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

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.