Analysis And Characterization Of Complex Mixtures Formed During Transition Metal-catalyzed Redistribution Reactions On Silicon.

Abstract

Small ring silicon-transition metal metallocycles previously were shown to catalyze the disproportionation of siloxanes. In this work, redistributions of the ligands bonded to the silicon moiety are shown to occur simultaneously with the disproportionation reactions. These redistributions are shown to arise from the breaking and reforming of Si-C, Si-H and Si-O bonds. Several transition metal complexes (Vaska's complex, Wilkinson's complex, (acac)Rh(C(,2)H(,4))) are shown to be catalyst precursors for this redistribution reaction. The presence of a hydrogen ligand on the silicon is shown to be necessary for the redistribution reaction to occur. A dinuclear mechanism is proposed to account for the product distribution found. Several other transition metal complexes are tested for activity as catalysts for the redistribution reaction. Pentamethyldisiloxane is shown to react with benzene in the presence of catalytic amounts of Vaska's complex to yield phenylsiloxanes. Extremely complex mixtures of silanes and siloxanes are generated during the redistribution reactions. These mixtures are characterized by GC/MS, IR, and NMR. The electron-impact (EIMS) and chemical-ionization mass spectra (CIMS) of linear dimethylsiloxanes are examined in detail. Ammonia is shown to be a better choice than methane as a reagent gas during the chemical-ionization mass spectrometry of the linear dimethylsiloxanes. A mechanism is proposed to explain the formation of the major ion series in the mass spectra (both EIMS and CIMS) of the linear siloxanes. This mechanism consists of loss from silicon of either a methyl radical or hydrogen atom followed by expulsion of small neutral silanes or cyclic neutral dimethylsiloxanes. An interesting polycyclic ion series is observed in the mass spectra of linear phenylmethylsiloxanes.