-) (y = 1, 3, 4), depending on the stoichiometric ratio of the reactants and the reaction temperature (25-85 °C). Below -40 °C the open-chain oligosilane chloride adducts [Si3Cl9](-), [Si3Cl10](2-), [Si4Cl11](-), and [Si6Cl15](-) are formed, again depending on the reaction conditions chosen. All species were characterized by X-ray crystallography. The underlying reaction mechanism is elucidated by DFT calculations. It incorporates all experimental findings and involves a few key elementary steps: 1) chloride-induced liberation of SiCl3(-) or higher silanides, 2) their addition to neutral silanes yielding larger oligosilane chloride adducts, 3) dimerization of larger silanides to (substituted) cyclohexasilane dichloride adducts with inverse sandwich structure.