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

Abstract

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 > CH2ICH2 ≈ 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: C2O42- > 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.