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 a-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(mu-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 a-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(mu-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 a-bond metathesis is evidenced by identical experimental rate laws (rate proportional to [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.