Si–H Bond Activation with Bullock’s Cationic Tungsten(II) Catalyst: CO as Cooperating Ligand

An in-depth investigation of the reaction of tertiary
hydrosilanes
with [CpW­(CO)2(IMes)]+[B­(C6F5)4]− reveals a fundamentally
new Si–H bond activation mode. Unlike the originally proposed
oxidative addition of the Si–H bond to the tungsten­(II) center,
there is strong experimental and NMR spectroscopic evidence for the
involvement of one of the CO ligands of the cationic complex in the
Si–H bond breaking event. The Si–H bond is heterolytically
cleaved to form a tungsten­(II) hydride and a silylium ion, which is
stabilized by one of the CO ligands. This reactive hydrosilane adduct
was eventually isolated and characterized by X-ray diffraction analysis.
Quantum-chemical calculations support a cooperative mechanism, but
a stepwise process consisting of oxidative addition and subsequent
tungsten-to-oxygen silyl migration in the tungsten­(IV) silyl hydride
is also energetically feasible. However, our combined spectroscopic
and computational analysis favors the cooperative pathway. The newly
identified hydrosilane adduct is the key intermediate of Bullock’s
ionic carbonyl hydrosilylation.