Electrochemical oxidation of cyclic polysilanes

During the past decade, the electrochemical properties of cyclic polysilane derivatives, namely [Mes2Si]3 (I), [t-Bu(Me)Si]4 (IIa), [Et2Si]4 (IIb), [(n-Pr)2Si]5 (IIIa), [Et2Si]5 (IIIb), [Me2Si]6 (IVa), [Et2Si]6 (IVb), [Et2Si]7 (V), [Me2Si]8 (VI), and [Me2Si]9 (VII), have been explored in our laboratory. Various parameters have been investigated, such as anodic peak potentials, the effect of anode material, nature of supporting electrolyte, atmosphere under which electrolyses were conducted, extent of electricity consumption, solvent, ring size, applied potential, and more. It has been found that the type of products emerged from electrolysis was highly sensitive to the nature of electrolyte. Thus, in the presence of BF4− salts, cyclic polysilanes underwent ring opening followed by SiSi bond cleavage to afford linear silanes with F [one or two (major)], H or OH moieties at the terminal positions. In the presence of ClO4− salts, mostly cyclic siloxanes with different number of oxygen atoms were formed, in addition to linear ones, in some cases. The origin of oxygen atoms in the oxygen insertion process was postulated to stem from both ClO4− anions and molecular oxygen. Both mechanisms are discussed. In the presence of non-oxidizing electrolytes, such as HSO4− and acetate salts, mostly mono- and dihydroxy linear polysiloxanes and polysilanes, respectively, were generated. When CF3COO− (TFA) salt was utilized under “oxygen-free” and “water-free” conditions, in the dry box, the reaction became quite selective since the major products were cyclic siloxanes containing only one oxygen atom each. The source of oxygen is assumed to originate from the TFA electrolyte, and a plausible mechanism is suggested. The net effect of ring size was studied with cyclic silanes bearing the same substituents (IIb, IIIb, IVb, and V). Each derivative afforded a distinctive major five- or six-membered siloxane, containing either one or two oxygen atoms. In addition, whereas IIb and IIIb gave products with retention of the original number of silicon atoms, IVb and V gave also cyclic siloxanes with a loss of 1–3 Si atoms, due to fragmentation.