Factors Favoring Efficient Bifunctional Catalysis. Study of a Ruthenium(II) Hydrogenation Catalyst Containing an N-Heterocyclic Carbene with a Primary Amine Donor

Abstract

An alcohol-assisted outer-sphere bifunctional mechanism is proposed for the H2 hydrogenation of ketones catalyzed by the ruthenium(II) precatalyst [RuCp*(C–NH2)py]PF6 (2; Cp* = pentamethylcyclopentadienyl ligand, C–NH2 = N-heterocyclic carbene (NHC) with a tethered primary amine donor, py = pyridine) when activated by an alkoxide base. It has a high activity (turnover frequency (TOF) of up to 17 600 h–1) and selectivity for the H2 hydrogenation of ketones at 25 °C and 8 bar of H2 pressure in the presence of alcohols. Computational studies of a model neutral ruthenium(II) hydride amine complex, RuCp(C–NH2)H (Cp = cyclopentadienyl ligand), using density functional theory (DFT) methods reveal a low free energy barrier for the transfer of a proton/hydride couple to the ketone in the outer coordination sphere. In contrast, the related iridium(III) hydride amine complex [IrCp(C–NH2)H]+ is predicted to have a high barrier for hydride transfer to the ketone due to the poor nucleophilicity of this cationic hydride...