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(MesNP i Pr 2 ) 3 CoN 2 ( 1 ) has been investigated as a catalyst for the hydrosilylation of ketones with PhSiH 3 . 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 PhSiH 3, 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 [(Ph 2 CO)Zr(MesNP i Pr 2 ) 3 CoN 2 ] 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 PhSiH 3 to the Zr-bound ketyl-radical.