Activation of arene C-H bonds by a cationic hafnium silyl complex possessing an α-agostic Si-H interaction

The reaction of Cp2Hf(SiMes2H)Me (1) with B(C6F5)3 produces zwitterionic Cp2Hf(n2-SiHMes2)(μ-Me)B(C6F5)3 (2), which is stable for >8 h at -40 °C in toluene-d8. Spectroscopic data provide evidence for an unusual a-agostic Si-H interaction in 2. At room temperature, 2 reacts with the C-H bonds of aromatic hydrocarbons such as benzene and toluene to produce Cp2Hf(Ph)(μ-Me)B(C6F5)3 (3), isomers of Cp2Hf(C6H4Me)(μ-Me)B(C6F5)3 (4-6), and Cp2Hf(CH2Ph)(μ-Me)B(C6F5)3 (7), respectively. The reaction involving benzene is first-order in both 2 and benzene; rate = k[2][C6H6]. Mechanistic data including activation parameters (ΔH = 19(1) kcal/mol; ΔS = -17(3) eu), a large primary isotope effect of 6.9(7), and the experimentally determined rate law are consistent with a mechanism involving a concerted transition state for C-H bond activation. Copyright © 2002 American Chemical Society.