Chloride-Induced Aufbau of Perchlorinated Cyclohexasilanes from Si2Cl6: A Mechanistic Scenario

A surprisingly simple preparative procedure, addition of Si2Cl6 to a solution of [nBu(4)N] Cl in CH2Cl2, leads to the formation of the chloride-complexed cyclic dianions [Si6Cl12 center dot 2Cl](2-), [(SiCl3)Si6Cl11 center dot 2Cl](2-), or [1,y-(SiCl3)(2)Si6Cl10 center dot 2Cl](2-) (y=1, 3, 4), depending on the stoichiometric ratio of the reactants and the reaction temperature (25-85 degrees C). Below -40 degrees 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.