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-d(5) 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(mu-H)B(C6F5)(3), and oligomeric silane products. Reactions of CpCp*Hf(SiR3)Me (SiR3 = SiBuPh2, 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), Si'BuPh2 (9), SiPh3 (10)) with B(C6F5)(3) quantitatively produced the corresponding cationic hafnium silyl complexes 12-14. The complex Cp2Hf-(Si'BuPh2)(mu-Me)B(C6F5)(3) (13) was isolated by crystallization from toluene at -30 degreesC and fully characterized, and its spectroscopic properties and crystal structure are compared to those of its neutral precursor 9. The g-bond metathesis reaction of 13 with Mes(2)SiH(2) yielded HSi'BuPh2 and the reactive species Cp2Hf(eta(2)-SiHMes(2))(mu-Me)B(C6F5)(3) (16, benzene-d(6)), which was also generated by reaction of Cp2Hf(SiMeS2H)Me (11) with B(C6F5)(3). Spectroscopic data provide evidence for an unusual alpha-agostic Si-H interaction in 16. At room temperature, 16 reacts with benzene to form Cp2Hf(Ph)(mu-Me)B(C6F5)(3) (17), and with toluene to give isomers of Cp2Hf(C6H4Me)(mu-Me)B(C6F5)(3) (18-20) and Cp2Hf(CH2Ph)(mu-Me)B(C6F5)(3) (21). The reaction with benzene is first order in both 16 and benzene. Kinetic data including activation parameters (DeltaH(double dagger) = 19(1) kcal/mol; DeltaS(double dagger) = -17(3) eu), a large primary isotope effect (k(H)/k(D) = 6.9(7)), and the experimentally determined rate law are consistent with a mechanism involving a concerted transition state for C-H bond activation.