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

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.
A 2-propanol molecule acts as a proton shuttle in the heterolytic
splitting of the η2-H2 ligand on ruthenium
according to deuterium labeling and computational studies. The carbonyl
stretching wavenumbers of the complexes [RuCp*­(D–NH2)­CO]­X, including the new complexes [RuCp*­(C–NH2)­CO]­PF6 (4) and [RuCp*­(P–NH2)­CO]­PF6 (5; P–NH2 = 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–NH2)­py]­PF6 (3).
We conclude that complex 2 outperforms other ruthenium­(II)
NHC complexes and some other phosphine–amine complexes for the H2 hydrogenation of ketones due to the following factors: (1)
the ease of formation of a highly reactive neutral hydride, (2) the
presence of an NH2 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.