Reversible Silylium Transfer between P-H and Si-H Donors.

The Mo=PR2 π* orbital in a Mo phosphenium complex acts as acceptor in a new PIII -based Lewis superacid. This Lewis acid (LA) participates in electrophilic Si-H abstraction from E3 SiH to give a Mo-bound secondary phosphine ligand, Mo-PR2 H. The resulting Et3 Si+ ion remains associated with the Mo complex, stabilized by η1 -P-H donation, yet undergoes rapid exchange with an η1 -Si-H adduct of free silane in solution. The equilibrium between these two adducts presents an opportunity to assess the role of this new LA in catalytic reactions of silanes: is the LA acting as a catalyst or as an initiator? Preliminary results suggest that a cycle including the Mo-bound phosphine-silylium adduct dominates in the catalytic hydrosilylation of acetophenone, relative to a putative cycle involving the silane-silylium adduct or "free" silylium.