An Yttrium Hydride-Silane Complex as a Structural Model for a σ-Bond Metathesis Transition State

摘要

The transition-metal-promoted Si—H bond cleavage is a pivotal step in several important catalytic processes, such as hydrosilylation, hydrosilane dehydropolymerization, and dehydrogenative silylation. There are two different mechanisms for the metal-mediated Si—H bond cleavage: a) oxidative addition of Si—H bond toward a low-valent electron-rich metal, in which a σ-complex is believed to be the key intermediate; and b) σ-bond metathesis of Si—H and M—E bonds (E = C, N, H, etc.) via a four-center transition state (Scheme 1). As evidence for the oxidative addition mech- anism, σ-silane transition-metal complexes have been well documented. For d~0-transition-metal complexes, the oxidative addition is impossible, and the Si—H bond cleavage proceeds through the a-bond metathesis mechanism. However, to the best of our knowledge, there has been no report of such a d~0-transition-metal complex, which can be considered as a model for the σ-bond metathesis transition state. On the other hand, β-agostic interactions between metal ions and Si— H bonds are commonly observed in the coordinative unsaturated d~0-transition-metal complexes.