Enhanced Reactivity of Cationic Hafnocene Complexes toward σ-Bond Metathesis Reactions. Si−H and Si−C Bond Activations in Stoichiometric and Catalytic Organosilane Conversions

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.