Early transition metal complexes in sigma-bond methathesis: The activation of silicon-hydrogen, silicon-carbon, and carbon-hydrogen bonds

Chapter 1. Enhanced activity in catalytic σ-bond metathesis reactions of PhSiH₃ was observed with zwitterionic hafnium hydride complexes. Neutral CpCp*HfHCl reacts slowly with PhSiH₃ to give oligomers, while zwitterionic CpCp*HfH(μ-H)B(C₆F ₅)₃ reacts ca. ten times faster, activating both Si–C and Si–H bonds of PhSiH₃, to give redistribution and dehydrocoupling products.; Chapter 2. Reactions of Cp₂Hf(SiR₃)Me (SiR₃ = Si(SiMe₃)₃, SitBuPh ₂, SiPh₃, SiHMes₂) with B(C₆F ₅)₃ quantitatively produced the corresponding cationic hafnium silyl complexes Cp₂Hf(SiR₃)(μ-Me)B(C₆F ₅)₃. The reactive species Cp₂Hf(η2 -SiHMes₂)(μ-Me)B(C₆F₅)₃ contains an unusual α-agostic Si-H interaction, and it reacts with benzene to form Cp₂Hf(Ph)(μ-Me)B(C₆F₅)₃ and Mes₂SiH₂. Kinetic data including activation parameters (ΔH‡ = 19(1) kcal/mol; ΔS‡ = −17(3) eu), a large primary isotope effect (k _H/k_D = 6.9(7)), and the rate law (rate = k[16][C₆H₆]) are consistent with a σ-bond metathesis mechanism for C–H bond activation.; Chapter 3. The scandium dihydrosilyl complex Cp* ₂SCSiH₂Mes was synthesized by addition of MesSiH₃ to pentane solutions of Cp*₂ScMe. Cp*₂ScSiH₂Mes reacts with methane to form a mixture of Cp*₂ScH, MesSiH₃ , and MesMeSiH₂. In contrast, reactions of Cp*₂ScMe with PhSiH₃ to form Cp*₂ScH and the product of Si–C bond formation, PhMeSiH₂. Reaction of Ph₂SiH₂ and CH₄ (∼150 atm) is catalyzed by Cp*₂ScH (80°C, cyclohexane) and gives 4 equiv of Ph₂MeSiH after 1 week (relative to Cp*₂ScH).; Chapter 4. The scandium alkyl Cp*₂ScCH ₂CMe₃ (2) was synthesized by the addition of a pentane solution of LiCH₂CMe₃ to Cp*₂ScCl at low temperature. Compound 2 reacts with the C–H bonds of hydrocarbons including methane, benzene, and cyclopropane to yield the corresponding hydrocarbyl complex and CMe₄. High selectivity towards methane activation suggested the participation of this chemistry in a catalytic hydromethylation, which was observed in the slow, Cp*₂ScMe-catalyzed addition of methane across the double bond of propene to form isobutane.