Cationic Hafnium Silyl Complexes and Their Enhanced Reactivity in σ-Bond Metathesis Processes with Si−H and C−H Bonds

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.