378791-41-4Relevant academic research and scientific papers
Enhanced reactivity of cationic vs neutral hafnocene complexes in stoichiometric and catalytic σ-bond metathesis reactions involving Si-H and Si-C bonds
Sadow, Aaron D.,Tilley, T. Don
, p. 4457 - 4459 (2001)
A comparison of the cation-like complex CpCp*HfH(μ-H)B(C6F5)3 with the neutral analogue CpCp*HfHCl reveals increased activity toward σ-bond metathesis for the more electrophilic, cationic complex. A catalytic transformation of PhSiH3 to Ph2SiH2 and SiH4 by CpCp*HfH(μ-H)B(C6F5)3 appears to occur by σ-bond metathesis of the Si-C bond, which occurs concurrently with dehydropolymerization to highly cross-linked polysilanes.
Cationic hafnium silyl complexes and their enhanced reactivity in σ-bond metathesis processes with Si-H and C-H bonds
Sadow, Aaron D.,Tilley, T. Don
, p. 9462 - 9475 (2007/10/03)
Reaction of the mixed-ring silyl methyl complex CpCp*Hf[Si(SiMe3)3]Me (4) with B(C6F5)3 in bromobenzene-d5 yielded the zwitterionic hafnium silyl complex [CpCpHfSi(SiMe3)3][MeB (C6F5)3] (7), which is stable for at least 12 h in solution. Addition of PhSiH3 to 7 rapidly produced HSi(SiMe3)3, CpCp*HfH(μ-H)B(C6F5)3, and oligomeric silane products. Reactions of CpCp*Hf(SiR3)Me (SiR3 = SitBuPh2, SiHMes2) with B(C6F5)3 rapidly produced HSiR3 in quantitative yield along with unidentified hafnium-containing species. However, reactions of Cp2Hf(SiR3)Me (SiR3 = Si(SiMe3)3 (8), SitBuPh2 (9), SiPh3 (10)) with B(C6F5)3 quantitatively produced the corresponding cationic hafnium silyl complexes 12-14. The complex Cp2Hf(SitBuPh2) (μ-Me)B(C6F5)3 (13) was isolated by crystallization from toluene at -30 °C and fully characterized, and its spectroscopic properties and crystal structure are compared to those of its neutral precursor 9. The σ-bond metathesis reaction of 13 with Mes2SiH2 yielded HSitBuPh2 and the reactive species Cp2Hf(η2-SiHMes2) (μ-Me)B(C6F5)3 (16, benzene-d6), which was also generated by reaction of Cp2Hf(SiMes2H)Me (11) with B(C6F5)3. Spectroscopic data provide evidence for an unusual α-agostic Si-H interaction in 16. At room temperature, 16 reacts with benzene to form Cp2Hf(Ph)(μ-Me)B(C6F5)3 (17), and with toluene to give isomers of Cp2Hf(C6H4Me) (μ-Me)B(C6F5)3 (18-20) and Cp2Hf(CH2Ph)(μ-Me)B (C6F5)3 (21). The reaction with benzene is first order in both 16 and benzene. Kinetic data including activation parameters (ΔH? = 19(1) kcal/mol; ΔS? = -17(3) eu), a large primary isotope effect (kH/kD = 6.9(7)), and the experimentally determined rate law are consistent with a mechanism involving a concerted transition state for C-H bond activation.
Enhanced reactivity of cationic hafnocene complexes toward σ-bond metathesis reactions. Si-H and Si-C bond activations in stoichiometric and catalytic organosilane conversions
Sadow, Aaron D.,Tilley, T. Don
, p. 3577 - 3585 (2008/10/08)
Interest in developing highly active catalysts for processes involving σ-bond metathesis steps led to a study directly comparing neutral, zwitterionic, and cationic derivatives of CpCp*Hf- in reactions with silanes. In stoichiometric transformations, comparisons of the reactivities of CpCp*HfMe2 (1), CpCp*HfMe(OTf) (2), CpCp*HfMe(μ-Me)B(C6F5)3 (3), and [CpCp*HfMe] [B(C6F5)4] (4) with organosilanes indicate that the cationic character in the complex influences the activity of the Hf-Me bond toward reactions with Si-H bonds. Enhanced activity in catalytic σ-bond metathesis reactions of PhSiH3 was also observed with zwitterionic hafnium hydride complexes. Neutral CpCp*HfHCl (7) reacts slowly with PhSiH3 to give oligomers, while zwitterionic CpCp*HfH(μ-H)B(C6F5) 3 (6) reacts ca. 10 times faster, activating both Si-C and Si-H bonds of PhSiH3, to give redistribution and dehydrocoupling products. A mechanism based on σ-bond metathesis is evidenced by identical experimental rate laws (rate ∝ [catalyst] [PhSiH3]2) for both the neutral and zwitterionic systems. The cationic complex 4 is an extremely reactive catalyst precursor and undergoes rapid redistribution and dehydropolymerization of PhSiH3 to give highly cross-linked insoluble materials of the type H-(PhSiH)n-(SiH)m-H.
