Factors favoring efficient bifunctional catalysis. Study of a ruthenium(II) hydrogenation catalyst containing an N-heterocyclic carbene with a primary amine donor

摘要

An alcohol-assisted outer-sphere bifunctional mechanism is proposed for the H_2 hydrogenation of ketones catalyzed by the ruthenium(II) precatalyst [RuCp*(C-NH_2)py]PF_6 (2; Cp* = pentamethylcyclopentadienyl ligand, C-NH_2 = 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 H_2 hydrogenation of ketones at 25 °C and 8 bar of H_2 pressure in the presence of alcohols. Computational studies of a model neutral ruthenium(II) hydride amine complex, RuCp(C-NH_2)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-NH_2)H] ~+ is predicted to have a high barrier for hydride transfer to the ketone due to the poor nucleophilicity of this cationic hydride. A 2-propanol molecule acts as a proton shuttle in the heterolytic splitting of the η ~2-H_2 ligand on ruthenium according to deuterium labeling and computational studies. The carbonyl stretching wavenumbers of the complexes [RuCp*(D-NH_2)CO]X, including the new complexes [RuCp*(C-NH_2)CO]PF_6 (4) and [RuCp*(P-NH_2)CO]PF_6 (5; P-NH_2 =2-(diphenylphosphino) benzylamine), decrease as D is changed in the order phosphine, NHC, 2′-pyridine, amine. Thus, the NHC complex 2 is more electron rich and more active as a hydrogenation catalyst than the phosphine complex [RuCp*(P-NH_2)py]PF_6 (3). We conclude that complex 2 outperforms other ruthenium(II) NHC complexes and some other phosphine-amine complexes for the H_2 hydrogenation of ketones due to the following factors: (1) the ease of formation of a highly reactive neutral hydride, (2) the presence of an NH_2 donor ligand required for bifunctional catalysis, and (3) the presence of an NHC donor to achieve the right balance of hydricity of the metal hydride and acidity of the protic amine group for the efficient H ~+/H~- transfer to the polar double bond.