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

Abstract

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.