Catalytic Hydrosilylation of Ketones Using a Co/Zr Heterobimetallic Complex: Evidence for an Unusual Mechanism Involving Ketyl Radicals

The tris­(phosphino­amide)-linked
heterobimetallic Co/Zr complex
(THF)­Zr­(Mes­NPiPr2)3­CoN2 (1) has been investigated
as a catalyst for the hydrosilylation of ketones with PhSiH3. Catalytic activity superior to monometallic Co or Zr analogues
has been observed, demonstrating the importance of cooperative reactivity
between Co and Zr. Upon examining stoichiometric reactions, complex 1 was found to be unreactive toward PhSiH3, implying
that the mechanism diverges from the typical Chalk–Harrod-type
hydrosilylation pathway. In contrast, 1 reacts readily
with ketones, and in the case of benzophenone, a radical coupling
product [(Ph2CO)­Zr­(MesNPiPr2)3­CoN2]2 (3) was isolated, implying the intermediacy of a Zr-bound ketyl
radical fragment. A radical-based hydrosilylation mechanism is proposed
involving hydrogen atom transfer from PhSiH3 to the Zr-bound
ketyl-radical.