Mass Spectrometric Studies of Reductive Elimination from Si(IV) Anions

Reductive elimination (RE) is one of the most significant obstacles to developing main-group catalysts for bond coupling processes based on conventional oxidative addition/reductive elimination steps. Here, we report an electrospray ionisation mass spectrometry (ESI-MS) study demonstrating RE from Si^IV hydrosilicates [(tmim)SiH(OAr^R)]^- (tmimH₃ = tris(3-methylindol-2-yl)methane) generated in solution by oxidative addition of phenols (Ar^RO─H) to an anionic Si^II centre (silanide) incorporated in a bicyclic cage structure ([(tmim)Si]^-). The activation energy for the reductive elimination linearly decreased with the increasing electron-withdrawing character of the aryl substituents, suggesting that a negative charge develops in the para position in the rate-limiting step (RLS). In agreement with experiments, a DFT study supports an ionic mechanism in which the formation of an ion/molecule complex by transient weakening of the Si─O bond is the RLS, as would be intuitively expected from the developing negative charge in the para position. This is a rare example of direct observation of reductive elimination from Si^IV to Si^II and, to our knowledge, the first involving anionic species.