Substitution Reactions at a Bridging Silicon Ligand. Formation of a Bis(μ-silylene) Complex Containing a Trifluoroacetoxy Group. Mechanistic Studies of the Site-Exchange Process of the Hydride Ligands

Treatment of a bis(μ-diphenylsilyl) complex, {Cp*Ru(μ-η2-HSiPh2)}2(μ-H)(H) (2a, Cp* =
η5-C5Me5), with trifluoroacetic acid in toluene resulted in introduction of a trifluoroacetoxy
group on a bridging silicon atom with concomitant loss of one phenyl group as benzene,
affording a mixed-bridging bis(μ-silylene) complex, {Cp*Ru(μ-H)}2{μ-SiPh(OCOCF3)}(μ-SiPh2)
(3b), in moderate yield. Nucleophilic addition of the acetoxy group took place in the
intermediate μ-silyl,μ-silylene complex (Cp*Ru)2(μ-η2-HSiPh2)(μ-SiPh2)(μ-CF3CO2)(μ-H)(H)
(7), containing a bridging carboxylate group. The trifluoroacetoxy group of 3b underwent
nucleophilic displacement at the bridging silicon atom, which afforded the mixed-bridging
bis(μ-silylene) complexes {Cp*Ru(μ-H)}2(μ-SiPhX)(μ-SiPh2) (3c, X = OMe; 3d, X = OH) upon
treatment with methanol and aqueous KOH, respectively. VT NMR studies of 3b, 3c, 3d,
{Cp*Ru(μ-H)}2(μ-SiPhMe)(μ-SiMe2) (3e), and {Cp*Ru(μ-H)(μ-SiPhMe)}2 (3f-syn) revealed a
linear correlation between the electronic nature of the substituents on the bridging silicon
atom and the activation parameters of the hydride site-exchange process occurring in the
bis(μ-silylene) complex.