Bifunctional Mechanism with Unconventional Intermediates for the Hydrogenation of Ketones Catalyzed by an Iridium(III) Complex Containing an N-Heterocyclic Carbene with a Primary Amine Donor

Abstract

An alcohol-assisted outer-sphere bifunctional mechanism for the H2 hydrogenation of ketones catalyzed by an iridium system is presented on the basis of experimental and theoretical evidence. An iridium(III) complex containing an N-heterocyclic carbene (NHC) with a tethered primary amine donor (C–NH2), [IrCp*(C–NH2)Cl]PF6 (3; Cp* = pentamethylcyclopentadienyl ligand), when activated by an alkoxide base, catalyzed the H2 hydrogenation of acetophenone and benzophenone under 25 bar of H2 pressure at 50 °C, achieving a maximum turnover frequency (TOF) of 416 h–1. The presence of 2-propanol accelerates catalysis only when the alkoxide base is used in large excess with respect to iridium. This system has activity for ketone hydrogenation that is inferior to that of the structurally similar ruthenium(II) complex [RuCp*(C–NH2)py]PF6 (2; py = pyridine, TOF of up to 17 600 h–1 for the hydrogenation of acetophenone). On the other hand, an iridium(III) complex that contains a cyclopentadienyl ligand (Cp), [IrCp(C–NH2)Cl]PF6 (5), has activity that is superior to that of its Cp* analogue (3) in the catalytic H2 hydrogenation of acetophenone when activated by an alkoxide base, reaching a TOF of up to 687 h–1. Consistent with our previous computational studies, an iridium(III) hydride–amine complex, [IrCp*(C–NH2)H]PF6 (6), was isolated and was found to be inactive as a catalyst for ketone hydrogenation. The cationic charge is thought to contribute to a diminished hydricity and reactivity of the iridium(III) hydride in comparison to the neutral ruthenium(II) analogue. The important role of the N–H group is illustrated as well by the poor catalytic activity of the structurally similar iridium(III) complex [IrCp*(C–NMe2)Cl]PF6 (8), which does not contain an N–H group. Nevertheless, we present evidence that complex 3, in the presence of an excess alkoxide base, does efficiently hydrogenate ketones via an outer-sphere bifunctional mechanism involving the novel, neutral hydride intermediate Ir(η4-Cp*H)(C–NH2)H. The formation of this intermediate relies on the uncommon migration of a hydride ligand to the η5-Cp* ligand which appears to be promoted by the unique NHC ligand of the present system.