Specific Features of Catalytic Hydrosilylation in Siloxane Systems in the Presence of Sulfoxide-Containing Platinum(II) Complexes

The rate of addition of MeSiH(OSiMe₃)₂, Me₂SiHOSiMe₃, and (Me₂SiH)₂O to vinylsiloxanes ViMeSi(OSiMe₃)₂, ViMe₂SiOSiMe₃, and (ViMe₂Si)₂O in the presence of square-planar platinum complexes [Pt(LL')X₂] (where L and L' are neutral ligands, and X is an anionic ligand) decreases in the following series of L and L': Ph₃PS > MeS(O)Tol-p > MeCOD > CH₂ÍCH₂ COD > Et₂SO > Et₂S > Me₂SO 2-aminopyridine > Py > 2-methylpyridine. Variation of X, the ligands L and L' remaining unchanged, leads to decrease in the reaction rate in the series: C₂O₄^2- > NO₃^- > Cl^- Br^-. Mixed-ligand complexes like (-)-[Pt(MeSOTol-p)PyCl₂] having a cis structure are more efficient catalysts then the corresponding trans isomers in the hydrosilylation of siloxanes. Reactions of sulfoxide platinum(II) complexes with vinylsiloxanes and silicon hydrides result in isomerization of the metal complex and dissociation of the sulfoxide; bis-sulfoxide complexes undergo deoxygenation of the sulfoxide ligand with formation of colloidal platinum. It was presumed that the active form of the catalyst is its trans isomer; it reacts with silicon hydride, leading to replacement of the sulfoxide ligand in the inner sphere of platinum(II) complex. The reactivity of silicon hydrides toward vinylsiloxanes in the presence of sulfoxide platinum(II) complexes decreases in the series (Me₂SiH)₂O > Me₂SiHOSiMe₃ > MeHSi(OSiMe₃)₂, and the vinylsiloxanes form the following reactivity series: ViMe₂SiOSiMe₃ > ViMeSi(OSiMe₃)₂ > (ViSiMe₂)₂O. These series conform to increase of the positive charge on the silicon atom and decrease in steric hindrance created by the substituents thereon.